A calorimetric investigation of A2[(UO2)2(WO5)O] compounds with AÂ =Â K, Rb and Cs and calculated phase relations in the K2WO4-UO3-H2O and K2MoO4-K2WO4-UO3-H2O systems

A calorimetric and thermodynamic investigation of three alkali-metal uranyl tungstates of general formula A2[(UO2)2(WO5)O], with A = K, Rb and Cs, was undertaken. All three phases were synthesized by high-temperature solid-state reaction of a mixture of the respective alkali-metal nitrates and tungsten (VI) and gamma uranium (VI) oxide. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of the three phases was determined using HF-solution calorimetry giving ΔfHm°(T = 298 K, K2[(UO2)2(WO5)O], cr) = −4185 ± 8 kJ·mol−1, ΔfHm°(T = 298 K, Rb2[(UO2)2(WO5)O], cr) = −4185 ± 10 kJ·mol−1, and ΔfHm°(T = 298 K, Cs2[(UO2)2(WO5)O], cr) = −4198 ± 9 kJ·mol−1. Their low-temperature molar heat capacity, Cp,m°, was measured using adiabatic calorimetry from T = 9 to 329 K for K2[(UO2)2(WO5)O], from T = 9 to 321 K for Rb2[(UO2)2(WO5)O] and from T = 6 to 320 K for Cs2[(UO2)2(WO5)O]. The respective molar third law entropy at T = 298.15 K, Sm°, was calculated giving 380 ± 1 J·K−1·mol−1 for K2[(UO2)2(WO5)O], 406 ± 1 J·K−1·mol−1 for Rb2[(UO2)2(WO5)O] and 419 ± 1 J·K−1·mol−1 for Cs2[(UO2)2(WO5)O]. These new calorimetric results, combined with literature data, have been used to calculate the Gibbs free energy of formation, ΔfGm°, for all the studied phases giving: ΔfGm° (T = 298 K, K2[(UO2)2(WO5)O], cr) = −3914 ± 8 kJ·mol−1, ΔfGm° (T = 298 K, Rb2[(UO2)2(WO5)O], cr) = −3915 ± 10 kJ·mol−1 and ΔfGm° (T = 298 K, Cs2[(UO2)2(WO5)O], cr) = −3926 ± 9 kJ·mol−1. Smoothed Cp,m°(T) values between 0 K and 320 K are presented along with the values for Sm° and the functions [Hm°(T) − Hm°(0)] and [Gm°(T) − Hm°(0)] for all three phases. The phase relations of various solid phases and solution complexes in the K2WO4-UO3-H2O and K2MoO4-K2WO4-UO3-H2O systems with and without CO2 at T = 298 K were calculated using a Gibbs energy minimization approach. The results show that there is no uranium-containing solid phase stable at pH < 2.2 in either CO2-free or CO2-bearing systems. Here, uranium (VI) is bound in solution complexes. Various solid phases are stable from pH 2.2 to pH 10 in the CO2-free system, but at pH > 10 and in the presence of CO2 no solid is present.