Environ. Sci. Technol. 2003, 37, 268-274
Measured Pore-Water Concentrations Make Equilibrium Partitioning Work s A Data Analysis RIK KRAAIJ, PHILIPP MAYER,† FRANS J. M. BUSSER, MAARTEN VAN HET BOLSCHER, WILLEM SEINEN, AND JOHANNES TOLLS* Institute for Risk Assessment Sciences, University of Utrecht, Yalelaan 2, PB 80176, 3508 TD Utrecht, The Netherlands ANGELIQUE C. BELFROID‡ Institute for Environmental Studies, Vrije Universiteit, De Boelelaan 1115, 1081 HV Amsterdam, The Netherlands
There is an increasing body of evidence that the bioaccumulation of sediment-associated hydrophobic organic compounds (HOCs) is strongly influenced by sequestration. At present, it is not known how equilibrium partitioning theory (EqP), the most commonly employed approach for describing sediment bioaccumulation can be applied to sediments with sequestered contaminants. In this paper, we present freely dissolved pore-water concentrations of HOCs. These data were employed to interpret sediment bioaccumulation and sequestration data in order to arrive at a process based evaluation of EqP. The data analysis suggests that sediment bioaccumulation of compounds up to log KOW 7.5 in Tubificidae can be described as bioconcentration from pore-water. In addition, the pore-water concentrations of HOCs (4.5 < log KOW < 7.5) are established by equilibrium partitioning between the rapidly desorbing HOCs fraction in the sediment and the pore-water. Taken together, these findings indicate that EqP is a conceptually correct representation of sediment bioaccumulation, provided that sequestration is accounted for. This implies that the risk assessment of sedimentassociated HOCs can be significantly simplified: With a method at hand for measuring freely dissolved pore-water concentrations of HOCs, it appears that HOCs’ body residues in sediment dwelling organisms can be estimated on the basis of concentrations in pore-water and bioconcentration factors.
Introduction Hydrophobic organic chemicals (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are ubiquitous in sediments (1-3). Due to their persistence and their hydrophobicity they tend to accumulate in benthic deposit-feeders, an important sub-
group of sediment dwelling organisms. The equilibriumpartitioning model (EqP) (2, 4) is frequently used for the assessment of bioaccumulation of HOCs from sediments. According to this model, HOCs are distributed between the lipids of these organisms, the pore-water and the organic carbon of the sediment. Furthermore, it is assumed that partitioning between these three compartments is at equilibrium. Biota to sediment accumulation factors (BSAF) calculated according to the EqP assumptions and using log KOW based prediction equations are constant at about 1 to 2 kg organic carbon (OC) per kg lipid. Wong et al. (5) reported median field BSAF-values for fish and bivalves of 3.3 and 2.8 kg OC per kg lipid, respectively, and concluded that a BSAFmodel, based on the EqP, can indeed be used as a screening tool. However, the conclusions pertaining to fish and bivalves cannot be extrapolated to deposit-feeders because of differences with regard to physical contact to pore-water and ingestion of sediment particles. Tracey et al. (6) reported in an earlier review that field and laboratory BSAF-values may vary up to 2 to 3 orders of magnitude. This variability in BSAF-values limits the utility of the model for site-specific assessments of bioaccumulation. Laboratory studies have demonstrated that sequestration is one of the processes controlling the bioavailability of hydrophobic organic compounds (HOCs) such as PAHs (7, 8) and linear alkylbenzenes to benthic deposit-feeders. In a recent investigation, we provided direct evidence of a causal relationship between sequestration and BSAF (9). This relationship means that BSAF values decrease with increasing degree of sequestration. In most applications of EqP however, sequestration is neglected and log KOW-log KOC-relationships are employed which lead to prediction of constant BSAFvalues. Therefore, it is desirable to improve approaches to prediction of sediment bioaccumulation by taking sequestration into account. Earlier efforts to validate the concept of equilibrium partitioning of HOCs between pore water, biota, and sediment (10, 11) have been limited by the lack of reliable methods for measuring concentrations of freely dissolved concentrations in pore-water as the key parameter in this three phase partitioning system. As a result, data sets on the concentrations of HOCs in deposit-feeders, in sediment, and in porewater obtained in one experiment have been incomplete. Here, we report measurements of the freely dissolved porewater concentrations using matrix solid phase microextraction (SPME), a technique recently developed in our laboratory (12). These measurements were performed with sediments from Lake Oostvaardersplassen, that had been spiked 960 days prior to the measurements and that have been employed in a related study in which we investigated the relationship between sequestration and bioaccumulation (9). Combining the latter data with the freely dissolved concentrations yields a consistent set of data that is employed for a detailed analysis of the distribution of HOCs between sediment, pore-water, and sediment deposit-feeders. In this analysis we evaluate the relationships between sequestration and bioavailability from an equilibrium partitioning perspective. We aimed at refining the traditional application of the EqP such that sitespecific risk assessment of sediments can be improved.
Model * Corresponding author phone: (49) 211 797 7860; fax: (49) 211 798 7505; e-mail:
[email protected]. Present address: Henkel KGaA, D-40191 Duesseldorf, Germany. † Present address: National Environmental Research Institute of Denmark, P.O. Box 358, DK-4000 Roskilde, Denmark. ‡ Present address: Royal Haskoning, PB 8520, 3009 AM Rotterdam. 268
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The EqP model describes the distribution of HOCs between sediment and biota with three compartments, biota, one homogeneous sediment organic carbon pool, and pore-water, and assumes that the three compartments are in equilibrium (2). The model does not account for biomagnification of HOCs 10.1021/es020116q CCC: $25.00
2003 American Chemical Society Published on Web 12/06/2002
from sediments, which can occur as a result of selective feeding of labile organic matter as encountered in post-algal bloom situations (13) and selective retention of sedimentassociated HOCs in the digestive fluid of for instance Arenicola polychaetes (14). Biomagnification from sediment could lead to BSAF-values higher than expected based on equilibrium partitioning and would thus limit the applicability of the equilibrium partitioning approach. When no magnification from sediment organic carbon occurs, equilibrium partitioning prevails and the concentration in the organism can be described as
Cb ) BCF ‚ Cp
(1)
with Cb being the lipid-normalized steady-state concentration in the biota (µg/kg lipid), Cp (µg/L) the concentration in water, and BCF (L/kg lipid) being the lipid-normalized, aqueous bioconcentration factor. In addition, the EqP assumes that concentrations of HOC in the pore-water and in the organic carbon are in equilibrium and that the freely dissolved concentration in the pore-water (Cp) is related to the HOC concentration (organic carbon normalized) in the sediment (Cs,OC) according to eq 2, with Koc being the partition coefficient between sediment organic carbon and pore-water (L/kg organic carbon).
Cp ) Cs,OC/Koc
(2)
Conventionally, Koc is assumed to be independent of sediment properties and for risk assessment purposes it is often estimated on the basis of empirical relationships, e.g. that of Karickhoff et al. (15). Sediment sorption studies from recent years, however, provide evidence that the sediment cannot be viewed as one compartment (16). Investigations of the desorption kinetics demonstrate the presence of two or three subcompartments in the sediment that release HOCs with characteristic desorption rate constants (17). The fraction of the sediment-associated contaminant molecules, which resides in the slowly desorbing subcompartments, is considered sequestered (18). McGroddy et al. (19) found that only a fraction of 0.01 to 0.4 of sediment associated PAHs appears to be involved in equilibrium partitioning with the pore-water. Desorption kinetic studies in field and laboratory contaminated sediments have revealed highly different sequestration for different compounds and sediments (8, 20, 21): the rapidly desorbing fractions were found to range between 0.06 and 0.87. In a number of studies bioaccumulation of sediment-associated HOCs was shown to decrease with an increasing degree of sequestration. Recently we concluded that the decrease of BSAF with decreasing rapidly desorbing fractions (Frap) is due to a causal relationship between sequestration and biological availability (9). Van Noort and co-workers (22, 23) suggested that the rapidly desorbing fraction of sediment-associated HOCs exchanges sufficiently rapid with the water such that there is equilibrium between CP and the rapidly desorbing fraction of the HOCs in the sediment. As a result, CP is established by equilibrium partitioning between the rapidly desorbing fraction of HOCs in sediment and the water. In eq 3, we adopt their suggestion in order to refine the description of sediment bioaccumulation of HOCs. It explicitly accounts for sequestration by relating Cp to Frap and the concentration in sediment via the partitioning coefficient Koc,rap that pertains to the equilibrium between the rapidly desorbing fraction and the pore-water. The term Cs,OC ‚ Frap specifies the concentration of rapidly desorbing compounds.
Cp ) Cs,OC ‚ Frap/Koc,rap
(3)
FIGURE 1. Model of distribution of hydrophobic organic chemicals in sediments according to the adapted concept. BCF ) bioconcentration factor (L/kg lipid); rap ) rapidly desorbing compartment; slow ) slowly desorbing compartment; Koc,rap ) partition coefficient between rapidly desorbing compartment and pore-water (L/kg oc). The arrows symbolize transport processes. From comparison of eqs 2 and 3 it follows that, according to the adapted concept, Koc varies with sequestration (eq 4).
Koc ) Koc,rap/Frap
(4)
A schematic picture of the refined model is shown in Figure 1. The system consists of three compartments, lipid of the deposit feeders, pore-water and sediment organic carbon, the latter being subdivided into two kinetically distinct fractions. The pairs of solid arrows indicate equilibrium partitioning processes between biota and pore-water and pore-water and the rapidly desorbing fraction of HOCs in sediments. The respective equilibria are characterized by the partitioning coefficients BCF and Koc,rap. The broken arrows pointing from the slow to the rapid compartment in the sediment and vice versa indicate the exchange processes between these compartments.
Experimental Section Chemicals. Test compounds phenanthrene (PHE), fluoranthene (FLU), benz[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), 1,2,3,4-tetrachlorobenzene, pentachlorobenzene, hexachlorobenzene, p,p′-DDE (2,2-bis(4chlorophenyl)-1,1-dichloroethylene), 2,2′,5,5′-tetrachlorobiphenyl (PCB 52), 2,3,5,6-tetrachlorobiphenyl (PCB 65), 2,2′,4,5,5′-pentachlorobiphenyl (PCB 101), 2,3,3′,4,4′-pentachlorobiphenyl (PCB 105), 2,3′,4,4′,5-pentachlorobiphenyl (PCB 118), 2,2′,3,4,4′,5′-hexachlorobiphenyl (PCB 138), 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB 153), 2,3,3′,4,4′,5-hexachlorobiphenyl (PCB 156), and 2,2′,3,4,4′,5,5′-heptachlorobiphenyl (PCB 180) and internal standards 2-ethylanthracene, 7-methylbenzo[a]pyrene, 2,3,3′,5,6-pentachlorobiphenyl (PCB 112), and 2,2′,4,4′,5,6′-hexachlorobiphenyl (PCB 154), were at least 98% pure, obtained from various commercial sources (PCB numbering according to IUPAC) and used as received. Optical fiber with a glass core diameter of 200 µm and a poly(dimethylsiloxane) (PDMS) coating of 15 µm was supplied by Fiberguide Industries (Stirling, NJ). The fiber was cut to 100 mm long pieces and purified by washing twice with methanol and Millipore water prior to use in the experiments. Sediments. Sediment originating from the relatively unpolluted lake Oostvaardersplassen (The Netherlands) was contaminated in the laboratory with the test chemicals and allowed to age for almost 32 months. This sediment was used in bioaccumulation studies reported earlier (9), and a subsample of it was split in two portions for the present study. The first portion was kept untreated. The second portion was suspended in the presence of a large amount of TENAX (30 g of TENAX/30 g of organic carbon) for 48 h. In that manner, a treated sediment aliquot was obtained in VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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which the rapidly desorbing fraction of HOC in the sediment was reduced while the sediment quality remained unaltered. Measurements of the Pore-Water Concentrations. Concentrations of freely dissolved HOCs in the pore-water of the untreated sediment were measured with matrix-SPME, developed by Mayer et al. (12). To that end, three 40 mL glass vials were filled with 35 g of wet untreated sediment. In each vial two fibers were brought in contact with the sediment by inserting the fibers through the silicone/PTFE septum of the vial caps. The vials containing the sediment and the fibers were then placed on a shaking device (200 rpm, 3-mm orbit) at 25 °C. Fibers were sampled after 33 days. This duration exceeds the time that is necessary for equilibration of HOCs between sediment and fibers (12). Upon withdrawing the fiber from the sediment-filled vial, it was inserted into the injector of the gas chromatograph within 10-20 s for gas chromatographic analysis. Removal of the rapidly desorbing fraction the TENAX treatment induces transport of molecules from the slowly to the rapidly desorbing fraction (9). This might be reflected in the nondepletive-SPME measurements if they are performed over more than a few days. To obtain the freely dissolved pore-water concentration in the sediment at the end of the bioaccumulation experiment, we performed 48 h experiments in which two fibers were brought into contact with 11 g of wet sediment in 15 mL glass vials in the same manner as described above. The vials were subsequently agitated on the shaking device and sampled after 48 h. Steady-state had not been reached for a major part of the compounds during this limited contact time. These short-term measurements were performed in the treated and the untreated sediment such that a ratio of treated vs untreated (rT/U) can be obtained. GC-MS Analysis. A Varian 3400 CX gas chromatograph equipped with a 1078 programmable injector, a 30-m DB5MS capillary column, with an I.D. of 0.25 mm and a film thickness of 0.25 µm, and a Saturn 2000 Ion Trap massspectrometric detector were employed for the analysis of the fibers. The insert liner of the injector had an internal diameter (I.D.) of 0.8 mm. The injector was operated in splitless mode, with a splitless time of 15 min. The initial temperature of the injector of 60 °C was held for 0.2 min, then increased at 150 °C/min to 250 °C, and remained at that temperature for 15 min before cooling down. The oven temperature was initially held at 70 °C for 15 min and then increased to 290 °C at a heating rate of 10 °C/min. The final temperature was held for 3 min. The mass spectrometer was operated in the EI (Electron Impact) (electron energy 70 eV) and SIS (Selected Ion Storage) cluster analysis mode with a scan time of 0.6 s. The ions employed for detection of the test compounds are listed in Table 1. Quantification. Injection of standard solutions was employed to calibrate the amounts of test compounds introduced by fibers into the GC. The concentration in the fiber was calculated as the ratio of the mass of compounds desorbing from the PDMS coating and the volume of the part of the PDMS coating that was thermally desorbed () lower part of 56 ( 2 mm). Subsequently, freely dissolved pore-water concentrations of the test compounds in the untreated sediment (CP,U) were calculated as the ratio of the concentration in the PDMS coating and partition coefficients for the PDMS and water (KPDMS, water) that were determined in our laboratory (24). For compounds for which experimental KPDMS, water values were not available, KPDMS, water was estimated using the regression equation reported by Mayer et al. (24). Limits of detection (LOD) were estimated using the lowest standard solution giving a signal at least three times higher than the background. The pore-water concentrations in the treated sediment CP,T were calculated as CP,T ) CP,U × rT/U. This approach gives an estimation of the freely dissolved concentration at the end of the bioaccumulation experiment. 270
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TABLE 1. Mass over Charge Ratio (m/z) Used for GC-MS Detection m/z and log Kow Values of the Chemicals Tested, the Concentrations of Freely Dissolved Compounds in Pore-Water (Cp) (pg/L) in Untreated (U) and Treated (T) (48 h Suspension with Tenax) Lab-Contaminated Sedimenta Cp (pg/L)
m/z
log Kowb
1,2,3,4 penta hexa
216 250 285
Chlorobenzenes 4.62d 1200 (170) 5.18b 1500 (270) e 5.62 400 (40)
52 65 101 105 118 138 153 156 180
292 292 326 326 326 361 361 361 396
6.3c 6.1c 6.55 6.6c 6.8c 6.9 f 6.9 f 7.5c 8.1c
PHE FLU BaA CHR BbF BkF BaP
178 202 228 228 252 252 252
4.57e 5.23e 5.91e 5.81e 6.1e 6.11e 6.13e
p,p′-DDE
318
Other Compounds 6.96b 660 (40)
U
PCBs 2900 (140) 5200 (480) 560 (4) 520 (30) 800 (80) 270 (5) 420 (30) 27 (8) 120 (10) PAHs 3600000 (300000) 650000 (40000) 51000 (1000) 73000 (2000) 10000 (7000) 8000 (6000)
