Fact sheet: Electro-oxidation – ex-situ

From: Public Services and Procurement Canada

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Description

Ex-situ electro-oxidation is a technology that applies to soil, sediment and groundwater. Electro-oxidation consists of the application of an electric field between two electrodes, the anode and the cathode, in order to remove contaminants by direct or indirect oxidation. This technology reduces concentrations in ammoniacal nitrogen and of certain organic contaminants, including recalcitrant organic contaminants.

In a direct oxidation reaction, oxidation occurs directly on the anode, either by partial conversion of non-biodegradable organic compounds to more biodegradation compounds, or by a complete conversion of organic contaminants to carbon dioxide and water. In the case of an indirect oxidation reaction, oxidizing compounds are produced on the anode and allow the degradation of organic contaminants in the medium (water). Oxidizing compounds which may be formed are mainly hydrogen peroxide, hydroxyl radicals, ozone, peroxydisulfuric acid, hypochlorous acid and hypobromous acid. The presence of chlorides can generate chlorine gas. The latter, depending on the pH, may then be converted into hypochlorous acid and hypochlorite ion, which are in equilibrium. These substances can then be used to degrade organic contaminants. At the cathode, hydrogen is produced during the electrolysis of water. This production of hydrogen must be quantified, and safety measures must be put in place on site to guarantee a non-explosive environment.

This technology is at the demonstration stage for soil remediation. Although this technology has existed for several years in the treatment of industrial water, it is an emerging technology in the field of land remediation.

Sources:

Implementation of the technology

For the treatment of groundwater, extraction structures are implemented to collect the contaminated groundwater and convey it to the treatment system where it is treated and then discharged. The implementation of this technology can include:

  • Mobilization, access to the site and setting up temporary installations;
  • Installation of wells, collection trenches or the installation of permeable drains;
  • Installation of pumps and supply lines (often underground or in trenches designed to withstand frost and traffic)
  • Installation of a treatment system, including a transfer tank, supply pumps, electro-oxidation reactors, electric power sources, a hydrogen ventilation system, and if required, a mixing tank for adjusting or neutralizing the pH of the water, tanks and systems for adding other reagents. This system is normally installed in a small building or container[ST3] ;
  • Installation of a discharge system (evacuation to existing pipes, new outlet in surface water, reinjection, infiltration field or infiltration basin) to receive the liquid discharges from the treatment;
  • The implementation of effluent quality monitoring in order to ensure its environmental compliance with applicable regulation;
  • The establishment of an electrical energy supply system.

For the treatment of contaminated soil or sediment, conventional excavation equipment is used to remove the contaminated soil or mix to carry out the treatment on site. This may include:

  • Mobilization, access to the site and setting up temporary installations;
  • Temporary storage and mixing of soils to homogenize the soils to be treated, including the installation of an impermeable surface to protect the underlying soils;
  • Addition of additives to improve the conductivity of the soil;
  • Management of treated soil in compliance with applicable regulation (off-site disposal, spreading on the site or backfilling of excavated areas);
  • Restoration of the land surface;
  • Vapour and gaseous effluent controls may be required if the contaminants present in the treated soil or groundwater are volatile.

Successful implementation of ex-situ electro-oxidation requires some elements to be selected judiciously:

  • Type of electrode;
  • Current density;
  • Inter-electrode distance;
  • Type and concentration of electrolytes;
  • Targeted final concentration based on the initial concentration of contaminants to be treated.

During the feasibility study, an analysis of the equipment cost as a function of the operating cost is required, particularly for the anodes which can represent a considerable cost.

Materials and Storage

  • This technology is implemented using traditional and commonly available methods and equipment for excavation or construction of wells, trenches or drains;
  • Treatment units can be built on site or pre-assembled and transported in shipping containers, trailers or on pallets;
  • Treatment equipment requires the installation of an electrical power source;
  • Treatment equipment includes electrodes, which may require special storage, other equipment for their handling and lifting, such as cranes, a forklift or a pulley or automated lifting system;
  • Hazardous materials stored on site may include compressed gases, strong oxidizers, acids or bases. The separation and containment of these products is very important;
  • In some cases, the reaction between reagents and contaminants, including unexploded ordnance and explosives, is sufficient to cause combustion;
  • 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 would be impracticable. The costs of using electricity generated by diesel are, however, generally higher. In some areas with strong solar radiation, solar panels with trackers and a battery can be used;
  • The addition of polar chemical compounds or compounds serving as reactive intermediates may be necessary for the treatment of soils or sediments;
  • Extraction, treatment and effluent evacuation systems can be built on site or previously assembled and sent to the site.

Residues and Discharges

Electro-oxidation can produce solid (precipitation in the presence of iron, formation of metal complexes, formation of polymers) and gaseous (formation of chlorine gas) residues which must be treated or disposed of properly. Hydrogen is produced at the cathode and must be properly monitored, captured and disposed.   

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
  • Suspended solids concentrations
  • Turbidity
  • Electrical conductivity
  • Coloring

Recommended trials for detailed characterization

Physical trials

  • Evaluation of electrical conductivity and temperature of water, soil, sediment or sludge

Other information recommended for detailed characterization

Phase III

  • Volume of contaminated material to treat
  • Volume or flow of water to be treated

Notes:

A small-scale pilot test to verify the effectiveness of the technology and to determine the number of pairs of electrodes required, their positioning, their required active area, the intensity of the electric current to be applied, etc., can improve the technology performance.

Applications

  • Treatment of pumped groundwater;
  • Treatment of soils or sediments accessible by excavation;
  • Allows the treatment of organic compounds and certain metals and non-metallic inorganic compounds such as ammoniacal nitrogen and nitrates.

Applications to sites in northern regions

The application of this technology in a northern environment may be difficult because of the monitoring that such a system requires. For remote sites, this implies greater mobilization and leads to higher on-site monitoring costs. The availability of equipment is limited and requires additional mobilization. The working windows are relatively short, considering that this technology can involve pumping groundwater. This activity, as well as the routing of water to the treatment unit, could require effort and generate additional costs in low temperatures or simply when there is a risk of frost. In addition, this rehabilitation technique requires a source of electrical energy, so it is not well suited to northern and remote areas.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
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
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)

Following the treatment of soil, whether it is used for backfilling excavations or whether it is imported material, an environmental and geotechnical control of the materials must be carried out to ensure that these soils do not exceed criteria applicable for the site and that they do not create problems of geotechnical stability or differential settlement. For groundwater treatment, long-term considerations are related to pumping technology and its potential impacts on site hydrogeology, and not to electro-oxidation treatment technology as such.

Secondary by-products and/or metabolites

Some unwanted by-products can be formed during the numerous redox[ST4]  reactions generated by electro-oxidation. In presence of chloride ions, the formation of chlorine gas is possible, which must lead to its conversion to hypochlorite. It can also lead to the production of toxic organochlorine products.

It is also possible that solid deposits form (precipitate) which must be removed and disposed of. Soil near the electrodes also needs to be removed and disposed of after treatment since their chemistry is altered by the precipitation process (change in pH, for example).  Significant changes in pH at the periphery of the electrodes can induce the mobilization or the formation of by-products such as the mobilization of heavy metals.

Limitations and Undesirable Effects of the Technology

  • The cost of electrodes can be considerable for certain materials such as boron-doped diamond electrodes, but have longer lifetimes than other electrodes formed by mixture of oxidized metals;
  • Each type of electrode produces different oxidants at the anode, which is important to understand before selecting an anode type for a given application;
  • The real lifespan of certain electrodes remains to be established beyond the supplier’s guarantee, since little on site data is available;[ST5] 
  • The formation of potentially harmful subspecies such as organochlorines;
  • The use of improper settings such as an inadequate electrical current can cause corrosion of electrodes or colouring of the solution;
  • Some unwanted reactions can occur which may reduce the contaminants degradation efficiency;
  • Matrix heterogeneity makes the optimization of electro-oxidation treatment more complex;
  • The electro-oxidation process requires an electricity input, which can make it a difficult solution to implement in the absence of energy sources;
  • Technology only applicable for soils with a high proportion of clay (negative surface charge);
  • Presence of (light and dense) non-aqueous phase liquids 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;
  • In some cases, the reaction between reagents and contaminants, including unexploded ordnance and explosives, is sufficient to cause combustion.

Complementary technologies that improve treatment effectiveness

  • Electro-oxidation can be combined with other technologies to optimize rehabilitation. For example electro-oxidation can be combined with chemical oxidation as in the presence of ferrous iron, it can be used to carry out Fenton reactions. 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 treatment;
  • electro-oxidation can be combined with other electrochemical processes such as electro-coagulation, where hydroxide complexes are formed with the dissolved and insoluble pollutants by phenomena and lead to their precipitation; or electrokinetics which mobilizes pollutants towards the electrodes where oxidation reactions allow their degradation;
  • 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.

In the case of groundwater, a regular and automated system for cleaning lime deposits on the cathodes is required and, in some cases, secondary treatment for unwanted by-products is also required. The gaseous effluents from the treatment may also require treatment if they are toxic and in sufficient concentration.

Application examples

The following websites provide application examples:

Performance

This technology is relatively complex and expensive. It mainly targets the treatment of pumped groundwater which is contaminated with unusual compounds that conventional technologies cannot treat.

For soil, this technology can be useful for fine soils (clay, silt, clay silt) and for recalcitrant contaminants for which few restoration technologies are effective. There have been few commercial applications for electro-oxidation technology in North America. However, this ex-situ technology has shown its effectiveness in a few decontamination cases in Europe.

In Montluçon, France, an ex situ electro-oxidation treatment was carried out on the sludge from a wastewater treatment plant during its reconstruction. The treatment had to be carried out in 7 days and the regulatory threshold was set at less than 5 mg/kg of dry mass. The initial content varying between 15 and 54 mg/kg of dry mass, with an average of 28 mg/kg of dry mass. The treatment was carried out in a wooden reactor insulated with plastic. Two electrodes were installed at opposite ends of the reactor. The electrodes consisted of a 1 x 2 m steel cathode, and an anode composed of 4 non-ferrous rods 30 cm in diameter and 1 m long. An electric current with an instantaneous power of 2.3 kW was applied to the sludge for 7 days. After 7 days of treatment, the concentrations in the sludge were between 0.02 mg/kg and 0.35 mg/kg dry mass, with an average of 0.126 mg/kg dry mass. The treatment objective was therefore largely achieved in 7 days with low electricity consumption.

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;
  • Wise choice of electrodes to reduce maintenance costs and optimize treatment;
  • 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

Applies only for soils treatment

Monitoring of conditions favorable to dispersal during the excavation of the soil to be treated

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

Applies

Monitoring of the discharge point or the perimeter, choice of parameters, types of sample and frequencies according to the source, risk and general requirements, minimize the generation and migration of water.

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

Does not apply (by-products are managed by the treatment system)

N/A

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

Applies

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

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 : Imad E. Touahar ing.jr, M.Sc.A., M.Sc., Nathalie Arel ing. M.Sc., Valérie Léveillé ing., M.Sc.A., PhD, Christian Gosselin ing., M.Sc., Golder Associés Ltd

Latest update provided by : Imad E. Touahar ing.jr, M.Sc.A., M.Sc., Nathalie Arel ing. M.Sc., Valérie Léveillé ing., M.Sc.A., PhD, Christian Gosselin ing., M.Sc., Golder Associés Ltd

Updated Date : December 1, 2021

Version:
1.2.1