## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**CS-Containing** & **Crosslinking** & **Shaping** & **Contaminant** & \begin{tabular}{c} **Absorption** \\ **Conditions** \\ \end{tabular} & **Absorption** & **Ref.** \\ \hline ECS@Ca@CTA & GA & Hollow spheres & SDBS & Room temperature, & 2430 mg/g & [118] \\ \hline CS–bentonite clay & GA & Capsules & Cr(VI) & 25 \({}^{\circ}\)C, pH 2 & 106.444 mg/g & [119] \\ \hline CS–FeO/TiO\({}_{2}\) & No & Beads & Naphthalene & \begin{tabular}{c} 25 \({}^{\circ}\)C, pH \\ Not indicated \\ \end{tabular} & 149.3 mg/g & [120] \\ \hline HC–H60 & SDS & Capsules & CR & 30 \({}^{\circ}\)C, pH 4 & 4705.6 mg/g & [121] \\ \hline Zr@BDC–CS & No & Spheres & Fluoride ion & Room temperature, & 4982 mg/kg & [122] \\ \hline Ag@beads & \begin{tabular}{c} Oxidized \\ dextran \\ \end{tabular} & Beads & \begin{tabular}{c} CR \\ Cu (II) \\ \end{tabular} & \begin{tabular}{c} 30 \({}^{\circ}\)C, pH 5 \\ 30 \({}^{\circ}\)C, pH 5 \\ \end{tabular} & \begin{tabular}{c} 7 mg/g \\ 9 mg/g \\ \end{tabular} & [123] \\ \hline \multicolumn{6}{c}{**Magnetic composites**} \\ \hline CS–Fe\({}_{3}\)O\({}_{4}\) & No & Film & As(III) & 25 \({}^{\circ}\)C, pH 3 & 10.4 mg/g & [126] \\ \hline MGC (guanidinylated CS) & No & \begin{tabular}{c} Core(Fe\({}_{3}\)O\({}_{4}\))–shell \\ particles \\ \end{tabular} & Pb(II) & 60 \({}^{\circ}\)C, pH 7 & 22.64 mg/g & [127] \\ \hline CS–PPIMB & No & Beads & Pb(II) & 303 K, pH 5.5 & 230.48 mg/g & [128] \\ \hline Fe\({}_{3}\)O\({}_{4}\)/CS/PEI & TPP & Core–shell particles & Phosphate ion & 25 \({}^{\circ}\)C, pH 3 & 48.2 mg/g & [129] \\ \hline rGO–ZF@CB & GA & Beads & \begin{tabular}{c} PFOA \\ PFOS \\ \end{tabular} & \
\begin{tabular}{c} 25 \({}^{\circ}\)C, pH 3 \\ 25 \({}^{\circ}\)C, pH 3 \\ \end{tabular} &
\begin{tabular}{c} 16.07 mg/g \\ 21.64 mg/g \\ \end{tabular} & [130] \\ \hline CS-covered C/Fe\({}_{3}\)O\({}_{4}\) & GA & Core–shell particles & MO & 45 \({}^{\circ}\)C, pH 3–11 & 425 mg/g & [131] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._catalytic processes in which CS-based systems can be tested [141; 144; 147; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161]. It is noteworthy that CS-containing materials are known as prospective adsorbents for the removal of heavy metals and dyes from wastewater; thus, in many works, their adsorption ability was successfully combined with catalytic properties. The shaping of such catalytic systems can be varied: films, fibers, beads or capsules. For example, Cu NPs were stabilized in CS-SiO\({}_{2}\) nanocomposite fibers [141]. CS-SiO\({}_{2}\) fibers were prepared by mixing SiO\({}_{2}\) NPs (5 wt.% of CS) with CS solution in acetic acid with subsequent precipitation of fiber via extrusion of the mixture into NaOH solution using a syringe. Dried CS-SiO\({}_{2}\) composite fibers were then treated with solutions of metal salts (Cu, Co, Ag and Ni) and reducing agent (NaBH\({}_{4}\)) that allowed for the formation ofmetal NPs 20-40 nm in diameter on the fibers' surface. The resulting metal-containing composites were used for the reduction of 4-nitroaniline (4-NA) and decolorization of CR in aqueous medium using NaBH\({}_{4}\) as a hydrogen source. Cu/CS-SiO\({}_{2}\) revealed high catalytic efficiencies in the reduction of 4-NA and CR compared to other metals: the apparent rate constants of 6.17 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) and 1.68 \(\times\) 10\({}^{-2}\) s\({}^{-1}\) were observed for the reduction of 4-NA and CR, respectively. A percentage of 94.53 of 4-NA was converted in 9 min. The percent reduction of CR in 5 min was found to be 92.7%. The catalytic activity of Cu/CS-SiO\({}_{2}\) was also examined in the reduction of nitrophenols (2-NP, 3-NP and 4-NP), which were successfully converted in a short time of 8-10 min, and other dyes (MO, MB and acridine orange (AO)). More than 90% of MO, MB and AO were decolorized in 6, 9 and 14 min, respectively. [PERSON] et al.