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 - Publisher Copyright:
© 2016 Elsevier Ltd.
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
SN - 0016-7037
VL - 189
SP - 269
EP - 281
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -