Securing a sustainable supply of rare earth elements (REEs) requires efficient separation and recovery from secondary streams. Zeolites are attractive, low-impact sorbents; however, the atomistic origins of REE selectivity under hydrated conditions remain insufficiently resolved. Here, we combine high-resolution synchrotron X-ray powder diffraction (XRPD) with density functional theory (DFT) to elucidate the exchange and recovery mechanisms of Ce3+, La3+, Eu3+, and Y3+ in ammonium-exchanged faujasite X (NH4–X). Monoelement solutions mimicking the concentrations of leachates of spent fluorescent phosphors were contacted with NH4–X; exchanged solids and their NH4+ back-exchanged counterparts were probed structurally. XRPD/Rietveld refinements show that Ce3+, La3+, and Eu3+ preferentially occupy site II* in the supercage, where mixed coordination to framework O atoms and H2O molecules stabilizes the cations, and the concomitant lattice responses are consistent with increased extra-framework interactions. In contrast, Y3+ partitions between a II*-like position and a distinct, more external site (III), where its coordination is with H2O and the interaction with the framework is weaker. DFT corroborates the hydrated nature of Y3+ at site III and reveals transient proton transfer from the coordinated water to the framework, forming Y(H2O)5(OH), which further attenuates the cation–framework bonding. These site-specific behaviors rationalize the experimentally observed selectivity and recovery trends: Y3+ shows a lower exchange capacity and is more readily released upon NH4+ back-exchange, whereas Ce3+/Eu3+ are strongly retained, and La3+ exhibits intermediate behavior. By linking the ionic radius and hydration enthalpy to site occupancy and lattice response, this study provides a mechanistic basis for rationally tailoring FAU-type zeolites to enhance REE selectivity, capacity, and recyclability in circular-economy recovery schemes.
Colombo et al. (2026) studied this question.