Effect of Coexisting Fe(III) (oxyhydr)oxides on Cr(VI) Reduction by Fe(II)-Bearing Clay Minerals

Fe(II)-bearing clay minerals are important electron sources for Cr(VI) reduction in subsurface environments. However, it is not clear how iron (oxyhydr)oxides impact Cr(VI) reduction by Fe(II)-bearing clays as the two minerals can coexist in soil and sediment aggregates. This study investigated Cr(VI) reduction in the mixed suspensions of reduced nontronite NAu-2 (rNAu-2) and ferrihydrite (Fe(II)/Cr(VI) = 3:1). When the mineral premixing time increased from 0 to 72 h, Cr(VI) reduction was accelerated prominently in the initial stage, while Cr(VI) sorption was inhibited drastically. Mineral premixing led to electron transfer from structural Fe(II) in rNAu-2 to ferrihydrite with formation of reactive-surface-associated Fe(II), which catalyzed ferrihydrite transformation to lepidocrocite. Reactive-surface-associated Fe(II) accelerated Cr(VI) reduction initially, and ferrihydrite transformation to lepidocrocite was responsible for the inhibited sorption. When the reactive-surface-associated Fe(II) was consumed in the initial stage, the Cr(VI) reduction rate decreased dramatically due to the limitation of slow electron transfer from structural Fe(II) in rNAu-2 to surface-reactive sites. The main reduction sites shifted from rNAu-2 to ferrihydrite/lepidocrocite when rNAu-2 coexisted with ferrihydrite. Our findings demonstrate that electron transfer between minerals has important implications for Cr(VI) and other high-valence contaminant reduction by Fe(II)-bearing clay minerals in subsurface environments.