Fact sheet: Soil washing and solvent extraction—ex situ

From: Public Services and Procurement Canada

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Description

Ex situ soil washing, also known as solvent or chemical extraction, combines technologies that involve mixing contaminated soils with an extraction solution to dissolve or mobilize organic and inorganic contaminants, such as organic compounds and heavy metals from soils to the extraction solution.

Before the washing process, contaminated soils, sediments or sludge are excavated and generally separated according to their particle size. Subsequently, the contaminated soil, sediment or sludge is washed with an extraction solution that allows the extraction of contaminants until the desired concentrations are reached. Soil washing can be carried out using various agents selected according to the contaminants present. The extraction solution and the contaminants are then recovered and the active compounds (surfactant, alcohol, etc.) are recycled. Soils, sediments or treated sludge can then be returned to the site if the site-specific environmental criteria are met.

This technology Doesn’t destroy contaminants, but solubilizes, mobilizes, suspends and concentrates them in a reduced soil volume or extraction solution.

Soil washing can be done using a mobile washing unit that can be used directly on the site or at a local facility that can perform such treatment.

Sources:

Implementation of the technology

A soil washing unit, including the extraction solution treatment system, may be temporarily constructed directly on-site, or the contaminated materials may be transported to a centre with the soil washing facilities. In both cases, the contaminated soils, sediments or sludge are first segregated into grain size classes (the contaminants are usually concentrated in the finest grain size classes) and are inserted into a cell and combined with an extraction solution, which usually flows counter currently. The extraction solution can then be recovered by distillation and reused for further processing. When implementing the treatment system, it is important to consider the system's water and air management.

The implementation of a soil washing system may include:

  • Preparation of the site for the installation of the washing unit and other equipment (e.g. preparation of the site, installation of fences, construction of a platform, etc.).
  • Recovery of the materials to be treated (e.g. excavation of soil, sediments, sludge).
  • The transport of materials to be treated is required if the washing unit is not installed on the site.
  • The installation of all the equipment necessary for the soil washing treatment.
  • Recovery and management of the treated materials and extraction solution.
  • The dismantling of the facilities and the restoration of the site.

Materials and Storage

  • The washing unit can be built on-site or pre-assembled and delivered to the site inside a container, trailer or truck.
  • The operation of the unit requires energy and products such as extraction solutions, water, etc.
  • Temporary facilities for the storage of segregated soils or sludge, as well as those treated, should be provided at the site.
  • Installation activities for this type of system generally have little impact but may require on-site storage, for residues produced, notably extraction solutions.

Residues and Discharges

The treated soil, sediment or sludge must be analyzed following the treatment, in order to evaluate the remaining contaminant concentrations and dispose of them appropriately. Soils that meet site-specific environmental criteria can be returned to the site.

A significant amount of water is used in the process and must be treated. Residues that are generated during this process may be considered hazardous materials.

Atmospheric emissions resulting from treatment may also be released from the treatment system.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic matter content
  • Cation exchange capacity (CEC)
  • Chemical analysis of each particle size fraction
  • Tessier's sequential extraction for metals
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Soil water content
  • Soil granulometry
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Chemical trials

  • Laboratory adsorption testing
  • Soil washing/flushing trials

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)

Phase III

  • Volume of contaminated material to treat

Notes:

Laboratory tests are recommended to determine optimal extraction solution composition, contact time and treatment cost. It is also important to analyze the impact of temperature and pH on the extraction reaction.

Applications

  • Treatment of organic and inorganic contamination simultaneously (sequential treatments are also possible).
  • Treatment of soils, sediments and sludge.
  • Effective treatment for free phase organic compounds.
  • Effective treatment for metals and radionuclides.
  • Possible treatment for a wide range of organic compounds.
  • Time of treatment may range from weeks to months.
  • Optimal treatment efficiency is achieved when the soil matrix is composed of 50 to 70% of sand.

Applications to sites in northern regions

Ex situ soil washing is not recommended in northern regions because the treatment system requires a certain level of equipment maintenance and residue management, which may be more difficult in an environment where access to public services or local labour is limited. In addition, the cold climate in northern regions could cause equipment breakage.

Remote sites also require greater mobilization, resulting in higher on-site monitoring costs. Equipment availability is limited, and work windows are relatively short. The volume of contaminants to be treated must be significant to justify the mobilization of a treatment unit.

Treatment type

Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Does not exist
Chemical
Applies
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Applies
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Applies
Metals
Applies
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Applies
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

Treatment time

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

Long-term considerations (following remediation work)

None.

Secondary by-products and/or metabolites

Soil washing Doesn’t transform contaminants, but rather concentrates the contaminants within the washing solution or a soil fraction. Treated soil must be analyzed and the extraction solution must be treated and disposed of appropriately.

Limitations and Undesirable Effects of the Technology

  • Removal of contaminated soil is limited by maximum depth of excavation.
  • High organic matter content in treated materials.
  • Space is needed for on-site treatment unit installation and equipment mobilization costs are high.
  • Infrastructure on or near the site can prevent soil excavation, eliminating the ex situ treatment option.
  • Ex situ treatment costs may be higher than in situ treatment costs because of increased handling of the contaminated material.
  • Large volumes of extraction solution must be treated.
  • Residual extraction solution in the soil after treatment may be unacceptable. It is therefore important to consider the toxicity of solvents when selecting them.
  • Control of dust and vapours during soil handling.
  • Consumption of a large volume of water.
  • Atmospheric emissions that may contaminate the environment.

The complex mixture and heterogeneous composition of contaminants in the material may complicate the formulation of an appropriate extraction solution to effectively remove the different types of contaminants. A sequential wash that uses different extraction solution formulations or different material ratios to be treated, may be necessary in this case.

Complementary technologies that improve treatment effectiveness

Ex situ soil washing technology can be combined with many other remediation technologies such as free phase separation and removal or vacuum/distillation methods. Soil washing is generally preceded by soil particles size separation, which reduces the amount of soil to be treated.

Required secondary treatments

  • Soil washing requires subsequent treatment of the extraction solution.
  • In the case of volatile compounds, vapour and gas emissions from excavated soils, if present, need to be collected and treated.
  • Residual organic solvent concentrations in the treated material should be removed. Ex situ technologies, such as hot air or steam volatilization, thermal desorption or hot gas injection, are suitable for this type of operation.

Application examples

Many private companies have developed their own soil washing procedures and extraction solutions. Many offer examples of their technologies.

The following sites provide application examples:

Performance

Ex situ soil washing is an effective remediation technology for a wide variety of contaminants. The performance of this technique depends upon the characteristics of the extraction solution and the soil to be treated, as well as the types of contaminants present in the soil.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and low-energy equipment.
  • Process optimization to reduce waste and consumables.
  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Optimization of the water consumption of the treatment system.
  • Reuse of extraction solutions.

Potential impacts of the application of the technology on human health

Main exposure mechanisms

Applies or Doesn’t 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

Applies

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

Applies

Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Runoff

Applies

Monitoring of stored materials, and of favourable conditions for runoff.

Groundwater—displacement

Doesn’t apply

N/A

Groundwater—chemical/ geochemical mobilization

Doesn’t apply

N/A

Groundwater—by-product

Doesn’t apply

N/A

Accident/Failure—damage to public services

Applies

File checks and licensing prior to excavation, development of excavation and emergency procedures

Accident/Failure—leak or spill

Applies

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

Accident/Failure—fire or explosion

Applies

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

Other—Manipulation of contaminated soils, sludge, sediments or other solids

Applies

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

Other—Production of hazardous materials

Applies

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

Other—Handling of toxic materials

Applies

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

References

Author and update

Composed by : Serge Delisle, Eng. M.Sc., National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : April 16, 2013

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

Date Modified: