U(VI) Sorption and Reduction by Fe(II) Sorbed on Montmorillonite

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• 作者：Sudipta Chakraborty ; Fabienne Favre ; Dipanjan Banerjee ; Andreas C. Scheinost ; Martine Mullet ; Jean-Jacques Ehrhardt ; Jocelyne Brendle ; Loc Vidal ; Laurent Charlet
• 刊名：Environmental Science & Technology
• 出版年：2010
• 出版时间：May 15, 2010
• 年：2010
• 卷：44
• 期：10
• 页码：3779-3785
• 全文大小：348K
• 年卷期：v.44,no.10(May 15, 2010)
• ISSN：1520-5851

文摘

The influence of surface-bound Fe(II) on uranium oxidation state and speciation was studied as a function of time (6 min−72 h) and pH (6.1−8.5) in a U(VI)−Fe(II)-montmorillonite (Ca-montmorillonite, MONT) system under CO₂-free, anoxic (O₂ <1 ppmv) conditions. The results show a rapid removal of U(VI) from the aqueous solution within 1 h under all pH conditions. U L_III-edge X-ray absorption near-edge structure spectroscopy shows that 96% of the total sorbed U(VI) is reduced at pH 8.5. However, the extent of reduction significantly decreases at lower pH values as specifically sorbed Fe(II) concentration decreases. The reduction kinetics followed by X-ray photoelectron spectroscopy during 24 h at pH 7.5 demonstrates the presence of partially reduced surface species containing U(VI) and U(IV). Thermodynamically predicted mixed valence solids like U₃O₈/β-U₃O₇/U₄O₉ do not precipitate as verified by transmission electron microscopy and extended X-ray absorption fine-structure spectroscopy. This is also supported by the bicarbonate extraction results. The measured redox potentials of Fe(II)/Fe(III)−MONT suspensions are controlled by the Fe(II)/hydrous ferric oxide [HFO(s)] couple at pH 6.2 and by the Fe(II)/lepidocrocite [γ-FeOOH(s)] couple at pH 7.5. The key finding of our study is the formation of a sorbed molecular form of U(IV) in abiotic reduction of U(VI) by sorbed Fe(II) at the surface of montmorillonite.