Modification of agricultural wastes to improve sorption capacities for pollutant removal from water – a review

Abstract

Water pollution is a great risk to aquatic ecosystems and human health. Among water pollution remediation strategies, adsorption mechanisms provide strong efficacy for a wide variety of pollutants. Several recent efforts examined the development of low cost adsorbents utilizing commonly available agricultural wastes. However, raw (i.e., unaltered) agricultural wastes typically exhibit low sorption capacity for pollutants due to their non-reactive structural/composition properties. Hence, modifications of raw agricultural wastes to enhance their sorption capacities for various aquatic pollutants are necessary to optimize their performance for pollutant removal. Numerous modification techniques are effective in altering agricultural wastes for improved sorption performance. This paper reviews the development of modified agricultural waste materials for pollutant removal from water. We compiled an extensive inventory of modification techniques applied to agricultural wastes to enhance their adsorption capacities for removal of a wide range of pollutants. Modification strategies and their effects on sorption properties were rigorously examined to highlight key advancements in the preparation of adsorbents from agricultural wastes. Finally, the costs of these materials were examined, along with the benefits and drawbacks concerning their use. This review provides a thorough assessment for advancing the utilization of agricultural wastes for preparing adsorbents for water treatment.

Graphical Abstract

Highlights

1. Modification methods and their effects on adsorption performance are summarized.

2. The roles of introduced functional groups and their retention mechanisms are investigated.

3. Benefits and drawbacks of low-cost agricultural waste adsorbents are assessed.

1 Introduction

Developments in industry and agriculture have greatly improved people’s living standards, but they often cause negative impacts on environmental health. Water pollution caused by human activities is a progressively growing threat, especially in rapidly developing countries (Mo et al. 2018). A wide variety of pollutants (e.g., nutrients, heavy metals, dyes, phenols, antibiotic, pesticides) are released into natural waters where they pose a serious threat to drinking water safety, human health and aquatic ecosystems (Abdolali et al. 2014; Ling et al. 2006). These pollutants may be further transmitted from the water to soils when the polluted water is used for agricultural irrigation, thereby entering plants and threatening food safety (Afroze and Sen 2018). Accordingly, the treatments for remediation of contaminated wastewaters are necessary to protect human and ecosystem health. Various techniques have been developed for pollutants removal, including adsorption, biodegradation, coagulation/flocculation, oxidation, membrane filtration etc. (Qu et al. 2022a; Rashid et al. 2021). Among the various methods applied to water purification/remediation in recent decades, sorption techniques display strong efficacy due to their high adsorption capability for selected compounds and their ease of preparation and operation (Mo et al. 2018).

Commercial sorbents demonstrate high removal rates for selected pollutants from wastewater, however, their high production cost, intricate design and complex operational conditions often limit widespread application. Hence, there is a distinct need for low-cost and effective adsorption materials to remove pollutants from waters (Abdolali et al. 2014). Nowadays, there is considerable interest in utilization of cheap alternatives based on the sorption potential of agricultural wastes, which provide favorable economic and eco-friendly (i.e., green chemistry) options (Kwon et al. 2020; Yang et al. 2019). Widespread agricultural and industrial activities generate large amounts of solid wastes. Some of these materials are reused, whereas other materials find no commercial utilization resulting in their ultimate disposal, which often causes environmental damage (Shi et al. 2020; Wang et al. 2013). Agricultural wastes are ubiquitous and nearly free of cost under most circumstances. Thus, conversion of these waste materials into low-cost adsorbents for water pollutant remediation is a compelling alternative that also solves a solid-waste disposal problem. There are now several examples of the utilization of raw agricultural wastes to successfully remove a myriad of water contaminants (Bhatnagar et al. 2015).

Raw (unaltered) agricultural wastes usually do not have high adsorption capacity for pollutants due to their inherent chemical structure/composition (Kaçakgil et al. 2021). The intermingled chemical structures comprising agricultural wastes and the low specific surface area of industrial wastes limit their physical and chemical interactions with many pollutants (Liu et al. 2020a). Consequently, several studies have investigated modification of agricultural wastes to enhance their adsorption capacity for various contaminants (Ding et al. 2014). Modification processes alter several physicochemical properties of agricultural wastes, such as their hydrophilic/hydrophobic characteristics, surface area and porosity, reactive functional groups, resistance to microbiological attack and thermal stability, thereby improving the adsorption performance for pollutant removal (Abdolali et al. 2014).

To date, a limited number of reviews exist on the use of agricultural wastes to adsorb pollutants and most of them take a holistic look at a variety of pollutants by focusing on either adsorbate-specific or adsorbent-specific applications (Bhatnagar et al. 2015; Dai et al. 2018). Notably, there are currently no systematic and comprehensive syntheses exploring the use and modification of agricultural wastes for pollutant abatement. Thus, in this review, we compiled and evaluated the various methods used to prepare agricultural waste adsorbents. Further, we compiled the literature on the extensive use and efficiency of these modified adsorbents for removal of various pollutants. The primary goals of this review were to: (i) present a comprehensive summary on the use of modified agricultural wastes as adsorbents in water pollution treatment; (ii) examine the myriad of mechanisms enabling pollutant removal; (iii) compile adsorption efficiency metrics for typical agricultural wastes; and (iv) provide perspectives on future applications and research needs to facilitate the efficient use of agricultural wastes in water pollution remediation.

2 Agricultural waste materials and their modification

Agricultural wastes, particularly those containing cellulose, are a by-product of agricultural and forestry production and mainly composed of lignin, cellulose and hemicellulose (Bhatnagar et al. 2015). These components contain several functional groups (e.g., alcohols, aldehydes, ketones, ether groups, and aromatic rings) in their molecular structures that can adsorb pollutants (Dai et al. 2018). The type of functional groups and chemical components comprising different agricultural wastes are similar, but occur in appreciably contrasting amounts. Some functional groups are shielded or sterically hindered inside the bulk structure of the organic matrix rendering them inaccessible to interact with pollutants directly, thereby hindering adsorption performance.

Modification alters the nature of surface functional groups, with the specific modification methods selected to optimize products for the intended application and chemical characteristics of the targeted contaminants. Modification methods for agricultural wastes can be classified into physical and chemical modifications. Chemical modification usually creates better physicochemical properties (functional group and surface area) for adsorbents, which enhance the adsorption performance more than that of physical modification techniques (Aryee et al. 2021). Thus, chemical modification is the most common way to modify agricultural wastes. Various chemical methods have been used to modify agricultural wastes in recent decades. Among these methods, alkalization, acidification, esterification, etherification, carbonization and magnetization are common modification procedures for agricultural wastes, along with surfactant modification and grafting (Fig. 1). The following discussion provides an overview of the common modification treatments employed to alter the physicochemical properties of agricultural wastes along with a synthesis of supporting studies demonstrating the efficacy of these modified products for pollutant retention/removal.

Fig. 1

Classification of modification techniques for agricultural wastes

2.1 Alkalization

Conventionally, alkaline treatment for modification of agricultural wastes is performed by reaction with NaOH or KOH (Dai et al. 2018). The treatment reduces the content of amorphous constituents, such as lignin and hemicellulose, thereby increasing the surface area and exposing additional functional groups within the treated agricultural wastes (Kovacova et al. 2020; Liu et al. 2020a). After alkaline modification, the mechanical properties and thermal stability of agricultural wastes are also improved, further facilitating enhanced pollutant removal (Sahu and Gupta 2020).

Asadi et al. (2008) demonstrated increased BET surface area of rice hulls following NaOH treatment from 0.7 to 4.3 m² g^− 1, which played an important role in increasing interactions between adsorbent and adsorbates. Some studies demonstrated that modified agricultural waste surfaces have augmented pores compared to the virgin feedstock (Ding et al. 2014), which increased pollutant adsorption through improved diffusion into pores. For example, KOH alkalization appears to unblock some pores resulting in the generation of more visible pores than the untreated material, which strongthens the retenion ciprofloxacin and tetracycline on the surface of adsorbent through hydrophobic interactions, H-bonding or van der Waals forces (Egbedina et al. 2021). Moreover, hydroxyl groups are generated in new adsorbents after NaOH treatment via hydrolysis of carbonyl groups and dissolution of lignin and hemicellulose, which enhance chemical reactivity for pollutants (Zafar et al. 2020). Furthermore, alkaline treatment releases cations (e.g., K⁺, Ca²⁺) from the treated residues that increase the ion exchange capacity contributing to increased retention of cationic pollutants (Azzaz et al. 2017). As a result, NaOH treated rice husk showed strong complexation, ionic exchange and H-bonding interactions with Cu²⁺ due to increased hydroxyl groups content, cation exchange capacity and porosity.

Notably, NaOH treatment may confer differing effects for different types of agricultural wastes. For example, sawdust generated from poplar and fir wood displayed similar adsorption capacities for Zn²⁺, while the adsorption capacity for Zn²⁺ increased by approximately 17 and 13 times for poplar and fir sawdust after NaOH treatment (Sciban et al. 2006). The contrasting sorption capacities induced by NaOH treatment were attributed to differences in the anatomical structure and chemical composition of the contrasting woody materials.

Alkaline treatment is an efficient methodology for exposing/generating functional groups on organic materials. The removal rate of Cu²⁺ by NaOH-thermal treated wheat straw increased to 21.9% (288 K), 25.8% (298 K), and 28.3% (308 K) compared with raw wheat straw (Guo et al. 2016). An increased intensity of the -C-O- group and Cu²⁺-complexed functional groups in the FTIR spectrum of the treated wheat straw indicated that an increase of -C-O- functional groups was the main reason for enhanced Cu²⁺ adsorption. Similarly, NaOH treatment increased the removal rate of UO₂²⁺ by wheat straw from 32.64% to 84.45% due to an increase of -OH and C=O groups (Xiao-Teng et al. 2019). Moreover, the alkaline treatment exposed more Si = O and P-O on the treated surface, which further promoted UO₂²⁺ adsorption (Xiao-Teng et al. 2019).

Structural changes to alkali modified agricultural wastes promote pollutant retention. Table 1 summarizes the effects of alkalization on the removal of pollutants by modified adsorbents in terms of their maximum adsorption capacities, specific functional groups and adsorption mechanisms. NaOH modified rice straw exhibited excellent adsorption capacity for ciprofloxacin (Huang et al. 2020). Adsorption equilibrium with ciprofloxacin was achieved at ~ 35 min for NaOH modified rice straw and the maximum adsorption capacity was 93.5 mg g^− 1. The sorption improvements were attributed to NaOH breaking down the lignin encapsulating cellulose or hemicellulose, thereby increasing the exposure of cellulose for reaction. Creation of tiny fissures on the treated rice straw surfaces enlarged the surface area, thereby increasing ciprofloxacin retention via physical adsorption. In an analogous study, NaOH treatment of wheat straw increased sulfonylurea herbicide retention resulting in a maximum adsorption capacity as high as 337.22 mg g^− 1 (Cara et al. 2017). Alkaline treatment increased both the surface roughness and functional surface activity (carboxyl and hydroxyl groups) due to hydrolysis of esters. These changes strengthened the interactions between the alkaline-treated straw and chlorsulfuron through H-bonding, ion exchange and complexation reactions. Notably, alkali treatment is operationally straightforward; however, excessive alkali concentrations should be avoided as excess alkali can degrade the functional group content as demonstrated for wheat straw (Guo et al. 2016).

Table 1 Sorption performance of alkali modified agricultural wastes as adsorbents for pollutant removal from water

2.2 Acidification

Acidification is a wet oxidation process that can remove mineral impurities from agricultural wastes and further improves the acidic behavior and hydrophilic nature of the adsorbent surface (Yanyan et al. 2018). Common reagents for acid modification include H₃PO₄, HCl, HNO₃ and H₂SO₄. During treatment, acids dissolve constituents reducing the tortuosity of the porous structure and increase the O content of the material. In particular, acid treatment promotes cellulose hydrolysis of agricultural wastes creating a more reactive material (Yoon et al. 2014).

Thevannan et al. (2011) demonstrated that HCl reduced the mineral content of barley straw; Al, P, Mn, Cu and Zn concentrations decreased by 2.28% ~ 9.80% after acid treatment. This reduction of mineral content contributed to increased adsorption of Ni²⁺ from solution due to decreasing competition among cations for adsorption sites. Generally, lignocellulosic adsorbents have low adsorption capacities for anionic pollutants due to their negatively charged surfaces. However, case studies have demonstrated that acidification can improve the adsorption performance for anionic pollutants (Acid Red 111) as well (Ayan et al. 2012). They ascribed this phenomenon to HNO₃ treatment promoting non-electrostatic interactions between adsorbent and adsorbate, such as van der Waals and H-bonding mechanisms.

Dilute acids increase the amount of C-H, O-H and C-O groups on agricultural wastes. The main functional groups in modified rice straw after HCl treatment were O-H and C-O groups, which facilitated the adsorption of 2-chlorophenol (Balarak et al. 2017). Concentrated acids can effectively convert hydroxyl and aldehyde groups to more oxidized groups, such as the carboxyl moiety. H₂SO₄-oxidized coconut shell had relatively high C (51.90%) and O (44.63%) contents due to the release of volatile compounds during acid oxidation (i.e., conversion of organic matter to carbonaceous compounds through destructive distillation) (Jawad et al. 2020). The surface of the treated material was surrounded by COO⁻ and SO₃⁻ groups, which were highly effective for the adsorption of methylene blue. Further, the O content of HCl- and HNO₃-treated agave bagasse increased by 4.9% compared with the raw material due to an increase of carboxyl groups (Velazquez-Jimenez et al. 2013). Carboxyl groups are highly effective in the adsorption of heavy metals (Cd²⁺, Pb²⁺ and Zn²⁺) through formation of strong bridging complexes. Notably, concentrated acid oxidation was shown to decrease the surface area of oxidized coconut shell due to strong corrosion, which may reduce the porosity and efficacy of the adsorbent material for retention of some pollutants (Jawad et al. 2020).

Several studies have demonstrated the efficacy of acid-treated agricultural wastes for heavy metal ions, such as Zn²⁺, Pb²⁺ and Cr⁶⁺ (Cr₂O₇²⁻) (Argun et al. 2007; Velazquez-Jimenez et al. 2013). Acid treatment alters functional groups and several surface area/porosity characteristics to enhance adsorption performance (Table 2). The adsorption capacity of natural corncob for Cd²⁺ increased from 4.7 to 19.3 mg g^− 1 when the material was acidified by HNO₃ (Leyvaramos et al. 2005). The Cd²⁺ was adsorbed mainly by the carboxylic sites through ion exchange and the adsorption capacity increased directly proportional to the concentration of carboxylic sites. The glycosidic bonds of cellulose and hemicellulose are broken down to produce aldehyde groups and eventually carboxylic groups during acidic modification. Similarly, the maximum Cu²⁺ sorption capacity of HNO₃-treated corn cob (0.325 mmol g^− 1) was 3-fold higher than that of the raw corn stalk (0.113 mmol g^− 1) (Vafakhah et al. 2014). The pH_PZC (i.e., point of zero charge) for modified corn stalk was 3.3, which was lower than that of untreated material due to the increase of O-containing groups. The acid-modified adsorbent also showed a lower separation factor (R_L = 0.224) than raw corn stalk (0.716), further contributing to enhanced adsorption properties.

Table 2 Sorption performance of acidification modified agricultural wastes as adsorbents for pollutant removal from water

Acid-treated agricultural wastes are also effective for removal of several organic pollutants. Attainment of adsorption equilibrium for Basic Red 18 and methylene blue by HNO₃- and H₃PO₄-modified oreganum stalks was much shorter (2 h) than that of the corresponding non-treated stalks (4 h). The more rapid kinetics were ascribed to increased surface area and reactive functional groups after acid treatment, which created enhanced electrostatic and hydrophobic interactions between adsorbents and adsorbates (Ayan et al. 2012). The results showed that the maximum adsorption of Victazol orange 3R dye to dilute HCl-treated mango seed increased from 36.9 to 63.3 mg g^− 1 with much faster sorption kinetics than the untreated seeds owing to increased BET surface area (8.5 ➔ 11.5 m² g^− 1) and average pore diameter (4.31 ➔ 5.78 nm). Overall, acid treatment contributes greater functional group reactivity and structural properties (e.g., surface area, porosity) to enhance adsorption performance.

2.3 Esterification

Esters are generated from the esterification of free hydroxyl groups in cellulose by reacting with one or more carboxyl groups (anhydrides), whereby cellulose reacts as a trivalent polymeric alcohol (Hokkanen et al. 2016). Succinic anhydride, EDTA (Ethylenediaminetetraacetic acid) dianhydride, citric acid anhydride and maleic anhydride are widely used for esterification reactions, thereby adding functional groups to the surface of agricultural wastes. In addition, the hydrophobicity and mechanical strength of adsorbents are improved by esterification as well (Pang et al. 2019). These beneficial changes from esterification contribute to enhanced adsorption performance of agricultural wastes for application in remediation of aqueous systems.

Crop straws are widely used as a feedstock for esterification treatment. For instance, soybean straw and citric acid (0.6 M) were mixed at a solid:liquid ratio of 1:10 (w/v) and reacted at 50 °C for 24 h and 120 °C for 90 min to prepare an esterified adsorbent (Zhu et al. 2008). After modification, a strong stretching vibration at 1742 cm^− 1 (assigned to C=O) occurred in the FTIR spectrum, indicating a successful esterification process (Zhu et al. 2008). The -OH of cellulose reacted with citric acid to form ester linkages and imparted carboxyl groups onto the straw surface. With regard to the adsorption mechanisms, the carboxyl groups introduced by citric acid reacted with Cu²⁺ via a complexation reaction. Similarly, the etherification procedure was adopted to prepare esterified rice straw using EDTA as a modifying agent. The EDTA etherification processes generated both amino groups and carboxyl groups as adsorbents (Moyo et al. 2017). The relative peak shifts and signal strength alterations of FTIR spectrums revealed the combined actions of carboxyl, ester and amine groups of the grafted EDTA in Pb²⁺ binding. Additionally, some studies determined that the carboxyl esterification not only introduced more carboxyl groups on wheat straw, but also roughened the surface, thereby increasing surface area and porosity (Han et al. 2010). As a result, the increased -COO⁻ content and porous surface properties enhanced methylene blue sorption through improved ion exchange and intra-particle diffusion.

The type of esterification reagent strongly affects the adsorption properties of agricultural wastes. For example, citric acid modified sesame straw fixed more methylene blue than that formed by tartaric acid modification (Feng et al. 2017). The contrasting effects of these two reagents were attributed to differences in their molecular structures. Citric acid possesses more carboxyl groups than tartaric acid, which resulted in generation of more adsorption sites following citric acid treatment (72.61 cmol kg^− 1) than for tartaric acid treatment (17.46 cmol kg^− 1). The FTIR spectrum of the modified materials confirmed a large increase in the intensity of the C=O stretching peak resulting from citric acid treatment. Catalyst addition could further promote the esterification efficiency during modification. The esterification between citric acid and hydroxyl groups achieved in high efficiency using a mild catalyst (NaH₂PO₂·H₂O) in a N, N-dimethylformamide medium (Li et al. 2010), NaH₂PO₂·H₂O, as a catalyst, increased the speed of the esterification reaction and allowed for a simplified procedure compared to traditional methods without catalyst. The catalyzed process created ester linkages within the spent grain complex and increased carboxylic acid groups on the adsorbent surface.

Several studies demonstrated improved adsorption performance for pollutants following esterification of agricultural wastes (Table 3). Adsorption of Cu²⁺ by citric acid modified soybean straw was rapid during the first 10 min and reached a maximum adsorption capacity of 0.69 to 0.76 mmol g^− 1 based on a Langmuir model (Zhu et al. 2008). The adsorption mechanism also changed due to the increase of functional groups. The biosorption energy of citric acid modified barley straw for Cu²⁺ adsorption was 8.513 kJ mol^− 1, indicating a chemical adsorption mechanism, as opposed to a physical adsorption mechanism for the raw biomass (Pehlivan et al. 2012). Similarly, Pb²⁺ adsorption capacity increased from 125.84 mg g^− 1 (raw grain) to 293.30 mg g^− 1 (citric acid esterified grain) due to the formation of both ester linkages and grafting of carboxyl groups (Pehlivan et al. 2012). For organic pollutants, the intensity of chemisorption was closely related to the number of functional groups. Methylene blue adsorption capacity increased from 170 mg g^− 1 (raw biomass) to 650 mg g^− 1 (citric acid treatment) and 280 mg g^− 1 (tartaric acid treatment) (Feng et al. 2017). The larger methylene blue adsorption capacity for the citric acid treatment was due to a larger increase of C=O in carboxyl groups, which interacted with methylene blue via complexation. In general, carboxyl and amino functional groups are the most common groups introduced onto adsorbents by esterification, which consequently improve adsorption performance through complexation reactions.

Table 3 Sorption performance of esterification modified agricultural wastes as adsorbents for pollutant removal from water

2.4 Etherification

Ethers are synthesized through etherification, whereby -OH groups on agricultural wastes are substituted by other functional groups (Hokkanen et al. 2016). Reaction of -OH groups with ethylene oxide or other epoxides is a typical etherification reaction yielding several reactive sites for further functionalization to introduce adsorption groups. Triethyleneteramine, diethylenetriamine and ethylenediamine are usually used to generate amine groups for adsorbents during the functionalization process. Generally, carboxyl, thio and amino functional groups are introduced to the biomass surfaces by the etherification process (Kong et al. 2018; Wang et al. 2017).

Etherification may generate positively-charged functional groups to augment sorption sites for retention of anions, such as PO₄³⁻, NO₃⁻ and SO₄²⁻ (de Lima et al. 2012; Keranen et al. 2015; Qiao et al. 2019). The interaction between epichlorohydrin and -OH groups of agricultural wastes is a common etherification process to generate new functional groups. For example, epoxy and amino groups were introduced onto raw rice straw by reaction between epichlorohydrin and trimethylamine (Cao et al. 2011). An FTIR peak associated with the C-N bond at 1470 cm^− 1 and a peak for quaternary ammonium salt at 1062 cm^− 1 appeared on the surface of rice straw following modification, thereby indicating generation of positively charged amino groups. This etherification process increased the total exchange capacity of the adsorbent from 0.32 to 1.64 mEq g^− 1 and rapid adsorption of sulfate via an ion exchange mechanism. Similarly, ethylenediamine-cross-linked wheat straw was utilized to introduce amine groups for use in removing HCrO₄⁻ and H₂PO₄⁻ from solutions (Xu et al. 2011). Although the BET surface area of the modified biomass decreased from 6.5 to 5.3 m² g^− 1, the modification process increased the quantity of positive charge. After modification, the zeta potential of the modified wheat straw was in the range of + 39.3 ~ − 7.0 mV compared with + 5.2 ~ − 45.8 mV for the raw wheat straw. The higher zeta potential for the modified material was attributed to the presence of -CN⁺, which possessed a stronger electrostatic attraction for anionic pollutants. Several studies have documented the efficacy of etherification for increasing the reactivity of agricultural wastes for retention of anionic pollutants through electrostatic interactions (Chen et al. 2011; Wu et al. 2016).

Etherification processes are also utilized to improve the removal rate of cationic pollutants by introducing amino groups for modified adsorbents. Kong et al. (2018) introduced amino and carboxyl groups on the surface of wheat straw to investigate Cu²⁺ sorption behavior. They found that the introduced -NH₂ groups shared their lone pair of electrons to form R-NH₂Cu²⁺ complexes as the adsorption mechanism. Moreover, the introduced -COOH groups facilitated charge transfer to the O to attract the Cu²⁺, thereby further promoting Cu²⁺ retention. Etherification can also generate thio groups on adsorbents to attach cationic pollutants. Moreover, ethylenediamine and CS₂ treatment generated S-containing functional groups on sugarcane bagasse, which played an important role in adsorbing Pb²⁺, Cu²⁺ and Zn²⁺ (Wang et al. 2017). Characterization of the etherification product indicated formation of -S = metal bonds formed through coordination bonds with the S atom in the biosorbent owing to the lone pair of S electrons sharing a bond with the metal.

A summary of etherification modified agricultural wastes as adsorbents for the removal of pollutants from aqueous solution were presented in Table 4. The maximum Cu²⁺ sorption capacity of etherified wheat straw cellulose reached up to 130 mg g^− 1, which were attributed to complexation reactions of Cu²⁺ with the amino and carboxyl groups generated by etherification (Kong et al. 2018). Additionally, adding an activated bond of -CN (acrylonitrile) to the cellulose -OH groups greatly improves Cd²⁺ adsorption performance (Zheng et al. 2010a). The Langmuir sorption model provided a better fit to the Cd²⁺ adsorption equilibrium data than a Freundlich model, and determined a maximum uptake of 12.73 mg g^− 1 for modified corn stalks, compared to 3.39 mg g^− 1 for raw corn stalks. With respect to anionic retention, etherification of rice straw increased the maximum adsorption capacity of sulfate from 11.68 to 74.76 mg g^− 1 by introducing amino groups (Cao et al. 2011). Generation of positively charge, −C-N⁺ moieties, was responsible for the removal of sulfate by electrostatic interactions. Furthermore, triethylamine-etherification significantly accelerated the adsorption rate of wheat straw for anionic constituents (Xu et al. 2011). Adsorption equilibrium was achieved in 35 min for HCrO₄⁻ and 15 min for H₂PO₄⁻, compared with 3 hr for the unaltered biomass. Accordingly, the adsorption capacity of modified wheat straw for HCrO₄⁻ and H₂PO₄⁻ were 0.98 and 1.15 mmol g^− 1, respectively. Electrostatic attraction, complexation and ion exchange mechanisms were the most prominent sorption mechanisms contributing to cationic and anionic retention by etherified adsorbents.

Table 4 Sorption performance of etherification modified agricultural wastes as adsorbents for pollutant removal from water

2.5 Carbonization

Carbonization is a thermal decomposition process of organic materials resulting in the production of a carbonaceous residue with a concomitant removal of distillates. Conversion of agricultural wastes into biochar (a heterogeneous charcoal material) is a common carbonization process that has been extensively studied (Jung et al. 2015; Yang et al. 2019). Biochar products have a larger specific surface area, greater porosity and more functional groups than the raw agricultural wastes from which they were formed (Divband Hafshejani et al. 2016). The alkaline nature (causd by the formation of alkaline carbonate during the pyrolysis) and the existence of mineral constituents on biochar surfaces also promote the formation of metal precipitates on the biochar surface (Liu et al. 2020b; Wu et al. 2018; Dai et al. 2014). The fixation ability of original biochar for pollutants is relatively limited and many studies have explored altering pyrolysis conditions (e.g., temperature, feedstock, chemical amendments) to generate activated biochars with altered functional groups and rougher surfaces to enhance sorption capacities for various pollutants.

Carbonization temperature has an especially strong effect on the properties of biochar generated from agricultural waste (Qiu et al. 2021). In general, biochar produced at lower temperatures (< 500 °C) contains a greater functional group content, whereas those produced at higher temperatures (500 ~ 700 °C) have a more porous surface and overall porosity (Oliveira et al. 2017). For example, aldehyde and ketone functionalities formed at ~ 200 °C and became dominant at 300 ~ 500 °C, increasing the adsorption capacity for various pollutants (Zhang et al. 2020). However, the drastic fusion of the ring structures in biochar occurred from 550 to 650 °C, which decreased the content of functional groups and weakened the chemical fixation for pollutants. The influence of feedstock source (i.e., type of biomass material) is also an important factor determining the efficiency of pollutant removal by agricultural waste-derived biochars. The type of functional groups and chemical components comprising various agricultural wastes are different, thereby significantly affecting the adsorption performance. The most important of these properties in relation to the adsorption capacity were the O/C ratio, the P content and the ash content (Arán et al. 2016). The capacity of the biochar to retain Cu present in solution depended on the size of the inorganic fraction and varied in the following order: rice biochar > chicken manure biochar > olive mill waste biochar > acacia biochar > eucalyptus biochar > corn cob biochar.

The addition of chemical amendments to the biomass feedstock has an appreciable effect on biochar characteristics and the adsorption performance of the biochar material. The selective introduction of functional groups, heteroatoms, metal atoms into biochar can improve its specific properties. In-situ synthesis of biochar refers to the preparation of modified biochar in only one step by simultaneously pyrolyzing reagents and agriculture wastes. As a result, the modified biochar has the optimized physicochemical structures and properties. Post-optimization of biochar is useful to further enhance its potential value after pyrolysis, a special biochar could be designed via the further surface modification and pore regulation.

Addition of acidic reagents has been shown to markedly increase functional group content. For example, H₂SO₄ addition during carbonization generated excellent sorption performance for methylene blue, brilliant green, crystal violet and orange G by formation of -SO₃ groups on the biochar surface that increased the chemical affinity for pollutants through covalent bonding (Goswami and Phukan 2017). Similarly, HNO₃ addition to the feedstock generated N-containing functional groups on biochar, with the positive ≡N⁺ group interacting with the negative -O≡, N=C and heterocycle N-C groups on methylene blue to facilitate adsorption (Yu et al. 2018). In addition, the weakly acidic FeCl₃ would be transformed to Fe₂O₃ during the preparation process of biochar, increasing the Fe-O functional groups strongly on the surface of biochar (Qu et al. 2022b). The peaks of Fe 2p_1/2 and Fe 2p_3/2 shifted after the stabilization of Cd, suggesting that the iron-bound functional groups participated in the Cd retention actively.

The addition of alkaline reagents was shown to increase both surface area and functional group content of biochars. Activation of orange peel biochar with CO(NH₂)₂ and KOH strongly altered several physicochemical properties (Xiao et al. 2020). During pyrolysis, CO(NH₂)₂ reacted with K₂CO₃ (an intermediate of KOH activation) to generate NH₃ and create a more porous structure with increased N-containing groups, which correspondingly increased the retention of methylene blue. Addition of oxidizing agents is a common technique for generating higher concentrations of oxygenated functional groups on biochar surfaces (Yang et al. 2019). Co-pyrolyzed peanut hulls with H₂O₂ resulted in an increase in the O content (16.4 ➔ 22.3%) and surface carboxyl functional groups, improving chemical adsorption (Xue et al. 2012). As another example of oxidation effects, a KMnO₄-treated hickory wood biochar surface was covered with ultrafine MnO_x particles exhibiting a surface enrichment of O-containing functional groups and a higher surface area (Wang et al. 2015). Metal retention by this engineered biochar mainly resulted from surface adsorption mechanisms involving both the surface MnO_x particles and O-containing functional groups.

A myriad of studies have demonstrated the efficacy of carbonization as an effective method for improving the adsorption performance of agricultural wastes, especially for modified biochar adsorbents (Table 5). The total adsorption capacity for Cd²⁺ increased due to the introduction of C-S complexes on biochar that facilitated the retention of Cd²⁺ through a strong bond with S (Tajar et al. 2009). Wang et al. (2018) showed that maximum acid red 18 dye sorption capacities for NH₄Cl/CH₃COONH₄-modified adsorbents were 1.41 and 1.18 times higher than for non-N-doped biochar. The enhanced sorption affinity was mainly attributed to π-π EDA (electron donor-acceptor) interactions between pyridine-N groups and acid red 18. The N functional groups enhanced the surface polarity, thereby increasing interactions with adsorbates, such as acid red 18. In addition, the amino functional group (−NH₂) greatly improved the adsorption behavior of Cr⁶⁺ (Cr₂O₇²⁻) by polyethylenimine-treated rice husk biochar (Ma et al. 2014). A pseudo-second-order kinetic model indicated a maximum Cr⁶⁺ (Cr₂O₇²⁻) adsorption capacity of 435.7 mg g^− 1 compared to only 23.09 mg g^− 1 for natural biochar. Moreover, this new material exhibited excellent cyclic adsorption ability (recycled use) making it a low-cost, ecofriendly adsorbent.

Table 5 Sorption performance of carbonization modified agricultural wastes as adsorbents for pollutant removal from water

In summary, carbonized agricultural wastes are an economical and effective approach for pollutant removal from water owing to their intrinsic physicochemical characteristics. Abundant functional groups, high surface area and a porous structure allow biochars to rapidly interact with pollutants through a variety of adsorption mechanisms (e.g., complexation, ion exchange, covalent bonding, π-π interactions, H-bonding and precipitation). Furthermore, introduction of chemical amendments (e.g., acids, bases, oxidizers, metals) to the carbonization process can generate specific chemical moieties that selectively target the removal of specific pollutants.

2.6 Magnetization

Magnetization of modified agricultural wastes, such as biochar, is a strategy to introduce transition metals or their oxides into the organic matrix to create a material (e.g., FeCl₃, Fe₃O₄, Fe₂O₃ and zero-valent iron nanoparticles) that is easily separable with an external magnet. Importantly, magnetic adsorbents can be easily removed making them highly effective for the removal of pollutants from aqueous solutions. Moreover, the doping of metals or metal oxides onto the surface of agricultural wastes can target specific functionalities to enhance adsorption properties, such as the modification of lignocellulose by magnetic materials to improve sorption of chloramphenicol while simultaneously allowing easy recovery and reuse of the material during adsorption applications (Dai et al. 2016).

Nanoscale-zero-valent, iron-coated wheat straw exhibited better adsorption performance for removal of Cu²⁺ than the raw adsorbent (Shao et al. 2020). A portion of the Cu²⁺ was directly adsorbed by the wheat straw, whereas another fraction of the Cu²⁺ was first reduced to zero-valent Cu and Cu₂O, which were subsequently attached to the surface of the material as part of the crystalline Fe-oxide structure. Further, doping agricultural wastes with Fe oxides was shown to promote the removal of anionic As⁵⁺ (H₂AsO₄⁻) due to enhanced surface interactions associated with the Fe oxides (Pehlivan et al. 2013). The Fe-oxide modified sugarcane bagasse rapidly removed H₂AsO₄⁻ oxyanions by electrostatic interactions, ligand exchange and chelation reactions between the positively charged surface ≡FeOH²⁺ group and the negatively charged H₂AsO₄⁻.

Loading Fe₃O₄ (magnetite) on the surface of organic materials is a common technique for magnetization that can be achieved by ferrofluids, microwave assisted and mechanochemical techniques (Safarik et al. 2020; Safarik and Safarikova 2014). Co-precipitation is also widely adopted to synthesize and load Fe₃O₄ on material surfaces, the changes for -OH and Fe-O groups in Fe₃O₄-modified straw resulting from the combination of O in the straw with the Fe of Fe₃O₄ (Haghighat and Ameri 2016). The modification further increased the BET surface area from 3.37 to 23.56 m² g^− 1. Pb²⁺ was retained on the Fe₃O₄-modified straw through diffusion adsorption and chemical fixation with O-containing functional groups. Similarly, Khandanlou et al. (2015) demonstrated an increase in the pore volume and BET surface area by 12 and 22 times after Fe₃O₄ modification, respectively. The enhanced porosity and functional group activation prominently promoted the adsorption of Cu²⁺ and Pb²⁺. Further, a Fe₃O₄₋doped organic adsorbent (sugarcane bagasse) exhibited excellent sorption capacity for Cd²⁺ and Pb²⁺, with Fe-OH groups playing an important role in the adsorption mechanism (Liu et al. 2020a). The modified adsorbent also had a more robust regeneration efficacy than the raw bagasse as the Fe₃O₄ stabilized the surface properties. The effect of SnO₂/Fe₃O₄ doping on the adsorption affinity of reactive blue 4 and crystal violet by treated rice bran was investigated by Ma et al. (2020a). They found that pore size was increased by 2 ~ 4 nm and the FTIR peaks for Sn-O and Fe-O strengthened upon doping with SnO₂/Fe₃O₄. The numerous -OH groups associated with the SnO₂/Fe₃O₄-adsorbent interacted with the reactive blue 4 and crystal violet to effectively remove these polluting dyes from solution.

The removal efficiencies of magnetic biosorbents for pollutants are listed in Table 6. Magnetization of wheat straw increased the adsorption capacity for Pb²⁺ by 23%, with an optimal contact time of 30 min facilitating rapid processing (Haghighat and Ameri 2016). Baldikova et al. (2016) found that magnetized barley straw increased adsorption of methylene blue and crystal violet by 16.7% and 54.9%, respectively, due to creation of a larger surface area. Further, the SnO₂/Fe₃O₄-doped rice bran showed a high adsorption affinity for reactive blue 4 and crystal violet with sorption capacities of 218.8 and 159.2 mg g^− 1, respectively (Ma et al. 2020a). Characterization of the SnO₂/Fe₃O₄-absorbent indicated that the surface was covered with abundant -OH functional groups, which played an important role in increasing the adsorption capacity through strong electron attraction. Moreover, Fe oxide-modified sugarcane bagasse showed a superior potential for H₂AsO₄⁻ removal (up to 22.1 mg g^− 1) compared to several other modification methods (Pehlivan et al. 2013). Positively charged ≡FeOH₂⁺ and negatively charged H₂AsO₄⁻ rapidly interacted through electrostatic interactions and ligand exchange reactions. Overall, magnetic adsorbents have demonstrated the capacity to efficiently remove heavy metals, anionic constituents, antibiotics and various dyes from water. The most prominent feature of magnetic modification is that a product can be easily recovered and reused, with improved the utilization efficiency and reduces production/use costs.

Table 6 Sorption performance of magnetization modified agricultural wastes as adsorbents for pollutant removal from water

2.7 Surfactants

Surfactants are compounds that consist of hydrophilic heads and hydrophobic tails, which can reduce the surface tension between different media when used as detergents and dispersing agents (Lee et al. 2018). The charge characteristics of the hydrophilic head classify the surfactants as ionic and non-ionic surfactant groups (Abrankó-Rideg et al. 2015). Various functional groups (e.g., phosphates, sulphonates and quaternary ammonium salts) occur in the structure of ionic surfactants, favoring the selective adsorption of various pollutants in solutions. Thus, surfactant modifications improve the surface hydrophobic/hydrophilic properties and enrich the variety and quantity of functional groups comprising agricultural wastes. Linear alkyl benzene sulphonates, secondary alkane sulphonates, alkyl trimethyl ammonium halides and quaternary ammonium-based compounds are common surfactants with widespread applications (Ying 2006).

Cationic surfactant treated agricultural wastes demonstrate a good potential for removal of anionic pollutants. Cetyl trimethyl ammonium bromide modified wheat straw obtained higher N (0.44%), C (46.32%) and H (6.49%) contents than that of the origin wheat straw (0.22% N, 42.34% C, and 5.81% H) due to the loading of the surfactant (Zhang et al. 2014). FTIR spectra indicated that the -CH₂ peak strengthened while the -NH₂ and -OH bands broadened in wheat straw after modification. Changes in these characteristics indicated that modification increased the number of ammonia functional groups, which combined with Congo red dye through ionic interactions. Moreover, the adsorption results indicated that π-π dispersion interactions between the surfactant and dye played an important role in the removal of the Congo red dye. Furthermore, Thamilarasi et al. (2018) found that cetyl trimethyl ammonium bromide formed a surfactant bilayer containing anion exchange properties due to reaction with acidic functional groups on palm fruit husk surfaces. The surfactant loading on the surface of the palm fruit husk surface reached a maximum at a cetyl trimethyl ammonium bromide concentration of 1.0% in solution. The cationic surfactants created numerous positive charges on the adsorbent surface, which increased its retention of negatively charged VO₄³⁻ in solution. Notably, some studies showed that surfactant modification decreased the BET surface area of adsorbents. For example, tetradecyl trimethyl ammonium bromide treated corn straw had a lower BET surface area (4.21 m² g^− 1) than that of the original straw (7.29 m² g^− 1) (Soldatkina and Zavrichko 2019). This phenomenon might be caused by the blockage of pores and loss of access to internal surface area; however, the loss of surface area did not affect the enhanced adsorption performance of the modified agricultural waste for acid red, acid orange and VO₄³⁻ due to a concomitant increase in the number of positive sites.

Sodium dodecyl sulfate is a common anionic surfactant used to improve the adsorbent performance for positively charged pollutants. Pirbazari et al. (2016) determined that surfactant molecules created aggregates on the surface of rice straw, which promoted the formation of a porous adsorbent structure during the modification. SEM characterization showed that the surface of modified straw was rougher with a specific surface area of 150 m² g^− 1 compared with 58 m² g^− 1 for the original straw, which provided an enhanced adsorption capacity for methylene blue. In addition, a -SO₃ group was attached to the surface of the modified straw further increasing the adsorption capacity. Similarly, studies have demonstrated that sodium dodecyl sulfate treated peanut husk exhibited a high removal efficiency for rhodamine B dye in solution (Fatimah et al. 2018). Kinetic studies demonstrated that the adsorption of rhodamine B by the modified peanut husk was best described with a pseudo-second-order kinetic model, indicating that chemical interactions played a dominant role in the retention of the rhodamine B dye.

Surfactant-modified agricultural waste products for the removal of a diverse range of aquatic pollutants are summarized in Table 7. The maximum adsorption capacity of sodium dodecyl sulfate treated peanut husks for rhodamine B was 240.0 mg g^− 1, which was about a 100-fold increase compared to the pristine husks (Fatimah et al. 2018). Pollutant adsorption onto the modified adsorbent was mainly controlled by a hydrophobic interaction mechanism, along with a considerable contribution from a cation exchange mechanism (Pirbazari et al. 2016). In addition, cetyl trimethyl ammonium bromide strengthened π-π interactions for organic pollutants due to the increasing role of benzene rings in the adsorption process (Zhang et al. 2014). As a result, the adsorption performance of modified wheat straw for Congo red dye was about a factor of two times higher than for the untreated straw. Although a decrease in BET surface area may occur in surfactant-modified adsorbents due to the constriction of pore channels by attachment of surfactant moieties, the overall adsorption performance for several pollutants increased due to an increased number of positively charged surface sites (Thamilarasi et al. 2018). Overall, surfactant modification is a feasible method for improving adsorption performance due to significant changes in surface properties of the adsorbent, such as the hydrophobicity, hydrophilicity and functional groups.

Table 7 Sorption performance of surfactant modified agricultural wastes as adsorbents for pollutant removal from water

2.8 Grafting

Grafting of polymers onto the surface of agricultural wastes is an important technique for selectively imparting the chemical characteristics of the polymer to the waste structure. Therein, the backbone of a polymer is linked to a side chain of lignocellulose in agricultural waste to form a branched copolymer (Hokkanen et al. 2016). Grafting-to, grafting-from and grafting-through are the three main linkage types, which are based on the type of polymer. Grafting-from is the most common method for introducing polymers onto agricultural waste surfaces. The process involves the addition of monomer units to form copolymers on the adsorbent surface, which transforms the surface functionality (Carlmark. 2013). Usually, grafting-from treatment occurs in an aqueous environment and a suitable initiator is required to induce the modification reaction. Chemical, radiation and photochemical processes are the main techniques employed to initiate the grafting process. For example, a chemical initiator generates free radicals to react with -OH groups on lignocellulose and is widely used due to its ease of operation and low cost. Potassium permanganate (KMnO₄), potassium persulfate (K₂S₂O₈) and ceric ammonium nitrate (NH₄)₂Ce(NO₃)₆ are common initiators for polymerization of monomers on agricultural wastes (Li et al. 2012; Zheng et al. 2010b). Many monomeric species, such as acrylic acid, acrylamide, acrylonitrile and aniline, are used for the synthesis of a polymeric adsorbent surface (Kumar et al. 2018). These monomers usually contain acidic (−COOH, −SO₃H, −COONa, −SO₃Na) or basic (−NH₂) functional groups.

Corn stalk was modified with an acrylonitrile monomer in the presence of KMnO₄ as the initiator (Zheng et al. 2010b), resulting in the -OH groups of cellulose reacting with the monomer to form copolymers on the surface. A FTIR spectrum indicated the presence of -CN functional groups and the N content of the corn stalk increased from 1.11% to 4.02%. The grafted material sorbed Cd²⁺ yielding an activation energy (E_α) of 9.43 kJ mol^− 1, indicating that the Cd²⁺ retention mechanism was a chemical process. Further, the adsorption behaviors of Indosol orange RSN and Indosol black NF by polyethyleneimine-grafted peanut husks were studied by Sadaf et al. (2015). The -NH and -C=O functional groups were introduced by the modification process and the adsorbent surface became rougher (i.e., more porous) as determined by FTIR and SEM analyses, respectively. As a result, the Indosol dyes reacted with the carboxylic and carbonyl functional groups leading to their efficient removal. Additionally, Fotsing et al. (2020) demonstrated that polyethyleneimine-grafted cocoa shell was an effective material for the removal of NO₃⁻ and Cr⁶⁺ (Cr₂O₇²⁻) from solutions. The pH_pzc value of modified cocoa shell was 9.3, much higher than untreated adsorbent (3.4) due to the large number of amino groups grafted on the material surface. Amino groups on the surface of the modified adsorbent reacted with H⁺ during acidic treatment, producing a surficial-NH₃⁺ moiety, which subsequently adsorbed NO₃⁻ and Cr⁶⁺ (Cr₂O₇²⁻) through electrostatic attraction.

Polyaniline grafting to agricultural wastes strongly alters their physiochemical properties, which in turn strongly alters adsorption performance for various pollutants. For instance, Soldatkina and Zavrichko (2017) prepared a new composite by chemical polymerization of aniline on the surface of corn stalks using ammonium persulfate (NH₄)₂S₂O₈ as an oxidation agent and H₃PO₄ as a dopant. H-bonding and π-π interactions between aniline and cellulose in the corn stalk structure anchored aniline firmly to the surface of the material. Moreover, the specific surface area of the adsorbent increased from 15.1 to 46.9 m² g^− 1 after modification, facilitating physical adsorption of acid red and acid orange dyes. In addition, π-π interactions between aniline and the two dyes promoted chemical adsorption. Some reagents can also exhibit excellent adsorption performence for pollutants without changing adsorbents surface structure srongly due to the unique properties of reagents. For instance, β-cyclodextrin has a well-developed three-dimensional structure (cavity) and numerous -OH, which made modified rice husk adsorb Pb and bisphenol A quite effectively (Liu et al. 2022). β-cyclodextrin grafting accomplished synergetic Pb and bisphenol A elimination through averting their competitive behaviors owing to diverse capture mechanisms for Pb (ion exchange, complexation and electrostatic attraction) and bisphenol A (hydrogen bonding and host-guest inclusion).

Grafting treatment improves agricultural waste adsorption for several pollutants as summarized in Table 8. The adsorption capacity of Congo red dye by polyphenolic tannin treated jute fiber was much higher than for the untreated fibers (Roy et al. 2013). The N atoms in -N=N- and -NH₂ and S and O atoms in the -SO₃⁻ moiety of the modified adsorbent participated in removal of Congo red dye via formation of intermolecular hydrogen bonds with the -OH moiety of the dye. Further, aniline modified corn stalk exhibited high adsorption capacity and efficiency for acid red and acid orange compared with unmodified corn stalk (Soldatkina and Zavrichko 2017). Adsorption equilibrium occurred within 120–150 min and the adsorption capacity of the modified corn stalk was ~ 2 times more than that of raw stalk (Soldatkina and Zavrichko 2017).

Table 8 Sorption performance of grafing modified agricultural wastes as adsorbents for pollutant removal from water

Grafting treatment of agricultural wastes has also been shown to enhance adsorption performance for inorganic pollutants. Acrylonitrile-modified corn stalk showed a maximum Cd²⁺ uptake of 12.7 mg g^− 1 compared with 3.4 mg g^− 1 for raw corn stalk, owing to an increase of -CN groups that promoted metal complexation (Zheng et al. 2010b). Similarly, polyethyleneimine-modified peanut shells increased the number of positive charges on the modified surface, thereby strengthening electron attraction for Cr⁶⁺ (Cr₂O₇²⁻) in the pH range of 2.0–11.0 (Ma et al. 2020b). Sorption of Cr⁶⁺ (Cr₂O₇²⁻) followed 2nd-order kinetics and a Freundlich-type isotherm, with a maximum adsorption capacity of 24.8 mg g^− 1. Overall, the primary adsorption mechanisms for grafted adsorbents were complex formation and electrostatic attraction between the grafted functional groups and pollutants in the solution.

3 Functional groups and their roles on adsorption mechanisms

3.1 O-containing functional groups

O-containing functional groups play a predominant role in surface reactions, hydrophobic/hydrophilic characteristics and electrical properties of adsorbents (Yang et al. 2019). The -OH and -COOH moieties are among the most abundant functional groups and participate in a wide range of adsorption mechanisms with various types of pollutants (Kurup 2012; Zafar et al. 2020) (Fig. 2). The adsorption of heavy metals (Cd²⁺, Pb²⁺ and Zn²⁺) and dyes (basic red 18, methylene blue, and acid red 111) by HNO₃ modified agave bagasse and oreganum stalks increased due to generation of -OH and -COOH on the surface of the modified materials (Velazquez-Jimenez et al. 2013). The -OH and -COOH groups may dissociate to form of -O⁻ and -COO⁻ upon interaction with water (Azzaz et al. 2017; Han et al. 2010) (Fig. 2). These acid ionic functional groups can interact with cationic pollutants through ion exchange or electron attraction mechanisms.

Fig. 2

Functional groups resulting from the modification of agricultural wastes

The -C=O and -C-O groups are commonly associated with modified agricultural waste products (Li et al. 2010; Ogata et al. 2015; Wang et al. 2017) (Fig. 2). For example, NaOH treatment increased the -C-O- content of wheat straw, which enhanced Cu²⁺ adsorption (Guo et al. 2016). The Cu²⁺ adsorption occurred primarily through a complexation interaction between -C-O- and the metal (Yu et al. 2013). Given the multifunctionality of modified agricultural wastes, most complexation reactions involve interactions between the pollutant and multiple functional groups, such as -OH, −COOH, −C=O and -C-O. For instance, NaOH-modified orange tree sawdust resulted in generation of -OH-, −COO- and -C-O groups, thereby promoting the retention of methylene blue through complexation reactions with each of the different functional groups (Azzaz et al. 2017). Similarly, −C=O, −C-O and -COOH groups in corn straw biochar interacted with triazine via electrostatic interactions resulting in effective removal of triazine from solutions (Suo et al. 2019). In summary, modification of agricultural wastes to generate additional O-containing functional groups increases pollutant adsorption, primarily through complexation reactions, hydrogen bonding, Van der Waals interactions and electrostatic interactions.

3.2 N-containing functional groups

N-containing functional groups are introduced to the surface of the agricultural material through etherification modification resulting in generation of -NH₂, −NH and -C-N (Fig. 2). The N atom has a lone pair of electrons that serves as an activation site to trigger interactions with pollutants. Qu et al. (2022c) showed that N doping introduced graphitic/pyridinic N on adsorbents for serving as reactive sites during the reaction, Pyridine N could transfer electrons from phenol to persulfate, while graphite N trigger nucleophilic addition of persulfate to generate ¹O₂ to degradate phenol further. In general, the N-containing functional groups on modified agricultural wastes increase the basic properties of adsorbents, thereby generating negatively charged surfaces to enhance the sorption of cationic pollutants (de Lima et al. 2012; Keranen et al. 2015; Qiao et al. 2019). For example, NaOH treatment exposed more -NH groups on the adsorbent surface, which strengthened the material’s affinity for Hg²⁺ and Cu²⁺ through complexation and electrostatic attraction (Song et al. 2014; Zafar et al. 2020). Similarly, modification with triethylamine increased -NH₂ concentration on modified adsorbents resulting in enhanced Ni²⁺ retention via electron sharing with the N atom (Wu et al. 2016).

N-containing functional groups can also form positive charges to attract anionic pollutants following protonation of the -C-N group in acidic environments to form -C-N⁺. For instance, a modified coconut shell fiber with increased -C-N group content exhibited strong attraction for nitrate, sulfate and phosphate with maximum adsorption capacities of 33.7, 31.2 and 200.6 mg g^− 1, respectively (de Lima et al. 2012; Keranen et al. 2015). Similarly, −NH₂ can be converted to -NH₃⁺ upon acidification, which effectively retained Cr⁶⁺ (Cr₂O₇²⁻) through electrostatic attraction (Wu et al. 2016). Thus, N-containing functional groups provide modified adsorbents with the capacity to retain both cationic and anionic pollutants depending on the pH of the reaction environment. Additionally, N-containing functional groups often interact synergistically with O-containing functional groups for retention of pollutants, such as toluene, methylene blue and acid red 18 (Xiao et al. 2020; Yu et al. 2018).

3.3 S-containing functional groups

S-containing functional groups (e.g., −S-O, −S=O, −C-S, −C=S, −S-S) on modified agricultural wastes can enhance the sorption capacity for several pollutants (Fig. 2). Sulfur is loaded onto waste materials through a sulfuration treatment, such as solution infiltration (H₂SO₄) or gas activation (SO₂, H₂S) (Tajar et al. 2009; Jawad et al. 2020). The sulfuration process introduces -S=O and -S-O moieties to the modified surface to enhance pollutant adsorption. For example, H₂SO₄-treated coconut shell generated numerous -S=O and -S-O functional groups that substantially increased the adsorption of methylene blue through electrostatic attractions, H-bonding interaction, and π-π interaction (Tajar et al. 2009; Jawad et al. 2020). Further chlorosulfonic acid activation of matured tea leaf biochar generated additional -S=O and -S-O functional groups, which participated in retention of Rhodamine B and orange G through complexation and electrostatic interactions (Goswami and Phukan 2017). Grafting of -C-S, −C=S and -S-S to adsorbent surfaces has also been used to improve adsorption properties. For example, sugarcane bagasse reacted with ethylenediamine and CS₂ produced a material with exceptionally high adsorption capacities for heavy metals (Pb²⁺, Cu²⁺ and Zn²⁺), with -C-S functional groups participating in the metal complexation reaction (Wang et al. 2017). Similarly, SO₂ or H₂S gas modified agricultural wastes biochars produced -C=S and -S-S groups, which formed strong complexes with Cd²⁺ and Hg²⁺ (Tajar et al. 2009; Krishnan and Anirudhan 2002).

Depending on experimental conditions, methods to introduce S-containing functional groups can either increase or decrease specific surface area and pore volume of carbonaceous adsorbents (Tajar et al. 2009; Yang et al. 2019); however, any reduction in sorption capacity associated with the loss of surface area and porosity is generally offset by the increased S-containing functional groups. Sulfur-functional groups also interact with other functional group types (e.g., O-containing or N-containing groups) in synergist adsorption mechanisms. When -COO-, −NH and -C-S were co-introduced to the surface of sugarcane bagasse during modification procedures, these groups interacted to retain metals (Pb²⁺, Cu²⁺ and Zn²⁺) through ion exchange and complexation reactions (Wang et al. 2017). The interactions among these functional groups greatly improved the overall sorption capacity for many environmental pollutants.

3.4 Other functional groups

While O-, N- and S-containing functional groups are the predominant reactive groups controlling adsorption performance, other functional groups (e.g., −C=C, Fe-O and -P-O) may also enhance the adsorption performance of agricultural waste products (Tables 1, 2, 3, 4, 5, 6, 7 and 8, Fig. 2). Most of the -C=C groups exist in benzene rings and react with pollutants through π-π interactions. Aniline addition modification enhanced the -C=C content of corn stalk and strengthened π-π interactions between adsorbent and adsorbates, thereby increasing the adsorption capacity for acid red and acid orange dyes (Soldatkina and Zavrichko 2017).

Fe-containing functional groups are typically introduced onto agricultural waste materials to prepare magnetic adsorbents, especially in the preparation of biochar. Doping with magnetic materials (e.g., Fe₃O₄, Fe₂O₃) creates an adsorbent that can be easily separated from solution to remove biochar-bound pollutants. These Fe oxides were highly efficient for the removal of As³⁺ (AsO₂⁻) and As⁵⁺ (AsO₄³⁻) through specific sorption reactions (Wu et al. 2018). Additionally, Fe₃O₄ reacts with H₂O creating a polar structure that can retain pollutants by hydrogen bonding.

P-containing functional groups (e.g., −P-O) are generated on adsorbents by H₃PO₄ modification and are capable of retaining pollutants primarily by ion exchange mechanisms. Tang et al. (2019) demonstrated the presence of a large number of -P-O functional groups on corn stalks after H₃PO₄ treatment. The -P-O group was effective in retention of methylene blue through ion exchange. Additionally, the -P-O group efficiently retained U⁶⁺ (UO₂²⁺) from solution via ion exchange (Xiao-Teng et al. 2019). Adsorption of uranium changed the morphology and the structural characteristics by decreasing the distributed interstices, voids and crystal structure in the surface of the modified rice stem through interaction with -P-O.

3.5 Effects of functional groups on adsorption mechanisms

Adsorption mechanisms are primarily affected by specific surface area, pore volume/connectivity and the amount/type of surface functional groups. Physical adsorption takes place by weak Van der Waals’ attraction forces and H-bonding, whereas chemisorption occurs through stronger chemical bonding between adsorbates and adsorbents. Chemisorption (e.g., ion exchange, complexation, electrostatic interaction, precipitation) generally plays a more prominent role in pollutant removal from aqueous solution than physisorption for modified agricultural adsorbents (Tables 1, 2, 3, 4, 5, 6, 7 and 8). Notably, although no functional groups or chemical bonds take part in the fixation of pollutants during the precipitation process, chemical reaction occurs during the formation of the solid products casused by the adsorbents. The retention of pollutants through precipitation is still a chemisorption process. Many O-, N- and S-containing functional groups are suitable for the removal of cationic pollutants, whereas O- and N-containing groups are more suitable for adsorption of anionic pollutants. Fe- and S-containing functional groups often coexist with modified adsorbents surface and were used to binding divalent cationic heavy metals via complexation (Qu et al. 2022b). Moreover, chemical precipitation usually occurred as well for the pollutants retention due to the formation of stable crystal structures.

Complexation and ion exchange are the main adsorption mechanisms for O-containing groups (Azzaz et al. 2017; Li et al. 2010; Qiao et al. 2019; Song et al. 2014), whereas N- and S-containing functional groups interact with pollutants primarily through complexation (Goswami and Phukan 2017; Yang et al. 2019). Studies involving modifications to increase O- and N-functionality of agricultural wastes are much more common than those addressing enhancement of S-functionality, due to the prominence of O- and N-functional groups and their higher adsorption affinity compared to S-functional groups, especially for anionic pollutants.

The main mechanisms for pollutant interactions with modified agricultural wastes are portrayed in Fig. 3. Complexation and ion exchange are often the most important adsorption mechanisms for interactions between functional groups and various pollutants. These chelate-forming or electron-donating functional groups readily react with pollutants to form complexes. Electrostatic attraction is a prominent adsorption process, especially under acidic or alkaline conditions where dissociation/ionization of functional groups initiates interaction with oppositely charged pollutant compounds. For carbonization-treated agricultural wastes, π-π interactions with organic pollutants and alkaline precipitation mechanisms are improved due to the increased C=C content and high alkalinity (leading to decreased metal solubility) after modification (Bashir et al. 2018; Wang and Wang 2019). Moreover, carbonization generally increases the specific surface area of the modified materials, which favors physical adsorption (Chen et al. 2008), H-bonding and van der Waals’ attraction. Given the multi-functionality of most modified adsorbents, the overall adsorption process will involve a combination of several mechanisms.

Fig. 3

Schematic of mechanisms for pollutant interactions with modified agricultural wastes

4 Cost analysis

Cost is an important consideration when selecting an agricultural waste and modification treatment. Currently, few studies have assessed cost estimates of modified adsorbent, thereby limiting an evaluation of material and processing costs. A few studies demonstrated that modified agricultural wastes showed a good price advantage compared with commercial adsorbents. For example, the cost of citric acid modified sugarcane bagasse was 4.76 $ kg^− 1 (Gupta et al. 2018), much lower than commercial activated carbon (56.06 $ kg^− 1). Further, Kannan and Sundaram (2001) determined that the price of carbonised adsorbents prepared from rice husk or straw was ~ 5 times cheaper than commercial activated carbon. In addition, some modified adsorbent materials exhibited excellent pollutant removal efficiency over at least three reaction/recovery cycles (Huang et al. 2020; Liu et al. 2020a; Xu et al. 2019), allowing for reuse potential that further reduces their lifetime costs. Moreover, many feedstocks for bio-adsorbents originate from agricultural wastes that would otherwise be discarded. Hence, the reuse of these solid waste materials is beneficial for utilization of waste materials as well as reducing waste disposal costs, which is often a more substantial cost than the modification costs. Other studies demonstrated that biochar preparation could generate energy (bio-oil and biogas), thereby offsetting production costs partly (Frank et al. 2020). Based on the available studies, almost all cost-benefit analyses indicated that agricultural waste modification provides a low-cost option for preparation of bio-adsorbents for environmental remediation.

Most studies of bio-adsorbent preparation were limited to laboratory evaluations of synthesis costs, which do not account for several associated costs. The costs of raw materials, transportation, modification chemicals and energy are the main factors determining the final preparation cost. Many of these costs are site and time (i.e., inflation) specific making it difficult to produce a universal cost estimate (Gkika et al. 2019). For example, Salam et al. (2011) calculated that the total cost (transportation, handling, chemicals, electrical, energy, drying, etc.) of HCl-modified peanut husk was L.E. 5000 t^− 1 (about $ 838.55 t^− 1 in 2011), a factor of two lower than commercial activated carbon (L.E. 10,000 t^− 1, about $ 1679.10 t^− 1 at 2011) at that time. Additionally, bio-adsorbents will have variable efficiencies for retention of a given pollutant, wherein a higher cost, but higher efficiency product will actually have a lower application cost per unit of pollutant removal. More research is required to assess the costs of modified agricultural waste materials, not only for their production costs, but also for the cost per unit of pollutant removal (including recycling/reuse of materials) and including a complete life-cycle analysis that accounts for externalities, such as greenhouse gas impacts.

5 Future perspectives

The main purpose of current modification procedures is to alter the physical (e.g., surface area, porosity) and chemical (e.g., O-, N- and S- functional groups) characteristics on the surface of agricultural wastes to enhance pollutant adsorption. Future studies will undoubtedly devise new techniques to improve the efficiency and selectivity of the modified adsorbents for targeted applications. In addition, some adsorbent modification processes generate a series of potentially toxic by-products that must be addressed to minimize negative environmental impacts. Thus, optimization studies are required to generate low-cost adsorbents that are tunable for efficient and selective retention of targeted pollutants using green-chemistry principals. This will require systematic studies followed by physicochemical characterization of the modified materials and finally testing of their efficacy for retention of various pollutants under a wide range of environmental water conditions. Rigorous physicochemical characterization is a critical intermediate step in this overall process. First, characterization provides the details for how various modification steps affect the physicochemical properties of the modified products. Second, understanding the physicochemical properties of the modified products is critical for mechanistically understanding the materials ability to retain various pollutants. Once sufficient characterization data has been systematically compiled, it will serve as a toolbox for fine-tuning modification techniques to optimize materials for efficient and selective retention of specific pollutants.

Although the modification methods discussed above are effective to improve the adsorption performance of agricultural wastes, there are some drawbacks that need attention. High alkali concentration may cause an excess elimination of covering materials from the cellulose surface and delignify the fibre extremely due to the hydrolysis, which can negatively affect the strength of the fibre. Concentrated acid oxidation has strong corrosiveness and was shown to decrease the surface area of oxidized adsorbent, which may reduce the porosity and efficacy of the adsorbent material for retention of some pollutants. Esterification and etherification creat high cost of the organic solvents used, the catalyst would also need to be recovered after modification. Carbonization not only consumes a lot of electricity, but also reduces the content functional groups of adsorbent at a high temperature. Magnetization can reduce surface area due to the block of magnetive particles in the pore of adsorbent. Surfactant and grafting have strict requirements for preparing environment and are prone to produce formation of undesirable block co-polymers. In all, the effectiveness of the treatment depends not only on the adsorption environment, but also on various preparation conditions. The conditions (i.e., temperature, atmosphere, reagent, reaction time, concentration, etc.) for the production of low-cost adsorbents after surface modification for higher uptake of pollutants need to be optimized further. Therefore, we still need to refine these modification methods to reduce their shortcomings and make them exhibit greater potential in preparing agricultural waste based adsorbents.

Although modified agricultural wastes have been proven effective in retention of various pollutants in water, remediation actions that leave the pollutant-adsorbent compounds in the water environment have not been fully investigated. Over time, pollutants retained by bio-adsorbents and accumulated in aquatic sediments may be released back to the water column upon changes in pH, temperature, ionic strength, redox conditions and bio-adsorbent degradation. Thus, bio-adsorbents that can be effectively extracted from the water environment, such as magnetic compounds, will provide a more permanent solution for pollutant retention and extraction from environmental waters.

While modified agricultural wastes show great potential for the removal of various pollutants under laboratory conditions, far fewer studies have demonstrated their efficacy in real-world environmental waters, such as industrial/municipal wastewaters. The composition of wastewaters is much more complicated than that of synthetic wastewaters used in many laboratory studies, which undoubtedly will affect the adsorption performance of the bio-adsorbents. Testing common bio-adsorbents across a wide range of environmental waters (e.g., industrial wastewater, municipal wastewaters, aquaculture tailwaters, urban storm runoff, livestock waste runoff) will provide fundamental knowledge to optimize applications for real-world use.

Finally, most previous studies focused on the single adsorbent-adsorbate system, while there is a paucity of studies evaluating simultaneous use of multiple bio-adsorbents for retention/extraction of multiple pollution types. The coexistence of different pollutants is a common scenario in environmental waters and interactions among pollutants often affect the adsorption performance of various adsorbents. Moreover, there is a distinct paucity of research on the removal of rare and emerging pollutants (e.g., endocrine disruptors, radionuclides, pharmaceuticals, antibiotic resistance genes, persistent organic pollutants), which are of considerable concern for human and aquatic ecosystem health. Finally, greater effort is required to rigorously investigate adsorption/retention mechanisms. In addition to traditional sorption isotherm and kinetic studies, the emergence of advanced analytical methods, such as spectroscopic, microscopic, isotopic and molecular techniques, offer great potential for advancing our understanding of adsorption mechanisms. We are currently at the cross-roads of advancing from a more trial-and-error approach to developing a cook-book approach for designing modification techniques to achieve targeted materials for selective removal of pollutants.