Insights into behavior and mechanism of tetracycline adsorption on virgin and soil-exposed microplastics

Microplastics (MPs), as vectors of pollutants, have attracted extensive attention because of their environmental effects. However, the adsorption behavior and antibiotic mechanism of environmentally exposed MPs is limited. Here, the adsorption of tetracycline (TC) onto virgin and soil-exposed polylactic acid (PLA), polyvinyl chloride (PVC) and polyethylene (PE) MPs showed that the adsorption capacity of MPs for TC increased after soil exposure, and PLA showed the strongest increase. Soil exposure increased the time to reach equilibrium, and the adsorption rate was controlled by both intraparticle diffusion and membrane diffusion. The isothermal adsorption results of soil-exposed PE and PLA indicated that TC adsorbed on heterogeneous surfaces was affected by the physicochemical adsorption process. The equilibrium absorption capacity of MPs for TC increased by 88% (PLA), 26% (PVC) and 15% (PE) after soil exposure. Soil dissolved organic matter promoted the desorption of TC from MPs, and TC speciation changed with pH. Soil-exposed MPs have the potential to promote TC degradation in solution without the addition of biological inhibitors. Moreover, density functional theory calculations verified that PE and PVC adsorbed TC through physical interactions, while hydrogen bonds were formed on PLA with TC. These results clarified the behavior and mechanisms of TC adsorption on virgin and soil-exposed MPs, which can help in the risk assessment of concomitant pollution of MPs and antibiotics.