 |
GeoInsighter Winter/Spring 2007 Newsletter INNOVATIVE REMEDIATION TECHNOLOGY ENHANCES RECOVERY OF NAPL Return to the Newsletter Index Non-aqueous phase liquids (NAPLs) are compounds that exist as a separate, immiscible phase when in contact with water and/or air. NAPLs in the subsurface are recognized as long-term sources of ground water impacts at many sites. Conventional efforts to remove NAPL include "pump and treat" methods such as total-fluids recovery or dual-pumping configurations, soil vapor extraction (for light NAPL), multi-phase extraction, and in situ chemical oxidation. The effectiveness of these remedial technologies is limited by the geochemistry of the site and the physical properties of the NAPL. The total time required to reach closure for many of these sites is measured in decades. 
An innovative NAPL removal method that has been shown to be effective is surfactant enhanced recovery. Surfactants (surface active agents) are amphiphilic compounds having a hydrophilic (polar, water soluble) head and a hydrophobic (non-polar, water repelling) tail that gather at the interface between two fluids. Surfactants increase the solubility of organic compounds and decrease the interfacial tension, which allows for more effective in situ or pump and treat technologies. Surfactant enhanced recovery involves the injection of a surfactant into the subsurface that is designed to mobilize the NAPL for subsequent removal by creating an emulsion. Injecting surfactants into the subsurface enhances NAPL mobility and solubility, thereby improving NAPL removal and reducing the cost for remediation. NAPL is solubilized through the formation of micelles, which are colloidal clusters of nonionic surfactants in ground water. The micelles are withdrawn together with the solubilized NAPL. At certain characteristic concentrations, surfactants exhibit marked changes in several physical and chemical properties, including electrical conductivity, interfacial tension, and detergency. At this concentration, the polar and non-polar groups become oriented such that they form colloidal clusters of molecules in solution. The concentration of the surfactant at which these phenomena occur is known as the critical micelle concentration. Surfactant enhanced recovery involves the injection of micellar surfactant solutions to solubilize and mobilize NAPL constituents. The surfactant and contaminants are subsequently extracted through strategically-placed pumping wells. Above ground processes are typically utilized to treat the extracted ground water and recycle the surfactant, if necessary. Various design approaches may be utilized, depending upon the hydrogeological conditions and the physical-chemical properties of the NAPL. The end result is a process that allows for significantly quicker and, therefore, less expensive remediation. This new technology offers several benefits over conventional remedial measures, including accelerated removal of the NAPL source, a 100-fold reduction in the number of pore volumes of ground water that must be withdrawn from the aquifer to remove NAPL, and a significant cost and time savings compared to current removal methods. Gerald H. Cresap, Jr., P.E. Return to the Newsletter
Index
|
