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

摘要

Layered, poorly crystalline Mn(IV)O_2 phases are abundant in the environment. These mineral phases may rapidly oxidize Cr(m) to more mobile and toxic Cr(VI) in soils. There is still, however, little knowledge of how Cr(III) oxidation by Mn(IV)O_2 proceeds at the microscopic and molecular levels. Therefore, the sorption mechanisms of Cr(III) and Cr(VI) on Random Stacked Bimessite (RSB), δ- MnO_2, and Acid Birnessite (AB) were determined by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These three synthetic Mn(IV)O_2, which are poorly crystalline phases and have layered structures, were reacted with SO mM Cr(III) at pH 2.5, 3, and 3.5 before being analyzed by EXAFS. The results indicated that Cr(VI) was loosely sorbed as an outer-sphere complex on Mn(IV)O_2, while Cr(III) was tightly sorbed as an inner-sphere complex. Further research is needed to understand why Cr(III) stopped being significantly oxidized by Mn(IV)O_2 after 30 min. This study, however, demonstrated that the formation of a Cr surface precipitate is not necessarily responsible for the cessation in Cr(III) oxidation. Indeed, no Cr surface precipitate was detected at the microscopic and molecular levels on Mn(IV)O_2 surfaces reacted with Cr(III) for 1 h, although the Cr(III) oxidation ceased before 1 h of reaction at most employed experimental conditions.