Efficient adsorption removal of 2,4-dichlorophenoxyacetic acid using amine-functionalized metal–organic frameworks (MOFs): Performance and mechanisms

Abstract

2,4-dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide and plant growth regulator worldwide; however, its overdose and improper use would cause severe eco-toxicological effects on aquatic life, plants, and human life. This work synthesized high-quality MIL-101(Cr)-NH2 nanoparticles via a one-pot hydrothermal reaction and used it in pioneer as an amine-functionalized mesoporous metal–organic frameworks (MOFs) adsorbent for 2,4-D removal. The adsorption kinetics results confirmed that MIL-101(Cr)-NH2 obeys the pseud-second-order kinetic model, and the isotherm results imply that it shows multilayer adsorption following the Freundlich model. Compared to state-of-the-art benchmark MOF adsorbent UiO-66-NH2, MIL-101(Cr)-NH2 achieved a superior 2,4-D adsorption capacity of 348.5 mg/g at pH = 7 with a nearly 50 % increase in adsorption removal rate under ambient conditions. Spectroscopy analysis and density functional theory (DFT) calculations revealed that the distinct large pore size, high surface area and porosity of MIL-101(Cr)-NH2 are the essential physical factors for enhancing the diffusion ability of 2,4-D in the framework. Meanwhile, the electrostatic interactions of the metal clusters and aminated benzene ring with the dissociated 2,4-D anion are the dictated binding forces during adsorption. This mechanism was further validated by the adsorption behaviors of other contaminants over the two MOF adsorbents. Furthermore, the MIL-101(Cr)-NH2 nanoparticles can be chemically anchored onto chitosan to form MIL-101(Cr)-NH2/Chitosan foams with feasible recovery ability and it was found that the MIL-101(Cr)-NH2/Chitosan (1/3) foam showed a comparable 2,4-D removal rate to UiO-66-NH2 (52.2 % vs. 57.7 %). This study reported a high-performance aminated MOF adsorbent for 2,4-D removal and the revealed adsorption mechanisms offer an excellent platform for rational design of advanced adsorbents for acidic contaminants adsorption.