Abstract
Reactivity of U (super VI) with pure, oxidized, and Ti-substituted magnetites
Mineralogical magazine, Vol.76(6), p.1978
Goldschmidt 2012
2012
Abstract
Reduction of U (super VI) to U (super IV) through coupled biotic-abiotic processes can significantly decrease uranium mobility in subsurface environments. To understand the abiotic factors contributing to this process, we investigated the reduction of U (super VI) to U (super IV) by magnetite, a common rock forming mineral and product of microbial Fe (super III) respiration. U (super VI) reactivity with pure, stoichiometric magnetite (Fe (super II) Fe (super III) (sub 2) O (sub 4) ,Fe (super 2+) /Fe (super 3+) = 0.5) is compared to that with a series of oxidized (Fe (super 2+) /Fe (super 3+) < 0.5) and Ti-substituted magnetites. Ti (super 4+) -for-Fe (super 3+) substitution is common in natural magnetite [1, 2] and results in a solid solution (Fe (sub 3-x) Ti (sub x) O (sub 4) , 0 < x <1 ) where Ti (super 4+) incorporation is charge balanced by proportional increases in Fe (super 2+) . Using x-ray absorption spectroscopy (XANES and EXAFS) at the U L (sub III) -edge we observe that the Fe (super 2+) /Fe (super 3+) ratio in magnetite is a major control on its ability to reduce U (super VI) . Stoichiometric and partially oxidized magnetites (Fe (super 2+) /Fe (super 3+) > or = 0.38) reduce U (super VI) to uraninite (U (super IV) O (sub 2) ) nanoparticles, whereas more oxidized magnetites (Fe (super 2+) /Fe (super 3+) < 0.38) adsorb U (super VI) as an inner-sphere complex without transferring electrons. The observed redox reactivity between magnetite and U (super VI) can be correlated with measured reduction potentials for magnetite and with published thermodynamic parameters for U (super IV) /U (super VI) redox couples [3]. Titanomagnetite nanoparticles with Ti formula contents up to x = 0.5 and Fe (super 2+) /Fe (super 3+) ratios between 0.5 and 1.2 can also reduce U (super VI) to U (super IV) . EXAFS spectra indicate that the reduced U (super IV) atoms are not incorporated in uraninite. The speciation of U (super IV) appears to be controlled by Ti-content and not by the Fe (super 2+) /Fe (super 3+) ratio, as the reduction of U (super VI) by partially oxidized x = 0.5 titanomagnetite results in the same non-uraninite U (super IV) species. This work highlights previously unexplored thermodynamic and geochemical factors that may influence the speciation and solubility of uranium in the subsurface. The observation of non-uraninite U (super IV) species in this study, as well as in carbonate and phosphate bearing systems in previous studies [4-6] suggests the need for a better understanding of the stability of reduced U (super IV) .
Details
- Title: Subtitle
- Reactivity of U (super VI) with pure, oxidized, and Ti-substituted magnetites
- Creators
- Drew E Latta - Argonne National LaboratoryCarolyn I PearceChristopher A GorskiKevin M RossoEdward J O'LoughlinKenneth K KemnerMichelle M SchererMaxim I Boyanov
- Resource Type
- Abstract
- Publication Details
- Mineralogical magazine, Vol.76(6), p.1978
- Conference
- Goldschmidt 2012
- Publisher
- Mineralogical Society
- ISSN
- 0026-461X
- eISSN
- 1471-8022
- Alternative title
- Goldschmidt 2012 abstract volume
- Language
- English
- Date published
- 2012
- Academic Unit
- IIHR--Hydroscience and Engineering; Civil and Environmental Engineering
- Record Identifier
- 9984202247202771
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