Volatilization, 2,3,7,8-TCDD

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... 

Because TCDO has a very low volatility, TCDD uptake via inhalation is directly related to the concentration of airborne dust due to wind-blown soil. It has been estimated that of all airborne, respirable particulates, only about 30-50% comes from soil, while the rest is apparently due to products of combustion, tire wear and other sources (32). Of the total suspended particulates, usually no more than 50% are respirable (i.e., particles less than 10 urn). Of these, about 50% of the respirable particles are deposited in the upper airways and ultimately swallowed while the rest reach the alveoli or are expired. An analysis of CDC s data indicates that CDC assumed that 100% of the TCDD present on all the inhaled particles would be retained and absorbed in the respiratory tract. In contrast, the EPA assessment (2) assumed that only 25% of the inhaled particles would be absorbed in the lower airways since at least 50% of the particles would be non-respirable (especially by weight) and these will be swallowed due to impaction in the throat and only about 50% of the respirable particles would be absorbed. In any assessment, it is important to recognize that of those particles swallowed, no more than 10-30% should be absorbed since they will pass through the G.I. tract (assuming 10-30% oral bioavailability). 

Some hquid defoamers are preemulsified relatives of paste defoamers. In addition to the fatty components mentioned above, kerosene [8008-20-6] or an organic cosolvent such as 2-propanol have been used to enhance stabiUty of the oil—water emulsion and the solubiUty of the defoamer s active ingredients. These cosolvents are used less frequently as concerns increase about volatile organic emissions (VOCs) from the paper machine. Additionally, the use of ultrapure mineral oil in defoamers has become commonplace. Concern about the creation of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) in the pulping process has led to the discovery of unchlorinated precursor molecules, especially in recycled mineral oil and other organic cosolvents used in defoamer formulations (28). In 1995 the mineral oil that is used is essentially free of dibenzodioxin and dibenzofuran. In addition, owing to both the concern about these oils and the fluctuating cost of raw materials, the trend in paper machine defoamers is toward water-based defoamers (29). 

Similar results were obtained when TCDD in methanol was exposed to natural sunlight in sealed borosilicate glass tubes or beakers (Figure 3). After about 36 hours exposure, a yellow non-volatile gum was obtained as the sole product by evaporation of the solvent. It showed no UV absorption and did not seem to retain the benzenoid chromophore ... 

Next, we attempted to deal with translocation of foliar-applied TCDD. Labeled dioxins were applied to the center leaflet of the first trifoliate leaf of 3-week-old soybean plants and the first leaf blade of 12-day-old oat plants. All compounds were applied in an aqueous surfactant solution (Tween 80) to enhance leaf adsorption and to keep the water insoluble dioxins in solution. Plants were harvested 2, 7, 14, and 21 days after treatment, dissected into treated and untreated parts, and analyzed separately. Neither dioxin nor chlorophenol was translocated from the treated leaf. A rapid loss of the dichlorodioxin and dichlorophenol occurred from the leaf surface. This loss may have resulted from volatilization. Very little TCDD was lost from soybean leaves while a gradual loss (38% in 21 days) did occur from oat leaves. 

Several facts have emerged from our studies with 2,7-DCDD and 2,3,7,8-TCDD. They are not biosynthesized by condensation of chloro-phenols in soils, and they are not photoproducts of 2,4-dichlorophenol. They do not leach into the soil profile and consequently pose no threat to groundwater, and they are not taken up by plants from minute residues likely to occur in soils. Photodecomposition is insignificant on dry soil surfaces but is probably important in water. Dichlorodibenzo-p-dioxin is lost by volatilization, but TCDD is probably involatile. These compounds are not translocated within the plant from foliar application, and they are degraded in the soil. 

NP NPEC OC OP OPEC PCB PCDBT PCDD PCDF PCP PFB RA TCA TCDD TCF TCMTB TOC VSC VOC Nonylphenol Nonylphenol ethoxycarboxylate Organo chlorine Octylphenol Octylphenol ethoxycarboxylate Polychloroinated biphenyls Polychlorinated dibenzothiophene Polychlorin ated dib enzo-p - dioxins Polychlorinated dibenzo-p-furans Pentachlorophenol Pentafluorobenzyl Resin acids 2,4,6-Trichloroanisole Tetrachloro dibenzo dioxin Totally chlorine- free 2-(Thiocyanomethylthio)-benzothiazole Total organic carbon Volatile sulphur compounds Volatile organic compounds... 

Surface Water. Plimmer et al. (1973) reported that the photolysis half-life of TCDD in a methanol solution exposed to sunlight was 3 h. Volatilization half-lives of 32 and 16 d were reported for lakes and rivers, respectively (Podoll et ah, 1986). 

The product used was a low-volatile 2,1, 5-T propylene glycol butyl ether ester formulation containing 0.0U ppm 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It was applied as an emulsion in water at a rate of 1.6 pounds of 2,1, 5-T acid equivalent per acre (lb/A) in the backpack study, and 2 lb/A in the other studies. These were low-volume applications at 10 gal/A in the ground studies and 5 gal/A by air. Thus, the workers in the aerial studies were exposed to about 5 2,1, 5-T spray solutions compared to the 2 or 3 used in the earlier studies by Dow and EPA described above (3, 7). 

Eduljee, G. (1987) Volatility of TCDD and PCB from soil. Chemosphere 16, 907-920. 

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). 

A model has been developed to describe the vertical transport of low-volatility organic chemicals in soil (Freeman and Schroy 1986). The model was used to make predictions on the transport of 2,3,7,8-TCDD at the Eglin Air Force Base Agent Orange biodegradation test plots (Freeman and Schroy 1986). 

Environmental Fate. CDDs are subject to atmospheric transport and both wet and dry deposition (Kieatiwong et al. 1990). They are partitioned to air, water, sediment, and soil, and they accumulate in both aquatic and terrestrial biota. CDDs can volatilize to the atmosphere from water and soil surfaces. They adsorb strongly to soils and are not likely to leach into groundwater (Eduljee 1987b). In the aquatic environment, CDDs partition to sediment or suspended particulates. TCDD, HpCDD, and OCDD are subject to photolysis in air, water, and soil (Plimmer et al. 1973). 2,3,7,8-TCDD is biodegraded very slowly in soil and thus is likely to persist in the soil. A better understanding of environmental behavior of CDDs is needed with respect to the importance of vapor-phase versus particulate transport, the... 

Freeman RA, Schroy JM. 1986. Modeling the transport of 2,3,7,8-TCDD and other low volatility chemicals in soils. Environ Progress 5 28-33. 

For low volatility components such as polychlorobiphenyls or tetrachloro-para-dibenzodioxins (TCDD), it is possible to make use of a solvent effect in order to perform on-column enrichments. Thus, Poy [ 14 ] injected 4 times 2 pi of a sample containing TCDD isomers without destroying the separation performance. The low boiling components were stron y retained in the thick solvent layer formed when the solvent recondensed so that all 4 injections refocused in the column inlet before the chromatographic process took place. 

TCDD, except for a "yellow, non-volatile gum" that was formed after extended exposure of methanolic solutions to fluorescent lamps (38). 

Plimmer (44) also found that 63% of 2,3,7,8-TCDD degraded on silica gel exposed to 20 hours of sunlight and produced a polar product. Only 68% of the original radioactivity was recovered after the 20-hour exposure, suggesting that volatility may be a factor In the loss. On soils, no difference was observed between exposed and non-exposed areas, suggesting that soil exerts a protective effect against the photolysis of 2,3,7,8-TCDD. 

Measurement of the vapor phase photolysis of 2,3,7,8-TCDD Is experimentally a major challenge due to the low volatility. Podoil and coworkers (3 ), however, have estimated the half life of 2,3,7,8-TCDD In the vapor phase to be 55 minutes. This estimate assumed that the quantum yield Is the same as In hexane and Is Invariant with wavelength, that the spectral properties are the same as In solution, and that the mechanisms for transformation are the same. This photolysis half-life Is approximately 200 times smaller than that expected for reaction with hydroxyl radical. However, both vapor phase photolysis and reactions with OH radicals would be unimportant If TCDD Is predominantly sorbed on particles In the atmosphere. Experimental verification of these estimates Is needed, because atmospheric transformation may be an Important fate of 2,3,... 

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. 

In the top several centimeters of soil, photolysis, volatilization, mass transport in water either dissolved, sorbed on particles, or complexed with other molecules, and bioturbation are potential processes that affect chemical behavior. Freeman and Schroy (22) have developed a model for movement of 2,3,7,8-TCDD in soils based... 

It is now obvious that atmospheric transport of persistent toxic organic substances is the major pathway between ecosystems. For dioxin, volatilization of residues from contaminated soils was first noted as a concern at Seveso, Italy (4). The National Research Council of Canada reported that atmospheric emissions were the major source of chlorinated dioxins in the Canadian environment (5). A recent Ontario report estimates that from 8 10 kg of 2,3,7,8 -TCDD equivalents enter the Ontario environment annually from combustion of municipal refuse and sewage sludge and that all other combustion sources contribute from 20 -50 kg annually (6). The only other major source considered was from the use and disposal of chlorinated phenols. 

The environmental behavior of chemicals with a very low volatility, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has been the subject of several studies over the last few years. The focus of these studies has varied from the development of physical property data ( 1,2) to lab and field tests of the mobility and/or fate of TCDD. 

Previous work by Freeman and Schroy (4) indicated that over 67 percent of the 2,3,7,8-TCDD contained in the top 1 cm of soil should volatilize during the first summer after application. The TCDD concentration in the top 1 cm of soil taken from experimental plots A and C-Control was found to have decreased by 20 percent and 18 percent respectively over the 15 months of elapsed time. The apparent rate of loss of TCDD from plots A and C-Control is lower than previously estimated by Freeman and Schroy (4). The TCDD concentration of the top 1 cm of soil taken from plot D-Shaded was found to have decreased 7 percent over the same 15 months. A loss of 7 percent is not statistically significant and it was concluded that plot D-Shaded remained unchanged over the course of the experiment. 

Laboratory and field testing determined the effectiveness of a new decontamination process for soils containing 2,4-D/2,4,5-T and traces of dioxin. The process employs three primary operations - thermal desorption to volatilize the contaminants, condensation and absorption of the contaminants in a solvent, and photochemical decomposition of the contaminants. Bench-scale experiments established the relationship between desorption conditions (time and temperature) and treatment efficiency. Laboratory tests using a batch photochemical reactor defined the kinetics of 2,3,7,8-TCDD disappearance. A pilot-scale system was assembled to process up to 100 pounds per hour of soil. Tests were conducted at two sites to evaluate treatment performance and develop scale-up information. Soil was successfully decontaminated to less than 1 ng/g... 

PROBABLE FATE photolysis, will not be an important process if reactive substrates are available, atmospheric photolytic half-life 1.1-3.4 days, aqueous photolytic half-life 1.1-3.4 days, photolysis half-life at the water s surface 21 hr (summer), 118 hr (winter) oxidation, not an important process hydrolysis does not occur, if released to atmosphere, vapor phase TCDD may be degraded by reaction with hydroxyl radicals and direct photolysis volatilization not an important process, volatilization half-life 46 days without considering adsorption, considering adsorption effects >50 yrs sorption an important process biologicaiprocesses bioaccumulation is possibly an important process... 

The rate constant for volatilization of 2,3,7,8-TCDD from water can be predicted using the general formulas of Liss and Slater (1974), Mackay (1978), and Southworth (1979). The rate of volatilization is given by the following equation ... 

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