Principle

For in-situ electro-reclamation of soil and groundwater contaminated with e.g. heavy metals and/or polar components, use is made of direct current (DC). This causes the following electrokinetic phenomena to occur:

• Electrolysis or movement of ions and ion complexes (ion migration) 
• Electro-osmosis or movement of water from anode to cathode (hydromigration) 
• Electrophoresis or movement of charged particles

The electrokinetic phenomena result in displacement of ions and water soluble pollutants, thereby disturbing the existing electrochemical equilibrium between the solid phase (metal salts, clay and other organic soil particles) and the liquid phase (ground- and porewater). This results in desorbtion, for example, of heavy metals (cations) or cyanides (anions) by ion exchange from the solid (immobile) phase to the liquid (mobile) phase. A second effect is the generating of H+ ions at the anode and OH- ions at the cathode as a result of the dissociation of water. H+ ions move twice as fast as OH- ions and 5 to 6 times faster than heavy metals. As a result, the soil acidifies from the anode into the direction of the cathode, leading to an increase in solubility of the pollutants and thus to accelerated desorption. However, a similar but slower drift of hydroxyl ions moves from the cathode displacing anions adsorbed on the soil particles. This results in increased alkalinity into the direction of the anode. If no measures are taken, the soil around the anode will, after a certain period, acidify (pH reduction) to such an extent that practically all energy is used for the transport of the H+ ions. Likewise pH around the cathode will increase to such an extent that precipitation of metal hydroxides will take place in the soil. These negative effects are avoided by actively managing pH around the anodes and cathodes.

Application

The electric current is induced into the ground by means of rows of alternating anodes and cathodes. The distance between electrodes of both equal and opposite charge depends on site specific conditions but, in general, amounts to 1.5 or 2 m. Anodes as well as cathodes are integrated into seperate closed loop pump systems, wherein electrolytes are circulated. Via these electrolytes pH is controlled at a predetermined level and the pollutants desorbed and mobilized under the influence of the applied potential are collected. Conditioning of the electrolytes as well as periodical removal of the contaminants from the electrolytes is effectuated in a special installation. Anodes and cathodes can be installed vertically as well as horizontally. Conditioning and purification systems of the electrolytes as well as the energy supply are accommodated in containers. If necessary, electricity cables and extraction ducts and pipes can be installed subsurface.

Site characteristics

Active in situ clean up of soil and groundwater polluted with a variety of contaminants, including heavy metals, arsenic, nitrates, phosphates, halogenides, and polar and/or water soluble organic compounds such as cyanides, PAH , PCB’s, phenols, and nitroaromatics (such as TNT). The technology is applicable for diffusely dispersed pollutants both in the unsaturated and saturated zone and in clay, sand, and peat soils. Minimal moisture content about 15% to 20%. In addition, the technology can be applied in the form of an electrokinetic fence as a passive in-situ cleanup method for containment and remediation of groundwater plumes contaminated with e.g. heavy metals. In-situ. In this application, the heavy metal ions transported with the groundwater are captured by a vertical row of electrodes placed perpendicular to the groundwater flow direction and are subsequently removed from the electrolytes above ground.

Combination with other techniques

As an electro-kinetic fence (see electro-reclamation of organic pollutants).

Advantages

Functions independently of the lithological composition of the subsurface and is applicable to relatively great depths and under houses and buildings. There is no disturbance of the groundwater flow regime, and with the applied current density and voltage there is no destruction of microbiological life.

Limitations

Not economically applicable to heavy metals in metallic form, such as metal grindings, slag and cinder, concretions, and paint particles (putty).

Supplementary Provisions

Availability of an electric power supply is a prerequisite. If no connection via the grid is possible, a generator can be used. Because of the high noise levels, a generator can only be deployed outside residential areas. Temperatures > 40 °C can have an adverse effect on some coatings of subsurface cables. The presence and location of these cables must be known so that they may be insulated. Cathodic protection of subsurface metal cables, pipes, and objects such as tanks may be necessary, when they are directly in the electric current zone).

Cleanup Duration

Is dependent on the nature, concentration, and extent of the pollution. In theory, 1,500 to 2,000 m³ of contaminated soil can be treated with the present installations. More extensive polluted sites are cleaned up in sections. Cleanup duration varies from a few months to a few years.

Costs

Dependent on specific factors such as the nature and extent of the pollution, the geohydrological situation, specific soil resistivity, nature of the site (developed or undeveloped), availability of electric power, etc