Dissolution study of metatorbernite: thermodynamic properties and the effect of pH and phosphate.

The uranyl copper-phosphate, metatorbernite, has been identified in the shallow vadose zone of the 300 A area at the Hanford site, WA, USA. Consequently, modeling the evolution of U concentrations in vadose zone porewaters driven by meteoric water recharge requires accurate knowledge of metatorbernite solubility. Previous determinations of the solubility constant for metatorbernite were under constrained. In the present contribution, the dissolution of natural metatorbernite crystals was studied at target pH 2.5 and 3.0, using both nitric and phosphoric acid. Steady state was approached from under- and supersaturation. The experiments and calculations yielded a preferred log K(sp) = -28.0 ± 0.1 that is significantly different than previously determined values. Further, both stoichiometric and nonstoichiometric dissolution was observed as a function of pH and aqueous phosphate concentration.