Up to 17% of the surface oil from the Deepwater Horizon may have been dissolved by solar light.
Following the 2010 Deepwater Horizon oil leak, sunlight may have helped clear as much as 17% of the oil slicking the Gulf of Mexico's surface. Researchers indicate in Science Advances on February 16 that sunlight has a larger role in clearing up such spills than previously considered.
When sunlight shines on oil that has been spilled in the sea, it can start a cascade of chemical reactions that change the oil into new substances (SN: 6/12/18). Photodissolution is a term used to describe how easily oil dissolves in water as a result of certain of these events. However, there is limited information on how much of the oil becomes water-soluble.
To test this, environmental chemists Danielle Haas Freeman and Collin Ward of the Woods Hole Oceanographic Institution in Massachusetts placed samples of Macondo oil from the Deepwater Horizon spill on glass disks and irradiated them with light using LEDs that emit wavelengths similar to those found in sunlight. The team next used chemical analysis to determine how much of the irradiated oil had been converted into dissolved organic carbon.
The thickness of the slick and the wavelengths of light was discovered to be the most relevant elements in photodissociation. Longer wavelengths (toward the red end of the spectrum) dissolved less oil than shorter wavelengths, probably because longer wavelengths are more easily dispersed by water. It didn't matter how long the oil was exposed to light.
Though the researchers didn't test for seasonal or latitude differences, computer simulations based on lab data revealed that those factors, as well as the chemical makeup of the oil, have a role.
Irradiation helped dissolve between 3 and 17 percent of the surface oil from the Deepwater Horizon leak, according to the study, which is comparable to evaporation and stranding on beaches. The influence of the sunlight-produced chemicals on marine ecosystems, on the other hand, remains unknown.
D.H. Freeman and C.P. Ward. Sunlight-driven dissolution is a major fate of oil at sea. Science Advances. Published online February 16, 2022. DOI: 10.1126/sciadv.abl7605.