Organic Compounds - Contaminant Geochemistry - page 84

4.2 Synthetic Halogenated Organic Substances
Halogenated hydrocarbons constitute a widely used family of products. Some of
the more common halogenated hydrocarbons are brominated flame retardants and
the chlorinated derivatives of methane, ethane, and benzene, which are used
mainly as solvents and chemical intermediates. Broad production and use of these
compounds began in the early 1900s when chlorinated solvents replaced other
flammable substances in a variety of industrial process. These compounds became
popular because the progressive halogenation of a hydrocarbon molecule yields a
succession of liquids or solids of increasing density, viscosity, and improved
solubility for a large number of inorganic and organic materials. Other physical
properties such as flammability, specific heat, dielectric constant, and water sol-
ubility decrease with increasing halogen content (Marshall 2003 ).
In addition to use as dry-cleaning and degreasing solvents, many of the halo-
genated organic solvents have been used in adhesives, pharmaceuticals, and textile
processing; as extraction solvents, paint solvents, and coating solvents; and as
feedstocks for production of other chemicals. The widespread use and subsequent
disposal of chlorinated solvents has led to their being among the most commonly
found contaminants at hazardous waste disposal sites. In general, these compounds
are considered persistent in the environment, having long half-lives in soil, air, and
water in comparison with other, nonhalogenated hydrocarbons. The health effects
of these compounds have been studied extensively, as a result of concerns raised
about their toxicity and their carcinogenic nature. Due to the large diversity of this
group, only a few major ubiquitous substances are discussed further.
4.2.1 Chlorinated Hydrocarbons
Chlorinated derivatives of methane include methyl chloride, methylene chloride,
chloroform, carbon tetrachloride, and several chlorofluorohydrocarbons (CFCs).
We discuss carbon tetrachloride (CT) as a representative example of this group.
CT was originally prepared in 1839 and was one of the first organic chemicals to
be produced on a large scale by the end of the nineteenth century and beginning of
the twentieth century. CT is the most toxic of the chloromethanes and the most
unstable on thermal oxidation (Holbrook 2000 ).
In the past, the main uses of CT were for dry-cleaning, fabric-spotting, and fire
extinguisher fluids; as a grain fumigant; and as a solvent in various chemical
processes (DeShon 1979 ). Until recently, CT was used as a solvent for the
recovery of tin in tin-plating waste, for metal degreasing, in the manufacture of
semiconductors, as a petrol additive and a refrigerant, as a catalyst in the pro-
duction of polymers, and as a chemical intermediate in the production of fluoro-
carbons and some pesticides (HSDB 1995 ).