Understanding and Controlling the Trace Iron Incorporation during Th-Rhabdophane Crystallization

Rhabdophane has an outstanding capacity of accommodating radioactive nuclides and is recognized as a significant host phase for the enrichment of actinides from radioactive waste liquid. Thus, assessing the impact of impurities present in such waste streams on rhabdophane crystallization is critical. This study systematically investigates the effects of Fe³⁺/Fe²⁺ coincorporation with actinides on the precipitation behavior of rhabdophane. Some specific issues, including lattice occupation of Fe³⁺, precipitation reaction kinetics, crystal growth affected under the influence of trace iron, and leaching stability, are discussed in detail. The results reveal that the presence of Fe³⁺, rather than Fe²⁺, can lower the energy barrier for nucleation and accelerate the transformation of Th-rich nucleus into rhabdophane. The limited uptake of Fe³⁺ with a suggested substitution ratio below 1%, contributes to establishing the stability domain of the rhabdophane phase, whereas excess Fe³⁺ promotes the formation of the FePO₄·2H₂O phase, which synergistically enhances the grain growth kinetics of rhabdophane/monazite through an aggregation-adsorption effect. Based on density functional theory of structural energetics, Fe³⁺ occupation is energetically favorable at nonhydrated [LaO₈] sites in rhabdophane, despite the fact that two-thirds of lattice sites are associated with [LaO₈·H₂O] sites. Intriguingly, amorphous FePO₄·2H₂O can act as active sites to facilitate the formation of a surface alteration layer with improving the leaching stability of Th⁴⁺ in pH = 5-7 solutions.