1798
Anal. Chem. 1987, 5 9 , 1798-1802
Determination of Alkylbenzenesulfonate Surfactants in Groundwater Using Macroreticular Resins and Carbon- 13 Nuclear Magnetic Resonance Spectrometry E. M. T h u r m a n , * T.Willoughby, L. B. Barber, Jr., and K. A. T h o r n U.S. Geological Survey, Water Resources Division, Denver Federal Center, Box 25046, Lakewood. Colorado 80225 Alkylbenzenesulfonate surfactants were determlned In groundwater at concentrations as low as 0.3 mg/L. The method uses XAD-8 resin for concentration, followed by elution wlth methanol, separation of anionic and nonlonlc surfactants by anion exchange, quantitatlon by tltratlon, and identification by I3C nuclear magnetic resonance spectrometry. Laboratory standards and field samples containing straight-chaln and branched-chain alkylbenrenesulfonates, sodlum dodecyl sulfate, and alkylbenrene ethoxylates were studied. The XAD-8 extractlon of surfactants from groundwater was completed In the field, whlch slmpllfled sample preservation and reduced the cost of transportlng samples.
Generally, surfactants are measured in water by colorimetric methods, such as t h e methylene-blue-active substances (MBAS) for anionic surfactants or the cobaltothiocyanateactive-substances (CTAS) for nonionic surfactants ( 1 , 2). These methods are nonspecific and may have interferences from naturally occurring compounds, such as humic substances ( 2 ) . Although desulfonation has been used for a number of years to identify alkylbenzenesulfonates ( 3 ) ,only recently has a micromethod been developed ( 4 ) that determines alkylbenzenes on milligram amounts of surfactant by gas chromatography. Furthermore, there is an extensive literature on the chemical analysis of linear-chain alkylbenzenesulfonate (LAS) surfactants (5-9), but much less on the analysis of branched-chain alkylbenzenesulfonate (ABS), which was removed from the market in 1965 because of its lack of biodegradation and subsequent foaming in rivers and streams ( 3 ) . In this study we developed a method to determine both branched-chain and linear-chain alkylbenzenesulfonate surfactants in groundwater, without degradation of the surfactant. We used a macroreticular resin (XAD-8) t o concentrate the surfactants in their ionic form, with the native cation from groundwater as the counterion. The surfactants were sorbed directly onto XAD-8 in the field, which simplifies sample processing. We separated the anionic and nonionic surfactants by weak anion exchange on A-7 resin in methanol, measured the amount of anionic surfactants by titration ( I ) , and determined their structure by I3C nuclear magnetic resonance (NMR). We applied our method on both standard solutions that contained a mixture of LAS, ABS, sodium dodecyl sulfate (SDS), and alkylbenzene ethoxylate (ABE) and on groundwaters from Cape Cod, MA, which have been contaminated for 40 years by secondary-treated sewage (IO). We found both LAS and ABS surfactants present in the groundwater. We used the chemical structure of the surfactants (ABS or LAS, Figure 1) as indicators of the age of the groundwater (before or after 1966). E X P E R I M E N T A L SECTION Reagents a n d Chemicals. All solvents were reagent grade (Burdick €& Jackson, Muskegon, MI), and the SDS (Fluka,
Ronkonkoma, NY), ABE (Amersham/Searle, Arlington Heights, IL), and LAS (LAS reference solution lot no. 1139, EMSL-EPA, Cincinnati, OH) surfactants were reagent grade. The ABS surfactant was technical grade (Association of American Soap & Glycerine Producers, Inc., New York). The 60-80 mesh XAD-8 macroreticular resin (Rohm and Haas, Philadelphia, PA) and the 2C--80 mesh A-7 ion-exchangeresin (Diamond Shamrock, Redwood City, CA) were Soxhlet extracted with methanol, rinsed with 0.1 N NaOH and 0.1 N HC1, and stored in methanol prior to use (11). Recovery of Standards. One liter of a 1 mg/L aqueous standard solution of each surfactant was passed through a 20-mL column of XAD-8 resin at a rate of -4 mL/min. The XAD-8 column was back eluted with 20 mL of methanol to determine sorption efficiency from aqueous solution. Standard solutions of surfactants (in methanol) were also passed through a 20-mL column of A-7 resin in hydrogen form. The A-7 column was back eluted with 20 mL of a 0.1 N aqueous NaOH/methanol solution (10/90 (v/v)) to determine the recovery of a mixture of anionic and nonionic surfactants. Finally, a mixture of aqueous standards was separated by the combined procedure of XAD-8 and A-7 resins. Field Samples. Two groundwater samples were collected; one sample was 500 m and the other 3000 m downfield of sewageinfiltration ponds located on an air base near Falmouth, MA (IO). Samples were collected with a peristaltic pump equipped with Teflon tubing (12). Three hundred liters of water was pumped through a 1.2-L column of XAD-8 at 0.3 L/min. The XAD-8 resin was eluted in a Soxhlet extractor with methanol, concentrated to 10 mL in a rotary evaporator, and centrifuged to remove precipitated salts (sodium sulfate). From this point the groundwater samples received the same treatment as the standard solutions, except that the samples were rotary evaporated to dryness and dissolved in D,O for I3C NMR. The sample from 500 m required a drop of deuteriated methanol and dimethylformamide (DMF) to solubilize. Analysis. The resin extracts were analyzed for anionic surfactants by the mixed indicator method ( I ) and nonionic surfactants were determined by the CTAS method ( I ) . The mixed indicator method involves the titration of the anionic surfactant with a quaternary ammonium surfactant in a chloroform/water mixture (1). The end point is indicated by a color change from rose to gray. The mixed indicator is a combination of dimidium bromide and erioglaucine in ethanol. The mixture of anionic surfactants was titrated before and after hydrolysis (1) to distinguish between SDS and alkylbenzenesulfonates (LAS + ABS). The concentration of anionic surfactants in groundwater was measured in the field with a MBAS test kit (Hach Chemical Co., Loveland, CO). Natural abundance 13CNMR spectra were recorded on a Varian (Palo Alto, CA) XL-300 NMR spectrometer at 75.4 MHz, using 10-mm NMR tubes with dioxane as internal reference at 67.4 ppm. Concentration of ABS was 200 mg/0.5 mL of D20 and 1.5 mL of H20, LAS was 120 mg/2.0 mL of D20, the 3000-m well was 100 mg/2.0 mL of D20, and the 500-m well was 68 mg/0.5 mL D,0/CH30D and a drop of deuteriated DMF (5-mm tube). Acquisition parameters for broad band decoupled spectra were sweep width equal to 250 ppm (18850 Hz), acquisition time equal to 0.795 s, pulse width equal to 45O, pulse delay equal to 1.0 s, continuous broad band decoupling and line broadening of 1.0 Hz, 47 000 transients for the ABS and LAS standards, 128000 transients for the 3000-m sample, and 218000 transients for the 500-m sample. The attached proton test (APT) (13) spectra were ac-
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987 cy3
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anionic surfactants had poor recoveries with front elution (62436%)but had better recoveries with back elution (82-84 f 5%). None of the anionic surfactants passed through the column; surfactants were sorbed by ion exchange. The difference in recovery between front and back elution is attributed to strong anion exchange sites that are distributed throughout the resin column and irreversibly bind the anionic surfactants. Thus, careful addition of the sample to the A-7 resin and use of the minimum amount of resin will improve recovery. The nonionic surfactant, ABE, did not sorb on the A-7 resin from methanol and was recovered in the effluent of the A-7 column (84 f 5%). There was a loss of ABE (15 f 5%) probably because of strong sorption sites on the A-7 column. Sorption Sites. Sodium salts of ABS and LAS surfactants were efficiently sorbed on XAD-8, probably as anions, rather than ion pairs (15-17). We propose that the surfactant orients with the sodium sulfonate group into the water phase and the hydrocarbon chain sorbed to the resin, which converts the nonionic XAD-8 resin to a cation exchange resin. This mechanism has been called the dynamic ion-exchange model in ion-pair chromatography (15-18), in which the organic anion sorbs and behaves as an ion-exchange site. One interesting result of the sorption of ABS onto XAD-8 is that the native cation in groundwater, which is an inorganic ion, is the counterion, rather than a large organic cation (tetrabutylammonium phosphate), which is typically used (15-1 7). In past studies (11,19-21),we have concentrated humic and fulvic acids from water on XAD-8 by lowering the pH to 2.0, in order to protonate ionized carboxyl groups; thus, it was useful to find that organic anions, which were 18 carbons long, were efficiently sorbed without pH adjustment. We now realize that our humic and fulvic acid isolation method (11, 21) also isolates surfactants, if they are present. However, because adjustment of the water sample to pH 2.0 is required for humic isolation ( I I ) , the majority of the humic and fulvic acids in the groundwater were not coisolated with the surfactants. Field Sample Results. We measured surfactants as MBAS, SDS, ABS, LAS, and CTAS on groundwater samples taken from monitoring wells 500-m and 3000-m downfield from the sewage-infiltration ponds (Table 11). MBAS was 0.4 mg/L in the 500-m well and 2.5 mg/L in the 3000-m well, which we thought contained ABS surfactants (12). We recovered 75% of the MBAS surfactants from the 500-m well and 88% from the 3000-m well on the XAD-8 resin, as determined by mixed indicator titration of the methanol eluate (Table 11). The XAD-8 efficiently removed the anionic surfactants in the field, based on the MBAS test, which we used to monitor the column effluent. No breakthrough of MBAS substances occurred on either sample. The SDS surfactants were not detected in either of the wells, possibly because of poor recovery on XAD-8. Nonionic surfactants by CTAS and I3C NMR (see discussion in following section) were present in trace quantities (0.01 mg/L) at the 500-m well but were not detected in the 3000-m well (Table 11). I3C Nuclear Magnetic Resonance. The presence of ABS and LAS in the groundwater was determined with 13C NMR spectrometry. Normal broad band decoupled spectra and
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Flgure 1. Chemical structure of SDS and isomers of LAS, ABS, and ABE surfactants.
Table I. Recoveries of Anionic and Nonionic Surfactant Standards from XAD-8 and A-7 Resins (Average of Duplicates) % recovered from
resin
surfactant alkylbenzenesulfonate (ABS) alkylbenzenesulfonate (LAS) sodium dodecyl sulfate (SDS) alkylbenzene ethoxylate (ABE)
98*5 100 = 5 50 i 5 100 5
*
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quired with conditions similar to broad band decoupled spectra except T delay was 8.0 ms, no delay was used between the spin echo sequences, and there were 45000 transients for the ABS and LAS standards, 183000 transients for the 3000-m sample, and 374 000 transients for the 500-m sample. RESULTS AND DISCUSSION Recovery of Standards. Table I gives the recovery results for ABS, LAS, SDS, and ABE on XAD-8 from 1 L of distilled water. The effluent from the XAD-8 column was analyzed by titration for each compound. The analyses showed that the XAD-8 resin had retained the ABS, LAS, and ABE surfactants with 99 f 5% sorption efficiency. Methanol was an efficient eluent for the surfactants, and titration analyses of the eluate indicated a recovery of 99 f 570,when back elution was used. The SDS had only 50 f 5% sorption efficiency on XAD-8, but was efficiently eluted from the resin. The shorter chain length of the SDS (12 carbons vs. 18 carbons for ABS and LAS) decreased sorption efficiency (14). The A-7 weak anion exchange column recovered the surfactants less efficiently than the XAD-8 (Table I). The three
Table 11. Titration Analysis of Surfactant Fraction from the 500- and 3000-m wells (ND is not detected)a sample
MBAS mg/L
recovery, %
SDS, mg/L
ABS, mg/L
LAS, mg/L
CTAS, mg/L
500 m 3000 m
0.4 2.5
75 88
ND ND
ND
0.3
0.01
ND
ND
2.3
"MBAS is for the original groundwater, other data are of the resin isolates. Recovery is for the amount of surfactant recovered from resin isolate.
1800
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APT spectra (13)were recorded for both the ABS and LAS standards and for the groundwater isolates from the 500-m and 3000-m wells. In the broad band decoupled spectrum of the ABS standard (Figure 2A), the aliphatic methyl, methylene, methine, and quaternary carbons of the branched alkyl chains occur in the region from 9 to 50 ppm. The APT spectrum of the ABS standard (Figure 2B) allows further assignment of the 13C resonances. Methylene and quaternary carbons are positive, and methine and methyl carbons are negative. The resolution of the many aliphatic carbons into methyl and methine carbons, in the region from 9 to 30 ppm, indicates the highly branched nature of the alkyl chain. Methylene and some quaternary carbons occur from 30 to 50 ppm (Figure 2B). The peaks centered at 126 ppm are the protonated aromatic carbons of the benzene ring (22). The peak at 142 ppm is the aromatic carbon attached to the sulfonate group; the peak at 154 ppm is the aromatic carbon bonded to the alkyl side chain (22). The broad band decoupled and APT spectra of the sample from 3000 m (Figure 3A) are virtually identical with the spectra of the ABS surfactant. Thus, the surfactant from the 3000 m well is unequivocally identified as ABS surfactant. Spectra of the LAS standard are shown in Figure 4. Methyl carbons in the aliphatic region of the spectrum occur at 13, 16, and 23 ppm. Methylene carbons occur from 24 to 40 ppm and methine carbons occur at 41,47 and 49 ppm. The alkyl
Figure 3. Broad band decoupled and APT % NMR spectra for 3000-m groundwater sample.
chains of the LAS standard are a less complicated mixture of isomers (five isomers for a 12-carbon alkyl chain) than the alkyl chains of the ABS standard (-100 isomers for a 12carbon alkyl chain) because of methyl branching of propylene tetramers (23),resulting in a simpler spectrum of the LAS standard compared to the ABS standard. This difference in the aliphatic region, especially the 9-30 ppm shifts, is the main spectral difference between LAS and ABS surfactants. The methine carbons a t 41-49 ppm in the LAS standard indicate a minor degree of branching in the alkyl chain, probably at the attachment of the alkyl chain to the aromatic ring (Figure 1). The protonated aromatic carbons of the benzene ring are a t 127-129 ppm, the benzene ring carbon bonded to the sulfonate group a t 142 ppm, and the benzene ring carbons bonded to the alkyl chains at 151 and 152 ppm. The spectrum from the surfactant isolate from the 500-m well (Figure 5) appears to contain LAS; however, the 13C NMR spectra do not match the standard LAS spectra identically. The aromatic and aliphatic regions of the 500 m sample spectrum closely resemble the LAS standard. The sulfonate carbon a t 142 ppm and the three sharp aromatic resonances from 127 to 129 ppm indicate a similar aromatic structure for the sample and LAS. Furthermore, the predominance of the methylene carbons in the aliphatic region are indicative of the presence of LAS in the 500-m sample. The broad band decoupled spectrum from the 500-m well (Figure 5A) exhibits resonances a t 70-72 ppm, which are identified as methylene carbons in the APT spectrum (Figure
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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5B) and are probably bonded to oxygen. These peaks may possibly be explained by the presence of oxyethylene units (-OCH&H,O-) in a nonionic surfactant, such as ABE. The assignments for peaks 1, 2, 3, and 4 in Figure 5A are listed above the chemical structure of ABE in Figure 1. Peak 4 is the aromatic carbon that is ortho to the oxyethylene chain. Peaks 1, 2, and 3 are methyl carbons attached to oxygen in the oxyethylene chain. The presence of nonionic surfactant in the 500-m sample would mean that they were not completely separated from the LAS anionic surfactant on the A-7 resin. Alternatively, a complex anionic surfactant may be present that contains oxyethylene units. Because the amount of sample isolated from the 500-m well was small (68mg),the signal to noise ratio of the N M R spectra was poor. When the concentration of LAS in the groundwater is less than 0.5 mg/L, as it was in the 500-m well, a larger volume of sample should be processed (>300 L). Finally, we concluded that ABS surfactants accounted for the majority of the anionic surfactant (2.3 mg/L) in the 3000-m well but were not detected in the 500-m well (Table II). Conversely, the LAS surfactanta were present in the 500-m well and were not detected in the 3000-m well. CONCLUSIONS The advantage of the 13C NMR method for determining surfactants in water is that it provides direct chemical evidence of the structure of the surfactant without degradation and will work even on a simple mixture of surfactants. The main
limitation of the method is that large volumes of water must be processed (>lo0 L) in order to use I3C NMR. Finally, we had hypothesized from hydrologic evidence (10, 12)that ABS was present in the 3000-m well, which indicated a groundwater age older than 1965 (the last year of domestic use of ABS). The XAD-8 isolation procedure coupled with 13C NMR confirmed this earlier hypothesis and indicated a minimum groundwater velocity of 0.3 m/day for the 3000-m flow path. ACKNOWLEDGMENT We thank Denis LeBlanc of the U.S. Geological Survey for help in field sampling. Registry No. Duolite A7, 37251-30-2; Amberlite XAD-8, 11104-40-8;ABS, 107820-74-6; LAS, 2211-99-6; SDS, 151-21-3; ABE, 9002-93-1;HzO, 7732-18-5;CsHSSOSH, 98-11-3. LITERATURE CITED (1) Miklwidsky, B. M.; Gabriel, D. M. Detergent Analysis; Gocdwin: London, 1982. (2) APHA, American Public Health Association, Standard Methods for the Exarnlnation of Water and Wastewater, 16th ed.; American Public Health Association: Washlngton, 1985. (3) Swisher, R. D. Surfactant BMegradatiOn; Marcel Dekker: New York, 1970. (4) Osburn, a. W. J . Am. OilChem. SOC. 1988, 63, 257. (5) Cross, J. Anbnk Suffactants-Chemical Analysis; Marcel Dekker: New York, 1977. (6) Gabriel, D. M. J . Soc.Cosmet. Chem. 1974, 25, 33. (7) Davidsohn, A.; Milwldsky, 8. M. Synthetic Detergents, 2nd ed ; Goodwin: London, 1978. (8) Llenado, R. A.; Jarnieson, R. A. Anal. Chem. 1981, 5 3 , 174R. (9) Llenado. R. A.; Neubecker, T. A. Anal. Chem. 1983, 5 5 , 93R. (IO) LeBlanc, D. R., U S .Geol. Water-Supply Pap. 1984, No 2218.
1802
Anal. Chem. 1987, 5 9 , 1802-1805
(11) Thurman, E. M.; Malcolm, R. L. Environ. Sci. Techno/. 1981, 15,463. (12) Thurman, E. M.; Barber, L. B., Jr.; LeBlanc, D. R. Contam. Hydro/. 1986, 1 . 143. (13) Patt, S. L.; Shoolery, J. N. J . Magn. Reson. 1982, 46, 535. (14) Thurman. E. M.; Malcolm, R . L.; Aiken, G. R. Anal. Chem., 1978, 50, 775. (15) Bidlingmeyer, B. A.; Deming, S.N.; Price, W. P.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 186, 419. (16) Bidlingmeyer, B. A. J . Chrornatogr. Sci. 1980, 18, 525. (17) Bidlingmeyer, B. A . LC Mag. 1983, 1 , 344. (18) Kissinger, P. T. Anal. Chem. 1977, 4 9 , 883. (19) Aiken, G. R.; Thurman, E. M.; Malcolm, R . L.; Walton, H. F. Anal. Chem. 1979, 5 1 , 1799.
(20) Thurman, E. M.; Malcolm, R. L. U S . Geol. Surv. Water-Supply Pap. 1979, No. 1817-G. (21) Thurman, E. M., U S . Geol. Sum. Water-Supply Pap. 1984, No. 2262. (22) Kosugi, Y.; Yoshida, Y.; Takeuchi, T. Anal. Chern. 1979, 5 1 , 951. (23) Swisher, R. D. J . Am. Oil Chem. Soc. 1963, 40 648.
RECEIVED for review January 9, 1987. Accepted March 25, 1987. The use of brand names is for identification purposes Only and does not represent endorsement by the U.S. Geelogical Survey.
Determination of Rhenium in Marine Waters and Sediments by Graphite Furnace Atomic Absorption Spectrometry Minoru Koide, Vern Hodge, Jae S. Yang, and Edward D. Goldberg* Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093
A graphite furnace atomic absorption method for the determination of rhenium at picomolar levels in seawater and parts-per-billion levels in marine sediments Is based upon the isolation of heptavaient rhenium specks upon anion exchange resins. All steps are followed with '*'Re as a yield tracer. A crucial part of the procedure is the separation of rhenium from molybdenum, which significantly interferes with the graphite furnace detection when the Mo/Re ratio is 2 or greater. The separation Is accomplished through an extraction of tetraphenylarsoniumperrhenate into chloroform in which the molybdenum remains in the aqueous phase.
Rhenium is one of the last stable elements discovered, one of the least abundant metals in the Earth's crust, and one of the most important sentinels of reducing aqueous environments through its abundance in sediments. Although its chemistry is fairly well circumscribed, its marine chemistry is as yet poorly developed. In addition, the understanding of rhenium's marine chemistry will provide an entry to the understanding of the marine chemistry of technetium, an element which is just above rhenium in group VIIA (group 7 in 1985 notation) of the periodic table. Technetium has only unstable isotopes whose origins are primarily in nuclear weapon detonations and in nuclear reactor wastes. These two elements have remarkably similar chemistries. Previously , we have carried out some preliminary analyses of technetium in the marine environment ( I ) . Rhenium's solution chemistry primarily involves anionic species in the IV, V and VI11 oxidation states ( 2 ) . The oxo anion perrhenate is especially stable. There have been some analyses of rhenium is seawater (3-6) usually employing activation analysis techniques in which the rhenium is isolated from seawater before irradiation. The reported concentrations range from slightly under 3 to slightly under 11 ng/L. These values are remarkably higher than those of neighboring elements like iridium, platinum, and gold whose seawater concentrations do not exceed 0.3 ng/L (7). On the other hand, the crustal rock and deep sea sediment concentrations of rhenium are usually a t least an order of magnitude less than its periodic table neighbors (7). Thus,
Table I. Effect of Added Molybdenum upon the 5-ng Signal of Rhenium
Re signal
amt of added Mo, ng 0 2 5 10 25
Re signal
normalized to 100% for 0 Mo added
Mo, ng
100 100
50 75
100 91 67
100
amt of
added
150 250
normalized to 100% for 0 Mo added 47 40 33 9 0
during the major weathering cycle, seawaters serve as a major reservoir for rhenium. The unusual seawater concentration is attributed to the the nonreactivity of the perrhenate ion (7). Removal of rhenium from its dissolved state in seawater to precipitated solid phases probably involves reduction to lower valence states. There have been a few analyses of rhenium in marine organisms (5,8) and in marine sediments (6). Our laboratory has made a survey of rhenium in oxic and anoxic deposits and has found remarkable enrichments of rhenium in anoxic sediments, especially hydrothermal sulfides (7). On the other hand, rhenium is usually depleted, relative to crustal abundances, in sediments that accumulate under oxidizing conditions. EXPERIMENTAL SECTION The following rhenium technique by atomic absorption spectroscopy using a graphite furnace was developed subsequent t o an initial observation that the rhenium signal was attenuated by as little as 10 ng or less of molybdenum in the isolate (Table 1). Thus, importance is placed upon molybdenum decontamination steps. In seawaters as well as in many marine sediments the Mo/Re varies about 1000 (7) (see also Table V). In addition, a clean separation of rhenium from other elements (the salt effect) is required. Otherwise, false peaks result upon atomization due to the high background generated by impurities. Seawater Collection. Open ocean seawaters were collected in 30 L, modified Go-Flo bottles (General Oceanic),coated with Teflon and suspended on a Kevlar line. The water was immediately pressure filtered through 0.4-gm Nuclepore filters and acidified with 10 mL of 6 M HC1 (G. Frederick Smith and Co.) per liter. The samples were stored in 8-L acid-cleaned poly-
0003-2700/87/0359-1802$01.50/00 1987 American Chemical Society
