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

Abstract

ABSTRACT 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 (UO2 2+, Th4+ and Eu3+) from aqueous pH medium exhibiting the trend in sorption efficiency as: Kd Th(6.19 × 103) > Kd U (5.20 × 103) > Kd Eu (1.58 × 103) 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: k2 U ~0.089 g mg−1 min−1, k2 Th ~0.011 g mg−1 min−1 and k2 Eu ~0.04 g mg−1 min−1. The processes were found to be spontaneous as revealed from negative ΔG values; and endothermic in nature (ΔHU ~10.5 kJ mol−1; ΔHTh ~12.8 kJ mol−1; ΔHEu ~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+ ~ -60.90 eV, ZIF-67-Th4+ ~ -70.78 eV, ZIF-67-Th4+ ~ -69.52 eV) is in line with the experimental selectivity of Th4+ > UO2 2+ > 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.