Enhanced removal of iodine ions using Zn-based metal-organic framework/polypyrrole adsorption membrane

Metal-organic framework membranes have been proven to possess outstanding iodine adsorption capacity, but many MOFs are highly sensitive to water, limiting their use in aqueous solution. Herein, polypyrrole (PPy) molecules are deposited onto Zn-based metal-organic framework (Zn-MOF) membranes to enhance water stability and the adsorption performance for iodine ions in aqueous solution. The hydrophobic nature of PPy molecules and the nitrogen atoms in the pyrrole ring, which have lone electron pairs, allow for the doping of iodine to form a charge-transfer complex. This effectively slows down the hydrolysis of the Zn-MOF and improves the iodine scavenging ability. For the iodine aqueous solution (I₂/KI), the saturated capacity of iodine adsorption of the Zn-MOF@PPy membrane reaches up to 397 mg/g, compared to that of the Zn-MOF membrane (183 mg/g). Under dynamic adsorption processes, the Zn-MOF@PPy membrane achieved a removal efficiency of 93.89 % within 6 min. Molecular simulations reveal that Zn-MOF exhibits a water molecule binding energy of -4.6 kcal/mol, while Zn-MOF@PPy shows a weaker binding energy of -1.3 kcal/mol. Based on the Monte Carlo simulations, the atomic distance between I₃^- and Zn-MOF@PPy is 2.8 Å, representing a reduction compared to that of Zn-MOF (4.3 Å). The Zn-MOF and Zn-MOF@PPy have binding energies of -3.4 kcal/mol and -4.6 kcal/mol with I₃^-, respectively. Those results confirm that the hydrophobic PPy layer effectively repels water molecules, creating a low-dielectric environment that substantially mitigates direct water attack on the Zn-MOF framework. The presence of strong interaction sites in the PPy modified layer, which tightly anchors the iodine species on the framework surface. This attempt provides new insights into improving the MOF adsorption membranes' capture performance and water stability for iodine in aqueous solution.