Environ. Sci. Technol. 2008, 42, 1109–1116
Enhanced Sorption of Polycyclic Aromatic Hydrocarbons to Tetra-Alkyl Ammonium Modified Smectites via Cation-π Interactions XIAOLEI QU, PING LIU, AND DONGQIANG ZHU* State Key Laboratory of Pollution Control and Resource Reuse and School of the Environment, Nanjing University, Jiangsu 210093, China
Received July 1, 2007. Revised manuscript received October 24, 2007. Accepted November 19, 2007.
The objective of this study was to characterize molecular sorptive interactions of polycyclic aromatic hydrocarbons (PAHs) by organoclays modified with quaternary ammonium cations. Three PAHs, naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR), and three chlorobenzenes, 1,2-dichlorobenzene (DCB), 1,2,4,5-tetrachlorobenzene (TeCB), and pentachlorobenzene (PtCB), were sorbed from aqueous solution to reference montmorillonite clays (SWy-2) exchanged respectively with tetramethyl ammonium (TMA), tetraethyl ammonium (TEA), tetran-butyl ammonium (TBA), and hexadecyltrimethyl ammonium (HDTMA) cations. Solute hydrophobicities are compared between PAHs and chlorobenzenes using the solute n-octanol–water partition coefficient, n-hexadecane-water partition coefficient, and polyethylene-water distribution coefficient. The PAHs show several- to more than 10-fold greater sorption than the chlorobenzenes having close hydrophobicities but fewer delocalized π electrons (NAPH/DCB, PHEN/TeCB, and PYR/ PtCB) by TEA-, TBA-, and HDTMA-clays. Furthermore, the PAHs show greater trends of solubility enhancement than the compared chlorobenzenes by TMA, TEA, and TBA in aqueous solution. The enhanced sorption and aqueous solubility of PAHs are best described by cation-π interactions between ammonium cations and PAHs relative to chlorobenzenes that are incapable of such interactions. Cation-π complexation between PAHs and tetra-alkyl ammonium cations in chloroform was verified by ring-current-induced upfield chemical shifts of the alkyl groups of cations in the 1H NMR spectrum.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous, nonionic hydrophobic organic compounds (HOCs) of great concern due to their toxicity and suspected carcinogenity (1–5). Environmental sources of PAHs include anthropogenic inputs from activities such as energy production and the transportation, storage, and refining of fuels. PAHs and other HOCs associated with soil particles in surface and subsurface environments may transfer to groundwater aquifers by leaching or colloid-enhanced transport, resulting in extensive contamination and great exposure of the * Corresponding author phone (fax): +86 025-8359-6496; zhud@ nju.edu.cn. 10.1021/es071613f CCC: $40.75
Published on Web 01/15/2008
2008 American Chemical Society
pollutants. Organoclays modified with quaternary ammonium surfactants are effective sorbents of HOCs and may find promising applications as low-permeability contaminant barriers to protect groundwater from contamination (e.g., slurry cutoff walls and landfill liners) (6–8). Therefore, understanding the molecular mechanisms that control the sorption of HOCs to organoclays is essential for predicting the activity and mobility of the sorbed solutes, as well as for designing effective sorbents with maximized sorptive capacities. It is well-recognized that the sorptive characteristics of HOCs by organoclays are highly dependent on the size and structure of organic cations. Organoclays treated with trimethylphenyl ammonium (TMPA) in general show greater sorptive capacities and more nonlinear isotherms than those treated with hexadecyltrimethyl ammonium (HDTMA) (9–12). To account for the observations, Gullick and Weber (10) proposed two types of sorption sites of HDTMA-clay, the dehydrated clay surface for adsorption and the organic phase formed by the alkyl chains of HDTMA for partition. Alternatively, the small, rigid organic cations (e.g., TMPA) may exist in the clay interlayer as pillars to create micropores that are somewhat hydrophobic for HOC adsorption (10, 13). To be consistent with this pseudo-pore-filling mechanism, greater sorption of naphthalene was observed on TMPAmontmorillonite compared to tetramethyl ammonium (TMA)- and HDTMA-montmorillonite due to the closeness of the TMPA-clay interlayer distance (5.1 Å) to the width (5.6 Å) of naphthalene molecules (9). Solute structural properties also greatly influence HOC sorption to organoclays. A remarkable finding of such structural dependence is that sorptive affinities may not fully comply with solute hydrophobicities, especially for compounds of diverse electronic structures. For example, Sheng et al. (14) found that aromatic solutes (benzene, nitrobenzene, and chlorobenzene) show greater sorption than aliphatic solutes (trichloroethylene and carbon tetrachloride) on HDTMA-clay despite the lower hydrophobicities of the aromatics. The investigators argued that aromatic solutes invoke stronger sorptive interactions because the planar shape and delocalized π bonds of these solutes favor the solvation of HDTMA. This hypothesis was supported by the fact that an increase in clay d-spacing accompanied the sorption of chlorobenzene but not trichloroethylene. Fourier transform infrared studies in alachlor sorption to TMPA-montmorillonite also indicated various sorptive forces, including ion-dipole interactions of the CdO group with the cation, and interactions of the alachlor aromatic ring with the mineral surface and the cation (15). Compared with chlorine-substituted aromatics [e.g., chlorobenzenes and chlorinated biphenyls (PCBs)], PAHs are rich in delocalized π electrons and thus may interact strongly with electron-deficient or positively charged species via electron-donor–acceptor (EDA) interactions or electrostatic attractions (cation-π bonding). Recent studies suggested that, besides hydrophobic effects, specific π-π EDA interactions also exist between π-donor compounds (e.g., pentamethyl benzene and phenanthrene) and π-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple electron-withdrawing groups (e.g., carboxyl) and heterocyclic aromatic amines (16). The π-acceptor ability of such aromatic moieties would increase with protonation as the pH decreases. Furthermore, Zhu et al. (17) observed that phenanthrene shows stronger sorption than 1,2,4,5tetrachlorobenzene by 2 orders of magnitude to Ag+-saturated montmorillonite, despite the close hydrophobicities of these VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Solute Water Solubility (SW), n-Octanol–Water Partition Coefficient (KOW), Hexadecane-Water Partition Coefficient (KHD), and Polyethylene (PE)-Water Distribution Coefficient (Kd,PE) compound (abbreviation)
SWa (µM)
KOWa (L L-1)
KHDb (L L-1)
Kd,PEc (L kg-1)
naphthalene (NAPH) 1,2-dichlorobenzene (DCB) phenanthrene (PHEN) 1,2,4,5-tetrachlorobenzene (TeCB) pyrene (PYR) pentachlorobenzene (PtCB)
251 891 6.31 5.89 0.692 2.63
2140 2510 37200 52500 135000 151000
2570 3310 55000 89100 153000 ( 8000d 160000 ( 30000d
700 ( 10e 550 ( 20e 9700 ( 400 11100 ( 400 69000 ( 2000 51000 ( 3000
a From Schwarzenbach et al. (26). b From Abraham et al. (25) except where noted. c Measured by single-point sorption except where noted, with ( standard deviation calculated from four replicates. d Measured by single-point partition, with ( standard deviation calculated from four replicates. e From Zhu and Pignatello (23).
two solutes. This was attributed to the strong cation-π interactions between phenanthrene and Ag+ on the mineral surface. The tetra-alkyl ammonium cation contains a positively charged cationic center in addition to hydrophobic hydrocarbon tails. Therefore, it is reasonable to hypothesize cation-π interactions between the exchangeable organic cations and the sorbed PAHs on clay surfaces. The mechanism of hydrophobic partition has been widely used for the sorption of PAHs to organoclays. However, structurally dependent, molecular sorptive forces with exchangeable ammonium cations in the sorption process are not well understood. In this study, we examined the sorption of a series of PAHs and chlorobenzenes to a reference montmorillonite exchanged by different quaternary ammonium cations. The selected PAHs and chlorobenzenes are paired for comparing sorptive affinities in terms of solute hydrophobicity and delocalized π-electron density. The relationship of solute aqueous solubility versus tetra-alkyl ammonium concentration in solution was also obtained to reflect the association affinity of ammonium-PAH complexes. The disparities shown in sorption and speciation between PAHs and chlorobenzenes were explored to delineate the mechanism of cation-π bonding that might operate specifically on PAHs. Results from solution-phase proton nuclear magnetic resonance (1H NMR) experiments also supported the hypothesis of cation-π bonding.
Experimental Section Materials. A reference montmorillonite (SWy-2; Source Clays Repository of The Clay Minerals Society) was used to prepare organoclays for sorption experiments. The physical/chemical properties of the reference clay, including chemical composition, cation exchange capacity, and surface charge (ζ potential) were given elsewhere (18–20). The clay was mixed with 1.0 L of 0.5 M NaCl aqueous solution for 24 h. The obtained suspension was then washed free of excess salts, and the 99.8% deuterium) or dimethyl sulfoxide (DMSO; >99.8% deuterium) at room temperature on a Bruker-DRX 500 MHz spectrometer (Germany). The spectrometer was run locked on the deuterated solvent with a pulse length of 3 µs at 90°, an acquisition time of 1.6 s, a VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Ratio of aqueous solubility (SW) to water solubility (SW0) vs concentration (CW) of TMA, TEA, and TBA shown separately for PAHs and chlorobenzenes. Error bars, in most cases smaller than the symbols, represent standard deviations calculated from four replicates. recycle delay of 1.0 s, and a time domain of 32 K. The chemical shifts (δ, ppm) were internally referenced to TMS.
Results and Discussion Batch Sorption. Sorption isotherms of different solutes to montmorillonite clays exchanged with TMA, TEA, TBA, and HDTMA are shown in Figure 1. The sorption isotherms were fitted to the Freundlich model: q ) KFC en
(1)
where q (mmol/kg) and Ce (mmol/L) are equilibrium sorbed and solution concentrations, respectively, KF (mmol1-n Ln/ kg) is the Freundlich affinity coefficient, and n (unitless) is the Freundlich linearity index. The fitting parameters are summarized in Table S1 of the Supporting Information. In general, the Freundlich model provides good fits. The clay-solution distribution coefficient, Kd (L/kg), for each individual sorption experiment was calculated, and the lowest and highest values are also listed in Table S1. The different sorbate-sorbent combinations exhibited varied degrees of 1112
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linearity, with n values from 0.77 (PHEN to TEA-clay) to 3.33 (NAPH to TMA-clay). However, no apparent trends of linearity can be summarized on the basis of either chemical properties (e.g., hydrophobicity, π-electron-donor property) or types of organic cations. Furthermore, it should be noted that generalization on sorption linearity in this study would be premature because the examined concentration ranges were narrow (Ce generally in only 1 order of magnitude). Nonetheless, a clear trend can still be observed that sorption isotherms of the three PAHs to TMA-clay are highly concaved-up (i.e., Freundlich n > 2.6, Table S1). The PAHs show 4- to more than 10-fold higher sorption than the chlorobenzenes paired for comparison to TEA-, TBA-, and HDTMA-clays (Figures 1b–d). The sorption trend of TMA-clay, however, becomes more complicated. Sorption of NAPH is close to or slightly lower than that of DCB when Ce is less than 0.03 mM but is higher by up to 6 times at larger Ce values; sorption of TeCB and PtCB is higher than that of PHEN and PYR, respectively, by up to 1 order of magnitude in the examined concentrations, while the order is likely to
be reversed at higher Ce values due to the concaved-up isotherms of PHEN and PYR (Figure 1a). Sorption of HOCs to the surface of organoclays is a complex and cumulative function of multiple processes. In this study, sorption isotherms were used mainly for comparing sorption affinities between PAHs and chlorobenzenes to probe potential sorption mechanism(s) that operate specifically on PAHs. However, the protocol would not work without normalizing for hydrophobicities of different solutes because hydrophobic effects often control the sorption of nonionic HOCs to sorbents containing fairly high OC contents. The OC content of organoclays prepared in this study varies between 7.25% and 16.05%. The hydrophobicity of organic solutes has been historically referenced by the solute n-octanol–water partition coefficient (KOW) and pure or subcooled liquid water solubility (SW or SWL). The predominance of hydrophobic effects in the sorption of low-polar HOCs (PAHs, PCBs, and methylated and chlorinated benzenes) to natural organic matter is demonstrated by the good linear free energy relationships between KOC and solute KOW or SW (24). To eliminate possible polar interactions (e.g., H-bonding) with the reference solvent and solute-dependent reference, the dilute solution in an inert hydrocarbon solvent, n-hexadecane, has been proposed as a universal thermodynamic reference state for justifying solute hydrophobicity (25–27). This reference state provides a common dispersion force field while affording the lowest possible opportunity for polar/polarizable, donor–acceptor interactions. Upon normalization for hydrophobic effects by the solute n-hexadecane-water partition coefficient (KHD), Borisover and Graber (27) compared sorption affinities among over 20 organic solutes of varied hydrophobicities and electronic structures for a better illustration of nondispersive sorptive interactions with SOM. More recently, Zhu and Pignatello (23) reported that a good correlation exists between the polyethylene-water distribution coefficient (Kd,PE) and KHD for a series of polar and nonpolar organic solutes. The results are understandable because the solute experiences almost the same chemical environment in polyethylene and nhexadecane dictated by the inert methylene structure in the process of sorption or partitioning. Herein, the PAHs and chlorobenzenes are paired (NAPH vs DCB, PHEN vs TeCB, and PYR vs PtCB) for comparing hydrophobicity using three independent parameters, KOW, KHD, and Kd,PE. Because PAHs and chlorobenzenes are considered nonpolar solutes, which invoke negligible polar interactions with the reference solvent, KOW and KHD are expected to be exchangeable when they are used as hydrophobicity indexes in the regression of free energy relationships. On the basis of the values in Table 1, it can be concluded that the three chlorobenzenes have very close hydrophobicities compared with their PAH counterparts. Therefore, the results from batch sorption experiments indicate that, besides hydrophobic effects, other mechanisms also significantly contributed to the sorption of PAHs to TEA-, TBA-, and HDTMA-clays. PAHs have strong permanent quadrupoles due to the highly delocalized π electrons and may interact with cations via electrostatic attractions (cation-π bonding). On the contrary, due to chlorine substitution, chlorobenzenes have only localized π electrons and thereby cannot invoke strong cation-π bonding. It appears that the enhanced sorptive affinities of PAHs over chlorobenzenes were due to cation-π interactions between PAHs and ammonium cations on the clay surface. Cation-π interactions have been found to play an important role in the formation of host–guest complexes (e.g., protein–ligand binding and ion channels) (28, 29). Since the demonstration of the cation-π bonding between the ammonium subunit of acetylcholine with the aromatic tryptophan side chain (30), the complexation of tetra-alkyl
ammonium cations with synthetic receptors, including calixarenes and cyclophane, has been intensively studied (28, 31–33). These studies revealed the significance of noncovalent binding forces, including cation-π bonding, CH-π bonding, and size-fit in ligand recognition processes. In aqueous solution, “soft” cations having larger sizes and lower hydration energies (e.g., Cs+ and Ag+) invoke stronger cation-π interactions compared with those “hard” cations (e.g., Na+) (17, 28). The “desolvation penalty” from the strong hydration of “hard” cations is highly energy-costly and seriously impairs possible cation-π interactions in aqueous solution. In this study, TMA is the hardest cation among all tested tetra-alkyl ammoniums, and the cation-π interactions between PAHs and TMA in aqueous solution are expected to be the weakest. This helps rationalize the different trends observed for the sorption of PAHs and chlorobenzenes between TMA-clay and other organoclays. At low solute loading, the sorption of PAHs and chlorobenzenes to TMA-clay would be mainly controlled by hydrophobic partition into micropores pillared by cations in the clay interlayer. At high solute loading, the more enhanced sorption of PAHs relative to chlorobenzenes is probably due to “conditioning” of the sorption sites. In previous studies (17), Zhu et al. attributed the concaved-up sorption isotherm of phenanthrene with Ag+-exchanged montmorillonite to the conditioning effect, where the clay interlayer is progressively opened up with increasing solute loading. In this study, PAH molecules sorbed on the clay surface solvate TMA cations, making microenvironments around the cations more hydrophobic and therefore favoring cation-π interactions with subsequently sorbed PAH molecules. To further evaluate the effect of cation solvation on sorption, PHEN and TeCB were sorbed from a pure nonpolar organic solvent of hexanes to freeze-dried, water-free clays exchanged with TMA (results shown in Figure S1, Supporting Information). Lacking in the driving force from hydrophobic effects, the sorption of both solutes in hexanes is significantly lower than that in aqueous solution. In agreement with the mechanism of cation-π bonding, PHEN shows much stronger sorption than TeCB to TMA-clay; that is, Kd varies between 42 and 60 L/kg for PHEN and between 1 and 16 L/kg for TeCB. In the absence of water, the “desolvation penalty” from cation hydration no longer exists, and the TMA cations exchanged on the clay surface thus invoke relatively strong cation-π interactions with PHEN. The exchange of the tetra-alkyl ammonium cations might also modify the hydration energy of the clay surface, therefore allowing direct interaction (π-H bonding) of the PAH molecules with protonated silicate sites on the clay surface. In fact, previous studies have suggested that π-electron-rich compounds (e.g., methylated benzenes) may interact with protonated silicate sites via π-H bonding (34). While reasonable to postulate, π-H bonding can be ruled out as an alternative for the enhanced sorption of PAHs after considering pH effects on sorption to TEA-clay (Figure S2, Supporting Information). Increasing the pH from 3.4 to 8.1 decreases the sorption of the π donor (PHEN) and non π donor (TeCB) to similar levels; that is, Kd lowered by 42% for PHEN and by 48% for TeCB. One possible reason for the observation is that protonation of the negatively charged clay surface groups slightly increases the overall hydrophobicity of the clay surface and thus facilitates hydrophobic effects in sorption. Aqueous Solubility Enhancement. Results of aqueous solubility enhancement of the PAHs and the chlorobenzenes by TMA, TEA, and TBA are shown in Figure 2. Clearly, the PAHs show much greater trends of solubility enhancement than the chlorobenzenes despite their close hydrophobicities. For instance, TMA, TEA, and TBA increase the aqueous solubility of PHEN up to 1.42 ( 0.06 times (measured as a VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. 1H NMR chemical shift (δ) of ammonium cations vs concentration (C) of π donors (NAPH, PHEN, PYR) and the non π donor (DCB) in chloroform-d. (a) TMA (N+CH3, 0.003 M). (b) TEA (N+CH2-, 0.01 M). (c) TBA (N+CH2-, 0.01 M). (d) HDTMA (N+CH3, 0.01 M). Lines are for visual clarity only. ratio to water solubility, with standard deviations calculated from four replicates), 3.0 ( 0.1 times, and 10.5 ( 0.2 times, respectively, and increase the aqueous solubility of TeCB up to 1.27 ( 0.02 times, 1.25 ( 0.06 times, and 1.73 ( 0.03 times, respectively. The maximum solubility enhancement by TBA is 24 ( 1 times for PYR and only 2.1 ( 0.2 times for PtCB. The results strongly suggest that the PAHs form cation-π complexes with the three nonsurfactant cations in aqueous solution. In contrast, the chlorobenzenes cannot form such complexes because they do not have delocalized π electrons due to chlorine substitution. The order of solubility enhancement of chlorobenzenes is consistent with hydrophobicities of the three nonsurfactant cations; that is, hydrophobicity increases with the length of the alkyl group (TBA > TEA > TMA), suggesting solvation by hydrophobic effects. However, the cation effect on solubility enhancement is much larger for the PAHs than for the chlorobenzenes, which cannot be fully ascribed to hydrophobic effects. The results can be explained by the inhibitive effect of cation hydration on cation-π interactions in aqueous solution. The larger the size of the cation (i.e., TBA relative to TMA), the smaller the “desolvation penalty” from cation hydration, and thereby the stronger the cation-π interaction. As a result, in aqueous solution, the intensity of cation-π interactions with PAHs follows an increasing order of TMA < TEA < TBA. Previous studies have shown that cation-π bonding in aqueous solution is indeed facilitated when the cation is surrounded by hydrophobic subunits of a water-soluble host (macrocyclic polyethers, cryptands, and cyclophanes) (28, 29). The solubility enhancement of PHEN and TeCB by HDTMA (Figure S3, Supporting Information) is consistent with the surfactant effect, where HOC molecules partition into surfactant micelles when the surfactant concentration is equal to or above the CMC. HDTMA is a typical cationic surfactant with a CMC of 1.2 mM (21). TeCB shows greater solubility enhancement than PHEN because TeCB is 1114
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slightly more hydrophobic and the examined concentrations of HDTMA are above the CMC. Solution-Phase 1H NMR. Molecular complexation between ammonium cations (TMA, TEA, TBA, and HDTMA) and π-electron-rich PAHs (NPAH, PHEN, and PYR) in chloroform is demonstrated by the 1H NMR spectrum. Placing a nucleus above or below the plane of an aromatic structure results in electronic shielding due to the “ring current” effect (35). Thus, one may expect upfield chemical shifts (shielding) of proton/carbon resonances of the alkyl groups of ammonium cations complexed with PAHs. Earlier studies explored 1H and 13C NMR upfield shifts of the phenyl and methyl groups of trimethylphenyl ammonium to demonstrate inclusion of the cation into the cavity of coneshaped p-sulfonatocalix[4]arene in water (31). The structure of cation-π complexes is optimized for the binding when the cation points perpendicularly to the plane of the aromatic ring. However, due to the steric effect, the PAH molecule intercalated between the alkyl groups of cation is expected to tilt in certain angles when interacting with the cationic center (N+). Consistent with the postulate of cation-π bonding, significant 1H NMR upfield shifts (up to 1.36 ppm) were observed for N+CH3 or N+CH2- of TMA, TEA, TBA, and HDTMA in mixtures of PAHs (NAPH, PHEN, and PYR) in chloroform (Figure 3). The shifts also positively correlate with the π-donor strength of PAH (PYR > PHEN > NAPH). In contrast, the upfield shifts of the ammonium cations in mixtures of DCB, a non-π-donor control, were negligible ( PHEN > NAPH). The results indicate that cation-π interactions also occur between PAHs and tetra-alkyl ammonium cations in polar solvents, but with lower interaction intensities due to cation solvation. Interestingly, for TBA, the upfield shifts dramatically decrease when the length between the probed alkyl group and the cationic center (N+) increases, that is, 0.78 ppm for N+CH2- and 0.27 ppm for -CH3 when mixed with PYR (Figure S5, Supporting Information). The same trend was also observed for TEA but with smaller differences between the two alkyl groups (i.e., 1.36 ppm for N+CH2- and 1.07 ppm for -CH3 with PYR). It is thus derived that the PAH molecule approaches and interacts with the cationic center (N+) via cation-π bonding rather than with the alkyl group via CH-π bonding when forming a complex with the ammonium cation. However, in situ spectroscopic evidence will still be needed in future studies to verify the cation-π interaction of PAH molecules with tetra-alkyl ammonium cations on the clay surface.
Acknowledgments We greatly appreciate Dr. Hui Li at Michigan State University for granting us the reference clay. This work was supported by the China National Science Foundation (grant 20637030 and grant 20777031) and Jiangsu Province Science Foundation (BK2006128).
Supporting Information Available Table S1 lists Freundlich fitting parameters of sorption isotherms. Figures S1-S5 present the sorption of PHEN and TeCB to water-free TMA clay in hexanes, pH effects on sorption of PHEN and TeCB to TEA clay in aqueous solution, aqueous solubility enhancement of PHEN and TeCB by HDTMA, 1H NMR chemical shifts of TEA versus solute concentration in DMSO, and 1H NMR chemical shifts of different moieties of TBA alkyl groups versus solute concentration in chloroform. This information is available free of charge via the Internet at http://pubs.acs.org.
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