Proposing two-dimensional covalent organic frameworks material for the capture of phenol molecules from wastewaters

Phenol is a common and highly toxic aromatic pollutant in industrial wastewaters from sectors such as electroplating, mining, dyeing, and petrochemicals. Even at low concentrations, phenol poses significant risks to aquatic life and human health, making its removal prior to discharge essential. However, phenol extraction from complex aqueous matrices is challenging due to coexisting salts, low concentrations, and polarity issues. Among available treatment methods (e.g., catalytic/thermal oxidation, condensation, ion exchange), adsorption on porous solids is efficient and economical. Two-dimensional covalent organic frameworks (COFs) are crystalline, highly porous, mechanically robust materials with tunable structures and high surface areas, making them attractive adsorbents. This study examines, via molecular dynamics (MD) and well-tempered metadynamics, how phenol adsorbs onto a 2D COF and how external electric fields (EFs) affect adsorption strength and dynamics. The work aims to determine conditions favoring phenol removal, quantify EF effects on adsorption/release, and assess the suitability of COFs as adsorbents for wastewater remediation.

COFs have been explored for contaminant removal due to their ordered porosity, stability, and functional tunability. Prior works demonstrated: sp2 carbon-conjugated COFs for uranium adsorption and detection in radioactive wastewater (high capacity and structural stability); MD simulations showing TpPa-OH COFs offering strong adsorption sites for various nanoplastics; superhydrophobic sponges coated with COFs enabling ultrahigh adsorption of oils/solvents and rapid oil–water separation; ionic COF membranes with rigid porous structures for nanofiltration and charge-controlled removal of organic pollutants. Three-dimensional COFs with diamond topology have also captured iodine vapor via charge-transfer interactions, and cationic COFs with quaternary ammonium groups efficiently removed PFAS substitutes (HFPO-TA and GenX) at capacities exceeding activated carbons and resins. Related MD studies on carbon-based adsorbents (graphene oxide, nanotubes) indicate that external electric fields can weaken adsorption of phenolic compounds. These findings motivate evaluating COFs’ phenol adsorption and EF-tunable release.

Systems: A 2D COF composed of TpAPH monomers (from 1,3,5-triformylphloroglucinol, Tp, and 4-aminobenzohydrazide, APH) was modeled as a four-layer structure (776 atoms) with pore vdW diameter ~27.08 Å. Three setups were simulated: COF/phenol under EF = 0, 0.5, and 1.0 V nm⁻¹. Each system contained the COF centered in a 4 × 4 × 10 nm³ periodic box with 50 phenol molecules, water (TIP3P), and 0.15 M NaCl (Na+ and Cl− ions present). MD details: GROMACS 2019.2 with the CHARMM36 force field was used. Temperature 310 K (Nose–Hoover thermostat), pressure 1 bar (Parrinello–Rahman barostat). PME for electrostatics; Lennard-Jones cutoff 1.4 nm. Bonds constrained with LINCS. Systems were energy-minimized (steepest descent) and simulated for 105 ns with a 1.5 fs timestep. Analyses included interaction energies (electrostatic and vdW), mean squared displacement and diffusion coefficients (Einstein relation), radial distribution functions (RDFs) for phenol–COF and phenol–water, and hydrogen-bond (HB) counts using a 3.5 Å distance and 30° angle cutoff. Metadynamics: Well-tempered metadynamics (PLUMED 2.5.2 with GROMACS 2018) over 105 ns per system to obtain free-energy surfaces as a function of the distance between the centers of mass of phenol and COF (COF COM used as the reference collective variable). Initial Gaussian height 1.0 kJ mol⁻¹, width 0.25 Å, bias factor 15, deposition every 500 timesteps. Free energy set to zero at large separations.

• Adsorption energetics: Both vdW and electrostatic interactions contribute, with vdW predominant. Interaction energies (phenol–COF, kJ mol⁻¹) decreased in magnitude with increasing EF, indicating weakened adsorption: - EF 0: Total −396.047; vdW −215.391; Elec −180.656 - EF 0.5: Total −328.357; vdW −176.000; Elec −152.357 - EF 1.0: Total −86.125; vdW −37.569; Elec −48.556 • Structural behavior: At EF = 0 and 0.5 V nm⁻¹, phenol molecules adsorb on the surface and infiltrate COF nanochannels; at EF = 1.0 V nm⁻¹, phenols largely fail to enter pores and aggregate, consistent with weakened adsorption. • Diffusion: Phenol self-diffusion coefficients (×10⁻⁵ cm² s⁻¹) decreased with EF, reflecting reduced mobility and stronger solvation/H-bonding with water under strong EF: 0 V nm⁻¹: 0.1172; 0.5 V nm⁻¹: 0.0715; 1.0 V nm⁻¹: 0.0155. • RDFs: Phenol–COF g(r) peak heights decreased with EF; at EF = 0 the main peak is ~0.5 nm (indicative of strong phenol–COF interactions including H···N contacts and π–π interactions). Phenol–water g(r) peaks centered at ~0.36 nm and increased with EF, showing enhanced solvation that competes with adsorption. • Hydrogen bonds: Phenol–COF HB counts are highest and increase over time at EF = 0, but decrease markedly under EF = 0.5 and 1.0 V nm⁻¹. Conversely, phenol–water HBs increase with EF, indicating EF-induced shielding of phenol from the COF surface. • Metadynamics FES: Global free-energy minima (kJ mol⁻¹) for phenol–COF binding were −290.13 (EF 0), −248.33 (EF 0.5), and −264.68 (EF 1.0), demonstrating that external fields reduce binding strength relative to zero-field conditions and facilitate desorption/release. • Mechanistic insights: Adsorption is driven mainly by Lennard-Jones interactions, with NH (COF) and OH (phenol) contributing electrostatic/HB stabilization. Strong EF promotes phenol–water H-bonding and phenol–phenol π–π stacking, hindering pore infiltration and reducing COF binding.

The simulations address the research questions by revealing that, in the absence of an external electric field, phenol exhibits strong adsorption to the COF surface and effective infiltration into nanochannels, stabilized by vdW interactions and hydrogen bonds between phenol OH and COF nitrogen sites. Applying external electric fields weakens both electrostatic and vdW components of the interaction, reduces hydrogen bonding between phenol and the COF, and enhances phenol–water interactions, collectively diminishing adsorption affinity and pore penetration. The free-energy profiles confirm that EFs act as a stimulus to lower binding free energy and facilitate controlled release of phenol from the COF. These findings demonstrate that COFs are promising adsorbents for phenol removal under zero-field conditions and that external fields can modulate adsorption/desorption, offering a potential lever for regeneration or controlled release in practical treatment systems.

Two-dimensional COFs (TpAPH-based) are effective adsorbents for phenol, with adsorption dominated by vdW interactions and reinforced by electrostatic hydrogen bonding between phenol OH and COF nitrogen sites. External electric fields weaken phenol–COF interactions, reduce infiltration into pores, and lower binding free energies, indicating a viable strategy for EF-triggered release/regeneration. Overall, COFs show strong potential for phenol removal from contaminated waters, and EF-tunable behavior offers operational flexibility. Future work could include experimental validation of EF-controlled adsorption/desorption, exploration of COF functionalization to optimize binding sites, assessment across broader EF magnitudes and orientations, investigation of competitive adsorption in multisolute systems, and extension to other phenolic and aromatic pollutants.

Findings are based on atomistic simulations of a specific COF (TpAPH) and selected field strengths in an idealized model system; generalizability to other COFs, wastewater matrices, and operating conditions may vary. No experimental validation is provided. Only one COF architecture and limited EF magnitudes were explored, and long-term stability or fouling effects were not assessed.