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4.5.4 In-sito chemical oxidation (CT)

In-situ chemical oxidation is based on the delivery of chemical oxidants into the vadose zone and/or groundwater to oxidize contaminants into carbon dioxide and water. This technique is best applied at highly contaminated sites or directed at source areas to reduce contaminant concentrations. In general this technique is not cost effective for plumes with low contaminant concentrations. The effectiveness of in-situ chemical oxidation is sensitive to variations in the hydraulic conductivity of the soil as well as to the distribution of contaminant mass. Therefore, performance is improved by detailed site characterization.

To date the most common oxidant delivery methods involve injection of oxidants only. Should a significant hydraulic gradient exist, targeted delivery of oxidant to the contaminant zones may require injection and extraction wells. The major benefits of a passive oxidant delivery mode are that treatment of groundwater and disposal of secondary hazardous wastes are avoided.

The common oxidants are hydrogen based Fenton’s reagent and potassium permanganate. In the application of Fenton’s reagent, hydrogen peroxide is applied with an iron catalyst creating a hydroxyl free radical. This hydroxyl free radical oxidizes complex organic compounds. Residual hydrogen peroxide decomposes into water and oxygen in the subsurface and any remaining iron precipitates out. Fenton’s reagent is produced on-site by adding an iron catalyst to a hydrogen peroxide solution. A 50% solution is common for this application. A pH adjustment may be required as Fenton’s reagent is more effective at acidic pH. The main difference to the oxidation techniques discussed in Section 4.3.2.1 is that here the contaminants are oxidized directly, rather than being broken down in an aerobic microbial process.

The volume and chemical composition of reactants are based on contaminant levels and volume in addition to subsurface characteristics, and may be derived from pre-application testing results. The methods for delivery of the oxidants vary; they can be injected through a well or directly into the subsurface through an injector head; they can be mixed with a catalyst and injected, or combined with groundwater extracted from the site and then re-injected. In the case of hydrogen peroxide, stabilizers are needed because of the inherent instability of this compound.

In-situ oxidation is being used for groundwater, sediment and soil remediation. It can be applied to a variety of soil types (silt and clay). It is used to treat volatile organic chemicals including dichloro-ethene (DCE), trichloro-ethene, trichloro-ethylene (TCE), benzene, toluene, ethylbenzene and xylene, as well as semi-volatile organic chemicals including pesticides, polycyclic aromatic hydrocarbons and polychlorinated biphenyls (PCB) [IAEA-2006b].

The limitations of the in-situ chemical oxidation technique include [IAEA-2006b]:

  • Target contaminants may be difficult to oxidize.
  • Areal extent of contamination may be too large; in-situ oxidation is best applied to ‘hot spots’ and source zones rather than very large groundwater plumes.
  • Geotechnical and hydraulic characteristics of the site may restrict drilling and limit ability to inject oxidant.
  • Presence of underground human made structures (i.e., buried pipelines and other utilities) can create short circuits for injected fluids.
  • High natural organic content will create a high oxidant demand, thus requiring larger amounts of oxidant that will increase the cost of treatment.
  • Inadvertent mobilization of co-contaminant metals, including radionuclides, from increased oxidation states.