From: Public Services and Procurement Canada
In situ dehalogenation is a process by which a halogen, such as chlorine, is removed from a contaminant in order to make it less toxic. This groundwater remediation technology involves the reduction of halogenated or volatile organic contaminants by a reducing agent (electron donor) combined with a catalyst. This very specific application using a metal catalyst is performed to treat the groundwater from a pumping well without bringing the water to ground surface. Water is pumped through the reactive zone into the well casing, where hydrogen is present and a commercial palladium-on-alumina catalyst is placed. This concept can be used to construct a permeable reactive barrier placed perpendicularly to the flow of contaminated groundwater through a reactive well system. The catalyst (palladium) is necessary, considering the short amount of time that the water to be treated spends in the pumping well.
One of the advantages of using this technology is the reduction of the risk of surface contamination by water containing another contaminant at risk of exposure, such as radioactive products. In addition, dehalogenation in a reactive well allows a reactive barrier to be installed deeper than the conventional trench reactive barrier system.
In situ dehalogenation using reactive wells involves the development of double-screen wells. Contaminated groundwater is pumped by one of the screens and then put in contact with the reducing agent. The treated water is then reinjected into the groundwater table by the second screen (McNab et al., 2000).
The implementation of this technology may include:
Installation of reactive wells forming the reactive barrier;
Installation of impervious walls or high permeability underground drains to channel groundwater to the well area forming the reactive zone;
Insertion of reactive products (reducing agent and catalyst);
Renewal or periodic replacement of reagent (reducing agent and catalyst).
Implementation of this technology may require on-site storage. On-site storage procedures are related to the types of reactive products and alternative processes used. Projects where periodic replacement is planned should not require on-site storage, as reactive products can be brought to the site as and when required.
System installation typically requires drilling or excavation in contaminated areas, resulting in the handling and disposal of contaminated soils, typically placed in containers and disposed off-site.
There is usually no liquid discharge, other than the continuous flow of treated water from treatment areas to downstream soils.
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.
Laboratory and field testing are still recommended to test the feasibility and suitability of the technology for each site specific conditions. On-site treatment tests allow the determination of optimal groundwater pumping rates, hydrogen injection rates, required residence times for dehalogenation in one single pass and other parameters.
Permeable reactive barriers have advantages over other groundwater treatment methods in northern and remote areas that do not have access to utilities or specialized local labor to operate and maintain the system. Initial barrier design can be expensive because of mobilization, limited local building capacity, and relatively short work windows. Similarly, the systems installed must be adapted to the climate (deep freeze, permafrost, spring melt and frost heave). However, the maintenance of such a system can be limited and periodic, depending on the injection methods of the reagent products.
Long-term treatment (3 to 5 years and more than 5 years) applies if the technology is used as an interception or containment barrier.
Hydraulic effects produced by the barrier on groundwater flow (for example, raising the water table upstream of a sealed permeable barrier) may create new bypass zones over time. Performance monitoring must be carried out throughout the operation of the barrier. Well maintenance to sustain hydraulic performance is necessary in the long term.
When hydrogen and palladium are available in sufficient quantities, catalytic reductive dehalogenation transforms chlorinated contaminants into their non-chlorinated derivatives, without the production of by-products such as vinyl chloride. However, the formation of toxic by-products is still a possibility to consider.
There is currently no complementary technology to improve effectiveness of in situ catalytic reductive dehalogenation.
None.
The following sites provide application examples:
The in situ catalytic reductive dehalogenation technique in reactive wells is able of removing almost 100% of the contaminants after one passage through the system. The time of reaction is very fast, and the presence of oxygen does not affect the dehalogenation process. During field testing, the technique has shown 99% removal efficiency for both trichlorethylene and tetrachloroethylene in water, without the formation of toxic by-products (McNab et al., 2000).
Main Exposure Mechanisms
Applies or Does Not apply
Monitoring and Mitigation
Dust
Doesn't apply
N/A
Atmospheric/Steam Emissions—Point Sources or Chimneys
Atmospheric/Steam Emissions—Non-point Sources
Air/steam—by-products
Does not apply
Runoff
Groundwater—displacement
Applies (variation of groundwater level)
Modelling required barrier effects, pressure sensor monitoring, groundwater migration monitoring and gradient change monitoring
Groundwater—chemical/geochemical mobilization
Groundwater—by-product
Applies
Modelling of contaminant degradation and transport processes, model validation, groundwater quality monitoring and pilot test
Accident/Failure—damage to public services
File checks and obtaining pre-drilling licences, development of drilling procedures and emergency response
Accident/Failure—leak or spill
Accident/Failure—fire or explosion
Other—Accident/Failure—treatment zone, barrier, drain
Hydrogeological modelling by indicating well locations, parameters and frequency of monitoring
Other—handling and management of contaminated soil
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions
Composed by : Josée Thibodeau, M.Sc, National Research Council
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
