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Solid State 15N NMR Study of Pyridine Adsorption on Clay Minerals L J E R K A U K R A I N C Z Y K * ,† A N D KAREN ANN SMITH‡ Department of Agronomy and Department of Chemistry, Iowa State University, Ames, Iowa 50011
A solid state nuclear magnetic resonance (NMR) method is presented for spectroscopic adsorption studies of organic pollutant compounds on surfaces of powders under environmentally relevant hydration conditions. The applicability of the solid state NMR spectroscopy was investigated using pyridine as a model adsorbate and a clay mineral, hectorite, as an adsorbent. Solid state 15N magic angle spinning (MAS) and cross-polarization (CP)/MAS NMR methods were used to study adsorption of [15N]pyridine on fully hydrated and on dehydrated homoionic K-, Ca-, Mg-, and Al-hectorites. Powder X-ray diffraction (XRD) was used to determine the expansion of basal spacings and to confirm intercalation of pyridine into clay interlayers. With the exception of hydrated Alhectorite, more intense signals were obtained with MAS than with CP/MAS, and cross-polarization was less efficient on dehydrated clay/pyridine samples than on hydrated ones, indicative of a higher mobility of pyridine in the interlayers of dehydrated clays compared to hydrated ones. Pyridine interacted primarily through hydrogen bonding to the interlayer water on the hydrated hectorites. On the dehydrated K-hectorite (d001 ) 10.7 Å before pyridine adsorption), pyridine solvated the interlayers. Dehydrated Caand Mg-hectorites (d001 ) 12.9 and 12.8 Å, respectively, before pyridine adsorption) retained one layer of hydration water to which pyridine hydrogen bonds. Intercalation of pyridine was facilitated by the presence of water, the extreme case being Al-hectorite where almost no intercalation was observed without water. Spinning sidebands were only observed for the pyridine resonance between -90 and -94 ppm. This anisotropic species is assigned to the pyridine keying into the ditrigonal hole, hydrogen bonded to the proton of structural OH. The results clearly show that 15N NMR of an adsorbed N-containing organic compound is a powerful tool for identifying and distinguishing various surface complexes and adducts on hydrated mineral surfaces.
S0013-936X(95)00735-8 CCC: $12.00
1996 American Chemical Society
Introduction The importance of adsorption/desorption processes occurring on the hydrated surfaces in soils and sediments in determining the fate of the organic pollutant compounds in the environment has prompted numerous studies to determine the sorption mechanisms and the nature of the surface adsorption sites. Many of the techniques used in these studies (for example, sorption isotherms) provide only macroscopic information on sorption processes. Sorption mechanisms have rarely been studied on a molecular level under environmentally relevant conditions because most of the traditional surface spectroscopic methods are wellsuited only for single crystal surface studies under ultrahigh vacuum (UHV) conditions. Neither UHV conditions nor single crystals are representative of the hydrated, aggregated systems found in soils and sediments. Infrared spectroscopy (IR) is the only well-established method for surface adsorption studies of organic contaminants on mineral and organic matter surfaces. Although IR studies have provided considerable information on adsorption processes, this technique has several limitations. First, the IR spectrum is dominated by the absorption of the framework vibrations of the adsorbent, especially strong for layer silicate clays and organic matter, and is less of a problem for oxides. Second, at high relative humidities, representative of conditions found in soils, the IR spectra are obscured by water absorption bands. Third, the simulation of environmental conditions is limited by sample preparation (thin self-supported film or 0.1-0.2% adsorbent in KBr matrix) although some of the newer IR accessories, such as attenuated total reflectance, can overcome this problem (1). There is still a need for better and more quantitative understanding of the adsorption of organic pollutant molecules in soils. For example, one of the poorly understood problems is the progressively increased resistance to desorption and degradation of the aged organic chemicals in soils (2). As recently pointed out by Aochi and Farmer (3), there are numerous examples of pollutant organic compounds resisting desorption from both organic and mineral soil fractions. Solid state NMR spectroscopy is unique among other surface techniques because it can be applied to powders without any sample preparation under the conditions of interest (4). In general, NMR spectroscopy is well-suited for the study of organic compound-solution or organic compound-adsorbent interactions because it is an element-specific method that is extremely sensitive to the electron density (shielding) near the nucleus of interest. NMR techniques can be used to obtain a variety of information about an adsorbed molecule, including mechanisms of adsorption and surface sites involved, the dynamics (molecular diffusion, chemical exchange) of an adsorbed molecule on a mineral surface, and its orientation on a surface (5). Solid state NMR techniques have been used for in situ spectroscopic investigations of adsorption * Corresponding author fax: (515)294-3163; e-mail address: l
[email protected]. † Department of Agronomy. ‡ Department of Chemistry.
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processes (4, 5) and heterogenous catalytic processes (4, 6) and for surface characterization of catalysts and various organic and inorganic adsorbents (7, 8). The application of NMR in soil science and geochemistry has been limited to structural characterization of minerals, soil organic matter, and whole soils (9) and to studies of water structure on clay surfaces and interlamellar regions (10, 11). Even though there are numerous NMR studies of organic molecules adsorbed to zeolites and various oxides used in catalysis (4), there are very few NMR studies of organic adsorbates on clay minerals. Resing et al. (12) used 13C NMR to study the orientation of a benzene molecule in the interlayers of montmorillonite. Carrado et al. (13) used solid state 13C NMR to investigate the reactivity of anisoles on pillared smectites. They found that anisoles were highly mobile in pillared clay galleries and did not cross-polarize prior to the catalytic reaction while the product crosspolarized, indicating decreased mobility. Studies of organoclays (tetraalkylammonium exchanged on clays) by NMR indicate that the interlayer environment is fluid-like (14). Pratum (14) used 13C NMR to study tetramethylammonium- and hexadecyltrimethylammonium-exchanged vermiculite and smectite and reported that these organocations adsorbed on clays cross-polarized very inefficiently, consistent with a high degree of mobility. O’Brien et al. (15) studied triethylphosphate adsorption and intercalation on montmorillonite (5-55% w/w organic on clay) using 1H, 13C, and 31P MAS and CP/MAS NMR. They were able to distinguish one liquid-like species and at least two species in a motion-restricted environment. They found that relative ratios of the three species at all surface coverages were the same, possibly due to the formation of interstratified clay intercalates or seggregation of adsorbate into domains at low coverages. In kaolinite intercalation complexes, where the interlayer environment is much more restricted and strong hydrogen bonding with the gibbsitic surface can occur, 13C cross-polarization is more efficient (16-18). For example, Sugahara et al. (16) used 13C CP/ MAS NMR to study the polymerization reaction of acrylamide intercalated in kaolinite. It has also been shown that information on adsorbed organic molecules can be obtained from shifts of structural cations in clay layers (17, 18). In a study (17) of pyridine orientation in the interlayers of kaolinite by IR, XRD, and 29Si CP/MAS NMR, it was shown that pyridine intercalation causes a high field NMR shift of structural 29Si in kaolinite. 15N NMR has so far not been used to study organic compound/clay mineral interactions; however, in several studies ,15N NMR was used to evaluate acid sites on catalysts such as zeolites (19-23), silica (24), silica-alumina (25, 26), and alumina (27). 15N NMR of enriched compounds is also a very common technique in biochemistry and biotechnology studies (28) because the natural abundance of 15N is negligible (0.365%) so the 15N NMR signal from the matrix does not contribute to the spectra. In environmental studies, 15N NMR has been used in the study by Ginwalla and Mikita (29) on 15N-labeled chloramine reactions with aquatic fulvic acid. Besides avoiding the contribution from the matrix to NMR signal, 15N NMR has been shown to be more sensitive than 13C NMR in adsorption studies of molecules with lone-pair electrons on N (24, 25). This is due to larger density changes upon adsorption around N than around C and to the large chemical shift range of N (∼900 ppm) (30). There are many pesticides and other toxic organic pollutants that contain N with lone-pair
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electrons that may react directly with the electron-deficient surface sites (Lewis acid-base interactions), may accept a proton (Bronsted acid-base interactions), and may form hydrogen bonds with surface groups or water of hydration. Many of the N-containing organic compounds [e.g., triazine herbicides (31, 32), nitrobenzenes (33, 34), nitrophenols (34), amides (35), aminoacids (36), pyridines (35-39)] are known to adsorb on clay mineral surfaces; however, the nature of these organomineral interactions is still not fully understood. In this study, the applicability of 15N NMR to adsorption studies of organic compounds on hydrated clay minerals exchanged with cations common in soils (K, Ca, Mg, Al) was investigated. Pyridine was chosen as a model compound because it has been extensively studied by nitrogen NMR (40-44) and because many pesticides are substituted pyridines. Pyridine is often used as a probe molecule in the surface characterization of metal oxide and zeolite catalysts (19, 22-27), and as such, it has been the adsorbate most studied by 15N NMR. There are also several spectroscopic studies of the pyridine/clay system (36-39) to which the NMR studies can be related.
Materials and Methods Natural hectorite [M(I) 0.67(Mg5.33Li0.67)(Si8)O20(OH,F)2] from Hector, CA was obtained in a spray-dried form from Baroid Division of NL Industries. The homoionic Na-hectorite was prepared as described previously (45). The N2 BET surface area of this clay is 30 m2 g-1; however, this does not include the interlayer surfaces that are inaccessible to N2 molecules. When interlayer surfaces are included, the theoretical surface area of this clay is 750 m2 g-1 (46). Potassium-, Ca-, Mg- and Al-exchanged hectorites were prepared by exchanging Na-hectorite with the appropriate chloride salt. The clays were then dialyzed free of salt and freeze-dried. Dry, 99% 15N-enriched pyridine (Isotec) was obtained in sealed glass ampules and stored in glass vials sealed with Teflon-coated rubber septa over P2O5. The pyridine was added to the clays with a microsyringe. The amount added to all the clays was 0.286 g of [15N]pyridine per/g of clay, which is equivalent to the monolayer surface coverage assuming the surface area of clay is 750 m2 g-1 and the surface area of pyridine is 34.4 Å2 (26). Fully hydrated pyridine/water/clay samples were prepared by adding a 1:1 (v/v) water/[15N]pyridine mixture to approximately 50 mg of air-dried clay in custom-made Teflon containers with a flat, tightly fitting seal. The Teflon containers were made to fit into standard Bruker zirconia rotors. Dehydrated pyridine/clay samples were prepared by adding dry pyridine with a microsyringe to a previously dried clay sample in a Teflon container over P2O5. The dried clay sample was prepared by heating the clay in a vacuum furnace (10-1 Torr) at 80 °C for 36 h. This treatment is known (47) to remove all the interlayer water from K-hectorite while in Ca- and Mg-hectorite interlayer cations usually remain in the form of trihydrate or tetrahydrate. All the clay pastes were allowed to equilibrate with [15N]pyridine for 24 h in sealed containers prior to obtaining NMR spectra. Experiments were not performed at lower than monolayer coverages, because it was anticipated that the adsorbate will seggregate into domains rather than evenly distribute on the surface and preferentially adsorb on high energy sites.
TABLE 1
Interlayer Spacings of Hectorites before and after Intercalation of Pyridine d-spacing (Å) exchangeable cation
treatment
no pyridine >50
K
hydrated
K
dehydrated
10.7
Ca
hydrated
19.0
Ca
dehydrated
12.9
Mg
hydrated
19.5
Mg
dehydrated
12.8
Al
hydrated
14.7
with pyridine 29.6 14.7 20.8 11.8 29.6 14.1 brb 21.3 11.8 23.1 12.6 20.1 13.2 23.3
Al a
dehydrated
See text for structure.
b
13.9
14.3 14.4
assignment (with pyridine) complex 1a 002 of 29.6-Å peak intercalated pyridine, no water no pyridine in the interlayers complex 1eqa complex 3a 002 of 21.3-Å peak complex 3a 002 of 23.1-Å peak complex 3a no pyridine in the interlayers hydroxy interlayered pyridine intercalated between hydroxy interlayer and silicate sheet (4a and 5a) no pyridine in the interlayers pyridine intercalates only on clay edges
br ) broad.
15N NMR spectra of pyridine-hectorite complexes were obtained with a Bruker MSL-300 spectrometer at a resonance frequency of 30.4 MHz. No signal could be detected on the static samples. Thus, all spectra were acquired with magic angle spinning at 3.3-3.6 kHz. Spectra were obtained on the same sample by two techniques: Bloch decay and cross-polarization. In Bloch decay, magnetization was generated by a π/2 pulse at the 15N resonance frequency. The repetition time between the scans was 10 s, sweep width was 18 519 Hz, and 720 free induction decays were co-added. On selected samples, repetition times up to 107 s were tested and found to give the same signals as 10-s repetitions. Spectra obtain by this method will be referred to as MAS spectra. In cross-polarization experiments, the spin-locked polarization from the protons was employed. Radiofrequency (rf) amplitudes were matched to provide a π/2 pulse of ∼10 µs, CP contact times were 3 ms, the repetition time between successive scans was 2 s, and about 35 000 scans were accumulated. The decoupler offset was 38.280 kHz from the nominal proton frequency of 300.13 MHz, which sets the decoupler near the center of the spectral region for protons. In preliminary experiments, a series of contact times ranging from 0.1 to 5 ms were tested, and the contact time of 3 ms was found to give the strongest magnetization of adsorbed [15N]pyridine. Spectra obtained by this method will be referred to as CP/MAS spectra. The spectra were externally referenced to neat liquid nitromethane with upfield shifts being negative (28). The precision of the measurements was 0.3 ppm, but the actual accuracy of chemical shifts may be lower because there is no convenient solid reference for 15N and data were not corrected for bulk susceptibilities. Therefore, the data interpretation is primarily based on chemical shift differences for MAS and CP/MAS spectra of the same sample and on the fact that the large chemical shift range (>110 ppm) of [15N]pyridine allows for distinguishing wellseparated chemical shift ranges for various pyridine species (19-27). All the 15N chemical shifts referenced in this paper from the various literature sources are relative to liquid nitromethane (28). Spinning sidebands were observed in some of the spectra, as indicated in the Results and
Discussion section; however, they often corresponded to poorly resolved peaks and/or their intensity was too low to extract meaningful chemical shift anisotropy tensors. The 15N NMR shifts of adsorbed pyridine are very strongly influenced by the hydration state of the surface, with the chemical shift moving upfield as the water content is increased (19, 22-27). The relative peak intensities in 15N NMR cannot be interpreted quantitatively because there are a number of factors, including instrumental parameters, that affect magnetization of 15N nuclei in different environments (28). X-ray diffraction (XRD) measurements were performed on a Siemens D5000 diffractometer with Cu KR radiation at 45 mV/40 mA power on both the randomly oriented clay pastes investigated by NMR and on oriented self-supporting clay films. The basal spacings of the clay complexes were determined by comparing the observed positions of multiple orders of the basal peak with one-dimensional diffraction simulations performed by NEWMOD (48).
Results and Discussion The XRD results, confirming the expansion of d001 spacing for both dehydrated and hydrated clays, are presented in Table 1. Consistent with previous studies of pyridine/clay intercalates, hydrated clays swell to a greater extent than dehydrated clays (37-39). The solid state MAS and CP/ MAS 15N NMR spectra of adsorbed pyridine on all the clays studied are presented in Figure 1. With the exception of hydrated Al-hectorite, CP/MAS resulted in spectra with lower signal to noise ratio (S/N) as compared to MAS spectra. The cross-polarization was least efficient on the dehydrated clay/pyridine samples. 15N MAS spectra were obtained by a single pulse sequence that generates magnetization from spin-lattice relaxation. Spin-lattice relaxation is dependent on the presence of a motional component near the Larmor frequency of 15N (30 MHz), so magnetization by this technique is most efficient if fast molecular motion is present. The sharp pyridine resonances in MAS spectra of pyridine/clay complexes are indicative of pyridine species with rapid isotropic motion. In the CP/MAS experiments,
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FIGURE 1. 15N MAS (a, c, e, g, i, k, m, o) and CP/MAS (b, d, f, h, j, l, n, p) spectra of pyridine/hectorite complexes: (a, b) hydrated and (c, d) dehydrated K-hectorite; (e, f) hydrated and (g, h) dehydrated Ca-hectorite; (i, j) hydrated and (k, l) dehydrated Mg-hectorite; and (m, n) hydrated and (o, p) dehydrated Al-hectorite.
magnetization is generated only from 15N, which is coupled with 1H and which is rigid on the time scale of 15N-1H dipole-dipole coupling (on the order of 10-3 s). In order to efficiently cross-polarize, pyridine molecules must either be tightly bound or exhibit highly anisotropic motion (49).
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The observation of CP/MAS spectra for fully hydrated pyridine/clay complexes shows that the diffusion and chemical exchange of the adsorbed pyridine molecules are slow or that the motion of adsorbed pyridine has an anisotropic component (nonrandom motion). The inef-
ficient cross-polarization, observed on the dehydrated clays, is indicative of the high mobility of pyridine molecules in the interlayers. When considering the conditions under which cross-polarization may occur in the dehydrated clays, it is important to remember that the interlayer clay surface is a (001) plane of siloxane oxygens (50). The structural hydroxyl group sits in the middle of the ditrigonal hole, and in hectorite (trioctahedral clay) it is perpendicular to the clay layers. It follows that on an anhydrous clay surfaces there are very few surface sites to which pyridine can be “tightly bound”: it can be hydrogen bonded to hydroxyl groups on clay edges and to structural hydroxyls of the octahedral sheet. K-Hectorite. The diffraction pattern of the hydrated K-hectorite/pyridine complex has two broad peaks at 29.6 Å and 14.7 Å. These values are similar to the 29.3 and 14.8 Å reported for the Na-montmorillonite-pyridine complex in the presence of water (37-39). The MAS and CP/MAS spectra of hydrated K-hectorite after adsorption of [15N]pyridine are depicted in Figure 1, panels a and b, respectively. The MAS spectrum in Figure 1a has one resonance at -84.0 ppm suggesting that all of the pyridine is in the same, mobile environment or that the exchange between different environments is fast on the NMR time scale. The peak is in the chemical shift range where the resonance for physically adsorbed and hydrated pyridine is expected (23). In aqueous solutions, the chemical shift of hydrated pyridine depends on the amount of water present ranging from -84.6 ppm at infinite dilution of pyridine in water to -63 ppm for neat pyridine (43). The CP/MAS spectrum (Figure 1b) exhibits a strong peak at -84.6 ppm and a broad weak resonance at -93 ppm. Solid state NMR studies of [15N]pyridine attribute chemical shifts in this range to physically adsorbed pyridine in the channels of mordenite (-89 ppm; 23), physically adsorbed and hydrated pyridine on alumina (-81 ppm; 23) and on silicaalumina (-88 ppm; 26), and pyridine in the pores of NaY zeolite (-89 to -91 ppm; 19). Pyridine adsorption on K-smectites has not been studied previously; however, this clay has similar hydration and swelling properties as Na-smectites. Na-montmorillonite/ pyridine complexes have been studied extensively as model systems for diffusion in clay minerals (38, 39). It has been proposed that in the 29 Å phase four pyridine molecules are in the first coordination sphere of the interlayer cation (36, 39). In this structure, two of the four pyridine molecules are standing edge-on with respect to clay layers and have water of hydration adjacent to the clay sheets. That would expand clay layers by approximately 18.2 Å [assuming the diameter of the pyridine is 6.6 Å (26) and the diameter of water is 2.5 Å], close to the value calculated by subtracting the thickness of the clay layer (9.6 Å) from the d001: 29.3 - 9.6 ) 19.7 Å. This arrangement seems unlikely for two reasons. First, it would imply water that is interacting with the aromatic CH of the pyridine ring. Second, spectroscopic studies of pyridine adsorbed on Na-montmorillonite (33, 37) and Na-zeolites (51) show that pyridine is hydrogen bonded to water and forms water-bridged complexes with cations. The IR spectra of Na-montmorillonite/pyridine complexes (37) are consistent with the formation of complex 1 and show no evidence of direct coordination of pyridine to Na.
The proposed interpretation here is that the expansion of clay layers to 29.6 Å is due to 1 and that none of the pyridine is in the first coordination sphere of K. Because the hydration energy of K is low, its water of hydration is not highly polarized which implies that the pyridine in complex 1 is not bonded very strongly. Therefore, the MAS resonance at -84.0 ppm most likely corresponds to a mobile hydrated pyridine that is rapidly exchanging between pyridine in complex 1 and pyridine hydrogen bonded to interlayer water outside the coordination sphere of the cation (2).
The chemical shift of pyridine in bulk water at the same water/pyridine ratio (1:1) is lower (-80.9 ppm; 43) than that observed for the K-hectorite/pyridine complexes, indicating that pyridine is more shielded when hydrogen bonded to interlayer water. This implies that hydrogen bonding between pyridine and interlayer water is stronger than in bulk water. The observation of a relatively strong CP/MAS resonance at -84.6 ppm (Figure 1b) indicates that some of the pyridine is in a less mobile and/or anisotropic environment. It is conceivable that two pyridine molecules in complex 1 (axial pyridines) exhibit anisotropic motion and/or slow exchange and thus cross-polarize more efficiently than pyridine in 2. Because the exchangeable K is usually a 4-6 coordinate in hydrated smectites, there are 2-4 water molecules in the equatorial positions of 1 (not shown) where pyridine may hydrogen bond as well. However, such a complex is likely to be short-lived because the hydration energy of K is low. The chemical shift difference between the peak in MAS and the peak in CP/MAS spectrum (Figure 1, panels a and b) is 0.6 ppm, which suggests either that these resonances correspond to the same pyridine species or that the exchange between mobile and immobile species is on the order of the inverse of the chemical shift difference: ∼0.5 × (0.6 ppm × 30 Hz ppm- × 2π)-1 ) 4 ms. This motion is on the same time scale as the spin-lattice relaxation time of water on hydrated clay (T1 ) 10-3-10-4 s) (52). Dehydrated K-hectorite, equilibrated with dry pyridine for 24 h, expands from 10.7 to 20.8 Å. The XRD pattern has an additional peak at 11.8 Å. If the clay is left in excess pyridine for 3 days, the peak at 20.8 Å increases in intensity while that at 11.8 Å almost disappears. This rules out the peak at 11.8 Å being a second order of the 20.8 Å peak. Thus, the clay has domains with layers that are not swelled by pyridine and with the swelled layers expanded to 20.8
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Å. The 20.8-Å basal spacing is somewhat different from that observed for Na-montmorillonite, where 23.3- and 19.4-Å phases (36-39) have been observed and assigned to pyridine coordinated directly to Na with either the pyridine plane perpendicular to clay layers (23.3 Å phase) or the pyridine plane at an angle to clay layers (19.4 Å phase; 36, 38, 39). However, such K-pyridine complexes are unlikely on dehydrated K-hectorite, because K is adsorbed directly on the siloxane surface in the ditrigonal hole, rather than in the middle of the interlayer (53). A 15N MAS spectrum of pyridine on dehydrated Khectorite (Figure 1c) has two peaks at -63.6 and -67.4 ppm. These are in the chemical shift range of neat liquid pyridine (-63 ppm; 23) and physically adsorbed pyridine on γ-alumina (-69 ppm; 23), respectively. The sample cross-polarized very inefficiently as evidenced by low S/N of the spectrum in Figure 1d. The peaks at -63.9 and -67.0 ppm in the CP/MAS spectrum are most likely equivalent to those observed in the Bloch decay spectrum. Although other features can be seen in the CP/MAS spectrum, the S/N is too low to extract chemical shifts. The inefficient cross-polarization of 15N pyridine in this clay suggests that no long-lived pyridine complexes are formed in the interlayers of dehydrated K-hectorite. The spectra in Figure 1c are clearly generated by two different pyridine species that are not exchanging on the NMR time scale, most likely pyridine that is not intercalated but physically sorbed on the outer clay surfaces or condensed in pores (-63.6 ppm) and highly mobile intercalated pyridine (-67.4 ppm). The nonrigid environment and inefficient cross-polarization have been observed previously for organic molecules intercalated in dry smectites (1315). As can be seen from the presence of un-intercalated pyridine in the NMR spectrum as well as from the interstratified XRD pattern, the diffusion of pyridine into interlayers is much slower under dehydrated conditions. Since most IR studies on the sorption of organic molecules on clay surfaces are done at very low relative humidities and without long equilibration periods, the importance of the intercalation of organic pollutants in soil smectites during aging of chemicals in soils may be underestimated in IR studies. Spinning sidebands were not evident in any of the spectra obtained from K-hectorite, indicating that residual chemical shift anisotropy (CSA) is Ca-hectorite > Mg-hectorite > Alhectorite.
Acknowledgments This research was supported by the Iowa State University Research Grant and Iowa Agricultural Experiment Station Project No. 3254 (IAHEES Journal Paper J-16552).
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(35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47)
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Received for review October 3, 1995. Accepted June 11, 1996.X ES950735H X
Abstract published in Advance ACS Abstracts, September 15, 1996.
