Polyacrylic acid-based and chitosan-based hydrogels for adsorption of cadmium: Equilibrium isotherm, kinetic and thermodynamic studies

The aim of present study was to determine the efficacy of polyacrylic acid-based and chitosan-based hydrogels (synthesized via chemical crosslinking) for the adsorption and removal of cadmium contained in aqueous solutions. Reactive functional groups involved in the adsorption of cadmium were confirmed by Fourier-transform infrared spectroscopy and cadmium adsorbed to the hydrogel surface was confirmed by scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy. The cadmium adsorption mechanism was evaluated using non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherms, with the best fits found using the non-linear Sips and Redlich-Peterson isotherms for adsorption to polyacrylic acid-based and chitosan-based hydrogels, respectively. Maximum cadmium adsorption capacities of the polyacrylic acid-based and chitosan-based hydrogels were respectively 197.92 and 234.84 mg of metal per g of dried hydrogel at pH 6.0 and 343.0 +/- 1.0 K. The best kinetic fit for adsorption to the chitosan-based hydrogel was found with the pseudosecond order kinetic model, whereas the best kinetic fit for adsorption to the polyacrylic acid-based hydrogel was found with the pseudo-first order kinetic model. The thermodynamic results indicated that the adsorption of cadmium to both hydrogels is a favorable, spontaneous process. The desorption studies suggested the formation of reversible chemical bonds between cadmium and active groups in the hydrogel network.