Incorporation of Th4+ and Sr2+ into Rhabdophane/Monazite by Wet Chemistry: Structure and Phase Stability

Abstract

Rhabdophane is an important permeable reactive barrier to enrich radionuclides from groundwater and has been envisaged to host radionuclides in the backend of the nuclear fuel cycle. However, understanding of how An⁴⁺ and Sr²⁺ precipitate into rhabdophane by wet chemistry has not been resolved. In this work, Th⁴⁺ and Sr²⁺ incorporation in the rhabdophane/monazite structure as La_1-2xSr_xTh_xPO₄·nH₂O solid solutions is successfully achieved in the acid solution at 90 °C. Some specific issues such as lattice occupation of Th⁴⁺ and Sr²⁺, precipitation reaction kinetics, and crystal growth affected by starting stoichiometry are discussed in detail, along with investigating the chemical stability of La_1-2xSr_xTh_xPO₄·nH₂O precipitations and associated La_1-2xSr_xTh_xPO₄ monazite. The results reveal that the excess of Sr²⁺ appears to be a prevailing factor with a suggested initial Sr: Th ≥ 2 to obtain the stability domain of La_1-2xSr_xTh_xPO₄·nH₂O (x = 0∼ 0.1). A rapid ion removal associated with a nucleation process has been observed within 8 h, and Th⁴⁺ can be removed more than 98% after 24 h in 0.01 mol/L solutions. From structural energetics based on density functional theory, the lattice occupation of Th⁴⁺ and Sr²⁺ is energetically favorable in nonhydrated lattice sites of [LaO₈], although two-thirds of lattice sites are associated with [LaO₈·H₂O] hydrated sites. Intriguingly, the crystal transformation from rhabdophane to monazite associated with the transformation from [SrO₈] to [SrO₉] polyhedra can greatly improve the leaching stability of Sr²⁺.