Oxidation and mobility of trivalent chromium in manganese-enriched clays during electrokinetic remediation.

摘要

The extent and effect of oxidation of Cr(III) in Mn-enriched clays on the electrokinetic remedial efficiency, and the effects of changes in chromium redox chemistry on the migration of the coexisting nickel and cadmium were studied. Bench-scale electrokinetic experiments were conducted using kaolin (a low buffering soil), and glacial till (a high buffering soil). Tests were performed with 1000 mg Cr(III)/kg, 500 mg Ni(II)/kg, and 250 mg Cd(II)/kg, both with and without the presence of 1000 mg Mn/kg, under a constant voltage gradient of 1.0 V d.c./cm. The results showed that in the presence of Mn, percentages of oxidation of Cr(III) into Cr(VI) ranged from 67% in kaolin to 28% in glacial till even before the application of induced electric potential. The low extent of oxidation of Cr(III) in glacial till may be attributed to the initial precipitation of Cr(III) as Cr(OH)3 resulting from high soil pH, reducing aqueous Cr(III) concentrations present within the soil. In kaolin, Cr(III), Ni(II), and Cd(II) under electric potential migrated toward the cathode and precipitated near the cathode due to high soil pH. When Mn was present in kaolin, Cr(VI) that was formed due to the oxidation of Cr(III) migrated toward anode and adsorbed to the soil surfaces near the anode region due to low soil pH. However, remaining Cr(III) as well as Ni(II), and Cd(II) migrated towards and precipitated near the cathode due to high soil pH. In kaolin, the migration of Ni(II) and Cd(II) was retarded in the presence of Mndue to a larger soil zone of elevated pH near the cathode. In glacial till, the migration of Cr(III), Ni(II) and Cd(II) was insignificant due to precipitation resulting from high soil pH caused by the high buffering capacity of the soil. Cr(VI) that resulted from the partial oxidation of Cr(III) in the presence of Mn, however, migrated toward the anode. The effects of Mn on Cr(III) oxidation in low buffering soils can be significant, which can in turn affect the extent and direction of Cr migration under induced electric potential.