In-Situ Gaseous Reduction
When present in soil, the oxidized forms of certain metallic contaminants (e.g., hexavalent chromium) are quite mobile and can be easily carried by waters percolating through the unsaturated zone. To avoid the excessive costs and risks associated with excavation, transportation, and disposal of the chromium contamination at locations such as Pantex, Hanford, and the Sandia National Laboratory, the Department of Energy (DOE) is testing the In-Situ Gaseous Reduction System (IGRS). In IGRS, the oxidized metals are reduced and immobilized. For chromium-contaminated sites, the primary chemical reaction involves the reduction of hexavalent chromium [Cr (VI)] to trivalent chromium [Cr (III)], with subsequent precipitation as a nontoxic, insoluble solid.
Reduction and immobilization of hexavalent chromium or other redox-sensitive metals in soils is achieved by injecting a low-concentration hydrogen sulfide (H2S) gas mixture into the soil. The gas mixture is injected into a central well, and gases are extracted by applying a vacuum in wells located at the plume boundary. The breakthrough of H2S at the extraction wells is monitored over time to provide a basis for assessing treatment progress. In some anaerobic environments (i.e., environments devoid of oxygen) there are bacteria that produce hydrogen sulfide, and thus naturally reduce the mobility of metals.
Limitations and Concerns
Treatment results obtained from a demonstration at the White Sands Missile Range indicated that geologic heterogeneity limits the effectiveness of ISGR. Test results revealed that channeling of the treatment gas occurred through strata having higher relative permeability. To adequately address this limitation, collection of permeability data is needed to support well-field design.
When a soil-fixation gas such as hydrogen sulfide is injected, the contact time between the gas and the metal being immobilized is critical. High soil density, which may impede the dispersion of the gas, may be a limiting factor.
The gas is dangerous to handle. Efforts must be made to ensure low occupational exposure to the reactive gas.
The amount of H2S consumed during the test was greater than the amount predicted in laboratory studies, probably due to interfering reactions in the field. More field data is needed to optimize the amount of gas that is needed for a specific application.
The depth of contaminants may limit some types of application.
The technology under development uses a gaseous reagent in unsaturated soils to reduce the toxicity and mobility of hexavalent chromium. Diluted mixtures of hydrogen sulfide or nitrogen can potentially treat soils contaminated with heavy metals and radionuclides. Tests also indicate that this technology may be applicable to TNT and VOCs.
Technology Development Status
The technology is under development. Further research could lead to technologies to immobilize other redox-sensitive metals (e.g., uranium and lead) above and below the water table.
Other Resources and Demonstrations
See related descriptions of Solidification/Stabilization (S/S)—Chemical.
ISGR was demonstrated during the spring and summer of 1998 at the White Sands Missile Range, New Mexico, in a cooperative effort between DOE and the Department of Defense. H2S gas was injected into chromate-contaminated soils. 70% of the Cr (VI) present at the site was reduced to Cr (III) during the demonstration, thus verifying the effectiveness of the approach. See http://costperformance.org/pdf/InSituGaseous.pdf.
See http://www.pnl.gov/main/publications/external/technical_reports/PNNL-13107.pdf for report on this technology at Hanford.