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The knowledge gap on vadose zone colloid transport limits predicting contaminant transport at many DOE sites, and remains an outstanding scientific challenge. Although the process of contaminant sorption at mineral surfaces has received much recognition as a major mechanism controlling contaminant behavior in subsurface environments, virtually little attention has been given to the possibility of contaminant sorption at gas-water interfaces, a major interface in the vadose zone. Moreover, little effort has yet been advanced to optimize such interactions for the purpose of facilitating in-situ remediation. Gas-water interfaces, unlike water-solid interfaces, are mobile. Therefore, associations of contaminants with gas-water interfaces can be very important not only in subsurface contaminant distributions, but also in contaminant mobilization, and potentially in remediation. The first objective of this project was to develop a fundamental understanding of interactions between contaminants and gas-water interfaces. For surface-active molecules, surface excesses can be determined through the Gibbs equation combined with measuring changes in surface tension with respect to changes in their solution concentration. However, for surface-active colloids, surface tension changes are too small to measure. Until initiation of this research project, there were no techniques available for quantifying sorption of colloids at gas-water interfaces. The second purpose of the proposed research, based on improved understanding gained in the first phase studies, was to develop a sorptive microbubble remediation technique, using surfactant stabilized microbubbles (fine gas-bubbles, 1-15 {micro}m in diameters) for subsurface in-situ remediation.. In the saturated zone, both pump-and-treat, and air sparging remediation methods are ineffective at displacing contaminants in zones that are ''advectively inaccessible''. Stable microbubbles might be able to migrate beyond preferential flow pathways through buoyant rise. Oxygen and nutrient delivery for promoting aerobic degradation of organic contaminants, and surfactant delivery for emulsifying NAPLs are potential benefits of microbubble injection

Sorption of Colloids, Organics, and Metals onto Gas-Water Interfaces: Transport Processes and Potential Remediation Technology