Environmental Fate of HHCs

Aliphatic HHCs are used mostly as solvents, and their production, use, and disposal can pose ecological and health (environmental and occupational) problems. Chlorination of drinking water produces chloroform and carbon tetrachloride, which, in USA alone, can continuously expose 29 million humans including infants and children. Tri- and tetrachloroethylenes used in dry cleaning are common environmental contaminants, especially at hazardous waste sites. These are present in ground-water and ambient and indoor air as well as in drinking water. Highly stable perfluoroalkyl acids, perfluoroocta-noic acids, and perfluorooctane sulfone are present in environment, wildlife, and humans.

Aromatic HHCs have a wide range of physical properties, which determine their environmental fate and effects. These HHCs range widely in their distribution in different environmental compartments and phases, some of which is related with differences in their persistence. Low molecular weight HHCs are more volatile than higher molecular weight HHCs and can be present in occupational, urban, and industrial air. The major reactions during their transport in air involve hydroxyl radical and photolysis.

Estimates of sources of HHCs and of their concentrations can be more accurately predicted by using quantitative structure-activity relationship (QSAR) models. For example, the inputs of carbon tetrachloride in the troposphere should be 1-2 million tons while anthrapo-genic sources amount to only one-tenth of this.

Environmetal sources of mono-, di-, and trichloro-acetic acids are not fully known. Some of these can be biological and environmental products of perfluoro-octanoic acids. These are present globally in ppb (mgP ) concentrations in rain and surface water, which are greater than any other low molecular weight HHC.

Trends in release of HHCs and by-products at manufacturing sites are more effectively controlled now and this has reduced their emissions and disposition. There is a dramatic reduction in PCDDs, PCDFs, and chloro-phenol discharges from the paper and pulp industry due to the new delignification and bleaching technologies. Legislative restrictions and new technologies have also reduced emissions of PCDDs/PCDFs from waste incineration plants. HHC pesticide usage has declined

Table 6 Biological concentration of HHC in biota

Bioaccumulation:

x in

water

HHC

Algae

Fish

PCP

1 240

350

2,4,6-TCP

800

230

HCB

24000

400

2,4,2',4'-Pentachlorobiphenyl

11 500

770

2,5,4-Trichlorobiphenyl

7 700

650

2,2'-Dichlorobiphenyl

3 050

830

Pentachloronitrobenzene

3600

380

Hexachlorocyclopentadiene

1 140

308

2,4-D

6

2

p-Chlorobenzoic acid

63

tremendously. However, nonbiodegradable mirex is now distributed all over North America. Kepone had contaminated James River, VA. In the 1970s, the usage of these insecticides was minimized and most of these were banned by 1980.

Volatility and lipophilicity of polychlorinated aro-matics (PCB, PCDD, PCDF, etc.) are associated with the degree of chlorination. Chlorination increases lipophilicity but also reduces volatility. Thus, low concentrations of HHCs, especially less chlorinated ones, are present in surface waters, while highly chlorinated aromatics remain in sediments and are picked up by invertebrates in high concentrations, as has been reported for 4-, 5-, and 6-chlorinated congeners of PCBs. In surface waters, photolytic orho-dechlorination is common. The octanol:-water partition coefficients for PCB 101, 128, 136, and 1 are 2.5, 5, 10, and 0.002, respectively. Bioconcentration factors for these chlorinated aromatics relate with their partition coefficients (Table 6).

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