Why plastic lingers: Water chemistry slows nature’s cleanup

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Why plastic lingers: Water chemistry slows nature's cleanup

Natural water chemistry shields plastics from sunlight and microbes

• In lab settings, ultraviolet light deteriorates plastic so microbes can break it down further
• But, in nature, plastic often persists in waterways for decades despite direct sunlight
• While lab experiments often use purified water or unrealistic water chemistry, engineers found salts and other chemical constituents in natural waters suppress sunlight-driven degradation
• Organic matter further slows down plastic degradation in freshwater and seawater

EVANSTON, Ill. --- Scientists have long known that sunlight helps break down plastic. So, why do plastic products linger for decades and even centuries in rivers, lakes and oceans - even when bathed in direct sunlight?

Northwestern University engineers have uncovered an unexpected answer. The surprising culprit is the water itself.

In a new study designed to mimic real environmental conditions, researchers found that the chemical makeup of natural waters - especially combinations of salt and organic matter - significantly delays the breakdown of polystyrene, a common plastic used in packaging and food containers.

Because sunlight cannot effectively initiate the degradation process, microbes cannot finish the job. That means nature's cleanup process slows down, allowing plastics to accumulate and persist in waterways around the world.

The findings show that solving plastic pollution isn't only about the material itself but also about the environment it enters. These insights could be used to design new types of plastic that degrade even in salty, complex environments or that don't rely on sunlight to jump-start the breakdown process.

The study was published today (June 10) in Materials Degradation, a Nature Portfolio journal.

"In the scientific literature, it's widely reported that ultraviolet light can facilitate significant breakdown of plastics in a process called photodegradation," said Northwestern's Ludmilla Aristilde, who led the study. "If that's the case, then why is plastic debris so slow to degrade in our surface waters? It turns out a lot of lab experiments use pure water or water that does not represent natural water chemistry and artificial light that does not reflect full spectrum of solar radiation. We wondered whether those studies were missing part of the story by overlooking the complexity of natural environments."

An expert in the dynamics of organics in environmental processes, Aristilde is a professor of civil and environmental engineering at Northwestern's McCormick School of Engineering and a member of the Center for Synthetic Biology, the International Institute for Nanotechnology and the Paula M. Trienens Institute for Sustainability and Energy. This work was led by postdoctoral researcher Nasrin Naderi Beni and doctoral student Cara Flynn, both members of the Aristilde Research Group.

Recreating nature in the lab

To find the missing piece to the puzzle, Aristilde and her team created multiple realistic scenarios in the lab. For ocean-like water, the researchers added salt and other dissolved ions like chloride, bromide, bicarbonate and sulfate. To mimic freshwater, the team created a solution with lower salt levels and a different mix of dissolved ions found in lakes and rivers. In some experiments, the team also added organic matter - similar to what's found in natural rivers - from decaying plants and microbial material.

For comparison and to reflect past studies, the researchers also set up an experiment with purified water that contained none of these ingredients. Then, they added thin strips of polystyrene plastics to each mixture and exposed them to simulated, full-spectrum sunlight for roughly three months.

Across all conditions, sunlight triggered the early stages of degradation. The surfaces of the plastic became rougher, cracked and chemically altered. But the extent of that damage varied dramatically depending on the water. Plastic degraded the most in purified water, less in freshwater and the least in seawater.

"We used multiple techniques to examine the plastic surfaces at the microscopic and molecular levels," Aristilde said. "For the samples in the pure water, we saw really obvious 'mountains and valleys' emerging on the plastic surface after exposure to continuous sunlight simulation. We captured evidence of the polymer breaking down, new surface chemistry occurring from oxidation of plastics, and release of products into the water."

Competing for sunlight

When researchers added natural organic matter, it further suppressed degradation in solutions representing both freshwater and seawater chemistries. In other words, the more realistic the water became, the less the plastic broke down. Aristilde concluded that, although sunlight triggers reactive chemistry in plastics, salts dampen that effect. Meanwhile, the combined effect of organic matter and salts can block sunlight or neutralize reactive molecules.

"We think that, in pure water, sunlight goes through the water and directly to the plastic," Aristilde said. "But when you have water with dissolved ions and organic matter floating around, the sunlight reacts with those components. So, those components compete with the plastics for the reactions driven by sunlight."

While sunlight doesn't fully degrade plastic materials, it can prepare them for further degradation by microbes. As polystyrene breaks down, it releases fragments and small molecules that microbes can use as food, and rougher surfaces give bacteria a foothold. After examining the plastic's reaction to sunlight alone, Aristilde's team introduced an environmental bacterium, known to degrade plastics, to each water sample.

The team found that plastic exposed to sunlight in freshwater solution showed more microbial breakdown than the plastic exposed in seawater solution. So, because seawater stifled initial damage from sunlight, microbes have less to work with.

"The sun does the first step of the degradation process to help the microbes chew up the plastic," Aristilde said. "By changing the chemistry of plastic surface, it becomes more attractive to bacteria."

Implications for plastic pollution

The study not only provides an explanation for why plastics remain in the environment for so long, but it also highlights a broader challenge. Solutions that appear promising under simplified lab conditions may not perform the same way in complex natural systems. Understanding true, real-world constraints, Aristilde said, is essential for predicting what will happen to plastic materials, designing degradable versions of plastic materials and developing effective strategies to address plastic pollution.

"Our first motivation was to understand natural processes that we observe outside the lab," Aristilde said. "But we're also motivated to find new processes that can inform engineering solutions. This new information could help engineers design plastics with characteristics that make them biodegradable eventually. For example, if we can make plastic more susceptible to sunlight, it might make it easier for microbes to take care of the rest."

The study, titled "Polystyrene photooxidation in natural waters as a precursor to microbial degradation," was funded by the National Science Foundation.

Study graphic

Please credit graphic to Ludmilla Aristilde/Northwestern University

Ludmilla Aristilde

Corresponding author

Professor of civil and environmental engineering

• [email protected]