Spectroscopic Study of Carbaryl Sorption on Smectite from Aqueous

In addition, the amount of carbaryl sorbed was strongly dependent on the nature of the exchangeable cation and followed the order of Ba ∼ Cs ∼ Ca ...
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Environ. Sci. Technol. 2005, 39, 9123-9129

Spectroscopic Study of Carbaryl Sorption on Smectite from Aqueous Suspension MAURILIO FERNANDES DE OLIVEIRA,† C L I F F T . J O H N S T O N , * ,† G. S. PREMACHANDRA,† BRIAN J. TEPPEN,‡ HUI LI,‡ DAVID A. LAIRD,§ DONGQIANG ZHU,† AND STEPHEN A. BOYD‡ Crop, Soil and Environmental Sciences, Purdue University, 915 West State Street, West Lafayette, Indiana 47907-2054, Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824-1325, and USDA-ARS National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011

Sorption of carbaryl (1-naphthyl-N-methyl-carbamate) from aqueous suspension to smectite was studied using Fourier transform infrared (FTIR), high-performance liquid chromatography (HPLC) (for batch sorption), and quantum chemical methods. The amount of carbaryl sorbed was strongly dependent on the surface-charge density of the smectite with more sorption occurring on the two “low” surface-charge density smectites (SHCa-1 and SWy-2) compared to that of the high surface-charge SAz-1 smectite. In addition, the amount of carbaryl sorbed was strongly dependent on the nature of the exchangeable cation and followed the order of Ba ∼ Cs ∼ Ca > Mg ∼ K > Na ∼ Li for SWy-2. A similar trend was found for hectorite (SHCa1) of Cs > Ba > Ca > K ∼ Mg > Na ∼ Li. Using the shift of the carbonyl stretching band as an indicator of the strength of interaction between carbaryl and the exchangeable cation, the observed order was Mg > Ca > Ba ∼ K > Na > Cs. The position of the carbonyl stretching band shifted to lower wavenumbers with increasing ionic potential of the exchangeable cation. Density functional theory predicted a cation-induced lengthening of the CdO bond, resulting from the carbonyl group interacting directly with the exchangeable cation in support of the spectroscopic observations. Further evidence was provided by a concomitant shift in the opposite direction by several vibrational bands in the 1355-1375 cm-1 region assigned to stretching bands of the carbamate N-Ccarbonyl and Oether-Ccarbonyl bonds. These data indicate that carbaryl sorption is due, in part, to site-specific interactions between the carbamate functional group and exchangeable cations, as evidenced by the FTIR data. However, these data suggest that hydrophobic interactions also contribute to the overall amount of carbaryl sorbed. For example, the FTIR data indicated that the weakest interaction occurred when Cs+ was the exchangeable cation. In contrast, the * Corresponding author phone: (765) 496 1716; fax: (765) 496 2926; e-mail: [email protected]. † Purdue University. ‡ Michigan State University. § USDA-ARS National Soil Tilth Laboratory. 10.1021/es048108s CCC: $30.25 Published on Web 10/27/2005

 2005 American Chemical Society

highest amount of carbaryl sorption was observed on Csexchanged smectite. Of all the cations studied, Cs has the lowest enthalpy of hydration. It is suggested that this low hydration energy provides the carbaryl with greater access to the hydrophobic regions of the siloxane surface.

Introduction N-Methyl-carbamates are among the most widely used insecticides in both agriculture and horticulture. Members of the N-methyl-carbamate family of insecticides include carbaryl, carbofuran, aldicarb, pirimicarb, oxamyl, and methomyl. Within this group, carbaryl is the most widely used N-methyl-carbamate insecticide in use today. In 1997 an estimated 4.9 million pounds of carbaryl were used to treat over 3 million acres in the United States alone. Carbaryl is a nonionic, moderately mobile compound in the environment, and although it is degraded through both abiotic and microbially mediated processes, carbaryl is the second most widely detected insecticide in surface water (1). At present, there is increased concern about the environmental fate and toxicity of these compounds because of their high toxicity to both humans and wildlife.

For the past 25 years, soil organic matter (SOM) has been considered the dominant soil constituent responsible for the sorption of neutral organic solutes, such as carbaryl, which are believed to partition into hydrophobic domains with SOM (2-5). Confirmation of this nonspecific mechanism is difficult because spectroscopic methods are not sensitive to the subtle surface interactions and generally only provide negative evidence to rule out other site-specific interactions. A comprehensive study of carbaryl sorption using a wide range of soils (48 soils from Australia, Pakistan, and the United Kingdom) showed that carbaryl sorption was not correlated with soil organic carbon content (6). Expandable clay minerals are strongly hydrophilic on a macroscopic scale (7-9) and therefore it has been incorrectly assumed that clays have a low affinity for neutral organic solutes. However, Laird and co-workers showed that Caexchanged smectites with a low surface-charge density had an unexpectedly high affinity for atrazine (10) with Freundlich isotherm constants (Kf values) in excess of 1300 (µmol/kg)/ (µmol/L)1/n. Although atrazine is a weakly ionizable compound (pKa ) 1.7), under the pH conditions of their experiment (4.5-6), atrazine could only be sorbed as the neutral species. Additional support for the role of clay minerals in atrazine sorption has been reported using different metal cations (11, 12) and with soil clays of mixed mineralogy (13). In fact, smectites can adsorb more than 1% neutral atrazine by weight when Cs+ is the exchangeable cation (14). Similarly, nitroaromatic compounds (15) were shown to have a higher than expected affinity for soil clays that were saturated with weakly hydrated cations (12, 1521). Examples include 1,3,5-trinitrobenzene (1,3,5-TNB) and 6-methyl-2,4-dinitrophenol with adsorption coefficient Kd values > 105 L kg-1 (20). Smectites are strongly hydrophilic on a macroscopic scale, but on a molecular scale the uncharged regions between charge sites on siloxane surfaces have recently been shown VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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to have a partial hydrophobic character (22). Sheng et al. (12) measured sorption isotherms of seven pesticides, including carbaryl, on two end-member sorbents: K-exchanged SWy-2, a low surface-charge density reference smectite, and a muck soil. The K-saturated SWy-2 smectite was found to be the dominant sorptive phase for dinitro-o-cresol, dichlobenil, and carbaryl. Atrazine sorption occurred on both solid phases and the muck soil was the dominant sorbent for biphenyl, diuron, and parathion. Several recent combined spectroscopic-sorption studies have shown that the high affinity of K-smectites for nitroaromatic compounds (NACs) in aqueous suspension is due primarily to site-specific interactions between the interlayer K ions and NACs (18, 20). By contrast, little is known about the mechanisms of carbamate sorption on clay minerals in aqueous suspension. Therefore, the purpose of this study is to examine the sorption mechanisms of carbaryl by smectites in aqueous suspension through the combined application of spectroscopic methods, sorption isotherms, and quantum chemical modeling.

Materials and Methods Four smectite clays were studied: the Wyoming montmorillonites (SWy-1 and SWy-2) collected from Crook County, WY, a hectorite (SHCa-1) collected from San Bernardino County, CA, and the Arizona montmorillonite (SAz-1) collected from Apache County, AZ. All clays were obtained from the Source Clays Repository of The Clay Minerals Society located at Purdue University. The physical and chemical properties of these clays have recently been described (23). For the Wyoming montmorillonite, the spectroscopic data were obtained from the SWy-1 smectite. This clay has been depleted from The Source Clays Repository and it is no longer available. Sorption data were obtained using SWy-2, which has chemical and physical properties similar to those of the SWy-1. To prepare the clays, portions (20 g) of the raw clay were placed in 1.0 L of 0.5 M NaCl for 24 h, then the suspensions were washed free of excess salts and the Na ∼ Li for SWy-2, and Cs > Ba > Ca > K ∼ Mg > Na ∼ Li for SHCa-1. The final VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. FTIR spectra of a self-supporting clay film of K-saturated SWy-1 smectite (A) and a self-supporting clay film of carbaryl sorbed on K-saturated SWy-1 (B), ATR-FTIR spectra of an 80 mg/L solution of carbaryl in aqueous solution containing 0.1 M KCl (C), and carbaryl in a KBr pellet (1 mg of carbaryl in 200 mg of KBr) (D). pH for all of the sorption experiments ranged from 5.0 to 5.7. Similar to the data for the K-saturated smectites shown in Figure 1, higher sorption occurred on the SHCa-1 than on SWy-2 for all of the metal cations used in this study (Figures 2 and 3). It is interesting to note that for both the SWy-2 and SHCa-1 clays, the Ca-exchanged smecites had higher carbaryl sorption than did the K-exchanged clays. In our previous work on nitroaromatics, the opposite trend was found, with K-exchanged clays having a higher affinity for the NACs over the Ca-exchanged clays (12, 18, 19, 21, 33). Numerous sorption studies have shown that sorption, and sometimes degradation, of organic solutes, including pesticides, on clay minerals is dependent on the nature of the exchangeable cation (11, 12, 18, 19, 21, 33, 36-39). In the case of atrazine, for example, significantly more sorption occurred on K-saturated SWy-2 smectite than for Ca-SWy-2 (12). Similarly, in sorption studies on NACs, significantly more sorption occurred on smectites exchanged with weakly hydrated monovalent cations (i.e., Cs+ and K+) relative to more strongly hydrated divalent cations (e.g., Ca2+). The preference of organic solutes for K- over Ca-saturated clays has also been observed for thiazaluron (40), imazamethabenz-methyl (41), and the nitroaromatic compounds dinitro-o-cresol and dinoseb (19). For the NACs, FTIR spectra of the clay-organic complexes showed that the -NO2 groups interacted directly with weakly hydrated exchangeable cations (i.e., K+ and Cs+) (18, 20, 33). Furthermore, the strongest spectral perturbations correlated directly with the amount of NAC sorption, and thus provided direct support of the site-specific interaction between the NO2 substituents and the exchangeable cations. Significantly less NAC was sorbed on clays saturated with more strongly hydrated cations, such as Ca2+ and Mg2+, and this was attributed to the potential binding sites being blocked by the water molecules hydrating the interlayer of Ca2+ and Mg2+ ions (18, 20, 33). Ab initio quantum calculations confirmed the spectroscopic findings. Spectroscopic Study of Carbaryl Sorption. In this study, the Ca-exchanged smectites show a greater preference for carbaryl as compared to K-smectites (Figures 2 and 3). In related work on nitroaromatics (18, 21, 33), we found that K-exchanged smecites exhibited a considerably greater sorption affinity over Ca-smecites. Thus, it would appear that a different sorption mechanism may be at work for carbaryl. Using an approach similar to earlier studies on nitroaromatics (18, 21, 33), FTIR spectroscopy was used to study the interaction of carbaryl with the clay surface. The FTIR spectrum of carbaryl sorbed to a K-exchanged SWy-1 smectite in the 1800-1200 cm-1 region is shown in Figure 4B. For comparison, an ATR-FTIR spectrum of carbaryl in 9126

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FIGURE 5. Quantitative FTIR analysis of carbaryl sorbed to K-exchanged SWy-1 smectite. The absorbance of the 1727 and 1599 cm-1 bands is plotted as a function of the amount of carbaryl sorbed determined using HPLC expressed as nanomoles of carbaryl sorbed/ cm2 of clay. FTIR spectra were normalized using the structural OH stretching band of the clay at 3630 cm-1. aqueous solution (spectrum C) and a spectrum of solid carbaryl in a pressed KBr pellet (spectrum D), along with the FTIR spectrum of the K-SWy-1 clay-only spectrum (spectrum A) are included in Figure 4. The strongest carbaryl band in this spectral region is the carbonyl stretching band (i.e., ν(CdO) at ∼1727 cm-1). The shape and position of the carbonyl stretching band in the carbaryl-K-SWy-2 smectite complex are distinct from those present in the carbaryl-only spectra (spectra C and D). In addition to the carbonyl stretching band, the bands at 1526 and 1358 cm-1 in the spectrum of carbaryl sorbed to K-SWy-1 smectite (spectrum B) were perturbed compared to those in the carbaryl-only spectra (spectra C and D). IR spectra of carbaryl sorbed to homoionic smectites (M ) Al, Cu, Na, and Ca) were reported previously (39). In this prior study, carbaryl was sorbed to self-supporting clay films exchanged with different cations from a concentrated organic (CCl4) solution. Although water was not present in this earlier study, the overall spectral observations found in this study are in good agreement with the aqueous results presented here. For example, the positions of the carbaryl bands sorbed to Na- and Caexchanged smectites were within a few wavenumbers of the positions observed in the previous study (39). Similar to the data shown in Figure 4B, bands were observed in the 13681378 cm-1 region for carbaryl sorbed to montmorillonite that were not present in the KBr matrix (39), these bands were assigned to be the carbamate C-N stretch. Quantum chemical calculations (discussed later in this paper) were used to confirm this assignment. Quantitative FTIR methods were used to determine if a linear relationship existed between the amount of carbaryl sorbed, determined using HPLC, and the intensities of the FTIR bands of carbaryl. Two bands were selected for this analysis: the carbonyl stretching band at approximately 1727 cm-1 and the weaker band at 1599 cm-1 assigned to a ring stretching vibration. Coupled spectroscopic-sorption isotherms were obtained for carbaryl sorbed to K-exchanged SWy-2 smectite and the results are shown in Figure 5. The absorbance values of the 1727 and 1599 cm-1 bands are plotted against the HPLC-derived surface concentration of carbaryl, expressed in nanomoles of carbaryl/cm2. The data were fit to a linear curve forced through the origin. The slopes of the two curves, shown in Figure 5, correspond to the molar absorptivities of the two bands, with values of 0.00193 and 0.00024 cm2/nmol for the 1727 and 1599 cm-1 bands. The highly correlated relationship between the FTIR and HPLC

FIGURE 6. FTIR spectra of self-supporting clay films of carbaryl sorbed to SWy-1 saturated with different metal cations in the 1800-1500 cm-1 region (left side) and in the 1410-1320 cm-1 region (right side). Self-supporting clay films were prepared from clay suspensions that contained 26 mg of smectite in 25 mL of solution with an initial carbaryl concentration of 82.6 mg/L. The amount of carbaryl was not determined and the spectra were analyzed by changes in band position. results confirms that the carbaryl detected by FTIR reflects the amount of carbaryl sorbed determined using HPLC. These results are in good agreement with our recent quantitative FTIR-HPLC studies of NAC sorption on smectites (18-21, 33). FTIR spectra of carbaryl sorbed to SWy-1 self-supporting clay films exchanged with different exchangeable cations are shown in Figure 6. As shown, the position and relative intensity of the carbonyl stretching band at 1725-1695 cm-1 were influenced by the nature of the exchangeable cation (left side of Figure 6). In addition to the carbonyl shift, a concomitant shift in the position of the carbamate C-N band at 1350-1375 cm-1 was observed (right side of Figure 6). These shifts are in good agreement with the prior IR study of carbaryl sorbed to montmorillonite from an organic solvent (39). Fusi and co-workers (39) observed that the two bands most influenced by the exchangeable cations were the carbonyl stretching bands (1708-1670 cm-1) and the C-N stretching band (1368-1380 cm-1) in agreement with spectra shown in Figure 6. The strongest shifts occurred for carbaryl sorbed to montmorillonite exchanged with Al3+ and Cu2+ and resulted in a decrease in frequency of the carbonyl band and a shift of the ν(CN) band to higher frequency. As shown in Figure 7, the position of the carbonyl stretching band was inversely related to the position of the carbamate band at 1350-1375 cm-1. Furthermore, the extent of the respective red- and blue-shifts depends on the valence of the exchangeable cation. As shown in Figure 7, the band shifts of the carbonyl and C-N stretching bands are clearly related to the nature of the exchangeable cation. The position of the carbonyl stretching band of carbaryl in aqueous solution (Figure 4C, 84 mg/L in 0.1 M KCl) occurs at 1722 cm-1; the corresponding position in the KBr pellet is 1712 cm-1 (Figure 4D), where reasonably some intermolecular hydrogen bonding occurs, resulting in a small shift in the position of this band to lower energy (i.e., a shift to lower wavenumber). As shown in Figure 6, the position of the carbonyl stretching band decreases in the order Cs > K > Na and Ba > Ca > Mg. The position of the

FIGURE 7. Band position of the 1350-1375 cm-1 band plotted as a function of the position of the carbonyl stretching band for carbaryl sorbed to SWy-2 smectite. Band positions were determined using a peak-fitting algorithm in the Grams v 6.0 program (Galactic Software). carbonyl stretching band is plotted as a function of the ionic potential (Z2/reff) of the exchangeable cations in Figure 8. As the ionic potential of the cation is increased, the position of the carbonyl band shifts to lower energies. A shift of the carbonyl stretching band to lower energy indicates a stronger interaction between the exchangeable cation and carbonyl group resulting in a longer bond length of the CdO bond. The spectral band shifts were not noticeably influenced by clay type; similar cation-induced effects were founds for both SWy-1 and SHCa-1. The shifts of the bands shown in Figure 6 provide direct evidence that the carbamate functional group of carbaryl interacts directly with the exchangeable cations. The ionic potential of a cation combines the properties of both charge and ionic radius into one numerical value. The small, more highly charged cations (Mg2+ for the alkaline earths and Na+ for the alkali metals) can interact more strongly with the partial negative charge of the oxygen atom of the carbamate VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Position of the carbonyl stretching band as a function of ionic potential of the exchangeable cation (Z2/reff) for SWy-2 and SHCa-1. Ionic potential values were obtained from Huheey (44). group. Within the group of alkaline earth and alkali metal cations, linear but distinct trends are evident in the order of Na+ > K+ > Cs+ and Mg2+ > Ca2+ > Ba2+. Similar trends were reported almost 40 years ago by Mortland and Meggitt (42) who studied the IR spectra of ethyl N,N-di-n-propylthiolcarbamate sorbed to smectite exchanged with different exchangeable cations (Mn+ ) Li, Na, Ca, Mg, Al, Cu, and Co). As the complexing ability of the exchangeable cations increased, the position of the C-O stretching band decreased with a concomitant increase in the position of the C-N stretching band. Further support was provided by Nakamoto (43) who suggested that compounds with similar functionality would form complexes through the C-O/CdO group rather than through the nitrogen atom. Quantum Chemical Calculations. These observations are supported by quantum chemical calculations. In the K+carbaryl complex, the OdCcarbonyl bond length increased by 0.014 Å with a concomitant 0.044 Å decrease in the N-Ccarbonyl bond length compared to noncomplexed carbaryl. These changes are consistent with the observed red-shift of the carbonyl stretching band (left side of Figure 6) and the observed increase (i.e., blue-shift) in energy of the 13501375 cm-1 band. The specific assignments of the complex bands in the 1355-1375 cm-1 region (Figure 6) are more difficult than the carbonyl stretching band. Ab initio calculations predict the occurrence of three mixed-mode carbamate bands in this spectral region: A vibrational band predicted at 1383 cm-1 (scaled frequency 1342 cm-1) contains substantial contributions of the N-Ccarbonyl and Oether-Ccarbonyl stretches. Thus, we have no reason to disagree with the previous assignment (39) of this band as the carbamate C-N stretch. The inverse relationship between the CdO band and this C-N band at 1355-1375 cm-1 (Figure 7) is physically reasonable since there are just three bonds around the carbonyl C. As the CdO bond is weakened via interaction with hydrated cations, the N-Ccarbonyl and/or the Oether-Ccarbonyl bonds must strengthen to maintain the bond-valence sum around the Ccarbonyl atom. Our quantum calculations show that the OetherCcarbonyl bond lengthens upon complexation, so the N-Ccarbonyl bond is forced to strengthen to compensate for the weakening of the other two bonds around Ccarbonyl. Similar to recent sorption studies using triazines and nitroaromatics, higher than expected sorption of carbaryl was observed in this study. FTIR spectra of carbaryl sorbed to three different smectites saturated with different exchangeable cations provide unambiguous evidence that the carbamate functional group interacts directly with hydrated exchangeable cations. As the ionic potential of the exchangeable cation is increased, the extent of the spectral perturbations of the sorbed carbaryl compound increases. This is shown by systematic decrease in frequency of the carbonyl 9128

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stretching band as the ionic potential of the exchangeable cation is increased. As the sorption data indicate, however, there appears to be a tradeoff between the electrostatic interaction between carbamate functional group and the hydrated cation,with the effective hydrated radius of the exchangeable cation. For example, the strongest molecular interaction between the carbamate functional group and metal-exchanged SWy-1 clay occurs for the Mg-exchanged smectite. The FTIR spectra indicate that Mg2+ forms a stronger complex with the carbonyl group resulting in red-shift of the carbonyl band to ∼1697 cm-1. In the case of Ca, the shift in position of the carbonyl band is less than ∼1705 cm-1. Considerably more carbaryl is sorbed to the Ca2+-exchanged clay than in the case of Mg2+. Of all the cations studied, however, Mg has the largest enthalpy of hydration (-2000 kJ/mol compared to a value of -315 kJ/mol for Cs+). Thus, Mg will be the most strongly hydrated cation with between 8 and 12 water molecules per Mg2+ cation (7). Water competes strongly for coordination sites around the Mg2+ interlayer cations and it is argued that much of the hydrophobic siloxane surface is covered by overlapping hydrations shells around the Mg2+ cations. Thus, one interesting feature of the combined spectroscopic and sorption data presented here is that the strongest spectral changes do not correspond to the situations where the highest sorption occurs. In fact, the opposite is true. For both sets of cations, for example, the strongest spectroscopic interactions follow the order of Mg2+ > Ca2+ > Ba2+ and, similarly, Li+ > Na+ > K+ > Cs+. The opposite trends are evident for sorption with the order of carbaryl sorption following Ba2+ > Ca2+ > Mg2+ and Cs+ > K+ > Na+ > Li+.

Acknowledgments This research was funded in part by the USDA National Research Initiative Competitive Grant 2003-35107-12899 and the Purdue Agricultural Research Programs.

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Received for review November 30, 2004. Revised manuscript received September 15, 2005. Accepted September 19, 2005. ES048108S

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