Microplastics Meet Metoprolol in Natural Water: Sorption Behavior and Mechanism

As an ideal carrier for the spread of pollutants in the aquatic environment, microplastics (MPs) can adsorb pharmaceutical β-blockers, which can affect their migration and lead to some unpredictable adverse consequences. In this paper, the sorption behaviors and mechanism of MPs (polyvinyl chloride (PVC) and polypropylene (PP)) for typical β-blocker metoprolol (MTL) were investigated. The effects of pH, salinity and humic acids (HAs) on the sorption were studied, which proved that the sorption behavior was different under different environmental conditions. Both low pH and high salinity inhibited the sorption of MTL by the MPs. Specifically, the sorption capacity of MTL increased, with pH increase from 3 to 10. When pH = 10, the sorption capacities of MTL on PVC (1.75 mg/g) and PP (3.34 mg/g) reached the maximum. After pH > 10, the amount of MTL adsorbed on PVC was slightly decreased, while that on PP was essentially the same. The addition of salt ions inhibited the sorption in the concentration range of 5–250 mg/g for both NaCl and CaCl2, with the inhibitory effect of Ca2+ being stronger than that of Na+. Moreover, the presence of HAs promoted the sorption of MPs for MTL. In the absence of HAs, the sorption capacities of PP and PVC for MTL were 0.34 mg/g and 0.79 mg/g, respectively. When HA concentration was 100 mg/L, the highest sorption capacities of PP and PVC reached 0.79 mg/g and 1.37 mg/g, respectively. This indicated that the promoting effect of HAs on PP was stronger than that on PVC. In general, based on the study of the sorption behavior of MTL and the characterization of the MPs, the sorption mechanism was speculated to consist mainly of electrostatic interactions, cation exchange, hydrophobic interaction and halogen bonding. The sorption kinetics of MTL on the two MPs were well-fitted by the pseudo-second-order model with R2 > 0.99. The sorption isotherms both fitted the Freundlich model, which substantiated that the sorption of MTL on the MPs (PVC and PP) was multilayered and heterogeneous. Collectively, these findings provided a theoretical basis for revealing the complex interactions between MPs and MTL in natural water and a new insight into the fate and migration of MPs and β-blockers in the environment.