Crown ether resins modeled for precise gadolinium isotope separation

by Riko Seibo
Tokyo, Japan (SPX) Jan 05, 2026

A new theoretical study shows how functionalized crown ether resins can selectively capture lighter gadolinium isotopes, offering a controlled route to isotope separation for nuclear and planetary science applications. Gadolinium, a rare-earth element, supplies isotopes that act as neutron absorbers in reactors and as tracers of early solar system evolution, but growing environmental releases and demand for highly purified neutron-rich isotopes such as 155Gd and 157Gd have increased the need for efficient, controllable separation methods.

The research team, led by Professor Liu Yun at the Institute of Geochemistry, Chinese Academy of Sciences, and Chengdu University of Technology, combined advanced quantum chemical simulations with isotopic fractionation theory to examine how functionalized crown ether resins influence gadolinium isotope partitioning. The work, published in Planet (2025, Volume 1, Issue 1), targets the molecular-scale mechanisms that govern fractionation of heavy-element isotopes.

The investigation concentrated on four functionalized crown ether resins-PMADB15C5, PMADB18C6, PMADB21C7, and PMADCH18C6-in complexes with Gd3+ ions. Using density functional theory calculations in Gaussian09 and DIRAC19, the researchers optimized molecular geometries, calculated vibrational frequencies, and obtained single-point energies for the complexes in gas phase and aqueous solution.

A central feature of the study is the inclusion of both mass-dependent fractionation and the nuclear volume effect, a relativistic correction arising from differences in nuclear size among isotopes that becomes important for heavy elements such as gadolinium. By combining reduced partition function ratios with nuclear volume corrections, the team derived total isotopic fractionation factors that extend earlier work focused only on mass-dependent contributions.

The results show that, relative to the hydrated complex [Gd(H2O)9]3+, all four functionalized crown ether resins preferentially enrich the lighter gadolinium isotopes 155Gd and 157Gd. Among these materials, PMADB15C5 displays the strongest total fractionation, with 1000lnatot values of 0.8786 for 160Gd/155Gd and 0.5065 for 160Gd/157Gd at 298.15 K.

Structural analysis links this enhanced fractionation to shorter average Gd - O bond lengths and a smaller cavity around the metal center in the PMADB15C5 complex compared with the other resins. These features correlate with stronger selectivity for lighter isotopes and connect coordination geometry and cavity size directly to isotope separation performance.

The work separates the contributions from mass-dependent and nuclear volume effects to the total fractionation behavior. In the aqueous reference complex, mass-dependent fractionation favors heavier isotopes, while in the crown ether complexes the nuclear volume effect acts in the opposite direction and promotes enrichment of lighter isotopes.

Although the nuclear volume effect is significant for heavy elements, the study concludes that mass-dependent fractionation still dominates overall isotope behavior in these systems because gadolinium remains in the +3 oxidation state throughout. Even so, accounting for both effects is necessary to reproduce the full partitioning pattern between water and the resin phase.

To link equilibrium calculations with practical separation, the researchers applied a Rayleigh fractionation model under standard conditions using PMADB15C5 as the adsorbent. They found that when the resin achieves a gadolinium uptake of at least 95 percent, isotope fractionation during separation can be kept within 0.138 per mille for 160Gd/155Gd and 0.080 per mille for 160Gd/157Gd, levels that support accurate isotopic analysis and purification.

The study identifies PMADB15C5 as a strong candidate for gadolinium isotope purification and provides molecular-level guidance for developing new lanthanide separation materials based on cavity size and coordination geometry. By quantifying nuclear volume effects alongside mass-dependent terms, the work sets a standard for theoretical modeling of heavy-element isotope fractionation and supports more precise use of gadolinium isotopes in nuclear technology, geochemistry, and planetary science.

Research Report:Theoretical estimations of equilibrium isotope fractionations between aqueous and crown ether coordinated Gadolinium (III)