Sorptive Bioaccessibility Extraction (SBE) of Soils ... - ACS Publications

Aug 30, 2012 - Varvara Gouliarmou , Chris D. Collins , Ellen Christiansen , and Philipp Mayer. Environmental Science & Technology 2013 47 (2), 941-948...
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Sorptive Bioaccessibility Extraction (SBE) of Soils: Combining a Mobilization Medium with an Absorption Sink Varvara Gouliarmou and Philipp Mayer* Department of Environmental Science, Aarhus University, PO Box 358, 4000 Roskilde, Denmark S Supporting Information *

ABSTRACT: In principle, soil bioaccessibility extraction methods are simple dissolution experiments, where the fraction of compounds that is transferred to the extraction medium is measured and considered to be bioaccessible. For hydrophobic organic chemicals (HOCs) such techniques can lead to underestimation of bioaccessibility when the capacity of the extraction medium is insufficient to provide infinite sink conditions for the target compounds. A sorptive bioaccessibility extraction (SBE) method was thus developed and validated, which integrates the key processes of desorption from the matrix and subsequent consumption or depletion. Cyclodextrin was used as a diffusive carrier to enhance desorption from the matrix, while a silicone rod was used as a dominating sink that continuously absorbed the HOC molecules from the cyclodextrin solution. The silicone rod was then solvent extracted and the HOCs measured by GC-MS. For wood soot, the SBE method yielded PAH bioaccessibility estimates that were 3−24 times higher compared to a cyclodextrin extraction without a sink. The study demonstrated that the inclusion of an absorption sink into an established bioaccessibility extraction method (1) is rather simple, (2) can have a major impact on the obtained results, especially for the more hydrophobic compounds and (3) can simplify the analytics.

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INTRODUCTION Sorption, entrapment, and desorption are often the controlling processes for the environmental mobility, partitioning, biouptake, and toxicity of soil pollutants.1−5 This makes sorption crucial for environmental and human risks, and also for various remediation technologies, where it can limit the substrate supply for degrader microorganisms and the mass transfer of pollutants to plants during phytoremediation.1−3 Risk assessment and management of soil pollution need thus to take sorption into account. When approaching this challenge from the perspective of the biological target organism, this requires exposure parameters that go beyond total concentrations. Freely dissolved concentration and chemical activity can be measured and applied to study and predict partitioning processes, whereas bioaccessibility can be used to determine the substrate availability for depletive processes.6 The present study focuses on the bioaccessibility parameter, which quantifies the mass of contaminant molecules in the soil that either are or can become mobilized. Thus it characterizes the pool of contaminants that can become available for biological uptake and degradation.6,7 Over the last decades, many methods to assess bioaccessibility have been developed within the areas of microbial biodegradation8,9 and digestive uptake by for example earthworms.10 Most of these methods are mild extraction techniques and aim at simulating or accelerating the mobilization of the bioaccessible pollutant fraction without attacking the soil matrix. The two determining steps for the pollutant transfer © 2012 American Chemical Society

from matrix to organism are (1) mobilization from the matrix and (2) the subsequent consumption or depletion. It is important to realize that the dynamics of the first mobilization step is strongly influenced by the second depletion step. Without such a depletion process, the capacity of the extraction medium needs to be much higher than that of the sample in order to ensure infinite sink conditions and avoid underestimations of bioaccessibility.11 Sequential extractions can be applied to deal with this problem,11−13 but they are laborious and cost time, materials and multiply the number of analysis. The challenge of integrating sorption into the risk assessment and management of soil pollution can also be approached from the perspective of the matrix rather than the target organism. Cornelissen and co-workers determined desorption rate constants for the fast desorbing fraction and the slow desorbing fraction.14,15 Variations of this approach are the separation into readily desorbing and desorption resistant fractions,16,17 and the characterization and application of complete desorption curves.18 All these approaches require desorption experiments, which are operated under infinite sink conditions in order to maintain a chemical activity gradient for the desorption process. The mixing of adsorption particles (e.g., Tenax) into soil and sediment suspensions is a very efficient approach to ensure such Received: Revised: Accepted: Published: 10682

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infinite sink conditions, and desorption can then be deduced from the pollutants measured on the adsorption particles.14,15,19−21 This Tenax method provides high analytical sensitivity, but has also some disadvantages related to the phase separation of Tenax from the soil suspension.12 The more recently developed contaminant trap method provides also infinite sink conditions, but without requiring a phase separation step.22 In this method, a silicone elastomer containing activated carbon serves as infinite sink and cyclodextrin is applied to ensure efficient mass transfer from the soil suspension to the sink. While the contaminant trap appears very practical and efficient for the isolation and quantification of the desorption resistant fraction, it seems difficult if not impossible to back-extract the pollutants from the trapping phase.22 There is therefore a need for a desorption approach that (1) provides (near) infinite sink conditions, (2) requires no phase separation steps, and (3) facilitates the chemical analysis. In the current study, a method that combines a mobilization medium and a new absorption sink is proposed. Silicone was selected as the absorption polymer due to its well-known partitioning properties and very low internal diffusive resistance for hydrophobic organic chemicals.23 A low internal diffusive resistance within the polymer is crucial during bioaccessibility extraction and the subsequent solvent back-extraction of mobilized pollutants. Several silicone formats were considered including thin silicone coatings,24 silicone cast at the bottom of glass vessels, silicone O-rings25 and thin silicone sheets.23,26 However, the most practical format appeared to be the silicone rod, which is also used in various silicone rod extraction methods.27−29 Silicone rods are available at a moderate price, allow practical sampling with a large polymer volume, it is easy to physically clean their surface and allow quantitative back extraction of the absorbed compounds.29 The first aim of the study was to develop and dimension a sorptive bioaccessibility extraction (SBE) method, which ensures quantitative absorption of the mobilized HOCs within a practical time span. The second aim was to evaluate the performance of the method by applying it to wood soot, which was selected as an environmentally relevant and worst-case material for bioaccessibility extractions. The results were then compared to a cyclodextrin extraction method.8 The underlying working hypothesis of this study was that for high capacity samples (i.e., high Kd value) it is necessary to incorporate a high capacity absorption sink into bioaccessibility extraction methods. Otherwise the desorption gradient might not be maintained, and bioaccessibility then underestimated.

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EXPERIMENTAL SECTION

Materials. The following polycyclic aromatic hydrocarbons (PAHs) were selected as model HOCs: naphthalene, NAPH (99+%, Aldrich); phenanthrene, PHEN (98%, Merck); anthracene, ANTH (99%, Fluka); fluoranthene, FLU (99%, Aldrich); pyrene, PYR (>99%, Sigma); and benzo[a]pyreneBaP (98%, Cerilliant). Other chemicals used were: methanol (HPLC-gradient grade, Merck, Darmstadt, Germany), acetone (glass distillated grade, Rathburn, Walkerburn Scotland), toluene (glass distillated, Rathburn, Walkerburn Scotland), ethyl acetate (Merck, Darmstadt, Germany), dichloromethane (HPLC grade, Rathburn, Walkerburn Scotland), n-pentane (glass distillated, Rathburn, Walkerburn Scotland) and Milli-Q water (Super Q treated, Millipore, MA). Materials used for passive dosing vials preparation and loading were described in detail in Gouliarmou et al.31 The aqueous cyclodextrin solution was prepared by adding 75 g of β-cyclodextrin (HPCD, hydroxylpropyl-β-cyclodextrin, water solubility 2300 g/L at 24 °C, Wacker-Chemie, Burghausen, Germany) to a 1 L volumetric flask and fill it up with Milli-Q-water. Silicone Rod. A flexible silicone rod from Altec (Altec, Cornwall, United Kingdom) with a diameter of 3 mm (2.87− 3.13 mm) was used as the absorption sink. Based on the supplier’s information the relative density of the silicone was 1.2 g/mL and the hardness was 60° Shore A. The mass of the rod was gravimetrically determined to 8.0 g/m, whereas the volume was calculated based on geometry to 7.3 mL/m. Before use, the rod was cleaned by soaking once overnight in ethylacetate, followed by soaking three times overnight with methanol and then three times overnight soaking with acetone. Finally, any adhering solvent was removed by at least four times overnight washes with Milli-Q water. Cleaned rods were stored until use in a sealed bottle with Milli-Q water. Wood Soot. The applicability of the method was tested with wood soot that (1) contains endogenous PAHs32 and (2) is a frequently encountered carbonaceous geosorbent (CG), especially in urban soils of northern areas where its continuous production from residential combustion processes is followed by atmospheric deposition.33,34 According to Cornelissen et. al 2005,35 at environmentally relevant PAH concentrations nonlinear sorption to CG often completely dominates PAH sorption in soils/sediments, with sorption to CG exceeding sorption to amorphous organic carbon by 100−1000 times.35 Soot was selected as a worst-case material for soil bioaccessibility extractions, since it has high soot to water partition ratios36 and a tendency to stick to surfaces. A chimney sweeper collected the soot material in 2009 from approximately 8−10 one family houses in a small village near Roskilde (Sealand, Denmark). All soot originated from stoves with a steel lined chimney, which apparently were fired solely with wood. The soot was sieved (150 μm) and the collected fraction was well mixed. Finally, through representative mass reduction, subsamples were taken for bioaccessibility extractions and for determination of total PAH concentration. For total PAH concentration 50 mg of soot (n = 3) were spiked with a recovery standard and then Soxhlet extracted for 24 h with 500 mL of toluene/methanol (1:6). This solvent mixture provided higher extraction yields compared to pure toluene (Supporting Information (SI) Table S1), which is in good agreement with the findings by Jonker et al.32 Then solvent was switched to n-pentane, eluted through activated silica and activated alumina-B with n-pentane/dichloromethane. The final

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WORKING PRINCIPLE A soil sample is suspended in a cyclodextrin solution and then incubated together with a silicone rod (see TOC figure). The cyclodextrin serves as a diffusive carrier30 that enhances desorption of HOCs from the soil and transfers them to the silicone rod. The silicone rod is applied as an absorption sink29 that is dimensioned to continuously and quantitatively absorb the HOCs from the cyclodextrin solution. This ensures that HOCs concentrations in the cyclodextrin solution remain low and that the chemical activity gradient that drives the desorption is maintained. Finally, the HOCs absorbed by the silicone rod are solvent extracted and measured by conventional instrumental analysis. 10683

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PAHs. The initial PAH concentrations in the HPCD solution ranged between 70 and 170 μg/L. HPCD solution with 3 m of silicone rod was placed into a 100 mL Pyrex bottle closed with Teflon (PTFE)-lined screw caps (SI Figure S2). Bottles (n = 2) were shaken at 300 rpm with a Heidolph Unimax 1010 orbital shaker at 20 ± 2 °C. Care was taken that the silicone rod was fully covered by the solution and all bottles were wrapped with aluminum foil to avoid photodegradation. Subsamples of 0.5 mL HPCD solution were taken at predetermined time points, then mixed with 0.5 mL methanol and analyzed by HPLC. The PAH fraction remaining in the HPCD solution at extraction time t (Fsolution(t)), was plotted against time. A one phase elimination model with a plateau was fitted to data:

extract was evaporated under nitrogen, redissolved in 1 mL toluene and mixed with volume spike before GC-MS analysis. Passive Dosing Experiment. Passive dosing was applied to determine (1) speciation and binding of PAHs within the cyclodextrin solution and (2) silicone to cyclodextrin solution partition ratios (Ksilicone,solution). The method was described in detail by Gouliarmou et al.31 Briefly, 500 ± 5 mg of medical grade silicone was cast into the bottom of 10 mL gastight glass vials. The silicone was then loaded by equilibrium partitioning from a loading solution containing a mixture of NAPH, PHEN, ANTH, FLU, and PYR at 10% of their solubility in methanol. Each vial was then used to equilibrate in sequence: Milli-Q water, then cyclodextrin solution (75 g/L) and again Milli-Q water. Equilibration was achieved by shaking overnight at 1000 rpm with an IKA Vibrax VXR orbital shaker (IKA- Werke, Staufen, Germany) at 20 ± 2 °C. Finally, the concentration of PAHs in HPCD solution (Csolution) and in water (Cwater) was measured. Cwater served as a surrogate for the freely dissolved PAH concentration in the solution and the free fraction (f f) of PAHs in the cyclodextrin solution was quantified according to equation: f f = Cwater/Csolution.31 At the end of the experiment, the silicone of the passive dosing vials was extracted to determine silicone to solution partition ratios. Extraction was performed in two overnight steps, using 8 mL of methanol in each step. All samples were analyzed by HPLC as described later. Sink Dimensioning. We define the absorption efficiency of the sink (F) as the fraction of the analyte that is absorbed by the sink from the cyclodextrin solution. The predicted absorption efficiency at the thermodymamic equilibrium is given by eq 1: F=

mPAH(silicone) mPAH(total)

=

Fsolution(t ) = (1 − Fsolution(eq)) ·e−k1·t + Fsolution(eq)

where Fsolution(eq) is the PAH fraction left in solution at equilibrium and k1 is the rate constant that characterizes the absorption kinetics into the silicone rod. Data were fitted by least-squares using Graphpad Prizm 5 (San Diego, CA), the time to reach 95% of Fsolution(eq) was calculated (t95% = ln(20)/ k1) while the experimental absorption efficiency of rod was determined using equation: Frod = 1 − Fsolution(eq) Solvent Extraction of the Sink. The rods used in the sink validation experiment were washed with Milli-Q-water and wiped with a lint free tissue. Rods (n = 2) were extracted with acetone and without shaking. Acetone was chosen as the extraction solvent since it causes less swelling of the silicone compared to other nonpolar solvents29 and is easier to evaporate compared to other polar solvents. Initially, 50 mL of acetone was added for 5.5 h, following addition of 50 mL fresh acetone for one day. Finally, to verify extraction efficiency, 60 mL of fresh acetone was added for 11 days and extracts were analyzed using HPLC. Silicone rods and acetone extracts were kept in the dark in order to avoid photodegradation of PAHs. Mass Transfer Kinetics from Soot to Rod. The PAH mass transferred from soot to rod was determined in parallel batch extractions. For that purpose 18 Pyrex 100 mL bottles were prepared by adding 3 m silicone rod, 50 mL HPCD solution (75 g/L) and 200 mg of soot in each. Bottles were closed with Teflon (PTFE)-lined screw caps, wrapped with aluminum foil and shaken at 300 rpm with Heidolph Unimax 2010, Merck orbital shaker at 20 ± 2 °C. At 3 h, 9 h, 18 h, 24 h, 7 and 14 days time points, rods from three bottles were removed. These rods were then thoroughly rinsed with Milli-Qwater and the smooth surface was physically cleaned by wiping with a lint free tissue. After addition of recovery standard, silicone rods were extracted twice with 100 mL of acetone (>7 h and then overnight). The two extracts were combined and evaporated under nitrogen, redissolved in 1 mL toluene and mixed with volume spike before GC-MS analysis. Results were expressed as the PAH mass absorbed by rod (mrod(t)) at extraction time (t) divided by PAH mass (msoot(0)) present in soot at t = 0. An one phase desorption model was fitted to the data obtained for t < 24 h:

K silicone,solution ·Vsilicone Vsilicone·K silicone,soution + Vsolution

(1)

Where mPAH(silicone) is the PAH mass absorbed by the rod and mPAH(total) is the initial PAH mass added in cyclodextrin solution. Vsolution and Vsilicone are the volumes of solution and silicone rod respectively. The volume of sink required to obtain a given absorption efficiency can then be calculated based on the required absorption efficiency (Frequired) from eq 2: Vsilicone =

Frequired·Vsolution (1 − Frequired) ·K silicone,solution

(2)

In the present study we aimed for a silicone sink that absorbs 90% or more of the mobilized analytes from a 50 mL HPCD solution (75 g/L). The inclusion of silicone rod increases the overall capacity of the system (HPCD solution + silicone) to receive the PAHs. The enhanced capacity (EC) of the SBE method relative to a cyclodextrin extraction with the same solution volume can be calculated according to eq 3: EC =

Vsolution + Vsilicone·K silicone,solution Vsolution

(4)

(3)

mrod(t )

Sink Validation. The new absorption sink had to be characterized before its application within the SBE method. Thus, an experiment was carried out to assess (1) the elimination kinetics of PAHs from the HPCD solution into the silicone rod and (2) the absorption efficiency of the sink. For that purpose, 50 mL of HPCD solution (75 g/L) were spiked with 125 μL of a methanol solution containing the 6

msoot(0)

=

mrod(fast) msoot(0)

·(1 − e−k 2·t ) (5)

using Graphpad Prizm 5 (San Diego, CA). Where, mrod(fast) is the PAH mass transferred from soot to rod during the first fast desorbing phase, k2 is a rate constant that characterizes the kinetics of the entire mass transfer during the fast desorbing 10684

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phase and t95% is the time to reach 95% of the fast desorbing phase (t95%= ln(20)/k2). Extraction of Wood Soot without the Absorption Sink. In parallel to the previous SBE desorption experiment, soot was extracted with the cyclodextrin method.8 For that purpose 100 mL PYREX bottles (n = 3) containing only 200 mg of soot and 50 mL cyclodextrin solution (75 g/L) were shaken simultaneously in the same shaker as the bottles from the above experiment. After two weeks of extraction time, cyclodextrin solutions were filtered through a Millipore glassfiber prefilter (2 μm) to remove soot particles. Then the filtrates were transferred to 100 mL glass bottles containing 3 m silicone rod and sealed with a Teflon-lined screw cap. Bottles were covered with aluminum foil and shaken at 300 rpm overnight. In this case silicone rod was used as a conventional sorptive extraction tool29 to absorb the PAHs from the cyclodextrin filtrate. Finally, each rod was rinsed and extracted in the same way as described above. The obtained extracted fractions were compared to the two weeks extracted fractions from the above SBE experiment in order to examine the effect of the sink and to test the working hypothesis that a sink is needed. Analytical Procedures. HPLC Analysis. All HPCD and water samples from the passive dosing and sink validation experiments were mixed 1:1 with methanol for preservation and extraction of analytes from the cyclodextrin cavity. These samples were then kept at −18 °C until analysis by HPLC with multiband fluorescence detection (Agilent 1100 HPLC equipped with a G1321A FLD operated at Ex: 260 nm and Em. 350, 420, 440, and 500 nm). Details are given in Gouliarmou et al. 31 GC-MS Analysis. PAHs in the rod and soot extracts were quantified using a Thermo Finnigan Trace 2000 gas chromatograph with a 30 m × 0.25 mm × 0.25 μm 5% - Phenylmethylpolysiloxane capillary column. A Combi-Pal autosampler introduced 1 μL into the inlet (280 °C), which was operated in splitless mode. The temperature program was: 90 °C for 1 min, ramped at 10 °C min−1 to 200 °C, then ramped at 5 °C min−1 to 240 °C and held for 4 min finally ramped at 20 °C min−1 to 270 °C and held for 10 min. The carrier gas was helium at 1 mL min−1. PAHs were detected by a Thermo Electron Automass III MS operated in the SIM-mode (275 °C, 70 eV). Quantification by relative response factors was accomplished with X-calibur software (Thermoquest-Finnigan, Bremen, DE).

Figure 1. (A) Free fraction of PAHs in cyclodextrin solution (75 g/L) and B) silicone to cyclodextrin solution (75 g/L) partition ratios were plotted against log Kow.37 Data show mean values with error bars denoting ±95% CI, n = 3.

silicone can act as an absorption sink of the PAHs and simultaneously increase the overall capacity of the system (HPCD solution + silicone) to receive the PAHs. Sink Dimensioning. With the silicone to solution partition ratios, the absorption efficiency of the sink under equilibrium conditions can easily be predicted using eq 1. Such predicted absorption efficiencies will increase with the length of the silicone rod (SI Figure S4), and 3 m (22 mL) of rod were estimated using eq 2 to be sufficient to absorb at least 90% of total PAHs from 50 mL of HPCD solution. The silicone surface area (A) in contact with the sample suspension is then 283 cm2 and the A/V ratio is 12.86 cm−1. Both values are considerably higher than for the recently published contaminant trap (82 cm2; 1.64 cm−1),22 thus the silicone rod is expected to provide faster elimination of PAHs from the solution compared to the contaminant trap. Based on eq 3, the enhanced capacity (EC) of the SBE method relative to the 50 mL cyclodextrin solution ranged from 7.7 (NAPH) to 96.7 (PYR) and seems to increase with compound’s hydrophobicity (SI Figure S5). This practically means that the capacity of the SBE method, for example, for pyrene corresponds to the capacity of 4.9 L (=50 mL × 97) of cyclodextrin solution. However the capacity increase depends on the analyte under consideration. For the less hydrophobic PAHs, these enhanced capacities are smaller, but the need to extend the capacity of the solution is also much less for these chemicals.11 Sink ValidationExchange Rate and Sorption Efficiency. The sink validation experiment confirmed that the elimination kinetics were very fast. Within 10−20 min of shaking the major fraction of the PAHs was transferred from the spiked solution into the rod (Figure 2a). Such kinetics seem to be sufficient and a good starting point for bioaccessibility extractions with incubation times of days, and much faster compared to the contaminant trap method that removed the major PAH fraction from a cyclodextrin solution within 1 day.22 Additionally, high absorption efficiencies of the rod were

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RESULTS AND DISCUSSION Passive Dosing Experiment. The free fractions of the PAHs in the 75 g/L HPCD solutions were plotted against log Kow37 in Figure 1a and ranged from 0.3% for ANTH to 2.4% for NAPH. This implies that the HPCD complexed PAHs were the dominant form in the HPCD solution and it implies enhanced capacities of the solution compared to water.31 There was no clear relationship between free fractions and Log KOW, and the lowest free fraction was observed for the three ringed PAH anthracene. This is consistent with that HPCD complexation is not solely controlled by the hydrophobicity of the analyte but depends also on steric factors.31 The silicone to solution partition ratios (SI Table S3) were plotted against log Kow37 in Figure 1b. Values ranged between 15.3 and 31.0 (L/L) for the less hydrophobic PAHs (NAPH, PHEN, and ANTH) while they were somewhat higher for the more hydrophobic ones (PYR: 117.3 ± 15.0 (L/L) and FLU: 217.4 ± 26.7 (L/L)). These partition ratios indicate that 10685

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Figure 2. (A) Elimination kinetics of PAHs from a spiked cyclodextrin solution into silicone rod. Data show mean PAH fraction left in cyclodextrin solution after time (t) of shaking ± SEM (n = 2). The shaking time (t95%) to reach 95% of equilibrium ±95% CI (n = 2) is given in parentheses. (B) Measured absorption efficiencies of the silicone rod plotted against the values predicted using eq 1. The x-axis shows the mean predicted efficiency with the error bars denoting ± SEM based on propagation of uncertainties in Ksilicone,solution values (n = 3) and the y-axis shows the mean measured efficiency ±95% CI (n = 2).

Figure 3. (A) Application of SBE to PAH desorption from wood soot. Data show mean (±SD, n = 3) of the PAH fraction (%) transferred from soot to the silicone rod against extraction time. (B) Data show the time to reach 95% of apparent equilibrium (t95%) for PAH elimination from spiked cyclodextrin solution (○, n = 2) and for SBE of PAHs from soot (□, n = 3). Data are mean values (±95% CI) that are plotted against the Log Kow37 of the target PAHs.

confirmed. At equilibrium, 93% of total PAHs (86% NAPH 98% BaP) was removed from the solution, which was in good agreement with the model predictions (Figure 2b). These high absorption efficiencies imply that the silicone rod can act as a high capacity absorption sink that continuously extracts the PAHs and in this manner keeps the PAH concentration in the solution at a very low level. Hence, during application to a real sample it is possible to maintain the desorption gradient between the matrix and the solution. If desired, it is also possible to correct for absorption efficiencies below 100% by the addition of “absorption efficiency standards” to the cyclodextrin solution at the beginning of the SBE incubation. Such standards can either replace or complement recovery standards that are spiked to the acetone at the initiation of the solvent extraction of the rods. Finally, it was verified that it is possible to quantitatively (>99%) back-extract the mobilized PAHs from the silicone rod. Back-extraction can easily be achieved in two extraction steps by using 50 mL acetone in each, while a third extraction step verified the high efficiency (SI Figure S6). Mass Transfer Kinetics from Soot to Rod. The developed SBE method was applied to wood soot and the mass transfer of PAHs from soot to rod was measured. Mean values and standard deviation (n = 3) of individual PAHs fractions were plotted against extraction time in Figure 3a, for clarity reasons only 10 PAHs are shown. Measured PAH fractions were precise, typically with relative standard deviations of less than 13% (mean RSD: 12.9%; range 1.8−26.2%). The mass transfer from soot to rod followed two-phase kinetics. The

time to reach 95% (t95%) of the first fast phase ranged between 8 and 21 h for all compounds (Figure 3a), which was much longer than the time required for the silicone rod to absorb the PAHs from the cyclodextrin solution (10−20 min) as shown in Figure 3b. The transfer of PAHs from the soot to the rod continued during the second slower phase and 20 to 70% of PAHs were transferred within one week (SI Figure S7). A detailed discussion on PAH desorption kinetics from soot is beyond the scope of the present study. However, the results clearly show that the silicone rod efficiently and continuously absorbed the PAHs from the HPCD solution and that the temporal resolution of the SBE method was sufficient to study PAH desorption kinetics of the rapidly desorbing PAHs from soot. Extraction of Wood Soot with SBE Method and Cyclodextrin Extraction Method. Wood soot was extracted with the SBE method and a cyclodextrin extraction without absorptive sink8 for two weeks under identical experimental conditions. The extracted fractions for the individual PAHs were 2.9−24.5 times higher with the SBE method (Figure 4), which clearly demonstrated the intended effect of the absorption sink. This confirms the working hypothesis that a high capacity sink is necessary to improve current bioaccessibility extraction methods. Furthermore, these results also emphasize the two different roles HPCD plays in the two bioaccessibility extraction methods. In cyclodextrin extractions 10686

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important for maintaining the desorption gradient. Sink dimensioning depends on the partitioning behavior of the analyte between the mobilization medium and the silicone rod. In the current study, we applied cyclodextrin solution as the mobilization medium, since bioaccessibility extractions with cyclodextrin have been widely used in the last two decades.40,41 However, depending on the research question, the sink could be combined with other mobilization media, such as surfactant solutions or artificial digestive fluids. Results from wood soot indicated that SBE can be utilized for urban soils, where soot is expected to be the main carbonaceous form33 and also to sediments, considering the ubiquitous presence of soot in sedimentary materials.42,43 Finally, other possible matrixes are potential soil amendments, such as activated carbon, biochar, compost and also sludge. In summary, Sorptive Bioaccessibility Extraction is an analytical approach that integrates the key processes of mobilization and depletion. Partitioning into the absorption sink is used for the continuous depletion of the mobilization medium to ensure (near) infinite sink conditions. This makes SBE suited even for pollutant-sample combinations that are characterized by high KD values. The current study aimed at improving bioaccessibility extractions in terms of soil risk assessment and therefore aims at measuring, in a practical and fast way, the maximum mobilized contaminant mass from soil. Additionally, the high partitioning to the silicone provides sorptive enrichment of the target analytes, which results in extracts that can be quantified with conventional instrumental analysis. Hopefully, this will facilitate the extension of bioaccessility research to more contaminants as well as the transfer of methods to more laboratories. More importantly, the inclusion of the silicone sink will lead to more conservative and better defined bioaccessibility estimates, which hopefully will increase the regulatory acceptance of such measurements.

Figure 4. PAH fraction extracted from wood soot after two weeks extraction time, using cyclodextrin extraction method8 and sorptive bioaccessibility extraction method under identical experimental conditions. Data show mean ± SE (n = 3).

without a sorptive sink, HPCD is mainly used as a molecular scale hydrophobic phase that enhances the capacity of the solution to receive hydrophobic compounds,8 whereas in the SBE method HPCD is used as diffusive carrier that enhances the mass transfer to the silicone rod.30 Characteristics of SBE Method. The SBE method resembles to some degree SPME38,39 and the coated vials method,24 where a PDMS phase is used for absorptive enrichment of organic contaminants from an aqueous suspension. However, the main difference between the methods is that the PDMS phases of SPME and coated vials have a very small capacity and only extract a negligible fraction of the tested compounds, because they aim at measuring freely dissolved concentrations (i.e., chemical activity) without depleting them. In the SBE method, the silicone was dimensioned to act as a depleting sorbent in order to quantitatively extract the readily desorbing contaminants from the tested soil. Results showed that in case of samples with high KD values, such as soot, the sink is necessary. However, the analytical advantages that the sink offers can be used to all kind of soil matrixes. The large volume of PDMS applied in this study (22 mL) has the advantage of being able to accumulate a large absolute mass of contaminants. By extracting the PDMS and concentrating the extract to a small volume (1 mL) even very low concentrations of accessible PAHs can be measured. For instance, in the present study the final 1 mL extracts even had to be diluted 50 times before GC injection. Additionally, sorptive extraction is a selective process for the target compounds, which isolates them to a high degree from matrix constituents that might interfere with the instrumental analysis. In the present study, the obtained extracts were analyzed without additional clean up steps. Applicability Domain of SBE. The applicability of the developed method to other organic pollutants is probably mainly restricted by the ability of HPCD to act as diffusive carrier. However, HPCD will be suited for many other pollutants and might if needed be replaced by other cyclodextrins or even other carriers. The silicone rod is expected to be applicable as depletive sink for most hydrophobic organic compounds (Log Kow > 3−4),29 which includes for instance brominated flame retardants, dioxins, chlorinated insecticides and pyrethroids. However, it should always be kept in mind that proper dimensioning of the sink is

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ASSOCIATED CONTENT

S Supporting Information *

Additional graphs and tables as referenced in this article. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 45 87 15 86 63; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Stefan Trapp and Killian E.C. Smith for valuable comments on the manuscript, Ellen Christiansen for performing the GC-MS analysis, and Leif Nielsen for collecting the soot material. This research was financially supported by the Danish Council for Strategic Research (REMTEC) and the European Commission (MODELPROBE, no. 213161).

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REFERENCES

(1) Ahn, S.; Werner, D.; Luthy, R. G. Physicochemical characterization of coke-plant soil for the assessment of polycyclic aromatic hydrocarbon availability and the feasibility of phytoremediation. Environ. Toxicol. Chem. 2005, 24 (9), 2185−2195. (2) Alexander, M. Aging, bioavailability, and overestimation of risk from environmental pollutants. Environ. Sci. Technol. 2000, 34 (20), 4259−4265.

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measuring the desorption resistant fraction of soil pollutants. Environ. Sci. Technol. 2011, 45 (7), 2932−2937. (23) Rusina, T. P.; Smedes, F.; Klanova, J.; Booij, K.; Holoubek, I. Polymer selection for passive sampling: A comparison of critical properties. Chemosphere 2007, 68 (7), 1344−1351. (24) Reichenberg, F.; Smedes, F.; Jönsson, J. Å.; Mayer, P. Determining the chemical activity of hydrophobic organic compounds in soil using polymer coated vials. Chem. Cent. J. 2008, 2, 8. (25) Smith, K. E. C.; Oostingh, G. J.; Mayer, P. Passive dosing for producing defined and constant exposure of hydrophobic organic compounds during in vitro toxicity tests. Chem. Res. Toxicol. 2009, 23 (1), 55−65. (26) Bruheim, I.; Liu, X. C.; Pawliszyn, J. Thin-film microextraction. Anal. Chem. 2003, 75 (4), 1002−1010. (27) Albuquerque, J. S.; Pimentel, M. F.; Silva, V. L.; Raimundo, I. M.; Rohwedder, J. J. R.; Pasquini, C. Silicone sensing phase for detection of aromatic hydrocarbons in water employing near-infrared spectroscopy. Anal. Chem. 2004, 77 (1), 72−77. (28) Popp, P.; Kalbitz, K.; Oppermann, G. Application of solid-phase microextraction and gas chromatography with electron-capture and mass spectrometric detection for the determination of hexachlorocyclohexanes in soil solutions. J. Chromatogr., A 1994, 687, 133−140. (29) van Pinxteren, M.; Paschke, A.; Popp, P. Silicone rod and silicone tube sorptive extraction. J. Chromatogr., A 2010, 1217 (16), 2589−2598. (30) Mayer, P.; Fernqvist, M. M.; Christensen, P. S.; Karlson, U.; Trapp, S. Enhanced diffusion of polycyclic aromatic hydrocarbons in artificial and natural aqueous solutions. Environ. Sci. Technol. 2007, 41 (17), 6148−6155. (31) Gouliarmou, V.; Smith, K. E. C.; de Jonge, L. W.; Mayer, P. Measuring binding and speciation of hydrophobic organic chemicals at controlled freely dissolved concentrations and without phase separation. Anal. Chem. 2012, 84 (3), 1601−1608. (32) Jonker, M. T. O.; Koelmans, A. A. Extraction of polycyclic aromatic hydrocarbons from soot and sediment: Solvent evaluation and implications for sorption mechanism. Environ. Sci. Technol. 2002, 36 (19), 4107−4113. (33) Park, S.-J.; Cheng, Z.; Yang, H.; Morris, E.; Sutherland, M.; McSpadden Gardener, B.; Grewal, P. Differences in soil chemical properties with distance to roads and age of development in urban areas. Urban Ecol. 2010, 13 (4), 483−497. (34) Ribes, S.; Van Drooge, B.; Dachs, J.; Gustafsson, O.; Grimalt, J. O. Influence of soot carbon on the soil-air partitioning of polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 2003, 37 (12), 2675− 2680. (35) Cornelissen, G.; Gustafsson, O.; Bucheli, T. D.; Jonker, M. T. O.; Koelmans, A. A.; Van Noort, P. C. M. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: Mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environ. Sci. Technol. 2005, 39 (18), 6881− 6895. (36) Bucheli, T. D.; Gustafsson, O. Quantification of the soot-water distribution coefficient of PAHs provides mechanistic basis for enhanced sorption observations. Environ. Sci. Technol. 2000, 34 (24), 5144−5151. (37) Hilal, S. H.; Karickhoff, S. W.; Carreira, L. A. Prediction of Chemical Reactivity Parameters and Physical Properties of Organic Compounds from Molecular Structure Using SPARC, publication no. EPA/600/R-03/030; U.S. Environmental Protection Agency: Athens, GA, 2003. (38) Mayer, P.; Vaes, W. H. J.; Wijnker, F.; Legierse, K.; Kraaij, R. H.; Tolls, J.; Hermens, J. L. M. Sensing dissolved sediment porewater concentrations of persistent and bioaccumulative pollutants using disposable solid-phase microextraction fibers. Environ. Sci. Technol. 2000, 34 (24), 5177−5183. (39) ter Laak, T. L.; Agbo, S. O.; Barendregt, A.; Hermens, J. L. M. Freely dissolved concentrations of PAHs in soil pore water: Measurements via solid-phase extraction and consequences for soil tests. Environ. Sci. Technol. 2006, 40 (4), 1307−1313.

(3) Bosma, T. N. P.; Middeldorp, P. J. M.; Schraa, G.; Zehnder, A. J. B. Mass transfer limitation of biotransformation: Quantifying bioavailability. Environ. Sci. Technol. 1997, 31 (1), 248−252. (4) Kelsey, J. W.; Kottler, B. D.; Alexander, M. Selective chemical extractants to predict bioavailability of soil-aged organic chemicals. Environ. Sci. Technol. 1996, 31 (1), 214−217. (5) Pignatello, J. J.; Ferrandino, F. J.; Huang, L. Q. Elution of aged and freshly added herbicides from a soil. Environ. Sci. Technol. 1993, 27 (8), 1563−1571. (6) Reichenberg, F.; Mayer, P. Two complementary sides of bioavailability: Accessibility and chemical activity of organic contaminants in sediments and soils. Environ. Toxicol. Chem. 2006, 25 (5), 1239−1245. (7) Semple, K. T.; Doick, K. J.; Jones, K. C.; Burauel, P.; Craven, A.; Harms, H. Defining bioavailability and bioaccessibility of contaminated soil and sediment is complicated. Environ. Sci. Technol. 2004, 38 (12), 228A−231A. (8) Reid, B. J.; Stokes, J. D.; Jones, K. C.; Semple, K. T. Nonexhaustive cyclodextrin-based extraction technique for the evaluation of PAH bioavailability. Environ. Sci. Technol. 2000, 34 (15), 3174−3179. (9) Cuypers, C.; Grotenhuis, T.; Joziasse, J.; Rulkens, W. Rapid persulfate oxidation predicts PAH bioavailability in soils and sediments. Environ. Sci. Technol. 2000, 34 (10), 2057−2063. (10) Liste, H. H.; Alexander, M. Butanol extraction to predict bioavailability of PAHs in soil. Chemosphere 2002, 46 (7), 1011−1017. (11) Hartnik, T.; Jensen, J.; Hermens, J. L. M. Nonexhaustive βcyclodextrin extraction as a chemical tool to estimate bioavailability of hydrophobic pesticides for earthworms. Environ. Sci. Technol. 2008, 42 (22), 8419−8425. (12) Cuypers, C.; Pancras, T.; Grotenhuis, T.; Rulkens, W. The estimation of PAH bioavailability in contaminated sediments using hydroxypropyl-β-cyclodextrin and Triton X-100 extraction techniques. Chemosphere 2002, 46 (8), 1235−1245. (13) Reichenberg, F.; Karlson, U. G.; Gustafsson, O.; Long, S. M.; Pritchard, P. H.; Mayer, P. Low accessibility and chemical activity of PAHs restrict bioremediation and risk of exposure in a manufactured gas plant soil. Environ. Polut. 2010, 158 (5), 1214−1220. (14) Cornelissen, G.; Rigterink, H.; Ferdinandy, M. M. A.; van Noort, P. C. M. Rapidly desorbing fractions of PAHs in contaminated sediments as a predictor of the extent of bioremediation. Environ. Sci. Technol. 1998, 32 (7), 966−970. (15) Cornelissen, G.; vanNoort, P. C. M.; Govers, H. A. J. Desorption kinetics of chlorobenzenes, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls: Sediment extraction with Tenax(R) and effects of contact time and solute hydrophobicity. Environ. Toxicol. Chem. 1997, 16 (7), 1351−1357. (16) Chen, W.; Cong, L.; Hu, H. L.; Zhang, P.; Li, J.; Feng, Z. Z.; Kan, A. T.; Tomson, M. B. Release of adsorbed polycyclic aromatic hydrocarbons under cosolvent treatment: Implications for availability and fate. Environ. Toxicol. Chem. 2008, 27 (1), 112−118. (17) Sander, M.; Pignatello, J. J. Sorption irreversibility of 1,4dichlorobenzene in two natural organic matter−rich geosorbents. Environ. Toxicol. Chem. 2009, 28 (3), 447−457. (18) Rhodes, A. H.; McAllister, L. E.; Semple, K. T. Linking desorption kinetics to phenanthrene biodegradation in soil. Environ. Pollut. 2010, 158 (5), 1348−1353. (19) Pignatello, J. J. Slowly reversible sorption of aliphatic halocarbons in soils.1. Formation of residual fractions. Environ. Toxicol. Chem. 1990, 9 (9), 1107−1115. (20) Pignatello, J. J. Slowly reversible sorption of aliphatic halocarbons in soils. II. Mechanistic aspects. Environ. Toxicol. Chem. 1990, 9 (9), 1117−1126. (21) Braida, W. J.; White, J. C.; Pignatello, J. J. Indices for bioavailability and biotransformation potential of contaminants in soils. Environ. Toxicol. Chem. 2004, 23 (7), 1585−1591. (22) Mayer, P.; Olsen, J. L.; Gouliarmou, V.; Hasinger, M.; Kendler, R.; Loibner, A. P. A Contaminant trap as a tool for isolating and 10688

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Environmental Science & Technology

Article

(40) Macleod, C. J. A.; Semple, K. T. Influence of contact time on extractability and degradation of pyrene in soils. Environ. Sci. Technol. 2000, 34 (23), 4952−4957. (41) Muijs, B.; Jonker, M. T. O. Does equilibrium passive sampling reflect actual in situ bioaccumulation of PAHs and petroleum hydrocarbon mixtures in aquatic worms? Environ. Sci. Technol. 2012, 46 (2), 937−944. (42) Gustafsson, Ö .; Haghseta, F.; Chan, C.; MacFarlane, J.; Gschwend, P. M. Quantification of the dilute sedimentary soot phase: Implications for PAH speciation and bioavailability. Environ. Sci. Technol. 1997, 31 (1), 203−209. (43) Jonker, M. T. O.; Koelmans, A. A. Polyoxymethylene solid phase extraction as a partitioning method for hydrophobic organic chemicals in sediment and soot. Environ. Sci. Technol. 2001, 35 (18), 3742−3748.

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