Properties of PCDD

As polychlorinated 1,4-dibenzodioxins (PCDDs) are highly toxic chemicals, and well-known environmental pollutants and environmental estrogens, a lot of attention has been paid to 1,4-dibenzodioxins and their halogenated derivatives. Optical spectra and photophysical properties of PCDD derivatives have been thoroughly reviewed <2000RCR1037>. 

Optical spectra and photophysical properties of PCDD derivatives have been thoroughly reviewed <2000RCR1037>. 

Information is lacking or scarce on the biological properties of PCDD isomers, except... 

Properties of PCDD and PCDF are similar to other POPs, whose aromatic rings are substituted with halogens. 

Some characteristic physical properties of PBDD/F have been determined and can be found elsewhere 8-14. An important property for environmental behaviour is the vapour pressure. Vapour pressures of some PBDD has been measured by a special new technique 30. pig. 12 shows extrapolated vapour pressures of PCDD and PCDF and of corresponding PBDD and PBDF. 

Toxic and other interaction effects of PCDDs with other groups of polychlorinated chemicals having similar structure and properties, such as biphenyls, dibenzofurans, and biphenylenes. 

Although these reservoirs may be highly contaminated with PCDD/PCDFs, the chemical and physical properties of these compounds imply that dioxins and furans will stay adsorbed to organic carbon in soils or other particles. On the other hand, mobilization can occur in the presence of lipophilic solvents (leaching into deeper layers of soils and/or groundwater) or in cases of erosion or run-off from topsoil (translocation into the neighbourhood). Experience has shown that transport of PCDD/PCDFs due to soil erosion and run-off does not play a major role in environmental contamination and human exposure (Fiedler 1995, 1999). 

The physicochemical properties of both PCDD/PCDFs and coplanar PCBs enable these compounds to be readily absorbed by organisms. The high lipid solubility and low water solubility of all congeners lead to the retention of the compounds in... 

PCDD/F and other chlorinated hydrocarbons observed as micropollutants in incineration plants are products of incomplete combustion like other products such as carbon monoxide, polycyclic aromatic hydrocarbons (PAH), and soot. The thermodynamically stable oxidation products of any organic material formed by more than 99% are carbon dioxide, water, and HCl. Traces of PCDD/F are formed in the combustion of any organic material in the presence of small amounts of inorganic and organic chlorine present in the fuel municipal waste contains about 0.8% of chlorine. PCDD/F formation has been called the inherent property of fire. Many investigations have shown that PCDD/Fs are not formed in the hot zones of flames of incinerators at about 1000°C, but in the postcombustion zone in a temperature range between 300 and 400°C. Fly ash particles play an important role in that they act as catalysts for the heterogeneous formation of PCDD/Fs on the surface of this matrix. Two different theories have been deduced from laboratory experiments for the formation pathways of PCCD/F ... 

There is a pronounced difference in biological and toxicological effects between different PCDD and PCDF isomers which is contradictory to the chemical and physical properties of these compounds discussed above. The isomers with the highest acute toxicity are 2,3,7,8-tetra-CDD, 1,2,3,7,8-penta-CDD, 1,2,3,, 7 8-, 1,2,3,6,7,8- and 1,2,3,7,8,9-hexa-CDD, 2,3,7,8-tetra-CDF, 1,2,3,7 8 and 2,3,, 7 8-penta-CDF and 2,3,U,6,7,8-hexa-CDF, see Figure 2. All these isomers have their four lateral positions substituted for chlorine, and they all have LD,-0 values in the range 1-100 yg/kg for the most sensitive animal species (15-17) The same isomers have been reported to have the highest biological potency (l8). 

Chlorophenols may contain a variety of by-products and contaminants such as other chlorophenols, polychlorinated phenoxy-phenols and neutral compounds like polychlorinated benzenes and diphenyl ethers, PCDDs and PCDFs (39). Some of these contaminants may also occur in chlorophenol derivatives like phenoxy acids, other pesticides and hexachlorophene. The possible presence of PCDDs and PCDFs is of special significance because of their extraordinary toxicological properties. 

Chen, J., Xie, Q., Zhao, Y., Yan, Y., Yang, F. (2001) Quantitative structure-property relationship studies on n-octanol/water partitioning coefficients of PCDD/Fs. Chemosphere 44, 1369-1374. 

Table 1 Physicochemical properties of selected PCDD/Fs and PCBs at ambient temperature...

As described above, wet and dry particle-bound deposition are likely important for the accumulation of the higher chlorinated PCDD/Fs in aerial vegetation. The accumulation of particle-bound PCDD/Fs in plants is a function of a myriad of factors. The deposition rate itself is influenced by the particle size spectrum in the atmosphere and the distribution of the PCDD/Fs on the different particle size fractions, and further by the atmospheric turbulence, the canopy and plant properties, and the frequency and intensity of precipitation. The retention of the contaminants on the plant surface depends on the degree to which the particles are permanently retained on the plant and, for those particles which are not retained, the degree of transfer of PCDD/Fs from the particles to the plant cuticle. This is a very complex system that is not yet well understood. One approach that... 

Properties of PCDEs, including physicochemical ones, are not well known as the literature reviews of PCDEs have shown [4, 11,40,46]. PCDEs resemble PCBs structurally and in their chemical and physical properties, which, like PCDDs, PCDFs, and related compounds, are known to be stable and resistant to breakdown by heat, hydrolysis, bases, and acids. PCBs are also quite stable to oxidation under moderate conditions [3], but there is not much data about PCDEs concerning their stability. There is some evidence that PCDEs are resistant to bases and acids and the occurrence of PCDEs in the environment indicates that PCDEs are persistent and bioaccumulating compounds. The study of Firestone et al. [37] already showed that PCDEs are quite stable, since PCDEs could be measured in chlorophenol extracts after sulfuric acid treatment. Tetra- and octachlorinated PCDE congeners were later proven resistant in treatment with... 

Data on the physical and chemical properties of PCDTs and PCTAs are scarce. Due to their structural similarity to PCDFs and PCDDs they are also supposed to possess some likeness in their physical and chemical properties. Sulfur and oxygen are both Group VI elements with two outer shell electrons available for covalent bonding. Structures of thiophene and furan with benzene carbon-sulfur (Cb-S) and carbon-oxygen bond (Cb-0), in PCDTs and PCDFs respectively, suggest similar chemical behavior. The bond dissociation energies (AH) show that less energy is required to break the Cb-S bond than the Cb-0 bond [17,36,37]. 

Indeed, Dachs et al. (2002) demonstrated, using a combination of field measurements of atmospheric PCBs, PCDDs, and PCDFs, and remote sensing data of ocean temperatirre, wind speed and chlorophyll, that deposition in mid-high latitudes is driven by sinking marine particulate matter, rather than by cold condensation. However, the relative contribution of this process is highly dependent on the physical chemical properties of the POPs under consideration. 

Chen, J., Quan,X., Peijnenburg, W.J.G.M. and Yang, F. (2001a) Quantitative structure-property relationships (QSPRs) ondirectphotolysis quantum yields of PCDDs. Chemosphere, 43, 235-241. 

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