## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Magnetically Separable Chitosan Composites
[127] synthesized a CS-containing magnetic nanobiocomposite. The composite was prepared using the chemical coprecipitation method with several significant differences: (i) before synthesis, the CS was guanidinylated, as shown in Figure 19; (ii) a modified CS was mixed with Fe(II) and Fe(III) salts with the subsequent formation of precipitate by adding ammonia solution. The resulting nanobiocomposite had a core-shell structure, where the core was represented by Fe\({}_{3}\)O\({}_{4}\) NPs with average sizes of 30-34 nm, while the shell consisted of the modified CS. The application of this nanobiocomposite was evaluated in two different fields: the adsorption and in vitro hyperthermia procedures [127]. Adsorption capacity was studied with respect to copper (Cu\({}^{+2}\)), lead (Pb\({}^{+2}\)) and chromium (Cr\({}^{+6}\)) ions. Different parameters including time, temperature and amount of biosorbent were optimized for each heavy metal ion. In optimized conditions, the adsorptions of Cu\({}^{+2}\), Pb\({}^{+2}\) and Cr\({}^{+6}\) were, respectively, 100%, 98.64% and 33.76%. Concerning the in vitro hyperthermia process, the maximum specific absorption rate value (70.59 w/g) was obtained by using 1 mg/mL of the nanobiocomposite, with very promising results. Figure 19: Guanidinylation of CS [127]. Another example of a modified CS loaded with magnetic NPs is CS functionalized with an aromatic Schiff's base ligand (3,3-diphenylpropylimine methyl benzaldehyde--PPIMB) (Figure 20). Such a treatment allowed for a high number of binding sites for the effective adsorption of lead ion in the aqueous medium [128]. It is noteworthy that CS was modified with PPIMB directly after the beading procedure. A magnetic CS-PPIMB adsorbent was prepared by dispersion of Fe\({}_{3}\)O\({}_{4}\) NPs in acetic acid with the subsequent addition of CS-PPIMB to this solution. The effect of pH, collision time, adsorbent dosage and initial concentration of Pb(II) was studied. The adsorption capacity of magnetic CS-PPIMB against Pb(II) was 230.48 mg/g, which was higher than that of many similar adsorbents. Moreover, it was shown that magnetic CS-PPIMB adsorbent was highly selective toward Pb(II) compared to other metal ions. The strong binding energy of the Pb atoms with aromatic rings and imine groups of the synthesized adsorbent was confirmed using theoretical computations. The synthesized magnetic CS-PPIMB also revealed good regeneration capacity towards Pb(II) ions after five successive adsorption-desorption cycles [128]. CS-based magnetic composites can be applied not only for the removal of cations, but also for anions from aqueous solutions. As an example of recent research, PEI-grafted CS core-shell magnetic NPs (Fe\({}_{3}\)O\({}_{4}\)/CS/PEI) were synthesized and applied for adsorbing phosphate in water [129]. This magnetic composite was obtained by using the embedding method (see above). No shaping of the resulting mixture was carried out. Instead, an aqueous solution of a gelating agent (TPP) was added directly to the mixture containing CS and Fe\({}_{3}\)O\({}_{4}\) NPs. Thus, ionotropic gelation of CS occurred directly on the surface of the Fe\({}_{3}\)O\({}_{4}\) NPs. Then, PEI was grafted onto the surface of Fe\({}_{3}\)O\({}_{4}\)/CS preliminarily treated with ECH, which reacted with the hydroxylmethyl groups of the CS (the molar ratio of total amino groups of PEI to the hydroxyl groups of the CS was fixed at 2.5). The composite Fe\({}_{3}\)O\({}_{4}\)/CS/PEI had a maximum adsorption capacity of 48.2 mg/g with respect to phosphate ions at an equilibrium pH = 3.0 and 25 \({}^{\circ}\)C. The primary mechanism for phosphate adsorption on the Fe\({}_{3}\)O\({}_{4}\)/CS/PEI particles was electrostatic attraction. The preferential adsorption of phosphate in the presence of other excess anions including chloride, nitrate, carbonate and sulfate was also shown [129]. Regeneration experiments revealed that more than 90% of the phosphate was desorbed by 0.05 mol/L of the NaOH solution.