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 ciprofoxacin 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 337.22mg g−1 . Alkaline treatment increased both the surface roughness and functional surface activity 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 .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 . 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 . Tevannan et al. 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,25 liter pot case studies have demonstrated that acidification can improve the adsorption performance for anionic pollutants as well . 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 . 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 and O contents due to the release of volatile compounds during acid oxidation . 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 . 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 . Several studies have demonstrated the efficacy of acid treated agricultural wastes for heavy metal ions, such as Zn2+, Pb2+ and Cr6+ . Acid treatment alters functional groups and several surface area/porosity characteristics to enhance adsorption performance . The adsorption capacity of natural corncob for Cd2+ increased from 4.7 to 19.3mg g−1 when the material was acidifed by HNO3 . 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. Te 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 was 3-fold higher than that of the raw corn stalk . The pHPZC 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 than raw corn stalk , 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 than that of the corresponding non-treated stalks . 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 . The results showed that the maximum adsorption of Victazol orange 3R dye to dilute HCl-treated mango seed increased from 36.9 to 63.3mg g−1 with much faster sorption kinetics than the untreated seeds owing to increased BET surface area and average pore diameter . Overall, acid treatment contributes greater functional group reactivity and structural properties to enhance adsorption performance.Esters are generated from the esterification of free hydroxyl groups in cellulose by reacting with one or more carboxyl groups , whereby cellulose reacts as a trivalent polymeric alcohol . Succinic anhydride, EDTA 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 . 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 were mixed at a solid:liquid ratio of 1:10 and reacted at 50°C for 24h and 120°C for 90min to prepare an esterified adsorbent . After modification, a strong stretching vibration at 1742 cm−1 occurred in the FTIR spectrum, indicating a successful esterification process . 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 . 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 . As a result, the increased -COO− content and porous surface properties enhanced methylene blue sorption through improved ion exchange and intraparticle 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 . 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 than for tartaric acid treatment . 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 esterifcation efficiency during modification. The esterification between citric acid and hydroxyl groups achieved in high efficiency using a mild catalyst in a N, N-dimethylformamide medium , NaH2PO2·H2O, as a catalyst, increased the speed of the esterification reaction and allowed for a simplified procedure compared to traditional methods without catalyst. Te 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 . Adsorption of Cu2+ by citric acid modified soybean straw was rapid during the first 10min and reached a maximum adsorption capacity of 0.69 to 0.76mmol g−1 based on a Langmuir model . 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 8.513kJ mol−1 , indicating a chemical adsorption mechanism,25 liter plant pot as opposed to a physical adsorption mechanism for the raw biomass . Similarly, Pb2+ adsorption capacity increased from 125.84mg g−1 to 293.30mg g−1 due to the formation of both ester linkages and grafting of carboxyl groups . For organic pollutants, the intensity of chemisorption was closely related to the number of functional groups. Methylene blue adsorption capacity increased from 170mg g−1 to 650mg g−1 and 280mg g−1 . 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 esterifcation, which consequently improve adsorption performance through complexation reactions.Ethers are synthesized through etherification, whereby -OH groups on agricultural wastes are substituted by other functional groups .

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 . Etherification may generate positively-charged functional groups to augment sorption sites for retention of anions, such as PO4 3−, NO3 − and SO4 2− . 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 . 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 1.64mEq 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 . 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~−7.0mV compared with +5.2~−45.8mV 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 . Etherifcation processes are also utilized to improve the removal rate of cationic pollutants by introducing amino groups for modified adsorbents. Kong et al. 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+ . 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 130mg g−1 , which were attributed to complexation reactions of Cu2+ with the amino and carboxyl groups generated by etherification . Additionally, adding an activated bond of -CN to the cellulose -OH groups greatly improves Cd2+ adsorption performance . The Langmuir sorption model provided a better ft to the Cd2+ adsorption equilibrium data than a Freundlich model, and determined a maximum uptake of 12.73mgg−1 for modified corn stalks, compared to 3.39mg g−1 for raw corn stalks.