Adsorption Interactions of Aromatics and Heteroaromatics with

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Environ. Sci. Technol. 2000, 34, 259-265

Adsorption Interactions of Aromatics and Heteroaromatics with Hydrated and Dehydrated Silica Surfaces by Raman and FTIR Spectroscopies STEVEN C. RINGWALD AND JEANNE E. PEMBERTON* Department of Chemistry, University of Arizona, Tucson, Arizona 85721

Raman and FTIR spectroscopies are used to investigate the sorption mechanisms of benzene, toluene, and 2- and 4-picoline onto silica as models for volatile aromatic pollutant interactions with a soil constituent. Benzene and toluene vapor adsorption on silica occurs via weak π-system-hydrogen bonding with silanols on the silica surface. This weak interaction would likely result in low vadose zone retention, especially in wet conditions where water adsorption would successfully compete for surface sites. The vapor adsorption of 2- and 4-picoline (2- and 4-methylpyridine) is studied to model aza-arene environmental contaminants. These species adsorb to surface silanols by a more specific and stronger hydrogen-bonding mechanism involving the lone pair electrons on the N atom. The strength of these interactions is probably sufficient to result in their retention by dry or damp vadose zone soil, slowing their transport. This work illustrates the utility of these vibrational spectroscopic techniques in elucidating specific surface interactions of pollutants with mineral oxides and in helping to predict the fate of pollutants in the environment.

Introduction A problem of considerable importance is the presence of volatile organic contaminants in soil and groundwater. The transport of such species through the vadose zone to the groundwater or the atmosphere has been indicated, and modeling of the process has been extensive (1-3). The mechanisms controlling fate and transport of pollutants in the vadose zone involve both vapor and solution transport processes. However, the mechanisms controlling vapor sorption in unsaturated soils are not completely understood (1-6) and, thus, are not usually incorporated into chemical transport models (2). The sorption of organic vapors on mineral surfaces is generally thought to occur via one of four processes: (a) adsorption from the vapor phase onto the mineral surface, (b) adsorption on the surface of an adsorbed water film, (c) dissolution into adsorbed water, and (d) partitioning into soil organic matter (5, 7). Distinguishing between these mechanisms requires careful control of the state of hydration of the mineral surface. The influence of processes c and d are minimized on dehydrated minerals, since such systems * Corresponding author phone: (520)621-8245; fax: (520)621-8248; e-mail: [email protected]. 10.1021/es980970a CCC: $19.00 Published on Web 12/02/1999

 2000 American Chemical Society

contain little adsorbed water. In the investigation reported here, the contributions of these processes to the adsorption mechanisms of model volatile aromatic pollutants with hydrated and dehydrated silica are studied. Silica is used as a model soil adsorbent due to prevalence in the environment and well-characterized surface properties (8). Most previous research on sorption of organic pollutants at solid-liquid and solid-vapor interfaces has focused on adsorption isotherm determination using chromatography. Such studies provide insight into the interaction strength of organics with soil and mineral surfaces from which some information about interaction mechanisms can be inferred (9-11). Vibrational spectroscopy is a useful complement to chromatographic studies, because molecularly specific information about interfacial chemistry can be ascertained. The work reported here describes investigations of the interactions of benzene, toluene, and 2- and 4-picoline as model volatile aromatic contaminants with a model soil (silica) using Raman and FTIR spectroscopies. These molecules span a range of expected interaction strengths and represent a useful set for categorizing the factors that influence the interaction of aromatics with silica-based soils. Moreover, the picolines belong to the class of compounds known as aza-arenes, which are nitrogen-containing analogues of polycyclic aromatic hydrocarbons. Aza-arenes are of particular concern due to their known carcinogenic and mutagenic properties and their numerous sources in the environment (12-17). They are found in automobile exhaust (18), air pollution source effluents (19), recent lake sediments (20), wood preservative wastewater (21), contaminated groundwater (22-28), and wastewater treatment biosludge (29). The picolines have been specifically identified as environmental contaminants in groundwater near underground coal gasification sites and in oil shale retort water (23, 28). The diversity of vapor- and liquid-phase sources of aza-arenes predicts that their environmental fate will be determined by their interactions with soil from these different phases. In addition to providing molecules of rich vibrational activity for study, it was believed that the results from these studies would complement the already extensive literature (30-36) on the related adsorbate system of pyridine on silica. Early studies used vapor-phase pyridine adsorption and established that the pyridine totally symmetric ring breathing mode and trigonal symmetric ring stretch are particularly sensitive to the strength of interaction between the pyridine lone pair electrons and the silica surface (30-33). Physisorption, hydrogen-bonding, and Brønsted acid-base interactions between pyridine and various oxides were distinguished by the frequency shifts of these modes (30-33). Several studies on liquid-phase pyridine adsorption have also appeared (33-36). In one such study (35), the effect of pH on the sorption of pyridine, acridine, and quinoline on silica from aqueous solutions was reported. Neutral azaarenes were found to hydrogen bond to water on the silica surface through nitrogen lone pair electrons at pH values above the pKa of the respective aza-arene. However, a protonated aza-arene/ClO4- ion pair adsorbed on the silica surface when the pH was below the pKa of the aza-arene. Simpson and Harris recently studied the adsorption of pyridine onto silica from carbon tetrachloride solutions using Raman spectroscopy (36). They found that the initial pyridine interaction was of a Brønsted acid-base nature and that the degree of proton transfer decreased as the surface coverage increased. Sayed and Cooney also studied pyridine sorption to silica from carbon tetrachloride (33). They concluded that VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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laser-assisted desorption of pyridine from silica occurs after an initial adsorption to Brønsted acid sites of silica. In the work described here, benzene and toluene are used to establish the basic interactions of aromatic and alkylated aromatics with silica. The picolines are used to assess the perturbation to these interactions resulting from incorporation of a N heteroatom. Although both techniques provide information about the bonding of these aromatics with silica surfaces, Raman spectroscopy is highlighted because of its relative immunity to interferences from silica and water. FTIR spectroscopy is used as a corroborative tool.

Experimental Section Materials. Benzene (EM Science, ACS spectroscopic grade), toluene (Mallinckrodt 99.9%), 2-picoline (Aldrich 98%), and 4-picoline (Aldrich 99%) were used as received. The silica gel (Aldrich, high purity grade) used has a specific surface area of 500 m2 g-1 as reported by the manufacturer. Dehydrated silica was prepared by heating in a furnace at 550 °C for at least 48 h followed by cooling to room temperature in a desiccator. The hydrated silica was stored indefinitely in the ambient laboratory environment at 10-30% relative humidity levels. KBr (Aldrich) was dried and stored in an oven at 120 °C. Water was purified with a Milli-Q UV system (Millipore Corp.). Instrumentation. Raman spectra were acquired using 514.5 nm radiation on a system described previously (37, 38). Laser powers of 100 and 50 mW were employed for neat liquids or aqueous solutions and adsorbates on silica, respectively. Raman spectra were acquired on samples contained in either NMR tubes or melting point capillaries with a precision of (2 cm-1. FTIR spectra were acquired on a Nicolet Magna 550 series II spectrometer equipped with a liquid N2-cooled MCT detector. Transmission experiments were performed on oxides pressed into KBr pellets or on neat solutions between NaCl windows. Spectra were acquired using 64 co-added scans of both sample and reference at 2 cm-1 resolution with Happ-Genzel apodization. A KBr spectrum was used as the reference for experiments involving adsorbates on silica, while NaCl was used as the reference for spectra of neat solutions. Procedure. Organics were adsorbed to hydrated and dehydrated silica by vapor deposition in a closed glass cell. The 100-mg silica samples were spread evenly over the bottom of 50-mL beakers that were placed in closed glass jars and exposed to solvent vapor. Surface coverage was controlled using exposure time, which varied depending on the adsorbate due to differences in vapor pressure. The silica was then immediately packed into a glass sampling tube, and Raman spectra were acquired. Exposure times on hydrated silica were typically longer than on dehydrated silica due to the significantly larger Raman background intensity from the former. Exposure times for individual spectra are provided in the figure captions. Due to differences in surface coverages and background intensities for individual systems, comparisons between spectra based on S/N are not straightforward. FTIR spectra were acquired only on the sorption systems formed from neat 2- and 4-picoline due to strong interference from water. Spectra of toluene and benzene on silica could not be acquired due to unavoidable desorption during sample handling and pellet pressing. Spectra of neat 2- and 4-picoline were acquired from sample sandwiched between two NaCl disks. Pellets were prepared by mixing the picoline/silica sample with KBr in a ratio of 1:10 wt/wt and grinding briefly in an agate mortar and pestle. The 50-mg aliquots of this mix were pressed in a pellet die for 5 min. After formation, the sample pellet was placed immediately into the FTIR sample 260

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compartment, which was subsequently purged with dry air for 2.5 min before spectra were acquired.

Results and Discussion In wet vadose zone environments, silica surfaces are hydrophilic and hydrated by several layers of water. Water in the first monolayer, which is hydrogen-bonded directly to surface silanols, is particularly strongly held (35). In order for adsorbates to interact directly with surface silanols, this strongly held water must be displaced. For most adsorbates, such displacement is unlikely; thus, one must consider both the hydrating layers of water and the surface silanols as potential adsorption sites (35). The hydrated and dehydrated silicas used in this study are best considered models for damp and dry vadose zone soils containing surface silanols with little or no water based on the conditions under which they were prepared. Hydrated silica was equilibrated in ambient laboratory conditions of low relative humidity (10-30%). FTIR spectra of the hydrated and dehydrated silica used in this study are presented in Figure S1 (Supporting Information). The spectrum of hydrated silica contains ν(OH) bands at 3400-3600 cm-1 and a δ(OH) band at ∼1620 cm-1, which indicate the presence of hydrogen-bonded silanols with some water. The dehydrated silica was heated extensively, and the FTIR spectrum contains a sharp band at ∼3750 cm-1, which indicates the presence of free silanols. Evidence for some water on dehydrated silica is present in the low-intensity ν(OH) and δ(OH) bands that remain even after heating. Since the 1620 cm-1 band comes only from adsorbed water, it can be used as an indicator for the quantity of surface water. The area of the 1620 cm-1 band on dehydrated silica is ∼14% of the area on hydrated silica, indicating that a significant amount but not all water is desorbed upon heating. Benzene. Previous results suggest that benzene interacts with silica very weakly by hydrogen bonding through the π electron system (30, 39-41). Even weaker hydrophobic interactions of benzene with the bare siloxane surfaces of clay have also been identified (42). Despite the challenges presented by such weak adsorption, several researchers have reported surface Raman spectra from benzene adsorbed on silica-based systems. Buechler and Turkevich reported a Raman spectrum of benzene adsorbed to Vycor glass from the gas phase and observed no frequency shifts (39). Egerton and co-workers also used Raman spectroscopy to study benzene adsorbed from the gas phase on Vycor glass (30). The adsorbed benzene bands were observed to be close in frequency to those of neat benzene. Interestingly, a decrease in intensity of the ring breathing band relative to the other bands of adsorbed benzene was noted that was attributed to interaction of the ring electrons with surface hydroxyls. Jeziorowski and Knozinger reported Raman spectra for benzene adsorbed from the vapor phase on silica gel (40). They indicated that the frequencies for adsorbed benzene were similar to those in neat liquid benzene but that many of the vibrational bands increased in intensity relative to the ring breathing mode for the surface-confined species. Sayed and Cooney reported Raman spectral data for benzene adsorbed on silica from the gas phase and from aqueous solution (41). The ring breathing mode of adsorbed benzene was observed as a doublet in their study, with the stronger of the two bands appearing at the frequency of neat benzene and a second, much weaker band appearing 3 cm-1 higher in frequency. This doublet was interpreted as evidence for a monolayer of adsorbed benzene covered by condensed multilayers of benzene. A shift to higher frequency of the ν(CH)ring mode was also observed. The magnitude of this shift ranged from 13 cm-1 for low coverages of benzene to 1 cm-1 for high coverages and was attributed to decreased adsorbate-surface interaction with coverage. These re-

FIGURE 1. Raman spectra of (a, d) neat benzene; (b, e) benzene adsorbed to hydrated silica by vapor-phase exposure for 1 min; (c, f) benzene adsorbed to dehydrated silica by vapor-phase exposure for 10 s.

FIGURE 2. Raman spectra of (a, d) neat toluene; (b, e) toluene adsorbed to hydrated silica by vapor-phase exposure for 1 min; (c, f) toluene adsorbed to dehydrated silica by vapor-phase exposure for 1 min.

searchers proposed an interaction of the benzene ring π electrons with surface silanols as the cause of these shifts. Several studies of water-benzene clusters have identified weak hydrogen bonding between water and the polarizable π electron cloud of benzene (43-46). A binding energy of 1.78 kcal/mol for the water-benzene complex has been reported (46), which is consistent with weak hydrogenbonding or a dipole-dipole interaction. This energy is stronger than that expected for van der Waals interactions (j1 kcal/mol) of the type usually associated with purely hydrophobic interactions. Given the substantial improvements in Raman spectroscopic instrumentation since these studies were performed, we chose to reinvestigate the benzene-silica system. Raman spectra of neat benzene and benzene adsorbed on hydrated and dehydrated silica from the gas phase are shown in Figure 1 for the regions centered at 1100 and 3100 cm-1. The quality of these spectra for such a weakly adsorbed system is noteworthy and reflects the extremely high sensitivity of current Raman spectroscopy technology. Table S1 (Supporting Information) contains frequencies and band assignments for neat benzene (47-49) and benzene adsorbed to silica. Most adsorbed benzene bands occur at frequencies close to those of neat benzene, characteristic of weak adsorption. Relative intensities are also unchanged except for that of the ring breathing mode at ∼992 cm-1, which decreases slightly in intensity upon adsorption. Silica bands are observed in Figure 1, panels b and c (low benzene surface coverage), at ∼400, 600, 800, 1050, and 1100 cm-1. The only exceptional

spectral behavior is a shift of 9 cm-1 to higher frequency of the ν(CH)ring band of adsorbed benzene. The sensitivity of this band to weak electronic perturbation, such as would be caused by ring π-system-hydrogen bonding, is characteristic for aromatic systems as will be shown below. The intensities and peak frequencies of adsorbed benzene are consistent with results from earlier studies of benzene adsorbed to porous silica glass (Vycor) (30, 39) and silica (40, 41). The results are also consistent with those for thiophene on silica, which was proposed to interact through an identical mechanism (50). Toluene. Previous studies of toluene-water clusters by Li and Bernstein suggest that water weakly hydrogen bonds to one side of the toluene ring (51). On this basis, one can hypothesize that toluene will adsorb to hydrated silica via a similar ring π-system-hydrogen-bonding mechanism. Raman spectra of toluene adsorbed to hydrated and dehydrated silica are presented in Figure 2. Table S2 (Supporting Information) lists peak frequencies and band assignments for neat (47, 52) and adsorbed toluene. Most surface toluene bands are at frequencies similar to those of neat toluene, except for the 1380 cm-1 band, which shifts 4 cm-1 toward higher frequency upon adsorption. The ν(CH)ring shifts from 3055 to 3062 cm-1 on hydrated silica and to 3061 cm-1 on dehydrated silica. The ν(CH3) at 2919 cm-1 also shifts toward higher frequency by ∼10 cm-1 on both silicas. These shifts are consistent with ring π-system-hydrogen bonding with surface silanols in a manner similar to that discussed above for benzene. For a predominantly ring π-system-surface interaction, a small shift in the δ(CH3) is VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Raman spectra of (a, f) neat 4-picoline; (b, g) 4-picoline adsorbed to hydrated silica by vapor-phase exposure for 75 min; (c, h) 0.1 M aqueous 4-picoline, pH 7.60; (d, i) 4-picoline adsorbed to dehydrated silica by vapor-phase exposure for 10 min; (e, j) 1.0 M aqueous 4-picoline, pH < 3. expected since steric considerations force the methyl group to interact with the surface upon adsorption. The shift in the νs(CH3) band is consistent with this picture. The shift in the ν(CH)ring band is only slightly less than that observed for benzene on silica. Thus, a similar hydrogen-bonding interaction through the aromatic ring, resulting in a flat orientation, is proposed for adsorbed toluene. 4-Picoline. Raman spectra of neat and 0.1 M aqueous 4-picoline (pH 7.60) are shown in Figure 3, panels a,f and c,h, respectively. Peak frequencies and assignments are given in Table S3 (Supporting Information). The vibrational spectra of the isomeric picolines has been characterized previously by several researchers (53-59); assignments are based on these previous studies. Substituent-sensitive ring bands of 4-picoline occur at ∼516, 803, 1211, and 1221 cm-1. Ring stretching modes occur at 995, 1496, 1563, and 1605 cm-1, and a δ(CH)ip mode occurs at 1069 cm-1. The δ(CH3) mode occurs at 1381 cm-1, the ν(CH3) modes are observed at 2869 and 2923 cm-1, and the ν(CH)ring occurs at 3051 cm-1. The solubility of 4-picoline in water allows preliminary assessment of the impact of hydrogen bonding on its vibrational frequencies. Spectra of 0.1 M aqueous 4-picoline are shown in Figure 3, panels c and h, respectively. Shifts to higher frequency ranging in magnitude from ∼2 to 17 cm-1 for almost all modes occur upon dissolution. Similar shifts to higher frequency are observed for aqueous pyridine solutions (31, 33, 35, 60, 61). The extreme of a hydrogen-bonding interaction is protonation of the ring N resulting in the Brønsted acid species. Protonated 4-picoline (pKa 6.02) was prepared by acidifying with HCl to a pH < 3. Spectra of the 4-picolinium species are shown in Figure 3, panels e and j, respectively. Peak frequencies and assignments are listed in Table S3. Protonation leads to increases of the ring breathing and stretching modes to 1013 and 1645 cm-1, respectively. The ring deformation mode shifts 18 cm-1 toward lower frequency to 262

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653 cm-1. The ν(CH)ring mode shifts toward higher frequency to 3105 cm-1. Spectral changes observed for 4-picoline upon aqueous dilution and protonation are used to model the interactions between 4-picoline and silica. Figure 3, panels b,g and d,i show Raman spectra for 4-picoline adsorbed to hydrated and dehydrated silica, respectively. Band shifts of up to 8 cm-1 to higher frequency are observed for adsorbed 4-picoline relative to those for aqueous 4-picoline (i.e., these modes shift by up to 25 cm-1 upon adsorption relative to neat 4-picoline). Such large shifts are indicative of substantial interaction between 4-picoline and silica. Bands associated with the methyl group and the pyridine ring also shift to higher frequencies by ∼10 and 16 cm-1, respectively, upon adsorption, similar to the shifts induced by aqueous dilution. Collectively, these observations are consistent with 4-picoline adsorption through hydrogen bonding of the ring N. However, the shift of the ring breathing mode, which involves symmetric stretching of all ring carbon atoms, indicates an interaction between 4-picoline and silica that is slightly stronger than between 4-picoline and water. A similar shift of ∼12 cm-1 to higher frequency has been reported for aqueous pyridine. In contrast, this mode shifts toward higher frequency by ∼20 cm-1 for the pyridinium ion (31, 33, 35, 60, 61). The shift of ∼22 cm-1 to higher frequency for the first monolayer of 4-picoline on silica suggests an interaction that is stronger than the hydrogen bonding in aqueous solution; thus, this interaction is assigned to that with surface silanols on the hydrated silica. Since the shift of this mode on dehydrated silica is similar to that observed on hydrated silica, an identical interaction is proposed. Considering the large shift in the ring breathing mode for the adsorbed 4-picoline relative to aqueous 4-picoline, a spectrum of the 4-picolinium ion was acquired to rule out Brønsted acid-base surface interactions. Comparison of the spectra of adsorbed 4-picoline and the 4-picolinium ion shows that the spectral match is only moderately good. Thus,

FIGURE 4. Raman spectra of (a, f) neat 2-picoline; (b, g) 2-picoline adsorbed to hydrated silica by vapor-phase exposure for 15 min; (c, h) 0.1 M aqueous 2-picoline, pH 7.65; (d, i) 2-picoline adsorbed to dehydrated silica by vapor-phase exposure for 6 min; (e, j) 1.0 M aqueous 2-picoline, pH < 3. the spectra of adsorbed 4-picoline are not adequately modeled by aqueous 4-picoline or by 4-picolinium. Instead, the spectra suggest an environment between these two extremes, although closer to the hydrogen-bonding environment than the Brønsted acid. On this basis, we propose a hydrogen-bonding interaction of 4-picoline with silanols on the silica surface. This interaction might be expected to be slightly stronger than simple hydrogen bonding but not as strong as for proton transfer. FTIR spectral data for 4-picoline are presented and discussed in the Supporting Information. These data support the conclusions drawn on the basis of the Raman spectroscopy. 2-Picoline. Raman spectra for neat and 0.1 M aqueous 2-picoline (pH 7.65) are presented in Figure 4, panels a,f and c,h, respectively. Peak frequencies and assignments for these systems are listed in Table S5 (Supporting Information). The Raman spectrum of neat 2-picoline contains substituent-sensitive modes at 547, 800/811, and 1236 cm-1. An in-plane ring deformation R(CCC) mode occurs at 629 cm-1, and the ring breathing mode occurs at 998 cm-1. δ(CH)ip modes occur at 1050, 1102, 1150, and 1296 cm-1, and the δ(CH3) is observed at 1377 cm-1. The ring stretching modes occur at 1570 and 1591 cm-1. The ν(CH3) and ν(CH3)FR are observed at 2863 and 2924 cm-1, respectively, and the ν(CH)ring bands occur at 3048 and 3061 cm-1. The dissolution of 2-picoline in water causes shifts toward higher frequency of ∼3-20 cm-1 for almost all modes, as for 4-picoline. These shifts are attributed to hydrogen bonding of the ring N with water. These spectral changes are used to model the interaction between 2-picoline and silica. Protonated 2-picoline (pKa 5.97) was prepared by acidification; spectra of the 2-picolinium species are shown in Figure 4, panels e and j, respectively. Peak frequencies and assignments are listed in Table S5 (Supporting Information). Protonation of the 2-picoline ring leads to further increases in the ring breathing mode to 1016 cm-1, the ring stretching

modes to 1632 and 1649 cm-1, and the ν(CH)ring modes to 3097 and 3113 cm-1. The substituent-sensitive mode at 547 cm-1 shifts to lower frequency by 4 cm-1. Figure 4, panels b,g and d,i, show Raman spectra for 2-picoline adsorbed to hydrated and dehydrated silica, respectively. Adsorbed 2-picoline bands shift by up to 5 cm-1 to higher frequency relative to aqueous 2-picoline. Thus, shifts of up to 17 cm-1 occur for 2-picoline adsorbed to silica as compared to neat 2-picoline. These large shifts are analogous to those observed for adsorbed 4-picoline. Bands associated with 2-picoline in the ν(CH) region shift toward higher frequency upon adsorption. The ν(CH3) at 2863 and the ν(CH3)FR at 2924 cm-1 shift toward higher frequency by 7 and 11 cm-1 on hydrated silica and by 11 and 13 cm-1 on dehydrated silica. Similar shifts toward higher frequency are observed for the 3048 and 3061 cm-1 ν(CH)ring bands, which increase by ∼20 cm-1 on both hydrated and dehydrated silica. The shifts in the 2924 and 3048 cm-1 modes are similar to those observed upon aqueous dissolution. These observations support hydrogen bonding of 2-picoline with silica surface silanols. The spectra are similar to those of aqueous 2-picoline, except that the ring stretching modes suggest a slightly stronger interaction between 2-picoline and silica than between 2-picoline and water. The magnitude of the shift observed for the ring breathing mode of adsorbed 2-picoline implies an interaction that is slightly stronger than simple hydrogen bonding or an interaction with some Brønsted character. Other ring stretching modes increase in frequency upon adsorption as well. The magnitudes of these shifts are larger than observed between neat and aqueous 2-picoline, also implying a slightly stronger surface interaction than simple hydrogen bonding. However, the shifts of these two modes are not as large as for the 2-picolinium ion (1632 and 1649 cm-1, respectively), indicating that the surface interaction is not completely Brønsted in nature. VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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be determined, but the adsorption geometry can be envisioned based on the interactions described. The environmental fate of these volatile organic molecules in dry and damp silica-based vadose zone soils can be predicted based on these surface interactions. Under conditions of low organic matter content, the extent of their vapor sorption to soil and, hence, their retention in the soil phase, is a direct function of their interaction strength with silica. The strong hydrogen bonding described for the picolines with silica would result in these molecules being retained in dry or damp soil, since the strength of their hydrogen bonding is slightly greater than their hydrogen bonding in aqueous solution. However, the weaker hydrogen bonding of the π-systems of benzene and toluene suggests that these molecules weakly adsorb to soil from the vapor phase, resulting in some retention in dry or damp soil. However, in wet soil where competitive water adsorption is significant, the forces involved in this interaction are probably insufficient to result in significant retention. These results also demonstrate the utility of Raman and FTIR spectroscopies in elucidating surface sorption mechanisms for model pollutant-soil systems. These techniques are being extended to more complex systems in this laboratory to better model natural environmental conditions. Results of these studies will be forthcoming. FIGURE 5. Schematic of the proposed interactions of benzene, toluene, 4-picoline, and 2-picoline with dry silica. Shifts observed in the ν(CH) region for adsorbed 2-picoline are more consistent with hydrogen bonding. The shifts in the ν(CH3) and ν(CH)ring bands are similar for adsorbed and aqueous 2-picoline. The magnitude of the shift in the ν(CH)ring modes is about 20 cm-1, similar to the shift observed in the ν(CH)ring mode of pyridine in an aqueous hydrogen-bonding environment (33, 35, 55). For the 2-picolinium ion, the ν(CH)ring modes each shift toward higher frequency by ∼50 cm-1. Without further evidence for a Brønsted interaction, 2-picoline interaction with silica via strong hydrogen bonding with surface silanols through the ring N is proposed, analogous to that described above for 4-picoline. The literature contains only one other Raman spectral study of a 2-substituted pyridine on silica. The Raman spectrum of adsorbed 2-chloropyridine indicated only a physisorbed layer (32). The lack of hydrogen bonding was attributed to steric hindrance between the Cl atom and the silica. In contrast, clear evidence for hydrogen-bond formation is observed for 2-picoline. Steric hindrance from the methyl group does not inhibit hydrogen bonding. Given the similarity in size of a Cl and a methyl group, these results lead to the conclusion that factors other than steric hindrance prevent hydrogen bonding between 2-chloropyridine and silica. FTIR spectral data for 2-picoline are presented and discussed in the Supporting Information. These data support the conclusions drawn on the basis of the Raman spectroscopy. Adsorption Models and Environmental Ramifications. The results of these studies suggest that the major mechanism of benzene, toluene, 2-picoline, and 4-picoline interaction with silica is hydrogen bonding through the ring π-system for benzene and toluene and through the lone pair electrons on the N atom for the picolines. A schematic of the proposed adsorption mechanisms on dehydrated silica is shown in Figure 5. Despite the molecular detail of these pictures, certain aspects of the surface interaction cannot be determined from these studies. The van der Waals interaction contribution to the adsorption mechanism is difficult to determine from these data. The absolute geometry of adsorbed molecules cannot 264

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Acknowledgments The authors gratefully acknowledge the Department of Energy, Office of Health and Environmental Research through the DOE Idaho Operations Office (Contract DE-AC0794ID13223) for partial support of this work. Partial support of this work by the National Science Foundation (CHE9504345) is also gratefully acknowledged. We also acknowledge receipt of Associated Western Universities Graduate (S.C.R.) and Faculty (J.E.P.) Fellowships.

Supporting Information Available Tabulated Raman spectral data for neat and adsorbed benzene and toluene on silica; tabulated Raman and FTIR spectral data for neat, aqueous, and adsorbed 4-picoline and 2-picoline on silica; spectral FTIR data for hydrated and dehydrated silica; spectral FTIR data and discussion for neat and adsorbed 4-picoline and 2-picoline (12 pages). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review September 21, 1998. Revised manuscript received October 25, 1999. Accepted November 1, 1999. ES980970A

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