An integrated microbial process for the bioremediation of soil contaminated with toxic metals

Microbially catalyzed reactions, which occur in the natural sulfur cycle, have been integrated in a microbiological process to remove toxic metals from contaminated soils. Bioleaching using sulfuric acid produced by sulfur-oxidizing bacteria was followed by precipitation of the leachate metals as insoluble sulfides by sulfate-reducing bacteria. Metal contaminants including Cd, Co, Cr, Cu, Mn, Ni, and Zn were efficiently leached from an artificially contaminated soil. Mn, Ni, and Zn were the only target elements that were significantly leached from soil minerals. Pb leaching was slow and remained incomplete over a period of 180 days. Mineral components such as Fe, Ca and Mg were also leached but the eventual reduction in soil mass was only approximately 10%. An industrially contaminated soil was also efficiently leached and approximately 69% of the main toxic metals present, Cu, Ni, and Mn, were removed after 175 days. The leachate that resulted from the action of sulfur-oxidizing bacteria on contaminated soil was stripped of metals using an anaerobic bioreactor containing a mixed culture of sulfate-reducing bacteria which precipitated soluble metal species as solid metal sulfides. More than 98% of the metals were removed from solution with the exception of Mn, Ni, and Pb, where 80–90% were removed. The metal content of the resultant effluent liquor was low enough to meet European criteria for discharge into the environment.