Mechanisms of Hexavalent Chromium Reduction in Fe(II)-Bearing Clay Mineral and Fe (hydr)oxide Binary Systems: Competitive Adsorption versus Mediated Electron Transfer.

Fe(II)-bearing clay minerals are widespread reductants of hexavalent chromium (Cr(VI)) in subsurface environments. However, the influences of coexisting Fe (hydr)oxides on Cr(VI) reduction by Fe(II)-bearing clay minerals remain elusive. Here, Cr(VI) reduction was investigated in premixed systems of reduced nontronite NAu-2 (rNAu-2) and various Fe (hydr)oxides. Results showed that coexisting ferrihydrite significantly facilitated Cr(VI) reduction during the quick stage (0-2.5 min) across pH 5.5-9.0. During the subsequent slow stage (2.5-205 min), ferrihydrite inhibited Cr(VI) reduction at pH 5.5-7.0 but enhanced it at pH 9.0. In contrast, coexisting goethite had negligible effects. Without Fe (hydr)oxides, direct reduction by rNAu-2 dominated for Cr(VI) removal. In the presence of Fe (hydr)oxides, competitive adsorption was found to inhibit direct Cr(VI) reduction, yet Fe (hydr)oxide-mediated electron transfer effectively offset this inhibitory effect. Both competitive adsorption and mediated electron transfer were influenced by the type and dosage of Fe (hydr)oxides, as well as solution pH. Kinetic modeling revealed that ferrihydrite-mediated electron transfer contributed 1.8-71.5% of the total Cr(VI) reduction, with greater contributions observed at higher solution pH and ferrihydrite dosage. Our findings provide new insights into mineral-mineral electron transfer and its role in regulating Cr(VI) fate in subsurface environments.