Environ. Sci. Technol. 1987, 21, 370-373
Mehran, M. J . Chromatogr. Sci. 1985,24, 546-548. Duggleby, R. G.; Morrison, J. F. Biochim. Biophys. Acta 1977, 481, 297-312. Goring, A. I.; Hamaker, J. W. Organic Chemicals i n the Soil Environment; Dekker: New York, 1972; Chapter 4. Sulflita, J. M.; Robinson, J. A.; Tiedje, J. M. Appl. Enuirorz. Microbiol. 1983, 45, 1466-1473. White, D. C.; Davis, W. M.; Nickels, J. S.; King, J. D.; Bobbie, R. J. Oecologia 1979, 40, 51.
(16) Kobayashi, H.; Rittmann, B. E. Environ. Sci. Technol. 1982,
16, 170A-183A. Received for review December 6, 1985. Revised manuscript received July 21,1986. Accepted November 3,1986. This work was supported by the US.Environmental Protection Agency, Ground Water Research Branch, under Contract CR809994-02 to the Florida State University and Subcontract 281308-500 to the Florida International University.
Structure-Activity Relationships for Sorption of Linear Alkylbenrenesulfonates Vlncent C. Hand" and Glenwood K. Wllliams Human and Environmental Safety Division, Procter and Gamble Company, Cincinnati, Ohio 452 17
rn Sorption of ten radiolabeled linear alkylbenzenesulfonate (LAS) homologues and isomers onto four river sediments increased with increasing alkyl chain length and as the phenyl position approached the end of the chain. Initial solution concentrations of LAS were varied between 10 and 1000 ppb. The value of the sorption partition coefficient (Kd)increased by factor of 2.8 for each additional methylene group in the homologous series CloLAS to CI4LAS. The value of Kd varied by 4 orders of magnitude, from 3 to 26000 L/kg, as sediment type, chain length, and phenyl position were varied. Sorption and desorption were rapid (97% as determined by thinlayer chromatography (TLC) and/or gas-liquid chromatography (GLC). Isomer distributions and specific activities are described in Table I. Stock solutions were prepared in alcohol (methanol/ethanol/2-propanol,5:90:5) and stored at 4 "C. Sediment characteristics are reported in Table 11. Dry sediments EPA B1 and EPA 5, along with the corresponding characterization data, were kindly provided by Dr. Samuel Karickhoff of the U.S. Environmental Protection Agency. Sediments RC 3 and RC 4 were collected from Rapid Creek, SD, 0.8 and 7 km downstream from the Rapid City sewage treatment plant, dried, and characterized by standard methods at CTL Engineering, COlumbus, OH. These sites correspond to sampling sites 1 and 2 in reference 5. Proximity to the sewage outfall causes the RC 3 sediment to appear as a black ooze containing relatively high organic carbon. Natural waters (Table 111) were centrifuged (lOOOOg, 40 min) before use to remove particles with diameters above 0.1 fim.
0013-936X/87/0921-0370$01.50/0
0 1987 American Chemical Society
Table I. LAS Sample Characteristics material
abbreviation
sodium (1-methylnony1)benzenesulfonate sodium (1-butylhexy1)benzenesulfonate sodium (1-methylundecy1)benzenesulfonate sodium (1-pentylhepty1)benzenesulfonate sodium (1-methyltridecy1)benzenesulfonate sodium decylbenzenesulfonate
Clo 2-Ph LAS Clo 5-Ph LAS Clz 2-Ph LAS Clz 6-Ph LAS CI4 2-Ph LAS Clo mixed LAS
sodium undecylbenzenesulfonate
Cll mixed LAS
sodium dodecylbenzenesulfonate
Clz mixed LAS
sodium tridecylbenzenesulfonate
C13 mixed LAS
sodium tetradecylbenzenesulfonate
C14mixed LAS
phenyl distribution
%
2-Ph 5-Ph 2-Ph 6-Ph 2-Ph 2-Ph 3-Ph 4-Ph 5-Ph 2-Ph 3-Ph 4-Ph 5-Ph, 6-Ph 2-Ph 3-Ph 4-Ph 5-Ph 6-Ph 2-Ph 3-Ph 4-Ph 5-Ph 6-Ph, 7-Ph 2-Ph 3-Ph 4-Ph 5-Ph 6-Ph, 7-Ph
100 100 100 100 100 37.6 22.9 20.3 19.3 35.5 21.0 17.5 26.1 19.7 21.8 17.8 18.8 21.9 40.2 18.5 13.5 12.1 15.8 27.0 16.3 14.2 13.6 28.8
sp act. pCi/mmol
1%
F
12.90 8.51 13.44 8.99 12.91 8.46
1.23
9.14
1.26
4.18
1.96
8.70
2.52
9.69
2.73
"Octanol/water partition coefficients from reference 12. Log P for C16 mixed LAS is 3.62. Table 11. Properties of Sediments and Correlation of Properties with LAS Sorption
Table 111. Sorption of C1, 2-Ph LAS" in Various Waters to RC 4 Sediment water source
PH
hardness, mg/L
log Kd
Rapid Creek, SD Winton Woods Creek, Cincinnati Ohio River, Cincinnati Acton Lake, OH Brookville Lake, IN 0.01 M KC1
7.4 7.1 7.8 7.7 8.1 7.0
207 162 123 164 175 0
2.98 2.91 2.93 2.93 2.93 2.70
organic CEC, sediment sand, % silt, % clay, % carbon, % mequiv/100 g" EPA Blb 62.6 33.6 EPA 5* RC 4 2.2 0.2 RC 3 r2c -0.908
14.9 35.4 76.8 97.4 0.987
22.5 31.0 21.0 2.4 0.592
0.90 2.28 0.99 3.5 0.447
3.00 19.00 15.4 15.7 0.367
" CEC, cation-exchange capacity. From ref 15. Linear correlation coefficient for mean of log Kd on each sediment vs. sediment property Procedures. Sediment sorption was determined by the following procedure. Sediment (0.010-2.500 g), LAS (typically 100 Fg/L), and 150 mL of water were combined in a polycarbonate centrifuge bottle. To maximize precision, sediment concentrations (67-17 000 mg/L) were chosen so that roughly equal masses of LAS would be in each phase at equilibrium. The system was equilibrated by shaking at 24 f 3 "C for 3-8 h. Phases were then separated by centrifugation at lOOOOg for 40 min at 24 "C. The concentration of LAS in the aqueous phase was determined directly by liquid scintillation counting (LSC). The amount of LAS sorbed to sediment was determined by refluxing the sediment overnight with methanol, followed by LSC of the methanol solution. Though quite water soluble, LAS readily adheres to labware. Special care was required for accurate, reproducible results. Pipets used to sample the aqueous phase were rinsed with alcohol, and LAS in the alcohol was quantitated by LSC. Calculated aqueous-phase concentrations were adjusted to account for LAS found on the walls of the pipet. Preliminary experiments demonstrated that loss of material onto the centrifuge bottle did not affect the measured value of the sorption partition coefficient. Nevertheless, the LAS on the centrifuge bottle was
a Initial
concentration, 100 ppb.
carefully removed by rinsing with alcohol and scrubbing with a soft brush, so that a mass balance was obtained for each experiment. Desorption experiments were carried out as follows. Sediment, water, and LAS were allowed to come to equilibrium, and phases were separated as described above. The LAS concentration in the solution phase was determined as before. Fresh water from the same source used in the sorption experiment, but containing no LAS, was added to the sediment. After reequilibration, LAS was determined in both phases as described previously and a desorption partition coefficient calculated. No attempt was made to desorb LAS completely.
Results Using radiolabeled materials to measure sorption had several advantages in this study. First, the method is extremely sensitive, so that realistic environmental concentrations of 100 ppb or lower were readily quantitated. Second, these measurements were simpler than other methods for determining traces of LAS in environmental matrices. Consequently, many measurements could be performed concurrently, decreasing the effect of changes in the matrix occurring over time. Also because of the simplicity of radiochemical methods, thorough recovery Environ. Sci. Technol., Vol. 21, No. 4, 1987
371
1
35
i RC 3
c
. .
RC 4
. . . . . . .
4
.
1.
5
‘t 3.5
EPA 5
EPA B1
Table IV. Comparison of Sorption and Desorptiono
LAS ClO c 1 4
sediment RC 4 RC 4
EPA 5
sorption 1.6ijb 3.3gC
109 K A desorption 1.70b 3.58c
3.32
2.58
3.48 2.67d
3.70 3.07d
aFor 100 ppb of LAS, Rapid Creek water. bData not significantly different ( p < 0.05). cReplicate data are significantly different (paired t test, p < 0.05). dData are significantly different 0, < 0.05).
0.05) as phenyl position was changed from the 5- or 6position to the 2-position; Kd values increased by a factor of -2 on going from the 5-Ph isomer to the 2-Ph isomer. No significant change in sorption was observed in different natural waters (Table 111). The value of Kd in a solution of 0.01 M KC1 in laboratory water was less than 2-fold different from the values in natural water. The difference in Kd values determined in natural and artificial waters may be due to differences in hardness. In any case, differences in water composition are unlikely to have a profound effect on LAS sorption in the environment. Results of single-step desorption experiments are reported in Table IV. No significant difference exists in the sorption and desorption Kd values for Clo LAS. The difference between sorption and desorption Kd values for C14 LAS, on the other hand, is statistically different. In either case, a substantial fraction of the LAS desorbed from the sediment in a single step.
per minute (dpm) in the alcohol extract of the solid phase, d, is dpm in an aliquot of the water phase, d, is dpm on the pipet used to transfer the aliquot, m, is the mass of the solid phase, and V , is the volume of the water-phase aliquot. Sorption of LAS varied by 4 orders of magnitude across different LAS chain lengths, different phenyl positions, and different sediment types (Figure 1). For each sediment type, sorption increased significantly [p < 0.05 using analysis of variance followed by Duncan’s multiple range test (11)] as LAS chain length was increased; Kd values increased by a factor of 2.8 for each additional methylene group. For isomers, sorption increased significantly (p < 372
Environ. Sci. Technoi., Voi. 21, No. 4, 1987
Discussion Mechanism of LAS Sorption. The data in Figure 1 suggest that LAS homologues and isomers are sorbed to sedimeni primarily by a hydrophobic mechanism. Sorption of LAS changed consistently with alkyl chain length and phenyl position for all homologues and all sediments tested. Increasing LAS chain length increases the hydrophobicity of the molecule (12). In molecules with longer alkyl chains, the negative charge on the sulfonate group will have a relatively smaller effect on hydrophobic interactions, compared to shorter chain homologues. Furthermore, longer alkyl chains have more surface available for van der Waals interactions. This interpretation of LAS sorption is consistent with the octanol/water partitioning of LAS, known to be a hydrophobic mechanism (12). Octanol/water partition coefficients for LAS increase by a factor of 2.6 for each additional methylene group (Table I) (13), almost identical with the factor of 2.8 change in & reported here. This suggests a similar, hydrophobic, mechanism is operating for octanol/water partitioning and for sediment sorption. Within an isomeric series, the isomer containing the longest unsubstituted n-alkyl fragment is the most sorptive. For example, in the series C126-Ph LAS, C12mixed LAS, and C12 2-Ph LAS, CI2 2-Ph LAS contains a 10carbon n-alkyl fragment, while the longest unsubstituted chain in the less sorptive C12 6-Ph LAS is a 6-carbon nalkyl fragment. (The mixed phenyl “isomer” had a sorption Kd intermediate between these extremes, as would be expected.) This trend in sorption within an isomeric series is not surprising, since sorption increases with increasing chain length in the homologous series from Clo LAS to C14 LAS. Again, a hydrophobic mechanism is consistent with the observed trend. The increased sorption with increasing LAS chain length is also consistent with an earlier monitoring study. Hon-
nami and Takahisa (6) reported that LAS-containing sediment in the environment was enriched in longer chain LAS homologues compared to the parent distribution. Water overlying the sediment was depleted of longer chain homologues compared to the parent distribution. Data for positional isomers were not reported. The correlations between sorption to sediments and the sediment properties (Table 11) are not straightforward. Sorption did not correlate well with organic carbon content, as would be expected for a simple hydrophobic mechanism (1). Two previous studies of LAS sorption (7, 8) have reported a correlation with organic carbon, although both of these studies were conducted at 10-fold or greater LAS concentrations than used here. Sorption and percent silt are highly correlated, while sorption and percent sand are negatively correlated. This could be a simple case of smaller particles having a larger surface area per unit mass. If this were the only explanation, then LAS would sorb even more strongly to clay than to silt, with a correspondingly high correlation between Kd and percent clay. In fact, sorption does not correlate well with percent clay, implying that surface/volume ratio is not the only explanation for the results in Table 11. One could speculate that differences in mineralogy of the clay fraction and of the silt fraction obscure the effects of surface area and organic carbon on sorption. For example the clay fraction may contain a higher percentage of the "clay minerals", with an affinity for LAS substantially different from the affinity of silt particles for LAS. Clay minerals often have excess negative surface charge, which would potentially decrease LAS sorption by two mechanisms. First, the clay fraction might contain less organic carbon than the larger fractions. Second, the negative charge on the clay would repel the LAS sulfonate group. Consistent with this hypothesis, montmorillonite clay sorbs nonionic and cationic surfactants, but not LAS (14). Desorption. Desorption (Table IV) is rapid and nearly reversible at the concentration tested here. Since the value of the desorption Kd is not greatly different from the value of the sorption Kd, a substantial fraction of LAS was desorbed even in the one-step desorption experiments reported here. Additional desorption could be expected in multistep experiments (7). Apparent desorption equilibrium occurred rapidly. Rapid desorption is consistent with the fact that LAS biodegradation rates are unaffected by the presence of sediment (10).
Summary and Conclusions The sorption of LAS to river sediment increased predictably with chain length and phenyl position, increasing by more than 2 order of magnitude from the Clo 5-Ph LAS
isomer to the CI42-Ph LAS isomer. At the ppb concentrations used in this study, sorption isotherms were linear, and desorption was nearly reversible. Sediment characteristics significantly affected sorption, with the strongest correlation between sorption and percent silt in the sediment. Sorption increased with increasing LAS hydrophobicity, suggesting a hydrophobic sorption mechanism.
Acknowledgments We appreciate the technical assistance of Jean A. Staubach. We are grateful to Mark Boardman, Department of Geology, Miami University (Ohio), for helpful discussion. Registry No. Clo 2-Ph LAS, 73602-65-0; Clo 5-Ph LAS, 73602-67-2; Clz 2-Ph LAS, 2211-99-6; Clz 6-Ph LAS, 2212-52-4; C14 2-Ph LAS, 13419-31-3;Clo mixed LAS, 1322-98-1;Cll mixed LAS, 27636-75-5; C12 mixed LAS, 25155-30-0; C13 mixed LAS, 26248-24-8; C14 mixed LAS, 28348-61-0; C, 7440-44-0.
Literature Cited (1) Karickhoff, S. W. J. Hydraul. Eng. 1984, 100, 707-35. (2) Westall, J. C.; Leuenberger, C.; Schwarzenbach, R. P. Environ. Sci. Technol. 1985, 19, 193-98. (3) Schellenberg, K.; Leuenberger, C.; Schwarzenbach, R. P. Environ. Sci. Technol. 1984, 18, 652-7. (4) Human Safety and Environmental Aspects of Major Surfactants;Report to the Soap and Detergent Association (PB-301 193/9); Arthur D. Little, Inc.: Cambridge, MA, 1977; pp 31-5. ( 5 ) Games, L. M. In Aquatic Toxicology and Hazard Assessment: Sixth Symposium, A S T M S T P 802; Bishop, W. E.; Cardwell, R. D.; Heidolph, B. B., Eds.; American Society for Testing and Materials: Philadelphia, 1983; pp 282-99. (6) Hon-nami, H.; Takahisa, H. Rikusuigaku Zasshi 1980,41, 1-4. (7) Matthijs, E. Tenside Deterg. 1985 22, 299-304. (8) Urano, K.; Saito, M.; Murata, C. Chemosphere 1984,13, 293-300. (9) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1976. (10) Larson, R. J.; Payne, A. G. Appl. Environ. Microbiol. 1981, 41, 621-27. (11) SAS Institute Inc. S A S User's Guide: Statistics, 1982 ed.; SAS Institute Inc.; Cary, NC, 1982; p 151. (12) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-616. (13) Holman, W. F., Procter and Gamble Co., Cincinnati, OH, unpublished data, 1974. (14) Law, J. P.; Kunze, G. W. Soil Sci. SOC.Am. R o c . 1966,30, 321-7. (15) Means, J. C.; Woods, S. G.; Hassett, J. J.; Banwart, W. L. Environ. Sci. .Technol. 1980, 14, 1524.
Received for review August 8,1986. Accepted November 20,1986.
Environ. Sci. Technol., Vol. 21, No. 4, 1987 373
