Partitioning of Polycyclic Aromatic Hydrocarbons to Marine Porewater Organic Colloids

Environ. Sci. Technol. 1992,26, 1621-1626

Happel, J.; Brenner, H. Low Reynolds Number Hydrody- (37) Romero, C. A.; Davis, R. H. J. Membr. Sci. 1988, 39, namics; Noordhoff International: Leyden, The Nether- 157-185. lands, 1973; p 553. (38) Davis, R. H.; Leighton, D. T. Chem. Eng. Sci. 1987, 42, Cox, R. G.; Mason, S. G. Annu. Rev. Fluid Mech. 1971,3, 275-281. 291-316. (39) Leighton, D.; Acrivos, A. Chem. Eng. Sci. 1986, 41, Goldsmith, H. L.; Mason, S. G. J. Colloid Sei. 1962, 17, 1377-1384. 448-476. (40) Bhatty, J. I.; Reid, K. J.; Dollimore, D.; Shah,T. H.; Davies, Ho, B. P.; Leal, L. G. J. Fluid Mech. 1974, 65, 365-400. L.; Gamlin, G. A.; Tamini, A. Sep. Sci. Technol. 1989,24, Halow, J. S.; Wills, G. B. J. AZChE 1970, 16, 281-286. 1-14. Brenner, H. Chem. Eng. Sei. 1961, 16, 242-251. (41) Russel, W. B. J. Fluid Mech. 1978, 85, 209-232. Robertson, J. M. Hydrodynamics in Theory and Appli- (42) Green, G.; Belfort, G. Desalination 1980, 35, 129-147. cations; Prentice Hall: Englewood Cliffs, NJ, 1965; p 652. (43) Porter, M. C. Znd. Eng. Chem. Prod. Res. Develop. 1972, Tien, C. Granular Filtration of Aerosols and Hydrosols; 11,234-248. Butterworth: Boston, MA, 1989; p 365. (44) Adham, S. A.; Snoeyink, V. L.; Clark, M. M.; Bersillon, J.-L. Cox, R. G.; Brenner, H. Chem. Eng. Sci. 1968,23,147-173. J.-Am. Water Works Assoc. 1991,83, 81-91. Schonberg, J. A.; Hinch, E. J. J. Fluid Mech. 1989,203, (45) Wiesner, M. R.; Clark, M. M.; Mallevialle, J. J. Environ. 517-524. Eng. 1989,115, 20-40. Drew, D. A.; Schonberg, J. A,; Belfort, G. Chem. Eng. Sci. (46) Bhatty, J. I.; Reid, K. J.; Dollimore, D.; Shah,T. H.; Gamlen, 1991,46, 3219-3224. G. A.; Tamini, A. Sep. Sci. Technol. 1989, 24, 165-178. Gregory, J. J. Colloid Interface Sci. 1981, 83, 138-145. (47) O'Melia, C. R. In Aquatic Surface Chemistry, 1st ed.; Gregory, J. Crit. Rev. Environ. Control 1989,19,185-230. Stumm, W., Ed.; John Wiley and Sons: New York, 1987; Czarnecki, J. J. Colloid Interface Sci. 1979, 72, 361-362. Vol. I, Chapter 14. Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday (48) Barnes, H. A.; Edwards, M. F.; Woodcock, L. V. Chem. Eng. SOC.1966, 62, 1638-1651. O'Melia, C. R. J. Environ. Eng. 1985, 111, 874-890. Sci. 1987, 42, 591-608. 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 Received for review January 27, 1992. Revised manuscript re- Works Assoc. 1985, 77, 97-106. ceived April 7,1992. Accepted April 21,1992. Support for this Granger, J.; Dodds, J.; LeClerc, D.; Midoux, N. Chem. Eng. work was provided by the National Science Foundation, (Grant Sci. 1986,41, 3119-3128. BCS-8909722). Any opinions, findings, and conclusions or Chellam, S. M.S. Thesis, Department of Environmental recommendations expressed in this publication are those of the Science and Engineering, Rice University, Houston, TX, authors and do not necessarily reflect the views of the National 1991, p 175. 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

released back to the water column, organisms throughout Fluorescence quenching was used to measure the the ecosystem will be exposed to them. binding of pyrene and phenanthrene to marine interstitial In the past, it was widely believed that strongly sorbed water organic colloids from Boston Harbor, MA. Both pyrene and phenanthrene were sorbed by porewater col- NOCs were rendered immobile in sediment beds. How- loids (Koccolloid -105.0 and 104.3,respectively) from a ever, interstitial water organic colloids have recently been heavily contaminated nearshore site. Pyrene had a sig- shown to affect the distribution of particle-reactive organic nificantly lower affinity toward colloids from a cleaner substances between pore fluids and their associated sediments (6-10). A number of controlled studies have dem- location (KmmuoidN 104.5). Sediments from the former site were also observed to be especially effective sorbents for onstrated that humic substances, which we believe com- these compounds relative to expectations based on the prise a significant component of the organic colloids, can literature. The high sorption coefficients may be due to bind NOCs and therefore enhance the amount of nonpolar the high lipid content of these sediments and colloids. compounds in the aqueous phase beyond simple twephase Alternatively, they may be due to a very substantial non- equilibrium expectations (11-14). Thus, the presence of polar character of the natural organic matter there. colloids in porewaters could play a major role in the fate and transport of stable and toxic organic substances in nearshore sediments. Introduction Several hypotheses have been forwarded recently stating The sediments of many coastal environments have been that humic substances can bind NOCs in a manner that contaminated by refractory nonpolar organic compounds is analogous to partitioning by sediment organic matter; (NOCs). Such pollution has included pesticides, polycyclic that is, the binding phenomenon is largely controlled by aromatic hydrocarbons (PAHs), and polychlorinated bi- the hydrophobicity of the organic compounds. As a result, phenyls (PCBs) (1-5). While efforts are underway to limit empirical relationships between colloid-NOC binding or stop the influx of these deleterious substances into coefficients (Kmco"Oid)and solute physicochemical param- receiving waters, NOCs currently in the sediments may eters, such as the n-octanol-water partition coefficient and continue to be harmful to benthic organisms for years and aqueous solubility, have been established (15, 16). A decades to come. Furthermore, if these compounds are conceptual model that quantifies the molecular interac-

0013-936X/92/0926-1621$03.00/0 0 1992 American Chemical Society Environ. Scl. Technol., Voi. 26, No. 8, 1992 1821 tions between a nonpolar organic compound and the humic colloids from our porewaters (20). Each device was ex- matrix using the Flory-Huggins equation has also been tensively cleaned by rinsing with methanol-water and then developed (12,17,18). While this approach has success- distilled water prior to use to remove any organic impur- fully predicted for a number of NOCs with dif- ities associated with the membrane. The raw porewater ferent humic substrates, some critical information re- sample (2 mL) was added to the top reservoir, capped, and garding the physicochemical properties (i.e., solubility centrifuged until the volume of liquid from the upper parameters) of the humic polymer is still lacking. reservoir passed through the membrane into the lower As part of our efforts to understand the role of colloids compartment. Raw and ultrafiltered porewater was as- in the release of NOCs from sediments to overlying waters, sayed by organic carbon analysis and the colloid concen- we have studied the partitioning of pyrene and phenan- tration was determined by difference. Control studies threne to both marine interstitial water colloidal and using vitamin BIz showed that losses to the membrane were sediment organic matter from samples taken in Boston minimal (-6%). Harbor, MA. Colloid and sediment samples from different PAH-Colloid Binding Studies. Fluorescence depths and sites were used to ascertain spatial variabilities quenching (13, 21) was used to study the binding of in the organic-carbon-normalized partition coefficient, K,. phenanthrene and pyrene to porewater organic colloids Measurements to determine the properties of the colloidal taken from the two sites. Fluorescence measurements were and sediment organic matter (size, percent organic carbon, obtained using a Perkin-Elmer LS-5 fluorescence spec- lipid content, extinction coefficients) were also performed. trophotometer. Optimum excitation-emission wavelength Finally, we used data gathered from the above analyses pairs were determined to be 230 nm/390 nm for pyrene to elucidate porefluid colloid solubility parameters, using and 230 nm/373 nm for phenanthrene. Interfering an equilibrium partition model based on the Flory-Hug- fluorescence from the organic colloids was observed to be gins concept, and compared these estimates with results minimal at these wavelengths. Saline solutions (I = 0.6 from other investigators. M NaC1) were spiked with 10 pL of the probe dissolved Materials and Methods in either acetonitrile or methanol. Saline solutions con- Sampling Protocol. Sediment box cores were taken taining no PAHs were used as blanks to quantify the from two sites in Boston Harbor: (1) at the mouth of Fort amount of interfering fluorescence from the colloids. Both Point Channel (FPC) and (2) near Spectacle Island (SI). the blank and the PAH-spiked saline solution were The cores were sectioned under nitrogen, and a portion transferred to quartz cuvettes, and initial fluorescence of the sediments was transferred to nitrogen-purged 250- emissions were recorded. Following this, 100 pL of raw mL ground-glass-stoppered centrifuge tubes. The re- porewater was added to each cuvette and allowed to mainder was freeze-dried and stored for later use. Pore- equilibrate. An additional control cuvette containing only waters were separated from the wet sediments by centri- pyrene was spiked with the saline solution (at 100-pL in- fugation at 600g for 20 min. The supernatants were drawn crements) to determine dilution effects on the fluorescence off, purged with prepurified nitrogen, and stored in sealed of the compound. Previous investigators (13,21)observed 20-mL glass syringes standing plunger side down under short equilibration times (less than 1 min). We allowed water in the dark at 4 “C until further use. All noncolloidal our samples to sit for up to 10 min before making another particles were allowed to settle onto the syringe plungers, reading. This process was conducted five to six times, and subsequent subsampling was performed to avoid re- depending upon the amount of pore fluid available. In suspending these solids. Sample pH values ranged from between fluorometric readings, the cuvettes were trans- 7.7 to 8.1. ferred to a Beckman DU-7 UV/vis spectrophotometer for Sediment and Colloid Analysis. Sediment organic absorbance measurements to determine “inner-filter carbon was determined by weight loss on ignition at 450 effects”. The experiment was repeated using ultrafiltered OC for 24 h and by assuming the weight loss was half porewater to ascertain the effects of either dynamic or carbon. Porewater organic carbon was measured using an static quenching by noncolloidal chemical species. The Ionics 555 TOC analyzer (Ionics Inc., Watertown, MA). corrected fluorescence data along with measured colloid This instrument is based on the high-temperature plati- concentrations (expressed as organic carbon) were used to num catalyst method developed by Sugimura and Suzuki determine the organic-carbon-normalized equilibrium (19). This method proved to be precise (f5%) and was binding constants, Koplloid. not affected by major ions in the sample matrix. Sediment Sorption Experiments. Sorption experi- Sediments were extracted using methylene chloride to ments were carried out in 50-mL Corex (VWRScientific) determine their lipid content and background PAH con- centrifuge tubes with Teflon-lined screwcaps. Freeze-dried centration. Freeze-dried sediment (5 g) was Soxhlet-ex- sediment was added to each tube (25 mg), followed by a tracted for 72 h with methylene chloride and the extract 50-mL aliquot of bicarbonate-buffered (1 mM) saline so- concentrated using a Buchler rotoevaporator. Aliquots lution (0.6 M NaC1). These suspensions were allowed to from the extract (500 pL) were evaporated, and total lipids rehydrate for several hours. Pyrene or phenanthrene, were determined gravimetrically. The PAH fraction was dissolved in an organic solvent (acetonitrile or methanol), separated from the remainder of the extract using alumina was added to each vessel by direct injection to yield initial column chromatography and assayed by combined gas solute concentrations ranging from 20 to 100 (pyrene) or chromatography-mass spectrometry (GC-MS). Total 50 to 250 pg/L (phenanthrene). Each set of experiments pyrene and phenanthrene in our sediments were in the included a blank comprised of saline solution and sedi- range of 3.8-7.4 and 1.2-1.7 ppm, respectively, but these ments (no PAH). Control experiments, using only buffered levels were sufficiently small so as to not contribute sig- saline solution and the sorbates, were also conducted to nificantly to the total sorbate loads in our sorption pro- determine solute losses to walls and cap liners. Each tube tocol. was capped, wrapped in aluminum foil, and tumbled on Ultrafiltration using Centricon microconcentrators a rotary tumbler for 30 h. Following equilibration, each (Amicon, Danvers, MA) equipped with 3000 MWCO tube was centrifuged for 45 min at lOOOg to separate the membranes (600 MWCO based on the configuration of solid and liquid phases. A portion of the supernatant was random coil macromolecules) was used to separate organic pipeted into a quartz cuvette and assayed by fluorescence

1822 Environ. Sci. Technol., Vol. 26, No. 8, 1992 Table I. Sediment and Colloid Organic Matter Properties"

FPC SI property 7-9 cm 15-17 cm 25-29 cm 14-16 cm porewater total organic 19.9 24.4 31.8 37.9 carbon, mg of C/L e porewater L (mol of nd 150 211 92 2 C)-1 porewater OC, mg of 7.3 7.7 13 21.5 C/L organic carbon fractn 5.47 5.19 5.23 3.34 sediment, % lipid fractn sediment, 0.97 1.01 0.49 0.26 .95 j % t lipid fractn sediment 8.8 9.6 4.7 3.9 .9J -.. . -. 1 org matter, % 0 .5 1 1.5 2 2.5 3 3.5 4 "OC, organic colloid; organic matter is taken as twice organic OC (mglL) carbon. nd, not determined. e, molar absorptivity. FPC, Fort Point Channel site. SI, Spectacle Island site. bAt 280 nm. Flgure 1. Quenching of pyrene by unaltered (circles) and ultrafiltered (squares) Fort Point Channel porewater (7-9-cm interval). Solid lines spectrophotometry using the wavelength pairs described show best fi, whlle dashed lines indicate 95% confidence intervals. previously. The amount of interfering fluorescence from desorbed sediment organic matter was measured by as- values reported for continental shelf sediments, but it is saying the blank supernatant. PAH-sediment partition comparable to values in other polluted marine environ- coefficients were determined using ments (23). Recently, Boyd and Sun (25)observed that Kp = [(Fo- F?/pl/F' (1) soils and sediments contaminated with dielectric fluids or petroleum hydrocarbon residues sorbed NOCs better than where Fo is the PAH fluorescence in the control vial, F' uncontaminated soils. They hypothesized that an organic is the blank-corrected fluorescence of the PAH in the contaminant can partition more favorably into the oily sample bottle (with sediments), and p is the solids con- residues than the natural soil organic matter, and they were centration (g/mL). able to predict this enhanced partitioning phenomenon using a two-compartment equilibrium model. Therefore Results and Discussion the high lipid content, which is probably comprised of Sediment-Colloid Organic Matter Properties. The anthropogenic hydrocarbons in the Fort Point Channel Fort Point Channel site is located in an area in close sediments, may enhance the sorption of NOCs like PAHs proximity to intensive anthropogenic activity, while the to these solids. Spectacle Island station is offshore and less affected. Binding of PAHs to Porewater Colloids. Pyrene Porewater total and colloidal organic carbon from both fluorescence was quenched by organic colloids from the locations were present in milligram of C per liter quantities raw sedimentary porewater (Figure 1). A linear rela- and increased with depth at Fort Point Channel (Table tionship existed between inverse fluorescence of the probe I). Colloidal organic matter extinction coefficients (4 and the concentration of organic carbon added. We at- measured at 280 nm are consistent with values reported tribute this drop in fluorescence, when unaltered porewater by Stuermer (22)for marine humic substances but sub- is added, to interactions between pyrene and colloids. stantially lower (by as much as a factor of 10) than those Phenanthrene fluorescence exhibited similar behavior. determined for terrestrial humic substances (IO). This This correlation is described by the Stern-Volmer equation suggests that our interstitial water organic matter either (21),where the slope of the line is equivalent to KocColloid has a marine origin or is composed of a mixture of ter- if static quenching is the only operative mechanism. Fort restrial humic substances diluted by other organic mate- Point Channel colloid-PAH binding constants were very rials that do not absorbed at 280 nm. large and in some cases exceeded the compound's octa- The organic carbon contents for all samples fell within nol-water partition coefficient; the Spectacle Island col- the range of values reported by others for nearby marine loid-pyrene KO,was significantly smaller (Table 11). sediments (7, 23, 24). Sediments from the Fort Point Similarly high humic substance-PAH partition coefficients Channel site contained significant amounts of methylene have been reported by others (21,26,27)using fluorescence chloride-extractable compounds, referred to here as lipids quenching. We suspect other mechanisms besides colloid (Table I). The lipid fraction is considerably higher than binding could contribute to these observations.

Table 11. Observed PAHColloid and PAH-Sediment Organic-Carbon-Normalized Partition Coefficients"

site (cm interval)/PAH FPC FCP FPC SI (25-29)/ (7-9) /pyrene (15-17) /pyrene FPC (25-29) / pyrene (14-16) /pyrene phenanthrene (K, colloid),,b 151000 129 000 100 000 57 500 74 100 K, colloid 111 000 i 5100 100000 i 3700 75500 f 8900 51700 f 4500 26700 * 1800 (70 700) (64 500) (48 900) (33 100) (15400) K, sediment 160000 f 7880 153000 i 3680 98700 i 2350 169000 f 10,700 19800 954 (102 000) (97 700) (63 000) (109 000) (1 1400) K, sediment (lit.) 84 000," 62 700d 23 OOOc K, colloid (lit.) 5750e KO, 151000 36 300

" Data in parentheses adjusted for comparable freshwater result. b~~,uncorrected for dvnamic auenchinn. Reference 33. dReference 15. ~ ~~~~~~~~~~

Environ. Sci. Technoi., Vol. 26, No. 8, 1992 1623 0 50 100 I50 0.9 Time (hours) 0 1 2 3 Figure 3. Fyrene sorption time course study with Fort Point Channel Organic Colloids (mg CL) 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 / I 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 0 100 200 raw porewater must be evaluated before the true colloid- NOC partition coefficient can be determined. The observed inverse fluorescence of a probe in the presence of Figure 4. Sorption of pyrene (closed circles) and phenanthrene (open two different chemical quenchers varies as circles) by Fort Point Channel sediments (25-29-cm interval). FO/F = (1 + Ko,““lloid[OC])(l+ K,,[Q]) (2) and is a function of the composition of the sample matrix. On the basis of literature values of -0.3 M-l for both where OC is the organic colloid concentration, Kufis the phenanthrene and pyrene (29,31),we estimate that the ultrafiltrate Stern-Volmer constant, and [Q] is the con- Koc(sw)values we report here are -0.2 log unit higher than centration of noncolloidal quenchers. In the absence of comparable partition coefficients measured in freshwater. OC, decreases in the fluorescence of the probe with in- Our colloid-NOC partition coefficients adjusted for non- creasing Q can be linearly related by colloidal quenching effects and salting are still high com- F,’/F’ = (1 + Kuf[QI) (3) pared to other reported literature values for colloids in natural waters (Table 11). We suspect that the composition where FdIF’reflects the changing fluorescence of the probe of the colloidal organic material from our study sites may in the ultrafiltrate experiment. Equations 2 and 3 can be cause some of this effective sorption. combined to yield Sorption of PAHs by Sediments. Time course studies (F0/F)(F’/Fd) = 1 + Ko~uoid[OC] (4) showed that pyrene sorption to FPC sediments required -1 day to be largely equilibrated (Figure 3). Presumably, A plot of the product (F0/F)(F’/F() versus the organic phenanthrene would have been equilibrated with our colloid concentration (expressed as organic carbon) yields sediments within this time frame because its has a smaller a straight line with KoF1loidas the slope (Figure 2). sediment partition coefficient and higher free-liquid dif- Binding constants determined by use of eq 4 often yielded fusivity. To assess the reasonableness of these time scales, values that were substantially lower than when we ne- we applied a radial diffusion sorption kinetics model (32) glected the effects of noncolloidal quenchers (Table 11). to estimate the extent of pyrene equilibration in 24 h. The We suspect that some of the large colloid-NOC partition model calculation used a pyrene liquid diffusion coefficient coefficients reported in the literature obtained by of 7.7 X lo* cm2/s, an intraaggregate porosity of 0.17, a fluorescence quenching may not have taken into account geometric mean particle size of 100 pm, a partition coef- the presence of other quenchers in the sample matrix. ficient of 5000 L/kg (determined experimentally), a par- Inorganic electrolytes in sufficient quantities will affect ticle density of 2.5 g/mL, and a solids concentration of 5 both the solubility and sorption of NOCs (28-30). When x lo-* kg/L. This resulted in an effective intraparticle the nonpolar probe is salted out of the aqueous phase, it diffusion coefficient of approximately 2 X cmz/s fOE partitions more favorably into an organic sorbent. In pyrene. The model predicted 95% attainment of equi- marine systems the “salting effect” can be significant for librium after 24 h. Thus, the time allowed to equilibrate certain compounds and is dependent upon the magnitude our PAHs with Boston Harbor sediments (24-30 h) ap- of the organic compound’s Setschenow constant, K,: peared to be sufficient to yield an apparent K, within 10% log KO,= log - aK,(C,) (5) of a totally equilibrated system. Pyrene fluorescence was significantly higher in the where KO,,,,)is the organic-carbon-normalized partition control bottles than in the vials containing sediments coefficient in seawater, a is an empirical constant with a (Figure 4). This suggests that pyrene was strongly sorbed value of -0.9 for PAHs, and C, is the salt concentration by the Boston Harbor sediments, and the data fit eq 1 (-0.6 mol/L as NaC1). K, is unique for each compound quite well (Figure 4). Phenanthrene behaved in a similar

1624 Environ. Sci. Technol., Vol. 26, No. 8, 1992 manner. K, values for both probes were high and in some Table 111. Fraction of PAH Bound to Colloids in Boston cases approached their octanol-water partition coefficients. Harbor Porewaters Adjusting for “salting effects” made our observed partition coefficients commensurate with values reported by others depth,” cm fraction bound, ‘70 PAH bound (3S3.5). Surprisingly,sediment organic-carbon-normalized 14-16 (SI) 52 pyrene partition coefficients for both compounds were similar to 7-9 (FPC) 44 pyrene or only slightly higher than their comparable colloid-NOC 15-17 (FPC) 44 pyrene binding constants at the Fort Point Channel site, while 25-29 (FPC) 49 pyrene colloid K, for pyrene was considerably smaller than its 25 phenanthrene sediment partition coefficient at the Spectacle Island SI Spectacle Island. FPC, Fort Point Channel. station. Reported humic substance and KWmuoidvalues for a number of nonpolar organic compounds in the literature (~al/mL)O.~,reported by Chiou and co-workers (17,37) for have been lower than comparable sediment K, (9,11,14, soil organic matter. We have also calculated 6,, for Buz- 15,36). We suspect that the significant amounts of lipo- zards Bay, MA, water column colloidal material using the philic residues found in the Fort Point Channel sediments 2,4,4’-PCB-colloid binding data of Brownawell (6) and may also be associated with the colloidal phase, thereby obtained a value of 12.9 (~al/mL)O.~,like the value deter- enhancing the binding of our probes. This phenomenon mined for soil organic matter. The smaller 6,, value for is analogous to the enhanced sorption of NOCs by oil- our nearshore pore fluid colloids suggests that this material contaminated soils observed by Boyd and Sun (25).More may be comprised of especially nonpolar polymeric ma- recently, Chiou and co-workers (37) showed that the terials comparable to polysalicylate or polyoleate. presence of neutral oils (1.7% by weight) in linear alkyl- Environmental Implications. Our results show that benzenesulfonates (LAS) greatly enhanced the binding of colloids are able to bind and stabilize nonpolar organic NOCs by these surfactants below the critical micelle con- pollutants in interstitial waters. This conclusion corrob- centration. They applied a two-compartment model where orates the work of Brownawell and Farrington (7), who the NOC can associate with the surfactant monomers and observed enhanced amounts of PCBs in Buzzard Bay partition into the surfactant-oil emulsions to explain their sedimentary porewaters. The magnitude of this binding observations. Thus, the presence of excess lipophilic phenomenon appears to be dependent upon the nature of residues (possibly in the form of petroleum hydrocarbons) the colloidal organic matter. In areas where sediments are associated with both sediment and colloidal organic matter heavily influenced by anthropogenic activity, the colloidal may enhance the ability of these particular sorbents to material may be better sorbents. At these locations (which bind PAHs and other NOCs. would also contain most of the refractory pollutants) it is Porewater Organic Colloids as Sorbents for HOCs. possible to have substantial amounts of NOCs in the As part of our efforts to understand colloid-NOC inter- porewaters in excess of expectations for a two-phase so- actions, we estimated the overall solubility parameter (6,) lution-sorbent system. The fraction of NOC in porewater of the FPC colloids using our measured pyrene binding bound to this colloidal material can be estimated by constants (KoF1loid)and an equilibrium partition model which incorporates the Flory-Huggins equation (12, 17, 18): log Koccolloid = log (Ti”) + log (VJVi) - log (oc) - Using the data from Tables I and I1 we anticipate about log (P) - ((1- Vi/VJ + ~)/2*303(6) half of the py-rene in these porewaters is bound to colloidal material; colloids have a less significant effect on phen- where 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 ( N 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 where Ji is the solute solubility parameter. Elucidation of effects. This phenomenon has been reported for radio- 6, using eqs 6 and 7 yields a quantitative value of the nuclides associated with interstitial water colloidal material colloidal material polarity and propensity to bind NOCs. (43). Consequently, efforts to estimate the release of NOCs Increasing values for 6,, are generally indicative of de- from contaminated sediment beds should include an as- creases in Koccolloid. sessment of the colloidal enhancement of pore fluid con- 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 Channel colloids and a value of 11 (~al/mL)O.~for the 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 sediment organic matter (6 = 9.7 (~al/mL)O.~)examined by Literature Cited 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.

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