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Oxygen Isotope Indicators of Selenate Reaction with Fe(II) and Fe(III) Hydroxides

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journal contribution
posted on 18.06.2013, 00:00 authored by Alexandra E. P. Schellenger, Philip Larese-Casanova
Selenate (SeO42‑) reduction to elemental selenium is an important Se immobilization process in subsurface environments that could be mediated by Fe­(II)-rich minerals or selenate-respiring microorganisms. We report the kinetic isotope effects for 18O within selenate during abiotic reactions with iron-bearing hydroxides within laboratory experiments. Selenate was reduced to Se(0) by a green rust (chloride interlayer type) and ferrous hydroxide, the two known environmentally relevant mineral reductants for selenate. Reaction kinetics are described by a rapid, low-fractionating uptake step caused by diffusive exchange between selenate and chloride followed by a slower, high-fractionating reduction step caused by electron transfer from structural Fe­(II). The dual-phase kinetics cannot be described with the traditional Rayleigh fractionation model; however, well after the initial uptake step, the extent of selenate reaction is well correlated with δ18O values in accordance with the Rayleigh model. Selenate-18O enrichment (εO) was nearly identical for reaction with chloride green rust (22.7 ± 2.2‰) and ferrous hydroxide (22.1 ± 1.1‰) which suggests a common reduction mechanism by structural Fe­(II). The minor enrichment due to anion exchange alone (1.4 ± 0.2‰) was confirmed using iowaite, a nonredox active Mg­(II)–Fe­(III) layered double hydroxide. Our εO results may contribute to Se isotope forensics to identify selenate reduction within field sites and to possibly distinguish between abiotic and biotic reduction processes.