Arsenic Incorporation in Pyrite at Ambient Temperature at Both Tetrahedral S–I and Octahedral FeII Sites: Evidence from EXAFS–DFT Analysis

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• 作者：Pierre Le Pape ; Marc Blanchard ; Jessica Brest ; Jean-Claude Boulliard ; Maya Ikogou ; Lucie Stetten ; Shuaitao Wang ; Gautier Landrot ; Guillaume Morin
• 刊名：Environmental Science & Technology
• 出版年：2017
• 出版时间：January 3, 2017
• 年：2017
• 卷：51
• 期：1
• 页码：150-158
• 全文大小：493K
• ISSN：1520-5851

文摘

Pyrite is a ubiquitous mineral in reducing environments and is well-known to incorporate trace elements such as Co, Ni, Se, Au, and commonly As. Indeed, As-bearing pyrite is observed in a wide variety of sedimentary environments, making it a major sink for this toxic metalloid. Based on the observation of natural hydrothermal pyrites, As^–I is usually assigned to the occupation of tetrahedral S^–I sites, with the same oxidation state as in arsenopyrite (FeAsS), although rare occurrences of As^III and As^II have been reported. However, the modes of As incorporation into pyrite during its crystallization under low-temperature diagenetic conditions have not yet been elucidated because arsenic acts as an inhibitor for pyrite nucleation at ambient temperature. Here, we provide evidence from X-ray absorption spectroscopy for As^II,III incorporation into pyrite at octahedral Fe^II sites and for As^–I at tetrahedral S^–I sites during crystallization at ambient temperature. Extended X-ray absorption fine structure (EXAFS) spectra of these As-bearing pyrites are explained by local structure models obtained using density functional theory (DFT), assuming incorporation of As at the Fe and S sites, as well as local clustering of arsenic. Such observations of As^–I incorporation at ambient temperature can aid in the understanding of the early formation of authigenic arsenian pyrite in subsurface sediments. Moreover, evidence for substitution of As^II,III for Fe in our synthetic samples raises questions about both the possible occurrence and the geochemical reactivity of such As-bearing pyrites in low-temperature subsurface environments.