Environ. Sci. Technol. 1996, 30, 1923-1932
Isolation of Marine Sediment Colloids and Associated Polychlorinated Biphenyls: An Evaluation of Ultrafiltration and Reverse-Phase Chromatography ROBERT M. BURGESS* U.S. EPA, NHEERL, Atlantic Ecology Division & Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882
RICHARD A. MCKINNEY U.S. EPA, NHEERL, Atlantic Ecology Division, Narragansett, Rhode Island 02882
WILLIAM A. BROWN Science Applications International Corporation, Narragansett, Rhode Island 02882
JAMES G. QUINN Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882
Environmental colloids are suspected of having significant effects on nonpolar organic contaminant geochemistry, transport, and bioavailability. However, environmental data on colloid-contaminant interactions is limited because isolating colloids from the dissolved and particulate phases is problematic. In this study, two practical methods using ultrafiltration and reverse-phase chromatography were evaluated for isolating environmentally contaminated marine sediment interstitial water colloids and associated PCBs. In assessing each method, ultrafiltration demonstrated extensive sorption of radiolabeled nonpolar compounds (>90%) and a re-occurring breakthrough phenomena, both of which compromise the method for accurately assessing colloid-PCB interactions. Conversely, C18 reverse-phase chromatography, performed using laboratory-packed columns, generated reproducible organic carbon-normalized colloidal partitioning coefficients (Kcoc) that agreed with literature and theoretical considerations. Evaluations of sample flow rate and prefiltration size along with potential for C18 bed saturation indicated that these parameters have only a minor (e.g., less than a factor of 2) effect on the calculated contaminant distribution coefficients. Of the two methods evaluated, reversephase chromatography was the most promising for quantifying environmental colloid-PCB interactions. * Corresponding author telephone: (401) 782-3106; fax: (401) 7823030; e-mail address:
[email protected].
S0013-936X(95)00620-1 CCC: $12.00
1996 American Chemical Society
Introduction Natural waters can be viewed as consisting of three phases: the familiar dissolved and particulate phases and the often overlooked colloidal phase. Based on operationally defined sizes, colloids are located between the particulate phase (>1.0 µm) and the dissolved phase (63 µm. These particles (100 mg) were loaded into 22 mm i.d. glass columns containing coarse glass frits (40-60 µm) (Kontes K-420540-0224, Vineland, NJ). Columns were activated with 20 mL of methanol followed by 20 mL of DI water and 10 mL of reconstituted seawater. Reconstituted seawater was prepared to be within (5‰ of sample salinity. Columns were lightly tapped during activation to distribute C18 material evenly across the column frit. The 5-10-mL sample was introduced onto the column and then eluted sequentially with 20 mL of 50/50 acetone/hexane and 10 mL of methylene chloride, followed by 10 mL of hexane. All elutions were combined. Columns were operated under gravity (∼15 mL/min) unless otherwise noted. For comparisons with commercial products [e.g., 3M C18 Empore Disc (St. Paul, MN) and Waters Environmental Plus C18 Sep-Pak], similar flow rates were used following the manufacturer instructions. Interstitial Water Collection and Processing. Interstitial water was collected from six (i.e., three from each site) 30 cm long by 15 cm wide surface sediment cores sampled in Providence River (RI) and New Bedford Harbor (MA) in July 1993 and November 1994, respectively. Prior to use, sediments were stored at 4 °C in the dark for up to
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996
TABLE 1
Selected Physical Properties of Organic Compounds Used in Ultrafiltration Studies compound
mol wt
aq solubility (mg/L)
log Kow
lindane fluoranthene 2,2′,5,5′-tetrachlorobiphenyl
291 202.3 292
7.8a 0.26a 0.033b
3.89a 4.90a 5.84c
a
Ref 46.
b
Ref 47. c Ref 54.
2 months. Sediments were placed, using clean spatulas, into either 750-mL polypropylene or 100-mL glass centrifuge tubes, and interstitial water was collected via centrifugation using an International Equipment Company (IEC) Centra-8 refrigerated centrifuge with an IEC 218 rotor (Needham Heights, MA) at 4 °C at 1000g for 1 h. Following centrifugation, interstitial water was transferred to 500-mL borosilicate glass Erylenmeyer flasks and stored in the dark at 4 °C. Providence River interstitial waters were stored for up to 1 year and New Bedford Harbor interstitial waters up to three months. Before using interstitial water from New Bedford Harbor, samples were prefiltered, under a vacuum, through a 1.2- or 5-µm muffled Poretics Corp. silver filter to remove particles (Livermore, CA). Contaminant Sorption to Ultrafilter Studies. Sorption behavior of three organic contaminants, representative of pollutants often present on marine sediments, to ultrafilter equipment was evaluated using Sigma Radiochemicals pesticide 3.6 mCi/mmol [14C]lindane (γ-hexachlorocyclohexane), the polycyclic aromatic hydrocarbon (PAH) 55 mCi/mmol [14C]fluoranthene, and polychlorinated biphenyl (PCB) 12.2 mCi/mmol [14C]-2,2′,5,5′-tetrachlorobiphenyl (BZ 52; 45) (St. Louis, MO). Pre-ultrafiltered reconstituted seawater solutions (30‰) were spiked with 14C-labeled contaminants prepared in HPLC-grade methanol (2.75225 µL) and allowed to mix at room temperature for at least 24 h. Radiolabeled chemicals were spiked into the above solutions for nominal concentrations of either 4.0 and 40 µg/L for [14C]lindane, 0.5 and 5.0 µg/L for [14C]fluoranthene, and 0.3 and 3.0 µg/L for [14C]-2,2′,5,5′-tetrachlorobiphenyl. All concentrations were selected to be at least 1 order of magnitude below the reported aqueous solubility (46, 47) (Table 1). To ensure that reported concentrations were truly dissolved levels, in some cases ultrafilters were also tested at a second concentration at least 2 orders of magnitude below solubility. Ultrafiltration was initiated by processing 50 mL of spiked solution, which was then discarded. Next, 150 mL of solution was processed with collections for scintillation counting after 20, 50, and 130 mL had passed through the cell. Samples collected from the retentate (i.e., colloidal phase) were sampled after 50 and 130 mL had been ultrafiltered. After completing the ultrafiltration, glassware and the ultrafilter membrane were extracted with methanol. For the YM30 evaluation, a 50/50 mixture of acetone/hexane and pure methylene chloride were also used to extract the ultrafilter. To avoid damaging the stirred cell, it was not solvent extracted but was detergent washed. All samples were scintillation counted using a Packard Tri-Carb 1500 liquid scintillation analyzer (Downers Grove, IL). PCB Congener Extraction and Analysis. For environmentally contaminated samples, the internal standard 2,2′,3,3′,4,5,5′,6-octachlorobiphenyl (congener BZ 198; 45)
was added to isolated colloidal and dissolved phases before extractions were initiated. A volume of 50/50 acetone/ hexane equivalent to three times the sample volume was added to colloidal and ultrafilter-derived dissolved phase samples. Acetone/hexane was selected to enhance extraction of contaminants potentially trapped within the hydrated colloidal material. Sonication was performed three times for 1-min activations of the microtip (no. 406HWseries extender tips) on the colloidal and ultrafilter-derived dissolved samples with a Heat Systems Ultrasonics instrument at a setting of 5 (Model W-385, Farmingdale, NY). An earlier comparison of colloid extraction techniques, including sonication, microwave, separatory funnel, refluxing, and wrist-action shaking, demonstrated that sonication was most effective for extracting a suite of organic contaminants from colloids. Following sonication, samples were partitioned against deionizied water, and the solution was saturated with hexane-rinsed sodium chloride. Hexane fractions were then removed, treated with sodium sulfate, transferred to Zymark TurboVap evaporator tubes (Hopkinton, MA), and concentrated to 1 mL using a Zymark TurboVap system. Dissolved phases present in organic solvent from C18 studies were also treated with sodium sulfate and volume reduced. After volume reduction, extracts were treated overnight with concentrated sulfuric acid to oxidize interfering organic compounds. Samples were analyzed for the PCB congeners BZ 8, BZ 18, BZ 28,BZ 44, BZ 52, BZ 66, BZ 99, BZ 101, BZ 105, BZ 110, BZ 118, BZ 128, BZ 138, BZ 151, BZ 153, BZ 170, BZ 180, BZ 183, BZ 187, BZ 194, BZ 195, BZ 206, and BZ 209 (45). Analysis involved injecting 1.0 µL of sample in the splitless mode with inlet purge after 1 min into a Hewlett-Packard 5890 gas chromatograph equipped with an electron capture detector and a 30-m DB-5 fused silica capillary column (J & W Scientific, Folsom, CA). Helium was the carrier gas at a flow rate of 1.5 mL/min, and the flow of a 95/5 mixture of argon/methane to the detector was maintained at 35 mL/min. Oven temperature was held at 100 °C for 1 min and then programmed to 140 °C at 5 °C/min and held at 140 °C for 1 min; to 230 at 1.5 °C/min and held for 20 min; and finally to 300 °C at 10 °C/min and held for 5 min. Injection port temperature was 270 °C, and the detector was held at 325 °C. Data was collected and analyzed on a MicroVax-based computer software system (PE-Nelson, San Jose, CA). PCB congeners were quantitated against authentic standards using an internal standard method with a five-point calibration curve. Some samples were reanalyzed using the conditions described above with a 60-m DB-5 fused silica capillary column and a single-point calibration secondary standard containing 89 congeners described by Mullin (48). Laboratory blanks of deionized water showed no contamination. Internal standard recoveries, based on surrogate recoveries, were 62 ( 14% (n ) 8), 74 ( 16% (n ) 28), and 82 ( 13% (n ) 29) for blanks, colloidal, and dissolved samples, respectively. Aqueous samples were preserved in phosphoric acid, stored at 4 °C (i.e., whole, retentate, and permeate), and analyzed for organic carbon using a high-temperature (Dohrmann, Santa Clara, CA) or persulfate oxidation total organic carbon analyzer (O. I. Analytical, College Station, TX). Ultraviolet spectroscopic studies were conducted with a Bausch & Lomb Spectronic (Rochester, NY) or Perkin Elmer UV/VIS Lambda 2 (Norwalk, CT) spectrometer. Ultraviolet absorbance was quantified at 320 nm. Artificial interstitial water was prepared from Aldrich humic acid
(Milwaukee, WI) in 30‰ reconstituted seawater (5.6 mg of organic carbon/L).
Results and Discussion Ultrafiltration. In evaluating ultrafiltration, two significant problems became apparent: ultrafilter breakthrough and dissolved phase contaminant sorption to the ultrafilter. For environmental studies, these problems result in the need to invoke several capricious assumptions before using ultrafiltration to quantify colloid-nonpolar organic contaminant interactions. As a result, ultrafiltration cannot be recommended for isolating colloids, at least from contaminated sediment interstitial systems. During preliminary ultrafilter studies with Providence River interstitial water, it was visually apparent that colloidal material was breaking through the ultrafilter (i.e., last ∼25% of sample). This phenomena manifested itself as the dissolved phase, which initially exited the ultrafilter as a clear liquid, began to acquire the characteristic yellowish color associated with interstitial humic substances. This phenomena was quantified by ‘tracking’ the increase in presence of ultraviolet absorbing substances (i.e., humic materials) in the dissolved phase using UV spectroscopy. Regardless of the initial volume, breakthrough was observed to occur after approximately 75% of the sample had been processed using the 1000 mwu ultrafilter (Figure 1a). The phenomena also occurred with the 500, 1000, and 3000 mwu ultrafilters evaluated with 180-mL samples of Providence River interstitial water (Figure 1b). Studies with artificial interstitial water generated similar results (Figure 1c), as did studies performed at different ultrafiltering pressures (28 and 37 kPa) (Figure 1d). Significance of breakthrough is worthy of concern because it indicates that a complete mechanical separation of the dissolved and colloidal phases is not possible given that the colloidal phase appears to enter into the dissolved phase during later stages of ultrafiltration. Radiolabeled contaminants lindane, fluoranthene, and tetrachlorobiphenyl all demonstrated various degrees of adsorption to the ultrafilter and ultrafiltering equipment (Table 2). For all three compounds, the ultrafiltration studies showed an inverse relationship between retentate compound sorption and ultrafilter molecular weight cutoff with compound loss increasing with decreasing ultrafilter molecular weight cutoff. In the retentates of lindane and fluoranthene, samples collected at the end of the ultrafiltration process (i.e., 130 mL of retentate) showed greater sorption than did samples collected early in the ultrafiltration (i.e., 50 mL). This suggests that as sample retentate volume decreases sorption increases. This same trend is seen with tetrachlorobiphenyl for some ultrafilters (e.g., 3000 and 30 000 mwu). For both fluoranthene and tetrachlorobiphenyl, very little dissolved (
