Environ. Sci. Technol. 2001, 35, 3931-3935
Biomimetic Extraction of PAHs and PCBs from Soil with Octadecyl-Modified Silica Disks To Predict Their Availability to Earthworms MARTIN KRAUSS* AND WOLFGANG WILCKE Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
We developed a model to predict the availability of PAHs and PCBs to earthworms (Lumbricus terrestris L.) in soils. We related the uptake of PAHs and PCBs by earthworms to the extraction with octadecyl-modified silica disks (C18 disks). Eleven soils (data set A) were used to develop the model, and 14 additional soils (data set B) were used to validate it independently. The biota-to-soil accumulation factors (BSAFs) of higher molecular weight PAHs (g202 g mol-1) and PCBs were higher than the corresponding C18 disk-soil partition coefficients (Kdisk) in the soils of data set A. This suggested that PAHs and PCBs were not only partitioned between soil and earthworms but also selectively ingested or metabolized. Nevertheless, we could predict the BSAFs of individual PAHs and PCBs for data set A by regression equations of log BSAF on log Kdisk (r ) 0.47-0.87). To predict the BSAFs of all PAHs and PCBs, respectively, we derived a model for each compound class that uses Kdisk and Kow values. Both the compound and the compound-class specific model were suitable to predict the BSAFs of PAHs and PCBs in data set B within a factor of 10-15.
solid-phase extraction (SPE) or solid-phase microextraction (SPME) techniques (4). With the latter methods, organic contaminants are extracted from soil or sediment suspensions by hydrophobic polymers such as TENAX (14), XAD-4 (15), octadecyl-modfied silica (11, 16), or poly(dimethylsiloxane)coated glass fibers (17). With these methods, freely dissolved HOCs are removed from soil solution by the sorbent, resulting in a desorption of HOCs from soil solid phase until an equilibrium between soil and hydrophobic sorbent is reached. This is expected to “mimic” the distribution of these contaminants between solid phase and organisms analogously to the EPT. As compared to “mild” solvent extractions, these “biomimetic” methods offer several advantages. The depletion of HOCs in the soil solid phase is smaller than after mild solvent extraction and comparable to those by organisms (4). The alteration of the soil organic matter is smaller than during organic solvent extraction. Finally, these methods have the potential to be applied in situ (17). Despite the variety of methods to determine the bioavailability of HOCs suggested in the literature, there is still a lack of predictive models based on nonexhaustive extractions, which have been validated with independent data sets. Our objective was (i) to examine whether octadecyl-modified silica solid-phase extraction disks (C18 disks) can be used to describe the bioaccumulation of PAHs and PCBs by earthworms (Lumbricus terrestris L.) and (ii) to develop and validate two simple predictive models for the bioaccumulation of PAHs and PCBs by earthworms based on this extraction method. The first compound-specific model uses one equation for each PAH and PCB for the prediction of bioaccumulation. The second compound-class specific model requires only one equation for all PAHs and one for all PCBs, respectively.
Theoretical Considerations and Experimental Approach In a previous paper (10), we showed that the uptake of PAHs and PCBs from the investigated soils by the earthworm species Lumbricus terrestris L. can be described by the firstorder compartment model as proposed in refs 5 and 7:
dCworm/dt ) k1,wormCsoil - k2,wormCworm
(1)
Introduction To describe the bioavailability of hydrophobic organic contaminants (HOCs) in soils or sediments, the equilibrium partitioning theory (EPT) has frequently been applied. The EPT approximates the distribution of HOCs between solid phase, solution, and organisms by equilibrium partitioning coefficients (1-4). Although the EPT is useful for many applications, some of its shortcomings have become evident in recent years (5). Because the EPT considers the freely dissolved concentration in soil solution as the bioavailable fraction, the solid phase-solution distribution is crucial to predict the uptake by organisms. This distribution depends on HOC and soil properties and the residence time of HOCs in soil or sediment, which are not addressed in the EPT (46). The EPT does not account for biotransformation of HOCs or their uptake via food (5). As a consequence, more refined models, which are still based on the EPT, have been developed (e.g., refs 7 and 8). To assess bioavailable concentrations by extractions, the use of solvent-water mixtures (9-11), supercritical carbon dioxide (12), or hydroxypropyl-β-cyclodextrin solutions (13) have been proposed. So-called biomimetic approaches use * Corresponding author telephone: +49 921 55 2178; fax: +49 921 55 2246; e-mail:
[email protected]. 10.1021/es010081e CCC: $20.00 Published on Web 08/23/2001
2001 American Chemical Society
which can be expressed after integration as
Cworm ) KwormCsoil(1 - e-k2,wormt) + Cworm,0e-k2,wormt (2) where Cworm is the HOC concentration in earthworm lipid on a fresh weight base (mg of HOC (kg of lipid)-1). Csoil is the total HOC concentration normalized to soil organic carbon (mg of HOC (kg of SOC)-1), which is considered as constant, because the uptake of HOCs by earthworms only results in a negligible reduction of the HOC concentrations in soil. k1,worm is the uptake rate constant (d-1). k2,worm is the elimination rate constant (d-1). Elimination includes desorption to the surrounding solution, losses due to excretion, and metabolic transformation. The equilibrium constant Kworm is k1,worm/k2,worm. Cworm,0 is the initial HOC concentration in earthworms before they are exposed to contaminated soil. At equilibrium, dCworm/dt ) 0, and eq 1 may be written as
k1,worm/k2,worm ) Cworm/Csoil
(3)
Thus, the ratio Cworm/Csoil is equal to Kworm, which is also defined as biota-to-soil accumulation factor (BSAF) at steadystate conditions (5). VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Land Use, Sum of PAH and PCB Concentrations, Soil Organic Carbon (SOC) Concentrations, and pH of the Studied Soils site
land use
G2 G3 G7 A2 A4 R2 I3
house garden house garden house garden alluvial grassland alluvial grassland roadside grassland fallow land, former landfill I4 grassland, former gaswork site F2 forest P2 park AG1 arable land G1 G4 G5 G6 G8 A1 A3 R1 I1 I2
house garden house garden house garden house garden house garden alluvial grassland alluvial grassland roadside grassland industrial area former industrial plant F1 forest F3 forest P1 park, former industrial area AG2 grassland
∑20 PAHs ∑12 PCBs SOC pH (mg kg-1) (µg kg-1) (g kg-1) (KCl) Data Set A 13.0 15.4 12.2 7.2 0.4 186.0 48.9
15.4 45.7 157.8 58.1 3.3 91.9 2.3
18.6 32.6 35.7 39.9 16.8 47.8 42.0
5.5 7.1 6.3 6.9 6.7 6.9 6.5
37.3
12.9
28.5
6.6
0.5 3.5 0.3
3.5 5.5 1.2
29.8 10.3 21.4
4.4 4.4 6.6
15.1 12.4 2.9 13.3 10.3 9.7 17.7 10.9 22.6 40.5
25.3 33.4 50.4 34.9 35.9 22.9 31.4 41.1 62.0 6.0
7.2 7.1 6.8 6.3 6.9 6.9 7.1 5.7 6.2 4.9
19.6 0.8 3.4
24.3 4.8 12.8
105.8 33.3 22.0
6.7 3.5 5.1
0.2
1.1
26.5
5.0
Data Set B 17.7 11.9 0.7 8.3 9.8 2.8 2.7 1.3 7.3 5.6
The same model was applied to describe sorption to C18 disks, assuming that the C18 disks extracted only a negligible HOC fraction from soil:
dCdisk/dt ) k1, diskCsoil - k2,diskCdisk
(4)
Cdisk ) KdiskCsoil(1 - e-k2,diskt)
(5)
where Cdisk is the HOC concentration in C18 normalized to the C concentration of the disk (mg of HOC (kg of C18-C)-1) and the constants are the same as in eqs 1 and 2. At steadystate conditions, Kdisk is given by
Kdisk ) k1,disk/k2,disk ) Cdisk/Csoil
(6)
We showed that near-equilibrium conditions were reached in earthworm uptake experiments after 15 d of exposure (10). To determine the exposure time necessary to reach steadystate conditions for C18 disk extraction, we examined the kinetics of sorption to disk at 20 and 40 °C using one soil (experiment 1). On the basis of this experiment, we decided to extract the 25 soils for 15 d at 40 °C to determine the near-equilibrium Kdisk values in experiments 2 and 3. The 25 studied soils were divided into the two data sets, A and B. For data set A (11 soils), we derived equations to predict the BSAFs of each PAH and PCB in earthworms from their Kdisk values (experiment 2). With the help of the 14 soils of data set B, we validated the model by comparing predicted and measured BSAFs (experiment 3).
Materials and Methods Soils. We used 25 urban topsoils (0-5 cm) with a wide range of properties and PAH and PCB concentrations (Table 1). To determine sorption kinetics to C18 disks, we used soil A2. All 3932
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soils were air-dried and sieved to 1).
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Received for review March 19, 2001. Revised manuscript received June 21, 2001. Accepted June 27, 2001. ES010081E VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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