In the many, now-closed, uranium mines, exposure of the uranium minerals to air and moisture has resulted in the formation and subsequent discovery of many new uranium minerals – more than 22 have been discovered out of a single mine. Uroxite ([(UO2)2(C2O4)(OH)2(H2O)2] H2O) and metauroxite (α-[(UO2)2(C2O4)(OH)2(H2O)2]), were reported in 2020 and are the first uranyl oxalate minerals. In addition to being minerals, actinide oxalates are important in the nuclear fuel cycle and environmental transport of actinides. It is important to understand the formation and mobility of actinide oxalate complexes in aqueous systems. Thermodynamic data is needed for prediction of environmental speciation and migration of actinide oxalate complexes, however, this data currently exists only for very few actinide oxalates. Several actinide oxalates were prepared, including synthetic analogues of the two uranyl oxalate minerals. High-temperature oxide-melt drop-solution calorimetry was performed with a Setaram AlexSYS calorimeter to determine the enthalpies of formation from elements of these compounds. This data can be used to aid in predicting the speciation and migration of actinide oxalate compounds. It can also be used to investigate trends of stability across compounds with different actinides, oxidation states, counter cations, oxalate chelation, and crystallographic structure.

High temperature calorimetric study of synthetic actinide oxalate minerals and materials

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ACS Fall 2022

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