An oil dispersant is a mixture of emulsifiers and solvents that helps break oil into small droplets following an oil spill. Small droplets are easier to disperse throughout a water volume, and small droplets may be more readily biodegraded by microbes in the water. Dispersant use involves a trade-off between exposing coastal life to surface oil and exposing aquatic life to dispersed oil. While submerging the oil with dispersant may lessen exposure to marine life on the surface, it increases exposure for animals dwelling underwater, who may be harmed by toxicity of both dispersed oil and dispersant. Although dispersant reduces the amount of oil that lands ashore, it may allow faster, deeper penetration of oil into coastal terrain, where it is not easily biodegraded.
History
Torrey Canyon In 1967, the supertanker Torrey Canyon leaked oil onto the English coastline. Alkylphenol surfactants were primarily used to break up the oil, but proved very toxic in the marine environment; many types of marine life were killed. This led to a reformulation of dispersants to be more environmentally sensitive. After the Torrey Canyon spill, new boat-spraying systems were developed. Later reformulations allowed more dispersant to be contained (at a higher concentration) to be aerosolized.
Exxon Valdez
Alaska had fewer than 4,000 gallons of dispersants available at the time of the Exxon Valdez oil spill, and no aircraft with which to dispense them. The dispersants introduced were relatively ineffective due to insufficient wave action to mix the oil and water, and their use was shortly abandoned. A report by David Kirby for TakePart found that the main component of the Corexit 9527 formulation used during Exxon Valdez cleanup, 2-butoxyethanol, was identified as "one of the agents that caused liver, kidney, lung, nervous system, and blood disorders among cleanup crews in Alaska following the 1989 Exxon Valdez spill."
Early use (by volume) Dispersants were applied to a number of oil spills between the years 1967 and 1989.
Deepwater Horizon
During the Deep water Horizon oil spill, an estimated 1.84 million gallons of Corexit was used in an attempt to increase the amount of surface oil and mitigate the damage to coastal habitat. BP purchased all of the world's supply of Corexit soon after the spill began. Nearly half (771,000 gallons) of the dispersants were applied directly at the wellhead. The primary dispersant used were Corexit 9527 and 9500, which were controversial due to toxicity. In 2012, a study found that Corexit made the oil up to 52 times more toxic than oil alone, and that the dispersant's emulsifying effect makes oil droplets more bio-available to plankton. The Georgia Institute of Technology found that "Mixing oil with dispersant increased toxicity to ecosystems" and made the gulf oil spill worse. In 2013, in response to the growing body of laboratory-derived toxicity data, some researchers address the scrutiny that should be used when evaluating laboratory test results that have been extrapolated using procedures that are not fully reliable for environmental assessments. Since then, guidance has been published that improves the comparability and relevance of oil toxicity tests.
Rena oil spill
Maritime New Zealand used the oil dispersant Corexit 9500 to help in the cleanup process. The dispersant was applied for only a week, after results proved inconclusive.
Theory
Overview Surfactants reduce oil-water interfacial tension, which helps waves break oil into small droplets. A mixture of oil and water is normally unstable, but can be stabilized with the addition of surfactants; these surfactants can prevent coalescence of dispersed oil droplets. The effectiveness of the dispersant depends on the weathering of the oil, sea energy (waves), salinity of the water, temperature and the type of oil. Dispersion is unlikely to occur if the oil spreads into a thin layer, because the dispersant requires a particular thickness to work; otherwise, the dispersant will interact with both the water and the oil. More dispersant may be required if the sea energy is low. The salinity of the water is more important for ionic-surfactant dispersants, as salt screens electrostatic interactions between molecules. The viscosity of the oil is another important factor; viscosity can retard dispersant migration to the oil-water interface and also increase the energy required to shear a drop from the slick. Viscosities below 2,000 centipoise are optimal for dispersants. If the viscosity is above 10,000 centipoise, no dispersion is possible.
Requirements There are five requirements for surfactants to successfully disperse oil:
Dispersant must be on the oil's surface in the proper concentration Dispersant must penetrate (mix with) the oil Surfactant molecules must orient at the oil-water interface (hydrophobic in oil and hydrophilic in water) Oil-water interfacial tension must be lowered (so the oil can be broken up). Energy must be applied to the mix (for example, by waves)
Effectiveness The effectiveness of a dispersant may be analyzed with the following equations. The Area refers to the area under the absorbance/wavelength curve, which is determined using the trapezoidal rule. The absorbances are measured at 340, 370, and 400 nm. Area = 30(Abs340 + Abs370)/2 + 30(Abs340 + Abs400)/2 (1) The dispersant effectiveness may then be calculated using the equation below. Effectiveness (%) = Total oil dispersed x 100/(ρoilVoil)
ρoil = density of the test oil (g/L) Voil = volume of oil added to test flask (L) Total oil dispersed = mass of oil x 120mL/30mL Mass of oil = concentration oil x VDCM VDCM = final volume of DCM-extract of water sample (0.020 L) Concentration of oil = area determined by Equation (1) / slope of calibration curve
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