PCDF congeners

Polychlorinated Dibenzo-(p)-Dioxins and Dibenzo-Furans. Another group of compounds that we need to specifically address are the polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzo-furans (PCDFs) (Fig. 2.15). The PCDDs and PCDFs are not intentionally produced but are released into the environment from various combustion processes and as a result of their occurrence as unwanted byproducts in various chlorinated chemical formulations (e.g., chlorinated phenols, chlorinated phenoxy herbicides see Alcock and Jones, 1996). Because some of the PCDD and PCDF congeners are very toxic (e.g., 2,3,7,8-tetrachloro dibenzo-p-dioxin, see margin), there have been and still are considerable efforts to assess their sources, distribution, and fate in the environment. Similarly to the PCBs or DDT (see above), the PCDDs and PCDFs are highly hydrophobic and very persistent in the environment. It is therefore not surprising that they have also been detected everywhere on earth (Brzuzy and Hites, 1996 Lohmann and Jones, 1998 Vallack et al., 1998). Finally, we should note that polybrominated diphenylethers (PBDEs, see margin) that, like the PBBs (see above), are used as flame retardants, are of increasing environmental concern (de Boer et al., 2000). 

The fractional contribution of the major PCDD and PCDF congeners to their respective total concentrations in Niagara River sediments in Table 3 is shown in Fig. 9. Despite large variations in concentration among sites, the PCDF pattern is remarkably similar along the river. The major congeners are OCDF,... 

Fig. 9 Fractional contribution of major PCDD and PCDF congeners to E-PCDDs and E-PCDFs concentrations in Niagara River sediments in 2000 (Table 3). Adapted from Richman [38]...

Temporal trends of PCDD/F concentrations in Lake Ontario lake trout, 1977-1993 were determined by Huestis et al. [65]. Time trends of the mean concentrations of the major congeners are shown in Fig. 12. Very similar trends were found in lake trout eggs from Lake Ontario collected during much the same period, 1978-1991 [16]. There were no strong trends in concentrations of the major PCDF congeners, 2378-TeCDF, and 23478-PnCDF. 

Opperhuizen, A., Wagenaar, W.J., Van der Wielen, F.W.M., Van den Berg, M., Olie, K., O. Hutzinger, O., Gobas, F.A.P.C. (1986) Uptake and elimination of PCDD/PCDF congeners by fish after aqueous exposure to a fly ash extract from a municipal incinerator. Chemosphere 15, 2049-2053. 

Neubert D, Golor G, Neubert R. 1992c. TCDD-toxicity equivalencies for PCDD/PCDF congeners prerequisites and limitations. Chemosphere 25 65-70. 

The upper limits to the 03 reaction rate constants given in Table 1, and the expected upper limit to the rate constants for the gas-phase reactions of 03 with PCB, PCDD and PCDF congeners of <2 x 10-2Ocm3 molecule-1 s-1 (see above), lead to a calculated lower limit to the tropospheric lifetimes of PCB, PCDD and PCDF congeners due to gas-phase reaction with 03 of >2 years. Similarly, the upper limit to the measured bimolecular rate constants kabs for reactions of the N03 radical with PCBs, PCDDs and PCDFs under tropospheric conditions, of kabs < 1 x 10-15 cm3 molecule-1 s-1, leads to a lower limit to the tropospheric lifetimes of PCB, PCDD and PCDF congeners due to gas-phase reaction with the N03 radical of >45 days. 

H. P. Goorissen, Comparison of PCDD and PCDF Congener Patterns in Different Species and Trophic Levels Evidence for Metabolism , University of Amsterdam, Amsterdam, 1994 (in Dutch). 

Figure 2 Relative abundance of 2,3,7,8-substituted PCDF congeners to 2,3,4,7,8-pentaCDF in marine (a) and terrestrial mammals (b). For explanation, see legend to Figure 1...

Photochemical reactions of PCDEs have been studied in different solvents, but no difference in the amount of PCDFs has been observed [72,73], In the case of PCBs, the rate of dechlorination is faster in polar solvents than in hydrocarbon solvents [3]. Dehalogenated PCDEs have dominated in photolysis in -hexane [72], but photolysis of PCDEs in acetone solution inhibits dechlorination and favors photocyclization [73]. Therefore, it has been suggested that reactions in the environment in conditions near to acetone solution could have significance in the formation of PCDFs from PCDEs [73]. Photolysis in acetone solution can be used to synthesize PCDF congeners [74]. Carbonium ion is a possible intermediate in ring closure of PCDEs to PCDFs [75]. PCDFs can be synthesized from PCDEs... 

The stability was verified at -20 C, -i-20°C and +37°C. No instability could be demonstrated for the PCDD and PCDF congeners at these temperatures over a 12 months period [34]. 

Polychlorodibenzo-p-dioxins (PCDDs) and poly-chlorodibenzofurans (PCDFs) are mostly produced by incineration, although they are also a waste product of the pulp/paper and chemical manufacturing industries (Duarte-Davidson et al. 1997).There are potentially 75 PCDD and 135 PCDF congeners (see Fig. 7.17), but PCDDs from most biological samples contain only the tetra- to octachloro congeners with 2,3,7,8 chlorination the absence of the other congeners has been attributed to rapid metabolism and excretion (Ahlborg... 

Most of the analyses of samples from incinerators have been done using non-validated and non-isomer specific sampling and analytical methods. Recent studies show the presence of a multitude of PCDD and PCDF congeners, in fact all isomers seem to be present k) Moreover, a striking similarity in the isomeric pattern of PCDDs and PCDFs was found between samples from different incinerators... 

Usually in the context of environmental remediation, human potential hazard, or/and comparison of dioxin-like compounds, DLC, an estimation of the toxic potency of the whole sample is desired, rather than the concentrations of the sample s components. One such measure, the toxic equivalents, TEQ, is obtained from the analytical data by using Toxic Equivalency Factors, TEFs, relating the potency of DLC X to the potency of TCDD, taken as a reference toxicant (TEF = 1). In this TEF scheme, the toxicity of any PCDD or PCDF congener is related, using animal data, to the amount ofTCDD that would have equal toxicity (Table 3). 

By comparison with 2,3,7,8-TCDD, the toxicity of other PCDD and PCDF congeners are reduced if chlorine atoms are removed from the lateral (2,3,7, or 8 positions) and/or are present at positions 1,4,6, or 9. These structural changes reduce greatly the toxicity of considered chemicals (Table 6). 

Describe the general structural formulas of PCDD and PCDF congeners. Stress the role of chlorine substitution in toxicity of these compounds. 

CK on the fecal excretion of polychlorinated dibenzo-/ -dioxin (PCDD) congeners and polychlorinated dibenzofuran (PCDF) congeners in rats administered the dioxin mixture [3]. 

Figure 1 Effects of Chlorella diet on fecal excretion amounts of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in contaminated rice oil-fed rats. Values are means SD, n=4. Vhlues of acceleration index of fecal excretion amounts of PCDD and PCDF congeners due to Chlorella diet were calculated using the following equation Acceleration index of fecal excretion amounts of PCDD...

In this study, highly toxic PCDD and PCDF congeners were more easily... 

These results are critical for evaluating the efficancy of chlorophyll as an agent for decreasing intake of toxic PCDD and PCDF congeners. Thus, chlorophyll administration enhanced the fecal excretion of PCDD and PCDF congeners and reduced PCDD and PCDF congeners absorption in rats administered the dioxin mixture. 

Each value represents percent body burden ot PCDD and PCDF congeners against total ng ot TEO values m rats administered PCDD and PCDF congeners. 

Acceleration index of disappearance trom the body (shown in parentheses) equals (percent body burden ot PCDD and PCDF congeners ot rats in the chlorophyll diet) / (percent body burden ot PCDD and PCDF congeners of rats fed the control diet) x 100. 

PCDDs and PCDFs constitute a class of ubiquitous pollutants with aromatic structure, high chemical stability and extremely poor water solubility. They can occiu in the form of 75 PCDD congeners and 135 PCDF congeners (Table 1). At present, most PCDD and PCDF soinces are well characterized. These sources include chemical, thermal, photochemical and enzymatic reactions. Combustion processes, mainly incineration plants such as municipal solid waste incinerators, clinical waste incinerators and industrial waste incinerators are known to be some of the most important sources responsible for the presence of these contaminants in the environment. Soils and sedi-... 

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