Fact sheet: Catalytic reductive dehalogenation—in situ

From: Public Services and Procurement Canada

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Description

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.

Sources:

Implementation of the technology

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:

  • Mobilization, access to the site and setting up temporary facilities;

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).

Materials and Storage

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.

Residues and Discharges

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.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
  • Electron acceptor concentrations/reaction by-products including:
    • nitrate
    • sulfate
  • Redox potential

Physical analysis

  • Dissolved oxygen concentration

Recommended trials for detailed characterization

Hydrogeological trials

  • Pumping 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

  • Contaminant delineation (area and depth)

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
  • Hydrogeological modelling
  • Detailed knowledge of groundwater geochemistry

Notes:

Notes:

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.

Applications

  • Allows the restoration of groundwater containing volatile and semi-volatile halogenated compounds.
  • Allows the degradation of certain pesticides.
  • Suitable for deep groundwater contamination.
  • Treatment with a reactive well or a reactive barrier minimizes the potential of surface spills (underground treatment) when undesirable substances (e.g. tritium and other radioactive contaminants) are present in the groundwater.

Applications to sites in northern regions

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.

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
Applies
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Does not exist
Residual contamination
Does not exist
Resorption
Applies
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
With restrictions
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Does not apply
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Does not apply
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
Does not apply
Polychlorinated biphenyls
Does not apply

Treatment time

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

Notes:

Notes:

Long-term treatment (3 to 5 years and more than 5 years) applies if the technology is used as an interception or containment barrier.

Long-term considerations (following remediation work)

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.

Secondary by-products and/or metabolites

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.

Limitations and Undesirable Effects of the Technology

  • The rate of degradation decreases over time for some chlorinated contaminants.
  • The cost of the palladium-on-alumina catalyst system is significant and can be justified when treating groundwater containing harmful co-contaminants that are safer to leave in the ground (e.g. radionuclides).
  • Procedures must be established to properly maintain the catalyst in good working order.
  • Sulphides, even in low concentrations, can neutralize the palladium catalyser.

Complementary technologies that improve treatment effectiveness

There is currently no complementary technology to improve effectiveness of in situ catalytic reductive dehalogenation.

Required secondary treatments

None.

Application examples

The following sites provide application examples:

Performance

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).

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery when installing the barrier and wells.
  • Optimization of the barrier configuration to reduce the size and amount of reactive material to be used.
  • Use of recyclable reactive materials.

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Does Not apply

Monitoring and Mitigation

Dust

Doesn't apply

N/A

Atmospheric/Steam Emissions—Point Sources or Chimneys

Doesn't apply

N/A

Atmospheric/Steam Emissions—Non-point Sources

Doesn't apply

N/A

Air/steam—by-products

Does not apply

N/A

Runoff

Does not apply

N/A

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

Applies (variation of groundwater level)

Modelling required barrier effects, pressure sensor monitoring, groundwater migration monitoring and gradient change monitoring

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

Applies

File checks and obtaining pre-drilling licences, development of drilling procedures and emergency response

Accident/Failure—leak or spill

Does not apply

N/A

Accident/Failure—fire or explosion

Does not apply

N/A

Other—Accident/Failure—treatment zone, barrier, drain

Applies

Hydrogeological modelling by indicating well locations, parameters and frequency of monitoring

Other—handling and management of contaminated soil

Applies

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

References

Author and update

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

Version:
1.2.1