Adsorption of PFAS onto secondary microplastics: A mechanistic study

Microplastics (MPs) are abundant in aquatic systems. The ecological risks of MPs may arise from their physical features, chemical properties, and/or their ability to concentrate and transport other contaminants, such as per- and polyfluoroalkyl substances (PFAS). PFAS have been extracted from MPs found in natural waters. Still, there needs to be a mechanistic investigation of the effect of PFAS chemistry and water physicochemical properties on how PFAS partition onto secondary MPs. Here, we studied the influence of pH, natural organic matter (NOM), ionic strength, and temperature on the adsorption of PFAS on MPs generated from PET water bottles. The adsorption of PFAS to the MPs was thermodynamically spontaneous at 25°C, based on Gibb’s free energy (ΔG ≤ -16 to -23 kJ/mol), primarily due to increased entropy after adsorption. Adsorption reached equilibrium within 7-9h. Hence, PFAS will partition to the surface of secondary PET MPs within hours in fresh and saline waters. Natural organic matter decreased the capacity of secondary PET MPs for PFAS through electrosteric repulsion, while higher ionic strength favored PFAS adsorption by decreasing electrostatic repulsion. Increased pH increased electrostatic repulsion, which negated PFAS adsorption. The study provides fundamental information for developing models to predict interactions between MPs and PFAS. Environmental Implication Microplastics (MPs) are abundant in natural and engineered aquatic systems. They have been reported to cause adverse effects to organisms. Similarly, PFAS are ubiquitous in the environment and are potentially toxic. In this study, we investigated interactions between both classes of hazardous materials using commonly detected representatives. Attractive interaction can lead to elevated exposure of organisms to PFAS if they ingest MPs. Despite being an endothermic process, the adsorption of the studied PFAS to MPs is thermodynamically favorable at ambient conditions due to increased entropy (randomness). It also occurs fast, which makes it environmentally relevant.