Roughly 98% of plastic waste in the ocean dissolves due to sunlight

Around 98% of all the plastic waste going into the ocean is unaccounted for. A new paper looks into where it winds up, and its effect on marine life.

It’s hard to overstate just how much plastic humanity has dumped into the ocean. Trillions of bits of plastic float into massive “garbage patches” along the subtropical gyres (rotating ocean currents). These patches have a dramatic impact on ocean life, ranging from the largest mammals to the humble bacteria.

And yet, these immense plastic patches only account for 1% to 2% of all the plastic going into the ocean. Which is quite a scary thought. One promising theory is that sunlight-driven chemical reactions break the materials down until they lose buoyancy, or become too small to be captured by researchers. However, direct, experimental evidence for the photochemical degradation of marine plastics remains rare.

Where’s the plastic?

“For the most photoreactive microplastics such as expanded polystyrene and polypropylene, sunlight may rapidly remove these polymers from ocean waters. Other, less photodegradable microplastics such as polyethylene, may take decades to centuries to degrade even if they remain at the sea surface,” said Shiye Zhao, Ph.D., senior author of the paper.

“In addition, as these plastics dissolve at sea, they release biologically active organic compounds, which are measured as total dissolved organic carbon, a major byproduct of sunlight-driven plastic photodegradation.”

The team, which included members from Florida Atlantic University’s Harbor Branch Oceanographic Institute, East China Normal University, and Northeastern University wanted to verify the theory. They selected polymers that are often seen in the garbage patches, and plastic-fragments collected from the surface waters of the North Pacific Gyre, and irradiated them for approximately two months using a solar simulator.

During this time, the team captured the kinetics of plastic degradation. To assess degradation levels, they used optical microscopy, electron microscopy, and Fourier transform infrared (FT-IR) spectroscopy.

All in all, the team reports, plastic dissolution led to an increase in carbon levels in their surrounding water and reduced particle size of the plastic samples. The irradiated plastics fragmented, oxidized, and changed in color. Recycled plastics, overall, degraded more rapidly than polymers such as polypropylene (e.g. consumer packaging) and polyethylene (e.g. plastic bags, plastic films, and containers including bottles), which were the most photo-resistant polymers studied.

Based on the findings, the team estimates that recycled plastics tended to degrade completely in 2.7 years and that plastics in the North Pacific Gyre degrade in 2.8 years. Polypropylene, polyethylene, and standard polyethylene (which see ample use in food packaging) degrade completely in 4.3, 33, and a whopping 49 years, respectively, the team estimates.

The compounds leaching out of the plastic as it degrades seem to be broadly biodegradeable, the team reports. While levels of plastic-sourced carbon in ocean water pale in comparison to natural marine-dissolved organic carbon, the team found that it can inhibit microbial activity. The carbon from degraded plastics was readily used by marine bacteria, the team adds.

“The potential that plastics are releasing bio-inhibitory compounds during photodegradation in the ocean could impact microbial community productivity and structure, with unknown consequences for the biogeochemistry and ecology of the ocean,” said Zhao.

“One of four polymers in our study had a negative effect on bacteria. More work is needed to determine whether the release of bioinhibitory compounds from photodegrading plastics is a common or rare phenomenon.”

Samples in the study included post-consumer microplastics from recycled plastics like a shampoo bottle and a disposable lunch box (polyethylene, polypropylene, and expanded polystyrene), as well as standard polyethylene.

The paper “Photochemical dissolution of buoyant microplastics to dissolved organic carbon: Rates and microbial impacts” has been published in the Journal of Hazardous Materials.

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