Adsorptive removal of 4-chloro-2-methylphenoxyacetic acid from aqueous solution using UiO-66(Zr)-impregnated amino acid ionic liquid: experimental and molecular docking simulation.

Abstract

Water pollution caused by the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) raises significant environmental concerns, making it essential to explore efficient methods for its removal from aquatic systems. This study explores the development of hybrid adsorbents by impregnating the ionic liquid [Ch][Ala] into UiO-66(Zr) to enhance their adsorption efficiency. In this study, Metal-organic framework UiO-66(Zr)-impregnated with 5.0 wt.% of cholinium alanate ([Ch][Ala]) was synthesized via solvothermal method to investigate its potential for adsorbing 4-chloro-2-methylphenoxyacetic acid (MCPA). The hybrid MOF was characterized by XRD, FESEM, N₂ adsorption-desorption isotherms, FTIR, and TGA. The impregnation of ionic liquid inside the pores of UiO-66(Zr) and their interaction with the MOF's cavities were confirmed by FTIR, TGA, and molecular docking. The removal efficiency of MCPA was evaluated using a central composite design (CCD) of response surface methodology (RSM). The optimal MCPA removal percentages were 94.22% and 97.62% for pristine UiO-66(Zr) and UiO-66(Zr)/[Ch][Ala]@5%, respectively. The optimal conditions for the adsorption process were observed at 40 °C, a contact time of 35 min, an adsorbent dosage of 40 mg, and an initial MCPA concentration of 40 mg/L, except for UiO-66(Zr)/[Ch][Ala]@5%, where the optimum temperature was 30 °C and the initial concentration was 20 mg/L. The adsorption efficiency of MCPA was best predicted using pristine-UiO-66(Zr) at node 6, with an R² of 0.920, an adjusted R² of 0.910, and the lowest RMSE of 0.812. However, the modified UiO-66(Zr)/[Ch][Ala]@5% showed even better performance for MCPA removal, achieving a higher R² of 0.974, an adjusted R² of 0.9946, and a significantly lower RMSE of 0.344 at node 6 using the 4-6-2 topology, indicating improved adsorption efficiency and prediction reliability. Adsorption isotherms were best described by the Freundlich model, indicating multilayer adsorption on heterogeneous surfaces with adsorption sites of different energy levels and affinities towards MCPA molecules. The thermodynamic analysis revealed that the adsorption process was both exothermic and spontaneous. Moreover, docking simulations showed the presence of π-π, anion -π, and hydrogen bonding interactions, which contributed to the stronger interaction between MCPA and the UiO-66(Zr)/[Ch][Ala]@5%, in agreement with experimental findings.