Enhanced adsorption of tetracycline on polypropylene and polyethylene microplastics after anaerobically microbial-mediated aging process

Aging of microplastics (MPs) can greatly affect their behaviors with other pollutants in aquatic and terrestrial ecosystems. Effect of anaerobically microbial-mediated aging on the physicochemical properties of polypropylene (PP) and polyethylene (PE) MPs was studied, then the adsorption behavior of tetracycline on the MPs was investigated. The results indicated that microbial-mediated aging process caused an increase in crystallinity and apparent changes in the morphology and surface structure of MPs. Visible pits and grooves on PP-MPs, as well as cavities and broken edges on PE-MPs were observed, while the surface functional groups also changed. Adsorption kinetics of tetracycline were well-described by a pseudo-second model. The equilibrium adsorption capacities of the microbial-aged PP- (MA-PP, 427.7 μg g−1) and PE- (MA-PE, 1095.3 μg −1) MPs were more than 3.8 and 8.1 times higher, respectively, than virgin PP- (V-PP, 111.7 μg g−1) and PE- (V-PE, 133.6 μg g−1) MPs. Adsorption isotherms of tetracycline on V-PP and V-PE were well-fitted by the Langmuir model, but failed to describe the adsorption process of tetracycline on MA-PP and MA-PE, suggesting changes in adsorption mechanisms of tetracycline due to anaerobic biodegradation of MPs. Solution pH had an evident effect on the adsorption capacity of tetracycline due to changes in both tetracycline speciation and surface charge of MPs. This study clearly suggests that anaerobically microbial-mediated aging process may greatly change the morphology and surface structures of PP-MPs and PE-MPs, and thus act as carriers for enhanced accumulation and transport of antibiotics in the environment, posing an increasing risk to ecosystem.