## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Structured Chitosan Materials
The effect of pH and the kinetics of adsorption were also investigated. As a result, pH 3, 10% (\(w/v\)) TPP, 5% (\(v/v\)) ECH and 12 h immersion time in TPP were selected as the optimum conditions for preparing CS beads. After removing the protective group from the beads, NH\({}_{2}\) groups of CS freely attracted the anionic dye MO, resulting in an adsorption capacity of 79.55 mg/g. Moreover, it was found that covalent crosslinking with ECH provided stability forCS in acidic conditions, whereas the concentration of TPP and immersion time controlled the pore size and morphology of the CS beads [107]. Figure 14: Chemical modifications to CS microspheres. Reproduced with permission from authors of [106], Elsevier, 2019. Interesting work was carried out by [PERSON] et al. [108], who developed new, low-cost support material for multimodal chromatography using CS and Orange R-HE triazine dye. CS-based minispheres (CMS) with a mean diameter of 1.4 \(\pm\) 0.2 mm were prepared using nonsolvent (acid-based) precipitation (dissolution of CS in acetic acid with subsequent dripping in NaOH solution) and crosslinked with ECH. Synthesized CMS were shown to be prospective sorbents for proteins, i.e., lactoferrin and lactoperoxidase--high value proteins found in diluted concentrations in sweet whey. Utilizing simple operating process, these two proteins were adsorbed onto the CMS-Orange R-HE matrix, even though their concentrations were low in sweet whey (adsorption \(\geq\) 90%). After adsorption, the differential elution process can be carried out, which results in two separate proteins with good yields and high purity. CMS crosslinked with ECH at 60 \({}^{\circ}\)C for 4 h and modified with sulfanilic acid (4-aminobenzzenenesulfonic acid) or glycidyl trimethylammonium chloride (GTMA) were synthesized (Figure 15) [109]. Crosslinked CMS modified with sulfanilic acid was used for the purification process of lactoferrin from cheese whey, while CMS treated with GTMA were designed for recovering whey protein isolate. The developed CMS allowed for 76.51% of lactoferrin adsorption for 4 h at a pH of 6.57. It is noteworthy that at the chosen pH of 6.57, the matrix and most of the proteins were charged negatively, while the target protein had the opposite charge, which allowed for high selectivity with respect to lactoferrin. In the case of sequential purification of lactoferrin and whey protein isolate, a yield of 62.31% was obtained for lactoferrin, while for whey proteins about 52% of adsorption was achieved; that is, 2.71 mg of protein per mL of whey processed. The developed CMS allowed the cheese whey to be processed without any previous conditioning, to have good mechanic resistance, and to be easily recovered after the adsorption/washing and elution steps. Moreover, modified CMS can be used as new chromatographic matrices that can be applied to direct protein isolation [109]. Structured CS in the form of both beads and films is widely applied for the adsorption of inorganic anions [110] and cations [111; 112]. For example, CS microspheres synthesized usingthe emulsification method and crosslinked with GA were used as an adsorbent for iodide anions [110]. The removal efficiency ofCS microspheres was limited to 0.8792 mmol/g. Better adsorption efficiency  
Figure 15: Simplified representation of modification of crosslinked CMS with sulfanilic acid or GTMA [109]. (\(>\)90% in 20 min with a maximum adsorption capacity of 1.5267 mmol/g) was achieved while using a modified CS-based sorbent, AgCl@CS, in a wide pH range (from pH 2 up to pH 10). For AgCl@CS, chemisorption from AgCl and physisorption from CS were responsible for I\({}^{-}\) removal. Recently, [PERSON] et al. [111] published a review on the application of CS and its derivatives for heavy metal removal from wastewater. Structured, CS-based materials modified by grafting and crosslinking can be used to remove Cu, Pb, Zn, Ni, Hg, Cd, Cr, Pt, Pd, Co, etc.