Environ. Sci. Technol. 1988, 22, 677-685
Environmental Control;CRC Press: Boca Raton, FL, 1981; Vol. 11, pp 219-276. (12) O'Brien, R. J.; Crabtree, J. H.; Holmes, J. R.; Hoggan, M. C.; Bockian, A. H. Environ. Sei. Technol. 1975,9,577-586.
Brackett, J. M.; Azarraga, L. V.; Castles, M. A.; Rogers, L. B. Anal. Chem. 1984,56,2007-2010. Kubelka, P.; Munk, F. Zh. Tekh. Fiz. 1973,12,593. Ip, W.M.; Gordon, R. J.; Ellis, E. C. Sei. Total EnviFon. 1973,36,203-208.
Gordon, R. J.; Bryan, R. J. Enuiron. Sci. Technol. 1973, 7,645-647.
Simoneit, B. R. T.; Mazurek, M. A. Critical Reviews in
Received for review June 4,1987.Revised manuscript received December 14, 1987. Accepted December 29, 1987.
Intercomparison Study of Liquid-Liquid Extraction and Adsorption on Polyurethane and Amberlite XAD-2 for the Analysis of Hydrocarbons, Polychlorobiphenyls, and Fatty Acids Dissolved in Seawater Josep I. Gbmez-Bellnchbn, Joan 0. Grimalt," and Joan AlbalgOs
Department of Environmental Chemistry (CID-CSIC), Jorge Girona Salgado, 18, 08034-Barcelona, Spain
w Liquid-liquid extraction and adsorption on polyurethane foam and on Amberlite XAD-2 resin have been compared for the analysis of aliphatic, aromatic, and chlorinated hydrocarbons and fatty acids dissolved in seawater. The application of these methods sampling in parallel the same body of water has resulted in significant differences related to the proportion of higher molecular weight components in the complex mixtures of aliphatic and aromatic hydrocarbons. These occur irrespective of the operating conditions and are attributed to selective interactions of these hydrophobic species with macromolecular organic matter such as fulvic and humic acids and to the resulting effects on adsorbent properties. In contrast, the quantitative results for most hydrocarbons and fatty acids depend on the total sampled volume, resulting in major losses when volumes larger than 300-400 L are collected with the adsorption systems (3000 bed volumes for the Amberlite XAD-2). This parameter is, however, of small importance for polychlorobiphenyls and does not influence the observed concentrations of unsubstituted polycyclic aromatic hydrocarbons. Introduction
The understanding of the molecular composition of the dissolved organic matter (DOM) in the marine environment deserves great interest from the geochemical and environmental standpoints. In this sense, marine DOM is a major pool for the biogeochemical cycle of carbon, representing a much larger amount than all the continental vegetation (1). However, despite the importance of this pool, the understanding of the composition of DOM is small. For example, only recently has molecular evidence for a terrestrial component of organic matter dissolved in ocean water been obtained (2). The molecular characterization of DOM is also needed for the ascertainment of the impact of urban, industrial, and agricultural activities onto the oceans, which may be of high relevance on a global scale. For instance, the estimated input rate of petroleum hydrocarbons to the marine environment is similar to the rate of biosynthetic hydrocarbons generated by the overall marine primary productivity (1-10 million metric tons per year in both cases) (3). Other man-made hydrocarbons, such as the extremely persistent polychlorinated biphenyls (PCB), were estimated in 1981to account for 6000 metric tons in oceanic water and biota (4). Consequently, in the recent years an important effort is being conducted in order to study the DOM of coastal and open sea environments and for the characterization of their source inputs. The lipid 0013-936X/88/0922-0677$01.50/0
fraction, especially the hydrocarbons, has been regularly focused on for this purpose (5). From the analytical point of view this study requires the isolation of trace components from the seawater and their identification and quantitation, usually accomplished by capillary gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). Therefore, a concentration step is needed where the organics dissolved in large volumes of seawater (100-1000 L) are concentrated into a small volume of an organic solvent (down to 50 pL) for adequate analysis. This can be fulfilled by solvent extraction (6, 7) or adsorption onto solid sorbents (8,9). However, when comparing the analytical results reported by different laboratories for the same region, important discrepancies are observed. For example, in the northwestern Mediterranean Sea the concentrations of total dissolved aliphatic hydrocarbons have been reported to range between 0.30 and 0.36 pg/L in surface waters from approximately 2 km off the coast of Monaco (IO),but the results from another group (11, 12) are in the range of 5.8-240 pg/L for waters sampled at 3.5 km offshore Villefranche Bay (11) and at a level of 20 pg/L for water collected about 35 km offshore the French coast, in the Ligurian Sea (12).There is an obvious discrepancy, especially when considered that the lower values are found in waters closer to seashore. On the other hand, a range of 0.007-0.67 pg/L has been reported by us for the total dissolved aliphatic hydrocarbons of marine waters sampled around the Ebro delta, an area where high levels of DOM should be expected (13). Since all these values correspond in general to repeated sampling and analysis, temporal or spatial variability cannot be quoted as an explanation for these differences. Contrarily, it is reasonable to question whether the source of this disagreement lies on the sample handling, since different methods were used for lipid concentration and isolation [adsorption on Amberlite XAD-2 resins (IO),liquid-liquid extraction (11,12),and adsorption on polyurethane foam (13)]. At this respect, it is worth mentioning that, despite the large extent of literature data presently available on these extraction and concentration techniques for trace organics in water, their validation for seawater analysis still remains to be clarified. Background
The performance of extraction and adsorption methods for the analysis of hydrophobic components in water has generally been tested with spiked water samples. In these studies, good recoveries have been found in the determination of polynuclear aromatic hydrocarbons in water by
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liquid-liquid extraction (14-16), adsorption on polyurethane plugs (17-20), or on Amberlite XAD-2 resins (18, 21-24). The same is the case for other hydrocarbons such as many chlorinated pesticides and PCBs (6, 7, 21, 22, 24-32) and n-alkanes (16,28).These adsorbents have also performed well for carboxylic acids in acidified waters (21, 22). Several of these tests have been carried out on variable volumes and flows of water and at different analyte concentrations (17, 20, 26, 31, 33-36) so that the chromatographic properties of these adsorbents are also known. But unfortunately, these experiments are usually carried out on low ionic strength and particle colloid free aqueous samples, like distilled, deionized, tap, or finished waters (6, 14, 15, 17-23, 25-32) representing a matrix rather different from natural waters and particularly from seawater. In this sense, significant losses have been observed in recovery tests of PCBs with XAD-2 when waters with higher content of organic matter have been analyzed (37) because a competition for the active sites of the adsorbent is established between the chlorinated hydrocarbons and other hydrophobic groups present in the samples. The problem is not only related with these effects but also with the influence of some constituents of the dissolved organic matter, such as fulvic or humic acids or other colloidal aggregates, on the distribution of trace components in water, which may also affect the partition or adsorption processes. All these aspecta point to the use of seawater for such recovery experiments. However this is not an easy task; it has been described that when spiking either fdtered (28) or unfiltered (28,38)seawater important amounts of the added compounds are not truly dissolved, being retained in glass fiber filters during analysis. In addition, irrespective of the feasibility of seawater for recovery tests, it must be considered that there will always be major differences between the distribution of trace organics in real marine water and those added by laboratory procedures (Le., solution in acetone and shaking) so that their analytical behavior may differ significantly. In fact, seawater is a complex dynamic system containing truly dissolved organics and a continuous range of particle sizes, from the smallest colloids up to larger organic aggregates, bacteria, and plankton (39). The crossed interactions between some of these materials are very relevant in terms of solubilities (40-43) or sediment sorption processes (44-47), two major physical properties determining transport and fate in the aquatic environment (47-49). Therefore, it is apparent that the system cannot be modeled only by spiked solutions; direct determinations of the selected trace organics in real samples are necessary. Such an objective compels the analyst to sample large volumes of water (100-1000 L), and this due to the variability of analyte distributions in the sea represents a severe problem for the intercomparison of methodologies. Sampling in parallel allows one to overcome this difficulty, although it requires an adequate instrumental setup. To the best of our knowledge, only one attempt has been carried out in this direction, involving a comparison of liquid-liquid extraction and polyurethane foam adsorption in open sea waters with a system installed on a research vessel (50). To this end, we report in this paper the results corresponding to several intercomparison experiments of liquid-liquid extraction and adsorption on polyurethane foam and Amberlite XAD-2 resins for the analysis of hydrocarbons (aliphatic, aromatic, and chlorinated) and fatty acids in seawater. A large-volume sampling assembly was prepared for the simultaneous utilization of these methods 078
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Flgure 1.
study.
Schematic diagram of the sampling assembly used in this
(see Figure 1). Volumes of seawater of about 4ooo L were taken, kept in indoor tanks,and sampled after several days of storage. The adsorbents and liquid extracts were later analyzed in the laboratory. With this approach the three methods can be tested by application of diverse sampling conditions (flow rate, volume, ... ) to the same large and homogeneous body of water, affording a significant improvement in the knowledge of their performance for seawater analysis. Experimental Section Materials. Glass microfiber filters (GF/C) were purchased from Whatman Ltd. (Maidstone, England). Polyurethane foam of density 0.0285 g/cms was from United Foam Corp. (Hayward, CA). Amberlite XAD-2 was obtained from Rohm and Haas Espaiia, S.A. (Tudela, Navarra, Spain). All solvents were glass distilled (Carlo Erba, Milan, Italy). Potassium hydroxide and sodium sulfate (analysis grade) were purchased from Scharlau-Ferosa (Barcelona, Catalunya, Spain). Analysis-grade sodium chloride and hydrochloric acid (25% w/w) were from Merck (Darmstadt, FRG). Boron trifluoride in methanol solution (10%w/w) was obtained from Fluka AG (Buchs, Switzerland). Neutral silica gel (Kieselgel40,70-230mesh) and alumina (aluminum oxide 90 active, 70-230 mesh) were from Merck (Darmstadt, FRG). Tetradecane, docosane, dotriacontane, hexatriacontane, phenanthrene, pyrene, chrysene, methyl heptadecanoate, and methyl heneicosanoate were purchased from Fluka AG (Buchs, Switzerland). Aroclor 1254 and hexachlorobenzene were obtained from Analabs Inc. (New Haven, CT). Apparatus. The water sampler (Figure 1)is adapted from the design of de Lappe et al. (50). It encompasses a priming circuit, an impeller pump, a filter holder, and a plumbing system enabling various combinations of polyurethane and XAD-2 columns, either in series or in parallel. A circuit of liquid-liquid extraction units can also be assembled. A noncontaminating Teflon impeller pump (Jupiter, Fluorocarbon Corp., Anaheim, CA) pushes the water through the system. When priming is necessary, a suction
device (Fortuny Ltd., Barcelona, Catalunya, Spain) is mechanically connected to the engine of this pump. The water is first filtered through a microfiber fiiter (rated pore sized 0.5 pm) placed in a 142 mm diameter stainless steel filter holder (Millipore Iberica, S.A., Barcelona, Catalunya, Spain). After this step, the dissolved organic matter is obtained by one of these processes: (a) adsorption on polyurethane, by means of Teflon columns (30.5 cm long, 6.35 cm o.d., 5.08 id.) packed with five polyurethane foam plugs; (b) adsorption on Amberlite XAD-2, using 20 cm x 2.7 cm i.d. glass cylinders filled with XAD-2 resin beads; (c) liquid-liquid extraction, by means of a circuit of twostage extraction units constructed in the design of Ahnoff and Josefsson (6). Flow rates and water volumes passing through these devices were individually monitored with turbine flowmeters (Pepperl-Fuchs, Mannheim, FRG) and totalizers (Koyo Electronic Industries, Japan). All connection tubing was made of 3/4 in. Teflon pipe and was externally protected with a stainless steel coil. Prior to sampling, 50 L of distilled water was pumped through the plumbing system with a clean filter and no adsorbents in the machine. The tubing was then additionally rinsed with the following series pf organic solvents: methanol, acetone, n-hexane, and acetone and methanol, leaving the machine ready for use after passing 20 more L of distilled water. The filters were kilp-fired at 350 "C, and the adsorbents and the extraction solvent were cleaned as follows: (a) The individual polyurethane plugs were extracted with acetone in a large Soxhlet apparatus during 72 h and packed into columns. These were then rinsed with cold acetone and n-hexane. A final n-hexane rinse was concentrated by rotary evaporation, reconstructed to 10 pL, and submitted to gas chromatography for blank testing. Blank requirements were as follows: splitless injection of 2.5 pL should result in chromatograms with no unresolved GC envelope and only very few peaks, representing up to 2 ng in terms of their FID response. This threshold, under the above dilution factor, is equivalent to a level of artifacts below 0.1 ng/L when referred to 100 L of water. The whole procedure was repeated until satisfactory blanks were obtained. Finally, the columns were primed for seawater sampling with a rinse of 500 mL of methanol and wrapped in Teflon bags for transport. (b) The XAD-2 resin beads were introduced in a glass cylinder and suspended in Milli-Q-grade water in order to fill up their internal pores. At the same time, a moderate flow of water (from bottom to top) was established for external bead rinsing, and the process was maintained until the supernatant was particle free. After this step, the resin beads were packed into glass columns and refluxed with acetonitrile for 3 h in a specially designed apparatus where, instead of siphoning, continuous percolation of solvent through the bed of solid sorbent was performed (51). Extraction was resumed with fresh acetonitrile (12 h) and then with three successive steps of acetone-water (9:l) (3 h each). In the last extraction, the acetone retained in the column was collected, dried over anhydrous sodium sulfate, and concentrated for gas chromatographic analysis. Further extractions with acetone-water (9:l) were performed if blanks were unsatisfactory according to the requirements indicated above. Finally, water (Milli-Qgrade) was filled into the cartridges to prevent shrinking of the resin beads, the cartridges were capped and kept in a refrigerator until use. Storage periods were shorter than 1 week. (c) The cyclohexane used in the liquid-liquid extraction tests was glass distilled on a 1-m packed (Rashig) column
equivalent to 12 theoretical plates, with a reflux ratio of 12:l. The purity was checked by concentrating under vacuum 100 mL of solvent to 10 pL and following the procedure indicated above. In addition to the sampling system the other materials were cleaned as follows: glassware was washed by sonication in a detergent solution and rinsed with Milli-Q-grade water and acetone. Sodium sulfate, silica gel, and alumina were extracted with methylene chloride-methanol (2:l) in a Soxhlet apparatus for 24 h. After solvent evaporation, sodium sulfate, potassium hydroxide, and alumina were heated at 350 "C and silica gel at 120 "C. A total of 5% (w/w) of Milli-Q-grade water was then added to the chramatographic adsorbents for deactivation. All organic solvents were distilled and analyzed for residues as described above for the cyclohexane. Analysis Methods. Volumes of about 4000 L of seawater were accumulated for each set of analyses. This was carried out in the Institute of Marine Sciences (CSIC) where seawater is regularly collected, filtered for gross particles, and stored in tanks. Three days prior to Sampling, a tank was emptied and filled with new seawater for 2 times and then left untouched. After this period the apparatus was situated at ground level and the Teflon intake tubing placed inside the tank, with the entrance fixed at 15 cm above the bottom. Liquid-liquid extraction and adsorption on XAD-2 and polyurethane columns were performed in parallel. The water flow for each extraction device was regulated independently with the corresponding valves and flowmeters. Volumes were also totaled individually. After sampling the polyurethane columns were kept wet at 4 "C in the dark and analyzed within a period of 24 h. According to the literature data (8, 19), postsampling degradation processes were avoided with these conditions of storage. The columns were extracted with 500 mL of acetone, followed by 500 mL of n-hexane. Solvent volumes were reduced by rotary evaporation, combined, and partitioned in a 2-L separatory funnel with 700 mL of Milli-Q-grade water to remove the acetone; the organics were collected in the n-hexane layer. The acetone-water mixture was back extracted once with 300 mL of n-hexane. The combined n-hexane extracts were reduced to approximately 1 mL prior to analytical handling. The Amberlite XAD-2 columns were also kept wet at 4 "C in the dark. In less than 24 h they were introduced in the continuous percolation refluxing apparatus mentioned above and extracted with 220 mL of acetone-water (91) for 4 h. The solution was then reduced under vacuum to a volume of about 50 mL, and sodium chloride was added until saturation. The organics were recovered from this mixture with n-hexane (3 X 30 mL) and concentrated under vacuum to -1 mL. The cyclohexane volumes used in the liquid-liquid extractors (250 mL per unit) were decanted and combined. Additional rinsing was performed with 100 mL of cyclohexane, and finally the combined extracts were concentrated under vacuum to about 1 mL. All these extracts were hydrolyzed overnight with 35 mL of 6% KOH/MeOH, and the neutral and acidic fractions were successively recovered by n-hexane extraction (3 X 30 mL), the latter after acidification (pH 2) with aqueous HC1,6 N. The acidic fractions, previously reduced to 0.5 mL, were esterified overnight with 15 mL of 10% BF,/ MeOH. Then, the boron-methanol complex was destroyed with 15 mL of water, and the fatty acids were recovered as methyl esters by extraction with 3 X 30 mL of n-hexane. Before gas chromatographic analysis, the extract was Environ. Sci. Technol., Vol. 22, No. 6 , 1988
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vacuum evaporated to 100 pL. The chromatographic separation of the neutral lipids was adapted from a previously established method (52). A column was filled with 800 mg of 5% deactivated silica (bottom) and 800 mg of 5 % deactivated alumina (top). The following fractions were collected: (a) saturated hydrocarbons (3 mL of n-hexane), (b) monoaromatic and organochlorinated hydrocarbons [3 mL of methylene chloride-n-hexane (1:9)], and (c) aromatic hydrocarbons [6 mL of methylene chloride-n-hexane (2:8)]. All these fractions were vacuum evaporated to a suitable volume (between 50 and 300 pL) before instrumental analysis. Gas chromatographic analysis of all these fractions and the fatty acids was performed with a Carlo Erba FTV 4160 GC instrument; equipped with a flame ionization detector and a splitless injector. A column of 25 m X 0.25 mm i.d. coated with SE-52 was used (surface film thickness 0.15 pm). Hydrogen was the carrier gas (50 cm/s). The temperature was programmed from 60 to 310 "C at 6 deg/min. Injector and detector temperatures were respectively 300 and 330 "C. The injection was in the splitless mode (solvent, isooctane; hot needle technique), keeping the split valve closed for 35 s. Organochlorinated hydrocarbons were analyzed with a Carlo Erba FTV 4130 series equipped with a splitless injector and an electron capture detector. A SE-52 column was also used (25 m X 0.25 mm id.). The chromatographic conditions were essentially the same as described above except for the final oven and detector temperatures, respectively, 290 and 300 "C. The fractions of aromatic hydrocarbons were also analyzed by UV fluorescence in a Perkin-Elmer MPF-3 UV spectrofluorimeter. Emission spectra from 320 to 500 nm (slit 2.5 nm) were obtained with an excitation wavelength of 300 nm (slit 20 nm). Synchronous excitation spectra between 280 and 480 nm were recorded with a AA (Ae,,, Aex) of 20 nm and both slits set at 5 nm. n-Alkanes and isoprenoid hydrocarbons (pristane and phytane) were quantitated by comparison with an external standard mixture of n-Ellr,n-Czz, n-C3z, and n-C,. A standard solution containing phenanthrene, pyrene, and chrysene was used for determining the concentrations of the resolved aromatic compounds. The unresolved GC envelopes were measured by planimetry and quantitated by reference to n-CZ2.All these operations were carried out semiautomatically with a Hewlett-Packard 86 microprocessor equipped with a digital planimeter. Fatty acids were quantitatively determined with reference to a mixture of heptadecanoate and heneicosanoate methyl esters. PCBs were quantitated by comparison with a standard of Aroclor 1254 and hexachlorobenzene with respect to a standard solution of this compound. Injections of both standards and samples were repeated until a reproducibility better than 5% in the peak areas was observed. Results and Discussion
Design of the IntercomparisonExperiments. Figure 1 illustrates the sampling system assembled for the analysis
of dissolved hydrocarbons and fatty acids in water with liquid-liquid extraction (cyclohexane) and adsorption on Amberlite XAD-2 and polyurethane foam operating simultaneously. From the diversity of designs and applications developed with these three methods, we have selected those specifically used for concentration of organics from large volumes of water. Thus, the liquid-liquid extractors (two units connected in series) were constructed in the design of Ahnoff and Josefseon (6),and the polyurethane foam columns were built following the model used by de Lappe et al. (50). A total of 28 g of poly680
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L1-1
L1-2
L2-1
I
P1-1
,
Al-1
P1-2
I
A1-21
P2-1
A2-1
Flgure 2. Gas chromatograms for the allphatic hydrocarbon fractions corresponding to seawater concentrates obtained by pafailel liquidliqukl extraction (L), adsorption of polyurethane foam (PI, and Amberlite XAD-2 resin (A).
urethane (five foam plugs) was fitted inside each cylinder. Likewise, the Amberlite XAD-2 columns were constructed according to the design of Ehrhardt (51). They were filled up with 74 g of resin, corresponding to a bed volume of 100 mL. Table I summarizes the sampling conditions of the experiments. All samples having the same numerical code were obtained in parallel. All samples labeled with the same first number were taken from the same body of water. In the first experiment the Amberlite XAD-2 and polyurethane foam were tested at variable flow rates and volumes of water. In the second experiment only the influence of flow rate was evaluated, and the third experiment was focused on the study of sample volume effects. In all cases the collection conditions were chosen within the range of those already recommended for large-volume water sampling. Thus, for the Amberlite XAD-2 resins, flow rates in the interval of 2.5-8 bed volumes per minute and volumes of 100-1000 L have been selected according to the indications of Harvey (38),Ehrhardt (51),and Webb (53). Likewise, the rates and volumes chosen for the polyurethane foam columns are comparable to those used by de Lappe et al. (50) (1000 mL/miq, -1000 L). Liquidliquid extraction was taken in parallel as a reference method, therefore the same flow rate (100 mL/min) and volume (-50 L) were selected in each sampling, according to the operating conditions recommended elsewhere (6,50). Qualitative Results. The composition of the aliphatic hydrocarbon fraction of the samples obtained in the different experiments is reported in Table 11. Representative chromatograms are displayed in Figure 2. The presence of an unresolved complex mixture of branched and cyclic hydrocarbons and a lack of n-alkane odd-to-even carbonnumber preference (CPI 1) evidence a predominant petrogenic origin (3,54). These hydrocarbons are present in three levels of concentration averaging values of 6600, 310, and 23 ng/L of total aliphatics for the first, second, and third experiments, respectively. However, as it is illustrated in Figure 2, the profiles supplied by the three methods contain significant qualitative differences. With liquid-liquid extraction the range of compounds eluting at longer retention times (Le., higher molecular weight hydrocarbons) appears in the highest proportion whereas with Amberlite XAD-2 their proportion is the lowest, polyurethane foam representing an intermediate case. This feature is observed independently
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Li-1
L1-1
P1-1
,/'I,
Al-1
A 1- 1
1'
Flgure 3. Gas chromatograms and synchronous excltatlon UV fluorescence spectra of the aromatic hydrocarbon fractions corresponding to seawater concentrates obtained by parallel sampling as descrlbed In Flgure 2.
of the sampling conditions and concentration of hydrocarbons, and to the best of our knowledge, it has not been previously reported. This selective effect is also observed in the aromatic fraction. In Figure 3 the gas chromatograms of the aromatic hydrocarbons of some representative samples are displayed. Again liquid-liquid extraction is the method concentrating a major proportion of high molecular weight components, and Amberlite XAD-2 is the one where these hydrocarbons are obtained in the lowest amount. These differences may also be detected by synchronous UV fluorescence. With this technique the PAH present in complex mixtures of hydrocarbons such as those of petroleum-related origin can be classified according to their number of fused aromatic rings (55). The spectra also displayed in Figure 3 are characterized by a major band between 300 and 340 nm (emission wavelength) corresponding to a mixture of aromatic hydrocarbons of two fused rings. There is also an additional band (340-380 nm) representing PAH of higher fused ring number (3 and 4) and consequently of higher mean molecular weight. This second band is relatively more predominant in the liquid-liquid extracts and lower in the samples obtained with Amberlite XAD-2. This phenomenon can be tentatively interpreted in terms of the modification of the adsorption behavior of these hydrocarbons by binding interactions with complex organic materials present in seawater (55),such as fulvic and humic acids (56) or colloidal aggregates (47). The magnitude of these interactions is inversely dependent on the aqueous solubility of the organic components (40-43, 57,58). Thus, among aliphatic and aromatic hydrocarbons, the binding affinity will generally increase according to their molecular weight (as shown in ref 41), determining a stronger association of the heavier components with those macromolecular materials. In this respect, studies of sediment sorption processes have shown that the sediment-water distribution coefficients corresponding to associated hydrophobic components change in the sense that their solubilization in water is increased and their adsorption onto solid surfaces is decreased (44-47). A parallel effect is also observed for chromatographic adsorbents of hydrophobic surface, like ODS silica, which have been used for the separation of the apolar components freely dissolved in water from those associated to humic acids (57). Amberlite XAD-2 is also known for its low retention of humic and fulvic acids, especially in nonacidified natural waters (59-61). Therefore its adsorption capacity of hy882
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drophobic components such as hydrocarbons will be high for those freely dissolved (18, 21-24) and low for those associated to polar macromolecules. Since the higher molecular weight hydrocarbons are those that tend to be more abundant in the associated state, they will be proportionally less retained, giving rise to the depletion effect o'bserved in Figures 2 and 3. A similar phenomenon can be expected for the polyurethane foam. In this case, the increase in the proportion of hydrocarbons eluting at longer retention times may reflect a higher capacity for the adsorption of polar macromolecular orgadic matter, which is consistent with the weakly base anion-exchange characteristics of the foam (20). In contrast, since the associations between hydrophobic components and polar organic materials are essentially reversible (41,621,in liquid-liquid extraction the binded hydrocarbons may be dissociated from the aggregates by the organic solvent (63). Consequently, liquid-liquid extraction is the method affording the major proportion of high molecular weight hydrocarbons. QuantitativeResults. The good reproducibility obtained for samples L-1-1, L-1-2, and L-1-3 (Table 11) confirms the homogeneous composition of the large volumes of water used in each experiment. These results are in agreement with some of those corresponding to the other methods, but evident discrepancies are also produced. In the first experimept, samples P-1-2 and P-1-3 show lower concentrations than the rest, and in the third experiment, samples A-3-2 and P-3-2 also show lower levels than samples A-3-1 and P-3-1. These differences cannot be attributed to the use of different flow rates (see Table I) because in the third experiment similar rates were used for the Amberlite (approximately 550 mL/min, 5.5 bed volumes/min) and polyurethane (- 1030 mL/min) samples; despite this the concentration in aliphatic hydrocarbons of A-3-1 and P-3-1 is about 4 times higher than, respectively, A-3-2 and P-3-2. Furthermore, in the second experiment different flow rates were used with both adsorbents for the concentration of similar volumes of water, and reproducible quantitative values have been obtained despite these rate changes. The cases where a decrease in concentration is observed correspond to the analysis of seawater volumes larger than 300-400 L, showing that the capacity of the columns is exceeded for volumes of this size. This phenomenon occurs independently of the concentration of aliphatic hydrocarbons in the waters, as indicated by the results from the first and third experiments, one dealing with alkane concentrations 250 times higher than the other. A detailed study of the results corresponding to samples P-1-1, P-1-2,and P-1-3 provides additional insight into this phenomenon. In principle, saturation of the adsorbents by the alkanes would give rise to the presence of a constant amount of these hydrocarbons in the column, irrespectively of the volume of water sampled. In such situations erroneous low concentrations would be reported when referring the alkanes retained by the adsorbents to the volume of water passed through the column. But the observed depletion for the concentrations of both n-alkanes and unresolved aliphatic hydrocarbons in these three samples is even higher than the values expected from such sampling error. Therefore, no constant level of hydrocarbons is established in the column despite the fact that water with a constant concentration of hydrocarbons is collected. On the contrary, it seems that after a period of accumulation further sampling results in desorption of alkanes. The presence of other organic materials dissolved in the water samples, such as humic and fulvic acids, may serve
as an explanation. The interaction between the humic molecules and Amberlite XAD-2, although low, involves primarily hydrophobic bonding (59),therefore leaving the polar groups of the molecule oriented in the opposite direction, toward the stream of seawater. After passing large volumes of water through the column, a higher amount of these macromolecules may be retained, leaving an important proportion of the hydrophobic sites of the adsorbent covered by polar groups. This, in turn, may result in a major change of its surface characteristics, and as soon as the degree of polarity is increased, a lower proportion of hydrophobic molecules will be retained. Furthermore, in nonpolar polymers such as XAD-2, these adsorption processes are reversible (€9, and since retention of the humic and fulvic acids generally involve more than one hydrophobic site, smaller apolar species already adsorbed in the resins may be displaced. Such interchanges involve a gain in entropy resulting from the extension of the hydrophobic interaction to the maximum available apolar sites of the macromolecule binded onto the chromatographic adsorbent, That is, an interaction paralleling the “octopus”effect described for the sorption of water-soluble oligomers on sediments (64). The quantitative results of the aromatic fractions are also influenced by the size of the volumes collected for each sample (see Table 11). The unresolved mixture of aromatics and the alkylaromatics show a decrease in concentration for the samples corresponding to volumes larger than 300-400 L, although the differences are not so important as in the case of the alkanes. This effect, however, is not observed for the parent PAH (phenanthrene, fluoranthene, pyrene), which may indicate that these compounds are better retained than their alkylated homologues. The same decreasing trend is also observed for some polychlorinated aromatic hydrocarbons identified in these waters. This is the case for hexachlorobenzene,with a similar degree of depletion compared to that of the unresolved aromatic hydrocarbons, but in contrast, this effect is of small significance for PCBs. The different behavior of the aromatic compounds is likely due to their ability, as donors of a-electrons, to form donor-acceptor complexes (65). Thus, they can bind to electron-acceptor groups of the humic substances and become retained in the column, despite the progressive coverage by these macromolecules when sampling large volumes of water. The presence of ring substituents may, in turn, modify their binding capability by steric hindrance (65) and therefore explain the differences in behavior between parent, alkylated, and chlorinated components. Collection volumes are also significant when analyzing other lipid components such as fatty acids (Table 111). Again, a decrease in concentration is observed when determining these biogenic compounds in large volumes of water with either Amberlite XAD-2 or polyurethane. As it has been stated above, the important differences between artificially spiked water and real seawater represent a major difficulty for the comparison of the results obtained here and those coming from recovery experiments. However, these can be compared with those of de Lappe et al. (50) for the cases where similar operating conditions were used. That is, the differences corresponding to liquid-liquid extraction and adsorption on polyurethane foam when analyzing, respectively, -50 and -1000 L of water in this study by contrast to those observed by these authors (50) in the analysis of 100 and 1400 L of seawater with the same respective methods. No major differences in analytical behavior are found from the concentrations of PCBs obtained in both papers,
-
-
but for other components the results are not always coincident. For instance, the depletion effect in the concentration of aliphatic hydrocarbons when sampling large volumes of water with polyurethane foam adsorption is not always observed by these authors (two cases out of six). There is, however, a better agreement in the results corresponding to the aromatic hydrocarbons showing that the decrease in concentration of these components, when observed, is lower than that of the alkanes. It is difficult at this stage to give an explanation for the disparent results; obviously the composition of the dissolved organic matter is not uniform, and seawater samples from diverse origin may behave differently in the concentration systems considered here. In any case, these observations should be taken into account for the interpretation of the results obtained with the large-volume sampling systems already in use.
Conclusions Liquid-liquid extraction and adsorption on polyurethane foam and Amberlite XAD-2 have been compared for the analysis of dissolved hydrocarbons and fatty acids in seawater. The evaluation of these methods has been carried out by simultaneous sampling of seawater stored in large tanks (-4000 L) with the three systems operating in parallel. The agreement of the quantitative results has been observed to be dependent on the collected volumes of water. Thus, when the sampled volumes were smaller than 3000 bed volumes for Amberlite XAD-2 (300 L)and 400 L for polyurethane foam, consistent results with those obtained by liquid-liquid extraction were achieved for all compounds analyzed, namely aliphatic, aromatic, and chlorinated hydrocarbons and fatty acids. Larger volumes yielded lower concentrations, depending on the type of compounds considered. Aliphatic hydrocarbons and fatty acids exhibited the stronger deviation whereas the aromatic hydrocarbons were less sensitive to this effect, especially in the case of PCBs and parent polycyclic aromatic hydrocarbons. In contrast, flow rates in the range of 100-2000 mL/min do not affect the recoveries. On the other hand, significant qualitative differences in the extraction of the high molecular weight aliphatic and aromatic hydrocarbons were observed among the three systems. Thus, liquid-liquid extraction is the method providing these components in higher relative amounts, and Amberlite XAD-2 adsorption is the method yielding a lower proportion, adsorption on polyurethane foam represents an intermediate case. This trend appears to be independent of the operational conditions used. These effects are presumably due to the interaction of colloidal materials, such as humic and fulvic acids, with the dissolved hydrocarbons and with the hydrophobic sites of the adsorbents. Despite the abundance of recovery studies carried out with these adsorbents such features have not been reported previously, stressing the importance of intercomparison studies with different methods and real seawater samples for the reliability of further analysis of hydrocarbons and other lipid components dissolved in seawater. Finally, as far as the selection of a method and operational conditions is concerned, the results obtained here show that the three techniques give similar quantitative results for collection volumes smaller than 300 L (XAD-2) or 400 L (polyurethane foam), but the qualitative differences suggest that in principle liquid-liquid extraction should be the method of choice. However, the field applications of this technique present many difficulties, such Environ. Sci. Technol., Vol. 22, No. 6 , 1988
683
as the large size of the extractors, the need for a constant control of the stirring process, and the low flow rates required for good recovery (6). Polyurethane foam adsorption is the method giving closer qualitative results to solvent extraction, and its application is faster and much easier. Therefore, when field conditions prevent the use of the liquid-liquid technique, adsorption on polyurethane appears as the better alternative method for the analysis of hydrocarbon mixtures ranging over a wide span of molecular weight homologues. Acknowledgments
We are most grateful to R. W. Risebrough (University of California at Santa Cruz) for his valuable help in the construction of the sampling assembly and in the use of polyurethane foam columns, to M. Ehrhardt (Department of Marine Chemistry, Institute of Marine Sciences, University of Kiel, FGR) for his information on the continuous percolation apparatus for the extraction of XAD-2 columns, and to P. Art6 (Marine Science Center, CSIC, Barcelona) for facilitating the use of large-volume tanks for collection and storage of seawater before sampling. Registry No. Amberlite XAD-2,9060-05-3; HzO, 7732-18-5; cyclohexane, 110-82-7; phenanthrene, 85-01-8; methylphenanthrene, 31711-53-2; fluoranthene, 206-44-0; pyrene, 12900-0;hexachlorobenzene, 118-74-1; tetradecenoic acid, 26444-03-1; tetradecanoic acid, 544-63-8; isopentadecanoic acid, 50973-09-6; anteisopentadecanoic acid, 5502-94-3; pentadecanoic acid, 100284-2; hexadecenoic acid, 25447-95-4; hexadecanoic acid, 57-10-3; heptadecanoic acid, 506-12-7; octadecenoic acid, 26764-26-1; octadecanoic acid, 57-11-4. Literature Cited Bolin, B. The Major Biogeochemical Cycles and Their Interactions;SCOPE 21; Wiley: Chichester, 1983; Chapter n
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1981. Received for review July 1,1987. Accepted December 11,1987.
Assessing the Regional Effects of Sulfur Deposition on Surface Water Chemistry: The Southern Blue Ridge Keith N. Eshleman" Environmental Research Laboratory, Northrop Services, Inc., 200 S.W. 35th Street, Corvallis, Oregon 97333
Philip R. Kaufmannt Utah Water Research Laboratory, Utah State University, Logan, Utah 84322
A method was developed for quantifying the regional chronic acidification of surface waters which uses synoptic survey data and a conceptual titration model of acidification. The principal assumptions of the model are that streamwaters have been titrated by an amount of sulfuric acid equivalent to their current SO-: concentration and historical pH and acid-neutralizing capacity (ANC) can be calculated from current chemical data. The model allows for increases in S042-concentration to be compensated by increased production of base cations through use of a regional coefficient. Making "worst case" assumptions, the median historical decline in ANC and pH in streams in the Southern Blue Ridge was estimated to be 23 pequiv/L and 0.09 unit, respectively. An inverse correlation between the Sod2to base cation ratio and ANC is shown to be consistent with the use of the titration model.
H
Introduction Quantifying the regional acidification of surface waters in the United States by acidic deposition remailns an important objective of environmental assessment. Analyses of data from historical surveys of lake water and streamwater chemistry have been undertaken as a means of assessing the regional extent of acidic or low acid-neutralizing capacity (ANC) surface waters (1-12). While the results of all of these studies are consistent with the hypothesis that surface waters in areas of North America have experienced declines in pH and ANC over the past half century, quantitative regional estimates of historical acidification by acidic deposition have been hampered by questionable analytical and sampling bias in these data sets (13). Estimates of historical acidification and predictions of future changes in aquatic chemistry are thus being made through application of state of the art mechanistic models on a site-specific basis (14-16). Data limitations have prohibited the explicit application of these models to the scores of systems that would comprise a reasonable sample from a regional population, although a regionalization method has been developed that uses Monte Carlo techniques to select combinations of input parameters for a mechanistic model of catchment geochemistry (17, 18). Another possible assessment technique is a combination ~~
Present address: Environmental Research Laboratory, U S . Environmental Protection Agency, 200 S.W. 35th St., Corvallis, OR 97333. 0013-936X/88/0922-0685$01.50/0
Table I. Chemical Characteristics of the National Stream Survey Population in the Southern Blue Ridge
parameter 20th percentile 40th percentile median 60th percentile 80th percentile
variable" SO:-
ANC
pH
86.6 102.6 119.6
6.86 6.97 7.03 7.06 7.23
18.6 22.4 22.9 27.5
134.3 58.9 197.7 "All variables expressed in bequiv/L except pH.
NOS3.0 4.4 7.6 10.9 23.4
of an empirical or conceptual acidification model with current regional water quality data sets. The ultimate goal of such an analysis would be to provide answers to the following types of questions with known confidence: (1)What is the likely proportion of surface waters in a region that has been acidified to a given extent by acidic deposition? (2) What is the median change in pH and ANC that has likely occurred in a region? (3) With 95% confidence, what is the maximum proportion of surface waters that has experienced chronic depressions in pH of at least 0.2 pH unit? (4) What proportion of surface waters are presently acidic as a result of atmospheric sulfur deposition? The purposes of this paper are to (i) derive an aquatic chemical model that is applicable to answering questions such as those stated above and (ii) apply the model so as to make quantitative,-statistically unbiased estimates of the historical acidification of streams in the Southern Blue Ridge of the United States (Figure 1)potentially attributable to deposition of atmospheric sulfur. Prior to the completion of Phase I of EPA's Eastern Lake Survey in 1984 (19) and a pilot of the National Stream Survey (NSS) in 1985 (20),this physiographic region had been shown to contain low ANC (
