Electronic and structural perturbations of microporous ZIF-67 nanoparticles and Cr(VI) molecule during adsorptive water decontamination unveiled by experimental and quantum computational investigations

Abstract

Metal organic frameworks (MOFs) have attracted great attention due to their unique characteristics and accordingly found their way to a multitude of applications including water remediation. Co-based zeolite imidazole framework (ZIF-67) has recently shown interesting adsorptive decontamination capabilities towards metallic and organic water pollutants. Yet, ZIF-67 adsorption mechanism and stability have not been studied enough. In this study, ZIF-67 nanocrystals (∼40 nm) were synthesized and applied as an adsorbent for Cr2O72− from water. The Cr2O72−−ZIF-67 adsorption mechanism and both of Cr2O72− and ZIF-67 consequent structural changes were investigated by analyzing the elemental composition and oxidation states, the adsorption isotherms, kinetics and thermodynamics, and the optimized quantum computational models. The experimental results fitted well with pseudo-second order and Langmuir models, and gave ΔH° and ΔG° of −86.4 ± 17.6 and 12.8 ± 2.4 kJ/mol, respectively, suggesting exothermic spontaneous monolayer chemical adsorption, with maximum capacity of 370 mg/g. Kinetics also show concurrence surface adsorption with intra-particle diffusion within the micropores. XPS results reveal that both Cr6+ and the less-toxic Cr3+ co-existed in Cr2O72−−ZIF-67, whilst the Co3+/Co2+ ratio increased, referring to an oxidation–reduction reaction between Cr and Co. Computational analysis concede that the Cr2O72−−ZIF-67 high affinity is based on H-bonding between the Cr2O72− oxygens and the imidazole hydrogens, van der Waals attraction between the Cr2O72− lone pairs and the imidazole π cloud, the Cr2O72−−ZIF-67 electron transfer, and CrN coordination interaction. However, these computations show that the Cr2O72−−ZIF-67 interaction causes internal destabilization within each of Cr2O72− and ZIF-67, as Cr2O72− tends to dissociate and ZIF-67 integrity weakens.