Polyoxymethylene Solid Phase Extraction as a Partitioning Method for

Aug 8, 2001 - The authors circumvented the two above-mentioned practical problems by applying partitioning methods that do not require soot−water ph...
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Environ. Sci. Technol. 2001, 35, 3742-3748

Polyoxymethylene Solid Phase Extraction as a Partitioning Method for Hydrophobic Organic Chemicals in Sediment and Soot MICHIEL T. O. JONKER* AND ALBERT A. KOELMANS Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands

During the past few years, the presence of soot in sediments has received growing interest. Soot is thought to serve as a strong partitioning medium for specific organic contaminants (PAHs). The precise extent of sorption to this material, however, is poorly known because soot/water distribution coefficients for native PAHs have not been determined yet. Measuring these coefficients using existing partitioning methods is problematic due to the nature of soot. Therefore, the objective of this study was to develop a method for the determination of distribution coefficients for organic contaminants in soot/water (but also sediment/ water) systems. The method is based on solid phase extraction (SPE) of chemicals onto the plastic polyoxymethylene (POM). Sorption experiments with POM showed monophasic sorption kinetics, linear isotherms covering several orders of magnitude, and a linear relationship between distribution coefficients for POM and the octanol/water distribution coefficient. Therefore, the sorption process can be considered to be true partitioning. Application of POM for the determination of distribution coefficients for soot and sediment (POM-SPE method) resulted in highly reproducible values. The method was validated by comparing values for sediment with results for the same sediment determined using the cosolvent method. This comparison resulted in an almost 1:1 relationship, proving the method’s validity.

Introduction Sorption of hydrophobic organic chemicals to sediments has been studied extensively during the last two decades. Currently, much is known about kinetics, equilibrium conditions, and factors influencing both aspects of sorption, such as contact time and the presence of dissolved organic macromolecules. Recently, the existence of a possible new strong influencing factor was suggested, i.e., the presence of soot in sediments (1-4). Soot has been detected in several sediments (5) and is thought to serve as an additional sorptive phase for specific contaminants, next to natural organic matter (3). Moreover, there are indications that soot is a very strong (activated carbon like) partitioning phase in sediments for planar chemicals such as polycyclic aromatic hydrocarbons (PAHs) and coplanar polychlorinated biphenyls (PCBs) * Corresponding author e-mail: [email protected]; phone: +31-317-485485; telefax: +31-317-484411. 3742

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(6) and that it might control the sorption of these contaminants in natural environments (3, 6). Therefore, characterization of the sorption process to soot is needed. Sorption is generally characterized by measuring the equilibrium distribution of chemicals between a sorbent and water. The determination of distribution (“partition”) coefficients for compounds in soot-water systems, however, is problematic because (a) the coefficients are expected to be extremely high (3, 6), which implies extremely low aqueous concentrations at realistic soot-adsorbed concentrations, and (b) soot is known to consist of particles spanning several orders of magnitude in size, i.e., a few nanometers to several micrometers (3). The lower range particles cannot be separated from water using common separation techniques such as filtration or centrifugation. Depending on the type of soot, these very small particles can make up a substantial part of the entire soot mass. Several “partitioning methods” for measuring distribution coefficients are known from sorption studies with sediments and/or soils, i.e., the classical “batch (shake) method” (7), the cosolvent method (8), headspace analysis (9), methods using dialysis membranes (10), and methods derived from the batch shake method using different techniques for isolating or detecting the aqueous concentration, such as ultracentrifugation (11), gas-purging (12), sorption to XAD or C18 material (12, 13), fluorescence quenching (14), and solid-phase microextraction (SPME) (15). None of these methods can however be used to measure distribution coefficients for soot-adsorbed PAHs as a result of foundering on one or both of the aforementioned problems. Even the SPME method (15) is not sufficiently sensitive for the measurement of desorbed concentrations of several PAHs from soot. Very recently, Bucheli and Gustafsson (16) determined the first distribution coefficients for four volatile lesshydrophobic PAHs to diesel soot, providing support for the hypothesis of soot being an important sorbent for these compounds. The authors circumvented the two abovementioned practical problems by applying partitioning methods that do not require soot-water phase separation, i.e., an “air-bridge” and a “cosolvent-column method”, and using added PAHs spiked at detectable dissolved concentrations (16). However, the reported coefficients provide no direct insight into the sorption of natively bound PAHs, all the more since they relate to very high (unrealistic) soot adsorbed concentrations (grams per kilogram). The determination of distribution coefficients for natively sorbed PAHs requires a method that is capable of detecting extremely low desorbed concentrations. The “air-bridge” method does not meet this requirement. Moreover, it is only applicable to volatile chemicals. Although the “cosolvent-column” method may possibly be qualified for the measurement of distribution coefficients for native PAHs, it has limitations because of the inevitable use of high fractions of methanol that probably alter the soot matrix and its sorption properties (16). The objective of the present study was to develop a sensitive method for the determination of distribution coefficients for nonvolatile and natively sorbed PAHs in soot/ water systems. However, the method should also be applicable to other sorbents and chemicals such as sediments and PCBs, respectively. Traffic soot was selected as testing material because of its widespread occurrence in the environment (17). Distribution coefficients for PAHs and PCBs to sediment which were obtained using the cosolvent method as part of an earlier study (6) were used to validate the method. 10.1021/es0100470 CCC: $20.00

 2001 American Chemical Society Published on Web 08/08/2001

Method Design. The two aforementioned difficulties in determining soot/water distribution coefficients can be circumvented by using a third additional phase next to soot and water. This phase must be a sorbent for the contaminants of interest and it should easily and fully be separable from soot and water. The mass balance of such a three-phase system is given by:

Qtot ) CSMS + CWVW + CPMP

(1)

where Qtot is the total amount (µg) of contaminant in the system, and CS,CP, and CW are the concentrations in soot and the additional partitioning phase (µg/kg) and in water (µg/ L), respectively. MS, MP, and VW are the masses of soot and the additional partitioning phase (kg) and the volume of water (L), respectively. Substitution of the relationships for the distribution coefficients (i.e., KS ) CS/CW and KP ) CP/CW, where KS is the soot/water distribution coefficient, and Kp is the additional phase/water distribution coefficient) and rewriting gives:

KS )

(

1 KPQtot - MPKP - VW M S CP

)

(2)

From eq 2, it appears that if the partitioning behavior of the contaminants over the additional phase and water (KP) as well as the total mass of the compounds in the system (Qtot) are known (KP and CS determined in advance; initial CP and CW are 0), measuring the concentration in the additional phase after equilibration (CP) will be sufficient to determine the soot/water distribution coefficient. In this way, problems related to soot/water phase separation and extremely low aqueous concentrations are avoided. The selection of a suitable additional phase was not selfevident. Prior tests showed that soot strongly binds to wellknown partitioning phases, such as Tenax, Empore Discs, C18 material and octanol, and cannot easily be separated from them anymore. The problem was resolved by using the plastic polyoxymethylene (POM). Hydrophobic organic chemicals appear to show reproducible and sufficiently strong partitioning to this material. Furthermore, because of its hard and smooth surface, soot can be wiped off easily from the plastic by just using a moist tissue. This fact was confirmed by microscopic analysis and appeared not to apply to other tested commercial plastics such as polystyrene, poly(vinyl chloride), polypropylene, polyethylene, and (silicon) rubber because of their somewhat rougher surfaces. Tests showed that wiping the plastic had no effect on the distribution coefficient (data not shown). This is in accordance with findings of Mayer et al. (18) who reported that fluoranthene partitioned into the plastic poly(dimethylsiloxane). Finally, POM is resistant to organic solvents which makes it perfectly (Soxhlet-) extractable. The method outline described above will be referred to as polyoxymethylene solid-phase extraction (POM-SPE) since a solid phase (POM) is used to extract contaminants from a sorbent.

Material and Methods Chemicals. The PAHs phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[e]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo[g,h,i]perylene were obtained from Sigma-Aldrich or Acros Organics, The Netherlands, and all had a purity of g 98%. 2-methylchrysene (purity 99.2%) was supplied by The Community Bureau of Reference (BCR), Geel, Belgium. PCBs used (IUPAC no. 18, 28, 52, 72, 77, 101, 118, 126, 138, 156, 169, and 209) all had a declared purity of g 98% and were obtained from Promochem, Wesel, Germany, except for PCB-72 which was purchased from Ultra Scientific, North

Kingstown, USA. Other chemicals used were: hexane and acetone (Promochem; picograde), methanol (Mallinckrodt Baker, Deventer, The Netherlands; HPLC gradient grade), ethanol (Merck, Darmstadt, Germany; p.a.), acetonitrile (LabScan, Dublin, Ireland; HPLC grade), isooctane (Promochem; nanograde), calcium chloride (Merck; p.a), sodium azide (Aldrich; 99%), aluminum oxide - Super I (ICN Biomedicals, Eschwege, Germany), silica gel 60 (Merck; 70-230 mesh) and Tenax-TA (Chrompack, Middelburg, The Netherlands; 60-80 mesh). Prior to use, Tenax-TA was Soxhlet-extracted with acetone (1.5 h) and hexane (1.5 h) and dried at 80 °C. Furthermore, silica gel was conditioned (activated) at 180 °C during 16 h, and aluminum oxide was deactivated with 10% (w/w) Nanopure water (Barnstead). Other chemicals were used as received. Polyoxymethylene. Polyoxymethylene (trade name: (poly)acetal; mol. formula: (-CH2O-)n; density: 1.41 g/cm3) was obtained from Vink Kunststoffen BV, Didam, The Netherlands. The plate which had a thickness of 0.5 mm was cut into strips of the desired dimensions. Before use, the strips were washed (cold-extracted) with hexane (30 min) and methanol (3 × 30 min), after which they were air-dried. Environmental Samples. A sediment core was taken from Lake Ketelmeer, The Netherlands, as part of an earlier study (6). The 40-120 cm layer from this core was collected, sieved (500 µm), and mechanically homogenized. A subsample was freeze-dried and used in the present study. The organic carbon content was determined to be 6.48% (for more details see ref 6). Soot was collected from used exhaust pipes (n ≈ 70) at a local garage using a large test tube brush. The material was suspended in water and metal particles were removed by using magnets. Subsequently, the suspension was centrifuged for 20 min at 2600g. To gain soot that was representative for the fraction found in sediments, the floating part was discarded and the settled fraction was washed with Nanopure water containing 0.01 M CaCl2 (shaking for 1 day followed by centrifugation, a procedure which was repeated during two weeks). Finally, the remaining soot fraction was dried at 80 °C, carefully grinded in a mortar, and sieved through a 50 µm sieve. Characterization of Sorption to POM. Sorption of PAHs and PCBs to POM was characterized by measuring sorption isotherms and adsorption kinetics. All experiments were carried out in 300-mL all-glass brown-colored bottles, with the exception of the lowest isotherm measurements which were performed in 1-L all-glass brown-colored bottles. The aqueous phase consisted of Barnstead Nanopure water containing 25 mg/L sodium azide and 0.01 M calcium chloride. The mass of the POM strips varied between 0.1 and 2 g, depending on the experiment. Isotherms were determined as follows. Bottles were filled with the aqueous solution, leaving approximately 10 mL of headspace. POM strips were added, and the systems were spiked with 25-200 µL of a cocktail solution of 11 PAHs or 11 PCBs in acetone. All bottles were horizontally shaken (100 rpm) at 20 (( 0.2) °C during 28 days. The POM strips were then removed and dried with a tissue prior to extraction. The dissolved contaminants were adsorbed to Tenax by adding 0.2 g of this material and shaking (110 rpm) during 20 h. Isotherms were measured in triplicate and consisted of six data points each, spanning at least a 150-fold concentration range. Sorption kinetics were determined in approximately the same way as the isotherms. However, equilibrium times were 3 h, 1, 4, 10, 20, or 28 days. Furthermore, the measurements were performed singular at three different POM/water ratios to investigate the influence of this ratio on sorption kinetics. All measurements were done at one fixed isotherm point. For both isotherms and kinetic experiments, mass balances for all chemicals were calculated. These measured 98 VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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( 4% (PAHs) and 100 ( 4% (PCBs) for the isotherms and 97 ( 10% (PAHs) and 101 ( 5% (PCBs) for the kinetic experiments (n ) 198 for each value). Sorption to Soot and Sediment using POM-SPE. Distribution coefficients for soot (PAHs) and sediment (PAHs and PCBs) were determined as follows: 0.6 g of sediment or 100 mg of soot [both with replicated (n ) 3) predetermined contaminant concentrations] was added to the 300-mL bottles. Subsequently, the bottles were filled with the previously described aqueous phase, again leaving 10 mL of headspace, and POM strips of 1.0 g were added. After 28 days of equilibration at 20 (( 0.2) °C, the POM strips were removed, cleaned with moist tissues, and extracted. The remaining soot suspensions were discarded, but the sediment suspensions were filtered through glass fiber filters (GF/F; Whatman, Maidstone, England) to recover the sediments. The filters were extracted to be able to calculate mass balances (PCBs only). These measured 100 ( 6% (n ) 33). A period of 28 days proved to be sufficient to reach the equilibrium state. In a pilot study, distribution coefficients for both soot and sediment were determined after 2, 4, 6, and 10 weeks. The coefficients for the last three determinations appeared to be indistinguishable. Extractions and Cleanup. Methanol was selected as extractant for POM. This solvent was tested to be very efficient in recovering contaminants from the plastic. Therefore, POM strips were Soxhlet-extracted with 70 mL of methanol during 3 h. The extracts were concentrated to 1 mL, exchanged to hexane, and cleaned-up over Al2O3 (PAHs) or Al2O3/silica gel (PCBs) columns. The eluates were reduced to 1 mL, exchanged to acetonitrile (PAHs) or isooctane (PCBs), and rereduced to 1 or 0.5 mL, after which internal standards were added. Tenax was extracted by transferring the beads to a column and eluting them with 15 mL of ethanol and 50 mL of hexane, successively. The extracts were then reduced to 1 mL (azeotropically removing small amounts of water) and exchanged to hexane. Finally, they were exchanged to acetonitrile (PAHs) or isooctane (PCBs) and concentrated to 1 or 0.5 mL, after which internal standards were added. Sediments and filters were extracted and cleaned-up as described in ref 6. Soot was Soxhlet-extracted using 70 mL of toluene/methanol (1:6) mixture during 16 h. The extract was concentrated to 1 mL and further treated as POM extracts for PAH analysis (see above). All extraction and cleanup activities for PAHs were carried out in brown or covered glassware to minimize PAH photolysis. Furthermore, numerous cleanup blanks and recoveries (for all chemicals) were determined. All samples were corrected for both blanks (always negligible) and recoveries. Instrumental Analysis. PAHs were analyzed on a HewlettPackard model 1100 HPLC equipped with a 4.6 mm Vydac guard and analytical reverse phase C18 column (201GD54T and 201TP54, respectively) which were kept at 22.00 °C. Detection was performed by using an HP 1100 multiwavelength fluorescence detector operating in the multi-emission wavelength mode. The mobile phase consisted of methanol/ water (mixture and flow gradient). The injection volume was 20 µL. PCBs were measured by splitless injection of 1 µL of sample in an upgraded HP 5890 series II gas chromatograph equipped with an HP 7673A autosampler system, two fused silica capillary columns, CP Sil-8 CB and CP Sil-5 CB (both 50 m; d.i. 0.15 mm; d.f. 0.20 µm), and two 63Ni electron capture detectors. The injector and detector temperatures were 250 and 325 °C, respectively. Carrier gas was N2 (1 mL/min). 3744

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FIGURE 1. Sorption kinetics for PCB-101 at three POM/water ratios: 0.35 (diamonds), 1.75 (squares), and 3.5 (triangles) g/L. CPOM and CW are the concentrations in POM (ng/kg) and water (ng/L), respectively. Solid lines are model fits (one compartment model) for the given POM/water ratios.

Results and Discussion Kinetics of Sorption to POM. As discussed in the method design section, the additional sorbing phase (POM) has to be characterized before the POM-SPE method can be applied. Information is needed on the equilibrium distribution of chemicals over POM and water (KPOM), but also on sorption kinetics to be able to determine the required equilibration time in POM-SPE experiments. Therefore, sorption kinetics for PAHs and PCBs were measured at three different POM/ water ratios, i.e., 0.35, 1.75, and 3.5 g/L. An example of the results is given in Figure 1 for PCB-101. From this figure, it appears that the rate of sorption increases with the POM/ water ratio, a fact that applies to all investigated chemicals and that can easily be explained by an increasing available sorption surface. For all ratios, however, equilibrium is certainly attained after 10 days for PCB-101 and also for the other PCBs and all PAHs. Nevertheless, isotherms and distribution coefficients for soot and sediment (see below) were determined after 28 days of equilibration. Moreover, the latter experiments were always performed using the highest (i.e., fastest) POM/water ratio for which equilibrium is attained within 5 days for all compounds. As compared to other sorbents such as Tenax or activated carbon, an equilibration time of days may seem rather long because these sorbents are known to reach equilibrium within a couple of hours. This difference, however, is mainly caused by a difference in sorption surface that is several orders of magnitude higher per volume unit for the suspended sorbents. For sorbents with smaller surface-to-volume ratios such as SPME fibers and Empore disks, equilibration times are within the same time scale as for POM (19). For semipermeable membrane devices (SPMDs) even much longer equilibration times of months up to years have been reported (20). A one-compartment model (21) was fitted to the kinetic data. The model assumes simple reversible sorption between the aqueous and solid phase, which is characterized by means of a kinetic first-order ad- and desorption constant. The results for PCB-101 are also shown in Figure 1. The model appears to fit well to the experimental data, which is in accordance with the hypothesis of sorption being a (onestep) partitioning process into the plastic matrix (18). Fitting of the model to the experimental data yields ad- and desorption rate constants (ka and kd, respectively, with units h-1) of which the logarithms are plotted against the chemicals’ octanol/water partition coefficient (log KOW) in Figure 2.

FIGURE 3. Examples of logarithmic isotherms for phenanthrene (squares), PCB-18 (triangles), and PCB-101 (diamonds). Solid symbols are used for KPOM determinations. CW is the aqueous concentration (ng/L), and CPOM is the concentration absorbed to POM (ng/kg). pears and the data can be pooled to obtain the equation:

log kd ) - 0.76 log KPOM + 0.75 (r2 ) 0.81, n ) 66) (3)

FIGURE 2. Log ka (a) and log kd (b) (both h-1) against log KOW for PAHs (closed symbols) and PCBs (open symbols) at different POM/ water ratios (diamonds represent 0.35, squares represent 1.75 and triangles represent 3.5 g/L). Log KOW values for PAHs are adopted from ref 20. Log KOW values for PCBs are from ref 30. Again, this figure shows that the adsorption constants increase as the POM/water ratio increases. Furthermore, for all three ratios, the ka’s remarkably show an increase with log KOW toward an optimum and then decrease. For PAHs, this optimum is located at log KOW ) 5.9 and for PCBs at log KOW ) 6.8. Interestingly, the optimum for both congener groups corresponds to a molar volume of approximately 300 cm3/mol. In other words, when the molecular size of chemicals exceeds a certain threshold, the adsorption rate to POM is significantly reduced. Possible explanations for this phenomenon are the occurrence of steric hindrance for the large molecules during uptake in the polymeric matrix and/or a reduced aqueous diffusivity (threshold at Dw ≈ 4.65 × 10-6 cm2/s). The limiting process, however, only affects the rate of adsorption and does not result in exclusion of large molecules from the plastic since the final equilibrium state is not influenced by the molar volume, i.e., a linear relationship between the logarithm of the POM/water distribution coefficient (KPOM; L/kg) and log KOW is observed, as is discussed in the next section. Log kd values show a linear decrease with log KOW (see Figure 2), which is independent of the POM/water ratio. Desorption of PCBs appears to be significantly faster than that of PAHs (F-test, P < 0.01). The regression lines of the relationships (both r2 ) 0.90, n ) 33) have slopes of -0.54 and -0.81 and intercepts of 0.67 and 1.94, for PCBs and PAHs, respectively. When the log kd’s are plotted against the accompanying log KPOM values (see next section), the significant difference between both congener groups disap-

Similar relationships with slopes between -0.63 and -0.98 and intercepts of 0.75-1.5 were derived for sediments (2224) using the two compartment model. Note that these relationships concern the desorption rate constant for the second compartment, i.e., for the “slowly desorbing fraction” (for details on the two-compartment model, see refs 22-24). Verbruggen et al. (25) investigated sorption kinetics of several chemicals (including some PAHs and chlorobenzenes) to polyacrylate (PAc) coated SPME fibers. For compounds with a log KOW > 3, also a linear relationship between log kd and log KPAc (slope ) - 0.89; intercept ) 3.36) can be calculated from their data. Considering the larger intercept, desorption from PAc coated SPME fibers appears to be faster than from POM. Isothermic sorption to POM. Some examples of isotherms for POM sorption are presented in Figure 3 (log-log scale). Isotherms for PCB-138 and the less hydrophobic PCBs (e PCB-101), except PCB-77, are fully linear on a nonlogarithmic scale, whereas isotherms for the remaining PCBs show downward curvatures at the highest isotherm point (triplicate). For PCB-169 and all PAHs, even the two highest (triplicate) points deviate from linearity. Part of this behavior is probably due to the exceeding of the aqueous saturation concentrations for the highly hydrophobic PAHs and PCBs. The other part, however (less hydrophobic PAHs (< benzo[a]anthracene)), cannot be explained by this artifact because concentrations were less than 5% of saturation levels. For these compounds, possibly real nonlinear sorption occurs at high concentrations. Of course, this is also a plausible explanation for the other deviating chemicals. Note in this respect that all compounds that show curve nonlinearity are planar congeners (all PAHs and non- and mono-ortho PCBs (77, 126, 169, 118, and 156). Log KPOM values were determined by using the data points that make up the linear parts of the isotherms. This choice is justified because application of the POM-SPE method (see next section) will always result in concentrations in POM that are in the very lowest range of the isotherms. Each data point was considered as an individual KPOM determination and all values were averaged. This method was preferred to taking the slope of the nonlogarithmic isotherms, to prevent VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Averaged log KPOM, log KOC (sediment), and log KS (soot) Values with Standard Deviations for PAHs and PCBs compounda

log KOWb

log KPOM (L/kgPOM)

log KOC (L/kgOC)

log KS (L/kgsoot)

compound

log KOWc

log KPOM (L/kgPOM)

log KOC (L/kgOC)

Phen Ant Flu Pyr BaA Chr BeP BbF BkF BaP BghiPe

4.6 4.6 5.2 5.2 5.9 5.8 6.4 5.8 6.2 6.0 6.9

3.29 ( 0.07 3.47 ( 0.10 3.73 ( 0.04 3.76 ( 0.05 4.51 ( 0.07 4.51 ( 0.09 4.73 ( 0.10 4.88 ( 0.13 4.94 ( 0.17 4.99 ( 0.12 4.90 ( 0.14

5.70 ( 0.04 ndd 6.04 ( 0.01 6.05 ( 0.01 6.95 ( 0.01 6.84 ( 0.02 7.37 ( 0.01 7.42 ( 0.01 7.41 ( 0.02 7.96 ( 0.03 7.84 ( 0.02

6.24 ( 0.02 6.94 ( 0.03 6.96 ( 0.03 6.79 ( 0.02 8.26 ( 0.00 8.18 ( 0.02 8.28 ( 0.03 8.54 ( 0.00 8.66 ( 0.01 8.87 ( 0.07 8.89 ( 0.03

PCB-18 PCB-28 PCB-52 PCB-72 PCB-101 PCB-77 PCB-118 PCB-138 PCB-126 PCB-156 PCB-169

5.2 5.7 5.8 6.3 6.4 6.4 6.7 6.8 6.9 7.2 7.4

3.90 ( 0.05 4.41 ( 0.05 4.44 ( 0.08 4.69 ( 0.09 4.91 ( 0.10 5.01 ( 0.15 5.05 ( 0.08 5.18 ( 0.11 5.20 ( 0.16 5.25 ( 0.13 5.26 ( 0.24

5.54 ( 0.00 6.28 ( 0.01 6.03 ( 0.02 6.01 ( 0.00 6.55 ( 0.01 6.86 ( 0.02 6.85 ( 0.03 7.15 ( 0.05 nd 7.38 ( 0.06 nd

a Explanation of abbreviations: Phen ) phenanthrene, Ant ) anthracene, Flu ) fluoranthene, Pyr ) pyrene, BaA ) benzo[a]anthracene, Chr ) chrysene, BeP ) benzo[e]pyrene, BbF ) benzo[b]fluoranthene, BkF ) benzo[k]fluoranthene, BaP ) benzo[a]pyrene, BghiPe ) benzo[g,h,i]perylene. b Log K c d nd ) not determined. OW values for PAHs are adopted from ref 20. Log KOW values for PCBs are from ref 30.

relationship between log K′POM and log KOW (r2 ) 0.88, n ) 22) is given by:

log K′POM ) 0.72 log KOW + 0.39

FIGURE 4. Relationship between log K′POM and log KOW (both in L/L) for PAHs (triangles) and PCBs (squares). dominance of the high concentrations. Nevertheless, both methods usually resulted in almost identical values. The averaged log KPOM values with standard deviations (SDs) are given in Table 1. The SDs are rather high, which is due to the averaging of measurements covering several orders of magnitude. Multiple determinations (n ) 6) at a “single” isotherm point, i.e., a smaller range, resulted in identical averaged KPOM values but with significantly (2.5 times) smaller SDs (data not shown). In Figure 4, the averaged log KPOM values are plotted against log KOW. Note that in Table 1 the values concern logarithms of KPOM’s with the units L/kg (essential for application of eq 2), whereas in Figure 4 the underlying units are L/L (values from Table 1 plus the logarithm of the density of POM). These values will be referred to as log K′POM and are introduced to be able to compare the resulting relationship with those for other sorbents which are all presented on a volume base; see below. The behavior of PCBs and PAHs appears not to be significantly different (F-test, P ) 0.17). The regression line for the pooled 3746

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(4)

This linear relationship indicates that sorption to POM is probably driven by hydrophobicity only. This, in combination with the observed linear isotherms covering several orders of magnitude and one-step kinetic sorption, leads to the conclusion that sorption to the plastic can be regarded as a true partitioning process. PAHs and PCBs therefore are absorbed by the plastic and thus will not suffer from competitive effects (i.e., competition for specific adsorption sites). For that reason, the sorption will be independent of the concentrations in and the composition of the surrounding aqueous phase, which makes the plastic an excellent (additional) sorbing phase in environmental matrixes. Sorption to the plastic poly(dimethylsiloxane) (PDMS) was also proven to be a partitioning process, based on proportionality between log KPDMS and log KOW, and fluorescence microscopy images of sorbed fluoranthene (18). Although the regression line for the relationship between log KPDMS and log KOW was found to be steeper than for POM (slope ) 1.00), the intercept measured -0.91 (18), which overall results in partitioning similar to that to POM. Moreover, from the data of Poerschmann et al. (26), a regression line for PDMS similar to eq 4 can be calculated (slope ) 0.82, intercept ) 0.32; pooled data for PAHs and PCBs). Finally, for the plastic PAc, the relationship between log KPAc and log KOW was characterized by a slope of 0.92 and an intercept of 0.01 (25). Sorption to this material therefore seems to be stronger. However, KPAc values tend to be constant or even decrease for chemicals with log KOW > 5 (25). Therefore, it can be concluded that sorption of hydrophobic organic chemicals to POM resembles the sorption to the two plastics (PDMS and PAc) that are commercially applied as partitioning phase for these chemicals on SPME fibers. Sorption to Sediment and Soot Using the POM-SPE Method. POM was used to determine distribution coefficients for soot and sediment by equilibrating it in the sorbent suspensions. The coefficients were calculated according to eq 2, substituting the averaged KPOM values from Table 1 for “KP”. The use of this equation is justified since the mass balances matched (see Material and Methods section). The resulting coefficients for distribution over soot and water (KS) for PAHs and for distribution over sediment and water (normalized to the sediment organic carbon content) (KOC) for PAHs and PCBs are presented in Table 1. From the standard deviations (SDs), it appears that the reproducibility of the POM-SPE method is very high. The differences between triplicate batches are very small. Note that the SDs represent

FIGURE 5. Comparison of log KOC values for PAHs (triangles) and PCBs (squares) in sediment from Lake Ketelmeer determined using the POM-SPE method with values determined using the cosolvent method. The dotted line represents the 1:1 relationship. the spreading around the mean of the triplicate KS or KOC determinations and not the real error. The value for the analytical error can be estimated using eq 2 and the error propagation equation of Gauss. Having an averaged error for Qtot of 5.9 (range 2.2-8.6) %, for CPOM of 2.7 (range 2.13.2) %, and for KPOM of 11.8 (range 4.6-24.2) % (i.e., the SDs for determinations at a small isotherm range; see former section) and assuming the weighting errors to be negligibly small, the averaged analytical error measures 0.08 (range 0.03-0.15) log units. Because KPOM is represented twice in eq 2, the error in this term determines the total error. Therefore, the largest total error is observed for the more hydrophobic chemicals (having the largest SDs for KPOM). The KOC values from Table 1 can be used to validate the POM-SPE method because for the same sediment, values are available that were determined using the cosolvent method (6). Comparison of the coefficients for chemicals considered in both studies (7 PCBs, 10 PAHs) shows very similar values. In fact, the methods are only statistically different at a significance level of < 0.14 (paired t-test, P ) 0.14). Hence, the methods can be considered indistinguishable. In Figure 5, both series are plotted against each other, clearly showing the 1:1 relationship. Since the cosolvent method is a well-established partitioning method (see refs in ref 6), these results can be interpreted as strong support for the validity of the POM-SPE method. Values similar to results obtained with the cosolvent method further implies that the POM-SPE method is capable of circumventing the DOC effect (27) because the cosolvent method was designed for this purpose. This is in line with expectations, considering the fact that POM is fully separable from water, and thus from DOC. The KS values from Table 1 illustrate the very strong sorption of native PAHs to soot. Values up to 1.3 log units higher than distribution coefficients for the organic carbon in the sediment and up to 2.9 log units higher than log KOW values are measured. Comparison of the coefficients with those determined by Bucheli and Gustafsson (16) shows

similar values for the common PAHs (phenanthrene and pyrene). Note, however, that the values from Table 1 are not normalized to carbon content and concern a different type of soot. A detailed discussion on the KS values is beyond the scope of this paper, and for now they primarily serve the purpose of demonstrating the applicability of the POM-SPE method to complex soot/water systems. In a following paper, the coefficients will be compared to those for other types of soot and related sorbents. Method Characteristics of POM-SPE. The POM-SPE method conceptually resembles the SPME method (15): an additional artificial partitioning phase is used to absorb organic contaminants from the aqueous phase. However, an important difference between the methods is that the SPME fiber has a very small capacity and thus extracts a negligible fraction of the test compounds from the natural sorbent. The POM-SPE method is capable of strongly extracting the natural sorbent due to the larger capacity (volume) of POM. Therefore, SPME is a “negligible depletion” method (19, 28) for the natural sorbent, whereas POM-SPE may be depletive. Negligible depletion was stated as a condition for the application of SPME in order not to disturb the equilibrium. At most, 5% of the total mass of chemicals in the natural sorbent is allowed to be extracted (28). In the present study, the soot experiments meet this 5% criterion, but application of the POM-SPE method to the sediment leads to extraction of up to 30% of the total sorbed contaminant mass. Nevertheless, the results from Figure 5 prove that this is not problematic for the determination of reliable distribution coefficients. Distribution is just measured at a new established equilibrium. The 5% criterion can however also be met for the sediment by just measuring at a 6 times higher sediment/water ratio (12 g/L). The large volume of POM applied in this study (0.71 mL) has the big advantage of being able to accumulate a rather large absolute amount of contaminants. By extracting the plastic phase and concentrating the extract to a small volume (0.5 mL), the POM-SPE method is capable of detecting very low aqueous concentrations, even while only 4 or 0.2% of the extract is injected in the HPLC or GC, respectively. By simply using a larger injection volume and/or by reducing the extract volume to 100 µL (mini-vials), detection limits as low as 275 fg/L for the highly hydrophobic PCBs can be reached. This makes the method up to 400 times as sensitive as a standard 7 µm PDMS-SPME. Another advantage of POM-SPE over SPME is that the method uses an analytical cleanup step. Although the absence of this step in the SPME method is claimed to be an advantage since it reduces solvent use and speeds up the analysis, it might change into a disadvantage in case of very complex environmental matrixes such as soot suspensions. After all, the extraction phase (plastic) is not a selective one and all compounds with any affinity for the polymer (anthropogenic and natural) are absorbed. Cleanup serves to remove interfering chemicals and the absence of this analytical step can result in, for example, coelution of “target” and “nontarget” chemicals (and thus overestimation of concentrations) or the well-known interference of elemental sulfur during GC-ECD applications (29). Also note that “bleeding” of the SPME polymer coating at elevated temperatures in the injector can cause interference (29). Soot in particular is a very “dirty” material with good chances of causing chromatographic problems. Soot(-POM) extracts after cleanup usually still show very complex chromatograms. Therefore, cleanup is essential or at least strongly advisable for sootrelated measurements. In summary, polyoxymethylene solid phase extraction appears to be a very simple, reproducible, and inexpensive (only 17 US $/L POM) partitioning method which, because of its cleanup possibility and extreme sensitivity, is suitable VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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for the determination of distribution coefficients for hydrophobic organic chemicals natively sorbed to soot and sediment.

Acknowledgments This study was financially supported by The Netherlands Organization for Applied Scientific Research, Institute of Environmental Sciences, Energy Research and Process Innovation (TNO-MEP). Kwik-Fit Wageningen is acknowledged for kindly supplying the exhaust pipes.

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Received for review February 21, 2001. Revised manuscript received June 13, 2001. Accepted June 19, 2001. ES0100470