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Deciphering the carrier potential of microplastics aged in freeze-thaw soils: Multiscale insights into mechanisms for antibiotic adsorption amplification
Summary
Freeze-thaw cycles (the natural freezing and thawing soil goes through in cold seasons) roughen the surface of microplastics and make them better at soaking up antibiotics from soil, like a sponge with more nooks and crannies to trap chemicals. This matters because it means microplastics in cold-climate farmland could become more effective carriers of antibiotic pollutants, potentially spreading them further through soil, water, and crops than previously thought, an overlooked risk as climate change alters freeze-thaw patterns.
Aged microplastics (MPs) have attracted growing concern about their carrier effects. Yet, the impact of soil freeze-thaw cycles (FTCs) on MPs as contaminant carriers remains poorly understood. This study assessed changes in the carrier potential of FTC-aged MPs for antibiotics (ATs) through adsorption experiments, and disentangled their underlying mechanistic pathways by innovatively integrating characterization, batch adsorption and density functional theory (DFT) data within a partial least squares structural equation modeling (PLS-SEM) framework. Results showed that soil FTCs roughened surfaces, altered wettability and surface charges, and increased oxygen-containing functional groups (OCFGs) of MPs. The adsorption capacities for sulfamethoxazole (SMX) and enrofloxacin (ENR) were generally enhanced on FTC-aged MPs across MP types and environmental conditions, indicating strengthened carrier potential. Mechanistically, the dominant driver was the FTC-altered surface morphology of MPs that promoted pore-filling, while FTC-generated OCFGs played a limited and polymer-specific role by remodeling the electronic structure of MPs, thereby modulating their electrostatic and hydrogen-bonding interactions with ATs. This study establishes a transferable framework for adsorption research, advances understanding of the carrier potential of MPs undergoing FTCs in soils, and calls for greater attention to the overlooked role of FTCs in shaping co-contamination risks in cold region ecosystems under climate change.