Aged polyamide microplastics enhance the adsorption of trimethoprim in soil environments

Microplastics (MPs) in the environment typically age. However, the influence of aged MPs on the adsorption of antibiotics in soil remains unknown. In this study, the adsorption behavior of trimethoprim (TMP) on soil and soil containing aged polyamide (PA) was investigated using batch and stirred flow chamber experiments. The adsorption of TMP on the tested soil with and without PA was fast, with the k_a values ranging from 50.5 to 55.6 L (mg min)^-1. The adsorption of TMP on aged PA was more than 20 times larger than that on the tested soil, which resulted in an "enrichment effect." Furthermore, aged PA altered the pH of the reaction system, thereby enhancing the adsorption of TMP. Consequently, the K_d values of TMP for soil, soil containing 5%, and 10% aged PA were 5.64, 12.38, and 23.65 L kg^-1, respectively. The effect of aged PA on the adsorption of TMP on soil depended on pH values. However, TMP adsorption on soil containing 10% aged PA was constantly higher (p < 0.01) than that on soil with NaCl concentrations ranging from 0 to 50 mmol L^-1. These findings provide new insights into the effect of environmental MPs on the fate and transport of antibiotics in soil environments.