Does plastic last for thousands of years in the environment?
WHOI marine chemist Collin Ward weighs in
Estimated reading time: 3 minutes
Plastic pollution is a serious—and growing—environmental problem, with millions of tons of bags, bottles, fishing gear and more piling up on land and floating out to sea. But just how long does it last—and how can we innovate new materials to address the problem moving forward? WHOI scientists are investigating both issues.
It’s easy to think the sole problem with plastic lies in its long life. Some estimates put the lifespan of some kinds of plastic as high as 1,000 years. But until recently, we didn’t really know how long plastic persists. Now that scientists have begun to investigate, they’re discovering plastic doesn’t last for millennia, but it also doesn’t break down fast enough, and we need to innovate new materials that cause less harm.
To be clear: there is no single “plastic” at the root of the issue. “Plastics are not numbers 1 through 7,” says WHOI marine chemist Collin Ward. Rather, they’re a complex mixture of base polymers and additives. Take the base polymer polyethylene, for example, the most widely used plastic on the planet. “There are hundreds of different types of polyethylene, and thousands of different chemicals are added to polyethylene to try to increase performance or increase the way it can be processed and shaped or molded into the goods we use,” Ward explains.
Additives might make a plastic stronger or more malleable, they might make it heat or fire resistant, or simply make it easier to work with during production. Those additives can change how easily a particular piece of plastic breaks down in the environment. And plastics do break down, recent research shows, through three main pathways.
Mechanical breakdown, or disintegration, happens when plastic is blown or pushed against other objects and eventually breaks into pieces. It’s “almost like scissors, where you're just chopping it into smaller pieces, but there's no change in its chemistry,” Ward says. It’s “a concern because that's how you end up with microplastics and nanoplastics.” These ultra-small pieces have spread to every part of the globe and even been found inside people.
The other two processes degrade the plastic by altering its chemistry. Photochemical breakdown happens when plastic is exposed to sunlight, which breaks the chemical bonds inside the plastic, releasing smaller, modified molecules. Breakdown can be complete, ultimately releasing carbon dioxide into the atmosphere, or it can be partial, releasing water-soluble molecules into the ocean or other bodies of water.
Biological breakdown occurs when microbes consume plastic. Some microbes can snack on certain plastics—but not the most common types. For those, microbes can only get to work after sunlight has begun the degradation process. Once photochemical degradation releases water-soluble molecules, microbes are quick to consume them. When the microbes later respire the plastic, they release carbon dioxide.
“Only biological and photochemical [pathways] result in the net production of carbon dioxide,” Ward says. “So those are the only ways by which you could remove plastic from the system.”
Given this new knowledge, how quickly are plastics breaking down in the ocean? It depends on the plastic and where it winds up. Floating plastics are exposed to sunlight and begin to degrade as they travel to the open ocean. Some degrade in months to years, rather than decades or longer, and the process happens faster when sunlight and microbes work together.
Dense plastics, on the other hand, sink to the seafloor. Without sunlight to break chemical bonds, these plastics are exposed only to mechanical breakdown. Since the bottom of the ocean lacks wave action, only a limited amount of disintegration can take place. “My guess is those plastics probably last a really long time. They’re probably going to get buried in the bottom of the ocean and eventually become part of the geologic carbon cycle,” Ward says.
It’s encouraging to know plastics can and do break down, but that doesn’t mean they aren’t a persistent problem. We don’t yet understand the impact microplastics are having on natural systems. And even if plastics are breaking down, they are doing so far too slowly.
“It's important for us to start thinking about designing new materials that degrade quickly if they leak into the environment,” Ward says. He and other WHOI researchers are doing just that by working with industrial partners to innovate new materials that break down in a matter of months, rather than years or decades.
