Adsorption of methyl parathion on four various microplastics in aqueous solution: Kinetics, isotherms and molecular dynamics simulations

Abstract

Microplastics (MPs) not only have biotoxicity but also affect the migration and bioavailability of many pollutants as their carriers, which pose potential ecological threats. This work mainly focused on the adsorption kinetics, adsorption isotherms, influencing factors (pH, temperature, salinity, particle size, and humic acid), and desorption (in four simulated environments) of an organophosphorus pesticide methyl parathion on four dominant MPs. In addition, MD simulations were used to analyze the adsorption progress of methyl parathion on MPs at a molecular level. The results showed that the methyl parathion reached equilibrium within 48h and the absorption capacity of various MPs for methyl parathion was as follows: polypropylene (0.8357 mg/g) > polyvinyl chloride (0.4285 mg/g) > polystyrene (0.2681 mg/g) > polyethylene (0.2119 mg/g). The adsorption data were described well by the pseudo-second-order kinetics model and Redlich-Petersen isotherms model, respectively, indicating a combination of monolayer and multilayer adsorption. The methyl parathion sorption onto MPs decreased with pH, due to the interaction between MPs and methyl parathion by effecting the surface charge effects of them. The adsorption capacity of methyl parathion on various MPs was also inversely proportional to other environmental factors such as temperature, salinity, and humic acid (HA). Moreover, smaller particle size was beneficial for the adsorption of MPs due to its large specific surface area. The desorption results showed Methyl parathion would desorb from the loaded MPs in all simulated environments such as intestinal juice, simulated intestinal fluid, artificial seawater, and artificial fresh water. Notably, the desorption was most pronounced in artificial freshwater, especially by PP with a desorption rate of 52.48 %. The results of the MD simulation effectively exhibited the adsorption process and characteristics of MPs at different temperatures, particle sizes, and types of MPs. Combine MD simulation, FTIR spectra, and the enthalpy analysis, indicated the adsorption processes occur on the surface of MPs and physical adsorption was the main mechanism. The results of this study may facilitate a better understanding of the MPs as sources and carriers for organic contaminants in various environments.