MATHEMATICAL-MODELING OF HEXAVALENT CHROMIUM DECONTAMINATION FROM LOWSURFACE CHARGED SOILS

A new electrokinetic technology has been developed for in-situ deconta mination of hexavalent chromium in sand. Imposition of a constant pote ntial gradient across the soil matrix through a graphite cathode and i ron anode resulted in successful migration of chromate towards the ano de. 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 wat er reduction flushes across the cell and favors faster transport of ch romate by enhancing its conductivity. The acidic front generated due t o 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 di ssolution reaction. The low pH at the anodic region favors the reducti on of hexavalent chromium to its trivalent state. The experimental res ults are compared with a theoretical model developed from first princi ples. The water electrolysis reactions at both electrodes, the sorptio n processes in sand and the water hydrolysis reaction have been includ ed in the model. Concentration profiles for the movement of ionic spec ies under a potential field were simulated for different times. The mo del predicts the sweep of the alkaline front across the cell due to th e transport of OH- ions. Comparison of the chromate concentration prof iles with experimental data after 28 days of electrolysis shows good a greement. 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 likel y to be an accurate estimation of the physical situation, within the l imits of the assumptions made. (C) 1997 Elsevier Science B.V.