Selenium Diffusion and Reduction at the Water−Sediment Boundary: Micro-XANES Spectroscopy of Reactive Transport

Mechanistic understanding of trace element cycling between surface waters and sediments can require highly spatially resolved information at water−sediment boundaries not obtainable by conventional sampling and analytical methods. This study demonstrates the application of micro-X-ray absorption near-edge structure (micro-XANES) spectroscopy for obtaining such direct, in-situ, spatially and temporally resolved information for selenium. The experiments determine the fate of soluble Se(VI) ponded over water-saturated oxidizing versus reducing sediments. Scanning micro-XANES analyses showed Se(VI) removal from ponded waters by diffusion into sediments and by conversion to Se(0) in the reducing sediment. Zones of accumulation of insoluble Se(0) were heterogeneously distributed. These results show that the scale over which volume averaging of chemical species is performed at sedi ment−water boundaries must be selected carefully and that the spatial resolution offered by micro-XANES is well-suited for these types of investigations.