Thermodynamics, Adsorption Kinetics, and Interfacial Partitioning of Tetracycline Antibiotics on Biodegradable Polyester versus Polyethylene Microplastics

Abstract Biodegradable mulch films, primarily poly(lactic acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT) blends, are increasingly applied as eco-friendly alternatives to conventional polyethylene (PE) films. However, the interfacial interactions and vector potential of commercial biodegradable mulch-derived microplastics toward typical agricultural antibiotics remain insufficiently understood. In this study, the adsorption thermodynamics, kinetics, and interfacial binding mechanisms of tetracycline (TC) and chlortetracycline (CTC) onto microplastics derived from two commercial biodegradable films (WHMPs and RAMPs) and a conventional PE film (PEMPs) were systematically investigated under controlled laboratory conditions. The results demonstrated that biodegradable microplastics exhibited markedly higher saturated adsorption capacities for both TC (5.20 mg/g for WHMPs and 6.18 mg/g for RAMPs) and CTC (11.46 mg/g for WHMPs and 15.01 mg/g for RAMPs) than those of PEMPs (1.22 mg/g for TC and 5.43 mg/g for CTC). Adsorption isotherms adhered closely to the Langmuir model, indicating a monolayer interfacial coverage. Thermodynamic analyses revealed that the adsorption was spontaneous and endothermic, driven predominantly by favorable entropy gain. The significantly higher adsorption affinity for CTC compared to TC was rationalized by its higher octanol–water partition coefficient and the electron-withdrawing induction of the C7–Cl substituent, which enhances localized dipole–dipole attractions with polymer ester carbonyls (C═O). High-resolution X-ray photoelectron spectroscopy and FT-IR analyses revealed that noncovalent physical partitioning, hydrophobic interactions, and dipole–dipole attractions dominated the interfacial binding without irreversible covalent transitions. Multicondition desorption and dynamic leaching assays further revealed that biodegradable microplastics exhibit moderate desorption reversibility and sustained release potential. These findings provide fundamental thermodynamic and kinetic benchmarks for understanding antibiotic retention on commercial mulch-derived microplastics, highlighting the need for future field-scale investigations to evaluate their environmental behaviors under complex soil matrixes and chronic weathering conditions.