From: Public Services and Procurement Canada
In situ adsorption is a process that consists of temporarily or permanently retaining organic contaminants and metals in the gaseous or dissolved phases on the surface of an absorbent material. The adsorbents may be made of materials such as activated carbon, activated aluminum, zeolites or polymers. In the case of dissolved contaminants, permeable walls are put in place within trenches or drill holes, or a combination of activated charcoal and amendments (chemical and/or biological) are injected into the ground surface.
Organic compounds are often hydrophobic and poorly soluble in water, so they have a tendency to partition onto to the adsorbent. When the adsorbent becomes saturated, it must be replaced. The saturated adsorbent is then recovered, processed or regenerated and reused in the system.
The combination of activated carbon and chemical and/or biological amendments is an emerging remedial technology applied to contaminants present in soil and groundwater. The mixture of activated carbon and amendments is injected via high pressure into the low permeability zone (blend of granular/powder activated carbon mixtures) and via low pressure into the permeable zone (blend of colloidal activated carbon mixtures). The activated carbon is responsible for the adsorption of contaminants, whereas the reactive amendments degrade adsorbed contaminants. Depending on the contaminants present, the activated carbon can be of various porosity, and the reactive amendments can consist of zero-valent iron, nutrients, electron acceptors, calcium peroxide, etc. The retention of contaminants in the activated carbon matrix allows for a longer residence time for contaminants subjected to degradation by reactive amendments. This technology is used primarily for the treatment of petroleum hydrocarbons as well as chlorinated solvents.
Sources:
In situ adsorption technology includes the installation of a permeable wall made from trenches or boreholes. In this case, the following steps should be considered:
With regard to in situ adsorption via the combination of activated carbon and amendments, the following steps should be considered:
Additional factors to consider when implementing the technology are:
The materials required for the installation of the permeable walls or the injection zone are progressively put in place on-site during construction. Once the systems are installed, minimal amounts of material are stored on-site.
The installation of wells or trenches generally requires drilling or excavation activities in contaminated areas. This results in the handling of contaminated soils that will have to be removed off-site.
When the use of the technology is no longer required, following the excavation of the system, few residues remain on the site. The system can, however, be built with the intention of leaving the materials in place indefinitely. Solid residues can also be generated when the system is replaced or repaired (trench-derived materials, for example).
The treated water as well as the gaseous effluents migrating through the absorbent constitute the two main discharges of the system. The quality of these discharges must be monitored to ensure the efficiency of the system.
Notes:
Laboratory testing may be required to verify the efficiency of the adsorbent for a given concentration of an organic compound or to select the most effective adsorbent.
In situ adsorption is possible in northern regions, however, remote sites require greater mobilization, resulting in higher on-site monitoring costs. Equipment availability is limited and work windows are relatively short.
The use of this technology in northern regions has advantages over conventional water treatments in remote northern areas, which do not have access to utilities or local labour for operation and maintenance. The system must be adapted to the climate and consider factors such as deep soil freeze and frost heave. Since the technology does not include complex equipment or materials, or regular maintenance, there is less risk of equipment breakdown by cold temperature or labour requirements.
In situ adsorption via a permeable wall can be used for many years and no restoration, as a result of the remediation process, would likely be necessary. However, the proper operation and maintenance of the technology must be ensured, as precipitation and fouling could lead to adsorbent usage over time and decrease the efficiency of the technology.
For the technology using activated carbon and amendments, there is limited data on the long-term effectiveness of the treatment. Further evaluation is needed for this technology, such as the possibility of salting-out the adsorbed contaminants in the long term in the case where the adsorbent material is left in place following the treatment.
A monitoring system must be put in place to ensure the quality of the effluent at the exit of the system or downgradient.
No secondary by-products are formed by the adsorption procedure.
When the adsorption technology is combined with biological amendments to stimulate the degradation of the adsorbed compounds, degradation by-products are likely to be generated.
In the case of contamination with volatile organic compounds, technologies such as sparging and volatilization are often used in combination with the in situ adsorption system.
In some cases, the adsorbent may be regenerated, such as activated carbon which can be regenerated by heating in the presence of organic contamination.
The following websites provide application examples:
The performance of an adsorption system varies according to the type of adsorbent used, the characteristics specific to the contaminated site and the target contaminants.
Main Exposure Mechanisms
Applies or Does Not Apply
Monitoring and Mitigation
Dust
Does not apply (minor during construction)
N/A
Atmospheric/Steam Emissions—Point Sources or Chimneys
Don’t apply
Atmospheric/Steam Emissions—Non-point Sources
Apply
Source, perimeter or receptor monitoring
Air/steam—by-products
Does not apply
Runoff
Groundwater—displacement
Applies
Modelling permeable wall effects, monitoring groundwater migration and monitoring gradient changes.
Groundwater—chemical/ geochemical mobilization
Groundwater—by-product
Contaminant behaviour and transport modelling, model validation, water quality monitoring and pilot testing.
Accident/Failure—damage to public services
File checks and licensing prior to excavation or drilling, development of excavation procedures and emergency response.
Accident/Failure—leak or spill
Accident/Failure—fire or explosion
Other—Manipulation of contaminated soils
Applies in the case of a renewal or repair of the adsorption system
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR
Updated by : Karine Drouin, M.Sc., National Research Council
Updated Date : June 8, 2016
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
