Physiochemical factors affecting chromate reduction by aquifer materials

Aquifer sediments from Norman, Oklahoma, were used to study the potential f or microbial reduction of Cr(VI) to Cr(III). Black, clay-like sediments rap idly reduced Cr(VI) in both autoclaved and viable microcosms, indicating an abiotic mechanism. Light-colored sandy sediments slowly reduced Cr(VI) onl y in viable microcosms, indicating a biological process. Cr(VI) reduction i n these sediments had a pH optimum of 6.8 and temperature optima of 22 degr eesC and 50 degreesC. Nearly complete inhibition of Cr(VI) reduction was ob served when sandy sediments were shaken in the presence of oxygen. The addi tion of nitrate but not sulfate, selenate, or ferrous iron to sandy sedimen ts inhibited Cr(VI) reduction. When electron acceptors were supplied in com binations with Cr(VI), reduction of Cr(VI) was greatest in the absence of n itrate. No loss of sulfate and no production of Fe(II) occurred in the pres ence of Cr(VI). The addition of molybdate to the microcosms did not affect Cr(VI) reduction in sandy sediments until very high concentrations (40 time s the Cr[VI] concentration) were used. Interestingly, the addition of bromo ethane-sulfonic acid in amounts less than, or slightly greater than, the Cr (VI) concentration partially inhibited Cr(VI) reduction in sandy sediments. In the absence of this bacterial inhibitor, the sandy sediments produced m ethane. A methanogenic enrichment capable of reducing Cr(VI) during growth was obtained from sandy sediments. However, the enrichment produced methane only when Cr(VI) was absent, indicating that a shift in electron flow from methane only when Cr(VI) was absent, indicating that a shift in electron f low from methane production to Cr(VI) reduction may have occurred. These st udies showed that Cr(VI) reduction in sandy aquifer sediments is a biologic ally medicated, anaerobic process that is inhibited by oxygen and partially inhibited by nitrate. The lack of sulfate reduction and sulfide production s, as well as a lack of inhibition of Cr(VI) reduction by molybdate, argues against an indirect mechanism for Cr(VI) reduction, in which the sulfide p roduced during sulfate reduction would chemically reduce Cr(VI). Rather, Cr (VI) reduction may be mediated by a community of microorganisms that ordina rily use methanogenesis as the terminal electron-accepting process.