Sunlight helps clean up oil spills in the ocean more than previously thought

Oil and water, known to be immiscible due to different densities make it harder to separate them. Though, scientists might have found an unsuspecting solution that transforms oil so it can be dissolved in water. 

A team of researchers from Woods Hole Oceanographic Institution (WHOI) set out to understand where much of the oil from the 2019 Deepwater Horizon oil spill ended up — an idea known as environmental fate.

The spill was the largest in U.S. history, releasing nearly 210 million gallons of crude oil into the Gulf of Mexico and causing 11 deaths. Researchers found that as much as 17 percent of the oil floating on the Gulf of Mexico was dissolved into seawater by sunlight.

“If this sizable fraction of oil is being transformed by sunlight and is dissolving into seawater, that might mean that less oil is ending up in other places, like sensitive coastal ecosystems. On the other hand, we have to consider the impacts of the compounds on marine organisms before we can decide if the net result is positive or negative,” said Daniella Haas Freeman, a joint program student at WHOI.

In their study published in Science Advances, researchers explained that sunlight exposure can initiate a chain of chemical reactions in floating oil slicks, that transform those clicks into a new compound.

Of those many transformations, one includes photo-oxidation, in which the Oxygen is incorporated into the long Carbon chains of those hydrocarbons, along with other photolytic processes.

Photo-oxidation impacts oil’s water solubility, which previous studies over the past 50 years have also shown, causing light exposure to generate water-soluble products out of crude oil from components that were originally insoluble—a reaction known as photo-dissolution. 

However, researchers said photo-dissolution hasn’t been evaluated for any past oil spills, with WHOI researchers emphasizing that photo-dissolution constitutes a removal process for oil slicks, potentially affecting the overall mass balance of surface oil. 

To come to these conclusions, researchers used custom-built light-emitting diode (LED) reactors to measure exactly how the rate of oil varies depending on the different types of light, like ultraviolet and visible light, while also allowing researchers to understand which conditions were most important in controlling photo-dissolution.

A series of hypothetical spill scenarios were created, all with varying oil slick thickness, time of the year, locations around the world, and types of light. They noticed that some of these changing conditions mattered more than others. 

Freeman explained that oil thickness is an incredibly important aspect of the process, as well as location. For example, the Arctic waters are an area that experiences a lot of cargo ship traffic and therefore is at a heightened risk of oil spills. 

Wavelengths of light are another important factor, as longer wavelengths were found to dissolve less oil, possibly because they are more easily scattered by water, in comparison to shorter wavelengths. 

“This kind of modeling is critical when forecasting spills and considering the impacts on marine ecosystems,” said Freeman. WHOI researchers’ study could create a new fate for how future oil spills are framed and how society responds to them, with Freeman and her co-authors encouraging more research on the impact of sunlight on oil spills. 




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