From: Public Services and Procurement Canada
Ex situ dehalogenation allows the rehabilitation of soils, sludge or sediments contaminated by chlorinated compounds such as polychlorinated biphenyls or dioxins and furans. This technology requires that the soil be excavated, crushed and/or separated as well as homogenized before being treated. It can be carried out using two different mechanisms, one involving sodium bicarbonate (base-catalyzed decomposition) and the other alkaline polyethylene glycol.
In the case of base-catalyzed decomposition, the soils are mixed with sodium bicarbonate and heated to 330 °C in a reactor, which allows a partial decomposition of the compounds as well as their volatilization. The gas emissions are collected and treated.
The use of an alkaline polyethylene glycol makes it possible, in turn, to replace the halogen of a chlorinated organic compound with polyethylene glycol, and thus reduce the toxicity of the treated compound. This reaction produces soluble compounds such as glycol ethers, hydroxyl and alkali metal salts. Soils are then washed and the leachate must be collected and treated. Once treated, soils can be reused to backfill the site.
Soils, sludge or sediments are excavated using conventional excavation equipment and sieved to separate and remove (or grind) overly coarse material (over 60 mm in diameter). If the water content is too high, drying may be necessary.
Base-catalyzed decomposition requires, after the treatment, the recovery and treatment of the gas emitted. This treatment may include adsorption using granular activated carbon, condensation or thermal oxidation (using direct flame, flame-free or catalytic oxides).
As for the use of alkali polyethylene glycol, the leachate produced must then be treated.
The implementation of this technology may include:
The implementation of this technology requires the use of construction equipment as well as civil engineering methods and conventional earthworks usually available for the excavation, sieving and homogenization of soils, sediments or sludge.
It also requires the installation of some specialized equipment. Transportable commercial equipment is available for both treatment processes. Chemicals include additives that are mixed with the soil.
If a thermal oxidizer is provided in the gas treatment chain during base-catalyzed decomposition, a source of natural gas or fuel oil is required. A connection to the local utility of natural gas can be made, but it is also possible to obtain a secondary electric combustion device.
Temporary piles of contaminated materials to be treated can be created on the site. In this case, they must be covered to limit the dispersion of dust, as well as the infiltration of water from precipitation, which could cause the runoff of contaminated soils to the surface as well as an increase of their water content which could affect the soil treatment.
Ex situ dehalogenation produces solid residues (treated soils or sediments) and liquid or gaseous discharges, depending on the treatment mechanism used. The management of these discharges and residues must be done on the basis of their environmental quality, as to whether they should be disposed off-site or if they can be reused.
Used adsorbent materials (granular activated carbon) or other products used in gas processing must be recovered and disposed off-site in an authorized center.
None.
Notes:
The implementation of this technology requires the development of several infrastructure as well as the use of specialized equipment. The costs associated with their mobilization and their monitoring at the time of treatment are much higher for sites in northern regions. In addition, equipment availability is limited and work windows are relatively short. Thus, this technology could be expensive and difficult to put in place efficiently.
Partial decomposition of chlorinated organic compounds can produce toxic by-products. Some ether glycol compounds produced with the alkaline polyethylene glycol dehalogenation technique may be toxic or recalcitrant.
Generally, there are no complementary technologies that improve dehalogenation treatment effectiveness. However, if there are contaminants present in the soil/sediment not targeted by this technology, such as metals, then solvent extraction can be added to the treatment processes.
The base-catalyzed decomposition technique requires the collection and the treatment of the gas emissions. The dehalogenation with alkaline polyethylene glycol technique requires the collection and treatment of the leachate.
Several private companies offer the dehalogenation technologies for the treatment of soil, sediment and sludge, and several examples specific to each company are available.
Application example of ex situ is available at the following site:
This technology is particularly useful for the remediation of soil contaminated with polychlorinated biphenyls. The alkaline polyethylene glycol technique successfully reduced polychlorinated biphenyls concentrations from 45,000 mg/kg to 2 mg/kg. Similarly, base-catalyzed decomposition has demonstrated reductions in polychlorinated biphenyls from 830 mg/kg to about 1 mg/kg.
Process optimization to reduce waste and consumables.
Main Exposure Mechanisms
Applies or Does Not Apply
Monitoring and Mitigation
Dust
Applies
Monitoring conditions favourable to dispersion during the excavation of the soil to be treated.
Atmospheric/Steam Emissions—Point Sources or Chimneys
Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements]).
Atmospheric/Steam Emissions—Non-point Sources
Doesn’t apply
N/A
Air/steam—by-products
Runoff
Monitoring of leachate water
Groundwater—displacement
Groundwater—chemical/ geochemical mobilization
Groundwater—by-product
Accident/Failure—damage to public services
Examination of records and pre-excavation authorizations, implementation of special excavation procedures, preparation and repetition of emergency interventions.
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—nonconforming backfill (processed or imported material)
Requires environmental and geotechnical control of materials used for backfilling
Other—soil leaching from stacks or open excavations
Reduction of leachate production, leachate collection and treatment, control of groundwater or surface water.
Other—Management of excavated soils, slurry and sediments
Composed by : Mélanie Bathalon, B.Sc, MCEBR
Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada
Updated Date : April 28, 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
