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Dioxins are a class of related chlorinated hydrocarbons that are structurally similar. The basic structure is a dibenzo-p-dioxin molecule that comprises 2 benzene rings joined at their meta and para carbons by oxygen atoms. There are 8 homologues of dioxins—monochlorinated through octachlorinated—that result in a total of 75 congeners. Common physical/chemical properties are found in Table 1.
Table 1. Physical and Chemical Properties of Chlorinated Dibenzo-ρ-Dioxins1
|Dibenzo-ρ-dioxin||Melting Point °C1||Solubility at 25°C mg/L||Log Kow||Vapor Pressure mm Hg||Henry's Law Constant atm-m3/mol|
|Monochloro-||89-105.5||0.278-0.417||4.52-5.45||9.0 X 10-5- 1.3 X10-4||82.7-146.2 X 10-6|
|Dichloro-||114-210||0.00375-0.0149||5.86-6.39||9.0 X 10-7- 1.1 X 10-6||21.02-80.04 X 10-6|
|Trichloro-||128-163||4.47-8.41 X 10-3||6.86-7.45||7.5 X10-7- 6.46 X 10-8||37.9 X 10-6|
|Tetrachloro-||175-306||7.9 X10-6 - 4.7 X 10-4||6.6-8.7||7.4 X 10-10- 7.5 X 10-9||7.01-16.1 X 10-6|
|Pentachloro||195-241||1.18 X 10-4||8.64-9.48||6.6 X 10-10||2.6 X 10-6|
|Hexachloro-||238-286||4.42 X 10-6||9.19-10.4||3.8 X10-11||44.6 X 10-6|
|Heptachloro-||265||1.9 X 10-3 - 2.4 X 10-6||9.69-11.38||5.6 X 10-12- 7.4 X 10-8||1.31 X 10-6 - 2.18 X 10-5|
|Octachloro-||330||7.4 X 10-8 - 2.27 X 10-9||8.78-13.37||8.25 X 10-13||37.9 X 10-6|
As the table illustrates, all chlorinated dibenzodioxins (PCDDs) are highly hydrophobic and would not be expected to be found in ground or surface water to any degree as freely dissolved molecules. The Log Kow values indicate that they are highly lipophilic and are likely to be strongly attached to organic matter found in soil and sediments (or incinerator/boiler stack particulates). Their lipophilic characteristic also means that they will bioaccumulate and biomagnify in the food chain.
The low vapor pressure of the PCDDs, like other semivolatile organic compounds, also tends to restrict their movement in the environment, as they do no easily evaporate into the atmosphere from soils and other solid matrices.
Dioxins biodegrade very slowly. The most rapid degradation process is through photolysis, with the estimated half life on the soil surface for 2,3,7,8-TCDD (one of the more toxic congeners) ranging from 9 to 15 years. For dioxins that are not exposed to sunlight, the half life is estimated to range from 25 to 100 years.
Toxicological Profile Chlorinated Dibenzo-p-Dioxins
Agency for Toxic Substances and Disease Registry, 1998.
For Further Information
Compilation of EU Dioxin Exposure and Health Data, Task 3: Environmental Fate and Transport
European Commission DG Environment, 26 pp, 1999.
Final Draft Report: Intermedia Transfer Factors for Contaminants Found at Hazardous Waste Sites 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD)
Dennis Hsieh, et al.
California Department of Toxic Substance Control, 44 pp, 1994.
Sorption, Mobility, and Fate of 1,4,7,8-Tetrachlorodibenzo-p-Dioxin in Soils
Gerald Larsen, Zhaosheng Fan, Francis Casey, Heldur Hakk.
Organohalogen Compounds, Vol 66, p 2317-2321, 2004 [from Dioxins 2004]
Technical Factsheet on DIOXIN (2,3,7,8-TCDD)
National Primary Drinking Water Regulations. U.S. EPA, 4 pp, 2002.