PNNL

Abstract

This study 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 – 9 under oxic or anoxic conditions. In the absence of Cr(OH)3(s), homogeneous Mn(II) oxidation by dissolved O2 was not observed at pH = 8.0 for 50 d. At pH 9.0, by contrast, dissolved Mn(II) was completely removed within 8 d 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. Our results suggest that Cr(OH)3(s) was readily oxidized by a newly formed Mn oxide as a result of Mn(II) oxidation catalyzed on Cr(OH)3(s) surface. XANES analysis of the residual solids after the reaction between 1.0 g/L Cr(OH)3(s) and 204 ?M Mn(II) at pH 9.0 for 22 d revealed that the product of surface catalyzed Mn(II) oxidation resembled birnessite. The rate and extent of Cr(OH)3(s) oxidation was likely controlled by those of surface catalyzed Mn(II) oxidation as the production of Cr(VI) increased with increasing pH and initial Mn(II) concentrations. This study evokes the potential environmental hazard of sparingly soluble Cr(OH)3(s) that can be a source of Cr(VI) in the presence of dissolved Mn(II).