Preparation and performance of MOF-808 (Zr-MOF) for the efficient adsorption of phenoxyacetic acid pesticides

The organic herbicides of chlorophenoxyacetic acids, including 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), have posed serious threats to aquatic ecosystems and human health due to extensive use, high solubility and degradation-resistant. In this study, the efficient adsorbent (MOF-808) was synthesized through solvothermal synthesis method and further evaluated for the adsorption performances of 2,4-D and MCPA. Results showed that MOF-808 exhibited the Type I nitrogen adsorption–desorption isotherm with a specific surface area of 2,155 m²·g⁻¹, pore volume of 0.59 cm³·g⁻¹ and pore size of 1.41 nm. It was suggested that the pseudo-second-order and Langmuir model were much better to illustrate the adsorption performances of MOF-808 for both 2,4-D and MCPA, and the maximum adsorption capacities of 793 and 775 mg·g⁻¹ were achieved, respectively; besides, the adsorption processes were spontaneous and primarily driven by chemical adsorption with contributions from physical-chemical adsorption. Additionally, the adsorption capacities for 2,4-D and MCPA were decreased with the increasing of pH and cations concentrations; the optimal pH was 3.0 for 2,4-D and 4.0 for MCPA, and the effects of four cations on pesticides adsorption followed the order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺. Reuse experiments demonstrated that MOF-808 could retain a high removal rate of 95.9 % (2,4-D) and 95.8 % (MCPA) at the fifth cycle. The spectral analysis revealed that the adsorption mechanisms of 2,4-D and MCPA by MOF-808 involved electrostatic interactions, π-π interactions, hydrogen bonding, and complexation. In conclusion, the well-defined physicochemical properties and outstanding adsorption performance of the synthesized MOF-808 may provide a potential application for the removal of chlorophenoxyacetic acid herbicides from real wastewater.