Dioxins
Chemistry and Behavior
2,3,7,8- Dibenzo-p-Dioxin Molecule
Dioxins, or polychlorinated dibenzo-p-dioxins (PCDDs), are a class of structurally similar chlorinated hydrocarbons made up of two benzene rings joined at their meta and para carbons by oxygen atoms. There are eight homologues of PCDDs — monochlorinated through octachlorinated — that result in a total of 75 congeners. Common physical and 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 high log Kow values in the table illustrate, all PCDDs are highly hydrophobic and would not be expected to be found in ground or surface water 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 not easily evaporate into the atmosphere from soils and other solid matrices.
While biodegradation is the principal process by which dioxins are removed from the environment, dioxins biodegrade very slowly. Breakdown by sunlight, or "photolysis" is a more rapid degradation process. The estimated half-life on the soil surface of 2,3,7,8-tetrachlorodibeno-p-dioxin (TCDD), one of the more toxic congeners, ranges 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, with degradation primarily a result of microbial anaerobic reductive dichlorination (Bunge and Lechner, 2009).
Adapted from:
2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN
National Center for Biotechnology Information. PubChem Database. CID=15625 (accessed on June 30, 2020)
Toxicological Profile Chlorinated Dibenzo-p-Dioxins (CDDs)
Agency for Toxic Substances and Disease Registry, 1998 with updated policy guideline in Appendix B for residential soil, September 2008
Anaerobic Reductive Dehalogenation of Polychlorinated Dioxins
Bunge, M. and U. Lechner | Applied Microbiology and Biotechnology volume 84, pages 429-444(2009)
For More Information
The Environmental Significance of Sediment Surface Area as a Controlling Factor in the Preservation of Polychlorinated Dibenzo-P-Dioxins and Dibenzofurans (PCDD/PCDF) in Sediments Adjacent to Woodfibre Pulp Mill, Howe Sound, British Columbia
Minerals 9:711(2019)
doi:10.3390/min9110711
Chalmers, G., R. Adams, A. Bustin, and M. Bustin.
A sediment core was retrieved from an area adjacent to a pulp and paper mill in Howe Sound to examine the accumulation of dioxins and furans.
Forensic Analysis of Polychlorinated Dibenzo-p-Dioxin and Furan Fingerprints to Elucidate Dechlorination Pathways
Environmental Science & Technology, 51(18):10485-10493(2017) DOI: 10.1021/acs.est.7b02705
Lisa A. Rodenburg, Yashika Dewani, Max M. Haggblom, Lee J. Kerkhof, and Donna E. Fennell.
This study evaluated a concentration data set of PCDD/F congeners with four or more chlorines along with all 209 PCB congeners in surface water, treated and untreated wastewater, landfill leachate, and biosolids to determine whether peri and peri/lateral dechlorination of PCDD/Fs occurs in these environments.
Compilation of EU Dioxin Exposure and Health Data, Task 3: Environmental Fate and Transport
European Commission DG Environment, 26 pp, 1999.
Evaluating Potential Exposures to Ecological Receptors Due to Transport of Hydrophobic Organic Contaminants in Subsurface Systems
Ford, R.G., M.C. Brooks, C.G. Enfield, and M. Kravitz. EPA 600-R-10-015, 69 pp, 2014
This technical paper recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs, as well as site artifacts that result from inadequate well installation and sampling procedures within a ground-water monitoring network.
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) Archived.
National Primary Drinking Water Regulations. U.S. EPA, 4 pp, 2002.