Many biodegradable plastics fully break down in water environments — though it can take decades

May 29, 2021

Miriam Weber of HYDRA Marine Sciences inspects an experiment that tested the underwater deterioration of biodegradable plastics. (HYDRA/Christian Lott)

The most comprehensive analysis of biodegradable plastics yet found that they fully break down in a wide range of marine environments at anywhere between roughly six months and 20 years, confirming that the plastics perform as intended but also demonstrating a wide difference in decomposition speeds.

The research, published May 6 in Frontiers of Marine Science, supports the premise that biodegradable plastics could help ease the ongoing crisis of plastic pollution in nature by replacing conventional plastics, which can take between centuries and millennia to fully decompose.

Concern has recently emerged over whether some "biodegradable" plastics actually last, and some materials have been observed leaving behind microplastic particles after partially breaking down. The new study shows that at least some biodegradable plastics work as intended in marine environments by completely decomposing.

"So about after five years, about after 10 years in this scenario, we are rid of it," said the paper's first author Christian Lott, referring to the longest breakdown times of one of the tested biodegradable plastics. "Whereas, the conventional plastic keeps accumulating, accumulating, accumulating, accumulating, and it will be the geologic record of humankind."

These eventually disappearing plastics, however, still persist long enough to do damage in natural ecosystems, warned the scientists behind the analysis, who said the resilience of these materials "underpins the urgency" to cease further plastic pollution in nature.

More than 8.3 billion metric tons (18.3 trillion pounds) of plastic have entered the environment in the last six decades, and harm from plastic exposure and consumption has been shown to negatively impact animals, ecosystems and possibly even humans.

Biodegradable plastics have emerged as an environmentally friendly alternative, though they comprise less than 1% of all plastics produced. They are generally made from materials that microorganisms, fungi or other microbes can digest with enzymes, such as starch or cellulose. A fully biodegradable carbon-based material will be partly consumed by naturally occurring microbes, while the rest is converted to carbon dioxide or methane and none is left behind, according to Lott and his co-authors.

Companies, regulators, material scientists and other groups were all interested in the biodegradation of these plastics in the open environment, according to Lott, the co-founder and a managing director of HYDRA Marine Sciences, a German research institute with a focus on testing biodegradable materials.

The scarcity of this data motivated the authors of the latest study, he said, despite the challenges of these experiments.

"It's some effort to go out in the sea, diving, exposing things with the risk of being lost, and get them back and analyzing them in a way that you get good numerical results that are statistically relevant, and can be acquired within a decent time," Lott said.

Over the last four years, he and his colleagues tested the performance of thin films of three biodegradable plastics: polyhydroxybutyrate, polybutylene sebacate and polybutylene sebacate co-terephthalate  — respectively abbreviated as PHB, PBSe and PBSeT. The materials were tested in the Mediterranean Sea and in tropical Southeast Asia, where water and sediment samples were also collected for laboratory tests and mid-scale simulations.

The speed of biodegradation is affected by the material's surroundings, so for each location, the tests and samples occurred in three kinds of environments: near the shore, in open waters and on the seafloor. 

All three plastics were found to fully biodegrade, but the type of plastic and location affected the degradation speeds, which were measured in half-lives — the amount of time for half of the sample to degrade. PHB had a half-life of less than eight weeks on the Southeast Asian seafloor but 3.4 years in open waters in the Mediterranean.

The half-lives of PBSe and PBSeT respectively ranged between from 14 weeks to 7.2 years, and from 21 weeks to 2.2 years.

The wide ranges were "not surprising at all" to Lott, who said he expected differences in factors including temperature, bacterial communities and available nutrients to affect the rate of degradation in the plastics.

Exponentially decaying samples never fully disappear in a mathematical sense, but to approximate reality, Lott tripled half-lives to get a useful rough estimate for how long the plastics will persist in nature. Applying this rule of thumb, PHB would be expected to entirely biodegrade in between six months and 10 years in the tested conditions.

The longest half-life, found in PBSe, would translate to a persistence of more than 20 years, demonstrating the large variance in how quickly biodegradable plastics actually break down, and how they can still cause harm as pollution before then.

"Plastic lost to the environment is pollution, even if biodegradable," the study's authors wrote, even though biodegradable plastics are less likely to stick around than their conventional counterparts. 

Lott said biodegradable plastics still need to be tested in a wider range of marine environments, including at the poorly studied deep seafloor, where a lot of plastic waste settles, and in water temperatures of -1 degree Celsius (30 degrees Fahrenheit) found in polar waters.

The study also doesn't cover the plastics' performance in artificial environments such as landfills, which can limit biodegradation due to conditions of low oxygen and sparse presence of microbes.

"We started just to shine a small torch into this big black hole," he said.

The study, "Half-life of biodegradable plastics in the marine environment depends on material, habitat, and climate zone," published May 6 in Frontiers in Marine Sciences, was authored by Christian Lott, Andreas Eich and Miriam Weber, HYDRA Marine Sciences; Dorothée Makarow and Boris Unger, HYDRA Fieldwork; Miriam van Eekert and Els Schuman, LeAF BV; Marco Segre Reinach, Coral Eye Outpost; and Markus T. Lasut, Sam Ratulangi University.

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