Adsorption of antibiotics on microplastics (MPs) in aqueous environments: the impacts of aging and biofilms

Microplastics (MPs) that accumulate in natural water bodies through various pathways inevitably undergo aging and biofilm attachment. These variations can alter the physicochemical properties of the MPs and thus affect their abilities to adsorb other pollutants. In this study, we chose virgin polyethylene (V-PE) as the experimental MP and generated aged, biofilm-developed and biofilm-developed with aged polyethylene (A-PE, B-PE and B-A-PE) via Fenton advanced oxidation and lake water incubation methods. Then, the adsorption of tetracycline hydrochloride (TC-HCl) and cefalexin (CFX) on V-PE, A-PE, B-PE and B-A-PE was investigated. The results showed that the capacities for adsorption of TC-HCl or CFX on the four polyethylene MPs (PEMPs) increased in the order V-PE < A-PE < B-PE < B-A-PE. The order was attributed to differences in the surface roughnesses, crystallinities, surface functional groups and biofilm biomass of the four PEMPs. Kinetic experiments and isotherms indicated that nonlinear multilayer adsorption occurred via physicochemical actions, such as diffusion, hydrophobic interactions, hydrogen bonding interactions, electrostatic interactions and biodegradation. Specifically, the adsorption of TC-HCl or CFX by V-PE and A-PE was predominantly controlled by diffusion, whereas B-PE and B-A-PE were chemically active. In addition, it is worth noting that biofilms may shield antibiotics from electrostatic interactions with the B-PE and B-A-PE surfaces, and an increase in ionic strength inhibited the adsorption of TC-HCl and CFX by the four PEMPs. Our study provides an important reference for determining the adsorption of antibiotics by MPs in natural waters.