Multiple Phenolic Hydroxyl-Modified Hyper-Crosslinked Polymers and Their Efficient Adsorptive Removal of o-Nitrophenol from Water

Herein, three multiple phenolic hydroxyl aromatic compounds, namely, p-dihydroxybenzene (PD), 1,3,5-trihydroxybenzene (TB), and gallic acid (GA), were applied as the chemical modifiers and incorporated into initial polymers via nucleophilic substitution, hence creating intermediates with different phenolic hydroxyls (−OH). Thereafter, Friedel–Crafts alkylation resulted in multiple phenolic −OH-modified hyper-cross-linked polymers (HCPs), namely, PS–PD-DCE, PS-TB-DCE, and PS-GA-DCE. The final polymers exhibited a high Brunauer–Emmett–Teller (BET) surface area (S_BET: 504–573 m²/g), large pore volume (V_total: 0.48–0.54 cm³/g), and plentiful phenolic −OH (acidic exchange capacity: 1.2–2.6 mmol/g). The adsorption experiments revealed that PS-GA-DCE with the highest phenolic −OH was the most efficient adsorbent for the adsorption of o-nitrophenol from aqueous solutions, and the predicted maximum capacity (q_max) at 298 K was 1052 mg/g according to the Langmuir model, surpassing the data of most synthetic adsorbents in previous studies. The micropore diffusion model and the intraparticle diffusion model effectively elucidated the kinetic data. 5% of sodium hydroxide desorbed o-nitrophenol from the polymers completely, and the equilibrium capacity dropped to 85.5% after five repeated adsorption–desorption cycles. The adsorption mechanism revealed that hydrogen bonding was primarily responsible for the adsorption, with micropore filling serving as an additional contributing factor. These results provide valuable insights into the synthesis and application of chemically modified polymers for the enhanced adsorption performance.