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