New Clues to the Local Atomic Structure of Short-Range Ordered Ferric Arsenate from Extended X-ray Absorption Fine Structure Spectroscopy

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• 作者：Christian Mikutta ; Petar N. Mandaliev ; Ruben Kretzschmar
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2013
• 出版时间：April 2, 2013
• 年：2013
• 卷：47
• 期：7
• 页码：3122-3131
• 全文大小：612K
• 年卷期：v.47,no.7(April 2, 2013)
• ISSN：1520-5851

文摘

Short-range ordered ferric arsenate (FeAsO₄路xH₂O) is a secondary As precipitate frequently encountered in acid mine waste environments. Two distinct structural models have recently been proposed for this phase. The first model is based on the structure of scorodite (FeAsO₄路2H₂O) where isolated FeO₆ octahedra share corners with four adjacent arsenate (AsO₄) tetrahedra in a three-dimensional framework (framework model). The second model consists of single chains of corner-sharing FeO₆ octahedra being bridged by AsO₄ bound in a monodentate binuclear ²C complex (chain model). In order to rigorously test the accuracy of both structural models, we synthesized ferric arsenates and analyzed their local (<6 脜) structure by As and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. We found that both As and Fe K-edge EXAFS spectra were most compatible with isolated FeO₆ octahedra being bridged by AsO₄ tetrahedra (R_Fe鈥揂s = 3.33 卤 0.01 脜). Our shell-fit results further indicated a lack of evidence for single corner-sharing FeO₆ linkages in ferric arsenate. Wavelet-transform analyses of the Fe K-edge EXAFS spectra of ferric arsenates complemented by shell fitting confirmed Fe atoms at an average distance of 5.3 脜, consistent with crystallographic data of scorodite and in disagreement with the chain model. A scorodite-type local structure of short-range ordered ferric arsenates provides a plausible explanation for their rapid transformation into scorodite in acid mining environments.