## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Magnetically Separable Chitosan Composites  
Magnetically separable composites based on various biopolymers are widely used for removal of different heavy metals from aqueous solutions [124]. Incorporating magnetic materials into the CS matrix and shaping CS in various forms such as beads, membranes, hydrogels, etc., greatly boosts the separation problem of CS [115]. In spite of the fact that many magnetic CS materials have been prepared for the adsorption of metal ions, [PERSON] et al. [125] noticed that, to date, there is no standard method for the comprehensive evaluation of material performance. Thus, two methods for evaluation of the overall performance of magnetic materials were established [125], which consider both heavy metal ion adsorption capacity (Q) and magnetic recovery of the material after use:  
1. Based on calculation of the adsorption recovery index using the values of Q and recovery time (t) needed for achieving 98% material recovery, a higher adsorption recovery indexvalue shows bettermagnetic material performance;
2. Based on effort vector data visualization, in which the position of a magnetic material is shown on a coordinate depicted using normalized Q and saturation magnetization value, the distance from the data point to the target indicates the performance of the material. Figure 18: Schematic diagram of the preparation of the Ag@beads (based on Ref. [123]). Moreover, [PERSON] et al. [125] used two methods for the preparation of magnetic CS beads. The first method (embedding method) was based on the most widespread approach--preliminarily synthesis of Fe\({}_{3}\)O\({}_{4}\) NPs, mixing of magnetic NPs with CS solution, and dripping of Fe\({}_{3}\)O\({}_{4}\)--CS solution in NaOH solution with further crosslinking with GA. The second method (chemical coprecipitation method) was based on the dissolution of CS and precursors of magnetic particles (Fe(II) and Fe(III)salts) in 3% acetic acid with subsequent dripping of this mixture in NaOH solution and crosslinking with GA. Thus, two series of magnetic CS beads with different Fe\({}_{3}\)O\({}_{4}\)/CS mass ratios were prepared. Synthesized magnetic CS beads were used for the adsorption of Ag\({}^{+}\), Cu\({}^{2+}\), Hg\({}^{2+}\), Cr\({}^{3+}\) and Cr\({}^{6+}\) from aqueous solutions and recovery studies. Compared to the adsorption recovery index, the effort vector data visualization was more straightforward and easier to use. While comparing the two magnetic CS bead synthesis methods based on the proposed effort vector data visualization, it was found [125] that both synthesis methods allow for similar efficiency of heavy metal adsorption and data distribution. Independent of the synthesis method, CS beads containing 0.3 g of CS and 0.7 g of Fe\({}_{3}\)O\({}_{4}\) NPs revealed best performance for the adsorption of the chosen heavy metal ions. Though most researchers prefer to synthesize CS-based materials in the form of beads, which possess low flow resistance and are hence convenient for continuous mode operation, for special applications, CS films and coatings can be obtained. CS nanocomposite films were prepared [126] using in situ precipitation of magnetic particles. The method of synthesis was based on the above-described chemical coprecipitation. The difference was in the shaping of the resulting material: a film was obtained by pouring the mixture (CS and iron salts) into Teflon Petri dishes and subsequently drying and rinsing it with NaOH solution; no crosslinking agents were added. The magnetic CS film was tested as an adsorbent for arsenic ions, revealing a higher adsorption capacity (10.4 mg/g) compared to common CS film (1.6 mg/g). The higher equilibrium adsorption capacity was explained by the synergic effects of CS and iron oxide particles regarding the adsorption capacities of the phases, film surface morphology (a much more irregular surface in the case of the nanocomposite), polymeric matrix crystallinity and electrostatic forces that prevail in acidic pH [126]. [PERSON] et al.