Reduction/oxidation (Redox) reactions chemically convert hazardous contaminants to less toxic compounds that are less mobile and/or inert. Redox reactions involve the transfer of electrons from one compound to another. One compound is oxidized (loses electrons) and one is reduced (gains electrons). Oxidizing agents most commonly used for treatment of hazardous contaminants are ozone, hydrogen peroxide, hypochlorite, chlorine, and chlorine dioxide. The reducing agents most commonly used for treatment of hazardous waste are ferrous sulfate, sodium bisulfite, and sodium hydrosulfite. Experimental systems have been used to oxidize trichloroethylene (TCE) and perchloroethylene (PCE).
Limitations and Concerns
Incomplete oxidation or formation of intermediate contaminants may occur. Oil and grease in the media decrease efficiency.
The process may not be cost-effective for high contaminant concentrations because of the large amounts of redox agent required.
Chemicals may be consumed by non-target compounds, increasing treatment cost and potentially forming undesirable byproducts.
Chemical redox has been commonly used to treat residual contaminants in wastewater systems. It is also used to treat pumped groundwater. The most common applications of chemical reduction are reducing chromium (VI) to chromium (III) in preparation for precipitation, oxidizing arsenic to reduce toxicity and improve removal by subsequent processes, and oxidizing cyanide. It is also used to treat metals in soils. The technology is less effective against non-chlorinated volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), fuel hydrocarbons, and pesticides.
Technology Development Status
Chemical redox is commercial technology used for disinfection of drinking water and wastewater. It is a common treatment for cyanide (oxidation) and chromium (reduction of hexavalent chromium to trivalent chromium prior to precipitation) wastes.
Other Resources and Demonstrations
See http://www.epa.gov/ada/download/reports/epa_600_r02_002.pdf Workshop on Monitoring Oxidation-Reduction Processes for Groundwater Restoration, EPA 600-R02-002, January 2002.