Volume II Chapter 2.0 Pages 1 of 3 page next page 2

2.0 PHYSICAL AND CHEMICAL PROPERTIES AND FATE 2-1

2.1. INTRODUCTION 2-1

2.2. GENERAL INFORMATION 2-2

2.3. PHYSICAL/CHEMICAL PROPERTY EVALUATION METHODOLOGY 2-4

2.4. PHYSICAL/CHEMICAL PROPERTIES - CHLORINATED COMPOUNDS 2-7

2.4.1. Water Solubility 2-7

2.4.2. Vapor Pressure 2-13

2.4.3. Henry's Law Constant 2-15

2.4.4. Octanol/Water Partition Coefficient 2-16

2. PHYSICAL AND CHEMICAL PROPERTIES AND FATE

2.1. INTRODUCTION

This chapter summarizes available information regarding the physical and chemical properties and fate of the CDDs, CDFs, BDDs, BDFs, and coplanar PCBs, with an emphasis on the subset of these chemicals defined as dioxin-like in Chapter 1. Physical/chemical properties addressed in this chapter include melting point, water solubility, vapor pressure, Henry's Law constant, octanol/water partition coefficient, organic carbon partition coefficient, and photochemical quantum yield. Fate and transport processes addressed include photolysis, oxidation, hydrolysis, biodegradation, volatilization, and sorption. Biologically-mediated transport properties (i.e., bioconcentration, plant uptake, etc.) are covered in the companion volume to this report, Volume 3: Site-Specific Assessment Procedures.

Knowledge of physical and chemical properties is essential to understanding and modeling the environmental transport and transformation of organic compounds such as the dioxin-like compounds. The properties most important for understanding the environmental behavior of the dioxin and dioxin-like compounds appear to be water solubility (WS), vapor pressure (VP), octanol/water partition coefficient (Kow), organic carbon partition coefficient (Koc), and photochemical quantum yield. The ratio of VP to WS (VP/WS) can be used to calculate the Henry's Law constant (Hc) for dilute solutions of organic compounds. Henry's Law constant is an index of partitioning for a compound between the atmospheric and the aqueous phase (Mackay et al., 1982).

To maximize and optimize the identification of information on the physical/chemical properties of these compounds, a thorough search of the recent literature was conducted. A computer literature search was conducted using the on-line Chemical Abstracts (CA) data base maintained by the Scientific Technical Network (STN). Printed abstracts were obtained and screened, and selected literature were retrieved and critically evaluated. The most definitive value for each physical/ chemical property for each congener was selected. The evaluation method used to select the most definitive physical/chemical property values is detailed in Section 2.3.

The property values obtained from the scientific literature are summarized in Appendix A. Sections 2.4 and 2.5 present the property values for the dioxin-like compounds that are considered to be the most definitive. These values are utilized in the modeling equations in the companion volume to this report, Volume 3-Site-Specific Assessment Procedures.

Appendix A lists all reported property values for the CDDs, CDFs, and coplanar PCBs. Where technically feasible, estimation procedures have been used to provide values where measured data are not available. For those compounds for which data could not be found and estimates are not appropriate, the field is left blank and a congener group average is presented as the property value for that congener group.

The values suggested in this document as most definitive are, in the authors' opinion, the best values derivable from current data. Since the document has undergone extensive review inside the Agency, by scientific community outside the Agency, and by the Science Advisory Board, the values can be interpreted as generally representative of the Agency and scientific community. The authors recommend that document users consider the values as defaults in the sense that users are encouraged to accept them as a starting point but should feel free to modify them as new data become available.

Brief summaries of the recent and relevant scientific literature on the environmental fate of the polychlorinated and polybrominated dibenzodioxins, dibenzofurans, and biphenyls are provided in Sections 2.6 and 2.7.

2.2. GENERAL INFORMATION

Polychlorinated dibenzodioxins (CDDs), polychlorinated dibenzofurans (CDFs), and polychlorinated biphenyls (PCBs) are chemically classified as halogenated aromatic hydrocarbons. CDDs and CDFs can be formed as unintentional by-products through a variety of chemical reactions and combustion processes. Both compound classes have a triple-ring structure that consists of two benzene rings connected by a third oxygenated ring. For CDDs, the benzene rings are connected by a pair of oxygen atoms. CDFs are connected via a single oxygen atom. (See structures below.) PCBs are a class of compounds formed by the chlorination of a biphenyl molecule.

There are 75 possible different positional congeners of CDDs and 135 different congeners of CDFs. Likewise, there are 75 possible different positional congeners of BDDs and 135 different congeners of BDFs. (See Table 2-1.) The basic structure and numbering of each chemical class is shown below. (Diagram 2-1)

table Table 2-1. Possible Number of Positional CDD (or BDD) and CDF (or BDF) Congeners. table Diagram 2-1
expand table Table V2 2-1 expand table Diagram V2 2-1

There are 209 possible PCB congeners. (See Table 2-1.) ...

table Diagram 2-2

... The physical/chemical properties of each congener vary according to the degree and position of chlorine substitution. The list of coplanar PCBs can be found in Table 1-2. PCBs assume a coplanar structure when the two benzene rings rotate into a position where the two rings are in the same plane.

The PCBs assume a dioxin-like structure when the substituent chlorines occupy the 3, 3', 4, 4', 5, or 5' positions, or possibly, one of the 2 or 2' positions, and the structure is not hindered from assuming the preferred planar configuration.

The basic structure and numbering of each chemical class is shown (Diagram 2-2).

expand table Diagram V2 2-2
2.3. PHYSICAL/CHEMICAL PROPERTY EVALUATION METHODOLOGY

As discussed above, a thorough search of the recent published scientific literature was conducted to maximize and optimize the identification of measured physical/chemical properties.

For the purpose of identifying the most definitive of two or more physical/chemical property values reported in the literature for a given dioxin-like compound, a ranking methodology was developed to evaluate the degree of confidence in the reported values.

A property value with a ranking of 1 is considered to have the highest level of confidence; a property value with a ranking of 6 is considered to have the lowest level of confidence. The ranking scheme assumes that measured values are more definitive than estimated values. The ranking scheme is based on five ranking criteria or factors. These factors are described below:

Factor 1:
Confirmation.
Value, measured or derived, confirmed by at least one other laboratory, or different experimental technique. Confirmation was assumed if the reported values were within 50 percent of the highest value (within 5 percent for values reported in logarithmic units).

Factor 2:
Measurement Technique.
Direct measurement technique used. No measurements reported less than 10 times the method detection limit.

Factor 3:
GLP Followed.
Good Laboratory Practice was followed in the experimental work. This includes the use of traceable, pure standards; sensitive, selective detection technique was employed; repeatability of measurements demonstrated; all experimental details sufficiently documented so others could reproduce experiments; sources of determinate error considered - error analysis conducted.

Factor 4:
Derived Value.
Value derived from other directly measured physical/chemical properties by use of known physical/chemical relationships developed for structurally similar chemicals (e.g., other dioxin, furan, and PCB congeners, multiple-ring halogenated compounds). The input value (i.e., the independent variable) used to derive the property value of interest from the equation (i.e., the physical/chemical relationship) is a directly measured value.

Factor 5:
Estimated Value.
Value estimated using a physical/chemical relationship that was developed using estimated values or a combination of estimated and measured values; this includes QSAR (Quantitative Structure Activity Relationship) methods. Also includes values derived from other directly measured physical/chemical properties by use of known physical/chemical relationships developed, in large part, for structurally dissimilar compounds.

Although this ranking scheme is subjective in nature, it is a reasonable method for identifying the most definitive physical/chemical property value. The ranking scheme has several advantages. First, it identifies where more work is needed to obtain a more definitive p-chem property value. Second, it allows for later adjustments in these values when more definitive studies are conducted. A low ranking for a study does not mean that a particular reported value is incorrect - only that insufficient evidence exists to determine its accuracy.

The ranking scheme is as follows:

Rank 1: Confirmed Measured Values.
The reported value has met Factors 1, 2, and 3. (See Table 2-2.) This value is considered definitive.

Rank 2: Unconfirmed Measured Values.
The reported value has met Factors 2 and 3. The value is considered accurate; it could be definitive subject to confirmation.

Rank 3: Confirmed Derived Values.
The reported value has met Factors 1, 3, and 4. The value is considered to be a close approximation.

Rank 4: Unconfirmed Derived Value.
The reported value has met Factors 3 and 4. The value is considered to be an approximation.

Rank 5: Estimated Value.
The reported value has met Factor 5 only. The value is considered to be an "order-of-magnitude" estimate.

table Table 2-2. Ranking Scheme for P-Chem Property Evaluation .

If two or more values have the same ranking, then the value that has been peer reviewed by other EPA offices, other government agencies, or scientific data bases (e.g., the Syracuse Research Corporation Environmental Fate Data Bases) and chosen by that office, agency, or data base as the most accurate, was deemed to be the most definitive value for this document.

If two or more values with the same ranking have not been peer reviewed as above, typically the most current value was chosen as the most definitive value.


This decision was made on the assumption that the most current value would have been developed by the latest scientific method.

expand table Table V2 2-2
If two or more values had the same ranking, then some evaluation of the techniques used to derive the value were also considered in choosing the more definitive value.

The ranking of the literature can be found in Table A-2 in Appendix A. Table 2-3 lists the property values for the dioxin-like compounds that are considered to be most definitive.

2.4. PHYSICAL/CHEMICAL PROPERTIES - CHLORINATED COMPOUNDS

Limited research has been conducted to determine physical and chemical properties of CDFs and CDDs. The congeners having 2,3,7,8-chlorination have received the most attention, with 2,3,7,8,-TCDD being the most intensely studied compound. All 2,3,7,8-substituted congeners are now available commercially, but many of these isomers have not been prepared in pure form. Some of the isomers that have been prepared may not be available in sufficient quantities for testing. Another factor which is likely to have limited research on these compounds is the high toxicity of these compounds, which necessitates extreme precautions to prevent potential adverse effects.

2.4.1. Water Solubility

Although water solubility data are not directly used in the exposure scenario equations in Volume 3, water solubility data can be used to estimate Henry's Law constants (using the VP/WS ratio technique) that are used in the equations in Volume 3.

Very few measured water solubility values are available in the literature. Marple et al. (1986a) reported the water solubility of 2,3,7,8-TCDD as 19.3 3.7 parts per trillion (nanograms per liter, ng/L) at 22 C. Marple et al. (1986a) used a procedure of equilibrating thin films of resublimed 2,3,7,8-TCDD with a small volume of water followed by gas chromatography (GC) analysis with 63Ni electron capture detection. Other water solubility values for 2,3,7,8-TCDD have been reported in the literature and are summarized in U.S. EPA (1990).

 
table Table 2-3. P-Chem Properties for Dioxin-Like Congeners.
expand table Table 2-3 Page 1 of 4 expand table Table 2-3 Page 2 of 4
Table V2 2-3 page 1 of 4 Table V2 2-3 page 2 of 4
expand table Table 2-3 Page 3 of 4 expand table Table 2-3 Page 4 of 4
Table V2 2-3 page 3 of 4 Table V2 2-3 page 4 of 4

Values ranging from 7.9 ng/L to 483 ng/L are reported in U.S. EPA (1990) with 19.3 ng/L selected as the recommended value. The value of 19.3 ng/L was confirmed by Marple et al. (1987) using both radio-labeled and unlabeled 2,3,7,8-TCDD. Marple et al. (1987) reported values of 10.6 ng/L and 10.4 ng/L for the labeled and unlabeled compounds respectively. Because the value of 19.3 ng/L was confirmed by other techniques and was recommended by U.S. EPA (1990), it was chosen as the most definitive value.

Friesen et al. (1985) and Shiu et al. (1988) used HPLC generator column techniques to measure the water solubilities of a series of chlorinated dioxins (1,2,3,4-, 1,2,3,7-, and 1,3,6,8-TCDD; 1,2,3,4,7-PeCDD; 1,2,3,4,7,8-HxCDD; 1,2,3,4,6,7,8-HpCDD; and OCDD). Reported water solubilities ranged from 320 ng/L to 0.074 ng/L for the 1,2,3,7-TCDD and OCDD congeners, respectively. The only congener with more than one value was OCDD.

The value of 0.074 ng/L (Shiu et al., 1988) was chosen because it was the most current. Friesen et al. (1990) used a gas chromatography/mass spectrometry detection (GC/MSD) generator column technique to measure the water solubilities of a series of chlorinated furans (2,3,7,8-TCDF; 2,3,4,7,8-PeCDF; 1,2,3,6,7,8- and 1,2,3,4,7,8-HxCDF; and 1,2,3,4,6,7,8-HpCDF) and reported a decrease in water solubility with an increase in the number of chlorine substituents.

The reported water solubility values ranged from 1.37 x 10-9 mol/L (419 ng/L) for the 2,3,7,8-TCDF isomer to 3.30 x 10-12 mol/L (1.35 ng/L) for the 1,2,3,4,6,7,8-HpCDF congener. The dioxin-like furans only had one value reported for water solubility.

Values for the various congener groups ranged as follows: TCDDs 0.47-596 ng/l, PeCDDs 120-166 ng/l, HxCDDs 4.4 ng/l, HpCDDs 2.4 ng/l, OCDD 0.074-0.4 ng/l, TCDFs 4.2 ng/l, PeCDFs 236 ng/l, HxCDFs 8.2-17.7 ng/l, HpCDFs 1.35 ng/l, and OCDF 1.16 ng/l. The range for CDDs covers nearly four orders of magnitude, and for the CDFs two orders of magnitude.

The reported water solubility values for the coplanar PCB compounds are comparable to those for the CDD and CDF compounds. The reported values range from 11,400 ng/L for 3,3',4,4'-TeCB to 0.74 ng/L for 2,3,3',4,4',5-HxCB. Measured water solubility data chosen as the most definitive were those reported by Dunnivant and Elzerman (1988) and Murphy et al. (1987).

The value of 549 ng/L (Dunnivant and Elzerman, 1988) for 3,3',4,4'-TCB was confirmed by Dickhut et al. (1986) with a value of 569 ng/L. Therefore, 549 ng/L was chosen as the most definitive value for 3,3',4,4'-TCB. Murphy et al. (1987) proved the only measured values for the other congeners.

For those compounds without reported measured water solubility values, estimations were calculated by the congener group-average method. For example, for the tetra-chlorinated dioxins, values reported in the literature were averaged to yield an estimated water solubility value for the tetra-chlorinated dioxin congener group. A similar procedure was used to develop the average value for each of the other CDD and CDF congener groups.

The most definitive value for each isomer was used to derive the congener group average. Estimating the water solubility values from measured log Kow values using the estimation procedure of Lyman et al. (1982) did not yield satisfactory results; the estimated water solubilities for 2,3,7,8-TCDD and 1,3,6,8-TCDD were at least two orders of magnitude greater than the measured values in Tables 2-3 and A-1. Compounds that have water solubility values in the ranges reported for these chlorinated compounds are considered to have very poor solubility in water.

2.4.2. Vapor Pressure

Vapor pressure data are not directly used in the exposure scenario equations in Volume 3. However, vapor pressure data can be used to estimate Henry's Law constant using the VP/WS ratio technique. Very few measured vapor pressure values are available in the literature for the CDDs and CDFs. The majority of the measured vapor pressures are for the 2,3,7,8-substituted compounds.

U.S. EPA (1990) presented the range of measured vapor pressure data for 2,3,7,8-TCDD and selected a recommended value of 7.4 x 10-10 mm Hg at 25 C.

This value was reported by Podoll et al. (1986) who used radiolabeled 2,3,7,8-TCDD and a gas saturation technique with combustion to 14CO2. Rordorf (1987, 1989) reported a higher vapor pressure value for 2,3,7,8-TCDD, 1.49 x 10-9mm Hg. SRC (1991) reported this same value by extrapolating the vapor pressures measured by Schroy et al. (1985) at four higher temperatures, 30 , 55 , 62 , and 71 C. The value recommended in U.S. EPA (1990) is reported in Table 2-3.

Rordorf (1987, 1989) reported experimental vapor pressure values for 1,2,3,4-TCDD (4.8 x 10-8 mm Hg), OCDD (8.25 x 10-13mm Hg), and OCDF (3.75 x 10-12 mm Hg) (Table A-1).

These values were chosen as the most definitive because they were the most current directly measured values. Rordorf (1987, 1989) used a gas-flow method in a saturation oven, with integrated gas chromatographic analysis, to measure vapor pressure values for ten CDDs and four CDFs. Rordorf (1987, 1989) also used a vapor pressure correlation method to predict the vapor pressures of 15 other CDDs and 55 CDFs based on the measured vapor pressures for the 10 CDDs, 4 CDFs, and the deduced boiling point and enthalpy data for the larger series of CDDs and CDFs.

Measured boiling point and enthalpy data are in good agreement with the deduced data used in the correlation method. Of the CDDs studied by Rordorf (1987, 1989), only three of the ten, 1,2,3,4-TCDD, 2,3,7,8-TCDD, and OCDD, are in the dioxin-like compound group of chemicals studied in this report. The other CDDs with measured values were monochloro-, dichloro-, and trichloro-dibenzo-p-dioxins.

Eitzer and Hites (1988) reported experimental vapor pressure values for several of the dioxin-like compounds utilizing GC capillary column retention time data. The values were reported as subcooled liquids and then converted to solid-phase vapor pressures. The solid-phase vapor pressures ranged from 2.16 x 10-12 mm Hg to 9.48 x 10-10 mm Hg for the CDDs and from 1.07 x 10-10 mm Hg to 8.96 x 10-9 for the CDFs. The values from Eitzer and Hites (1988) were considered the most definitive, except for OCDD, because they were the only values that were derived (i.e., Rank 4); all other values were estimated (i.e., Rank 5).

diagram V2 2-3

The range for CDDs covers over six orders of magnitude, and for the CDFs, four orders of magnitude.

The measured vapor pressure values reported for the coplanar PCBs are comparable to those reported for the CDD and CDF compounds; the estimated values are higher by several orders of magnitude. (See Table 2-3.) The directly measured values of Murphy et al. (1987) and the derived value of Dunnivant and Elzerman (1988) were considered the most definitive.

All other values were estimated. The values reported in Tables 2-3 and A-1 by Foreman and Bidleman (1985) are an average of the 0V-101 RI and Dexsil 410 RI correlation methods because both methods were determined to be equally valid. As with the other groups, the vapor pressures of the PCBs decrease with an increase in the number of chlorine substituents. The highest reported value for the coplanar PCBs is 2.90 x 10-6 mm Hg for 3,3',4,4',5-PeCB, and the lowest value reported is 2.80 x 10-10 mm Hg for 3,4,4',5-TeCB.

Estimated vapor pressure values for those CDDs and CDFs for which measured values were not found in the literature were calculated by the congener group-average method using the literature-reported values within a congener group. For example, the literature values for the TCDDs were averaged to obtain an estimated vapor pressure assumed to apply to the TCDD congeners that did not have literature values.

A similar procedure was used to develop a congener-average for each of the other congener groups. The most definitive value for each isomer was used to derive the congener group average. Compounds with vapor pressures in the ranges reported for these compounds are considered to have very low vapor pressures.

2.4.3. Henry's Law Constant

Henry's Law constant data are used in Volume 3 to estimate the volalitization of the dioxin-like compounds from soil. They are also utilized in estimating the vapor-phase bioconcentration factor from air to plant leaves. Directly measured data for Henry's Law constant have been reported for only two compounds, one TCDD congener, and one PCB congener.

The measured values for 1,3,6,8-TCDD, 6.81 x 10-5 atm-m3/mol (Webster et al., 1985), and for 3,3',4,4'-PCB, 9.4 x 10-5 atm-m3/mol (Dunnivant and Elzerman, 1988) were determined by the gas-purging technique. These two values were considered the most definitive. Other values reported in the literature for CDDs, CDFs, and PCBs were calculated by the vapor pressure/water solubility (VP/WS) ratio technique or by structure-activity relationship techniques. A derived VP/WS ratio value, Rank 4, was determined to be more definitive than an estimated value, Rank 5.

Group-average Henry's Law constants were estimated for each congener group based on the reported data for that group. The Henry's Law constant values for the PCBs are similar to those for the CDDs and CDFs.

Lyman et al. (1982) offers guidelines, though not specific to these compounds, for comparing the degree to which organic compounds volatilize from water. These guidelines suggest that volatilization of polycyclic aromatic hydrocarbons and halogenated aromatics (which includes all the dioxin-like compounds) from water represents a significant transfer mechanism from the aqueous to the atmospheric phase.

2.4.4. Octanol/Water Partition Coefficient

The octanol/water partition coefficient is used in several exposure estimation procedures in Volume 3. It is used to estimate log Koc when measured data are not available, and it is utilized in estimating the root concentration factor (RCF). The RCF is used to estimate the uptake of contaminants by plant roots. Log Kow is also used to estimate the vapor-phase bioconcentration factor from air to plant leaves.

Marple et al. (1986b) reported the octanol/water partition coefficient of 2,3,7,8-TCDD as 4.24 ( 2.73) x 106 at 22 1 C, yielding a log Kow of 6.64 (Table A-1). Two similar experimental techniques were used, but the more reliable method involved equilibration of water-saturated octanol, containing the 2,3,7,8-TCDD, with octanol-saturated water, over 6 to 31 days. U.S. EPA (1990) reported that the available low Kow data ranged from 6.15 to approximately 8.5. The 6.64 value reported by Marple et al. (1988b) was the value recommended in that report. The 6.64 value was confirmed by Sijm et al. (1989) with a value of 6.42, and by Marple et al. (1987) with a value of 6.69; therefore, a log Kow value of 6.64 was considered the most definitive.

Burkhard and Kuehl (1986) used reverse-phase High Pressure Liquid Chromatography (HPLC) and Liquid Chromatography/Mass Spectrometry (LCMS) detection to determine octanol/water partition coefficients for 2,3,7,8-TCDD and a series of seven other tetrachlorinated planar molecules, including three other TCDD isomers (1,2,3,4-TCDD; 1,3,7,9-TCDD; 1,3,6,8-TCDD), 2,3,7,8-TCDF, and 3,3',4,4'-tetrachlorobiphenyl. The log Kow values for the four TCDD isomers ranged from 7.02 to 7.20. The log Kow for 2,3,7,8-TCDF was 5.82, and the log Kow for 3,3',4,4'-TCB was 5.81.

Burkhard and Kuehl (1986) also re-evaluated data on 13 CDDs and CDFs previously reported by Sarna et al. (1984) under similar experimental techniques. In the re-evaluation, Burkhard and Kuehl (1986) used experimental rather than estimated log Kow values in correlations with gas chromatographic retention times. This approach yielded log octanol-water partition coefficients ranging from about 4.0 for the nonchlorinated parent molecules to about 8.78 for the octa-chlorinated compounds, much lower than the values originally reported by Sarna et al. (1984).

Sijm et al. (1989) used a slow stirring method to obtain log Kow values for 73 CDD and CDF congeners. Values ranged from 6.10 to 7.92.

The most definitive values chosen were either a directly measured value or the most current derived value. Only 2,3,7,8-TCDD had more than one directly measured value.

Diagram V2 2-4

The range for the CDDs covers nearly six orders of magnitude, and for the CDFs nearly seven orders of magnitude.

The measured and literature-estimated log Kow values for the PCBs are similar to those reported for the CDDs and CDFs. The values range from 5.62 (measured) for 3,3',4,4'-TeCB to 7.71 (literature-estimate) for 2,3,3',4,4',5,5'-HpCB. The log Kow values increase with an increase in the number of chlorine substituents.

The log Kow for the 3,3',4,4'-TeCB was measured by Hawker and Connell (1988) using the generator column technique against the linear relationship of relative retention time on a nonselective gas chromatograph stationary phase. This was the only directly measured log Kow for the PCBs; therefore, it was considered the most definitive.

Log Kow values for the HxCBs were measured by Risby et al. (1990) using a high-performance liquid chromatographic (HPLC) system. The values reported in Tables 2-3 and A-1 are an average of the two techniques because both methods were determined to be equally valid. The values for the HxCBs were ranked 3 because both methods produced similar values. The most definitive values for the other PCBs were either derived values or the most current estimated value.

Partition coefficient values were calculated for those compounds for which no measured data were reported in the literature by averaging the literature values within congener groups, as were done for vapor pressure and water solubility. Literature values for the hexachlorodibenzofurans could not be found; thus, no congener group average could be calculated. Partition coefficients in the ranges of these reported values indicate that the substances tend to adsorb strongly to organic components in the soil and may bioconcentrate in those organisms exposed to the compounds.