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

Gadolinium (Gd), as one of the rare earth elements, plays a significant role in planetary science and studies of early solar system evolution, since its isotopic composition can record both fractionation processes within nebulae and preserve primordial nucleosynthetic heterogeneities inherited from presolar materials. Among the isotopes of gadolinium, 155Gd and 157Gd have exceptionally high neutron absorption cross-sections, making the separation of these isotopes highly valuable as excellent neutron absorbers. However, with increasing applications, it has become an emerging contaminant in aqueous environments. This study employs quantum chemical software packages Gaussian09 and DIRAC19 to systematically investigate the coordination behavior of four functionalized crown ether resins (abbreviated as PMADB15C5, PMADB18C6, PMADB21C7, and PMADCH18C6) with Gd3+ ions, and evaluates their contributions to Gd isotope fractionations caused by both mass-dependent and the nuclear volume effects in terms of 160Gd/155Gd and 160Gd/157Gd. Our results indicate that in aqueous solutions, functionalized crown ether resins preferentially enrich light Gd isotopes compared to the [Gd(H2O)9]3+. Among them, PMADB15C5 exhibits the greatest potential for Gd isotope fractionations and is identified as a more reliable adsorbent for gadolinium isotope purification. Using the Rayleigh fractionation model at standard conditions (298.15 K, 1 atm) and taking the 160Gd/155Gd pair as an example, the isotopic fractionations induced by the adsorption process can be limited to within 0.138‰ when PMADB15C5 is used as the adsorbent with yield ≥ 95%. This study elucidates, at the molecular level, the coordination mechanisms between functionalized crown ether resins and Gd3+ ions. The theoretical findings on isotopic fractionation provide important insights for designing and optimizing chemical procedures in Gd isotope analysis.