Development of surfactant-based immiscible displacement technologies for remediation of aquifers contaminated with dense non-aqueous phase liquids

Traditional pump-and-treat remediation is often ineffective for treatment of dense non-aqueous phase liquid (DNAPL) source zones. Several innovative technologies have been developed to address the inadequacies of conventional source zone treatment, one of which is surfactant enhanced aquifer remediation (SEAR). To date, SEAR technologies have typically relied upon micellar solubilization to recover DNAPLs, however, immiscible displacement offers the advantage of shorter treatment times and lower remediation costs. The major limitation of this approach, however, is the potential for uncontrolled, downward migration of the displaced DNAPL free product. To address this issue, the density modified displacement (DMD) method was developed, which involves the partitioning of alcohol into a DNAPL to lower the density and effectively convert the DNAPL to a light NAPL (LNAPL). Following density conversion, NAPL displacement and recovery is achieved by flushing with a low-interfacial tension surfactant solution.; The feasibility of using n-butanol for density conversion of three representative DNAPLs, chlorobenzene (CB), trichloroethene (TCE) and tetrachloroethene (PCE) was assessed in batch and 2-D aquifer cell systems. Results from liquid-liquid equilibrium studies indicated that DNAPL density conversion relative to water occurred at n-butanol mole fractions of 0.38, 0.50, and 0.56 in CB-, TCE- and PCE-NAPLs, respectively. Based on equilibrium alcohol distribution data, an aqueous solution of 6% (wt) n-butanol was evaluated for density conversion of CB-, TCE- and PCE-NAPLs in a series of 2-D aquifer cell studies. Subsequent low-interfacial tension displacement floods provided effective recovery (85% to 93%) of the density converted NAPLs. Visual observations and experimental measurements indicated that density conversion was successful for all three DNAPLs, however, greater than 5 pore volumes of the 6% n-butanol preflood solution were required for in situ density conversion of TCE- and PCE-NAPLs.; A surfactant-stabilized emulsion of n-butanol in water was developed to reduce the volume of preflood solution required for density conversion of TCE and PCE. Results from 2-D aquifer cell experiments indicated that use of the emulsion reduced the amount of preflood solution required for density conversion of TCE- and PCE-NAPLs to 1.2 pore volumes. Substantial mass recoveries (50% to 70%) of TCE and PCE were achieved during the emulsion preflood, prompting further investigation of DNAPL recovery via emulsification. However, cumulative recoveries of TCE and PCE in 2-D aquifer cell experiments were found to plateau at ∼80% after approximately one to two pore volumes of emulsion flooding. Refinement of the DMD method demonstrates the potential of immiscible displacement technologies to achieve efficient free product recovery from DNAPL source zones, while minimizing the risks of uncontrolled downward contaminant migration.