Imine functionalized cross-linked chitosan based bio-sorbent as a pH tunable platform for efficient dye adsorption

Chitosan, a biopolymer rich in amino and hydroxyl groups, is a promising material for the development of bio sorbents for dye removal from wastewater. In this study, chitosan was functionalized via imine formation with 3,4-dihydroxybenzaldehyde and subsequently crosslinked with glutaraldehyde to obtain a chitosan based crosslinked gel (ChDBAG). The material was characterized and compared with unmodified crosslinked chitosan gel (ChG) using different techniques, including FTIR, UV-DRS, XRD, TGA, FE-SEM, BET, and XPS. Adsorption studies under varying pH conditions demonstrated that ChDBAG exhibits high removal efficiency toward anionic dyes (CR, MO, and IC) in acidic media and cationic dyes (MB, MG, and CV) in basic media, with removal efficiencies exceeding 95%. At an optimized adsorbent dosage of 0.2 g/L and a dye concentration of 20 ppm, the maximum adsorption capacity for Congo Red was 105.26 mg/g. The adsorption data were analyzed using both linear and non-linear fitting of isotherm and kinetic models, revealing the best fit to the Langmuir isotherm and pseudo-second-order kinetic model. The mechanism of dye removal is attributed to electrostatic interactions, hydrogen bonding, and n-π interactions between the adsorbent and the dyes. Furthermore, desorption studies revealed that the adsorbent could be effectively regenerated and reused for up to ten cycles maintaining its high removal efficiencies for cationic and anionic dyes. The adsorption capacity was evaluated using dye mixtures under variable pH conditions as well as in real water samples. These results highlight the potential of ChDBAG as a sustainable and efficient adsorbent for dye-polluted water.