Selenium isotope fractionation during reduction by Fe(II)-Fe(III) hydroxide-sulfate (green rust)

We have determined the extent of Se isotope fractionation induced by reduction of selenate by sulfate interlayered green rust (GR(SO4)), a Fe(II)-Fe(III) hydroxide-sulfate. This compound is known to reduce selenate to Se(0), and it is the only naturally relevant abiotic selenate reduction pathway documented to date. Se reduction reactions, when they occur in nature, greatly reduce Se mobility and bioavailability. Se stable isotope analysis shows promise as an indicator of Se reduction, and Se isotope fractionation by various Se reactions must be known in order to refine this tool. We measured the increase in the Se-80/Se-76 ratio of dissolved selenate as lighter isotopes were preferentially consumed during reduction by GR(SO4). Six different experiments that used GR(SO4) made by two methods, with varying solution compositions and pH, yielded identical isotopic fractionations. Regression of all the data yielded an instantaneous isotope fractionation of 7.36 +/- 0.24parts per thousand. Selenate reduction by GR(SO4) induces much greater isotopic fractionation than does bacterial selenate reduction. If selenate reduction by GR(SO4) occurs in nature, it may be identifiable on the basis of its relatively large isotopic fractionation. Copyright (C) 2003 Elsevier Science Ltd.