Adsorptive Characteristics of the Siloxane Surfaces of Reduced

Stronger sorption of low-charge organoclays over high-charge organoclays suggests that the former owned more available internal siloxane surface (ISS)...
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Environ. Sci. Technol. 2008, 42, 7911–7917

Adsorptive Characteristics of the Siloxane Surfaces of Reduced-Charge Bentonites Saturated with Tetramethylammonium Cation XIUXIU RUAN, LIZHONG ZHU,* AND BAOLIANG CHEN Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, Zhejiang University, Hangzhou, Zhejiang 310028, China

Received April 14, 2008. Revised manuscript received August 11, 2008. Accepted August 12, 2008.

To elucidate interactions of neutral organic contaminants (NOCs) with siloxane surfaces (often referred to hydrophobic nanosites) found between cations in 2:1 phyllosilicates, adsorption of aliphatic and aromatic compounds onto both internal and external siloxane surfaces of tetramethylammonium-intercalated bentonite with a cation exchange capacity (CEC) of 108 cmol/ kg (108TMA) and its reduced-charge bentonite (CEC ) 65 cmol/ kg, 65TMA) were investigated. Reduction of the layer charge and saturation of bentonite interlayers with TMA+ modify the interlayer microenvironments, which dramatically promote adsorption of NOCs. Specific mechanisms (i.e., steric restriction and phenyl-effect) control the adsorption of NOCs onto internal siloxane surfaces of TMA+-bentonites from water. The adsorption sites of 108TMA can not provide sufficient space to accommodate NOCs, hence hindering adsorption. Adsorption mechanism on 65TMA varies with solute-loadings, from polarityselective at low loadings to aromaticity-preferable at high loadings. Significant contribution of phenyl-effect between adsorbedsolutes to aromatics adsorption on 65TMA is found. Solvent polarity effect on the aggregation of TMA+-bentonites and aniline adsorption demonstrated that the contribution of external siloxane surfaces to favor adsorption in n-hexane are actually exploited but generally omitted. These observations provide significant insights into distinguishing different uptake mechanisms as well as the potential means for the rational design of better organic sorbents.

Introduction Sorption of neutral organic contaminants (NOCs) onto modified bentonites has been of increasing concern in pollution control (1-13). Bentonites are popularly modified by intercalating organic cations (1-4, 8, 13) or weakly hydrated inorganic cations (e.g., K+) (14-18) into the interlayer in the manner of cation-exchange. Resultantly, the interlayer microenvironments are transited from a fully hydrated state to a partially hydrated state (9, 10). Adsorptive sites include organic medium, cationic head, and siloxane surfaces between cations (4, 8, 9, 13-18). The availability of * Corresponding author phone: 86-571-88273733; fax: 86-57188273733; e-mail: [email protected]. 10.1021/es801034h CCC: $40.75

Published on Web 10/03/2008

 2008 American Chemical Society

siloxane surfaces in the interlayer of bentonite is largely regulated by substrate surface charge density, exchangeable cation type, and sorbate structure (11, 12, 19, 20). Recently, the accessible siloxane surface of bentonite was exploited by reduction of the layer charge, and thus enhanced adsorption of water-soluble organic compounds to organoclay (19). Mechanism and its influential factors that control sorption of NOCs onto siloxane surfaces, particularly neutral hydrophobic regions (often referred to as hydrophobic nanosites) found between cations in low-charge 2:1 phyllosilicates are of major interest for various applications in wastewater treatment, soil remediation, hazardous waste landfills, and so on (1-13). Stronger sorption of low-charge organoclays over highcharge organoclays suggests that the former owned more available internal siloxane surface (ISS) in the interlayer of clay (21-24). High compatibility of aromatics over aliphatics with tetramethylammonium(TMA+)-exchanged clays was explained by additional interaction of aromatic ring with the ISS (13, 24-26). Contrarily, extremely weak sorption of aromatics to TMA-clay in water was also reported and illustrated by the unavailability of the ISS to accommodate solute (i.e., steric restriction (25-27),). Therefore, we hypothesize that interaction mechanisms of NOCs with siloxane surfaces can be regulated to favor adsorption by reduction of the layer charge and saturation of bentonite interlayers with TMA+. The aggregation of clay platelets and the formation of quasicrystal influence the accessibility of ISS and the availability of external siloxane surface (ESS) which consists of the edges of clay platelets and the outer surfaces of some open silicate sheets. The enhancement of nitroaromatic compounds sorption onto the interlayer of K+-smectite as KCl ionic strength increased was ascribed to the decrease of colloid particle sizes and the formation of better-ordered K-clay quasicrystals (14). Due to serious suppression by water, the contribution of ESS to the whole adsorption of solutes from water to organobentonites has been usually omitted in previous reports (13, 26). Similarly, the more effective removal of phenol by TMA-clay from n-hexane over water was supposedly explained by weakening hydration of TMA+ on ISS to lower the steric restriction (26), but the role of ESS was generally neglected. The aggregation states of clay platelets in various solvents and their effect on sorption of organic contaminants to TMA-bentonite are limited. The main objective of this study is to elucidate adsorption mechanisms of NOCs onto siloxane surfaces (hydrophobic nanosites) found between cations in 2:1 phyllosilicates. To this purpose, the ISS was exposed by reduction of the layer charge of bentonite with a cation exchange capacity of 108 cmol/kg to 65 cmol/kg and intercalation of the bentonite interlayer with TMA+, and the ESS was gradually exploited in nopolar solvents. Aromatic and aliphatic compounds were employed to probe adsorptive characteristics of the ISS and ESS. The modified bentonites were characterized by organic carbon contents, BET-N2 surface area, themogravimetric and derivative themogravimetric analysis (TG-DTG), and X-ray diffraction (XRD). Aniline adsorption and XRD of modifiedbentonites in water, methanol, dichloromethane, and nhexane with decreasing solvent polarity were analyzed. The selected XRD patterns and FTIR spectra of TMA-bentonites with and without the presence of organic contaminant were examined to further elucidate adsorption mechanisms of TMA-bentonites with NOCs. VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Selected Properties of the Tested Organic Compoundsa

a MW: molecular weight, g/mol; Sw: aqueous solubility at 25 °C, mmol/L; Kow: octanol-water partition coefficient. b The molecular dimensions is demonstrated as a × b × c, where a, b and c are the molecular length, width and thickness, respectively; and their unit is nanometer (nm). For details see Supporting Information Figure S-1.

Materials and Methods Materials. The raw material was a calcium bentonite (montmorillonite content >95%) from Inner-Mongolia, China. The total interlayer charge and cation exchange capacity (CEC) were 0.82 per formula unit and 108 cmol/kg, respectively. Lithium chloride and tetramethylammonium chloride were selected to modify bentonite. Aromatics (aniline, phenol and toluene) and aliphatics (cyclohexanone, cyclohexanol, and n-hexanol) with similar dimensions were used as model sorbates. Their selected properties and three dimensions are presented in Table 1 and Supporting Information Figure S-1, respectively. Four solvents with different polar index, i.e., n-hexane (0.0), CH2Cl2 (3.1), methanol (5.1), and water (10.2) were selected. All reagents were of analytical grade and used as received. Reduced-Charge Bentonite and TMA-Bentonites. Reduced-charge bentonite was prepared according to the Hofmann-Klemen effect (28). In brief, the original bentonite (108 cmol/kg) was saturated with 1.0 mol/L LiCl solution three times, and then washed with distilled water and airdried, and ground through 100-mesh. The product is denoted as 108Li-Bent. After heating at 130 °C for 24 h, the CEC of 108Li-Bent was reduced to 65 cmol/kg determined by an ion-exchange method (29). And the resultant named as 65Libent. 65Li-bent, and 108Li-bent were saturated with TMA+ to synthesize organoclay. Briefly, 5 g of the sample was dispersed in 200 mL of water, and a given amount of TMA+ was added to satisfy 100% CEC. The suspension was stirred at 50 °C for 2 h and aged at 60 °C for 10 h. After filtration, the residue was washed with distilled water five times, ovendried at 80 °C, and ground through 100-mesh. The organoclays are denoted as 65TMA and 108TMA, respectively. The exchanged extent of Li+ by TMA+ was evaluated by organic carbon content of 65TMA and 108TMA (see Table 2). Characterization of Modified-Bentonites. Oven-dried samples (i.e., 65Li-bent, 108Li-bent, 65TMA and 108TMA) were characterized by organic carbon contents (foc), X-ray diffraction (XRD) patterns, surface characteristics (BET-N2) and themogravimetric and derivative themogravimetric analysis (TG-DTG). Basal spacings (d001) were determined according to the peaks in XRD patterns. The total surface 7912

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areas of samples were calculated by Multipoints BET method. The internal surface area of the platelets and the external surface area of the particles consisting of the stacking platelets were estimated by t-method (30-32). XRD patterns of 65TMA and 108TMA in water-, methanol-, dichloromethane-, and n-hexane-wet states were recorded. The selected XRD patterns and FTIR spectra of TMA-bentonite with and without the presence of organic contaminant were also analyzed. The characterization methods are detailed in the Supporting Information. Batch Sorption Experiments. Sorption was conducted in 25 mL centrifuge tubes. After adding 0.1 g of sorbent and 20 mL of aqueous solution containing a series of concentrations of solute, the tubes were shaken at 25 °C and 150 rpm for 4 h on a gyratory shaker to apparent equilibrium. The solution pH values were controlled at ∼7.0, so the ionization of aniline (pKa ) 4.58) and phenol (9.95) was negligible. After centrifugation, solute concentrations in the supernatant were determined. Aniline and phenol were determined by UV spectrophotometer (Shimadzu UV-2450) at 230 and 270 nm, respectively. Toluene, extracted by carbon disulphide, was analyzed by GC-FID (Shimadzu GC-14B) with capillary column (30 m × 0.32 mm × 0.25 µm). The recovery rates were 90-98%, but sorption data presented were not corrected by the recovery rates. Cyclohexanone, cyclohexanol, and n-hexanol were directly analyzed by GC-FID with a packed PEG-2000 column. Aniline sorption to 65TMA and 108TMA from n-hexane, dichloromethane, and methanol was conducted. The aniline concentrations in organic solvents were determined by a HPLC-UV (Hitachi 7200) with a XDB-C18 column. The adsorbed-amount was calculated by the difference between initial and equilibrium concentration. All samples were run in triplicate.

Results and Discussion Characterization of Modified-Bentonites. The organic carbon contents (foc), surface areas (SA), and basal spacings (d001) of Li-bentonites and TMA-bentonites are presented in Table 2. The respective foc of 65TMA and 108TMA are 2.97% and 4.97%, approaching 100% CEC of 65Li-bent and 108Libent. The d001 values of TMA-bentonites (∼1.40 nm) are larger

TABLE 2. Structural Characteristics of Li-Bentonite and TMA-Bentonite Samples in Air-dry State parameter organic carbon content (foc), % CEC occupied by TMA+a, % basal spacing (d001), nm BET-N2 surface areac, m2/g external surface areac, m2/g internal surface areac, m2/g available internal siloxane surface per charged, nm2/charge estimated total internal surface areae, m2/g temperature for maximal water weight lossf, °C water contentf, %

65Li-bent

108Li-bent

65TMA

108TMA

0 1.23 63.8 44.4 19.4 0.052

0 1.25 53.9 41.3 12.6 0.020

63. 7(131) 9.05

52.6(135) 15.38

2.97 99 1.39 199 53.9 145 0.389 328 60.6 6.29

4.97 102 1.41 138 59.6 78.8 0.126 374 62.8 8.12

b

b

a The percentage of CEC occupied by TMA+ was calculated by converting foc to the adsorbed-amount of TMA+ (cmol/kg) and then divided by corresponding CEC value. b The foc values for 65Li-bent and 108Li-bent are less than 0.04%. c BET-N2 surface area is equal to the sum of external and internal surface areas, which were calculated by t-method. d Available internal siloxane surface area per charge) internal surface area/ (CEC × NA), where CEC is the cation exchange capacity; NA is Avogadro’s number, 6.022 × 1023. e The estimated total internal surface area ) measurable internal surface area + the occupied surface area by adsorbed-TMA+. Noting that the total internal surface area was estimated upon one side of the interlayer. f The temperatures for maximal water weight loss and water contents were detected upon DTG data in Supporting Information Figure S-3.

than those of Li-bentonites (∼1.24 nm), indicating that TMA+ was intercalated into the interlayer space of bentonite. The interlayer spacings are calculated by subtracting the measured d001 by the unit-cell thickness (0.96 nm), i.e., 0.43 nm for 65TMA and 0.45 nm for 108TMA, which are smaller than the size of TMA+ (Supporting Information Figure S-2(a)), suggesting that some hydrogen atoms of TMA+ insert into the silicate sheets and form a flat-monolayer in the interlayer (25). The DTG peaks (Supporting Information Figure S-3) at low temperature ( 108TMA (78.8 m2/g). These observations suggest that the availability of internal siloxane surface can be regulated by reducing layer charge and then intercalating bentonite interlayer with TMA+. The available ISS per charge, presented in Table 2, follows the magnitude order of 65TMA (0.389 nm2/charge) > 108TMA (0.126) > 65Li-Bent (0.052) > 108Li-Bent (0.020). The aggregation states of 65TMA and 108TMA in waterwet are characterized by XRD pattern (see Figure 1). Two diffraction peaks are observed for 65TMA at 2θ ) ∼1.75° and VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Sorption isotherms of toluene (O, b), aniline (0, 9) and phenol (∆, 2) onto 65TMA and 108TMA from water. Open symbols for 65TMA and solid symbols for 108TMA. Each data point is the mean of three replicates and the error bar is its standard deviation. ∼6.0°, and 108TMA at ∼1.20° and ∼6.0°. The XRD peaks at 2θ )∼6.0° are assigned to the diffraction of clay platelets (quasi-crystal), and the peaks at low-angle are attributed to the diffraction of clay particles (aggregates) typically consisting of stacks of parallel platelets (14). Therefore, for 65TMA many pairs of clay platelets in solution are roughly parallel to each other with center to center spacings of ∼5.0 nm (2θ ) 1.75°), which is smaller than that of 108TMA (∼7.5 nm, 2θ ) 1.20°), suggesting that the reduction of layer charge also tends to reduce the aggregation of clay platelets into particles in water. Based on the observed structures, the interlayer microenvironments of 65TMA and 108TMA in air-dry and waterwet states are schematized in three dimensions in Supporting Information Figure S-2. At air-dry state, TMA+ act just as pillars to open interlayer spacings and concurrently the siloxane surfaces (hydrophobic nanosites) between pillars functions potential adsorptive sites. For water-wet samples, the available ISS are reduced due to the hydrated TMA+ occupying more siloxane surface area. The available ISS are exploited significantly through reducing the layer charge of bentonite. Rationally, the interlayer microenvironments and aggregation states of TMA-bentonites will play a critical role in adsorption of NOCs to siloxane surfaces. Adsorption Mechanisms on TMA-Bentonites in Water. Sorption of aromatic compounds (toluene, aniline, and phenol) and aliphatic compounds (cyclohexanone, cyclohexanol, and n-hexanol) with similar molecular sizes onto 65TMA and 108TMA in water is presented in Figures 2 and 3. After intercalating with TMA+, sorption of aniline and phenol on 108TMA is still similar with that on 108Libentonite, whereas sorption of 65TMA is greatly enhanced in comparison with 108Li-bent and 65Li-bent (see Supporting Information Figure S-4). The steric restriction and phenyleffect in the nanointerlayer space are employed to elucidate the distinctive adsorption characteristics on TMA-bentonites. From Figure 2, sorption of toluene, aniline, and phenol to 108TMA is significantly lower than their monolayer internal surface coverage (∼22 mg/g) predicted in Supporting Information Table S-1. The bare siloxane surfaces between hydrated-TMA+ pillars of 108TMA can not provide sufficient space to accommodate the solutes (i.e., steric restriction) due to the available siloxane surface per charge (0.126 nm2/ charge) less than the solute molecular size (see Table 1). 7914

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FIGURE 3. Sorption isotherms of aromatic compounds and aliphatic compounds onto 65TMA from water. (a) Q-Ce plots for the whole tested concentration ranges; and (b) Q-Ce/Sw plots for low concentration ranges, where Ce/Sw is the ratio of equilibrium concentration to solute aqueous solubility (Sw). Each data point is the mean of three replicates and the error bar is its standard deviation. Sorption isotherms of aromatic compounds to 65TMA exhibit a sigmoid shape at low to medium concentration range and then demonstrate Freundlich-type shape at high concentration range (see insert in Figure 2, regression data not shown). The adsorptive intensities of aromatics increase with solute concentrations increasing and then the adsorbedamounts of solutes significantly exceed the theoretical monolayer interior surface coverage (∼40 mg/g, see Supporting Information Table S-1). For aliphatic compounds, sorption isotherms on 65TMA are Langmuir-type shape and reach plateau at high concentrations (Figure 3(a)). Stronger sorption of aromatics over aliphatics is observed at high solute-loadings, reported frequently in the literature (23, 25), suggesting that the 65TMA is an effective adsorbent for water-soluble NOCs and especially selective for aromatic compounds. In comparison with 108TMA, sorption on 65TMA is dramatically enhanced (Figure 2), i.e., 10-20 times for toluene, 50-1000 times for aniline, and 10-25 times for phenol. This cannot be explained by surface coverage of ISS which is determined by the internal SAs between 65TMA and 108TMA (145 vs 78.8 m2/g). There are three potential reasons to illustrate this observation: (i) The available ISS per charge are 0.126 and 0.39 nm2/charge, respectively, for 108TMA and 65TMA, and the latter is large enough to retain

aromatic contaminants to abate the steric restriction which occurs to 108TMA. (ii) Specific interaction of phenyl group with the ISS of 65TMA and the subsequent phenyl-phenyl interaction between adsorbed aromatics are involved. (iii) For 65TMA, the disaggregation of particles into platelets increases the accessibility the ISS (derived from XRD data). Therefore, it is practical to exploit the availability of ISS to enhance sorption of NOCs in water by reducing layer charge and saturation with TMA+. The abscissas of the isotherms of aromatics and aliphatics are described as the relative equilibrium concentrations (Ce/ Sw) to eliminate solute hydrophobic effects (Figure 3(b)). Adsorption on 65TMA with the tested solutes at low soluteloadings exhibits highly selectivity of solute polarity. Cyclohexanone with the largest dipole moment (D ) 3.08) exhibits the greatest sorption at Ce/Sw < 8.0 × 10-4; while toluene with the lowest polarity (D ) 0.36) shows the weakest. The rest of solutes (phenol, aniline, n-hexanol and cyclohexanol) owning similar dipole moment (1.45∼1.86) demonstrate almost identical sorption at low Ce/Sw. At low solute-loadings, high sorption of TMA+-clay with chlorobenzenes (high polarity) over PAHs (low polarity) was observed in previous report (13), although they own the identical hydrophobicity. This cannot be explained by their hydrophobic partition into micropores pillared by cations in the clay interlayer. The current study and previous result suggest that at low soluteloading, the initial sorption of organic compounds on 65TMA is characterized by solute polarity-selective rather than aromaticity-preferable as generally assumed (25). The subsequent adsorption of aromatics to 65TMA is sharply increased at moderate solute-loadings, which is attributed to the direct interaction between the bare siloxane surfaces and aromatic ring (25, 26). At high solute concentrations, adsorbed-amount of aromatics over their monolayer internal surface coverage (Figure 1) indicates that phenyl-phenyl interactions of adsorbed-solutes are involved across high solute-loadings. As we know, the changing of adsorption mechanisms with solute-loadings in interlayer microenvironments of TMA-bentonite is systematically elucidated for the first time. Interestingly, the distinctive adsorption of phenol and cyclohexanol with almost identical dimensions to 65TMA, presented in Figure 3(a), further proves that the phenyl group plays a critical role in solute-adsorption onto the ISS. Due to the lack of phenyl-effect, the respective saturation sorption of n-hexanol, cyclohexanone, and cyclohexanol to 65TMA are 15.8, 9.2, and 5.8 mg/g, which are significantly lower than the predicted monolayer internal surface coverage (46.4, 45.2, and 44.9 mg/g, see Supporting Information Table S-1). This phenomenon may be attributed to both steric restrictions and the lack of specific interactions with the siloxane surfaces of 65TMA (as expected for the phenylic compounds) although the available siloxane surfaces are large enough to accommodate aliphatics. In contrast, the strong specificinteraction of phenyl group with siloxane surfaces for aromatics favors the entire elimination of the steric restriction in the interlayer of 65TMA. Additionally, adsorption are favored to the solute with more hydrophobicity, e.g., toluene > aniline > phenol. To further elucidate the interaction of TMA-bentonites with organic pollutant, the selected XRD patterns and FTIR spectra of TMA-bentonites with and without the presence of organic contaminant are presented in Supporting Information Figures S-5 and S-6, respectively. The N-H stretching band (3439, 3376 cm-1) and C-H stretching band (3038 cm-1) of aniline are observed in the IR spectra of 65TMA with the presence of aniline, indicating the loading of aniline onto 65TMA (34). However, the characteristic bands of aniline are not observed for 108TMA with the presence of aniline. Comparing to the IR spectra of pure aniline, the N-H and

FIGURE 4. Sorption isotherms of aniline onto 65TMA and 108TMA in n-hexane (O), CH2Cl2 ()), methanol (∆) and water (0) solution. Each data point is the mean of three replicates and the error bar is its standard deviation. C-H bands of aniline on 65TMA are shifted to higher frequencies, which imply that the adsorbed-aniline molecules are limited to the nanoconfined interlayer environment. The intercalation of organic contaminants into the interlayer of organoclay with large organic cation detected by XRD in several papers was evidenced with the steep increase of basal spacing (35), which resulted from a rearrangement of the alkyl chains from tilted alkyl chains to erect. In the current study, however, the interlayer spacings (d001) of TMAbentonite platelets loading with organic pollutants are almost identical to the bulk TMA-bentonites which do not provide direct evidence for the intercalation because there is no rearrangement of the small cation (TMA+). Solvent Effect on Adsorption of Organic Solute on TMABentonites. In Figure 4, aniline sorption to 65TMA and 108TMA in four solvents with increasing polarity index, i.e., n-hexane (0), CH2Cl2 (3.1), methanol (5.1), and water (10.2) are compared to distinguish the roles of ISS and ESS in adsorption. Aniline is selected as a model solute to exclude different hydrophobic effects in various solvents due to its sufficiently low hydrophobicity (i.e., extremely high Cs and low Kow). In water (high polarity), strong sorption of aniline on 65TMA occurs at ISS due to the ESS suppressed by water molecules (13, 26); whereas little sorption on 108TMA is attributed to the steric restriction of the ISS. In moderate polar solvents (CH2Cl2 and CH3OH), aniline sorption to 65TMA and 108TMA is prohibited completely (Figure 4), suggesting that both the ISS and ESS of TMA-bentonites are preferably covered by these solvents, and then implying that the polarity of siloxane surfaces is at moderate extent. Therefore, the siloxane planes in bentonite are readily wet by moderate polar solvents. Similar observation was reported VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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that the siloxane plane in clay was more readily wet by EGME (ethylene glycol monomethyl ether, a polar liquid) than by water to inhibit adsorption (36). In a nonpolar solvent (n-hexane), aniline sorption on 108TMA is extremely enhanced in comparison with polar solvent (Figure 4(b)), and shows Langmuir-type shape (regression coefficient R2 ) 0.985, and the saturated sorption amount Qmax ) 28.18 mg/g). Effective removal of phenol by TMA-clay from n-hexane in comparison with water was reported and explained by the hypothesis that the steric restriction in the interlayer microenvironment of TMA-clay is largely eliminated due to less hydrated of TMA+ in n-hexane solvent (26), but the role of external surface of TMA-clay in n-hexane has been supposedly omitted (13, 26). In the current study, the maximum of available ISS per charge of 108TMA in n-hexane (0.126 nm2/charge), predicted according to the steric boundary condition known from the BET measurement under vacuum (the solvated state of TMA+ is negligible), which is still lower than the molecular size of aniline (planar area of 0.54 nm2 and side area of 0.27 nm2), suggesting that the steric restriction in the interlayer microenvironment of 108TMA should still remain in n-hexane. Furthermore, the saturated sorption amount (28.18 mg/g) based on Langmuir regression parameter exceeds the monolayer ESS coverage (17.17 mg/g) and the monolayer ISS coverage (22.70 mg/g, see Supporting Information Table S-1). For 65TMA, sorption of aniline is higher from n-hexane than from water at relative low concentrations (Figure 4(a)). These observations prove our hypothesis that the adsorptive sites on TMA-bentonites in n-hexane occur at the ISS and the ESS, which are potentially exploited in a nonpolar solvent. The aggregations of 65TMA and 108TMA in four solventwet states are deduced from XRD pattern, presented in Figure 1. In comparison with water-wet samples, the basal spacings (d001) of n-hexane-wet samples decreases according to XRD patterns, supporting that the TMA+ cations are less hydrated in n-hexane as generally assumed. The d001 values of 65TMA are identical (1.38 nm) to that of 108TMA, and the interlayer spacings (0.42 nm) are equal to the size of methylene group (-CH2, 0.42 nm) of n-hexane molecules. Interestingly, the diffraction peak assigning to clay particles (aggregates) consisting of the stacks of parallel platelets become weak with decreasing solvent polarity (water f n-hexane), and almost disappears for n-hexane-wet sample. Simutaneously, the diffraction peak of clay platelets at 2θ )∼6.0° is gradually sharpened. These observations strongly indicate that the clay particles are gradually disaggregated into clay platelets with better-ordered structure (formation of quasi-crystals) with reducing solvent polarity. Similar disaggregating was observed for K+-smectite suspensions with increasing KCl concentrations from 0.01 to 0.30 mol/L in aqueous solution, and the presence of KCl above 0.20 mol/L caused disappearance of the diffraction peak (2θ ) ∼ 2.2°, d001 ) 4.0 nm) of clay particles (14). Rationally, the disaggregation of clay particles into platelets in n-hexane makes the external siloxane surfaces of TMA-bentonite entirely available for aniline. In summary, the availability of siloxane surfaces in bentonites is significantly exploited to favor solute adsorption by reducing layer charge and subsequently saturating with TMA+ (65TMA). Significant contribution of a phenyl-effect between adsorbed-solutes to the adsorption of aromatics on 65TMA is found. The adsorption of organic compounds on the modified-bentonites in different solvent is concurrently regulated by the interlayer microenvironments of clay platelets and the external siloxane surfaces of clay particles actually exploited in nonpolar solvents. 7916

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Acknowledgments We are highly grateful to four anonymous reviewers for their valuable comments and constructive suggestions. This work is supported by National Natural Science Foundation of China (No. 20737002, 50378081, 20577041) and the Provincial Science and Technology Project of Zhejiang (No. 2006C13058).

Supporting Information Available Detailed method for characterization of Modified-bentonite. Table presenting maximal surface coverage by monolayer arrangement (Table S-1). Figures showing the schematic of molecular dimensions of sorbates (Figure S-1), the proposed microenvironments (Figure S-2), TG-DTG data of 65Li-Bent, 108Li-Bent, 65TMA and 108TMA (Figure S-3), and Q-Ce plots of aniline and phenol sorption onto Li-bentonites and TMAbentonites from water (Figure S-4), the selected XRD patterns (Figure S-5) and FTIR (Figure S-6) of TMA-bentonites with and without the presence of organic contaminant. This material is available free of charge via the Internet at http:// pubs.acs.org.

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