Fact sheet: Electrokinetics—in situ

From: Public Services and Procurement Canada

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Description

Electrokinetics is a technology that extracts or immobilizes organic and inorganic contaminants from soil, sediment, sludge or groundwater. Electrokinetics requires the application of a low-intensity electric current between pairs of electrodes (anodes and cathodes) located in and around a contaminated area or installed perpendicular to the direction of groundwater flow to create a barrier. The difference in potential between the electrodes creates physicochemical changes in soils or sediments and facilitates the migration of ions and water to the electrodes. Metallic ions, ammonium, and organic compounds with a positive charge migrate towards the cathode, while anions such as chloride, fluoride, nitrates, and negatively charged organic compounds migrate towards the anode. The application mechanisms of electrokinetics include dissolution, precipitation, volatilization, sorption and three principal transport mechanisms, namely electroosmosis, electromigration and electrophoresis.

In the presence of organic contaminants with a low polarity, separation by electrokinetics alone is not very effective. In these cases, chemo-electrokinetic (a chemical compound with a strong polarity such as carboxymethyl-ß-cyclodextrin) is added to the contaminated matrix to enhance the solubility of the hydrophobic organic compounds and the migration towards an electrode according to the variation of electrical potential, and can improve the application of the technology, especially in fine particle enriched soils (clayey or silty soils).

Electrokinetic technology can be combined with the extraction of contaminants. The processes of contaminant extraction may consist of: electro-deposition (formation of a deposit on the surface of a conductive extractor), precipitation near the electrodes, pumping of water near the electrodes, volatilization of contaminants combined with steam extraction or formation of a complex with ion-exchanging resins. When there is no extraction of contaminants, electrokinetics is used to move the contaminants to an in situ treatment zone, located between the pairs of electrodes where another remediation technology is applied.

Sources:

Implementation of the technology

Electrokinetics causes much oxidation-reduction reactions that can lead to the formation of some undesirable by-products, such as the formation of chlorine in gaseous form, acids, or strong bases at the periphery of the electrodes. The installation of a recovery and treatment system for gaseous effluents may be necessary.

The implementation of this in situ technology may include:

  • Mobilization, access to the site and setting up temporary installations;
  • The installation of an electricity supply system;
  • The installation of electrodes in the soil along with their connection;
  • The installation of a system for the recovery and treatment of gaseous effluents and the control of atmospheric emissions.
  • When installing the electrodes, a crane and other equipment such as a forklift may be needed to move the equipment to the site;
  • The necessary electricity can be supplied through a trailer containing diesel generators in cases where the construction of a connection to the power grid would be impracticable. The costs of using electricity generated by diesel, however, are generally higher;
  • The addition of a polar chemical compound may be required for treatment;
  • The recovery and treatment system of gaseous effluents can be built on-site or pre-assembled and brought to the site.

Other installations or systems may be required depending on the additional technology that will be put in place for the recovery or in situ treatment of soils or groundwater. These are specified in the fact sheets associated with each one of these technologies.

Materials and Storage

Other materials or storage may be required at the site, depending on the complementary technologies that will be put in place for the treatment of soil and groundwater.

Residues and Discharges

Electrokinetics does not allow the destruction of contaminants; it is used to migrate contaminants to an in situ treatment area where another remediation technology can be used. This technology can produce solid (precipitation) and gaseous residues that need to be processed or appropriately disposed.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Metals concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • etc.
  • Presence of non-aqueous phase liquids (NAPLs)
  • Soil buffering capacity

Recommended trials for detailed characterization

Physical trials

  • Evaluation of the radius of influence
  • Soil electrical conductivity

Hydrogeological trials

  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)

Phase III

  • Soil stratigraphy
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the hydraulic gradient

Notes:

Small-scale studies are recommended to determine the applicability of the technology and the best design plan, including the number of pairs of electrodes and the positioning of each pair, to adapt the technology to the characteristics of the contaminated site. Conducting or isolating materials on the site can change the electrical conductivity of the soil (pipeline, building, fence, etc.), and therefore must be verified before the application of the technology.

Applications

  • Applicable for treatment of contaminated soils, sediments, sludge and groundwater;
  • Suitable for low-permeability soil (clays and silt);
  • Suitable for saturated or partially saturated areas in the soil matrix;
  • Allows the migration of inorganic (ions and metals) and organic contaminants to recovery points for extraction or secondary treatment.

Applications to sites in northern regions

Since this rehabilitation technique requires high energy consumption, it is not well suited to northern and remote environments. In addition, this technology also requires the implementation of a complementary technology, which could be very expensive to install.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Applies
Control
Applies
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
Chlorobenzenes
With restrictions
Explosives
With restrictions
Metals
Applies
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Applies
Pesticides
With restrictions
Petroleum hydrocarbons
With restrictions
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
With restrictions

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Applies
More than 5 years
Applies

Long-term considerations (following remediation work)

For groundwater treatment, when a system is decommissioned, it is often possible to observe an increase in contaminant concentrations in the groundwater after the system shut down. Concentrations following system shutdown should be monitored to ensure they remain below the applicable criteria.

This technology induces significant geochemical changes in the aquifer (acidification and basification near the electrodes). Return to ambient conditions must be monitored.

Secondary by-products and/or metabolites

Numerous redox reactions can be generated by electrokinetics, which may create noxious gases such as chlorine gas that must be treated. The formation of solid deposits (precipitates) is also possible during this technology, which must be recovered and disposed. Soils near the electrodes should also be removed and disposed since their chemistry is altered by the precipitation process (e.g. pH change). Significant changes in pH around the electrodes can induce the formation and mobilization of secondary products, such as the mobilization of heavy metals.

Limitations and Undesirable Effects of the Technology

  • Electrokinetic technology, when applied without chemical compounds, is not an efficient technology to treat non-polar or non-ionic contaminants, such as most organic contaminants. However, chemo-electrokinetic technology is efficient for a wide range of organic compounds;
  • Acidic environments may cause corrosion of the electrodes;
  • This technology is not efficient when the soil water content is lower than 10%;
  • The heterogeneity of the soil matrix may affect the efficiency of the decontamination process;
  • Electrokinetic technology requires a lot of electricity and is may not always be economically viable;
  • Electrokinetics is more efficient in clays because clay particles have a negative surface charge;
  • The presence of non-aqueous phase liquids (light and dense) can obstruct the system and impair the application of the technology;
  • Geochemical changes at the periphery of electrodes can increase the solubility of unwanted chemical species such as heavy metals;
  • Potentially conductive infrastructure (above ground or underground) can hinder the effective implementation of the technology and/or require significant health and safety measures.

Complementary technologies that improve treatment effectiveness

Electrokinetic technology may be combined with other technologies to optimize remediation. For example:

  • Electrokinetics can be used to enhance the migration of organic contaminants towards an in situ treatment zone, where in situ remediation technology is being applied (example: biotreatments, permeable reactive walls, etc.).
  • Electrokinetics can be used to transport treatment agents required for other technologies, such as zero-valent iron nanoparticles, oxidants (peroxide, permanganate or other), or biological amendments.
  • The use of electrokinetics has previously been used in conjunction with a phytoremediation treatment, or during the use of surfactants or ultrasound treatments.

Required secondary treatments

A system for recovering and treating contaminants that can accumulate around the electrodes may be required, as well as a system for recovering and treating gaseous effluents, if required.

Application examples

The following websites provide application examples:

Performance

Electrokinetic technology, although expensive, is an interesting in situ remediation solution to treat contamination within fine particle soils (clayey and silty soils) where other remediation technologies are not suitable. There have been only a few commercial applications of electrokinetic remediation in North America. However, there have been successful demonstrations of the in situ electrokinetic remediation technology in Europe.

In the state of Wisconsin in the United States, a site consisted of high levels of trichloroethylene and dense non-aqueous phase liquid, with some areas having concentrations higher than 1000 mg/kg. The soil consisted of saturated clay and silt. Remediation was carried out with two anodes and a cathode and monitoring for this site was done remotely, thereby saving some operational costs. The entire project lasted over 2 years, until 2008. Post-treatment sampling showed that more than two thirds of the total trichloroethylene was withdrawn, and the dense non-aqueous phase liquid areas decreased

Measures to improve sustainability or promote ecological remediation

  • Optimized device installation that provides energy to reduce energy requirements.
  • Optimization of the time of year in which the process is in operation in order to reduce energy costs (avoiding work during winter months).
  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Operation in pulsed mode.
  • Use of renewable energies and low-energy equipment for secondary treatment (geothermal, wind, solar).

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Doesn’t apply

Monitoring and Mitigation

Dust

Doesn't apply

N/A

Atmospheric/Steam Emissions—Point Sources or Chimneys

Applies

Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Atmospheric/Steam Emissions—Non-point Sources

Applies

Monitoring of soil vapour migration (choice of parameters, types of samples and type of intervention [depending on source, risk or local requirements]), validation of the presence of potential preferential pathways.

Air/steam—by-products

Doesn’t apply

N/A

Runoff

Doesn’t apply

N/A

Groundwater—displacement

Doesn’t apply

N/A

Groundwater—chemical/ geochemical mobilization

Doesn’t apply

N/A

Groundwater—by-product

Doesn’t apply

N/A

Accident/Failure—damage to public services

Applies

Review existing records and obtain pre-drilling permits, establish special excavation procedures, preparation/practice of emergency interventions.

Accident/Failure—leak or spill

Doesn’t apply

N/A

Accident/Failure—fire or explosion (inflammable vapours)

Doesn’t apply

N/A

Other—Handling contaminated soils or other Solids

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

References

Author and update

Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR

Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada

Updated Date : April 23, 2014

Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1