Abstract
16 Single-phase monazite-type ceramics are discussed as waste forms for the safe disposal of 17 surplus plutonium or separated minor actinides. To gain a deeper insight into the mechanism 18 governing the dissolution of monazite-type compounds, the dissolution kinetics of synthetic 19 LaPO 4-monazite was systematically studied by dynamic dissolution experiments carried out 20 in the temperature range from 50 to 130°C in acidic media (0.01M HNO 3). The dissolution 21 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 22 g m-2 d-1 at 130°C. Two different temperature regions were observed, in which the normalised 23 dissolution rates of LaPO 4 have a diverging temperature dependence, indicating two different 24 mechanisms of dissolution: namely surface-controlled dissolution (T = 50-90°C; E a = 44 25 kJ mol-1) and transport-controlled dissolution (T = 90-130°C; E a = 7.5 kJ mol-1). 26 Complementary thermodynamic modelling studies of the dissolution of LaPO 4 at the 27 experimental conditions show that La-rhabdophane (LaPO 4 ·0.667H 2 O) is the 28 thermodynamically favoured phosphate phase in aqueous environments below about 100°C. 29 Apparently, the hydration of monazite and the formation of a thin surface layer consisting of 30 rhabdophane is an intermediate step controlling the dissolution kinetics of the LaPO 4 31 monazite ceramics in low temperature aqueous environments. 32 33