Polychlorinated biphenyl sorption by soils: Measurement of soil-water partition coefficients at equilibrium

Pergamon

PH: S0045-6535(97)00225-7

Chemosphere, Vol. 35, No. 9, pp. 2007-2025, 1997 © 1997ElsevierScienceLtd All rights reserved.Printedin GreatBritain 0045-6535/97 $17.00+0.00

P O L Y C H I ~ R I N A T E D BIPHENYL SORPTION BY SOILS: MEASUREMENT OF SOIL-WATER PARTITION COEFFICIENTS AT EQUILIBRIUM Don C. Girvin, AI J. Scott Battelle Pacific Northwest Laboratories Richland, Washington 99352 (Receivedin USA 23 December1996;accepted 19 March 1997) ABSTRACT The sorption of PCB congeners 24' (IUPAC number 8), 22'55' (52) and 22'44'55' (153) by eight soils differing in their organic carbon content (2.3 to 0.2% by Wt.) and expandable clay mineral content (17 to 53 % by wt.) has been investigated for 0.1 g/L soil-water suspensions. The time to reach sorption equilibrium, i.e. sorption-diffusion equilibrium, for a highly aggregated montmorillonitic soil containing 0.9% organic carbon for congeners 8, 52, 153 was determined to be 3, 5, and 10 months,respectively, using a gas-purge technique. For these sorbate-sorbent contact times soilwater sorption equilibrium partition coefficients on a soil-organic carbon basis (Koe) were measured and for these congeners ranged varied from 2.4 x 104 to 6.8 x 106 mL/g. Based on these data a correlation relation was derived, log Koc = 1.07 log Kow - 0.98, for estimating Koe from known values of octanol-water partition coefficient (Kow). From literature Kow values this regression equation was used to estimate Koe for 46 of the major PCB congeners in Aroclor 1242, 1254 and 1260. © 1997 Elsevier Science Ltd INTRODUCTION The PCB sorption literature contains scant data on equilibrium sorption coefficients (Kp or Koc) for individual PCB congeners for a range of soils differing in their organic carbon (OC) content (Sklarew and Girvin, 1987). The majority of Kp or Koc data are for Aroclor mixtures, and hence, less accurate for modeling the attenuation and transport of PCBs in soils than if data for individual congeners were used. In those few cases where adsorption of individual congeners had been measured, short (1- to 2-d) contact times were used and equilibrium conditions were not obtained. Therefore, measured values of Kp or Ko¢ were underestimated. Only a single study on the sorption of individual PCB congeners by sediments that addresses the issue of the slow approach to sorption equilibrium prior to measurement of Kp or Ko¢ has appeared in the literature (Coates and Elzerman, 1986). Mounting evidence suggests that the diffusion of hydrophobic compounds into the interior porous structure of soil and sediment particles slows the approach of sorption equilibrium to time scales of weeks to years, depending on the hydrophobicity of the compound (Karickhoff and Morris, 1985). For a montmorillonitic soil, six weeks were required to approach equilibrium for PCB congeners with up to four chlorine atoms, and for hexachloro congeners, estimates of the time to reach equilibrium are months to years (Coates and Elzerman, 1986). The results of the application by these investigators of the gas-purge technique (developed by Mackay et al., 1979) to measure both the rapid and the slower components of the sorption process for hydrophobic compounds cast grave doubt on earlier reports that PCB sorption equilibrium occurred within hours to days (Nau-Ritter et al., 1982; Nau-Ritter and Wuster, 1983; Weber et al., 1983; Horzempa and DiToro, 1983). It appears that in these reports of rapid equilibrium that only the initial rapid sorption step was observed experimentally and that true thermodynamic equilibrium was not attained. For soils and sediments where foe > 0.002, the carbon-reference model for describing sorption of hydrophobic compounds, that is, 2007

2008

KD=

I ~ foo

(1)

has been combined with linear regression equations of the type log Koc = a log Kow + b (2) to predict Kp from known values of Kow for which no direct sorption data for Kp exist. The general form of Equation 2 can be derived from thermodynamic considerations (see discussion below). The regression coefficients in Equation 2 derived for one family of organic compounds (e.g., polynuclear aromatics) are not generally applicable to other groups of organic compounds that differ significantly in structure and substitution (Karickhoff, 1984). No such regression equation, based on experimental equilibrium data, exists for PCB congeners in the open literature. To provide a basis for modeling PCB attenuation and mobility in soils, measurements of equilibrium Kp data have been made for three PCB congeners, 24', 22'55' and 22'44'55', on eight soils with foc >- 0.002. The approach taken in the research presented here was to use the gas-purge technique (Karickhoff 1980; Brussean et al. 1990; Cdrvin et al. 1997) to establish the required contact time for equilibrium [t(eq)] to be reached in the combined "sorption-diffusion" process. Measurements of Kp were made only on soil samples with contact times > t(eq). These Kp values will thus include both the fast and the slow (mass-transfer-limited) components of sorption and will thus be larger than if only the fast component is measured. A predictive equation is derived for estimating the Kp for individual congeners for which sorption experiments have not been performed. Using this predictive equation, estimates of Koc values are derived for the major congeners in PCB Aroclors 1242, 1254 and 1260, for which no previous equilibrium sorption data are available. THEORY Eouilibrium Sorotion The sorption of PCB congeners at low environmental concentrations by a wide variety of soils and sediments can be described at equilibrium by the linear isotherm model. This model can be derived from the thermodynamic condition that the sorbate fugacities (F) in the solution and sorbed phases are equal at equilibrium (i.e., Fs = Fw). The subscripts "s" and "w" refer to the sorbent and solution phases, respectively. In each phase, the fugacity and concentration are related by the equation Fi = ¢i Ci (3) where i is "s" or "w", ¢i is the fugacity coefficient, and Ci is the equilibrium solution concentration (molesYmL). Thus the equality of fugacities yields the isotherm S = Kp C

(4)

where S is the sorbed concentration (moles/g solid) and Kp = Cw/~s (5) In general, the fugacity coefficients are functions of solute concentration and thus the sorption isotherm (Equation 4) is nonlinear. However, for dilute environmental conditions and the conditions

of this study, the value of ~s approaches a limiting value and Equation 4 reduces to a linear isotherm with a constant partition coefficient, Kp. Sorption on natural sediments and soils typically yields linear isotherms when the organic sorbate is present in solution at concentrations below 10-5M or below 50% of the water solubility of the compound, whichever is lower (Karickhoff et al., 1979; Rogers et al., 1980; Brown and Flagg, 1981;

2009 Chiou et al., 1982; Hassett et al., 1980a,b; Means et al., 1980, 1982). Changes in solute speciation (e.g., protonation, chemical complexation, coagulation, or degradation) represent a significant source of isotherm nonlinearity for ionizable hydrophobic compounds. However, PCBs are relatively resistant to degradation and undergo virtually no chemical side reactions in aqueous solution. Thus, at low PCB concentrations the linear isotherm model adequately describes the sorption of individual congeners. Based on extensive data for pesticides and neutral hydrophobic organic compounds, Karickhoff (1984) concluded that soil OC content dominates observed variations in Kp for surface soils and sediments, and that particle-size distribution and expandable clay content (CM) are of secondary importance for most sorbents with foc -> 0.002. Thus to a first approximation Kp = ew foc/¢oc = Koc foc (la) where Koc is the carbon-referenced partition coefficient defined as the ratio of fugacity coefficients in water and soil organic carbon. For hydrophobic compounds, whose water solubility is <10-3 M, sorption is controlled by soil organic carbon (humin-kerogen), and sorption can be adequately described using the carbon-referenced partition coefficient, Koc. Thus for individual PCB congeners Koc should be independent of soil type. Because the octanol-water partition coefficient, Koc, is the ratio of solute concentration in octanol to that in water, the equality of solute fugacities in octanol and water at equilibrium allows Koc to be expressed as the ratio of fugacity coefficients Koe = CJCw = ¢w/¢o (6) when the pure supercooled liquid solution is taken as the standard state in each phase. The partition coefficient, Koe, can be related to Kow by combining Equations 5 and 6, Ko¢ = Kow (¢'o/¢'oc) (7) where the primes refer to water-saturated octanol and sorbent. It is important to note that the ratio of

fugacity coefficients in Equation 7 must not depend on congener if a linear relation between Koc and Kow is to exist (i.e., if a regression of the form given in Equation 2 is to hold). In addition, the accuracy of Koe estimated using Equation 2 depends on the predominance of organic carbon as the sorbent, the uniformity and origin of the sorbent organic carbon among soils, and sorption isotherm linearity. Sorption Kinetics: Approach to Sorotion Eauilibrium The sorption kinetics of hydrophobic nonionizible organic compounds by natural sediments typically consists of an initial rapid (labile) component followed by a slow (nonlabile) component that can persist for an extended period (Karickhoff, 1980, 1984; Karickhoff and Morris, 1985; Coates and Elzerman, 1986). The characteristic time scales of the labile and nonlabile components are hours to days and weeks to months, respectively. The effect of the nonlabile component on approach to equilibrium during sorption or desorption will depend on the aggregation of the sediment particle assemblage and the strength of the sorption (equilibrium Kp). Because this slow component can accommodate a significant quantity of additional sorbate mass per unit mass of sorbent on environmental time scales, measurements of Kp that fail to include this nonlabile component will underestimate the equilibrium Kp by factors of two to three (Karickhoff, 1984). Thus, estimates of the times required to reach sorption equilibrium [t(eq)] for PCB congeners and

2010 soils used in this study were experimentally determined as a prerequisite to measurement of equilibrium Kp values. For the experimental conditions in gas-purge apparatus, the two-compartment model predicts that the fraction of the sorbed congener released from the nonlabile component of the sorbent, fr, is given by fr = 1 - (1 - X 0 exp {-lq t}

(8)

where X1 is the labile fraction of the sorbent mass, ko (days-l) is the mass-transfer coefficient for diffusive release of the congener from the nonlabile component of the sorbent, and t (days) is purge time (Karickhoff and Morris, 1985). Plots of experimental values of fr versus the square root of t were used to determine the sorbate-sorbent contact times required to ensure that a reasonable approach to sorption equilibrium had been achieved.

MATERIALS AND METHODS Congener Selection The selection of individual PCB congeners was based on 1) the abundance of congeners in Aroelor mixtures 1242, 1254, and 1260; 2) the commercial availability of 14C-labeled congeners; and 3) the inclusion of congeners spanning the widest possible range of chlorine numbers for derivation of the Koc - Kow correlation. The three 14C-labeled congeners selected were 8, 52, and 153, which contained two, four, and six chlorine atoms, respectively. These congeners were obtained from Pathfinder Laboratories, St. Louis, MO, and gas chromatography/mass spectroscopy (GCMS) analysis in our laboratory showed them to be 98% pure. The abundance of these congeners in the Aroclor mixtures, their log Kow values and water solubilities are given in Table 1. Carbon-14 labeled congeners containing seven or eight chlorine atoms were not routinely available from commercial sources. Soils Selection The eight soils used in this study are from various locations throughout the United States and from various soil horizons at these locations (Table 2). All soils were air-dried, and the portion that passed through a 270-mesh sieve (silt plus clay <53-1im nominal diameter) was characterized and used for sorption experiments. These soils span a range of soil organic carbon and expandable clay Table 1. Physical properties of 14c-labeled PCB congener. IUPAC number of congener Chlorine substitution pattern on biphenyl

8

52

153

2 4'

22'55'

10.7 0 0

4.2 3.2 0

0 8.1 19

1000 + 400

30 + 20

1.0 5:0.4

5.1 ± 0.4

6.1 5:0.1

6.9 5:0.2

22'44'55'

Weight percent of aroclors(a) 1242 1254 1260 Aqueous solubility(b) (p.g/L) Octanol-water partition coefficient(b) (logKow)

(a) From Capel et al. 1985. (b) Values selected by Shiu and Mackay (1986) in critical review of published information on physical properties of PCB congeners.

2011 Table 2. Soil series, classification, horizon, depth, and sampling location (county, state) of soils used in sorption studies. Soil series name and classification

Horizon

Devth (cm~

Catlin silt loam (fine silty, mixed mesic, Typic Argiudoll)

Ap

0-15

Champaign, Illinois

Cloudland/Holston loam (coarse-loamy, siliceous, thermic Glossary Fragiudult/Typic Paleudult)

E

15-30

Hawkins, Tennessee

Kenoma silt loam (fine, montmorillinitic, thermic, Vertic Argiudoll)

Ap Bt2 C

0-20 58-82 120-155

Norbome silt loam (fine-loamy, mixed mesic, Typic Argiudoll)

Ap Btl Bt2

0-20 33-65 65-85

Linn, Kansas

Boone, Missouri

mineral content (Table 3). Use of the carbon reference model for describing PCB sorption requires a range of foe to be considered. The expandable clays, e.g. montmorillonite, are considered to represent the dominant mineral sorbent for hydrophobic organic compounds with decreasing foe (Karickhoff and Brown, 1978; Hassett et al., 1980a; Karickhoff, 1984). The montmorillonitic clay content of the soil also exerts a major influence on the aggregation of the soil particles and thus the diffusion of the hydrophobic sorbate into the structure of the soil particle and the subsequent approach to sorption equilibrium. Comparison of foe and the expandable clay fraction (fcm) (Table 3) shows for the Norborne soils that fern is essentially constant while foe decreases with increasing depth. On the other hand, fcm for the Kenoma soils is variable, being a factor of three greater than in the Norborne in the B horizon. Expandable 2:1 clays dominate the clay fraction of the Kenoma soils, with 60% or more identified as montmorillonite. This contributes to the highly aggregated structure of the Kenoma soils. Montmorillonite dominates the clay fraction of the Norborne soils; however, these soils are aggregated to a lesser extent than the Kenoma soils because of their lower total clay fractions. The Cloudland soil is a highly weathered acidic soil containing considerable crystalline iron and aluminum oxides with a purely kaolinitic clay fraction. The clay fraction of the Catlin soil is approximately 30% expandable clays. Preoaration of Samt)les Batch samples used for determination of the time required to reach sorption equilibrium and the measurement of equilibrium Kp values were prepared as follows: Aliquots of standards containing a single 14C-labeled congener in isooctane were added to narrow-neck glass bottles. Immediately following evaporation of the isooctane, 50 nag of pre-wet soil and 500 mL of electrolyte solution were added and the bottle was sealed with a Teflon-coated septum stopper. The electrolyte solution contained 0.01 M CaCI2 to maintain a uniform homoionic solution among all of the soil suspensions and 10 ppm HgC12 to inhibit microbial growth and/or degradation of the congener (Baxter et al., 1975). The initial mass of each congener added was a factor of 50 below the water solubility (Table 1) of congeners 8 and 52 and a factor of 10 below that of congener 153 for the 500 mL of electrolyte added. To eliminate the possibility of congener loss to a separate gas phase in the samples, zero headspace was achieved when sealing the bottles (Girvin et al. 1997). Samples were

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2013 rotated (2 rpm) end over end to maintain continuous suspension of soil without excessive abrasion of the soil particles during the period of soil-solution contact. Those samples which developed head space bubbles during the period of contact were discarded. The entire 500-mL sample was filtered for subsequent determination of aqueous and sorbed concentrations of the congener, unless the sample was chosen for determination of congener mass balance. In that case, two 50-mL aliquots of the suspension were analyzed for the total aqueous plus sorbed concentration, and the remaining 400 mL was filtered. An all-glass, high-pressure (35-psig) filtration system using glass-fiber filters (Fisher, 23-ram dia) was used to separate the solid and aqueous phases. This all-glass system was used to minimize the loss of dissolved PCB from solution. The nominal glass fiber filter pore size is 0.3 gin. However, loading of the filter with the first 50 mL of soil suspension significantly reduces the effective pore size. This was examined in a series of filtration experiments, in which the filtrate leaving the glass-fiber filter was passed through 0.02-I.tm Nucleopore membrane filters. The weight of sediment retained on the Nucleopore filter was measured for 25-mL increments of the soil suspension passed through both filters. For all soils studied, the following was observed: First, no detectable weight gain (0.02 rag) was measured on the Nucleopore filter after 50 mL of soil suspension had been filtered through the glass-fiber filter. Therefore, the initial 50 mL of filtrate was discarded and not included in the measurement of the aqueous phase concentration of PCB. Second, for the first 50 mL of soil suspension filtered the combined weight of sediment collected on the Nucleopore filter never exceeded 0.4 mg and it was typically less than 0.1 nag. Thus, the quantity of sediment passed (not retained) by the glass-fiber filter was in all cases less than 1% of the total sediment used in the experiments. Ouantification of H - ~ , : L k ~ l c ~ [ ~ g g ~ Extraction procedures for measuring aqueous, sorbed, and total (aqueous plus sorbed) PCB congeners are briefly described here; they have been described in detail elsewhere (Coates, 1984; Dunnivant, 1988). Extraction of the aqueous congeners was performed by collection of the filtrate (with the first 50 mL discarded) in a 1-L separatory funnel containing 25 mL of saturated NaC1 plus 50 mL of isooctane, followed by shaking for 3 minutes and separation of phases. A second extraction with 40 mL of isooctane was performed and the combined isooctane made up to 100 mL in a volumetric flask. If an additional concentration step was required, the isooctane was blown down and made up to 5 mL final volume. Three milliliters of isooctane was added to the scintillation cocktail for subsequent 14C counting. The sorbed congeners were extracted from soils by transferring the filter plus soil to a beaker containing 50 mL of acetone. The filter device was rinsed with acetone and the acetone was then added to the beaker. The sediment-filter acetone mixture was sonicated for 15 minutes at 75% power using a Fisher Model 300 sonicator. The sonicated mixture was covered and allowed to stand overnight and sonicated again. The acetone was filtered through a glass-fiber filter, made up to a fixed volume and 3 mL were added to the scintillation cocktail for subsequent 14C counting. Measurement of the total PCB congener was performed on two replicate 50-mL aliquots of the well-mixed soil suspension. Each aliquot was added to a beaker containing 50 mL of acetone and 15 mL of isooctane. The mixture was sonicated for 25 minutes at 75% power and allowed to stand

2014 for 3 hours, and then the sonication was repeated. The sample was filtered through a glass-frit trdter into a separatory funnel containing 175 mL of distilled water and 25 mL of saturated NaCI solution and shaken for 3 minutes, and then the isooctane was separated. A second 15-mL isooctane extraction of the aqueous phase was performed and the combined isooctane made up to 30 mL, 3 mL of which were added to the scintillation cocktail for 14C counting. The congener mass balance for filtrate, soil, and the total suspension were measured for all three congeners on samples with contact times and solid-to-solution ratios equal to those used for determination of Kp (Table 4). The mass balance was defined in terms of the activity measured in each phase using the expression [(filtrate + soil)/(total suspension)] x 100. In general recoveries were between 95 and 100%, with some exceptions. No trends with congener or foc were apparent. RESULTS AND DISCUSSION Approach to Eouilibfium The diffusional penetration of PCB congeners into the interior of aggregated soil particles limits the rapid attainment of sorpfion equilibrium in those regions of the soil particle assemblage not in direct contact with the bulk solution. Sorption equilibrium can only be obtained when this diffusional penetration is complete. The term "sorption equilibrium" is used here to refer to the Table 4. Percent mass balance of congeners for I ~ determinations in the filtered soil and filtrate and total suspension. (Percent mass baIance = [(filtrate + soil) + total suspension) x 100]. All soil water suspensions contained 100 mg soil/L with contact times for congeners 8, 52, and 153 of 3, 5, and 10 months, respectively.) Percent mass balance of con~,ener Soil name Catlin Ap

87 93 96

102 96

100 109 98 96

Kenoma Ap

94 101

92 96

100 97

Norborne Ap

96 91

Kenoma Bt2

104 97 95

Norborne Btl

92 87 96

Cloudland E

103 91 94

95 102

92 104 100 93

Kenoma C

101

87

100

103 97 94 86 91 93

94 101 90 93 87 94 96

2015 combined processes of diffusion and sorption within aggregated soils. To estimate the time required to approach sorption equilibrium for congeners 8, 52 and 153 and the soils used in this study, replicate samples of the Kenoma soil were contacted with PCBs for varying lengths of time. Given that expandable clays contribute significantly to the extent of aggregation in soils, the Kenoma soil was selected for the equilibration studies because its montmorillonitic clay fraction is a relatively constant weight percent of the total soil for all of the Kenoma horizons used in this study and because montmorillonite is the predominant clay, in all of the soils except the Cloudland (Table 3). Soil-water suspensions of the Kenoma Bt2 soil were purged using the gas-purge apparatus to determine the changes in congener release profiles as a function of the original sorbate-sorbent contact time. Release profiles for each of the congeners (Figure 1) were definedby plotting the cumulative fraction of the total congener released from the soil against the square root of purge time. The term "release" refers to the combined processes of congener desorption from the sorbent and congener diffusion through the aggregation of soil particles into the bulk aqueous phase. The asymptotic convergence of the release profiles toward a single curve with increasing contact time provides a sensitive indicator that diffusional penetration of the sorbate into the aggregate is complete, i.e., that sorption equilibrium is achieved (Karickhoff, 1984). Thus, the contact time for which minimal incremental change in the profiles occurs has been used to estimate the time required for sorption equilibrium to occur. For a six-day contact between congener 8 and the Kenoma Bt2 soil, a major fraction of the sorbed congener was present in labile form and released rapidly (in the first 9 to 16 days of the purge), as shown in the initial portion of Figure la. With increasing contact times, the labile fraction decreases and the linear region of slow release begins at an earlier stage of the purge, indicating more complete penetration of the congener into the soil. Little additional change in shape of the release curves is observed between the samples having 102- and 250-day contact times. The estimate of t(eq) based on these data was 90 _
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2017 respectively, for all soils. The Kp and Koc values obtained in this study were calculated from measured solution and sorbed concentrations of each congener and the fraction organic carbon in the <53-ttm fraction of each soil (Table 5). All of the data in Table 5 are for 100 mg soiFL. The general applicability of the carbon-reference model (Equation 1) is clearly indicated by the linear correlations between Kp and foc for all data in Table 5, shown by the solid lines in Figure 2. The carbon-reference model requires that the correlation have zero intercept. For all three congeners, the regression coefficient, r2, exceeds 0.96. The slope of the regressions for congeners 8, 52 and 153 are 3.59 + 0.04 mL/g, 2.20 + 0.06 mL/g and 2.80 :t: 0.12 mlJg, respectively, and represent the average Koc for the soils examined. The standard error in the estimation of the slope is given by the + sign. The 95% confidence intervals of the regression line are shown by the diverging dashed lines, that is the regression has a 95% probability (two-tailed) of falling within the dashed lines. Although some scatter of the data around the correlation lines through the origin and the 95% confidence intervals is apparent, the only consistent deviation is for the Cloudland E soil (labeled CE in Figure 2) which consistently falls above the correlation line for congeners 52 and 153. The Cloudland E and Kenoma C soils are both near the lower limit of applicability for the carbonreference model; however, Kenoma C data fall on the correlation line for congeners 8 and 52. Unfortunately, no sorption data for 100 mg soil/L were obtained to substantiate this trend with the Kenoma C soil for congener 153. It has been reported that expandable 2:1 clays (e.g., montmorillonite) dominate hydropbobic sorption in mixed mineral assemblages at low organic carbon levels (Karickhoff and Brown, 1978; Hassett et al., 1980a). However, the fact that the Cloudland E soil contains essentially no expandable clays, while the clay content of Kenoma C is dominated by expandable montmorillonitic clays (see fcm/focratios in Table 3), precludes the influence of expandable clays as the cause of the Cloudland E deviation from the carbon-reference correlation at low soil organic carbon values. Although the Cloudland soil is the only acidic soil containing several percent crystalline iron and aluminum oxides used in this study, there is no evidence in the literature that would warrant speculation that either of these factors contribute to the anomalous behavior observed for the Cloudland soil. The cause of this deviation remains unclear. Re~ression of IC~ Versus The average Koc values (Figure 2) increase with the chlorine number and show a definite correlation with the log Kow (Table 1). To derive the correlation coefficients for the linear log Koc log Kow model given by Equation 2, the experimental Koc data (Table 5) were used rather than the average Koc values for each congener (Figure 2). The Cloudland E data in Table 5 was not used here. The Kow values (Table 1) were selected from the critical review of Shin and Mackay (1986). To include the errors in both dependent and independent variables the geometric mean (GM) functional regression method (Halfon, 1985) was used to derive the following expression (solid line in Figure 3), log Ko¢ = 1.07 log Kow - 0.98 (r2 = 96.4)

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Figure 2. Linear regression between equilibrium sorption partition coefficient, Kp, and the weight fraction of soil organic carbon for congeners (a) 8, (b) 52, and (c) 153 on eight soils. Center solid line is regression, diverging dashed lines represent 95% confidence limits of the regression line. Data for Cloudland E soil is indicated by CE. For the soils used the average Koc for each congener is the slope of the regression.

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Figure 3. Linear regression between measured log Koc on eight soils and literature log Kow for congeners 8, 52, and 153 which includes errors in both dependent and independent variables. The regression (Eq. 9) is shown by the solid line, the dotted lines are the 95% confidence limits of the regression mean and the dashed lines are the 95% predicted interval of a future measurement. where the standard deviation for the slope and intercept are :1.-0.03 and i-0.18, respectively. The 95% confidence limits of the regression mean are shown as dotted lines and the 95% predicted interval for a future Koc measurement is indicated by the dashed lines (Figure 3). As noted in the derivation of the relation between Koe and Kow above (Equation 7), a linear relation between Koc and Kow exists only when the ratios of fugacity coefficients in octanol and soil organic carbon are independent of congener. The regression coefficient for Equation 9, r2 = 96.4, indicates that 95% of the variation in log Koc with log Kow is accounted for by this linear model. Thus, Equation 9 is consistent with the hypothesis that the ratio of fugacity coefficients in Equation 7 is independent of congener chlorine number in the range of two to six chlorine atoms. The linear correlation model represented by Equation 9 was used to estimate log Koc values from literature values of log Kow for those congeners for which no Koc data exist. The dominant congeners in three common Aroclor mixtures are given in Table 6, along with the weight percent of the congener in the mixture and the octanol-water partition coefficient for the congener. The values of log Koe in the last column of Table 6 were calculated from Equation 9. We have assumed that Equation 9 can be extrapolated to congeners containing seven and eight chlorine atoms. These estimations of Log Koc from Equation 9 are expected to agree with measured values within a factor of 2. Given these estimated Koc values and the measured foc value for a given soil, Equation I can be used to estimate Kp for the congeners of interest when foe >0.002.

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Because no other log Koc - log Kow relation exists in the literature that has been experimentally derived from PCB data, the information presented in Table 6 and Equation 9 represents a significant advance in estimating Kp for soil-water systems at a PCB spill site where the movement of a series of PCB congeners is of concern. When describing the influence of sorption upon PCBs transport in soils a distinction must be made between the equilibrium partition coefficient for an individual congener "i", Kp,i distribution ratio, Ko, for PCB mixtures such as Aroclors, which is defined by, ~Sj KD

i~ KpiCi

--

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ZC+

Sa n Cln "

i

Here San and Can are the total analytical concentrations of all sorbed and solution congeners, respectively, Si and Ci are the sorbed and solvent concentration of congener "i', respectively, and the sum is over all congeners present. There are numerous reports of KDS for Aroclor mixtures in the literature (Sklarew and Girvin 1987). Because Kp,i values of individual congeners in an Aroclor vary by a factor of 100 (Table 6) the KD will change as congeners are chromatographically separated from each other along their path of migration in the soil. That is, at a particular location in the soil the relative values of the Cis will vary as a function of time, thus effecting KD. For this reason the preferred approach when modeling PCB transport, is to use Kp,i rather than KDS for describing PCB attenuation or retardation via sorption. An additional and perhaps more important reason for treating PCB transport in terms of individual congeners is the variation in the kinetics of sorption-desorption among congeners in Aroclor mixtures (Coates and Elzerman 1986; Girvin et al. 1997). The observed variation in the sorption and desorption rates among congeners requires that PCB transport be represented in terms of individual congeners. CONCLUSIONS 1. For PCBs the gas purge technique provides a sensitive method for quantitatively evaluating the approach to sorption-diffusion equilibrium in soil-water systems because the shape of the sorbate release curves is highly sensitive to how complete the diffusion of the sorbate into the sorbent volume. This has been previously reported for other hydrophobic organic compounds (Wu and Gschwend 1986; Brusseau et al. 1990). 2. The times to reach sorption-diffusion equilibrium for a highly aggregated montmorillonitic soil containing 0.9% organic carbon for PCB congeners 8, 52, 153 were 3, 5, and 10 months, respectively. For less aggregated soils shorter times may be sufficient. 3. Measurements of Kps for PCB congeners 8 and 153 after 6 and 15 day contact times, respectively, yielded Kp values which were factors of 1.5 to 2 lower than equilibrium values obtained after 3 and 10 month contact times. 4. The log Koc - log Kow relationship derived here for PCB congeners 5, 52 and 153 using contact times of 3, 5 and 10 months, respectively, were used to estimate Koc form literature Kow data within a factor of 2. This relationship differs from that derived for other organic compound

2024 classes, e.g. polynuclear aromatic hydrocarbons, which differ in substitution and structure. Thus, this PCB log Koc - log Kow relation is not generally applicable to other classes of compounds. 5. When describing the sorption of Aroclor in soil-water systems the partition coefficient of individual PCB congeners, Kp, should be used rather than describing Aroclor sorption in terms of a distribution coefficient, KD, which will vary spatially and temporally because of the chromatugraphic separation of congeners as they move through sorbent materials. ACKNOWLEDGMENTS This research was supported by the Waste and Water Management Program, Generation and Storage Division of the Electric Power Research Institute, Palo Alto, CA. The support of Dr. M. McLearn and Dr.R. Komai is appreciated. We thank Dr. S. Karickhoff and J.T. Coates for their valuable advise and assistance. The Pacific Northwest Laboratory is operated by Battelle Memorial Institute. REFERENCES Baxter, R. A., P. E. Gilbert, R. A. Lidgett, J. H. Mainprize, and H. A. Vodden. 1975. "The Degradation of PCBs by Microorganisms." Sci. Tot. Environ. 4:53-61. Brassean, M. L. and Rao, P. S.C. 1989. "The Influence of Sorbate-Organic Matter Interactions on Sorption Nonequilibrium." Chemosphere. 18: 1691-1706. Brussean, M. L., R. E. Jessup, and P. S. C. Rao. 1990. "Sorption Kinetics of Organic Chemicals: Evaluation of Gas-Purge and Miscible-Displacement Techniques." Environ. Sci. Tech. 24(5):727-19735. Brown, D. S., and E. W. Flagg. 1981. "Empirical Prediction of Organic Pollutant Sorption in Natural Sediments." J. Environ. Qual. 10(4):382-386. Capel, P. D., R. A. Rapaport, S. J. Eisenreich, and B. B. Looney. 1985. "PCBQ: Computerized Quantification of Total PCB and Congeners in Environmental Samples." Chemosphere 14:439-450. Chiou, S. J., R. A. Griffin, and M. M. Chou. 1982. "Effect of Soluble Salts and Caustic Soda on Solubility and Adsorption of Hexachlorocyclopentadiene." In Proceedings of the Eighth Annual Research Symposium. Cincinnati, Ohio; U.S. Environmental Protection Agency. EPA-600/9-82-002, pp. 137-149. Coates, J.T. 1984. Sorption Equilibria and Kinetics for Selected Poly-chlorinated Biphenyls on River Sediments. Ph.D. Dissertation. Clemson, South Carolina: Clemson University. Coates, J. T., and A. W. Elzerman. 1986. "Desorption Kinetics for Selected PCB Congeners from River Sediments." J. Contain. Hydro. 1:191-210. Dunnivant, F. M. 1988. Congener-Specific PCB Chemical and Physical Parameters for Evaluation of Environmental Weathering of Aroclors. Ph.D. Dissertation. Clemson, South Carolina: Clemson University. Girvin, D. C. D. S. Sklarew, A. J. Scott, and J. P. Zipperer. 1997. "Polychlorinated Biphenyl Desorption From Low Organic Carbon Soils: Measurement of Rates In Soil-Water Suspensions." See companion article to this volume of Chemospbere. Hassett, J. J., J. C. Means, W. L. Banwart, and S. G. Wood. 1980a. Sorption Properties of Sediments and Energy Related Pollutants. Athens, Georgia: U.S. Environmental Protection Agency. EPA-600/3-80-041. Horzempa, L. M., and D. M. DiToro. 1983. "The Extent of Reversibility of Polychlorinated Biphenyl Adsorption." Water Res. 17:851. Karickhoff, S.W. 1984. "Organic Pollutant Sorption in Aquatic Systems." J. Hydraulic Eng. 10(6):708-735.

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Neter, J., and W. Wasserman. 1974. "Applied Linear Statistical Models: Regression, Analyses of Variance, and Experimental Designs." Illinois: Irwin-Dorsey Limited, 326 pp. Rogers, R. D., J. C. McFarlane, and A. J. Cross. 1980. "Absorption and Desorption of Benzene in Two Soils and Montmorilionite Clay." Environ. Sci. TechnoL 14(4):457-460. Shiu, W. Y., and D. Mackay. 1986. "A Critical Review of Aqueous Solubilities, Vapor Pressures, Henry's Law Constants and Octanol-Water Partition Coefficients of the Polychlorinated Biphenyls." J. Phys. Chem. Ref. Data 15:911-929. Sklarew, D. S., and D. C. Girvin. 1987. "Attenuation of Polychlorinated Biphenyls in Soils." Rev. Environ. Contam. Toxicol. 98:1-41. Weber, W. J., Jr., T. C. Voice, M. Pirbazari, G. E. Hunt, and D. M. Ulanoff. 1983. "Sorption of Hydrophobic Compounds by Sediments, Soils, and Suspended Solids. II. Sorbent Evaluation Studies." Water Res. 17:144 Wu, S. C. and P. M. Gschwend. 1986. "Sorption Kinetics of Hydrophobic Organic Compounds from Natural Sediments and Soils." Environ. Sci. TechnoL 20:717-725.