Terephthalaldehyde as better crosslinking agent in crosslinked chitosan hydrogel for selective removal of anionic dyes

Herein, we report the ultrasound wave-induced synthesis of crosslinked chitosan hydrogels (CAAT and CAAG) using terephthalaldehyde/glutaraldehyde as crosslinking agents and their comparison for the removal of synthetic anionic dyes from aqueous media. Synthesized hydrogels were characterized via spectral, thermal, X-ray crystallographic, morphological and elemental analysis. The swelling studies showed a higher percentage swelling (3287%) in the case of CAAT, and according to swelling kinetics, it followed the Fickian behavior. The adsorption performance of the crosslinked chitosan hydrogels was investigated for the removal of cationic dyes [malachite green (MG) and methyl violet (MV)] and anionic dyes [congo red (CR), methyl orange (MO), solochrome black (SB) and thymol blue (TB)]. Results showed that both CAAT and CAAG have high selectivity for the removal of anionic dyes, in particular the carcinogenic CR azo dye. The adsorption capacity of CAAT was higher than that of CAAG and the adsorption isotherm analysis revealed that the adsorption of CR on CAAT best fitted the Sips and Redlich–Peterson isotherm, while in the case of CAAG, the Sips isotherm was best fitted. The calculated rate constant value corresponds to the pseudo-second-order model, which varied in the range of 3.02 × 10−4–4.92 × 10−4 g mg−1 min−1. The high selectivity of CAAT was retained even in a multi-component system (anionic and cationic dyes). Desorption studies showed the favorable reusability potential of the CAAT hydrogel up to six adsorption–desorption cycles. Further, the stability of the CAAT hydrogel under operational conditions was evaluated using FTIR spectra recorded after adsorption–desorption cycles.