## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Chitosan-Based Organic/Inorganic Composite Adsorbents  
In most cases, composite CS-based materials represent the combination of CS with inorganic compounds such as metal oxides. Such CSoxide composites typically are obtained via preliminarily mixing CS solution with solid inorganic particles and the subsequent gelation of the resulting composite material. It is noteworthy that the same method can be used for the synthesis of CS-based composites with carbonaceous materials. For example, [PERSON] et al. [116] synthesized crosslinked CS/graphene nanoplate composite spheres (CS-GNPs) by a simple mixing of GNPs with CS solution followed by syringe dripping and CS crosslinking with GA. The adsorption activities of CS-GNPs were evaluated by the example of two azo acid dyes: MO and acid red 1 (AR 1). For the optimized composite material, the adsorption capacity of MO was 230.91 mg/g and 132.94 mg/g for AR 1, respectively. It was shown that CS-GNPs can be easily separated from the solution and efficiently regenerated after reaching saturated adsorption. After five cycles, its adsorption capacity for MO was about 90%. [PERSON] et al. [117] synthesized Schiff's base-crosslinked CS-GA loaded with TiO\({}_{2}\) NPs (Figure 16) for the removal of reactive red 120 (RR120) dye from aqueous solutions in batch mode at various RR120 concentrations, adsorbent dosages, contact times and pHs (3-12). CS-TiO\({}_{2}\) composites were prepared by mixing certain amounts of titania NPs with CS before the dissolution and formation of CS beads and further crosslinking. Different ratios of TiO\({}_{2}\) NPs were used, such as 25% of TiO\({}_{2}\) (CS-GA/TNC-25) and 50% of CS-GA/TNC-50. CS-GA/TNC-25 exhibited the best adsorptive property (103.1 mg/g at 303 K), which was likely due to the perfect balance between the surface area and available aminogroups in this composite. The kinetics and isotherm model results indicated [117] that the adsorption of RR120 was affected by chemisorption and multilayer adsorption. The adsorption mechanism of RR120 on the CS-GA/TNC-25 surface was assigned to various interactions, such as electrostatic attraction, \(n\)-\(\pi\) stacking and H-bonding. A novel CS material (ECS@Ca@CTA) was prepared [118] for the adsorption of sodium dodecylbenzene sulfonate (SDBS). CS was complexed with CaCl\({}_{2}\) and crosslinked into hollow spheres withGA (Figure 17). However, a part of the CS' amino groups was depleted. Therefore, the amino groups were reactivated with ethylenediamine; then, the ECH@Ca was quaternized with (3-chloro-2 hydroxypropyl) trimethylammonium chloride (CTA) and prepared asa hollow sphere structure with a wrinkled surface (designated ECS@Ca@CTA) in order to improve the effect of pH on adsorption of SDBS. pH has a great impact on the adsorption of SDBS. It was shown [118] that the pH dependence of CS-based adsorbents can be improved usingCTA. Ca\({}^{2+}\) and CTA causeCS to be positively charged at different pH levels, while ethylenediamineincreases the number of amino groups. Thus, the maximum adsorption capability of ECS@Ca@CTA was 2430, 1967 and 1116 mg/g at pH 3.0, 7.0 and 10.0 for SDBS, respectively. Crosslinking of CS into wrinkled hollow spheres resulted in an increase inthe specific surface area, chemical  
Figure 16: Simplified representation of the synthesis of composite CS–TiO\({}_{2}\)[117]. Figure 17: Mechanism of formation of ECS@Ca@CTA. Reproduced with permission from authors of [118], Elsevier, 2019.  
stability and capability of SDBS capture. The adsorption capacity of ECS@Ca@CTA can still reach over 80% after sixrepeated adsorption-desorption runs. A composite adsorbent made of CS and bentonite clay was prepared [119] and used for the adsorption of Cr(VI) ions from aqueous solutions. A composite CS-bentonite clay was obtained by mixing bentonite with CS solution in 2% acetic acid with further dripping in NaOH solution to form composite CS capsules, which underwent crosslinking with GA.