Oil Irradiation Experiments Document Changes in Oil Properties, Molecular Composition, and Dispersant Effectiveness Associated with Oil Photo-Oxidation

While chemical dispersants are a
powerful tool for treating spilled
oil, their effectiveness can be limited by oil weathering processes
such as evaporation and emulsification. It has been suggested that
oil photo-oxidation could exacerbate these challenges. To address
the role of oil photo-oxidation in dispersant effectiveness, outdoor
mesocosm experiments with crude oil on seawater were performed. Changes
in bulk oil properties and molecular composition were quantified to
characterize oil photo-oxidation over 11 days. To test relative dispersant
effectiveness, oil residues were evaluated using the Baffled Flask
Test. The results show that oil irradiation led to oxygen incorporation,
formation of oxygenated hydrocarbons, and higher oil viscosities.
Oil irradiation was associated with decreased dispersant efficacy,
with effectiveness falling from 80 to <50% in the Baffled Flask
Test after more than 3 days of irradiation. Increasing photo-oxidation-induced
viscosity seems to drive the decreasing dispersant effectiveness.
Comparing the Baffled Flask Test results with field data from the
Deepwater Horizon oil spill showed that laboratory dispersant tests
underestimate the dispersion of photo-oxidized oil in the field. Overall,
the results suggest that prompt dispersant application (within 2–4
days), as recommended by current oil spill response guidelines, is
necessary for effective dispersion of spilled oil.