Effects of clay minerals on Cr(VI) reduction by organic compounds.

The objective of this study is to examine the effect of clay minerals (illite, montmorillonite, and kaolinite) on chromate (Cr(VI)) reduction by several low molecular weight organic compounds. Batch experiments at pH ranging from 3.0 to 6.0 and 25 degrees C showed that 2:1 layered clays illite and smectite catalyzed Cr(VI) reduction by oxalate. The catalytic effect increased as pH was decreased. The 1:1 clay kaolinite had no catalytic effect under comparable conditions. Direct Cr(VI) reduction by reactive moieties associated with illite and montmorillonite was observed, but at a much slower rate than the catalytic pathway. Cr(VI) reduction by glyoxylic acid, glycolic acid, lactic acid, and mandelic acid was accelerated by illite, although aqueous phase reduction might occur in parallel. These results suggest that Cr(VI) reduction rates in subsurface environments rich in organic compounds may be elevated through catalysis of surface-bound metals and/or soluble species from the clay minerals, and as a result, higher than those expected from aqueous phase reaction alone. Such rate enhancement for Cr(VI) reduction needs to be accounted for when developing new remedial techniques for chromium site remediation or assessing its natural attenuation.