Adsorption–Desorption Behaviors of Enrofloxacin and Trimethoprim and Their Interactions with Typical Microplastics in Aqueous Systems

Microplastics can transfer antibiotics in water through adsorption and desorption, causing adverse effects on the water environment. Therefore, understanding the interaction between microplastics and antibiotics is important in order to assess their impact on the environment. In this study, the adsorption–desorption behaviors of two commonly used antibiotics [enrofloxacin (ENR) and trimethoprim (TMP)] in aquaculture and their interactions with three typical microplastics [polystyrene (PS), polyvinyl chloride (PVC), and polyethylene (PE)] were investigated through laboratory experiments. The results showed that the adsorption capacity of the three microplastics was 1.229–1.698 mg/g for ENR and 1.110–1.306 mg/g for TMP, correlating with the octanol–water partition coefficients (logKow) of antibiotics. Due to the larger specific surface areas and special functional groups of microplastics, the antibiotic adsorption capacity of PS and PVC was higher than that of PE. The adsorption behavior followed pseudo-second-order kinetics and a Freundlich isotherm model, indicating a non-uniform surface with multilayer adsorption. A thermodynamic analysis showed that these were all spontaneous endothermic adsorptions. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analyses indicated that the adsorption mechanism was dominated by physical adsorption, involving π–π conjugation, halogen bonds, hydrogen bonding, and electrostatic interactions. High salinity and alkaline environments were conducive to desorption, and the ENR and TMP desorption rates of the microplastics ranged from 20.65% to 24.95%. This indicates that microplastics adsorbed with antibiotics will desorb antibiotics when entering the seawater system, thereby affecting marine ecosystems. These findings reveal the interaction mechanism between microplastics and aquaculture antibiotics in aqueous systems, providing theoretical support for environmental protection and sustainable development.