Plastic is Forever. How long is forever?

I was taught that plastic is not biodegradable (which is certainly true), and, accordingly, we should assume that it will remain in the environment for a long time, effectively “forever”.  It is also clear that plastic materials can last decades in the wild, and can have devastating effects on wildlife.

Researchers from MIT and WHOI report this fall that there is more to this picture. In controlled experiments, they show that the ubiquitous polystyrene decomposes into (tiny amounts of) Carbon and CO2 when exposed to sunlight.  Polystyrene is pretty much indigestible by microbes (hence, “not biodegradable”), but is quite sensitive to UV light.

This finding is interesting because it explains the anomalous finding that there isn’t anywhere near as much plastic waste in the oceans as would be expected if they last thousands of years [1].  A lot of plastic probably degrades in the sun, turning into traces of carbon, and effectively disappearing.  Understanding this process in more detail will help refine estimates of the Earth’s Carbon budget.

Does this mean we are all clear to dump plastic?  Not really.

For one thing, this lab study doesn’t consider many details of what may happen in the wild, not least how much sunlight really hits the waste, and how gooky the plastic may be, and so on.  There are many unknowns, that could speed up or slow down decomposition.

“Multiple variables are not considered in these lifetime calculations that could shift our estimates to be shorter or longer. For example, it is unknown how the light absorption properties of PS change with increasing time in the environment (e.g., yellowing or fouling by organics and biofilms) or how the residence time of PS in sunlit environments varies” ([2], p. D)

Equally important, additives in the plastic have significant effects on the degradation.  It will be important to understand this photochemistry, not least because some additives may make the polystyrene much more or less degradable.  There may be additives that should be avoided, and others that have shorter lifetimes.

Finally, there are many plastics in wide use (with many additives), so similar studies are needed to understand the overall picture.  Again, some formulations may be much more degradable than others.  And some plastics emit greenhouse gasses in these conditions, so they may not be as benign as polystyrene when they solar degrade.

In the best case, we may discover some plastics that are relatively “solar degradable”, and therefore better for the overall environment.  For that matter, we may discover ways to promote this process, to get rid of unwanted plastic.  (How about microbes that eat sugary traces of soda and emit UV light to melt the empty bottle?)

It also occurs to me that this finding suggests that burying polystyrene might be a bad idea, because it might help preserve it.  Burial is a rich environment for microbes, but poor in UV and sunlight.  Perhaps polystyrene discards can be treated by exposure to sunlight, and should not be sent to landfills, where they really will last forever.

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1. William J. Broad, In the Sea, Not All Plastic Lasts Forever, in New York Times. 2019: New York. https://www.nytimes.com/2019/10/11/science/plastics-ocean-degrade.html
2. Collin P. Ward, Cassia J. Armstrong, Anna N. Walsh, Julia H. Jackson, and Christopher M. Reddy, Sunlight Converts Polystyrene to Carbon Dioxide and Dissolved Organic Carbon. Environmental Science & Technology Letters, 2019/10/10 2019. https://doi.org/10.1021/acs.estlett.9b00532