Environ. Sci. Techno/. 1995, 29, 1599-1606
Measurement of Octanol-Air Partition Coefficients for Chlorobenzenes, PCBs, and DDT T O M HARNER A N D D O N M A C K A Y * Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, M5S lA4, Canada
Novel methods are developed and tested for measuring the octanol-air partition coefficient (KOA),which is suggested to be a valuable descriptor of airvegetation and air-soil equilibrium. Data are reported for six chlorobenzenes (CBs), five polychlorinated biphenyls (PCBs), and DDT over the temperature range -10 to +20 “C with values approaching 10l2. KOA varies log-linearly with reciprocal absolute temperature and increases by a factor of approximately 30 over this temperature range, the temperature coefficient being approximately 62 kJ/mol for CBs and 70 kJ/ mol for PCBs: f o r PCBs, the values of KOAare within a factor of 4 of values calculated as the ratio of the octanol-water and the air-water partition coefficients, with a factor of 7.4 applying to DDT. It is suggested that for hydrophobic chemicals it is preferable to measure KOAdirectly since this avoids handling aqueous solutions.
Introduction It has become apparent that once-pristine regions, such as the Arctic, have become contaminated by atmospheric transport of hydrophobic, persistent, and toxic chemicals such as PCBs, DDT, and especially HCB (hexachlorobenzene) and HCH (hexachlorocyclohexane) (1). Relatively high concentrations are found in the biota, wildlife, and even humans that inhabit these regions. For instance, Dewailly et al. (2) found that Inuit women in northern Quebec had PCB levels in breast milk that were five times that of Caucasian women in southern Quebec. The terms “cold condensation” and “global distillation” have been used to explain the process by which chemicals deposit or absorb at different latitudes (and temperatures). Risebrough (3)compared it to a type of gas chromatographic migration in which hydrophobic chemicals partition from the mobile atmospheric phase to the stationary phases of soil and vegetation. As with gas chromatography, temperature strongly affects this partitioning. Although this analogy may oversimplify the real global situation, it does highlight the importance of quantlfylng chemical partitioning and transport between the atmosphere, soil, and vegetation as a function of temperature. We suggest that the single physical-chemical parameter which best describes this partitioning and its temperature dependence is the octanol-air partition coefficient, &A. Justification of t h i s assertion lies in the observation that most organic chemicals partition into organic carbon or lipid phases for which octanol is a recognized surrogate and is characterized by the octanol-water partition coefficient, KOW,Le. ColCw. Water to air partitioning is expressed as KAW, i.e. CA/CW.It follows that organic media to airpartitioning, i.e. ColCAis expressed byKoA, which can be regarded as &wlK~w. For example, foliage to air partitioning has been correlated with KOAby Paterson et al. (4). Presumably soil organic matter to air partitioning is similarly correlated. Mackay and Wania (5)suggest KOAas the preferred indicator of the susceptibility of a chemical to global fractionation. There is thus a case for measuring KOAdirectly and especially as a function of temperature, a strong temperature dependence being expected because of the large enthalpy change involved in octanol to air transfer. Another benefit of understanding the relationship between air and foliage is the potential use of foliage as an air monitoring or biomonitoring device. Foliage responds gradually to sudden changes in atmospheric levels and should therefore provide aless variable and more integrated air concentration than sampling a volume of air. Also, in some circumstances it may be difficult to sample large volumes of air, whereas foliage samples can be taken easily and quickly for subsequent laboratory analysis. As early as 1985 Gaggi (6) performed a series of experiments with pine needles in Italy, confirming the feasibility of evaluating air pollution levels through foliage data. Hermanson and Hites (7) correlated PCB levels in tree bark to ambient air concentrations. Simonich and Hites (8)have estimated that 30% of all PAHs entering the * Corresponding author.
0013-936)(/95/0929-1599$09.00/0 0 1995 American Chemical Society
VOL. 29, NO. 6, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY rn 1599
atmosphere of the Northeastern United States partition into vegetation and that this is an important pathway for atmospheric PAH deposition to soils. Alcock and coworkers (9)have estimated that in the U.K. over 93%of the contemporary PCB burden resides in soilswith the present burden shifted toward the heavier, more hydrophobic congeners (10). It is apparent that the &A can be a useful tool for predicting and monitoring chemical fate in foliage/soil/ atmosphere systems. There is thus a need to develop a reliable method for measuring KOAespeciallyfor persistent and toxic chemicals that are susceptible to atmospheric transport.
Theoty The octanol-air partition coefficient, KOA,is defined as
where COand CAare the concentrations (e.g., g/m3)of the solute in the octanol and air phases, respectively. It is also possible to calculate &A from the octanol-water partition coefficient, &w, and the dimensionless air-water partition coefficient, KAW:
There is, however, a possible error inherent in this estimation procedure in addition to the obvious combination of measurement errors in &W and KAW. &W represents the partitioning of chemical between octanol saturated with water and water saturated with octanol, whereas KAW represents partitioning between air and pure water. As a result of this difference in phases, there may be error especially for chemicals which interact with octanol in aqueous solution or with water in octanol solution. There is thus an incentive to measure KOAdirectly. It is also useful to examine the kinetics of air-octanol transfer since this influences the method of measurment. The transfer of chemical across the octanol-air interface can be described by the Whitman two resistance mass transfer coefficient (MTC) approach (111,which adds the resistances in the octanol and air boundary layers. Molecular diffusion probably controls mass transfer in these regions resulting in slower diffusion. The overall MTC, k, can be deduced from the individual MTCs as follows:
I l k = llk,
+ l/(k0hA)
Vo dColdt = -GCA
(4)
where VO(mL)is the volume of octanol; COand CArepresent the concentrations of the solute in the octanol and air phases, respectively; and G (mL/min) is the air flow rate. If CAachieves equilibrium with CO,this can be rewritten as
(5)
(3)
where kA and ko are the MTCs on the air side and octanol side of the interface. KOA relates the air and octanol concentrations on either side of the interface. For chemicals with a large KOAthe overall MTC becomes air-sidecontrolled and k equals k A . When KOAis small, it is expected that the octanol boundary layer will control mass transfer across the interface. Typically,k A w i l l exceed ko by2 or 3 orders of magnitude. Therefore,for chemicalswith large values of KOA(e.g., > io4) it is likely that the overall MTC will be air phase limited and for chemicals with small &A (e.g., “10) transfer will be octanol limited. The former case is more common for chemicals of environmental concern. A fundamental difficulty encountered in the measurement of KOAis its magnitude. For PCBs a value of the order of 1010 is expected. It follows that an octanol solution 1600
containing 1 g/L of chemical will be in equilibrium with air of 0.1ng/L. Large sample volumes of air are thus necessary to obtain sufficient analyte. If even a minute quantity of octanol is inadvertedly sampled with the air, considerable error results. Stripping techniques are used to measure KAW by measuring depletion from water (121, but to strip half the PCB from a 1-L volume of octanol would require on the order of 1Olo L of air, i.e. the ratio of the volume of air contacted to the volume of octanol must be approximately &A. At an air flow rate of 1L/min, this would require 20000 years. The preferred option is thus to equilibrate an airstream with as small a volume of octanol as possible in a generator column configuration, ensuring a high air-side MTC and either measure the concentration in the air CAor, if KOAis small enough, the change in CAor COwith time. Severalcontacting configurationswere tested, the major concern being that the mass transfer rate was sufficient so that equilibrium is achieved. As described later, the configuration ultimately selected consists of a generator column packed with glass wool, which is coated with athin film of octanol solution. Chemical in the octanol partitions into the flowing airstream to an extent governed by KOAand the MTC. There are two primary considerations that control the configuration of the system, ensuring an adequate mass transfer rate and determining the degree of mixing in the octanol phase. We examine first the conditions and equations that apply when mass transfer is infinitely fast and thus that equilibrium is achieved, there being two limiting conditions concerning the degree of mixing of the octanol in the generator column: (i) well-mixed or homogeneous and (ii) no mixing. (i) MTC Is Infinite, Equilibrium Conditions Exist, and Octanol IsWell Mixed. As shown in Figure 1A and assuming that there is no chemical in the input air, a mass balance on the octanol yields
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6. 1995
and integration from an initial exit air concentration Cg yields
h(CA/Cfi)= (-GI (VoIyoA))t KOAis then determined from the slope of the plot of In CA or In COversus time, since COand CAare always related by KOA. This is the equation used in the dynamic stripping method for determining KAW (12). The expected concen-
tration history in octanol and air are also shown in Figure 1A. The characteristic time of response of the system is KOAVi / G. (ii) MTC Is Infinite, Equilibrium Conditions Edst, and No Mixing Occurs. In this case, the air immediately achieves equilibrium with the immobile octanol, and a concentration front or “shock wave” moves through the column as shown in Figure 1B. As a result, the exit air
Glass wool coated with solute solution in octanol.
\
AreaA9MTC7k
slope = -G/(VOKOA)
octanol well mixed MTC is infinite
t=O
A
CA=CO/KOA
In CO t
length no mixing MTC is infinite
It=O
hCo1
lt=11t=2 / t = 3
B
In CO
r7;
CA = CO/KOA = VoKodG
I 1 J I
t
slope = -G~A/((VOKOA(G+~A))
well mixed MTC is finite
I
t=O
C
In CO t
no mixing MTC is finite InCo
m t=2 t=4 t=6
D
In CO
i\ t
plateau, CA= COKOA partial mixing MTC is finite I
t FIGURE 1. Schematic diagram to show the affect of mixing and mass transfer coefficient (MTC) on octanol-air partitioning regimes at and CA baing the solute concentrations in octanol and air. various times, (4,
concentration remains constant at Co/GAuntil the column is depleted, then it falls to zero. This occurs when
(111) MTC Is Finite, and Octanol Is Well Mixed. For this case, illustrated in Figure lC, it can be shown that the expression for the outlet air concentration becomes
+
CA= [kA/(G l~A)]Coi/&~ exp(-t/t) where VA is the total volume of air passed through the column q d is thus VOKOA. If this condition applies, &A can be estimated from CO/ CA or from the time (or air volume) corresponding to breakthrough, i.e. as VA/V0. In practice, some mixing is expected to occur in the column, and breakthrough will be less sudden. It is instructive to examine how these equations change if, as is inevitable, the MTC is finite. The reason is that it is important to ensure that mass transfer rates are adequate, it being inherently undesirable to measure an equilibrium quantity under nonequilibrium conditions in which mass transfer rates influence the concentrations.
(8)
where t=
V&A(G
+ kA)/(GkA)
(9)
A is the octanol-air interfacial area and Coi is the initial
octanol concentration. It is preferable to operate at or near equilibrium, which occurs when kA > G or when kAl(G kA) approaches 1, t approaches VOKOAIG,and eqs 8 and 9 reduce to eq 6. It is therefore useful to know the magnitude of the MTC-area group, kA,which can be estimated by varying the air flow rate, G. If during a test CO remains fairly constant (Le., t
+
VOL. 29, NO. 6,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1601
>+z), we can write
(10) If the air flow rate is small such that kA > G, CAwill be constant, and the system approaches equilibrium. As G increases, the air concentration declines, and equilibrium is no longer achieved. The approximate magnitude of the group kA can then be estimated as G when CAis half its maximum. It should be noted, however, that kwill depend on G. This procedure is useful to gain an appreciation of the magnitude of group kA and thus the degree of approach to equilibrium. Since the aim is to measure an equilibrium quantity, it is crucial that the approach to equilibrium is ascertained. (iv)MTC Is Finite, and No Octanol Mixing Occurs. In this case, the approach to equilibrium depends on kA,the gas flow rate, the concentration profile within the column as determined by the degree of mixing, and KOAas shown in Figure 1D. The differential equation for change in gas concentration is
where z is distance along the column, v is the velocity of the airstream, and k is the overall air-side mass transfer coefficient. Solutionsto this equation have been derived for systems such as gas absorption, ion exchange in futed packed beds, and the flow of water over sediments in rivers (13,14).The nature of the concentration profile and the breakthrough curve depend on the relative values of v, z, k, and KOA. Figure 1E depicts the most likely situation in which there is some intermediate mixing in the octanol and the MTC is finite. In practice, as is discussed later, it was found that the most reliable results were obtained by measuring the exit air concentration during the constant period prior to breakthrough as shown in Figure 1E and then deducing KOAas CO/CA.This was termed the plateau method. It also proved possible to estimate KOA,but less accurately, from the slope of the plot of In CAversus time using eq 6, which assumes complete mixing in the octanol phase. In reality, a concentration profile develops in the octanol, but mixing apparently does occur, probably by diffusion both in the octanol and in the air phases over the long time period of the experiment. This was termed the slope method. A third possible method is to estimate the time to breakthrough by integrating the areas under the concentrationtime curve and applying eq 7. This was termed the breakthrough time method. The aim of the experimental program was to determine how the system behaves in practice, identify the preferred method, and obtain experimental data for selected compounds. It is possible that different methods are appropriate for different ranges of KOA.
Experimental Section Development of Generator Column. Several designs of the glass wool generator column were tested before deciding on a preferred arrangement. Initially, attempts were made to keep the octanol well mixed by alternating the direction of the flow through the column at regular intervals using three-way valves. The data were too scattered to permit accurate measurement of KoAprobably because of poor 1602 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6, 1995
mixing. Another problem as discussed by Wania et al. (1.5) is the strong tendency for hydrophobic chemicals of low volatility to partition onto stainless steel, especially at low temperatures. It was thus desirable to minimize the exposed area in the section between the glass wool column and the Tenax trap. Attempts to maintain or increase homogeneity in the column were unsuccessful. Since a concentration gradient was inevitable, it was judged preferable to use the simplest system, i.e., a straight glass wool column. Advantages are that octanol-air contact and hence exchangewas improved with less chance for the air to short circuit through the column, and there was a constant outlet air concentration or plateau regime. The final apparatus is shown in Figure 2. Air from a compressed cylinder was passed through a regulator, a gas purifier (Matheson Model 4501, and then a heat exchanger coil (1m in length made of 1/8-in.stainless steel) immersed in a refrigerating bath (Endocal Model RTE-5B) at a temperature of approximately22 "C. Aflowmeter/regulator (Cole-ParmerModel N032-41,Serial no. 063931, glass float) was used to maintain a constant air flow through the system in the range 25-35 mL/min and was calibrated prior to each experiment. The air was then saturated with octanol by sparging through an octanol column approximately 30 cm in length and passed through another heat exchange coil, which reduced the air temperature from TI to the desired measurement temperature of -10 to +20 "C. T,, the generator column temperature, was always lower than Ti to ensure that the air remained saturated with octanol. Thermal equilibrium between the generator column air and the bath was confirmed by measuring constant air concentrations while varying the air flow rate. As the temperature was reduced from TI to Tz, octanol condensed, the excess liquid being collected in a 1/4-in., stainless steel U-shaped trap thus preventing it from entering the next stage. The air saturated with octanol was then passed through the glass wool column which consisted of a 114-in.(o.d.1glass tube approximately 11.5 cm in length. Prior to each experiment, clean glass wool was packed into the tube to a length of approximately 9 cm and was coated with 100 p L of a solution of the chemical(s) in octanol at typical concentrations of 0.1-2 g/L, which was sufficiently low to ensure near-infinite dilution. A gas-tight syringe was used to evenly apply the octanol solution to the glass wool. The procedure involved inserting the needle through one end of the generator column to the top of the packing and then withdrawing it slowly while evenly delivering the solution. Chemical in the outlet air was collected on an adsorbent trap containing Tenax TA (35/60 mesh). The traps were made by lightly packing 5 cm of the Tenax material into 6-mm (0.d.) glass tubes. The Tenax was held into place with 1-cm glass wool plugs at either end. Analysis. The traps were analyzed by thermal desorption (Envirochem Model 850) and gas chromatography (GC) (Hewlett Packard Model 5890) equipped with a J&W Durabond-17 30 m long column and an electron capture detector. For chlorobenzenes, the temperature program began at 65 "C for 1 min, ramped to 280 "C at 20 "Clmin, and maintained at that temperature for 4 min. For PCBs, the initial temperature was 85 "C, ramped to 175 "C at 30 "Clmin, then ramped to 220 "C at 5 "C/min, and then ramped to 280 "C at 30 "C/min, and maintained at that
Air Purifier
Q
Regulator
TENAX
Octanol Saturation
r
Stage
Temp. Bath #1
Compressed
Octanol Solution
Temp. Bath #2
TI
Air
T2
FIGURE 2. Schematic of apparatus used for measuring Koa. TABLE 1
Physical-Chemical Propelties of Selected CBs. PCBs, and DDT at 25 “C (16, 17) chemical
mol w l Iglmoll
vapor pressure IPal
aq. sol. (glmsI
log Kow
log Kol (calcdl
log KO,, (measdl
factor difference
147.0 181.5 215.9 215.9 250.3 284.8
196 28 5.2 0.72 0.22 0.0023
118 21 7.8 1.27
3.4 4.1 4.5 4.5 5.0 5.5
4.41 5.11 5.74 5.81
4.36 5.19 5.64 5.63 6.27 6.90
1.1 1.2 1.3 1.5 1.5 1.3
10.09
3.2 1.6 2.2 3.9 3.4 7.4
CBS
1.21.2.31.2.3.4. . . 1.2.4.5-
pentahexa-
0.65 0.005
6.46 6.78
PCRc
o.d:DiJT
’
’
354.5
0.00002
0.003
temperature for 4 min. Peak areas were determined using a Hewlett Packard (Model 3392 A) integrator.
Internalstandardswereinjectedintothetrapswithevery sample to account for changes in desorption efficiency or detector sensitivity. Peak areas were normalized by the internal standards, solutions in hexane of 1.2.4.5-tetrachlorobenzene and hexachlorobenzene (HCB) The Tenax traps were calibrated prior to each set of experiments. The calibration solutions were made by dissolvingthe chemical of interest in hexane and obtaining the desired concentration range by serial dilution. Chemicals. The octanol-air partition coefficient, KO,, was measured for several chlorobenzenes (CBs),PCBs, and p,p’-DDT. The PCBs were purchased from Ultra Scientific (NorthKingstown,RI), andp,p’-DDTwas obtainedthrough Chem Service West Chester, PA). The PCB structures and IUPAC numbers (in square brackets) are as follows: 4-chlorobiphenyl ICBPI 131. 4,4’-diCRP 1151. 2.4.5-lriCHI’ 1291.2,3.4.5-tetraCRP161I, and 2.2’.4.4’.6.6‘-hexaCRP 11551. lable 1 lists the relevant physical chemical propertiec as well as Kunvalues calculared as K,,wlKA,v.
6.2
9.22
Estimation of Approach to Equilibrium. Experiments were performed with pentachlorobenzene (QCB) to investigate the equilibrium relationship betweenoctanoland air and the effect of the air flow rate, G. The glass wool column was reduced in length from 9 cm to approximately 2.5 cm, thus reducing the contact time between the two phases. Figure3showsaplotofoutletairpeakareas(normalized to sample volume) against log Gat +10 and +20 “C. The resultssuggestthat equilibriumismaintainedforflowrates as high as 30 mllmin. Extrapolated to a 9-cm column, the maximum acceptable flow rate would be approximately 100 mllmin. The normal operating flow rate ranged from 25 to 35 mllmin and was thus well below this value.
Results Results of Combined Slope and Plateau Experiments. Figure 4 is a plot of log CAversus time for 1,2-dichlorobenzene (IICR)ai fourremperaturesthar showsboththeplareau and theslope rewons and their variation with temperature. Each symbol on rhe line represents a separate sample. At lower temperatures, Krlnincreases. rhus the air concentra-
5.6 r--
54 5.4
1:
gS2
[
3 4 8
1
& w S l
4 46
06
'-
L
3.3 10
1
3.4
(+I. 2
,-
~-
____
__
we
I 0
_ _ ~ 500
1000
2000 time (min)
1500
2500
3000
3500
FIGURE 4. Plot of log CA versus time for 12-DCB showiog the temperature dependence of the slope and the plateau phase concentration: +20 (O),+lo ( 0 ) , 0 (01,and -10 "C (D).
tions in the plateau region were reduced, and breakthrough time was increased. The air concentration also decreased more slowly as expected. Operating Considerations. The slope method is useful for chemicals having small values of KOA (i.e. log KOA< 4). In this regime, the plateau phase is short in duration, making it difficult to measure CAaccurately. The plateau method is preferred for more hydrophobic and less volatile chemicals such as the higher chlorinated benzenes and PCBs corresponding to log KOA> 5 . For these chemicals, the slope method is not practical since in the slope region CO declines at a much slower rate. At the experimental conditions of 0.1 mL of octanol (VO) and 30 mL of gaslmin (0, the breakthrough time can be estimated as Ko~Vo/GusingKo~valuesfrom Table 1. This ranges from 1.4 h for 1,2-dichlorobenzene to 13.9 days for hexachlorobenzene to as long as 668 days for 2,2',4,4',6,6'hexachlorobiphenyl. The slope method requires an experimental time about five times the breakthrough time, thus it is not feasible for most hydrophobic chemicals. To reduce the time required to complete a set of experiments, KOAcan be measured for several chemicals simultaneously. If conditions are dilute, it is unlikely that one chemical significantly alters the behavior of others. The chemicals must have similar K ~ ~ v a l u(within es a factor of 10)and/or the COis adjusted for each chemical such that the resulting peak areas are in the same range (within a factor of 10). The KOAvalues determined by the plateau and slope method are illustrated in Figure 5 . The slope method results 1604 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 6,1995
3.8
3.9
FIGURE 5. Log KOAversus l/Tfor 12-DCB determined by the plateau and the slope (A) method from the plot of log CA versus time.
(*I
TABLE 2
Regression Constants for Eq 12 and Temperature Coefficient of Phase Change from Octanol to Air for Chlorobenzenes, PCBs, and p,p'=DDT chemical
-2
3.7
Thousandths 1/T (1IK)
air flow (mL/min) FIGURE 3. Plot of log of peak areas normalized for sample volume against air flow rate for pentachlorobenzene at +10 (0) and f 2 0 "C
3.6
3.5
1000
IO0
chlorobenzenes 1,21,2,31,2,3,41,2,4,5PeCBHCBPCBs 44,4'2,4,52,3,4,52,2',4,4',6,6'p,p'-DDT
A
B
AHOA(kJ/mol)
0.99992 0.99970 0.99981 0,99991 0.99998 0.99998
-4.3 -4.6 -5.3 -5.0 -6.2 -6.3
2574 2909 3254 3176 3722 3928
49.3 55.7 62.3 60.8 71.3 75.2
0.999971 0.999922 0.999602 0.99642 0.99435 0.996586
-6.5 -5.1 -4.8 -2.9 -2.2 -3.2
3962 3792 3792 3464 3337 3954
75.9 72.6 72.6 66.3 63.9 75.7
R2
(R2= 0.98) showed greater variability,and the deduced KOA values were lower than the plateau method results (R2 = 0.999) by up to a factor of 2. In all experiments with 1,2DCB, the outlet air concentration was monitored in the declining phase over a range of two decades. However the slope, and thus the apparent KOA,varied with time. This phenomenon of a decreasing slope suggests that in the declining phase (slope region) the glass wool column deviates from well-mixed behavior, as is expected for flow through a column of this type. Indeed, the presence of the plateau region is evidence of poor mixing. The fairly good agreement between the KOAvaluescalculated from the first decade of the slope method and the plateau method suggeststhat back mixing in the column is substantial. This is possible since the air flow rate through the column is relatively slow. Since mixing does occur, the breakthrough time method is suspect. The plateau method results are regarded as most reliable. Results of Plateau Experiments. The plateau method was used to measure KOAfor several CBs, PCBs, and p,p'DDT in the range -10 to +20 "C. The results are shown in Table 2 and Figure 6. Each point represents three to six determinations. The standard deviations were 5 - 10%for the CBs and PCBs. For DDT, each determination is shown in Figure 6, the standard deviation being about 50%. The experimental data were regressed using the equation,
logKoA=A+ B I T = A + AHOAIRT
(12)
the coefficients A and B being given in Table 2. The temperature coefficient expressed as an enthalpy of phase
12 o DDT data
10
4
DDT
t
PCB-155
EIPCB-61 G
* PCB-15
e
3
3
PCB-29
+ HCB * PCB-3
8
m
t QCB
6
Y
1,2,3,4
4
1,2,4,5
* 1,2,3 e 1,2
4 3.3
3.4
3.5
3.6
3.7
3.8
3.9
Thousandths
lrr(l/K) FIGURE 6. Summary of plateau experiments for several CBs, PCBs, and DDT. log KOAwas measured at $20, +IO, 0, and -10 "C and plotted against 1/T.
change AHOAUlmol) is thus approximately2.303BRwhere R is the gas constant. Strictly KOAshould be calculated on amole fraction basis, not a masslvolume basis, to eliminate the effect of temperature on concentration in air; however, the effect is only a few kilojoules per mole over this temperature range. AHOAvalues for chlorobenzene ranged from 49.3 kJ/ mol for 1,2-DCBto 75.2 kJlmol for HCB, thus exhibiting a trend of increasing AHOAwith molecular weight. The average value for PCBs was approximately 70 kJ/mol.
Discussion Equation 12 was used to extrapolate the measured KOA values to 25 "C which were then compared with values calculated from reported measurements of KOWand KAWas shown in Table 1. The factor difference is KoA/(K&/KAw), i.e. where KAWis calculated from vapor pressure, solubility, and molecular weight. Generally, the measured and calculated values are within a factor of 4with the exception of DDT for which the factor is 7.4. There appears to be a trend of increasing difference as KOA increases with measured values exceeding the calculated values. This trend may be the result of the inherent inconsistency in phases caused by the presence of octanol in water and vice versa, as discussed earlier, or it may be experimental error. Many of the persistent, semivolatile hydrophobic organic chemicals that are of interest in this context are difficult to
handle experimentally because they sorb strongly to surfaces, especiallyfrom aqueous solution. Measurements of KAWand KOWare thus difficult and subject to error, and the reported values often vary considerably, as is apparent from examination of compilations of these data (16,181. Calculation of KOAfrom KOWand KAWthus combines these errors and introduces uncertainties inherent in handling aqueous solutions. It is also possible that the presence of octanol in water decreases the aqueous phase activity coefficient,thus decreasingKOw from its "pure water" value. These uncertainties can be avoided, and the errors can be reduced by measuring KOAdirectly. All of the chemicals showed a satisfactory correlation of log KOAwith reciprocal absolute temperature. Referring to Figure 6 for a chemical such as HCB, a 30 "C drop in temperature increases KOAby a factor of 30. There are insufficient data to justify a detailed thermochemical analysis at this stage, but certain general semiquantitative relationships can be identified. Examination of the reported enthalpies of fusion, sublimation, and dissolution of HCB and PCBs (19-30) suggests that AHOA is similar in magnitude to the enthalpy of liquid-vapor transition, which suggests that the enthalpy of solid dissolution in octanol is similar to the enthalpy of fusion, Le., 20-25 kJlmo1. Enthalpies of solution of PCBs in water are however larger, Le., 33-67 kJ/mol (191,which implies that the enthalpy of water solution to vapor transition is VOL. 29, NO. 6,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1605
smaller than AHoA. Air-octanol partitioning is thus more temperature sensitive than air-water solution partitioning. There is a clear need to determine and reconcile these enthalpies of transition in order that the effect of temperature on these partitioning phenomena can be quantified. An implication of these results is that the capacity of lipid or organic carbon phases such as soil, foliage, and tree bark is very sensitive to ambient temperatures. At lower ambient temperatures, soils and foliage can retain more chemical than their counterparts in warmer climates. Contaminants will also volatilize more slowly from these media at low temperatures.
Conclusions A system has been developed for direct measurement of
KOAfor hydrophobic chemicals. Several operating configurations were tested with the plateau method being selected for its greater reliability and consistency. KO* was measured for several chlorobenzenes, PCBs, and DDT. The results showed that I(oAincreased log-linearlywith 1I Tand that values at - 10 "C were about a factor of 30 greater than +20 "C values. The temperature coefficientof phase change from octanol to air, AHoA, had average values for chlorobenzenes and PCBs of 62 and 70 kJ/mol, respectively. A comparison with values calculated from GWand KAW showed differences of up to a factor of 4 for PCBs and of 7.4 for DDT. These differences were generally greater for chemicals of large KOA. The KOAfor DDT approaches 10l2at -10 "C. This may represent the largest environmentally relevant partition coefficient for an organic compound that has been measured. It is believed that the octanol-air partition coefficient is an important parameter, which determines partitioning from the atmosphere to vegetation, to soils, and possibly to aerosols. There is thus an incentive to measure it directly, especially as a function of temperature since it is very sensitive to temperature. Improvements in the method can undoubtedly be made, but it is believed that the basic principle described here is sound. It is hoped that this work will encourage such measurements, especially for persistent organochlorine compounds and PAHs, which are subject to long range transport and accumulation in cold climates.
Acknowledgments The authors are grateful to NSERC for financial support.
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Received for review October 6, 1994. Revised manuscript received February 17, 1995. Accepted February 21, 1995.@
ES940625V @Abstractpublished in Advance ACS Abstracts, April 1, 1995.
