Cr(OH)_3(s) Oxidation Induced by Surface Catalyzed Mn(Ⅱ) Oxidation

摘要

We examined the feasibility of Cr(OH),(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)_3(s) (1.0 g/L) at pH 7.0-9.0 under oxic or anoxic conditions. Homogeneous Mn(Ⅱ) oxidation by dissolved O_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)_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)_3(s) oxidation by a newly formed Mn oxide via Mn(Ⅱ) oxidation catalyzed on Cr(OH)_3(s) surface. XANES results indicated that this surface-catalyzed 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)_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)_3(s), which has been considered the most common and a stable remediation product of Cr(Ⅵ) contamination.