Acetaminophen adsorption mechanism on CuO and ZnO nanoparticles: DFT and MD insights for wastewater treatment

An integrated computational study combining density functional theory and molecular dynamics simulation were performed to elucidate the adsorption mechanism of acetaminophen on Copper Oxide (CuO) and Zinc Oxide (ZnO) nanoparticles in aqueous media. The global electronic descriptors of the optimized adsorption complexes show clear differences. CuO/acetaminophen exhibits EHOMO = −7.33, ELUMO = −6.87 (ΔEgap = 0.461 eV), low chemical hardness (η = 0.23), high softness (S = 4.33) and very large electrophilicity (ω = 109.35). In contrast, ZnO/acetaminophen has EHOMO = −5.76, ELUMO = −0.92 (ΔEgap = 4.84 eV), η = 2.42, S = 0.41 and ω = 2.30, indicating CuO as a much stronger electron acceptor. Fukui and MEP maps identify the carbonyl (O11) and phenolic (O7) oxygens as primary donors (O11 f − = 0.1398, f + = 0.0866, f° = 0.2264). NBO analysis reveals larger donor → acceptor stabilization for CuO (E2 = 37.7 kcal/mol) than ZnO (E2 = 8.5 kcal/mol), consistent with significant LP(O/N) → metal charge-transfer on CuO. MD simulations corroborate stronger binding on CuO (Eads = −276 to −277.6 kcal/mol) than ZnO (−219 to −230 kcal/mol) over 290-318 K. Energy decomposition shows that (Edeformation = −182 to −186 kcal/mol) dominates stabilization in both systems, while CuO displays a much larger Erigid (−93 to −95 kcal/mol) than ZnO (−36 to −45 kcal/mol). RDFs and non-bonded decomposition indicate short range, electrostatic coordination bonding on CuO (1.9–2.0 Å), and longer-range, dispersion H-bonding dominated contacts on ZnO (' 3.5 Å). These descriptors support a mechanism in which CuO favors charge transfer coordination (chemisorption), while ZnO favors relaxation assisted physisorption. This could guide the development of practical adsorbents for water treatment. CuO for strong sequestration and ZnO for reversible capture.