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 solid-solution were synthesized by solid state method. • Thermodynamic stability of charge-coupled substituted monazite solid-solution have been investigated using high temperature oxide melt calorimetry. • Thermodynamic parameters for the solid solution such as standard enthalpy of formation and heat capacity have been determined. • Leaching behaviour of solid solution were investigated under off-normal conditions. Monazite (LnPO 4 , 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, thermodynamic stability of Ba and Th charge-coupled substituted monazite solid-solution, (La 1- x Ba x /2 Th x /2)PO 4 (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 (Δ H f o) , heat capacity C p o (T) have been determined and Gibbs energy of formation (Δ G f o) have been derived as a function of temperature. Leaching behaviour of (La 1- x M x /2 Th x /2)PO 4 (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. [Display omitted] [ABSTRACT FROM AUTHOR]