Chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel for the adsorption and removal of Pb+2 and Ni+2 ions accommodated in aqueous solutions

A chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel were synthesized by copolymerization of radical chitosan, acrylic acid and N,N'-methylenebisacrylamide and tested with regard to the adsorption and removal of Pb2+ and Ni2+ ions accommodated in aqueous solutions. Fourier-transform infrared spectra confirmed the hydrogel synthesis. The swelling degrees of the chitosan-based hydrogel were 40.86, 20.97 and 4.16 g water per g dried hydrogel at pH 8.0, 6.5 and 4.5, respectively, after 1440 min of contact, whereas the swelling degrees for the chitosan/acid-activated montmorillonite composite hydrogel were 80.01, 25.97 and 14.76 g water per g dried composite hydrogel at pH 8.0, 6.5 and 4.5, respectively, after 5760 min of contact. The water absorption mechanism was strongly influenced by the pH of the aqueous solution, varying between Fickian and non-Fickian transports. The Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel ranged from 41.06 to 30.20 and 42.38 to 36.45 mg metal per g dried hydrogel in the pH range of 5.5-3.5, respectively. The adsorption capacities of the chitosan/acid-activated montmorillonite composite hydrogel ranged from 35.22 to 26.11 and 37.16 to 42.04 mg metal per g dried composite hydrogel in the pH range of 5.5-3.5, respectively. The Pb2+ and Ni2+ adsorption mechanism was evaluated using linear and non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherms, with the best fit determined for the nonlinear Redlich-Peterson isotherm with both hydrogels. The best kinetic fit to both hydrogels was observed using the non-linear pseudo-second order kinetic model and confirmed by Chi-square test statistics.