Summary:
BP has sprayed over 780,000 gallons of chemical dispersants to alleviate the spread of crude oil slick in the Gulf of Mexico (Johnson and Torrice, 2010). We can assume that the surfactant (sodium dioctyl sulfosuccinate [SDS]) and its delivery system (Corexit) were aerially sprayed over the slick at a constant concentration where the slick was visually apparent. The resulting concentration of the surfactants as it was incorporated in the marine system was not controllable, and the spray results in a surfactant being delivered at various concentrations throughout the surface and the upper water column, and from open ocean to the coast line.
Three questions arise from this process:
- How does the surfactant capability change as its concentration changes during its incorporation in the water column,
- How does the surfactant capability change as the concentration of the crude oil changes, and
- How does the chemistry of saltwater influence this surfactant capability?
This 12-month study conducted at the Department of Physical and Earth Sciences at Jacksonville State University (JSU) will focus on SDS and its delivery system and its changing capability as it enters the marine system. Schor (2010) reported the surfactant is an organic sulfonate (or organic sulfonic acid salt) which is a synthetic chemical detergent, which acts as a surfactant to emulsify oil and allow its dispersion into water. EPA disclosed the identity in June 2010 that the sulfonate used was SDS occurring in two forms of Corexit.
There are problems associated with the use of surfactants, however. The following issues need to be addressed when using SDS or any surfactant. As the surfactant spreads and approaches the coastline, the salinity of the marine system changes to a brackish one and eventually to a freshwater system. Does a change in salinity alter the surfactant capability? Also, interfacial tensions and the formation of critical micelle concentration in hydrocarbon mixtures are difficult to model as the molar fractions change (Yoon et al., 2009).
Surfactants act at liquid-liquid interfaces, but also at solid-liquid interfaces, where they may adsorb to the suspended solids in the water column (Rosen, 1989). Alternatively, they may precipitate under certain conditions (Stellner and Scamehorn, 1989; Jafvert and Heath, 1991). Both sorption onto a surface and precipitation will reduce surfactant availability. Temperature reduction can reduce surfactant effectiveness, critically so below the Krafft point (West and Harwell, 1992). Surfactants can partition into the crude oil and its derivatives if their solubility is high enough. They can also separate chromatographically. A study by Canevari and Fiocco (1990) found that the concentration of vanadium and nickel present in the porphyrins are a key component in the emulsification of a crude oil-sea water-surfactant mixture.
Finally, surfactants must be acceptable environmentally. Clearly, surfactant selection is a multifaceted issue (Vigon and Rubin, 1989), although guidelines for proper selection are readily available (Rosen, 1989; West and Harwell, 1992).
This study is directed towards characterizing the physical and chemical properties of the surfactant in terms of its surfactant capabilities and flocculent behavior in marine, brackish, and freshwater systems. Tests will be performed to gather information on the surface tension (ST) and the interfacial tension (IFT) of fluid mixtures. The test will evaluate SDS and the commercial products delivery systems called Corexit EC9500A, Corexit EC9527A, and Corexit EC9580A.
Crude oil will be tested in contact with SDS and its different delivery systems. These tests will be conducted under different controlled environments that represent the real world conditions. Such as, various salinities, pH’s, surfactant concentrations, delivery systems. Crude oil will be characterized using a specific polycyclic aromatic hydrocarbon (PAH) known to occur in crude oil. The Committee on Oil in Sea (2010) documents hydrocarbon components in seawater released in the Gulf of Mexico. A representative PAH will be selected from this list. In addition, degraded crude oil and tar balls will be investigated for surfactant effectiveness.