8

Separation of Humic Substances and Anionic Surfactants from Ground Water by Selective Adsorption

Ε. M. Thurman

U.S. Geological Survey, Campus West, Lawrence, KS 66046

Jennifer Field

Geochemistry Department, Colorado School of Mines, Golden, CO 80401

Surface-active organic compounds, surfactants, may be isolated from ground water effectively on polymeric resins with a procedure nearly identical to the isolation procedure for humic substances. Surfac­tants, such as alkylbenzenesulfonates and alkyl sulfates, are sorbed from both distilled water and ground water onto XAD-4 (sty­rene- divinylbenzene) resin at neutral pH (6-7). At this pH the ma­jority of the humic substances are not sorbed, but pass through the XAD-4 resin. The effluent from the column is collected and the pH adjusted to 2.0 with concentrated hydrochloric acid. The solution is then passed through a second column of XAD-8 (methyl methacry­late) resin, and humic substances are sorbed. The surfactants and some colored substances are eluted from the XAD-4 column with methanol, and the humic substances are eluted from the XAD-8 column with an aqueous solution of 0.1 Ν sodium hydroxide. The humic substances from the XAD-8 column are then ready for char­acterization without interference from anionic surfactants. Unfor­tunately, the surfactant fraction from the XAD-4 column does contain some colored humic material.

THE STUDY OF AQUATIC HUMIC SUBSTANCES in surface and ground water is an important area of environmental chemistry because they are so wide-

0065-2393/89/0219-0107$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

108 AQUATIC HUMIC SUBSTANCES spread (1-3) and they play an important role in chemical reactions such as trihalomethane production (4), trace-metal complexation (5), cosolubilization of pollutants (6, 7), and other environmental chemical reactions (3). Fur­thermore, studies of the structure of humic substances (J) require humic fractions that are free of synthetic organic contaminants. Isolation proce­dures, therefore, are the important first step in the study of aquatic humic substances.

The two general isolation approaches for aquatic humic substances are ion-exchange (8-10) and adsorption chromatography (II). Both approaches isolate 50-90% of the dissolved organic carbon (3) and most of the organic color from natural waters (3, I I , 12). However, both ion-exchange resins and polymeric resins also isolate nonionic contaminants from wastewater (13), as well as some anionic surfactants (14, 15) such as linear alkylben-zenesulfonate surfactants (LAS) and branched-chain alkylbenzenesulfonate surfactants (ABS). Thus, there is the potential for co-isolation of surfactants and humic substances in ground and surface waters contaminated with waste­water; sewage is a common contaminant source (15, 16). Because we were studying ground water contaminated by sewage at Otis Ai r Base, Falmouth, Massachusetts (16), we became interested in selective isolation of both an­ionic surfactants (15, 17) and humic substances.

Past studies have found that the sodium salts of ABS and LAS surfactants were efficiently sorbed onto X A D - 8 (a methyl methacrylate resin) as ions. The hydrophobic alkylbenzene chain interacts with the resin by the hydro­phobic effect (15), and the sulfonic acid salt is oriented into the water phase. This mechanism has been named the dynamic ion-exchange model in ion-pair chromatography (18-20).

X A D - 2 and X A D - 4 (styrene-divinylbenzene resins) have been used to isolate both nonionic (21-22) and anionic surfactants from water. They are effective resins for isolation of small molecules (less than 500 molecular weight), but are less effective for humic substances because their small pore size excludes humic substances (12). On the other hand, X A D - 8 effectively isolates humic substances from water (11-12).

We used both X A D - 4 for surfactants and X A D - 8 for humic substances to take advantage of the best characteristics of both for the selective isolation of humic substances and anionic surfactants. Results discussed here include laboratory and field experiments.

Experimental Procedures Reagents and Supplies. XAD-4 and XAD-8 resins (Rohm and Haas, Phila­

delphia), styrene-divinylbenzene and methyl methacrylate copolymers, were ex­tracted with 0.1 Ν NaOH, followed by methanol Soxhlet extraction. The resins, which had been stored in methanol, were rinsed with 50 bed volumes of distilled water prior to use to remove the methanol. Surfactant standards were reagent grade, including the sodium salt of linear alkylbenzenesulfonate (Fluka, Ronkonkoma, New

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8. THURMAN & FIELD Separation by Selective Adsorption 109

York), and sodium dodecyl sulfate (SDS) (Fluka, Ronkonkoma, New York). The ABS surfactant was technical grade (Association of American Soap and Glycerine Pro­ducers, New York). The humic material was Suwannee River fulvic acid, which was isolated by the method of Thurman and Malcolm (11) and has been discussed in previous work (23).

Procedures. The XAD-4 and XAD-8 resins were packed into 20-mL columns in water and rinsed with 50 bed volumes of distilled water. The individual solutions of surfactants in the form of sodium salts and humic substances, at 2 mg/L each in distilled water, were passed through the XAD resins at 15 bed volumes per hour. The anionic surfactant breakthrough curve was measured by determining the meth­ylene blue active substances (MBAS) in the column effluent (24). The fulvic acid concentration was measured by absorbance at 320 nm. The fulvic acid did not in­terfere in the MBAS test.

Field Analysis. The ground water on Cape Cod, Massachusetts (17), was sam­pled by stainless steel submersible pump at well F347-67 for surfactant and humic substances analysis. The water samples were collected in 4-L glass bottles and shipped to the laboratory for analysis.

The 8-L aliquots were pumped through a 20-mL column of XAD-4 at neutral pH (15 bed volumes per hour). The water sample was adjusted to pH 2 with con­centrated hydrochloric acid and passed through a 20-mL XAD-8 column.

The surfactants were measured on the XAD-8 and XAD-4 columns by the MBAS colorimetric test (24). This method uses the formation of a complex between the anionic surfactant and methylene blue (a cationic dye), which is extracted into chloroform and quantified by colorimeter. The humic substances were measured as color (absorbance at 320 nm).

Results and Discussion Surfactant Recovery on XAD. LAS and SDS are efficiently sorbed

on X A D - 4 , with 100% sorption of each (Table I). Although distribution coefficients were not measured on the resins, the minimum distribution coefficient can be calculated from the experimental conditions. This calcu­lation gives a distribution coefficient of greater than 166 for LAS and SDS

Table I. Percent Recoveries of Anionic Surfactants and Aquatic Humic Substances at Neutral pH (7.0)

Compound Percent Sorbed0

Percent Eluted with CH3OHb

Compound XAD-4 XAD-•8 XAD-4 XAD--8 Linear alkylbenzenesulfonate 100 ± 5 100 ± 5 77 ± 5 88 ± 5 Branched-chain

alkylbenzenesulfonate NDC 100 ± 5 NDC 88 ± 5 Sodium dodecyl sulfate 100 ± 2 41 ± 2 72 ± 2 86 ± 2 Suwannee River fulvic acid 0 0 — — "Percent sorbed is the percent of the total surfactant passed through the column. ''Percent eluted is the percent of the amount sorbed, which is then eluted. rND, experiment not done.

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110 AQUATIC HUMIC SUBSTANCES on the X A D - 4 . ABS was not determined on X A D - 4 , but because of its structure (nearly identical to LAS) and the complete sorption of SDS, it is reasonable to assume that ABS would also have large sorption efficiency on X A D - 4 .

A minimum value for the distribution coefficient, KD, can be estimated from the size of the column (6 g of resin dry weight) and the interstitial volume of water in the column (8.0 mL):

concentration on resin/g of resin D concentration in water/mL of water

Thus, for complete removal of 2 mg of surfactant from 1 L of water at a concentration of 2 mg/L, the calculation gives:

= 2 mg/6 g ° 0.002 mg/rnL

KD > 166

We realize that this calculation is not a true K D , but actually a point on the isotherm. Because the column was not saturated with surfactant, the actual capacity of the resin is much greater than this estimated value.

The sorption of ABS, L A S , and SDS on X A D - 8 was different. A l l of the ABS and L A S was sorbed, but only 41% of the SDS was removed. This result indicates a KD of less than 66. Because the SDS contains only 12 carbon atoms and ABS contains 18 carbon atoms, there is much less of a hydrophobic effect upon sorption for SDS. This conclusion is based upon previous studies of X A D sorption of hydrophobic organic compounds (25). On the other hand, the X A D - 4 has considerably more surface area than X A D - 8 (650 versus 140 m 2/g), and the SDS was completely sorbed to X A D - 4 . Although X A D - 4 and X A D - 8 have different functional groups (sty­rene-divinylbenzene and methyl methacrylate), past studies have shown that surface area is of key importance on sorption (12). These data suggest that X A D - 4 is more efficient for the removal of the more water-soluble anionic surfactants.

During elution the X A D - 4 released 72-77% of the anionic surfactants (Table I), and X A D - 8 released 86-88% of the sorbed surfactants. Larger pore size is credited with giving X A D - 8 more efficient elution properties (12) than X A D - 4 . Our data support that hypothesis. On the basis of these data, X A D - 4 seems to be a better sorbent for a wide range of water-soluble surfactants. On the other hand, if only the alkylbenzenesulfonates are of interest, then the X A D - 8 is more efficient.

The Suwannee River fulvic acid (II, 23) did not adsorb at neutral p H on either the X A D - 4 or the X A D - 8 resin. This result was expected because of previous studies (II, 12) and the initial isolation of the Suwannee River

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8. THURMAN & FIELD Separation by Selective Adsorption 111

fulvic acid on X A D - 8 resin at p H 2. Previous studies (IJ, 12, 23) have shown that, without p H adjustment, aquatic humic substances do not concentrate on the X A D resin because the many ionic carboxyl groups present on the humic material make the compounds water-soluble. When the p H is adjusted to 2.0, the carboxyl groups are protonated and the fulvic acid is much less soluble. Thus, a considerable enhancement in sorption occurs at p H 2.0.

However, some natural surface-active compounds in water (for instance, long-chain fatty acids and some pigments) may sorb onto X A D resins at neutral p H . Thus, many natural compounds may still be present in the surfactant fraction. The question of whether the pigments and other colored substances are humic substances is the subject of a different paper (26). The isolation of natural surfactants at neutral p H would be an interesting research project for future work.

H u m i c Isolation and Surfactant Co-isolat ion. Although humic material does not co-isolate with the surfactants at neutral p H , there may be a major problem with the co-isolation ofhumic substances and surfactants at p H 2.0. Humic substances are efficiently sorbed on X A D - 8 at p H 2.0 (II, 12), but anionic surfactants are also efficiently sorbed at this p H . The amount sorbed in both cases is 100 ± 5%, and the amount eluted is 2 ± 5%. Co-isolation, which would be most pronounced in surface waters receiving sewage effluent, is a minor problem if the eluent used is 0.1 Ν N a O H . We found that only a trace amount (2%) of the surfactant sorbed with the humic material co-eluted with the humic substance at alkaline p H . However, if a mixture of methanol and ammonium hydroxide is used as eluent, then the surfactants would co-elute. Methanol-ammonium hydrox­ide is a commonly used eluent for the removal of humic substances from sea water (27, 28). Harvey (29) reported that alkylbenzenes found in marine fulvic acid may originate from sewage (30). An alternative hypothesis, based on our results about surfactant removal, is that ABS and LAS surfactants would co-isolate with the marine fulvic acid and then be co-eluted in meth­anol-ammonium hydroxide. These substances would then interfere in the structural studies of the marine fulvic acid. Thus, we have designed the following simple two-column isolation scheme for aquatic fulvic acid.

Selective H u m i c Isolation. For selective isolation, the water sam­ple is passed through an X A D - 4 column at neutral p H ; the anionic surfac­tants, long-chain fatty acids, and some pigments are removed. However, the majority of the humic substances pass through the column. The amount sorbed of linear alkylbenzenesulfonates is 100 ± 5%, and 0 ± 5% of Su­wannee River fulvic acid. The effluent is recovered, then the p H is lowered to 2.0 with HC1. The sample is processed a second time through X A D - 8 ; during this process the humic substances are sorbed. The amount sorbed of linear alkylbenzenesulfones is 0 ± 5%, and 100 ± 5% of Suwannee River

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112 AQUATIC HUMIC SUBSTANCES ftilvic acid. The humic substances are then back-eluted with 0.1 Ν N a O H and treated with the procedure described by Thurman and Malcolm (11).

The anionic surfactants are efficiently separated and recovered in the first step (>90% recovery of anionic surfactants from a 2-mg/L solution of surfactants), and the humic substances are efficiently recovered in the second column (>90% recovery at 2 mg/L). Thus, the modification of the isolation procedure is an effective method to remove surfactant substances from the humic material. An added benefit of this procedure is that nonionic contam­inants are also effectively removed in this first step (13). Biodégradation products of surfactants (carboxylated alkylbenzenesulfonates) may also be present. These substances would most likely not isolate in either step 1 or step 2 in the humic fraction. This topic is the subject for future research on surfactants in surface and ground waters.

Selective Surfactant Isolation from Ground Water. This result, that surfactants may be isolated separately from the humic fraction, was tested on ground water contaminated by sewage on Cape Cod, near Fal­mouth, Massachusetts. The ground water contained anionic surfactants that had been mapped by M B A S in previous studies (16, 17).

Table II shows the recovery of anionic surfactant (MBAS) and of aquatic humic substances (color at 320 nm) for well F347-67, which is 300 m down gradient of the sewage disposal beds and known to contain a mixture of surfactants and humic substances (16, 17). Table II shows that 64% of the M B A S substances were isolated onto the X A D - 4 resin (0.105 mg/L). This fraction is currently being examined by linked-scan fast-atom bombardment to determine if L A S is present.

Approximately 18% of the color was isolated on the X A D - 8 resin and 11% on the X A D - 4 resin. The remainder of the color was not removed by either column. The small X A D - 8 column (20 mL) was overloaded with sample (8.0 L), which forced much of the colored organic matter through X A D - 8 . Only 2% of the M B A S co-isolated on X A D - 8 , with most sorbing to the X A D - 4 (62%). For future work a larger X A D - 8 column wil l be used, approximately 20 m L of resin per liter of water. The fact that color sorbed without p H adjustment indicates that a colored surfactantlike material is present in the wastewater. Thus, the method seems to be partially effective for the separation of anionic surfactants and humic substances during the

Table U . Recovery of Surfactants and Humic Substances from Well F347-67

Color Removed MBAS* Removed Resin mg/L % mg/L % XAD-4 XAD-8

0.316 11 0.515 18

0.105 0.004

62 2

"Methylene blue active substances.

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8. THURMAN & FIELD Separation by Selective Adsorption 113

sorption phase. However, the humic substances isolated on X A D - 8 are relatively free of surfactants.

Summary When water samples are passed through X A D - 8 resin at p H 2.0, anionic surfactants and humic substances are effectively isolated. Because of this success, we have modified a previously published isolation scheme for humic substances. The new procedure includes a precolumn of X A D - 4 that isolates the surfactants during sorption at neutral p H . The aquatic humic substances are isolated in a second step by passing the water through a X A D - 8 column at p H 2.0. The major improvement of this new method over previous humic-substances research methods is the finding that surfactants wil l isolate with humic substances, even in an ionic form. Thus, when large volumes of water are passed through X A D columns and eluted with methanol-ammonium hydroxide (commonly done in marine humic studies), there is the possibility of eluting surfactants into a humic fraction. Unfortunately, colored hydro­phobic organic acids are isolated on X A D - 4 without p H adjustment. Thus, it is not possible to separate humic substances and surfactants totally in a single adsorption-elution procedure.

References 1. Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P. Humic Sub­

stances in Soil, Sediment, and Water: Geochemistry, Isolation, and Character­ization; John Wiley and Sons: New York, 1985.

2. Stevenson, F. J. Humic Chemistry; John Wiley and Sons: New York, 1982. 3. Thurman, Ε. M. Organic Geochemistry of Natural Waters; Martinus Nijhoff:

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R., Eds.; Elsevier: Amsterdam, 1981; 179-224. 6. Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol.

1986, 20, 502. 7. Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735. 8. Sirotkima, I. S.; Varshal, G. M.; Lu're, Y. Y.; Stepanovas, N. P. Zh. Anal. Khim.

1974, 29, 1626. 9. Abrams, I. M. Ind. Eng. Chem. Prod. Res. Dev. 1975, 14, 108.

10. Leenheer, J. A. Environ. Sci. Technol. 1981, 15, 578. 11. Thurman, Ε. M.; Malcolm, R. L. Environ. Sci. Technol. 1981, 15, 463. 12. Aiken, G. R.; Thurman, Ε. M.; Malcolm, R. L.; Walton, H. F. Anal. Chem.

1979, 51, 1799. 13. Junk, G. Α.; Richard, J. J.; Grieser, M. D.; Witiak, D.; Witiak, J. L.; Arguello,

M. D.; Vick, R.; Svec, J. J.; Fritz, J. S.; Calder,G. V. J. Chromatogr. 1974, 99, 745.

14. Gabriel, D. M. J. Soc. Cosmet. Chem. 1974, 25, 33. 15. Thurman, E. M.; Willoughby, T.; Barber, L. B., Jr.; Thorn, K. A. Anal. Chem.

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114 AQUATIC HUMIC SUBSTANCES 16. LeBlanc, D. R. U.S. Geological Water-Supply Paper, 1984, No. 2218. 17. Thurman, Ε. M.; Barber, L. B., Jr.; LeBlanc, D. Contam. Hydrol. 1986, 1,

143. 18. Bidlingmeyer, Β. Α.; Deming, S. N.; Price, W. P.; Sachok, B.; Petrusek, M. J.

J. Chromatogr. 1979, 186, 419. 19. Bidlingmeyer, B. A. J. Chromatogr. Sci. 1980, 18, 525. 20. Bidlingmeyer, B. A. LC Mag. 1983, 1, 344. 21. Jones, P.; Nickless, G. J. Chromatogr. 1978, 156, 87. 22. Osburn, W. W. J. Am. Oil Chem. Soc. 1986, 25, 33. 23. Thurman, Ε. M.; Malcolm, R. L. In Aquatic and Terrestrial Humic Materials;

Christman, R. F.; Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, 1983; 1-23.

24. American Public Health Association. Standard Methods for the Examination of Water and Wastewater, 16th ed.; American Public Health Association: Wash­ington, DC, 1985.

25. Thurman, Ε. M.; Malcolm, R. L.; Aiken, G. R. Anal. Chem. 1978, 50, 775. 26. Thurman, Ε. M.; Aiken, G. R.; Ewald, M. J.; Fischer, W. R.; Forstner, U.;

Hack, A. H.; Mantoura, R. F. C.; Parsons, J. W.; Pocklington, R.; Stevenson, F. J.; Swift, R. S.; Szpakowska, B. Humic Substances and Their Role in the Environment, 1988 Proceedings of Dahlem Konferenzen; Frimmel, F. H.; Christman, R. F., Eds.; John Wiley and Sons: New York, 1988; pp 31-43.

27. Mantoura, R. F. C.; Riley, J. P. Anal. Chim. Acta 1975, 76, 97. 28. Stuermer, D. H.; Harvey, G. R. Mar. Chem. 1978, 6, 55. 29. Harvey, G. R. Mar. Chem. 1985, 16, 187. 30. Eganhouse, R. P.; Blumfield, D. L.; and Kaplan, I. R. Environ. Sci. Technol.

1983, 17, 523.

RECEIVED for review July 24, 1987. ACCEPTED for publication March 7, 1988.

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