Uranophane dissolution and growth in CaCl2-SiO2(aq) test solutions

The dissolution and growth of uranophane [Ca(UO2)2(SiO3OH)2·5H2O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10−2 M CaCl2 and ∼10−3 M SiO2(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction (logKspo=11.01±0.09) and standard state Gibbs free energy of formation for uranophane (ΔGfo=-6196.1±0.5 kJ mol−1).