Removal of Pb²⁺ from aqueous solutions using two Brazilian rocks containing zeolites

The removal of Pb²⁺ from aqueous solution by two Brazilian rocks that contain zeolites—amygdaloidal dacite (ZD) and sandstone (ZS)—was examined by batch experiments. ZD contains mordenite and ZS, stilbite. The effects of contact time, concentration of metal in solution and capacity of Na⁺ to recover the adsorbed metals were evaluated at room temperature (20°C). The sorption equilibrium was reached in the 30 min of agitation time. Both materials removed 100% of Pb²⁺ from solutions at concentrations up to 50 mg/L, and at concentrations larger than 100 mg/L of Pb²⁺, the adsorption capacity of sandstone was more efficient than that of amygdaloidal dacite due to the larger quantities and the type of zeolites (stilbite) in the cement of this rock. All adsorbed Pb²⁺ was easily replaced by Na⁺ in both samples. The analysis of the adsorption models using nonlinear regression revealed that the Sips and the Freundlich isotherms provided the best fit for the ZS and ZD experimental data, respectively, indicating the heterogeneous adsorption surfaces of these zeolites.