Assessment of microplastics production from waste plastics-modified asphalt pavement

Post-consumer recycled plastics (PCRP) have been increasingly utilized in asphalt pavement applications as the world moves towards a circular economy. However, it is unknown whether microplastics (MPs) could be generated in significant amounts during the subsequent pavement life cycle. This study developed methods to detect and quantify the potential microplastic generation from asphalt pavement materials containing PCRP (specifically, linear low-density polyethylene) using the Hamburg wheel-tracking test and a water permeability test. MPs were quantified by Nile Red staining followed by fluorescence microscope imaging. Environmental scanning electron microscopy (ESEM) and energy dispersive spectroscopy (EDS) were further conducted to characterize the nanoscale morphology of the embedded PCRP. The results showed that the generated MPs could reside at water/air interface, in bulk water, or with asphalt binder and aggregates in the sediment; and their amounts varied depending on the polyethylene dosage and type. The potential environmental impact of MP production from PCRP-modified asphalt pavement needs to be examined under the context of MP particle re-leases from other emission sources, notably, the tire wear particle generation when rubber tires meet the road. Our assessment indicated that while there was a potential of MPs release from the PCRP-modified asphalt, the up-scaled level of their release from a pavement was estimated to be about three orders of magnitude less than the tire wear rubber particles.