Hawaii is paving roads with abandoned fishing nets - and the microplastic risk appears minimal

American Chemical Society

Eighty-four tons. That is how much derelict fishing gear the Center for Marine Debris Research at Hawaii Pacific University has pulled from the Pacific Ocean through its Bounty Project, which pays commercial fishers a financial reward for every load of abandoned nets and lines they recover. The gear is mostly high-density polyethylene - the same plastic used in milk jugs and recycling bins. It is durable, it does not biodegrade, and once it is out of the ocean, it needs somewhere to go.

For an island state with overflowing landfills, limited recycling infrastructure, and the highest shipping costs in the nation, the question of what to do with recovered marine plastic is not academic. Shipping it to the mainland for recycling costs money Hawaii does not want to spend. Incinerating it produces emissions. Burying it fills landfill space that is running out.

So the Hawaii Department of Transportation asked an unusual question: what if we put it in our roads?

Melting plastic into the binder

Since 2020, most of Hawaii's roads have been paved with polymer-modified asphalt, or PMA. Standard PMA uses pellets of styrene-butadiene-styrene (SBS), a virgin copolymer, melted into the petroleum-based asphalt binder before it is mixed with rock aggregate. The polymer makes the pavement more elastic and resistant to cracking, rutting, and water damage - qualities that matter in a tropical climate where heat and rain take turns punishing road surfaces.

The concept behind the Hawaii experiment is straightforward: replace some or all of the virgin SBS polymer with recycled high-density polyethylene from two sources - residential recycling containers collected in Honolulu and derelict fishing nets recovered from the ocean. A US-based company converted the waste plastic into a form compatible with asphalt production, and a local paving company laid down test sections on a residential road on the island of Oahu.

The resulting road looks no different from any other. But underneath the surface, the binder contains polymers that were once clogging the ocean or sitting in a recycling bin waiting for a destination that might never come.

Tracking what the pavement sheds

The environmental concern with putting plastic in roads is obvious: does it just create a new source of microplastic pollution? Every road surface gradually wears down under traffic, weather, and time. If recycled-plastic asphalt sheds more microplastic particles than conventional asphalt, the environmental calculation could flip from beneficial to harmful.

Jennifer Lynch, director of the Center for Marine Debris Research, and researcher Jeremy Axworthy designed a testing program to answer this question. After approximately 11 months of regular traffic use, they collected road dust samples from each section of pavement - the control sections made with standard SBS, and the experimental sections made with recycled polyethylene from both waste streams.

The analysis used pyrolysis gas chromatography-mass spectrometry (Py-GC-MS), a technique that heats samples to extreme temperatures, breaks polymers into characteristic fragments, and identifies them by molecular weight. This allowed the researchers to distinguish styrene and butadiene from the standard PMA, polyethylene from the recycled-plastic sections, and isoprene and butadiene rubber from tire wear - all present in the same road dust.

Tire rubber swamps the plastic signal

The initial results were encouraging on two fronts. First, pavements made with recycled polyethylene did not release more polymers than the control pavement made with virgin SBS. The microplastic shedding rates were comparable across all pavement types, both in mechanical performance tests conducted in the laboratory and in simulated stormwater collected from the road sections.

Second, very few of the microplastic-sized particles identified in road dust were actually polyethylene, regardless of which pavement type they came from. This likely reflects the manufacturing process: when polyethylene is melted into asphalt binder at high temperatures, the polymer chains become integrated into the binder matrix. Particles that break off the road surface are not naked plastic - they are composites of rock, binder, and melted polymer, chemically different from the free-floating microplastics that concern marine biologists.

The most striking finding, though, involved tires. When Lynch's team compared the polymer contribution from pavement wear to the polymer contribution from tire wear in the same road dust samples, the tire signal was orders of magnitude larger. Lynch described having to search through the noise of the chromatogram to find signs of polyethylene, while tire-derived polymers produced massive peaks.

This finding has a broader implication beyond the Hawaii experiment. If tire wear is the dominant source of road-related polymer pollution - and other studies have reached similar conclusions - then the environmental impact of adding recycled plastic to asphalt may be negligible compared to the polymer load that vehicles deposit on roads regardless of what those roads are made from.

Durability remains the open question

The microplastic results are promising but address only half of the practical question. The other half is whether recycled-plastic asphalt holds up as well as conventional PMA over the years of use that a road surface must endure. Eleven months of traffic on a residential road is a start, but roads are expected to last a decade or more. Pavement engineers need to know whether recycled-polyethylene binder maintains its elasticity, resists cracking, and handles the thermal cycling of Hawaiian weather as well as virgin SBS.

Performance testing is ongoing but not yet conclusive. If recycled-plastic pavement degrades faster than conventional PMA, the economic and environmental benefits of using waste plastic could be offset by more frequent repaving - which itself consumes energy, materials, and money.

The chemical composition of ocean-recovered fishing nets is also more variable than that of controlled waste streams like residential recycling. Nets spend years exposed to UV radiation, saltwater, and biological fouling, which can alter the polymer's properties. Whether this weathering affects asphalt performance in ways that fresh polyethylene does not is still under investigation.

A local solution to a local problem

The Hawaii experiment is not a prescription for solving the global plastic crisis. It is a geographically specific response to a geographically specific problem. Hawaii's isolation means that waste disposal options available to mainland communities - regional recycling facilities, extensive landfill networks, rail transport to processing plants - simply do not exist at the same scale or cost.

Recycled-plastic asphalt offers an end-of-life fate for waste that otherwise has no good destination on the islands. If the durability data hold up and the microplastic risks remain negligible, it could become a standard practice for Hawaiian road construction - absorbing some fraction of the state's plastic waste into infrastructure that would be built anyway.

Whether the approach makes sense elsewhere depends on local economics and waste management realities. On the mainland, where recycling infrastructure is more developed and waste transport costs are lower, the calculus may favor conventional recycling over road incorporation. But for island communities, remote regions, and coastal areas grappling with marine debris, the Hawaii model offers a proof of concept worth watching.

Lynch put the broader significance in perspective: recycling can work when society prioritizes sustainability. The fishing nets that spent years tangling in Pacific currents are now part of a road that people drive on every day. That journey from ocean debris to infrastructure is not a complete solution, but it is a real one.