Environ. Sci. Technol. 2005, 39, 141-148
Importance of Black Carbon to Sorption of Native PAHs, PCBs, and PCDDs in Boston and New York Harbor Sediments R . L O H M A N N , * ,† J . K . M A C F A R L A N E , A N D P. M. GSCHWEND Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 48-415, Cambridge, Massachusetts 02139
The solid-water distribution ratios (Kd values) of “native” PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sedimentwater suspensions. Observed solid-water distribution coefficients exceeded the corresponding focKoc products by 1-2 orders of magnitude. It was hypothesized that black carbon (fBC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via Kd ) focKoc + fBCKBCCwn-1 with Cw in µg/L. Predictions based on published Koc, KBC, and n values for phenanthrene and pyrene showed good agreement with observed Kd,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbonnormalized adsorption coefficients, KBCs, were deduced for these contaminants. Log KBC values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log KBC ≈ 0.83 log γwsat - 1.6; R 2 ) 0.99, N ) 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.
Introduction The fate and effects of hydrophobic organic compounds (HOCs) in the environment are largely determined by their sorption to solid phases. For example, HOC releases to lake waters and coastal seawaters from contaminated sediment beds, accumulation by benthic animals, and biodegradation by microorganisms are processes that all depend on that HOC’s sorption in the situation of interest (1). For many years now, investigators have noted that sorption of HOCs to natural sediments is not always consistent with predictions based on an absorption model ( focKoc) that considers all organic matter as equally sorptive (2-15). Since investigators of polycyclic aromatic hydrocarbons (PAHs) long ago found * Corresponding author phone: +49-421-218-8683; fax: +49-421218-8664; e-mail:
[email protected]. † Present address: Research Center for Ocean Margins, Universita ¨t Bremen, D-28334 Bremen, Germany. 10.1021/es049424+ CCC: $30.25 Published on Web 12/02/2004
2005 American Chemical Society
that these HOCs were commonly associated with “active carbon particles” in sediments (16-18), we have hypothesized that this sorption modeling discrepancy is often due to the presence of soots or chars (hereafter referred to as black carbon, BC particles) derived from combustion processes. This might be especially important for compounds such as PAHs and polychlorinated dibenzo-p-dioxins (PCDDs) that are co-generated with BC. Using spiking experiments, various workers have found that soots, as well as coal dust and naturally occurring coal and coaly particles, can be important for the sorption of PAHs (10, 11, 19, 20). All these carbonaceous solids may also be important sorbents, analogous to granular activated carbon, for other HOCs like polychlorinated biphenyls (PCBs). This may be especially true for HOCs that can assume a planar conformation and thus maximize their contact with planar adsorbent surfaces (21). In this paper, we examine the solid-water distributions of naturally incorporated (“native”) PAHs, PCDDs, and PCBs, long present in sediment beds from Boston and New York harbors. By using the native sorbates, we sought to avoid the concern that spiked HOCs may not distribute into the solids in the same manner as the compounds long present in the environment. The Boston and New York harbor sediments were chosen to emphasize sorption to soots and chars since previous workers have shown the mostly pyrogenic origin of the PAHs in these locales (4, 22). Coal shipping, storage, and use have not been prominent in these regions for the last 50 years, suggesting that the BC present in the top sediments is primarily composed of sootlike BC rather than coal. Using these harbor sediments in a series of laboratory experiments, we determined the overall solid-water distribution coefficients (Kd,obs) of such compounds without adding “spiked compounds” to the sediment slurries. Equilibrium dissolved concentrations were deduced by including small passive samplers, pieces of polyethylene (PE), in the incubations (23-26). The PE takes up organic compounds from the solidwater system until phase equilibrium is reached. At equilibrium, the measured PE concentrations, together with the corresponding PE-water partition coefficients (KPEw), can be used to quantify the compounds’ freely dissolved concentrations. While previous observations have long indicated the importance of BC to PAHs (7, 16-18), we also sought to analyze other hydrophobic compounds in the same sediments including PCBs and PCDDs. Since these chlorinated, aromatic compounds differ in that PCDDs are planar molecules such as PAHs, while many PCB congeners often do not prefer planar conformations, we wanted to evaluate the role of sorbate shape.
Materials and Methods Overview. PEs were deployed as passive in situ equilibrium samplers (23-28) in the sediment incubations. Low-density 51-µm-thick polyethylene sheet (Carlisle Plastics, Minneapolis, MN) was cut into small pieces and precleaned by extracting for 24 h in dichloromethane (DCM) twice, then methanol, and finally milli-RO water (AriesVaponics, Rockland, MA). When a piece of PE is placed in a sediment-water slurry, the HOCs accumulate within the PE until phase equilibrium occurs. In previous work in our laboratory (23, 24), studies of the kinetics of the phenanthrene, pyrene, and 2,2′,5,5′-tetrachlorobiphenyl uptake into such cleaned PE from water revealed equilibration required less than a week, consistent with other literature reports (27). With this time frame in mind, the solvent-cleaned PE was characterized with respect to equilibrium partition constants for individual PAHs, PCBs, and PCDDs at room temperature (the same VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
141
TABLE 1. Partition Coefficients of Selected Compounds: Octanol-Water (Kow), Organic Carbon-Water (Koc), Polyethylene-Water (KPEw), and Aqueous Activity Coefficients at Saturation (γwsat) compd log log log log
Kowa γwsat b Kocc KPEwd
phen
pyr
bap
#44
#66
#95
2,7/8-Cl2DD
2,3,7-Cl3DD
4.5 6.2 4.2 4.5
5.0 6.8 4.7 5.0
6.0 8.2 5.7 6.3
5.8 7.7 5.1 5.3
6.2 8.1 5.5 6.1
6.1 8.0 5.4 5.9
5.7 7.7 5.4 5.5
6.3 8.5 6.0 6.2
a Mackay et al. (39) for PAHs. Hawker and Connell (37) for PCBs. Shiu et al. (38) for PCDDs. b Schwarzenbach et al. (1). log γ sat (PAHs) ) (log w Kow + 0.13)/0.75. log γwsat (PCBs) ) (log Kow + 0.70)/0.85. log γwsat (PCDDs) ) (log Kow + 0.79)/0.84. c Xia (36). log Koc (PAHs, PCDDs) ) 0.97 × log d Kow - 0.12. log Koc (PCBs) ) 0.97 × log Kow - 0.50. Adams (23). log KPEw (PAHs) ) 1.2 × log Kow - 0.97. log KPEw (PCBs) ) 1.8 × log Kow - 4.9. log KPEw (PCDDs) ) 0.99 × log Kow - 0.09.
temperature used in our sediment-water incubations). The partition coefficients (KPEw), defined as the ratio of the compound concentration in the PE (CPE) over the dissolved concentration (Table 1):
KPEw ) CPE/Cw (mol/g PE)(mol/mL water)-1
(1)
were reproducible to within 0.1 log units (24). Knowledge of the HOC’s PE-water partition coefficients allowed us to quantify the concentrations of the dissolved HOCs in equilibrium with the PE sampler and the sediments. Experimental Procedure. Wet sediment, water, and a small piece of PE were weighed into each of three foil-clad flasks and tumbled end-over-end in the dark for periods of up to 6 months. Each flask had a different size of PE (between 0.12 and 0.72 g). The masses of PE were chosen so that even the largest ones accumulated less than 5% of the individual sorbates at system equilibrium. At increasing time intervals, the PEs were removed, extracted, and analyzed for individual PAHs, PCBs, and PCDDs. After air-drying, the cleaned PEs were re-introduced into the flasks and subjected to further tumbling. In short incubations (less than 1 week), we saw that the smallest PEs exhibited greater HOC concentrations, CPE, than the larger-PE replicates. This implied uptake by the PE was not limiting the rate of approach to system equilibration since all PEs had the same surface-area-to-mass ratio. Hence, like-CPE values in small, medium, and large PE pieces were taken as evidence of sediment-water-PE equilibrium. Experiments were carried out with two different sediments: one from Quincy Bay in Boston Harbor (BH) and one from the Hudson River estuary in New York Harbor (NYH). Boston Harbor. Grab samples of surface sediments were collected in North Quincy Bay, near Boston, MA, in December 1999. The sediment sample is described in more detail in Accardi-Dey and Gschwend (13). Three 500-mL glass flasks were used with same amount of sediment (20.9 g dry weight, determined from subsampled wet sediments) and water (450 mL), but increasing PE sizes (0.115, 0.262, and 0.642 g). A fourth glass flask served as a blank, consisting of 500 mL water and PE (0.35 g). The PEs were removed and analyzed after a succession of 3, 4, 17, 125, and 182 days of sedimentwater exposures. After each extraction, the cleaned PEs were re-introduced into the flasks; this marked the beginning of a new tumbling experiment with the tumbling time set back to zero. The total amount removed by PE throughout the sequence of five tumbling experiments was around 10% for PAHs and 20% for PCBs. New York Harbor. A 50 cm × 50 cm box corer was deployed from the M/V Samantha Miller to obtain a sediment core in the Hudson River (NYH) on October 4, 2000. The site was situated at the estuarine turbidity maximum zone (40°49′20.9′′ N, 73°58′26.2′′ W; more details are given in ref 29, in which the site is designated S11). A composite sample made up of the top 4 cm of the core was homogenized and subsampled for the tumbling experiment. For the NYH sediment tumbling, a higher sediment-water ratio than used for the BH 142
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 1, 2005
experiment was chosen to shorten the times to reach phase equilibrium. Three 250-mL glass flasks were used with same amounts of sediment (40.0 g dry weight) and water (200 mL) but increasing PE sizes (0.269, 0.434, and 0.722 g). A blank consisting of water and PE (0.38 g) was also run in parallel. The PEs were removed, extracted, and analyzed for PAHs, PCBs, and PCDDs after three consecutive 14-day sedimentwater exposures. In the NYH sediment, the accumulated removals were around 20% of PAHs and PCBs and about 10% of PCDDs. Analytical Procedure. Upon recovery of the PEs from the slurries, they were rinsed with milli-RO water, dried with Kimwipes, and extracted consecutively for 24 h in 60 mL DCM and again in 60 mL hexane. Internal standards (25 ng individual deuterated PAHs, 5 ng individual PCB congeners, and 1.25 ng 13C12-labeled PCDDs per PE) were added prior to extraction. Prior to tumbling, and again after each sediment’s sequence of incubations, aliquots of sediment were analyzed for the PAHs, PCBs, and PCDDs. The sediment samples were Soxhlet-extracted consecutively for 24 h with hexaneacetone (90-10 v:v) followed by DCM. Internal standards (250 ng individual deuterated PAHs, 5 ng individual PCBs, and 12.5 ng individual 13C12-labeled PCDDs per sample) were added to the sediments prior to extraction. The combined extracts were dried over anhydrous Na2SO4, concentrated to ca. 1 mL, and analyzed for PAHs and PCBs. For PCDDanalysis, sediment sample extracts were cleaned up using a mixed silica gel column (silica gel from Merck, grade 60; 2-cm-diameter column containing, from bottom to top: anhydrous Na2SO4, 2.5 g activated silica gel, 2.5 g basic silica gel (33% w:w 1 M NaOH), 2.5 g activated silica gel, 5 g acid silica gel (44% w:w concentrated H2SO4), 2.5 g activated silica gel, and anhydrous Na2SO4. Extracts were eluted with 125 mL hexane. All samples were then fractionated on a basic alumina column (1-cm-diameter column containing 4.5 g Super alumina 1 B from ICN) and eluted with 20 mL DCMhexane (7:93 v:v, F1) followed by 25 mL DCM/hexane (50:50 v:v, F2-contained PCDDs). F2 was spiked with 50 µL of deuterated p-terphenyl at 125 pg/µL in decane (injection standard) and reduced to its final volume before GC-MS analysis. Quantification. All samples were analyzed by GC-MS. Analytes were separated on a 30 m × 250 µm i.d. DB5-MS capillary column with a 0.25-µm film thickness (J &W Scientific, Folsom, CA) in an HP 6890 GC connected to a JEOL GCmate MS operating at a resolving power of 500 (PAHs, PCBs) or 1000 (PCDDs). Samples (1 uL) were autoinjected in the splitless mode with the injection port held at 280 °C. The MS was operated in EI+ mode with selected ion monitoring (SIM). The following compounds were analyzed: phenanthrene (Phen), anthracene (An), sum of methylphenanthrenes (∑Me-phen), fluoranthene (Fluo), pyrene (Pyr), benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), perylene (Per), PCB congeners 2,2′,3,5′-tetrachloro-
TABLE 2. Sedimenta (ng/gdw) and Dissolved Concentrationsb (ng/L) Found in Laboratory Incubations of Boston Harbor (BH) and New York Harbor (NYH) Bed Sedimentsd compd
phen
pyr
mean Csed SD mean Cw SD
9000 2000 1100 420
11000 4500 560 91
mean Csed SD mean Cw SD
310 13 94 29
740 63 32 7
bap 3200 350 10 3 500 43 g0.71 0.04
PCB 44 4300 2100 180 49 13
