## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Structured Chitosan Materials  
The general method for preparation of CS beads consists of the dissolution of CS in acid medium and its subsequent dripping in alkaline medium. There are many beading parameters (CS molecular weight and concentration, acid concentration, NaOH concentration, temperature), which are the subject of optimization. Additionally, the subsequent crosslinking of CS beads with various agents allows for further increase in their permeability, stability and adsorption ability. For example, [PERSON] et al. [99] studied CS beads crosslinked with 1,2,7,8-diepoxyoctane (DEO) in the adsorption of RR2 dye. A maximum adsorption capacity of 788.6 mg/g was achieved at a pH of 2. The satisfying performance of the CS beads (597.6 mg/g at pH 4), compared to that of CS flakes (487.23 mg/g), implied that beading is a promising process for enhancing CS effectiveness as an adsorbent. It was also proven that CS and crosslinked CS beads can be recycled (by washing with NaOH solution) at least five times before sensible performance loss. DEO-crosslinked CS beads modified with spermine (SP) were synthesized (Figure 12) [100]. SP was chosen as an amination agent for the enhanced adsorption of Cr(VI) from aqueous solutions. As can be seen from Figure 12, DEO can interact with either -CH2 OH or -NH2 groups of CS; thus, free -CH2 OH groups can further interact with ECH and then with SP. Optimal CS-DEO-SP beads were prepared using 1.5 mg/L of SP for 5 h at 50 \({}^{\circ}\)C. The prepared beads exhibited heterogeneous and porous surfaces. The obtained CS-DEO-SP beads were macroporous and had increased surface area, pore volume and pore size by 4.9, 7.1 and 1.4 times, respectively, in comparison with initial CS beads. The experimental data revealed that the adsorption of Cr(VI) ions on the CS-DEO-SP beads was extremely dependent on the solution's pH level. A maximum adsorption capacity of 352 mg/g was found at acidic condition (pH 2.0). Regeneration of CS-DEO-SP was successfully performed via treatment with NaOH solution (1.0 mol/L). Regenerated CS-DEO-SP beads allowed for the achievement of an adsorption capacity of 192.31 mg/g after five cycles of repeated use. Thus, high adsorption capacity, acid stability and reusability render the CS-DEO-SP beads a promising and effective adsorbent for eliminating Cr(VI) from metal plating wastewater. Figure 11: Modification of CS with pyridine-2-carbaldehyde [98]. [PERSON] et al. [101] synthesized CS beads crosslinked with GA for the removal of cationic (malachite green (MG)) and anionic (indigo carmine (IC)) dyes. For the synthesis of CS beads, high molecular weight (360,000 g/mol) CS was used. CS beads were formed using the common method, including the dissolution of CS in acetic acid and precipitation by dripping in NaOH solution with subsequent freeze drying. The removal of dyes was based on the soaking time at the same initial concentration (60 mg/L) at varying temperatures (30-40 \({}^{\circ}\)C). It was shown that crosslinked CS beads in freeze-dried form have a high adsorption capacity (714.29 mg/g) for MG, allowing quick removal of the dye in 30 min independent of the temperature, but they are not suitable for indigo carmine. [PERSON] et al. [102] studied the influence of crosslinking agent (GA or ECH) on the adsorption ability of CS spheres with respect to diesel oil at concentration of 200-900 mg/L. It was found that the maximum amount of oil (1.82 mg/g) was adsorbed on CS crosslinked with GA. CS spheres crosslinked with ECH adsorb 1.642 mg/g of diesel oil. The difference in adsorption ability was explained by the modification of CS surface characteristics (porosity, roughness, etc.) after the crosslinking process. A biofilm of crosslinked CS-ethylene glycol diglycidyllether (CS-EGDE) with an average thickness of 4.24 \(\upmu\)m was prepared by [PERSON] et al. [103] and used as a biosorbent for the following dyes: reactive red 120 (RR-120) and methyl orange (MO). The CS-EGDE  
Figure 12: Synthesis of CS–DEO–SP beads [100].