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

The objective of this study is to examine the effect of clayminerals (illite, montmorillonite, and kaolinite) on chromate (Cr(VI)) reduction by several low molecular weightorganic compounds. Batch experiments at pH ranging from 3.0 to6.0 and 25 °C showed that 2:1 layered clays illite andsmectite catalyzed Cr(VI) reduction by oxalate. The catalyticeffect increased as pH was decreased. The 1:1 clay kaolinite hadno catalytic effect under comparable conditions. Direct Cr(VI)reduction by reactive moieties associated with illite andmontmorillonite was observed, but at a much slower rate than thecatalytic pathway. Cr(VI) reduction by glyoxylic acid, glycolicacid, lactic acid, and mandelic acid was accelerated by illite,although aqueous phase reduction might occur in parallel. Theseresults suggest that Cr(VI) reduction rates in subsurfaceenvironments rich in organic compounds may be elevated throughcatalysis of surface-bound metals and/or soluble species from theclay minerals, and as a result, higher than those expected fromaqueous phase reaction alone. Such rate enhancement for Cr(VI)reduction needs to be accounted for when developing new remedialtechniques for chromium site remediation or assessing its naturalattenuation.