Chemistry of chromium in soils with emphasis on tannery waste sites

Worldwide chromium contamination of soils has arisen predominantly from the common practice of land-based disposal of tannery wastes under the assumption that the dominant species in the tannery waste would be the thermodynamically stable Cr(III) species. However, significant levels of toxic Cr(VI) recently detected in surface water and groundwater in India, China, Australia, and elsewhere raise critical questions relating to current disposal criteria for Cr-containing wastes. It now appears that despite the thermodynamic stability of Cr(III), the presence of certain naturally occurring minerals, especially Mn oxides, can enhance oxidation of Cr(III) to Cr(VI) in the soil environment. This factor is of public concern because at high pH, Cr(VI) is bioavailable, and it is this form that is highly mobile and therefore poses the greatest risk of groundwater contamination. A review of the current literature indicates that extensive research has been performed on the speciation of Cr in soil, the effect of pH on soil solution concentrations of Cr(III) and Cr(VI), soil adsorption phenomenon of Cr species, redox reactions, and transformation of Cr(II) and Cr(VI) together with remediation strategies to decontaminate Cr-contaminated soils. Most of the studies were conducted using an uncontaminated soil artificially spiked with Cr, and very limited research has been conducted in the contaminated soil environment. Furthermore, studies on tannery waste contaminated soils are limited, and obviously a serious gap of knowledge exists in understanding the influence of long-term tannery waste contamination on Cr behavior in soil.