## 3 Chitosan-Containing Materials and Composites as Adsorbents  
CS is known to be an environment-friendly and cost-efficient adsorbent for dyes and inorganic ions from wastewater. However, CS has some drawbacks, including dissolution in anacidic environment and poor mechanical properties, resulting in unsatisfactory adsorption performances. Crosslinking allows for the enhancement of the mechanical properties and pH-stability of CS. At the same time, the grafting of CS with functional groups is an effective way ofimproving its adsorption performance [97].  
Figure 9: Types of CS-containing NPs: NP formed by crosslinking (**a**), liposome coated with CS (**b**), polymer-coated CS NP (**c**), CS nanocapsule (**d**) (based on Ref. [95]).  
In a work by [PERSON] et al. [97], divinylbenzene (DVB) was selected as a crosslinking agent in a two-step, one pot method based on the acylation reaction between maleic anhydride and CS to prepare a series of DVB-crosslinked CS/maleic anhydride (MAH) polymers (CS/MAH-DVB), which wasused to remove methylene blue (MB) from aqueous solutions. A simplifiedCS/MAH-DVB synthesis scheme is presented in Figure 10. Briefly, CS reacted with MAH in formamide medium in the presence of p-toluene sulfonic acid. Then, the modified CS reacted with DVB (vinyl groups were taken equimolar to MAH) in DMSO medium in the presence of 2,2\({}^{\prime}\)-azobisisobutyronitrile. As a result, three samples of CS/MAH-DVB polymers were prepared using 0.5, 1 and 2 CS/MAH molar ratios.  
The adsorption parameters (pH, contact time, initial concentration of dye and temperature) were varied, and the maximum adsorption capacities were 448 mg/g, 482 mg/g and 503 mg/g, respectively, for three CS/MAH-DVB polymers with different molar ratios at 298 K, which were superior to other reported bio-based adsorbents.  
The adsorption mechanism was attributed to electrostatic interaction and \(\pi\)-\(\pi\) stacking interaction between MB and CS/MAH-DVB polymers. Furthermore, the CS/MAH-DVB polymers could easily be recycled by washing with acidic acetone, demonstrating excellent stability and favorable recyclability over five cycles [97].  
Pyridine-modified CS was synthesized (Figure 11) and used for simultaneous adsorption of Cr(VI) and Cu(II) cations [98] at variation of concentration, contact time, and pH.The maximum adsorption capacities of pyridine-modified CS (at pH 4.5) in single and binary aqueous solutions were 1.84 mmol/g (at pH 5.5) and 1.13 mmol/g (at pH 4.5) for Cu(II) and 3.86 mmol/g (at pH 3.6) and 0.98 mmol/g (at pH 4.5) for Cr(VI), respectively. It was proposed that the imine and pyridine groups introduced on CS were able to adsorb oxyanions of Cr(VI) by the formation of outer-sphere complexes, while Cu(II) ions were adsorbed by the formation of inner-sphere complexes. The reuse of pyridine-modified CS  
Figure 10: Synthesis of CS/MAH–DVB polymers [97].  
was studied. It was shown that Cu(II) ions were almost completely desorbed (-96%) and fully re-adsorbed (-100%), while the desorption of Cr(VI) was incomplete (-49%),resulting in lower re-adsorption (-36%). The thermodynamic parameters of adsorption revealed that the adsorption of both metal ions was enthalpically driven, but entropically unfavorable.  
In spite of numerous CS-based polymers with enhanced adsorption ability, structured CS-containing materials, e.g., in form of beads, exhibit good mechanical and fluid dynamic properties that are suitable for large-scale adsorption processes. Moreover, CS beads present remarkably enhanced adsorption capacity, mainly due to increased specific surface area and porosity [99].