The adsorption and desorption behaviors of phenanthrene and pyrene onto microplastics in the aquatic environment and digestive fluids

The adsorption and desorption behaviors of Phenanthrene and Pyrene onto polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly (methyl methacrylate) (PMMA) microplastics in the simulated freshwater and digestive fluids (simulated gastic fluid (SGF), simulated intestinal fluid (SIF)) were studied. The second-order kinetic model described the sorption kinetics of polycyclic aromatic hydrocarbons (PAHs), and the Langmuir model fitted the sorption isotherms of PAHs better, which indicated the surface sorption as the main mechanism. The sorption capacity was in the order of PMMA > PS > PE > PP, which may due to the specific surface area and particle size of microplastics. Various environmental conditions were considered, including pH, salinity and dissolved organic matters (DOM). Although higher salinity would bring negative effects on the adsorption of PAHs onto microplastics, the adsorption behaviors were not significantly influenced by pH and DOM. The desorption rate of PAHs from microplastics in SGF was lower than that in the SIF, with the desorption equilibrium time of 12 h in SIF. For the desorption in simulated freshwater, a secondary adsorption occurred around 3 h. Our work for the first time comparing the desorption behaviors between freshwater and two kinds of digestive fluids, indicating the negative effects of microplastics bring to aquatic organisms through digestive tracts. Further studies are needed to quantify the toxicity threshold of aquatic organisms through ingestion of microplastics combined with persistent organic pollutions.