Dissolution kinetics of synthetic LaPO4-monazite in acidic media

Single-phase monazite-type ceramics are discussed as waste forms for the safe disposal of surplus plutonium or separated minor actinides. To derive a fundamental understanding of the long-term stability of these materials under repository relevant conditions, the dissolution kinetics of synthetic lanthanum monazite (LaPO4) were studied in dynamic dissolution experiments in the temperature range from 50 to 90°C under acidic conditions. The surface area normalised dissolution rates increased with temperature from 3.2•10-5 g m-2 d-1 at 50°C to 1.8•10-4 g m-2 d-1 at 90°C. The apparent activation energy Ea of the dissolution process was determined to be about 44 kJ mol-1, indicating a predominantly surface reaction controlled dissolution process in this temperature range. From thermodynamic considerations it can be inferred that the dissolution of the LaPO4 ceramics is governed by the dissolution of a thin layer of La-rhabdophane (LaPO4 • 0.667H2O) forming at the monazite surface in low temperature aqueous environments.