Behavior and mechanisms of ciprofloxacin adsorption on aged Polylactic Acid and Polyethylene microplastics

Abstract Microplastics (MPs) and antibiotics are emerging pollutants in aquatic environments. MPs can absorb antibiotics in the environment, resulting in compound pollution. The relationship between the physicochemical properties of aged microplastics and the adsorption of CIP was investigated to assess the control on the fate and transport of CIP in the environment. The adsorption capacity decreased in the following sequence: aged PLA > PLA > aged PE > PE. The experiment data matched the pseudo-secondary dynamics and the Langmuir isotherm model well, implying that adsorption was primarily governed by monolayer chemisorption. When the solution's pH was close to the dissociation constant (pKa) of CIP, the sorption capacity of MPs increased. The greater the concentration of Na+ and fulvic acid, the stronger the inhibition of adsorption. The adsorption effect of microplastics on CIP was more strongly inhibited by norfloxacin (NOR) at lower concentrations (CNOR＜CCIP). Based on the adsorption data, FTIR, and XPS spectra, we demonstrated that the adsorption process was a non-spontaneous heat absorption reaction, with chemical and physical mechanisms including hydrogen bonding, π-π conjugation, ion exchange, and electrostatic interactions controlling it. This study's discovery of the mechanism of interaction between MPs and CIP serves as a foundation for assessing the environmental risk of MPs and antibiotic complex contamination.