Environ. Sci. Technol. 1992,26, 1621-1626
Happel, J.; Brenner, H. Low Reynolds Number Hydrodynamics; Noordhoff International: Leyden, The Netherlands, 1973; p 553. Cox, R. G.; Mason, S. G. Annu. Rev. Fluid Mech. 1971,3, 291-316. Goldsmith, H. L.; Mason, S. G. J . Colloid Sei. 1962, 17, 448-476. Ho, B. P.; Leal, L. G. J. Fluid Mech. 1974, 65, 365-400. Halow, J. S.; Wills, G. B. J . AZChE 1970, 16, 281-286. Brenner, H. Chem. Eng. Sei. 1961, 16, 242-251. Robertson, J. M. Hydrodynamics in Theory and Applications; Prentice Hall: Englewood Cliffs, NJ, 1965; p 652. Tien, C. Granular Filtration of Aerosols and Hydrosols; Butterworth: Boston, MA, 1989; p 365. Cox, R. G.; Brenner, H. Chem. Eng. Sci. 1968,23,147-173. Schonberg, J. A.; Hinch, E. J. J . Fluid Mech. 1989,203, 517-524. Drew, D. A.; Schonberg, J. A,; Belfort, G. Chem. Eng. Sci. 1991,46, 3219-3224. Gregory, J. J . Colloid Interface Sci. 1981, 83, 138-145. Gregory, J. Crit. Rev. Environ. Control 1989,19,185-230. Czarnecki, J. J . Colloid Interface Sci. 1979, 72, 361-362. Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday SOC. 1966, 62, 1638-1651. O'Melia, C. R. J . Environ. Eng. 1985, 111, 874-890. Peters, M. H.; Gupta, D. AZChE Symp. Ser. 1984,80, No. 234, 98-105. Ridgway, H. F.; Rigby, M. G.; Argo, D. G. J.-Am. Water Works Assoc. 1985, 77, 97-106. Granger, J.; Dodds, J.; LeClerc, D.; Midoux, N. Chem. Eng. Sci. 1986,41, 3119-3128. Chellam, S. M.S. Thesis, Department of Environmental Science and Engineering, Rice University, Houston, TX, 1991, p 175.
(37) Romero, C. A.; Davis, R. H. J . Membr. Sci. 1988, 39, 157-185. (38) Davis, R. H.; Leighton, D. T. Chem. Eng. Sci. 1987, 42, 275-281. (39) Leighton, D.; Acrivos, A. Chem. Eng. Sci. 1986, 41, 1377-1384. (40) Bhatty, J. I.; Reid, K. J.; Dollimore, D.; Shah,T. H.; Davies, L.; Gamlin, G. A.; Tamini, A. Sep. Sci. Technol. 1989,24, 1-14. (41) Russel, W. B. J . Fluid Mech. 1978, 85, 209-232. (42) Green, G.; Belfort, G. Desalination 1980, 35, 129-147. (43) Porter, M. C. Znd. Eng. Chem. Prod. Res. Develop. 1972, 11,234-248. (44) Adham, S. A.; Snoeyink, V. L.; Clark, M. M.; Bersillon,J.-L. J.-Am. Water Works Assoc. 1991,83, 81-91. (45) Wiesner, M. R.; Clark, M. M.; Mallevialle, J. J . Environ. Eng. 1989,115, 20-40. (46) Bhatty, J. I.; Reid, K. J.; Dollimore, D.; Shah,T.H.; Gamlen, G. A.; Tamini, A. Sep. Sci. Technol. 1989, 24, 165-178. (47) O'Melia, C. R. In Aquatic Surface Chemistry, 1st ed.; Stumm, W., Ed.; John Wiley and Sons: New York, 1987; Vol. I, Chapter 14. (48) Barnes, H. A.; Edwards, M. F.; Woodcock, L. V. Chem. Eng. Sci. 1987, 42, 591-608.
Received for review January 27, 1992. Revised manuscript received April 7,1992. Accepted April 21,1992. Support for this work was provided by the National Science Foundation, (Grant BCS-8909722). Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Partitioning of Polycyclic Aromatic Hydrocarbons to Marine Porewater Organic Colloids Yu-Ping Chin and Phlilp M. Gschwend"
Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Fluorescence quenching was used to measure the binding of pyrene and phenanthrene to marine interstitial water organic colloids from Boston Harbor, MA. Both pyrene and phenanthrene were sorbed by porewater colloids (Koccolloid -105.0 and 104.3,respectively) from a heavily contaminated nearshore site. Pyrene had a significantly lower affinity toward colloids from a cleaner location (Kmmuoid104.5).Sediments from the former site were also observed to be especially effective sorbents for these compounds relative to expectations based on the literature. The high sorption coefficients may be due to the high lipid content of these sediments and colloids. Alternatively, they may be due to a very substantial nonpolar character of the natural organic matter there. N
Introduction The sediments of many coastal environments have been contaminated by refractory nonpolar organic compounds (NOCs). Such pollution has included pesticides, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) (1-5). While efforts are underway to limit or stop the influx of these deleterious substances into receiving waters, NOCs currently in the sediments may continue to be harmful to benthic organisms for years and decades to come. Furthermore, if these compounds are 0013-936X/92/0926-1621$03.00/0
released back to the water column, organisms throughout the ecosystem will be exposed to them. In the past, it was widely believed that strongly sorbed NOCs were rendered immobile in sediment beds. However, interstitial water organic colloids have recently been shown to affect the distribution of particle-reactive organic substances between pore fluids and their associated sediments (6-10). A number of controlled studies have demonstrated that humic substances, which we believe comprise a significant component of the organic colloids, can bind NOCs and therefore enhance the amount of nonpolar compounds in the aqueous phase beyond simple twephase equilibrium expectations (11-14). Thus, the presence of colloids in porewaters could play a major role in the fate and transport of stable and toxic organic substances in nearshore sediments. Several hypotheses have been forwarded recently stating that humic substances can bind NOCs in a manner that is analogous to partitioning by sediment organic matter; that is, the binding phenomenon is largely controlled by the hydrophobicity of the organic compounds. As a result, empirical relationships between colloid-NOC binding and solute physicochemical paramcoefficients (Kmco"Oid) eters, such as the n-octanol-water partition coefficient and aqueous solubility, have been established (15, 16). A conceptual model that quantifies the molecular interac-
0 1992 American Chemical Society
Environ. Scl. Technol., Voi. 26, No. 8, 1992 1821
tions between a nonpolar organic compound and the humic matrix using the Flory-Huggins equation has also been developed (12,17,18). While this approach has successfully predicted for a number of NOCs with different humic substrates, some critical information regarding the physicochemical properties (i.e., solubility parameters) of the humic polymer is still lacking. As part of our efforts to understand the role of colloids in the release of NOCs from sediments to overlying waters, we have studied the partitioning of pyrene and phenanthrene to both marine interstitial water colloidal and sediment organic matter from samples taken in Boston Harbor, MA. Colloid and sediment samples from different depths and sites were used to ascertain spatial variabilities in the organic-carbon-normalized partition coefficient, K,. Measurements to determine the properties of the colloidal and sediment organic matter (size, percent organic carbon, lipid content, extinction coefficients) were also performed. Finally, we used data gathered from the above analyses to elucidate porefluid colloid solubility parameters, using an equilibrium partition model based on the Flory-Huggins concept, and compared these estimates with results from other investigators.
Materials and Methods Sampling Protocol. Sediment box cores were taken from two sites in Boston Harbor: (1)at the mouth of Fort Point Channel (FPC) and (2) near Spectacle Island (SI). The cores were sectioned under nitrogen, and a portion of the sediments was transferred to nitrogen-purged 250mL ground-glass-stoppered centrifuge tubes. The remainder was freeze-dried and stored for later use. Porewaters were separated from the wet sediments by centrifugation at 600g for 20 min. The supernatants were drawn off, purged with prepurified nitrogen, and stored in sealed 20-mL glass syringes standing plunger side down under water in the dark at 4 “C until further use. All noncolloidal particles were allowed to settle onto the syringe plungers, and subsequent subsampling was performed to avoid resuspending these solids. Sample pH values ranged from 7.7 to 8.1. Sediment and Colloid Analysis. Sediment organic carbon was determined by weight loss on ignition at 450 OC for 24 h and by assuming the weight loss was half carbon. Porewater organic carbon was measured using an Ionics 555 TOC analyzer (Ionics Inc., Watertown, MA). This instrument is based on the high-temperature platinum catalyst method developed by Sugimura and Suzuki (19). This method proved to be precise (f5%) and was not affected by major ions in the sample matrix. Sediments were extracted using methylene chloride to determine their lipid content and background PAH concentration. Freeze-dried sediment (5 g) was Soxhlet-extracted for 72 h with methylene chloride and the extract concentrated using a Buchler rotoevaporator. Aliquots from the extract (500 pL) were evaporated, and total lipids were determined gravimetrically. The PAH fraction was separated from the remainder of the extract using alumina column chromatography and assayed by combined gas chromatography-mass spectrometry (GC-MS). Total pyrene and phenanthrene in our sediments were in the range of 3.8-7.4 and 1.2-1.7 ppm, respectively, but these levels were sufficiently small so as to not contribute significantly to the total sorbate loads in our sorption protocol. Ultrafiltration using Centricon microconcentrators (Amicon, Danvers, MA) equipped with 3000 MWCO membranes (600 MWCO based on the configuration of random coil macromolecules) was used to separate organic 1822 Environ. Sci. Technol., Vol. 26, No. 8, 1992
colloids from our porewaters (20). Each device was extensively cleaned by rinsing with methanol-water and then distilled water prior to use to remove any organic impurities associated with the membrane. The raw porewater sample (2 mL) was added to the top reservoir, capped, and centrifuged until the volume of liquid from the upper reservoir passed through the membrane into the lower compartment. Raw and ultrafiltered porewater was assayed by organic carbon analysis and the colloid concentration was determined by difference. Control studies using vitamin BIz showed that losses to the membrane were minimal (-6%). PAH-Colloid Binding Studies. Fluorescence quenching (13, 21) was used to study the binding of phenanthrene and pyrene to porewater organic colloids taken from the two sites. Fluorescence measurements were obtained using a Perkin-Elmer LS-5 fluorescence spectrophotometer. Optimum excitation-emission wavelength pairs were determined to be 230 nm/390 nm for pyrene and 230 nm/373 nm for phenanthrene. Interfering fluorescence from the organic colloids was observed to be minimal at these wavelengths. Saline solutions ( I = 0.6 M NaC1) were spiked with 10 pL of the probe dissolved in either acetonitrile or methanol. Saline solutions containing no PAHs were used as blanks to quantify the amount of interfering fluorescence from the colloids. Both the blank and the PAH-spiked saline solution were transferred to quartz cuvettes, and initial fluorescence emissions were recorded. Following this, 100 pL of raw porewater was added to each cuvette and allowed to equilibrate. An additional control cuvette containing only pyrene was spiked with the saline solution (at 100-pL increments) to determine dilution effects on the fluorescence of the compound. Previous investigators (13,21)observed short equilibration times (less than 1 min). We allowed our samples to sit for up to 10 min before making another reading. This process was conducted five to six times, depending upon the amount of pore fluid available. In between fluorometric readings, the cuvettes were transferred to a Beckman DU-7 UV/vis spectrophotometer for absorbance measurements to determine “inner-filter effects”. The experiment was repeated using ultrafiltered porewater to ascertain the effects of either dynamic or static quenching by noncolloidal chemical species. The corrected fluorescence data along with measured colloid concentrations (expressed as organic carbon) were used to determine the organic-carbon-normalized equilibrium binding constants, Koplloid. Sediment Sorption Experiments. Sorption experiments were carried out in 50-mL Corex (VWRScientific) centrifuge tubes with Teflon-lined screwcaps. Freeze-dried sediment was added to each tube (25 mg), followed by a 50-mL aliquot of bicarbonate-buffered (1mM) saline solution (0.6 M NaC1). These suspensions were allowed to rehydrate for several hours. Pyrene or phenanthrene, dissolved in an organic solvent (acetonitrile or methanol), was added to each vessel by direct injection to yield initial solute concentrations ranging from 20 to 100 (pyrene) or 50 to 250 pg/L (phenanthrene). Each set of experiments included a blank comprised of saline solution and sediments (no PAH). Control experiments,using only buffered saline solution and the sorbates, were also conducted to determine solute losses to walls and cap liners. Each tube was capped, wrapped in aluminum foil, and tumbled on a rotary tumbler for 30 h. Following equilibration, each tube was centrifuged for 45 min at lOOOg to separate the solid and liquid phases. A portion of the supernatant was pipeted into a quartz cuvette and assayed by fluorescence
Table I. Sediment and Colloid Organic Matter Properties" property porewater total organic carbon, mg of C/L porewater L (mol of C)-1 porewater OC, mg of C/L organic carbon fractn sediment, % lipid fractn sediment,
FPC SI 7-9 cm 15-17 cm 25-29 cm 14-16 cm 19.9
nd
24.4 150
31.8
37.9
211
e
2
92
7.3
7.7
13
21.5
5.47
5.19
5.23
3.34
0.97
1.01
0.49
0.26
8.8
9.6
4.7
3.9
.95
%
lipid fractn sediment org matter, %
j
.9J 0
"OC, organic colloid; organic matter is taken as twice organic carbon. nd, not determined. e, molar absorptivity. FPC, Fort Point Channel site. SI, Spectacle Island site. b A t 280 nm.
- .5.
.
. - 1.5.
1
t
2
3
2.5
3.5
1
4
OC (mglL) Flgure 1. Quenching of pyrene by unaltered (circles) and ultrafiltered (squares) Fort Point Channel porewater (7-9-cm interval). Solid lines show best fi, whlle dashed lines indicate 95% confidence intervals.
spectrophotometry using the wavelength pairs described previously. The amount of interfering fluorescence from desorbed sediment organic matter was measured by assaying the blank supernatant. PAH-sediment partition coefficients were determined using (1) Kp = [(Fo- F ? / p l / F '
values reported for continental shelf sediments, but it is comparable to values in other polluted marine environments (23). Recently, Boyd and Sun (25) observed that soils and sediments contaminated with dielectric fluids or petroleum hydrocarbon residues sorbed NOCs better than uncontaminated soils. They hypothesized that an organic contaminant can partition more favorably into the oily residues than the natural soil organic matter, and they were able to predict this enhanced partitioning phenomenon using a two-compartment equilibrium model. Therefore the high lipid content, which is probably comprised of anthropogenic hydrocarbons in the Fort Point Channel sediments, may enhance the sorption of NOCs like PAHs to these solids. Binding of PAHs to Porewater Colloids. Pyrene fluorescence was quenched by organic colloids from the raw sedimentary porewater (Figure 1). A linear relationship existed between inverse fluorescence of the probe and the concentration of organic carbon added. We attribute this drop in fluorescence,when unaltered porewater is added, to interactions between pyrene and colloids. Phenanthrene fluorescence exhibited similar behavior. This correlation is described by the Stern-Volmer equation (21),where the slope of the line is equivalent to KocColloid if static quenching is the only operative mechanism. Fort Point Channel colloid-PAH binding constants were very large and in some cases exceeded the compound's octanol-water partition coefficient; the Spectacle Island colloid-pyrene KO,was significantly smaller (Table 11). Similarly high humic substance-PAH partition coefficients have been reported by others (21,26,27)using fluorescence quenching. We suspect other mechanisms besides colloid binding could contribute to these observations.
where Fo is the PAH fluorescence in the control vial, F' is the blank-corrected fluorescence of the PAH in the sample bottle (with sediments), and p is the solids concentration (g/mL). Results and Discussion Sediment-Colloid Organic Matter Properties. The Fort Point Channel site is located in an area in close proximity to intensive anthropogenic activity, while the Spectacle Island station is offshore and less affected. Porewater total and colloidal organic carbon from both locations were present in milligram of C per liter quantities and increased with depth at Fort Point Channel (Table I). Colloidal organic matter extinction coefficients (4 measured a t 280 nm are consistent with values reported by Stuermer (22) for marine humic substances but substantially lower (by as much as a factor of 10) than those determined for terrestrial humic substances (IO). This suggests that our interstitial water organic matter either has a marine origin or is composed of a mixture of terrestrial humic substances diluted by other organic materials that do not absorbed at 280 nm. The organic carbon contents for all samples fell within the range of values reported by others for nearby marine sediments (7, 23, 24). Sediments from the Fort Point Channel site contained significant amounts of methylene chloride-extractable compounds, referred to here as lipids (Table I). The lipid fraction is considerably higher than
Table 11. Observed PAHColloid and PAH-Sediment Organic-Carbon-Normalized Partition Coefficients" site (cm interval)/PAH
(K, colloid),,b K, colloid K, sediment K, sediment (lit.) K, colloid (lit.) KO,
FCP (7-9) /pyrene
FPC (15-17) /pyrene
151000 111000 i 5100 (70 700) 160000 f 7880 (102 000)
129000 100000 i 3700 (64 500) 153000 i 3680 (97 700)
84 000," 62 700d 5750e 151000
" Data in parentheses adjusted for comparable freshwater result.
FPC
FPC (25-29) / pyrene
SI (14-16) /pyrene
phenanthrene
100000 75500 f 8900 (48 900) 98700 i 2350 (63 000)
57 500 51 700 f 4500 (33 100) 169000 f 10,700 (109 000)
74 100 26700 1800 (15400) 19800 954 (11400)
(25-29)/
*
23 OOOc 36 300 b
~
uncorrected ~ , for dvnamic auenchinn. Reference 33. dReference 15. ~
~~~~~~~~~~
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0
0.9 0
1
2
3
Organic Colloids (mg CL)
50
100
I50
Time (hours) Figure 3. Fyrene sorption time course study with Fort Point Channel sediments.
Flgure 2. Corrected pyrene quenching by Fort Point Channel colloids (7-9-cm interval).
Quenching of pyrene and phenanthrene fluorescence by the Fort Point Channel porewater ultrafiltrate was also observed (Figure 1, lower line). Quenching of both compounds by the ultrafiitrate was less intense and was linear as a function of organic carbon. Spectacle Island ultrafiltered interstitial water had virtually no effect on pyrene fluorescence. The ultrafiltrate from the former site may contain chemical species (organic and/or inorganic) that can statically and/or dynamically quench our probes. Contributions from this type of fluorescence quenching to the overall observed quenching of our PAH probes by the raw porewater must be evaluated before the true colloidNOC partition coefficient can be determined. The observed inverse fluorescence of a probe in the presence of two different chemical quenchers varies as
FO/F = (1 + Ko,““lloid[OC])(l + K,,[Q]) (2) where OC is the organic colloid concentration, Kufis the ultrafiltrate Stern-Volmer constant, and [Q] is the concentration of noncolloidal quenchers. In the absence of OC, decreases in the fluorescence of the probe with increasing Q can be linearly related by (3) F,’/F’ = (1 + Kuf[QI) where FdIF’reflects the changing fluorescence of the probe in the ultrafiltrate experiment. Equations 2 and 3 can be combined to yield (F0/F)(F’/Fd) = 1 + K o ~ u o i d [ O C ] (4) A plot of the product (F0/F)(F’/F() versus the organic colloid concentration (expressed as organic carbon) yields a straight line with KoF1loidas the slope (Figure 2). Binding constants determined by use of eq 4 often yielded values that were substantially lower than when we neglected the effects of noncolloidal quenchers (Table 11). We suspect that some of the large colloid-NOC partition coefficients reported in the literature obtained by fluorescence quenching may not have taken into account the presence of other quenchers in the sample matrix. Inorganic electrolytes in sufficient quantities will affect both the solubility and sorption of NOCs (28-30). When the nonpolar probe is salted out of the aqueous phase, it partitions more favorably into an organic sorbent. In marine systems the “salting effect” can be significant for certain compounds and is dependent upon the magnitude of the organic compound’s Setschenow constant, K,: log KO,= log - aK,(C,) (5) where KO,,,,)is the organic-carbon-normalized partition coefficient in seawater, a is an empirical constant with a value of -0.9 for PAHs, and C, is the salt concentration (-0.6 mol/L as NaC1). K , is unique for each compound 1624
Environ. Sci. Technol., Vol. 26, No. 8, 1992
I
/
0
100
200
Figure 4. Sorption of pyrene (closed circles) and phenanthrene (open circles) by Fort Point Channel sediments (25-29-cm interval).
and is a function of the composition of the sample matrix. On the basis of literature values of -0.3 M-l for both phenanthrene and pyrene (29,31),we estimate that the Koc(sw) values we report here are -0.2 log unit higher than comparable partition coefficients measured in freshwater. Our colloid-NOC partition coefficients adjusted for noncolloidal quenching effects and salting are still high compared to other reported literature values for colloids in natural waters (Table 11). We suspect that the composition of the colloidal organic material from our study sites may cause some of this effective sorption. Sorption of PAHs by Sediments. Time course studies showed that pyrene sorption to FPC sediments required -1 day to be largely equilibrated (Figure 3). Presumably, phenanthrene would have been equilibrated with our sediments within this time frame because its has a smaller sediment partition coefficient and higher free-liquid diffusivity. To assess the reasonableness of these time scales, we applied a radial diffusion sorption kinetics model (32) to estimate the extent of pyrene equilibration in 24 h. The model calculation used a pyrene liquid diffusion coefficient of 7.7 X lo* cm2/s, an intraaggregate porosity of 0.17, a geometric mean particle size of 100 pm, a partition coefficient of 5000 L/kg (determined experimentally), a particle density of 2.5 g/mL, and a solids concentration of 5 x lo-* kg/L. This resulted in an effective intraparticle diffusion coefficient of approximately 2 X cmz/s fOE pyrene. The model predicted 95% attainment of equilibrium after 24 h. Thus, the time allowed to equilibrate our PAHs with Boston Harbor sediments (24-30 h) appeared to be sufficient to yield an apparent K , within 10% of a totally equilibrated system. Pyrene fluorescence was significantly higher in the control bottles than in the vials containing sediments (Figure 4). This suggests that pyrene was strongly sorbed by the Boston Harbor sediments, and the data fit eq 1 quite well (Figure 4). Phenanthrene behaved in a similar
manner. K , values for both probes were high and in some cases approached their octanol-water partition coefficients. Adjusting for “salting effects” made our observed partition coefficients commensurate with values reported by others (3S3.5). Surprisingly,sediment organic-carbon-normalized partition coefficients for both compounds were similar to or only slightly higher than their comparable colloid-NOC binding constants at the Fort Point Channel site, while colloid K , for pyrene was considerably smaller than its sediment partition coefficient at the Spectacle Island values for station. Reported humic substance and KWmuoid a number of nonpolar organic compounds in the literature have been lower than comparable sediment K , (9,11,14, 15,36). We suspect that the significant amounts of lipophilic residues found in the Fort Point Channel sediments may also be associated with the colloidal phase, thereby enhancing the binding of our probes. This phenomenon is analogous to the enhanced sorption of NOCs by oilcontaminated soils observed by Boyd and Sun (25).More recently, Chiou and co-workers (37) showed that the presence of neutral oils (1.7% by weight) in linear alkylbenzenesulfonates (LAS) greatly enhanced the binding of NOCs by these surfactants below the critical micelle concentration. They applied a two-compartment model where the NOC can associate with the surfactant monomers and partition into the surfactant-oil emulsions to explain their observations. Thus, the presence of excess lipophilic residues (possibly in the form of petroleum hydrocarbons) associated with both sediment and colloidal organic matter may enhance the ability of these particular sorbents to bind PAHs and other NOCs. Porewater Organic Colloids as Sorbents for HOCs. As part of our efforts to understand colloid-NOC interactions, we estimated the overall solubility parameter (6,) of the FPC colloids using our measured pyrene binding constants (KoF1loid) and an equilibrium partition model which incorporates the Flory-Huggins equation (12, 17, 18):
Table 111. Fraction of PAH Bound to Colloids in Boston Harbor Porewaters depth,” cm
fraction bound, ‘70
PAH bound
14-16 (SI) 7-9 (FPC) 15-17 (FPC) 25-29 (FPC)
52 44 44 49 25
pyrene pyrene pyrene pyrene phenanthrene
SI Spectacle Island. FPC, Fort Point Channel.
(~al/mL)O.~, reported by Chiou and co-workers ( 1 7,37) for soil organic matter. We have also calculated 6,, for Buzzards Bay, MA, water column colloidal material using the 2,4,4’-PCB-colloid binding data of Brownawell (6) and obtained a value of 12.9 (~al/mL)O.~, like the value determined for soil organic matter. The smaller 6,, value for our nearshore pore fluid colloids suggests that this material may be comprised of especially nonpolar polymeric materials comparable to polysalicylate or polyoleate. Environmental Implications. Our results show that colloids are able to bind and stabilize nonpolar organic pollutants in interstitial waters. This conclusion corroborates the work of Brownawell and Farrington (7), who observed enhanced amounts of PCBs in Buzzard Bay sedimentary porewaters. The magnitude of this binding phenomenon appears to be dependent upon the nature of the colloidal organic matter. In areas where sediments are heavily influenced by anthropogenic activity, the colloidal material may be better sorbents. At these locations (which would also contain most of the refractory pollutants) it is possible to have substantial amounts of NOCs in the porewaters in excess of expectations for a two-phase solution-sorbent system. The fraction of NOC in porewater bound to this colloidal material can be estimated by
log Koccolloid = log (Ti”) + log (VJVi) - log (oc) log ( P ) - ((1- V i / V J + ~ ) / 2 * 3 0 3(6)
Using the data from Tables I and I1 we anticipate about half of the py-rene in these porewaters is bound to colloidal material; colloids have a less significant effect on phenwhere yiwis the NOC aqueous activity coefficient, Vw/Vi anthrene (Table 111). Thus, colloids can significantly is the water and solute molar volume ratio, V, is the molar enhance the concentration of particle-reactive compounds volume of the colloid, oc is the organic carbon content of (Kow>lo5) in porewaters. the colloid, p is the colloid density ( 1.2 g/mL) (17),and A number of investigators (40-42) have observed x is the Flory parameter. The value of x is comprised of transport of water-soluble geochemical tracers across the both entropy (x,) and enthalpy (xh)terms. The former sediment-water interface in excess of diffusional transport. is determined empirically and has a value of 0.34 for polar Much of this enhanced exchange has been attributed to polymers, while the latter is estimated by use of the bioirrigation processes. If colloids are stabilizing NOCs Scatchard-Hildebrand regular solution equation ( 17,38): in porewaters, then the transport of organic pollutants from sediments to overlying waters could be enhanced Xh = (Vi/RT)[(&oc - 6i)’ (7) through a combination of bioirrigation and colloid sorption effects. This phenomenon has been reported for radiowhere Ji is the solute solubility parameter. Elucidation of nuclides associated with interstitial water colloidal material 6, using eqs 6 and 7 yields a quantitative value of the (43). Consequently, efforts to estimate the release of NOCs colloidal material polarity and propensity to bind NOCs. from contaminated sediment beds should include an asIncreasing values for 6,, are generally indicative of desessment of the colloidal enhancement of pore fluid concreases in Koccolloid. Using a pyrene solubility parameter of 8.2 (~al/mL)O.~ centrations of such compounds. (estimated from heat of vaporization data) and a molar Acknowledgments volume of 159 mL/mol, we calculated a 6, of 10.3 f 0.17 (~al/mL)“~ (one standard deviation unit) for the Fort Point We gratefully acknowledge Connie Hart, John for the Channel colloids and a value of 11 (~al/mL)O.~ MacFarlane, Sue McGroddy, John Farrington, Gordon Spectacle Island colloidal matter. These values suggest Wallace, and Hovey Clifford for their invaluable assistance that the colloids from our sites are as nonpolar as octanol during sampling and data analysis. [ 6 = 10.3 (~al/mL)O.~] and were similar to the pond sediLiterature Cited examined by ment organic matter (6 = 9.7 (~al/mL)O.~) Freeman and Cheung (39). The solubility parameter of (1) Shiaris, M. P.; Jambard-Sweet, P. Mar. Pollut. Bull. 1986, our colloids is significantly smaller than the value, 13 17, 469. N
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Received for review November 26, 1991. Revised manuscript received April 23,1992. Accepted April 27,1992. The work was conducted with support by the National Science Foundation, Grant 8714110-CES, Dr. Edward H. Bryan Program Director, by a grant provided by M.I.T. Sea Grant Program (a part of the National Oceanic and Atmospheric Administration), by support from the Massachusetts Water Resources Authority, and by U S . Environmental Protection Agency, Grant R-817145-01-0.
