Hydrodynamically-enhanced transfer of dense non-aqueous phase liquids into an aqueous reservoir.

The use of surfactants represents a viable strategy to boost the removal yield of Dense Non-Aqueous Phase Liquids (DNAPLs) from groundwater and to shorten the operational timing of the remediation process. Surfactants, in general, help in reducing the interfacial tension at the DNAPL/water interface and enhance the solubility of the pollutant in the water phase through the formation of dispersed systems, such as micelles and emulsions. In this paper, we show that a suitable choice of a surfactant, in this case belonging to the bio-degradable class of ethoxylated alcohols, allows for the formation of hydrodynamic interfacial instabilities that further enhances the dissolution rate of the organic pollutant into the water phase. In a stratified configuration (denser organic phase at the bottom and lighter water phase on top), the instabilities appear as upward-pointing fingers that originate from the inversion of the local density at the interface. This inversion stems from the synergetic coupling of two effects promoted by the ethoxylated surfactant: i) the enhanced co-solubility of the DNAPL into the water (and viceversa), and (ii) the differential diffusion of the DNAPL and the surfactant in the aqueous phase. By dissolving into the DNAPL, the surfactant also reduces locally the surface tension at the liquid-liquid interface, thereby inducing transversal Marangoni flows. In our work, we carefully evaluated the effects of the concentration of different surfactants (two different ethoxylated alcohols, sodium dodecylsulphate, cetyltrimethyl ammonium bromide, N-tetradecyl-N, N-dimethylamine oxide and bis(2-ethylhexyl) sulfosuccinate sodium salt) on the onset of the instabilities in 3 different DNAPLs/water stratifications, namely chloroform, trichloroethylene and tetrachloroethylene, with a special emphasis on the trichloroethylene/water system. By means of a theoretical model and nonlinear simulations, supported by surface tension, density and diffusivity measurements, we could provide a solid explanation to the observed phenomena and we found that the type of the dispersed system, the solubility of the DNAPL into the water phase, the solubility of the surfactant in the organic phase, as well as the relative diffusion and density of the surfactant and the DNAPL in the aqueous phase, are all key parameters for the onset of the instabilities. These results can be exploited in the most common remediation techniques.