'Biodegradable' plastic bags often don’t break down | Science News for Students

'Biodegradable' plastic bags often don’t break down

Some 'green' plastics could still hold five pounds of groceries after three years outside
Jun 28, 2019 — 6:45 am EST
a broken down plastic bag floating underwater in the ocean

Biodegradable plastic bags could help reduce plastic pollution in the environment — but only if the material actually breaks down. Most don’t.

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Plastic bags are handy for carrying light items. But many are trashed after a single use. Some of these bags end up as litter that may harm animals (including those in the ocean). That’s one reason some companies have switched to biodegradable plastic. These are supposed to break down faster than regular plastics. But a new study in England shows that may not happen.

“Single-use plastic bags are a huge source of litter worldwide. We wanted to test whether biodegradable plastic bags could help reduce plastic pollution,” says Richard Thompson. He’s a marine biologist at the University of Plymouth in England. Thompson and a graduate student, Imogen Napper, decided to test that.

Materials break down through rot or decay. That’s usually a process whereby microbes feed on them, breaking big molecules into smaller, simpler ones (such as carbon dioxide and water). Other living things can now feed on these breakdown products to grow.

The problem: Ordinary plastic bags are made from oil, which few microbes can digest. So these plastics don’t decay easily.

Biodegradable plastics are sometimes made from materials that microbes do readily digest. Others may be held together with chemical bonds that break apart when exposed to water or sunlight. There’s also no one rule for how quickly biodegradable plastic bags should break down. Some plastics may even need special conditions — such as heat — to fully break down.

To study how well these bags live up to such claims, Thompson and Napper collected 80 single-use plastic bags from stores for testing.

Watching and waiting

The pair chose bags made from each of four different types of biodegradable plastic. They would compare these to a group of ordinary plastic bags. For the tests, they submerged some bags of each type in ocean water. They buried some of each type in garden soil. They tied others to a wall where the bags could flutter in the breeze. They placed still more of them in a closed, dark box in the laboratory.

Then the scientists waited. For three long years they observed what happened to these bags. At the end, they measured how well the plastic had broken down.

Most of the bags didn’t break down much in soil or seawater. Even after three years in such environments, three of the four types of biodegradable bags could still hold up to 2.25 kilograms (5 pounds) of groceries. The ordinary plastic bags could too. Bags marked “compostable” were the only ones that disappeared completely.

biodegradable plastic bags holding groceries three years after being exposed to the elements
Biodegradable plastic bags still hold groceries after three years submerged in the ocean (left) or buried in soil (right).
Richard Thompson

In the open air, the results were different. Within 9 months, all the types of bags began to break apart into tiny pieces.

But this is different from decay. Exposure to sun, water or air can help break chemical bonds that hold plastic molecules together. It does not, however, break big molecules into simpler ones. It just makes smaller and smaller bits of the starting plastic. “The object may disappear, but the material doesn’t,” says biochemist Taylor Weiss. He works at Arizona State University in Mesa. Although not involved in this study, he does work on biodegradable plastics.

This breaking of plastic into smaller fragments may be a good starting point, he says. It may make the plastic easier for microbes to digest. But any chunks not eaten can further break apart into microplastics. These bits — each smaller than a rice grain — can spread easily through the environment. Some travel long distances in the air. Others end up in the ocean. Animals even mistake these tiny pieces for food.

Chemist Marty Mulvihill says he’s “a little surprised” that most of the bags could still hold groceries after three years. But he's not surprised the bags didn’t decay completely. He’s a co-founder of Safer Made, a California company that aims to create products that are safer for people and the environment.

Different environments contain different types and numbers of microbes. Their physical conditions differ too. There’s less sunlight and oxygen underground, for instance. Such factors can affect how fast something decays, explains Mulvihill.

Overall, none of the types of plastic bags consistently broke down in all the environments, the researchers concluded. They shared their findings May 7 in Environmental Science & Technology.

Concludes Mulvihill, “Just because something says ‘biodegradable’ doesn’t mean you should litter it.”

Reduce and reuse

If biodegradable plastic bags aren’t actually breaking down in the environment, what should people do?

“Use fewer bags,” says Thompson. Reuse clean plastic bags more than once before throwing them out. Or take reusable bags with you when you go shopping, he suggests.

People have been carrying things around for thousands of years. Single-use plastic bags only became common in the 1970s. “We’ve become conditioned to expect the convenience everywhere we go,” he says. However, he adds, “That’s a behavior we need to reverse.”

Power Words

(more about Power Words)

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide (or CO2)     A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

 chemical bonds     Attractive forces between atoms that are strong enough to make linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

decay     The process (also called “rotting”) by which a dead plant or animal gradually breaks down as it is consumed by bacteria and other microbes.

digest     (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

environmental science     The study of ecosystems to help identify environmental problems and possible solutions. Environmental science can bring together many fields including physics, chemistry, biology and oceanography to understand how ecosystems function and how humans can coexist with them in harmony. People who work in this field are known as environmental scientists.

factor     Something that plays a role in a particular condition or event; a contributor.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

litter     Material that lies around in the open, having been discarded or left to fall where it may. (in botany) Decaying leaves and other plant matter on the surface of a forest floor.

marine     Having to do with the ocean world or environment.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microplastic     A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

physical     (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

seawater     The salty water found in oceans.


Journal: I.E. Napper and R.C. Thompson. Environmental deterioration of biodegradable, oxo-biodegradable, compostable, and conventional plastic carrier bags in the sea, soil, and open-air over a 3-year period. Environmental Science & Technology. Vol. 53, May 7, 2019, p. 4775. doi: 10.1021/acs.est.8b06984.