Cr(OH)^sub 3^(s) Oxidation Induced by Surface Catalyzed Mn(II) Oxidation

We examined the feasibility of Cr(OH)^sub 3^(s) oxidation mediated by surface catalyzed Mn(II) oxidation under common groundwater pH conditions as a potential pathway of natural Cr(VI) contaminations. Dissolved Mn(II) (50 μM) was reacted with or without synthesized Cr(OH)^sub 3^(s) (1.0 g/L) at pH 7.0-9.0 under oxic or anoxic conditions. Homogeneous Mn(II) oxidation by dissolved O^sub 2^ was not observed at pH ≤ 8.0 for 50 days. At pH 9.0, by contrast, dissolved Mn(II) was completely removed within 8 days and precipitated as hausmannite. When Cr(OH)^sub 3^(s) was present, this solid was oxidized and released substantial amounts of Cr(VI) as dissolved Mn(II) was added into the suspension at pH ≥ 8.0 under oxic conditions. Production of Cr(VI) was attributed to Cr(OH)^sub 3^(s) oxidation by a newly formed Mn oxide via Mn(II) oxidation catalyzed on Cr(OH)^sub 3^(s) surface. XANES results indicated that this surfacecatalyzed Mn(II) oxidation produced a mixed valence Mn(III/IV) solid phase. Our results suggest that toxic Cr(VI) can be naturally produced via Cr(OH)^sub 3^(s) oxidation coupled with the oxidation of dissolved Mn(II). In addition, this study evokes the potential environmental hazard of sparingly soluble Cr(OH)^sub 3^(s), which has been considered the most common and a stable remediation product of Cr(VI) contamination.