Environ. Sci. Technol. 2006, 40, 3260-3266
Investigating the Role of Mineral-Bound Humic Acid in Phenanthrene Sorption XIAOJUAN FENG, A N D R EÄ J . S I M P S O N , A N D MYRNA J. SIMPSON* Department of Physical and Environmental Sciences, University of Toronto, Toronto, Ontario, Canada M1C 1A4
Contaminant-soil interaction studies have indicated that physical conformation of organic matter at the solid-aqueous interface is important in governing hydrophobic organic compound (HOC) sorption. To test this, organo-clay complexes were constructed by coating montmorillonite and kaolinite with peat humic acid (PHA) in Na+ or Ca2+ dominated solutions with varying pH and ionic strength values. The solution conditions encouraged the dissolved PHA to adopt a “coiled” or “stretched” conformation prior to interacting with the clay mineral surface. Both kaolinite and montmorillonite organo-clay complexes exhibited higher phenanthrene sorption (Koc values) with decreasing pH, indicating that the coiled configuration provided more favorable sorption conditions. Evidence from 1H highresolution magic angle spinning (HR-MAS) nuclear magnetic resonance (NMR) indicated that polymethylene groups were prevalent at the surface of the organo-clay complexes and may enhance sorptive interactions. Preferential sorption of polymethylene groups on kaolinite and aromatic compounds on montmorillonite may also contribute to the difference in phenanthrene sorption by PHA associated with these two types of clay. This study demonstrates the importance of solution conditions in the sorption of nonionic, hydrophobic organic contaminants and also provides evidence for the indirect role of clay minerals in sorption of contaminants at the soil-water interface.
Introduction The transport and distribution of hydrophobic organic compounds (HOCs) in the environment is governed by sorption to soils and sediments (1-6). Organic matter, a key component of both soils and sediments, plays a major role in the sorptive fate of HOCs in terrestrial and aquatic environments (1, 7-12). Current studies of contaminantsoil interactions have focused on the quality of the organic matter and its relationship to HOC sorption. For instance, HOC sorption capacity has been reported to increase with sorbent aromaticity (4, 13, 14) and/or aliphaticity (12, 15, 16). However, there is little consensus in the literature regarding one specific sorbent characteristic that strictly determines HOC sorption behavior. Organic matter conformation is an emerging variable used to further explain HOC sorption phenomena (17-21). With in-situ atomic force imaging, Maurice and Namjesnik-Dejanovic (22) observed nanometer-scale ring-shaped humic aggregates at the basal* Corresponding author phone: 416-287-7234; fax: 416-287-7279; e-mail:
[email protected]. 3260
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plane surface of mica in 0.01 M CaCl2 at pH 5, which may represent hydrophobic domains for HOC sorption. Furthermore, Murphy et al. (23) observed different sorption maxima of HOCs (dibenzothiophene, carbazole, and anthracene) on mineral-associated peat humic acid (PHA) under different pH and electrolyte regimes, indicating that PHA conformation, as controlled by solution conditions, is an important consideration of HOC sorption. Schultz et al. (19) investigated phenanthrene sorption to soil organic matter (SOM) and found little correlation between sorption properties and organic matter chemical characteristics, suggesting that sorption phenomena are more dependent on organic matter-mineral interactions or organic matter conformation. Salloum et al. (17) also reported that the chemical characteristics of SOM alone could not explain the observed 1-naphthol sorption coefficients and employed a mass balance approach to demonstrate that organic matter physical conformation was playing a role in regulating the accessibility to specific sorption sites within the SOM matrix. Jones and Tiller (20) compared phenanthrene sorption to dissolved and clay-bound humic acid, and found that sorption values for the clay-bound humic acid were lower than that of the bulk humic acid prior to adsorption to clay mineral surfaces. Their results suggested that the conformation of the humic substance at the clay mineral surface determined the level of phenanthrene sorption. Detailed chemical characteristic studies have also indicated that sorption processes cannot be solely based on sorbent chemical properties and concluded that the conformation of organic matter at the solid-aqueous interface needs to be investigated further (18). Nuclear magnetic resonance (NMR) spectroscopy has been applied more readily recently to examine the role of organo-clay compounds in sorption interactions with HOCs. For example, Chen and Xing (21) demonstrated, with the use of solid-state 13C NMR, a phase transition from solidamorphous to mobile-amorphous domains of reconstituted cuticular waxes on montmorillonite in the presence and absence of sorbed HOCs (naphthalene, phenanthrene, or pyrene). They also suggested that the HOC sorption capacity depended on the conformation of organic matter due to partition-like mechanisms of mobile amorphous domains. Wang and Xing (24) observed that the aliphatic fraction of Amherst soil humic acid was preferentially adsorbed by montmorillonite and kaolinite by using solid- and liquidstate 13C NMR and attributed the elevated sorption of phenanthrene (i.e., higher Koc values) by mineral-associated humic acid to the fractionation and conformation changes in humic acid during adsorption to clay mineral surfaces (25). The preferential sorption of aromatic compounds from humic acids by montmorillonite and polymethylene organic matter (CH2 groups) by kaolinite was observed by NMR spectroscopy (25, 26), suggesting that “available” organic matter structures at the soil-water interface may be controlled by clay mineral type and solution conditions. These recent studies collectively suggest that the mineral phase indirectly regulates sorption reactions by controlling the distribution and/or the conformation of organic matter at the solid-aqueous interface. In this study, we aim to explore the role of organic matter physical conformation in sorption processes by using organoclay complexes that are constructed by coating montmorillonite and kaolinite with PHA in Na+- or Ca2+-dominated solutions with varying pH and ionic strength values. The solution conditions encourage the dissolved humic acid to adopt a “coiled” or “stretched” conformation prior to 10.1021/es0521472 CCC: $33.50
2006 American Chemical Society Published on Web 04/05/2006
interacting with the clay mineral surface (23, 25, 27). It is hypothesized that the conformation of humic acid on clay surfaces is influenced by its configuration in solution (1, 2) and will therefore determine the degree of phenanthrene sorption. To meet this objective, batch equilibration sorption studies are used to study phenanthrene sorption to constructed organo-clay complexes under different solution conditions. Details regarding mechanisms of PHA binding to kaolinite and montmorillonite have been published previously (25). In addition, 1H high-resolution magic angle spinning (HR-MAS) NMR, a novel technique which enables one to obtain liquid-state quality spectra on semisolid samples, is employed to determine the humic acid fractions present at the solid-aqueous interface (25, 28) and the structures that are most likely to interact with phenanthrene. It should be noted that organo-clay complexes may or may not fully represent the reactivity of organo-clay complexes found in nature, but here we use constructed complexes such that NMR can be applied to study the chemical nature of these compounds. Future studies will aim to investigate the efficiency of this NMR technique with whole soil samples.
Experimental Section Clays and Peat Humic Acid. SAz-1 Ca-montmorillonite and KGa-1b kaolinite were obtained from the Clay Minerals Society’s Source Clays Repository (West Lafayette, IN). Montmorillonite is reported to have a surface area (N2-BET) of 97.42 m2/g and a cation exchange capacity (CEC) of 120 meq/100 g, and kaolinite is reported to have a surface area of 10.05 m2/g and a CEC of 2.0 meq/100 g (29). Both montmorillonite and kaolinite were made homoionic by resuspension in a 0.01 M solution of NaNO3 or Ca(NO3)2 for 1 h to remove exchangeable cations prior to sorption experiments. The suspension was then centrifuged (1000g for 15 min), the supernatant was decanted, and the clays were washed with deionized water. The procedure was repeated once. PHA was extracted from a Florida peat sample (International Humic Substances Society, St. Paul, MN) based on the method by Salloum et al. (17). Elemental analysis and solidstate 13C cross polarization magic angle spinning (13C CP/ MAS) NMR spectroscopy were used to characterize the PHA (16, 17). The PHA was then dissolved with the aid of 0.1 N NaOH into distilled water containing 0.05 mM HgCl2 (except samples for NMR analysis) to prevent biological degradation without causing significant influences on PHA chemistry (30). The PHA solution was filtered through a 0.22 µm Millipore membrane filter before sorption experiments to remove bacteria and any undissolved constituents. 1 H Liquid-State and HR-MAS NMR. To examine the humic acid structures that were most likely to interact with phenanthrene, 1H liquid-state and HR-MAS NMR spectroscopy was employed to determine the chemical nature of the unbound humic acid fractions in solution and the structures sorbed at the mineral surfaces. Samples for NMR were prepared by mixing 150 mg of cation-exchanged kaolinite and montmorillonite with 150 mg of dissolved PHA in 40 mL of 0.01 M NaNO3 solution (initial pH of 7) for 48 h. The supernatant was retained after centrifugation, and the precipitate was washed 5 times with distilled water to remove any unbound PHA. The supernatant was then cationexchanged through an Amberlite 1200(H) ion-exchange resin (Fisher Scientific) and both fractions were freeze-dried and desiccated in the oven at 40 °C under vacuum and over P2O5 for at least 24 h. Supernatant samples (∼50 mg) were dissolved in DMSOd6 (0.75 mL) and examined by 1H liquid-state NMR, which was carried out on an Avance 500 MHz spectrometer equipped with a 1H-BB-13C 5 mm TBI probe. 1-D liquid-state 1H NMR experiments were performed with 128 scans, a
recycle delay of 3 s, 32 000 time domain points, and a sample temperature of 298 K. Spectra were apodized by multiplication with an exponential decay corresponding to 1 Hz line broadening in the transformed spectrum, and a zero filling factor of 2. Sample preparation for 1H HR-MAS entailed adding organo-clay complex (∼40 mg) into a 4 mm zirconium oxide rotor. Then, 60 µL of DMSO-d6 was added as a swelling solvent. After homogenization of the sample using a stainless steel mixing rod, the rotor was doubly sealed using a Kel-F sealing ring and a Kel-F rotor cap. 1H HR-MAS NMR spectra were acquired using a Bruker 500 MHz Avance spectrometer equipped with a 4-mm inverse 1H-13C-15N HR-MAS probe fitted with an actively shielded Z gradient and at a spinning speed of 10 kHz. Scans (512) were acquired using presaturation of the water signal at ∼3.3 ppm (note: the water was taken up from the atmosphere during preparation of the sample), 3 s delay between pulses, a sweep width of 20 ppm, 8192 time domain points, and a sample temperature of 298 K. The spectra were processed with a zero-filling factor of 2 and an exponential multiplication, which resulted in a line broadening of 1 Hz in the transformed spectrum. Organo-Clay Preparation for Sorption Experiments. Organo-clay complexes for phenanthrene sorption were constructed by mixing 2 L of 50 mg/L PHA solution with 20 g of homoionic clay (montmorillonite or kaolinite) under different conditions of pH, ionic strength, and solution cation type (Table 1). NaNO3 and Ca(NO3)2 solutions were used as background electrolytes to control ionic strength. The pH was controlled at 4.0 or 7.0 with NaOH and HNO3 (0.1 N). The suspension was mixed by hand for the first minute and then placed on a shaker for 24 h (apparent equilibrium was reached in this time) at room temperature. The samples were centrifuged (450g for 40 min) and the supernatant was decanted. The clay particles were resuspended in fresh electrolyte, shaken for 1 h, and centrifuged again to remove any unbound PHA. Preliminary tests indicated that no significant desorption was observed. The solid residue isolated after centrifugation is defined as the organo-clay complex. A 50 mg portion of each type of organo-clay was freeze-dried and sent for total organic carbon (TOC) analysis on a 2400 Series II CHNS analyzer to measure the organic C fraction (foc). Phenanthrene Sorption to Organo-Clay Complexes. Phenanthrene (>96%, Sigma-Aldrich, St. Louis, MO) has a reported aqueous solubility of 1.29 mg/L and a supercooled liquid-state solubility (Sscl) of 5.9 mg/L (31). An aliquot from a methanol stock solution of phenanthrene (500 mg/L) was dissolved in a solution with the same cation type, pH, and ionic strength values as the corresponding organo-clay suspension. HgCl2 (0.05 mM) was added to prevent biological degradation of the sorbate (30). Methanol concentrations represented 0.5% of the total solution (v/v) to avoid any cosolvent interferences. Preliminary experiments indicated that phenanthrene activity did not change significantly by adding 0.5% methanol. Phenanthrene solutions (10 mL) of varying concentration (0.2-1.0 mg/L) were added to constructed organo-clay suspension (dry weight of 100-150 mg) previously weighed into 13-mL Kimax glass test tubes. The dry weight of organo-clay complexes was determined by oven drying the sample at 105 °C, and the amount was controlled through preliminary tests to achieve a 20-80% sorption of phenanthrene (except B and D, which exhibited 15% phenanthrene sorption). Three replicates of each starting concentration were assembled, together with a blank without any organo-clay, for a total of 20 tubes per isotherm. Precleaned hollow glass beads (6 mm in diameter) were added into each tube to facilitate mixing and to minimize headspace to avoid losses by volatilization. The tubes were sealed with Teflon-lined screw caps and then shaken on an Eberbach VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Solution Conditions and Isotherm Parameters for Phenanthrene Sorption modified Freundlich model Kf′ ( SEa (µg/g) n ( SEa r2
Freundlich model Kf ( SEa (×10-3) (mg/g)/(mg/L)n n ( SEa
linear model KD ( SEa (L/kg)b r2
foc (%)
Koc ( ×103 L/kg)
cation
ionic strength
Na
0.01 M
pH 4 pH 7 0.001 M pH 4 pH 7 0.01 M pH 4 pH 7 0.001 M pH 4 pH 7
A B C D E F G H
27.4 ( 0.0 14.5 ( 0.0 5.6 ( 0.0 18.4 ( 0.0 74.2 ( 0.04 12.2 ( 0.0 40.3 ( 0.01 33.0 ( 0.02
Montmorillonite Clay 0.59 ( 0.06 0.98 10 ( 0.8 1.10 ( 0.1 0.97 2.0 ( 0.1 0.43 ( 0.11 0.85 3.0 ( 0.2 1.19 ( 0.10 0.99 2.0 ( 0.1 0.73 ( 0.17 0.93 21 ( 0.9 0.40 ( 0.12 0.82 6(1 0.73 ( 0.11 0.95 11 ( 2 0.75 ( 0.20 0.88 9(2
0.59 ( 0.1 1.09 ( 0.1 0.44 ( 0.1 1.19 ( 0.1 0.75 ( 0.2 0.40 ( 0.1 0.75 ( 0.1 0.75 ( 0.2
0.98 0.97 0.86 0.99 0.93 0.82 0.95 0.89
14.2 ( 0.4 1.90 ( 0.0 3.95 ( 0.0 1.90 ( 0.1 28.9 ( 0.6 12.4 ( 0.3 16.6 ( 0.4 11.3 ( 0.3
0.98 0.96 0.96 0.99 0.97 0.99 0.87 0.97
0.16 0.14 0.10 0.05 0.59 0.31 0.66 0.60
8.8 1.3 4.0 3.5 4.9 3.7 2.5 1.9
0.01 M
I J K L M N O P
37.6 ( 0.01 29.8 ( 0.01 44.1 ( 0.02 34.7 ( 0.01 63.7 ( 0.02 25.7 ( 0.01 301 ( 0.2 27.9 ( 0.01
0.61 ( 0.10 0.74 ( 0.15 0.80( 0.13 0.94 ( 0.05 0.74 ( 0.11 0.62 ( 0.12 1.2 ( 0.19 0.55 ( 0.14
Kaolinite Clay 0.94 13 ( 2 0.92 8 ( 0.1 0.94 10( 2 0.99 6 ( 0.1 0.95 17 ( 3 0.91 8(2 0.94 36 ( 1.4 0.91 11 ( 2
0.60 ( 0.1 0.74 ( 0.2 0.77 ( 0.1 0.91 ( 0.1 0.72 ( 0.1 0.62 ( 0.1 1.09 ( 0.2 0.56 ( 0.1
0.94 0.93 0.93 0.99 0.96 0.91 0.93 0.91
24.5 ( 0.1 10.5 ( 0.1 13.6 ( 0.4 7.20 ( 0.1 24.1 ( 0.1 14.7 ( 0.1 28.6 ( 0.4 17.4 ( 0.5
0.93 0.97 0.97 1.00 0.98 0.92 0.97 0.91
0.16 0.10 0.15 0.22 0.35 0.13 0.37 0.14
15.3 10.5 9.1 3.3 6.9 11.3 7.7 12.5
Ca
Na
label
pH 4 pH 7 0.001 M pH 4 pH 7 0.01 M pH 4 pH 7 0.001 M pH 4 pH 7
Ca
a
pH
SE ) Standard error, confidence interval of 95%.
b
Linear fit of isotherms was based on 5 data points, except E and F, which was on 4 points.
6010 shaker for 48 h (apparent equilibrium was reached before this time) at room temperature. The samples were centrifuged (160g for 40 min) and a 2-mL aliquot of the supernatant was removed into an amber vial for analysis by high-performance liquid chromatography (HPLC). Phenanthrene concentration was determined using an Agilent 1100 HPLC system equipped with an autosampler, a 5 µm Vydac C18 2.1 × 250 mm column, and a diode array detector. The HPLC parameters used were: 1 µL injection volume, a mobile phase of 80% acetonitrile and 20% water, a flow rate of 0.45 mL min-1, and an absorbance wavelength of 254 nm. Preliminary experiments tested the phenanthrene uptake to the glass tubes and to pure clays. These experiments did not detect any sorption to either the glass walls of the test tubes nor the pure clay minerals, thus phenanthrene uptake by organo-clay complexes was measured by difference. Sorption Isotherms. Phenanthrene sorption data were fit to both the linear partition model (x/m ) KDCe, where x/m and Ce are the equilibrium solid-phase and aqueous-phase solute concentrations, KD is the distribution coefficient (L/ kg)), and the Freundlich model (x/m ) KfCen, where Kf and n are the Freundlich model capacity factor and the isotherm linearity parameter, an indicator of site energy heterogeneity, respectively). Isotherm model fitting was performed using Origin Version 7.0 (Microcal Software, Northampton, MA) at a confidence level of p e 0.05. The fit of linear versus Freundlich model was tested by using an F-test (32), which justified the use of the linear isotherm for most samples. KD values for all samples (except E and F) were calculated from the linear fit of the isotherm data isotherm (i.e., based on 5 experimental points). Samples E and F did not produce linear isotherms and, therefore, the KD values were calculated from the linear part of the isotherm (the first 4 data points). The organic carbon normalized sorption coefficients (Koc) were calculated from KD (Koc ) KD/foc). To compare different isotherms, the modified Freundlich parameter, Kf′, was calculated after normalizing Ce by the supercooled liquidstate solubility (Sscl) of phenanthrene (x/m ) Kf′Crn, where Cr is the reduced concentration, i.e., solubility-normalized equilibrium concentration, 31).
Results and Discussion Characterization of PHA and Organo-Clay Complexes. The PHA was found to contain 55.4% C, 3.7% H, 34.6% O, 3.9% 3262
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N, and an ash content of 0.43%. The distribution of general carbon characteristics, as determined by 13C CP/MAS NMR, was found to be 22% aliphatic C (0-50 ppm), 23% substituted aliphatic C (50-110 ppm), 36% aromatic C (110-160 ppm), and 19% carboxylic and carbonyl C (160-220 ppm). A 13C CP/MAS NMR spectrum of the PHA sample used in this study is published in ref 25. The liquid-state 1H NMR spectra of the supernatant and1H HR-MAS NMR spectra of PHA-kaolinite/ montmorillonite complexes in 0.01 M Na+ (pH 7) are shown in Figure 1. It should be noted that specific PHA components that are tightly bound to a mineral surface and form true solid domains into which no solvent can penetrate may be underestimated by the HR-MAS technique. At the spinning speeds employed in the study, 1H-1H dipolar interactions will not be completely averaged, and very broad resonances will arise. Consequently, signals from true solid domains, if present, may be underestimated by the HR-MAS approach. NMR assignments were based on comparison with biopolymer standards, confirmation by two-dimensional NMR experiments (data not shown), and/or with literature assignments (25, 33-38). The labeled regions highlight chemical shift regions that are from amide groups, aromatics, amino acids and polysaccharides, and aliphatic compounds (25, 33-38). Both kaolinite and montmorillonite organo-clay complexes display a strong CH3 resonance from short sidechains found in amino acids. In addition, the presence of aromatic signals (∼6.5-8 ppm) suggests the presence of peptide material in both samples noting that the peptide signature is more prevalent in the montmorillonite sample (Figure 1). Comparison with a peptide standard (bovine serum albumin) and the results from the 13C CP/MAS also support the presence of peptide material in the PHA. However, this peptide material appears to have sorbed more strongly to the montmorillonite than to the kaolinite clay. Long-chain aliphatic structures, such as polymethylene groups, produce prominent CH2 peaks at ∼1.29 ppm and to a lesser extent terminal CH3 peaks (25, 33-38). The lack of signals in the 4-5 ppm region and the relatively low intensity of the amide peak centered at ∼8.1 ppm in the organo-clay complexes results from the presaturation needed to remove the intense and broad water resonance centered at ∼3.3 ppm in the 1H HR-MAS NMR spectra (this water is introduced into the sample from the atmosphere during the extensive sample preparation). This water stretched from 3 to 4.5 ppm
FIGURE 1. Comparison of 1H liquid-state and HR-MAS NMR spectra of the unbound PHA and PHA-mineral complexes in 0.01 M Na+, pH 7: (A) PHA-kaolinite complex, (B) PHA-montmorillonite complex. * indicates DMSO-d6. ** Note other moieties such as esters, ethers, and some amino acids may also contribute strongly to this region. *** The aromatic signatures and intensive CH3 resonance in this case result from peptide structures. Note that the region associated with r protons in peptides is reduced due to the presaturation employed in the 1H HR-MAS experiment (for further discussion, see text). and signals in this region were inadvertently lost while trying to suppress the water (suppression is critical in order to obtain a NMR spectrum). Hence, the R protons associated with the peptides (3-5 ppm) may be underestimated. CH2 groups at ∼1.29 ppm in 1H HR-MAS spectra were observed to bind preferentially to both minerals, while polysaccharides largely remained in the unbound fraction (supernatant). The 1H HR-MAS NMR spectrum of the montmorillonite organo-clay complex displays a considerable contribution from peptides, in addition to the CH2 (polymethylene) signals. An estimate based on the ratio of the CH3 versus CH2 signals in the NMR spectra of the PHAmineral complexes indicates that there is more peptide material sorbed to the montmorillonite than to kaolinite. While there is evidence for small amounts of peptides present in the kaolinite sample, CH2 signals clearly dominate the spectrum of PHA-kaolinite complex (Figure 1). This finding is consistent with previous observations that montmorillonite sorbs more aromatic moieties and kaolinite sorbs primarily cutin-derived, polymethylene structures (24-26, 37, 38). Polymethylene-rich organic matter is an important domain for hydrophobic sorption (16, 21, 39, 40) that may influence the ability of soils to retain HOCs. The spectra supporting the preferential sorption of PHA by clay minerals under varying solution conditions were not included in this study. However, our previous observation with 1H HR-MAS NMR (25) has indicated that the chemical moieties at the surface of montmorillonite-PHA complexes are similar in Na+ and Ca2+ solutions. Furthermore, lower pH values appear to inhibit sorption of peptide moieties by montmorillonite, which was observed by fewer bound CH3 groups at pH 4. The 1H HR-MAS data demonstrate that solution conditions and clay mineral type are important for the selective sorption of humic materials at the soil-water interface. Phenanthrene Sorption to Organo-Clay Complexes. The varying organo-clay complexes resulted in different isotherm parameters and sorption coefficients (Table 1, Figure 2). The n values for phenanthrene sorption isotherms were generally
less than 1. The Kf′ values (ranging from 5.57 to 301.54 µg/g) were similar to those reported for phenanthrene sorption to soil and microaggregates in 0.01 M CaCl2 (31) but lower than those reported for sorption to humic acids (39). Varying solution conditions resulted in Kf′ values that ranged over 2 orders of magnitude (Table 1) and suggests that future studies of HOC sorption should employ the same ionic strength, pH, and cation types. Both montmorillonite- and kaolinite-PHA complexes produced low Freundlich n values in Na+, pH of 4, and Ca2+, pH of 7; and high n values in Na+, pH of 7, and Ca2+, pH of 4 (Table 1). The chemical composition of PHA is uniform before its interaction with the clay and, therefore, the isotherm nonlinearity can be attributed to the selective sorption of PHA during its binding to clay surfaces and/or different PHA conformation at the solid-aqueous interface. Other studies (1, 2, 23) have indicated that mineral-associated PHA is uniformly extended in Na+ at a pH of 7 and coiled in Ca2+ at a pH of 4, resulting in high n values. In contrast, Na+ at a pH of 4 and Ca2+ at a pH of 7 produce partly coiled and partly elongated PHA configuration, thus resulting in a heterogeneous conformation (low n values). Ionic strength did not have a significant impact on n values, likely because the electrolyte concentrations in our experiment were too low to compete with the control of pH and solution cation. Intuitively, one would expect that fewer binding sites are available on humic acid at a lower pH (coiled configuration) because some hydrophobic domains are “hidden” or “protected.” However, the Kf′ values for phenanthrene sorption to the organo-clay complexes were generally higher at lower pH values (with the exception of sample C and D). Koc values also increased with decreasing pH, except those of kaolinitePHA complexes prepared in Ca2+, which resulted in an increase in Koc with increasing pH (Table 1). Laor et al. (41) described a decrease in sorption of phenanthrene at higher pH values due to the increased repulsive force due to a greater number of negative charges on humic material as it becomes less protonated. However, Kile et al. (9) found that polarity was a poor predictor of HOC sorption capacity. We therefore VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Phenanthrene sorption isotherms for PHA-montmorillonite complexes in NaNO3 (a) and Ca(NO3)2 (b) solutions, and PHAkaolinite complexes in NaNO3 (c) and Ca(NO3)2 (d) solutions. KD values are given in units of L/kg. The linear fit is based on five data points except for samples E and F which were calculated using four data points. All curves were forced through zero. hypothesize that when the PHA takes on a coiled configuration at pH 4, more favorable sorption sites become available at the mineral surface and thus enhance phenanthrene sorption. Furthermore, PHA has a higher loading on both clay surfaces at pH 4 (25). It should also be noted that PHA may form its own discrete domain on the mineral surface at higher PHA concentrations and these PHA “patches”, may also contribute to enhanced phenanthrene sorption. This is consistent with Chen and Xing’s phase transition model (21), where the proportion of plant cuticular waxes in the “expanded” or mobile phase on montmorillonite surfaces is greater at higher wax loadings than at lower wax loadings and thus enhances HOC sorption. Alternatively, at lower pH values fewer peptides are sorbed to clay surfaces (25) and, hence, relatively higher amounts of polymethylene structures are exposed at the solid-aqueous interface and results in enhanced phenanthrene sorption because sorbents rich in polymethylene organic matter have been reported to exhibit very high Koc values for phenanthrene (16, 39, 40). The binding pattern of PHA on kaolinite in Ca2+ dominated solution is different, demonstrating a higher sorption capacity for phenanthrene at higher pH values. Phenanthrene sorption was generally greater at lower ionic strengths (except montmorillonite-PHA complex at pH 4), which was similar to the observation by Murphy et al. (23). This phenomenon implied that partitioning was not the dominant mechanism in phenanthrene sorption because partitioning is a concentration-independent process that is not influenced by ionic strength (23). The 1H HR-MAS NMR 3264
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data show polymethylene groups at the surface of the PHAkaolinite/montmorillonite complexes resulting from long alkyl chains projecting from the surface (Figure 1). Therefore, interactions such as hydrogen bonding and/or π-bonding might be important in the uptake of HOCs by this aliphatic fraction (42). The type of solution cation had mixed effects on phenanthrene sorption. The Koc values were generally greater in Na+dominated solutions than in Ca2+-dominated solutions at pH 4, and at pH 7 the solution cation did not induce a uniform control on phenanthrene sorption. This is similar to the findings of Jones and Tiller (20), who reported that phenanthrene binding to the humic-coated kaolinite was greater in the presence of Na+ than that in the presence of Ca2+ at pH 4. They also observed that different cations did not produce a significant difference in phenanthrene sorption (Koc values) to illite-associated humic acids. Therefore, the solution cation has a different degree of influence on the conformation of humic acid associated with various clay minerals and/or the distribution of discrete PHA domains on the clay mineral surface. Ionic strength did not produce a consistent relationship with the Koc value (Table 1). Again, this is likely because the electrolyte concentrations in our experiment were not high enough to dominate other aqueous chemistry controls. Role of Different Mineral Surfaces. Although mineral surfaces do not contribute to phenanthrene sorption directly, their interaction with soil organic matter may play pivotal roles in the sorptive behavior of fulvic and humic acids (43).
With kaolinite-associated PHA, a statistically higher Koc value for kaolinite-associated PHA was observed than with montmorillonite-associated PHA (Table 1; except the similar Koc values for both organo-clays in 0.001M Na+, pH of 7). PHAkaolinite complexes provide more preferential binding sites for phenanthrene sorption at the solid-aqueous surface. This result is consistent with the findings of Wang and Xing (24), who observed that kaolinite-humic complexes had greater Koc values for phenanthrene sorption than montmorillonitehumic complexes in 0.01 M CaCl2 solutions. The mineral control of HOC sorption to organo-clay complexes may take place via the distribution of hydroxyl sites on mineral surfaces, which may determine the interfacial configuration of humic coatings through binding with the carboxyl groups on humic substances, and thus alter the size or accessibility of hydrophobic domains (1, 2). Alternatively, preferential sorption of different fractions of humic substances by different clays might play a part in governing sorption reactions. Wang and Xing (24) indicated that the humic acids sorbed on kaolinite surfaces are more aliphatic than those sorbed on montmorillonite surfaces, resulting in a more hydrophobic and less polar coating that would favor phenanthrene sorption. With 1H HR-MAS NMR, we directly observed that polymethylene signals dominated the spectrum of the PHA-kaolinite complex while montmorillonite sorbed more aromatic/peptide compounds (Figure 1). Peptides at the montmorillonite organo-clay surfaces may block sorbate access to polymethylene organic matter, which is reported to sorb high amounts (i.e., high Koc values) of phenanthrene (16, 39, 40). The dominance of polymethylene structures at the kaolinite organo-clay surfaces, as observed by 1H HRMAS NMR, supports the increased uptake of phenanthrene by kaolinite-PHA complexes in comparison to montmorillonite-PHA complexes. The mineral control of the sorption site distribution and chemical moieties at the solid-aqueous interface deserve further study.
Acknowledgments We thank Prof. George Arhonditsis (University of Toronto) for guidance with statistical analyses. We gratefully acknowledge the Natural Science and Engineering Research Council (NSERC) of Canada for supporting this research. M.J.S. also thanks NSERC for support via a University Faculty Award (UFA). We thank the Canada Foundation for Innovation and the Ontario Innovation Trust for support of the Environmental NMR Centre where the NMR studies were performed.
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Received for review October 27, 2005. Revised manuscript received February 3, 2006. Accepted March 13, 2006. ES0521472
