Sorption of Lead (Pb) from Aqueous Solutions by Sepiolite and Bentonite Modified with Chitosan Biopolymers: Isotherms and Kinetics

In this study, sepiolite and bentonite clay minerals were modified with a natural chitosan biopolymer and the modified-clays were characterized using XRF, XRD, FTIR, SEM, and TOC analyses. The isothermal and kinetic parameters of lead (Pb) sorption by both the minerals and the modified-minerals were determined in a batch mode under various conditions such as different contact times and initial concentrations of Pb. It was found that the Freundlich model described well the isotherm experimental data of Pb sorption by the sorbents. Modification with chitosan, however, decreased the Pb adsorption capacity of sepiolite from 83 to 27 mg g-1 and that of bentonite from 56 to 29 mg g-1. Kinetic results showed that more than 24 hours was required for Pb sorption by the natural clays to reach equilibrium, while the equilibrium time reduced to 16 and 4 hours for Pb sorption on chitosan-sepiolite and chitosan–bentonite, respectively. The pseudo-second-order model well described the time-dependent Pb sorption data by sepiolite, chitosan-sepiolite, and chitosan-bentonite, suggesting that chemical sorption is the rate-limiting step of Pb adsorption mechanism. The Pb sorption data by bentonite showed the best fit with Elovich model.