Styrofoam cups are made of polystyrene, which takes centuries, not millennia, to break down.
Until today, it was thought that polystyrene would pollute the environment for millennia. This widely-held assumption was based off the observation that microbes generally do not break down polystyrene. To the microbes, polystyrene is a complex food source that is energetically hard to consume.
A new study, released today in Environmental Science and Technology Letters by researchers at the Wood’s Hole Oceanographic Institution, shows that polystyrene is completely broken down over the course of centuries and partially broken down over the course of decades. This is orders of magnitude faster than breakdown of polystyrene by microbes.
In order to determine these new rates, the scientists put five commercially-available polystyrene samples in water under simulated sunlight. Sunlight eventually oxidizes the compounds into dissolved organic carbon or carbon dioxide. They calculated these rates for latitudes from the equator north to the U.S.-Canada border. Using the experiment and calculations, they determined that partial degradiation to dissolved organic carbon would take decades. Complete breakdown to carbon dioxide would take centuries. The low levels of carbon dioxide released would not impact climate change.
Breakdown rate was different between the five samples depending on which additives they contained. Additives alter polystyrene’s color, flexibility, and other characteristics. These additives could be changed in the future to encourage a shorter lifetime.
Collin Ward, lead author of the study said “right now, policy makers generally assume that polystyrene lasts forever in the environment.That’s part of justification for writing policy that bans it. One of our motivations for this study was to understand if polystyrene actually does last forever. We’re not saying that plastic pollution isn’t bad, just that the persistence of polystyrene in the environment may be shorter and likely more complicated than we previously understood. The chance for injury to the environment over decades is still available.”
Although polystyrene breaks down much faster than originally assumed, it is still a major pollution ... [+]
Looking at the structure of polystyrene offers insights on why sunlight is better than microbes for breakdown of the polymer. “Although the ring-based backbone of polystyrene makes it a difficult target for microbes, it’s the perfect shape and size to catch certain frequencies of sunlight,” says Ward. The sunlight provides energy that breaks apart chemical bonds. Chris Reddy, a co-author on the paper, adds that“different additives seem to absorb different frequencies of sunlight, which influences how fast the plastic breaks down.”
More research is needed to understand the impact of other products that dissolve into the water. The study is the start of helping us estimate how much plastic is in the natural environment, and how plastic breakdown will impact us.
