Performance and mechanism insights into a synergistic coagulation-adsorption process for high-concentration oily wastewater treatment

High-concentration emulsified oily wastewater is difficult to treat due to its high interfacial stability and complex composition. Herein, a sequential coagulation-adsorption process (CAP) integrating PAC, wood-based activated carbon (WAC), and 15-million-molecular-weight anionic polyacrylamide (15APAM) was developed for simulated and real oily wastewater treatment. Process parameters were optimized via response surface methodology (RSM), and the coupling mechanism was elucidated using ζ-potential, floc size evolution, adsorption kinetics/thermodynamics, fluorescence spectroscopy, and molecular dynamics (MD) simulation. Under optimized conditions (45 °C, pH 7, 5.2 min; PAC 30 mg·L⁻¹, WAC 0.75 g·L⁻¹, 15APAM 6 mg·L⁻¹), CAP achieved 98.73% COD, 99.38% oil, 99.35% turbidity, and 71.34% NH₃-N removal in simulated wastewater, with only 0.02-0.07% deviation between experimental and model-predicted COD removal. Compared with PAC-15APAM (PA) system, CAP produced larger shear-resistant flocs (max 504 μm, Sf = 24.6%, Rf = 93.4%). Adsorption followed pseudo-second-order model (R² = 0.999), with PAC pretreatment doubling equilibrium adsorption capacity (323.44-660.85 mg·g⁻¹), and thermodynamic analysis indicated that the adsorption was spontaneous and exothermic, with ΔHo = -34.07 kJ·mol⁻¹. For real wastewater at pH 4, CAP achieved 80.12% COD and 85.32% NH₃-N removal, validating it as an efficient, reliable strategy for high-concentration emulsified oily wastewater remediation.