Electro-reclamation Diagram
The term electro-reclamation consists of a number of concepts which can be summarised as follows:
- Electro-migration occurs via the potential difference between anodes and cathodes, whereby a negatively loaded particle migrates towards the anode and a positive particle towards the cathode. Electro-heating is the so-called Coulomb effect caused by a direct or alternative current in a conductor: Heating whereby the generated heat capacity is equal to the electric resistance multiplied by the square of the realised current flow. Due to the increased temperature, mainly in less water-permeable zones, VOCI’s are mobilised which can later be recovered via an extraction system
- Electrophoresis is the transport of loaded suspended particles, or colloids, under the influence of an electric field; any potential pollutants that may be bonded to the particles are thus able to travel through the soil (towards an electrode).
- Electro-osmosis is defined as the movement of soil moisture or ground-water from the anode to the cathode in an electrolytic cell. Pollutants that are solvent (not loaded) in soil moisture, such as VOCI’s, can be transported with the water via advection and can be pumped using an extraction system close to the cathode.
- Electro-bioremediation is a term which describes the clean up method where, based on the processes described above, micro-organisms are stimulated via electro-migration, using nutrients such as nitrate, thus allowing them to migrate towards polluted soil zones. The heating effect as a result of the electric current, will further stimulate pollutant-degrading bacteria.
An important additional feature for implementing an electric field in soil is that it induces the electrolysis of water:.
To the anode: 2 H2O ® 4 H+ + O2(g) + 4 e-
To the cathode: 2 H2O + 2 e- ® 2 OH- + H2(g)
An acid/base front is created in the soil and oxy-hydrogen gas is produced at the electrodes.
Electro-reclamation for organic compounds is a technique whereby the soil is heated using an alternative current field, up to 70 to 90 °C – whereby the solubility of pollutants is increased. Some of the pollutants will also become volatile as a result of the temperature increase. The pollutants are then transferred in the water phase and the air phase using extraction systems, and are then treated in above-ground purification installations.
If electro-reclamation is implemented in the saturated zone, the covering soil will also be heated. In this manner, by heating the saturated zone, the unsaturated zone can also be heated via convection and radiation (Van Deynze et al., 1998).
At present (2004), electro-reclamation is not a conventional clean up technique.
Implementation area and implementation conditions
In principle, the technique can be implemented on all types of ground, including heavy clay and peat soils. A large buffer capacity and a large tying capacity (to clay particles or organic matter) results in a longer clean up operation and increased energy costs. The soil may not contain metal substances or insulating substances (wood, plastic etc). To date, the maximum treatment depth for in-situ implementation of electro-reclamation has been 3m-mv. The moisture content is an important parameter for optimal processing and must be at least around 20% in order to ensure a certain level of pollutant mobility.
Costs
The costs per tonne ground are strongly determined by the time needed for the clean up. A large current input is required for fast clean ups, which greatly increase the (energy) costs per tonne. The speed of the clean up is also determined by the mobility of the pollutant in the matrix, as well as the soil’s electric resistance. Heavily polluted soils with a high exchange capacity and high buffering capabilities require a relatively high quantity of energy/m³ in order to clean the pollutant to the desired level (Van der Gun et al., 2000).
The average cost is around 90 euro per tonne.
These costs are generally divided as follows:
Catchment and disposal: 5 to 10 %
Installation: 8 to 20 %
Electro-reclamation: 50 to 80 %
Additional costs: 8 to 10 %
Environmental burden and measures to be implemented
Due to the rise in soil temperature, some volatile compounds could evaporate. This effect must be considered if there is a cocktail of pollutants. If there is a high voltage difference, Cl2-forming may occur at the anode. This emission can be prevented by accurately arranging the pH and redox potential of the anode.
In a purification installation, the polluted water is separated in two waste streams: One with cleaned electrode fluid and one with filter residue. The cleaned liquid can be re-used in the process and the filter residue must be processed. The quantity of filter residue in relation to the quantity of soil material, amounts to ca. 0.5-1 weight %.
Due to the high voltage, the terrain must be encircled by fencing.