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

We examined the feasibility of Cr­(OH)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)₃(s) (1.0 g/L) at pH 7.0–9.0 under oxic or anoxic conditions. Homogeneous Mn­(II) oxidation by dissolved O₂ 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