Co-precipitation behavior of Ba2+ and Th4+ in rhabdophane and its conversion to monazite ceramics for radioactive waste immobilization

Precipitation enrichment coupled with ceramic solidification represents a technically viable and promising engineering approach for volume reduction and immobilization of high-level radioactive waste. Rhabdophane has been proposed as a candidate host phase for immobilizing actinides from high-level liquid waste (HLLW). However, the wet-chemical co-precipitation mechanism governing the simultaneous incorporation of tetravalent actinides and divalent charge-compensating cations remains incompletely understood. This knowledge gap is particularly relevant because Ba 2+ is a major product in HLLW generated from commercial nuclear power reactors. In this study, we investigate the Ba 2+ /Th 4+ co-incorporation behavior and solubility limits in La 1–2 x Th x Ba x PO 4 ·nH 2 O rhabdophane synthesized at pH=1 and 90 °C. Our results suggest that an excess of Ba 2+ is required to stabilize a single-phase rhabdophane, with an optimal Ba:Th ratio of 5. Combined structural, spectroscopic, and microscopic analyses confirm effective substitution of Ba 2+ and Th 4+ at La 3+ lattice sites for x  ≤ 0.05. Higher doping levels lead to the crystallization of rhabdophane with nonstoichiometric compositions, accompanied by Ba 2+ vacancy defects. Over 99% of Th 4+ in 0.05 mol/L solution was removed within 12 h. Upon thermal treatment, the as-synthesized rhabdophane powders can be converted into dense monazite ceramics at 1250 °C.