Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

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• 作者：Drew E. Latta ; Bhoopesh Mishra ; Russell E. Cook ; Kenneth M. Kemner ; Maxim I. Boyanov
• 刊名：Environmental Science & Technology
• 出版年：2014
• 出版时间：February 4, 2014
• 年：2014
• 卷：48
• 期：3
• 页码：1683-1691
• 全文大小：431K
• ISSN：1520-5851

文摘

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where U^IV predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed U^IV species following reduction of U^VI. Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, U^IV forms inner-sphere complexes with two metal oxides, TiO₂ (rutile) and Fe₃O₄ (magnetite) (at <1.3 U nm^鈥? and <0.037 U nm^鈥?, respectively). The uraninite (UO₂) form of U^IV predominates only at higher surface loading. U^IV鈥揟iO₂ complexes remain stable for at least 12 months, and U^IV鈥揊e₃O₄ complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed U^IV results from U^VI reduction by Fe^II or by the reduced electron shuttle AH₂QDS, suggesting that both abiotic and biotic reduction pathways can produce stable U^IV鈥搈ineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on U^IV speciation helps explain the presence of nonuraninite U^IV in sediments and has important implications for U transport modeling.