The cationic surfactants created numerous positive charges on the adsorbent surface, which increased its retention of negatively charged VO4 3− 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 than that of the original straw . 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 VO4 3− 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. 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 . 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 240.0mg g−1 ,drainage pot which was about a 100-fold increase compared to the pristine husks . Pollutant adsorption onto the modified adsorbent was mainly controlled by a hydrophobic interaction mechanism, along with a considerable contribution from a cation exchange mechanism . 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 . 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 . 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.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 . Grafting-to, grafting-from and grafting-through are the three main linkage types, which are based on the type of polymer. The process involves the addition of monomer units to form copolymers on the adsorbent surface, which transforms the surface functionality . 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 , potassium persulfate and ceric ammonium nitrate 2Ce6 are common initiators for polymerization of monomers on agricultural wastes . Many monomeric species, such as acrylic acid, acrylamide, acrylonitrile and aniline, are used for the synthesis of a polymeric adsorbent surface . These monomers usually contain acidic or basic functional groups. Corn stalk was modified with an acrylonitrile monomer in the presence of KMnO4 as the initiator , 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 of 9.43kJ 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. . Te -NH and -C=O functional groups were introduced by the modification process and the adsorbent surface became rougher 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. demonstrated that polyethyleneimine-grafted cocoa shell was an effective material for the removal of NO3 − and Cr6+ from solutions. The pHpzc value of modified cocoa shell was 9.3, much higher than untreated adsorbent 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 surfcial-NH3 + moiety, which subsequently adsorbed NO3 − and Cr6+ 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 prepared a new composite by chemical polymerization of aniline on the surface of corn stalks using ammonium persulfate 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 frmly 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 and numerous -OH, which made modified rice husk adsorb Pb and bisphenol A quite effectively . β-cyclodextrin grafting accomplished synergetic Pb and bisphenol A elimination through averting their competitive behaviors owing to diverse capture mechanisms for Pb and bisphenol A . 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 fber was much higher than for the untreated fbers . The N atoms in -N=Nand -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 . Adsorption equilibrium occurred within 120–150min and the adsorption capacity of the modified corn stalk was ~2 times more than that of raw stalk . 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 12.7mg g−1 compared with 3.4mg g−1 for raw corn stalk, owing to an increase of -CN groups that promoted metal complexation . Similarly, polyethyleneimine-modified peanut shells increased the number of positive charges on the modified surface,drainage planter pot thereby strengthening electron attraction for Cr6+ in the pH range of 2.0–11.0 . Sorption of Cr6+ followed 2nd-order kinetics and a Freundlich-type isotherm, with a maximum adsorption capacity of 24.8mg 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.O-containing functional groups play a predominant role in surface reactions, hydrophobic/hydrophilic characteristics and electrical properties of adsorbents . Te -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 . The adsorption of heavy metals and dyes by HNO3 modified agave bagasse and oreganum stalks increased due to generation of -OH and -COOH on the surface of the modified materials . The -OH and -COOH groups may dissociate to form of -O− and -COO− upon interaction with water .
These acid ionic functional groups can interact with cationic pollutants through ion exchange or electron attraction mechanisms. The -C=O and -C-O groups are commonly associated with modified agricultural waste products . For example, NaOH treatment increased the -C-O- content of wheat straw, which enhanced Cu2+ adsorption . Te Cu2+ adsorption occurred primarily through a complexation interaction between -C-O- and the metal . Given the multi-functionality 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 . Similarly, − C=O, −C-O and -COOH groups in corn straw bio-char interacted with triazine via electrostatic interactions resulting in effective removal of triazine from solutions . In summary, modification of agricultural wastes to generate additional O-containing functional groups increases pollutant adsorption, primarilythrough complexation reactions, hydrogen bonding, Van der Waals interactions and electrostatic interactions.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 . Te N atom has a lone pair of electrons that serves as an activation site to trigger interactions with pollutants. Qu et al. 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 1 O2 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 . 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 . Similarly, modification with triethylamine increased -NH2 concentration on modified adsorbents resulting in enhanced Ni2+ retention via electron sharing with the N atom . 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 fber with increased -C-N group content exhibited strong attraction for nitrate, sulfate and phosphate with maximum adsorption capacities of 33.7, 31.2 and 200.6mg g−1 , respectively . Similarly, −NH2 can be converted to -NH3 +upon acidifcation, which effectively retained Cr6+ through electrostatic attraction . 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 .S-containing functional groups on modified agricultural wastes can enhance the sorption capacity for several pollutants . Sulfur is loaded onto waste materials through a sulfuration treatment, such as solution infltration or gas activation . 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 . Further chlorosulfonic acid activation of matured tea leaf bio-char generated additional -S=O and -S-O functional groups, which participated in retention of Rhodamine B and orange G through complexation and electrostatic interactions . 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 , with -C-S functional groups participating in the metal complexation reaction .