The feasibility of using anionic surfactants to remove organic contaminants located above the water table will depend on several criteria including mobility in unsaturated soil. Surfactants having limited mobility will not be practical regarding in situ cleanup operations. Unsaturated mobility characteristics also need to be considered for gauging possible adverse environmental impacts resulting from release of surfactant waste products into near-surface soil material. Because chemical structure may affect transport, transient unsaturated column tests in a loamy soil were conducted with a variety of anionic surfactants. The tested anionic surfactants differed with respect to molecular structural components such as linear alkyl chain length, number of oxyethylene groups, type of hydrophilic head group (sulfonate, -graphic, vs. sulfate, -O-graphic), and number of hydrophilic sulfonate head groups. Test results show that increasing the linear hydrocarbon chain length causes alkyl sulfate surfactant mobility to decrease. The presence of an oxyethylene group significantly increases the transport of alkyl ether sulfates over alkyl sulfates; however, the number of such groups is not overly important. Sulfates appear to be more mobile than sulfonates. Surfactants with two hydrophilic sulfonate head groups are transported substantially further than those having one. Relative mobility was then correlated with five different surfactant properties to determine which could potentially be used as an indicator of anionic surfactant transport under unsaturated conditions. The best correlation was found with percent hydrophilic molecular volume (r = 0.77), followed by hydrophilic lipophilic balance (r = 0.63), and next the critical micelle concentration (r = 0.48). Test results obtained in this study can be used as guidelines to qualitatively predict anionic surfactant mobility in unsaturated soil.

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