New insights into oxytetracycline (OTC) adsorption behavior on polylactic acid microplastics undergoing microbial adhesion and degradation

Biodegradable plastics have been evaluated as promising alternatives for conventional polymers in various fields. Although microplastics (MPs) have been reported as vectors for pollutants in the environment (e.g. antibiotics), knowledge about the chemicals carrying mechanisms of degradable MPs during the biodegradation process is limited. In this study, we investigated the adsorption behavior of oxytetracycline (OTC) on polylactic acid (PLA) MPs during its biodegradation process. Scanning electron microscopy (SEM) discovered the growth of scattered rod-shaped bacteria on the surface of PLA MPs. Two-dimensional correlation spectroscopy (2D-COS) combined with Fourier transform infrared spectroscopy (FTIR) identified the breakage of ester linkages and the generation of more oxygen-containing functional groups, indicating PLA biodegradation occurred. The presence of a biofilm coating increased the OTC adsorption capacity on MPs by 20.15%, mainly due to OTC complexing with chemical functional groups existing in biofilms (e.g. N–H functional group). More oxygen-containing functional groups were exposed on the surface of PLA MPs after removing biofilms, which further increased the adsorption quantity of OTC by 39.01% through enhanced hydrogen bonding compared with biofilm coating MPs. The adsorption quantity of OTC adsorbed onto MPs first increased and then decreased with the pH ranging from 3.0 to 9.0, which was dominated by electrostatic interaction and hydrophobic interaction. In addition, the adsorption capacity of MPs was suppressed by the presence of fulvic acid (FA) due to that the adsorbed FA blocked further OTC adsorption and OTC showed higher affinity to free FA compared with MPs. These results unravel the OTC adsorption behavior of PLA MPs undergoing microbial adhesion and degradation, which could be useful for understanding the environmental sorption behavior of degradable MPs.