Old fishing nets and household plastic could end up under Hawaii drivers’ tires.
Researchers in Hawaii are testing asphalt made with discarded fishing nets and recycled household plastic, and early results suggest those roads do not release more plastic particles than standard pavement.
Jeremy Axworthy, a researcher at the Center for Marine Debris Research at Hawaiʻi Pacific University, presented the findings at the spring meeting of the American Chemical Society.
“This work investigates whether it’s responsible to use recycled plastics in Hawaii’s roads,” shares Axworthy. “By reusing plastic waste that is already in Hawaii, we can reduce the environmental and economic impacts of transporting waste plastics from the islands, incinerating it or dumping it in Hawaii’s overflowing landfills.”
Hawaii has faced high recycling costs and ongoing marine debris washing ashore or remaining in nearby waters. Since 2020, most roads in Hawaii have been built using polymer-modified asphalt, or PMA, which is designed to improve strength and durability.
Compared with conventional asphalt, PMA is more flexible and better able to resist cracking, rutting and water damage, according to the American Chemical Society summary. In Hawaii, PMA is typically made by melting pellets of styrene-butadiene-styrene into a petroleum-based asphalt binder, then mixing that binder with heated rocks and sand.
Researchers wanted to test if some of that virgin polymer could be replaced with discarded plastics, and if roads made with recycled plastics would perform well and release microplastics or other chemicals into the environment. The Hawaii Department of Transportation partnered with environmental chemist Jennifer Lynch, director of the Center for Marine Debris Research, on the work.
HDOT asked Lynch’s team to supply abandoned fishing nets collected from Hawaii’s waters for use in experimental asphalt and to test if pavement made with recycled plastic released more microplastics than standard styrene-butadiene-styrene-modified asphalt.
“Foreign plastic derelict fishing gear is the largest contributor of Hawaii’s marine debris problem,” shares Lynch. “To date, CMDR’s Bounty Project, which pays a financial reward to licensed commercial fishers for marine debris removal, has removed 84 tons of large, derelict fishing gear from the Pacific Ocean.”
“CMDR’s laboratory is equipped with state-of-the-art chemical instrumentation for quantifying and characterizing microplastics in environmental samples,” explains Lynch. “This capability is incredibly unique and impactful, especially when coupled to our marine debris-removal project and our mission to recycle the debris into long-term, locally necessary infrastructure products.”
After a US company processed the recovered plastics into materials suitable for asphalt production, a local paving company resurfaced sections of a residential street on Oahu using three asphalt mixes. One used standard styrene-butadiene-styrene, one used recycled polyethylene from Honolulu’s residential recycling program, and one used polyethylene recovered from discarded fishing nets.
About 11 months later, Lynch’s team collected road dust from each section to measure any microplastic released into the surrounding environment.
The scientists separated polymers in the road dust, including microplastics, larger plastic fragments and tire rubber. They used pyrolysis gas chromatography-mass spectrometry to identify where the materials came from.
The analysis identified styrene and butadiene from standard polymer-modified asphalt, polyethylene from recycled plastic and fishing net pavements, and isoprene and butadiene rubber from vehicle tires.
Early findings showed pavement containing recycled polyethylene did not release more polymers than conventional styrene-butadiene-styrene pavement. The same pattern appeared in laboratory performance testing and in simulated stormwater collected from the experimental road sections.
Researchers detected microplastic-sized particles, but only a very small number were identified as polyethylene, regardless of pavement type. The researchers believe this is because the plastic becomes mixed into the asphalt binder, and particles that wear away are made up of rock, asphalt binder and polymer together rather than plastic alone.
The team is also comparing polymer release from the pavement with the amount of tire material found in road dust.
“In our initial Py-GC-MS data,” continues Lynch, “we saw tire wear swamps the signal of polyethylene by orders of magnitude, like gigantic peaks! We had to search the weeds of the chromatogram to find signs of polyethylene.”
More testing is still needed to evaluate how well the recycled plastic roads hold up over the long term. The research was funded by the Hawaii Department of Transportation.
“Some people think plastic recycling is a hoax, that it doesn’t work; it’s too challenging,” Lynch shares. “But this work demonstrates that recycling can work when society prioritizes sustainability.”
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