Oxygen Isotope Indicators of Selenate Reaction with Fe(II) and Fe(III) Hydroxides
journal contributionposted 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.
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ε O resultsselenate reductionOxygen Isotope IndicatorsSe isotope forensicslaboratory experimentsδ18 O valuesselenate reactionanion exchangeuptake stepelectron transferRayleigh fractionation modeldiffusive exchangeReaction kineticsfield sitesFeRayleigh modelchloride interlayer typeferrous hydroxiderustreduction mechanismabiotic reactionsSelenate Reactionbiotic reduction processesSe immobilization processisotope effects18 Oenrichmentmineral reductants