Dehalogenation
Description
Dehalogenation is the process of removing the chlorine molecules from a contaminant in the soil. Contaminated soil is screened, processed with a crusher, and mixed with chemicals (i.e., reagents).). The mixture is heated in a reactor. The dehalogenation (i.e., dechlorination) process is achieved either: by replacing the chlorine molecules or by partially decomposing and partially volatizing contaminants. There are two dehalogenation processes used to remove polychlorinated biphenyls (PCBs), dioxins, furans, and other chlorinated hydrocarbons—such as pesticides—from soil.
Base-Catalyzed Decomposition (BCD)
BCD remediates soils and sediments contaminated with chlorinated organic compounds, especially PCBs, dioxins, and furans. After contaminated soil is screened and crushed, it is mixed with sodium bicarbonate. The mixture is heated to 330¡ Celsius in a reactor and the chlorine molecules volatilize in reaction to the heat and the reagent. The volatilized contaminants are captured, condensed, and treated separately. Concentrations of PCBs as high as 45,000 parts per million (ppm) are reported to have been treated. The PCB concentrations were reduced to less than 2 ppm per individual PCB congener. Reagent substitutes (such as sodium orthosilicate, sodium borohydride, and sodium phosphate) are being tested to improve overall efficiency.
Glycolate/Alkaline Polyethylene Glycol (APEG)
This technique is similar to the one described above, except it uses an alkaline polyethylene glycol reagent to break the carbon-chlorine bond. Contaminated soils are mixed with the reagent and heated in a treatment vessel. The reaction causes the polyethylene glycol to replace chlorine molecules and render the compound non-hazardous or less toxic. The APEG reagent dehalogenates the pollutant to form glycol ether and/or a hydroxylated compound and an alkali metal salt, which are water-soluble byproducts.
Limitations and Concerns
The technologies are not as effective with chlorinated volatile organic compounds (VOCs), and they are used mostly for PCBs and pesticides.
Soil with elevated concentrations of chlorinated contaminants requires large volumes of reagent.
With the BCD process, the capture and treatment of residuals (e.g., volatilized contaminants and dust) is difficult, especially when the soil contains high moisture levels.
For the BCD process, off-gas and condensate must be collected and treated.
With the BCD process, debris greater than 60 mm in diameter typically must be removed prior to processing.
Some glycol ethers are very toxic and persistent. It is not clear what byproducts the APEG technology produces and how they are captured and treated.
The APEG technology is generally not cost-effective for large waste volumes.
High clay and moisture content make treatment more difficult and increase treatment costs.
Applicability
The target contaminant groups for dehalogenation treatment are halogenated semi-volatile organic compounds (SVOCs) and pesticides in soils. APEG dehalogenation is one of the few processes available for treating PCBs. The technology is amenable to small-scale applications.
Technology Development Status
The BCD process is commercial. The APEG system has been field tested.
Web Links
http://www.frtr.gov/matrix2/section4/4-17.html
http://www.clu-in.org/download/citizens/chem-dehalo.pdf
http://www.clu-in.org/download/remed/destruct_tech.pdf
Other Resources and Demonstrations
See http://www.clu-in.org/products/costperf/THRMDESP/Widebch.htm for a description of APEG at the Wide Beach Development Superfund Site in conjunction with thermal desorption.