Manganese(II)-Catalyzed and Clay-Minerals-Mediated Reduction of Chromium(VI) by Citrate

Unlike lower valent iron (Fe), the potential role of lower valent manganese (Mn) in the reduction of hexavalent chromium (Cr­(VI)) in soil is poorly documented. In this study, we report that citrate along with Mn­(II) and clay minerals (montmorillonite and kaolinite) reduce Cr­(VI) both in aqueous phase and in the presence of dissolved organic carbon (SDOC) extracted from a forest soil. The reduction was favorable at acidic pH (up to pH 5) and followed the pseudo-first-order kinetic model. The citrate (10 mM) + Mn­(II) (182.02 μM) + clay minerals (3% w/v) system in SDOC accounted for complete reduction of Cr­(VI) (192.32 μM) in about 72 h at pH 4.9. In this system, citrate was the reductant, Mn­(II) was a catalyst, and the clay minerals acted as an accelerator for both the reductant and catalyst. The clay minerals also serve as a sink for Cr­(III). This study reveals the underlying mechanism of the Mn­(II)-induced reduction of Cr­(VI) by organic ligand in the presence of clay minerals under certain environmental conditions