With respect to anionic retention, etherification of rice straw increased the maximum adsorption capacity of sulfate from 11.68 to 74.76mg g−1 by introducing amino groups . 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 . Adsorption equilibrium was achieved in 35min for HCrO4 − and 15min for H2PO4 −, compared with 3hr for the unaltered biomass. Accordingly, the adsorption capacity of modified wheat straw for HCrO4 − and H2PO4 − were 0.98 and 1.15mmol 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.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 bio-char is a common carbonization process that has been extensively studied . Biochar products have a larger specific surface area, greater porosity and more functional groups than the raw agricultural wastes from which they were formed . The alkaline nature and the existence of mineral constituents on bio-char surfaces also promote the formation of metal precipitates on the bio-char surface . The fixation ability of original bio-char for pollutants is relatively limited and many studies have explored altering pyrolysis conditions to generate activated bio-chars 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 bio-char generated from agricultural waste . In general, bio-char produced at lower temperatures contains a greater functional group content,black plastic plant pots whereas those produced at higher temperatures have a more porous surface and overall porosity .
For example, aldehyde and ketone functionalities formed at ~200°C and became dominant at 300~500°C, increasing the adsorption capacity for various pollutants . However, the drastic fusion of the ring structures in bio-char 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 is also an important factor determining the efficiency of pollutant removal by agricultural waste-derived bio-chars. 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 . The capacity of the bio-char to retain Cu present in solution depended on the size of the inorganic fraction and varied in the following order: rice bio-char > chicken manure bio-char > olive mill waste bio-char > acacia bio-char > eucalyptus bio-char > corn cob bio-char. The addition of chemical amendments to the biomass feedstock has an appreciable effect on bio-char characteristics and the adsorption performance of the bio-char material. The selective introduction of functional groups, heteroatoms, metal atoms into bio-char can improve its specific properties. In-situ synthesis of bio-char refers to the preparation of modified bio-char in only one step by simultaneously pyrolyzing reagents and agriculture wastes. As a result, the modified bio-char has the optimized physicochemical structures and properties. Postoptimization of bio-char is useful to further enhance its potential value after pyrolysis, a special bio-char could be designed via the further surface modification and pore regulation. 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 bio-char surface that increased the chemical afinity for pollutants through covalent bonding . Similarly, HNO3 addition to the feedstock generated N-containing functional groups on bio-char, with the positive ≡N+ group interacting with the negative -O≡, N=C and heterocycle N-C groups on methylene blue to facilitate adsorption . In addition, the weakly acidic FeCl3 would be transformed to Fe2O3 during the preparation process of bio-char, increasing the Fe-O functional groups strongly on the surface of bio-char .
The peaks of Fe 2p1/2 and Fe 2p3/2 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 bio-chars. Activation of orange peel bio-char with CO2 and KOH strongly altered several physicochemical properties . During pyrolysis, CO2 reacted with K2CO3 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 bio-char surfaces . Co-pyrolyzed peanut hulls with H2O2 resulted in an increase in the O content and surface carboxyl functional groups, improving chemical adsorption . As another example of oxidation effects, a KMnO4-treated hickory wood bio-char surface was covered with ultrafne MnOx particles exhibiting a surface enrichment of O-containing functional groups and a higher surface area . Metal retention by this engineered bio-char mainly resulted from surface adsorption mechanisms involving both the surface MnOx 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 bio-char adsorbents . The total adsorption capacity for Cd2+ increased due to the introduction of C-S complexes on bio-char that facilitated the retention of Cd2+ through a strong bond with S . Wang et al. 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 bio-char. The enhanced sorption affinity was mainly attributed to π-π EDA 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 greatly improved the adsorption behavior of Cr6+ by polyethylenimine-treated rice husk bio-char . A pseudo-second-order kinetic model indicated a maximum Cr6+ adsorption capacity of 435.7mg g−1 compared to only 23.09mg g−1 for natural bio-char. Moreover, this new material exhibited excellent cyclic adsorption ability making it a low cost, ecofriendly adsorbent.
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 bio-chars to rapidly interact with pollutants through a variety of adsorption mechanisms . Furthermore, introduction of chemical amendments to the carbonization process can generate specific chemical moieties that selectively target the removal of specific pollutants.Magnetization of modified agricultural wastes, such as bio-char, is a strategy to introduce transition metals or their oxides into the organic matrix to create a material 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 . Nanoscale-zero-valent, iron-coated wheat straw exhibited better adsorption performance for removal of Cu2+ than the raw adsorbent . A portion of the Cu2+ was directly adsorbed by the wheat straw, whereas another fraction of the Cu2+ was frst 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+ due to enhanced surface interactions associated with the Fe oxides . 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 on the surface of organic materials is a common technique for magnetization that can be achieved by ferrofuids,black plastic pots for plants microwave assisted and mechanochemical techniques . 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 . 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. 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 exhibited excellent sorption capacity for Cd2+ and Pb2+, with Fe-OH groups playing an important role in the adsorption mechanism . 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. .
They found that pore size was increased by 2~4nm 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 30min facilitating rapid processing . Baldikova et al. 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 159.2mg g−1 , respectively . 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 compared to several other modification methods . 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.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 . The charge characteristics of the hydrophilic head classify the surfactants as ionic and non-ionic surfactant groups . Various functional groups 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 . Cationic surfactant treated agricultural wastes demonstrate a good potential for removal of anionic pollutants. Cetyl trimethyl ammonium bromide modified wheat straw obtained higher N , C and H contents than that of the origin wheat straw due to the loading of the surfactant . 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, Tamilarasi et al. 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.