Microplastics Alter the Distribution and Toxic Potential of Typical Pharmaceuticals in Aqueous Solutions: Mechanisms and Theory Calculations

The aquatic risks associated with various pharmaceuticals can be significantly influenced by the ubiquitous presence of microplastics (MPs), leading to unforeseen environmental effects. Uncovering the interactions between MPs and pharmaceuticals with diverse functional groups is of crucial importance for accurate risk assessment. Here, the sorption behaviors and underlying mechanisms by which polystyrene (PS) MPs interact with pharmaceuticals having different functional groups were explored through experimental methods, site energy distribution theory, and density functional theory (DFT) calculations. Results indicated that PS MPs exhibited a notable sorption capacity for pharmaceuticals, with the order of sorption being naproxen (NAP), bezafibrate (BZF), norfloxacin (NOR), ibuprofen (IBU), sulfamethoxazole (SMX), and carbamazepine (CAB). A deeper analysis revealed that multiple factors, including hydrophobicity, electrostatic repulsion, π–π interactions, and hydrogen bonding, regulate the sorption process. Furthermore, the Dubinin–Astakhov (DA) model was employed to calculate the energy distribution. The adsorption affinity (E_m = 2.88–8.36 kJ/mol) and energy heterogeneity (σ_e* = 1.59–2.25) of PS MPs for different pharmaceuticals followed the order SMX > NOR > NAP > CAB > IBU > BZF. DFT calculations confirmed that the formation of n−π bonds between PS MPs and pharmaceuticals was also a primary sorption mechanism. The different sorption mechanisms of PS MPs for various pharmaceuticals can eventually alter their toxicity, such as increased toxicity of pharmaceuticals with carboxyl groups. Overall, this study offers a more comprehensive understanding of the interactions between MPs and pharmaceuticals, which can contribute significantly to the risk assessment of pharmaceuticals in the presence of MPs.