Hexachlorobenzene -- Health and Environmental Effects

Hexachlorobenzene,
or HCB, is a highly persistent environmental toxin that degrades slowly in air and, consequently, undergoes long-range
atmospheric transport. HCB bioaccumulates in fish, marine animals, birds, lichens, and animals that feed on fish or
lichens. Based on studies conducted on animals, long-term low-level exposures may damage a developing fetus,
cause cancer, lead to kidney and liver damage, and cause fatigue and skin irritation. HCB is considered a probable
human carcinogen and is toxic by all routes of exposure.

Chemosphere, Volume 43, Issue 2, April 2001, Pages 167-182

Global hexachlorobenzene emissions

Robert E. Bailey
Bailey Associates, 4115 Elm Court, Midland, MI 48642, USA
Received 24 November 1999; accepted 17 May 2000 Available online 5 March 2001.

Abstract
Information from a variety of sources has been assembled to give a global picture of hexachlorobenzene (HCB)
emissions in the mid 1990s. No single overwhelming source of HCB was identified. The best estimates of global HCB
emissions from different categories of sources are as follows: pesticides application – 6500 kg/yr; manufacturing –
9500 kg/yr; combustion – 7000 kg/yr, includes 500 kg from biomass burning. This adds up to total current HCB
emissions of approximately 23,000 kg/yr with an estimated range 12,000–92,000 kg/yr. A substantial portion of HCB
measured in the atmosphere is thought to come from volatilization of “old” HCB on the soil from past contamination
along with unidentified sources. No information on potential sources in developing countries was available.



1: Appl Environ Microbiol. 1988 Feb;54(2):327-30.
Reductive dechlorination of hexachlorobenzene to tri- and dichlorobenzenes in anaerobic sewage sludge.
Fathepure BZ, Tiedje JM, Boyd SA.
Department of Crop and Soil Sciences, Michigan State University, East Lansing 48824.

Hexachlorobenzene was dechlorinated to tri- and dichlorobenzenes in anaerobic sewage sludge. The complete
biotransformation of 190 microM hexachlorobenzene (approximately 50 ppm) occurred within 3 weeks. The calculated
rate of hexachlorobenzene dechlorination was 13.6 mumol liter-1 day-1. Hexachlorobenzene was dechlorinated via two
routes, both involving the sequential removal of chlorine from the aromatic ring. The major route was
hexachlorobenzene----pentachlorobenzene----1,2,3,5-tetrachlorobenzene--- -1,3,5- trichlorobenzene. Greater than
90% of the added hexachlorobenzene was recovered as 1,3,5-trichlorobenzene, and there was no evidence for further
dechlorination of 1,3,5-trichlorobenzene. The minor route was hexachlorobenzene----pentachlorobenzene----1,2,4,5-
tetrachlorobenzene--- -1,2,4- trichlorobenzene----dichlorobenzenes. These results extend reductive dechlorination to
poorly water soluble aromatic hydrocarbons which could potentially include other important environmental pollutants
like polychlorinated biphenyls.

Chemosphere, Volume 38, Issue 5, February 1999, Pages 1015-1023
Microbial dechlorination of hexachlorobenzene in anaerobic sewage sludge

S. Y. Yuana, C. J. Sub and B. V. Changb,  
a National Institute of Environmental Analysis, Environmental Protection Administration, Taipei, Taiwan, R.O.C.
b Department of Microbiology, Soochow University, Taipei, Taiwan, R.O.C.
Received 5 May 1998;  accepted 16 June 1998. ; Available online 1 March 1999.

Abstract
From this study of the dechlorination of hexachlorobenzene (HCB) in municipal sewage sludge by an anaerobic mixed
culture, we determined the dechlorination rate to be 0.29 mg/L/day under the optimal incubation conditions of pH M, 30
C temperature, and a concentration of total solids at 26.8 g/L. In addition, we found that dechlorination did not occur
during agitation. Biotransformation occurred along a pathway of HCB → pentachlorobenzene (QCB) → 1,2,3,4
tetrachlorobenzene (TecB) + 1,2,3,5-TeCB → 1,2,4-techlorobenzene (TCB) + 1,2,3-TCB + 1,3,5-TCB → 1,2-
(dichlorobenzenc (DCB) + 1,4-DCB. Sequential dechlorination was observed within a substrate concentration range of
2–50 mg/L, but slowed down significantly at concentrations of 50 mg/L or higher. High-to-low dechlorination rates
under three reducing conditions were identified as methanogenic conditions (0.30 mg/L/day) > sulfate-reducing
conditions (0.12 mg/L/day) > denitrifying conditions (0.08 mg/L/day). 01998 Elsevier Science Ltd.