Chlorinated compounds, degradation

Nevertheless, an anaerobic system may be the method of choice under certain conditions (/) contamination with compounds that degrade only or better under anaerobic conditions, (2) low yield aquifers that make pump and treat methods or oxygen and nutrient distribution impractical, (J) mixed waste contamination where oxidizable compounds drive reductive dehalogenation of chlorinated compounds, or (4) deep aquifers that make oxygen and nutrient distribution mote difficult and cosdy. 

The chloride anion is a major species in the oceans and plays an essential role in biochemistry. Compounds containing carbon-chlorine bonds occur much less frequently in nature. Volcanos emit some halocarbons, and marine algae generate chloromethane. Other marine species produce toxic organohalogen molecules that protect them from predators. Nevertheless, organic chlorine compounds are uncommon, and consequently there are few mechanisms that degrade them. 

Complete phase-out of chlorinated compounds is being resisted not only by chlorine producers, but also by the many industries that use chlorine compounds in the manufacture of products from paper to pharmaceuticals. Meanwhile, chemists seek ways to degrade dioxins to nontoxic substances. 

Microbial activity, which is often stimulated during bioremediation projects, can alter the external pH. For instance, the anaerobic degradation of chlorinated compounds produces organic acids and HC1 and the pH may drop to acidic values if the soil has a low buffering capacity. In this case, control of the external pH will be required in order to maintain biodegradation activity at... 

Bioremediation also has its limitations. Some chemicals are not amenable to biodegradation, for instance, heavy metals, radionuclides, and some chlorinated compounds. In some cases, the microbial metabolism of the contaminants may produce toxic metabolites. Bioremediation is a scientifically intensive procedure that must be tailored to site-specific conditions, and usually requires treatability studies to be conducted on a small scale before the actual cleanup of a site.13 The treatability procedure is important, as it establishes the extent of degradation and evaluates the potential use of a selected microorganism for bioremediation. A precise estimate on vessel size or area involved, speed of reaction, and economics can therefore be determined at the laboratory stage. 

All PCDD/F isomers are solids with high melting points, but low vapor pressure and low solubihty in water. The high octanol-water coefficients are an indication of the observed bioaccumulative behavior in plants and animals for these compounds. Detailed environmentally important physicochemical properties can be found in the literature. All higher chlorinated compounds are very persistent in the environment with half-lives of 5-10 years photolysis with sunlight is the only degradation process in the environment. 

Transformation and degradation of chlorinated compounds presumably via reductive dehalogenation and subsequent aerobic metaboAsm ... 

All three chloroacetic acids (chloroacetic acid [MCA], dichloroacetic acid [DCA], and trichloroacetic acid [TCA]) are naturally occurring (7), with TCA being identified in the environment most frequently (reviews (278, 405 108)). However, these chlorinated acetic acids also have anthropogenic sources. The major source of natural TCA appears to be the enzymatic (chloroperoxidase) or abiotic degradation of humic and fulvic acids, which ultimately leads to chloroform and TCA. Early studies (409) and subsequent work confirm both a biogenic and an abiotic pathway. Model experiments with soil humic and fulvic acids, chloroperoxidase, chloride, and hydrogen peroxide show the formation of TCA, chloroform, and other chlorinated compounds (317, 410-412). Other studies reveal an abiotic source of TCA (412, 413). 

Jardim, W., Moraes, S., and Takiyama, M., Photocatalytic degradation of aromatic chlorinated compounds using Ti02 toxicity of intermediates, Water Res., 31, 1728, 1997. 

Dechlorination is a surface reaction with the zero-valent iron serving as the electron donor. When there is a proton donor, such as water, chlorinated compounds will be dehalogenated. The reaction kinetics depends upon the mass transfer to the surface of the iron, the available surface area, and the condition of the surface. The reaction is pseudo first order, and direct contact with the surface of the iron is required for degradation to take place (Gillham and O Hannesin, 1994). The basic equation for dechlorination by iron metal is as follows ... 

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