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The AAPG/Datapages Combined Publications Database
Environmental Geosciences (DEG)
Abstract
Anionic Surfactant Mobility in Unsaturated Soil: The Impact of Molecular Structure
1
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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. Allreds 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.
ABSTRACT
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.
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