A study of the binding strength and thermodynamic aspects of cadmium and lead ions with natural silicate minerals in aqueous solutions

Batch sorption experiments were conducted with cadmium and lead ions at low equilibrium concentrations in 0.01 M of NaNO3 onto Petra clay in single component systems. The equilibrium isotherms were determined at pH 6 under constant ionic strength and at different temperatures. The experimental data were analyzed using the Langmuir, Temkin, Dubinin–Kaganer–Radushkevich, Freundlich and Redlich–Peterson isotherm models. From the Langmuir isotherm, the equilibrium adsorption capacity for Cd2+ is 74.074–144.927 mg g−1 and that for Pb2+ is 83.333–263.158 mg g−1. It therefore showed that Petra clay which was mainly composed of 20% of kaolinite and 55% of calcium montmorillomite exhibited higher selectivity for Pb2+, whereas its selectivity for Cd2+ was often lower at all concentrations applied. From the R2 values for the five isotherm models it was found that the sorption was good for the two metal ions and the good correlation confirms the formation of a monolayer of Cd2+ and Pb2+ on the surface of the clay. The derived constants and their temperature dependencies were used to calculate the corresponding thermodynamic parameters. The Langmuir isotherm, Temkin isotherm and Dubinin–Kaganer–Radushkevich isotherms analysis showed that the binding for these metal ions with Petra clay minerals was physisorption in its nature with an endothermic process derived from very favorable entropy.