Dispersants are one option for reducing damage from oil spills. By breaking up slicks, they can lessen effects from oil coating and smothering, reducing biological damage. However, dispersants are not a panacea.
This report considers when they should be used, and when they should not, and how the dispersant option relates to contingency planning.
Dispersants and how they work
It is common knowledge that oil and water do not mix easily. Spilled oil floats on the sea surface in calm conditions. The mixing action of the waves can cause oil and water to combine in two ways:
Natural dispersion
Waves break up the oil slick, forming oil droplets that become suspended in the water. The majority of these oil droplets will float back to the surface; but a small proportion of tiny droplets with neutral buoyancy will remain dispersed in the water almost indefinitely.
Water-in-oil emulsification
The mixing action of waves can cause water droplets to be incorporated into the oil, forming a water-in-oil emulsion which has a much higher viscosity than the oil from which it is formed. This emulsion is often referred to as ‘chocolate mousse’ and can increase in volume by up to four times that of the spilled oil.
Dispersants alter the balance between natural dispersion and emulsification, pushing the balance strongly towards dispersion and away from emulsification. By applying dispersant onto the spilled oil, it is possible to inhibit emulsion formation while promoting oil dispersion.
Dispersants – the active ingredients
Dispersants promote the formation of numerous tiny oil droplets, and delay the reformation of slicks because they contain surfactants with hydrophilic heads which associate with water molecules, and oleophilic tails which associate with oil (see diagram). Oil droplets are thus surrounded by surfactant molecules and stabilised. This helps promote rapid dilution by water movements.
The formation of droplets increases the exposure of oil to bacteria and oxygen, favouring biodegradation. However, the distribution of oil into the water column is increased.
How to weigh up these advantages and disadvantages is one of the main subjects of this report.
Advantages and disadvantages of dispersants
Dispersion of floating oil into the water column provides a number of advantages, including:
• Reducing risk of contamination of marine habitats and wildlife;
• Assisting with biodegradation of the oil by increasing exposure to naturally-occurring bacteria and oxygen;
• Reducing the amount of surface oil susceptible to drifting with the wind;
• Rapid treatment of large areas through application of dispersant from aircraft compared to alternative response methods;
• Dispersed oil droplets can become associated with suspended sediment, producing a neutrally buoyant ‘aggregate’ which is distributed naturally over large areas at very low concentrations.
The main potential disadvantage of dispersion of oil is the localised and temporary increase in oil in water concentration which could effect marine life in the immediate vicinity of the spill.
