Oil Weathering after the Deepwater Horizon Disaster Led to the Formation of Oxygenated Residues

Abstract

Following the Deepwater Horizon disaster, the effect of weathering on surface slicks, oil-soaked sands, and oil-covered rocks and boulders was studied for 18 months. With time, oxygen content increased in the hydrocarbon residues. Furthermore, a weathering-dependent increase of an operationally defined oxygenated fraction relative to the saturated and aromatic fractions was observed. This oxygenated fraction made up >50% of the mass of weathered samples, had an average carbon oxidation state of −1.0, and an average molecular formula of (C 5 H 7 O) n . These oxygenated hydrocarbon residues were devoid of natural radiocarbon, confirming a fossil source and excluding contributions from recent photosynthate. The incorporation of oxygen into the oil's hydrocarbons, which we refer to as oxyhydrocarbons, was confirmed from the detection of hydroxyl and carbonyl functional groups and the identification of long chain (C 10 −C 32 ) carboxylic acids as well as alcohols. On the basis of the diagnostic ratios of alkanes and polycyclic aromatic hydrocarbons, and the context within which these samples were collected, we hypothesize that biodegradation and photooxidation share responsibility for the accumulation of oxygen in the oil residues. These results reveal that molecular-level transformations of petroleum hydrocarbons lead to increasing amounts of, apparently recalcitrant, oxyhydrocarbons that dominate the solvent-extractable material from oiled samples.

Impact of Photooxidation and Biodegradation on the Fate of Oil Spilled During the Deepwater Horizon Incident: Advanced Stages of Weathering

“…15,18,20,21 Aeppli and colleagues noted the apparent recalcitrance of sand patties and characterized their chemical composition. 20 They reported that >50% of the mass of the material in these structures is not hydrocarbon at all, but oxygenated hydrocarbon-derived chemical features produced through a combination of weathering processes. We questioned the environmental fate and impact of these residual oil structures, and considering their deposition in the beach swash zone, explored the susceptibility of sand patties to the most likely forms of advanced decomposition -photoxidation and biodegradation.…”

“…Most notably, the transformation of the crude Macondo 252 oil resulted in the formation of a group of partially oxidized organic compounds collectively termed 'oxyhydrocarbons.' 20 The formation of the oxyhydrocarbons was coincident with a decrease in the fraction of n-alkanes and aromatic compounds in the residue. These oxyhydrocarbons are most obviously found in sand patties that wash up in the swash zone of northern GoM beaches due to prevailing winds and currents.…”

“…10,12,14-19 Not surprisingly, the environmental fate of the oil was influenced by many well documented weathering processes. [20][21][22][23][24][25] These processes resulted in the massive transformation of the oil components and a chemical footprint that differed substantively from the crude oil released at the wellhead. Most notably, the transformation of the crude Macondo 252 oil resulted in the formation of a group of partially oxidized organic compounds collectively termed 'oxyhydrocarbons.'…”

“…That is, sand patties are thought to be highly weathered oil deposits that combine with sediment particulates and migrate along the seafloor after a spill and eventually reach coastal beaches. 15,18,20 The oxyhydrocarbons constitute over 50% of the organic compounds in sand patties and of the residuum on 'oiled' rocks. 20,21 Sand patties derived from oil are likely exposed to both anaerobic conditions during their migration along the seafloor and aerobic conditions once they reach the beachfront.…”

“…15,18,20 The oxyhydrocarbons constitute over 50% of the organic compounds in sand patties and of the residuum on 'oiled' rocks. 20,21 Sand patties derived from oil are likely exposed to both anaerobic conditions during their migration along the seafloor and aerobic conditions once they reach the beachfront. Additionally, due to the nature of sand patties, it is possible that the patties themselves may develop an oxygen gradient within pores where diffusion may be limited.…”