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Investigating if Sunlight Degrades Microplastics

COS/CEE Associate Professor Aron Stubbins, in collaboration with Kara Lavender Law from the Sea Education Association (SEA), is leading a $420K NSF grant for “The role of sunlight in determining the fate and microbial impact of microplastics in surface waters”.


Source: NSF

Microplastics, plastic particles smaller than 5 mm in size that mostly originate from fragmentation of larger plastic objects, are now found in nearly all natural and managed waters, from drinking water to rivers, lakes and streams, and the oceans. An estimated 8 million tonnes of plastic waste enter the oceans from land each year, yet only a fraction of this material is accounted for by floating microplastics. One hypothesis for the “missing plastics” is that exposure to sunlight dissolves them. We will test whether sunlight-driven photochemistry releases dissolved organic carbon (DOC) from plastics, essentially removing the plastics from the water. We will also test whether the chemicals released as plastics and sunlight interact affect bacterial growth and survivability in natural waters. Results will improve basic understanding of the fate of floating plastics in natural waters, the impacts of microplastics upon microorganisms, and inform decisions about how to prioritize plastic waste management.

The oceans, other natural, and managed waters are contaminated with plastic fragments below 5 mm in size. These microplastics have been studied most in the ocean, where more than 98% of all inputs of buoyant microplastic go missing each year. Our preliminary work suggests sunlight is responsible for removing floating microplastics on relatively short time scales (years). We will test this hypothesis through a series of field, experimental and analytical studies. Our preliminary work also shows plastics to release dissolved organic carbon (DOC) as they photo-degrade with mixed impacts on microbes – from stimulating to inhibiting microbial growth. Therefore, bioassays with natural marine and freshwater microbes will assess how plastic-derived DOC impacts microbes in natural waters. This project will produce data for the size, chemistry and geometry of plastics afloat in the Atlantic Ocean. Experiments will assess the degradation kinetics (dissolution, fragmentation, and oxidation) of standard and ocean collected plastics in seawater and freshwater in the light and dark, with and without microbes. Rates and exponents of plastic decay will reveal how quickly sunlight and/or bacteria can breakdown plastics in surface waters, and the amount of DOC released as microplastics decay. Experiments with plastics of variable polymer chemistry, size and previous oxidation history will reveal the role of these factors in explaining variability in degradation rates and allow these factors to be included in future modeling studies of plastic loss. Fundamental knowledge about the process of plastic degradation, including oxidation, fragmentation to nanoparticles, and release of DOC, will be attained by applying a suite of analytical techniques to samples from the experiments. Bioassays will further explore the potential for the DOC released from plastics to impact microbes and the combined role of bacteria and sunlight in degrading microplastics. The project brings together scientists with complementary educational, field, experimental and analytical skills. We will transform understanding of the fate and impact of buoyant plastics in surface waters, communicate this learning to students and to colleagues in the broader field, and prepare young scientists to carry the field forward. The project will incorporate the education, training, and development of a postdoctoral research (PD) and an undergraduate (UG). The UG will partake in a ~1 year-long immersive field and laboratory research opportunity. Results will be disseminated to other scientists, the press, and incorporated into high school classroom learning via publication in the Science Journal for Kids, an open access journal which adapts primary, peer-reviewed research papers with age-appropriate language and illustrations geared towards middle and high school students.


Source: News @ Northeastern

Nearly eight million tons of plastic is missing from our oceans. Where did it go?

Aron Stubbins, associate professor of marine and environmental sciences, uses a solar simulator to inspect microplastics from the open ocean in the Mugar Life Sciences Building. Stubbins received a grant from the National Sciences Foundation to study the effect of sunlight on microplastics. Photo by Ruby Wallau/Northeastern University

by Kerry Benson – contributor, August 7, 2019

Northeastern professor Aron Stubbins has a mystery to solve: We seem to have misplaced nearly eight million metric tons of plastic.

That’s the estimated amount of plastic waste spilling into the world’s oceans each year, yet more than 98 percent of it is unaccounted for. How could it simply vanish? Stubbins has a culprit in mind: sunlight.

Over time, says Stubbins, the sun’s powerful ultraviolet rays strike large pieces of plastic floating in the ocean and break them into fragments about the size of a sesame seed or smaller, which scientists call microplastics. As sunlight continues to beat down on these microplastics, it degrades them into dissolved carbon. Eventually, they dissolve completely, and presto! No more plastic.

Together with doctoral student Lixin Zhu, Stubbins studied this phenomenon by floating microplastics in seawater under artificial ultraviolet rays produced by a “sun in a box,” a device that mimics sunlight. Sure enough, a couple months’ exposure to the light increased the amount of dissolved carbon in the water and made those tiny plastic particles even tinier.

Stubbins studied the phenomenon of vanishing plastic waste by floating microplastics in seawater under artificial ultraviolet rays produced by a “sun in a box,” a device that mimics sunlight. Photos by Ruby Wallau/Northeastern University

“This is good news,” says Stubbins, who was recently awarded a grant from the National Science Foundation to study the problem of the disappearing plastic, “but it should be tempered. Sunlight’s not going to solve all our problems.”

One issue is that the sun isn’t as effective at degrading certain types of plastics. For instance, while a five-millimeter piece of Styrofoam would disappear after just a few months of exposure, polyethylene—one of our most abundant plastics—would take “decades to centuries” to break down.

He also points out that only plastics that float will dissolve in this manner.

“Other [plastics] will just sink, and if they get buried in sediments—either at the bottom of the oceans or in soils, like plastics we put in landfills—they will probably stay as big plastics for much longer,” he says. “They’re much slower to degrade in the dark, so it might be thousands of years before they become microplastics. If plastic ends up in an environment that’s very stable, it essentially lasts forever.”

And although Stubbins’ experiments suggest that sunlight could be the main remover of the plastics in our oceans, the case isn’t closed yet: Uncertainties could partly explain the missing waste, too. Some of it may be washing up in remote, under-sampled areas, for example—and perhaps eight million tons is an overestimate.

Regardless, Stubbins says, “the better we know the [degradation] processes involved, the more we can understand whether those plastics are likely to build up in the open ocean, on beaches, and in places where they may have cosmetic, human health, or environmental health impacts.”

He’s also investigating how microplastics affect the ocean’s microbe population. Preliminary evidence suggests that some types of microplastic waste promote the growth of bacteria, while other microplastics inhibit that growth, but we still need to work out the details.

For that matter, we aren’t even sure how microplastics directly affect humans.

“They’re finding plastics everywhere,” Stubbins says. “You’re almost certainly breathing in microplastics as we speak, because these small fibers are in the air, too. There’s no strong evidence that they’re bad for us yet, but they’re certainly out there, and we don’t know if they’re having a negative effect in most systems.”

Ultimately, he plans to share his findings with schoolchildren to inspire future generations to think critically about the world around them.

“Hopefully we make the right choices for them now so that the future they inherit is good,” he says—but someday, those choices will be theirs to make.

Stubbins thinks back to his own childhood, as he gazed with wonder into the rivers surrounding his home in the Welsh countryside. At first, he thought only about the creatures he could see. But in time, his focus shifted to the invisible influences that shape the natural world: to the single-celled plants that float in our oceans and the legions of microbes that swim alongside them—and now, to vanishing plastic and ultraviolet rays.

Over the years, he has learned that just because something is out of sight, it shouldn’t be kept out of mind.

“The impact of the processes we can’t see,” he says, “is often greater than the impact of what we can see.”

Related Departments:Civil & Environmental Engineering