New Insight into Mechanism of Cr(VI) Migration and Transformation in Typical Soils of Chromite ore Processing Residue (COPR) Contaminated Sites

Chromite ore processing residue (COPR) storage sites are widely distributed all over the world, causing serious soil and groundwater pollution. However, the differences in soil constituents and properties between different regions are significant, and the dynamic migration and transformation of Cr(VI) in different types of soil under alkaline condition of the COPR site is still unclear. In this study, the typical black soil, red soil and loess in different regions of China were chosen to investigate the adsorption kinetics and thermodynamics of Cr(VI) under the original pH conditions of the soil, and then the alkaline Cr(VI) solution was introduced into the soil column to simulate the dynamic migration and transformation process of Cr(VI) at COPR sites. According to the results, the Cr(VI) breakthrough curve predicted by the solid-liquid distribution coefficient Kd based on the static isotherm adsorption experiments significantly underestimated and overestimated the retention effect of black soil and red soil on Cr(VI) dynamic migration, respectively. For the black soil, the retention of Cr(VI) was dominated by Cr(VI) reduction, which is a slow reaction compared with Cr(VI) adsorption. Therefore, the reduction kinetics process during the column experiment cannot be neglected. In respect to the red soil, the outlet Cr(VI) concentration turned to be higher than the inlet concentration with the soil alkalization, which indicated that the adsorbed Cr(VI) desorbed again, and this is the main reason for the overestimation of Cr(VI) retention effect by the red soil. This study shows that the environmental risks of Cr in different types of soil are quite different, mainly related to the valence and occurrence form of Cr that governed by the soil constituents and properties. In addition, the stable form of Cr in the black soil column after the reaction indicates that the soil organic matter can be used as a potential environmentally friendly remediation material for Cr(VI) contaminated soils at COPR sites.