Colloidal gas aphrons: generation, flow characterization and application in soil and groundwater decontamination
This study examines the fundamental properties of Colloidal Gas Aphrons, CGA, relevant to its application in soil and groundwater decontamination. It also presents the results of laboratory experiments on the applicability of CGA in treating soils contaminated with organic chemicals.
CGA is a collection of spherical, 10 to 100 micron-sized gas bubbles dispersed in an aqueous surfactant solution with a volumetric gas fraction (quality) of at most 0.74. It is characterized by its quality, bubble size, stability and apparent viscosity. The stability can be described by the half life, the time needed for 50% of its liquid phase to separate by gravitational drainage.
CGA is a non-Newtonian fluid. The apparent viscosity is higher than that of its liquid or gas phase and increases with increase in quality. At quality above 0.6, CGA is a pseudoplastic fluid and can be modelled by the power law. In this quality range, apparent viscosity decreases with increase in shear rate. Measured apparent viscosity can be as high as 25 centipoise at low shear rates for a CGA quality of 0.72. But at low quality, CGA is best described by the Bingham fluid model.
CGA flow through porous media follows the discontinuous fluid flow model with the liquid phase advancing faster than the gas phase. The presence of CGA bubbles causes considerable reduction of effective mobility compared to water flow. The bubbles act to reduce the effective flow area by first blocking the wider pores and re-directing flow to narrower pores. The tenacity of the bubbles in the presence of adequate surfactant molecules account for its flow characteristics in porous media.
CGA is more effective in flushing hydrophobic organics from saturated sand-packed columns compared with surfactant flushing. CGA provides the surface active agent needed to lower the interfacial tension of the non-wetting phase and at the same time creates enough viscous force to mobilize any trapped fluid. Its ability to preferentially block large pores in a heterogeneous media and direct fluid flow to smaller pores also contributes to the demonstrated effectiveness as a soil flushing agent.