Chitosan-based hydrogels for the sorption of metals and dyes in water: isothermal, kinetic, and thermodynamic evaluations

Abstract

The aim of the present study was to evaluate a chitosan-based hydrogel and chitosan/magnetite-based composite hydrogel as potential matrices for water treatment through sorption experiments involving metals and dyes. Cadmium and methylene blue were used as model pollutants. The best isotherm fits were found using three-parameter isotherms, indicating both the formation of a monolayer and multi-site interactions in the hydrogel networks due to the diffusion of the solutes and macromolecular relaxation of the polymer chains. Maximum methylene blue sorption capacities of the chitosan-based hydrogel and chitosan/magnetite-based composite hydrogel were 23.389 and 23.478 mg g−1, respectively. These values were respectively 90.038 and 80.383 mg g−1 for cadmium sorption. The best kinetic fit for the interaction of methylene blue and cadmium to the chitosan-based hydrogel was the nonlinear pseudo-second-order kinetic model, indicating that a chemical reaction controls the adsorption rate between the hydrogel and pollutant. The opposite was found for interaction with the chitosan/magnetite-based composite hydrogel, suggesting that the active-site occupation rate is proportional to the number of non-occupied active sites. The thermodynamic results revealed that the sorption processes were favorable and endothermic, with the possible occurrence of physical interactions. However, hydrogel swelling can alter the sorption isotherm, kinetics, and thermodynamics. The interaction of methylene blue and cadmium with both hydrogels was confirmed by Fourier-transform infrared spectroscopy and thermogravimetric analyses.