From: Public Services and Procurement Canada
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.
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:
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:
Successful implementation of ex-situ electro-oxidation requires some elements to be selected judiciously:
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.
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.
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.
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.
This technology has potential for specific application to certain emerging contaminants such as perfluoroalkyl substances and endocrine disruptors.
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.
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.
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.
The following websites provide application examples:
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.
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
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
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
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
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
Records checks and pre-excavation permits, development of excavation or drilling procedures and emergency response[ST7]
Accident/Failure - leak or spill
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Accident/Failure – fire or explosion (inflammable vapours)
Other - Handling contaminated soils or other Solids
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
