Pyrolusite surface transformations measured in real-time during the reactive transport of Co(II)EDTA(2-)

Oxidation of Co(II)EDTA(2-) to Co(III)EDTA(-) by manganese and iron hydrous oxide minerals enhances the transport of Co-60 in subsurface environments. Until now, reduction of the oxidant MnO2 has not been identified in hydrod ynamic systems, leaving the fate and transport mechanisms involving Co-60 i n natural environment!; unresolved. We investigated the transport of Co(II) EDTA(2-) through packed beds of beta-MnO2 and identified the reaction mecha nism using a novel hydrodynamic how cell coupled with X-ray absorption near edge structure (XANES) spectroscopy. Using this technique we are able to d etermine both solution and solid-phase species of cobalt and manganese in r eal-time. Co(II)EDTA(2-) is produced while Mn(IV) is reduced to Mn(III) whi ch forms an alpha-Mn2O3 layer on pyrolusite. This layer passivates the surf ace after an initial reaction period and ultimately limits the production o f Co(III)EDTA(-). As a consequence, the enhanced transport of Co-60 by oxid ative processes may be diminished by continual exposure to pyrolusite-an ad vantage from an environmental quality perspective. It has also been clarifi ed that Mn(III) is formed rather than Mn(II) resulting in formation of a st able trivalent manganese solid (alpha-Mn2O3). Copyright (C) 1999 Elsevier S cience Ltd.