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

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• 作者：Alexandra E. P. Schellenger ; Philip Larese-Casanova
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2013
• 出版时间：June 18, 2013
• 年：2013
• 卷：47
• 期：12
• 页码：6254-6262
• 全文大小：394K
• 年卷期：v.47,no.12(June 18, 2013)
• ISSN：1520-5851

文摘

Selenate (SeO₄^2-) 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 未¹⁸O values in accordance with the Rayleigh model. Selenate-¹⁸O enrichment (蔚_O) was nearly identical for reaction with chloride green rust (22.7 卤 2.2鈥? and ferrous hydroxide (22.1 卤 1.1鈥? which suggests a common reduction mechanism by structural Fe(II). The minor enrichment due to anion exchange alone (1.4 卤 0.2鈥? was confirmed using iowaite, a nonredox active Mg(II)鈥揊e(III) layered double hydroxide. Our 蔚_O results may contribute to Se isotope forensics to identify selenate reduction within field sites and to possibly distinguish between abiotic and biotic reduction processes.