Reaction of UVI with Titanium-Substituted Magnetite: Influence of Ti on UIV Speciation

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• 作者：Drew E. Latta ; Carolyn I. Pearce ; Kevin M. Rosso ; Kenneth M. Kemner ; Maxim I. Boyanov
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2013
• 出版时间：May 7, 2013
• 年：2013
• 卷：47
• 期：9
• 页码：4121-4130
• 全文大小：442K
• 年卷期：v.47,no.9(May 7, 2013)
• ISSN：1520-5851

文摘

Reduction of hexavalent uranium (U^VI) to less soluble tetravalent uranium (U^IV) through enzymatic or abiotic redox reactions has the potential to alter U mobility in subsurface environments. As a ubiquitous natural mineral, magnetite (Fe₃O₄) is of interest because of its ability to act as a rechargeable reductant for U^VI. Natural magnetites are often impure with titanium, and structural Fe³⁺ replacement by Ti^IV yields a proportional increase in the relative Fe²⁺ content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe²⁺/Fe³⁺ ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe_{3聽鈥撀?i>x}Ti_xO₄) reduce U^VI to U^IV. The U^VI鈥揊e²⁺ redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (x_Ti). However, in contrast to previous studies with pure magnetite where U^VI was reduced to nanocrystalline uraninite (UO₂), the presence of structural Ti (x_Ti = 0.25鈥?.53) results in the formation of U^IV species that lack the bidentate U鈥揙₂鈥揢 bridges of uraninite. Extended X-ray absorption fine structure spectroscopic analysis indicated that the titanomagnetite-bound U^IV phase has a novel U^IV鈥揟i binding geometry different from the coordination of U^IV in the mineral brannerite (U^IVTi₂O₆). The observed U^IV鈥揟i coordination at a distance of 3.43 脜 suggests a binuclear corner-sharing adsorption/incorporation U^IV complex with the solid phase. Furthermore, we explored the effect of oxidation (decreasing R) and solids-to-solution ratio on the reduced U^IV phase. The formation of the non-uraninite U^IV鈥揟i phase appears to be controlled by availability of surface Ti sites rather than R. Our work highlights a previously unrecognized role of Ti in the environmental chemistry of U^IV and suggests that further work to characterize the long-term stability of U^IV phases formed in the presence of Ti is warranted.