## 3 Chitosan-Containing Materials and Composites as Adsorbents
### Chitosan-Based Organic/Inorganic Composite Adsorbents
The effects of pH, adsorbent dosage, adsorbate concentration, temperature and contact time were studied in batch mode. The experimental data were well-fitted to a Scatchard adsorption isotherm and the adsorption capacity was calculated from theisotherm model. It was found that bentonite clay had a maximum adsorption capacity of 11,076 mg/g, while the composite revealed an almost ten times higher maximum adsorption capacity (106.444 mg/g) for the removal of Cr(VI) ions at 25 \({}^{\circ}\)C, pH 2 and a contact time of 60 min. The mechanism of adsorption included ion exchange, electrostatic interaction and complexation. CS beads modified with FeO and TiO\({}_{2}\) NPs were synthesized by [PERSON] et al. [120] using anionic crosslinking procedure, which allowed beads with semispherical shapes to be obtained. FeO and TiO\({}_{2}\) NPs were synthesized by performing coprecipitation and green chemistry methods, respectively, and then mixed with CS solution. The CS-FeO/TiO\({}_{2}\) mixture was then dripped in NaOH solution, washed and dried at room temperature. The resulting CS-FeO/TiO\({}_{2}\) beads showed high BET surface area (27.13 m\({}^{2}\)/g) and pore size (6.8 nm). These results were attributed to the presence of NPs, which promoted the formation of porosity and increased the surface area. After the incorporation of NPs, the semicrystalline nature of the CS changed into a completely amorphous structure due to the break intermolecular bonding. The CS-FeO/TiO\({}_{2}\) revealed excellent thermal stability (up to 270 \({}^{\circ}\)C) and superparamagnetic response, attributed to the presence of FeO and TiO\({}_{2}\) NPs. Adsorption experiments were performed using naphthalene as an example of polycyclic aromatic hydrocarbons (PAHs). The pseudo-second-order model properly fitted the adsorption data, indicating that chemisorption was the primary adsorption mechanism. CS-FeO/TiO\({}_{2}\) showed a higher adsorption capacity of 33.1 mg/g compared to the 29.8 mg/g of the unmodified CS beads. A maximum adsorption capacity of 149.3 mg/g for CS-FeO/TiO\({}_{2}\) was found. The high affinity toward PAHs was due to additional non-covalent forces. Strong chemisorption between naphthalene and the functional groups as well as electrostatic interaction with the NPs was found [120]. CS/hematite nanocomposite hydrogel capsules (HC-H) were developed by [PERSON] et al. [121] usinganionic surfactant gelation occurring simultaneously with the immobilization and impregnation of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) NPs within the gel matrix. Hematite NPs were mixed with CS solution in 2% acetic acid. The resulting mixtures were dripped into an aqueous solution of sodium dodecyl sulfate, SDS (50 g/L). SDS penetrated the drops, which allowed for the creation of CS hydrogel capsules. Thus, a series of HC-H samples were prepared with different loadings of hematite NPs varying from 10% up to 60% (\(p/v\)). The HC-H composites were highly stable in acidic media (even at pH 2) and revealed enhanced adsorptive capacity for Congo red (CR). The maximum adsorption capacity of HC-H60 was 4705.6 mg/g (26.7 times more than that of the pristine CS hydrogel capsule (176.5 mg/g)). Zr-organic frameworks (Uio-66, named Zr@BDC metal-organic frameworks (MOFs)) were prepared using Zr(IV) and 1,4-benzenedicarboxylic acid (BDC) for efficient fluoride removal [122]. Since the Zr@BDC MOFs possessed some limitations such as column blockage and pressure drop during field tests, the Zr@BDC MOFs were modified with CS (Zr@BDC-CS). The Zr@BDC-CS spheres were obtained viate common mixing of Zr@BDC MOFs with CS solution with subsequent dripping in NaOH solution. The synthesized Zr@BDC MOFs and Zr@BDC-CS spheres possessed the highest defluoridation capacities at 4935 and 4982 mgF/kg within 20 min, respectively. The defluoridation capacity of the Zr@BDC-CS was slightly dependent on the pH level and was majorly diminished in the presence of HCO\({}_{3}\)\({}^{-}\).