Solubility and Thermodynamic Investigation of Meta-Autunite Group Uranyl Arsenate Solids with Monovalent Cations Na and K

We investigated the aqueous solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids (UAs). The measured solubility products (log Ksp) obtained in dissolution and precipitation experiments at equili-brium pH 2 and 3 for NaUAs and KUAs ranged from -23.50 to -22.96 and -23.87 to -23.38, respectively. The secondary phases (UO2)(H2AsO4)2(H2O)(s) and tro''gerite, (UO2)3(AsO4)2 center dot 12H2O(s), were identified by powder X-ray diffraction in the reacted solids of KUA precipitation experiments (pH 2) and NaUAs dissolution and precipitation experiments (pH 3), respectively. The identification of these secondary phases in reacted solids suggest that H3O+ co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements (Delta Hf-el) of NaUAs is -3025 +/- 22 kJ mol-1 and for KUAs is -3000 +/- 28 kJ mol-1 derived from measurements by drop solution calorimetry, consistent with values reported in other studies for uranyl phosphate solids. This work provides novel thermodynamic information for reactive transport models to interpret and predict the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters affected by mining legacy and other anthropogenic activities.

Automated summary

We studied the solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids. The experiment results showed that the solubility products of NaUAs and KUAs ranged from -23.50 to -22.96 and -23.87 to -23.38, respectively. Secondary phases were identified in the reacted solids, indicating that H3O+ co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements for NaUAs and KUAs is -3025 +/- 22 kJ mol-1 and -3000 +/- 28 kJ mol-1 respectively, consistent with values reported in other studies for uranyl phosphate solids. This work provides important thermodynamic information for understanding the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters.