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Environmental Geosciences

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Anionic Surfactant Mobility in Unsaturated Soil: The Impact of Molecular Structure

¹ Agricultural Engineer, USDA-ARS Soil Drainage Research Unit, 590 Woody Hayes Drive, RM. 234, Columbus, OH 43210
² Associate Professor, Biosystems and Agricultural Engineering Department, Oklahoma State University, Stillwater, OK 74078

Barry Allred earned his Ph.D. in Agricultural Engineering at Oklahoma State University and is licensed as a Professional Engineer in the State of New Mexico. He is presently employed as an Agricultural Engineer with the USDA/ARS Soil Drainage Research Unit in Columbus, Ohio and has also held prior research positions at the New Mexico Bureau of Mines and Mineral Resources and at Oklahoma State University. In addition, he previously worked in the environmental consulting and petroleum exploration fields. Dr. Allred’s research is focused on effective design and management of Wetland Reservoir Subirrigation Systems (WRSIS) and in using geophysical/ geotechnical methods to locate buried drain pipe. Besides investigations on agricultural subsurface drainage, he is likewise interested in porous media fluid flow and contaminant transport.

Glenn O. Brown is an Associate Professor of Biosystems and Agricultural Engineering at Oklahoma State University. He has also held positions at Sandia National Laboratories and Gulf Oil Corporation performing environmental impact assessments. A graduate of Colorado State University.

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, -, vs. sulfate, -O-), 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.