Stimulation of microbially mediated chromate reduction in alkaline soil-water systems

Acetate was added to two closed soil-water systems that are representative of the subsurface environment close to chromium ore processing residue disposal sites; one had a pH of 7.7, the other 9.3. Cr(VI) reduction occurred in both systems as part of a cascade of microbially mediated terminal electron-accepting processes, occurring between nitrate and iron reduction. Cr( VI) and subsequently iron reduction took longer to start and were slower in the more alkaline system. At the point when Cr( VI) reduction was essentially complete, the microbial populations in both systems showed an increase in species closely related to beta-proteobacteria that are capable of nitrate reduction.