TY - JOUR
T1 - Structure and thermodynamics of uranium-containing iron garnets
AU - Guo, Xiaofeng
AU - Navrotsky, Alexandra
AU - Kukkadapu, Ravi K.
AU - Engelhard, Mark H.
AU - Lanzirotti, Antonio
AU - Newville, Matthew
AU - Ilton, Eugene S.
AU - Sutton, Stephen R.
AU - Xu, Hongwu
N1 - Funding Information:
This work was supported by the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy (US-DOE), Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089. The Mössbauer spectroscopic and XPS analysis were performed using Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is operated by Battelle for the U.S. DOE under contract DE-AC06-76RLO1930. The synchrotron XAS work was performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy - GeoSciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The last stages of this work was also supported by the laboratory-directed research and development (LDRD) program, through the G. T. Seaborg Institute, of Los Alamos National Laboratory, which is operated by Los Alamos National Security LLC, under DOE Contract DE-AC52-06NA25396, in that X.G. is now pursuing a Seaborg postdoctoral fellowship at LANL. We thank A.H. Tavakoli for participation in the initial stages of this project and P.S. Maram for helpful discussions.
PY - 2016/9/15
Y1 - 2016/9/15
N2 - Use of crystalline garnet as a waste form phase appears to be advantageous for accommodating actinides from nuclear waste. Previous studies show that large amounts of uranium (U) and its analogues such as cerium (Ce) and thorium (Th) can be incorporated into the garnet structure. In this study, we synthesized U loaded garnet phases, Ca3UxZr2-xFe3O12 (x = 0.5-0.7), along with the endmember phase, Ca3(Zr2)SiFe3+2O12, for comparison. The oxidation states of U were determined by X-ray photoelectron and absorption spectroscopies, revealing the presence of mixed pentavalent and hexavalent uranium in the phases with x = 0.6 and 0.7. The oxidation states and coordination environments of Fe were measured using transmission 57Fe-Mössbauer spectroscopy, which shows that all iron is tetrahedrally coordinated Fe3+. U substitution had a significant effect on local environments, the extent of U substitution within this range had a minimal effect on the structure, and unlike in the x = 0 sample, Fe exists in two different environments in the substituted garnets. The enthalpies of formation of garnet phases from constituent oxides and elements were first time determined by high temperature oxide melt solution calorimetry. The results indicate that these substituted garnets are thermodynamically stable under reducing conditions. Our structural and thermodynamic analysis further provides explanation for the formation of natural uranium garnet, elbrusite-(Zr), and supports the potential use of Ca3UxZr2-xFe3O12 as viable waste form phases for U and other actinides.
AB - Use of crystalline garnet as a waste form phase appears to be advantageous for accommodating actinides from nuclear waste. Previous studies show that large amounts of uranium (U) and its analogues such as cerium (Ce) and thorium (Th) can be incorporated into the garnet structure. In this study, we synthesized U loaded garnet phases, Ca3UxZr2-xFe3O12 (x = 0.5-0.7), along with the endmember phase, Ca3(Zr2)SiFe3+2O12, for comparison. The oxidation states of U were determined by X-ray photoelectron and absorption spectroscopies, revealing the presence of mixed pentavalent and hexavalent uranium in the phases with x = 0.6 and 0.7. The oxidation states and coordination environments of Fe were measured using transmission 57Fe-Mössbauer spectroscopy, which shows that all iron is tetrahedrally coordinated Fe3+. U substitution had a significant effect on local environments, the extent of U substitution within this range had a minimal effect on the structure, and unlike in the x = 0 sample, Fe exists in two different environments in the substituted garnets. The enthalpies of formation of garnet phases from constituent oxides and elements were first time determined by high temperature oxide melt solution calorimetry. The results indicate that these substituted garnets are thermodynamically stable under reducing conditions. Our structural and thermodynamic analysis further provides explanation for the formation of natural uranium garnet, elbrusite-(Zr), and supports the potential use of Ca3UxZr2-xFe3O12 as viable waste form phases for U and other actinides.
KW - Calorimetry
KW - Garnet
KW - Structure
KW - Uranium
KW - X-ray absorption spectroscopy
KW - X-ray photoelectron spectroscopy
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U2 - 10.1016/j.gca.2016.05.043
DO - 10.1016/j.gca.2016.05.043
M3 - Article
AN - SCOPUS:84976427865
VL - 189
SP - 269
EP - 281
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
ER -