Thermodynamic stability and leaching behaviour of Ba and Th charge-coupled substituted monazite solid solution (La1-xBax/2Thx/2)PO4 (x=0.1, 0.2 and 0.3) and its comparison with Sr and Th substituted monazite

Monazite (LnPO4, Ln = light rare-earth elements; La-Gd) and its solid-solutions are potential candidates for the immobilization of long-lived minor actinides. Thermodynamic stability and chemical durability of monazite -based solid solutions are important to evaluate the matrix for waste disposal. In the present work, thermody-namic stability of Ba and Th charge-coupled substituted monazite solid-solution, (La1-xBax/2Thx/2)PO4 (x = 0.1, 0.2 and 0.3) have been investigated for the immobilization of radioactive waste. Thermodynamic parameters for the solid solution such as standard enthalpy of formation (Delta Hof), heat capacity Cop(T) have been determined and Gibbs energy of formation (Delta Gof) have been derived as a function of temperature. Leaching behaviour of (La1-xMx/2Thx/2)PO4 (M = Sr and Ba) solid solution revealed that the release of Sr ions is lower than that of Ba ions and in accordance with their corresponding thermodynamic stability. Thermodynamic and leaching studies results established long-term stability of charge-couple substituted monazite solid solution.