One-pot chemical activation and pyrolysis process was developed for biochar production from red macroalgae residue of Gelidium sesquipedale. The macroalgae residue was activated by various catalysts (KOH, NaOH, H3PO4, and CH4ON2) with the two concentrations (2.5 wt% and 5 wt%) using a pulverization system followed by slow pyrolysis at 500 °C. The activated biochars showed a porous morphology with an increase of water holding capacity compared to the unactivated one. The properties of activated biochar observed by further characterization (i.e., FTIR, SEM, TGA) revealed their feasibility to be used as an adsorbent. The results of adsorption experiment confirmed that adsorption was dependent not only on the surface area but also on the surface charge, and functional groups. The sorption performance of activated biochars (AcBC), in terms of the adsorption of methylene blue, was comparable to commercial activated charcoal (Norit®). NaOH (2.5 wt%)-activated biochar had the removal efficiency of 87% versus 97% for commercial activated charcoal.
Over the years, there was an increase the presence of micropollutants in industrial effluents (e.g., dyes, antibiotics, etc.). The discharge of these contaminated effluents in the environment is worrying for both toxicological and esthetical reasons (Tan et al. 2008). To remediate this matter, many technologies for effluents treatment have been reported; for example, membrane technologies represent a potential solution for wastewater treatment, and their industrial applications have expanded considerably in the past 50 years (Aluigi et al. 2014). However, as these technologies are associated with high operational costs, this type of waste treatment can be prohibitively expensive (Thompson et al. 2016). Commercial activated charcoal (AC) can also effectively remove organic and inorganic micropollutants from wastewater. Nonetheless, the high cost of activated carbon has stymied its industrialization. The production of low-cost absorbents (e.g., biochars and/or activated biochars) that involve thermochemical conversion processing of agro-industrial residues have consequently received attention in recent years (Xu et al. 2011; Lee et al. 2013; Monlau et al. 2015). Slow pyrolysis represents one of the most conventional types of pyrolysis that have been used specifically for biochar production (Yaman 2004; Bahng et al. 2009). In parallel, slow pyrolysis also generates bio-oil and syngas that can be used as energy carriers, thus contributing to energy self-sufficiency of the overall process.
Several studies have already demonstrated the efficiency of biochars for environmental bioremediation based on their capacities to remove various organic/inorganic contaminants from aqueous media. For instance, furan compounds (e.g., furfural and 5-hydroxymethyl furfural) were reported to have an adsorption efficiency of 94% with a synthetic medium (Monlau et al. 2015), and it could, in fact, reach a removal efficiency of 100% (Li et al. 2014). Biochar also exhibits a high level of efficiency toward pesticides, with removal efficiencies of 90% and 95% for atrazine and simazine, respectively, using a greenwaste biochar (Zheng et al. 2010). Moreover, phenol and its derivatives have been evaluated in numerous studies, and 2,4-dichlorophenol was found to be fully adsorbed using a paper sludge/wheat husk biochar (Kalderis et al. 2017). Biochar has also been assessed as an adsorbent for many types of organic dyes, such as methylene blue (Li et al. 2016; Lonappan et al. 2016), green malachite (Leng et al. 2015), and methyl violet (Xu et al. 2011). Thus, biochars derived from wheat straw were able to remove between 86.7% and 91.4% of methylene blue. Aside from organic contaminants, inorganic pollutants such as heavy metals have also been widely studied to quantify the potential of biochar to capture and immobilize these types of pollutants. Biochar adsorption capacities of 11.0 mg/g for Zn (II), 12.52 mg/g for Cu (II) (Chen et al. 2011), 13.24 mg/g for Cd (II) (Kim et al. 2013), and 123.0 mg/g for Cr(VI) (Dong et al. 2011) have been reported.
Nonetheless, the removal efficiency performances vs. the amount of biochar used are still lower for commercial activated charcoal due to the higher specific surface area of commercial AC (1024 m2/g) (Alahabadi et al. 2017). This is why an activation process can be presumed to be a particularly suitable way to improve and enhance the surface properties and microporosity, and hence the removal efficiency of organic and inorganic contaminants in wastewater. Currently, there are several types of activation processes available: physical, chemical, and mechanical activation. Chemical activation of biochar using chemical catalysts (acidic, alkaline, or other) has been investigated in several studies.
Conventional chemical activation (Fig. 1) requires many steps, which again generate a certain amount of effluents and energy consumption: stirring, soaking followed by filtration, washing, and a thermal activation step that generally requires high temperatures (e.g., 875 °C, and 675 °C) (Dehkhoda et al. 2016). These additional costs are an impediment to industrial scale-up of this process and limit their deployment (Fig. 1). In this study, an activation/pyrolysis process was developed and referred to a “one-pot activation and pyrolysis process”. Moroccan Gelidium sesquipedale red macroalgae residue is abundant in Marine area of Morocco (3500 km of coastline) (Ainane and M’hammed 2011), which represents about 90% of the harvest of the marine algae treated locally and generated an important quantity of residues and by-products, which is estimated to be of 870 tons/years (Aboulkas et al. 2017). The algal residue was impregnated using different chemical catalysts and then directly pyrolyzed without washing and without generation of waste effluents. The objectives of this work were to study the influence of different “chemical activation” couplings to pyrolysis (Fig. 2) on the yield and the physicochemical properties of activated biochars. To evaluate the adsorption efficiencies of raw biochar and activated biochars, adsorption tests were then carried out and the results were modeled using pseudo-first-order (PFO) and pseudo-second-order (PSO) models to understand the methylene blue adsorption mechanism.
Materials and methods
Feedstock and preparation
The feedstock used in this study was red macroalgae residue (AG) collected from the industrial processing of Agar–Agar extraction from G. sesquipedale at the SETEXAM Company (Kenitra, Morocco). The residue was first ground using a knife mill (SM100, Retsch, Germany) with a screen size of 4 mm.
Chemical activation of the residue and pyrolysis
Prior to the pyrolysis process, 100 g of ground algae residue (4 mm) was treated and activated with different chemical catalysts using a spray system (Fig. 2). Potassium hydroxide (KOH), phosphoric acid (H3PO4), urea (CH4ON2), and sodium hydroxide (NaOH) (purchased from Sigma-Aldrich) at two different concentrations (2.5 wt% and 5 wt%) were applied at a solid loading of 1 kg TS/L. After 48 h of impregnation at room temperature, the chemically activated samples were dried at 105 °C for 24 h. Biochars from raw and chemically activated algae residues were produced in a lab-scale pyrolysis device developed in the laboratory (Fig. 2) under operating conditions adapted from (Francavilla et al. 2014; Aboulkas et al. 2017). All of the experiments were carried out using a horizontal stainless steel fixed-bed reactor. The purge gas (99.99% pure nitrogen) was injected into the reactor at a flow rate varied between 250 and 260 mL/min. For each chemically activated sample, 15 g of sample was loaded into a stainless steel weighing boat and placed in the middle of the reactor to ensure homogenous pyrolysis. It was then purged for 10 min before starting the pyrolysis to remove residual oxygen (air) from the reactor. Each sample was pyrolyzed at 500 °C with a heating rate of 10 °C/min and a residence time of 15 min at the final temperature. The furnace was then cooled under nitrogen flow to 25 °C and then the yield of biochar (BC) and activated biochar (AcBC) was determined.
Physicochemical properties of the biomass and biochars
Chemical and biochemical analysis
The C, H, N, and S contents of the macroalgae residue and the various unactivated/activated biochars were analyzed in duplicate using an elemental analyzer (varioMicro V4.0.2, Elementar®, Germany). The H/C and O/C molar ratios were calculated from the elemental compositions. For macroalgae residue, the carbohydrate, and the lignin composition of the algae residues were determined using the NREL protocol (Sluiter et al. 2008). The Klason lignin content was determined as the weight of the residue retained on the filter, and the soluble fractions were analyzed by high-pressure liquid chromatography (HPLC) to quantify the monosaccharide content (i.e., glucose, xylose, and arabinose). The HPLC (Waters Alliance® system) analysis was performed using a column (Aminex® HPX-87H, BioRad, France) at 40 °C and 0.3 mL/min of 0.005 M H2SO4. All of the measurements were performed in triplicate.
where Vtot and Mini are the total volume of the hydrolysis medium (24.65 mL) and the initial mass of sample in grams, respectively, while 1.11 represents the conversion factor between glucose and cellulose and 1.13 represents the conversion factor between monomers (xylose and arabinose) and hemicelluloses (Barakat et al. 2015).
Thermogravimetric analysis (TGA)
The moisture, volatile matter, fixed carbon, and ash contents of the macroalgae residue and of the various biochars (BC and AcBC) were determined using a thermogravimetric analyzer (TGA 2-LF, Mettler Toledo®, Switzerland). The moisture content was determined as the mass loss after the sample was heated to 110 °C under N2 for 60 min, with flow and heating rates of 50 mL/min and 5 °C/min, respectively. The amount of volatile matter, which is the mass loss between 110 and 900 °C under N2, was also determined. Fixed carbon is the solid lost after switching the medium of analysis from nitrogen to air at 900 °C with the same flow rate for 30 min; the amount of ash was determined as the remaining mass after combustion.
Fourier-transform infrared spectroscopy (FTIR)
All of the biochars that were produced were analyzed using an FT-IR spectrometer (Bruker tensor 27, Bruker Optics, USA) at wavenumber from 400 to 4000 cm−1 with a resolution of 4 cm−1, the accumulation of 16 scans, and in transmittance mode.
Scanning electron microscopy (SEM) analysis
The surface morphologies of macroalgae residue, selected biochars that had been produced (AG, BC, AcBC), and commercial activated biochar (AC) were investigated using a tabletop scanning electron microscope (5th generation-Phenom ProX, Phenom-World®, Netherlands) using a backscattered electron detector and the following parameters: acceleration voltage 10 kV and imaging mode.
Water holding capacity (WHC) and density measurements
The water holding capacity (WHC) is an important parameter indicating the adhesiveness and cohesive forces of biochar toward water (Song and Guo 2012). It was determined using 0.5 g of each biochar placed in glass pasteur pipettes. The pipettes were plugged to avoid loss of the biochar. Four milliliters of deionized water was then introduced gradually. After 20 min, the difference between the water introduced (4 mL) and the amount recuperated in the tubs determined the value of the water holding capacity per 0.5 g of biochar. The density of the biochars (BC and AcBC) was determined using a nitrogen pycnometer (Ultrapycnometer 1000, Quantachrome Instruments, USA) by flow mode for 1 min.
Zeta potential, specific surface area, and pH
The zeta potential represents the potential in a sliding plane of colloidal particles, and its values and charges are linked to the surface charge of the particles (Yuan et al. 2011). Therefore, the surface charge of selected biochars was determined using a modified protocol of Yao et al. (2011). The zeta potential was measured using a Nicomp Dynamic Light Scattering system (Z3000, Entegris, USA). The measurements were carried out using a capillary cell. Magnetic stirring for 30 min of 0.5 g of biochar and 50 mL of ultrapure water was done. Then, the mixture was sonicated for 5 min at an amplitude of 50 and then filtered with a sintered crucible (Ø = 25 mm) using vacuum pump filtration system. The following measurement parameters: electric field: 2 V/cm, electrical intensity: 0.8 mA were chosen after extensive testing to obtain nearly stable values for the zeta potential during the analysis. The specific surface area was determined using N2-BET analysis (3Flex, Micrometrics, Canada). The pH values of the biochars (BC and AcBC) were determined using a pH-meter (FE20 FiveEasy™, Mettler Toledo, Australia). Samples of 1 g of each biochar were mixed with 20 mL of deionized water; the mixture was agitated for 5 min and stand for 5 min before measurement (Song and Guo 2012).
Adsorption of methylene blue
To evaluate the adsorption efficiency of the biochars (BC and AcBC), adsorption experiments with methylene blue (50 mg/L of MB) were conducted in an aqueous medium. MB adsorption experiments are widely used for the evaluation of adsorbents because this dye is toxic for humans and animals and considered as a model for visible pollution and already used in literature to evaluated the efficiency of adsorbent material (Sun et al. 2013). Thus, MB was selected as a model organic contaminant in this study, and the first adsorption experiment was carried out using a fixed concentration of 40 g/L of unactivated/activated biochars compared to the commercial activated charcoal (AC) Norit® (steam-activated Norit GAC 1240 from coal, for potable water processing) purchased from Sigma-Aldrich. Based on the results of the above experiment, the effect of contact time on MB adsorption onto AcBC-NaOH (2.5 wt%) was investigated at three different biochar concentrations (10, 20, and 40 g/L) and at various times ranging from 0 to 1440 min. To quantify and compare the adsorption kinetics of MB onto AcBC-NaOH (2.5 wt%), pseudo-first-order PFO (Eq. 3) and pseudo-second-order PSO (Eq. 4) kinetic models were tested according to the following equations:
where qe (mg/g) is the adsorption capacity at equilibrium, qt (mg/g) is the adsorption capacity at contact time t (min), and K1 (1/min) and K2 (g/mg min) are the equilibrium rate constants for the PFO and PSO models, respectively.
Macroalgae residue composition and biochar production
Gelidium sesquipedale red macroalgae residue was used for biochar production. This biomass contained approximately 40.4 ± 0.1 wt% of carbon, 5.9 ± 0.2 wt% of hydrogen, 5.1 ± 0.1 wt% of nitrogen, 0.9 ± 0.0 wt% of sulfur, and 38.5 ± 0.0 wt% of oxygen. The red macroalgae residue also contained approximately 17.4 ± 5.1 wt% of cellulose, 4.3 ± 2.8 wt% of hemicelluloses, and 17.0 ± 4.7 wt% of lignin. Furthermore, the residue contained 10.9 ± 1.0 wt% of moisture, 65.8 ± 2.1 wt% of volatile matter, 18.7 ± 1.0 wt% of fixed carbon, and 9.2 wt% of ash. These findings are almost in accordance with the results reported by Francavilla et al. (2014) with Gracilaria gracilis red macroalgae residue. Indeed, this residue contained 31.7% of carbon, 5.2% of hydrogen, 4.0% of nitrogen, 1.6% of sulfur, and 36% of oxygen. However, the moisture (1.3 wt%) and ash (21.0 wt%) contents differed significantly from those of the G. sesquipedale macroalgae residue used in this study. The behavior of the macroalgae residue during thermogravimetric analysis is shown in Fig.S1. (Supplementary data). According to the curve, the main loss of mass started at approximately 240 °C onward and progressed extensively until 480 °C with a heating rate of 5 °C/min. This weight loss represents almost 66% of initial weight, and it is due to degradation of main components (fibers and lignin) of the macroalgae residues. Indeed, according to previous studies, hemicelluloses and cellulose consisting of sugar monomers (i.e., arabinose, glucose, and xylose) degrade at low temperatures (less than 480 °C), thereby generating volatile components and vapors (bio-oil and syngas). while lignin, an amorphous polyphenol, decomposes slowly and generates an aromatic solid condensed with reduced functional groups (Yang et al. 2007). This indicates that red macroalgae residues thermally degraded beyond 480 °C, which explains the choice of 500 °C as pyrolysis temperature. In addition, the fixed carbon represented only 19% (dry ash-free basis) of the initial weight.
The mass yields of the biochar (BC) and activated biochars (AcBC) are presented in Fig. 3. According to these results, the production yields of BC and AcBC varied from 31% to 43% of the initial dry weight. These variations in the mass yields were due to the effect of chemical catalysts on volatilization and solid formation reactions (Manyà et al. 2013). For the unactivated biochar (BC), the production yield was 6% higher than the 28% yield reported by Francavilla et al. (2014). Moreover, Fig. 3 shows that the nature and the concentration of the chemical catalyst affected the biochar yield. The yields of biochars activated by phosphoric acid and urea were higher compared to the ones with alkaline activation. These findings may be explained by the increase of the degradation of lignin polymer in the presence of alkaline catalysts (e.g., NaOH and KOH) compared to acidic catalysts (Putro et al. 2016). In addition, it was reported that during biomass pyrolysis, inorganic matter, especially alkali metals (i.e., K, Na, etc.), can catalyze both biomass decomposition and biochar-forming reactions (Manyà et al. 2013).
Physicochemical properties of biochar (BC) and activated biochars (AcBC)
The chemical composition and structural characterization of the different biochars that were produced (BC and AcBC) were determined using TGA, FTIR, SEM, and elemental analysis (CHNS). Table 1 shows that the carbon content ranged from 43% to 62% of dry matter for both BC and AcBC and that it was the most abundant element (Carpenter et al. 2014; Aghababaei et al. 2017). AcBC-NaOH (5 wt%) had the lowest carbon content, while AcBC-CH4N2O had the highest. The nature and the concentration of the chemical catalyst, as well as the pyrolysis temperature, significantly affected the O/C ratio more than the H/C ratio. According to Table 1, the O/C ratio of AcBC-NaOH increased from 0.44 to 0.72 when the NaOH concentration was increased from 2.5 wt% to 5 wt%. The O/C ratio of AcBC-KOH increased from 0.24 to 0.38 when the catalyst concentration was increased. On the other hand, the O/C ratio of AcBC-H3PO4 exhibited a decrease of 0.26 (going from 0.33 to 0.07) as the phosphoric acid concentration was increased, while there was not a significant difference between 2.5 wt% and 5 wt% AcBC-CH4N2O. In parallel, TGA analyses were performed on BC and AcBC to evaluate their stability (Fig.S1 Supplementary data). The curves revealed a clear difference compared to the BC, with a slight loss from 110 to 900 °C, which is an indicative of the high stability of biochar after pyrolysis in an inert medium. However, this weight loss was more pronounced for biochars activated by alkaline catalysts (KOH and NaOH), as was also the case when the catalyst concentration was increased (from 2.5 wt% to 5 wt%), compared to acidic and urea-based catalysts. This finding can be explained by the effect of the alkaline catalyst residues on AcBC (NaOH and KOH), which might lead to the volatilization of the non-complete charred phase of the biochar. The pH of the various biochars were also determined (Table 1). The pH values of the biochars (BC and AcBC) varied from 8 to 12, indicating that these biochars were generally alkaline. According to the data in the literature, the alkalinity of biochar is mainly determined by organic functional groups, carbonates, and inorganic alkalis (Lee et al. 2013). Compared to BC, the pH values of AcBC (NaOH and KOH) increased due to the enrichment of ash by the formation of carbonates such as K2CO3 and Na2CO3 (Oliveira et al. 2017). The pH can play an important role in adsorption tests as it can determine the ionic strength, thereby influencing the sorption of organic particles onto the biochar’s surface (Zhang et al. 2019). An increase in pH from 7.7 to 8.7 has been reported to enhance the adsorption capacity of methyl violet by increasing the net negative surface charge of biochar, thus enhancing the electrostatic binding of methyl violet (Xu et al. 2011).
The water holding capacities (WHC) of the different biochars were also determined (Table 1). The WHC values of BC and AcBC were found to range from 2.4 to 3.6 L/kg. In this study, AcBC-NaOH (2.5 wt%) had the highest WHC value, while the lowest values were obtained with acidic activation by H3PO4 (2.5 wt% and 5 wt%). The real densities were also measured for the BC and all of the AcBC, and the results are summarized in Table 1. The results show that the BC had a density of 0.84 g/cm3, which is slightly lower than that of water (1 g/cm3). This is still acceptable according to the study of Inyang and Dickenson (2015) which reported that at a density less than 0.45 g/cm3, biochar can cause problems in filtration applications due to poor settleability during backwashing in pilot or full-scale filters. Chemical activation had significant effects on density of AcBC, increasing from 0.8 to 3.2 g/cm3 for AcBC-H3PO4 (5 wt%) compared to 1.2, 1.3, and 2.5 g/cm3 for AcBC-KOH, AcBC-NaOH, and AcBC-CH4N2O (5 wt%), respectively. While none of all AcBC (2.5 wt%) exhibited significant changes compared to the BC. Direct information about changes in surface chemical functional groups could be obtained by FT-IR spectroscopy, which can be used for structural analysis of biomasses before and after pyrolysis (Monlau et al. 2015). The FTIR-spectra of red macroalgae residues, BC, and all of the activated biochars AcBC (2.5 wt% and 5 wt%) are presented in Fig.S2. (supplementary data) and the functional groups corresponding to each spectrum are presented in Table 2. The FTIR spectra clearly indicate that the pyrolysis significantly modified the chemical surface of the AG. A wide band at 3284 cm−1 attributed to the –OH stretching vibration of hydroxyl groups (Aboulkas et al. 2017; Zhu et al. 2018); this band disappeared for the BC and all of the AcBC after the pyrolysis process, due to dihydroxylation during the pyrolysis. These findings were confirmed by the O/C ratio. Hence, the O/C ratio was 0.95 for the AG and became 0.24 for the BC after pyrolysis. The two weak bands at 2920 cm−1 and 2840 cm−1 correspond to asymmetric and symmetric –C–H stretching vibrations of aliphatic groups (Mecozzi et al. 2011), and they indicate the presence of methyl and methylene groups (Zhang et al. 2014). These characteristic bands of the aliphatic groups disappeared after the pyrolysis process for the BC and all of the AcBC. The peak at 1635 cm−1, which can be assigned to C=O of carboxyl and C=C stretching vibrations, corresponds to carboxylic groups of hemicelluloses (Pandey 1999; Zhu et al. 2018). This peak disappeared for the BC and all AcBC, due to decarboxylation reactions. The presence of aromatic rings was proven by the presence of bands at approximately 1570–1590 cm−1 that reflect vibrations in condensed aromatic carbon skeletons (Zhang et al. 2014), and they occurred in AcBC-NaOH (2.5 wt%), AcBC-H3PO4 (2.5 wt%), and AcBC-KOH (5 wt%). However, the peak observed at 1430 cm−1 can be assigned to bending vibrations of C–H aliphatic bonds (Li et al. 2017). Although, there was a positional shift for all of the activated biochars, which was probably due to the carbonization process (González et al. 2013). The peak at 1300–1430 cm−1 may be assigned to bending vibration of O–H and C–H bonds which corresponded to phenol and hydrocarbon, respectively. A strong peak at 1028 cm−1, which can be attributed to the stretching vibration of –C–O of the hydroxyl functions of the carbohydrate of raw residues of AG, decreased in intensity for all of the activated biochars due to the degradation of carbohydrate (Liu et al. 2016). The peak around 1035–1050 cm−1 which is detected for AcBC-KOH (2.5 wt%), and AcBC-NaOH (2.5 wt%) corresponds to the angular deformation in plane of C–H bonds of the aromatic rings (Zhu et al. 2018). While, the peaks around 1143 cm−1 correspond to C–O stretching vibration of esters bonds (Li et al. 2017). An exception for AcBC-H3PO4 (2.5 wt% and 5 wt%) is that they showed a peak at 1076 and 1082 cm−1 which is ascribed to the symmetrical vibration in chain of P–O–P (Polyphosphate) (Puziy et al. 2002). Some peaks were observed at 700–950 cm−1 assigned for out-of-plane deformation mode of C–H in various benzene rings (Puziy et al. 2002) and alkene (Li et al. 2017). Due to the high ash amount in AcBC (Table 1) analysis, inorganic groups were also detected by FTIR, the peaks around 400–600 cm−1 could be associated with the Si–O-Si groups (Zhu et al. 2018), and those peaks were observed on all produced BC and AcBC except AG.
Efficiency of biochar adsorption: removal of methylene blue (MB)
The removal efficiency test in fixed condition (40 g/L for 10 min) of BC and all AcBC was carried out and the results are presented in Fig. 4. It is clear that AcBC activated with NaOH and KOH seems to be the most effective AcBC to remove MB dye compared to the others. For alkaline-activated AcBC, the removal efficiencies ranged from 65.5% to 92% and were comparable to commercial activated charcoal (97% removal efficiency). These results highlight the efficiency of the “one-pot” activation pyrolysis approach to produce activated biochars that have the capacity to remove MB. In this study, AcBC-NaOH (2.5 wt%) was selected as the material for the contact time and concentration optimizations study. As from an economic point of view, such catalyst and concentration are the most relevant regarding the efficiency to remove MB. The effect of the contact time on the MB adsorption by AcBC-NaOH (2.5 wt%) at different concentrations (10, 20, and 40 g/L) is presented in Fig.S3 (Supplementary data). The results clearly show that the MB adsorption rate was dependent on the biochar concentration. Thus, the rate of MB removal increased as the biochar concentration increased.
This can be explained by the accessibility and the availability of adsorption sites, which increased with the adsorbent concentration (Fayoud et al. 2016). For 40 g/L of AcBC-NaOH (2.5 wt%), the MB was almost completely removed from the synthetic medium (94.4%) after 15 min; however, at the stationary phase > 60 min (Fig.S3), the MB removal efficiencies were 92.6%, 96.1%, and 97.7% at AcBC-NaOH (2.5 wt%) concentrations of 10, 20, and 40 g/L, respectively. Consequently, the removal efficiencies of MB with 20 g/L and 40 g/L of AcBC-NaOH (2.5 wt%) were almost identical.
Therefore, a biochar dose of 20 g/L and a contact time of 1 h could be considered as the optimal condition for MB removal (50 mg/L). As shown in Table 3, these results are comparable with the data in the literature (Mahmoud et al. 2012; Sun et al. 2013; Inyang et al. 2014; Sun et al. 2015). Indeed, Sun et al. (2013) reported an MB removal efficiency ranging from 78% to 96% using 8 g/L of each biochar produced from various biomasses (i.e., palm bark, eucalyptus, and anaerobic digestate) in MB solution of 5 mg/L which equals to 80 g/L for 50 mg/L. However, Mahmoud et al. (2012) reported an MB removal efficiency of 95% using only 0.5 g/L of HCl-activated biochar produced from Kenaf fiber in 50 mg/L of MB solution. This finding could be explained partially by the high SBET, which was 347 m2/g compared to only 3.3 m2/g for the AcBC-NaOH (2.5 wt%) (Table 3). Furthermore, the SBET of CNT-modified biochar (1%) reported by Inyang et al. (2014) was estimated to be 390 m2/g, which is 118 times higher than the AcBC-NaOH (2.5 wt%). However, the CNT-modified biochar only has a removal efficiency of 64% in 20 mg/L MB solution. Same results have been found by (Lyu et al. 2018).
For the surface reactions, the PFO and PSO models were evaluated to determine the nature of interactions between MB and active sites on the surface of activated biochar. The resulting model parameters and linear plots (R2) are presented Fig.S4 (Supplementary data). According to the obtained data, the adsorption of MB onto AcBC-NaOH (2.5 wt%) is best described by the PSO kinetic model (R2 ~ 1). The correlation coefficient values were higher compared to those obtained with the PFO model. This confirms that the MB adsorption rate is monitored by chemical interactions with the active functional groups, which are largely independent on the accessible surface area.
This is in agreement with the low accessible surface area of the BC and AcBC, as reported in Table 3.
The zeta potential represents the potential in the sliding plane of colloidal particles, and its value and sign are related to the surface charge of the particles. Protonation and deprotonation of functional groups can, therefore, create a net charge on the surface of the solid particles, which can form an electrical double layer in the solution phase in proximity of the surface (Xu et al. 2011), and this property is required in adsorption materials. In this study, the surface of all of the selected biochars BC/AcBC was negatively charged. Increasing the NaOH concentration from 2.5 wt% to 5 wt% has led to an increase of zeta potential (absolute value) from − 34.12 to − 41.45 mV, respectively. The commercial activated charcoal had the lowest zeta potential value of − 20.60 mV, which is similar to the zeta potential of non-activated BC (− 22.77 mV). Scanning electronic microscopy (SEM) was used to characterize the morphology of the macroalgae residue (AG) and the porosity of selected biochars: BC, AcBC-NaOH (2.5 wt% and 5 wt%), AcBC-KOH (5 wt%), as well as commercial activated charcoal (AC), as shown in Fig. 5. According to the SEM images, alkaline activation with KOH (Fig. 5e) and NaOH (Fig. 5c, d) generated porous structures compared to the non-activated one (Fig. 5a).
These findings are in accordance with the SBET results presented in Table 3, which reveal an improvement of the SBET after alkaline activation, with a slight enhancement of the accessible surface (Table 3). However, these values of SBET are very low compared to the 947.2 m2/g value for commercial activated charcoal (AC). Nonetheless, based on the adsorption test presented in Fig. 4, the removal efficiencies of AcBC-NaOH (2.5 wt%) and AC appear to be almost similar. Therefore, the adsorption kinetics described by the PSO model confirmed that the rate of adsorption of MB on AcBC-NaOH (2.5 wt%) was mostly controlled and depended strongly on chemisorption, which involves valency forces through sharing or exchange of electrons. In this case, the affinity of biochar to adsorb MB depends on the surface chemistry of biochar. The types and levels of surface-functional groups determine the adsorption rate (Li et al. 2018). As it was detected on FTIR analysis, which highlight the presence of –C–O, C=C and aromatic rings, these functional groups gave the biochar the ability to attract cationic molecules (MB in this study), and it was proved by the determination of zeta potential (negatively charged). Therefore, and according to the literature (Inyang and Dickenson 2015; Li et al. 2018), the interaction mechanism could be suggested through: π–π stacking electrostatic interaction, hydrophobic interaction, and hydrogen bonding interaction (Li et al. 2018). In addition, the porous structure of AcBC also has an advantage on the diffusion of particles of MB into biochar (Li et al. 2018).
The present study investigated the production of chemically activated biochars using one-pot activation/pyrolysis process. The results show that the activated biochar with NaOH (2.5 wt%) represents the most promising adsorbent due to its high zeta potential and the highest SBET, compared to the other selected biochars. The results of adsorption tests on the various biochars indicate that the removal efficiency of methylene blue holds considerable promise compared to commercial activated charcoal. The removal efficiency of activated biochar-NaOH (2.5 wt%) was estimated to be 87% compared to 97% for activated commercial carbon in fixed adsorption condition (40 g/L, 15 min). The adsorption of methylene blue depends not only on the surface area, but also on the surface charge and functional groups.
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The authors would like to thank the SETEXAM Company in Morocco for providing the algae residues. This work was based on a formal collaboration between the INRA Montpellier, Mohammed VI Polytechnic University (UM6P), OCP group and the APESA-Pole Valorization (ATLASS Project).
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Tayibi, S., Monlau, F., Fayoud, N. et al. One-pot activation and pyrolysis of Moroccan Gelidium sesquipedale red macroalgae residue: production of an efficient adsorbent biochar. Biochar 1, 401–412 (2019). https://doi.org/10.1007/s42773-019-00033-2
- Red macroalgae residue
- Chemical activation
- Slow pyrolysis