How long do different everyday plastics take to biodegrade?

Question

There are so many different types of plastic - how long do different plastics take to biodegrade?

Answer 1

The major problem with plastics is that current knowledge suggests that plastics may never biodegrade. A study reviewing plastic pollution in 2020 suggests that

• Plastic will first break down into smaller pieces known as microplastics (typically defined as <5mm in size) and then likely into even smaller pieces known as nanoplastic particles. The rate of this process known as fragmentation is controlled by various natural factors: sunlight (UV radiation), mechanical breakdown (by wind or water erosion) and the chemical structure of the plastic in question. For background on the chemistry of plastics, see What is the process for making plastic?.
• Theoretically, it may be possible that plastic will degrade to an atomic size where it will biodegrade. This a process called mineralization. However, this process is likely on the timescale of thousands to millions of years.
• Current estimates suggest that all plastic waste produced ever is still in a form too large to biodegrade. Unless, this plastic has been burnt first.

Ultimately, a lot more research is required to be able to categorically state the long-term fate of plastic in the natural environment. But the current state of knowledge suggests that plastic biodegradation occurs on very long timescales. This differs to traditional materials such as paper, metals, wood and glass which on long timescales, are incorporated into natural recycling processes. Therefore, humans are taking a considerable risk by using such a long-lasting material as a single-use, disposable product. The outcome is likely a long-term environmental pollution problem.

 

References:

Napper, I.E. and Thompson, R.C., (2020). Plastic Debris in the Marine Environment: History and Future Challenges. Global Challenges, p.1900081.

Answer 2

Biodegradation is a process when a substance or an object is being decomposed by bacteria or other living organisms. Therefore plastics don’t really “biodegrade” as bacteria are not “interested” in them. This means that plastics can’t be converted into useful compounds for life. They photodegrade instead. It means that photons from the sun pulverise the plastic polymers until they are broken into individual molecules. But even then they remain PLASTICS!
This makes them even more dangerous for us as they can easily enter the food chain.
For example a plastic bottle will degrade in this way in around 500 years (up to a 1000 years).
Recently the scientists discovered a mutated bacteria that produces an enzyme called PETase which can break down PET (Polyethylene Terephtalate) mainly used to produce plastic bottles. It means (to my understanding) that this particular type of plastic could be recycled in a much easier way but it doesn’t mean it would completely “biodegrade”.
Therefore an answer to the question “how long does it take to plastics to biodegrade” is: never.

Bibliography

Ransford M, “Why Trashing the Oceans Is More Dangerous Than We Imagined” (Popsci.com, 2008)