Surfactant enhanced soil washing can result from two distinct mechanisms that are an outgrowth of fundamental surfactant properties: The first mechanism, solubilization, occurs above the CMC (i.e. critical micelle concentration), and the other occurs below the CMC (roll-up mechanism). Above the CMC, surfactant molecules aggregate into micelles, which are basically spheres with a water-like exterior and an oil-like interior. Surfactant enhanced solubilization results from contaminant partitioning into the hydrophobic interior of these surfactant micelles.
Oil based cuttings generated by Marathon Oil Company and Baroid Drilling Fluids Company have initial oil content in the range of 10-20% by weight. The first quarter research showed 99% removal of oil from drill cuttings via solubilization by using appropriate anionic surfactant (50 x CMC), hydrotrope, and electrolyte concentration as demonstrated in Figure 2. Figure 2 shows near 99% removal of the oil from the cuttings at a surfactant concentration of 4 wt. %, [NaCl] = 6.0%, and [hydrotrope] to [surfactant] ratio equal to 5. Thus supra-CMC surfactant solutions with appropriate hydrotrope and [electrolyte] reduce the oil content of the cuttings to 0.1 wt %.
The batch washing studies are conducted by placing 3g of contaminated soil or oil based drill cuttings to 15 ml of the optimal surfactant formulation in a 40 ml EPA vial in a temperature-controlled environment. The vials are shaken for 20 min on a wrist action shaker. Immediately after shaking the samples are centrifuged at 1790(g. The wash solutions are quantified for both micellarly solubilized oil and free phase oil. The results are plotted in Figure 1 against the surfactant concentration. The surfactant is
Figure 1: Percent of oil removed from drill cuttings using solubilization
a branched alcohol propoxylate sulfate with four PO groups and a branched alkyl chain containing 14 to 15 carbons. The commercial name of this surfactant is Isofol 145 4 PO purchased from Condea Vista. In Figure 1 the initial oil content of the cuttings is 10 wt% (300 mg). At low surfactant concentrations ([surfactant]( 0.5 wt%) the majority of liberated oil is free phase oil. For [surfactant]( 1.0 wt % the majority of liberated oil is solubilized in micelles.