Chromium(III) Oxidation\nby Three Poorly Crystalline Manganese(IV) Oxides. 2. Solid Phase Analyses

Abstract

Layered, poorly crystalline Mn­(IV)­O₂ phases\nare abundant in the environment. These mineral phases may rapidly\noxidize Cr­(III) to more mobile and toxic Cr­(VI) in soils. There is\nstill, however, little knowledge of how Cr­(III) oxidation by Mn­(IV)­O₂ proceeds at the microscopic and molecular levels. Therefore,\nthe sorption mechanisms of Cr­(III) and Cr­(VI) on Random Stacked Birnessite\n(RSB), δ-MnO₂, and Acid Birnessite (AB) were determined\nby Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS).\nThese three synthetic Mn­(IV)­O₂, which are poorly crystalline\nphases and have layered structures, were reacted with 50 mM Cr­(III)\nat pH 2.5, 3, and 3.5 before being analyzed by EXAFS. The results\nindicated that Cr­(VI) was loosely sorbed as an outer-sphere complex\non Mn­(IV)­O₂, while Cr­(III) was tightly sorbed as an inner-sphere\ncomplex. Further research is needed to understand why Cr­(III) stopped\nbeing significantly oxidized by Mn­(IV)­O₂ after 30 min.\nThis study, however, demonstrated that the formation of a Cr surface\nprecipitate is not necessarily responsible for the cessation in Cr­(III)\noxidation. Indeed, no Cr surface precipitate was detected at the microscopic\nand molecular levels on Mn­(IV)­O₂ surfaces reacted with\nCr­(III) for 1 h, although the Cr­(III) oxidation ceased before 1 h\nof reaction at most employed experimental conditions.