Environ. Sci. Technol. 1996, 30, 89-96
Organic Cosolvent Effects on Sorption Equilibrium of Hydrophobic Organic Chemicals by Organoclays VALENTINE A. NZENGUNG* Georgia Institute of Technology, Atlanta, Georgia 30332
EVANGELOS A. VOUDRIAS School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
PETER NKEDI-KIZZA Soil and Water Science Department, University of Florida, Gainesville, Florida 32611
J. M. WAMPLER AND CHARLES E. WEAVER School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332
Isotherms were measured for sorption of naphthalene and diuron by four organoclays in equilibrium with various mixtures of methanol and water. The organoclays were prepared from Wyoming montmorillonite by replacing the natural exchangeable cations of the clay by the quaternary ammonium ions TMA (tetramethylammonium), TMPA (trimethylphenylammonium), HDTMA (hexadecyltrimethylammonium), and BDTDA (benzyldimethyltetradecylammonium). TMPA-clay showed the greatest sorptive capacity for naphthalene, while BDTDA-clay was the most effective sorbent for diuron. The sorption mechanism for each sorbatesorbent combination was related to the arrangement of the quaternary ammonium cations in the exchanged clay and the volume fraction of methanol in solution (f c). As expected from the solvophobic theory, the linear sorption coefficients decreased loglinearly with increasing f c in the binary solvent mixture, except for TMPA-clay at f c > 0.5. In addition to solute-solvent and solvent-sorbent interactions, an additional effect involving solute-organoclay interactions influenced the sorption of naphthalene and diuron by organoclays from aqueous and mixed solvents.
Introduction The aluminosilicate sheets of common clay minerals possess a net negative electrical charge compensated for by * Corresponding author present address: Department of Geology, University of Georgia, Athens, GA 30602-2501; telephone: 706-5422652; fax: 706-542-2425.
0013-936X/96/0930-0089$12.00/0
1995 American Chemical Society
inorganic exchangeable cations (e.g., Na+ and Ca2+), which are strongly hydrated in the presence of water. Surface properties of natural clays can be modified by simple ion exchange with organic cations. Montmorillonite clays (smectites) are easily modified by exchanging their inorganic cations with quaternary ammonium cations (1-6). This may result in an increase in the inter-lamellar spacing and exposure of new sorption sites of the clays (1-3, 7-9). More importantly, the substituted organic cations are weakly hydrated. As the inorganic cations are progressively replaced by the organic cations, the surface properties of a clay may change considerably from highly hydrophilic to increasingly organophilic (hydrophobic). Depending on the hydrophobicity of the substituents, the clays resulting from the exchange reactions have been classified into two categories (2, 3, 9): (1) organophilic clays, those produced from substituted ammonium cations having one or more long-chained alkyl substituents, and (2) adsorptive clays, those produced from quaternary ammonium ions having relatively small substituents (4, 5). HDTMA- and BDTDA-smectites represent organophilic clays, while TMA- and TMPA-smectites represent adsorptive clays (Figure 1). Sorption studies from aqueous solution with natural smectites and organosmectites as sorbents have been conducted with a variety of hydrophobic organic chemicals (HOCs). Boyd et al. (2, 3) and Jaynes and Boyd (5, 10) showed that sorption of nonionic organic chemicals by HDTMA-smectite is essentially due to solute partitioning into the organic phase created by the large C16 alkyl chains of the HDTMA ions. Meanwhile, nonlinear isotherms indicative of adsorption or cosorption were observed when TMA- and TMPA-clays were used as sorbents (4, 11, 12). In many waste disposal sites and in industrial wastewater treatment systems, solvents may be present that enhance movement of sparingly soluble organics. Rao et al. (13) presented the solvophobic theory for sorption of HOCs by soils from aqueous organic binary solvent mixtures. The theory that predicts an exponential decrease in the sorption coefficient as the fraction of cosolvent increases because of increased HOC solubility in the binary solvent was applied to several HOCs and soils (14-17). The solvophobic theory has not been tested for organoclays in mixed solvent systems. This is an important task because organoclays have been suggested for increasing the adsorptive capacity of clay barriers, such as landfill liners and slurry walls, and as sorbents in water treatment applications (2-6, 9, 10, 18). The overall objective of this work was to determine whether the solvophobic theory could be applied to mixed solvent systems, using organoclays as sorbents for HOCs. The specific objectives were (1) to study the sorption of naphthalene and diuron from methanol-water mixtures by a Na-montmorillonitic clay exchanged with four kinds of surfactant cations, selected to provide a wide range in molecular size and configuration; (2) to evaluate the solvophobic model proposed by Rao et al. (13) for sorption of naphthalene and diuron by these organoclays; and (3) to compare the organoclays in terms of their sorption capacities for naphthalene and diuron, respectively.
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TABLE 1
Solute Properties solute
hydrocarbonaceous surface Area (HSA) (Å)2
MW
147a
diuron 233.1 naphthalene 128.2
log Kowb 2.8 3.37
156b
a Determined from percent effective HSA reported by Nkedi-Kizza et al. (15). lengths.
log Kocb 2.60 2.94
b
aq solubility (mg/L)b
length (Å)
width (Å)
42 31.7
8.4d
6.6d 5.6c
Karickhoff (25). c Jaynes and Boyd (4).
7.3c d
Estimated from bond
Theoretical Framework The solvophobic model presented by Rao et al. (13) forms a basis for quantifying the cosolvency of HOCs. According to this model, the equilibrium sorption coefficient (Km) of an HOC decreases exponentially with increasing volume fraction of the cosolvent (f c) in a binary solvent mixture:
ln
( )
Km ) -aRσcf c w K
(1)
where Kw is the equilibrium sorption coefficient from water (mL/g); Km is the equilibrium sorption coefficient from mixed solvent (mL/g); a is the empirical constant accounting for water-cosolvent interactions (note that for watermethanol a ) 1, implying ideal water-cosolvent interactions); R is the empirical constant accounting for solventsorbent interactions; and σc is the the cosolvency power of a solvent for a solute accounting for solvent-solute interactions. The value of σc can be approximated when hydrophobic interactions are dominant (13, 15):
σc )
(∆γcHSA) kT
(2)
where HSA is the hydrocarbonaceous surface area of the solute molecule (Å2); ∆γc is the the difference in interfacial free energy density (erg/Å2) between HSA and the solvents; k is the Boltzmann constant (erg/K); T is the temperature (K). At a given temperature, the parameter σc is dependent only on the sorbate and solvent properties and not on the sorbent characteristics (15). The value of σc for a sorbate estimated from data for different sorbents (organoclays, soils, sediments) is expected to be constant if the model assumptions are valid (13, 15).
Experimental Materials and Methods Materials. Naphthalene and diuron were obtained as reagent grade chemicals from Fisher Scientific (Pittsburgh, PA). Naphthalene was selected as an ideal hydrophobic sorbate without polar functional groups for which the total surface area (TSA) and HSA are equal. Diuron has polar functional groups; so for it, HSA is less than TSA. These compounds also represent two classes of chemicals of current environmental concern present in industrial and agricultural wastewaters and in contaminated sites, and a large volume of sorption data is available for these compounds (15, 16, 19-22). Some important properties of these chemicals are presented in Table 1, and their structures are presented in Figure 1. Radiolabeled (14C) naphthalene and diuron were obtained from Sigma Chemical Co. (St. Louis, MO) and had >99% radiochemical purity. Scintiverse II scintillation
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FIGURE 1. Chemical structures of naphthalene, diuron, and quaternary alkylammoniumsalts used to prepare organoclays.
liquid, obtained from Fisher Scientific, or its equivalent was used in the radioassays. Methanol was chosen as the organic cosolvent because it is completely miscible in water and is expected to be found in many waste streams from industrial wastes. Highperformance liquid chromatography (HPLC) grade methanol and deionized or distilled water were used in all experiments. The Na-montmorillonite was separated from a reference clay, SWy-1, obtained from the Source Clay Repository of the Clay Minerals Society (Columbia, MO). Reagent grade tetramethylammonium chloride, trimethylphenylammonium chloride, and hexadecyltrimethylammonium bromide were obtained from Fisher Scientific, and benzyldimethyltetradecylammonium chloride dihydrate was purchased from Aldrich Chemical Co. (Milwaukee, WI). The molecular structures of the organic cations are presented in Figure 1. Preparation of Organoclays. The organoclays were prepared as described in detail by Nzengung (23). The 0.5, TMPA-clay slope deviated from the log-linear relationship. Regression lines for TMPA- and BDTDA-clays at f c e 0.5 were essentially the same and were also described by a single line with a slope of -8.0. Data for TMA- and HDTMA-clay were described by slopes that were statistically the same within their 95% confidence limit (Table 6). The slope of the regression lines in Figure 4A, according to eq 1, represent the product Rσc. Using a ∆γc value of 2.36 × 10-15 erg/Å2 (27), HSA ) 156Å2 (Table 1), k ) 1.38 × 10-16 erg/K, and T ) 298 K, the value of σc for naphthalene in methanol-water mixtures was independently estimated from eq 2 to be 8.95. The value of σc is independent of the sorbent (eq 2). From this value and the slope determined
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TABLE 6
Solute-Solvent-Sorbent Interaction Parameters and Koc values for Four Organoclaysa sorbent
Koc naphthalene
Koc diuron
measd rσc (naphthalene)
rapp (naphthalene)
measd rσc (diuron)
rapp (diuron)
TMA-clay TMPA-clay HDTMA-clay BDTDA-clay
743 114524 3624 13983
4228 4905 3497 6158
5.4 (4.4-6.4) (a) 8.0 (7.2-8.8) (b) 5.4 (4.4-6.4) (a) 8.0 (7.2-8.8) (b)
0.60 (0.5-0.7) 0.90 (0.8-1.0) 0.60 (0.5-0.7) 0.90 (0.8-1.0)
10.1 (8.4-12) (c) 10.1 (8.4-12) (c) 5.6 (5.0-6.2) (d) 5.6 (5.0-6.2) (d)
1.2 (1-1.4) 1.2 (1-1.4) 0.66 (0.59-0.73) 0.66 (0.59-0.73)
a 95% confidence interval are in parentheses. (a-d) denote data pooled for the indicated solute-sorbent systems bearing the same letter because they were statistically not different.
A
B
FIGURE 4. (A) Log-linear relationship between relative sorption coefficients (Km/Kw) and volume fractions of cosolvent (f c) for naphthalene and four organoclays. (B) Log-linear relationship between relative sorption coefficients (Km/Kw) and volume fractions of cosolvent (f c) for diuron and four organoclays.
by linear regression of Km vs f c, the value of Rapp was calculated for each clay (Table 6). The notation Rapp, rather than R, is used because the data clearly show solute-sorbent effects that influence the slopes. Values of Rapp for naphthalene range between 0.6 and 0.9. For TMPA-clay at f c e 0.5 and BDTDA-clay at f c e 0.9, the Rapp values for naphthalene were the same and close to unity (Rapp ) 0.9). This suggests that solvent-sorbent effects were negligible over the relevant range of experimental conditions for each of these organoclays. Each of these two organoclays have a benzyl ring and were also most effective as sorbents of naphthalene. The Rapp values for TMA- and HDTMA-clay, which have no benzyl ring and were not relatively effective as sorbents of naphthalene, were significantly less than unity. It appears that the sorbate-sorbent and solventsorbent interactions for naphthalene were related to the sorption extent in the organoclays. In studies on sorption of HOCs by soils from solutions containing varying fractions of organic cosolvent, R < 1 has usually been obtained (14-17), which indicates that sorption from solvent mixtures were greater than that
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predicted from increased solubility alone (16). This behavior has been attributed to the swelling of soil organic matter in solvents. The gel swelling of sorbent organic matter results in enhanced permeation of HOCs, leading to greater sorption (16, 28). For R > 1, sorption of HOCs should be less than that predicted from increased solubility (29). Values of R should be near unity if the sorbent properties (such as swelling) are independent of change in solution phase compositions (16, 21). In Figure 4B, the dependence of diuron sorption coefficients on the volume fraction of methanol in the binary solvent mixture is presented. A log-linear relationship exists between Km and f c for diuron sorption on each organoclay, except for TMPA-clay when f c > 0.5. Except for TMPA-clay at f c g 0.7, the slopes for the organoclays with no long-chained alkyl groups (TMA- and TMPA-clays) were essentially the same, but different from the slopes for HDTMA- and BDTDA-clays, which have long-chained alkyl groups. Using an effective HSA of 147Å2 (15) and ∆γc value of 2.36 × 10-15 erg/Å2 (27), k ) 1.38 × 10-16 erg/K, and T ) 298 K, an independently estimated σc value of 8.4 was estimated for diuron in methanol-water mixtures from eq 2. For f c e 0.5, the average Rapp value for TMA- and TMPAclay was 1.2. Meanwhile, the Rapp value observed for the pooled data of HDTMA- and BDTDA-clay was significantly less than unity (i.e., Rapp ) 0.66). Apparently, Rapp for diuron was related to the length or complexity of the alkyl chain of the alkylammonium cation. According to the model predictions, we expected to obtain similar R values for both HOCs sorbed on the same organoclay in methanol-water mixtures. This was the case for TMPA-clay with Rapp close to unity and for HDTMAclay with Rapp < 1 (Table 6). However, for TMA-clay, the R value for naphthalene was 0.5 in TMPA-clay experiments. Of the four organoclays, HDTMA-clay swelled most, with a maximum swelling of 2.8 Å, which could account for the low Rapp values of 0.60 and 0.66 (i.e., significantly less than unity) for naphthalene and diuron data, respectively. The degree of swelling of organoclay organic carbon has been linked to the size of the organic ions, the dielectric constant of the solvating liquid, and the extent of the surface coating of the clay particles by organic carbon. Jordan (31) observed that swelling in various organic solvents is negligible until a chain length of 10 carbons is reached or until approximately half of the clay surface is coated. Maximum swelling was observed in his studies at a chain length of 12 carbons. This is consistent with the increasing trend in swelling from TMA- to TMPA-clay with the maximum swelling shown by HDTMA-clay, which has a chain length of C16 (Table 3). We cannot explain the failure to observe any swelling in BDTDA-clay except to speculate that the presence of a benzyl group makes solvation by methanol less favorable in this case. Previous studies have indicated that gel swelling is also inhibited at both low and higher surfactant doses (31, 32). The effects of organoclay swelling were generally greater for the organoclays with small organic ions than for those having long chains. Our X-ray data (Table 3) show clearly that adsorption of polar organic cosolvent molecules by TMA- and TMPA-clays tends to further separate the clay plates, thereby rendering the interlayers more organophilic and the organic coating more accessible. Thus, one needs more than the solvophobic theory to understand the effects of the solvent methanol on HOC sorption by organoclays. The independent organoclay swelling data obtained by X-ray diffraction must be considered along with the solvophobic theory in order to adequately rationalize the observed sorption behavior in these systems. Sorbate-Organoclay Interactions. As proposed by Rao et al. (13), the solvophobic model considered solute-solvent interactions, but solvent-sorbent interactions were considered in later studies (15, 16, 29). Sorbate-sorbent interactions were not considered in previous studies because the effects could not be determined, or they were assumed negligible for simplicity. Rao et al. (29) have suggested the need to address this additional facet of the solvophobic theory. The difference in independently determined σc values of 8.95 and 8.44 for naphthalene and diuron, respectively, do not account for the differences in the estimated slopes presented in Table 6. If solvophobic model predictions are
correct, R values for naphthalene and diuron for the same organoclay should be constant. Except for HDTMA- and TMPA-clays, the values of Rapp presented in Table 6 are different for the same organoclay. This means, for example, that if σc for naphthalene and diuron are known, then the estimated Rapp value for TMA-clay can not be used to predict the slope (Rσc) of ln (Km/Kw) vs f c for diuron and TMA-clay (eq 1). It is evident that an additional effect, presumably differences between sorbate-sorbent interactions for naphthalene and diuron caused the differences in Rapp for TMAand BDTDA-clays. Further evidence on the influence of sorbate-organoclay interactions comes from the structural differences between naphthalene and diuron that caused the observed differences in sorption mechanisms for these compounds by the organoclays. Because naphthalene and diuron are very different in structure (Figure 1; Table 1), their respective molecules interacted differently with the organoclays. The layered nature of clays, with most of the sorption sites (80%) located between the clay layers, makes it especially important to consider the role of sorbate size in their sorption process. This means that the sorbate-sorbent interactions would also depend on the organoclay interlamellar spacing. Sorbate-sorbent interactions of this type should be important even at infinite dilution. The size exclusion effects may, however, change due to organoclay swelling leading to effective solute movement into and within the clay layers (e.g., TMPA-montmorillonite). The extent of such enhanced movement of solute molecules in the different organoclays should be considered on a case by case basis according to the d001 spacing relative to the size of the solute molecules. The sorbate-sorbent interactions should result in either less sorption due to solute exclusion or higher sorption when the solute molecules are readily accessible to sorption surfaces in the organoclay interlayer. Experimental data presented in this paper for the sorption of naphthalene and diuron by organoclays in methanol-water mixtures suggest that sorbate-sorbent interactions may depend on the following: (1) the size and arrangement of the surfactant cation exchanged for the clay inorganic cations, (2) the extent of swelling of the organoclay organic carbon with increasing volume fraction of organic solvent in the system, and (3) the physicochemical properties of the solutes. Currently, no independent approach to estimate such solute-sorbent interactions (or exclusion) is available. Comparative Effectiveness of Organoclays as Sorbents. Of all sorbents used in this study, Ca-clay had the lowest sorption coefficient for both solutes in water (Tables 4-6). This was followed in order of increasing sorption by TMA-, HDTMA-, BDTDA-, and TMPA-clay in naphthalene sorption experiments and by TMA-, TMPA-, HDTMA-, and BDTDAclay in diuron sorption experiments. Except in one case (naphthalene on TMA-clay), Koc values for the sorption of naphthalene and diuron from water by these organoclays are considerably higher than Koc values for the sorption of these chemicals by natural sediments and soils. For naphthalene, the organoclays range from no better (TMAclay) to a little more than 100 times better (TMPA-clay) than sediment and soils. For diuron, the organoclays are consistently about 10 times better than sediments and soils. If partitioning was controlling the sorption phenomena, Kw should increase with increasing fraction of organic carbon (foc) of the sorbent (25, 34, 35). This expectation was realized in sorption of diuron but was not observed for
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naphthalene (Table 6). The clay with the highest sorption capacity for naphthalene (TMPA-clay) had an foc of 7.4%, which is lower than the foc of BDTDA-clay (foc ) 17.7%) by a factor of 2.3 (Table 2). The basal spacings (d001) from X-ray diffraction analysis were used to explain this observation. The measured d001 values of 13.6 and 14.5 Å for TMAand TMPA-clay, respectively (Table 2), indicate that the relatively small organic ions function as “pillars” to keep the interlayers apart. The basal spacing of TMPA-clay is larger by 0.9 Å than that of TMA-clay, which is consistent with the larger size of TMPA ions compared to TMA ions. The basal spacings in Table 2 include the 2:1 aluminosilicate sheet (9.4 Å), giving an interlayer separation (∆) of about 4.2 Å for TMA-clay and of 5.1 Å for TMPA-clay (Table 3). This observation is in agreement with the diameter of the TMA ion, which is 4.9 Å, and suggests some keying of hydrogen atoms into the aluminosilicate sheets (4). Jaynes and Boyd (5, 10) suggested that the triangular bases of TMA and TMPA cations could be aligned parallel to the clay sheets, and presumably the bases could adhere alternately to the upper and lower clay layers. The greater naphthalene uptake by TMPA-clay may be partly attributed to the closeness of the ∆ value of TMPA-clay to the width (5.6 Å) of naphthalene molecules, whereas the ∆ value of TMAclay is smaller than the width of naphthalene molecules. The sorption of TMPA-clay may also have been enhanced by a site-specific attraction of naphthalene by the benzyl groups in TMPA-clay. This was not the case for the branched and larger sized molecules of diuron (6.6 Å). It is evident that solute size and shape may significantly affect sorption by organoclays having small organic ions. The larger basal spacings (d001) of HDTMA- and BDTDAclay (d001 ) 17.7 Å) correspond to the formation of bilayers (24) in which the long-chained surfactant cations are in direct contact with each other (10), leading to the formation of organic phases consisting mostly of the C16 and C14 hydrocarbon groups into which solutes are partitioned. Thus, the degree of uptake of naphthalene, unlike diuron, depends not only on the amount but also on the arrangement of the surfactant ions in the montmorillonite interlayer sites.
Acknowledgments The authors thank Professor E. M. Perdue for valuable discussions. The research described in this paper has been funded in part by a grant from the United States Department of Interior, Geological Survey, through Agreement ERC 0693 of the Georgia Institute of Technology Environmental Research Center. However, the contents of this publication do not necessarily reflect the views and policies of the United States Department of the Interior nor does mention of trade names or commercial products constitute their endorsement by the United States Government. Author-Supplied Registry Numbers: Methanol, 67-561; diuron, 330-54-1; naphthalene, 91-20-3.
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Received for review February 14, 1995. Revised manuscript received July 27, 1995. Accepted August 7, 1995.X ES9501225 X
Abstract published in Advance ACS Abstracts, November 1, 1995.
