Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Reexamination Certificate
1997-06-13
2003-06-10
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
C205S766000, C205S769000, C204S515000, C204SDIG008, C588S253000
Reexamination Certificate
active
06576116
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to extracting contaminants from soil, particularly to extracting contaminants from low-permeability soil layers, and more particularly to a hybrid joule heating/electro-osmosis process for extracting water soluble and non-aqueous phase liquid contaminants from saturated, low-permeability soil layers.
Contaminants migrating from various types of facilities, accidental spills, and industrial operations threaten health and ground water supplies. Such contamination often covers large volumes of soil underlying several acres of surface area. In view of the high cost of land, limited resources, and the fact that contamination can occur in densely populated areas, such as from leakage of fuel or gas storage tanks or lines, or industrialized areas adjacent dense populated areas, there exists a need to find economical and efficient technologies of remediation for rapid reclamation and rehabilitation of such areas.
Many facilities have suffered contamination of the vadose and saturated regimes by the spilling, or leakage, for example, of dense non-aqueous phase liquids (DNAPLs), such as trichlorethylene (TCE) and other solvents to produce localized sources of contamination. Pump-and-treat methods applied to the source may only dilute the contamination but not remove or even reduce it. Soil removal may be impractical owing to the large volumes that have become contaminated at some sites. Techniques that remediate by either in situ contaminant mobilization and extraction or by in situ breakdown of the contamination into harmless products avoid some of the disadvantages that characterize the pump-and-treat or soil-removal schemes. Steam injection, extraction and air-sparging remediation processes already exist for in situ treatment of moderate-to-high permeability soils. However, the techniques are generally inappropriate for application to low permeability clay layers that have suffered contamination. Cleaning up the higher permeability layers of the soil while neglecting the contamination in adjacent lower permeability formations may permit federal water quality standards to be achieved for several years following cleanup. However, eventually the low permeability contaminated soils will provide a contamination source for the surrounding soil layers, as leaching occurs which results in a decrease of water quality with time.
A process involving ohmic or joule heating of low permeability soil by passing alternating electrical currents through the soil, as a means of in situ contamination mobilization has been studied in some detail (see R. Newmark, Dynamic Underground Stripping Project LLNL Gasoline Spill Demonstration Report 6, UCRL-ID-113521, 1994; and C. R. Carrigan et al., A Fully Coupled Model for 3-D, Partially Saturated Flow and Transport in Soil Ohmically Heated by Application of Multiphase A. C. Electrical Potentials, UCRL-JC-120954, 1996). In addition to mobilizing contamination, ohmic dissipation also provides a potential source of heat for destroying contaminants in situ by hydrous pyrolysis (see K. G. Knauss et al., TCE: Thermodynamic measurements and destruction via hydrous pyrolysis/oxidation, Geol. Soc. Am. Abstr., Vol. 27, No. 6, p. 249, 1995). Electrical heating as a means of either contaminant mobilization or destruction appears to offer significant advantages over steam heating when low permeability clay layers are present. The heating of such soil layers is accomplished by implanting two or more alternating current (AC) electrodes on the edge of the targeted zone of contamination. Two electrodes are the minimum number, but the heating distribution will have little uniformity between the two electrodes. Thus, heating arrangements have used six or more electrodes in a circle to produce more uniform heating of the targeted area. In addition phase-shifting the alternating current applied to each electrode (e.g., the current applied to each electrode of a six-electrode array would be electrically phase shifted by 60 degrees), enhances the heating uniformity of the target zone at the center of the circle (see U.S. Pat. No. 5,330,291 issued Jul. 19, 1994 to W. O. Heath et al., for example). Also, using a six-electrode array, the electrical connections can also involve a three-phase heating arrangement with the six electrodes grouped in three pairs. While the six phase arrangement produces the greatest initial uniform heating, the most serious heating uniformity issues arise when the heating electrodes have been in operation long enough (typically an hour to a day depending on the current applied) to dry out the low permeability soil immediately adjacent to the electrodes. This presents a very serious problem for the ohmic or joule heating technique since groundwater in the soil is a major determining factor of the electrical conductivity of the soil. Thus, drying out of the soil immediately around an electrode is comparable to losing that electrode from the heating circuit, whereby maintaining current carrying capability by resaturation of the soil around the electrodes is necessary.
Another and different electro-remediation phenomenon, known as electro-osmosis, which has been utilized for various applications for about five decades, has been recently considered by researchers as a means of transporting across a porous regime either contamination or solutions intended to mobilize contamination. (See A. P. Shapiro et al., Removal of Contaminant From Saturated Clay by Electro-osmosis, Environ. Sci. Technol., 27, 283-291, 1993; and R. F. Probstein et al., Removal of Contaminants From Soils by Electric Fields, Science, 260, 498-503, 1993.) In addition experiments have demonstrated the ability of electro-osmosis to remove soluble organics from clays. (See Y. B. Acar et al, Phenol Removal From Kaolinite by Electrokinetics, J. Geotech. Eng., 118, 1837-1852, 1992; and U.S. Pat. No. 5,137,608 issued Aug. 11, 1992 to Y. B. Acar et al.), Electro-osmosis is the flow of an ion-containing liquid with respect to a charged surface (i.e., porous medium) in response to an applied electric field across the porous medium. Several models exist that describe the dynamic relationship between the ions in the fluid and the applied field that results in the flow. However, the often-assumed Helmholtz-Smoluchowski Model (See A. T. Young, Electro-kinetic Flow Processes in Porous Media and Their Applications (Chap. 5) in Advances in Porous Media (M. Y. Corapcioglu, ed. 2, Elsevier Amsterdam, pp. 309-395, 1994) provides the simplest and quantitatively adequate understanding of the process. The application of an electric field to a non uniformly distributed charge distribution in a fluid causes the fluid to be more or less dragged through to pore space. Clay pore walls tend to have a negative residual charge which produces the required non uniform charge distribution in the adjacent ion filled fluid. As a remedial technique, the phenomenon has significant potential for restoring low permeability, small-pore, contaminated soils (e.g., clays) since the induced flow does not depend strongly on pore size. On the other hand, for flow in a porous medium that is induced by a simple hydraulic head, there is a strong dependence on pore size with the flux being proportional to the cube of the effective pore diameter. Estimates (see A. P. Shapiro, et al., supra) indicate that electric field strengths of 100 V/m can give rise to electro-osmotic velocities (pore velocities) of about 10 cm/day on a saturated clay. Application of the electro-osmotic process is not new, as pointed out above, and has been applied successfully in civil engineering, separation science and physiological contexts. It has also been used on a large scale for the dewatering of saturated clays to provide a stable foundation for overlying structures. If transport in a low permeability contaminated layer is a necessary component of a remediation scheme, an electro-kinetic mechanism appears to be the only viable possibility.
The present invention involves hybrid processes or techniques for reme
Carrigan Charles R.
Nitao John J.
Carnahan L. E.
Phasge Arun S,.
The Regents of the University of California
Thompson Alan H.
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