Understanding the sorption behavior of tri, tetra and hexavalent f cations on metal-organic framework (MOF), ZIF-67: experimental and theoretical investigation

Metal-organic frameworks (MOFs) are a class of compounds with modern-age technological importance due to diverse structural topologies, porosity, and modularity. ZIF-67 has been synthesized for efficient extraction of f cations in different oxidation states (UO22+, Th4+ and Eu3+) from aqueous pH medium exhibiting the trend in sorption efficiency as: K-d(Th)(6.19 x 10(3)) > K-d(U) (5.20 x 10(3)) > K-d(Eu) (1.58 x 10(3)) in accordance to their chemical potential. Langmuir isotherm was found to be predominate with sorption capacity 85.9 mg g(-1), 137.4 mg g(-1) and 50.6 mg g(-1) for U, Th and Eu, respectively. The sorption predominantly proceeded via pseudo 2nd order rate kinetics with rate constants: k(2)(U) similar to 0.089 g mg(-1) min(-1), k(2)(Th) similar to 0.011 g mg(-1) min(-1) and k(2)(Eu) similar to 0.04 g mg(-1) min(-1). The processes were found to be spontaneous as revealed from negative ?G values; and endothermic in nature (Delta HU similar to 10.5 kJ mol(-1); Delta HTh similar to 12.8 kJ mol(-1); Delta H-Eu similar to 8.3 kJ mol(-1)). The sorption proceeded via the formation of an inner sphere complex as indicated from their enhancement in the entropy values on sorption due to the release of water molecules. The Density Functional Theory (DFT) predicted selectivity using binding energy (ZIF-67-Eu3+ similar to -60.90 eV, ZIF-67-Th4+ similar to -70.78 eV, ZIF-67-Th4+ similar to -69.52 eV) is in line with the experimental selectivity of Th4+ > UO22+ > Eu3+. Total density of states (TODS) and Projected density of States (PODS) were estimated in order to understand the interaction between the ZIF-67 and f cations.

Automated summary

Metal-organic frameworks (MOFs) are a class of compounds with diverse structural topologies, porosity, and modularity, which are of significant technological importance. ZIF-67 has been synthesized to efficiently extract ions in different oxidation states from aqueous solutions, with sorption efficiency influenced by their chemical potential. The sorption process follows pseudo 2nd order rate kinetics, and is spontaneous and endothermic. The binding energy predicted by Density Functional Theory (DFT) aligns with the experimental selectivity of ions, demonstrating the interaction between ZIF-67 and ions.