Controlled Exposure Chamber Study of Uptake and Clearance of

Nov 13, 2007 - Department of Environmental Toxicology, University of. California, 1 Shields Avenue, Davis, California 95616, Arizona. State University...
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Environ. Sci. Technol. 2007, 41, 7934-7940

Controlled Exposure Chamber Study of Uptake and Clearance of Airborne Polycyclic Aromatic Hydrocarbons by Wheat Grain REIKO KOBAYASHI,† THOMAS M. CAHILL,‡ ROBERT A. OKAMOTO,§ RANDY L. MADDALENA,| AND N O R M A N Y . K A D O * ,†,§ Department of Environmental Toxicology, University of California, 1 Shields Avenue, Davis, California 95616, Arizona State University, West Campus, P.O. Box 37100, Phoenix, Arizona 85069, California Air Resources Board, 1001 “I” Street, P.O. Box 2815, Sacramento, California 95812, and Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 90R-3058, Berkeley, California 94720

Polycyclic aromatic hydrocarbons (PAHs) can partition from the atmosphere into agricultural crops, contributing to exposure through the dietary pathway. In this study, controlled environmental chamber experiments were conducted to investigate the transfer of PAHs from air into wheat grain, which is a major food staple. A series of PAHs ranging in size from naphthalene to pyrene were maintained at elevated gas-phase concentrations in the chamber housing mature and dry wheat grain both on the plant and with the husk removed. The PAHs did not achieve equilibrium between the air and grain over the 6.5 month monitoring period used in this study. Therefore, PAH uptake under field conditions is expected to be kinetically limited. A clearance study conducted for the grain showed the half-life of clearance was approximately 20 days for all compounds studied. The results suggest that atmospheric contaminants that partition into grain may remain in the grain long enough to contribute to dietary exposure for humans. Mass transfer across the air/grain interface appeared to be limited by grain-side resistance. The grain may act as a multicompartment system with rapid exchange at the surface followed by slower transfer into the grain. A grain/air concentration relationship was derived for the uptake time that is relevant to field conditions.

Introduction Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants that are emitted primarily from combustion sources. A number of PAHs are considered a health risk since they are mutagenic and/or carcinogenic to mammals (1-4) and bioavailable through inhalation, ingestion, and dermal contact (4). One potential route for human exposure to PAHs is through ingestion. Measurements of different food items have * Corresponding author phone and fax: +1 530 752 2457; e-mail: [email protected]. † University of California, Davis. ‡ Arizona State University at the West Campus. § California EPA, Air Resources Board. | Lawrence Berkeley National Laboratory. 7934

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shown that broiled meats, leafy vegetables, and unrefined grains had elevated concentrations of PAHs (5, 6). Although broiled meat typically has the highest concentrations of PAHs, grains contribute a large part of PAH intake in the average diet because of higher intake rates of grain-based products (5, 6). PAHs in raw grains and vegetables are thought to be the result of atmospheric rather than soil sources because of the hydrophobic nature of these compounds (7). Plant uptake studies for airborne semivolatile compounds such as PAHs have been conducted in controlled environments to investigate relationships between concentrations in air and in terrestrial plants (8-11). These studies have largely focused on nonedible plant leaves to incorporate vegetation into environmental fate models, whereas controlled studies on edible grains for human exposure risk assessment are limited. The objective of this research was to assess the partitioning of gas-phase PAHs into agricultural grains. Wheat was used because it is widely consumed and recent data have indicated potential for atmospheric deposition of PAHs into wheat grain (12). Mature and dry wheat grain, both with and without husks, were placed in a chamber where PAHs were maintained at constant elevated concentrations. The concentrations of PAHs in the air and grain were monitored for 196 days. A second experiment monitored the clearance of PAHs from contaminated grain in a “clean” atmosphere. The results were used to calculate the overall air/grain mass transfer coefficients for PAHs and the expected concentration ratios between grain and air for an exposure duration that is relevant to field conditions.

Materials and Methods Uptake and Clearance Chambers and PAH Generation Systems. Two identical plant growth chambers (model PGR15, Conviron, Canada) were located in the Controlled Environment Facility (CEF) at the University of California, Davis, for the uptake and clearance studies. Temperature was maintained at 20 °C in the chambers, which is about the average temperature in California when grain maturation occurs. The relative humidity ranged from 30 to 70% in the chambers. The chamber floor space was 1.4 m2, and the height was 1.68 m. For the exposure chamber, room air in the CEF was drawn directly through carbon-impregnated polyurethane foam into the temperature-controlling compartment of the chamber. For the clearance chamber, the air was drawn through a HEPA filter and a granular activated-carbon filter (3 kg carbon loading/m2) in series before entering the carbonimpregnated polyurethane foam to remove background PAHs. The conditioned air circulated in the chamber (25 m3/min), entering through the floor apertures and exiting near the ceiling (Figure 1). The air exchange rate with outside air was 0.56 air changes/min for both chambers. Uptake and clearance studies were conducted under dark conditions. Naphthalene, 1-methylnaphthalene, 2-methylnapthalene, 2,6-dimethylnaphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene (Sigma-Aldrich St. Louis, MO) were used for this study. These were the most commonly detected PAHs in California wheat from an earlier study (12) and are more prevalent in the gas phase than the particle phase of the atmosphere (13, 14). Fluorene, phenanthrene, anthracene, fluoranthene, and pyrene were introduced into the uptake chamber air through a 1 m long stainless-steel column (1.27 cm diameter) filled with PAH-coated glass beads (1 mm diameter). A small amount of pure chemicals was placed at the front part of the column, and both ends were plugged with glass wool. The 10.1021/es071459x CCC: $37.00

 2007 American Chemical Society Published on Web 11/13/2007

FIGURE 1. Uptake chamber with PAH generation system, plants, and air-monitoring equipment.

column was placed inside the chamber and connected to a nitrogen cylinder (Figure 1). The column flow rate (346 mL/ min) was sufficient to elevate gas-phase concentrations more than 10-fold above background while still saturating the nitrogen exiting the column with these compounds. The rest of the PAHs were delivered into the chamber air by diffusion vials. Each 3.7 mL amber glass vial was filled one-quarter full with each pure PAH. A 9 mm hole in the vial cap allowed PAHs to diffuse into the chamber air at a rate controlled by the diffusion resistance of the air inside the vial. The PAH generation column and diffusion vials were placed near the edge of the chamber below the air circulation intake slot. A divider wall made of Tedlar PVF film (70 cm wide × 67 cm high) was placed between the PAH generation systems and wheat plants to facilitate the homogeneous distribution of PAHs in the chamber (Figure 1). Due to the rapid air circulation and the divider wall, PAH levels measured inside the chamber did not vary by location. Wheat Plants and Analysis. Wheat plants (hard-red spring wheat) were grown in pots in the clearance chamber until they matured and dried out. The soil surface was covered by a thin layer of white sand to minimize PAH sorption to the soil, and the plant containers were moved to the uptake chamber to start the uptake study. In addition, grain of the same cultivar that had been removed from the plant and the husk (bare grain) were introduced in the uptake chamber to better understand the controlling parameters for uptake kinetics for the grain itself. The bare grain was obtained from a wheat farm in Los Banos, California. The bare grain was one to two layers deep in stainless-steel screen baskets that were elevated 5 cm above the chamber floor to allow air to mix below and flow through. The vertical air velocity as measured by an anemometer ranged from 0.1 to 0.5 m/sec below the baskets and from 0.1 to 0.3 m/sec above the baskets. Above the wheat plants, the air was so turbulent that the vertical velocity could not be measured. The concentrations in the grain for the mature wheat plants (husk grain) and bare grain were monitored for 196 days. After 35 days of the uptake study, a portion of bare grain was moved to the clearance chamber where grain concentrations were monitored for 56 days. Approximately 10 g of grain consisting of 3 to 4 plants was used for each measurement. Grain was carefully removed

from husk by tweezers working on a chilled stainless-steel dish to minimize volatilization losses. PAH analyses were conducted mainly by isotope dilution using naphthalene-d8, acenaphthylene-d8, acenaphthened10, fluorene-d10, phenanthrene-d10, anthracene-d10, fluoranthene-d10, and pyrene-d10 (Cambridge Isotope Laboratories, Andover, MA) as internal standards. Analytes that did not have a deuterated analogue were quantified off an internal standard with a similar response. Grain samples were spiked with the internal standards and extracted in 50 mL dichloromethane by a horizontal shaker (Lab-Line Instruments, Melrose Park, IL) for 24 h at 110 rpm. Extraction was repeated one more time for 24 h with another aliquot of dichloromethane. The combined extract was filtered through sodium sulfate, concentrated under nitrogen stream at 35 °C (TurboVap II, Zymark, Hopkinton, MA), solvent-exchanged to hexane, and then reduced to 0.5 mL of volume. The hexane extract was loaded onto a glass column filled with 3 g of silica (100-200 mesh) saturated with hexane. The silica gel was eluted with 9 mL of hexane followed by 25 mL of a hexane/dichloromethane mixture (9:2 v/v). The second fraction was collected and concentrated to 0.75 mL of volume for analysis. Analysis was conducted by gas chromatography/mass spectrometry (GC/MS) using a Hewlett-Packard (HP) 5890 Series II gas chromatograph interfaced to a HP5972 mass selective detector run in selective ion monitoring mode. The injector was operated in splitless mode. The GC was equipped with a DB-5ms fused silica capillary column (30 m × 0.25 mm i.d., 0.25 µm film thickness). PAH standard reference material (SRM) 2260 (National Institute of Standards and Technology, Gaithersburg, MD) was used to prepare calibration solutions. Biphenyl-d10 and p-terphenyl-d14 were added just prior to analysis to measure recoveries of internal standards. The recoveries were approximately 50% for naphthalene-d8, 80% for acenaphthylene-d8, acenaphthene-d10, and fluorene-d10, and over 90% for the rest of the compounds. The isotope dilution compensated for recoveries, so no further corrections were made. At least one process blank was run for every three samples, and the reported results were corrected for the matched blank. Concentrations of 300 ng/kg in grain or higher were quantified for all compounds. The blank concentrations of naphthalene and 2-methylnaphthalene were significant compared to the sample concentrations during the initial period of the uptake study and naphthalene during the entire period of the clearance study. The data in the results were flagged when the blank concentration exceeded 20% of the measured sample value. The water content of grain was determined before and after the uptake study by measuring the weight loss of the grain from drying at 110 °C for 24 h. Lipid content was determined by extracting the grain with a mixture of hexane and acetone following the method in ref 15. Grain density was obtained from the weight and volume of the grain measured by water displacement. The average surface area and volume of the grains were estimated from measurements of grain length and width, assuming that the grain shape was that of a prolate spheroid. Air Sample Collection and Analysis. PAH concentrations in air just above the wheat canopy were monitored in the chambers throughout the uptake and clearance studies. Total suspended particulates were collected by 47 mm PTFE membrane filters (2.0 µm), and gas-phase PAHs were collected by 1 g of precleaned XAD-4 adsorbent filled into a 10 cm long stainless-steel tube (4.8 mm diameter). For gas-phase PAHs, approximately 3 m3 of air in the uptake chamber and 10-50 m3 of air in the clearance chamber was sampled each time. For particle associated PAHs, 25-260 m3 of air was collected per filter. VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The same internal standard solution as for grain analyses was used for filter and XAD analyses. A filter was spiked with the internal standard and extracted in dichloromethane by sonication (Branson Ultrasonics, Danbury, CT) three times for 15 min each time period. The combined extract was concentrated to 0.75 mL under a nitrogen stream at 35 °C prior to GC/MS analysis. The XAD was transferred to a separatory funnel, spiked with the internal standard, and extracted in dichloromethane three times. The extract for GC/MS analysis was filtered through a PTFE membrane filter (0.45 µm) and concentrated under a nitrogen stream to 0.75 mL of volume. Eleven blank XAD samples were extracted throughout the study, and the results were corrected for blank levels. Validation of Method. Extensive validation was conducted on the methodology. To evaluate extraction efficiency, the extraction method was compared to Soxhlet extraction with grain that had been ground, and the results were similar. For air sampling, initial samples were equipped with a backup XAD cartridge to determine that no significant breakthrough occurred.

Results PAH Concentrations in Chamber Air. All PAHs except for 2,6-dimethylnaphthalene and acenaphthene were maintained at elevated and stable levels in air throughout the uptake study. Coefficients of variation for the measured air concentrations ranged between 8 and 32% depending on compounds (Table S1 in Supporting Information). The diffusion vials did not supply enough 2,6-dimethylnaphthalene and acenaphthene to the uptake chamber air, so these results are not reported. The PAH concentrations in the particle phase were less than 1% of the gas-phase concentrations for all compounds in both chambers. The total concentration of five PAHs (fluorene, phenanthrene, anthracene, fluoranthene, and pyrene) in the uptake chamber was 1250 ng/m3. The concentration ranges of the sum of seven PAHs (fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, and triphenylene) reported in North America, Europe, and Asia are from 0.05 to 1158.5 ng/m3, 0.1 to 353.6 ng/m3, and 10.5 to 11 700 ng/m3, respectively (16). Given that chrysene and triphenylene are generally in much lower atmospheric concentrations than phenanthrene or fluorene, the air concentrations in the uptake chamber were similar to those observed in the field. Grain Characterization. Water contents (mass fraction) for bare grain before and after the uptake study were 8.6 ( 0.7% (average ( standard deviation, n ) 3) and 11 ( 0.06% (n ) 3). For husk grain, they were 9.4 ( 0.5% (n ) 3) and 11 ( 0.06% (n ) 3). Grain lipid contents (mass fraction) for bare grain and husk grain were 1.2 ( 0.1% (n ) 5) and 1.3 ( 0.07% (n ) 5), respectively, on a dry weight basis. The densities of bare grain and husk grain were 1.4 ( 0.08 g/cm3 (n ) 4) and 1.3 ( 0 g/cm3 (n ) 3), respectively. The bare grains had a length of 0.77 ( 0.06 cm and a width of 0.34 ( 0.05 cm (n ) 24), giving 0.69 cm2 and 0.047 cm3 as average surface area and volume. For the husk grains, the length was 0.72 ( 0.08 cm and the width was 0.31 ( 0.05 cm (n ) 24), giving 0.59 cm2 and 0.036 cm3 as average surface area and volume. Any apparent changes in grain such as fungal infection were not observed during the experiments. No further analyses for change in grain composition over time, such as types of proteins, lipids, and carbohydrates, were conducted. However, considering the low moisture content of grain and the low ambient temperature that are comparable to those used during the storage of grain (17), we assume that any change in grain composition was negligible. Uptake Study with Bare Grain and Husk Grain. The results from naphthalene, fluorene, and pyrene, selected to represent the higher, intermediate, and lower volatility PAHs 7936

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FIGURE 2. Naphthalene, fluorene, and pyrene uptake by bare grain. (b) Grain concentrations (CG) normalized to air concentrations (CA) (O marks indicate that the sample levels were less than 5 times the matched blank level). (×) Air concentrations in ng/m3. (---) Average air concentration in ng/m3. in this study, are shown in Figure 2 for bare grain and Figure 3 for husk grain. At the end of the uptake study, grain concentrations for naphthalene, fluorene, and pyrene reached 3.0 µg/kg (4.3 µg/L of grain), 120 µg/kg (160 µg/L of grain), and 5.7 µg/kg (8.0 µg/L of grain), respectively, for bare grain. For husk grain, they were 1.4 µg/kg (1.8 µg/L of grain), 32 µg/kg (41 µg/L of grain), and 1.9 µg/kg (2.4 µg/L of grain), respectively. Results for the other compounds are shown in the Supporting Information. The uptake trend was clearly observed for all compounds except for anthracene in husk grain due to low measured concentrations. The uptake was slower in husk grain than bare grain, indicating that the husk slows the rate of PAH uptake by increasing the resistance to diffusion. Concentration measurements for the husk grain were more variable than those for the bare grain. This may be partly due to the morphological variance in husk structure. Also, the husk removal process may have contributed to increased analytical variance. Analytical error may be significant for naphthalene because of low grain concentrations relative to blank levels. For the bare grain, all compounds were still increasing in concentration at the end of the study. In the husk grain, slower uptake rates and more scatter in the data points make it difficult to judge whether PAHs in the husk grain were in

FIGURE 4. PAH clearance in bare grain where grain concentrations are normalized to the concentrations at the start of the clearance study

Discussion Model Interpretation of PAH Uptake in Grain. A model that has been applied to the air-leaf system (8, 10, 18, 19) was applied to the air-grain system by assuming that grain behaves as a single homogeneous compartment for the uptake of gas-phase PAHs and that the loss processes of PAHs in grain, other than diffusion to the air, are negligible. The model is given by

(

CG dCG ) kAGaAG CA dt BCF

)

(1)

where CG is the concentration in the grain (mol/m3), aAG is the grain surface area normalized to grain volume (m2/m3), t is time (h), BCF is the grain/air bioconcentration factor ((mol/m3)/(mol/m3)), and kAG is the overall air-grain mass transfer coefficient referenced to the air (m/h) (20, 21). aAG was assumed to be constant since the experiments were conducted on matured and dried grain. kAG values were calculated from the initial uptake phase of the experiment where CG/BCF in eq 1 is negligible such that eq 1 reduces to

dCG ) kAGaAGCA dt FIGURE 3. Naphthalene, fluorene, and pyrene uptake by husk grain. (b) Grain concentrations (CG) normalized to air concentrations (CA) (O marks indicate that the sample levels were less than 5 times the matched blank level). (×) Air concentrations in ng/m3. (---) Average air concentration in ng/m3. equilibrium with air after 196 days of exposure. However, the grain composition for the husk grain and the bare grain is thought to be similar since they are from the same cultivar, and measured water and total lipid concentrations were similar, so the equilibrium concentrations are expected to also be similar, indicating that uptake by husk grain is still occurring over the duration of this study. Given that the exposure duration for wheat grain in the field is approximately 1 month from grain formation to harvest, PAH uptake under field conditions is expected to be kinetically limited and to not attain equilibrium concentrations. Clearance Study with Bare Grain. The clearance of naphthalene, fluorene, and pyrene from the bare grain is shown in Figure 4 by normalizing grain concentrations to the concentrations at the start of the clearance experiment. The other compounds are shown in the Supporting Information. Fluorene and pyrene showed a similar trend. Naphthalene did not have a clear trend due to the large variability in the grain concentrations. The scatter in the naphthalene data was likely due to the relatively low concentrations in grain where the levels of naphthalene in the grain were less than 5 times the matched blank levels for many data points of the naphthalene clearance experiment.

(2)

The calculated kAG values are given for both bare grain and husk grain in Table 1. At the initial phase of clearance, eq 1 is reduced to eq 3 by assuming that CA is negligible.

CG kAGaAG ln )t CG0 BCF

(3)

where CG0 is grain concentration at the start of the clearance. Good linear correlation between ln(CG/CG0) and t for the initial 22 days was observed with a R2 value ranging from 0.85 to 0.95 for all PAHs except for naphthalene, for which a linear regression was not conducted due to low concentrations relative to the blank levels. The slope factors were similar among compounds (Table 1). The average was 1.5 × 10-3 (h-1) with a standard deviation of 2.1 × 10-4 (h-1), giving 20 days as the average clearance half-life. Primary Resistance to Diffusive Mass Transfer. According to the two resistance model, kAG includes mass transfer at the air side of the interface (kAS) and the grain surface side (kSG) as follows (18-21, 24):

1 1 1 ) + kAG kAS kSG‚BCF

(4)

When mass transfer is limited by grain-side resistance (i.e., 1/(kSG‚BCF) . 1/kAS), then kAG would be proportional to BCF. In this case, kAG would increase for compounds with a higher VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Estimated Overall Air-Grain Mass Transfer Coefficients Referenced to the Air (kAG) and Slope Factors from the Clearance

compounds naphthalene 2-methylnaphthalene 1-methylnaphthalene fluorene phenanthrene anthracene fluoranthene pyrene

log KOAa (20 °C) 5.30b

kAG (m/h)e bare grain

husk grain

slope factor from clearance in baregrain (h-1)f

d

d

d

5.73b

5.3 × 10-3

d

1.6 × 10-3

5.80b

6.7 × 10-3

4.8 × 10-3

1.3 × 10-3

7.13c 7.90c 6.97b 9.12c 9.16c

7.0 × 10-2 1.7 × 10-1 4.7 × 10-1 9.4 × 10-1 1.2

2.1 × 10-2 8.5 × 10-2

1.2 × 10-3 1.4 × 10-3 1.3 × 10-3 1.7 × 10-3 1.7 × 10-3

d

4.5 × 10-1 5.2 × 10-1

aK b Temperature corrected OA ) octanol-air partitioning coefficients. to 20 °C from: log KOA(20 °C) ) log KOA(25 °C) + ∆Hvap / 2.303R (1 / 293 - 1 / 298). KOA(25 °C) was calculated from selected values given in ref 14. ∆Hvap is enthalpy of vaporization (kJ/mol) from ref 22. c Measured by Harner and Bidleman (23). d Values not calculated because measured concentrations were too low. e Overall air-grain mass transfer coefficient. f Slope of linear relation between ln(CG/CG0) and time (h) observed in clearance where CG ) grain concentration and CG0 ) grain concentration at the start of clearance.

octanol-air partitioning coefficient (KOA) since log BCF and log KOA are expected to be proportional (19, 24). This is seen in the results for both bare grain and husk grain. Considering the clearance data, if the grain-side resistance is limiting mass transfer, then the slope factor in eq 3 would be approximately kSGaAG and independent of BCF given that kAG is approximated by kSG‚BCF. On the contrary, if air-side mass transfer is limiting, the slope factor would be inversely related to BCF, resulting in large differences among compounds. The slope factors in Table 1 are similar across compounds, indicating that for bare grain the overall mass transfer rate for air-to-grain transport of PAHs is mainly limited by grain-side resistance. For husk grain, the observed kAG values were smaller than those for bare grain, which is likely due to the additional resistance to diffusion created by the husk where the diffusion path length in the air side is considerably longer and the area of diffusion is considerably limited by the presence of the husk. This indicates that for the husk grain the contribution from the air-side resistance may also be significant as well as for the grain side. Barber et al. reported that for PCB uptake by plant leaves that air-side resistance is significantly reduced by wind speed (25). The wind speed measurements in the chambers used in this study were approximately 0.1-0.5 m/sec, which is typical of calm field conditions. Earlier work found that the air-side resistance to mass transfer was dominant for the uptake of semivolatile compounds to leaves (18, 19) although studies with PCBs found plant-side resistance can also have an important effect (25, 26). If mass transfer is limited by grain-side resistance, then the uptake and clearance would be slower for grain than for leaves provided that the air boundary layer over grain and leaves are similar. Mass transfer coefficients for uptake of gas-phase PAHs into bell pepper leaves were determined to be between 1.8 and 4.4 m/h, which are 3.7-11 times higher than the estimates for the bare grain and 8.5-21 times higher than the estimates for the husk grain (10). Reported clearance half-lives for PCBs in ryegrass are in the range of 0.9 days for dichlorobiphenyls to 6 days for octachlorobiphenyls (27). A clearance study on evergreen shrub leaves and PCBs ranging from trichloro- to octachlorobiphenyls has shown that the clearance half-lives were less than 1.5 h to 10 days for all compounds except for one tetrachlorobiphenyl compound (>38 days) (26). These values are generally much smaller than those found in this study for PAHs in grain (20 days). 7938

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Slower mass transfer rates between air and grain relative to the air-leaf interface mean contamination levels in grain may not become as high as those in leaves when placed in the same environment for the same amount of time. However, once chemicals are taken up into the grain they can remain there for a much longer period of time relative to that of leaves. Calculated Grain/Air BCFs. Since PAH uptake did not appear to reach equilibrium during the study period (196 days), BCFs could not be determined directly from grain/air equilibrium concentration ratios. However, if eq 1 holds, then the BCFs can be calculated from kAG and the slope factors during the initial phase of clearance (Table 1). BCFs thus calculated for 1-methylnapthalene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene were 7.3 × 103, 8.4 × 104, 1.9 × 105, 5.5 × 105, 7.9 × 105, and 1.0 × 106. These values are much smaller than the CG/CA ratios actually observed in the uptake studies. For example, the calculated BCF for pyrene is similar to the CG/CA ratio observed at 40-50 days of uptake in bare grain (Figure 2). This brings into question the relevance of eq 1 for the air-grain system. Equation 1 assumes that the loss of PAHs from grain is only by diffusion to air and that the grain acts like a single well-mixed compartment. Biotransformation and direct photolysis are expected to be negligible because the experiments were conducted on dry plants under dark conditions. Reactions with atmospheric oxidative species should also be minimal because the intake air to the chamber came from indoors and was preconditioned through carbon-impregnated polyurethane foam, so concentrations of reactive species should be low. In addition, monitoring data shows that air concentrations of PAHs in the uptake chamber were at a steady state, so the assumption that diffusion is the dominant loss pathway seems valid. However, the uptake curves in both bare grain and husk grain appeared to be considerably steeper in the initial phase than in the later phase. This is similar to the results seen earlier with PCB uptake into leaves where a fast reacting surface compartment and a slow reacting inner compartment were suggested (2527). If the grain is comprised of multiple interconnected layers, then eq 1 would apply only to the initial phase of uptake into the surface layer. If we assume that during the initial phase of uptake the PAH is primarily in the surface layer with volume VGS and F is the volume fraction (VGS/VG), then equals F, then the concentration in the grain is given by

(

dCG 1 aAG CG 1 ) kAG CA dt F F BCF F

)

(5)

If CG/(BCF‚F) is negligible at the initial phase of uptake, eq 5 becomes eq 2 from which kAG values in Table 1 were estimated. However, if F is very small, then the assumption that CG/(BCF‚F) is negligible may not be valid, leading to uncertainties in the estimates of kAG values, particularly for compounds with lower KOA values. Likewise, when CA is assumed negligible for the clearance, eq 5 is reduced to

CG kAGaAG 1 ln )t CG0 BCF F

(6)

which shows that the actual BCF value would be much higher than the value calculated by assuming a single well-mixed compartment. Unfortunately, there is no way to estimate F, so the actual BCF values for PAHs in grain cannot be calculated directly. Field Relevant Grain/Air BCFs. Since the uptake of gasphase PAHs into grain under field conditions is kinetically

the UC Davis Jastro Shields Research Award, and the U.S. Department of Energy Fossil Energy Program through Grant No. DE-AC02-05CH1123 to Lawrence Berkeley National Laboratory. We thank Eve Kwan, Yvonne Ho, and Paul Kuzmicky for the laboratory assistance.

Supporting Information Available Chamber air concentrations and uptake and clearance curves for the rest of the compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited FIGURE 5. Logarithm form of grain/air concentration ratios (mol/ m3)/(mol/m3) for husk grain at day 30 of uptake plotted against log KOA values estimated for 20 °C. limited, grain/air concentration ratios (CG/CA) for a realistic time frame may be more useful than an equilibrium-based expression such as BCF for predicting the grain contamination in support of human exposure assessment. The CG/CA values at day 30 of uptake for husk grain, representing typical field conditions, were determined for this purpose assuming that CG/CA values at day 30 in the real environment where grain changes in size and water content during these 30 days are similar to the CG/CA values obtained from this study that used dry wheat from the beginning of the exposure. The log-transformed concentration ratios, log(CG/CA) at day 30 of uptake for husk grain, were plotted against log KOA values (Figure 5). Although the system was not at steady state, a good linear relationship was observed. A similar finding has been reported for the nonequilibrium state of PCB uptake by leaves (25). The slope of the linear relationship for husk grain was much smaller than unity (0.63), which may partly indicate that compounds with higher log KOA values were more strongly limited by kinetics because of their higher BCF values. The observed linear relationship may apply to other PAHs with KOA values in a similar range. Although the log CG/CA vs log KOA relationship may be affected by environmental factors such as temperature, wind speed, atmospheric concentrations, and wheat cultivar, the data in Figure 5 were collected at a moderate temperature under realistic air concentrations and the chemical transport is thought to be largely limited by the grain side (the influence of wind speed may be minor). Therefore, the regression presented in Figure 5 provides an initial estimate of a field grain concentration at a given air concentration for gas-phase PAHs. Grain concentrations may change during harvest, transportation, and storage depending on the concentration of ambient air PAH in contact with the grain. However, grain that was harvested using a diesel-fueled harvester had PAH concentrations similar to those for grain collected by hand, indicating that the combination of slow uptake kinetics and short exposure duration to exhaust during harvest did not influence the concentration in the grain (12). Furthermore, the clearance half-life of approximately 20 days and kinetically limited uptake found in this paper indicate that the field contamination of grain can remain long enough to contribute to dietary exposure. A dynamic model that accounts for the kinetics of grain uptake would be desired to better predict the grain concentration at harvest. Further, studies on the fate of these atmospherically derived PAHs in grain as it is processed and then cooked are needed to relate field concentrations to human exposure.

Acknowledgments This study was supported by UC Toxic Substances Research & Teaching Program, the UC Davis Ecotoxicology Program,

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Received for review June 15, 2007. Revised manuscript received September 4, 2007. Accepted September 7, 2007. ES071459X