Effect of Microbially Induced Anoxia on Cr(VI) Mobility at a Site Contaminated with Hyperalkaline Residue from Chromite Ore Processing

This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyper alkaline, chromium contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) disposal site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite the pH value of 10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter. With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ~300uM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occuredoccurs over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH value of 10.5 on a time scale compatible with engineering intervention at COPR contaminated sites.