Environ. Sci. Technol. 1997, 31, 1735-1741
Sorption of Nonionic Surfactants on Sediment Materials BRUCE J. BROWNAWELL,† HUA CHEN, WANJIA ZHANG,‡ AND JOHN C. WESTALL* Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331-4003
The distribution of a series of nonionic surfactants between sediments and water was studied as a function of surfactant structure, ionic strength, pH value, Ca2+ concentration in solution, sediment composition, and concentration of suspended solids. The surfactants were monotridecyl ethers of poly(ethylene glycol), also known as alcohol ethoxylates, AnEx ) CH3(CH2)n-1(OCH2CH2)xOH), with n ) 13 and x ) 3, 6, and 9. Isotherms were nonlinear, with the degree of nonlinearity and the extent of sorption increasing with the number of oxyethylene (-OCH2CH2-) groups. Freundlich isotherms represented the data well. The pH and ionic strength of the solution had a small effect on sorption, but the effect increased with the number of oxyethylene groups. These observations suggest that the oxyethylene chain is specifically adsorbed, presumably through a hydrogen-bond mechanism. Addition of Ca2+ did not affect the distribution of AEs. The distribution ratios of the AEs did not correlate well with the fraction organic carbon of the sediments; the amount of swelling clay in the sediment may affect distribution ratios. The concentration of solids had no effect on the extent of sorption.
Introduction Polyoxyethylene alkyl and alkyl-aryl ethers, which are also known as alcohol ethoxylates (AE) and alkylphenol ethoxylates (APE), are nonionic surfactants that find widespread use in cleaning, industrial, agricultural, and personal care products (1-3). These compounds interact with cell membranes (4) and are moderately toxic to fish and aquatic invertebrates (2, 5-7). Furthermore, recent interest in the environmental behavior of these surfactants has been stimulated from the roles that they have on the mobility (8-11) and rates of biodegradation (12-15) of hydrophobic organic contaminants in soil and aquifer environments. However, comparatively little has been published about the factors that affect the interactions of these nonionic surfactants with environmental surfaces. Several questions exist: What are the interactions of both the hydrophobic and polar parts of the molecules with environmental surfaces? What are the properties of the surfactant, sorbent, and solution that affect the distribution? How can the distribution be modeled as a function of these properties? In this study, we address these questions for a series of n-alkyl ethers of poly(ethylene glycol), which have the general formula CH3(CH2)n-1(OCH2CH2)xOH (AnEx). The pure compounds rather than the mixture of homologs found in commercial products were used in this study. * Corresponding author e-mail:
[email protected]. † Present address: Waste Management Institute, Marine Sciences Research Center, SUNY, Stony Brook, NY 11794-5000. ‡ Deceased.
S0013-936X(96)00692-X CCC: $14.00
1997 American Chemical Society
Sorption to Environmental Sorbents. In one study of the interaction of nonionic surfactants with environmental sorbents, an AE and an APE exhibited nonlinear sorption isotherms, and the extent of sorption tended to increase with increasing organic carbon content of the sorbent (16). Liu and co-workers (17) studied the sorption of three APEs and one AE with a soil containing approximately 0.96% organic carbon. Their study examined the sorption of nonionic surfactants at concentrations above and below the critical micelle concentration (cmc) of the surfactant (equilibrium solution concentrations in the range of 1 µM-4 mM). The surfactants were commercial mixtures of several homologs, and an indirect surface-tension method was utilized to determine sorption in the lower concentration range. Sorption isotherms were found to be distinctly nonlinear. The results from that study for the effect of increasing the number of methylene and oxyethylene moieties on sorption provide an instructive comparison to sorption results for similarly sized AE compounds determined at lower concentrations (0.3-900 nM) in this study. Other, less detailed, studies of APE sorption on environmental surfaces have shown nonlinear isotherms for concentrations below the cmc (8, 18). The sorption of poly(ethylene glycol) oligomers to sediments has also been examined, primarily at relatively high concentrations, resulting in near-saturation of the surface (19). Adsorption of two pure radiolabeled AE compounds (20) and a mixture of AE compounds (as they occur in commercial products) (21) to four sediment materials has been reported recently as well as the sorption of 11 non-labeled AE compounds to a single sediment (22). Sorption to Homogeneous Surfaces. The sorption of AE and APE to surfaces of pure materials has been studied more extensively. The sorption of nonionic surfactants onto both relatively hydrophobic and hydrophilic surfaces has been reviewed by Clunie and Ingram (23). The surfaces that have been studied include those of oxides, such as silica (24-27), alumina (28), and titania (29); activated carbon (30-34); polystyrene-based latexes (35-37); clays (38, 39); elemental Hg (40); silver iodide (41); CaCO3 (42); and textile fibers (23). Many of these studies deal with the orientation of the surfactant at the surface and the relation between orientation and surface coverage. Most of the studies dealt with high surface concentrations, and a few dealt with low surface concentrations. Sorption at High Concentrations. The sorption of nonionic surfactants reaches a maximum on the surface when the solution concentration is near or just at the critical micelle concentration (cmc) of the surfactant in solution. The maximum surface coverage (mol/m2) decreases with increasing size of the oxyethylene chain (43) and, less dramatically, with increasing polarity of the surface (28, 35, 37); maximum surface coverage increases slightly with increasing alkyl chain length (43, 44). These observations have been explained by steric effects, sorbate-sorbate interactions, and the effect that sorbate-sorbent interactions have on orientation at the interface. An increase in ionic strength and pH are found to decrease the sorption of nonionic surfactants at concentrations near saturation on quartz (45) and silica (24, 25, 27), but 0.2 M NaCl had no effect on sorption of three nonionic surfactants of the formula A8PE8-12 on active carbon (34). These effects have been variously interpreted as being due to interaction of the terminal hydroxyl group with SiO- groups, hydrogen bonding of the ether groups with protonated surface groups, competitive sorption, and changes in surfactant monomer activity as a function of salt concentration. Sorption at Low Concentrations. There is much less known about sorption of nonionic surfactants at concentra-
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TABLE 1. Surfactants Used in This Study IDa
name
structure
Mb
elemental composition
CAS Registry No.
AE A13E3 A13E6 A13E9
alcohol ethoxylate monotridecyl ether of triethylene glycol monotridecyl ether of hexaethylene glycol monotridecyl ether of nonaethylene glycol
CH3(CH2)12(OCH2CH2)3OH CH3(CH2)12(OCH2CH2)6OH CH3(CH2)12(OCH2CH2)9OH
332 464 596
C19H40O4 C25H52O7 C31H64O10
4403-12-7 930-09-6 7300-80-3
a
Abbreviation used in this study.
b
Molar mass, g/mol.
TABLE 2. Properties of Sedimentsa sediment
organic carbon (%)
sand (%)
silt (%)
clay (%)
CEC (mmol/g)
pHb (1:1)
pHc (1:20)
surface aread (m2/g)
clay mineralogye
EPA-16 EPA-13 EPA-12 EPA-25
1.20 3.04 2.33 0.76
0.5 20.3 0 41.9
60.5 27.1 64.6 37.6
39.0 52.6 35.4 20.5
0.110 0.119 0.135 0.089
6.50 6.90 7.63 7.65
6.76 7.08 7.52 7.57
18 13 12 8
kao, ver, ill kao, ill kao, ill kao, sm, ill
a Reported by Hassett et al. (48). b Solution was 0.01 M NaCl, 1 g of sediment to 1 mL of solution. c Solution was 0.01 M NaN , 1 g of sediment 3 to 20 mL of solution. d Determined courtesy of Jeff Fahey, Teledyne Wah-Chang, Albany, OR. e Major components as determined by X-ray diffraction: ill, illite; kao, kaolinite; sm, smectite; ver, vermiculite (49).
tions below 10 µM or at concentrations low enough that sorbate-sorbate interactions are not significant. Klimenko (33) studied the effect of surfactant structure on the sorption of AnEx to acetylene black. Isotherm data were extrapolated to infinite dilution to examine the relative contributions of alkyl and oxyethylene chains on the distribution ratio. The distribution ratio at low concentration depended strongly on alkyl chain length, approximately according to Traube’s rule (46): ∆ log D/∆n ) 0.48, where D is the apparent distribution ratio of the surfactant. The distribution ratio also increased with length of the oxyethylene chain: ∆ log D/∆x ) 0.11. The distribution ratio of AE between Hg/water also increases with an increasing number of oxyethylene groups at low concentration (40), contrary to decreases that are always observed at high concentration. Objectives of This Study. The objectives of this study were:(i) to determine the sediment-water distribution of AE, at low environmental concentrations, as a function of solution composition, sediment properties, AE structure, and sediment to solution ratio and (ii) to elucidate the mechanism of sorption of AEs to sediments and soils. The availability of purified, 14C-labeled homologs of AE has enabled us to work at very low (0.3-900 nM) aqueous concentrations, which are typical of the environment. Some of the results have been presented in a review paper in which we contrast the sorption of ionic and nonionic surfactants (47). This work focuses on our understanding of nonionic surfactant sorption with environmental sorbents.
Experiments and Methods Materials. Three homologs of A13Ex (x ) 3, 6, 9) were used in this study, as described in Table 1. The compounds were obtained through the Soap and Detergent Association from Shell Development Company, Westhollow Research Center, Houston, TX, and were synthesized by reacting 1-bromo[1-14C]-tridecane with the tri-, hexa-, or nonaethylene glycol. The resulting specific activities for each of the three homologs were 6.9, 21.3, and 6.6 Ci/mol, respectively. The radiochemical purities, determined by TLC and RP-HPLC, were 9598%. Solutions of primary and spiking standards of AE were stored in methanol and 95% ethanol, respectively, at a temperature 1) in the sorption isotherms
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FIGURE 2. (a) The dependence of observed distribution ratio, D, on concentration of sediment in water (Cs(w)) for the three AE homologs. The sorbent was EPA-12 in 0.01 M NaN3. (b) Raw data from panel a (open symbols) are replotted as isotherms along with the data and Freundlich isotherms from Figure 1 (solid symbols). The correspondence indicates that most of the apparent dependence of D on Cs(w) observed in panel a is due to the nonlinearity of the isotherms. (23). The possibility of cooperative sorption of some nonionic surfactants on environmental sorbents is suggested by data presented by Liu et al. (17) for sorption of A12E4. The Freundlich n for that compound was 2.12 at surface concentrations between 20 and 100% of saturation. As mentioned above, the isotherms of more hydrophilic APEs were described by values of n from 0.56 to 0.75. Further work is needed to elucidate at what point cooperative effects are manifested as a function of alkyl chain length and the hydrophobic/ hydrophilic balance of amphiphilic compounds. The fraction of the surface covered by AE can be estimated from the surface area of EPA-12 (Table 2) and the surface area of an adsorbed A12E6 molecule, which was estimated to be 0.95 nm2 from the saturation sorption of A12E6 on acetylene black (30). According to this calculation, the fraction of a monolayer surface coverage never exceeds 0.013 for A13E9, suggesting that these experiments were conducted far from saturation, if the AE was distributed uniformly over the surface. Effect of Concentration of Solids. There are several reports in the literature concerning the existence of a “solidsconcentration” effect on the sorption of nonpolar organic compounds (51, 55). The apparent distribution ratio is often observed to decrease with increasing concentration of sediment in the slurry. It has been disputed whether this correlation is due to experimental artifacts or to a general phenomena that occurs in particle suspensions. To address this issue for this study, a set of experiments was conducted in which the total amount of AE in every tube was identical, but the mass of sediments was varied. The sediment to water ratio is denoted by Cs(w) (kg/L). Results from those experiments are presented in Figure 2a. The dependence of D on
a
FIGURE 3. Effect of log [H+] on the distribution ratios of the three AE homologs between water and sediment EPA-12 (0.024 kg/L in 0.01 M NaN3). The pH was measured before and after the 12-h equilibration period; the average change in log [H+] was 0.24 unit. The log [H+] at the end of the period is used in the plot. the sediment to water ratio is negligible for A13E3, slight for A13E6, and pronounced for A13E9. All of these results can be explained by the constant amount of surfactant in every tube and the nonlinearity of the sorption isotherms. For greater concentrations of sediments, there is more surfactant associated with the surface, lower aqueous phase concentrations, and consequently greater values of D (the dependence of D on aqueous phase concentration can be deduced from the isotherms in Figure 1). This explanation can be confirmed by replotting the data in Figure 2a as an isotherm in Figure 2b along with the data from Figure 1. The two types of isotherms are almost indistinguishable. Thus, although there appears to be an effect of solids concentration on D, the effect is largely explained by a simple reversible equilibrium model and a nonlinear isotherm. Effect of pH. The effect of pH on the sorption of the three AE homologs on EPA-12 sediment is seen in Figure 3. The pH value has a small but measurable effect for A13E9 (∆ log D/∆ log [H+] ≈ 0.07), and the magnitude of the effect decreases with decreasing oxyethylene chain length. Large decreases in sorption with increasing pH have been observed for APE compounds on materials with pH-dependent surface charges such as quartz (45), silica (27), and a carboxyl-rich fiber (56). An interaction of the ether oxygens, behaving as weak H-bond acceptors, with the protons from surface carboxylic acid or silanol groups has been suggested. Savintseva et al. (24, 25) have presented evidence from IR spectroscopy that the terminal hydroxyl group of APE interacts with the silanol group of silica through chemisorption and that this interaction is favored at lower pH. Effect of Ca2+ and NaN3. Experiments were conducted with A13E3 and A13E9 homologs and EPA-12 sediment to examine the effects of added Ca2+ and NaN3 on sorption. Figure 4a shows that there was no significant effect of up to 1 mM added Ca2+; the background concentration of Ca2+, which came into solution from dissolution of the sediments or ion exchange with the Na+ electrolyte, was 0.1-0.2 mM under the conditions of these experiments. Small but measurable changes in D with NaN3 concentration were observed in this study, and the effect is different for the two homologs, as shown in Figure 4b. The value of D for A13E3 increases almost 2-fold over the range from 0.010 to 0.3 M NaN3. The value of D for A13E9 decreases from 1850 to 1280 over the range from 0 to 0.3 M NaN3. Added NaN3 could affect the value of D in many ways, and it is difficult to attribute the observations to any one cause. Increases in salt concentration can act to salt-out (54) nonionic surfactants, as evidenced by the decrease in cmc with increasing ionic strength (1). This effect would favor increases
b
FIGURE 4. Effect of added Ca2+ (a) and NaN3 (b) on adsorption of A13E3 and A13E9 on EPA-12 sediment. In the calcium experiments, Cs(w) was approximately 0.010 kg/L (in 0.01 M NaN3), and the measured pH was 7.10-7.39. In the NaN3 experiments, Cs(w) ) 0.029 kg/L, and the pH was 7.39-7.70. in sorption with increased NaN3. Addition of certain supporting electrolytes can decrease sorption of the polar part of the AE by blocking sorption sites in some fashion; Savintseva et al. (25) report that sorption of an APE on silica is decreased by the addition of NaCl and even more so by sodium phosphate. Sorption of AE on Hg is stronger in NaCl than in Na2SO4 (35), but sorption of APEs on active carbon is unaffected by 0.2 M NaCl (34). Another effect of NaN3 is to inhibit adventitious biodegradation of the AEs. Thus, in the absence of effective levels of NaN3, biodegradation could lead to more soluble degradation products and lower values of D. Effect of Different Sediments. The sorption isotherms of A13E6 on four different sediments, the properties of which are given in Table 2, are shown in Figure 5. The affinity of the sediments for the surfactants follows the order EPA-13 > EPA25 ≈ EPA-16 > EPA-12. This order does not correspond to the order of fraction of organic carbon of the sediments as was observed by Urano et al. (16). Sorbent EPA-13 had the greatest organic carbon content and the greatest affinity for the surfactants, but this correspondence is not seen for the other sorbents. One possible perturbation to the order of affinities could be the presence of appreciable quantities of swelling clays found in EPA-16 and EPA-25, but not in EPA12. Swelling clays, such as montmorillonite, are known to intercalate AEs (39, 40). Podoll and co-workers (19) found that the sorption of poly(ethylene glycols) to natural sediments was related to the fraction of montmorillonite + vermiculite and not directly to fraction organic carbon for all systems studied. Such a mechanism would still not explain the especially high energy of sorption found for EPA-13 sediment (Table 2, Figure 5). The isotherms are slightly nonlinear, and the data can be described very well by the Freundlich equation, for which the parameters are given in Table 3. The values of n are in the range 0.74-0.88 for the four sediments. The nonlinearity
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FIGURE 5. Logarithmic sorption isotherms of A13E6 on four sediments (0.024 kg/L in 0.01 M NaN3), the properties of which are given in Table 2. The solid lines are Freundlich isotherms (eq 2), for which the parameters K and n are given in Table 3. is attributed to the heterogeneity of sorption sites that interact with the oxyethylene chain. Nature of Interaction with Surface. Both the alkyl group and the oxyethylene chain affect the sorption behavior of the AEs. Although we did not specifically examine the effect of alkyl chain length on sorption, we can estimate the effect by comparison to other work. For sorption of AE on acetylene black, ∆ log D/∆n-CH2- ) 0.48 (33), and for sorption of linear alkylbenzene sulfonantes (LAS) on sediments, ∆ log D/∆n-CH2) 0.40-0.45, based on linear approximations of the isotherms at low concentrations (47, 58). These values are in the range expected for the hydrophobic effect. The effect of added methylene groups on APE sorption on soil can be inferred from the data in Liu et al. (17) by examining the difference in sorption between A8PE12 and A9PE10.5. At a surface concentration of 1 µmol/g, the distribution ratio, D, of the latter compound was 0.26 log units greater than the former. At the same surface concentration, it was found that increasing the number of oxyethylene groups increased D by 0.11 log unit per oxyethylene unit. Thus, one can infer a value of ∆ log D/∆n-CH2- ) 0.43, matching expectations. The effect of additional oxyethylene units is to increase sorption, but the effect is smaller than it is for additional methylene units. This result is significant, since the oxyethylene group also promotes the solubility of the AE in water, as indicated by the increase in cmc with the number of oxyethylene groups (1), for example. In contrast, the effect of the entire hydrophilic chain is to decrease sorption. For example, the value of the distribution ratio of dodecane (unpublished data) between water and several low organic carbon aquifer materials is over an order of magnitude greater than that seen here for A13Ex. Podoll et al. (19) also reported an increase in sorption of poly(ethylene glycol) with oxyethylene chain length. However, a quantitative characterization of this effect with our data is not straightforward because the AE isotherms are nonlinear and there is no single value of D. One practical way to compare sorption behavior of different homologs under these circumstances is to compare the values of D at constant CAE(s), since the heterogeneity of the sorbent is thought to be the major source of nonlinearity. For example, at CAE(s) ) 100 µmol/kg, the value of ∆ log D/∆n-(OCH2CH2)is 0.06 between A13E3 and A13E6 and 0.16 between A13E6 and A13E9, as calculated from the parameters in Table 3. Due to isotherm nonlinearity, the magnitude of this effect increases with decreasing concentration. These magnitudes are similar to the value of 0.11 found for both sorption of AE onto carbon black at infinite dilution (33) and for sorption of two APEs with soil at a constant surface concentration of 1 µmol/g (17). However, our data and that of Liu and co-workers (17) indicate that the effect of the oxyethylene chain on D depends on
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both concentration and oxyethylene chain length of the homolog. The oxyethylene chain interacts with specific binding sites on the sediment, and this specificity increases with increasing chain length. This observation is supported by the increase in isotherm nonlinearity and increasing effects of pH and ionic strength as the number of oxyethylene groups is increased from 3 to 9. The interactions of the polar oxyethylene chain with the sediment may be through the ether groups or the hydroxyl group. An increase in the length of the oxyethylene chain allows a greater number of these favorable sorption interactions to occur and greater possibility for the flexible chain to reorient itself to maximize these favorable interactions. Also, there is increasing likelihood of multi-site sorption as the oxyethylene chain length increases. Information concerning the sorption of the oxyethylene chain is also obtained from the effects of pH and ionic strength that we determined for the AEs on EPA-12 sediment. Increasing [H+] increases the sorption; the effect is more pronounced for A13E9 than for the A13E3 homolog. This might be due to interactions of an increasingly protonated surface with several of the ether oxygens, the effect being cumulative and increasing with chain length, or it may be due to pHdependent specific interactions with the terminal hydroxyl group (24). Again, the larger effect observed as the oxyethylene chain is increased may be due to increased degrees of freedom and the ability to conform to energetically favorable sites. The predominant binding mechanisms and whether the interactions are through the ether or terminal hydroxyl groups of the AEs cannot be elucidated from our study. Sorption Behavior of AE in the Environment. Finally, we summarize the results of this study relative to the sorption behavior of AEs in the environment. Solution chemistry variables such as pH and salt content had a small but detectable effect on AE sorption, while Ca2+ (hardness) had no detectable effect up to the 1 mM level. Among sediment properties, sorption did not reflect a strong correlation with organic carbon content of the sorbent, perhaps because the effect of the organic carbon was masked by the effect of expandable 2:1 clay minerals. Further work to clarify these correlations would be of interest. Perhaps the most significant result from our study with pure compounds is that the extent and degree of nonlinearity of sorption increased with an increasing number of oxyethylene groups, although the effect of the oxyethylene unit on the energy of sorption is weaker than that for a methylene unit. Brownawell et al. (47) have compared the sorption behavior of the nonionic surfactant AE to that of anionic surfactants (linear alkylbenzene sulfonates) and cationic surfactants (alkyl pyridinium) of similar hydrocarbon chain length, with the same set of sediments used in this study. Consistent with the negative charge of the sediments at the pH values of the experiments, the affinity of the sediments was strongest for the cationic surfactant and weakest for the anionic surfactant, with the nonionic surfactant in-between.
Acknowledgments The authors gratefully acknowledge the assistance of Vincent Hand (Miami University) and Keith Booman, Richard Sedlak, and Alvaro DeCarvalho of the Soap and Detergent Association. This research was sponsored by the Soap and Detergent Association.
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Received for review August 12, 1996. Revised manuscript received December 15, 1996. Accepted December 30, 1996.X ES960692K X
Abstract published in Advance ACS Abstracts, March 1, 1997.
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