Development of a Method to Identify and Quantify Microplastics Generated from the Roadway Environment

About the report

Microplastics (MPs) are increasingly recognized as an emerging environmental contaminant originating from the fragmentation of larger plastic items. MPs have been identified in roadway environments related to conventional sources (e.g., particles from tires and brakes, road markings, erosion control netting, and litter) and emerging sources such as recycled plastic-modified asphalt pavements. Still, significant knowledge gaps persist regarding consistent analytical methods and the relative contributions of these diverse sources. This study developed a method to identify these sources of MPs and quantify their concentrations and then demonstrated this method at a field site.

Based on a review of academic literature and technical documents, a three-step method was created that incorporates hydrogen peroxide digestion, zinc chloride density separation with centrifugation, and thermogravimetric analysis-Fourier transform infrared (TGA‑FTIR) spectroscopy. Testing in the laboratory with polymer‑spiked sediment showed high recovery efficiency (≈ 99.9%) in extraction and reliably identified polyethylene based on characteristic thermal and spectral signatures. The method was then applied to field‑collected stormwater and sediment samples from a rural roadway site in Palmyra, Virginia. TGA-FTIR evaluation of field samples indicated that eight of the 12 samples contained MPs associated with acrylic road markings and tire wear particles. One sample was found to contain traces of degraded erosion control netting. MP concentrations in these samples ranged from 0.135 mg/g to 1.75 mg/g, with acrylic road markings and tire wear particles being the predominant type. In this regard, this method is reproducible and thus may characterize MPs in diverse roadway environments.

The study also showed that one limitation of the method is that it cannot identify MPs in samples containing an unknown mixture of polymer types. This limitation is due to some polymers having overlapping thermal degradation curves and overlapping infrared spectra. Therefore, the TGA-FTIR evaluation cannot distinguish which types of polymers are necessarily present in a given sample. A modification is proposed that replaces or complements the TGA-FTIR evaluation with pyrolysis-gas chromatography-mass spectrometry.

The Virginia Department of Transportation can thus use this method, or a proposed enhancement thereof, to evaluate existing maintenance practices should regulatory conditions necessitate such work. This method may also inform the development of mitigation strategies for MPs required to comply with potential future regulatory requirements.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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