Physico-chemical limitations during the electrokinetic treatment of a polluted soil

The influence of the ion-exchange solid/liquid reaction on electrokinetic transport phenomena was studied and modelled for a non-permeable porous medium composed of kaolin. Kaolin was selected to model a low-permeable medium, even though it is usually considered to have low ion-exchange capacity and weak selectivity. The influence of this reaction on the characteristic time of the process was demonstrated by studying the lithium (Li+) electrokinetic transport on a sodium (Na+) pre-saturated kaolin. Experimental results were obtained using apparatus developed specifically for the study of the electrokinetic transport and which has been well characterized in previous works. The tests were performed by introducing a solution containing Li+ at the anode, and measuring the Na+ and Li+ concentrations at the cathode outlet. Under operating conditions, local equilibrium is assumed in the medium. The equilibrium isotherm of Li+/Na+ exchange was determined by independent experiments on a laboratory column. Ion-exchange equilibrium can be satisfactorily described by a linear isotherm. A theoretical model based on the tanks-in-series model was used for modeling the experimental results of the Li+ electrokinetic transport. This model considers electro-migration and electro-osmosis as the only transport mechanisms, and takes into account the solid/liquid reaction by a retardation factor, as is usually the case for linear equilibrium models. The comparison between experimental and theoretical results shows that the chemical solid/liquid reaction at least doubles the characteristic time of the transport.