PRODUCTS OF MICROBIAL CR(VI) REDUCTION IN BIOSTIMULATED ALLUVIAL AQUIFER SEDIMENTS

The bioremediation of hexavalent chromium, Cr(VI), is premised on its reduction to relatively insoluble Cr(III) species by indigenous microorganisms in soils and aquifers. Reducing conditions are promoted by the injection of a soluble electron donor into the subsurface to stimulate the growth of the indigenous microbial community and the concomitant consumption of electron acceptors. Although the enzymatic and chemical redox pathways for chromium reduction were documented, factors controlling the speciation of Cr(III) products formed in natural sediments has not yet been examined in detail.

In this study, alluvial sediments from a chromium-contaminated site in Thun, Switzerland were amended with four electron donors to determine their relative effectiveness in enhancing Cr(VI) reduction: lactate, acetate, vegetable oil and molasses. Following the discovery that molasses was the best substrate, a series of sediment bioreactors were enriched with molasses and sacrificed as a function of redox regime (i.e., iron- vs. sulfate-reducing conditions). Changes in Fe and Cr speciation and mineralogy in the sediments were characterized using X-ray absorption spectroscopy, X-ray fluorescence microscopy, and electron microscopy. The results indicate that the formation of micron-scale Fe(III)/Cr(III) precipitates during iron reduction play a major role in sequestering bioreduced Cr(III). Microbial community analyses (16S rRNA) further support the contention that Cr(VI) reduction by biogenic Fe(II), instead of direct enzymatic Cr(VI) reduction, is a major pathway for Cr(III) production.