Sorption of Ionizable Organic Solutes by Surfactant-Modified Zeolite

Department of Earth and Environmental Science,. New Mexico Institute of Mining and Technology,. Socorro, New Mexico 87801. We determined the sorption ...
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Environ. Sci. Technol. 2000, 34, 3756-3760

Sorption of Ionizable Organic Solutes by Surfactant-Modified Zeolite Z H A O H U I L I , * ,† T O D D B U R T , ‡ A N D ROBERT S. BOWMAN‡ Geology Department and Chemistry Department, University of WisconsinsParkside, Kenosha, Wisconsin 53141, and Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801

We determined the sorption of ionizable organic solutes on a natural zeolite modified with hexadecyltrimethylammonium (HDTMA), a cationic surfactant. The sorption of benzene and its ionizable analogues phenol and aniline by surfactant-modified zeolite (SMZ), prepared at different HDTMA surface coverages, was affected by solution pH. All of the sorption isotherms were linear and could be described by a distribution coefficient (Kd). At neutral pH, the Kd values of benzene, phenol, and aniline on SMZ increased with HDTMA loading up to monolayer coverage of 100 mmol/kg. Beyond monolayer coverage, further increases in HDTMA loading did not increase the Kd values of the solutes at pH 7.0, where all species exist primarily in their neutral forms. The Kd values were consistent with the relative octanol-water partition coefficients of the three compounds, indicating that sorption of the neutral species was primarily by partitioning into the bound HDTMA organic pseudophase. Phenol sorption by SMZ treated to bilayer coverage increased as the pH, and hence fraction of anionic phenolate, increased. The counterion balance indicated that the increased retention of phenol was due partially to anion exchange of phenolate for bromide, the same mechanism responsible for sorption of inorganic anions by SMZ. In contrast, decreases in pH resulted in reduced aniline sorption due to a lower concentration of the neutral species and repulsion of the positively charged anilinium from SMZ treated to bilayer coverage. The results demonstrate that sorption of target species can be maximized by tailoring the HDTMA surface coverage to account for species and solution characteristics.

Introduction Natural zeolites possess permanent negative charges in their crystal structures, making them suitable for surface modification using cationic surfactants. When aqueous surfactant concentrations are greater than the critical micelle concentration (CMC) and sufficient surfactant is present in the system, the sorbed surfactant molecules primarily form a bilayer on zeolite external surfaces (1). Recent studies on the properties of surfactant-modified zeolite (SMZ) indicate that it is an effective sorbent for multiple types of contaminants, * Corresponding author phone: (262)595-2487; fax: (262)595-2056; e-mail: [email protected]. † University of WisconsinsParkside. ‡ New Mexico Institute of Mining and Technology. 3756

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such as chromate and perchloroethylene (PCE) (1-5). While the sorption of oxyanions by SMZ was attributed to anion exchange on the positively charged surfactant bilayer (1, 5), the sorption of hydrophobic organic contaminants was due to partitioning of the organics into the organic pseudophase created by the surfactant tail groups (3, 6). Sorption of PCE by SMZ at different surfactant surface coverages revealed that the PCE Kd values were affected by the surface concentration of surfactant (6). Partitioning also appears to be the mechanism for sorption of benzene, toluene, and xylene by SMZ (3). The sorption of ionizable organic solutes such as phenol and aniline by SMZ has not been studied however. Sorption of aniline on montmorillonite was shown to be pH dependent, and the sorption reached a maximum at a pH close to aniline’s pKa value of 4.6 (7). In a separate study, it was found that aniline sorption was greater at acidic pH as compared to neutral pH for kaolinite, montmorillonite, and vermiculite, an indication of cation exchange of the positively charged anilinium ion (8). In one study, no phenol sorption was observed on smectite modified with hexadecyltrimethylammonium (HDTMA) at an HDTMA loading of 900 mequiv/kg (9). In contrast, in another study, phenol sorption was greatly enhanced by HDTMA treatment of montmorillonite at approximately the same loading level (10). No pH control was provided in either of these studies. Phenol sorption on organoclay modified with dimethyldistearylammonium to a level of 88% of the cation exchange capacity was found to be greatest when the pH was low enough to ensure that the uncharged species was predominant (11). However, further lowering the pH from 7.4 to 5 resulted in a decrease in phenol sorption (11). Stapleton et al. (12) provided one of the clearest demonstrations of the effects of speciation on sorption of ionizable organic chemicals. Their work showed that sorption of pentachlorophenol (PCP) by surfactant-modified montmorillonite decreased as the fraction of the deprotonated (anionic) form of the solute increased with increasing pH. Stapleton et al. (12) hypothesized that the modified clay sorbed the neutral form of PCP via a partitioning mechanism, while the much less strongly sorbed phenolate was retained via coadsorption with a counterion. Since most surfactant retention occurs in the interlayer space of montmorillonite and other layer silicates, these modified minerals provide a primarily hydrophobic environment for retention of solutes. Three-dimensional framework silicates such as zeolites, in contrast, retain high molecular weight surfactants primarily on their outer surfaces, where at sufficient surfactant loading a bilayer forms. This bilayer formation results in a charge reversal on the external zeolite surface, providing sites where anions will be retained and cations repelled, while neutral species can partition into the hydrophobic core (6). Thus, the retention of ionizable organic solutes by SMZ is expected to be fundamentally different from their retention by surfactant-modified clay minerals. Since SMZ’s anion exchange and partitioning characteristics vary with surfactant loading, there is an opportunity to tailor SMZ to optimally retain mixed contaminants whose properties may vary with solution conditions such as pH. The objective of the present research was thus to determine the effects of surfactant coverage and solution pH on SMZ sorption of benzene and its ionizable analogues phenol and aniline.

Materials and Methods A clinoptilolite-rich zeolitic tuff (“zeolite”), with a particle size range of 0.4-1.4 mm, was obtained from the St. Cloud 10.1021/es990743o CCC: $19.00

 2000 American Chemical Society Published on Web 08/04/2000

Mine in Winston, NM. The mineral content of this zeolite, based on internal standard XRD analysis (13, 14), was 74% clinoptilolite, 5% smectite, 10% quartz plus cristobalite, 10% feldspar, and 1% illite. The zeolite had an internal (zeolitic) cation exchange capacity (CEC) of 800 mequiv/kg and an external (nonzeolitic) cation exchange capacity (ECEC) of 90-110 mequiv/kg, as determined using a method modified from that of Ming and Dixon (1, 15). The external surface area determined from nitrogen adsorption was 15.7 m2/g (14). Hexadecyltrimethylammonium bromide (HDTMA-Br) was used for surface modification. An HDTMA aqueous solution (360 mL) with initial concentrations varying from 0 to 67 mmol/L was mixed with 120 g of raw zeolite to produce SMZ with surface coverages of 0, 25, 50, 75, 100, 150, and 200 mmol/kg. Since the ECEC of the zeolite is about 100 mequiv/ kg and the HDTMA sorption plateau is 200 mmol/kg (1), the surface coverages encompassed submonolayer, full monolayer, partial bilayer, and full bilayer formation. The HDTMAzeolite mixture was shaken for 8 h at 25 °C and 150 rpm on a shaker table followed by centrifuging and washing with two portions of Type I water from a Milli-Q system. Earlier work showed that final HDTMA surface coverages for SMZ prepared in this manner were 98-100% of the target values (1). The surfactant-modified samples were then air-dried prior to further use. Sorption isotherms were prepared for benzene, phenol, and aniline, using SMZ with HDTMA loading levels of 25200 mmol/kg, with solutions buffered at different pH values. Air-dried SMZ (2.00 g) was put into a 12-mL glass headspace vial, 10.0 mL of benzene, phenol, or aniline solution (initial concentrations 120-1200 mg/L) added, and the vial was crimp-sealed using a Teflon-lined septum. The samples were shaken at 25 °C and 150 rpm for 24 h on a shaker table and then centrifuged at 3800g for 25 min to yield a clear supernatant for analysis of the equilibrium solute concentrations. Research reported earlier showed that 24 h was sufficient to attain sorption equilibrium for nonionic benzene derivatives, chlorinated aliphatic compounds, and inorganic anions (1, 3, 6) and was assumed sufficient for phenol and aniline sorption. For phenol sorption, the pH was controlled using Na2CO3/NaHCO3 buffer solutions. The aqueous Na2CO3/ NaHCO3 solutions with concentrations of 10 mM/3 mM, 12 mM/1 mM, and 30 mM/3 mM had pH values of 10.6, 11.0, and 11.2. After 24-h contact with SMZ the final solution pH values were 8.4, 9.8, and 11.2, respectively. For aniline sorption, the pH was controlled using CH3COOH/CH3COONa buffer solutions at concentrations of 4 mM/93 mM, 4 mM/ 40 mM, 7 mM/4 mM, and 37 mM/3 mM, yielding final solution pH values of 6.2, 5.2, 4.3, and 3.6, respectively, after 24-h contact with SMZ. SMZ equilibrated with Type I water yielded a final pH of 7.0; the pH remained at 7.0 when benzene, phenol, or aniline was present in the water. Benzene sorption was determined at pH 7.0 only. Control samples (no SMZ added) were run under the same conditions. All initial conditions were prepared in duplicate. Equilibrium benzene concentrations were determined using a Hewlett-Packard 5890 GC with an HP-5 0.53 mm × 30 m capillary column and an FID detector. The oven, injector, and detector temperatures were 40, 240, and 300 °C. The retention time was 1.5 min. Equilibrium phenol and aniline concentrations were analyzed by HPLC using a Rexchrom S5-100-ODS column (Regis, Morton Grove, IL) and a model 510 pump, model 481 detector, and model 717 autoinjector (all from Waters, Milford, MA). The mobile phase was 50/50 water/acetonitrile. At the detection wavelength of 254 nm and flow rate of 0.7 mL/min, the retention times were 3.3 and 4.3 min for phenol and aniline, respectively. Equilibrium bromide concentrations were determined by HPLC using a method described earlier (1, 5).

FIGURE 1. Sorption of benzene, phenol, and aniline by SMZ at pH 7.0 with an HDTMA surface coverage of (a) 100 or (b) 200 mmol/kg. Also shown is benzene sorption by raw zeolite. Linear calibration curves for chemical analyses were based on six standards; in all cases the coefficients of determination exceeded 0.99. The amount of solute sorbed was calculated from the difference between the initial and equilibrium solution concentrations. Corrections were made for loss of analyte from the controls, which was generally negligible.

Results and Discussion Sorption of benzene, phenol, and aniline by SMZ in the unbuffered solutions (pH 7.0) at monolayer (100 mmol/kg) and full bilayer (200 mmol/kg) HDTMA surface coverages can be seen in Figure 1. All three compounds followed linear sorption isotherms at both surfactant loading levels. Such linear isotherms were observed for all three compounds at all pH values and surfactant loadings, with coefficients of determination typically in the range of 0.96-0.99. Linear sorption of benzene has also been observed on surfactantmodified clays (16-18). This linear sorption behavior may be described by

Cs ) K d Cw

(1)

where Cs is the amount of solute sorbed per unit mass of sorbent, Cw is the equilibrium aqueous solute concentration, and Kd is the distribution coefficient. Reflecting the dominant influence of solid-phase organic carbon on the sorption of organic solutes, Kd is often normalized by the sorbent’s organic carbon content:

Koc )

Kd foc

(2)

where foc is the fractional organic carbon content of the sorbent and Koc is the organic carbon-based partition coefficient. At monolayer coverage (Figure 1a), the order of distribution coefficients followed Kd,benzene > Kd,phenol > Kd,aniline, VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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corresponding to the compounds’ relative octanol-water partition coefficients (Kow) of 135, 28, and 8 (19). The Koc values calculated from eq 2 using foc values based on the amount of sorbed HDTMA were 570, 480, and 220 L/kg for benzene, phenol, and aniline, respectively. Doubling the amount of HDTMA to full bilayer coverage had little effect on the Kd values for the three compounds (Figure 1b). However, since foc for bilayer coverage is twice the value for monolayer coverage, the experimentally obtained Koc values decreased respectively to 260, 240, and 90 L/kg for benzene, phenol, and aniline. This decrease in Koc at greater than monolayer coverage by HDTMA was also noted for PCE sorption by SMZ (6). Phenol and aniline each exist in solution as both protonated and deprotonated species, with the proportions being a function of pH. Phenol is a weak acid that dissociates into phenolate and a proton:

Aniline is a weak base that can protonate to form the anilinium ion:

Since the speciations of phenol and aniline are pH dependent, sorption of phenol and aniline by SMZ should vary with pH. In addition, since the net charge on SMZ varies from negative to positive depending upon surfactant loading (5-6, 20), phenol and aniline sorption should also vary with HDTMA treatment level. The Kd values for benzene, phenol, and aniline sorption as a function of HDTMA surface coverage at several pH values can be seen in Figure 2. Each symbol in Figure 2a-c represents the Kd of a linear sorption isotherm at the indicated coverage. Below HDTMA monolayer coverage (100 mmol/ kg), the Kd values for benzene increased linearly with the increase in foc (Figure 2a). Above monolayer coverage, further increases in surfactant loading resulted in no further increase in the Kd for benzene. This plateau in Kd values above a threshold surfactant coverage was also observed by Yu and Lobban (21) for the adsolubilization (solubilization by adsorbed micelles) of trimethyl orthobenzoate and its reaction product methyl benzoate by sodium dodecyl sulfatemodified alumina. Li and Bowman (6) likewise saw no increase in PCE sorption on SMZ once equivalent monolayer coverage by HDTMA was exceeded. The lack of increased sorption beyond monolayer coverage likely reflects a change in HDTMA configuration and packing density in the bilayer system, which affects the partitioning of nonpolar organic compounds (6, 20). The phenol Kd values with respect to different HDTMA surface coverages at pH 7.0 and pH 9.8 can be seen in Figure 2b. At pH 7.0, 100% of the phenol is in its neutral form, but at pH 9.8, 50% of the phenol is present as the deprotonated phenolate anion (Table 1). Similar to the case for benzene, the Kd for phenol sorption remained constant when the HDTMA loading increased from 100 to 200 mmol/kg at pH 7.0. In contrast, at pH 9.8 the Kd for phenol increased as 3758

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FIGURE 2. Distribution coefficients (Kd) of (a) benzene, (b) phenol, and (c) aniline on surfactant-modified zeolite as a function of HDTMA surface coverage at different pH values. The lines are linear regressions of the data up to an HDTMA coverage of 100 mmol/kg. In panel b, solid and dashed lines are for phenol at pH 7.0 and pH 9.8. In panel c, solid and dashed lines are for aniline at pH 5.2 and pH 7.0. The standard deviations of Kd fell within the areas of the plotted points.

TABLE 1. Ionic Strengths and Species Distributions for Isotherms Prepared at Different pH Values pH

ionic strength of buffer (mM)

3.6 4.3 5.2 6.2 7.0 8.4 9.8 11.2

6.8 12.2 8.2 38.2 0.0 23.0 27.8 80.2

% neutral species (activity ratio) aniline phenol 9 33 80 98 100

100 97 50 5

HDTMA loading increased beyond monolayer coverage (Figure 2b). The additional retention reflects anion exchange of the phenolate by the positively charged SMZ treated to 200 mmol/kg, along with continued partitioning of phenol into the HDTMA bilayer. For aniline sorption below monolayer coverage, the same relationship between the Kd and surface coverage follows as

FIGURE 3. Bromide desorbed vs phenol sorbed at pH 8.4, pH 9.8, and pH 11 by SMZ at (a) 100 and (b) 200 mmol/kg HDTMA surface coverage. with benzene and phenol sorption. However, the Kd values at all pH values decreased as HDTMA loading increased beyond monolayer coverage (Figure 2c). The decrease in Kd was most pronounced at the lowest pH of 3.6, where positively charged anilinium represented more than 90% of the aniline in solution (Table 1). The decreased aniline sorption can be attributed to the repulsion of cationic anilinium from the positively charged headgroups of the bilayer system. These results are consistent with reported effect of pH on aniline sorption by alfisols, where it was shown that sorption was greatest when the pH exceeded aniline’s pKa (22). At pH 8.4, no bromide was released upon phenol sorption in the monolayer system (Figure 3a). At pH 9.8 and pH 11.2, bromide release was proportional to phenol sorption and reached a maximum of 18 mmol/kg. This bromide release suggests that a fraction of the sorbed HDTMA was present in the form of admicelles, with bromide counterions, even in the “monolayer” system. Much more bromide was released per mole of phenol retained in the bilayer system (Figure 3b) as compared to the monolayer system (Figure 3a). The greater phenol sorption in the bilayer system, along with increased counterion desorption at higher pH values in both systems, suggests significant sorption of the anionic phenolate species under the higher pH conditions. Figure 4 shows the variation in phenol and aniline Kd values as a function of pH for the monolayer and bilayer systems. Phenol sorption was greater for the 200 mmol/kg HDTMA loading than for the 100 mmol/kg loading at all pH values (Figure 4a), reflecting the additional anion exchange mechanism for phenolate sorption when a surfactant bilayer is present. For the bilayer system, the maximum phenol Kd values was achieved in the pH 9.8 buffer. Although a greater Kd might have been expected at pH 11.2, the much greater concentration of carbonate, bicarbonate, and hydroxyl anions in the higher ionic strength buffer (Table 1) likely reduced

FIGURE 4. Changes in distribution coefficients (Kd) with pH at different HDTMA loadings for (a) phenol and (b) aniline. The standard deviations of Kd fell within the areas of the plotted points. phenolate sorption at this pH due to competition for anion exchange sites. In fact, high pH carbonate solutions are effective for desorbing anions from SMZ (23). Phenol sorption generally decreased with pH increases for the 100 mmol/kg HDTMA treatment. The SMZ treated to monolayer coverage had fewer sorption sites for the phenolate anion, and the fraction of neutral species decreased, so sorption became less as the phenol speciation shifted toward phenolate at higher pH values. The sorption of phenol on SMZ is in agreement with the results of Zhang and Sparks (10) and Dentel et al. (11), but contrary to the results of Mortland et al., who reported no sorption of phenol by HDTMA-modified smectite (9). Mortland et al. (9) added only enough HDTMA to satisfy the smectite’s CEC and did not control the pH; their systems may have contained primarily phenolate anions, which would not have been retained by their hydrophobic smectite. In contrast to the case for phenol, aniline sorption was always greater on zeolite treated with HDTMA to 100 mmol/ kg rather than to 200 mmol/kg. This trend primarily reflects the repulsion of the anilinium cation from SMZ treated to bilayer coverage. As the concentration of anilinium increased with decreasing pH (Table 1), aniline sorption decreased for both HDTMA surface treatments, reflecting the lower concentration of neutral species available to sorb to the SMZ via partitioning and repulsion of anilinium from bilayer-treated HDTMA. The decreased aniline sorption at pH 3.6 also suggests that anilinium could not compete effectively with HDTMA for cation exchange sites on the zeolite. For both the 100 and 200 mmol/kg HDTMA treatments, aniline sorption was greater at pH 5.2-6.2 than at pH 7.0, even though the anilinium concentration at pH 7.0 was negligible. The greater sorption at pH 5.2-6.2 probably represents some “salting out” of aniline onto SMZ from the higher ionic strength buffer solutions at pH 5.2-6.2 as compared to the unbuffered pH 7.0 system. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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In summary, sorption of benzene, phenol, and aniline by SMZ followed linear sorption isotherms in the pH range of 3.6-11.2. At neutral pH, the Kd values of the three compounds reflected their relative octanol-water partition coefficients and increased with surfactant loading up to equivalent monolayer coverage of 100 mmol/kg. Beyond monolayer coverage, further increases in surfactant loading did not increase Kd at neutral pH. This phenomenon suggests that zeolite modified to a monolayer surfactant surface coverage may achieve maximum economic and remediation goals if the target contaminants are nonpolar. The sorption of the ionizable organic compounds phenol and aniline varied as functions of both pH and surfactant loading on the zeolite. At monolayer surfactant coverage changes in sorption reflected changes in the fraction of the organic solute present in the neutral form. At bilayer coverages, changes in sorption also reflected attraction or repulsion of the ionized form of the organic solute to or from the SMZ bilayer surfaces. The results of this study demonstrate that surfactantmodified minerals such as SMZ are effective in sorbing ionizable organic compounds in addition to hydrophobic nonionic compounds. When solution pH is such that the neutral form of an ionizable species dominates, no sorption enhancement occurs in treating the SMZ beyond monolayer coverage. At pH values where ionized species are important, sorption to SMZ at bilayer coverage will be enhanced or depressed depending upon the ionic charge. For solutes such as aniline that protonate at low pH, optimal sorption may occur at submonolayer coverages of SMZ, where some cation exchange sites of the zeolite still remain available to sorb other cationic species.

Acknowledgments This research was supported through U.S. DOE Federal Energy Technology Center under Contract DE-AR2195MC32108. Shawn Williams performed some of the experimental work.

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Received for review July 1, 1999. Revised manuscript received March 24, 2000. Accepted June 9, 2000. ES990743O