Modeling Releases of Polymer Additives from Microplastics into the Aqueous Environment

Abstract

Microplastics (MPs) are becoming an emerging problem due to increased consumption of plastic. Despite research on MPs acting as sinks of contaminants, the potential of leaching additives out of MPs has been given little attention. Given challenges like the slow release rates of additives and the variety of physical chemical properties of MPs, mathematical models are great tools to explore this problem. In this study, the internal controlled diffusion model was used to describe release behaviors of additives from MPs into aqueous environments. This model was then applied to study Bisphenol A (BPA) and 4-t-butylphenol (TBP) leaching from epoxy MPs. Simulations on the influence of properties of microplastics and various temperatures were completed. Calculated diffusion coefficients of BPA and TBP based on leaching experiments data range from 10-13.3 cm2/s to 10-14.3 cm2/s and 10-12.1 cm2/s to 10-12.7 cm2/s, respectively. Though at low release rates, the release process was accelerated significantly by smaller sizes and irregular shapes of MPs. With a particle radius reduced from 1mm to 100 nm, the half-life of BPA changes from 3000 years to several minutes, and from 30 years to several seconds for TBP. Also, temperature dependence of migration obeys the Arrhenius equation and activated energies for BPA and TBP are 48.9 kJ/mol and 27.0 kJ/mol, respectively. To sum up, simpler plastic structures, smaller sizes, rough surfaces, smaller additive molecules, and the higher temperature facilitate the release process. This model contributes to the risk assessment of additives releasing from MPs. Yet the real problem might be far more complex considering special properties of plastics materials and environmental conditions. Thus, more research is required for a deeper understanding of this problem.