## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Structured Chitosan Materials
biofilm had low swelling index, good mechanical strength on glass plate and relatively high (64.4%) free amino groupcontent. The maximum adsorption capacity of CS-EGDE at 303 K was 165.3 mg/g and 131.2 mg/g for RR-120 and MO, respectively. The mechanism of adsorptionmainly included hydrogen bonding interaction, electrostatic attractions and \(n\)-\(\pi\) stacking interaction. Novel spherical adsorbents were synthesized by [PERSON] et al. [104] via crosslinking of CS beads grafted by polyethylenimine (PEI) with GA (composite GACS@PEI) or ECH (composite EPCS@PEI, in which ECH is denoted as EP) (Figure 13). CS beads were formed using the precipitation of drops of CS solution in glacial acetic acid with sodium hydroxide dissolved in a methanol-water mixture. CS beads were crosslinked with GA or ECH. In order to graft PEI to the surface of crosslinked CS beads, PEI was treated with 3-glycidyloxypropyltrimentoxysilane (EPTTMS). Such a treatment was shown to increase the number of adsorption sites of the resulting materials. Synthesized composites of GA/EPCS@PEI were used to remove toxic contaminant--diclofenac sodium (DS)--from water. EPCS@PEI showed a maximum adsorption capacity of 253.32 mg/g and a removal efficiency of nearly 100% for the DS in the initial 50 mg/L solution. It was also shown that after the five repeated cycles of EPCS@PEI, the final removal rate is more than 80%, indicating excellent properties, where good adsorption capacities were retained over repeated cycles [104]. The adsorption capacity of EPCS@PEI was higher than for some other CS-containing composites, i.e., ZnFe\({}_{2}\)O\({}_{4}\)/CS [105], which was only used allow DS concentrations and allowed for 75% of the adsorption removal rate. [PERSON] et al. [106] synthesized an innovative adsorbent in the form of CS microspheres crosslinked with ECH and modified with PEI. In contrast to the majority of the described CS-based adsorbents, CS microspheres were obtained via inverted-phase emulsification crosslinking technique by using a one-pot method. Briefly, CS solution was added to liquid paraffin (oil phase) under mechanical agitation and addition of ethyl acetate and surfactant (span-80). Then, formaldehyde (FA) was added to protect the amino groups of the CS; after that, crosslinking was carried out at a pH of 8-10. After separation and drying, the resulting  
Figure 13: Preparation of GA/EPCS@PEI composites. Reproduced with permission from authors of [104], Elsevier, 2020.  
crosslinked CS microspheres were modified with PEI using the surface grafting reaction (see Figure 14) by adding PEI solution to the suspension of microspheres at a pH of 10 (sample PEI-ECH-FA-CS MPs). Additionally, the carboxymethylation of PEI-ECH-FA-CS MPs was carried out (sample PEI-ECH-FA-CMCS MPs) by subsequent treatment with HCl solution in order to eliminate the protecting groups (sample PEI-ECH-CMCS MPs). As a result, the synthesized magnetic adsorbent (PEI-ECH-CMCS MPs) contained free amino- and carboxyl groups. The specific surface area of the PEI-ECH-CMCS MPs was about 29 m\({}^{2}\)/g, with porous 3D architectures, uniform spherical surfaces and narrow size distribution (19-33\(\upmu\)m) [106]. PEI-ECH-CMCS MPs allowed for the capture of Cr(VI) and Pb(II) with a maximum adsorption capacity of 331.32 (at pH = 3) and 302.56 mg/g (at pH = 5), respectively, in batch experiments. Excellent reusability and feasibility were evidenced by an adsorption capacity loss < 12.10% and high removal efficiency for Cr(VI) (95.79%) and Pb(II) (91.40%) in synthetic effluents [106]. [PERSON] et al. [107] synthesized CS beads crosslinked withboth TPP and ECH for the adsorption of MO. It was shown that TPP could act as aprotecting agent forthe amino groups of CS, while ECH reacted with the primary hydroxyl groups of the polymer. Various concentrations of TPP, ECH and immersion time in the TPP solution were studied. It was proposed that TPP prevents chemical binding between the NH\({}_{2}\) of CS and ECH, which reduced the adsorption capacity of CS beads.