Sorption behavior of diclofenac sodium and venlafaxine on microplastics: a mechanistic study based on spectroscopy and density functional theory

Diclofenac sodium (DCF) and venlafaxine (VLF), as widely used and environmentally persistent pharmaceutical pollutants, are frequently detected in water bodies and exhibit bioaccumulation and ecotoxicity, which may threaten the stability of ecosystems by disrupting the endocrine systems of aquatic organisms and neural signaling. The combination of these pollutants with microplastics (MPs) not only enhances the migration of the pollutants, but also amplifies the risk of exposure to high trophic level organisms via the food chain, which is an important warning significance for the study of the safety of the aquatic environment and the mechanism of pollutant coupling in the global “plastic circle”. This study delved into the sorption behavior and mechanisms of DCF and VLF on two different MPs (i.e., polystyrene (PSMPs) and polylactic acid (PLAMPs)). Batch sorption experiments were used to quantify sorption capacity under controlled solution chemistry conditions, examining sorption kinetics, isotherms, and thermodynamics. The maximum sorption capacity of PLAMPs and PSMPs for DCF was 2.994 and 3.296 mg g⁻¹, respectively, while that of PLAMPs and PSMPs for VLF was 5.864 and 6.234 mg g⁻¹, respectively. The sorption of DCF and VLF on both MPs mainly depended on physical interactions, with the kinetic model in agreement with the pseudo-first-order model, the isotherms were consistent with the Langmuir model. Fourier infrared spectroscopy and X-ray photoelectron spectroscopy demonstrated variations regarding surface functional groups upon MPs before and after sorption. Both MPs showed a significant increase in oxygen and nitrogen-based functional groups after sorption of DCF and VLF, with the most obvious variations being the increase in C-O and C=O. Density-functional theory was used to detect the main sorption mechanisms and the binding energies between PhACs and MPs. In summary, our study found the sorption of DCF and VLF on MPs is controlled by electrostatic interactions, followed by some weaker interactions (i.e., van der Waals forces, hydrogen bonding, and hydrophobic interactions). This study provides new insights into the mechanisms of effective separation and interaction between complex pollutants.