Influence of temperature on the dissolution kinetics of synthetic LaPO4-monazite in acidic media between 50 and 130 °C

Single-phase monazite-type ceramics are discussed as waste forms for the safe disposal of surplus plutonium or separated minor actinides. To gain a deeper insight into the mechanism governing the dissolution of monazite-type compounds, the dissolution kinetics of synthetic LaPO4-monazite was systematically studied by dynamic dissolution experiments carried out in the temperature range from 50 to 130 °C in acidic media (0.01 M HNO3). The dissolution rates at far from equilibrium conditions increased from 3.2 × 10−5 g m−2 d−1 at 50 °C to 2.5 × 10−4 g m−2 d−1 at 130 °C. Two different temperature regions were observed, in which the normalised dissolution rates of LaPO4 have a diverging temperature dependence, indicating two different mechanisms of dissolution: namely surface-controlled dissolution (T = 50-90 °C; Ea = 44 kJ mol−1) and transport-controlled dissolution (T = 90-130 °C; Ea = 7.5 kJ mol−1). Complementary thermodynamic modelling studies of the dissolution of LaPO4 at the experimental conditions show that La-rhabdophane (LaPO4·0.667H2O) is the thermodynamically favoured phosphate phase in aqueous environments below about 100 °C. Apparently, the hydration of monazite and the formation of a thin surface layer consisting of rhabdophane is an intermediate step controlling the dissolution kinetics of the LaPO4 monazite ceramics in low temperature aqueous environments.