Chlorinated Volatile Organic Compounds (CVOCs)
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Various water-soluble formulations of EHC have been specially designed for injection via existing wells or hydraulic injection networks. EHC-A (Aqueous) is composed of controlled-release food-grade organic carbon plus a soluble reduced iron source (ferrous sulfate). EHC-F (Fine) is composed of controlled-release food-grade organic carbon plus fine-grained particles of actual ZVI, of which 90% will have diameter less than 45 microns:
ZVI Particle Size in EHC-F (% Fraction Finer than Value Indicated) |
< 150 µm |
< 75 µm |
< 45 µm |
100 |
97.0 |
92.0 |
The organic components of EHC-A/EHC-F are hydrophilic and soluble in cold water. As described above, the addition of soluble carbon to the subsurface will support the growth of indigenous heterotrophic bacteria in the groundwater environment. As they feed on the soluble carbon, the bacteria consume dissolved oxygen, thereby reducing the redox potential in groundwater. In addition, as the bacteria ferment organic carbon, they release a variety of volatile fatty acids (VFAs, for example acetic, propionic, butyric), which diffuse from the site of fermentation into the groundwater plume and serve as electron donors for other bacteria, including dehalogenators and halorespiring species.
The soluble ferrous iron in EHC-A or generated from EHC-F ZVI will travel with the groundwater to stimulate Fe(II)-utilizing bacteria yielding an additional drop in the redox potential of the groundwater via chemical oxygen scavenging. As the dissolved iron travels into areas with higher redox potential, it will precipitate out as a number of ferrous and ferric precipitates, including magnetite, iron sulfides and green rust. These precipitates have also been proven to be reactive with CVOCs and will hence provide substantial reactive surface area that stimulates direct chemical dechlorination.
EHC-A/F will generate the following break-down products: ferrous iron, sulfate, hydrogen, dissolved organic carbon (volatile fatty acids) and methane. Information about these environmentally innocuous end-products is presented in the following references:
Ferrous iron (Vogan, J.L., Focht, R.M., Clark, D.K. and Graham, S.L., 1999. Performance evaluation of a permeable reactive barrier for remediation of dissolved chlorinated solvents in groundwater. J. Hazard. Mater. Vol. 68, pp. 97-108.)
Hydrogen (O’Hannesin, S.F., and Gillham, R.W., 1998. Long-term performance of an in-situ “iron wall” for remediation of VOCs. Ground Water, Vol. 36, No. 1, pp. 164-170.)
Dissolved organic carbon (Hoeks, J., Borst, R.J., 1982. Anaerobic Digestion of Free Volatile fatty Acids in Soils Below Waste Tips. Water, Air, Soil Pollut.,17,165-173.)
Methane (Hoeks, J., Borst, R.J., 1982. Anaerobic Digestion of Free Volatile fatty Acids in Soils Below Waste Tips. Water, Air, Soil Pollut.,17, 165-173.
