Microplastics play a minor role in tetracycline sorption in the presence of dissolved organic matter

Microplastics have a great potential to sorb organic pollutants from the adjacent environment. In this study, the sorption of tetracycline, a polar and ionizable antibiotic, on three types of microplastics (polyethylene (PE), polypropylene (PP) and polystyrene (PS)) were investigated in batch sorption experiments. The sorption isotherms were well fitted by the Langmuir model, indicating that not only hydrophobic interactions but also other interactions (e.g. electrostatic interactions) played important roles in the sorption process. PS had the maximum sorption capacity, following the order PS > PP > PE, which can be attributed to polar interactions and π-π interactions. The sorption of tetracycline on microplastics was significantly influenced by pH, with sorption capacity increasing gradually, peaking at pH 6.0 and then decreasing, likely due to the influence of tetracycline speciation with the change of pH. Fulvic acid was selected as representative dissolved organic matter (DOM) to examine the effect on sorption. The increasing concentration of fulvic acid inhibited the sorption of tetracycline on three microplastics, decreasing them by more than 90% at the fulvic acid concentration of 20 mg/L, which implied a greater affinity of tetracycline to fulvic acid than to microplastics. Increasing salinity from 0.05 to 3.5% had negligible effects on the sorption of tetracycline on the three microplastics. Our results highlight the importance of pH and DOM on the sorption of tetracycline on microplastics, and suggest the relatively minor role of microplastics in the fate and transport of tetracycline in the aquatic environment in the presence of DOM.