Impact of hydrodynamic conditions on ofloxacin adsorption by microplastics: roles of turbulence and equilibrium capacity

Understanding the complex interplay between hydrodynamic conditions and the dynamics of antibiotic adsorption by microplastics (MPs) is essential for accurately assessing environmental risks in aquatic systems. This study systematically investigated the adsorption mechanisms of ofloxacin (OFL) onto polystyrene (PS) and polyvinyl chloride (PVC) MPs under varying hydrodynamic conditions. Batch adsorption experiments demonstrated that the adsorption of both materials for OFL follows pseudo-second-order kinetic profiles (<i>R</i>² > 0.99) and exhibits Freundlich isothermal behavior (<i>R</i>² > 0.98), suggesting that heterogeneous surface-driven multilayer adsorption is the predominant mechanism. Material characterization revealed that the physicochemical properties of PS featured a significantly higher specific surface area (2.10 m² g^-1) than PVC (0.87 m² g^-1), yet their equilibrium adsorption capacities were comparable (29.33 µg g^-1 for PS <i>vs.</i> 30.47 µg g^-1 for PVC under high agitation). This discrepancy implies that factors such as surface roughness and micropore architecture, rather than merely specific surface area, play a dominant role in determining adsorption efficiency. Fourier-transform infrared (FTIR) spectroscopy confirmed the absence of new covalent bonds, indicating that physical interactions-such as hydrophobic interactions, van der Waals forces, and micropore filling-are the primary adsorption mechanisms. Hydrodynamic conditions emerged as a critical regulator of adsorption dynamics. Increasing turbulence intensity (40-200 rpm) shortened the equilibrium attainment time by more than 83% and enhanced equilibrium adsorption capacities (with a maximum increase of 16.2% for PS and 6.5% for PVC). These findings highlight that hydrodynamic forcing caused by natural flow regimes and anthropogenic disturbances can exacerbate microplastic-antibiotic composite contamination through enhanced adsorption processes.