Phenol, a common and highly toxic aromatic pollutant in industrial wastewater, poses significant threats to aquatic life and human health. Its removal is challenging due to its persistence and the presence of coexisting minerals. Various methods exist for phenol removal, including adsorption, catalytic oxidation, thermal oxidation, condensation, and ion exchange; however, adsorption is considered one of the most effective and economical. Two-dimensional covalent organic frameworks (COFs) are emerging as promising adsorbents due to their high porosity, modifiable skeletons, physicochemical properties, nano-porous structure, mechanical robustness, design flexibility, solvent and thermal stabilities, well-proportioned cavities, low densities, high specific surface areas, and strong interactions with nanoparticles, maintaining stability in various mediums. While several studies have explored COFs for wastewater remediation (e.g., uranium removal, nanoplastic adsorption, oil-water separation, and cationic organic contaminant removal), this study focuses on the use of self-assembled COFs as biocompatible adsorbents for phenol removal under the influence of an external electric field. The research questions addressed are: (1) under what conditions are phenol molecules removed from the environment? (2) How does the electric field affect phenol adsorption on COFs? (3) Can COFs be applied as a pollutant removal system for industrial wastewater treatment?

Several recent studies have highlighted the potential of COFs and their derivatives for wastewater remediation. Zhang et al. demonstrated the effectiveness of COFs-PDAN-AO in adsorbing uranium from radioactive wastewater. Shang et al. used molecular dynamics simulations to investigate the adsorption mechanisms of various nanoplastics on different COFs, finding TpPa-OH to be the most effective. Li et al. synthesized superhydrophobic sponges incorporating COFs for oil removal, showcasing high adsorption capacity and rapid oil-water separation. Chen et al. explored a 2D sulfonate anionic COF membrane for removing cationic organic contaminants. Wang et al. utilized 3D COFs for radioactive vapor removal and cationic COFs for removing HFPO-TA and GenX from aqueous solutions. These studies demonstrate the versatility of COFs in addressing various pollutant types and highlight the need for further investigation into their application in phenol removal.

Classical molecular dynamics (MD) simulations were conducted using the GROMACS software package (version 2019.2) with the CHARMM36 force field. Three systems were simulated: (a) COFs/phenol without an external electric field (EF); (b) COFs/phenol with an EF of 0.5 V nm⁻¹; and (c) COFs/phenol with an EF of 1 V nm⁻¹. Each system contained a COFs nanocarrier, 50 phenol molecules, 3795 TIP3P water molecules, and 0.15 M NaCl. The simulation box dimensions were 4 × 4 × 10 nm³. Periodic boundary conditions were applied. Non-bonded electrostatic and Lenard-Jones interactions were treated using the particle mesh Ewald method with a 1.4 nm cutoff. Temperature (310 K) and pressure (1 bar) were maintained using the Nose-Hoover thermostat and Parrinello-Rahman barostat, respectively. All bonds were constrained using the LINCS algorithm. The system was initially energy-minimized using the steepest descent algorithm, followed by 105 ns MD simulations with a 1.5 fs time step. Well-tempered metadynamics simulations were also performed for 105 ns to calculate the free energy surface (FES) as a function of the distance between the centers of mass (COMs) of the phenol molecules and the COFs. Analyses included interaction energies, mean squared displacement (to calculate the diffusion coefficient), radial distribution functions (RDFs), and hydrogen bond (HB) analysis.

MD simulations revealed that both van der Waals (vdW) and electrostatic interactions contribute significantly to phenol adsorption onto COFs. The strength of these interactions decreased with increasing electric field strength. Interaction energies were significantly lower under an EF of 1 V nm⁻¹ compared to 0 and 0.5 V nm⁻¹. The diffusion coefficient (D) of phenol decreased with increasing electric field strength (0.1172, 0.0715, and 0.0155 × 10⁻⁵ cm² s⁻¹ for 0, 0.5, and 1.0 V nm⁻¹, respectively), indicating hindered diffusion at higher field strengths. RDF analysis showed a reduction in the maximum height of RDF peaks with increasing EF, indicating weaker interactions between phenol and COFs at higher field strengths. The strongest RDF peak at EF=0 was located at ~0.5 nm, suggesting strong interactions between the phenol hydroxyl group and COFs nitrogen atoms, as well as π-π interactions. Hydrogen bond analysis revealed a significant reduction in the number of hydrogen bonds between phenol and COFs with increasing EF, while the number of hydrogen bonds between phenol and water increased. Well-tempered metadynamics simulations showed that the free energy values at the global minima were approximately -290.13 kJ mol⁻¹ (EF=0), -248.33 kJ mol⁻¹ (EF=0.5 V nm⁻¹), and -264.68 kJ mol⁻¹ (EF=1 V nm⁻¹), indicating that the electric field acts as a driving force for phenol release from the COFs. The overall results demonstrate that the COF nanocarrier is an effective adsorbent for phenol molecules, and the electric field can be used to control phenol release.

The findings demonstrate that COFs are promising adsorbents for phenol removal from wastewater. The dominant role of both vdW and electrostatic interactions in the adsorption process was confirmed. The application of an external electric field effectively controlled the adsorption process, acting as a driving force for phenol release. This tunability offers potential for controlled release applications. The decrease in the diffusion coefficient with increasing electric field strength is consistent with the observed weakening of interactions between phenol and COFs. The observed increase in phenol-water hydrogen bonds at higher electric field strengths indicates that the electric field enhances the solvation of phenol, further hindering its adsorption onto the COF surface. The results are consistent with other studies that have investigated the effect of electric fields on adsorption processes. This study provides valuable insights for the design and optimization of COF-based materials for efficient and controlled phenol removal from contaminated water sources.

This study successfully demonstrated the potential of two-dimensional COFs as efficient adsorbents for phenol removal from wastewater. Both molecular dynamics and metadynamics simulations confirmed the efficacy of COFs, while also revealing the impact of external electric fields on adsorption and release. The tunability offered by the electric field provides a novel approach for controlled release applications. Future research could focus on exploring other COF structures and functional groups to further enhance adsorption capacity and selectivity. Investigating the long-term stability and reusability of COFs under various environmental conditions would also be valuable for practical applications.

The study was limited to simulations and did not involve experimental validation. The simulations considered a simplified model of wastewater, without accounting for the complexity of real industrial wastewater which may contain other competing adsorbates. The accuracy of the simulation results relies on the chosen force field and parameters. Further studies incorporating experimental validation and more realistic wastewater models are needed to fully assess the practical applicability of this approach.