Environ. Sci. Technol. 1997, 31, 2944-2948
Interspecies Variability of the Plant/Air Partitioning of Polychlorinated Biphenyls PETER KO ¨ MP AND MICHAEL S. MCLACHLAN* Ecological Chemistry and Geochemistry, University of Bayreuth, 95440 Bayreuth, Germany
The plant/air partition coefficients (KPA) of a range of PCB congeners were determined in five different grass and herb species common to Central Europe [ryegrass (Lolium multiflorum), clover (Trifolium repens), plantain (Plantago lanceolata), hawk’s beard (Crepis biennis), and yarrow (Achillea millefolium)]. The measurements were conducted between 10 and 35 °C using a solid-phase fugacity meter. Large differences in the partitioning behavior between the plants were observed, with the partition coefficient varying by up to a factor of 20 between the five species. There was also considerable interspecies variability in the enthalpy of phase change (plant to air), but these differences were not related to the differences in the partition coefficients. A good linear relationship between log KPA and experimentally determined log KOA values (octanol/air partition coefficients) was obtained for each plant species (r 2 between 0.86 and 0.98). However, the slope of the regression lines ranged from 0.57 to 1.15. Using a solvent analogy, this indicates that the lipophilicity of the contaminant storage compartment is often different from octanol and varies widely among plants. The variability in the partition coefficients was primarily due to these differences in the quality of the lipophilic contaminant storage plant compartment, not to differences in its quantity or size. Most current models of organic contaminant accumulation in plants assume that the lipophilic storage compartment behaves like octanol, but this was not the case for any of the plants studied here. Before we can create models capable of accurately predicting concentrations of lipophilic trace organics in plants, more research into the nature of the plants’ contaminant storage properties is necessary.
Introduction The fate of persistent organic pollutants in terrestrial ecosystems has been attracting growing interest in recent years, in part because of the crucial role played by agricultural food chains in human exposure to many of these compounds. For instance, background exposure to polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) has been shown to be dominated by the pathway atmosphereffodderfcattlefcows’ milk/beef (1-5). Atmospheric deposition to pasture land and meadows is thus a key process in the environmental fate of SOCs, and experiments with ryegrass, an important agricultural grassland species in Central Europe, have shown that dry gaseous deposition is the primary mechanism of atmospheric deposition for many of these compounds (6). * Corresponding author fax: +49 921 55 2334; e-mail:
[email protected].
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Dry gaseous deposition is a diffusive process that can be understood as chemical partitioning between the gas phase and the vegetation, with chemical being deposited to bring the vegetation into equilibrium with the gas phase (7, 4, 5). Field studies, again with ryegrass, indicate that equilibrium is not approached for very involatile compounds due to slow uptake kinetics compared to the contaminant storage capacity of the vegetation (8). However, for persistent chemicals of intermediate volatility such as the higher chlorinated benzenes or the lower chlorinated biphenyls, an approximate equilibrium can be expected for many plant species (8). In those situations where the vegetation approaches a partitioning equilibrium, the plant concentration will be determined by the plant/air partition coefficient (8). The literature on plant/air partition coefficients (KPA) has borrowed heavily from aquatic environmental chemistry, postulating that KPA should be proportional to the octanol/ air partition coefficient (KOA) (7, 9, 10). This is a logical extension of the well-established relationship between water/ organic phase partitioning and the octanol/water partition coefficient (KOW). There is experimental evidence supporting this approach, obtained from laboratory experiments with azalea leaves (9, 10) and ryegrass (11), and, less directly, from studies with aquatic macrophytes (12) and isolated cuticles (13) in aqueous systems. Currently most predictive models of organic contaminant behavior in plants use a linear or near-linear relationship between KPA and KOA to describe the influence of physical/chemical properties on plant/air partitioning (8-10, 14). However, the experimental justification for this approach remains meager, especially given the importance of this process for contaminant accumulation in agricultural food chains and hence human exposure. In this paper, we present measurements of plant/air partition coefficients (KPA) for polychlorinated biphenyls in five grass and herb species that are frequently found in agricultural grasslands in Central Europe. The partitioning behavior in the five species is compared, and the suitability of KOA as a predictor for KPA is discussed. Since there is a strong temperature influence on the partition coefficient, the temperature dependence of KPA was also measured, allowing the calculation and discussion of enthalpies of phase change.
Experimental Section The plant/air partition coefficients for a range of di- to heptachlorinated PCB congeners (see Table 1 for a list of those congeners studied) in different plant species at different temperatures were investigated using the solid-phase fugacity meter method. The fugacity meter measures the concentration of a chemical in air at equilibrium with a solid surface. An airstream is passed through a glass column packed with, for instance, plant material in such a way that a chemical equilibrium between the surface of the vegetation and the air is established. After leaving the column, the SOCs in the air are trapped on an adsorbent cartridge which is later extracted and analyzed. If a contaminant equilibrium was present between the surface and the interior of the vegetation, the quotient of the volume-based concentrations in the vegetation and the air gives the plant/air partition coefficient. A detailed description of the fugacity meter method may be found elsewhere (11, 15, 16). Plant Species. Five herb and grass species common to agricultural grassland in Central Europe were employed: Italian ryegrass (Lolium multiflorum), white clover (Trifolium repens), ribwort plantain (Plantago lanceolata), rough hawk’s beard (Crepis biennis), and yarrow (Achillea millefolium). They were chosen to reflect a diverse range of leaf shape and surface morphology. Pure cultures of the plants were grown from seed outdoors in flower boxes.
S0013-936X(97)00141-7 CCC: $14.00
1997 American Chemical Society
TABLE 1. Plant/Air Partition Coefficients (KPA) and Enthalpies of Plant-to-Air Phase Change for PCB Congeners in 5 Grass and Herb Species ryegrass
d
clover
plantain
hawk’s beard
yarrow
PCB IUPAC no.
log KOAa (25 °C)
log KPA (25 °C)
∆HPAb (kJ/mol)
log KPA (25 °C)
∆HPAb (kJ/mol)
log KPA (25 °C)
∆HPAb (kJ/mol)
log KPA (25 °C)
∆HPAb (kJ/mol)
log KPA (25 °C)
∆HPAb (kJ/mol)
4+10 8+5 18 16+32 31+28 52 44 64 95 84+101 110 149 153 163c+138d 187 180
7.18 7.40 7.60 7.72 7.92 8.22 8.36 8.41 8.71 8.80 9.06 9.27 9.37 9.51 9.87 9.88
4.73 4.92 5.25 5.46 5.58 5.86 6.12 6.08 6.24 6.54 6.94 7.01 7.31 7.59 7.52 8.11
56.7 67.9 73.1 66.9 84.8 89.1 86.2 92.4 94.3 100.3 109.6 111.3 119.2 125.8 119.7 131.3
nde 5.39 5.34 5.75 5.58 5.77 5.84 5.92 6.27 6.45 nd nd nd nd nd nd
nd 74.8 77.7 77.4 86.0 89.3 91.8 96.9 99.7 104.2 nd nd nd nd nd nd
nd 5.18 5.31 5.51 5.62 5.84 5.89 5.98 6.32 6.47 nd nd nd nd nd nd
nd 81.5 86.7 86.0 89.2 92.2 96.6 96.6 100.6 103.4 nd nd nd nd nd nd
nd 5.42 5.57 5.65 5.93 6.06 6.13 6.20 6.34 6.53 6.75 6.62 6.87 7.16 nd nd
nd 84.9 87.3 86.2 90.1 93.0 94.4 94.4 99.2 100.6 105.7 107.2 107.6 112.2 nd nd
nd 6.22 6.07 6.14 6.33 6.33 6.44 6.49 6.69 6.76 7.03 6.94 7.23 7.29 nd nd
nd 64.3 62.2 70.2 81.2 94.0 95.4 99.3 85.7 102.9 95.1 104.4 105.7 109.3 nd nd
a Octanol/air partition coefficients were obtained from ref 17. b Enthalpy of the plant-to-air phase change ∆H . c Includes congener PCB 164. PA Includes congeners PCB 158 and PCB 160. e nd, not determined.
Plant Contamination. In this experiment, the vegetation was precontaminated with PCBs in order to facilitate the fugacity measurements. The plant cultures were placed in a contamination chamber containing high levels of gaseous PCBs (a unit mixture of Aroclors 1248, 1254, and 1260) as described in ref 15. After 3 days of contamination, the grass leaves were cut and allowed to equilibrate in a sealed vessel. Previous work has shown 20 h to be sufficient for the equilibration of PCBs in ryegrass (11). To be on the conservative side, 50 h was used for all plant species in this work. Fugacity Meter. The fugacity meter used is described in ref 15. The fugacity measurements were conducted at six temperatures between 5 and 50 °C for ryegrass (see ref 15), at four temperatures between 10 and 50 °C for plantain, and at three temperatures between 10 and 35 °C for clover, yarrow, and hawk’s beard. At all temperatures, a relative humidity of 100% was employed. Samples of the plant material were taken for analysis before the contaminated plants were packed in the sample chamber. The concentrations of the PCB congeners in the plants ranged from 3 to 3000 ng/g. The equilibrium air concentrations were determined by trapping the PCBs at the outlet of the sample chamber on a Florisil cartridge spiked with internal standards (six 13C12-labeled PCB congeners). A minimum of three air samples were collected and analyzed at each temperature. The air concentrations ranged between 1 pg/L and 2 ng/L and were in all cases at least 2 orders of magnitude below the vapor pressure. Analytical Methodology. The Florisil traps were eluted with n-hexane/diethyl ether (4:1). Plant samples (3-5 g fresh weight) were spiked with a known amount of a 13C12-labeled internal standard solution and extracted in n-hexane/acetone (1:1) by sonification. The plant extracts were then dried using Na2SO4 and cleaned up using Florisil and C18. All samples were concentrated to 30 µL and analyzed using HRGC (a Varian 3400 GC fitted with a 30 m J&W DB-5 MS column) coupled with MRMS (Finnigan MAT 8230 operating in the EI mode at 70 eV and a mass resolution of 2000). Two masses in the M+ isotope cluster were monitored for each analyte and each internal standard. KPA was calculated from the quotient of the volume-based concentrations in the plants and in air. The specific plant volume needed for this calculation was determined by water displacement.
Results and Discussion Demonstration of Plant/Air Equilibrium. The presence of an equilibrium between the air leaving the sample chamber
FIGURE 1. Log KPA (25 °C) vs log KOA (25 °C) for a range of PCB congeners in ryegrass. and the plant surface was verified for each species by varying the residence time of the air in the sample chamber. The absence of such an equilibrium is reflected in decreasing air concentrations with decreasing residence times (15, 16). At the maximum temperatures, it was verified that the air concentrations did not decrease when the residence time in the sample column was decreased beyond the value used to measure KPA. Reproducibility of the Measurements. A minimum of three fugacity (air) measurements were conducted at each temperature. The coefficient of variation (%) for a given compound and temperature ranged from 0.3 to 45%, lying under 15% in 84% of the cases. Three parallel plant samples were analyzed. The coefficient of variation (%) of the measurements ranged from 0.6 to 32%, lying under 15% in 94% of the cases. Mass balance calculations indicated that the amount of chemical removed with the airstream during the fugacity measurements did not influence the plant concentrations. This was confirmed by repeating the measurements at the first temperature point after all other points had been completed. The air concentrations from this final set of measurements fell within the range measured at the outset. In summary, the reproducibility of the measurements of KPA was very good. Influence of Plant Species on the Plant/Air Partition Coefficient (KPA). The measured KPA values (interpolated to 25 °C) for the five plant species are listed in Table 1. In Figure 1, the measured values of log KPA for ryegrass are plotted against experimentally determined log KOA values, both interpolated to 25 °C. The KOA values were measured in our
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TABLE 2. Results of the Linear Regression of log KPA vs log KOA at 25 °C for the Five Plant Species
a
plant species
Aa
Ba
r2
ryegrass clover plantain hawk’s beard yarrow
-3.56 0.15 -1.30 -0.07 1.80
1.15 0.70 0.87 0.74 0.57
0.98 0.86 0.98 0.97 0.93
Coefficients of the equation log KPA ) A + B log KOA.
FIGURE 2. Log KPA (25 °C) vs log KOA (25 °C) for a range of PCB congeners showing the linear regression lines for all five plant species. The regression coefficients are given in Table 2. laboratory using the fugacity meter method (see ref 17). An excellent correlation between log KPA for ryegrass and log KOA was found (r 2 ) 0.98). The linear regression is somewhat different than the one reported in ref 15, due to the fact that calculated and not experimentally determined values of KOA were used in the earlier paper. Excellent correlations between log KPA and log KOA were also obtained for the other plant species, with r 2 ranging from 0.86 to 0.98 (see Table 2). However, the partitioning behavior differs widely among the plants (see Figure 2). Whereas the regression lines converge for the pentachlorobiphenyls at a log KOA value of about 8.5, reflecting similar plant/air partition coefficients, they deviate from one another at lower KOA values, and the difference in KPA between ryegrass and yarrow for the dichlorobiphenyls at a log KOA of 7.2 exceeds an order of magnitude. These large differences in partitioning behavior at lower KOA values are of particular environmental significance. Compounds with log KOA values of less than 8 can be expected to approach a plant/air partitioning equilibrium (8). In this case, the plant concentrations are determined by the partition coefficients. Hence, the wide range in KPA values for compounds with low KOA should be reflected in the field in a similarly wide range in plant concentrations for different grassland species exposed to the same ambient air levels. On the other hand, for compounds with log KOA values of 9 or more, a partitioning equilibrium will not be approached due to the kinetic limitations imposed by the air-side resistance (the plant simply does not “see” enough air) (8). In this case, the KPA values, which are more similar between species, will have practically no influence on the plant concentrations in the field. It is notable that Buckley (18) found the ∑PCB concentrations in six plant species from the same site to range over a factor of 8. Information about the nature of the interspecies differences in partitioning behavior is given by the slopes of the regression lines in Figure 2/Table 2. They range from 0.57 for yarrow to 1.15 for ryegrass. A slope of 1 means that there is a linear relationship between KPA and KOA (not to be confused with the linear relationship between log KPA and log KOA in Figure 2). As has been argued by Schwarzenbach and Westall
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for aqueous systems (19), a slope of 1 indicates that octanol is a good model for the partitioning properties of the test solvent, whereas slopes deviating from 1 indicate that the lipophilicity of octanol differs from the lipophilicity of the test solvent, in this case vegetation. In no case was a slope of 1 obtained, and hence octanol is not a good model for any of the plants studied. This work contrasts with a considerable body of literature suggesting that octanol is a good model compound for the partitioning properties of plants. A linear relationship between KPA and calculated KOA values was reported for azalea (9, 10). In studies of enzymatically isolated cuticles of four terrestrial plants in aqueous system, it was found that the cuticle/water partition coefficients were approximately linearly related to the octanol/water partition coefficients, and that the partition coefficients did not vary widely between the four species tested (13). Octanol was also found to be a good model for the partitioning properties of PCBs in Myriophyllum spicatum, an aquatic macrophyte (12). However, evidence that this is not always the case is found in the earlier work of Briggs and co-workers, who measured partitioning between macerated barley shoots and water. They found a nonlinear relationship between the barley/water partition coefficients KPW and KOW, with KPW being proportional to KOW0.77 for macerated roots (20) and to KOW0.95 for macerated stems (21). In a similar study, Trapp and Pussemier found KPW proportional to KOW0.75 for cut pieces of roots and stems from bean plants (22). The reasons for the interspecies differences in KPA are not yet known. A disequilibrium artifact in the determination of KPA is one possible explanation. Although the presence of an equilibrium between the plant surface and the air in the fugacity meter was demonstrated for all plants, this does not rule out the possibility of an internal disequilibrium within the plant leaves. If the smaller, lower chlorinated PCBs were able to penetrate into the leaf storage compartments faster than the larger, higher chlorinated PCBs, an internal disequilibrium within the leaves would result in a greater underestimation of KPA for the higher chlorinated PCBs than for the lower chlorinated congeners, yielding a lower slope for the KPA vs KOA plot. A disequilibrium would also mean that KPA would increase with time in the fugacity meter as the contaminant diffused from the plant surface into the storage compartments. However, the repeat measurements of the first temperature point at the end of the fugacity measurements (see Reproducibility of the Measurements) showed no change in KPA over a time period of 4 days, indicating that an internal disequilibrium is an unlikely explanation for the differences in plant behavior. The variable slopes in Figure 2 show that the interspecies differences in KPA are not primarily due to the quantity or size of the lipophilic contaminant storage compartment in the plant, but rather to the quality or properties of the compartment. Returning to the solvent analogy of Schwarzenbach and Westall (19), ryegrass with a slope >1 behaves (qualitatively) like a more lipophilic solvent than octanol, whereas the other plants with slopes
