SPECTROSCOPIC STUDY OF SURFACE REDOX REACTIONS WITH MANGANESE OXIDES

Redox reactions involving soil minerals and materials are important processes in environmental chemistry, but unfortunately they only have been characterized in the solution phase. The lack of a suitable method has prevented investigations of the mineral surface component of redox reactions. We present a model system in which ultraviolet-visible spectroscopy is employed to observe a surface redox reaction. A synthetic MnOx is reacted with Cr(III), resulting in Cr oxidation to the hexavalent state and the reduction of the MnOx mineral. A scattered transmission accessory allowed us to observe spectroscopically both the appearance of Cr(VI) and the attenuation of MnOx in a suspension. Loss of absorbance intensity by MnOx suspensions may be useful as a general method to follow redox reactions as a function of the solid phase rather than relative to a specific reactant. This spectroscopic approach provides the capability to monitor in situ reactions of solid-phase components and is useful for examining transformations of metallic and organic wastes and naturally occurring materials in the environment.