(332d) Laboratory Investigation of Combined Chemical/Biological Treatment of Hexavalent Chromium, Carbon Tetrachloride and Trichloroethylene in Aqueous System

Among the hundreds of chromium contaminated sites in the United States are Hanford and Savannah River Sites, which are under the purview of the U.S. Department of Energy (DOE). At the Hanford Site, which is the most contaminated nuclear site in the United States, aquifer and groundwater contamination represents a complex mixture of inorganics including hexavalent chromium , Cr(VI), radionuclides and nitrates, and organic contaminants, e.g., trichloroethylene (TCE) and carbon tetrachloride (CCl₄). Conventional treatment of chromium contamination focuses on immobilization of the highly toxic and water-soluble form Cr(VI), by reducing it to the relatively less toxic and less mobile trivalent form, Cr(III) while organic chlorinated compounds and nitrates can also be reduced using ferrous sulfate. Both types of contaminants (inorganic and organic) also respond to biodegradation. In this paper, results will be presented from a two-step comprehensive approach: (i) to chemically reduce hexavalent chromium, carbon tetrachloride and trichloroethylene using ferrous sulfate and zero valent iron (ZVI) amendments, which leads to nontoxic compounds, but also results in oxidation of ferrous to ferric iron. (ii) In the second step, ferric iron is regenerated to ferrous through biotic transformation using metal-reducing bacteria, Shewanella oneidensis MR-1. The microorganisms support biodegradation of contaminants, and convert ferric to ferrous iron in the process, leading to a self-containing sustainable process. The results can be used in designing an at a hazardous waste site.