Chlorinated PCDD

TCDD has been more widely studied than other PCDDs, and will be taken as an example for the whole group of compounds. It is a stable solid with a melting point of 306°C. Its water solubility is very low, which has been estimated to be 0.01-0.2 pg/L its log is 6.6. More highly chlorinated PCDDs are even less soluble in water. 

Their mobility may be increased by the simultaneous presence of organic solvents such as mineral oil. The half-life of TCDD in soil has been reported as 10-12 years, whereas photochemical degradation seems to be considerably faster but with a large variation that might be explained by experimental differences (solvents used, etc.). Highly chlorinated PCDD/PCDFs seem to be more resistant to degradation than those with just a few chlorine atoms. 

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 ... 

In analyses using high-resolution GC/MS and MS confirmation, Rappe et al. (35.1 and Norstrom et al. (30) have reported that in other samples of Herbicide Orange, as well as in European and US 2,U,5-T formulations from the 1950 s and 1960 s, 2,3,7,8--tetra-CDD was the dominating compound of this group. Only minor amounts of other PCDDs and PCDFs could be found, primarily lower chlorinated PCDDs in samples of Herbicide Orange, see ref 35. 

Giesy et al. [66] compared PCDD/F concentrations in dorsal muscle and eggs of coho salmon, chinook salmon and lake trout from Lakes Michigan, Superior, and Huron, 1987 and 1988. Data for the three species were presented as concentrations of homologs plus 2378-TeCDD and 2378-TeCDF. The homolog approach renders comparison of higher chlorinated PCDD/F concen-... 

HpCDD > 123678-HxCDD were > 95% of E-PCDD/Fs in all groups. The dominance of these congeners is a very different pattern than found in fish and birds from the Great Lakes (Tables 5-10), a reflection of the broad-based incorporation of atmospheric-sourced PCDD/Fs into the human food chain from meat and dairy products [83]. Bioavailability of the higher chlorinated PCDD/Fs is much less of a factor in modifying source patterns in the atmosphere-plant-livestock route than it is in the atmosphere-water-sediment-fish route. 

Much of the effort on environmental chemicals that contaminate food has concentrated on a small range of chlorinated chemicals. In addition to the chlorinated PCDDs, PCDFs and PCBs already mentioned, other chlorinated compounds can be separated into two groups chlorinated aromatic compounds and chlorinated aliphatic compounds. Although there is a number of organochlorine pesticides that are persistent in the environment, these will not be considered here, as they comprise an extensive field of study in their own right. 

This states that the concentration of the lower chlorinated PCDD/Fs in vegetation is the time integral of the product of the air-side mass transfer coefficient (the inverse of the air-side resistance), the specific surface area of the vegetation and the gaseous air concentration. 

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... 

Ambient air studies39,78,80,81 show that the particle-phase (and hence more chlorinated) PCDDs and PCDFs are more efficiently removed from the atmosphere by wet deposition than are the gas-phase PCDDs and PCDFs, analogous to the case for the PCBs. 

Adriaens P., Chang P. L., and Barkovskii A. L. (1996) Dechlorination of chlorinated PCDD/F by organic and inorganic electron transfer molecules in reduced environments. Chemosphere 32, 433-441. 

As for the PCBs, the lower chlorinated PCDDs and PCDFs exhibit higher aqueous solubilities and vapour... 

In addition to tetra- up to hexachlorodibenzo-p-dioxins/furans with chlorine atoms at positions 2, 3, 7, 8 (8 substances transferred from the Ordinance on Dangerous Substances, App. V,3), the amendment of 6th July 1994 extends the number of congeners of this type by some additional penta- up to octachlorodibenzo-p-dioxins/furans. Now the total number of restricted chlorinated PCDD/PCDF-Congeners amounts to 17 substances. The threshold concentrations for these chemicals in substances preparations and products were decreased considerably, depending on their toxic potentials. However, plant treatment agents, intermediates, and a few more products were excluded from this regulation. 

High-chlorinated PCDD and PCDF are generally characterised by considerable bioaccumulation potential and high persistence. Their exclusion from the exposed organism is very slow. Typical ranges of half-Kfe values for elimination of toxic congeners from human... 

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