Fact sheet: Electro-oxidation – in situ

From: Public Services and Procurement Canada

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Description

In situ electro-oxidation consists of applying an electrical field in the soil in order to oxidize contaminants present in both soil and groundwater. The application of an electric current allows, among other things, to partially transform (reduction of toxicity) or to completely decompose the contaminants by oxidation processes. This technology is at the demonstration stage only.

This remediation technique is particularly effective in low permeable saturated soils containing contaminants strongly adsorbed to soil particles.

Sources:

Implementation of the technology

Electro-oxidation is implemented by the direct application of a low-voltage electric current (energy from 0.2 to 1.5 kWh/tones of soil) using electrodes of about 10 cm in diameter driven into the ground at depths varying between 0.5 and 2.0 m.

Different configurations can be used for an in situ application of the technology. The electrodes can be arranged vertically or horizontally directly in the ground. Generally, the anodes and cathodes are spaced a few metres apart. The electrodes can be designed with different materials such as platinum, lead dioxide (PbO2), diamond doped with boron or titanium compounds. However, less expensive materials such as stainless steel or carbon are generally used.

The implementation of this in situ technology can include:

  • mobilization, access to the site and the establishment of temporary facilities;
  • the establishment of an electrical energy supply system by connection to the electrical network or by the use of generators on the site according to the needs and accessibility of the site;
  • the installation of electrodes in the soil and their connection;

Other amenities or treatment systems may be required depending on the complementary technology that will be implemented for the recovery or treatment of soil or groundwater. These arrangements are specified in the Fact sheets associated with each technology.

Materials and Storage

  • When installing the electrodes, a forklift may be necessary to move the equipment around the site;
  • Required electricity can be supplied by means of a trailer containing diesel generators in cases where the construction of a connection to the electricity network is impracticable.

Other materials or storage may be required on site depending on the additional technologies that will be implemented for the treatment of soil and groundwater.

Residues and Discharges

Electro-oxidation causes many redox reactions which can lead to the formation of certain undesirable by-products such as the formation of hydrogen fluoride, chlorine gas, bromate, perchlorate or adsorbable organic halides, but also of strong acids or bases around the electrodes. Installation of a recovery and treatment system for these by-products may be necessary.

In most cases, recovery of the contaminants is not necessary. These are mineralized or considerably altered to no longer pose a risk to the environment.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Oxidation reduction potential (Eh)
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
  • Dissolved oxygen
  • Alkalinity and hardness
  • Concentration of metals, particularly iron
  • Concentration of chloride, sulfates, and bromides ions
  • Chemical oxygen demand or total organic carbon as indicators of the concentration of organic compounds
  • Concentration of oxidants such as chlorine, chloroamine, bromine and broamine
  • Concentration of chlorination by-products such as perchlorate, trihalomethanes and halo acetic acid

Physical analysis

  • Temperature
  • Soil water content
  • Soil granulometry
  • Suspended solids concentrations
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • etc.
  • Presence of non-aqueous phase liquids (NAPLs)
  • Soil buffering capacity
  • Turbidity
  • Electrical conductivity
  • Coloring

Recommended trials for detailed characterization

Physical trials

  • Soil electrical conductivity
  • Evaluation of the radius of influence of the electrodes

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

  • Concentration and nature of contamination

Phase III

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

Notes:

Small-scale trials (laboratory / pilot) to verify the efficiency of the technology and to determine the number of pairs of electrodes required, their positioning, etc. is recommended due to the state of development of the technology (demonstration).

The presence of conductive or insulating elements which can affect the electrical conductivity of the ground (pipe, building, fence, etc.) must be verified before the application of this technology.

Applications

  • Allows the treatment of soil, sludge, sediment and groundwater;
  • Designed for poorly permeable soils (clay, silt and clay silt);
  • Allows the treatment of saturated soils.

Applications to sites in northern regions

Considering the risk of frost and the temperature conditions in northern environments and since this rehabilitation technology requires high energy consumption, significant mobilization and the work windows being relatively short, it is not well suited to northern and remote areas.

Treatment type

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

State of technology

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

Target contaminants

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

Notes:

This technology has potential for specific application to certain emerging contaminants such as perfluoroalkyl substances and endocrine disruptors.

Treatment time

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

Long-term considerations (following remediation work)

This technology induces significant geochemical changes in soil and groundwater (acidification and basification near the electrodes, formation of by-products). The return to the initial conditions must be followed up by groundwater quality monitoring campaigns and one or more soil characterization campaigns.

Secondary by-products and/or metabolites

Depending on the contaminants treated, certain undesirable by-products can be formed. It is important to learn about the different by-products linked to the degradation of certain contaminants to effectively treat the affected matrix.

In addition, heavy metals can precipitate prematurely in soil near the electrodes. Therefore, this soil may need to be removed and treated, if required. Finally, the soil at the periphery of the electrodes can be affected by the increase in pH and will therefore potentially have to be treated.

Limitations and Undesirable Effects of the Technology

  • Acid environments can cause corrosion of the electrodes;
  • This technology is not very effective for soil with low water content;
  • The heterogeneity of the environment can affect the efficiency and / or the design complexity of this technology;
  • Technology only applicable for soil with a high proportion of clay (negative surface charge);
  • The presence of non-aqueous phase liquids (light and dense) can obstruct the system and interfere with the application of the technology;
  • Geochemical changes at the periphery of the electrodes can increase the solubility of unwanted chemical species such as heavy metals;
  • Electricity costs can be high depending on the geographic position and the availability of an electrical network;
  • Potentially conductive infrastructure (above ground or underground) may hinder the effective implementation of the technology and / or require significant health and safety measures.
  • Hydrogen is produced during the electrolysis of water at the cathode. This production must be quantified and security measures must be put in place on the site to ensure a non-explosive environment.
  • The choice of electrodes to reduce maintenance costs and optimize treatment.

Complementary technologies that improve treatment effectiveness

Electro-oxidation can be combined with other technologies to optimize rehabilitation. For example:

  • In the presence of heavy metals or certain organic contaminants, this technology can be combined with electro-kinetics and the addition of surfactants, solvents or oxidants to improve the mobility of the contaminants and thus migrate the contaminants to an area of in situ treatment where another restoration technology can be used;
  • electro-oxidation can be combined with chemical oxidation as in the presence of ferrous iron, it can be used to carry out Fenton reactions. The combination of an electrochemical process such as electro-oxidation and the Fenton process is called electro-Fenton.This electro-Fenton process is based on the reaction of hydrogen peroxide with the ferrous ion to form the hydroxyl radical which is a powerful oxidant capable of degrading most of the organic compounds until the final stage of the mineralization. The combination of these two technologies allows local generation of hydrogen peroxide by electro-oxidation and benefits from the efficiency of the Fenton reactions;
  • electro-oxidation can be combined with biological treatments in the presence of recalcitrant organic compounds, thus taking advantage of the low operating cost of biological processes and the short treatment times of the electro-oxidation process.

Required secondary treatments

A system for cleaning 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

This technique is effective in environments with fine soils (clay, silt, clay silt) with high adsorption capacity where few restoration technologies are effective. It applies to both organic and inorganic contaminants. It allows to simultaneously treat several metals as well as a wide variety of organic contaminants.

In Europe and the United States, relatively large-scale projects have already been carried out. For example, the application of this technology to treat mercury in the soil has produced convincing results. Indeed, tests at the Union Canal in Scotland, where an average concentration of total mercury of 243 mg/kg was present in silts, showed a reduction in concentrations of 124 mg/kg after 12 days of treatment.

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;
  • During the electrolysis of water at the cathode, hydrogen is produced at the cathode. The production of this hydrogen must be quantified and safety measures must be implemented on the site to ensure a non-explosive environment;
  • Use of renewable energies and low-energy equipment for secondary treatment (geothermal, wind, solar) and if possible, for primary treatment.

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Does Not apply

Monitoring and Mitigation

Dust

Does not 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 vapor 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

Applies

Estimation of the potential for steam emissions and emissions monitoring (choice of parameters, types of samples and intervention levels depending on source, risk and local requirements) to confirm predictions

Runoff

Does not apply

N/A

Groundwater - displacement

Applies in the case of groundwater treatment

Modeling and monitoring using pressure sensors.

Groundwater - chemical/ geochemical mobilization

Applies in the case of groundwater treatment

Geochemistry modeling, laboratory test and/or pilot tests. 

Groundwater quality monitoring

Groundwater - by-product

Applies

Estimation of potential by-products and groundwater quality monitoring

Accident/Failure - damage to public services

Applies

Records checks and pre-excavation permits, development of excavation or drilling procedures and emergency response[ST7] 

Accident/Failure - leak or spill

Does not apply

N/A

Accident/Failure – fire or explosion (inflammable vapours)

Applies

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

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 : Maïté Faubert, M.Sc., Nathalie Arel ing. M.Sc., Valérie Léveillé ing., M.Sc.A., PhD, et Christian Gosselin ing., M.Sc., Golder Associés Ltd

Latest update provided by : Maïté Faubert, M.Sc., Nathalie Arel ing. M.Sc., Valérie Léveillé ing., M.Sc.A., PhD, et Christian Gosselin ing., M.Sc., Golder Associés Ltd

Updated Date : December 1, 2021

Version:
1.2.1