Efficient Removal of Metronidazole from Water using Pyrolyzed ZnO-Polyethylene Waste Nanocomposites as an Adsorbent

Metronidazole (MNZ) is a widely used antibiotic that frequently contaminates aquatic environments, posing risks of antibiotic resistance and toxicity to ecosystems and human health. In this study, zinc oxide–modified polyethylene waste nanocomposites (ZnO-MPEW-NCs) were synthesized using polyethylene waste (PEW) as a matrix and modified with zinc oxide (ZnO) and concentrated sulfuric acid (H₂SO₄). The composites were thermally treated at 300, 400, and 500 °C to produce P300-, P400-, and P500-ZnO-MPEW-NCs. Comprehensive characterization using Fourier-transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), and Brunauer–Emmett–Teller (BET) surface area analysis confirmed successful structural modification, chemical stability, and compositional integrity. Pyrolysis at 500 °C significantly enhanced the textural properties, increasing the specific surface area from 10 to 106 m²/g. Batch adsorption experiments demonstrated rapid MNZ removal, with equilibrium achieved within 30 min. The P500-ZnO-MPEW-NCs exhibited the highest adsorption capacity (133.75 mg/g) and removal efficiency (99.4%). Kinetic analysis showed that the adsorption process followed the pseudo-second-order model (R² = 0.99941, qcal = 9.88 mg/g), while intraparticle diffusion analysis indicated contributions from both surface adsorption and pore diffusion. Equilibrium data were best described by the Freundlich isotherm (R² = 0.99958), suggesting multilayer adsorption on a heterogeneous surface dominated by physisorption. Thermodynamic analysis confirmed that adsorption was spontaneous and exothermic, with Gibbs free energy changes (ΔG° = −2.90 to −10.44 kJ/mol), enthalpy changes (ΔH° = −10.50 to −42.88 kJ/mol), and entropy changes (ΔS° = −25.5 to −102 J/(mol.K)). These results demonstrate the strong potential of P500-ZnO-MPEW-NCs as sustainable adsorbents for pharmaceutical removal from water.