Preparation of Ag-MOFs Regulated by Imidazole-Based Ligands and Study on Their Adsorption Performance and Mechanism for Br- and I-

To develop highly efficient adsorbents for removing Br⁻ and I⁻ from aqueous solutions, three silver-based metal-organic frameworks (Ag-MOF-1, Ag-MOF-2, and Ag-MOF-3) were synthesized via a solvothermal strategy using imidazole, 2-methylimidazole, and benzimidazole as organic ligands, respectively. The regulatory effect of ligand structure on the physicochemical properties and adsorption performance of the Ag-MOFs was systematically investigated through a combination of structural characterization techniques (XRD, SEM, FT-IR, etc.) and density functional theory (DFT) calculations. Batch adsorption experiments were conducted to evaluate the effects of contact time and solution pH on Br⁻ and I⁻ removal behavior, and the adsorption mechanism was further revealed. The results demonstrate that variations in ligand structure include significant differences in crystal structure and morphology, yielding lamellar, irregular blocky, and nanorod-like architectures of the three Ag-MOFs. Among them, Ag-MOF-1 exhibited the highest adsorption capacities for Br⁻ and I⁻, reaching 5.56 mmol·g⁻¹ and 5.97 mmol·g⁻¹, respectively. All three Ag-MOFs displayed a stronger adsorption affinity toward I⁻ than Br⁻, and the adsorption kinetics were well described by the pseudo-second-order model, indicating a chemisorption-dominated process. Acidic to neutral conditions (pH < 9.50) were favorable for adsorption, while alkaline environments inhibited adsorption due to electrostatic repulsion and the formation of Ag-based precipitates. In actual brine systems, the adsorption rates of the materials for Br⁻ and I⁻ exceeded 81% and 98%, respectively, without significant interference from high-concentration competing ions such as Cl⁻ and SO₄²⁻. The adsorption mechanism mainly involved two key processes: the formation of silver halide precipitates between Ag⁺ and Br⁻/I⁻, and the electrostatic interaction between protonated N atoms in ligands and anions. This study provides a theoretical basis and technical reference for the design of high-selectivity halide ion adsorbents.