Fact sheet: Chemical oxidation with ozone—in situ

From: Public Services and Procurement Canada

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Description

In situ ozone oxidation is a treatment option for soil and groundwater which consists of injecting ozone gas either into the saturated or vadose zone to partially or completely oxidize the contaminants.

Oxidation of contaminants by ozone can occur either directly by the ozone molecule (O3) or indirectly by hydroxyl radicals (OH•). Indirect oxidation of contaminants occurs when ozone is injected into a contaminated aquifer and decomposes into dioxide (O2) and hydroxyl radicals (OH•). These hydroxyl radicals are more reactive and less selective than ozone which allows for the oxidation of a wide range of organic contaminants. Ozone treatment is effective against organic compounds which are toxic or do not biodegrade easily and can also be used against inorganic compounds. Ozone also reacts with organic matter, producing carbon dioxide and water.

Ozone oxidation is a rapid reaction and treatment time ranges from several weeks to several months.

Internet links:

Implementation of the technology

Ozone gas injection is used to bring a strong oxidant in contact with an organic contaminant at the level of the plume and/or the contamination source, mineralizing the contaminant to carbon dioxide and water. The primary issue in chemical oxidation with ozone is the gas distribution in the subsurface and the contact with the contaminants. In the saturated zone, ozone sparging is performed, while in the unsaturated zone, ozone is injected in wells screened above the water table. In order to carry the ozone gas in the subsurface and obtain a uniform distribution, air is often used as a carrier gas. Gaseous ozone, being unstable and highly reactive, must be produced directly at the site prior to injection.

Projects may include:

  • Pilot tests: Laboratory trials are not generally required since they are complex to realize. Some test may be performed to determine optimal operational parameters and treatment efficiency. Furthermore, pilot-scale field tests are essential for selecting the type and position of injection wells, establishing the radius of influence of the injection wells and to calculate optimal ozone injection rates.
  • Mobilization, site access and temporary facilities
  • On-site ozone generation equipment
  • Reagent delivery, which could entail such measures as:
    • Groundwater sparging wells or venting wells installation
    • Monitoring
  • Decommissioning of installed equipment (including injection, venting and groundwater wells)

In the saturated zone, ozone is applied as an oxidant, either by itself (for example, injected in sparging wells) and air is added as the carrier gas as needed. In the vadose zone, ozone is injected in venting wells, and sometimes air extraction wells are used to control the migration of the ozone in the subsurface.

Ozone reactions are exothermic (generate heat), but the effect is typically diffuse.

Materials and storage

  • Ozone gas generators are used on-site. The gas generators consist of an oxygen generator using atmospheric air followed by an ozone generator. Ozone is then injected by itself or using an air compressor or blower which provides air as the carrier gas. Ozone gas storage is not usually performed since ozone is a toxic gas.
  • Underground injection method relies on traditional sparging or venting wells construction methods.
  • Pressurized piping network to carry ozone to the injection wells are required. Activators such as hydrogen peroxide if needed may be stored on-site.
  • In some instances, oxidant reactivity with subsurface contaminants, including unexploded munitions and explosives, may be sufficient to result in combustion.

Waste and Discharges

  • Ozone leave few residual other than oxygen. Complete mineralization of organic compounds leaves carbon dioxide, water, and inorganic ions (like chloride). However, complete mineralization doesn’t always occur. In most instances, chlorinated methane are not oxidizable.
  • In some instances, there may be concerns with respect to the presence of an oxygen-rich vapour where there are significant quantities of a combustible contaminant (such as petroleum hydrocarbons).
  • System installation typically requires drilling or excavating in contaminated areas, resulting in the handling and disposal of contaminated soils, typically containerized and disposed of off-site.
  • There is not a large risk of oxidant rich groundwater to travel out of the treatment zone since it is not stable nor persistent in the environment. Properly designed injections do not result in uncontrolled flow of oxidants along preferential pathways.

Recommended analyses for detailed characterization

Chemical analysis

  • Organic matter content
  • Concentration of oxidant-consuming substances includes:
    • natural organic matter not considering the contaminants
    • reduced minerals
    • carbonate
    • other free radical scavengers
  • Reaction parameters include:
    • kinetic
    • stoichiometry
    • thermodynamic parameters
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Chemical trials

  • Evaluation of the matrix oxidant demand

Notes:

On-site treatment trials will establish the efficiency of the technology and the parameters that influence the treatment time and cost (e.g. residence time, pump flow rate, requirements for pre-treatment, etc.).

Physical trials

  • Vapour survey
  • Evaluation of the radius of influence
  • Evaluation of operating pressure/vacuum

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Notes:

On site treatment trials are recommended to select the proper type of injection well, the radius of influence of the well, the positions of wells and the ozone gas injection rate.

Applications

  • Allows for vadose zone treatment by injection of ozone gas
  • Allows for treatment of a contaminated aquifer by direct injection of ozone using ozone gas sparging or dissolved ozone injection

Applications to sites in northern regions

In situ chemical oxidation with ozone, is potentially applicable to remote northern sites, however, significant impediments to material transport and injection equipment mobilization (such as ozone generators) must be overcome. The electrical power required to operate the ozone generators on-site may be an issue since the ozone generation can require a lot of energy. Alternatively, ozone can be used with the objective of reducing concentrations to levels amendable to monitor natural attenuation or other less energy intensive technology. Northern systems require climate-appropriate design, including consideration of permafrost and of seasonal changes in ground conditions.

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
Does not exist
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Does not exist
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

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
With restrictions
Chlorobenzenes
Applies
Explosives
Applies
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
With restrictions

Notes:

Initial treatment time is relatively short and may vary from weeks to months. However, in short to medium terms (weeks to months) contaminant concentrations may increase or “rebound,” if the injected oxidants did not reach all of the contamination, or if the oxidant is consumed before all the contamination is treated. Additional injections may be needed in the weeks to months following the first treatments.

Treatment time

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)

Long-term considerations of in situ chemical oxidation with ozone include:

  • Redistribution of the contaminant. The oxidants are consumed relatively quickly and do not persist long enough to penetrate the layers of impermeable materials. At several sites, contaminants continue to spread outside impermeable structures such as silts or clay lenses for several years.
  • Increased contaminant concentrations in soil gases can be observed.
  • Soil sterilization is not a major problem. Numerous studies have shown that microbial communities rapidly recolonize oxidation zones.

Secondary by-products and/or metabolites

Chemical oxidation reactions may transform petroleum hydrocarbons into carbon dioxide and water (complete mineralization). This technique increases the amount of dissolved oxygen in contaminated soils and groundwater, which can promote the aerobic biodegradation of residual contaminants after the oxidation treatment.

By-product formation can be a concern if complete reaction cannot be obtained. Bench top and/or pilot testing, as well as strict quality control for injected materials, can be required. Oxidation products are usually (but not always) less toxic, more mobile and more biodegradable than parent compounds. For example, MTBE may degrade into acetone of tert butyl formate. Petroleum hydrocarbons may generate acetone or alcohols. Explosives (RDX and HMX) may create elevated levels of nitrate.

Limitations and Undesirable Effects of the Technology

  • Low soil matrix permeability and heterogeneity influence migration of contaminants, hence, treatment efficiency. Oxidants are consumed relatively quickly and do not persist long enough to penetrate low permeability materials. At many sites, contaminants continue to diffuse out of untreated low permeability structures, such as silt or clay lenses, for many years.
  • Presence of oxidant consumers (in other words high organic matter content) in the soil matrix can reduce treatment efficiency
  • Partial oxidation of organic compounds may produce toxic by-products
  • Risk of ozone gas migration through preferential pathways and the risk of infiltration into nearby infrastructure
  • On-site production of ozone is required
  • Ozone is a very toxic gas; A minimal soil layer above the injection zone is typically required to avoid risks of ozone exposure. Recovery of gases produced (ozone and volatile compounds) may also be required. Design of ozone injection systems must address health and safety risk associated with the potential leaks of ozone gas.
  • Chemical oxidation deliberately causes extreme changes in geochemistry and creates a strongly oxidizing environment (to the extent that incompatible conduit, pipe or other underground materials can be damaged).
  • Metals can be mobilized within the treatment zone due to a change in oxidation states (such as chromium) and/or pH. However, metals can attenuate by various mechanisms within a short distance of the injection site(s).
  • Non-target organic carbon may be mineralized, changing the fate and transport of hydrophobic compounds (contaminants may be de-sorbed as total organic carbon is mineralized; in some cases this increases the effectiveness of the oxidant application by making more contaminant accessible for destruction).
  • Injected fluids may displace (or “push”) contaminated pore water ahead of the injection front, leading to short-lived but dramatic changes in the distribution of groundwater contamination.
  • Oxidation reactions are exothermic. Oxygen gas bubbles can be formed. Due care must be used to prevent damage to underground utilities, fire hazard, or the mobilization of contaminated soil vapour along preferential pathways such as utility trenches.
  • Energetic oxidation can ignite flammable materials; thermal decomposition can release oxygen and heat to intensify the blaze. Explosions can result because of the accumulation of ozone gas and/or oxygen.
  • On-site generation of ozone may entail working around high voltage equipment, ambient air monitoring/alarm systems. Safety precautions related to oxygen generators/concentrators and/or tanks of oxygen gas must be applied.

Complementary technologies that improve treatment effectiveness

  • Increasing fractures in the soil matrix to increase permeability
  • The ozone can also be mixed with hydrogen peroxide in the injection process, also called Perozone oxidation. Injection of ozone gas in combination with hydrogen peroxide is sometimes more effective than ozone oxidation treatment alone. A number of proprietary systems and patents for adding ozone with hydrogen peroxide exists on the market.
  • The installation of a gas extraction system may be necessary, especially when the treatment is applied under or near a building due to the risks of intrusion of toxic vapor.
  • Since ozone degrades in oxygen, aerobic bioremediation can be used as a polishing step following ozone injection. Soil sterilization post oxidation is not a major issue, as repeated investigations have demonstrated that microbial communities quickly recolonize the treatment areas.
  • Free phase contaminants in soil must be removed prior to ozone oxidation to optimize treatment efficiency.

Required secondary treatments

A soil venting system including vacuum extraction wells and vapor treatment system using nickel to catalyze ozone decomposition may be necessary to minimize atmospheric ozone release.

Application examples

Application example is available on the following websites:

Performance

According to the FRTR (2002), in situ chemical oxidation techniques can achieve high treatment efficiency (for example > 90 percent) for unsaturated chlorinated aliphatic (such as trichloroethylene [TCE]) with very fast reaction rates (90% destruction in minutes).

  • Chemical oxidation with ozone injection can result in complete decontamination within a short period of time
  • Appropriate technology for small sites but large sites can result in large ozone generation systems and large energy power requirements and consumption.
  • Minor production of volatile organic compounds (VOCs)
  • Possible to use automated systems to control the procedure

Measures to improve sustainability or promote ecological remediation

  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Use of renewable energy for the operation of the ozone generators.
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions.
  • Use of direct-push injection techniques.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

Composed by : Josée Thibodeau, M.Sc, National Research Council

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

Updated Date : March 1, 2009

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1