Thermodynamic studies of zippeite, a uranyl sulfate common in mine wastes

Zippeite is a potassium uranyl sulfate mineral that forms on uranium mine wastes and that may be important in nuclear waste disposal. Little is known about its thermodynamic properties. In this study, we synthesized zippeite, K₃(H₂O)_3.3[(UO₂)₄(SO₄)₂O₃(OH)], and measured its thermodynamic properties. The standard enthalpy of formation of zippeite from the elements, − 8655.97 ± 12.55 kJ/mol, was calculated from the measured enthalpy of dissolution in a high-temperature oxide melt solution calorimeter. Solubility measurements were conducted from both undersaturation and supersaturation in order to constrain the equilibrium state. Using the solubility product (K_sp) obtained from the solubility data, the standard Gibbs free energy of formation of zippeite at 298 K was calculated to be − 7783.44 ± 6.87 kJ/mol. This value, coupled with the standard enthalpy of formation value determined from the calorimetric data, was then used to calculate the standard entropy of formation, − 2926.49 ± 45.64 J/mol. Our studies yield the first complete set of thermodynamic properties of zippeite, and the results allow us to predict the conditions under which its formation is favorable relative to other uranyl minerals under a wide range of conditions of geologic and engineering interest.