Molecular Interactions of Pesticides at the Soil−Water Interface

Jun 27, 2008 - to analyze various modes of soil interaction (4, 9, 10) and also allows for the unique opportunity to study chemical processes at the s...
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Environ. Sci. Technol. 2008, 42, 5514–5520

Molecular Interactions of Pesticides at the Soil-Water Interface AZADEH SHIRZADI, MYRNA J. SIMPSON, RAJEEV KUMAR, ANDREW J. BAER, ´ J. SIMPSON* YUNPING XU, AND ANDRE Department of Chemistry, University of Toronto, Scarborough College, 1265 Military Trail, Toronto, Ontario, M1C 1A4 Canada

Received February 23, 2008. Revised manuscript received April 22, 2008. Accepted May 20, 2008.

High-resolution magic angle spinning (HR-MAS) NMR spectroscopy combined with saturation-transfer double difference (STDD) NMR can be used to analyze the molecularlevel interactions of pesticides and whole soils occurring at the soil-water interface. Here 1H HR-MAS STDD NMR has been applied to some common pesticides (trifluralin, acifluorfen, and (4-nitro-3-(trifluoromethyl) phenol) and a pesticide degradation product (1-naphthol). Results indicate that dipolar interactions, H-bonding, hydrophobic associations, and potentially π-π interactions are the predominant sorption mechanisms for these molecules at the soil-aqueous interface. It is evident that the physical and chemical characteristics of soil are highly influential in determining the mechanisms of pesticide sorption, as they significantly affect soil conformation. In particular, different binding mechanisms were observed for 1-naphthol in soil swollen using a buffer versus D2O, indicating that the Koc alone may not be enough to accurately predict the behavior of a molecule in a real soil environment. Preliminary kineticbased studies suggest that both the swelling solvent and soil moisture content significantly influence the sequestration of trifluralin. These studies demonstrate that HR-MAS and STDD NMR are powerful and versatile tools which can be applied to expand our knowledge of the mechanistic interactions of agrochemicals at the molecular level.

Introduction Pesticide sorption to soil organic matter (SOM) plays an important role in reducing pesticide bioavailability and complicates bioremediation efforts (1). Elucidation of the mechanistic interactions of anthropogenic chemicals in whole soil (2, 3) as well as understanding the physical and chemical factors which influence their sorption (4, 5) is critical to understanding and eventually predicting their behavior in the environment. In some cases, pesticides are overapplied, as a large fraction interacts with the soil instead of entering the target organism. The overapplication of agrochemicals is costly to farmers, highly detrimental to the environment, and increases the chances of human exposure. Expanding our knowledge of mechanistic interactions of agrochemicals at a molecular level should, in theory, allow the design and manufacture of novel chemicals, which are more effective and require lower application rates, creating a beneficial situation for agricultural practice and the environment. * Corresponding author tel.: 1-416-287-7547; fax: 1-416-287-7279; e-mail address: [email protected]. 5514

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Recently, a relatively novel NMR method, saturation transfer double difference (STDD) NMR spectroscopy (6), has been shown to be a powerful tool for probing the interactions of contaminants with natural organic matter in solution (7). High-resolution magic angle spinning (HR-MAS) NMR allows NMR spectra of semisolid and swellable materials to be acquired (8). HR-MAS NMR has previously been used to analyze various modes of soil interaction (4, 9, 10) and also allows for the unique opportunity to study chemical processes at the soil-water interface (11). HR-MAS has been combined with saturation transfer difference (STD) to study protein-ligand interactions (12) and molecularly imprinted polymer binding interactions (13) but never, to our knowledge, for environmental samples. In this study, the molecular interactions of four common pesticides with a peat soil are investigated. Trifluralin is a selective herbicide used to control a wide range of annual grasses and broadleaf weeds; it is bioaccumulative and persistent and is a possible human carcinogen (14). Acifluorfen is an herbicide, used in the selective control of broadleaf weeds; it is persistent in soils and aquatic environments, and it is relatively mobile (15). 4-Nitro-3-(trifluoromethyl) phenol (TFM) is used to control sea lamprey; it is chemically and biologically stable, and it is known to be toxic for >80 days in the aquatic environment (16). 1-Naphthol is the primary degradation product of the neurotoxic insecticide carbaryl and has been shown to be more harmful than its parent compound in some cases (17). In this study, HR-MAS STDD is employed to analyze the binding mechanisms of the chemicals with whole peat soil swollen in both a buffer and D2O. Additional kinetic-based NMR studies were performed with trifluralin to investigate the influence of basic chemical and physical soil conditions on its sequestration.

Materials and Methods The following sections provide a brief summary of the experimental protocols. For full details, see the Supporting Information S1. Sample Preparation. Samples were prepared by first dissolving 10 mg of pesticide in 30 µL of acetone-d6, then adding this solution to 40 mg of finely ground Florida Pahokee peat soil. The contents of the rotors were homogenized, and acetone-d6 was then evaporated using nitrogen gas. In most cases, peat soil was used as-is, termed “air-dried peat soil”. In one of the kinetics experiments, the peat soil was further dried in a vacuum desiccator over phosphorus pentoxide for 2 weeks, and this sample is referred to as “desiccator-dried peat soil”. Two different swelling solvents were used to swell the peat soil samples: 60 µL of a pD 7 sodium phosphate D2O buffer solution or 60 µL of pure D2O. The reference rotors, which contained only the peat soil, were prepared in the exact same manner as above, with the exception that no pesticide was added to the rotor. Before addition to the soil, the buffer and D2O had pD values of 7.0 and 7.4, respectively. After mixing with the soil, the pD values dropped to 5.9 and 5.8, respectively. NMR Spectroscopy. Full details of NMR experiments are provided in the Supporting Information. Briefly, STD experiments were carried out using the approach described by Mayer and Meyer (18), although without the use of relaxation filters to suppress the background signals. The background signals were subtracted using a double difference approach (6). Selective saturation was 10.1021/es800115b CCC: $40.75

 2008 American Chemical Society

Published on Web 06/27/2008

performed at a chemical shift of 3.79 ppm and off-resonance at 300 ppm. Unless otherwise stated, STD experiments with 1-naphthol, TFM, and trifluralin employed an effective field of 34 Hz for irradiation (∼70 db attenuation on a 60 W amplifier). Readers should note that the effect of varying the irradiation frequency was tested for 1-naphthol, acifluorfen, and TFM, and no changes in the epitope maps were observed (see Supporting Information section S2). As such, it was concluded that for the contaminants tested the irradiation frequency produced no quantifiable change in the binding epitope, and a single irradiation point reflected the interactions of the soil surface as a whole. Readers should be aware that this may not be the case for different soils and contaminants and that varying the point (and power) of irradiation may yet prove to be a valuable tool in unravelling the differing interactions of specific soil components in future studies (see Supporting Information S2). Note that it is impossible to irradiate peat/trifluralin at different frequencies due to the extensive overlap of the pesticide and peat signals. As such, it is possible that irradiation at 3.79 ppm may bias the interactions of the saturated protons (mainly lignin methoxyl and carbohydrates (19)) and that in the case of trifluralin, which could not be tested, these components may not represent the interactions of the soil interface as a whole. For kinetic studies, trifluralin was introduced into a rotor, homogenized, sealed, and immediately transferred to the NMR spectrometer. The total time between spiking and the collection of the first data point was ∼3 min. 1H spectra acquisition was performed every minute for the first 45 min, and then every 8 min for the next 15 h. Unless referenced otherwise, all reported Koc and pKa values have been calculated using Advanced Chemistry Development Physico-Chemical Laboratory (20). Note that, in the case of Koc, the values do not account for the specific soil used in this study. These values are provided only as a rough guide such that a reader can gauge how the Koc values are expected to change with pH for the various pesticides.

Results and Discussion Saturation Transfer Double Difference (STDD) Epitope Mapping. Detailed explanations of the STD and STDD NMR techniques have been previously discussed (6, 7, 18). Briefly, STDD NMR epitope mapping involves the selective saturation of the peat soil (i.e., the “receptor”). Saturation is then transferred to any pesticide bound or interacting with the soil, with the pesticide nuclei closest to the soil receiving the greatest amount of saturation and those furthest from the soil receiving the least. The experimental design ensures that all other signals (e.g., water or unbound pesticide) cancel and are not observed. Thus, information regarding molecular binding is encoded in the integrals of the pesticide signals in the STD spectrum. Before this information can be extracted, it is important to correct for the soil background, which can be accomplished by using a double difference subtraction approach (6, 21). With the background signals successfully eliminated, a quantitative comparison of the double difference reference spectrum (DDRS), which is a spectrum of the pesticide-soil mixture without saturation transfer (Figure 1A), and the STDD spectrum (Figure 1B) is used to produce an epitope map, which describes the binding mechanism of the molecule. Readers should be aware that the epitope maps presented here will generally reflect the average binding orientation of the pesticide and that in a soil environment there could be many different orientations that give rise to this “average binding epitope”. It also should be noted that in “conventional STD” experiments a relaxation filter is used to remove the background from the macromolecule (in this study, the soil), as such conventional results (STD) strongly bias toward

the interaction of ligands in fast exchange. In this study, relaxation filters were not used, and the soil background was removed via spectral subtraction (STDD). This strategy was purposely employed such that both tightly bound and fastexchanging species will contribute to the final binding epitope. Indeed, this is clear from studies of 1-naphthol (see later) in which tightly bound pesticide can be observed. However, it should be pointed out that, if both pesticides in fast exchange (sharp lines) and tightly bound (broad lines) are present, the signals from the tightly bound fraction could be underestimated, mainly due to the preferential relaxation of the tightly bound fraction during the delays of the water suppression technique. Therefore, while every effort has been taken to detect both strongly interacting and fast exchanging molecules, the very nature of the NMR experiments themselves may bias the results toward the fast-exchanging component when both bound and fast-exchanging components coexist in the mixture. In the STDD spectrum (Figure 1B), the protons of trifluralin most closely associated with the soil receive the greatest amount of saturation and, thus, have higher relative signal intensities when compared to the DDRS. The epitope map is created by setting the proton signal showing the strongest interaction with the soil to 100%, with all other protons reported relative to it (18). As such, the epitope map provides a quantitative description of the degree of interaction of each of the pesticide protons with the soil. The insert in Figure 1A shows the epitope map of trifluralin as an example. For a more in-depth explanation of this technique, please refer to refs 12, 18, and (21). It is important to note that only the interacting fraction of the pesticide is observed using STD-NMR, and molecules not in direct contact with the soil are not observed (18, 21). Trifluralin Interactions with Soil. The HR-MAS STDD epitope map of trifluralin binding to the swollen peat soil (using a pD 7 sodium phosphate D2O buffer) is depicted in Figure 1C1. Protons (Ha) on the aromatic ring, which are located between the CF3 and the nitro groups, show the highest binding affinity (100%). This high affinity may be the result of the dipoles created by the polar, covalent C-F bonds (22). Interactions with the soil may include H-bonding, dipole-dipole interactions, and various other interactions based on partial charge. This is consistent with previous findings, which demonstrated that halogen groups at pD 7 play a key role in the interactions of pesticides with humic acid (HA) (21). At pD 5.9 (pD measured in soil-buffer), trifluralin has a very high organic carbon absorption coefficient (Koc) of 20,898 (ACD laboratories Inc., ACD/Adsorption Predictor, V7.04), and it is likely that the CF3 group contributes in part to this sorption coefficient. The nitro groups may also play a role in the high affinity of protons Ha for the peat soil; however, as the binding affinities of protons Hb are not very high (37%), it is more likely that the CF3 group dominates in trifluralin’s binding mechanism. This is supported by the results with acifluorfen and TFM (see later), in which the nitro groups appear to play a less dominant role than the COOH and CF3 groups at pD values of 5.8-5.9. At the other end of the molecule, it appears that the aliphatic N plays a significant but less influential role, as binding affinity decreases along the alkane chains Hb(37%) > Hc(16%) > Hd(14%). It is feasible that π-π interactions between the aromatic ring in trifluralin and soil aromatics play a role in its binding. However, the benzene ring in trifluralin will be electrondeficient due to the presence of electron-withdrawing groups (-F and -NO2). The peat soil contains ∼25% aromatics (based on 13C Ramp-CP/MAS, see Supporting Information Figure S5), a large portion of which is also likely to be electrondeficient due to substituted electron-withdrawing groups (19). As such, π-π interactions between trifluralin and much of the SOM are likely to be weak. Previous studies (21) and VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Trifluralin in peat soil swollen with the buffer, (A) DDRS and (B) STDD NMR. The inset on spectrum A shows the epitope map for trifluralin. (C-F) Epitope maps of the four pesticides in the buffer or D2O as labeled. The percentage values for each epitope map are all relative to the strongest binding proton in each pesticide, which is expressed as 100%. To be consistent with previous work (7), protons which have 80-100% binding are highlighted by red circles, protons with 65-79% with blue, and 64% and lower with green. The lower-case letter “a” next to a proton indicates that it has the highest chemical shift; “b” refers to the second-highest chemical shift, and so on. The shaded region in 1D1 and 1D2 simply highlight the side of the molecule that has the strongest interaction with the soil surface. An asterisk (*) indicates that absolute quantification was not possible at these positions due to spectral overlap; percent ranges based on spectral deconvolution are provided (see the text). results from other compounds discussed later in this manuscript suggest that, while π-π interactions may be important, interaction with electronegative groups such as halides and carboxyl groups are more significant. Trifluralin shows a similar binding epitope when sorbed to peat soil swollen with D2O (Figure 1C2). When in D2O, however, the protons further from the CF3 group show higher affinities for the soil (i.e., higher percentages/greater interaction), Hb(90%) > Hc(58%) > Hd(52%), than their buffered counterparts. This is indicative of a more nonspecific binding mechanism in D2O compared to when a buffer is used as the 5516

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swelling solvent, possibly due to differences in soil conformation. It has previously been demonstrated that, in the presence of the background electrolyte Na+, HA takes on a “stretched” or elongated conformation (9, 23). It is possible that, when in D2O, the soil has a more condensed structure, which would cause an increase in steric hindrances, preventing the aliphatic tails of trifluralin from being free to take on their ideal conformation (i.e., the tails are forced closer to the surface, hence the stronger interaction). Note that the pD values for the soil swollen in a buffer and D2O are very similar (5.9 and 5.8, respectively), but this small

difference may still play a role in the arrangement and hydration of components at the soil/aqueous interface. Similarly, increases in the epitope map percentages are also observed for acifluorfen when peat soil is swollen using D2O in comparison to a buffer (as discussed later). Acifluorfen Interactions with Soil. The epitope map of acifluorfen in a buffer (Figure 1D1) shows that Hf and Hb are the protons which display the highest affinity for peat soil, 100 and 71%, respectively. It is evident that the side of the acifluorfen molecule containing the Cl atom and the carboxylic acid group (shaded) has a stronger affinity for the peat soil than the other side. This planarity in the sorption mechanism has also been observed previously in the solutionstate analysis of acifluorfen (21). The proton Hf is in close proximity to the COOH group and can potentially interact with the soil via hydrogen-bond formation and dipole interactions. Proton Hb, which is flanked by the group of three fluorine atoms and a chlorine atom, shows the secondhighest binding affinity. Both F and Cl have high electronegativities, and when bound to carbon, the resulting bond is considered to be polar covalent. Similar to trifluralin, this dipole can interact with other polar species; potential interactions may include H-bonding, dipole-dipole interactions, and interactions based on partial charge. The protons on the other side of acifluorfen (unshaded) interact to a much lesser degree (Ha(41%), Hc(41%), Hd(25%), and He(42%)), further supporting that one side of the molecule has a higher affinity for the soil and further implies a planar interaction. When the peat soil is swollen using D2O, the binding epitope of acifluorfen remains similar (Figure 1D2). However, as observed with trifluralin, the relative interactions of the protons further removed from the points of contact are higher, suggesting these moieties are forced closer to the soil surface, likely due to the different conformation of the soil in D2O. The binding epitope of acifluorfen to peat HA has been previously determined using solution-state 1H STDD NMR experiments (21), and the same side of the acifluorfen molecule (shaded) displayed a higher binding affinity. The major difference in the binding mechanisms of HA and peat soil is that Hb (close to the CF3 group) shows the highest affinity for HA (21), while in soil, Hf (close to the COOH group) shows the highest affinity. The stronger affinity of the COOH in the soil versus HA may in part be a reflection of the different pD’s in the two studies, which were 5.8-5.9 and 7.0, respectively. The pKa of the acid is ∼1.9 (ACD pKa predictor V8.09) and, thus, at both pDs will be highly dissociated on average (note that the pKa in water is not identical to that in fully deuterated solvents (24)). At the lower pD (5.8-5.9 in soil vs 7.0 in HA study), the equilibrium between the deuterated and nondeuterated forms will alter, and a slightly larger population of deuterated acid will be present. In its deuterated form, acifluorfen may likely “hydrogen”-bond to other soil components, which in turn may explain the stronger interaction of the carboxyl group in the soil (pD 5.8-5.9) than was observed from HA (pD 7.0). However, this is just one of many feasible hypotheses. Others may include the following: the chemical constituents at the soil interface are different from those in the HA fraction; the physical arrangement of components at the soil interface, for example carboxyl groups exposed to the solvent (11), alter the reactivity at the soil interface; the soil contains a range of domains and microenvironments to which the carboxyl preferentially binds which are not preserved after HA extraction. At this point, it is not clear if the different bindings of acifluorfen in soil and HA observed here are due to fundamental differences between the two materials or whether the differences are more simply explained by pD effects. Readers should be aware that “adjusting” the pH/pD of a whole soil is not trivial, and both the pH/pD and chemical interface (observed by HR-

MAS) are dynamic over many days, which requires extensive investigation and will be the basis of future studies. 1-Naphthol Interactions with Soil. The epitope map of 1-naphthol in peat soil swollen using a pD 7 sodium phosphate D2O buffer is displayed in Figure 1E. Before a detailed interpretation of the epitope map can be undertaken, it is important to explain that spectral overlaps of signals for Hc, Hd, He, and Hf make it very difficult to obtain reliable quantitative information for these protons (Figure 2A,B). Semiquantitative information, however, can be extracted in the form of an approximated range, via qualitative inspection and spectral deconvolution. When the STDD (red) and DDRS (black) of 1-naphthol are compared visually in Figure 2C, it is clear that the peaks corresponding to Hc and Hd show a higher degree of saturation than He and Hf. Protons Ha and Hb (which are fully resolved) flanking Hc and Hd show the highest binding, 100 and 91%, respectively. Taking the values for Ha and Hb into consideration, it is possible to estimate that the saturation percentages of Hc and Hd are approximately 90-100%. Similarly, the values for He and Hf can be estimated to be in the range of approximately 60-80%. The epitope map (Figure 1E) suggests that the hydrophobic portion of 1-naphthol is primarily responsible for sorption. The alcohol group could potentially partake in hydrogen bonding, since its pKa is 9.4, but on the basis of the epitope map, this does not appear to be as influential as hydrophobic interactions. 1-Naphthol is a small and spherical molecule with a relatively large portion of the molecule devoid of polar functionalities. It is possible that this molecule is able to penetrate into hydrophobic domains that are known to exist in soils and thus associate via hydrophobic interactions. SOM is a matrix known to have internal and external hydrophobic surfaces (25), and hydrophobic interactions of 1-naphthol with hydrophobic subunits of SOM fractions have been previously observed (26). The peat soil contains ∼25% aromatic carbon (see Supporting Information Figure S5), and thus it is feasible that π-π interactions are participating in the sorption (27). The epitope map however, indicates that the nonsubstituted ring displays the stronger interaction. Thus it is most likely that the binding is simply driven by the low solubility of 1-naphthol in water with its most hydrophobic portion partitioning with hydrophobic pockets in the soil. In soil swollen with D2O, no distinct binding motif was observed for 1-naphthol (Figure 2D,E), with all protons strongly interacting with the soil (i.e., all protons ∼100%). Furthermore, considerable broadening in line shape is observed for 1-naphthol in D2O (Figure 2D,E) versus the buffer (Figures 2A,B). NMR signals from TFM, trifluralin, and acifluorfen were broadened by ∼10% in D2O (data not shown) versus the buffer but not to the extent observed for 1-naphthol (broadening of >200% at the base of the peaks). This extremely large change in line shape suggests a tight and stable association with the soil (11, 28) in D2O. In such cases, the bound molecules become very rigid (behaving more like the soil than a small molecule), and thus the propagation of saturation by spin diffusion within the tightly bound 1-naphthol molecule may become very efficient (28, 29). As such, one reason a binding epitope cannot be observed may be that the saturation cannot “accumulate” at the “points of contact” but is distributed evenly via a dipole network which could form in rigidly bound 1-naphthol. Conversely, 1-naphthol may simply become entrapped in the soil structure, which is less expanded in D2O versus the buffer, such that all protons are forced to interact with the soil surface. Interestingly, the predicted Koc for 1-naphthol at pD 5.9 (buffer-soil) and pD 5.8 (∼soil-D2O) are identical at 711 (ACD/Adsorption Predictor, V7.04), making it difficult to explain the different interactions on the basis of the contaminant alone. Irrespective of the reason for the absence VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) The DDRS and (B) STDD of 1-naphthol in peat soil swollen using a pD 7 sodium phosphate buffer. (C) Expansion of the crowded region from A and B. (D) DDRS and (E) STDD of 1-naphthol in peat soil swollen using D2O. (F) Trifluralin kinetic study; for the first 45 min, data were collected every minute, after which data were collected every 8 min. of a binding epitope in D2O, the considerable difference in the behavior of 1-naphthol in the buffer versus D2O strongly suggests that physiochemical properties of the soil have strong influences in the sorption of 1-naphthol (30). TFM Interactions with Soil. The epitope map of TFM in the buffer (Figure 1F) shows that all protons have a relatively high affinity for the peat soil. Proton Hb has the highest affinity (100%) and is flanked by a CF3 group and an alcohol group. As in trifluralin and acifluorfen, it appears that the highly polar covalent fluorine atoms play an important role in the binding mechanism. Protons Ha next to the nitro group and Hc next to the OH group follow closely behind (97 and 95%, respectively). TFM has a pKa of 6.07 (31); however, it is not clear if the OH group plays an important role in binding under the conditions employed in this study. It has been reported that, as pH increases, TFM’s toxicity, bioaccumulation, and adsorption to sediment decrease (31). Thus, it would be valuable to assess differences in binding mechanisms at various pH/pD’s in future studies. It is likely that, due to its small size and the abundance of polar groups, TFM does not show a very distinct orientation with respect to the soil surface, as all protons are essentially close in proximity to the soil in most orientations. It is also feasible that, due 5518

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to its smaller size, it can enter physically smaller spaces, in which it can more readily become encapsulated. When the epitope map of TFM was determined in peat soil swollen using D2O, all protons displayed 100% interaction, similar to 1-naphthol, demonstrating that, under these conditions, the molecule displays a very nonspecific association with the soil. It is important to note that, unlike 1-naphthol, the line shapes of TFM in D2O only broaden slightly (∼10% data not shown) and do not suggest extremely tight binding to the soil. Given the small size of TFM, the lack of epitope is likely due to the molecules entering micropores in which their motion and thus orientation are restricted (11). Sorption Kinetics across the Soil-Water Interface. HRMAS NMR affords the unique opportunity to look at processes occurring directly at the soil-water interface. In HR-MAS NMR, soluble and semisolid fractions are easily detected. However, if the contaminant becomes sequestered into a solid domain, into which the aqueous solvent cannot penetrate, its signals will broaden due to 1H-1H dipolar interactions and become unobservable. Thus, the loss of a signal over time in HR-MAS NMR can be used to follow the kinetic migration of a contaminant across the soil-water

interface and into rigid soil domains. Considering the importance of physiochemical properties on the uptake of contaminants by soil (32, 33), three different soil conditions were tested with trifluralin, in large part to demonstrate the applicability of HR-MAS for such studies. Specifically, the parameters examined included the type of swelling solvent used (D2O versus buffer) as well as the moisture content of the peat soil (desiccator-dried versus air-dried). Figure 2F shows a plot of signal intensity versus time. It is evident that the air-dried soil swollen using the buffer shows the fastest binding kinetics, with 50% of the trifluralin sequestered within 1.5 h, while desiccator-dried soil and air-dried soil, swollen using D2O, required 4.2 and 5.6 h, respectively. Kinetic experiments show sorption to be much faster in the buffer than in D2O, indicating that the contaminant is crossing the solid-aqueous interface more rapidly when in the buffer. A possible explanation for the differences in kinetics between the different swelling solvents is that, when in a sodium phosphate buffer, peat soil is expanded to a greater degree, and therefore a greater number of sorption domains are available or accessible for contaminant binding (9, 23, 33). The rate of transfer of trifluralin across the soil-water interface appears to decrease with hydration, with 50% becoming “sequestered” within 4.2 and 5.6 h with desiccatordried and air-dried soil, respectively (Figure 2F). However, at the early stages of the experiment, the opposite may be true (see Supporting Information Figure S6). It has previously been observed that drying treatments can modify the sorption affinity of soils for organic molecules, with dried and rehydrated soils having an order of magnitude greater affinity for the pesticide, atrazine, relative to soils that have never been dried (34). However, the mechanisms of sorption can vary depending on contaminant and soil physiochemical properties. For example, Borisover et al. (35) suggested that some sorbates can efficiently enter dry SOM binding domains and that hydration introduces a competitor for those domains causing a decrease in sorption. This is consistent with the observations in this study with trifluralin. It is possible that the presence of additional water in the air-dried soil may “repel” or inhibit the migration of trifluralin into aqueous occupied regions. This suggests that soil-water content may influence pesticide sorption processes and warrants further investigation, as this information may have implications for the application and management of pesticides. In addition, STDD NMR experiments were performed on replicate samples prepared in rotors to assess if the binding mechanism of trifluralin changed over time (STD recorded every 30 min over 24 h); however, no significant change in the binding of trifluralin with time was observed. This study demonstrates that HR-MAS STDD permits one to access unique information pertaining to the soil-water interface, as well as allowing the elucidation of mechanisms of pesticide binding to whole soils. Preliminary kinetic information indicates that the physical and chemical characteristics of soils are highly influential in determining the mechanism of contaminant sorption, as they significantly alter soil conformation. In particular, the differential binding of 1-naphthol in the buffer versus D2O indicates that the Koc alone may not be enough to accurately predict the behavior of contaminants in a natural soil environment. Further molecular-level studies that investigate physiochemical parameters such as ionic strength, pH/pD, the presence of various background electrolytes, and temperature on sorption will be imperative to expanding our understating of contaminant toxicity, transport, fate, bioavailability, and bioaccumulation in the environment.

Acknowledgments We would like to thank the Natural Science and Engineering Research Council (NSERC) for support via a Strategic Grant (STPGP 336534). M.J.S. also thanks NSERC for a University Faculty Award (UFA), and A.J.S. thanks the government of Ontario for an Early Researcher Award and NSERC for a discovery grant.

Supporting Information Available Numerous additional NMR spectra, further experimental details, and explanations/consideration as to the points of irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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