Environ. Sci. Technol. 2009, 43, 6171–6176
Role of Interlayer Hydration in Lincomycin Sorption by Smectite Clays CUIPING WANG,† YUNJIE DING,† BRIAN J. TEPPEN,† STEPHEN A. BOYD,† C U N Y I S O N G , ‡ A N D H U I L I * ,† Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, and Department of Environmental Engineering, University of Science and Technology Beijing, China, 100083
Received March 11, 2009. Revised manuscript received June 18, 2009. Accepted July 6, 2009.
Lincomycin, an antibiotic widely administered as a veterinary medicine, is frequently detected in water. Little is known about the soil-water distribution of lincomycin despite the fact that this is a major determinant of its environmental fate and potential for exposure. Cation exchange was found to be the primary mechanism responsible for lincomycin sorption by soil clay minerals. This was evidenced by pH-dependent sorption, and competition with inorganic cations for sorptive sites. As solution pH increased, lincomycin sorption decreased. The extent of reduction was consistent with the decrease in cationic lincomycin species in solution. The presence of Ca2+ in solution diminished lincomycin sorption. Clay interlayer hydration status strongly influenced lincomycin adsorption. Smectites with the charge deficit from isomorphic substitution in tetrahedral layers (i.e., saponite) manifest a less hydrated interlayer environment resulting in greater sorption than that by octahedrally substituted clays (i.e., montmorillonite). Strongly hydrated exchangeable cations resulted in a more hydrated clay interlayer environment reducing sorption in the order of Ca< K- < Cs-smectite. X-ray diffraction revealed that lincomycin was intercalated in smectite clay interlayers. Sorption capacity was limited by clay surface area rather than by cation exchange capacity. Smectite interlayer hydration was shown to be a major, yet previously unrecognized, factor influencing the cation exchange process of lincomycin on aluminosilicate mineral surfaces.
Introduction Lincomycin, a lincosamide antibiotic, is one of the most widely used antibiotics administered to animals against grampositive bacteria. Lincosamide antibiotics are reported to be recalcitrant to degradation in the environment (1-3). This persistence leads to the frequent detection of lincomycin in surface waters of North America and Europe (4-6). Kolpin et al. (5) reported that lincomycin was present in 19% of 104 water samples collected from streams in 30 states of the U.S. with concentrations up to 730 ng L-1. In Italy lincomycin was found in all samples collected from eight selected sampling sites along the River Po (4). In Canada 92% of 125 samples from the Grand River, Ontario were found to contain * Corresponding author e-mail:
[email protected]. † Michigan State University. ‡ University of Science and Technology Beijing. 10.1021/es900760m CCC: $40.75
Published on Web 07/17/2009
2009 American Chemical Society
lincomycin with concentrations ranging from 0.2 to 355 ng L-1, attributed to inputs from agricultural husbandry (6). Although lincomycin is frequently found in water, little is known about its environmental fate, including immobilization in soils and exposure of at-risk populations. Sorption by soils is a major process influencing transport, fate, and bioavailability of antibiotics in the environment. A widely accepted view of sorption of neutral nonpolar organic compounds is that soil organic matter (SOM) functions as the predominant sorptive domain whereas soil mineral fractions play a relatively minor role ((7)and references therein). However, this view inadequately describes sorption of many antibiotics because these bioactive agents usually contain several polar and/or ionic functional groups, which can develop multiple interaction mechanisms with both SOM and soil minerals, especially clays (8-14). Among soil minerals, 2:1 aluminosilicate smectite clays are especially important as sorptive components owing to their wide distribution in soils, high cation exchange capacity (CEC), large surface areas (∼800 m2 g-1), and reversible interlayer expandability. Lincomycin, an organic base with a pKa value of 7.6 (6), manifests both cationic and neutral species in natural waters. Plausibly, both species interact with soils; the organic cation could replace inorganic cations associated with cation exchange sites of SOM and clays, and the neutral species could partition into SOM. Previous studies of organic bases such as aromatic amines and quinoline demonstrated the predominant role of cation exchange in overall sorption even at high soil-solution pH (e.g., pH > pKa) (15-17). Owing to high CEC and interlayer accessibility, smectite clays are expected to be a major soil mineral domain for retention of organic cations via cation exchange reactions, and the cation selectivity of clay mineral sorptive (cation exchange) sites often favors organic cations (18-21). The driving forces for the high selectivity of organic cations for mineral surfaces are attributed to the combination of electrostatic interactions, van der Waals forces, and hydrophobic incompatibility of organic compounds with the aqueous phase. The classic view of cation exchange holds that selectivity is determined primarily by electrostatic forces between negative charges within the clay mineral lattice and positive charges of exchangeable cations; these interactions are controlled by the size of hydrated exchangeable cations and the distance between the positive- and negative-charged centers (22, 23). For organic cations such as alkylammoniums, the unfavorable dissolution of the hydrophobic moieties in water facilitates their adsorption by mineral phases (24). Recently, Teppen and Aggrawal (25) argued that cation exchange selectivity is not simply determined by electrostatic forces, but that affinity depends largely on the hydration energies of the exchanged cations, and the differential state of water in the clay interlayers vs bulk solution. This is supported by the fact that greater sorption is observed for more hydrophobic alkylammonium compounds by smectite clays from water while the reverse order is observed for sorption from organic solvents,e.g., dimethyl sulfoxide and acetonitrile (26). During cation exchange, the more weakly hydrated cations (i.e., organic cations vs inorganic cations) pay a smaller energy penalty for partial dehydration in the subaqueous clay interlayers where many properties of water molecules are restricted compared to those properties in bulk solution. Furthermore, for organic cations containing hydrophobic moieties, additional energy is gained by removing these moieties from bulk solution and depositing them in partially dehydrated clay interlayers. VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6171
TABLE 1. Characteristics of Clay Minerals Used in This Study clay
mineralogy
cation exchange capacity (cmolc kg-1)
tetrahedral charge (%)
surface area (m2 g-1)
surface charge density (µmolc m-2)
SWy-2 SapCa-2 SAz-1 KGa-1
montmorillonite dioctahedral saponite trioctahedral montmorillonite dioctahedral kaolinite
83.6 97.4 130 8.7
3.6 100 ∼12
766 750 768 8.4
1.09 1.30 1.69
The objectives of this study were to understand the mechanism responsible for lincomycin sorption by clay minerals from aqueous suspension, and to evaluate the influence of hydration status of smectite clay interlayers on sorption of lincomycin. The hydration status of the clay interlayer environment is hypothesized to be a determinant of the adsorption of cationic organic compounds by clays. To test this hypothesis, lincomycin sorption from water was measured for clay minerals saturated with exchangeable cations possessing different hydration propensities (i.e., Ca2+, K+, and Cs+) at varying pH and ionic strength. The smectite clays used included those with isomorphic substitution occurring in tetrahedral vs octahedral layers; together these factors provide a means to systematically alter the hydration status of the clay interlayers. X-ray diffraction (XRD) patterns of clay films were used to quantify interlayer distances which providefurtherinsightintothelincomycinsorptionmechanism.
Experimental Section Chemicals and Sorbents. Lincomycin was purchased from Sigma-Aldrich Chemical Co. with a reported purity of >95%. The water solubility of lincomycin is 927 mg L-1, and the octanol/water partition coefficient (log Kow) is 0.56. Reference smectites (SWy-2, SAz-1, SapCa-2) and kaolinite (KGa-1) were purchased from the Source Clays Repository of the Clay Minerals Society. These clays were chosen to provide a range of surface charge densities, differences of origin of charge deficit, and structure of octahedral sheet (dioctahedral vs trioctahedral) (Table 1). The clay-sized fractions (4.0 was coincident with the decrease in the organic cation fractions present in the aqueous phase, implicating cationic lincomycin as the predominant species for sorption. That is, cation exchange is the primary mechanism responsible for lincomycin sorption by clays. The comparably low sorption at pH 3.2 is due plausibly to the competition of hydronium ions for adsorptive sites. In addition, the dissolution of the clay minerals under acidic conditions may release Al3+ which could occupy the cation exchange sites and thereby inhibit sorption of lincomycin. It was noted that ∼30% of Ca2+ associated with clay was released into solution at pH 3.2, whereas 7). Concentration of Released Cation. Sorption of lincomycin by cation exchange should be accompanied by the release of inorganic cations associated with cation exchange sites of the clay surfaces, e.g., Ca2+ for homoionic Ca-SWy-2. As such, the addition of the cationic species released via cation exchange (e.g., Ca2+) at increasingly higher concentrations with cationic lincomycin should cause a shift in the cation exchange equilibrium toward reduced sorption of lincomycin. Lincomycin sorption by Ca-SWy-2 was measured at pH 6.0 in 0.01 and 0.05 M CaCl2, and compared with sorption from solutions without added CaCl2 (Figure 2). The presence of CaCl2 significantly inhibited lincomycin sorption. The sorption coefficients decreased to 263 and 191 L kg-1 when lincomycin sorption was from the 0.01 and 0.05 M
FIGURE 3. Sorption isotherms of lincomycin by homoionic Cs-, K-, and Ca-smectites at pH ) 6.0. CaCl2 solutions, respectively, whereas the sorption coefficient was 3145 L kg-1 in the absence of added CaCl2 in the solution. For the sorption isotherms reported here the maximum aqueous lincomycin equilibrium concentration was 1.9 µmol L-1, so even 0.01 M Ca2+ was more than 5000 times the lincomycin concentration in aqueous solution. Such large discrepancies between Ca2+ and lincomycin concentrations in the solution phase resulted in a shift of the cation exchange equilibration toward less release of Ca2+ from clay surfaces, thereby manifesting significantly reduced sorption for lincomycin, in accordance with Le Chatelier’s principle. Lincomycin sorption from 0.01 and 0.05 M CaCl2 solutions at pH 6.0 was even less than its sorption at pH 8.7 without added CaCl2 (Figure 1A). The presence of relatively high but still environmentally relevant concentrations of inorganic cations such as Ca2+ could effectively compete with trace levels of organic cations for sorptive sites on mineral surfaces, reducing the contribution of cation exchange reactions to sorption of organic cations, e.g., lincomycin. Effects of Exchangeable Cations. To examine whether interlayer hydration influenced sorption of lincomycin, smectite sorbents were prepared by saturating with exchangeable cations (i.e., Cs+, K+, Ca2+) with varying strengths of hydration. The hydration energy for Cs+ is -284 kJ mol-1, less than that of K+ (-397 kJ mol-1) and much less than that of Ca2+ (-1580 kJ mol-1). The relatively strong hydration of Ca2+ attracts more water molecules surrounding the cation, leading to larger expansion of the clay layers; the basal spacing of Ca-SWy-2 was >18 Å corresponding to ∼3 layers of water in the interlayer. The weakly hydrated exchangeable cations Cs+ and K+ manifest comparatively narrow interlayer spacings, i.e., usually only one layer of water is present in the interlayers of Cs-smectite, and one to two layers of water in the K-smectite interlayers. Smectites usually maintain interlayer distances at ∼3 to 9 Å (dependent on type of exchangeable cation) under ambient environmental conditions. These distances do not allow the formation of fully hydrated exchangeable cations in interlayers. The freedom of movement of water molecules in clay interlayers is much lower than that in bulk solution (28-30). This perspective provides a view of the clay interlayer environment as partially dehydrated, hence organic molecules sequestered therein exist in a somewhat dehydrated state (31-34). Sorption of lincomycin from water at pH 6 and no background electrolyte decreased in the order of Cs- > K- > Ca-SWy-2 (Figure 3). At the aqueous concentration of 50 µg L-1, sorption by Cs-SWy-2 was two times more effective than K-SWy-2, and 40 times more effective than Ca-SWy-2. Sorption by K-SAz-1 was 130 times more effective than that by Ca-SAz-1. VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6173
The greatest sorption was observed for smectite saturated with Cs+, which provides the least hydrated interlayer regions among the exchangeable cations evaluated. A less hydrated interlayer domain has been shown to be favorable for intercalation of many nonpolar or semipolar neutral organic compounds such as tricholorethene, dioxin, and nitrobenzenes, though the interaction mechanism(s) did not involve cation exchange (31, 35, 36). In the current study, the effect of exchangeable cation hydration on sorption of lincomycin was qualitatively the same as the previously noted effect on sorption of neutral organic molecules. Interestingly, the same effect is observed even though lincomycin sorption involves a totally different mechanism, i.e., chemisorption of an organic cation via ion-exchange reactions. It is of interest to note that lincomycin sorption by Cssmectite was greater than that by K-smectite. In general, Cs+ is more selective for negatively charged sites on mineral surfaces than K+ (22). This view of cation exchange would predict, a priori, reduced sorption of lincomycin by Cssmectite (vs K-smectite) since Cs+ is a less favorable “leaving cation” compared to K+. However, greater lincomycin sorption was observed for Cs-smectite compared to Ksmectite. This result indicates that the less hydrated Cssmectite interlayer facilitates the sequestration of lincomycin. We hypothesize that the hydration status of smectite clay interlayers appears to be an important factor influencing the ion exchange reactions of organic cations such as lincomycin, and the propensity of the leaving inorganic cation plays a comparatively smaller role. The hydration property of Cs+ as an exchangeable cation favorably controls the interlayer environment for sorption of organic compounds via its effect on the amount of interlayer water, the size of sorption domains consisting of unobscured siloxane surface between exchangeable Cs+ ions, and the interlayer distance. The latter allows for the simultaneous interaction of intercalated lincomycin with both of the opposing siloxane sheets of smectite clays, and maximal dehydration of lincomycin. The somewhat more hydrated environment within the K-smectite interlayers manifested reduced sorption compared to that by Cs-clay. In a similar but more dramatic fashion, the highly hydrated interlayer environment of Ca-smectite is one contributing factor to its much lower affinity for lincomycin. In addition, the divalent cation Ca2+ develops relatively strong electrostatic interactions with negatively charged sites of the mineral, making it an unfavorable leaving cation compared to monovalent species such as K+ or Cs+. Unlike the effects of exchangeable cations on lincomycin sorption, cationic surfactant hexadecyltrimethylammonium (HDTMA) sorption by montmorillonite manifested a different sorption order Ca- > Cs- > Na-clay when HDTMA sorption was SWy-2 . SAz-1 > KGa-1 ∼ muck soil. The less hydrated interlayer environment for saponite (SapCa-2) resulted in greater sorption than that by the montmorillonite clays SWy-2 and SAz-1, consistent with our hypothesis. For saponite the negative charge deficit originates primarily from isomorphic substitution in tetrahedral layers, which results in the distribution of negative charges over fewer oxygens compared to montmorollite (e.g., SWy-2) where charge deficit originates in the octahedral layer (38). It has been reported that due to tetrahedral layer isomorphic substitution, saponite develops relatively strong interactions with inorganic exchangeable cations involving formation of inner-sphere complexes (28, 39). Isomorphic substitution of montmorillonite occurs in the central octahedral layer resulting in the negative charges being distributed over more oxygens on both sides of the 2:1 clay layers, but with a smaller portion of negative charge associated with each oxygen. The more distributed structural charge on montmorillonite surfaces manifests comparatively weak interactions with exchangeable cations as evidenced by formation of fully hydrated outersphere complexes (39). As a result, saponite displays a somewhat smaller interlayer distance, less water retention, and overall a less hydrated subaqueous interlayer environment compared to montmorillonite (40, 41). Thus, the saponite interlayers intercalate organic compounds (both neutral and charged) more favorably compared to montmorillonite. The reduced intercalation of lincomycin in more hydrated clay interlayers could also explain its lower sorption by SAz-1 than by SWy-2 (Figure 4). Among the smectites investigated SAz-1 has the highest cation exchange capacity (and surface charge density) resulting in a larger number of hydrated
FIGURE 5. X-ray diffraction patterns and basal spacing (angstroms) of K-SWy-2 (A-E) and Ca-SWy-2 montmorillonites (F-J) at several lincomycin loadings (mg g-1). cations per unit cell. This manifests a more hydrated clay interlayer environment and smaller distance between hydrated exchangeable cations, leading to lower sorption of lincomycin by the high-charged SAz-1 clay. Sorption by kaolinite (KGa-1) was less than that by smectite clays due to the much lower cation exchange capacity and surface area, as well as the lack of expandable clay interlayer regions. Interestingly, lincomycin sorption by the muck soil (devoid of 2:1 swelling clay minerals) was substantially smaller than that by smectite clays, implying that soil organic matter likely plays a secondary role in the sorption of lincomycin by whole soils. Intercalation of Lincomycin in Clay Interlayers. X-ray diffraction patterns for K- and Ca-SWy-2 at several lincomycin
loadings are presented in Figure 5. The basal spacing of K-SWy-2 increased gradually from 12.70 to ∼16.98 Å with increasing lincomycin sorption from 0 to 170 mg g-1 (Figure 5A-E), establishing that lincomycin entered the clay interlayers. Compared to the blank sample of K-SWy-2, an increase in lincomycin sorption up to 121 mg g-1 caused XRD peaks to become sharper and more symmetrical. The alterations in XRD peak shape and increased basal spacing with lincomycin sorption suggest that the intercalated lincomycin molecules are randomly interstratified in clay interlayers. Increasing basal spacings were also observed for Ca-SWy-2 with increasing lincomycin sorption (Figure 5F-J). The replacement of inorganic exchangeable Ca2+ with monovalent lincomycin cation resulted in a significant reduction in XRD VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6175
intensities, implying that the well-oriented parallel layers of Ca-SWy-2 structures were somewhat disrupted by the cation exchange reactions involving lincomycin. At the highest lincomycin loadings (170 mg g-1 for K-SWy2, 157 mg g-1 for Ca-SWy-2) measured in this study, the two corresponding XRD peaks became broad indicating less ordering in the clay interlayers. At these high loadings the intercalated lincomycin molecules may be forced into a more vertical orientation (relative to the clay siloxane sheets) causing more expansion of the clay sheets. The surface area of lincomycin is ca. ∼371 Å2. Assuming the smectite surface (800 m2 g-1) is fully available for sorption of lincomycin, it is estimated that 159 mg of lincomycin would provide full surface coverage for 1 g of smectite. At this loading, only ca. ∼50% of cation exchange sites of SWy-2 would be occupied by lincomycin. Hence the broad XRD peaks and increased basal spacings (Figure 5E and J) reveal that intercalated lincomycin molecules were forced to tilt vertically due to steric constraints at high loadings. These results suggest that for lincomycin sorption, the cation exchange capacity of the swelling clays was not the limiting factor for sorption. Rather, smectite surface area limits sorption of cationic lincomycin to ∼50% of the cation exchange capacity of the smectite clays, despite their very large surface area of ∼800 m2 g-1.
Acknowledgments This research was funded by National Research Initiative Competitive Grant 2007-35107-18353 from the USDA Cooperative State Research, Education, and Extension Service, Pfizer Inc., and the Michigan Agricultural Experiment Station.
Literature Cited (1) Andreozzi, P.; Canterino, M.; Lo Giudice, R.; Marotta, R.; Pintob, G.; Pollio, A. Lincomycin solar photodegradation, algal toxicity and removal from wastewaters by means of ozonation. Water Res. 2006, 40, 630–638. (2) Loftin, K. A.; Adams, C. D.; Meyer, M. T.; Surampalli, R. Effects of ionic strength, temperature, and pH on degradation of selected antibiotics. J. Environ. Qual. 2008, 37, 378–386. (3) Oesterling, T. Aqueous stability of clindamycin. J. Pharm. Sci. 1970, 59, 63–67. (4) Calamari, D.; Zuccato, E.; Castiglioni, S.; Bagnati, R.; Fanelli, R. Strategic survey of therapeutic drugs in the Rivers Po and Lambro in northern Italy. Environ. Sci. Technol. 2003, 37, 1241–1248. (5) Kolpin, D. W.; Furlong, E. T.; Meyer, M. T.; Thurman, E. M.; Zaugg, S. D.; Barber, L. B.; Buxton, H. T. Pharmaceuticals, hormones, and other organic wastewater contaminants in U.S. Streams, 1999-2000: A National Reconnaissance. Environ. Sci. Technol. 2002, 36, 1202–1211. (6) Lissemore, L.; Hao, C.; Yang, P.; Sibley, P.; Mabury, S.; Solomon, K. R. An exposure assessment for selected pharmaceuticals within a watershed in southern Ontario. Chemophere 2006, 64, 717–729. (7) Chiou, C. T. Partition and Adsorption of Organic Contaminants in Environmental Systems; John Wiley and Sons: Hoboken, NJ, 2002. (8) Tolls, J. Sorption of veterinary pharmaceuticals in soils: A review. Environ. Sci. Technol. 2001, 35, 3397–3406. (9) Sassman, S. A.; Lee, L. S. Sorption of three tetracyclines by several soils: Assessing the role of pH and cation exchange. Environ. Sci. Technol. 2005, 39, 7452–7459. (10) Gu, C.; Karthikeyan, K. G. Interaction of tetracycline with aluminum and iron hydrous oxides. Environ. Sci. Technol. 2005, 39, 2660–2667. (11) MacKay, A. A.; Canterbury, B. Oxytetracycline sorption to organic matter by metal-bridging. J. Environ. Qual. 2005, 34, 1964–1971. (12) Mackay, A. A.; Seremet, D. E. Probe compounds to quantify cation exchange and complexation interactions of ciprofloxacin with soils. Environ. Sci. Technol. 2008, 42, 8270–8276. (13) Carrasquillo, A. J.; Bruland, G. L.; Mackay, A. A.; Vasudevan, D. Sorption of ciprofloxacin and oxytetracycline zwitterions to soils and soil minerals: Influence of compound structure. Environ. Sci. Technol. 2008, 42, 7634–7642. (14) Gao, J.; Pedersen, J. A. Adsorption of sulfonamide antimicrobial agents to clay minerals. Environ. Sci. Technol. 2005, 39, 9509– 9516.
6176
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 16, 2009
(15) Fabrega, J. R.; Jafvert, C. T.; Li, H.; Lee, L. S. Modeling shortterm soil-water distribution of aromatic amines. Environ. Sci. Technol. 1998, 32, 2788–2794. (16) Zachara, J. M.; Ainsworth, C. C.; Felice, L. J.; Resch, C. T. Quinoline sorption to subsurface materials: Role of pH and retention of the organic cation. Environ. Sci. Technol. 1986, 20, 620–627. (17) Li, H.; Lee, L. S.; Fabrega, J. R.; Jafvert, C. T. Role of pH in partitioning and cation exchange of aromatic amines on watersaturated soils. Chemosphere 2001, 44, 627–635. (18) Cowan, C. T.; White, D. The mechanism of exchange reactions occurring between sodium montmorillonite and various nprimary aliphatic amine salts. Trans. Faraday Soc. 1958, 54, 691–697. (19) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; John Wiley and Sons: New York, 1974. (20) Xu, S. H.; Boyd, S. A. Cationic surfactant adsorption by swelling and nonswelling layer silicates. Langmuir 1995, 11, 2508–2514. (21) Margulies, L.; Rozen, H.; Nir, S. Model for competitive adsorption of organic cations on clays. Clays Clay Miner. 1988, 36, 270–276. (22) McBride, M. B. Environmental Chemistry of Soil; Oxford University Press: New York, 1994. (23) Evangelou, V. P. Environmental Soil and Water Chemistry: Principles and Applications; Wiley-Interscience Publications, John Wiley & Sons: New York, 1998. (24) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry, 2nd ed; John Wiley and Sons: Hoboken, NJ, 2003. (25) Teppen, B. J.; Aggarwal, V. Thermodynamics of organic cation exchange selectivity in smectites. Clays Clay Miner. 2007, 55, 119–130. (26) Mizutani, T.; Takano, T.; Ogoshi, H. Selectivity of adsorption of organic ammonium-ions onto smectite clays. Langmuir 1995, 11, 880–884. (27) Arroyo, L. J.; Li, H.; Teppen, B. J.; Boyd, S. A. A simple method for partial purification of reference clays. Clays Clay Miner. 2005, 53, 511–519. (28) Sposito, G.; Prost, R. Structure of water adsorbed on smectites. Chem. Rev. 1982, 82, 553–573. (29) Chang, F. R. C.; Skipper, N. T.; Sposito, G. Computer-simulation of interlayer molecular-structure in sodium montmorillonite hydrates. Langmuir 1995, 11, 2734–2741. (30) Sutton, R.; Sposito, G. Molecular simulation of interlayer structure and dynamics in 12.4 angstrom Cs-smectite hydrates. J. Colloid Interface Sci. 2001, 237, 174–184. (31) Boyd, S. A.; Sheng, G.; Teppen, B. J.; Johnston, C. T. Mechanisms for the adsorption of substituted nitrobenzenes by smectite clays. Environ. Sci. Technol. 2001, 35, 4227–4234. (32) Haderlein, S. B.; Weissmahr, K. W.; Schwarzenbach, R. P. Specific adsorption of nitroaromatic explosives and pesticides to clay minerals. Environ. Sci. Technol. 1996, 30, 612–622. (33) Li, H.; Pereira, T. R.; Teppen, B. J.; Laird, D. A.; Johnston, C. T.; Boyd, S. A. Ionic strength-induced formation of smectite quasicrystals enhances nitroaromatic compound sorption. Environ. Sci. Technol. 2007, 41, 1251–1256. (34) Li, H.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A. Thermodynamics of nitroaromatic compound adsorption from water by smectite clay. Environ. Sci. Technol. 2004, 38, 5433–5442. (35) Aggarwal, V.; Li, H.; Boyd, S. A.; Teppen, B. J. Enhanced sorption of trichloroethene by smectite clay exchanged with Cs+. Environ. Sci. Technol. 2006, 40, 894–899. (36) Liu, C.; Li, H.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A. Mechanisms associated with the high adsorption of dibenzop-dioxin from water by smectite clays. Environ. Sci. Technol. 2009, 43, 2777–2783. (37) Xu, S.; Boyd, S. A. Alternative model for cationic surfactant adsorption by layer silicates. Environ. Sci. Technol. 1995, 29, 3022–3028. (38) Bleam, W. F. The nature of cation-substitution sites in phyllosilicates. Clays Clay Miner. 1990, 38, 527–536. (39) Greathouse, J.; Sposito, G. Monte Carlo and molecular dynamics studies of interlayer structure in Li(H2O)(3)-smectites. J. Phys. Chem. B 1998, 102, 2406–2414. (40) Lantenois, S.; Nedellec, Y.; Pre´lot, B.; Zajac, J.; Muller, F.; Douillard, J. M. Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water. J. Colloid Interface Sci. 2007, 316, 1003–1011. (41) Suquet, H.; de la Calle, C.; Pezerat, H. Swelling and structural organization of saponite. Clays Clay Miner. 1975, 23, 1–9.
ES900760M
