Electrokinetic demonstration at Sandia National Laboratories: Use of transference numbers for site characterization and process evaluation Page: 4 of 9
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The scope of this paper is to discuss methods for estimating the transference number of
chromate in unsaturated soil and make comparisons with the chromate transference number realized
in a large scale electrokinetic extraction process. The methodology should be useful for
characterizing electrokinetic remediation of any water soluble ionic contaminant from saturated or
unsaturated soil. Water soluble heavy metal contaminants are typically oxyanions such as
chromate. Other heavy metals which can exist as water soluble oxyanions include arsenic,
molybdenum, selenium, technetium and uranium. Some heavy metal contaminants, such as lead,
mercury or plutonium, typically exist in soil as a precipitated solid phase. Such contaminants must
first be dissolved into the porewater before removal by an electric field is possible. The current
efficiency for this case depends additionally on the dissolution process and is therefore more
complicated to determine.
The Chemical Waste Landfill was the chemical disposal site for Sandia National
Laboratories in Albuquerque New Mexico from 1962 to 1985. During this time, chemicals were
separated by type and disposed in unlined trenches. It is estimated that over 4290 gallons of
chromic sulfuric acid solution were disposed into unlined chromic acid pits. The chromium was
disposed in the form of hexavalent chromium and the very low organic fraction present in the
native soil suggests the chromium should stay in the hexavalent form. Such anionic hexavalent
chromium adsorbs weakly to soil beneath the CWL. (1) Thus, in its present form, the chromium is
mobile in the environment and has apparently migrated to a depth of at least 23 meters below the
An electrokinetic process was demonstrated in one of these unlined chromic acid pits
during the summer and fall of 1996. The purpose of the demonstration was to show that an
electrokinetic process could be used to extract chromate contamination from in situ unsaturated
soils without significantly altering the soil moisture content. The treatment zone of the
electrokinetic field demonstration was located in a 3.7 meter by 3.7 meter area over the most highly
contaminated known area (see Figure 1). The electrode layout and spacing were chosen to produce
as uniform (or planar) an electric field as practical while minimizing soil heating effects. The active
treatment horizon was 1.8 meters thick and placed at 2.4 to 4.3 meters below the surface where the
greatest amount of contamination was located. A total of fifteen electrodes were installed for the
demonstration. During the installation of the electrodes numerous soil samples were collected to
characterize the site. One row of five anodes were placed in the center of the pit. Two rows of
five cathodes were placed six feet north and six feet south of the row of anodes near the edge of the
The electrode assemblies used are a unique, patented design (2,3) which uses porous
ceramic casings filled with electrolyte solution in which a drive electrode is deployed. The
electrolyte solution is held under tension inside the porous ceramic housing by an applied vacuum
which prevents saturation of the adjacent soil. Application of an electric potential to the electrodes
cause chromate anions in the soil porewater to migrate toward the anode electrode assemblies. The
chromate migrates through the porous ceramic into the electrolyte solution. A small amount of the
electrolyte solution is periodically pumped out of the anode assembly and into waste barrels. This
effluent stream was sampled and analyzed daily to determine the concentration and removal rate of
The fraction of current carried by a particular ionic species in response to an electric field is
defined as the transference number (sometimes called the transport number) (4,5):
Ti = I; I.,= i, / L (1)
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Lindgren, E.R. & Mattson, E.D. Electrokinetic demonstration at Sandia National Laboratories: Use of transference numbers for site characterization and process evaluation, article, March 1, 1997; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc675667/m1/4/: accessed February 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.