Removal of heavy metals from aqueous solutions by adsorption using natural and ammonium carbonate modified kaolinite clays

Natural kaolinite obtained from Share, Nigeria was treated with ammonium carbonate to enhance the adsorption capacity. The optimum operating conditions for the adsorption of metal ions on the natural clay (SK clay) were employed on the ammonium carbonate-treated clay (SK-AC clay). The adsorption data were analysed using adsorption isotherm and kinetic models. The statistical p-values were less than 0.05, indicating that the model terms are significant for the adsorption of the metal ions. The pseudo-second-order kinetic model best describes the metal ions adsorption mechanism. The activation energy values less than 40 kJ mol-1 obtained indicated physisorption. Langmuir adsorption isotherm model fitted the adsorption data for Fe2+, Zn2+ and Cr6+ ions while the Freundlich model fitted Pb2+ ions adsorption. Analysis of thermodynamic parameters shows the adsorption is spontaneous and endothermic. The ammonium carbonate-modified clay exhibited an increase in adsorption capacity from 10.74 to 13.60 mg g-1, 16.48 to 17.38 mg g-1 and 11. 96 to 13.8 mg g-1 for Fe2+, Pb2+ and Zn2+ ions while uptake of Cr6+ decreased from 8.75 to 7.25 mg g-1. The results showed that the adsorption of Fe2+, Pb2+ and Zn2+ ions onto kaolin-containing cowlesite can be enhanced by treating it with ammonium carbonate.