High-temperature calorimetric measurements of thermodynamic properties of uranyl arsenates of the meta-autunite group

Five uranyl arsenates of the meta-autunite group with mono (Li, K, Cs) and divalent (Sr and Cu) cations were synthesized at room temperature and characterized. Their thermodynamic properties were measured using high-temperature oxide melt calorimetry in a molten 3Na2O·4MoO3 solvent at 976 K. Measured enthalpies of drop solution of each compound were used to calculate their respective enthalpies of formation from oxides and elements. Calculated standard state enthalpies of formation from the elements are −3301 ± 14 kJ/mol for LiUAs, −3089 ± 14 kJ/mol for KUAs, −5607 ± 15 kJ/mol for CsUAs, −6638 ± 21 kJ/mol for SrUAs, and −6161 ± 24 kJ/mol for CuUAs. These values are evaluated relative to acid-base interactions using oxide acidity expressed on Smith's scale, as well as the normalized charge deficiency per anion (NCDA), which probes relationships between structural units and interstitial complexes. Linear relationships exist between the heats of formation, acidity of binary oxides, and bond requirements within these meta-autunite type structures. The new thermodynamic data combined with published results yield insights into the thermodynamic behavior of uranyl arsenates. Thermodynamic properties presented here are important for understanding the genesis of uranium deposits and uranium mobility in the subsurface.