Sunlight degrades polystyrene much faster than expected

Polystyrene persists in the environment for millennia, according to some international governmental agencies. This estimate is based on the amount of time required for microbes to break down the plastic. But now researchers have challenged this common assumption with the finding that sunlight can break down polystyrene over a much shorter time scale, from centuries to decades. They report their results in Environmental Science & Technology Letters.

Used in many consumer and industrial products, such as food containers, protective packaging, and building materials, polystyrene widely contaminates the environment. Common microbes cannot degrade the polymer because of its aromatic backbone, leading scientists to estimate that it persists for tens of thousands of years.

Collin Ward and colleagues at Woods Hole Oceanographic Institution wondered whether sunlight absorbed by polystyrene could transform it into carbon dioxide and dissolved organic carbon in a much shorter time.

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To find out, the researchers placed five commercially available polystyrene samples in water and then exposed them to simulated sunlight that was three times brighter than sunlight at the equator. The researchers found that the simulated sunlight partially oxidized all five samples to dissolved organic carbon. They calculated that, for latitudes 0° to 50° N (extending from the equator to about the southern border of Canada), this process would take decades.

Complete oxidation of polystyrene to carbon dioxide by sunlight would require centuries, they estimate. The polystyrene samples degraded at different rates depending on the additives they contained, which in the future could be manipulated to control the lifetimes of the plastics, the researchers say.