Uranium solubility and speciation in reductive soda-lime aluminosilicate glass melts
Résumé
Uranium solubility in aluminosilicate melts of the Na$_2$O-CaO-Al$_2$O$_3$ -SiO$_2$ system with two different Na/Ca ratios was studied at temperatures of 1250–1400 °C and under various redox conditions. A closed thermochemical reactor was used to control the alkali metal activity (sodium oxide content) and the oxygen fugacity imposing the reducing environment on the glass melt (10$^{−5}$ atm < fO$_2$ < 10$^{−15}$ atm). The compositions of the quenched glasses were analyzed by scanning electron microscopy and electron probe microanalysis. It appeared that uranium solubility decreased with decreasing oxygen fugacity, elucidating the roles of the different valences of uranium. To account for the respective effects of theses valences, we proposed a method to determine the proportion of each uranium oxidation state in the glass sample. The coexisting U$^{VI}$, U $^V$, and U$^{IV}$ species have been characterized for the first time in glass samples using U M$_4$ edge high energy resolution X-ray absorption near-edge structure. Results showed that the lowest solubility values, of approximately 1 mol% UO$_2$ , were obtained under strongly reducing conditions, and thus with U$^{IV}$ as the main valence. Under higher oxygen fugacity, uranium solubility was controlled and drastically enhanced by the U$^{VI}$ concentration in the melt.
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