Vapor pressure 2,3,7,8-TCDD

No quantitative data were located regarding absorption of CDDs in humans following dermal exposure. However, based on data from studies with structurally related chemicals it is reasonable to assume that CDDs are absorbed by this route. Furthermore, data on levels of CDDs in blood from populations with above-background exposures (i.e., occupational, accidental) also suggest that dermal absorption occurs in humans. Due to the relatively low vapor pressure and high lipid solubility, dermal uptake of 2,3,7,8-TCDD in the workplace may be a significant source of occupational exposure (Kerger et al. 

Those chemicals with vapor pressures between these values can be found in both the vapor phase and associated with particulates (Eisenreich et al. 1981). With a reported vapor pressure ranging from 7.4xlO 10 to 3.4xl0"5 mm Hg, 2,3,7,8-TCDD falls into the intermediate category. 

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). 

Volatilization of 2,3,7,8-TCDD from soils is expected to be very slow, due to the low vapor pressure and high octanol-water partition coefficient. Mill (47) estimates that the half-life for vaporization of 2,3,7,8-TCDD from soils will range from many months to years, in the absence of intervening transformation processes. 

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. 

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... 

Figure 24.9. Vapor pressure of selected compounds versus temperature. (Note Atmospheric pressure is 760mmHg.) (Copyright, 2001, CRC Press, used with permission) (Stegemeier and Vinegar, 2001). TEL, tetra-ethyl lead PCP, pentachlorophenol TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin PCB, polychlorinated biphenyls.

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-... 

Naphthalene (NAPH), pyrene (PYR), and 2,3,7,8-tetrachlorodibenzodioxin (TCDD) were chosen as typical air contaminants. For purposes of human exposure and risk assessments, the concentrations in films at 25°C and 1 atm total pressure assuming air concentrations are at the saturated vapor levels. Choose any realistic parameters needed and justify any key assumptions. 

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