Effects of soil composition on the removal of chromium by electrokinetics

Electrokinetic experiments were conducted on three different types of soil: glacial till, kaolin and Na-montmorillonite, in order to investigate the effect of soil mineralogy and naturally occurring hematite (Fe2O3) on the removal of chromium from these soils. Batch tests were also performed to characterize Cr(VI) adsorption onto these soils. This study has shown that soils which contain high carbonate buffers, such as the glacial till, hinder the development of an acid front, which results in alkaline conditions throughout the soil during electrokinetic remediation. However, soils possessing low buffering capacity, such as kaolin and Na-montmorillonite, favor the development of an acid front which results in a distinct pH gradient with pH values varying from 2 near the anode to over 11 near the cathode. The results from the adsorption tests showed that Cr(VI) adsorption onto soils depends on the soil type and soil pH. The adsorption of Cr(VI) was found to be governed by soil surface complexation reactions and was significant in Na-montmorillonite, moderate in kaolin and low in glacial till. The Cr(VI) adsorption was found to be pH dependent, with low adsorption occurring at high pH values and high adsorption occurring at low pH values. The low adsorption of Cr(VI) under alkaline conditions in the glacial till resulted in high Cr(VI) removal during electrokinetics. Moderate Cr(VI) adsorption in the acidic regions in kaolin resulted in lower Cr(VI) removal than in the glacial till, High Cr(VI) adsorption in acidic regions of Na-montmorillonite resulted in low Cr(VI) migration. The presence of hematite or iron oxide in soils on the removal of Cr(VI) by electrokinetics depends on the soil mineralogical composition. In sells such as glacial till. the presence of iron oxide creates complex geochemistry and retards Cr(VI) removal. However. in homogeneous clays such as kaolin and Na-montmorillonite, the presence of iron oxide does not significantly affect Cr(VI) removal by electrokinetics. (C) 1997 Elsevier Science B.V.