Effect of surface equilibria on the electrokinetic behaviour of Pb and Cd ions in kaolinite

BACKGROUND: Electrokinetic remediation of fine-grained soils polluted by heavy metals is affected by the equilibria between dissolved and sorbed ions; pH can also influence the surface equilibria. Geochemical models which describe the behaviour of different surface sites and their interactions with heavy metal ions have to be considered for a good mathematical representation of an electrokinetic process. RESULTS: Cd2+ and Pb2+ sorption isotherms were performed at different pH values using kaolinite as sorbent. Electrokinetic tests were performed with samples of kaolinite spiked with Cd2+ and Pb2+ at 0.7 and 2Vcm-1: the concentration profiles were obtained experimentally at different treatment times. The pH in the compartments was conditioned by acetate buffer. On the basis of the experimental results, a numerical model was formulated to simulate ion transport under an electric field combining the diffusion-advection equations with a two-sites geochemical model taking into account ion exchange and surface complexation of the metal ions, as well as the protonation-deprotonation reactions of the surface sites. CONCLUSIONS: The process was effective for cadmium removal, while a strong dependence on pH was observed for lead removal. The model allowed prediction of time/space profiles in solid and liquid phase: a good agreement with the experimental data was obtained under all the conditions adopted.