Mathematical modeling of hexavalent chromium decontamination from low surface charged soils

A new electrokinetic technology has been developed for in-situ decontamination of hexavalent chromium in sand. Imposition of a constant potential gradient across the soil matrix through a graphite cathode and iron anode resulted in successful migration of chromate towards the anode. The hexavalent chromium ions are reduced to the harmless trivalent form by chemical reaction with the anodic electrochemical dissolution product, Fe2+. The alkaline front generated at the cathode due to water reduction flushes across the cell and favors faster transport of chromate by enhancing its conductivity. The acidic front generated due to water oxidation at the anode remains adjacent at the electrode-sand interface due to its consumption by the corrosion reaction with iron. The lower production rate of H+ is also due to the competing anodic dissolution reaction. The low pH at the anodic region favors the reduction of hexavalent chromium to its trivalent state. The experimental results are compared with a theoretical model developed from first principles. The water electrolysis reactions at both electrodes, the sorption processes in sand and the water hydrolysis reaction have been included in the model. Concentration profiles for the movement of ionic species under a potential field were simulated for different times. The model predicts the sweep of the alkaline front across the cell due to the transport of OH- ions. Comparison of the chromate concentration profiles with experimental data after 28 days of electrolysis shows good agreement. The potassium cations are positively charged and remained at the cathode where they had been placed initially. The good agreement between the model and the data demonstrates that the analysis is likely to be an accurate estimation of the physical situation, within the limits of the assumptions made.