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Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
* Corresponding author (martin.krauss{at}uni-bayreuth.de)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20 PAHs concentrations ranged from 0.3 to 186 mg kg-1, the 12 PCBs concentrations from 1.2 to 158 µg kg-1. In most soils, the PAH concentrations decreased in the order, silt > clay 
fine sand > coarse sand, and those of the PCBs in the order, clay > silt fine sand > coarse sand. The distribution of PAHs among particle-size fractions was more heterogeneous than reported in the literature because the soils received PAH-contaminated wastes (ashes, slags, rubble) with varying texture. In all soils, the proportions of two- or three-ring PAHs decreased with decreasing particle size, indicating that the PAH mixture was increasingly altered. The KOC values of the PAHs were three to 10 times higher than those of the PCBs with similar octanol-water partition coefficients (KOW). The mean KOC values of all individual PAHs were highest in silt. For all individual PCBs, mean KOC values were highest in clay. The KOC values of PAHs and PCBs varied up to a factor of 100 among the studied soils and particle-size fractions. Particle-size fractions with highest PAH and PCB concentrations also showed highest KOC values indicating low bioavailability.
Abbreviations: ACE, acenaphthene • ACY, acenaphthylene • ANT, anthracene • BAA, benz(a)anthracene • BAP, benzo(a)pyrene • BBJK, benzo(b + j + k)fluoranthenes • BEP, benzo(e)pyrene • BGHI, benzo(ghi)perylene • CT, chrysene + triphenylene • DBAH, dibenz(a,h)anthracene • ECEC, effective cation-exchange capacity • fc, methanol fraction • FLA, fluoranthene • FLU, fluorene • IND, indeno(1,2,3-cd)pyrene • KOC, organic C-water partition coefficient • KOC, mix, KOC in methanol-water mixtures • KOW, octanol-water partition coefficient • NP, naphthalene • PAH, polycyclic aromatic hydrocarbon • PCB, polychlorinated biphenyl • PER, perylene • PHE, phenanthrene • PYR, pyrene • SOC, soil organic C • SOM, soil organic matter • TSA, total molecular surface area • 
, deviation factor from ideal sorption in methanol-water systems • 
, difference of the interfacial free energies in the methanol-water system • Hdes, enthalpy of desorption
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In recent years, it has been widely recognized that the bioavailability or degradability of persistent organic pollutants like PAHs and PCBs in soils is not constant, but decreases with increasing residence time of the compounds in soil and depends on SOM properties (Alexander, 2000; Reid et al., 2000). Several approaches have been suggested to assess pools of different availability, e.g., by chemical extraction methods (Alexander, 2000; Reid et al., 2000). The distribution of organic contaminants among particle-size fractions is another approach to characterize such different pools, because organic contaminants are closely associated with SOM.
Polycyclic aromatic hydrocarbons and PCBs are unevenly distributed among particle-size fractions (Guggenberger et al., 1996a; Wilcke et al., 1997; Müller et al., 2000). The concentrations of soil organic C (SOC), and PCBs increase in most soils with decreasing particle size (Wilcke et al., 1996, 1997; Wilcke and Zech, 1998; Müller et al., 2000).
In studies of rural soils receiving PAHs mainly from the atmosphere a homogeneous PAH distribution among particle-size fractions has been observed (Guggenberger et al., 1996a; Wilcke et al., 1996, 1997). In urban soils the PAHs were less homogeneously distributed and showed maximum concentrations in different particle-size fractions (Müller et al., 2000). This may be the result of a deposition of contaminated materials, which may be a more important PAH source than deposition from the atmosphere in urban soils.
For PAHs, silt is a preferential sorbent (Guggenberger et al., 1996a; Wilcke et al., 1996), possibly because of the high affinity of PAHs to aromatic structures that are enriched in this fraction. The PAH and PCB pattern seem to be altered simultaneously to soil organic matter, because less degradable PAHs and PCBs accumulate preferentially with decreasing particle size (Guggenberger et al., 1996a; Wilcke and Zech, 1998).
Persistent organic pollutants may show different bioavailability in different particle-size fractions. This is particularly important for organisms which selectively feed on smaller particle-size fractions such as earthworms (e.g., Lumbricus terrestris) (Shipitalo and Protz, 1989; Zhang and Schrader, 1993).
Although in recent years substantial evidence was presented that the equilibrium partitioning model is not sufficient to describe sorption of persistent organic pollutants in soils, and alternative theories have been presented (e.g., the nonlinear, dual-mode sorption model; Pignatello and Xing, 1996; Weber et al., 2001), the KOC is still a useful measure for sorption strength and also bioavailability. The KOC increases with increasing aromaticity of the organic matter (Chiou et al., 1998; Kile et al., 1999). Polycyclic aromatic hydrocarbons have higher KOC values than PCBs with similar KOW (Chiou et al., 1998; Krauss and Wilcke, 2001). This is attributed to the higher affinity of PAHs for aromatic structures in SOM by 
– interactions (Gauthier et al., 1987) or enhanced sorption because of the planar molecular structure (Chin et al., 1997). In particular, PAHs sorb strong to highly aromatic sorbents like black C (McGroddy et al., 1996; Gustafsson et al., 1997; Jonker and Smedes, 2000; Karapanagioti et al., 2000).
The objective of this study was (i) to examine the distribution of PAHs and PCBs among particle-size fractions of urban soils and (ii) to determine KOC values of PAHs and PCBs in particle-size fractions as a measure of bioavailability. To determine KOC values, we used a method that is based on a sequential extraction with different methanol-water mixtures at elevated temperatures (Krauss and Wilcke, 2001).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Particle-size Fractionation
We fractionated the samples according to the method of Amelung et al. (1998) into coarse sand (250–2000 µm), fine sand (20–250 µm), silt (2–20 µm), and clay (<2 µm). We suspended 30 g of soil in 150 mL of distilled water and dispersed it with an energy input of 60 J mL-1 using a Branson Sonifier 450 (Branson Ultrasonics, Danbury, CT). The coarse-sand fraction was separated by wet sieving. The remaining suspension was diluted to a soil/solution ratio of 1:10 and dispersed with 440 J mL-1. The clay fraction was obtained from the supernatant by repeated centrifugation at 40 x g. The remaining silt and fine-sand fractions were separated by wet sieving at 20 µm. The clay fraction was flocculated with MgCl2. All particle-size fractions were freeze-dried.
Polycyclic Aromatic Hydrocarbon and Polychlorinated Biphenyl Analyses
We quantified 20 PAHs and 12 PCBs by isotope dilution analysis with eight deuterated PAHs and seven 13C-labelled PCBs: naphthalene (NP), acenaphthylene (ACY), acenaphthene (ACE), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benz(a)anthracene (BAA), chrysene + triphenylene (CT), benzo(b + j + k)fluoranthenes (BBJK), benzo(a)pyrene (BAP), benzo(e)pyrene (BEP), perylene (PER), indeno(1,2,3-cd)pyrene (IND), dibenz(a,h)anthracene (DBAH), benzo(ghi)perylene (BGHI), and the PCBs 8, 20, 28, 52, 101, 118, 138, 153, 180, 199, 206, and 209 (numbers according to Ballschmiter and Zell, 1980). Total PAH and PCB concentrations of bulk soils and particle-size fractions were extracted with an Accelerated Solvent Extractor (ASE 200, Dionex, Sunnyvale, CA) using hexane/acetone 2:1 (v/v) at 120°C. The compounds were separated by gas chromatography using a Hewlett-Packard 5890 II and identified/quantified by mass selective detection (Hewlett-Packard 5971A). Details of the method are given in Krauss et al. (2000a). The recoveries of the internal standards ranged from 74 ± 15% (mean ± standard deviation) to 93 ± 8% for the total concentrations and 69 ± 14 to 91 ± 16% for the sequential extraction for KOC determination.
Determination of Organic Carbon-Water Partition Coefficients
Details of the method including the theoretical background are given in Krauss and Wilcke (2001). Between 2 and 6 g of sample were extracted in 70-mL centrifuge tubes with teflon-lined screw caps at a soil/solution ratio of 1:10 with methanol–water (methanol fraction, fc = 0.35) for 24 h at 60°C, followed by methanol–water (fc = 0.65) for 24 h at 60°C. The residual PAH and PCB concentrations after the second step were extracted with methanol for 24 h at 80°C. All solution phases were separated by centrifugation for 20 min at 3000 x g, spiked with internal standards, and liquid-liquid extracted twice with 20 mL of hexane. The hexane fraction was dried over anhydrous Na2SO4 and evaporated to 200 µL prior to measurement of PAHs and PCBs.
The sum of concentrations of both extraction steps and the subsequent exhaustive methanol extraction represented approximately the total concentrations in bulk soils or particle-size fractions as extracted with the accelerated solvent extractor (88 ± 22% for 20 PAHs and 109 ± 25% for 12 PCBs).
The KOC values in the soil-water system at T0 = 293 K were calculated using the equation
 | [1] |
is the difference of the interfacial free energies of the methanol-water system (2.16 x 10-20 J nm2; Krauss and Wilcke, 2001), TSA is the total molecular surface area (nm2) taken from Pearlman et al. (1984) for the PAHs and Hawker and Connell (1988) for the PCBs assuming a planar configuration.
With estimated values for the enthalpy of desorption Hdes (kJ mol-1), we were able to calculate log KOC and , which is the deviation factor from an ideal sorption, from the KOC, mix values at two fc-temperature combinations obtained from the sequential extraction. Because Hdes was found to vary for different soils (Krauss and Wilcke, 2001), a proper choice of this parameter is essential. To test, whether Hdes values differed between the compounds or particle-size fractions, we additionally extracted samples of coarse sand (A4, AG1, P2), fine sand (G3, A4, P2), silt (A2, A4), and clay (G3, A2, A4) sequentially as described above, but at 40°C. From the data of the sequential extractions at both 40 and 60°C, we obtained KOC, mix values at four different fc-temperature combinations and were able to estimate log KOC, , and Hdes from the nonlinear multiple regression model (Eq. [1]).
Data Analysis
For statistical analyses we used the software STATISTICA for Windows 5.1 (StatSoft of Europe, Hamburg, Germany). Normal distribution of the data was tested with the Kolmogorov-Smirnov test (Liliefors probabilities). Linear regression analyses were conducted using the least squares method. Nonlinear equations were fitted with the module Nonlinear Estimation using Quasi-Newton estimation and least squares method. We used the t-test for dependent samples to detect significant differences of KOC values among particle-size fractions. The significance level was set at P < 0.05.
The KOW values of the PAHs for standard conditions (101.3 kPa, 25°C) were taken from Mackay et al. (1992), those of the PCBs from Hawker and Connell (1988). Because a gas chromatographic separation of chrysene and triphenylene and of the benzo(b, j, k)fluoranthenes was not possible, we used as an approximation the KOW and TSA values of the most abundant compounds in these mixtures, i.e., chrysene and benzo(b)fluoranthene, respectively.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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coarse sand (20 ± 6) < silt (17 ± 5) < clay (10 ± 2). This was similar to results reported by Guggenberger et al. (1994) and Amelung et al. (1998) and indicates an accumulation of more decomposed organic matter in finer fractions (Christensen, 1992).
In most samples, the sum of PAH concentrations decreased in the order, silt > clay fine sand > coarse sand (Table 2), which is in line with results of other studies (Guggenberger et al., 1996a; Wilcke et al., 1996, 1997; Müller et al., 2000). The sum of PCB concentrations decreased in the order clay > silt fine sand > coarse sand (Table 2), which is comparable with the results reported by Wilcke and Zech (1998) and Müller et al. (2000). The same was true for the concentrations of individual PAHs and PCBs, which showed a very similar distribution among particle-size fractions like the sums of PAH and PCB concentrations, respectively.
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In the Soils G2, G3, G7, I3, I4, and R2, that were mainly contaminated by deposition of PAH-containing waste materials, the distribution of PAHs among particle-size fractions was probably controlled by the properties of these materials. The house garden Soils G2, G3, and G7 received PAH inputs by ashes from domestic heating, compost, garden wastes, and rubble for at least 65 yr. The Soil I3 on a former landfill contained contaminated ashes, blacktop material, and rubble, while the Soil I4 at a former gaswork site was contaminated with PAHs by ashes and coal tar. Soil R2 was contaminated by slags or ashes used for road construction.
For Soils AG1, F2, P2, A2, and A4 with lower PAH concentrations, we found no indication for a PAH input with wastes. Therefore, we assume that deposition from the atmosphere was the main source of the PAHs in these soils, resulting in an accumulation in fine-sand and silt fractions. This agrees with findings of Guggenberger et al. (1996a), and Wilcke et al. (1996)(1997), who studied soils in rural areas, receiving PAHs mainly by deposition from the atmosphere.
The sum of the SOC-normalized PCB concentrations was in general highest in fine sand or silt and lowest in coarse sand (Table 3). A similar distribution was reported by Wilcke and Zech (1998) and Müller et al. (2000). The result indicates that the main PCB source for all soils was atmospheric deposition, or that the PCBs were faster redistributed between particle-size fractions than PAHs.
Polycyclic Aromatic Hydrocarbon and Polychorinated Biphenyl Patterns
In all soils, the proportions of two- or three-ring PAHs decreased with decreasing particle size and those of five- or six-ring PAHs increased (Fig. 1)
. These results agree with findings of Guggenberger et al. (1996a), Wilcke et al. (1996)(1997), and Müller et al. (2000). One explanation may be that the PAH mixture is altered together with the organic matter. Therefore, the more recalcitrant and less volatile higher molecular weight PAHs are preferentially enriched with increasing degree of SOM decomposition from coarse sand to silt and clay. A similar enrichment pattern was found in organic horizons of forest soils, where higher molecular weight PAHs were enriched relative to SOM with increasing degree of decomposition from Oi to Oa horizons (Pichler et al., 1996; Krauss et al., 2000b). Another explanation may be that the distribution of PAHs among particle-size fractions reflects the pattern of deposited particles, because in atmospheric aerosol the proportion of higher molecular weight PAHs increases with decreasing particle size (Smith and Jones, 2000).
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Organic Carbon-Water Partition Coefficients of Bulk Soil and Particle-size Fractions
The Hdes values of individual PAHs and PCBs calculated by nonlinear regression from the sequential extraction data at 40 and 60°C using Eq. [1] did not differ significantly between different particle-size fractions and between different PAHs and PCBs. The mean Hdes values were -7.9 ± 4.3 kJ mol-1 for the PAHs and -7.1 ± 3.7 kJ mol-1 for the PCBs. The KOC values determined by the sequential extraction at 60°C with the mean Hdes values for PAHs and PCBs, respectively, and those determined by nonlinear regression from the sequential extraction at 40 and 60°C were closely correlated (PAHs: r = 0.92; PCB: r = 0.89). The corresponding regression equations had slopes of 0.99 (PAHs) and 1.02 (PCBs) and constants of -0.30 (PAHs) and -0.07 (PCBs) and where thus not much different from the 1:1-line. Therefore, we conclude that the sequential extraction at 60°C and use of the mean Hdes values for PAHs and PCBs was a suitable method to compare KOC values of PAHs and PCBs in particle-size fractions and bulk soils. Nevertheless, the determined KOC values are operationally defined and subject to the assumptions and simplifications underlying the approach (Krauss and Wilcke, 2001).
The KOC values of the PAHs in particle-size fractions were in most soils (except for A4 and F2) significantly lower than those in bulk soil. On average, the differences ranged from a factor of two for BGHI, DBAH, and IND up to a factor of ten for FLU, PHE, ANT, FLA, and PYR (Fig. 2) . This was probably caused by the disaggregation or stretching of organic aggregates by ultrasonic dispersion and freeze-drying of the particle-size fractions, which increased extractability of the compounds (Guggenberger et al., 1996b). Because this should primarily affect humified organic matter (Guggenberger et al., 1996b), the KOC values of silt and clay fractions could have been underestimated. Another explanation may be that we lost PAHs with floating particles during fractionation. Müller et al. (2000) distinguished a floatable fraction consisting mainly of plant debris and soot-like material, which showed high PAH and PCB concentrations although the mass contribution to total soil was low. The loss of this fraction could explain the different KOC values of bulk soil and particle-size fractions, because PAHs sorbed to soot show high KOC values (Jonker and Smedes, 2000).
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coarse sand clay > fine sand (Fig. 2), but for the individual soils, a heterogeneous pattern was observed. The KOC values of individual PAHs varied by a factor of five to 50 among different particle-size fractions of one soil and by a factor of 10 to 100 for a particular particle-size fraction among different soils.
The mean KOC values of individual PCBs decreased in the order silt = clay fine sand > coarse sand (Fig. 3). The KOC values of PCBs in individual soils showed in general a more homogeneous pattern than those of the PAHs. The coarse-sand fraction had in nine of 11 soils the lowest KOC values, the fine-sand fraction in 8 of 11 soils the second lowest KOC values. Nevertheless, the KOC values of the PCBs varied in the same order of magnitude as those of the PAHs.
In the particle-size fractions, highest KOC values did not consistently coincide with highest SOC-normalized concentrations. For the PAHs, the silt showed high SOC-normalized concentrations and high KOC values (Table 3 and Figure 2). The fine sand showed the lowest KOC values of all particle-size fractions although the SOC-normalized concentrations were similar to those in silt. For the PCBs, the low SOC-normalized concentrations in coarse sand coincided with the low KOC values (Table 3 and Fig. 3)
The KOC values of the PAHs were higher than those of the PCBs at similar KOW values in most particle-size fractions and bulk soil samples. The differences increased in general in the order, silt = clay < fine sand < coarse sand. These results agree with previous findings that PAHs sorb stronger to soil and sediment organic matter than nonplanar PCBs with similar KOW values (McGroddy et al., 1996; Chiou et al., 1998; Jonker and Smedes, 2000).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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While the higher molecular weight PAHs were preferentially enriched with increasing degree of SOM decomposition, and the lower molecular weight PAHs depleted, PCBs were mainly partitioned among particle-size fractions according to the SOM concentrations. The reason for this difference was the stronger sorption to SOM of PAHs than of PCBs. This is illustrated by the finding that the KOC values of PAHs were higher than those of PCBs.
The mean KOC values of PAHs and PCBs were significantly different among particle-size fractions. For PAHs, they were highest in silt where PAHs accumulate preferentially. For PCBs, they were highest in clay where the highest PCB concentrations were found. The KOC values of PAHs and PCBs varied by a factor of 100 among the studied soils and particle-size fractions illustrating that they are not a compound-specific constant but depend on properties of the soils and particle-size fractions. Our results demonstrate that the sorption strength of PAHs and PCBs varies much among different soils and particle-size fractions. Particle-size fractions with highest PAH and PCB concentrations also showed highest KOC values indicating low bioavailability.
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ACKNOWLEDGMENTS |
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Received for publication April 11, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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