Adsorption of sodium diclofenac on vinylbenzyl trimethylammonium chloride-modified covalent organic polymer

Abstract

The effective removal of diclofenac sodium (DCF) has become an important topic in waste management because of its poor efficiency in wastewater treatment. In this study, covalent organic polymer (COPs) were modified with vinylbenzyl trimethylammonium chloride (VTC), an organic compound containing abundant aromatic rings and quaternary ammonium groups. The structure of the resulting adsorbent (VTC-COPs) was investigated via FTIR, XRD, TGA, SEM, BET, and XPS. The modified VTC-COPs exhibited a mesopore structure with a pore size of 27.9 nm and a specific surface area of 38.5 m²/g. The DCF removal performance of VTC-COPs in aqueous solutions was evaluated by a batch adsorption experiment. At pH 6, the saturated adsorption capacity of VTC-COPs reached 458 mg/g, whereas that of the original CC-COPs was 346 mg/g. The results show that the Langmuir adsorption isotherm and pseudo-second-order kinetic models can effectively describe the equilibrium adsorption process. According to the Langmuir model, the maximum adsorption capacity of the adsorbent VTC-COPs at 298 K was 616 mg/g. The negative ΔH and ΔG values obtained through thermodynamic parameters confirmed that the DCF adsorption of VTC-COPs was a spontaneous exothermic process. Coexisting ions influenced adsorption in the following order: PO₄³⁻ > SO₄²⁻ > CO₃²⁻ > NO₃⁻ > Cl⁻. The desorption experiment shows that DCF desorption can recover the material in a 0.1 mol/L NaOH solution. After 10 sorption-desorption cycles, the performance loss of VTC-COPs was only 5 %. The post-adsorption FTIR, BET, SEM, and XPS analysis results show that the nitrogen functional groups and aromatic rings of VTC-COPs interact with DCF. The adsorption mechanism is primarily based on electrostatic interaction, π–π interaction, and pore filling. According to these results, VTC-COPs can be considered a viable adsorbent for removing DCF from wastewater.