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Removal of chromium, nickel and cadmium from clays by in-situ electrokinetic remediation

Reddy, K.R.; Parupudi, U.S.

Journal of Soil Contamination 6(4): 391-407

1997


DOI: 10.1080/15320389709383574
Accession: 009335813

This article presents the results of a research study that investigated the use of the in situ electrokinetic process for removing chromium, nickel, and cadmium from contaminated clays. For this study, electrokinetic experiments were conducted on two types of clays: kaolin, a commercial-grade soil consisting mainly of kaolinite clay mineral, and glacial till, a field derived clay that possesses a complex mineralogical composition. Two sets of experiments were performed. The first set of experiments consisted of clays contaminated with both chromium in its hexa valent form, Cr(VI), and nickel, Ni(II). Each of these contaminants was in a concentration of 500 mg/kg. The second set of experiments consisted of clays contaminated with Cr(VI), Ni(II), and cadmium, Cd(II), in concentrations of 500 mg/kg, 500 mg/kg, and 250 mg/kg, respectively. These specific contaminants and contaminant concentration levels were selected to simulate typical electroplating waste conditions. In each test, the contaminated clay was subjected to an external voltage gradient of 1.3 VDC/cm for a duration of 4 d. The results of these experiments determined that in kaolin, a sharp pH gradient, ranging from approximately 2 near the anode to 12 near the cathode, was developed. However, in the glacial till, alkaline conditions (pH gt 6) existed throughout the soil because of its high carbonate-buffeting capacity. These pH variations affected the adsorption-desorption and dissolution-precipitation of the contaminants and, consequently, affected the contaminant migration in the soil. The migration of the cationic metallic contaminants, Ni(II) and Cd(II), toward the cathode was higher in kaolin when compared with the glacial till. Because of high pH conditions near the cathode, Ni(II) and Cd(II) were precipitated in kaolin. In glacial till, however, because of alkaline conditions throughout the soil, most of Ni(II) and Cd(II) precipitated, consequently, the migration of these contaminants was restricted. The migration of the anionic contaminant Cr(VI) toward the anode was more efficient in the glacial till and also near the cathode region of kaolin because of low adsorption of Cr(VI) in high pH environments. Adsorption of Cr(VI) in low pH regions near the anode in kaolin caused low Cr(VI) removal. The synergistic effect of the co-contaminants was insignificant on both cationic and anionic contaminant migration in kaolin. However, in glacial till, the synergistic effects of co-contaminants on anionic contaminant Cr(VI) was found significant, while the effect on cationic contaminant Ni(II) was insignificant. Overall, this study demonstrated that adsorption, precipitation, and reduction were the significant fixation mechanisms of nickel, cadmium, and chromium in the contaminated soils during the electrokinetic process. In order to enhance contaminant removal by the electrokinetic process, conditioning of the electrodes must be implemented.

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