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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.
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:
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.
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.).
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.
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.
Long-term considerations of in situ chemical oxidation with ozone include:
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.
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 example is available on the following websites:
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).
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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