Emulsion based recovery of multicomponent hydrocarbons using nonionic surfactants

Remediation of source zones containing non-aqueous phase liquids (NAPL) often involves aggressive mass removal to reduce mass discharge and create favorable conditions for follow-on treatments. One option for aggressive mass removal is the use of surfactants. Surfactants can aid subsurface remediation through three primary mechanisms - solubilization, mobilization or emulsification. While surfactant selection and flow field design can promote one mechanism over the others, the three are rarely mutually exclusive. Solubilization-based approaches rely on increases in apparent solubility of the contaminant in the surfactant solution due to partitioning of the organic contaminant into the hydrophobic interiors of surfactant self-assemblies (e.g. micellar solubilization). Mobilization-based approaches draw on the use of surfactants that greatly reduce the interfacial tension at the NAPL-water interface, thereby reducing the capillary forces and freeing entrapped NAPL. Emulsion-based approaches rely on the mechanical energy associated with the flow-field in combination with more limited-reductions in interfacial tension to create and stabilize NAPL droplets in macroemulsions (i.e., oil-in-water emulsions having average droplet diameters in the range of 1-10 mum). Among these mechanisms, emulsification has been studied far less, perhaps due to concerns about the mobility of emulsions in porous media.;Studies examining surfactant-based recovery processes largely focus on pure-component NAPLs. Multicomponent NAPLs, however, increase the complexity of remediation, especially when treatment technologies preferentially recover certain contaminants from the NAPL. This sort of preferentially removal, or fractionation of the NAPL, can increase the recalcitrance of the remaining NAPL.;Here emulsification is examined within the context of LNAPL recovery, specifically a five-component synthetic gasoline. The objective of this research is to elucidate the physical and chemical nature of the emulsification process when applied to a multicomponent NAPL. Batch and column experiments were used to examine emulsification of the synthetic gasoline mixture containing approximately 50% 2, 2, 4-trimethylpentane, 30% 1, 2, 4-trimethylbenzene, 12% m-xylene, 6% toluene and 2% benzene. The surfactants used to promote emulsification fall within two classes of non-ionic surfactants currently used in remediation (nonylphenol ethoxylates and alcohol ethoxylates). The alcohol ethoxylate formulation was designed by the manufacture as a more environmentally-friendly version of the nonylphenol ethoxylate formulation. The surfactant formulations have similar hydrophile-lipophile balances and are therefore anticipated to have similar emulsification behavior with petroleum hydrocarbons.;Results from the batch experiments were used to quantify the potential for preferential dissolution and/or solubilization of the synthetic gasoline components, as well as mobilization of the synthetic gasoline mixture -NAPL for each of the surfactant solutions. Batch experiments also provided important data related to the density and viscosity of the emulsions formed when the solutions are contacted with synthetic gasoline mixture. This batch-derived information was then used to interpret the recovery of the synthetic gasoline mixture from column experiments containing approximately 15% NAPL saturation entrapped within a medium-to-fine grain Ottawa sand. Results from experiments conducted with the nonylphenol ethoxylate formulation suggest that emulsification was responsible for recovery of the synthetic gasoline mixture. In contrast, results from the experiments conducted with the alcohol ethoxylate suggest this recovery occurs by both emulsification and mobilization. Mobilization observed with the alcohol ethoxylate formulation enhanced recovery, suggesting the combination of mobilization and emulsification could be beneficial when treating multicomponent LNAPLs. For both formulations, results suggest that emulsion-based recovery of the multicomponent NAPL used herein was a physical process that limits fractionation of the synthetic gasoline NAPL. Results also imply that the alcohol ethoxylate formulation is an effective, more environmentally conscious replacement for the nonylphenol ethoxylate formulation.