PYROLUSITE SURFACE TRANSFORMATIONS MEASURED IN REAL-TIME DURING THE REACTIVE TRANSPORT OF CO(II)EDTA2- - THE EFFECT OF PH AND LIGHT

Abstract

Oxidation of Co(II)EDTA2- to Co(III)EDTA- by manganese and iron hydrous oxide minerals enhances the transport of 60Co in subsurface environments. Until now, reduction of the oxidant MnO2 has not been identified in hydrodynamic systems, leaving the fate and transport mechanisms involving 60Co in natural environments unresolved. We investigated the transport of Co(II)EDTA2- through packed beds of β-MnO2 and identified the reaction mechanism using a novel hydrodynamic flow cell coupled with X-ray absorption near edge structure (XANES) spectroscopy. Using this technique we are able to determine both solution and solid-phase species of cobalt and manganese in real-time. Co(II)EDTA2- is produced while Mn(IV) is reduced to Mn(III) which forms an α-Mn2O3layer on pyrolusite. This layer passivates the surface after an initial reaction period and ultimately limits the production of Co(III)EDTA-. As a consequence, the enhanced transport of 60Co by oxidative processes may be diminished by continual exposure to pyrolusite-an advantage from an environmental quality perspective. It has also been clarified that Mn(III) is formed rather than Mn(II) resulting in formation of a stable trivalent manganese solid (α-Mn2O3).