Published in Soil Sci. Soc. Am. J. 67:1370-1377 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-2—SOIL CHEMISTRY
Sorption of Pyridine to Suspended Soil Particles Studied by Deuterium Nuclear Magnetic Resonance
Dongqiang Zhu*,a,
Bruce E. Herberta and
Mark A. Schlautmanb
a Dep. of Geology and Geophysics, Texas A&M Univ., College Station, TX 77843
b Dep. of Agricultural & Biological Engineering, Clemson Univ., Clemson, SC 29634-0357, and Dep. of Environmental Toxicology and the Clemson Institute of Environmental Toxicology, Clemson Univ., Pendleton, SC 29670
* Corresponding author (Don.Zhu{at}po.state.ct.us).
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ABSTRACT
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Spin-lattice relaxation times (T1) and chemical shifts (
) of perdeuterated pyridine (
, ß, 
deuterium of d5–pyridine) in aqueous solution (e.g., water, 0.001 M benzoic acid and phenol) are found to decrease with increasing pH values. This is because the concentration distribution of the two pyridine species, protonated and unprotonated, varies with solution pH. The T1 values of are lower than and ß deuterium in water and methanol compared with n-hexane, indicating more anisotropic molecular movements of pyridine resulting from interactions between amine and the polar solvent. Spin-spin relaxation times (T2) and of d5–pyridine in aqueous suspensions of water-dispersible clay (WDC) soil components are highly pH dependent. The lowest T2 and the most downfield-shifted compared with aqueous solution show that the strongest sorption occurs at the weak acidic condition (pH 6). The downfield shifts of observed in WDC suspensions are directly caused by the increased mole ratio of protonated pyridine through sorption. However, no significant changes in are observed for organic free minerals (H2O2–treated WDCs and a standard clay mineral) compared with aqueous solution, indicating interactions with mineral surfaces are negligible in pyridine sorption. A sorption mechanism of cation exchange between protonated pyridine and charged sites of soil organic matter (SOM) is inferred based on the measured values.
Abbreviations: FTIR, Fourier transform infrared • NHC, nitrogen-heterocyclic compounds • NMR, nuclear magnectic resonance • SOM, soil organic matter • T1, spin-lattice relaxation times • T2, spin-spin relaxation times • TCE, trichloroethylene • TOC, total organic C • WDC, water-dispersible clay • , chemical shifts
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INTRODUCTION
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PYRIDINE and other nitrogen-heterocyclic compounds (NHCs) have received considerable environmental attention because of their frequent existence at waste sites generated from coal gasification, shale oil extraction and pesticide production. Despite their high water solubilities (e.g., pyridine is miscible with water), sorption to soils and sediments significantly affect the fate of these chemicals, including biodegradation and chemical oxidation (Fetzner, 1998; Thomsen and Laturnus, 2001).
Sorption mechanisms of organic contaminants to soils and sediments have been studied mainly through batch sorption experiments (Chiou et al., 1979; Karickhoff et al., 1981; Young and Weber, 1995; Xing and Pignatello, 1997). However, batch sorption experiments are unable to provide information directly on the molecular-level interactions between sorbates and sorbents. On the other hand, studies of sorption by spectroscopic techniques are very limited. Fourier transform infrared (FTIR) spectroscopy has been used to study sorption of organic chemicals to humics and soils (Martinneto et al., 1994; Landgraf et al., 1998; Suetsugu et al., 2001). However, observing sorbate–sorbent interactions through wavelength shifts of existing bands or appearance of new absorbence bands sometimes can be ambiguous because of high background interferences (e.g., water absorbance). Compared with FTIR, nuclear magnetic resonance (NMR) spectroscopy is able to differentiate signals of interest from the others by probing isotopically enriched chemicals. High-frequency solid state NMR has been used to characterize sorption of organic chemicals to dry humics or soils. For example, chemical shifts () of 13C-labeled chemicals (e.g., acetone, trichloroethylene [TCE], carbon tetrachloride) have been used to study their sorption on dry minerals, humic acids and soils (Jurkiewicz and Maciel, 1995). More recently, local motions of organic pollutants including TCE, pyridine, and benzene adsorbed on dry soil components were studied through 2H NMR using a quadrupole-echo technique (Xiong et al., 1999).
Deuterium NMR has the advantage of being sensitive to relatively weak solute interactions, and is well suited for NMR relaxation studies because deuterium relaxation is dominated by the quadrupolar relaxation mechanism (Smith, 1983). Recently, solution-phase, noncovalent interactions between perdeuterated monoaromatic compounds (e.g., phenol, pyridine, benzene) and natural humic acids have been characterized by T1 measurements (Nanny and Maza, 2001). Spin-spin relaxation times or the equivalent line-broadening has also been used to characterize sorption of d5–fluorobenzene to organic materials including surfactant micelles and humic acids (Herbert and Bertsch, 1997).
Pyridine has an aromatic ring containing amine with a lone-pair of electrons. This structure allows several potential mechanisms of pyridine sorption to SOM. Unprotonated pyridine can readily form H bonds with hydroxyls of functional groups (e.g., carboxyl and phenol) in SOM. On the other hand, protonated pyridine can be sorbed to soils through cation exchange. A recent study (Weber et al., 2001) proposed the cation-exchange mechanism by showing that the adsorbed 14C-labeled pyridine was nearly completely recovered after treatment by NH4AC. However, none of these sorption mechanisms have been verified by spectroscopic data. Additionally, the charge-transfer complexation between a n-electron donor such as pyridine (unprotonated) and aromatic structures of SOM that have high electron acceptability (e.g., quinone like structures) (Melcer et al., 1989; J. J. Pignatello, personal communication, 2002) may also work as a sorption mechanism, though few relative studies have been reported.
In this study, high-resolution 2H NMR spectroscopy has been obtained for perdeuterated pyridine (d5–pyridine) in aqueous suspensions of WDC soil components. Based on combined measurements of T1, T2 and of d5–pyridine in aqueous solutions and clay suspensions, we have obtained useful information on sorption mechanisms of pyridine to high organic C soils.
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MATERIALS AND METHODS
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Chemicals
Pyridine (Aldrich) and perdeuterated compounds including d5–pyridine (C/D/N Isotopes, Pointe-Claire, Quebec, Canada), d6–benzene (Cambridge Isotope Laboratories, Andover, MA), d12–cyclohexane (Aldrich Chemical Co., Gillingham, Dorset, UK), and d2–water (Aldrich Chemical) were used as received. Phenol (Fisher Science, Pittsburgh, PA), benzoic acid (Fisher Science, Pittsburgh, PA), methanol (HPLC grade, Fisher Science, Pittsburgh, PA), and n-hexane (HPLC grade, Fisher Science, Pittsburgh, PA) were also used as received. All aqueous solutions were prepared using doubly distilled water.
Water Dispersible Clay Preparation
The soil used in this study (collected from Montgomery County, southeastern Texas) is classified as the Tuscumbia Series, consisting of nearly level, deep, poorly drained, clayey soils (USDA Soil Conservation Service and Forest Service, 1972). To prepare a water-dispersible soil fraction, 100 g of soil was mixed with 10 L of water (1:100 soil/water mass ratio) and shaken on an orbital shaker at 200 rpm for 1 h. After a settling period of 4 h, the fraction with size <2 µm (calculated from Stoke's law) was siphoned off from the top 5-cm suspension. The WDC fraction obtained was air dried and x-ray diffraction (XRD) showed that it was nearly all from the clay fraction (smectite mixed with very small amounts of kaolinite and quartz). The WDC collected was then further divided into fractions based on different chemical pretreatments. One WDC fraction was treated with 0.5 M NaOH to extract humics (Hayes, 1985), while a different fraction was subjected to H2O2 (30%, Fisher) treatment to remove SOM (Kunze and Dixon, 1986). A subfraction of the H2O2–treated WDC was further treated by 0.5 M NaOH. All WDCs and a standard clay (SAz-1, Ca-montmorillonite, Apache County, Arizona) (Source Clay Minerals Repository, Univ. of Missouri-Columbia, MO) were saturated with Na+ by washing four times with 1.0 M NaCl, four times with 0.1 M NaCl, and one time with 0.01 M NaCl. The various WDC fractions and standard clay were then freeze-dried and stored in a desiccator until use. Na+ saturation was used to increase the suspendibility, and therefore the uniformity, of the aqueous suspensions for subsequent NMR analysis. WDC fractions receiving no treatment, treated with NaOH, and treated with H2O2 had total organic carbon (TOC) contents of 1.41, 1.32, and <0.03% (i.e., below the detection limit), respectively. The standard clay also had a TOC content below the detection limit. Specific surface areas of the H2O2–treated WDC (9.5 m2 g-1) and standard clay (16.1 m2 g-1) were measured using the three point N2–Brunauer, Emmett, and Teller (BET) isotherm method on a Quantasorb Jr. (Quantachrome Corp., Greenvale, NY).
Sample Preparation
Aqueous solutions of pyridine were prepared by adding 50 µL of d5–pyridine to 25 mL (0.2% v/v, 0.0025 M) of water, 0.001 M of benzoic acid, 0.001 M of phenol, and 0.01 M of NaCl (background solution for aqueous suspensions) aqueous solutions, respectively. After pH adjustment by 0.1 M NaOH or HCl, a 2.0-mL aliquot was transferred to a 2-mL vial (Teflon-backed septa, Fisher, Pittsburgh, PA). Two microliters of d6–benzene was added directly to 2 mL of water (0.1% v/v) with a preadjusted pH value. For organic solvent samples, 4 µL of d5–pyridine, d6–benzene or d12–cyclohexane was added to 2 mL (0.2% v/v) of methanol or n-hexane. For aqueous suspension samples, 100 µL of d5–pyridine was added to 50 mL (0.2% v/v) of suspensions of untreated and NaOH-treated only WDCs (0.005 mass ratio of WDC/0.01 M NaCl) and pH was then adjusted to desired values. To prepare suspensions of organic-free clays (H2O2–treated WDCs and standard montmorillonite), nondeuterated pyridine was used instead to avoid quadrupolar splitting (i.e., doublet peak interferes with measurement) resulting from ordering of the deuterated probe on charged mineral surfaces of smectite interacting with the NMR magnetic field (Zhu, 2001). Given the same experimental condition, values should be identical in 1H and 2H NMR spectra. Buffer solutions were not used because the solutes themselves adequately buffered all samples. All samples were kept in the dark at ambient temperature and shaken overnight (20–24 h) before NMR analysis.
NMR Experiments
Deuterium NMR spectra were recorded at ambient temperature (21 ± 0.5°C) using a Varian 400 spectrometer operating at 61.35 MHz, an acquisition time of 2.73 s, and a spectra width of 3000.3 Hz with a 5-mm switchable probe. This high-frequency spectrometer provides better shimming and signal/noise ratio, especially for samples of clay suspensions. After locking and primary shimming on a reference sample containing pure d2–water or suspension of d2–water and clay (1:200 mass ratio) for solution and clay samples, respectively, the spectrometer was run unlocked after fine shimming based on the shape of the peak of interest. A standard inversion-recovery pulse sequence for T1 and the Carr-Purcell Meiboom-Gill (CPMG) pulse sequence for T2 measurements were used. A recycle delay of at least 5T1 was applied, and at least 10 transients were used to give a good signal/noise ratio. Spectra were processed with an exponential multiplication corresponding to line broadening of 2 Hz. Chemical shifts were internally referenced to the natural abundance of deuterated water for soil suspension samples ( <0.02 ppm at different pH values, checked with an external reference of deuterated chloroform) or deuterated chloroform for aqueous solution samples. T1 and T2 were calculated by exponential regressions and reported with instrument-recorded errors, which were typically <1% for solution samples and <10% for soil suspension samples.
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RESULTS AND DISCUSSION
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Aqueous Solutions
Figure 1a
shows T1 values of , ß, and deuterium of d5–pyridine in water. Figure 1b shows T1 values of deuterium in 0.001 M benzoic acid and phenol solutions. Note that the concentrations of different solutes added to the aqueous solutions were very low, and thus changes in ionic strength resulting from pH adjustment had no observable effect on and T1 measurements (verified by independent experiments, data not shown). These results also indicate that the potential effect of changing solution viscosity, which affects deuterium relaxation via molecular correlation (Glasel, 1969), was negligible in this study. The T1 values of are clearly lower than and ß deuterium in water (also in phenol and benzoic acid solutions, data not shown) at all pH values. A decreased T1 value measured in solution 2H NMR generally indicates a slowed rotation characterized by a higher molecular correlation time (
c) (Abragam, 1961). The interaction between amine and water through hydration of protonated pyridine or H bonding of unprotonated pyridine results in anisotropic molecular movements of pyridine (i.e., and ß rotate faster than deuterium along the c2 axis). This result is in good agreement with a previous study by Nanny and Maza (2001).
Figure 1 show that T1 values of pyridine in aqueous solutions decrease with increasing pH (data for ß and deuterium of pyridine in phenol and benzoic solutions not shown). Protonated pyridine and unprotonated pyridine predominate below and above the pKa of 5.25 (Lide, 1998), respectively. The protonated species is hydrated and becomes more mobile, while the unprotonated species is less mobile because its molecular rotation is perturbed by hydrophobic effects (i.e., pyridine caged in a H-bonded water network). As a result, the protonated species has a higher T1 value than does the unprotonated form in aqueous solutions. Support for this can be seen by comparing the T1 values of d6–benzene in water versus organic solvents (Table 1). d6–benzene in water has a much lower T1 value than in methanol or n-hexane, indicating slowed molecular rotations in water due to hydrophobic effects.
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Table 1. Spin-lattice relaxation times (T1) (s) and associated errors (shown in parentheses) for , ß, and deuterium of d5–pyridine (C5D5N), d6–benzene (C6D6) and d12–cyclohexane (C6D12) in solvents of water, methanol, and n-hexane.
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Other useful information can also be obtained from T1 measurements of d5–pyridine, d6–benzene, and d12–cyclohexane in water and/or organic solvents (Table 1). Compared with d6–benzene and d12–cyclohexane, the difference in T1 of d5–pyridine between n-hexane and water or methanol is larger. This is because pyridine (unprotonated at pH 7) can interact strongly with water and methanol through H bonding. Also, the difference in T1 values among , ß, and deuterium of pyridine decreases with decreasing solvent polarities (i.e., water > methanol > n-hexane), indicating more isotropic molecular motion in solvents with lower polarities. In n-hexane, which has a very low polarity, molecular motion of pyridine is nearly completely isotropic, which is demonstrated by the close T1 values of , ß, and deuterium. Interestingly, the T1 values of all deuterium of pyridine are higher than those for d6–benzene but lower than d12–cyclohexane in n-hexane. These results can be explained by the difference in aromaticity of the three solutes (i.e., benzene > pyridine > cyclohexane). Deuterium attached to a carbon with higher aromaticity tends to be less mobile (lower T1) because of the increased restraint on molecular mobility (i.e., deuterium of benzene is inhibited from moving out of the planar ring).
More detailed information on solute interactions of pyridine in phenol and benzoic acid solutions can also be derived from Fig. 1(b). Although the difference is small, it is clear that the T1 values of deuterium of pyridine in benzoic acid solutions are higher compared with water and phenol solutions (also true for ß and deuterium, data not shown). This is most likely caused by the electrostatic interaction between the protonated pyridine and the dissociated benzoic acid (i.e., formation of an ion pair), which drives the equilibrium toward slightly higher production of the protonated species, which has a much higher T1 value than the unprotonated form. Because the pKa values of benzoic acid and pyridine are 4.19 and 5.25 (Lide, 1998), respectively, the neutral species of the two solutes cannot coexist at high concentrations. Thus, decreasing T1 values of pyridine through forming H bonds between these two species is not favored. Also, a slight increase in T1 of deuterium is observed when pH increases from 8 to 9 for the different solutions. This is because when pH increases to 8, the ratio of protonated to unprotonated pyridine is very small (<0.2%, calculated from the pKa), thus the observed T1 is mainly determined by speciation of the latter in the aqueous phase. Unprotonated pyridine tends to move more freely with increasing pH values because H bonds become weaker at higher pH values.
Assuming the observed T1 value is only a function of the ratio of the two pyridine species, T1 at an arbitrary pH value can be calculated through data fitting using T1 values of protonated and unprotonated pyridine as the two end members, expressed as:
 | [1a] |
 | [1b] |
where Rsol is the mole ratio of the protonated to unprotonated species in water, calculated from the pKa and pH value, T1,cal is the calculated overall T1, and T1,cal-p (0.714 s) and T1,cal-u (0.516 s) are two constants obtained from data fitting to represent the ideal T1 values of protonated and unprotonated pyridine in water, respectively. Thus, T1,cal can be expressed as a function of solution pH only as shown in Fig. 1(b). The model fits the observed data very well between pH 3 and 6, although it is less accurate at higher pH values. This is because with increasing pH the unprotonated form of pyridine becomes more dominant and its speciation (i.e., H bonding) contributes more to the observed overall T1, which the model fails to include. However, the simplified model still provides a relatively good prediction (relative std deviation = 3.94%) that solute interactions other than pyridine protonation have insignificant effects on the overall T1 of pyridine. This may explain why the measured T1 values of pyridine in phenolic solutions are so close to that in water, though phenol is a good candidate for H bonding with unprotonated pyridine at a wide pH range (i.e., phenol pKa = 9.89) (Lide, 1998).
The measured values of ß deuterium of pyridine in different aqueous solutions as a function of pH are shown in Fig. 2
. The ß deuterium is reported because it has the highest sensitivity to pH variation. Compared with T1, is less sensitive to changes in relatively weak solute interactions (i.e., H bonding becomes weaker at higher pH values) because the difference in of pyridine among these different solutions is so small. Similar to the method used for T1 prediction in Eq. [1], at an arbitrary pH value can also be calculated using values of protonated and unprotonated pyridine in water as the two end members, expressed as:
 | [2] |
where cal is the calculated overall , cal-p (7.70 ppm) and cal-u (7.08 ppm) are two constants obtained from data fitting that represent the ideal values of protonated and unprotonated pyridine in water, respectively, and Rsol has the same definition as in Eq. [1b]. Again, cal can be expressed as a function of the solution pH by solving cal-p and cal-u through data fitting. Calculated cal values as a function of pH for pyridine in water are shown in Fig. 2 together with the measured data. The model fits the data extremely well (relative std deviation = 0.15%), indicating that the ratio of the two pyridine species in aqueous solutions (Rsol) totally controls the observed value. This model is useful with two aspects: (i) any significant change in is most likely caused by the ratio of protonated to unprotonated forms of pyridine, and (ii) the ratio of the two pyridine species can be calculated from the observed value.
Aqueous Water-Dispersible Clay Suspensions
T2 values of deuterium of d5–pyridine in aqueous suspensions of untreated and NaOH-treated WDCs are shown in Fig. 3
as a function of pH. Because the WDC was present at a very low level (1:200 solid/solution mass ratio), any differences in ionic strength resulting from changing speciation of SOM functionalities with pH adjustment should have a negligible effect on and T2 measurements. The T2 values at pH 6 and 7 were calculated from line-broadening (i.e., T2 = 1/
1/2, where 1/2 is the half line width) because the CPMG pulse sequence was no longer appropriate for measuring such extremely low T2 values (i.e., very large line-broadening). Recorded 2H NMR spectra of pyridine in aqueous suspensions of untreated and NaOH-treated WDCs at several pH values are also shown in Fig. 4
. Similar to aqueous solutions, T2 values of pyridine in WDC suspensions were highly pH dependent. T2 decreased with increasing pH when pH < 6, while the opposite trend was observed when pH > 6. The lowest T2 value was found at pH 6. In systems where the deuterium nucleus is rapidly exchanging between two environments with different correlation times, which can be caused by sorption, the relationship between the observed T2 can be described by Eq. [3] (Zens et al., 1976):
 | [3] |
where 
is the resonance frequency (61.35 MHz), e2Qq/
is the quadrupolar coupling constant in radian per second, f and s are the molecular correlation times for free and sorbed solutes, respectively, and 
is the mole fraction for sorbed solutes. An decreased T2 value in general indicates a higher s (slower molecular rotation) and a higher mole fraction of sorbed solute. However, it is important to note that the magnitude of T2 can be significantly decreased by paramagnetic-induced relaxation. No effort was made in the present study to eliminate this effect because of its impracticality (e.g., removal of structured Fe in clay). However, since all relaxation times were measured with the same amount of WDC in suspension, the same paramagnetic effect would have been present for all samples, thereby resulting in a constant baseline effect. Therefore, it can be concluded that the strongest pyridine sorption occurred at pH approximately 6 for both the untreated and NaOH-treated WDCs based on results shown in Fig. 3 and 4.
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Fig. 4. 2H nuclear magnetic resonance (NMR) spectra of d5–pyridine and deuterated water at natural abundance (the most upfield-shifted peak) in aqueous suspensions of untreated and NaOH-treated WDCs at different pH values. (a) untreated at pH 4 (, ß, and water). (b) untreated at pH 6 (unresolved , ß, and water). (c) untreated at pH 7 (unresolved , ß, and water). (d) untreated at pH 9 (, , ß, and water). (e) treated at pH 4 (, ß, and water). (f) treated at pH 6 (, ß, and water). (g) treated at pH 7 (, ß, and water). (h) treated at pH 9 (, , ß, and water).
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Measured values of ß deuterium of d5–pyridine in aqueous suspensions of untreated, NaOH-treated, H2O2–treated, and H2O2 + NaOH-treated WDCs, and the standard clay are shown in Fig. 5
as a function of pH. Data of pyridine in 0.01 M NaCl background solution are also shown for comparison. Downfield shifts of are observed for the untreated and NaOH-treated WDCs compared with 0.01 M NaCl, and the untreated compared with the NaOH-treated WDC. Negligible difference in values was observed among the H2O2–treated, the H2O2 + NaOH-treated WDCs, the standard clay, and background solution. This result indicates that (i) sorption of pyridine to mineral surfaces is insignificant compared with that to SOM, and (ii) the NaOH treatment affects pyridine sorption through varying structures of SOM rather than mineral surfaces. for the untreated WDC at pH 6 and 7 is not shown because and ß peaks merge to one unresolvable peak (Fig. 4). From the previous discussion, the model (Eq. [2]) correlates the observed value and the ratio of the two pyridine species in aqueous solutions. We consider this model to still be valid for the WDC suspensions because of pyridine is also determined by the ratio of the two pyridine species. No interactions stronger than H bonding (e.g., no covalent bonds) are expected in pyridine sorption whereas H bonding contributes little to no variation in in the aqueous phase. All strong interactions involved in pyridine sorption in this study would be expected to be amine-induced because without the amine moiety pyridine would have a benzene-like structure, which clearly does not involve any covalent interaction. As a result, although sorbed pyridine is much less mobile than its unsorbed counterpart (e.g., lower T2 or higher line-broadening due to sorption), both forms should have very similar values given the same speciation state (e.g., protonated and unprotonated). Thus, without losing validity, Rsol in Eq. [2] can be replaced by Rsus, the ratio of protonated (including sorbed and free) to unprotonated pyridine (including sorbed and free) in WDC suspensions, expressed as:
 | [4] |
where p, ps, u, and us are the moles of free protonated, sorbed protonated, free unprotonated, and sorbed unprotonated pyridine, respectively. After replacing Rsol in Eq. [2] by Rsus, Rsus can be calculated using the measured value for WDC suspensions, expressed as,
 | [5] |
where cal-p (7.10 ppm) and cal-u (7.08 ppm) have the same definitions as in Eq. [2]. Note the 0.01 M difference in ionic strength between the background solution and water makes little to no change in these two values based on measurements. Finally, the mole fraction of protonated pyridine (sus%) of the total pyridine added in WDC suspensions can be expressed by equation (6a):
 | [6a] |
 | [6b] |
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Fig. 5. Measured chemical shifts () of ß deuterium of d5–pyridine in aqueous suspensions of untreated, NaOH-treated, H2O2–treated, and H2O2 + NaOH-treated WDCs, and the standard clay as a function of pH. Data for background solution (0.01 M NaCl) is shown for comparison.
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Equation [6b], combined with Eq. [1b], calculates the mole fraction of protonated pyridine in water from the pKa and pH values. The calculated sol and sus are listed in Table 2. Given the same pH value, sus for untreated and NaOH-treated WDCs is always higher than sol, indicating an increased ratio of protonated pyridine due to sorption. Note the same trend of sus observed for the NaOH-treated WDC is expected to hold for the untreated WDC at pH 6 and 7 (sus is not calculated at these pH values due to unresolvable peaks). The further downfield shift of ß deuterium at pH 6 or 7 is one reason to cause merging of peaks of and ß deuterium. The errors of sus are calculated by considering errors for cal-p and cal-u from data fitting (0.15%) and an assumed error of 0.02 ppm for measurements. The errors are extremely high at low or high pH values (data for pH 4 and 9 not shown for this reason) because the measured value approximates one of the two boundaries, cal-p or cal-u, in Eq. [5]. It is worth noting that the ratio of the free to sorbed protonated pyridine cannot be resolved without additional data (e.g., concentrations of free pyridine in solutions by batch sorption experiments). However, it is certain that sorption of protonated pyridine is the driving force for the increased mole ratio of the total protonated pyridine. Additionally, it can be concluded that H bonding of unprotonated pyridine is relatively less important because otherwise the ratio of the total unprotonated pyridine would increase, resulting in upfield shifts of . The calculated sus values also show that pyridine sorption is low at very high and very low pH values, which is consistent with the result from T2 measurements.
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Table 2. Mole fractions (in percent) of protonated pyridine in water (sol) and in suspensions (sus) of untreated and NaOH-treated water-dispersible clays (WDCs) as a function of pH.
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Based on the calculated sus values, it is interesting to note that for the untreated and NaOH-treated WDCs, the strongest sorption of protonated pyridine occurs at pH 6. Assuming a carboxyl content of 9.1 mmol g-1 after Reuter and Perdue (1981) in the untreated WDC, the amount of carboxyl content is approximately 0.64 µmol mL-1 for WDC suspension samples (calculated from the 1.41% soil TOC). At pH 6 and 7, there are approximately 3.8 and approximately 0.44 µmol mL-1 protonated pyridine, respectively, in solutions before sorption (calculated from the pKa). Because most SOM carboxyl groups would be dissociated when pH > 6 (i.e., the average carboxyl pKa of humic acids is approximately 4.5; Perdue, 1985), the maximum amounts of protonated pyridine and dissociated carboxyl groups available for sorption can be obtained at pH approximately 6 (at 0.64 µmol mL-1) rather than at pH approximately 7 (at 0.44 µmol mL-1). When pH decreases from 6 to lower values, sorption of protonated pyridine becomes less favorable because the number of dissociated carboxyl groups decreases, while when pH increases from 6 to higher values the sorption also becomes less favorable because there is less protonated pyridine available for sorption. Note the boundary values of the assumed carboxyl content can be relatively loose to allow for the strongest sorption occurring at pH 6 in this study. It is worth noting that for pyridine with trace concentrations existing at most contaminated sites, pyridine protonation is the key process, and thereby a lower pH value than 6 is expected for the highest pyridine sorption through the cation-exchange mechanism.
One question brought about by this study is to estimate the relative importance of mineral surfaces versus SOM in pyridine sorption. Previous studies (Laird et al., 1994; Barriuso et al., 1994) by batch sorption/desorption methods have suggested that atrazine(6-chloro-N2–ethyl-N4–isopropyl-1,3,5-triazine-2,4-diamine), a NHC chemical, is mainly sorbed to mineral surfaces through weak van der Waals or H bonds in low-organic soils. However, based on the very close values of pyridine observed for the organic-free clays and background solution given the same pH value, the interaction of pyridine with mineral surfaces is not likely to be important compared with the contribution from SOM in this study. Extra support can be obtained by comparing the T2 values (Fig. 3) or line-broadening in 2H spectroscopy (Fig. 4), and the sus values (Table 2) of pyridine for the untreated and NaOH-treated WDCs. The effects of extracted humics (6.4% TOC of untreated WDC) on pyridine sorption are significant, indicating an important role of humics in pyridine sorption. This is because NaOH-extracted humics contain a large fraction of functionalities of SOM, which can sorb protonated pyridine through the cation-exchange mechanism.
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ACKNOWLEDGMENTS
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We thank Dr. Joseph J. Pignatello for his critical review of early manuscript drafts, Ms. Lai Man Lee for preparation of water-dispersable soils, and the U.S. Department of Energy for financial support.
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NOTES
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D. Zhu is currently at: The Connecticut Agricultural Experiment Station, 123 Huntington St., New Haven, CT 06504.
Received for publication April 1, 2002.
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