Environ. Sci. Technol. 2007, 41, 153-159
Variation in Phenanthrene Sorption Coefficients with Soil Organic Matter Fractionation: The Result of Structure or Conformation? JULIA L. BONIN AND MYRNA J. SIMPSON* Department of Physical and Environmental Sciences, University of Toronto, Scarborough College, Toronto, Ontario M1C 1A4, Canada
Sorption of phenanthrene to varying soil types was investigated to better understand sorption processes. Humic acid and humin fractions were isolated from each soil sample, and sorption coefficients were measured by batch equilibration. Samples were characterized by carbon analysis and 13C cross polarization magic angle spinning (CP/ MAS) nuclear magnetic resonance (NMR) spectroscopy. Measured organic carbon-normalized sorption coefficients (Koc) of the fractions were greater in all cases when compared to the soils. The humin fractions exhibited greater Koc values than did source samples, suggesting that fractionation may reorganize organic matter in humin resulting in an increased availability of and/or more favorable sorption domains. Mass balance calculations revealed that the sum of sorption to the fractions is greater than sorption to the whole sample. The greatest difference between sorption values was found to occur with the mineral soils, suggesting that clay minerals influence the physical conformation of soil organic matter (SOM) and availability of sorption domains. The mass balance, sorption data, and a lack of consistent trends between observed Koc values and solid-state 13C NMR data suggest that the physical conformation of SOM and chemical characteristics both play important roles in sorption processes.
Introduction Soil organic matter (SOM) plays a dominant role in the sorption of polycyclic aromatic hydrocarbons (PAHs) in the environment (1-15). Sorption phenomena are of concern due to the strong affinity of PAHs to natural organic matter (1-5, 7-15). In addition, PAHs exhibit toxicity at low concentrations, and, hence, there is a potential risk associated with long-term persistence in the environment (3, 12). Therefore, it is imperative to improve the fundamental understanding of PAH sorption processes in soil. The chemical properties of the solid phase (sorbents) have been correlated to sorption capacity (4-9) to better identify structures that are responsible for the sorption of contaminants. For example, Xing et al. (4) and Chen et al. (5) documented a correlation between xenobiotic sorption and aromaticity using solid-state 13C nuclear magnetic resonance (NMR). Ahmad et al. (6) investigated the sorptive properties of 27 soil samples using carbon types derived from NMR * Corresponding author phone: (416)287-7234; fax: (416)287-7279; e-mail:
[email protected]. 10.1021/es061471+ CCC: $37.00 Published on Web 12/02/2006
2007 American Chemical Society
data and found that the variation in Koc values was proportional to the SOM aromaticity. Gauthier et al. (7) examined the binding of pyrene to 14 different humic acid (HA) and fulvic acid (FA) samples and attributed the magnitude of Koc values to the degree of aromaticity of the humic material. More recently, Chefetz et al. (8) and Salloum et al. (3) found a positive relationship between phenanthrene Koc values and aliphatic (paraffinic) carbon content of SOM. Similarly, Kopinke et al. (9) reported that the sorption potential of aromatic ester moieties in model polymers and humic substances was similar to those with aliphatic moieties and concluded that aromatic substituents are not superior to aliphatic substituents in the sorption of pyrene to the examined model polymers (13). Simpson et al. (2) also reported that aromaticity and H/C atomic ratios were not sufficient for assessing the degree of phenanthrene sorption to humic acid. Consequently, there is increasing evidence that chemical characteristics of SOM cannot solely be used to determine or predict sorption behavior of PAHs. Many studies have emphasized that organo-mineral complexes potentially play an important role in governing SOM conformation and are becoming more important in the study of sorption mechanisms (2, 11, 13, 15-18). Organo-mineral complexes are widely distributed in the environment (10), and SOM has been reported to be intimately associated with the clay mineral fraction (19-21). Sorption studies conducted with organic matter and organoclay complexes have demonstrated that the extent of sorption decreases when organic matter is associated with minerals (13, 15, 17, 18). In addition, several studies have suggested that sorption of contaminants to mineral-bound organic matter is influenced by the configuration of organic matter on minerals by governing accessibility to sorption domains (11-13). For example, Chen and Xing (18) suggested that PAH sorption capacity varied with the conformation of organic matter, which in turn regulated the access of contaminants to mobile amorphous domains. Wang and Xing (24) reported that the clay minerals preferentially sorbed aliphatic organic matter, which indirectly governed the degree of sorption. Feng et al. (15) reported similar results and found that preferential sorption of organic matter by clay minerals and the resulting control of organic matter physical conformation on the clay mineral surface greatly varied phenanthrene sorption coefficients. These studies and others (1218, 24, 25) collectively highlight the importance of clay minerals in governing the accessibility of sorption domains at the soil-water interface. In this study, we measured the sorption of phenanthrene to soil and soil fractions using a batch equilibration method. Sorbents were characterized using solid-state 13C cross polarization magic angle spinning (CP/MAS) NMR to gain semiquantitative insight into the chemical nature of SOM in the soil and soil fractions. The objective of this study is to examine the sorption of phenanthrene to whole soils and respective humin and HA fractions in an attempt to gain insight into sorption processes in soil. In addition, we examine the use of a mass balance approach proposed by the authors in ref 11 to elucidate how SOM fractionation in addition to the SOM chemical characteristics impacts physical conformation and resulting sorption coefficients. It should be noted that the method used to measure phenanthrene sorption (batch equilibration method) only measures total retention and not sorption to specific sites. Hence, “sorption” in the context of this study is used to describe total retention of phenanthrene to the sorbent. VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Experimental Section Soil Samples, Fractionation, and Characterization. Soil samples were collected from Alberta and chosen on the basis of similarities in clay mineralogy (11) and differences in SOM content. These soils (classified as Chernozems in the Canadian System of Classification) develop over fine textured, calcareous parent material under the influence of grassland vegetation. The surface (Ah) horizons were collected from pristine and native grassland environments. The brown soil was sampled south of Lethbridge, Alberta. The dark brown soil was sampled at the Agriculture and Agri-Food Canada research station in Lethbridge. The black soil was sampled from the University of Alberta Ellerslie Research Station, located south of Edmonton. For comparison with an organicrich soil, the Pahokee peat sample was purchased from the International Humic Substances Society (IHSS; Minneapolis, MN). All soil samples were air-dried, passed through a 2 mm sieve, and ground to pass a 106 µm sieve. Soil samples were fractionated into humin, HA, and FA fractions using the classical base (NaOH) extraction procedure as outlined in ref 11. HA was isolated by dialysis (Fisherbrand 6000-8000 molecular weight cut off) against distilled water and then freeze-dried. Humin, the fraction of humic substances that is insoluble in aqueous solution under any pH (14), was isolated by repeatedly extracting the soil with 0.1 M NaOH until the removal of organic matter ceased and the NaOH solution remained clear after mixing with the solid residue. The humin then was rinsed repeatedly with distilled water to remove excess salts and then freeze-dried. Preliminary studies suggest that SOM fractions did not require presaturation with Ca2+ prior to sorption experiments (sorption values for Ca-saturated HA and humin were equivalent to those for H-saturated HA and humin samples) and that the background electrolyte was sufficient in saturating the exchange complex with Ca2+. Total carbon was measured using a Perkin-Elmer model 2400II CHN analyzer with Perkin-Elmer AD-6 autobalance at Laboratory Services, University of Guelph (Guelph, Ontario). Inorganic carbon was measured by the method of Bundy and Brenner (22) and was not detectable in these samples. All HA, humin, and whole samples underwent deashing via six cycles with 0.1 M HCl/0.3 M HF to remove paramagnetic ions, concentrate the organic matter, and increase NMR sensitivity (23). Samples were dialyzed to remove excess salts and then freeze-dried. The removal of minerals by de-ashing invalidates any attempts to obtain conformational information from the NMR experiments. Solid-state 13C CP/MAS NMR data were acquired on a Bruker Avance 200 MHz NMR spectrometer, equipped with a 4 mm H-X MAS probe, and using a ramp-CP pulse program. The following acquisition parameters were employed: spinning rate of 10 kHz, ramp-CP contact time of 2 ms, 1 s recycle delay, number of scans of 25 000, and line broadening of 50 Hz. Carbon assignments were made using chemical shift ranges (24, 25): 0-50 ppm alkyl carbon, substituted aliphatic (50-110 ppm), aromatic carbon (110-165 ppm), and carboxyl and carbonyl carbon (165-215 ppm). Batch Sorption Experiments. Phenanthrene (>96%, HPLC grade) was purchased from Sigma-Aldrich and used as received. All soil and peat samples were weighed into 8 or 15 mL KIMAX vials with Teflon-lined screw caps along with 5, 6 mm glass beads to ensure sufficient mixing. A background solution of 0.01 M CaCl2 and 10-4 M of HgCl2 biocide was maintained at a pH ) 6.5 for soil and humin samples and pH ) 4 for HA samples. Preliminary experiments demonstrated that phenanthrene sorption to whole soil and humin samples did not change with the pH of the background electrolyte (pH ) 6.5 versus pH ) 4); thus pH ) 4 was used for HA samples to ensure that the HA remained in the solid 154
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phase (11). Phenanthrene solutions were made in methanol and then diluted using the background solution. Methanol concentrations were maintained at 0.08% of the total solution volume to avoid any cosolvent effects (26). Aqueous phenanthrene concentrations ranged from 0.2 to 1 ppm. Preliminary experiments were conducted to estimate apparent equilibrium and ensure that final phenanthrene solution concentration remained in the range of 20-80%, such that analysis errors were minimized. Sorption isotherms consisted of five concentration points each in triplicate. The vials were placed on a shaker for 48 h at room temperature (preliminary tests indicated that apparent equilibrium was reached before 36 h). The vials were centrifuged at 1000g for 20 min, and then a 2 mL aliquot was removed from the supernatant and transferred into 2 mL amber vials. Phenanthrene concentration was determined by high performance liquid chromatography (see Supporting Information for details). Phenanthrene sorption to test tube and HPLC vial walls, and to kaolinite and montmorillonite (using the same sorbate to sorbent ratios used for soil, HA, and humin samples), was not detected, so, consequently, phenanthrene sorption was calculated by difference (15). Sorption Coefficients and Mass Balance Equations. The soil-water distribution coefficients (Kd) were calculated from the slope of the sorption isotherms, and the organic carbon normalized sorption coefficients (Koc) were calculated by dividing the Kd value by the fraction of organic carbon (foc) in the sample (27). Linear regression analysis of isotherms was performed using a graphical software package (Origin v7.0). A reconstituted Koc was calculated from: Koc,sample ) Koc,huminXhumin + Koc,HAXHA + Koc,FAXFA, where X represents the fraction of total carbon in that isolate (Table S1, Supporting Information) and Koc,humin, Koc,HA, and Koc,FA represent the carbon normalized sorption coefficient for respective sorbents (11). Once extracted with NaOH, the FA fraction is soluble at all pH values, and thus experimental Koc values could not be obtained with the batch equilibration method used with HA and humin and would have entailed using either a dialysis method (28) or a fluorescence method (29). Therefore, the contribution of the FA fraction (Koc,FA) was estimated for comparative purposes from the octanol-water partitioning coefficient (Kow) (phenanthrene log Kow ) 4.45, ref 18) using the equation [graphic2] log Koc ) 0.989 log Kow - 0.346 (27).
Results Sorbent Characteristics. Solid-state 13C CP/MAS yields semiquantitative information on SOM composition (30), which may be used to better understand sorption phenomena. Absolute quantification of individual SOM structures by solid-state 13C NMR is difficult due to the broad lines from the multitude of compounds in SOM, and thus the data presented here should be considered as semiquantitative estimates that are used for comparative purposes only (3032). Figure 1 shows the solid-state 13C CP/MAS NMR spectra of the whole sample and respective humin and HA fractions, and the integration results are listed in Table 1. The spectra display similar chemical shifts; however, the distribution of the components varies among the samples. The unsubstituted aliphatic region (0-50 ppm) contains signals from: methine groups in alkyl chains (24-26 ppm), methylene carbons (28-34 ppm), R-carbon in aliphatic acids (41-42 ppm), and amine carbon (45-46 ppm) (24). Amorphous (2930 ppm) and crystalline (32-33 ppm) methylene carbon have been differentiated in SOM and are believed to arise from recalcitrance and accumulation of plant cuticles (3, 33). Furthermore, it has been proposed that amorphous methylene carbon is important in sorption processes (3, 8, 18, 34, 35). The substituted aliphatic region (50-110 ppm) includes signals from methoxyl carbon (56 ppm), oxygen substituted
FIGURE 1. 13C Cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra of (A) brown, (B) dark brown, (C) black, and (D) peat soil, humic acid (HA), and humin samples.
TABLE 1. Integration Results from Solid-State 13C CP/MAS NMR Analysisa relative distribution of C asb
a
samples
aliphatic (0-50 ppm)
substituted aliphatic (50-110 ppm)
aromatic and substituted aromatic (110-165 ppm)
carboxylic and carbonyl (165-215 ppm)
brown soil brown humin brown humic acid dark brown soil dark brown humin dark brown humic acid black soil black humin black humic acid peat soil peat humin peat humic acid
24 27 26 23 21 23 23 12 17 28 31 20
41 44 33 40 41 34 28 45 29 37 33 25
22 18 27 26 29 29 39 37 41 26 25 37
13 11 14 11 9 14 10 6 13 9 11 18
Integration was performed on a relative basis rather than an absolute basis. b % of total
carbon, ring carbons in carbohydrates, and carbons in ethers (65-95 ppm) and the anomeric carbon in carbohydrates (105 ppm) (24). The aromatic and substituted aromatic region (110-165 ppm) contains signals from aromatic carbon (110145 ppm) and phenolic carbon (145-160 ppm) (24). Signals from carboxylic, amide, and ester carbon (160-190 ppm) were also observed (24). The mineral soil samples (brown, dark brown, and black) have similar quantities of aliphatic carbon, and, in contrast, the peat soil contains more aliphatic carbon than the mineral soils. All soil samples contain chemical shifts consistent with the presence of amorphous (29-30 ppm) and crystalline (3233 ppm) methylene carbon. The characteristic “doublet” is most resolved in the peat soil sample. The brown soil has less aromatic carbon (22%) than do the dark brown (26%)
13C
NMR signal (0-215 ppm).
and black soils (39%). The peat soil contains 26% aromatic carbon, which is similar to the aromatic content of the dark brown soil. The HA spectra (Figure 1) are similar to spectra published for other HA samples (2, 11, 36). The brown and dark brown HA have similar characteristics; however, the dark brown HA is slightly enriched in aromatic carbon (Table 1). The black HA has the least amount of aliphatic carbon and is rich in aromatic carbon (41%). The peat HA also contains appreciable amounts of aromatic carbon (37%) in addition to a clear distinction between amorphous (29-30 ppm) and crystalline (32-33 ppm) methylene carbon. The mineral soil humin samples (brown, dark brown, and black) contain more substituted-aliphatic carbon than do the whole soils (Table 1). The aromatic carbon content is highest in the black humin (37%); however, the brown and VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Sorption isotherms of phenanthrene to soil, humic acid (HA), humin, and de-ashed humin samples.
TABLE 2. Comparison of Measured Phenanthrene Kd and Koc Values with Reconstituted Phenanthrene Sorption Values
sample
Kd ((SE) (mL/g)
foc (%)
Koc (mL/g) ((SE)a
∆Koc (mL/g)b
r2
brown soil brown humin brown humic acid brown humin (de-ashed) dark brown soil dark brown humin dark brown humic acid dark brown humin (de-ashed) black soil black humin black humic acid black humin (de-ashed) peat soil peat humin peat humic acid
102 ( 4 84.0 ( 2 1523 ( 100 4547 ( 155 182 ( 9 134 ( 5 2019 ( 70 6629 ( 219 465 ( 37 380 ( 7 2338 ( 147 12 605 ( 131 4224 ( 70 7123 ( 326 4860 ( 245
2.08 0.49 18.55 19.34 2.77 0.93 23.48 22.97 5.26 0.83 25.37 23.54 48.35 46.84 55.36
4890 ( 221 17 100 ( 449 8210 ( 541 23 500 ( 801 5410 ( 332 14 400 ( 626 8600 ( 300 28 900 ( 955 6550 ( 708 45 800 ( 895 9220 ( 581 53 700 ( 558 8740 ( 145 15 200 ( 697 8780 ( 443
na 12 210 3320 230 110 na 8990 3190 23 490 na 39 250 2670 47 150 na 6460 40
0.998 0.981 0.993 0.998 0.994 0.997 0.983 0.994 0.965 0.964 0.984 0.991 0.996 0.985 0.986
reconstituted Koc (mL/g) without fulvic acid fractionc
reconstituted Koc (mL/g) with fulvic acid fractiond
9750
13 160
9180
11 340
21 080
22 670
11 150
12 510
a K b ∆K c d K oc ) Kd/(%foc/100). oc ) Koc(fraction) - Koc(source) from ref 11, na ) not applicable. Koc,sample ) Koc,huminXhumin + Koc,HAXHA. oc,sample ) Koc,huminXhumin + Koc,HAXHA + Koc,FAXFA, where log Koc,FA ) 0.989 log Kow - 0.346, from ref 27; and log Kow ) 4.45, from ref 18.
black humin have less aromatic carbon than do the whole soils. The black humin also has less aliphatic carbon (12%) than do the brown (27%) and dark brown (21%) humin samples. The peat humin has slightly more aliphatic carbon, less substituted aliphatic carbon, similar aromatic carbon, and more carboxylic/carbonyl carbon than does the whole peat sample. The peat and peat humin have similar solidstate 13C CP/MAS NMR spectra, suggesting that they are both chemically similar sorbents. 156
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Phenanthrene Sorption Coefficients. The sorption isotherms for the soil, HA, and humin samples are shown in Figure 2, and the sorption coefficients are listed in Table 2. The isotherms were fit with the linear sorption model (r2 values of 0.964-0.998; Table 2). Comparisons of Koc values of soils versus the fractions within each sample set (brown, dark brown, black, and peat) revealed that the humin fraction consistently produced the highest Koc value and indicate an increased affinity of phenanthrene for the humin and HA
fractions than for the soil samples. The ∆Koc (∆Koc ) Koc,fraction - Koc,source) was used to compare differences in sorptive behavior with soil fractions (11), and the greatest ∆Koc was observed for the humin fractions of the mineral soils (brown, dark brown, and black). Consequently, the humin samples from the mineral soils were subjected to extensive de-ashing to investigate if soil minerals are playing a role in the accessibility of sorption domains in SOM. The de-ashed humin samples were found to have greater Koc values than did the untreated humin samples (Table 1). The reconstituted Koc values were found to be greater than the measured phenanthrene Koc values for the soil samples (Table 2). Inclusion of the FA fraction in the reconstituted Koc calculation increased the calculated Koc value by approximately 10-30%. However, it is important to note that reconstituted Koc values determined without the FA term are consistently greater than the measured values for the whole sample. Furthermore, the estimated Koc may not be fully representative of FA sorptive behavior because FA is more polar than both HA and humin; consequently, the estimated Koc may overestimate the contribution of the FA fraction, and thus the contribution of FA to the reconstituted Koc value should be interpreted with caution.
Discussion A comparison of phenanthrene sorption coefficients with SOM characteristics in the soils, HA, and humin samples did not reveal any uniform trends in this study. For example, the phenanthrene Koc values of the three mineral soils (brown, dark brown, and black) increase with SOM aromaticity. However, the Koc value for the peat (Koc ) 8.74 × 103 mL/g) is greater than that of the black soil sample (Koc ) 6.55 × 103 mL/g), yet the peat contains less aromatic-C than does the black soil. The soils all have similar quantities of aliphatic carbon (Table 1) with the peat sample containing the most aliphatic carbon (28%), and thus it is possible that this slight enrichment of aliphatic carbon may contribute to the observed Koc value; however, it is difficult to make this conclusion due to the heterogeneity of soil samples and semiquantitative nature of solid-state 13C NMR. Thus, the observed Koc values for the whole soil samples cannot be explained by SOM aromaticity or aliphaticity alone. Similarly, HA structural characteristics could not explain the differences in observed Koc values as the phenanthrene Koc values for the HA samples did not vary considerably (Table 2); yet there are distinct differences in the percentages of aromatic and aliphatic carbon in the samples (Table 1, Figure 1). Furthermore, the HA samples contain varying amounts of aliphatic carbon, substituted aliphatic carbon, and carboxylic/carbonyl carbon, indicating that they each have a distinct composition and polarity; yet they produced similar Koc values. Salloum et al. (11) also observed that chemically unique HA produced similar 1-naphthol Koc values and concluded that organic matter physical conformation was an important variable in sorption processes. The soil humin fractions exhibit greater sorption coefficients than the soils (Table 2) and demonstrated the greatest difference in Koc values in comparison to the whole soils (i.e., ∆Koc values). Gunasekara and Xing (36) reported that naphthalene sorption was 3 times greater for humin than for the respective whole soil. Salloum et al. (11) reported that 1-naphthol Koc values were higher for humin fractions as compared to whole soil samples. These studies along with others (37-39) have also observed an increase in sorption to the humin fraction. The solid-state 13C CP/MAS NMR spectra (Figure 1) do not provide any conclusive insight into the observed trends; however, Simpson and Johnson (23) proposed that increased sorption coefficients for soil humin is due to the large concentration of mobile polymethylene carbon in humin samples. They also hypothesized that
fractionation exposed polymethylene domains that are tightly associated with clay minerals that are typically not accessible at the soil-water interface. Consequently, although we did not observe an increase in the aliphatic carbon content with some samples, the observed increase may be due to accessibility of more favorable sorption domains in soil humin that were made available during the organic matter fractionation process. For example, the peat and peat humin (Figure 1) have comparable 13C NMR spectra, but the Koc value for the humin is almost double that measured for the peat. Therefore, we hypothesize that both organic matter structure in addition to organic matter physical conformation are important considerations for sorption processes. To further test this hypothesis, humin samples were de-ashed using HF/HCl to remove minerals. The resulting de-ashed humin Koc values (Table 2) were all greater than the respective humin Koc values and suggest that minerals may be “blocking” or physically protecting favorable phenanthrene sorption domains. This trend is consistent with the observations of Wang and Xing (14), who found increases in phenanthrene sorption when soil humin samples were de-ashed with HF/ HCl. These results suggest that soil minerals play an important role in regulating the accessibility and/or distribution of sorption sites for organic chemicals. However, as we did not measure SOM conformation directly, we can only hypothesize that physical conformation is an important variable in sorption processes. The results of the mass balance calculations further suggest that SOM physical conformation is an important consideration as indicated by other researchers (2, 11, 13, 15-18). The reconstituted Koc values were found to be greater than the measured phenanthrene values for the whole sample. The sum of the parts exceeds that of the whole and implies that accessible carbon, rather than total carbon, governs the extent of sorption. Furthermore, it was found that the peat sample yielded the least variation in measured and reconstituted Koc values (Table 2). This sample is low in mineral content and is not expected to contain many SOMmineral associations (11), and, therefore, the influence of minerals on organic matter physical conformation may be less pronounced. This concept is corroborated by other published works (2, 11, 13, 15, 17, 18, 36, 40). For example, Salloum et al. (11) also found that 1-naphthol reconstituted Koc values were larger as compared to measured whole Koc values. Kohl and Rice (40) reported that with the removal of lipids from SOM, the sorptive capacity of mineral soils doubled, and they hypothesized that PAHs and lipids were competing for sorption domains and the removal of lipid resulted in the opening up of sorption domains. Simpson et al. (2) observed increases in phenanthrene sorption when rigid structures such as aromatics and carbohydrates were selectively extracted from a series of HA samples. Furthermore, Gunesakara et al. (36) found that with the removal of carbohydrate components from HA samples, a decrease in isotherm linearity occurred, suggesting that with the removal of these rigid structures, mobile sorption domains were “freed” and became more accessible for contaminants. Furthermore, studies that have focused on PAH sorption to humic-minerals have consistently found that all Koc values of humic-mineral bound sorbents were lower than those of the humic materials alone (13, 15, 17, 18). In this study, phenanthrene sorptive behavior could not be solely attributed to a specific SOM chemical characteristic (i.e., aliphaticity or aromaticity), and this corroborates the findings from other studies that conclude that both structure and organic matter physical conformation are important in sorption processes (2, 3, 8, 11, 36, 41). Furthermore, reconstituted Koc data suggest that fractionation of organic matter alters the accessibility of sorptive domains. Hence, chemical characteristics in addition to organic matter physical VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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conformation are both important considerations when examining sorption processes in soils. In addition, the increased sorption of humin fractions as compared to whole soils samples (∆Koc) is most pronounced in the mineral soils (brown, dark brown, and black soils). This suggests that clay minerals influence the physical conformation of SOM and the distribution or availability of sorption domains (12, 23, 36); however, as we did not measure SOM physical conformation directly, we can only speculate that clay minerals play both a direct role and an indirect role in sorption of contaminants in soil. Recent studies (15, 16, 42) using highresolution magic angle spinning (HR-MAS) NMR have revealed that not all SOM structures are accessible on the soil-water interface and that the conformation of humic materials is governed by interactions with clay minerals surfaces. In addition, Simpson et al. (15, 16, 42) demonstrated that soil colloid surfaces are predominantly aliphatic in nature and aromatic structures were only visible when a penetrating solvent (DMSO) was used. Consequently, clay minerals likely play an important role in governing SOM accessibility at the soil-water interface, and the role of organo-mineral complexes in contaminant sorption processes warrants further attention.
Acknowledgments We thank the Ontario Premier’s Research Excellence Award (PREA) and the Natural Science and Engineering Council (NSERC) of Canada for supporting this research. M.J.S. also thanks NSERC for a University Faculty Award (UFA).
Supporting Information Available Details regarding the HPLC method used to quantify phenanthrene, and Table S1 (gravimetric organic carbon (OC) distribution in the fulvic acid (FA), humic acid (HA), and humin fractions). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review June 20, 2006. Revised manuscript received September 27, 2006. Accepted September 28, 2006. ES061471+
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