Nickel oxalate metal-organic framework-like material for efficient adsorptive removal of 2,4-dichlorophenoxyacetic acid from aqueous environments

• Ni-Ox was synthesized and structurally regulated via a facile hydrothermal method. • Maximum adsorption capacity for 2,4-D was determined to be 384.6 mg/g. • Adsorption process was revealed by studying kinetics, isotherm, and thermodynamics. • Adsorption performance was conducted both in simulated and lake water samples. The use of herbicides in farming to manage growth, combat pests, and enhance crop production ultimately contributes to environmental pollution. However, the removal of these herbicides remains challenging due to high costs, limited adsorption efficiency, complex synthesis, and poor stability of the adsorbents. In this study, we synthesized a nickel oxalate metal-organic frameworks like coordination compound (Ni-Ox) using a straightforward hydrothermal method for the adsorptive removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from wastewater samples. The adsorbent exhibited a unique morphology with a mesoporous structure, abundant functional groups, and a homogeneous elemental composition and distribution, as characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and Brunauer-Emmett-Teller (BET) analysis. Ni-Ox demonstrated an exceptionally high removal efficiency of 99.7% for 2,4-D under optimal adsorption conditions: pH 2, contact time of 60 minutes, dose of 80 mg, stirring speed of 120 rpm, batch volume of 50 mL, and an initial concentration of 100 mg L⁻¹. The adsorption of 2,4-D on Ni-Ox was best described by the Langmuir model, with a maximum theoretical adsorption capacity of 384.6 mg g⁻¹ at 298 K. The kinetic data were best fitted by the pseudo-second-order model, yielding a rate constant of 2.8 × 10⁻³ g mg⁻¹ min⁻¹ and a q e value of 60.98 mg g⁻¹. Thermodynamic analysis indicated that the adsorption process was both endothermic and spontaneous, as evidenced by a positive ΔH° (+65.9 kJ mol⁻¹) and negative ΔG° values. The adsorption mechanism of 2,4-D on Ni-Ox was elucidated through post-characterization using XPS, FTIR, and XRD, and was found to be primarily driven by electrostatic attraction, hydrogen bonding, and van der Waals forces. Furthermore, Ni-Ox maintained an efficiency of over 80% even after multiple regeneration and reuse cycles. These findings provide valuable insights for the design of effective adsorbents for the herbicide removal from aquatic environment.