Adsorption behavior of crude oil hydrocarbons on polyethylene microplastics in batch experiments

This study examines the impact of microplastics on the fate of spilled crude oil in water. Batch adsorption experiments were conducted using polyethylene microplastics ranging in size between 300 and 600 μm. Environmentally relevant concentrations of crude oil and microplastics were tested. Samples processing involved liquid-liquid extraction (LLE) followed by quantitative analysis using Gas-Chromatography coupled to Mass Spectrometry. Kinetic analyses employed the most commonly used models in microplastic adsorption studies, including the pseudo-first order, pseudo second-order, Elovich, and intra-particle diffusion models. Results mainly conformed to the Elovich model, followed by the pseudo-second order model, suggesting chemisorption. Isotherm evaluations involved the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models, selected for their effectiveness in describing the behavior of microplastics in adsorption studies. These models revealed diverse behaviors: alkanes from nC11-nC21 conformed to the Freundlich isotherm, suggesting multilayer adsorption. While nC10, nC27-nC29, nC33, and nC34 were best described by the Langmuir model, and nC22-nC26 and nC30-nC32 adhered to the Temkin model, both indicative of monolayer adsorption. Notably, nC35 adsorption was best described by the Dubinin-Radushkevich model. The different PAHs exhibited preferences for either the Freundlich or the Langmuir model. The maximum adsorption capacities of the contaminants onto polyethylene were 263.12 and 101.57 mg.g^-1 for the targeted alkanes and PAHs, respectively, corresponding to a maximum adsorption of 5.75 mg of targeted hydrocarbons per m² of polyethylene. The study highlighted the potential role of microplastics in influencing the environmental fate of selected crude oil hydrocarbons and provided insights into their interaction and partitioning behavior in water.