Water solubility 2.3.7.8- TCDD

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. 

There are 210 different isomeric possibilities, 75 of which are PCDDs and 135 are PCDFs. The toxicity of these isomers varies greatly, and only 15 exhibit extreme toxicity, the most toxic of which is 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD). The toxicity of the other isomers is therefore expressed as a toxicity equivalent of 2,3,7,8-TCDD. The PCDDs and PCDFs are poorly water soluble but are fat soluble and are therefore able to accumulate in tissue fat, thus allowing them to bio-accumulate in living organisms. The origin of dioxins in the pulp and paper industry is not entirely clear. They may be produced from the chlorination of dibenzodioxin which may be present in recycled oils used to make defoamers, but they may also arise from wood chips which have been treated with polychlorophenol to prevent sap stain formation. It is also possible that they are derived from lignin by chlorination. Dioxins are also known to be formed naturally by combustion of material such as wood, and forest fires have been particularly identified as a likely major cause of dioxin emissions. 

Dougherty et al. (1993) conducted a theoretical analysis of a proposed in situ method for decontaminating soil by photodegradation. Up to 86% of TCDD in the soil can be degraded by this process (Zhong et al. 1993). Because of its extremely low water solubility and volatility, TCDD is a very persistent soil contaminant. With the method, based on the physical properties that facilitate photolysis... 

TrCDD). Thus, volatilization from the water column is not expected to be a very significant loss process for the TCDD through OCDD congeners as compared to adsorption to particulates. In general, the Henry s law constants decrease with increasing chlorine number as a result of the decrease in vapor pressure and water solubility (Shiu et al. 1988). Volatilization half-lives for 2,3,7,8-TCDD were calculated for ponds and lakes (32 days) and for rivers (16 days) (Podoll et al. 1986). The primary removal mechanism for CDDs from the water column is sedimentation, with 70-80% of the CDDs being associated with the particulate phase (Muir et al. 1992). The remainder was associated with dissolved organic substances. CDDs bound to sediment particles may be resuspended in the water column if the sediments are disturbed. This could increase both the transport and availability of the CDDs for uptake by aquatic biota (Fletcher and McKay 1993). 

Adams WJ, Blaine KM. 1986. A water solubility determination of 2,3,7,8-TCDD. Chemosphere 15 1397-1400. 

The major purpose of biotransformation is to chemically modify (metabolize) poorly excretable lipophilic compounds to more hydrophilic chemicals that are readily excreted in urine and/or bile. Without metabolism, lipophilic xenobiotics accumulate in biota, increasing the potential for toxicity. Examples of such compounds are highly halogenated polychlorinated biphenyls (PCBs) and polychlorinated dibenzofu-rans (TCDD and dioxins) that occur as tissue residues in humans. On the contrary, biotransformation is normally not required for xenobiotics with high water solubility because of rapid excretion in urine. 

TCDD. Early studies by Matsumura and Benezet (20) demonstrated that microbial metabolism was very slow if it occurred at all. Further work confirmed that although 2,3,7,8-TCDD is indeed relatively stable to microbial metabolism, two Isolated microorganisms that gave water soluble metabolites were identified. In both aquatic sediment and terrestrial soil systems, metabolism of 2,3,7, 8-TCDD was demonstrated. Addition of nutrients such as glucose, mannitol, and bactopeptone stimulated the transformation (21). 

Podoll and coworkers O) have redetermined the vapor pressure of 2,3,7,8-TCDD. Using this information together with the aqueous solubility, octanol/ water partition coefficient and photolysis quantum yields, investigators have estimated the half-lives for movement and transformation of 2,3,7,8-TCDD in water and air. Even though the vapor pressure (P) of 2,3,7,8-TCDD is low, the water solubility (S) is also very low and the Henry s law constant is therefore significant, and allows vaporization from water. 

The extensive amount of research on this compound has indicated that 2,3,7,8-TCDD is both remarkably stable and is almost exclusively associated with particulate material. Because of the low water solubility and high sorption on soils, it moves only very slowly in natural soils. Human exposure to 2,3,7,8-TCDD in soils is thus a function of movement and degradation (primarily photochemical) on soil particles. Enhanced movement of 2,3,7,8-TCDD in soils also contaminated with oils and/or solvents potentially can increase concentrations near the surface, and this process, along with photolysis on particles, requires further investigation. 

Laboratory photolysis experiments were designed to confirm that 2,3,7,8-TCDD contained in the selected scrubber solvent could be reduced to 1 ng/g and to determine the reaction rates of the primary HO constituents and 2,3,7,8-TCDD in that solvent matrix. A previous photolysis process for 2,3,7,8-TCDD used hexane as a solvent (8). The solvent selected for use in the TD/UV process was different - a high boiling (kerosene-like) mixture of isoparaffins. This hydrocarbon solvent was selected because of its very low vapor pressure and water solubility, nontoxic and nonflammable characteristics, relatively low cost, chemical stability, and good solvent properties for HO constituents. A second major difference from earlier IT photolysis studies was the presence in the scrubber solution of significant concentrations of other chlorinated organic reactants (2,4-D and 2,4,5-T) which were also subject to photolysis. In fact, the typical concentration ratio between 2,4-D or... 

PHYSICAL PROPERTIES colorless to light-tan crystalline solid odorless usually has 2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD) as a minor component soluble in alcohol insoluble in water very slightly soluble in petroleum ether forms water-soluble sodium and atkanolamine salts commercial products are usually in the form of esters or amines, often in mixture with 2,4-dichlorophenoxyacetic acid (2,4-D) MP (151-153°C, 304-307°F) BP (decomposes above melting point at 760 mmHg) DN (1.80 g/cm at 20°C) SG (1.80) VD (NA) VP (< 1 x 10 mmHg at 20°C). 

TCDD has a very low vapor pressure of only 1.7 x 10 mm Hg at 25°C, a high melting point of 305°C, and a water solubility of only 0.2 pg/L. It is stable thermally up to about 700°C, has a high degree of chemical stability, and is poorly biodegrad-... 

Tetrachlorodibenzo-p-dioxin (TCDD) (I), an occasional contaminant in 2,4,5-T and other trichlorophenol derivatives, is the most toxic of the commonly-encountered dioxins (8) and it received most of our attention. Its low solubility in common solvents and water (ca. 2 ppb) limited our experiments since the products were difficult to identify by the conventional techniques of organic chemistry. However, TCDD has an absorption maximum at 307 nm in methanol—well within the solar spectrum observed at the earth s surface and near the region of maximum intensity (310-330 nm) of the UV lamps used in previous experiments (H 29). 

However, some of the studies were limited by using 2,3,7,8-TCDD concentrations in excess of its solubility in water. Only two early studies reported positive results (Hussain et al. 1972 Seiler 1973). However, the results were limited by failure to demonstrate a dose-response relationship and by low bacterial survival rates. In addition, 2,3,7,8-TCDD exposure induced reverse mutations in Escherichia coli (Hussain et al. 1972) and in Saccharomyces cerevisiae (Bronzetti et al. 1983). The conflicting data obtained in the above studies may result from technical difficulties in testing 2,3,7,8-TCDD rather than from a lack of biological activity. Testing difficulties arise from an extreme insolubility of this compound and a high toxicity observed in some test systems, which would be anticipated to result in a very narrow window for effective genotoxic doses. 

TP [2-(2,4,5-trichlorophenoxy propionic acid], or silvex, is an herbicide that has been used for weed and brush control on rangeland and rights of way. It is soluble in water and its environmental resistance is expected to be relatively short. 2,4,5-TP is contaminated to varying extents with 2,3,7,8-TCDD, a toxic polychlorinated dibenzo-/j-dioxin. It has been classified in the EPA s Group D (not classifiable). 

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