CHROMIUM REDUCTION IN BIOSTIMULATED SOIL FROM OAK RIDGE, TENNESSEE

The adsorption and reduction of Cr(VI) onto unaltered soil from the Oak Ridge Reservation was studied to distinguish biogeochemical versus geochemical pathways of metal reduction. The Oak Ridge soil under investigation is a saprolite sequence of interbedded weathered shale and limestone obtained at the capillary fringe with a pH near 7.6. Experiments were conducted on unaltered soils under anaerobic and aerobic conditions using both batch and column systems. Soils were biostimulated using 25mM ethanol, lactate, or acetate. Initial Cr(VI) concentrations ranged from 1 to 20 ppm.

Results from batch and column studies indicate that significantly more chromium was associated with the solid phase in soils stimulated with lactate compared to both ethanol- and acetate-induced soils. Distribution coefficients calculated from batch data imply an order of magnitude increase in solid phase chromium in lactate-induced soils. Preliminary results from x-ray absorption near edge spectroscopy (XANES) suggest the chromium associated with the solid phase is sparingly soluable Cr(III). Results from batch experiments indicate that direct reduction of chromium by lactate cannot account for the majority of the solid phase chromium.

Analyses of column effluents indicate that different microbial populations developed in soils stimulated with different carbon sources. Plate counting revealed differences in both the number and species of cells active within the soils. More detailed analyses will be completed using 16s rRNA technique to identify the major bacterial species present. Future experiments using XANES should indicate if chromium reduction is due to direct metabolism by indigenous bacteria or is indirectly related to the amount of reduced iron in the system.