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

• Published in: Environmental science & technology Vol. 48; no. 18; pp. 10760 - 10768
• Main Authors: Namgung, Seonyi, Kwon, Man Jae, Qafoku, Nikolla P, Lee, Giehyeon
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.09.2014; American Chemical Society (ACS)
• Subjects: anaerobic conditions; Catalysis; Chemical Precipitation; chromium; Chromium - chemistry; Cr(III); Cr(III) oxidation; Cr(VI) contamination; dissolved oxygen; Environment; environmental hazards; ENVIRONMENTAL SCIENCES; groundwater; manganese; Manganese - chemistry; manganese oxides; Mn(II) oxidation; oxidation; Oxidation-Reduction; remediation; Solutions; surface sorbed Mn; Suspensions; Time Factors; toxicity; X-Ray Absorption Spectroscopy; X-Ray Diffraction
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/es503018u

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.