Objective The aims were to clarify the role of microplastics (MPs) in the environmental behavior of antibiotics, and to elucidate the influence mechanisms of MPs on the adsorption and migration behaviors of antibiotics in soil.

Method The polyamide MPs (PA-MPs) and polystyrene MPs (PS-MPs) along with the typical antibiotic trimethoprim TMP were selected as research subjects. The physicochemical properties of the two MPs were characterized using a combined approach of microscopic imaging and spectroscopic analysis techniques. Batch experiments and soil column experiments were conducted to examine their impacts on the adsorption and migration behavior of TMP in soil. Additionally, a solute transport model was employed to analyze the underlying mechanisms of these effects.

Result PA-MPs had a rougher surface and contain oxygen-containing functional groups, thereby providing more sorption sites for TMP. Consequently, PA-MPs exhibited significant TMP affinity, which resulted in enhanced TMP adsorption capacity upon introduction into soil. Conversely, PS-MPs exhibited a low affinity for TMP and caused a dilution effect upon introduction into the soil, thereby reduced the soil's capacity to adsorb TMP. The reduction of soil porosity was the primary mechanism through which PS-MPs inhibit TMP migration in soil column. In contrast, PA-MPs exerted a stronger inhibitory effect on TMP transport compared to PS-MPs by the synergistic effects of enhanced adsorption and reduced soil porosity.

Conclusion The results have confirmed that PA and PS microplastics in soil affect the adsorption and transport behavior of antibiotics through different mechanisms, which provides a scientific basis for assessing and controlling the combined toxic effects and environmental risks of MP-antibiotic co-pollution.