Reduction/oxidation (Redox)
Description
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.
Applicability
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.
Web Links
http://www.frtr.gov/matrix2/section4/4-16.html
http://t2.serdp-estcp.org/t2template.html#tool=DCE&page=b2.1
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.