A new study in Organic Geochemistry shines light on how sunlight likely changed Deepwater Horizon oil chemically and potentially made it more toxic and longer lasting. When the Deepwater Horizon well blew in 2010, it spewed 4.9 million barrels of crude oil into the Gulf of Mexico—subsequently covered beaches and marshes across the northern gulf with a coating of oily brown mousse. The foamy oil was a result of weathering from the wind, waves and sunlight. Weathering from sunlight, called photooxidation, was a significant process that changed the oil and until now scientists did not know exactly how the oil changed chemically. Less than 25% of crude oil can be analyzed to the molecular level with traditional techniques, and analysis on weathered oil is technically difficult.
The recently published study provides insights into the transition and the transformed molecular structure of oil from photooxidation using sophisticated molecular tools. The lead author, Qing Wang, a visiting student in Dr. Zhanfei Liu’s laboratory at The University of Texas Marine Science Institute (UTMSI), and his colleagues were able to both separate and quantify oil before and after photooxidation using adsorptive chromatography. In addition, the researchers were able to provide even more detail on how photooxidation changes the chemical structure of oil by using mass spectrometry and nuclear magnetic resonance.
Their results indicate that photooxidized oil, or asphaltenes, are sourced from alkylated aromatic hydrocarbons, and have a molecular structure with small aromatic cores as the bases and long alkyl tails. This finding strongly supports one of the two widely referenced models, the archipelago model, for the structure of asphaltenes. This molecular structure is part of the reason that photooxidized oil can be heavier, insoluble and hard for bacteria to degrade, and certain components can be more toxic than the source oil. These research results about chemical formation pathways and the resulting molecular structure offer insights into toxicity assessment and developing response strategies for oil spills.
Co-authors include Zhanfei Liu, Meredith Evans Seeley and Kaijun Lu from UTMSI, Qing Wang from Anhui Normal University, Nathaniel Adegboyega from Baylor University and Southern Illinois University, and Burke Leonce and William Hockaday from Baylor University. The study was supported by the ACS Petroleum Research Fund and the Gulf of Mexico Research Initiative.