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 degrees C in acidic media (0.01 M HNO3). The dissolution rates at far from equilibrium conditions increased from 3.2 x 10(-5) g m(-2)d(-1) at 50 degrees C to 2.5 x 10( -4) g M(-2)d(-1) at 130 degrees 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 degrees C; E-a = 44 kJ mol(-1)) and transport-controlled dissolution (T = 90-130 degrees C; E-a = 7.5 kJ mol(-1)). Complementary thermodynamic modelling studies of the dissolution of LaPO4 at the experimental conditions show that La-rhabdophane (LaPO4 .0.667H(2)O) is the thermodynamically favoured phosphate phase in aqueous environments below about 100 degrees 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. (C) 2018 Elsevier B.V. All rights reserved.