Selenium isotope ratios as indicators of selenium sources and oxyanion reduction

Selenium stable isotope ratio measurements should serve as indicators of so urces and biogeochemical transformations of Se. We report measurements of S e isotope fractionation during selenate reduction, selenite sorption, oxida tion of reduced Se in soils, and Se volatilization by algae and soil sample s. These results, combined with previous work with Se isotopes, indicate th at reduction of soluble oxyanions is the dominant cause of Se isotope fract ionation. Accordingly, Se isotope ratios should be useful as indicators of oxyanion reduction, which can transform mobile species to forms that are le ss mobile and less bioavailable. Additional investigations of Se isotope fr actionation are needed to confirm this preliminary assessment. We have developed a new method for measurement of natural Se isotope ratio variation which requires less than 500 ng Se per analysis and yields +/-0.2 parts per thousand precision on Se-80/Se-76. A double isotope spike techni que corrects for isotopic fractionation during sample preparation and mass spectrometry. The small minimum sample size is important, as Se concentrati ons are often below 1 ppm in solids and 1 mu g/L in fluids. The Se purifica tion process is rapid and compatible with various sample matrices, includin g acidic rock or sediment digests. Copyright (C) 1999 Elsevier Science Ltd.