Environmental Fate of Chemical Dispersant Corexit®9500 in Seawater by High-resolution Mass Spectrometry

• Main Author: Choyke, Sarah
• Other Authors: Ferguson, P. Lee
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: 2018
• Subjects: Environmental science; Analytical chemistry; Corexit 9500; Deepwater Horizon Oil Spill; High resolution mass spectrometry; polysorbate surfactants; Surfactant degradation; Arctic
• Online Access: https://hdl.handle.net/10161/17508

Over 7 million liters of Corexit® series dispersants were applied in the Gulf of Mexico during the 2010 Deepwater Horizon oil spill to facilitate the dispersion of crude oil into the water column. At the time of application, the composition, fate, and transformation kinetics of the surfactants in Corexit®9500 were relatively unknown. Recent advances in high-resolution mass spectrometry, such as resolving power, mass accuracy, and acquisition rates, have allowed for comprehensive characterization of complex surfactant mixtures in environmental matrices. The objective of this dissertation was to develop a method for comprehensive characterization of Corexit in the marine environment using high-resolution mass spectrometry. Specifically, I assessed the kinetic and biodegradation rates and transformation products of Corexit®9500 in under environmentally relevant conditions. In Chapter 2, I identified individual nonionic polysorbate surfactants in Corexit using ultra high-resolution mass spectrometry. The method allowed for greater confident in structural assignment and systematic differentiation of isobaric and isomeric compounds. I examined the heterogeneity of Corexit based on differences in the hydrophilic core and hydrophobic tail groups. Specifically, the nonionic surfactants exhibited variability in the degree of esterification, fatty acid chain length and saturation as well as different core groups and ethoxymer distribution. The composition of Corexit has implications on the utility, fate, and persistence during an oil spill emergency response. In Chapter 3, I investigated Corexit degradation under biotic and abiotic conditions using temperate seawater collected from the Pivers Island Coastal Observatory in Beaufort, NC. Corexit degraded under both biotic and abiotic conditions, although biodegradation rates were two to four times faster. Kinetic degradation rates were highly dependent on the degree of esterification and fatty acid chain length. However, they were not dependent on the core group, fatty acid saturation, or degree of ethoxylation. The only observed transformation products were the nonesterified ethoxylates generated by ester hydrolysis. Abiotic hydrolysis rates were also dependent on temperature. Increased temperature had a greater influence on the monoester surfactants degradation rates than surfactants that contained two or more ester moieties. I determined the kinetic degradation rates for individual surfactants, ethoxymer series, and ester components and compared degradation rates to polysorbate mixtures. In Chapter 4, I examined the impact of crude oil on Corexit partitioning and degradation in Arctic seawater collected from the Chuckchi Sea. Components of Corexit did not partition into the crude oil layer based on surfactant hydrophobicity. Instead, the total abundance of Corexit increased in the water column in the presence of dispersed oil. The transformation products detected were the nonesterified ethoxylates, which suggested ester hydrolysis was the main pathways of degradation. The biodegradation rates of the ester components were slightly faster with dispersed oil due to the greater concentrations in the water column. In addition, observed abiotic hydrolysis rates at cold temperatures agreed with estimated rates calculated from the kinetic variable determined in Chapter 3. Overall, Corexit followed the same degradation pathway with slower transformation kinetics in Artic seawater in the presence and absence of crude oil. Finally, Chapter 5 presented results from the microbial community analysis in the Corexit only and Corexit dispersed crude oil treatments. In the presence of Corexit, Bacteroidetes dominated the microbial community within 24-hours. Bacteroidetes were also present of dispersed oil and untreated water, however, in lower abundance. We did not observe a dramatic change in the microbial structure after the monoester component of Corexit was removed after 24-hours. The communities maintained a large degree of diversity in the presence of Corexit and dispersed crude oil, however after 5 days Proteobacteria (specifically Oceanospirillales and Altermonadales) and Flavobacterilia accounted for 75% of the community in the Corexit treatment. The inclusion of both dispersant and oil had significant effects on microbial community structure, which can be correlated to the degradation of dispersants. High resolution mass spectrometry analysis allowed me to perform a complete characterization of Corexit and its related polysorbates, as well as determine the degradation rates and identify transformation products under various environmental conditions. I detected individual ethoxymers and evaluated the heterogeneity of the mixtures as well as the kinetic rates of degradation. I showed that the components of Corexit degrade on varying time scales based on structural similarities. All nonionic components of Corexit were removed within 10 days in temperate seawater and after 20 days in Arctic seawater. These studies are among the most comprehensive analysis of nonionic polysorbate surfactants under environmental conditions and will help to profile other dispersants and inform the application of Corexit series dispersants in future oil spill disasters.