Microbial role in the failure of natural attenuation of chromium(VI) in long-term tannery waste contaminated soil

Bioremediation of Cr(VI) in long-term contaminated soil environment has not been very successful due to the known reoxidation of Cr(III) to Cr(VI). Microbial role on the presence of Cr(VI) in soil samples from a long-term tannery waste contaminated site, even 25 years after cessation of waste disposal was examined. The contaminated soil, unamended or amended with 6.6% (w/w) cow manure, was incubated under 70% water holding capacity and saturated conditions. Water-soluble and exchangeable Cr(VI) declined to undetectable levels within 20 days only in saturated soil, irrespective of the presence or absence of cow manure. When the saturated soil with no Cr(VI) present was subjected to drying, Cr(VI) reappeared in cow manure-amended and unamended soil within 10 days of drying, but at levels far less than that present in the soil before saturation. When the saturated soil was autoclaved and then dried, Cr(VI) was not formed. Evidently, microorganisms are involved in the reoxidation of chromium and the resulting negligible attenuation of Cr(VI) at the long-term tannery waste contaminated site.