Surfactant solutions developed for NAPL recovery in contaminated aquifers

Phase diagrams can be used to select surfactant solutions for diesel dissolution and extraction in contaminated aquifers. They indicate the active matter concentrations which can be injected into an aquifer. Due to surfactant precipitation or fine particles dispersion in sediments, surfactant solutions concentration greater than 5% and less than their solubility limit can be used for injections. Surfactant solution effects on interfacial tension (IFT) are consistent with predictions from phase diagrams. For similar active matter concentrations, IFT are lower for surfactant solutions that dissolve more diesel. IFT values between equilibrated diesel and surfactant solutions and observation of the oil-in-water microemulsions in column effluents confirm that solubilization of oil can occur in a Winsor type I system without oil mobilization. More than 50 sand column experiments were done for this study and a good agreement is observed between diesel dissolved in sand column experiments (with downward injection) and diesel dissolution predicted from phase diagrams. However, sand columns experiments showed significant decrease in efficiency with a drop in temperature from 25 degrees C to 8 degrees C (phase diagrams predict no change) and they are essential to check efficiencies of washing solution selected by phase diagrams. Optimal injection concentration of solutions decreases as the alcohols ratio (n-AmOH:n-BuOH) increases and as the temperature decreases. In columns, optimal concentration of surfactant solutions has a maximum efficiency of 0.4 (dissolved diesel/active matter). This limited performance is explained by: the use of water to push a slug of washing solution creates dilution and thus demixing of the oil-in-water microemulsion in the column; the limited interfacial area between diesel and washing solution in the porous media; surface effects on the solid phase. Diesel extraction in sand columns is strongly influenced by the flow direction (up/down), the volume of the solution, and to a lesser extent by water salinity. Small modifications of flow velocity (three times faster) do not have a significant effect on diesel dissolution. Efficiency of diesel dissolution by surfactant solutions decreases as washing progresses because interfacial area (diesel/surfactant solution) decreases. The selection of a surfactant for diesel recovery is not based on efficiency alone but must also consider the costs of the ingredients.