Pollution

Are biodegradable plastics too good to be true? Here’s what actually happens to them in the ocean

Reviewing: Lott, Christian, et al. “Half-life of biodegradable plastics in the marine environment depends on material, habitat, and climate zone.” Frontiers in Marine Science 8 (2021): 426. https://doi.org/10.3389/fmars.2021.662074

 

“Biodegradable plastics are better for the environment!” You might have heard this a lot, but is this true?

Biodegradable plastic is a type of plastic designed to be broken down completely into water and compost by microbes. And they are becoming a popular alternative to regular plastic .Throwing away a biodegradable food container or a coffee cup into your trash feels almost comforting – they will break down quickly and won’t stay forever in the environment, right? What is not widely marketed is that biodegradable plastic can only be disintegrated under the “right” conditions. These plastics actually need to be in the right temperature, humidity and oxygen level for bacteria to feed on them. Would the ocean be considered the “right” environment for biodegradable plastic to be degraded? A group of scientists decided to answer this question by testing how long biodegradable plastic last in different parts of the ocean.

A vegetable container made of biodegradable plastic (Wikimedia Commons)
A biodegradable plastic cup made from corn starch (Flickr)

 

 

 

 

 

 

 

 

 

Putting biodegradable and regular plastic in the ocean

Lott and colleagues compared how quickly biodegradable plastic broke down relative to regular plastic, by comparing a biodegradable plastic material (polyhydroxybutyrate) to two other common plastic materials (polybutylene sebacate and polybutylene sebacate co-terephthalate) in three different ocean environments – beach, seafloor and open water.

For the “beach test”, the team filled huge 60-liter plastic bins with sediments from a beach at Island of Elba, Italy. They then put film made from either biodegradable or regular plastic on top, and put the bins in the ocean. The “seafloor test” was conducted in both the Mediterranean (Tuscan Archipelago, Italy) and Southeast Asia (Sahaong Bay, Indonesia), where plastic films were put on a flat panel fixed to the seafloor. Finally, the “open water test” took place at the Mediterranean Sea, where plastic samples were attached to an anchored frame at a depth of 20 meters.

To determine how much plastic was degraded in the different ocean conditions, the scientists measured how much the total plastic film area decreased. They then put these measurements into a statistical model to calculate the half-life of plastic (the time that it had taken for the plastic film to lose half of its area). Why half the area? Comparing the half-life of different plastic materials is better than comparing how much plastic was left after the experiment, because plastic does not break down at the same speed over time. Let’s say a piece of plastic in the ocean completely breaks down and disappears after 100 days. It is very unlikely that a hundredth of plastic disappeared every day. Instead, plastic may disappear more quickly over the first 50 days, when microbes are starting to break it down and feed on it, but as the more easily degradable material gets consumed first by microbes, the remaining piece will take longer to disappear.

Left: “beach test” setting, where plastic films were on top of sand-filled bins and put in a rack on the Island of Elba; center: “open water test” setting, where plastic films were put on a rack at 20m-deep water in the Mediterranean sea; right: “seafloor test” setting, where plastic films were fixed to the sandy seafloor on both the Mediterranean Sea and Southeast Asia (Lott et al. 2021)

Biodegradable plastic breaks down faster than plastic, but still takes a long time to break down

The test results showed that biodegradable plastic films degrade faster than regular plastic films on the beach and at the seafloor. The half-life of biodegradable plastic film on the beach was 14 months, which was much shorter than the two types of plastic (23 and 40 months each). Similarly, the half-life of biodegradable plastic at the seafloor of the Mediterranean Sea was 22 months, again much shorter than regular plastic (27 and 47 months each). In contrast, the half-lives of biodegradable plastic and the two other plastic types at the seafloor in Southeast Asia were 2, 7 and 4 months. Because microbes that break down plastic are much more active at higher temperatures, they degraded plastic faster in the tropical waters in Southeast Asia (28.5°C) than in the Mediterranean Sea (17.5°C).

However, both biodegradable and regular plastic films barely broke down at all in the open Mediterranean waters, even though the scientists monitored them for almost 2 years. Because there isn’t much microbial activity going on in the open waters, even biodegradable plastic didn’t break down over 2 years.

Plastic litter on the beach (Flickr)

Takeaway? Biodegradable plastic may not be “biodegradable”.

Even though biodegradable plastic breaks down faster than plastic, it will still stay in the environment for a long time. The term “biodegradable” can be quite misleading, because the biodegradability of plastic depends so much more on which environment it is in. So the next time you buy a biodegradable plastic bag, consider how really “biodegradable” and “environmental-friendly” that bag is.

 

 

 

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