Effects of initial form of chromium on electrokinetic remediation in clays

摘要

This paper presents the results of a laboratory investigation performed to evaluate the effects of the initial form of chromium on the electrokinetic remedial efficiency for contaminated clays. Electrokinetic experiments were conducted by contaminating clays with chromium in three different forms: Cr(III) alone, Cr(VI) alone, and a combination of Cr(III) and Cr(VI). The same total chromium concentration of 1000 mg/kg was maintained in all cases. Ni(II) and Cd(II) in concentrations of 500 mg/kg and 250 mg/kg, respectively, were also introduced into the clays as co-contaminants to simulate typical elecroplating waste constituents. Two different clays, kaolin, a typical low buffering clay and glacial till, a typical high buffering clay, were tested. All tests were conducted with a constant voltage gradient of 1.0 VDC/cm. The test results showed that chromium migration was highest when it was present in kaolin in the Cr(III) form and in glacial till in the Cr(VI) form. When chromium was present in Cr(III) form, migration occurred towards the cathode due to the existence of Cr(III) as cation and cationic hydroxide complexes. Cr(III) migration was not observed in glacial till because of precipitation that resulted from high pH conditions that existed throughout the glacial till. However, when chromium was present in Cr(VI) form, the migration occurred towards the anode, due to the existence of Cr(VI) as soluble oxyanions. The migration of Cr(VI) was higher in glacial till as compared to kaolin due to alkaline conditions that existed in the glacial till, resulting in negligible Cr(VI) adsorption to soil solids. When chromium was present as a combination of Cr(VI) and Cr(III), Cr(VI) migrated towards the anode, while Cr(III) migrated towards the cathode. For these cases, the total chromium migration was lower than the migration observed when only Cr(III) was present in kaolin or when only Cr(VI) was present in glacial till. No migration was observed for the co-contaminants, Ni(II) and Cd(II), in glacial till due to precipitation as a result of alkaline conditions. In kaolin, however, Ni(II) and Cd(II) migrated towards the cathode. Overall, the test results show that significant removal of contaminants from the soils was not achieved for the processing periods utilized. This study clearly demonstrated that the efficiency of the electrokinetic removal of chromium, nickel and cadmium from the contaminated clays depends on the initial form of chromium as well as the soil chemistry. Enhancement strategies should be investigated in order to enhance contaminant migration and to achieve high removal efficiencies.