EFFECT OF TREATMENT TIME AND VOLTAGE GRADIENT ON MIGRATION OF HEAVY METALS IN SOILS DURING ELECTROKINETICS

This article presents a bench-scale study performed to investigate the effect of applied voltage gradient and time treatment on electromigration behavior of chromium, nickel and cadmium combined in soils. For this study, electrokinetic experiments were conducted on two types of clay: kaolin, a commercial-grade soil consisting mainly of kaolinite clay mineral, and glacial till, a field derived clay that possesses a complex mineralogical composition. On each type of soil, six elctrokinetic experiments were performed: four with 1, 2, 4, and 10 days of treatment time and 1 VDC/cm applied voltage gradient, and two with applied voltage gradient of 0.5 and 2 VDC/cm and 4 days treatment time. In all tests, the contaminants were Cr(VI), Ni(II), and Cd(II) combined in the soil with concentrations of 1000 mg/kg, 500 mg/kg, and 250 mg/kg, respectively. This study has shown that a change in treatment time or/and the induced voltage gradient let the dynamics during electrokinetics process in soils continuously alter the soils conditions, causing a change in the metal behavior throughout the soil. In kaolin, the applied voltage gradient and the test duration have a pronounce effects on the migration of nickel and cadmium. The Ni(II) and Cd(II) migration towards the cathode increases in kaolin as the applied voltage gradient or/and the treatment time increase. However, the time effect is noticeable to a certain extend after which this effect diminishes because of the generated soil conditions by the cathode that hinder the migration of the nickel and cadmium. Unlike in kaolin, treatment time and applied voltage gradient have no effect on nickel and cadmium migration because of the high buffering capacity of glacial till. In both kaolin and glacial till, Cr(VI) migration towards the anode increases as the applied voltage gradient and treatment time increase. However, Cr(VI) migration was higher in glacial till as compared to kaolin because of the high pH conditions that exist in glacial till. In all tests, some Cr(VI) reduced to Cr(III). The Cr(VI) reduction rate to Cr(III) as well as the Cr(III) migration are significantly affected by the applied voltage gradient and treatment time, leading to different chromium distributions throughout the soil. Overall, this study showed that the electroosmotic flow, the energy expendure, the direction and extend of contaminant removal depend on the polarity of the contaminant, the type of soil, the applied voltage gradient, and the treatment duration.