Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Patent
1996-04-18
1998-03-10
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
204515, 204516, 204517, 204600, 204648, C25C 122
Patent
active
057257523
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the decontamination of land using an applied DC electric field applied across electrodes buried in the land or other material comprising soil including landfill sites and soil heaps. It is well known that, when a DC current is applied across a porous non-conducting matrix made up of fine pores containing water, the water phase is drawn towards the cathode. This phenomenon is termed electroosmosis. The transport of the water phase does not obey Faraday's Laws but is related to the zeta potential of the solid phase, in this case soil. The zeta potential is a function of conductivity, viscosity and dielectric constant of the aqueous phase contained within the soil. When ionic species are present in the aqueous phase cations are moved through the soil with the water phase towards the cathode under the influence of the applied field. There will also be a simultaneous movement of anions towards the anode in the applied field against the electroosmotic flow. Electrophoretic movement of colloids suspended in the aqueous phase will also occur. Electroosmotic and electrophoretic processes are termed electrokinetic phenomena. The electroosmotic transport process is maximised when the conductivity is low.
As a result of the electroosmotic process, dissolved cations are moved to the cathode and by virtue of water movement past the soil interface, absorbed ions will be leached from the soil and will pass into the aqueous phase. Similarly, water-soluble organic species contained within the soil matrix will also pass into the aqueous phase. The extent of the leaching process will depend on the affinity of the ions or organic species for the water phase relative to that for the soil phase.
The use of water soluble electrolytes, surfactants, complexing agents or polyelectrolyte leachants and mineral or organic acids which have an affinity for absorbed/adsorbed species, can aid removal of contaminants when introduced in solution into the soil.
It is well known that when an electrode operates as a cathode in a near neutral aqueous solution, as would generally be the case for soil groundwater, the pH increases in the region of the cathode. If metal cations drawn with the aqueous phase to the cathode form insoluble hydroxides at the cathode, these hydroxides will precipitate and cause problems of removal from the vicinity of the cathode. Examples of cations forming insoluble hydroxides are iron, copper and chromium. A solution to this problem is to maintain the pH of the cathode solution (catholyte) below a threshold pH limit to prevent hydroxide formation and hence precipitation. This can be achieved by placing the cathode in a permeable chamber through which the catholyte is circulated, preferably slowly. The pH can be adjusted conveniently at or above ground level to prevent susceptible metal species precipitating. The dissolved species can be removed by a variety of means e.g. solvent extraction, electrochemical treatment, ion-exchange, activated carbon adsorption or alkaline precipitation. It is also common practice in some electrokinetic devices to control the pH adjacent to the anode. This is disclosed by Drinkard in U.S. Pat. No. 3,956,087 for the extraction of copper from ore, and by Hydroconsult B.V. for the removal of metal contaminants from soil as disclosed in EP-A-312 174.
As a consequence of the electroosmotic process water is drawn towards the cathode, causing the resistivity of the soil to increase around the anode. In order to prevent the soil from becoming too resistive adequate electrical contact between the soil and the anode must be maintained; this can be achieved by bringing electrolyte into contact with the soil, for example, by using a porous delivery means, e.g. an unglazed pipe or some other porous material so that electrolyte is provided in the vicinity of the anode. Although it may be thought advantageous to enclose the anode in a porous housing containing electrolyte and control the pH, as is required in the method described by Hydroconsult, there are disadvantages to this
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Roberts Edward Pelham Lindfield
Sunderland John Garry
EA Technology Ltd.
Phasge Arun S,.
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