We know that plastic waste is overwhelming the ocean, sea life is dying from ingesting it and some even ends up in seafood. But scientists also now worry that plastic trash is coming with a side helping of toxic metals that latch onto plastic surfaces and enter the marine environment and food chain—and, eventually, what people eat.
Metals, such as cadmium and lead, are often used in manufacturing plastic and, over time, can enter coastal waters. Once floating in the ocean or discarded on a beach and washed by the tides, plastics can also attract and concentrate a variety of metals already present in the environment that attach themselves, or "sorb," to the surface. In both cases, the worry is that these metals—often toxic ones such as cadmium that are health concerns for both wildlife and humans—can contaminate waters or harm wildlife that ingest plastics, especially those that live in intertidal zones near sources of plastic pollution.
Researchers, however, are only just starting to understand how metal-tainted plastics interact in coastal environments, said Leah Bendell, professor of marine ecology and ecotoxicology at Simon Fraser University in British Columbia.
Bendell led one recent study, published in February in the online journal PLoS One, that examined how four metals—cadmium, lead, zinc, and copper—both attach onto and are released from plastics found on Canada's beaches. She said her results show how a whole host of metals can enter the marine food chain or coastal waters.
"Not only were these plastics serving as a way of metal getting into these lower trophic levels, but also they were a source of the metal into the water column and they can be acutely toxic," Bendell said. "It was a little bit of an eye-opener to the multifaceted role the plastics played."
For the study, Bendell's graduate student, Bertrand Munier, picked up every bit of plastic waste from transects on nine Vancouver-area beaches, gathering 144 unique plastic items, mostly food packaging and takeout containers. They sorted the plastics into 11 types and then used a weak acid to extract and separate the four metals—this kind of analysis is often used to estimate levels of toxins that could enter the tissues of wildlife if eaten. As a point of comparison, they also did the same for newly manufactured plastic samples. The goal was to distinguish metals that came from the plastic itself and those that had sorbed to the surface of the beach debris from the environment.
Of the collected items, five samples released what the study said were "extreme" high levels of metals—including a plastic tampon applicator tested for high levels of zinc—and all had at least trace amounts of the four metals tested. Different kinds of plastic also released different levels of metals. For example, PVC, the most commonly found plastic, had high levels of lead and copper attached to its surface. The comparison of the new and debris plastic also showed how some of the chemicals used in plastic production may release over time—including cadmium, which is used to make plastic rigid and resistant to UV light. The researchers found that new PVC releases zinc and cadmium.
A previous study examining metals sorbing onto plastics has found that the age of the material also matters. Chelsea Rochman, an assistant professor at the University of Toronto's department of ecology and evolutionary biology, led a study when she was at San Diego State University in which her team dropped mesh bags of various kinds of plastic pellets into three areas around San Diego Bay in California. They measured how much aluminum, chromium, manganese, iron, cobalt, nickel, zinc, cadmium, and lead from the environment sorbed onto their samples.
The year-long study, also published in PLoS One, found that metal levels increased the longer the plastic samples were in the water. That's probably because surface area increases as the plastics degrade over time and biofilms form, Rochman said.
Biofilms are collections of unattached microorganisms that put down roots on surfaces and can act as a surface for metals to latch onto. Fungi are a type of biofilm, as are bacteria. "Basically, over time there's more space for these metals to bind to," Rochman said.
There's still a lot scientists don't know. For example, it's unclear how big a role biofilms play in the concentration of metals on plastics and the ultimate effects of the metals on wildlife that ingest plastics. It's possible, for example, they may digest the biofilm, metals, or chemicals—even if they ultimately expel the plastic itself. "If the metals are bound on the biofilm, the question is, Are they even more bioavailable than we think?" Rochman asked.
The presence of a toxic metals-saturated biofilm on plastics could be both an ecological and human health problem, Bendell said. The bacterial growth on the biofilm could potentially pick up pathogens in and around coastal areas. And as these plastics break down into smaller and smaller pieces, they're more easily ingested by marine life, and now it looks like they're bringing dangerous metals along for the ride. While the studies were conducted in North America, the environmental risks may be far greater in regions like Southeast Asia that lack waste management infrastructure and where more plastic pollution makes its way to the coast.
The actual risk of metals associated with plastics to human health is unknown, Bendell said. But as plastic pollution grows, it's concerning to scientists like Bendell. "We need to change from thinking everything can be thrown away to you are accountable and responsible for every piece of plastic that comes into your house," she said.