Degradation of Dispersants and Dispersed Oil

Chemical oil dispersants are proprietary mixtures of surfactants and solvents which are directly applied to a spill in order to reduce the natural attractive forces of the oil. When oil treated with dispersants is exposed to mixing energy, typically from wind and wave action, it is broken up into small droplets which may then become entrained in the water column (Li et al., 2009a; Li et al., 2009b; Li, 2008; Lunel, 1995). Many of these droplets are small enough to be neutrally buoyant, and therefore, advection and diffusion forces dilute the plume and transport the droplets far from the site of the original spill. As compared to a surface oil slick or larger and more buoyant physically dispersed oil droplets, these chemically dispersed droplets are much easier for oil-degrading bacteria to colonize and break down (Venosa and Holder, 2007; Venosa and Zhu, 2003). In addition, small droplets enhance dissolution of soluble and semi-volatile compounds into surrounding waters, wherein biodegradation is carried out by aqueous phase microbes. Under these conditions, oil concentration are effectively reduced below toxicity threshold limits, and biodegradation becomes the most important process in reducing the total mass of petroleum hydrocarbons in the environment. By enabling rapid dispersion and biodegradation of surface oil slicks at sea, the use of chemical oil dispersants can be effective in preventing heavy oiling of sensitive coastal environments such as beaches and wetlands, and consequently mitigates risk associated with marine and terrestrial wildlife coming into direct contact with a slick.