Abstract
Access to drinkable water is becoming more and more challenging due to worldwide pollution and the cost of water treatments. Water and wastewater treatment by adsorption on solid materials is usually cheap and effective in removing contaminants, yet classical adsorbents are not sustainable because they are derived from fossil fuels, and they can induce secondary pollution. Therefore, biological sorbents made of modern biomass are increasingly studied as promising alternatives. Indeed, such biosorbents utilize biological waste that would otherwise pollute water systems, and they promote the circular economy. Here we review biosorbents, magnetic sorbents, and other cost-effective sorbents with emphasis on preparation methods, adsorbents types, adsorption mechanisms, and regeneration of spent adsorbents. Biosorbents are prepared from a wide range of materials, including wood, bacteria, algae, herbaceous materials, agricultural waste, and animal waste. Commonly removed contaminants comprise dyes, heavy metals, radionuclides, pharmaceuticals, and personal care products. Preparation methods include coprecipitation, thermal decomposition, microwave irradiation, chemical reduction, micro-emulsion, and arc discharge. Adsorbents can be classified into activated carbon, biochar, lignocellulosic waste, clays, zeolites, peat, and humic soils. We detail adsorption isotherms and kinetics. Regeneration methods comprise thermal and chemical regeneration and supercritical fluid desorption. We also discuss exhausted adsorbent management and disposal. We found that agro-waste biosorbents can remove up to 68–100% of dyes, while wooden, herbaceous, bacterial, and marine-based biosorbents can remove up to 55–99% of heavy metals. Animal waste-based biosorbents can remove 1–99% of heavy metals. The average removal efficiency of modified biosorbents is around 90–95%, but some treatments, such as cross-linked beads, may negatively affect their efficiency.
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Introduction
Over the past few years, water pollution has become an increasingly urgent global issue due to industries discharging significant amounts of pollutants into water bodies. This situation poses substantial risks to human health and the environment, creating multiple treatment technologies. Among these methods, adsorption holds great promise as it involves removing contaminants from water by adhering them to an adsorbent material (Abdel Maksoud et al. 2022; Osman et al. 2022a). Due to their unique properties and cost-effectiveness, biosorbents and magnetic sorbents have emerged as cost-effective and environmentally friendly alternatives to conventional adsorbents (Abdel Maksoud et al. 2020). Biosorbents are derived from renewable sources, such as plant-based materials, animal waste, and marine biomass, and can be easily modified to increase their adsorption capacity, making them versatile and efficient for various pollutants (Crini et al. 2019a, b; Osman et al. 2020a). Meanwhile, magnetic sorbents are produced by incorporating magnetic nanoparticles into adsorbent materials, allowing easy separation from water using a magnetic field. This makes them ideal for continuous water treatment, as they can be recovered and reused multiple times.
This review aims to provide a comprehensive overview of recent advancements in biosorbents and magnetic sorbents for water treatment. The topics covered include preparation methods, adsorption isotherms, mechanisms, applications, and economic evaluation of these materials, as shown in Fig. 1. To the best of the authors' knowledge, this is the first study to critically evaluate adsorbents, with particular emphasis on biosorbents, magnetic sorbents, and other economical sorbents, from their preparation to their ultimate application. Furthermore, it provides a better understanding of adsorption mechanisms and facilitates the effective regeneration of contaminated adsorbents.
Main topics presented in this review article. Various materials can be used to prepare these sorbents cost-effectively. By utilizing these sorbents, improved pollutant removal can be achieved, resulting in a more efficient technology for water treatment. The sorbents can be regenerated and reused to maximize these approaches' cost-effective and sustainable benefits
Methods to prepare biosorbents
The industrial and technological progress of the past century has resulted in the release of substantial quantities of various organic and inorganic pollutants into the environment (Hosny et al. 2022; Mahmoud et al. 2022a). In order to remove these pollutants, different treatment techniques are continuously researched and implemented. Adsorption is considered one of the most effective techniques for removing pollutants due to its simplicity, low energy demand, and adaptability to various types of pollutants (Abdelfatah et al. 2021; Eltaweil et al. 2022a; Osman et al. 2023). However, the production cost of conventional synthetic adsorbents is prohibitively high, which limits their usage. This has increased the demand for natural adsorbents, also known as biosorbents (Salleh et al. 2011). Various materials can be utilized as biosorbents, including wood biomass, agro-waste such as crop and forest residue, and animal waste materials, e.g., fish scales, crab shells, and chicken feathers. The materials undergo various processes before being used for pollutant removal, as illustrated in Fig. 2. Subsequent sections will provide a detailed discussion of these materials.
Biosorbents from synthesis to application. The production and application of biosorbents involve several stages. First, the desired substrates are selected and subjected to washing and drying. Subsequently, physical techniques are employed to increase the surface area of the feedstock. This is followed by the adsorbent process, which can be carried out using pyrolysis, torrefaction, and gasification methods. The resulting biosorbent can remove contaminants from wastewater, producing purified water suitable for various applications
Preparation of wooden, herbaceous, bacterial, and marine-based biosorbents
Adopting the circular economy concept in the national development plans of many countries has facilitated the use of waste products for various purposes (Stjepanović et al. 2021). In the case of wood-based production, most waste wood biomass has been used to generate heat and electricity, replacing non-renewable sources with renewable, eco-friendly, and more sustainable alternatives (Uasuf and Becker 2011). However, a significant amount of wood biomass waste remains underutilized or disposed of inadequately, resulting in hazardous environmental issues (Stjepanović et al. 2021). Therefore, recent research articles have focused on utilizing wood biowaste in multiple applications, mainly as adsorbents for various pollutants.
In order to adsorb phosphates from wastewater, aspen wood particles were treated in two stages with carboxymethyl cellulose and ferrous chloride solutions (Eberhardt and Min 2008). The study's novel aspect was using carboxymethyl cellulose, a nontoxic anionic polymer, to create additional binding sites for iron ions, thereby increasing phosphate adsorption capacity. The wood particles were first ground, then treated with a carboxymethyl cellulose solution in deionized water, and left for several hours. They were then filtered via vacuum filtration and dried at 60 °C. The sample was subsequently soaked in an aqueous ferrous chloride solution, filtered, washed, and processed similarly. The positive effect of carboxymethyl cellulose was evident in the adsorption capacity of approximately 4.14 mg g−1, compared to approximately 2.78 mg g−1 for the wood sample treated solely with ferrous chloride.
Similarly, Yang et al. (2022) developed two wooden-based biosorbents for removing congo red and crystal violet using a one-step ball milling approach with Hickory wood feedstock. The biosorbents were synthesized via acidic and alkaline ball milling, while a neutrally-prepared biosorbent served as a control. In the synthesis process, 100 g of 6 mm agate spheres and 1 g of biomass feedstock were added to agate jars. Each jar was filled with 20 ml of sulfuric acid, sodium hydroxide, or deionized water (control). The jars were subjected to a 12-h ball-milling process at 300 rpm in ambient air, changing the rotation direction every three hours. After ball milling, the remaining biosorbents were rinsed with deionized water, dried for 12 h at 80 °C, ground, and stored in airtight containers. The resulting biosorbents exhibited high removal efficiencies of 87.9% for Congo red using the acidic biosorbent and 76.9% for crystal violet using the alkaline biosorbent, even after five regeneration cycles. The increased oxygen-containing functional groups and pore channels of the biosorbents due to the acidic/alkaline ball milling process resulted in electrostatic interaction, ion exchange, and surface complexation between the biosorbents and the polar dyes. It is worth noting that this low-cost biosorbent production method did not require extensive heat treatment, unlike the biochar preparation method.
Another study targeted the removal of crystal violet from water using a biosorbent prepared by the valorization of jujube shell wood (El Messaoudi et al. 2017). The jujube core was initially crushed to extract the shell, which was then processed using procedures similar to those used for the biosorbent mentioned earlier. To create the modified shell powder, the jujube shell was mixed with sodium hydroxide, stirred at room temperature for one day, filtered and rinsed with water, and finally dried and sieved similarly to the raw powder. The removal efficiency achieved using 0.2 g of raw shell powder was 95.84%, while only 0.1 g of the modified shell powder was required to achieve 98.16% removal efficiency. The modified powder was also found to have a significantly higher adsorption capacity than the raw powder, with a capacity of 288.18 mg g−1 compared to only 59.84 mg g−1 for the raw powder. This enhanced adsorption capacity was attributed to irregular cavities and carboxylate groups on the surface of the modified powder, which facilitated the adsorption of crystal violet molecules.
Chemical modifications used in current biosorbent synthesis methods can lead to secondary pollution, making it essential to search for eco-friendly and green alternatives (Zhang et al. 2012). In this concern, a recent study by Zhang et al. (2022b) investigated using an ultrasound-assisted and hydroalcoholic-freezing combination modification method for preparing a biosorbent made of waste peach wood branches to remove methylene blue. Initially, the peach wood branches were ground into powder (control sample), mixed with ethanol, stirred for a few minutes, sonicated in a water bath at 70 °C, centrifuged, and subjected to freeze-drying at − 60 °C. The resulting biosorbent had a porous structure with fewer impurities than the control sample, providing more adsorption sites and improving the overall removal efficiency to 94.91%. After removing impurities with an ultrasound-assisted hydroalcoholic cleaning process, the remaining cellulose, lignin, and hemicellulose were responsible for methylene blue adsorption via electrostatic, ion–dipole interactions, and π electrons transfer from the biosorbent to methylene blue. Additionally, the stability of this biosorbent was confirmed by observing a removal efficiency of 81.79% after five recycling times.
A green and cost-effective method for producing a highly efficient and easily recyclable biosorbent for removing nickel ions from wastewater was achieved through the esterification of paulownia wood using eco-friendly chemicals, phosphoric acid, and urea, commonly used in food and fertilizer production (Huo et al. 2022). The Paulownia wood was first made into chips, mixed with sodium hydroxide, and heated for one hour at 100 °C, followed by rinsing with hot deionized water to produce the control wood biosorbent. The chemically modified (phosphorylated) biosorbent was prepared by suspending the control biosorbent in water and mixing it with phosphoric acid and urea. The mixture was then dried at 80 °C for 12 h, rinsed with hot water, and left to dry in the open air. The positive effect of phosphorylation was evident in the significant increase in the adsorption capacity, from 18.5 mg g−1 for the unmodified biosorbent to 130.2 mg g−1 for the phosphorylated biosorbent. This increase was attributed to the complexation and electrostatic attraction between the negatively charged phosphate groups on the phosphorylated biosorbent surface and the positively charged nickel ions. The high adsorption capacity was also attributed to the protection and stabilization of the phosphorylated biosorbent achieved by urea.
In contrast to biosorbents prepared by Richards et al. (2019) from wood biomass, natural softwood chippings exhibited very low efficiency in removing copper and zinc ions from aqueous solutions, with only about 3% efficiency for both metals. This was due to the low cation release from the wood biomass in both metal solutions, resulting in a very low cation exchange rate. However, when using Fucus vesiculosus algal biomass as a biosorbent, the removal efficiencies were much higher, with 67% and 55% for copper and zinc ions, respectively. To prepare the unmodified biosorbent, the algae were collected, rinsed with water, air-dried, crushed, rewashed, and air-dried. A chemically treated algal biosorbent was produced to enhance the removal efficiency by shaking the dried algal powder with a calcium nitrate solution for one day, followed by rinsing with water and air drying.
Furthermore, modified algal biosorbents were also prepared for the same purpose. A chemically modified biosorbent was produced by treating the dried algal powder with a calcium nitrate solution for one day, then rinse it with water and air drying. A thermally modified biosorbent was also prepared by drying the crushed algae at 100 °C for two days after rinsing it with water. Surprisingly, both modified biosorbents showed lower efficiencies than the untreated biosorbent without any explanation provided by the authors. The chemically modified biosorbent exhibited 62% and 52% efficiency against copper and zinc ions, respectively, while the thermally modified biosorbent exhibited 59% and 42% efficiency against copper and zinc ions, respectively. The high efficiency of the untreated biosorbent was attributed to the presence of functional groups such as alcohols, carboxylic acids, esters, and amides that have a high affinity for both metals.
Various herbaceous materials have been investigated as efficient biosorbents in multiple research articles. One such material is Phytolacca americana L., which Wang et al. (2018a) used to remove lead ions (Pb2+) from aqueous solutions. To prepare the biosorbent, the plant material was processed similarly to the biosorbents discussed previously. A modified biosorbent was created by soaking the plant material in a diluted nitric acid solution and shaking it for six hours at room temperature. This was followed by washing to neutralize the pH and drying the modified biosorbent similarly to the raw biosorbent. The modification with nitric acid played a crucial role in improving the adsorption efficiency from 81.97 to 93.29% by increasing the surface porosity and creating cracks that served as additional adsorption sites for lead ions. The adsorption mechanism was electrostatic interaction and ion exchange through the hydroxyl, carboxyl, and amine functional groups.
Phragmites australis, also known as the common reed, is a cosmopolitan aquatic plant species that pose various environmental issues, including depletion of dissolved oxygen and reduction of biological diversity (El-Borady et al. 2021; Hosny et al. 2021). Nevertheless, due to its high cellulose and lignin content, it was utilized by Southichak et al. (2006) as a biosorbent to remove heavy metals, such as nickel ions. The biosorbent was prepared in the same way as previously mentioned. The raw biosorbent was then modified by treating it with sodium hydroxide, washing it with water to neutralize the pH, and oven-drying it at 90 °C. The adsorption efficiency of the raw biosorbent was only 14% against a nickel concentration of 1.1 ppm, but it significantly increased to 81% for the treated biosorbent. This improvement was primarily attributed to the enhanced surface area of the treated sorbent (20 m2 g−1), compared to only 3.49 m2 g−1 for the raw sorbent. Surprisingly, the removal efficiency of the treated biosorbent against a nickel concentration of 0.9 ppm increased from 92% in the first cycle to almost 99% in the second and third cycles. This behavior was attributed to the slight increase of the biosorbent's negative zeta potential after being recycled using hydrochloric acid, resulting in improved adsorption of positively charged nickel ions.
Shahnaz et al. (2020) conducted a study to remove hexavalent chromium using a chemically modified biosorbent made from Acacia auriculiformis. The plant material was processed like the previously mentioned biosorbents, then treated with sulfuric acid and pyrolyzed at 400 °C. The resulting powder was neutralized with a sodium bicarbonate and water mixture, dried, and mixed with ethylenediaminetetraacetic acid for two days at room temperature before being washed and dried. The modified biosorbent achieved a higher removal efficiency of 57.51% compared to 40.65% for the pristine biosorbent. This increase was attributed to the complexation of the pristine biosorbent with chelating agents, which increased the number of active functional groups, such as carboxyl and hydroxyl groups, that interacted with chromium ions.
Due to their high surface area and other positive properties, nanomaterials have found numerous applications. In a recent study, Herrera-Barros et al. (2020) investigated using alumina nanoparticles to modify an oil palm-based (bagasse) biosorbent to remove cadmium and nickel from wastewater. The preparation process involved shredding, rinsing, drying, and grinding the oil palm into fine pieces, followed by mixing the pristine powder with dimethyl sulfoxide, tetra ethyl-o-silicate, and alumina nanoparticles while stirring for 12 h. The biosorbent was then rinsed and dried at room temperature. However, the chemical modification of the bagasse biosorbent did not significantly improve its adsorption efficacy. While the removal efficiency increased slightly for cadmium and nickel, the improvement was insignificant enough to justify the modification process's additional cost and environmental impact. Therefore, it may be more cost-effective and environmentally friendly to rely on unmodified biosorbents.
To enhance the adsorption efficiency of biosorbents, surfactants can be utilized to reduce surface tension and increase the rate of ionic exchange. Bhatti et al. (2007) conducted a study using Moringa oleifera as a biosorbent to remove zinc ions from aqueous solutions. The plant was processed by being rinsed, dried, and ground into a fine powder before being mixed with three different surfactants: Triton, sodium dodecyl sulfate, and acetyl trimethyl ammonium bromide. After each surfactant was added and washed to reach a pH of approximately 7, the biosorbent was air-dried. The results showed that acetyl trimethyl ammonium bromide had the highest removal efficiency at 85.6%, followed by Triton (84.12%), sodium dodecyl sulfate (80.82%), and the unmodified biosorbent (74%). This was attributed to the high number of carbon atoms in acetyl trimethyl ammonium bromide, which created more positive sites for improved ionic exchange with zinc ions than other surfactants.
Yeast, including commercial dry baker's yeast (Saccharomyces cerevisiae), has been used by researchers as a category of biological material for preparing biosorbents. do Nascimento et al. (2019) utilized this yeast and successfully removed 76% of copper ions. Similarly, De Rossi et al. (2020) used the same yeast to remove hexavalent chromium ions from real wastewater. Encapsulating the yeast with alginate beads led to an efficacy of approximately 95%. To prepare the biosorbent, the yeast was first rinsed with water and mixed with a sodium alginate solution in phosphate buffer for 10 min. The mixture was then added to a calcium chloride solution and left for two hours for the capsules to form. These capsules were then rinsed and oven-dried at 50 °C for 24 h. The significant removal efficiency was attributed to the surface pores formed by the dried alginate beads.
Using the same biosorbents, baker’s yeast was treated with ethylenediamine tetra-acetic acid di-anhydride by Yu et al. (2008) to remove lead and copper ions. The preparation procedures involved two main steps; firstly, the baker’s yeast was mixed with a glutaraldehyde solution for 24 h at room temperature, rinsed, and subjected to freeze drying. Secondly, ethylenediamine tetra-acetic acid di-anhydride was mixed with a solution of N, N-dimethylacetamide containing yeast crosslinked biomass that was prepared in the first step. Subsequently, the mixture was washed to remove the unreacted reagent after being stirred for 4 h at 60 °C and freeze-dried. This treatment improved the active adsorption sites, leading to a removal efficiency of more than 90% for both metals through complexing these ions with ethylenediamine tetra-acetic acid of the modified biosorbent.
In another study, Chwastowski and Staroń (2022) investigated how modifying a biosorbent made from coconut fibers with Saccharomyces cerevisiae impacted the removal of lead ions. Initially, the fibers were rinsed and dried at 50 °C for 72 h. The researchers then mixed the fibers with sterilized yeast cells and left the mixture to incubate at 28 °C for 24 h. The study found no significant difference in the removal efficiency before and after the modification, with both the unmodified and modified biosorbents achieving a removal efficiency of 99.32% against a lead concentration of 1000 ppm. However, the modified version demonstrated a slightly improved efficiency of 99.87%, attributed to the accumulation of lead ions in the yeast cells. As the lead concentration increased, the efficiency of the modified biosorbent decreased and became lower than the unmodified version. This was due to the toxic effect of lead ions on the yeast cells, resulting in their death and the subsequent blocking of active adsorption sites.
Jalali et al. (2002) utilized Sargassum hystrix, a type of brown algae, as a source of biosorbents for removing lead ions. Marine biomass has become a popular and environmentally friendly option due to its simplicity and cost-effectiveness. The algal biomass was prepared conventionally by rinsing it with water, drying it under the sun, oven-drying it, and finally grinding it into powder. The study found that the obtained removal efficiency was 98%, and it is worth noting that the efficiency remained constant even after ten regeneration cycles. This high efficacy was attributed to the algal polysaccharides, particularly alginates, which contain carboxyl and sulfate functional groups that act as adsorption sites (Volesky 1994).
Several research studies have confirmed the effectiveness of modifying biosorbents, including marine-based ones, with surface functional groups like amine, carboxyl, and phosphate to enhance their removal efficacy (Jayakumar et al. 2021). For example, Oscillatoria princeps was modified using three different amine ligands; tetraethylene tetramine, para-amino benzamidine, and polydopamine to enhance the removal efficiency of reactive red 120 dye (Bayramoglu et al. 2022). The study found that the removal efficiency of the unmodified biosorbent was approximately 33.1% but significantly increased to 99.7%, 81.4%, and 59.8% after being modified with tetraethylene tetramine, para-amino benzamidine, and polydopamine, respectively. This increase in efficiency was attributed to the increased number of adsorption sites after modification and the electrostatic attraction between the amine groups on the aliphatic chains of the ligands and the dye's sulfonyl groups. The higher efficiency of the tetraethylene tetramine-treated biosorbent compared to the other modified biosorbents was due to its numerous primary and secondary amine groups. Overall, various biosorbents have been found to be effective in removing dyes, heavy metals, and other pollutants, as listed in Table 1, with algal biomass exhibiting the highest efficiency even after ten regeneration cycles.
Preparation of agricultural waste-based biosorbents
Due to their wide availability, eco-friendliness, and facile conversion to biosorbents by straightforward processes such as washing, drying, and grinding, agricultural waste materials are an efficient candidate for removing various organic and inorganic pollutants (Bhatnagar et al. 2015). The utilization of such a type of waste materials offers numerous positive aspects, such as minimizing the number of released waste materials, producing materials that could be of a high value as well as treating wastewater by removing organic and inorganic contaminants (Escudero et al. 2019; Peng et al. 2023). In this regard, peels of overripe Cucumis sativus, commonly known as cucumber, were used by Lee et al. (2016) to remove acid blue 113 dye. The biosorbent was conventionally prepared by washing the whole plant with water, followed by removing the plant's outer layer, cutting it into small pieces, oven drying, and finally, the peels were ground into fine powder. The obtained removal efficiency was 97.6%, as indicated in Table 2, at an acidic pH level of 2 due to the electrostatic attraction between the positively charged adsorption sites and the negatively charged sulfonate groups of the dye molecules.
Similarly, Ali (2018) used banana peels to support silicon dioxide nanoparticles to produce a nanocomposite of cellulose/silicon dioxide and employed this composite in the biosorption of methylene blue. Silicon dioxide nanoparticles were first synthesized through the alkaline hydrolysis of tetraethylorthosilicate in ethanol. Then the peel powder was similarly prepared to the abovementioned article. Subsequently, the powder was mixed with the nanoparticles solution and stirred for 16 h, dried at 80 °C, and eventually calcined at 550 °C for one hour. Upon characterizing both biosorbents, the surface area of the raw biosorbent was found to be 1.9 m2 g−1, whereas, for the modified biosorbent, the surface area significantly increased to 8.9 m2 g−1. Consequently, the adsorption capacity increased from 21 to 78.75 mg g−1 by the modified biosorbent. Also, the removal efficiency increased from 86.5 to 99.6% by increasing the pH from 4 to 10. Such a result indicated the quintessential role of the electrostatic attraction between the negatively charged biosorbent and the cationic dye molecules and the chemical adsorption.
Similarly, sugarcane bagasse was treated with phosphoric acid to produce a biosorbent for methyl red dye adsorption (Saad et al. 2010). The bagasse powder was added to a phosphoric acid solution and heated at 150 °C for one day, followed by water rinsing and overnight mixing with sodium carbonate to remove acid residues. Lastly, the mixture was neutralized by washing with deionized water, 24 h-drying at 150 °C, and ground into fine powder. This treatment increased the adsorption efficacy from 64.2 to 83.2%, which was attributed to the enhanced anion-exchange reaction between the dye and functional groups on the biosorbent surface.
Treating biosorbents with surfactants to enhance their removal performance of various organic and inorganic pollutants was indicated in previous research works, such as the one conducted by Karaman et al. (2022) on Congo red dye. The raw biosorbent was made of orange peel waste, similar to the abovementioned studies. The modified biosorbent was prepared in two steps; chemical activation accompanied by carbonization and modification with the surfactant cetyl trimethyl ammonium bromide. Firstly, the powder was magnetically stirred with zinc chloride solution and then dried at 110 °C, followed by a carbonization process in a muffle furnace at 500 °C for one hour. Secondly, the carbonized powder was rinsed with hydrochloric acid followed by water, oven-dried at 110 °C, mixed with the surfactant, shaken for one day, rinsed, and dried. The removal efficiency was boosted from 61% by the pristine biosorbent to almost 98% using the treated biosorbent. Such a significant difference was related to the enhancement of surface area from 102 to 1169 m2 g−1 and the improved pores' formation after the modification process. Also, van der Waals forces between the dye and the surfactant molecules were considered one of the main factors for increasing the adsorption efficacy.
One more study targeted the removal of eight different heavy metals by a chemically-modified biosorbent made of tangerine peels (Abdić et al. 2018). The peel powder was prepared using the same steps mentioned above and then mixed with nitric acid at the average room temperature for one day, followed by thorough washing to neutralize the pH and air-dried. Secondly, the powder underwent a further alkaline treatment by mixing the neutralized powder with sodium hydroxide for three to four hours and then processed in the same manner as the first step. The observed removal efficiencies using the modified biosorbent were 97.9%, 94.7%, 88.9%, 97.0%, 92.4%, 93.5%, 93.0%, and 96.8% for cadmium, cobalt, hexavalent chromium, copper, manganese, nickel, lead, and zinc, respectively. These removal efficiencies were about 40% higher than the unmodified biosorbent for all eight metals. Such an improved performance was accredited to the increased adsorption sites and the enhanced ionic exchange between the negatively charged functional groups in the cell wall of tangerine peels and the positively charged metal ions upon chemical modification.
Another sort of common agricultural waste material that could be harnessed in producing biosorbents for wastewater treatment is corn husk. In this line, a recent study (Rwiza et al. 2018) targeted the adsorption of lead ions using a thermochemically treated corn (Zea mays) husk biosorbent. Conventional experimental procedures, from rinsing and drying to grounding, were employed to prepare the raw corn husk biosorbent. Subsequently, biochar material was produced by pyrolyzing at 500 °C for 1 h. Afterwards, the biochar powder was chemically treated using two different reagents; firstly, mixed in an equal ratio with zinc chloride in water, followed by stirring, heating, filtration, and drying. The resulting powder was re-carbonized using the same conditions, removed excess zinc chloride by nitric acid, and finally washed to have a pH range of 5–6. Secondly, another chemically modified biosorbent was prepared similarly using potassium hydroxide. The observed removal efficiency for the raw biosorbent was about 82% but reached almost 100% using the two thermochemically biosorbents. Such a result was accredited to the improved chemical cation exchange because of the increased oxygen-containing functional groups, including carboxyl and hydroxyl groups (Gaskin et al. 2008), along with the enhanced adsorption sites after chemical treatment.
Fungi-based materials constitute another important type of biosorbent that showed high efficacy and well applicability in many research works, such as the one carried out by Contreras-Cortés et al. (2019) to adsorb copper ions on the surface of crosslinked composites beads containing copper-tolerant Aspergillus australensis biomass with an efficiency of 79%. The dead fungi powder was prepared in a broth containing potassium hydrogen phosphate, sodium nitrate, sucrose, yeast extract, and others. Subsequently, the broth was inoculated, and the produced biomass was filtered, rinsed, sterilized in an autoclave, dried, and eventually ground into fine powder. To prepare the chitosan beads, chitosan solution was mixed with the prepared fungal powder, then sodium alginate solution was added to synthesize the polymerization gel. Furthermore, the gel was mixed with a tripolyphosphate solution to formulate the crosslinked composite beads. A comparison between the unmodified composite beads and the crosslinked composite beads revealed higher removal efficiency for the unmodified beads since the crosslinking step blocked some active adsorption sites. However, the mechanical stability of the crosslinked beads was higher, particularly in the acidic medium, which is better for avoiding chitosan beads dissolution that could result in forming other secondary pollutants (Sánchez-Duarte et al. 2017).
Applying the ultrasound energy in modifying biosorbents is another option to produce highly efficient biosorbents, such as the ultrasound seaweed-based (Kappaphycus alvarezii) biosorbent synthesized by Kumar et al. (2022) to remove methylene blue. The seaweed powder was prepared similarly to the aforementioned raw biosorbents. Subsequently, the powder was mixed with ethanol and subjected to a probe-based ultrasound treatment at 30 °C with a variation in ultrasound power, solid–liquid ratio, and contact time. Such treatment created more cavities on the biosorbent surface and increased the surface area from 0.77 m2 g−1 to almost 1 m2 g−1, leading to complete removal efficiency (100%).
Forest waste materials are another category of eco-friendly biosorbents that attracted the researchers’ interest due to their wide availability, low cost, simple preparation techniques, facile modification, and high efficacy (Gemici et al. 2021). In this regard, Franco et al. (2021) investigated the removal of gentian violet dye using the pods of two forest species; Inga marginata and Tipuana tipu. The pods were rinsed separately and then dried at 60 °C inside a greenhouse for two days. Subsequently, the dried materials were ground, washed with ethanol followed by water, and ultimately dried one more time inside the greenhouse. Some negatively charged functional groups, including carbonyl and hydroxyl groups on the biosorbents’ surface led to the adsorption of the cationic dye molecules by 77.65% and 68.71% using the biosorbents of Inga marginata and Tipuana tipu, respectively.
Within the same biosorbents category, Razak et al. (2020) examined the adsorption of copper ions by a phosphoric acid-treated biosorbent based on Kenaf fiber. The pristine biosorbent was produced similarly to the abovementioned biosorbent. However, the pristine powder was mixed with dimethyl formamide solution for one day at room temperature to prepare the modified one. Subsequently, the solution was filtered, followed by adding urea in a fresh dimethyl formamide solution and phosphoric acid addition. Furthermore, the mixture was refluxed, filtered, and washed with ethyl alcohol, followed by water. Finally, the filtered powder was mixed with sodium hydroxide, rinsed with water, and dried. The observed adsorption capacity of the treated biosorbent was 25.2 mg g−1 with an efficiency of 88.2% compared to just about 15 mg g−1 using the untreated biosorbent when the target sample was real wastewater released from the electroplating industry. A substantial difference was attributed to the increased surface area from 78.38 to 150.82 m2 g−1, accompanied by an increased number of pores that serve as adsorption sites and the presence of oxygen-containing functional groups bind the target pollutant.
In conclusion, acid treatment is the most common modification technique for improving the efficacy of various biosorbents. Additionally, ultrasound and thermochemical treatments positively influence the performance of biosorbents. However, some treatment types might have a negative effect on the biosorbents' efficiency, such as using cross-linked beads. Such a conclusion could be supported or refuted by undertaking further research work.
Preparation of animal waste-based biosorbents
The industrial revolution, advanced medical care, overpopulation, economic growth, and welfare are all interrelated reasons behind the substantial food and animal consumption rates worldwide. Subsequently, it releases loads of animal waste into the environment, such as fish scales, crab shells, and chicken feathers. Therefore, it is essential to make the utmost use of these wastes by turning them into biosorbents and employing them in wastewater treatment.
In this regard, Ighalo and Eletta (2020) conducted experiments using biosorbents derived from fish scales to eliminate zinc and lead ions from aqueous solutions. Scaling scales involved a few simple steps, beginning with a thorough rinsing with water and then a detergent. Subsequently, the scales were mixed with nitric acid to remove metal oxides and other waste materials on the scales' surface and then washed with water. Lastly, the scales were oven-dried and ground into fine powder. The detected removal efficiencies for zinc and lead ions were 96.4% and 98.7%, respectively, as indicated in Table 3. Such a result was accredited to many adsorptive constituents, including esters and carboxylic acids.
Regarding applying the same sort of biosorbents in the remediation of sediments, Pal and Maiti (2020) investigated the adsorption of cadmium and lead using chemically, hydrothermally, and chemically hydrothermally biosorbents made from fish scales. The raw powder was prepared as mentioned above and then chemically modified using two different reagents, hydrochloric acid and sodium hydroxide, by simply mixing and stirring each one, followed by washing and drying. The powder was dissolved in deionized water for the hydrothermal treatment and heated for 3 h at 150 °C in a stainless-steel autoclave, followed by rinsing and oven drying. Furthermore, the raw powder was dissolved in hydrochloric acid instead of water and processed similarly to the hydrothermally treated biosorbent to produce the hydrothermally-acidic biosorbent. The observed efficiencies were 43.3%, 60.9%, 76.5, 77.4%, and 95.9% for cadmium ions using raw biosorbent, alkaline-treated, acid-treated, hydrothermally-treated, and hydrothermally-acidic treated biosorbents, respectively. While for lead ions, the obtained efficiencies were 51.2%, 65.3%, 78.8%, 83.3%, and 97.2% using the same order of biosorbents.
Continuing with the abovementioned study, the enhanced efficacies of all treated biosorbents indicated the positive effect of treatments in removing the inorganic and the organic materials blocking the active adsorption sites on the biosorbent surface. The chemical treatments were specifically responsible for blocking lipids and proteins, creating more pores, and increasing the surface area. Additionally, the hydrothermal treatment resulted in slightly better adsorption efficiencies attributed to the enhanced adsorption sites as a consequence of removing inorganic minerals from the biosorbent surface. As a result, it was expected that the combination of the acid and hydrothermal treatments would yield greater removal efficiencies for both metals.
Extracting several materials from biological waste materials and using them as biosorbents is another viable option by which a product with the added value could be produced from such waste. In this respect, Aziz et al. (2022) extracted hydroxyapatite from two different bio-waste materials, fish scales, and camel bones, and employed it to remove bisphenol A. Both scales and bones were firstly and separately processed using the abovementioned experimental procedures and then dissolved in hydrochloric acid, followed by adding sodium hydroxide solution while keeping the mixture heated and stirred for 12 h. Subsequently, white precipitates started settling out of the solution and were collected, washed, oven-dried, and ground. This heat treatment helped improve the porosity of both biosorbents allowing the bisphenol A molecules to get adsorbed on their surfaces. Therefore, efficient adsorption performance was detected with removal efficacies of 83.51% and 79.38% using scales-based and bone-based biosorbents, respectively.
Another class of animal waste materials that are released from the seafood industry, restaurants, and homes daily is crab shell waste. Crab shells contain various organic and inorganic constituents that help produce biosorbents with high adsorption efficacy. Therefore, many research studies were focused on investigating these waste materials, such as the one that targeted the adsorption of copper and lead using a crab shell-based biosorbent (Cancer pagurus) (Richards et al. 2019). The shells were rinsed with a surfactant, followed by water, and ground into fine powder. The observed removal efficacy of copper was 31%, while 18% was against zinc ions. The formation of coordination bonds between the metals and the amino acids and carboxyl functional groups on the biosorbent surface governed the removal of these ions. Moreover, the increased concentration of calcium ions released from the biosorbent surface into the solution indicated the ionic exchange mechanism. Also, it was concluded that chitin molecules, a major constituent of crab shells, acted as binding sites for the metal ions.
Concomitantly with the same biosorbents category, the removal of lead and zinc ions was targeted by the crab shells of Clistocoeloma sinensis (Zhou et al. 2016). Four different biosorbents were prepared from the same material, including a fresh biosorbing sample that was prepared via rinsing with water and air drying. Additionally, a portion of this sample was then ground to prepare the boiled biosorbent. At the same time, to produce the deproteinated biosorbent, the powder was mixed and stirred with sodium hydroxide for six hours. Lastly, the demineralized biosorbent was prepared by mixing the boiled one with hydrochloric acid at average room temperature for 48 h. The obtained removal efficiencies of lead ions were 99.95%, 99.90%, 99.99%, and 10.17% using the fresh, boiled, deproteinated, and demineralized biosorbents, respectively. At the same time, the efficiencies of these biosorbents in the same order against zinc ions were 99.72%, 99.70%, 99.89%, and 1.50%.
The abovementioned percentages indicated that the presence of minerals with a biosorbent structure is a quintessential factor in boosting the removal efficacy against both metals. The lowest effective biosorbent was the demineralized one. The primary mechanism for lead adsorption was the precipitation of lead ions on the surface of the biosorbent as lead carbonate with the aid of the dissolved calcium carbonate from the shells. At the same time, the chelation of zinc ions with carbonyl and hydroxyl functional groups, along with the coordination between zinc and carbonate components of the crab shell, was considered the main removal mechanism.
Extracting biopolymers from shrimp shells is another sustainable route for preparing biosorbents such as chitosan, extracted from shrimp shell waste material and used as a biosorbent to remove heavy metal ions (Mohanasrinivasan et al. 2014). Firstly, the shrimp shells were demineralized using hydrochloric acid, followed by acid removal using water. The second step was producing chitin via deproteinization, which was conducted by mixing the demineralized shells with 5% sodium hydroxide for one day at 90 °C and then drying. Subsequently, the chitin was subjected to deacetylation to produce chitosan by mixing with a higher concentrated sodium hydroxide (70%) under stirring for three days. Lastly, the pH of the residual solid was neutralized by washing with water and then filtering, drying, and crushing. The detected removal efficiencies were 98.97%, 86.15%, 65.2%, and 37.51% for copper, zinc, ferrous iron, and hexavalent chromium ions, respectively. This result was accredited to the main role of polysaccharides existing in chitosan as active adsorption sites for these metal ionic species.
In line with the abovementioned technique, chicken feathers constitute another renewable, widely available, and sustainable source of biopolymers and proteins, such as keratin (Tesfaye et al. 2017). Accordingly, a recent study used keratin to produce an efficient nano-biosorbent by modifying it using graphene oxide nanoparticles to remove several cationic and anionic heavy metals with efficacies of more than 90% (Zubair et al. 2022). The feathers were pretreated in a similar procedure to other types of biosorbents. Then they were stirred with ethylenediamine tetra-acetic acid, urea, tris-base, and sodium sulfite at 90 °C for two days. Subsequently, they were sonicated for half an hour to extract keratin. Secondly, keratin was precipitated with hydrochloric acid and mixed with graphene oxide, and then the precipitate was centrifuged, rinsed, dried, and ground into a fine powder biosorbent. Keratin has high stability due to its crosslinking structure, so many functional groups in its internal structure and side chains are unavailable for adsorbing pollutants. Consequently, they became more exposed and exfoliated upon modification by graphene oxide, which was in line with the substantial increase in the surface area of the modified biosorbent (19.50 m2 g−1) compared to the unmodified one (1.06 m2 g−1). Thus, the observed removal efficacies against both anionic and cationic metals were higher than 90%, as indicated in Table 3, whereas the efficacies of the unmodified one ranged from 20 to 70%.
Eggshell wastes are commonly produced in most of the world's countries regardless of their economic well-being. They represent another green, facile, and cost-effective biosorbent material, whether prepared in a pristine or modified form. Therefore, such a kind of waste material always attracts researchers' interest, such as Slimani et al. (2014), who examined the adsorption efficacy of physically treated egg shells by calcination in removing basic yellow 28 dye with an efficiency of 93.2%. The raw biosorbent was prepared by the same conventional experimental procedures used for most biosorbents, including washing, drying, and grinding. In contrast, the calcined biosorbent was prepared by heating the ground powder for two hours at 900 °C. The adsorption mechanism was found to be governed by van der Waals along with dipole forces by observing that the adsorption capacity decreased from 9 mg g−1 at 15 °C to 5 mg g−1 at 45 °C.
Mechanical ball milling is a commonly used technique for preparing various nanomaterials and nano-biosorbents. Mohammad et al. (2022) in preparing an eggshell-based biosorbent to remove toxic copper ions from water. The raw biosorbent material was prepared similarly to the previously mentioned biosorbents. In contrast, for the nano-biosorbent, the raw biosorbent powder was crushed into nano-sized powder using a ball milling machine. Although the obtained removal efficiencies were primarily comparable, as they were 91.36% and 97.21% for the raw and the mechanically treated biosorbents, respectively, the removal rate of the latter was about 17 times faster than the former. The main reason behind that difference was the high surface area of the modified biosorbent (21.2 m2 g−1) which was 1.6 times higher than the unmodified powder. The major driving force of biosorption was the electrostatic attraction between positive copper ions and negative biosorbent particles at an almost neutral pH (6).
This section summarizes the preparation methods of different biosorbents using various source materials, including wooden, bacterial, algal, and herbaceous materials. Additionally, agricultural wastes, fungi, and forest waste materials, as well as animal wastes (e.g., fish scales, camel bones, crab and shrimp shells, eggshells, and chicken feathers), as shown in Fig. 3, could be employed in the adsorption of several organic and inorganic pollutants. Such a fact is based on their availability worldwide, eco-friendliness, low cost, facile modification, and high adsorption efficacy. As shown in Fig. 4, numerous modification techniques could be applied to enhance the performance of biosorbents, and they are mainly categorized into chemical, physical, and mechanical techniques. Therefore, the main target of biosorbent preparation is two-fold: first, to use the unwanted biological waste materials that always end up being released into the environment and polluting water systems. The second target is to advocate circular economy principles and sustainable development by using pollutants (i.e., biosorbents) to remove other pollutants.
Categorization of common biosorbent materials. Biosorbent materials can be broadly categorized into woody and herbaceous-based biosorbents, crop residue and biomass-based biosorbents, and animal waste-based biosorbents. Examples of animal waste-based biosorbents include fish scales, eggshells, and chicken feathers. Among bacteria-based biosorbents, commercial dry baker's yeast (Saccharomyces cerevisiae) and blue-green algae-based biosorbents are commonly used. Biosorbents from agricultural waste, such as fruit peels, sugarcane bagasse, and corn husk, are also widely used
Classification of major modification techniques of biosorbents' preparation. Mechanical techniques such as ball milling are used—physical techniques involving ultrasound treatment and calcination. Chemical techniques include acid treatment, alkaline treatment, treatment with surfactants, and others. A combination of multiple treatment types is also applied in biosorbents' preparation
Methods to prepare magnetic sorbents
Due to magnetic sorbents' outstanding properties, sustained studies have been executed to foster their preparation approaches. Notably, the preparation method mainly controls the magnetic sorbents' shape, morphology, magnetic property, and particle size. Generally, the most applied approaches to synthesizing magnetic sorbents are coprecipitation, hydrothermal, thermal decomposition, polyol, microwave, sol–gel, and micro-emulsion, as revealed in Table 4 and Fig. 5.
Preparation approaches of magnetic sorbents. Magnetic sorbents could be prepared via coprecipitation using a precipitating agent. Further, the sol–gel method is a popular method for preparing magnetic sorbents. Heating in a reactor (hydrothermal process), microwave-assisted process, or direct thermal decomposition could be used to prepare magnetic sorbents. Other methods, such as micro-emulsion, could also prepare magnetic sorbents. M3+ and M2+ refer to the metal valences
Coprecipitation approach
Coprecipitation is one of the most straightforward conventional approaches that is widely utilized for fabricating magnetic sorbents. Typically, the approach is proceeded by mixing a divalent metal ions (M2+) precursor with a trivalent metal ions precursor (M3+) in fundamental conditions. Furthermore, the fabricated magnetic sorbents by the coprecipitation approach could be easily functionalized by adding functional substances to the reaction mixture. Notably, the magnetic nature, structure, particle size, and morphology of the synthesized magnetic sorbents by coprecipitation are controlled by some key parameters, including the type of metal salts, M2+/M3+ ratio, and the process temperature and pH (Wu et al. 2008). Also, the precipitating agent type impacts the magnetic sorbent specifications since the crystallinity and magnetic saturation of the fabricated particles by ammonium hydroxide are higher than those prepared by sodium hydroxide (Faraji et al. 2010). Coprecipitation possesses remarkable merits, such as producing a high yield, having a short reaction time, and utilizing water as an eco-friendly solvent.
On the other hand, a high base condition is one of the demerits of the coprecipitation method since such conditions decrease the uniformity of the produced magnetic particles. In addition, the low reproducibility is another disadvantage of the coprecipitation method (Husnain et al. 2017). The coprecipitation method is a simple, efficient, economical procedure for producing magnetic sorbents but lacks uniformity and reproducibility.
Hydrothermal approach
Hydrothermal is an approach for fabricating magnetic sorbents by chemical reactions in an autoclave. Hydrothermal needs specific conditions such as elevated temperature ranging from 150 to 200 °C and high pressure reaching 2000 psi (Lv et al. 2009). Noteworthy, the saturation magnetization of magnetic sorbents could be enhanced by adjusting the reaction temperature (Attallah et al. 2016). In addition, elevated temperature leads to fast nucleation and growth, resulting in the fabrication of small particles, which increase the surface area and the adsorption efficacy of the magnetic sorbents. Whereas the generated high pressure in the reactor could reduce the volatilization of the reactants as well as improve the purity of the produced particles (Sari et al. 2018).
Furthermore, the metal salt precursors significantly influence the size and morphology of the fabricated magnetic substances. Interestingly, the synthesized magnetic materials via the hydrothermal method have higher crystallinity and narrower size distribution than those manufactured by the other preparation approaches. The long hydrothermal time increases the size of the magnetic particles, while the short time produces uniform particles ranging from 10 to 50 nm (Xu and Teja 2008). Nevertheless, the hydrothermal approach has shortcomings, such as a slow reaction rate and high temperature and pressure requirements (Eltaweil et al. 2022b). In brief, the hydrothermal method needs high temperature and pressure, and it overcomes the metal salt effect on the produced size and morphology. Still, the reaction time should be well-adjusted to control the particle size.
Thermal decomposition approach
Thermal decomposition is one of the popular approaches for preparing monodispersed magnetic sorbents via the decomposition of organometallic precursors in boiling organic liquids (Duan et al. 2015). The size and morphology of produced magnetic particles by the thermal decomposition method depend on the ratio between metal precursor, surfactant, and solvent, in addition to the domination of the other factors, such as annealing temperature and reaction time on the structural and magnetically nature of the fabricated magnetic substances by thermal decomposition method (Patsula et al. 2016). However, thermal decomposition suffers the safety issue since it requires harsh conditions like boiling organic liquids at elevated temperatures and using vapor phases without air. Nevertheless, how metal oxides can be synthesized without oxygen is still questionable. Pioneering studies pointed out that the protons of water could oxidize the formed metal hydroxide to metal oxide (Gul et al. 2019). It could be concluded that thermal decomposition suffers extreme operating conditions; moreover, the produced magnetic particles' size and morphology depend on metal precursor ratios, surfactant, and solvent.
Microwave-assisted pyrolysis approach
Microwave-assisted pyrolysis involves using heat to shorten the preparation time, yield a high product, and enhance the adsorption property (Abdel Maksoud et al. 2021). Such an approach could heat the prepared substances without transferring the heat throughout the furnace, saving time and energy (López-Quintela et al. 2004). Notably, the characteristics of the fabricated magnetic adsorbents via microwave-assisted pyrolysis are controlled by the power of the microwave and the radiation time. The surface area of magnetic adsorbents can be ameliorated by raising the power of the microwave owing to the increase in the quantity of volatile matter (Vidal-Vidal et al. 2006). Furthermore, the increment in the radiation time and microwave power increase the pores' volume, enlarging the micro-pores, decreasing the micro-pores number, and increasing the total pores' volume (Chin and Yaacob 2007). More importantly, the microwave-assisted pyrolysis method has an advantage over the other preparation approaches, which is rapid crystallization since the microwave provides central heating to the reaction solution.
Chemical reduction approach
Chemical reduction is the simplest solution-phase approach to fabricating magnetic adsorbents, especially metal nanoparticles (Eltaweil et al. 2020). The process proceeds as follows; simple reduction to the metal precursor by reducing agent at room temperature. Chemical reduction distinguishes by its safety, simplicity, and ease of being performed in the laboratory (El-Monaem et al. 2021, 2022). Nevertheless, the metals could be oxidized during the fabrication via chemical reduction, which is the major disadvantage of this approach. To overcome this limitation, adjusting the ratio between metal precursor and reducing agent, utilizing freshly prepared reducing agent, performing the process quickly, and washing the fabricated metal particles well with an ethanolic solution is essential.
Polyol approach
Polyol is a liquid phase approach that proceeds by the divalent metal ions precursor (M2+)/trivalent metal ions precursor (M3+) oxidative alkaline hydrolysis in a polyol solvent. Although the particle size of the produced magnetic substances by the polyol method is poly-dispersed, the magnetism of the particles is pretty high. In addition, their unique morphology, "flower-like structure", renders them suitable for performed applications at high temperatures (Hugounenq et al. 2012). Interestingly, the increment in the precursors' concentration increases the particle size. Furthermore, the crystallinity of the particles is controlled by raising the reaction temperature (Yang et al. 2014). Advanced heating sources like ultrasonic and microwaves have revealed great results in the polyol process. However, the preparation of metal oxides by the polyol method is limited due to the production of elemental metals (Wee et al. 2017). Overall, the polyol method is a liquid-phase approach that can produce magnetic substances with high magnetism and a unique flower-like structure suitable for high-temperature applications. However, the method's limitations in producing metal oxides due to the production of elemental metals must be considered.
Micro-emulsion approach
Micro-emulsion method involves the fabrication of magnetic sorbents via the precipitation in an oil/water medium. Notably, the oil/water ratio could adjust the micro-emulsion medium since oil acts as a surfactant and water is a co-solvent. Micro-emulsion produces magnetic substances with unique advantages such as relatively small particles' size, uniformity, and superior magnetic property. Nonetheless, there is a restriction to applying the micro-emulsion approach in the laboratory owing to its low yield, high cost, and the need for large quantities of emulsifiers (Liu et al. 2020b). In conclusion, the micro-emulsion method is promising for producing magnetic sorbents with small particle sizes, uniformity, and superior magnetic properties. Still, its limited yield, high cost, and high emulsifier requirements restrict its applicability in the laboratory.
Arc discharge approach
The arc discharge is a common approach for preparing magnetic sorbent encapsulated into a carbon layer. The arc discharge is executed by placing the metal precursor into the cavities of the graphite electrode, and then arc discharge is applied to evaporate the solvents of the precursor (Ansari et al. 2022). The quality and quantity of the yielded magnetic sorbent by arc discharge are controlled by the process temperature, power supply, applied pressure, and electrode geometry (Arora and Sharma 2014). The arc discharge method possesses economic and environmental merits; efficient, cheap, eco-friendly, and nontoxic (Aljohani et al. 2021). However, this approach cannot be applied in the industry since controlling the magnetic particle size is impossible. Excessive studies have focused on overcoming this bottleneck to exploit the unique advantage of arc discharge.
Other preparation methods to prepare cost-effective sorbents
Nowadays, cost-effective sorbents prepared from industrial wastes have drawn particular attention due to their abundant resources, low cost, and excellent adsorption efficiency. Many industrial wastes that cost nothing have exhibited an auspicious adsorption behavior toward various contaminants. Interestingly, most of these industrial wastes could be used as sorbents after a simple treatment via cleaning with water and acid like dolochar, bagasse fly ash, and others. While there are wastes that need some processing before use as sorbents, as follows.
Cost-effective sorbents-derived cotton gin trash
Figure 6 represents the pretreatment approaches of cotton gin trash for material fabrication. Raw cotton gin trash consists of many fractions like sticks, leaf parts, and motes, so the first step involves milling these fractions to a powder (Haque et al. 2020). The milled cotton gin trash could also easily interact with chemicals such as solvents, polymers, and others, facilitating its surface modification. Then, the size of the milled cotton gin trash could be reduced by "mechanical processing", in which the speed of the shaft and impellers is relatively high in the presence of water as a solvent to form a slurry (Chranioti et al. 2016). Interestingly, the drying step impacts the morphology of the fabricated cotton gin trash since the produced cotton gin trash powders by spray drying are more spherical than those dried in an oven (Haque et al. 2021). Next, after the mechanical processing, "chemical processing" is sometimes conducted to facilitate the cotton gin trash properties. For example, the hydrophilicity of cotton gin trash could be improved by succinylation, while acetylation is performed to enhance cotton gin trash hydrophobicity. In addition, the mechanical strength of cotton gin trash is reinforced by polyvinyl alcohol. Also, "thermal processing" could be carried out to diminish the moisture content in cotton gin trash powder and increase the interaction with polymers. Notably, the thermal processing could be performed at low temperatures (50 °C) or high temperatures (150 °C), taking into consideration reduces the treatment time at high temperatures and vice versa (Holt et al. 2012; Sutivisedsak et al. 2012). In summary, various pretreatment methods, including milling, chemical and mechanical processing, drying, and thermal processing, can convert raw cotton gin trash into material suitable for fabrication.
Cotton gin trash consists of fractions like sticks, leaf parts, and motes. These parts should be milled into powder. Thermal processing is used to eliminate moisture content in cotton gin trash powder. Milled cotton gin trash is treated with chemicals through a chemical modification process. Polyvinyl alcohol could be used to enforce cotton gin trash mechanical strength
Cost-effective sorbents-derived red mud
Red mud is a rich resource for fabricating efficient adsorbents, but it needs excessive neutralization to reduce its dangerous environmental effects. Several neutralization or activation routes have been adopted to fabricate a cost-effective sorbents-based red mud, as clarified in Fig. 7.
Treatment approaches to fabricate cost-effective sorbents-derived red mud. Red mud is a good resource for the efficient adsorbent. Red mud needs excessive treatment to make it safe for use. Acid, microbial, gypsum, carbon dioxide, and seawater neutralizations could be employed for red mud before use. Thermal activation below 700 °C can lead to a higher crystallinity and an increase in the surface area of red mud
Seawater neutralization is a simple and popular route for red mud neutralization. This approach exploits the soluble ions into seawater (mainly calcium and magnesium) to neutralize the red mud via the precipitations of the hydrotalcite compounds and carbonate and hydroxide minerals (Kannan et al. 2021). The increase in the washing of red mud by seawater increases its surface area, reaching 31 m2 g−1, and decreases pH to ~ 8.5 (Rai et al. 2013). In brief, seawater neutralization is an effective and straightforward method for neutralizing red mud using soluble ions present in seawater.
Combined treatment involves the enhancement of the adsorbability of red mud via a series of treatment routes. For instance, in the first stage, red mud is treated by seawater (Bauxsol™), and the second stage involves the combined acid-thermal treatment of Bauxsol™. Finally, sulfate salts of ferric and aluminum are used for further treatment of Bauxsol™ (Taneez and Hurel 2019). It was found that the combined treatment route significantly enhances the surface area of red mud, reaching about 130 m2 g−1 (Sahu et al. 2013). It could be concluded that the combined treatment route can significantly increase the surface area of red mud through a series of treatment routes involving seawater, acid-thermal treatment, and sulfate salts of ferric and aluminum.
Thermal activation means the treatment of red mud at an elevated temperature ranging from 200 to 1000 °C for a pyrolysis time in the range of 1–3 h. The X-ray diffraction analysis revealed hematite, bayerite, gibbsite sodalite, quartz, calcite, and titanium dioxide crystalline phases. In addition, the crystallinity of hematite enhanced with the raising in pyrolysis temperature above 600 °C. Moreover, the surface area of the red mud improved at 500 °C, which is most likely due to the water exclusion. Nonetheless, the adsorption performance declined when the pyrolysis temperature exceeded 700 °C (Smiljanić et al. 2011; Taneez and Hurel 2019). To summarize, the thermal activation of red mud at elevated temperatures can lead to the formation of various crystalline phases and an increase in the surface area of red mud. Still, the adsorption performance is negatively impacted at temperatures above 700 °C.
Acid treatment includes neutralizing red mud with acids such as sulphuric, nitric, and hydrochloric acid to remove the alkali salts and inorganic and organic impurities. The acid neutralization of red mud can be executed via two approaches; the first is simple treatment by acid (0.1–1.0 M), followed by washing with distilled water and drying at 100 °C. At the same time, the second approach involves the reflux of red mud in an acidic solution for 1–2 h and then precipitating the red mud in an ammonia solution. Finally, the precipitated red mud was washed with distilled water and dried at 100 °C (Joseph et al. 2019; Qi 2021). Generally, acid treatment is an effective method to remove impurities from red mud. There are two approaches to achieving acid neutralization, the simple treatment and reflux method, followed by washing and drying.
Adsorbent classifications and their wastewater treatment applications
The sorption application and uptake efficiency are often a function of a given adsorbent's class and physicochemical features. Accordingly, these adsorbents vary in selectivity, adsorption capacity, shelf life, and synthesis cost (Syeda and Yap 2022). Also, there are obvious distinctions in their respective active surface, pore diameters, quality of pore distribution, and surface functional groups. The following subsection discusses applying an array of adsorbents in the adsorption of varying pollutants. The different adsorbents used in wastewater treatment were broadly classified as activated carbon, biochars, lignocellulosic biomass, clay minerals, silica, zeolite, and peat and humic soil (Chen et al. 2019b; Osman et al. 2020b, 2022b).
Similarly, the common pollutants encountered in wastewater treatment were also classified. Figure 8 presents the different classes of adsorbents and pollutants considered in this review. The applicational benefits and drawbacks of the different adsorbent classes considered in this review are outlined in Table 5.
Broad classification of adsorbents and pollutants. The adsorption capacity and ultimate success of the adsorption process depend on the degree of adsorbent-adsorbate compatibility. The respective physicochemical properties and the structural/ionic composition of the adsorbent are significant for establishing the desired adsorbent-adsorbate affinity
Activated carbon adsorbents
Activated carbons are porous carbon materials produced by carbonizing chemically dehydrated carbonaceous materials or by oxidizing chars (Heidarinejad et al. 2020). Both fossil and renewable carbonaceous materials serve as activated carbon precursors. Other non-conventional activated carbon precursors, such as ionic liquids (Zhang et al. 2014a), organic salts (Xu et al. 2012), and deep eutectic solvents (Iwanow et al. 2017, 2020) still exist. To obtain high-quality and low-cost activated carbon adsorbents, non-fossil and renewable materials with high carbon and volatile components and low ash content are recommended. However, coal-based activated carbons exhibit superior mechanical properties to those from lignocellulosic materials.
Dyes are one of the most typical aquatic pollutants successfully adsorbed using activated carbon and are often classified following their chemical structure and industrial application (Liu et al. 2022b). The adsorption of different classes of dye, including cationic (Ahmed 2016; Corda and Kini 2018), dispersed (Mittal et al. 2022; Shukla and Dhiman 2017), reactive (Giannakoudakis et al. 2016; Silva et al. 2016), direct (Ho 2020; İmran et al. 2012), vat (Ho 2020; Nagy 2018), and acid (Biglari 2017; Naraghi et al. 2017) dyes onto activated carbon have been extensively studied. However, it was noted that irrespective of the dye class, efficient adsorption is always a function of high surface area. The larger the surface area, the more available active sites for binding dye molecules. The weak adsorptive interaction between the π-electrons of the activated carbon basal planes and the loose electrons of the aromatic rings of the dye molecule also accounts for the efficient dye adsorption (Chatterjee et al. 2005; Giannakoudakis et al. 2016). Thus, efficient adsorption of dye molecules onto activated carbon is significantly influenced by the surface area present for physisorption and active sites for chemisorption.
Lead (Largitte et al. 2016; Shi et al. 2018), chromium (Ugwu and Agunwamba 2020; Yang et al. 2015), copper (Natrayan et al. 2022; Ugwu and Agunwamba 2020), zinc (Tuomikoski et al. 2019; Ugwu and Agunwamba 2020), cobalt (Kakavandi et al. 2018; Peres et al. 2018), manganese (Tran et al. 2018), iron (Goher et al. 2015), and arsenic (Yin et al. 2019) have been reportedly adsorbed using activated carbon. However, for improved adsorption efficiency, the adsorbate solution pH, the activated carbon surface charge, textural characteristics, and surface chemistry should be considered (Li et al. 2010; Liu et al. 2022c). It was noted that when the activated carbon surface is composed mainly of alkaline/basic groups, the point of zero charges (pHpzc) value is expected to be higher than pH 7.0. In such a situation, electrostatic interaction and exchange of ions between the negative activated carbon surface groups, π–electrons of the activated carbon, and the heavy metal cations are realized at the acidic pH ranges (Aniagor et al. 2022). Similarly, in the presence of abundant acidic surface groups, the pHpzc is less than pH 7.0, and efficient cation adsorption via an electrostatic interaction mechanism is achieved when the solution pH tends to the alkaline pH ranges (Hashem et al. 2021b). Also, the issue of ionic speciation and precipitation strongly affects the effective adsorption of heavy metals, and to circumvent such a limitation, the appropriate solution pH must be adopted. In conclusion, the heavy metals' adsorption onto activated carbon is a function of the adsorbents' surface area, pore volume, and the number of surface groups.
The effectiveness of activated carbon for adsorbing different pharmaceuticals and personal care products (PPCPs) (Bursztyn Fuentes et al. 2022; El Naga et al. 2019; García-Rosero et al. 2022; Kim et al. 2022) and other emerging pollutants (Ahn et al. 2022; Moreno-Marenco et al. 2020) have been documented. The adsorption generally occurs via pore-filling/size exclusion, hydrogen bonding, π–π electro-donor acceptor, electrostatic interactions, and hydrophobic interaction mechanism (Ahn et al. 2022; Liu et al. 2022a; Pamphile et al. 2019). Accordingly, the pore-filling mechanism is a function of the adsorbate’s molecular size and the adsorbent pore properties (Moreno-Marenco et al. 2020). Aside from the pore-filling, all the mechanisms mentioned above are controlled by the activated carbon’s surface charge, acid dissociation constant (pKa) of the organic pollutants, and pH variation within the bulk adsorbate phase (Kim et al. 2022). During the comparative adsorption of caffeine, ibuprofen, and triclosan, due to the size exclusion mechanism, a higher adsorption capacity was recorded when there is a high correlation between the molecular size of the adsorbate and pore diameter of the activated carbon (Kaur et al. 2018).
Furthermore, El Naga et al. (2019) noted that the phenolic and carboxylic acid groups on the activated carbon, respectively, donated hydrogen bonds to the oxygen atoms in diclofenac molecules up to pH ∼ 10 and pH ∼ 4.9, hence the improved uptake via hydrogen bonding. Also, strong electron acceptor groups like the chlorine atoms in most PPCPs and other emerging organic pollutants lower the π-electron density on their respective phenyl ring, thus improving π–π interactions with the activated carbon surface (El Naga et al. 2019). Therefore, the sorption efficiency of the PPCPs and other emerging organic pollutants on activated carbons depends on the solution pH, the adsorbate's molecular features, and the nature of the mutual adsorbate-adsorbent interaction, as well as the surface and pore properties of the activated carbon.
Activated carbon adsorbent has been applied for the adsorption of different radionuclides such as cobalt-60 (Hamed et al. 2016), europium (Hamed et al. 2016; Moloukhia et al. 2016), cesium (Khandaker et al. 2021; Moloukhia et al. 2016), radon (Karunakara et al. 2015), thoron (Karunakara et al. 2015), uranium (El-Magied et al. 2021; Nezhad et al. 2021), radioactive iodine (Chien et al. 2011; Li et al. 2014), selenium (Elgazzar et al. 2020), molybdenum (Elgazzar et al. 2020), strontium (Elgazzar et al. 2020; Moloukhia et al. 2016), lanthanum (Elgazzar et al. 2020), and technetium-99 (Li et al. 2014). All the studies highlight the importance of oxygen-based functional groups and activated carbon π-electrons. Consequently, the radionuclides could either form complexes/cation exchange with the oxygen-based groups (Khandaker et al. 2021), interact with aromatic π-electrons of the activated carbon basal planes (Cho et al. 2020), or physically bind onto the activated carbon pores (Khandaker et al. 2018). During the adsorption of uranium ions, Nezhad et al. (2021) and Rout et al. (2016) noted that improved adsorption is achieved at decreasing solution acidity since the electron lone pair from the negatively charged oxygen-based groups form complexes with empty uranium ions orbitals. Therefore, it is concluded that highly porous activated carbon with either positive or negative oxygen-containing surface groups can efficiently adsorb arrays of radionuclides from wastewater.
In conclusion, adsorption onto activated carbon is a function of the adsorbents' surface area, pore volume, and the number of surface groups, irrespective of the pollutant class. Furthermore, the weak adsorptive interaction between the π-electrons of the activated carbon basal planes and the loose electrons of the aromatic rings also accounts for the efficient adsorption of organic pollutants.
Biochar
The term “char” refers to the residue obtained from the pyrolysis of carbonaceous materials (Armah et al. 2022), while biochar strictly implies the solid products from the pyrolysis of agro and animal waste materials (Chen et al. 2019c). Biochar is distinguished from activated carbon because of its low pyrolysis temperature requirement, which is always below 700 °C, and the absence of any activation step (Ighalo et al. 2022a). The high porosity of biochar is comparable to that of activated carbon, but biochars also have a high ion exchange capacity compared to activated carbons. Such high biochar ion exchange capacity is linked to residual carboxylic, phenolic, hydroxyl, and carbonyl groups on the graphitic backbone (Zhao et al. 2021). Meanwhile, the percentage composition of each functional group on the biochar varies as the precursor material and thermochemical conversion technique are adopted (Amin et al. 2016). Thus, the creation of efficient and affordable adsorbents for wastewater decontamination is greatly aided by the physicochemical and porous features of biochar.
The application of biochar for azo (Gurav et al. 2021; Guy et al. 2022), reactive (Acemioğlu 2022; Muralikrishnan and Jodhi 2020), vat (Gupta et al. 2022; Vigneshwaran et al. 2021a), basic (Praveen et al. 2021; Silva et al. 2021), disperse (Das et al. 2021), and acid (Gao et al. 2021; Jose et al. 2022) dye adsorption has been reported in the literature. Notably, dye adsorption onto biochar generally proceeds via physical interaction (Guy et al. 2022), ion exchange (Goswami et al. 2016), and electrostatic interaction (Nguyen et al. 2021) mechanisms. The physical interaction mechanisms include pore filling, hydrophobic effect, π–π interactions, and n–π interactions. The pore-filling mechanism is always facilitated and favored by biochar's enhanced surface area and pore volume (Goswami et al. 2016). Similarly, the abundant carbonyl, hydroxyl, carboxyl, phenyl, and amine functional groups on the biochar are advantageous for π–π interactions with the aromatic rings of the different dye molecules (dos Reis et al. 2021). Siddiqui et al. (2019) reported efficient adsorption via a hydrogen bond donor–acceptor interaction between the hydroxyl group of modified biochar and the hydrogen bond receptor present in the methylene blue molecules. Also, non-covalent π–π and n–π interactions were reported between the aromatic rings of biochar and the Evans blue dye (Guy et al. 2022). Electrostatic attraction and ions exchange mechanism was noted as important sorption mechanisms during the adsorption of methylene blue dye (Pirbazari et al. 2014; Yang et al. 2019a). Consequently, electrostatic interaction predominates when the dye molecules and biochar surface contain opposite charges, while ion exchange mechanism involves ionic exchange between the surface groups of the biochar and the charges on the dye molecules. In conclusion, dye adsorption onto biochar occurs significantly through the hydrophobic effect, electrostatic attraction, hydrogen bonding, and π–π interaction. However, for biochar with highly developed porosity and specific surface area, efficient dye adsorption is mostly via a pore-filling mechanism.
Heavy metal adsorption onto modified and unmodified biochar has been successfully demonstrated (Amabilis-Sosa et al. 2022; Li et al. 2022b; Sun et al. 2022). Several properties, such as elemental distribution, surface area, and surface charge, have been identified as key factors influencing adsorption (Bai et al. 2023; Liu et al. 2023). Ion exchange, surface complexation, electrostatic interactions, precipitation, and π–π interaction mechanisms are some main processes during cation adsorption. According to Chu and Nguyen (2023), the adsorption of hexavalent chromium onto magnetic biochar involves ion exchange, surface complexation, and redox reactions. Consequently, the sorption mechanism mentioned above is possible following the involvement of the biochar's carboxyl, hydroxy, and carbonyl groups. Qi et al. (2023) also affirmed that the surface complexation involving the oxygen-based functional groups of the biochar was critical to the successful adsorption of lead, cadmium, and copper ions. Conversely, the influence of pore-filling and electrostatic attraction between ionic copper species and the surface of magnetic biochar have been reported (Sun et al. 2023b). Therefore, biochar with improved porosity, specific surface area, and large oxygen group amounts exhibit high adsorption capacity for heavy metal species.
PPCPs and other emerging organic pollutants are adsorbed onto modified and non-modified biochar via chemical and physical adsorption (Nayyar et al. 2022; Zhang et al. 2022a). Chemisorption interaction occurs via electrostatic interactions and ion exchange, while physisorption occurs via pore-filling, π–π interaction, and other weak interaction mechanisms (Keerthanan et al. 2020; Liu et al. 2022a). Choudhary and Philip (2022) ruled out the role of electrostatic interaction during the sorption of methylparaben, carbamazepine, ibuprofen, and triclosan onto biochar. Instead, the predominance of non-electrostatic interaction (such as hydrogen bonding, electron donor–acceptor, and hydrophobic interactions) that is driven by the hydrophobic and hydrophilic moieties on the respective PPCPs, was proposed. Mayakaduwa et al. (2017) reported the role of chemisorption and physisorption interaction during the adsorption of carbofuran onto biochar. The study further noted that during chemisorption, the carbonyl groups interact with the nitrogen atoms of the carbofuran molecule via a nucleophilic addition reaction.
Similarly, the hydroxyl and carbonyl groups of the biochar can also interact with the heterocyclic ring of the carbofuran via π–π and hydrogen bonding-driven physisorption. The occurrence of an n–π interaction between the nucleophile-like oxygen groups on the biochar and π electron cloud of caffeine molecules was reported by Keerthanan et al. (2020). Thus, it is concluded that high π-electron density on the biochar and abundant electrophilic groups in the PPCPs makes for efficient PPCPs uptake.
The adsorption of different radionuclides such as europium (Frišták et al. 2017), uranium (Ahmed et al. 2021a, b; Guilhen et al. 2019), and thorium (Chen et al. 2019d) onto raw and modified biochar has been reported. The studies generally highlighted the involvement of chemisorption, electrostatic interaction, and surface complexation reaction mechanisms. Also, oxygen-containing functional groups' significance in facilitating the abovementioned mechanisms was identified. For improved adsorption of radionuclides, different authors (Li et al. 2019b; Liu et al. 2021b) incorporated iron and sulfur into their biochar samples. It is believed that the iron and sulfur species promoted the reduction of most radionuclides, especially uranium, and enhanced their removal from wastewater. Ahmed et al. (2021a) and Li et al. (2019b) applied magnetic biochar for uranium adsorption. They suggested the possibility of surface complexation between the oxygen-based groups, such as the hydroxyl, carbonyl, iron oxide, and uranium molecules. Based on spectroscopic evidence, Ahmed et al. (2021b) also verified the binding of uranyl ions on surface groups of oxidized biochar.
In conclusion, the creation of efficient and affordable biochar for wastewater decontamination is greatly aided by the physicochemical and porous features of the adsorbent. The high porosity of biochar is comparable to that of activated carbon, but biochars also have a high ion exchange capacity compared to activated carbons. Such high ion exchange capacity of biochar is linked to residual carboxylic, phenolic, hydroxyl, and carbonyl groups on the graphitic backbone.
Lignocellulosic wastes
This class of adsorbent is generally sourced from plant materials and by-products of agro-material processing in various industries (Syeda et al. 2022). Many authors have previously developed efficient lignocellulosic adsorbents from tree bark, fruit peels, seeds, husk, and leaves. According to Aniagor et al. (2022), these lignocellulosic wastes are generated in large quantities, and their disposal is often challenging. Consequently, successfully utilizing these lignocellulosic wastes as adsorbents in wastewater treatment offers the advantage of cost-effectiveness and mitigates the environmental problems associated with their indiscriminate disposal. The high presence of relevant organic functional groups at the surface of these agricultural wastes makes them efficient in adsorbing varying classes of pollutants (Wakkel et al. 2019).
The adsorption potentials of the raw lignocellulosic wastes have been previously investigated (Hashem et al. 2022b; Ighalo et al. 2022b). However, it has been reported that this adsorbent class's functional group and sorption site could be greatly enhanced through surface chemical functionalization and other pre-treatment methods (Hashem et al. 2021a). Cavalcante et al. (2022) utilized grape winery waste functionalized with 3-aminopropyl tri ethoxy silane as an adsorbent for methyl orange dye uptake. The study noted the positive impact of chemical grafting on the adsorption performance of the modified material, as significant electrostatic interactions occurred between the ammonium group of the grafted adsorbent and the sulfur trioxide group on methyl orange dye species. Asides from electrostatic interaction, van der Waals, π−π interactions, and hydrogen bonds between the remaining functional groups of the dye molecules and the modified adsorbent were also reported. Other biomasses such as Dodonaea viscosa (Hopbush) plant, apricot (Prunus armeniaca L.) seed shell wastes, lupine seed powder, Lepidium sativum seed powder and olive waste have been respectively utilized in the adsorption of methyl red (Gul et al. 2022), acid blue 193 (Hashem et al. 2022a, c), acid orange 142 (Hashem et al. 2022b) and methylene blue (Ferkous et al. 2022) dyes. Incidentally, all the studies mentioned above affirmed the predominance of the electrostatic interaction mechanism.
Studies have shown that heavy metal adsorption onto lignocellulosic waste biosorbents occurs mostly via chelation, ion exchange, electrostatic interaction, and complexation with relevant surface groups (Aniagor et al. 2021; Mohamed et al. 2021). Also, the role of sodium, potassium, calcium, and magnesium in the adsorbent towards improved ion exchange process has been highlighted (Akar et al. 2012). Furthermore, certain functional groups, like the acetamide, carboxyl, phenolic, amino, alcohols, and ester groups, can substitute hydrogen ions with cations or donate electron pairs to form complexes with the metal ions in solutions (Kwikima et al. 2021). The predominance of electrostatic interaction and ion exchange mechanism was also reported during the adsorption of lead ions onto sugarcane waste (Hashem et al. 2021b) and respective adsorption of four different heavy metals onto husk cedar cones, pine nut oil cake, baffle walnut, pectin (Salishcheva et al. 2021). Therefore, the organic functional groups on lignocellulosic wastes play a crucial role during heavy metal adsorption from wastewater.
Lignocellulosic waste has also proven effective for PPCPs adsorption. Among the PPCPs investigated are bisphenol A, tetracycline, oxytetracycline, chlortetracycline, diclofenac, trimethoprim, and aflatoxin B1 species (Abdullah et al. 2021; Li et al. 2019c; Vázquez-Durán et al. 2021). It was, however, observed that the efficient sorption occurs majorly via the electrostatic and non-electrostatic mechanisms. Vázquez-Durán et al. (2021) used unmodified lignocellulosic adsorbents prepared from kale and lettuce wastes to adsorb carcinogen aflatoxin B1. The study concluded that adsorption occurred mainly via electrostatic, hydrophobic, and dipole–dipole interactions and hydrogen bonding.
Furthermore, complexation between the aflatoxin B1 and the chlorophyll content of the agro-waste also drove the sorption process. Similarly, an ion exchange mechanism could occur involving the carbon and oxygen-based functional groups on the acid-treated banana bunches, coconut bunches, and bisphenol A (Abdullah et al. 2021). Conde-Cid et al. (2019) also reported the influence of electrostatic attractions between different antibiotics and variable charge components in the mussel shell and pine bark. Also, cationic bridges between antibiotics and non-crystalline minerals of the respective adsorbents were reported. In conclusion, the adsorption of PPCPs and other emerging organic contaminants reportedly occurred mainly through various electrostatic and non-electrostatic interactions between the individual drug molecules and the adsorbent’s surface functional groups. Notably, no recent studies were found on the adsorption of radionuclide onto lignocellulosic waste adsorbent, and the reason for this observation is unknown. Therefore, it is suggested that the researcher further explore the potential of lignocellulosic waste for radionuclide adsorption. Also, effective surface functionalization and pretreatment could be necessary to improve the sorption capacity of lignocellulosic wastes for radionuclides adsorption.
Overall, lignocellulosic adsorbents have been successfully synthesized from tree bark, fruit peels, seeds, husk, and leaves. The successful utilization of these lignocellulosic wastes as adsorbents in wastewater treatment offers the advantage of cost-effectiveness and mitigates the environmental problems associated with their indiscriminate disposal. However, the functional group and sorption site of this type of adsorbent could be greatly enhanced through surface chemical functionalization and other pre-treatment methods.
Clay minerals
The classification of clay minerals is based on the respective layer type, interlayer order, and layer net charge (Bergaya and Lagaly 2013). However, the extensive application of clay minerals is hinged on their low cost, ready availability, ecofriendliness, high specific surface area, and ion exchange capacities (Uddin 2017). Thus, clay minerals constitute the active adsorptive groups on natural clay. However, despite their abundance, naturally occurring clay minerals have limited and variable adsorptive capacities due to their inherent impurities and mineralogical composition variations (Jaber et al. 2013; Zhang et al. 2010). Controlled fabrication of synthetic clay minerals is being explored to obtain a pure and homogenous clay mineral phase. These synthetic clay minerals are applied as advanced functional materials in specialized systems and the synthesis of specialized consumer goods. Therefore, regarding the layer structure, natural and synthetic clay minerals are widely used in many process industries and as an adsorbent in water treatment.
Several researchers have recorded massive success in the adsorption of dyes using modified and unmodified clay minerals. Dyes such as methylene blue (Amrhar et al. 2021; Çiftçi 2022), crystal violet (Cao et al. 2020; Sarma et al. 2019), brilliant green (Sarma et al. 2019), malachite green (Sevim et al. 2021), basic blue 9 (El Kassimi et al. 2021; Lawchoochaisakul et al. 2021), basic yellow 28 (El Kassimi et al. 2021; Lawchoochaisakul et al. 2021), acid green 25 (Yap and Priyaa 2019), methyl orange (Akbour et al. 2020; Lawchoochaisakul et al. 2021), rhodamine B (Ouachtak et al. 2020; Yu et al. 2019), and congo red (Yu et al. 2019) have been successfully investigated and reported in the literature. Notably, dyes have charged and neutral parts, but Haounati et al. (2021) noted that clay adsorbent provides suitable sites for both species. The cationic dye molecules interact with the negative charge sites of the clay minerals through electrostatic attraction. At the same time, the neutral species are adsorbed onto the external surface of the clay mineral via hydrogen bonding with the hydroxyl groups (de Queiroga et al. 2019). Aside from hydrogen bonding, this surface attachment can also occur via intermolecular π–π stacking attraction (Dobe et al. 2022; Thirumoorthy and Krishna 2020). Thus, dye adsorption onto clay minerals majorly proceeds via electrostatic attraction, hydrogen bonding and intermolecular attraction. Similarly, the sorption efficiency is a function of the solution pH, initial dye concentration and adsorbent dose.
The adsorption of different heavy metal cations, including trivalent chromium, hexavalent chromium (Essebaai et al. 2022; Mdlalose et al. 2021), lead (Jabłońska 2021; Jiang et al. 2021a; Sun et al. 2023a), zinc (Jabłońska 2021; Jiang et al. 2021a), nickel (Jabłońska 2021), cadmium (Szewczuk-Karpisz et al. 2022; Tonk et al. 2022; Zeng et al. 2023), and barium (Atun and Bascetin 2003; Mundim et al. 2022) have been reported. The efficiency of clay and clay minerals in heavy metal adsorption has been linked to the clay's high cation exchange capacity, high specific surface area, and high swelling properties (Essebaai et al. 2022). According to Szewczuk-Karpisz et al. (2022), heavy metals usually adsorb onto the inner-sphere complexes of the clay minerals via ionic exchange and on the silicon monoxide and aluminium oxide surface groups. Furthermore, at low pH (acidic sorption environment), negatively charged sites of the clay mineral adsorbent establish electrostatic interaction with the target heavy metal, forming outer-sphere complexes (Mundim et al. 2022). In summary, heavy metal adsorption onto clay and clay minerals mainly occurs via ion exchange and electrostatic attraction mechanisms. The solution pH also influences sorption efficiency since the charges on the surface groups of the clay minerals vary considerably with pH variation.
Clay minerals adsorbents also have proven effective for the adsorption of PPCPs. Among the PPCPs investigated are propranolol, ibuprofen, amoxicillin, diclofenac-sodium, imipramine, paracetamol, p-chlorophenol, and tetracycline species (Chauhan et al. 2020a, b; del Mar Orta et al. 2019; Ji et al. 2019; Martín et al. 2019; Obradović et al. 2022; Sun et al. 2017; Zhang et al. 2021). It was, however, observed that efficient sorption occurs majorly via an ionic exchange mechanism (de Farias et al. 2022; del Mar Orta et al. 2019; Martín et al. 2019; Obradović et al. 2022). Furthermore, a complex mechanism involving hydrophobic interactions between the nonpolar groups of the PPCPs and clay minerals and an electrostatic interaction mechanism was reported while adsorbing Ibuprofen and diclofenac-sodium (Obradović et al. 2022). Zhang et al. (2021) reported improved adsorption of tetracycline species onto montmorillonite via partial cation exchange, surface complexation, and hydrogen bond/Vander Waal interaction. Therefore, it is concluded that the ionic exchange mechanism accounts for most PPCPs adsorption onto clay minerals.
The adsorption of different radionuclides, including cesium, strontium, uranium, europium, plutonium, iodine, cobalt, zirconium, and selenium, onto modified and unmodified clay minerals, have been successfully investigated (Akemoto et al. 2021; Pavón González and Alba 2022; Soliman et al. 2019; Zabulonov et al. 2021). Generally, one or a combination of electrostatic interaction, surface complexation, and ionic exchange mechanisms play a significant role in the adsorption of radionuclides onto clay minerals (Akemoto et al. 2021; Alamudy and Cho 2018). In addition, clay minerals' expandability and basal spacing enhance their cation exchange capacity and affinity for radionuclides (Philipp et al. 2022; Soliman et al. 2019). The influence of environmental factors such as temperature, pH, organic matter content, contact time, initial adsorbate concentration, and ionic strength also impacts the adsorption of radionuclides (Fan et al. 2019). Zabulonov et al. (2021) reported a low distribution coefficient for cesium radionuclide at low solution pH and enhanced strontium uptake due to the high probability of strontium ionic fixation in diffuse clay ionic layers. Philipp et al. (2022) reported improved uranium adsorption with increasing ionic strength and solution alkalinity. According to the study, at pH < 8.0, the main sorption driving force was cation exchange, which depends on ionic strength. Beyond this pH, there was a sequential surface de-protonation, thus enhancing surface complexation. The positive impact of solution temperature on the clay expandability and the mobility and effective collision of the radionuclide molecules was also highlighted (Soliman et al. 2019). Thus, it is concluded that the sorption capacity of clay minerals for radionuclides is a function of the expandability of the clay particles, as well as the pH and the ionic strength of the adsorbate solution.
To sum up, clay minerals are widely used in adsorption due to their affordability, availability, eco-friendliness, high specific surface area, and ion exchange abilities. However, despite their abundance, their adsorptive capacities are limited and inconsistent due to inherent impurities and variations in mineralogical composition. Therefore, chemical modification is necessary to enhance their adsorption capabilities.
Silica
Silica is an inorganic solid material often applied as an efficient adsorbent in water treatment, either in its raw or chemically modified state. They exhibit a considerably high surface area, chemical inertness, improved pore properties, and many surface functional groups, which can provide grafting sites (Lahiri and Liu 2021). Many recent publications have demonstrated the versatility of silica-based adsorbents in wastewater treatment.
Different types of dye have been successfully adsorbed onto varieties of silica-based adsorbents (Arasi et al. 2021; Benvenuti et al. 2020; Koyuncu and Okur 2021); however, it was noted that the functionalization of these silica adsorbents greatly enhanced their dye adsorptive capacity. Consequently, different surface-modified silica-based adsorbents (Bensedira et al. 2022; Cao et al. 2020; Gomaa et al. 2022; Li et al. 2020d; Zein et al. 2020) have been used to adsorb different dyes. Specifically, Kalkan and Nadaroglu (2021) adsorbed acid fuchsin dye onto laccase-modified silica fume. The optimum uptake was established at pH 5.0, with a hydrogen bond forming between the adsorbent's sulfate and amine groups and the positively charged acid fuchsin dye molecules. Also, during the adsorption of reactive wool dyes, Gemeay et al. (2020) reported the role of the sulfonate and oxygenated groups for enhanced hydrogen bonds and electrostatic interaction. Pham et al. (2021) synthesized a silica-based adsorbent with a high positive surface charge via functionalization with poly-diallyl-dimethylammonium chloride. Based on the study, efficient dye uptake was achieved following a complex formation between the oxygen atoms on the sulfonic group of the azo dye and the adsorbents’ amine groups. Al-Shehri et al. (2021) investigated the impact of incorporating neodymium into the three-dimensional structure of mesoporous silica. The results show an improved adsorption capacity via electrostatic interaction, as the surface modification introduced a large number of negative binding sites that are necessary for adsorbing the positively charged nitrogen atoms of the dye species. Also, the possible formation of a coordination bond between the nitrogen lone pairs of the neutral amino groups and the neodymium atoms was further postulated. Therefore, irrespective of the nature of the silica-based adsorbent, the solution pH, which directly influences the degree of electrostatic interaction, plays a crucial role during dye uptake.
Different forms of raw and functionalized silica-based adsorbents have been successfully investigated for the adsorption of heavy metal ions. The surface functionalization techniques mainly involve introducing relevant surface groups to enhance interaction with heavy metal cations. Although many studies have demonstrated the usefulness of surface functional groups in pollutant binding, it should be noted that some functional groups exhibit superior cation binding capacities than others. Through successful silylating reactions, Shao et al. (2020) incorporated the sulfoacid, thiol, amino, carboxyl, and ethylenediamine tetra-acetic acid groups onto silicon dioxide nanoparticles. Further, they tested the lead ion adsorption potentials of the respective adsorbents. It was reported that the ethylenediamine tetra-acetic acid-decorated silicon oxide, with improved geometrical adaptability, depicted the highest adsorption capacity. Other studies by Vareda et al. (2020), Wang et al. (2020c), and Albayati et al. (2019) also highlighted the role of nitrogen-containing functional groups, having similar electrostatic interaction mechanisms in heavy metal adsorption. Radi et al. (2019) explored the potential of porphyrin-modified silica for adsorbing lead, zinc, cadmium, and copper ions. The study concluded that efficient uptake occurred through the direct complexation of porphyrins with the cations via a so-called ‘sitting-atop’ interaction. Conclusively, it was noted that efficient cation uptake is hinged upon successfully introducing sufficient negatively charged groups on the silica adsorbents that could interact with the different cations.
Studies exist on the adsorption of PPCPs and other emerging organic pollutants onto modified silica adsorbents (Ighalo et al. 2022c; Igwegbe et al. 2021). Modified nano-silica (Pham et al. 2020b) and polycation-modified nano-silica (Pham et al. 2020a) were used for the adsorption of ciprofloxacin and beta-lactam cefixime. Both studies reported that the main adsorption driving force was the electrostatic interaction between the anionic surface charge of the respective drugs and the protonated adsorbent surface. Besides, Jodeh et al. (2022) recorded high adsorptive capacity while adsorbing trimethoprim onto a chelating matrix synthesized via a reaction between 1.5-dimethyl-1H-pyrazole-3-carbaldehyde, 3-aminopropyltrimethoxysilane and silica gel. According to the study, efficient drug binding occurred via complexation between the nitrogen atoms of the adsorbent and π-electrons originating from the benzene and pyrimidine rings in the drug. Also, low adsorption enthalpy values were recorded throughout the entire sorption process due to the hydrogen bond formation and the formation of a water bridge between the nitrogen/oxygen-containing groups of the trimethoprim and the amine groups of the adsorbent (Jodeh et al. 2022). Dipyridyl-based organo-silica nanosheets were successfully synthesized and utilized for adsorbing clofibric acid, ketoprofen, and naproxen sodium from wastewater (Guo et al. 2020). It was noted that the adsorption of naproxen sodium occurred via π–π interaction, while partition and π–π stacking predominated during the ketoprofen adsorption. Conversely, clofibric acid uptake was controlled by multiple interaction mechanisms, including hydroxyl-π interaction, partition, π–π interaction, and π–π stacking. In conclusion, the adsorption of PPCPs and other emerging organic contaminants reportedly occurred mainly through various physical interactions between the respective drug molecules and the adsorbent’s surface functional groups.
The adsorption of different radionuclides such as cesium (Bu et al. 2019; El-Shazly et al. 2021b; Zhuravlev et al. 2022), uranium (Tripathi et al. 2018), europium (Garcia et al. 2019; Wang et al. 2020b), cobalt (El-Shazly et al. 2021a), and strontium (Zhuravlev et al. 2022) onto raw and modified silica-based adsorbent has been reported. Bu et al. (2019) investigated the adsorption of cesium ions onto the tobermorite 9 Å, tobermorite 14 Å, and jennite. Accordingly, cesium adsorption onto tobermorite 14 Å and tobermorite 9 Å, respectively, occurred via the ionic adsorption at the octahedral hexaaqua calcium (II) complex surface and cation exchange at the tetrahedral silicate surface. Similarly, electrostatic interaction via complexation with surface hydroxyl groups controlled the cesium adsorption onto jennite (Bu et al. 2019). The influence of the cation exchange mechanism was also highlighted during cesium adsorption onto insoluble ferrocyanide composites (El-Shazly et al. 2021b). Tripathi et al. (2018) evaluated the feasibility of uranium adsorption onto hollow amorphous silicon oxide nanotubes. A robust electrostatic attraction was established between the deprotonated silicon oxide nanotube surface and the uranyl hydroxyl cation during uptake. Using sodium lauryl sulfonate-modified silicon dioxide adsorbent, Wang et al. (2020b) reported that the oxygen-containing functional groups from sulfur trioxide were responsible for the europium ions adsorption. According to Jose et al. (2020), the europium ions often interact with sulfur trioxide anion in preference to the sodium lauryl sulfonate chain. It is thus concluded that the adsorbent’s surface charge, solution pH and structural orientation significantly impact the type of the dominant sorption mechanism during radionuclide sorption.
Overall, surface complexation, hydrogen bonding, and electrostatic interaction are the primary mechanism responsible for pollutant binding onto silica adsorbents. Furthermore, efficient binding of organic pollutants occurred via complexation between the silicate groups of the adsorbent and π-electrons originating from the benzene and pyrimidine rings in the respective organic pollutants.
Zeolite
Zeolites are porous materials with improved ion exchange capacity, mainly composed of hydrated aluminosilicate minerals (El-Nahas et al. 2020). This class of adsorbent exhibit a negatively charged framework, which stems from substituting tetra atomic silicate cation for aluminium cation within the structural backbone. Unmodified zeolite adsorbents have proven efficient for adsorbing different classes of dyes, except the anionic dyes, due to the anionic nature of the zeolite surface. However, different functional groups and complexes have been successfully incorporated into the zeolite structural framework for improved dye adsorption capacity.
Gollakota et al. (2020) investigated the potential adsorption of rhodamine 6G onto a novel zeolite material. During the study, two main sorption mechanisms were identified: electrostatic interaction between the deprotonated silanol groups and the cationic dye species and hydrogen bonding due to the interactions between the silanol and amine functional groups of zeolite and the cationic dye species. Similarly, while adsorbing basic fuchsin dye onto tetra propyl ammonium bromide-modified magnetic zeolite, Mohammed et al. (2020) observed the deprotonated silanol, silicon oxide groups of the adsorbent interacted with the cationic dye species, to entrench efficient uptake at increasing pH up to pH 5.0. Meanwhile, between pH 5.0–9.0, efficient dye uptake was achieved following the complexation between the neutral silanol groups of the adsorbent and electrons lone-pair from primary amine groups. Also, a robust electrostatic attraction was reported as the predominant mechanism during the sorption of acid red 66 dye onto Linde-type A zeolite (Al-dahri et al. 2022), methylene blue dye onto synthetic zeolite mixture (Youssef et al. 2021), methyl orange dye onto pillar(5)arene modified zeolite (Yang et al. 2019b) and methylene blue dye onto zeolite/cerium oxide nanocomposite (Nyankson et al. 2020). Furthermore, Phouthavong et al. (2020) reported the efficient binding of methylene blue species onto the 3-dimensional pore system of silica-rich magnetic zeolite via a pore-filling mechanism. Thus, the solution pH regulates the extent of dissociations and charge distributions of the zeolite functional groups.
Different studies have successfully investigated and established raw and modified zeolite's heavy metal adsorption capacity. Meanwhile, the strong influence of the electrostatic attraction mechanism during the cation binding was reported by most authors. Gollakota et al. (2023) evaluated the potential adsorption of cadmium and zinc ions onto ionic liquid functionalized zeolite X. They affirmed the dominance of electrostatic attraction to complexation between the silanol groups of the adsorbent and the cationic species. According to Guo et al. (2023), a coordination interaction with the nitrogen/oxygen-containing groups of the functionalized zeolitic imidazole frameworks-functionalized resulted in efficient sequestration of lead and copper ions. Similarly, the exchange of ions within the zeolitic structure by the heavy metal cations was also reported during the adsorption of copper (Dasgupta et al. 2021) and chromium ions (Huang et al. 2022) onto zeolite. However, the mechanism of heavy metal adsorption onto zeolite is mainly via cation exchange within the zeolitic structure and silanol groups-assisted electrostatic interactions.
The adsorption of PPCPs and other emerging organic contaminants onto zeolite-based adsorbents has been reported mainly through hydrophobic and electrostatic interaction mechanisms (Belviso et al. 2021; Goyal et al. 2018). However, a zeolite-based adsorbent capable of efficient adsorption of different organic pollutants was generally synthesized via chemical functionalization. Smiljanić et al. (2020, 2021) comparatively investigated the adsorption of ibuprofen, naproxen, diclofenac sodium, and ketoprofen onto natural zeolites that are respectively loaded with monolayer and bilayer of cationic surfactant. During sorption onto the loaded zeolite, the anionic drug species interact electrostatically with the cationic surfactant head. The alkyl chains of surfactants complex with the hydrophobic heads of the drugs via hydrophobic interaction. Asides from the interactions mentioned above, there was an ionic exchange between the counter chloride anion on the surfactant molecule and the anionic drug species in the case of zeolite with a bilayer of surfactant (Smiljanić et al. 2020). Using a two-step computational analysis, Lin et al. (2020b) investigated the possible adsorption of four linear siloxanes and derivates onto a myriad of hypothetical pure-silica zeolites. It was observed that the most robust adsorption energy and electrostatic interaction were recorded for the compound with the smallest particle sizes and significant electronegativity differences between the bonded atoms. The dominance of hydrophobic interactions was also reported during the adsorption of ketoprofen, hydrochlorothiazide, and atenolol in their neutral state with high-silica commercial zeolites (Sarti et al. 2020). Thus, the affinity of zeolite-based adsorbents for organic and nonpolar molecules could be improved through surface modification with the right reagent, noting that the adsorption of organic and non-polar molecules is a function of the adsorbent’s physicochemical properties.
The effectiveness of zeolite in the adsorption of different radionuclides has been established in the literature, and electrostatic interaction was noted as the major sorption mechanism involved (Li et al. 2022a). Salam et al. (2020) and Ai et al. (2022) investigated the sorption of uranium ions from wastewater. They observed that the complexation of thiol and hydroxyl with uranyl ions between pH 5.0–6.0 facilitated sorption. Adsorption of strontium radionuclide onto microporous Linde Type A zeolites of varying crystal sizes also proceeded with predominant electrostatic interaction (Kwon et al. 2020). The study also observed that adsorbents with a crystal size of 2 μm showed better adsorption capacity and selectivity for strontium ions than those with smaller crystal sizes of 100 nm and 500 nm. Li et al. (2020c) blended the aspartic acid molecules with the bilayer cetyltrimethylammonium bromide cations onto zeolite Y. The modified zeolite Y adsorbent was utilized for adsorbing cesium, strontium, and uranium ions. It was, however, reported that the monovalent cesium ions were adsorbed better and faster than the bivalent strontium and uranium ions. Notably, higher consumption of aspartic acid ligands occurred during the formation of a coordination compound with bivalent ions than with monovalent ions, hence the decreased adsorption capacity. However, the crystal size significantly affects the affinity between the zeolite adsorbent and radionuclides.
In conclusion, zeolites exhibit a negatively charged framework due to substituting tetra atomic silicate cation for aluminium cation within the structural backbone. The silanol and amine functional groups of zeolites facilitate the occurrence of hydrogen bonding and electrostatic interaction, which are the two main identified sorption mechanisms.
Peat and humic soil
Peats and humus are soil materials mainly comprising decaying organic matter, detrital minerals, and plant debris such as lignin, cellulose, and hemicellulose. Its different constituents' polar organic functional groups confer high pollutant adsorption and ion exchange capacity on peat soil. It has been shown that the predominant physical and chemical properties of any given peat soil are a function of the origin and nature of the decayed plant materials and the moisture relations during and after the peat formation (Kolay and Taib 2018).
The presence of cellulose, hemicellulose, and lignin compounds, originating from decaying plant biomass makes for the efficient uptake of dye molecules by peat soil. Rahmayanti et al. (2021) studied the adsorption of naphthol blue black and indigo sol blue dyes onto peat soil humin. The dominance of electrostatic interaction via protonation of the carboxyl and hydroxyl groups of the peat humin was recorded between pH 2.0 and 5.0. At the same time, the hydrogen bonding mechanism predominated between pH 7.0 to pH 8.0. Dzieniszewska et al. (2019) adsorbed five different dyes: reactive blue 19, reactive blue 81, reactive black 5, acid black 1, and acid blue 9, onto low-moor peat in the presence of sodium chloride, sodium carbonate, and ethanoic acid auxiliaries within the adsorbate solution. In the presence of ethanoic acid, the peat surface charges protonated, thus enhancing the anionic dye adsorption via an electrostatic interaction mechanism (Dzieniszewska et al. 2019).
Similarly, the reactive dye species' increased intermolecular force and dimerization were observed in the presence of salts. The dominance of the electrostatic interaction and ion exchange mechanisms was also highlighted during the adsorption of malachite green dye onto coco peat (Kumari and Dey 2019) and modified sphagnum peat moss samples (Abu-Saqer and Lubbad 2019). Therefore, it is concluded that the different functional groups inherent in the peat soil's cellulose, lignin, and hemicellulose content are responsible for both electrostatic and non-electrostatic interactions involved during the uptake of different classes of dyes.
Different studies have successfully investigated and established the heavy metal adsorption capacity of raw and modified peat soil. Kasiuliene et al. (2019a) studied the adsorption of hexavalent chromium, trivalent chromium, copper, arsenic, and zinc onto raw and iron-modified peat. It was reported that peat magnetization negatively impacted the adsorption of chromium, copper, and zinc due to the screening/shielding effect of the iron coating on some surface organic groups and active sites. Conversely, due to their high affinity for iron hydroxide and improved surface area, arsenic was better adsorbed onto the iron-modified peat. Furthermore, the solution pH for optimal adsorption of hexavalent chromium, trivalent chromium, copper, and zinc was established at pH 1.5–3.0, 4.0–5.5, 5.0, and 7.0–9.0, respectively. Bartczak et al. (2018) recorded the optimum adsorption of nickel and lead ions onto peat at pH 5.0. The electrostatic attraction between the negative surface charge on the peat and the metal cations, as well as the progressive exchange of adsorbent cations by the nickel and lead ions, was also highlighted in the study. Asides from magnetization, Perelomov et al. (2021) also observed that the oxidation of peat soil humin with potassium persulfate improved its capacity for adsorbing zinc, lead, copper, and nickel. According to the study, the oxidative treatment increased the amount of carboxylic, ketone, and quinoid groups on the adsorbent structure. In addition to the already stated studies, other literature on the successful adsorption of iron (Ashraf et al. 2019), chromium (Ashraf et al. 2019), nickel (Ashraf et al. 2019), copper (Lodygin 2019; Naymushina and Gaskova 2019), zinc (Lodygin 2019), lead (Lodygin et al. 2020; Pelinsom Marques et al. 2020), and cadmium (Lodygin et al. 2020; Pelinsom Marques et al. 2020) onto peat-based adsorbent still exist. In summary, heavy metal adsorption potentials of raw and modified peat mostly occur via ion exchange and/or electrostatic interaction mechanisms. However, a proper pH adjustment on the respective heavy metal wastewater can greatly enhance uptake.
The adsorption of PPCPs and other emerging organic contaminants onto peat and humic-based adsorbents is primarily a function of the available functional groups and organic carbon content of the peat soil, as well as the adsorbate speciation and solution pH (Chen et al. 2017). Guo et al. (2017b) reported the successful adsorption of sulfamethazine antibiotics onto peat soil humin. During the study, the interactions between the sulfonamide group of the sulfamethazine antibiotics, which act as π-electron-acceptors, and the benzene rings on the adsorbent ensured efficient sorption. Chen et al. (2017) investigated the adsorption of sulfamethoxazole and sulfapyridine antibiotics onto peat. For the neutral sulfamethoxazole species, the dominant mechanism was the hydrophobic partitioning between the sulfonamide group of the adsorbate and the organic matter of the peat soil. At the same time, cation bridging and slight contribution of van der Waals forces controlled the sorption of deprotonated sulfamethoxazole species (Chen et al. 2017).
Furthermore, the dispersive and electrostatic interactions between the sulfapyridine's pyridine group and the peat's aromatic carbon ring were responsible for efficient adsorption. Only a few earlier studies were found on the adsorption of PPCPs and other emerging organic contaminants on peat and humic-based adsorbent. This observation could be related to the fact that this class of adsorbent is open to and compatible with a wide range of surface functionalization required for the efficient sorption of structurally complex pollutants such as PPCPs and other emerging organic contaminants.
Due to the abundant presence of carboxyl and phenolic hydroxyl groups on peat and humic soil, the adsorption of varying radionuclides mainly occurs via cation exchange and electrostatic interaction mechanisms (Belousov et al. 2021). Komissarov and Ogura (2019) and Belousov et al. (2021) observed a variation in the adsorption of cesium and strontium radionuclides onto different peat soils at varying solution pH. The studies reported enhanced uptake via electrostatic attraction between the sorbent's deprotonated carboxyl and phenolic functional group and the radionuclides cations at acidic pH ranges. Meanwhile, when the solution pH was increased beyond pH 6.0, Belousov et al. (2021) recorded a decreased adsorption for both radionuclides due to decreasing adsorbate-adsorbent bond strength. Similarly, the possible formation of stable complexes between the oxygen-based groups of the peat soil and the respective radionuclide cations was postulated (Belousov et al. 2021). Bordelet et al. (2018) also highlighted the key role of carboxylic and phenolic functional groups of peat soil during the adsorption of uranium and radium radionuclides. The study further reported that the optimum uptake of uranium and radium onto peat soil occurred between pH 3.0 to 6.0. This observation aligns with the result obtained by Belousov et al. (2021). Thus, the interaction of the deprotonated groups on the peat soil and the cationic species of the radionuclide remains the dominant sorption mechanism.
In summary, the polar organic functional groups of the peat adsorbent's cellulose, hemicellulose, and lignin constituents are responsible for high pollutant adsorption and ion exchange capacity. Also, the main adsorption mechanisms are the electrostatic interaction, hydrogen bond formation, and possible formation of stable complexes between the oxygen-based groups of the peat soil and the respective pollutants.
Adsorption isotherms
Isotherm modeling is an applicable technique for expecting adsorption behaviors and studying the correlation between adsorbents and contaminants at an equilibrium medium. A comprehensive understanding of the isotherm models would significantly enhance the behavior of the adsorption method and pollutant-adsorbent design (Dada et al. 2021). Moreover, isotherm modeling could provide insight into the removal method of pollutants, such as maximum adsorption capacity, strength, and adsorption state (Majd et al. 2021). Adsorption isotherms are produced when an adsorbent and an adsorbate come across for a sufficiently extended period for the interface concentration to be in equilibrium with the absorption of the contaminant at a constant temperature (Al-Ghouti and Da'ana 2020). Adsorption isotherm is critical for fundamental studies for designing, optimizing, and troubleshooting manufacturing adsorption experiments. Various factors could be influenced during the adsorption process, such as pH, temperature, initial concentration, and adsorbent characteristics (Ahmad et al. 2022; Huang et al. 2021b). These impacting parameters affect the adsorption modeling. For instance, temperature, pollutant dosage, and adsorbent properties could completely influence the level of selecting the proper isotherm model (Duan et al. 2020).
Various isotherm models with differing factors have been established and utilized in a broad series of manufacturing removal methods via adsorption involving oil and gas factories, food manufacturing, and water remediation activities. The proper isotherm model affords the necessary details for evaluating the adsorbent acts, incorporating the adsorption mechanism, adsorption capacity, and fundamental characteristics of the removal method. Numerous adsorption isotherm models have been applied throughout the previous years to review the thermodynamic equilibrium correlation among pollutants and adsorbents at steady temperatures.
Following the sum of factors, adsorption models are categorized as one-factor, two-factor, and three-factor models. The isotherm model is regularly employed to study adsorption processes, capability, and adsorbent characteristics in different pollutants studies. Nevertheless, because of some model constraints, not all isotherm models could be suitable for expressing the data and explaining the pollutant’s adsorption method.
Langmuir adsorption isotherm
The Langmuir isotherm model, one of the fundamental empirical models, presumes the pollutant and adsorbent material in a standard method utilized for homogenous surfaces (Al-Ghouti and Da'ana 2020). A further supposition of this model is the reversibility of the adsorption–desorption process (Langmuir 1916, 1918). The Langmuir isotherm could be gained from this equation, inferring that:
where qm, L is the maximum adsorption amount of Langmuir isotherm (mg g−1), and k is Langmuir isotherm constant (dm3 mg−1).
Due to its simplicity and homogenous monolayer adsorption, Langmuir is the most frequently used optimal isotherm in the adsorption of heavy metals. Most of the heavy metal adsorption research described in the literature used monolayer chemisorption techniques. Chitosan aerogel was synthesized by Fan et al. (2021) to remove copper from contaminated water, and the equilibrium data were compared using Langmuir and Freundlich models. The outcome showed that the Langmuir model is the most appropriate one to describe the adsorption process. Using Pinewood sawdust biochar, naphthalene, phenanthrene, and anthracene compounds were effectively removed from the contaminated aqueous solution. The isotherm models revealed the complete success of the Langmuir equation in elucidating the contaminants’ uptake (Rashad et al. 2022).
Freundlich adsorption isotherm
The Freundlich isotherm model is a broadly applied practical principle that relies on tentative findings since it explicitly explains the removal of organic/inorganic toxins on numerous adsorbent materials. Herbert Freundlich initially suggested the model in 1906 (Freundlich 1907) as an expansion of Henry's standard. The primary assumption in enhancing the Freundlich adsorption isotherm model in 1906 is that the adsorbent material has a heterogeneous surface comprising numerous active adsorption sites. Following this perspective, the quantity removed is the amount of adsorption on all active sites until the adsorption capacity reduces exponentially at the end of the adsorption process. The Freundlich model could also be employed for multilayer adsorption. When chemisorption is the major adsorption mechanism, the Freundlich model describes monolayer adsorption, whereas it explains multilayer adsorption when physisorption is the major mechanism. The Freundlich isotherm model has both linear and nonlinear models, which are defined as follows:
where Kf (mg L−1) and N are the Freundlich isotherm model's coefficients for the adsorption removal and strength coefficients, respectively, the infinite surface coverage that is anticipated mathematically indicates multilayer adsorption on the surface since the Freundlich model does not expect the overload of the adsorbent. The value 1/N is temperature-dependent and depends on the adsorption conditions like surface heterogeneity or adsorption capacity (Febrianto et al. 2009).
To examine experimental data of chromium, arsenic, cadmium, and lead adsorption with graphene oxide, Abbasi et al. (2021) employed linear and nonlinear regression of Freundlich models. The data analysis revealed that the nonlinear Freundlich adsorption isotherm model outperformed the linear form in estimating the quantity of heavy metals removed by graphene oxide.
Dubinin–Radushkevich isotherm
The Dubinin–Radushkevich isotherm model presupposes a multilayer structure with van Der Waal's forces relevant to physical adsorption (Al-Ghouti and Da'ana 2020). The Dubinin–Radushkevich adsorption model, an empirical isotherm, depicts the adsorption of vapors and gases via a pore-filling mechanism on micropore adsorbent surfaces, such as biochar, activated carbon, and metal–organic framework. To explain the pore size, adsorption capacity, and removal of microporous non-polar contaminants, Dubinin and Radushkevich designed the Dubinin–Radushkevich formula in 1946 (Dubinin 1947). The Dubinin and Radushkevich model is beneficial for different water pollutants removal applications, including measuring adsorption capability, evaluating the mechanism of the adsorption method, and figuring out the average adsorption energy (Chen et al. 2022b). The Dubinin and Radushkevich isotherm model depends on temperature variations. The following formula can be used to apply the Dubinin and Radushkevich adsorption model:
\(\varepsilon\) (kJ mol−1) is Polanyi potential, R is the gas constant 8.314 J mol−1 K−1, and T(K) is the absolute temperature.
Elgarahy et al. (2020) inspected the subsequent remediation of Congo red anionic dye and copper metal ions on the multifunctional alginate beads. Calculating the mean sorption energy permitted researchers to differentiate between mercury's physical and chemical adsorption utilizing the Dubinin- Radushkevich model.
Temkin isotherm
The Temkin model presumes a multi-layer chemosorption process containing a parameter that reflects relations between the adsorbent and the pollutant (Temkin 1940). Temkin isotherm model disregards great and little concentration amounts while accounting for adsorbent-contaminant contact. The Temkin isotherm is merely appropriate to specific ion concentrations. This model assumes that the binding energy is uniformly distributed and that the adsorption heat of all molecules in the layer drops linearly rather than logarithmically when the surface coverage upsurges as a function of temperature (Elgarahy et al. 2022). The Temkin model equation is stated as follows.
where A (L g−1) is the constant of equilibrium binding; b (J mol−1) is the adsorption heat constant. The above-mentioned equation can be solved by plotting qe versus lnCe, which yields a straight line, and A and b can be calculated from the slope and intercept.
Cadmium, Nickel, Copper, lead, and Zinc metal ions were discarded from the contaminated solution utilizing crab shell-derived chitin, and the experimental findings were modeled using the Langmuir, Freundlich, Temkin, and D-R isotherms to describe the adsorption mechanisms. It was revealed that the Temkin model successfully fitted the adsorption method. The heat of adsorption value from the Temkin isotherm shows that the chitin and heavy metal interaction is likely physisorption rather than ionic interaction (Boulaiche et al. 2019).
Brunauer, Emmett, and Teller isotherm
In 1938, Brunauer, Emmet, and Teller suggested a theoretical isotherm model that relied on multimolecular adsorption. This model was first applied in gas–solid equilibrium techniques (Brunauer et al. 1938). The Brunauer, Emmett, and Teller isotherm model is a Langmuir version with several additional assumptions, such as (1) the adsorption method is multilayer homogeneous; (2) the first layer's adsorption force is persistent (i.e., The adsorption energy of each additional layer is equivalent to the heat of condensation. The interactions between the adsorbent and the adsorbate have no impact on the fusion heat that is used to generate this energy.), and (3) adsorption and desorption rates are equal (Wang and Guo 2020). There are numerous variations of the Brunauer, Emmett, and Teller isotherm model, but the following describes its extinction model at the liquid–solid interface:
where qmBET (mg g−1) is the maximum adsorption capacity of Brunauer, Emmett, and Teller isotherm; Cs (mg L−1) is the adsorbate solubility; CBET (mg L−1) is the Brunauer, Emmett, and Teller isotherm adsorption constant, a parameter related to the binding intensity for all layers.
Saccharomyces cerevisiae that had been chemically and thermally modified was used by De Rossi et al. (2018) to examine chromium removal from contaminated solution. The Brunauer, Emmett, and Teller isotherm was effectively applied to model the chemically treated bio-sorption isotherms. The BET isotherm posits that the adsorbent material was soaked with chromium following the Langmuir isotherm; therefore, the monolayer had become saturated, and more biosorption took place on the numerous layers of the material, resulting in a more remarkable ability to absorb chromium. Nevertheless, relatively than a feasible adsorption isotherm, Brunauer, Emmett, and Teller isotherm model is currently considered a technique for characterizing various biosorbents (Majd et al. 2021). Additionally, it has been utilized to investigate the specific surface area, total pore volume, and pore size of various carbonaceous materials (Najaflou et al. 2021; Zeng et al. 2020).
Adsorption kinetics
Kinetic models help to define the functional performance, comprehend the adsorbent-pollutant correlations, and afford perceptions into the multifaceted adsorption method. Despite several compound equations that have been established, e.g., Elovich, Avrami, Bangham, or layer diffusion models, the utmost applied models are the pseudo-first-order and pseudo-second-order equations. The kinetic explanation is a critical dynamic process by removing different pollutants from contaminated solutions since it could describe the equilibrium period, reaction order, and reaction side and consequently define the behavior and the rate-governing phase of the contaminant catching from the wastewater onto the adsorbent material. Kinetic models stipulate vision with result data essential for operating a successful biosorption model.
Generally, the biosorption method that relies basically on time could be categorized into different kinetic models such as pseudo-first-order (Lagergren 1898), pseudo-second-order (Ho and McKay 1999), Weber and Morris model (Weber Jr and Morris 1963), Boyd model (Okewale et al. 2013), Bingham’s model (Malana et al. 2011), and Elovich model (Zeldowitsch 1934). Nevertheless, some challenges occur in the use of these kinetic models. The first is that the pseud-first-order and Pseudo second order models that are most commonly applied are empirical models with no apparent physical implication. With these empirical kinetic models, examining the molecule removal routes is difficult. It is essential to create the physical importance of observed kinetic patterns. The following is that while the physical implications of the differential kinetic models, such as the phenomenological external/internal and adsorption in active sites models, are well defined, the methods for explaining them are challenging. The mass transfer systems consuming these models have not been carefully examined. The complex explaining methods impede the applications of these models. The third one is that in some published papers, the kinetic models are employed in inappropriate manner due to some statistical interference.
Three phases make up the adsorption molecules transfer kinetics. External diffusion comes first. In this stage, the pollutant invades the aqueous medium around the adsorbent material—the concentration variations between the surface of the adsorbent and the overall contaminated solution fuel the external diffusion. Internal diffusion is the second phase. Internal diffusion defines the diffusion of the pollutant in the apertures of the used material. The third step is the adsorption of water pollutants in the active pore volumes of the adsorbent material (Elgarahy et al. 2021).
Pseudo-first-order reaction kinetic
This model is the utmost applied for the liquid–solid adsorption method and relies on the pseudo-first-order equation (Lagergren 1898). The pseudo-first-order model identifies the adsorption kinetics of molecules in an adsorbent surface by the following ordinary first-order differential equation Eq. (35) (Rodrigues and Silva 2016):
where Qe: the amount of adsorbate in the adsorbent at equilibrium (mg g−1); Qt: the amount of adsorbate in the adsorbent at time t (mg g−1); K1: constant rate of Lagergren’s first order; and t: time of contact (min).
Pseudo-second-order reaction kinetic
Adsorption takes place on two surface sites in pseudo-second order kinetics (some authors call it ‘‘Blanchard’s model”), and therefore, can be expressed by the following second-order differential equation Eq. (37) (Naderi et al. 2018).
where Qe is the amount of adsorbate in the adsorbent at equilibrium (mg g−1); Qt is the amount of adsorbate in the adsorbent at time t (mg g−1); K2 is a constant rate of the pseudo-second-order; and t: time of contact (min).
Intraparticle diffusion model
In general, the adsorption process by porous carbonaceous materials involves four stages; (1) the border layer surrounding the particle receives the solute from the aqueous solution, (2) transfer of the solute to the adsorbent surface from the boundary layer, (3) the solute is transported to the adsorbent sites by diffusion in the micro- and macropores, and (4) interactions between the particles of the solute and the active sites of the adsorbent: Adsorption, complexation, and precipitation. The following equation could interpret this model:
Qt is the amount of adsorbate in the adsorbent at time t (mg g−1); KID is the constant rate of intraparticle diffusion; t: time of contact (min); and I is the intercept of intraparticle diffusion kinetic model. To test this isotherm, it is suitable to graph Qt as a function of the square root of the contact time and to note the coefficient of determination R2.
Elovich model
In recent years, the adsorption of contaminants from aqueous solutions and the kinetics of gas adsorption on solids has been extensively described using the Elovich equation (Ho 2006). This model matches the following equation (Chien and Clayton 1980).
where Qt: the amount of adsorbate in the adsorbent at time t (mg g−1); b: the number of sites available for adsorption; a: the initial adsorption rate (mg g−1 min); and t: time of contact (min).
Adsorption thermodynamics
Thermodynamics provides insight into the intrinsic energetic variations connected to the adsorption method, which is essential to explain the method's adsorption behavior, irreversibility, and naturalness. The thermodynamic parameters, ΔG°, ΔH°, and ΔS°, are typically assessed after implementing equilibrium tests at changed heats, which include a vast quantity of tentative studies, restraining the obtainable information for thermodynamic factor approximation. The description of these thermodynamic functions is the following: Gibbs free energy is essential for identifying the spontaneity of the system. When ΔG° is negative, the adsorption is considered spontaneous, while ΔG° > 0 suggests non-spontaneity, demonstrating that the method involves energy from the environments to be expatriate to the equilibrium (Guo et al. 2017a).
Generally, the contaminant-biosorbent systems are spontaneous because an absorption slope mostly rules the adsorption process. Henceforth, its Gibbs-free energy is further beneficial in defining whether adsorption is preferred at advanced or lesser temperatures. Consequently, the enthalpy change indicates the outcome energy or the consumed energy. Adsorption is endothermic if the shift in Enthalpy is more significant than zero, while it is exothermic when it becomes less than zero (Vilela et al. 2019).
Most of the reactions are exothermic; nonetheless, one practicable identification of the endothermic mechanism is that when particles are nearby to be attached to the adsorbent material, they miss a portion of their hydration cover. This dehydration method uses energy that surpasses the exothermic behavior of connecting particles to the surface (Anastopoulos and Kyzas 2016). Ultimately, entropy change designates if the haphazardness rises or declines, subsequently, the adsorption method (Dirbaz and Roosta 2018). Gibbs free energy and Van’t Hoff models are the furthermost applied equations to evaluate thermodynamic factors (Akram et al. 2017; Lombardo and Thielemans 2019; Maity et al. 2018; Marques et al. 2019). The Clausius-Clapeyron model is not commonly applied because the change in enthalpy value could be deviated upon its use. Nevertheless, it is used favored in the solid–gas adsorption methods (Li et al. 2019a; Wang et al. 2018d).
Accurately calculating the thermodynamic values is essential for determining more details about the nature of the contaminant adsorption method (endothermic/exothermic/spontaneous/viability). Gibb’s free energy variation, enthalpy, and entropy could be calculated at various temperatures by the following equations;
Cs and Ce represent the equilibrium value of the contaminant onto the adsorbent surface and in aqueous media, correspondingly. In contrast, an equilibrium constant, Kc, states the proportion of pollutant concentration on the adsorbent to the dissolved in an aqueous medium.
Mechanistic understanding of biosorption
Biosorption as a multidimensionally successful method has become emerging recently. It is elaborated as an advanced substitute to other traditional approaches for wastewater treatment (Demey et al. 2019).In the physicochemical phenomenon, the sorbate particles (pollutants) gather on another particle (sorbent) surface, as shown in Fig. 9. High-quality effluents are generated in this manner. The term "biosorption" is also defined simply, even though the "bio" prefix indicates a biological entity's involvement.
Adsorption mechanisms of biosorbents. This figure depicts the accumulation of pollutant molecules on the surface of an adsorbent. The bond strength between the adsorbent and adsorbate, which occurs through specific mechanisms, will influence the number of cycles required in the adsorption and desorption processes
Regarding the sorption mechanism, both bioabsorption and biosorption dimensions are implicated. A material in such a state combined with another substance in a different state over absorption. It also covers the uptake of gases or liquids by solids or water. However, adsorption remains in a physical connection wherein a sorbent and sorbate interact to form a contact (Maksoud et al. 2020). Biosorption requires all aspects of the interaction between some pollutants and the biological environment and is a submissive, metabolically independent process (biosorbent). It is essential to numerous processes that naturally take place throughout several scientific fields (Elgarahy et al. 2021; Tee et al. 2022).
Selection of biosorbents
The most crucial factor influencing the choice of biosorbent is well known to be its compatibility. An essential factor to consider when choosing a biosorbent is the cost and source of the biomass. Dead biomass is preferred over living biomass when making various biosorbents (Adewuyi 2020). Utilizing dead biomass has several advantages. It can be summed up as follows; (1) no need for growth requirements to remain included in the aqueous solution (e.g., media, nutrients); (2) no toxicity restrictions; (3) potential for reuse of exhausted adsorbent and pollutants, respectively, and (4) simpler statistical modeling of contaminants removal (Chan et al. 2022; Thirunavukkarasu et al. 2021). The selected biosorbent would also meet several other requirements, including eco-friendliness, ease of application, feasibility, and sustainability. This guarantees its prospective capacity to eradicate various toxins from wastewater (Priya et al. 2022). There is a critical demand for more diverse excellent qualities that are described by biosorbents. This involves its excellent stability, high removal efficacy toward the targeted contaminants, and ease of regeneration (SafaÇelik et al. 2022). Regeneration and biosorbent adaptation to several models (such as batch and fixed bed reactors) would be heavily considered. Accessible waste should receive the utmost attention based on the concept of waste as wealth. Due to their environmental friendliness, their use offers several advantages. It is economically advantageous because it solves disposal issues and brings in money for several sectors. The abundant biological materials have structural differences that are notable (Sayin et al. 2021). Various ligands, including alcohol, aldehydes, carboxylic, hydroxyl, phosphate, thiol, ketones, phenolic, and ether compounds, make up their structure. Due to their variable levels of presence, they can interact with target pollutants in a variety of ways. A potential replacement for traditional methods is biosorption (Yaashikaa et al. 2021). It depends on how bio-wastes are used to remove various types of water pollutants. Its theory is designed to get two uses out of such bio-pollutants. This is accomplished by reusing them to actively contribute to trash minimization and maximize their usefulness (Gupta et al. 2019). As a result, it permits achieving the reduction concentrations supported by global or domestic rules and the World Health Organization (Hespanhol and Prost 1994). It distinguishes itself by exceptional qualities, including low production and running costs, adaptability, simplicity, and great efficiency. Several biosorbents based on activated carbon (Boulika et al. 2022), agriculture wastes (Bushra et al. 2021), bio-calcium carbonate (Arslanoğlu 2021), biochar (Xiang et al. 2022), bio-nanocomposites (Motaghi et al. 2021), bio-hydrogels (Wan et al. 2022), chitosan (Chen et al. 2022a), macroalgae (Elgarahy et al. 2019), and microalgae (Shalaby et al. 2021) were used to purify different water pollutants.
For the treatment of wastewater, marine algae are regarded as an effective and alternative base sorbent. It considers the availability of renewable resources worldwide. Based on its hue and colloid content, it can be grouped. Oceans typically contain three basic kinds of organisms algal phyla, Chlorophyta, Phaeophyta, and Rhodophyta (Mokhtar et al. 2017). Recent research has shown that using microalgae is a sustainable substitute. Afshariani and Roosta (2019), for instance, researchers identified the batch and continuous removal of methylene blue in a contaminated solution. The ultimate sorption stayed attained at pH 9 and 30 °C, reaching 87.69 3.22 mg g−1. To find a more effective leather dye adsorbent, defatted microalgae biomass (also known as microalgae biofuel waste) was inspected (da Fontoura et al. 2017). Acid Blue 161 solutions were applied in biosorption analyses. At room temperature and 40 °C, respectively, the highest concentration of a dye adsorbed was 75.78 mg g−1 and 83.2 mg g−1. The findings showed that biomass remarkably eliminated the dye amounts in real tannery waste effluents by 76.65%. Alginate, carrageenan, and polycolloid comprise the algal cell wall, primarily made up of polysaccharides. These ingredients are capable of removing several types of water contaminants (Daneshvar et al. 2017).
Macro and microalgae were used as strong contenders for eradicating heavy metals and synthetic dyes from aquatic systems (Chen et al. 2019a; da Rosa et al. 2018). Enteromorpha flexuosa, a green macroalga, was tested for its capacity to absorb crystal violet and methylene blue cationic dyes from aqueous media (Elgarahy et al. 2019). The findings demonstrated that the removal efficacy of 90.3% and 93.4% under ideal variable circumstances, respectively, were attained. The primary component of green algae is cellulose, coupled with many proteins and polysaccharides to form glycoproteins (Jayakumar et al. 2014). These compounds have several functional groups (e.g., amino, hydroxyl, carbonyl, and carboxyl). They are interestingly crucial to sorption (Rangabhashiyam et al. 2016). Brown algae are distinguished by having a wide variety of metabolites, including halogenated substances, polyphenols, terpenoids, laminarins, fucoidans, mannitol, and alginates (Saravana et al. 2018; Farghali et al. 2023). Salts of calcium, phosphate, and sodium make up alginates. The primary naturally occurring components of the cell wall of brown seaweed are sodium salts. Its weight is almost between 30 and 40%. These polysaccharides are linear anionic and water-soluble (Fernando et al. 2019). For extracting alginate from brown seaweed, various pre-extraction treatment techniques were employed (Saravana et al. 2018). Numerous researchers have tested the usage of biochar synthesized from microalgae as another type of sorbent for removing heavy metals from contaminated solutions (Amin and Chetpattananondh 2019). Many trials on the efficacy of cobalt eradication employing biochar have been implemented in a batch scheme. The equilibrium numbers matched the Freundlich, Temkin, and D-R isotherm models. The Langmuir biosorption amount became 1.117 mg g−1 (Bordoloi et al. 2017). It has been identified that biochar synthesized from water hyacinths (Eichhornia crassipes) was a successful sorbent to remove several heavy metals from contaminated water and prevent the harmful effects of the existing invasive species. The further value of harvesting aqueous hyacinth medium as biomass for biochar is that it has fewer invading species effect on delicate aquatic environments. The cadmium removal from a contaminated medium by applying biochar-alginate capsules was studied, with maximal sorption amounts varying from 24.2 to 45.8 mg g−1 (Liu et al. 2020a).
Recently, batch biosorption of the rare-earth element ytterbium from the aqueous media using alginate and sericin particles chemically crosslinked with poly(vinyl alcohol) was explored. The equilibrium point revealed that at 55 °C, ytterbium had a maximum biosorption capacity of 0.642 mmol g−1. Calcium carbonate was used as the pore-forming agent to create sodium alginate-based beads with varying amounts of the pore-forming agent to increase alginate gel beads' sorption capabilities. According to the experimental findings, copper (II) adsorption's capability rose by at least twofold (from 13.69 to 33.88 mg g−1). Alginate-PEI beads were functionalized by phosphorylation and used for the sorption of neodymium (III) and molybdenum in another recent method of alginate modification (VI). While the phosphorylation of molybdenum (VI) significantly limits the enhancement, it considerably increases the sorption of neodymium (III). Because molybdate species have a strong affinity for amine groups, the phosphorylation of alginate-polyethyleneimine (PEI) beads enhanced neodymium(III) maximal sorption capacity from 0.61 to 1.46 mmol g−1. However, the neodymium (VI) uptake increase is much less pronounced (from 1.46 to 2.09 mmol g−1).
Chitin is a precursor of chitosan. It is primarily extracted from waste shellfish, such as crab or shrimp shells (Elwakeel et al. 2018). Chitosan sorbent can be cross-linked by different crosslinking agents either (e.g., citric acid, sodium tripolyphosphate, and polyaspartic acid sodium salt) or covalent (e.g., epichlorohydrin, trimethylo propane, and triglycidyl ether) to improve its stability (Jóźwiak and Filipkowska 2020). The Reactive Black 5 dye removal efficacy was significantly impacted by ionic crosslinking using the chitosan hydrogel. After 24 h of the sorption procedure, chitosan cross-linked with sodium citrate and sulfosuccinate had a greater sorption capacity of 46.7% and 37.2%, respectively, than chitosan, which had not been cross-linked. Moreover, after 24 h of sorption, it was noticed that the removal efficiency of the chitosan cross-associated with glutaraldehyde and trimethylolpropane triglycidyl ether reduced to 35.3% and 26.6% lesser than that of unmodified chitosan. The unmodified chitosan displayed the maximum sorption removal (2307.0 mg g−1) once the sorption equilibrium had been reached.
In comparison, the sorption potential of the crosslinked hydrogels (ionically/covalently) ranged from 2005 to 2164 mg g−1 and from 2083.0 to 2183.0 mg g−1, respectively (Jóźwiak and Filipkowska 2020). Chitosan-based magnetic sorbent decorated with polypyrrole was synthesized to remove anionic methyl orange and cationic crystal violet dye from wastewater. The removal performance was measured at 88.11% and 92.89% under optimized operational conditions for crystal violet and methyl orange dyes. Though the pseudo-first-order kinetic model more closely matched methyl orange, the pseudo-second-order better-reflected crystal violet adsorption. The results from the reaction equilibrium for the two dyes closely fitted the Langmuir isotherm model, which owned a maximum sorption capacity of 62.89 and 89.29 mg g−1 for crystal violet and methyl orange dyes, respectively. Chitosan can also be altered by becoming immobilized on another polymer. For instance, ethoxy-functionalized 4-methyl-2-(naphthalen-2-yl)-N-propylpentanamide efficaciously modified chitosan (Jabli 2020) and then examined for the presence of methylene blue and acid blue. After applying multiple beads, the adsorption capacity increased 1.4 times for acid blue 25 and three times for methylene blue, in comparison with the capacity of the original chitosan beads.
Adding tiny volumes of chitosan and Yttrium trivalent to the acid-based fly ash created another chitosan hybrid composite with a saturation adsorption amount of 627 mg g−1 (Li et al. 2020a). The researchers put much effort into improving chitosan's selectivity towards metal ions. For instance, developing highly selective sorbents is possible by grafting 2-mercapto benzimidazole onto chitosan microparticles. Another tactical component for the feasible recovery process of small chitosan particles is the integration of magnetite particles (Elwakeel et al. 2021). The sorbent demonstrated high eligibility for valued metals over base metals. Chitosan and 2-mercapto benzimidazole can create a very effective sorbent for recovering valuable metals from acidic leachates. Batch experiments were performed to estimate the adsorption capacities of the heavy metal pollutants (manganese, iron, cobalt, nickel, copper, and zinc) on chitosan relative to their comparable anions, sulfate, chloride, and nitrate (Weißpflog et al. 2020). The various heavy metal ions removal efficacy was analyzed using column tests. Upon comparison, the chloride anions and nitrate salts, respectively, the heavy metal cations of the sulfate salts and the sulfate ions adsorb to a substantially greater amount (Weißpflog et al. 2020).
To effectively eradicate chromium from wastewater, Luo et al. (2021) produced fluorescent chitosan built on hydrogel that contained titanate and cellulose nanofibers improved with carbon dots. The sorbent's increased capacity for adsorbing chromium of 228.2 mg g−1 may be primarily attributable to its porous design and the addition of titanate and cellulose nanofibers treated with carbon dots. By inserting co-polymerization on the surface of the chitosan/iron oxide composite, a sequence of magnetically modified chitosan adsorbents in conjunction with core-brush topology was produced. These sorbents were consequently applied to remove two water contaminants (e.g., diclofenac sodium and tetracycline hydrochloride) from the contaminated medium (Zhang et al. 2016).
Calcium carbonate is regarded as one of the most adaptable substances ever created by humans. It is pervasive and makes up more than 4% of the earth's crust. The common forms of calcium carbonate include chalk, marble, and limestone. The primary source of bio-calcium is the skeletons of some marine creatures, such as shellfish, crustacean shells, coral, seaweed, bivalves, and snails (Elwakeel et al. 2020). Aragonite, calcite, and vaterite are the three primary types of carbonate (Hoque et al. 2013). Even though the chemical makeup of all these forms is similar, they differ in terms of whiteness, homogeneity, thickness, and purity. Calcium carbonate has a distinctive white hue and is widely used in cement. In coatings features, including plastics, dyes, and paper productions, it has a wide range of purposes as a padding and/or coating material (Thilagan et al. 2015). Additionally, due to its antacid qualities, calcium carbonate is utilized in industrial locations to neutralize acidic mediums in soil and water (Correa et al. 2013).
A different fast-developing class of materials called MXenes is based on carbides or nitrides of transition metals (e.g., Titanium, Niobium, and Vanadium) and is applied for various purposes. MXenes have a large specific surface area, are chemically stable, and have various effective adsorption sites originating from external functional groups such as hydroxyl (Jeon et al. 2020). Due to its negative surface charge and robust-sheet-like structure, which contains plenty of active surface areas, titanium carbide (Ti3C2Tx), for example, removed 180 and 225 mg g−1 of barium and strontium ions in fracturing effluent. Metal–organic frameworks are crystalline hybrid inorganic–organic compounds. They have incredibly high specific surface areas reaching up to 6500 m2 g−1 (Wang et al. 2015). They are excellent contenders for the adsorption of contaminants because they have a convenient pore volume and a spatial topology with an arranged porous composition originating from positive metal ions, metal groups, and organic connections (Huang et al. 2021a). Metal–organic frameworks are unstable in moist environments, but this can be remedied by changing the surface with appropriate functionalization, such as water-balanced. Metal–organic framework with a great specific surface area (1288 m2 g−1) called MIL-53 (MIL stands for Matériaux de l'Institut Lavoisier) was applied to remove amoxicillin, which from the primary concentration of 150 mg L−1, at a quantity of 0.1 g L−1 (Imanipoor et al. 2020). Another study found that a mercaptosuccinic-functionalized zirconium-based metal–organic framework with an adsorption amount of 1080 mg g−1 for Mercury ions and 510 mg g−1 for lead ions at pH 4.0 might potentially adsorb the harmful metals from wastewater (Wang et al. 2020a). Along with the substances mentioned overhead, carbonaceous materials such as carbon aerogels (Kalotra and Mehta 2022), carbon hydrogels (Yang et al. 2020b), and carbon xerogels (Girgis et al. 2012) also reported for the elimination of pollutants. These materials would share characteristics including an enormous specific surface area, great porosity, and organized structures even if they were created using various synthesis processes.
Graphene oxide and reduced graphene oxide, which are the graphene compound derivatives, own special sheet-like constructions, stand out among the numerous carbon nanostructures for their elevated specific surface area, high thermal stability, mechanical stability, and unique functional groups (Thakur et al. 2019). Although such materials exhibit strong adsorption capability, a significant problem that prevents various active sites from being available for contaminant remediation is their propensity to agglomerate. Aggregation is handled by including functional groups, such as oxygenated functional groups, or insertion constituents amongst graphene sheets (Baig et al. 2019). As an illustration, the reusability findings of fixed-bed removal revealed that a silica gel/graphene oxide-based adsorbent with an adsorption dosage of 147 mg g−1 for indium ions demonstrated successful regeneration (Li et al. 2021). The electrical characteristics, oxygen content, and interactions with contaminants of both graphene oxide and reduced graphene oxide-based adsorbents remain significantly influenced by structural intactness, which is sensitive to uncertain environments such as temperature, radiation, and specific pH media. The adsorption capacity of lead (Pb2+) improved with irradiation doses. Nevertheless, an opposite tendency happened for chromium hexavalent, according to a rapid, substantial-ion-band and electron-beam radiation that altered the oxygen content of graphene oxide (Bai et al. 2016; Yang et al. 2021).
Another derivative of carbon nanomaterials is carbon nanotubes, comprising functionalized carbon nanotubes. Carbon nanotubes have fascinating structures, for instance, elevated thermal constancy, great chemical steadiness, nano-composition, uniform pore volume, exclusive specific surface area, ease of functional group formation, and tube-shaped construction, all of which produce great records of adsorption pore sites for removing different contaminants such as heavy metals and synthetic dyes (Mashkoor and Nasar 2020; Sarkar et al. 2018).
A distinct category of carbon nanomaterials comprising functionalized carbon nanotubes is the diatomite-carbon nanotubes made through acid modification and chemical vapor deposition. It had a specific surface area of 50 m2 g−1 and demonstrated two different adsorption capabilities. The high thermal and chemical steadiness, nano-composition, uniform pore size, great specific surface area, easiness of functionalization, and tube-shaped construction of carbon nanotubes are altogether intriguing properties that result in a significant number of adsorption sites for contaminants like metal ions and dyes.
Regeneration of spent adsorbents
Adsorbents that exhibit superior aquatic permanence can be easily separated from wastewater effluents after removing contaminants. The potential of exhausted adsorbents to be recovered, decontaminated, and regenerated will decide how useful they can be again (Yang et al. 2020a). A quality sorbent can be recycled and recovered for manufacturing, greatly reducing the cost of making adsorbents (Gupta et al. 2020). The restoration of used adsorbents can be done repeatedly; nevertheless, the restored adsorbent's adsorption capacity is lesser than that of recent adsorbents (Reddy et al. 2017). The effectiveness of the contaminant's desorption can be increased using the appropriate regeneration procedure. The viability of the industrial-scale application relies on numerous aspects, involving the sort of adsorbent, the pollutant, the adsorbent's stability, the toxicity of used adsorbents, and the expense and energy supplies of the recyclability method. Several methods involving magnetic separation (Tamjidi et al. 2019), filtration (Da’na and Awad 2017), thermal regeneration (Hwang et al. 2020), solvent reusability (Jiang et al. 2018), microwave treatment (Zhang et al. 2014b), supercritical fluid restoration (Shahadat and Isamil 2018), advanced oxidation method (Acevedo-García et al. 2020), and microbial-assisted pathway (Abromaitis et al. 2016) are used to recover the saturated adsorbents. Evaluating various recovery and regeneration procedures is crucial to recognize the ultimate recycling and dumping of exhausted adsorbents. Regeneration efficiencies of different sorbents in removing heavy metals, dyes or other contaminants and their adsorption efficiency are summarized in Tables 6, 7 and 8.
Regeneration methods
Thermal regeneration
Thermal desorption is a new technique for recovering metal from the used adsorbent. Thermal regeneration involves raising a sorbent's temperature to a specified point to break the chemical and physical bonds that hold sorbate and sorbent together (Shahadat and Isamil 2018). Activated carbon is regenerated using this process on an industrial and commercial scale.
The carbon format and its volatile components will be eliminated from biochar when heated in the atmosphere at temperatures lower than 500 °C (Zhang et al. 2019a). Discarded-painting paper biochar with a superior preservative matter reduced lead metal ions from liquid media with the highest adsorption volume of 1555 mg g−1 (Xu et al. 2017). Moreover, the wasted biochar was pyrolyzed at around 350 °C, which assisted in the gain of lead metal ions and subsequent conversion to lead oxide nanoparticles on the nano-biochar’s surface with higher purity (greater than 96%). Turning exhausted adsorbents into beneficial products over thermal regeneration has received relatively little attention up to this point and is still in its beginning. However, the process's discharge of volatile compounds into the atmosphere could be a potential cause of secondary pollution. Potential impacts towards the ecosystem and human health can be observed in the emission of Polycyclic aromatic hydrocarbons and dioxin as process byproducts. As a result, the benefit of biochar in carbon immobilization is eradicated (Toński et al. 2021). Multi-walled carbon nanotubes were effectively recycled, and it was used to remove cyclophosphamide, ifosfamide, and 5-fluorouracil along with a superior adsorption potential. For the greatest recovery of carbon nanotubes, the temperature and duration of the thermal recyclability requirements are varied, and the ideal circumstances are discovered to be 300 °C for 2 h. Findings further demonstrate that the adsorption capacity is unaffected even after 5 sets of adsorption and desorption.
In a further analysis, heat-treated gilsonite was utilized as an efficient adsorbent for removing toluene from wastewater (Saffarian Delkhosh et al. 2021). After four thermal recyclability sets, 250 °C and 20 min are used for regeneration, with adsorption effectiveness of 62.12%. Thermal regeneration demonstrated a higher toluene removal efficiency than acetone and ethanol washing. However, due to its quickness, selectivity, and regulated heating, microwave treatment technology is currently used to replace thermal regeneration (Falciglia et al. 2018). This method entails the transformation of microwave radiation into heat at the molecular phase using adsorbent material (Falciglia et al. 2017). The sorbent is uniformly heated by microwave treatment from the exterior to the interior.
In contrast to traditional thermal heating, microwave heating preserved the sorbent's porous feature and the adsorbate's properties (Dai et al. 2019). Microwave irradiation technology effectively utilizes controlled heating to regenerate used sorbents. This technique takes advantage of the interaction between the delocalized electrons of activated carbon, the adsorbent material, and the microwave electrons. These interactions can facilitate the regeneration of activated carbon exhausted by perfluoroalkyl and polyfluoroalkyl substances. The dielectric properties of activated carbon, combined with the high volatility of the organic contaminants, contribute to the success of this regeneration process. In addition to being expensive to set up the plant, using microwave irradiation on an industrial scale to thermally desorb adsorbents is also an energy-intensive procedure. However, further research is necessary to determine whether microwave treatment may regenerate used sorbents (Gagliano et al. 2020).
Recently, pollutants loaded onto the exhausted adsorbents have been dissolved using a thermal process, producing a new material with a different porous composition and surface-chemical properties. Following this heat treatment of the used adsorbent, the resultant adsorbent was used to reabsorb similar contaminants with slightly reduced elimination capacities. For instance, Sonmez Baghirzade et al. (2021) suggested a thermally adjusted method for efficiently reusing granular activated carbon loaded with perfluoroalkyl and polyfluoroalkyl materials. This method involves mineralizing the persistent Perfluoroalkyl and Polyfluoroalkyl Substances to recover the used granular activated carbon. Perfluoroalkyl and polyfluoroalkyl substances have been shown to desorb and volatilize at temperatures as low as 175 °C. However, they can also be mineralized at much higher temperatures, up to 700 °C (Xiao et al. 2020). High-temperature thermal desorption, in particular, leads to significant energy requirements that impede its sustainable use and applicable industrial manufacture. Thermally treated, wasted granular activated carbon may exhibit a growth in specific surface area and micropore size with temperature, although extremely high temperatures (over 1200 °C) may permanently damage the pore structure. Following removing the antidepressant medication contaminant amitriptyline, Chang et al. (2021) used a 600 °C modification for 2 h to renew a montmorillonite substance. The regenerated adsorbent's physicochemical characteristics changed, showing a removal of amitriptyline of 71.7 mg g−1, or around 26% of the original montmorillonite. Therefore, using the thermal decomposition approach, it is crucial to use the right temperature and treatment conditions (such as a gaseous atmosphere) to successfully regenerate an adsorbent. Future research is necessary for scientific advancement and process scaling because the thermal treatment settings can fluctuate based on the nature of adsorbents, pollutants, and the intent of the resulting product.
Chemical regeneration
Chemical regeneration is one of the most common methods for renewing adsorbent materials (Alsawy et al. 2022; Wu et al. 2020). It primarily depends on the concentration of the adsorbate and the forces interacting with the adsorbent. Chemical regeneration may better suit organic adsorbents with low boiling points and limited thermal stability (Dai et al. 2019). Solvents and chemical reagents are used in the chemical regeneration process. Acids and alkalis like phosphoric acid, nitric acid, sulfuric acid, ethylenediaminetetraacetic acid, calcium nitrate, sodium hydroxide, and sodium nitrate have been employed as regenerating solvents (Gupta et al. 2020; Hassan et al. 2020; Yang et al. 2020a). Baig et al. (2014) demonstrated that applying 0.5 M sodium hydroxide allowed arsenic ions to be regenerated from magnetic sorbent materials and subsequent magnetic sorbents to be recycled (Baig et al. 2014). A sizable desorption efficiency is monitored when acids are employed as regeneration solvents. In an acidic environment, adsorbent renewals and causes metals desorption from the surface of the adsorbent. Notably, the sorbent should have chemical stability as the adsorbent structure may be distorted by the chemical's action (Dai et al. 2019).
Supercritical fluid desorption
A substance is transformed into a supercritical fluid when heated over its critical temperature and squeezed past its critical pressure (Shahadat and Isamil 2018). Supercritical fluid desorption to regenerate used adsorbents is widespread and is considered a replacement for chemical-solvent and incineration methods (Efaq et al. 2015). The supercritical fluid desorption behaves as a typical solvent in the soil matrix and speeds up the process of pollutant desorption. The pollutant is further condensed by lowering the pressure until it can eventually be collected into a small container. Due to its incombustibility, non-hazardousness, and affordability, carbon dioxide is the most preferred for supercritical fluid desorption and is commonly utilized (Noman et al. 2020).
Additionally, supercritical fluid desorption exhibits a rapid mass transfer rate and reduced surface tension. Despite its advantages, carbon dioxide has a lower capacity for regeneration with phenol-loaded adsorbents (Humayun et al. 1998). To tackle this problem, authors substituted carbon dioxide with supercritical water, which completely regenerated phenol from the phenol-loaded sorbent material and achieved effectiveness levels of about 100% (Salvador et al. 2013). The use of supercritical water has benefits and drawbacks. For example, it has a short process time, significantly lowering costs. Still, it also requires elevated pressure, which raises the method's cost and limits its industrial application. Supercritical water regeneration can only be used on a small scale. Zhang et al. (2019b) used an alkali metal catalyst and supercritical water regeneration to regenerate activated carbon using hydrogen peroxide. With a recyclability efficacy of 107%, the supercritical water regenerated trials had improved specific surface area (813 m2 g−1), contrary to the original samples (765 m2 g−1). Additionally, it has been revealed that the restoration temperatures (385 °C, 405 °C, and 425 °C), the hydrogen peroxide concentration, and the base metal behaving as a catalyst all impact the phenol contamination's capability to fasten to surfaces. In a unique study, Granular activated carbon is restored utilizing supercritical carbon dioxide (Carmona et al. 2014). Here, the pressure (e.g., 6, 15, 20, 31 MPa) and temperature (e.g., 45 °C, 60 °C) influenced the desorption yield of the contaminants. Intended for phenol, 2-chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol, respectively, desorption yields of up to 97.9%, 68.3%, 71.5%, and 64.5% were achieved at 31 MPa and 45 °C. To remove salicylic acid from bentonite that has been biologically functionalized, salgn used ethanol (Salgın et al. 2004). When no ethanol was used, the desorption efficiency was 76%, and when 10% (v/v) ethanol was applied, it was 98%. These results demonstrate the possible contribution of supercritical fluid desorption and the purpose of a co-solvent in the regeneration of toxins and recyclability of exhausted adsorbents. Nevertheless, for this procedure to be used on an industrial scale, creative methods for lowering the cost must be discovered.
Chemical versus thermal regeneration
The adsorption capabilities of such exhausted carbonaceous material are best enhanced by combining thermal and chemical regeneration. Compared to the temperature settings, acidic medium, and both treatment-washing steps, the latter brought more encouraging results in the regeneration process than the other recyclability treatments. The method was repeated five times to assess the consistency and removal efficacy of the carbonaceous material. The effectiveness and efficiency of the recycled adsorbent for removing various contaminants were demonstrated by the fact that this substance has nearly equal adsorption stability measurements and kinetic rate (Nahm et al. 2012). By using chemical, thermal, and electrochemical methods, the recyclability tests of phenol adsorbed on carbon-rich material (activated carbon) have been studied and evaluated. The sodium hydroxide-based alkaline medium improved phenol desorption efficiency compared to pure water. A clearance efficacy of greater than 60% cannot be attained with this procedure. In this work, the phenol compounds were eliminated after 3 h of electrochemical exposure; however, thermal recyclability requires at least 450–600 °C to produce equivalent results. This study found that, when run under ideal operational parameters, electrochemical regeneration could remove particles with an efficiency of about 80% and produce surfaces with more functionality and surface area than thermal regeneration testing (77%). As a result, it might be suggested that chemical regeneration (in the electric field) is a better method than thermal regeneration (Berenguer et al. 2010). When Enteromorpha Porifera-derived biochar polluted with polycyclic aromatic hydrocarbon compound (Pyrene) was regenerated at 80, 150, and 200 °C, the renewal effectiveness was 35%, 45%, and 48%, respectively, indicating a correlation between the heat and the removal capability.
In experiments using iron-based biochar to remove microcystin-LR from polluted aqueous media, quartet sets of persulfate recyclability experiments achieved a removal efficacy of 92.81% and a regeneration efficiency of 82.89% in lake water. Furthermore, increasing the temperature from 20 to 50 °C significantly enhanced microcystin-LR oxidation and the impact of persulfate during the persulfate recyclability tests on iron-based biochar (Zeng and Kan 2022). A peroxymonosulfate-enhanced novel electrochemical method was used to regenerate rice-based biochar. Regeneration was successfully achieved under suboptimal conditions using a 75:1 ratio of peroxy monosulfate (PMS) to fluoxetine, 150 mA of appropriate current, and 0.15 mM of citric acid. Multiple adsorption–desorption cycles were completed successfully with minimal heat while maintaining the biochar's consistency. This technique demonstrates the applicability of chemical regeneration, which can be used under optimal conditions without external heat (Escudero-Curiel et al. 2021).
The regeneration process using the (water cleansing + nitrogen purging + alkali soaking) method was successful. The iron-copper-potassium hydroxide/biochar reusability test was conducted under medium conditions of 500 °C and 13% potassium hydroxide soaking matter. After three cycles of this test, the findings showed that the sulfur desorption had reached 42.64 mg sulfur g−1, close to the adsorbent's initial removal capability of 48.58 mg sulfur g−1. The nitrogen-wide approach reduced the ferric sulfate (Fe2(SO4)3) into ferric oxide (Fe2O3), recovering a sizable amount of pre-diameter and effective components. However, the oxidation of the used-up biochar at a high temperature decreased regeneration effectiveness. The alkaline soaking procedure supplied the hydroxyl group. It reclaimed its alkaline strength, which helped with the desorption of sulfur. However, the high alkalinity levels in the medium resulted in the clogging of the pores of the employed biochar. It thus decreased the capacity for desorption. The block influence of the potassium hydroxide in the medium was blamed for the reduced removal capacity (Song et al. 2017).
Exhausted adsorbent management and disposal
Secondary contamination is the primary restriction on chemical regeneration. It should be thought about managing used adsorbents sustainably to protect the environment. Disposal methods were suggested for reuse, incineration, and landfilling (Baskar et al. 2022; Lata et al. 2015). The old adsorbents can be recycled for soil fertilizers, energy transfer, storage devices, capacitors, and catalyst/catalyst support. Findings showed that nutrient-enriched biochar is an organic fertilizer that might replace synthetic fertilizers (Liu et al. 2019). Shortly, metal-impregnated green materials could replace carbon nanotubes and be utilized as supercapacitors or to remove tar (Baskar et al. 2022). When exhausted adsorbent material is applied as a supplier of thermal power rather than coal, incinerating waste results in less corrosiveness and harmful gas emissions. This process is known as waste-to-energy (Martín-Lara et al. 2016). Landfilling is a method for disposing of exhausted adsorbents, where the concentration of pollutants in the used adsorbent is identified before dumping to determine the acceptability of this method. For materials contaminated with heavy metals, pretreatment is required before landfilling. Other strategies, such as microwave irradiation, phyto-capping, and phytoremediation, could also be applied (Alsawy et al. 2022; Fuke et al. 2021).
Perspective
In order to release wastewater into the environment, it must first undergo a cleaning process to remove various organic and inorganic impurities. Adsorption using organic and inorganic adsorbent materials such as charcoal, activated carbon, clay minerals, and zeolite is a standard method for removing unwanted substances from wastewater. Exhausted adsorbents can often be recycled for the circular economy through various techniques, including filtration, chemical and thermal regeneration, and advanced oxidation methods. The recovery and regeneration approaches are significantly impacted by the type of pollutant and the adsorbent used. Regenerated samples perform exceptionally well in terms of wastewater adsorption. There is a growing interest in developing advanced, high-capacity adsorbents for extracting and recovering pollutants from wastewater. Researchers are also focusing on the cost and safety of the adsorption process. The recovery of sorbed pollutants after disposing of end-of-life adsorbents has been a significant challenge until recently. The adsorbents can now be recycled as catalysts, capacitors, or soil amendments, or they can be safely disposed of through incineration or landfilling.
Reusing used adsorbents not only reduces application costs but also has environmental benefits. Chitosan-based materials have many applications, including tissue engineering, medication delivery, bioimaging, and wound healing (Ahmad et al. 2017, 2019, 2021b). Repurposing used chitosan-based materials for such applications would be interesting. Life cycle analysis is a valuable tool for assessing a system's environmental and financial viability, including wastewater treatment. This method considers both positive and negative effects, considering waste management, cost, energy use, and safety. For example, a life cycle analysis was conducted on activated carbon from discarded cherry and sour cherry kernels (Vukelic et al. 2018). This complex process involves various components: transportation, management, chemical handling, water and acid usage, energy consumption, rinsing, waste paper utilization, and wastewater treatment. The results indicate that utilizing waste cherry and sour cherry kernels for this process is economically and environmentally feasible, with minimal environmental impact when implemented at a production scale. Researchers must focus on life cycle analysis, especially for new adsorbents, to better understand their environmental impact. In order to successfully implement the regeneration technology for practical applications, several challenges need to be addressed in future research works:
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Many studies have focused on using adsorbents to remove a single contaminant from synthetic wastewater, which is impractical for real-world applications. Therefore, future studies should consider using genuine sewage wastewater with a mixture of contaminants to assess the effectiveness of the regeneration method.
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Developing cost-effective technologies for recycling pollutants from used adsorbents should be a key research focus.
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Characterizing the adsorbent material (e.g., surface structure, porosity, and functionality) after each regeneration cycle is essential to fully understanding the regeneration process. Therefore, artificial intelligence technologies can be developed to predict optimal recovery conditions and facilitate the long-term use of the adsorbent in wastewater treatment applications.
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The surface of many adsorbents can be modified to enhance their adsorption capacity while reducing regeneration problems. Therefore, researchers should suggest further modification methods that could aid in the complete chemical regeneration of the adsorbent material.
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The potential benefits of using biomass feedstocks in situ for functionalization and material improvement should be explored to reduce costs and improve sustainability.
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Previous experiments have demonstrated the effectiveness of using certain adsorbents to extract and recover precious metals from wastewater, but the scale of these experiments was limited. Therefore, further research is needed to scale up the use of adsorbents in wastewater treatment and to strengthen the technical feasibility of the adsorption/desorption process.
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The oxidative breakdown of organic contaminants is necessary for chemical regeneration, but it is still unclear whether harmful by-products may be produced. Thus, further research is required to ensure the safety and environmental impact of the regeneration process.
Conclusion
The success of biosorbents depends on their physical and surface chemistry characteristics, and understanding the removal mechanisms of each class of adsorbent is crucial to ensure high adsorption performance. The use of raw industrial waste and its functionalization offers a promising strategy for sanitation and water reuse, aligning with sustainable development objectives. This review has discussed various preparation and modification techniques to produce efficient biosorbents using various materials, including wood, bacteria, algae, herbaceous materials, agricultural waste, and animal waste. The preparation method to control the magnetic sorbents' shape, morphology, magnetic property, and particle size was also discussed.
Adsorption processes using different adsorbents were evaluated for removing various contaminants from wastewater and water, providing insights into isotherm and kinetic models for optimizing adsorption behavior and designing effective pollutant-adsorbent systems. The review also covers techniques for recovering, decontaminating, and regenerating exhausted adsorbents, emphasizing the importance of maximizing their reuse. Finally, future recommendations on biosorbents and magnetic sorbents are highlighted. Overall, this review emphasizes the crucial role of biosorbents and adsorption processes in promoting sustainable development and the circular economy.
Abbreviations
- PPCPs:
-
Pharmaceuticals and personal care products
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Acknowledgements
Dr. Ahmed I. Osman and Prof. David W. Rooney wish to acknowledge the support of The Bryden Centre project (Project ID VA5048), which was awarded by The European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB), with match funding provided by the Department for the Economy in Northern Ireland and the Department of Business, Enterprise and Innovation in the Republic of Ireland. The researcher (Mohamed Hosny) is funded by a full scholarship (MM32/21) from the Egyptian Ministry of Higher Education & Scientific Research represented by the Egyptian Bureau for Cultural & Educational Affairs in London.
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Osman, A.I., El-Monaem, E.M.A., Elgarahy, A.M. et al. Methods to prepare biosorbents and magnetic sorbents for water treatment: a review. Environ Chem Lett 21, 2337–2398 (2023). https://doi.org/10.1007/s10311-023-01603-4
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DOI: https://doi.org/10.1007/s10311-023-01603-4