DFT, MD simulations and experimental analysis of adsorptive complexation and isotope separation of gadolinium ion with macrocyclic crown ether embedded polymeric resin

Abstract

Di-benzo-18-crown-6 ether (DB18C6) functionalized polymethyl acrylic acid (PMA) resin was synthesized after screening by employing density functional theory (DFT) and molecular dynamics (MD) simulations for the enrichment of gadolinium isotopes, Gd-155 and Gd-157. Batch adsorption and column chromatography studies were performed at various pH, gadolinium ion concentration and temperatures to check its efficacy and validation of theoretical approach. The adsorption capacity of gadolinium ion with the PMADB18C6 resin was found to be quite modest with a value of 1.128 mg/g. Both MD simulations and adsorption experiments confirm the Temkin type of adsorption isotherm. The positive entropy change along with the positive enthalpy change indicates that sorption of the Gd3+ ion with PMADB18C6 is entropy driven process. From the chromatography experiments, the lighter isotope (Gd-155) is seen to be depleted in the breakthrough volume samples which specify that the resin phase is depleted with heavier isotope (e.g Gd-160) contrast to natural Gd composition. The separation coefficient (ε) for Gd155/158, Gd156/158, Gd157/158, Gd155/160 and Gd157/160 isotopic pairs were found to be 1.67 × 10−3, 2.65 × 10−3, 5.40 × 10−4, 3.18 × 10−3 and 1.87 × 10−3 respectively. The experimental isotopic separation factor corroborates the DFT findings. Further, both DFT and MD simulations confirm the experimentally observed endothermic complexation of Gd3+ ion with PMADB18C6 resin. The present findings employing combined DFT, MD simulations and experimental approaches are expected to benefit in the fundamental understanding of molecular complexation based isotope separation and design and development of future gadolinium isotopic enrichment technologies using chemical exchange methods.