HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Simpson, M. J. Search for Related Content PubMed Articles by Simpson, M. J. Agricola Articles by Simpson, M. J. Related Collections Humic Substances Organic Compounds Soil Pollution Soil Chemistry

Symposium: Meaningful Pools in Determining Soil C and N Dynamics

Nuclear Magnetic Resonance Based Investigations of Contaminant Interactions with Soil Organic Matter

Dep. of Physical and Environmental Sciences, Univ. of Toronto, Scarborough College, 1265 Military Trail, Toronto, ON M1C 1A4, Canada

^* Corresponding author (myrna.simpson{at}utoronto.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 
Contaminant interactions with soil organic matter (SOM) are central to understanding the fate and transport of chemicals in soil environments. Elucidation of sorption processes will facilitate the efficiency of passive remedial methods and improve the accuracy of risk assessment models. Early studies in the 1960s identified a relationship between SOM and the sorption of chemicals and laid the foundation for an area of research which is still active today. The onset of analytical instrumentation assisted the characterization of SOM chemical fractions, namely the fulvic acid (FA) and humic acid (HA) fractions. The employment of SOM chemical fractions in contaminant sorption studies has produced many empirical relationships between contaminant sorption behavior and SOM structure. More recently, molecular-level techniques such as nuclear magnetic resonance (NMR) spectroscopy have been applied to examine specific interactions between contaminants and SOM fractions. These methods enable direct studies and are likely to further improve the fundamental understanding of contaminant interactions with SOM in the near future. For instance, NMR techniques should produce mechanistic information that will enable the accurate explanation of sorption phenomena at the macroscopic and landscape level. In addition to SOM chemical structure, researchers must consider the organic matter physical conformation at the soil–water interface because chemical methods provide structural information of the whole sample but do not provide detail about their physical architecture within the soil. This manuscript highlights studies which have examined contaminant interactions at the macroscopic- and molecular-level and demonstrates the common themes stemming from different levels of investigation.

Abbreviations: CP/MAS, cross polarization magic angle spinning • FA, fulvic acid • HA, humic acid • HR-MAS, high resolution magic angle spinning • NMR, nuclear magnetic resonance • PAH, polycyclic aromatic hydrocarbons • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 
INTERACTIONS between problematic organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), pesticides, and herbicides with SOM have been the focus of scientific study for more than four decades. Hydrophobic organic contaminants have a strong tendency to associate with SOM, and this association may limit contaminant bioavailability and hinder remedial attempts (Luthy et al., 1997; Alexander, 2000). However, the precise mechanisms of uptake are poorly understood and hence an unspecific, umbrella term sorption is used to describe the movement of contaminants from the water phase to the soil phase. The use of the word sorption implies that one or more uptake process is occurring and includes: absorption, adsorption, and/or partitioning (noting that absorption and partitioning are often used interchangeably to describe the same process). The complex and heterogeneous nature of SOM makes it difficult to ascertain the precise mechanism of uptake, although many hypotheses have been formulated (Karickhoff et al., 1979; Pignatello and Xing, 1996; Luthy et al., 1997; Chefetz et al., 2000; Salloum et al., 2001a, 2001b; Ahmad et al., 2001; Chiou, 2002; Gunasekara and Xing, 2003). Nonetheless, improving the understanding of sorptive processes in terrestrial environments will facilitate the development of efficient and passive remediation strategies as well as improve our understanding of health risks associated with contaminated land.

Three main types of investigations are typically performed when attempting to delineate contaminant fate and transport in terrestrial environments: (i) landscape-level measurements, (ii) macroscopic-level measurements, and (iii) molecular-level measurements. As summarized in Fig. 1 , landscape-level studies typically address contaminant distribution across the landscape or in a particular area of interest (urban environments, industrial sites, etc.). These large-scale studies attempt to ascertain the quantity and dispersal of targeted contaminants. Macroscopic-level studies aim to elucidate sorption mechanisms through the measurement of sorption isotherms and contaminant distribution coefficients (K_d values). These types of investigations have resulted in many empirical relationships between contaminant distribution coefficients (specifically organic carbon normalized coefficients, K_oc values) and SOM characteristics (Xing et al., 1994a; Chen et al., 1996; Chin et al., 1997; Ahmad et al., 2001; Salloum et al., 2001a, 2001b; Chiou, 2002; Mao et al., 2002; Simpson et al., 2003; Kang and Xing, 2005). Molecular-level examinations employ state-of-the-art analytical tools to decipher the molecular composition of SOM and determine the precise nature of the contaminant–SOM interaction. The need to combine one or more scales of investigation is becoming apparent and necessary if we are to accurately describe contaminant fate and transport in the environment (Fig. 1). Furthermore, if one is to understand contaminant fate and transport at the fundamental level, a multitiered approach which includes at least two types of these investigations is required.

View larger version (31K): [in this window] [in a new window]   Fig. 1. A proposed multitiered approach to studying organic contaminant interactions with soils. Most investigations occur at the landscape or macroscopic-level; however, combining all three approaches will result in a fundamental understanding of contaminant fate in terrestrial environments.

View larger version (28K): [in this window] [in a new window]   Fig. 2. An illustration of (A) macroscopic-level and (B) molecular-level data. Macroscopic data has yielded a multitude of indirect hypotheses which are now being refined with the application of direct, molecular-level observations. Macroscopic methods commonly employ batch equilibration experiments that produce sorption isotherms (note: not all sorption isotherms are nonlinear; linearity varies with sorbate and sorbent properties and the type of mechanisms that are dominating the attenuation process). Molecular-level studies enable sorption mechanisms, such as those between trifluralin and SOM (adapted from Simpson et al., 2001). Two-dimensional molecular-level data, such as correlation spectroscopy NMR of a soil humic acid, displayed from Simpson et al. (2004), can provide detailed structural information about humic material and can be used to decipher specific SOM interactions with contaminants. Reprinted with permission from Simpson et al. (2004). Copyright (2004) Environmental Toxicology and Chemistry, Alliance Communications Group.

	INDIRECT MACROSCOPIC-LEVEL AND NMR STUDIES

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 
Early macroscopic sorption studies quickly identified that SOM was central to the attenuation of contaminants in soil environments. Lambert et al. (1965) proposed that an active fraction of SOM was responsible for sorption of chemicals to soil. Hance (1965) suggested that the variation in diuron sorption coefficients was due to the availability of SOM groups at the soil colloid surface. Doherty and Warren (1969) hypothesized that the relationship between herbicide binding and SOM was governed by some other physical or chemical factor to which SOM is correlated. These studies (Doherty and Warren, 1969; Hance, 1965; Lambert et al., 1965), followed by subsequent studies (Bailey and White, 1970; Hance, 1969; Lambert, 1967), concluded that SOM was playing a principal role in the sorption of chemicals to soil. A compilation of early sorption studies clearly demonstrated the prominent role of SOM in the sorption of many hydrophobic contaminants (Karickhoff et al., 1979) and established a linear relationship between sorption partition coefficients (K_d values) with soil carbon contents. Hence, the practice arose of reporting K_oc values in addition to other experimental parameters. Linear, empirical correlations between K_oc values and the octanol–water partition coefficient (K_ow) were also being proposed around the same time (Chiou et al., 1979, 1982, 1983, 1985; Chiou, 2002) and suggested that a soil or sediment K_oc value could be predicted from the K_ow value because sorption mechanisms were dominated by an entropy driven, partitioning process. The K_ow predictive method does not entail any experimental work and provides a quick and easy means for predicting K_oc values; however, experimental data emerged indicating that octanol was not a good surrogate for SOM (Mingelgrin and Gerstl, 1983; Xing et al., 1994a). Consequently, the emphasis to further understand how SOM chemical structure governed the sorption of organic contaminants in soil was reinforced by these studies and laid the foundation for future investigations.

Researchers began to employ SOM chemical fractions (FAs, HAs, and humin) to gain insight into SOM and contaminant interactions, with a main objective of elucidating sorption mechanisms. In addition, the advances made in analytical instrumentation, which are typically designed for pure chemicals, were often limited in their application to whole soils. Therefore, using SOM factions as sorbents enabled researchers to study macroscopic sorption mechanisms to individual SOM chemical fractions and apply analytical techniques more readily. Consequently, macroscopic studies with SOM chemical fractions which were characterized using different analytical techniques began to emerge with the specific goal to develop SOM structure–contaminant relationships. It should be noted that in 1970, Hayes (1970) indicated the value of employing chemical fractions in herbicide sorption studies, however pointed out that the use of fractions had not yet been explored to its full potential. Garbarini and Lion (1986) reported that toluene and trichloroethylene sorption to FAs, HAs, and soil humin could not be explained by carbon content alone. Their results suggested that oxygen content, in addition to carbon content, provides a more accurate prediction of toluene and trichloroethylene sorption (Garbarini and Lion, 1986). Grathwohl (1990) demonstrated that samples of different diagenetic origin, that is, coals vs. HAs, produced varying K_oc values for a series of chlorinated aliphatic hydrocarbons. Furthermore, a relationship between log K_oc values and the H/O atomic ratio was presented. Other studies have provided further evidence for structural relationships between SOM fractions and K_oc values (Chen et al., 1996; Xing, 2001; Kang and Xing, 2005).

Studies which employed solid-state cross polarization magic angle spinning (CP/MAS) ¹³C NMR spectroscopy to characterize sorbent structure proposed a relationship between K_oc values and the relative percentage of aromatic carbon in the sample (Ahmad et al., 2001; Chefetz et al., 2000; Chen et al., 1996; Chin et al., 1997; Ganaye et al., 1997; Xing, 1997; Xing et al., 1994a, 1994b). In a spectroscopic study of humic substances, Hu et al. (2000) reported the detection of amorphous and crystalline polymethylene carbon. Subsequent reports have indicated that the aliphatic carbon content, namely the amorphous polymethylene structures, may also be playing a major role in the uptake of contaminants (Boyd et al., 1990; Chefetz et al., 2000; Gunasekara et al., 2003; Kang and Xing, 2005; Salloum et al., 2002; Simpson et al., 2003). Some studies have concluded that it is a combination of structures that influence contaminant uptake because contaminant K_oc values could not be explained by humic material characteristics alone (Gunasekara and Xing, 2003; Salloum et al., 2001a, 2001b).

Two recent studies employed chemical modifications to selectively remove or change the HA structure (Gunasekara et al., 2003; Simpson et al., 2003). Both investigations included spectroscopic measurements to ascertain how sorption of phenanthrene, a commonly used sorbate, would be altered through the chemical modifications made to HA isolated from compost, peat, and soil. The solid-state ¹³C CP/MAS NMR spectra in Fig. 3 illustrate the changes made to HA isolated from peat and soil. The reported sorption values and their chemical characteristics are summarized in Table 1. The structural modifications invoked similar structural changes in all of the HAs. For instance, bleaching removed mostly aromatic structures (110–160 ppm) leaving behind a predominantly aliphatic rich HA. Acid hydrolysis removed carbohydrates and peptides (50–110 ppm) and resulted in an aromatic- and substituted aromatic-rich HA (110–160 ppm). Oximation yielded similar results as hydrolysis because only a subtle difference between the ¹³C CP/MAS NMR spectra is noticeable (Fig. 3). Subcritical water extraction resulted in aromatic-rich humic material that has structural characteristics similar to that reported for shales and coal (Salloum et al., 2001b; Simpson et al., 2003). Distinct trends between sample aromaticity and condensation (H/C ratio) could not be deciphered, and linear and exponential regression coefficients (r²) ranged from 0.20 to 0.54. However, increases in phenanthrene K_oc values could not be attributed to one specific group of structures (aromatic vs. aliphatic) or bulk characterization (H/C values), leading to the conclusion that HA structural characteristics should not be used solely to predict contaminant behavior because it appears that other factors, such as contaminant accessibility to sorption sites, are involved. A subsequent study employed ²H spin–spin relaxation (T₂) measurements by solid-state NMR (Gunasekara et al., 2003). This technique enables one to examine the degree of molecular motion of a sample as expanded domains require longer T₂ times than rigid/condensed domains to return to equilibrium. Bleaching produced a more expanded structure which exhibited a partitioning type sorption isotherm (Gunasekara et al., 2003). This observation is consistent with the emerging hypothesis that amorphous aliphatic domains (30 ppm) can partition appreciable amounts of hydrophobic contaminants (Chefetz et al., 2000; Kang and Xing, 2005; Mao et al., 2002; Salloum et al., 2002). These results collectively indicate that aliphatic and aromatic structures both play a role in contaminant uptake; however, sorption values could not be explained by each of these characteristics alone.

View larger version (28K): [in this window] [in a new window]   Fig. 3. Solid-state 13C CP/MAS NMR spectra of chemically modified humic acids from Simpson et al. (2003). The selective removal of SOM components suggests that structures such as polymethylene carbon may be blocked by other humic structures. Only through chemical modification do they become more available in contaminant interactions. Reprinted with permission from Simpson et al. (2003).

A number of relationships between contaminant sorption and SOM structural characteristics have been put forth in the literature, and a great number of these employ NMR spectroscopy. However, elucidating sorption mechanisms indirectly may not be ideal for complex systems such as SOM. Granted, the use of chemical fractions has enhanced our knowledge about contaminant–SOM interactions a great deal; however, hypotheses based on indirect correlations may not be applicable to a wide range of SOM types, and therefore are limited to the experimental parameters the correlation is based on. Furthermore, the application of molecular-level techniques may not necessarily yield molecular-level data. For example, a typical solid-state ¹³C NMR spectrum of humic material yields broad lines that stem across a 20-ppm or sometimes even a 50-ppm range (see untreated HAs in Fig. 3). Consequently, precise structural information is difficult to ascertain. Therefore, one may argue that in the case of humic material, molecular-level techniques may sometimes produce bulk molecular-level data, especially in the case of solid-state ¹³C NMR. Therefore, macroscopic studies can greatly benefit from more specific structural detail from other, direct NMR techniques.

	DIRECT NMR STUDIES

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 
The NMR investigations that examine interactions between contaminants and SOM or fractions thereof are advantageous because one can develop or test hypotheses from direct observations. Direct observations allow one to monitor interactions between compounds unequivocally. For instance, several NMR techniques enable the examination of interactions between contaminants (probe molecules) and SOM chemical fractions (Hatcher et al., 1993; Nanny et al., 1997; Guthrie et al., 1999; Kohl et al., 2000; Simpson et al., 2001) as well as specific associations between nuclei (Sachleben et al., 2004; Simpson et al., 2004). Direct molecular-level studies of contaminant interactions with SOM or humic material are also less prone to uncertainty because the data reveal the types of bonds that form and provide precise information regarding the molecular environment of the compound of interest. The majority of NMR based contaminant–SOM interactions employ isotopically enriched contaminants or focus on nuclei that are only found in the contaminant, such as ¹⁹F (Hatcher et al., 1993; Nanny et al., 1997; Dixon et al., 1999; Guthrie et al., 1999; Kohl et al., 2000; Simpson et al., 2001, 2004; Sachleben et al., 2004). By doing so, one can reduce the potential for overlapping signals between the contaminant and SOM and increase the sensitivity of detection of the contaminant molecular environment. For instance, several studies have employed ¹³C- or ¹⁵N-labeled compounds to overcome poor sensitivity of natural abundance ¹³C and ¹⁵N. Furthermore, the strong signals from ¹³C or ¹⁵N enriched compounds dominate the NMR spectrum, and signals from SOM or humic material are no longer prevalent and thus do not interfere. Carbon-13 and some ¹⁵N-labeled compounds are costly; consequently, some researchers have opted to use fluorinated or deuterated compounds (Dixon et al., 1999; Kohl et al., 2000; Nanny and Maza, 2001), which are less costly but still enable the direct detection of a contaminant's molecular environment. In addition to employing ¹³C-labeled contaminants, Simpson et al. (2004) measured ¹H contaminant–HA interactions. Delineating the ¹H signal of the contaminant was not problematic at low HA concentrations (<50 mg C L^–1), and the ¹H HA signal was spread out across a range of chemical shifts whereas the contaminant signals are concentrated in the aromatic region (Fig. 4 ; Simpson et al., 2004). The use of labeled compounds is commonplace when examining contaminant–SOM interactions and enables one to focus directly on one nucleus within the contaminant molecule, providing a greater potential to delineate specific interaction mechanisms such as covalent or noncovalent bond formation.

View larger version (18K): [in this window] [in a new window]   Fig. 4. (A) Data from (Simpson et al., 2004) displaying the decline in 1H T1 values of 1-naphthol with increasing concentration of soil humic acid (HA). The measurements indicate that all protons are equally interacting and hence, a specific interaction between individual nuclei (1H in this example) and the HA is not observed, suggesting a strong interaction with all protons in the molecule with HA. (B) The aromatic region from the 1H spectrum does not reveal a change in chemical shift, suggesting that this is a strictly noncovalent interaction. The increased line broadening with the addition of 50 mg C L–1 of HA is indicative of a strong, noncovalent association. Reprinted with permission from Simpson et al. (2004). Copyright (2004) Environmental Toxicology and Chemistry, Alliance Communications Group.

Noncovalent interactions, especially in studies which involve nonionic compounds, have also been reported (Nanny et al., 1997; Dixon et al., 1999; Guthrie et al., 1999; Kohl et al., 2000; Nanny and Maza, 2001; Simpson et al., 2004). Guthrie et al. (1999) observed that ¹³C-labeled pyrene, which had been incubated in sediment for 60 d, did not display a change in chemical shift and became noncovalently sequestered and remained structurally unaltered. Furthermore, during the time course of the experiment, increasing amounts of ¹³C-labeled pyrene became associated with the HA and humin fractions (Guthrie et al., 1999). Kohl et al. (2000) examined hexafluorobenzene sorption to two peat soils by solid-state ¹⁹F NMR. They observed both free and bound hexafluorobenzene and surmised that SOM is composed of mobile and rigid domains with which contaminants interact (Kohl et al., 2000). Recently, Sachleben et al. (2004) examined the interactions of ¹³C-labeled pyrene with cuticular material and reported that sorbed pyrene existed in a liquid-like state, suggesting the source (cuticles) and reasons for high sorption coefficients of amorphous aliphatic SOM. The researchers also used sophisticated spin diffusion experiments which confirmed that the ¹³C-labeled pyrene was associating strictly with the paraffinic carbon and not with other types of structures present in the sample such as carbohydrates (Sachleben et al., 2004).

Contaminant–SOM interactions are not limited to the solid phase, and several studies have been conducted between contaminants and humic material in the liquid phase (Nanny et al., 1997; Dixon et al., 1999; Nanny and Maza, 2001; Simpson et al., 2004). These types of studies investigate the molecular associations between a probe molecule and dissolved humic fractions (FA and HAs) and typically involve the measurement of molecular motion via spin-lattice relaxation. The spin-lattice relaxation time (T₁) is characteristic of the overall molecular motion of a compound and includes translational, rotational, and vibrational motion. T₁ values are dependent on molecular size, solvent properties such as viscosity, magnetic field strength, dipolar interactions, and temperature; however, when these are kept equal, the change in the T₁ is useful for monitoring solution phase molecular interactions. In general, lower molecular weight compounds such as contaminants will have longerT₁ values than higher molecular weight compounds such as FA and HA. Therefore, any change in the measured T₁ value can be attributed to contaminant associations with fulvic or humic material. Nanny and co-workers (1997) demonstrated a decline in ¹³C labeled acenaphthenone with subsequent additions of FA. The lack of a chemical shift change indicated that the interaction was strictly noncovalent but likely occurred within the hydrophobic regions of the FA (Nanny et al., 1997). Similarly, Dixon et al. (1999) reported the decline of ¹⁹F T₁ values of 4'-fluoro-1'-acetonaphthone with increasing concentration of dissolved FA. The results suggested a weak, noncovalent interaction between the contaminant and the FA (Dixon et al., 1999). A T₁ study with deuterated monoaromatic compounds and HAs indicated that the degree of interaction increased with HA aromaticity, suggesting that – interactions are also important in liquid-phase sorption studies (Nanny and Maza, 2001). A recent study examined interactions between naphthalene, 1-naphthol, and quinoline with a soil HA and found a steep decline in contaminant ¹³C T₁ values with increasing concentrations of HAs (Simpson et al., 2004). This study also examined ¹H T₁ values of each contaminant to determine if there was any interaction specificity between different protons within the contaminant molecule. As depicted in Fig. 4, the ¹H T₁ values declined equivalently for naphthalene, 1-naphthol, and quinoline, with increasing concentrations of HA (note: only data for 1-naphthol is shown). The ¹H chemical shifts did not change, thus it was concluded that only noncovalent associations were formed. However, ¹H line broadening was observed, indicating that the molecular association between 1-naphthol and soil HA was extremely strong (Simpson et al., 2004). Although T₁ based NMR measurements are informative, they are time consuming; thus, only researchers with ample amounts of NMR time may be able to conduct such investigations. However, T₁ curves can be used to construct sorption isotherms (Bortiatynski et al., 1997; Simpson et al., 2004) and when used in addition with T₂ (spin–spin relaxation) measurements, can be used to calculate molecular correlation times (Dixon et al., 1999).

High resolution magic angle spinning (HR-MAS) is a recently developed and commercially available NMR probe that has demonstrated great promise to study complex, environmental samples. The HR-MAS NMR allows the acquisition of liquid-like NMR spectra on samples that are not fully soluble. Furthermore, virtually any liquid-state experiment can be performed, thus making the technique highly advantageous because one- and two-dimensional experiments that include ¹H (a highly abundant and sensitive nucleus) can be executed. In addition, mineral surface structures can be probed by using a series of selective solvents. Simpson et al. (2001) demonstrated that aromatic components of SOM are not surface accessible when a whole soil is swollen in D₂O. When the sample was swollen with DMSO-d₆ (deuterated dimethylsulfoxide), a penetrating solvent which is capable of breaking hydrogen bonds, aromatic moieties were detected. Simpson et al. (2001) also employed HR-MAS to study herbicide–soil binding in situ and delineated binding mechanisms between SOM and trifluralin (2,6-dinitro-N,N-dipropyl-4-trifluoromethyl-aniline). Feng et al. (2005) applied ¹H HR-MAS NMR to study the molecular structure of HA sorbed to clay mineral surfaces. This study provided molecular-level information regarding the selective sorption of natural organic matter to different clay minerals (kaolinite and montmorillonite). Future HR-MAS NMR experiments will continue to provide informative, molecular-level data on the organic matter present at the soil–aqueous interface. Understanding the molecular structure of organic matter on colloid surfaces will unquestionably advance our comprehension of contaminant–SOM interactions and further refine hypotheses developed from macroscopic experiments.

	SYNTHESIS AND FUTURE PROSPECTS

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 
Chemical fractions are valuable when applying molecular-level techniques to study contaminant and SOM interactions. Undoubtedly, this pursuit will continue in the future and facilitate further advances in our understanding of contaminant sorption processes in soils. Although the number of indirect studies are greater than direct studies, future advances in technology and improved access to instrumentation will nonetheless support direct molecular-level based studies. Hypotheses that are based on direct observations are more likely to be applicable to a wider range of soil types and contaminant properties. Furthermore, these hypotheses may be able to explain phenomena observed at the macroscopic and landscape levels. Contaminant fate and transport in the soil environment is complex and necessitates several scales of investigation if we are to understand contaminant behavior on a fundamental level (Fig. 1).

When employing chemical fractions in sorption studies, one must be aware that the sample may not be fully representative of SOM as it exists in nature. For example, extractions typically involve harsh chemical treatments and those intending to perform NMR experiments must remove paramagnetic species and clay minerals through washing with hydrofluoric acid (Schmidt et al., 1997). Therefore, one must ask, how representative are these fractions of natural SOM? Furthermore, by eliminating clay minerals from the picture, how are we skewing our results? Studies that examine contaminant interactions with constructed HA–clay complexes have indicated that the mineral phase, which in comparison with SOM, sorbs significantly less amounts of nonionic hydrophobic contaminants, plays an indirect role by governing organic matter accessibility at the soil–water interface (Murphy et al., 1994; Jones and Tiller, 1999). A mass balance sorption approach by Salloum et al. (2001b) also revealed that the parts (chemical fractions) sorbed more than the whole (soil). They hypothesized that through chemical fractionation, more favorable or simply more sorption sites were exposed and resulted in larger K_oc values and suggested that SOM physical conformation in addition to SOM chemistry governs sorption mechanisms. In addition, ¹H HR-MAS NMR data suggests that some organic matter structures are buried and may not be surface accessible (Simpson et al., 2001; Feng et al., 2005). Consequently, total organic matter structures detected in SOM chemical fractions by other characterization techniques may not be participating in short-term surface interactions (fast sorption) but may be more important when delineating long-term interactions (slow sorption and diffusion). Furthermore, by investigating sorption phenomenon through fractionation, a great deal of information has been gained on SOM chemistry and the emergence of new hypotheses regarding organic matter physical conformation.

Many of the available techniques today necessitate the use of chemical fractions, and our understanding of SOM interactions with contaminants has advanced significantly. For example, characterized SOM fractions are by NMR spectroscopy more resolved than the data obtained with whole soils. Most NMR techniques are not amenable to whole soils; therefore, one must employ chemical fractionation or some type of pretreatment to apply the majority of NMR methods. By using NMR methods, researchers have been able to identify the types of SOM structures, such as aromatic and aliphatic carbon, that are likely participating in contaminant sorption processes. Research employing chemical fractions has greatly enhanced our understanding and has enabled researchers to devise several hypotheses about contaminant and SOM interactions. However, in our pursuit to understand contaminant and SOM interactions at the molecular level, the physical architecture of the soil has been overlooked, perhaps because of the emphasis placed on elucidating the structures of humic material that are responsible for contaminant attenuation. But, are contaminant interactions strictly governed by chemical processes, or does SOM physical conformation play a role? These questions must be kept in mind for future experiments. In addition, some research has suggested that clay minerals play a secondary role in contaminant sorption by regulating the physical conformation of organic matter at the soil–water interface (Murphy et al., 1994; Jones and Tiller, 1999; Salloum et al., 2001b, Simpson et al., 2001; Feng et al., 2005). Consequently, future research should focus on organo–clay associations and the role of clay minerals in regulating SOM structure and environmental reactivity at the soil–water interface.

The emergence of novel techniques such as HR-MAS NMR and future developments in analytical instrumentation will facilitate further advances made in the chemical characterization of SOM fractions. For instance, advanced mass spectrometry methods have been able to identify molecular structures of soil FA and HA constituents (Kujawinski et al., 2002; Kramer et al., 2004). Chemical characterization of SOM and its constituents will further enhance our understanding of contaminant interactions; in addition, future research should focus on the physical architecture of soil colloids. For instance, there should be more collaborative research between those who study SOM chemical fractions and those who study SOM physical fractions. Similarly, SOM researchers should collaborate more with clay mineralogists because recent literature suggests that in addition to SOM chemical characteristics, clay minerals may be involved in regulating surface accessible organic matter. Thus, greater significance should be placed on organo–mineral associations. Future scientific enquiry will undoubtedly benefit from an increased number of macroscopic- and molecular-level studies with chemical fractions; however, one must always keep in mind the physical arrangement of molecules and how this relates to macroscopic- and landscape-level processes (Fig. 1). With this in mind, molecular-level studies will be more applicable and able to explain sorption phenomena on a larger and more fundamental scale.

	ACKNOWLEDGMENTS

 
Many thanks to Prof. Michael H.B. Hayes (University of Limerick), Prof. André Simpson (University of Toronto), and Dr. Brian Kelleher (Dublin City University) for their helpful comments on earlier versions of this manuscript. The comments from two anonymous reviewers greatly enhanced the quality of the manuscript. I thank Drs. Cindy Cambardella, Ed Gregorich, and Dan Olk for their invitation to be part of this symposium on C & N pools. A University Faculty Award (UFA) from the Natural Science and Engineering Research Council (NSERC) of Canada is gratefully acknowledged.

Received for publication March 31, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION INDIRECT MACROSCOPIC-LEVEL AND... DIRECT NMR STUDIES SYNTHESIS AND FUTURE PROSPECTS REFERENCES

 

• Ahmad, R., R.S. Kookana, A.M. Alston, and J.O. Skjemstad. 2001. The nature of soil organic matter affects sorption of pesticides. 1. Relationships with carbon chemistry as determined by ¹³C CPMAS spectroscopy. Environ. Sci. Technol. 35:878–884.[Medline]
• Alexander, M.A. 2000. Aging, bioavailability and overestimation of risk from environmental pollutants. Environ. Sci. Technol. 34:4259–4265.[CrossRef]
• Bailey, G.W., and J.L. White. 1970. Factors influencing the adsorption, desorption and movement of pesticides in soil. Residue Rev. 32:29–92.[Medline]
• Bortiatynski, J.M., P.G. Hatcher, and R.D. Minard. 1997. The development of 13c labeling and ¹³C NMR spectroscopy techniques to study the interaction of pollutants with humic substances. p. 26–50. In M.A. Nanny et al. (ed.) Nuclear magnetic resonance spectroscopy in environmental chemistry. Oxford Univ. Press, New York.
• Boyd, S.A., J. Xiangcan, and J.-F. Lee. 1990. Sorption of nonionic organic compounds by corn residues from a no-tillage field. J. Environ. Qual. 19:734–738.[Abstract/Free Full Text]
• Bruns-Nagel, D., H. Knicker, O. Drzyzga, U. Butehorn, K. Steinbach, D. Gemsa, and E. Von Low. 2000. Characterization of ¹⁵N-TNT residues after an anaerobic/aerobic treatment of soil/molasses mixtures by solid-state ¹⁵N NMR spectroscopy. 2. Systematic investigation of whole soil and different humic fractions. Environ. Sci. Technol. 34:1549–1556.[CrossRef]
• Cardoza, L.A., A.K. Korir, W.H. Otto, C.J. Wurrey, and C.K. Larive. 2004. Applications of NMR spectroscopy in environmental science. Prog. Nucl. Magn. Reson. Spectrosc. 45:209–238.[CrossRef]
• Chefetz, B., A.P. Deshmukh, P.G. Hatcher, and E.A. Guthrie. 2000. Pyrene sorption by natural organic matter. Environ. Sci. Technol. 34:2925–2930.[CrossRef]
• Chen, Z., B. Xing, W.B. McGill, and M.J. Dudas. 1996. -naphthol sorption as regulated by structure and composition of organic substances in soils and sediments. Can. J. Soil Sci. 76:513–522.
• Chin, Y.-P., G.R. Aiken, and K.M. Danielsen. 1997. Binding of pyrene to aquatic and commercial humic substances: The role of molecular weight and aromaticity. Environ. Sci. Technol. 31:1630–1635.[CrossRef]
• Chiou, C.T. 2002. Partition and adsorption of organic contaminants in environmental systems. Wiley, Hoboken, NJ.
• Chiou, C.T., D.E. Kile, D.W. Rutherford, G. Sheng, and S.A. Boyd. 2000. Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fractions: Potential sources of the sorption nonlinearity. Environ. Sci. Technol. 34:1254–1258.[CrossRef]
• Chiou, C.T., L.J. Peters, and V.H. Freed. 1979. A physical concept of soil-water equilibriums for nonionic organic compounds. Science 206:831–832.[Abstract/Free Full Text]
• Chiou, C.T., P.E. Porter, and D.W. Schmedding. 1983. Partition equilibria of nonionic organic compounds between soil organic matter and water. Environ. Sci. Technol. 17:227–231.[CrossRef][Web of Science]
• Chiou, C.T., D.W. Schmedding, and M. Manes. 1982. Partitioning of organic compounds in octanol-water systems. Environ. Sci. Technol. 16:4–10.
• Chiou, C.T., T.D. Shoup, and P.E. Porter. 1985. Mechanistic roles of soil humus and minerals in the sorption of nonionic organic compounds from aqueous and organic solutions. Org. Geochem. 8:9–14.
• Delort, A.-M., B. Combourieu, and N. Haroune. 2004. Nuclear magnetic resonance studies of interactions between organic pollutants and soils components, a review. Environ. Chem. Lett. 1:209–213.[CrossRef]
• Dixon, A.M., M.A. Mai, and C.K. Larive. 1999. NMR investigation of the interactions between 4'-fluoro-1'-acetonaphthone and the Suwannee River fulvic acid. Environ. Sci. Technol. 33:958–964.[CrossRef]
• Doherty, P., and G. Warren. 1969. The adsorption of four herbicides by different types of organic matter and a bentonite clay. Weed Res. 9:20–26.
• Farenhorst, A. 2006. Importance of soil organic matter fractions in soil-landscape and regional assessments of pesticide sorption and leaching in soil. Soil Sci. Soc. Am. J. 70:1005–1012 (this issue).[Abstract/Free Full Text]
• Feng, X., A.J. Simpson, and M.J. Simpson. 2005. Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces. Org. Geochem. 36:1553–1566.[CrossRef]
• Ganaye, V., K. Keiding, T. Vogel, M.-L. Viriot, and J.-C. Block. 1997. Evaluation of soil organic matter polarity by pyrene fluorescence spectrum variations. Environ. Sci. Technol. 31:2701–2706.[CrossRef]
• Garbarini, D.R., and L.W. Lion. 1986. Influence of the nature of soil organics on the sorption of toluene and trichloroethylene. Environ. Sci. Technol. 20:1263–1269.[CrossRef]
• Grathwohl, P. 1990. Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: Implications on K_oc correlations. Environ. Sci. Technol. 24:1687–1692.[CrossRef]
• Gunasekara, A.S., M.J. Simpson, and B. Xing. 2003. Identification and characterization of sorption domains in soil organic matter using structurally modified humic acids. Environ. Sci. Technol. 37:852–858.[Medline]
• Gunasekara, A.S., and B. Xing. 2003. Sorption and desorption of naphthalene by soil organic matter: Importance of aromatic and aliphatic components. J. Environ. Qual. 32:240–246.[Abstract/Free Full Text]
• Guthrie, E., J. Bortiatynski, J. van Heemst, K. Hardy, E. Kovach, and P.Hatcher. 1999. Determination of ¹³C pyrene sequestration in sediment microcosms using flash pyrolysis-GC-MS and ¹³C NMR. Environ. Sci. Technol. 33:119–125.
• Hance, R. 1965. Observations on the relationship between the adsorption of diuron and the nature of the adsorbent. Weed Res. 5:108–114.
• Hance, R.J. 1969. An empirical relationship between chemical structure and the sorption of some herbicides by soil. J. Agric. Food Chem. 17:667–668.[CrossRef]
• Hatcher, P.G., J.M. Bortiatynski, R.D. Minard, J. Dec, and J.M. Bollag. 1993. Use of high-resolution carbon-13 NMR to examine the enzymatic covalent binding of carbon-13-labeled 2,4-dichlorophenol to humic substances. Environ. Sci. Technol. 27:2098–2103.[CrossRef]
• Hayes, M.H.B. 1970. Adsorption of triazine herbicides on soil organic matter, including a short review on soil organic matter chemistry. Residue Rev. 32:131–174.[Medline]
• Hu, W.-G., J. Mao, B. Xing, and K. Schmidt-Rohr. 2000. Poly(methylene) crystallites in humic substances detected by nuclear magnetic resonance. Environ. Sci. Technol. 34:530–534.[CrossRef]
• Huang, W., P. Peng, Z. Yu, and J. Fu. 2003. Effects of organic matter heterogeneity on sorption and desorption of organic contaminants by soils and sediments. Appl. Geochem. 18:955–972.[CrossRef]
• Jones, K.D., and C.L. Tiller. 1999. Effect of solution chemistry on the extent of binding of phenanthrene by a soil humic acid: A comparison of dissolved and clay bound humic. Environ. Sci. Technol. 33:580–587.[CrossRef]
• Kang, S., and B. Xing. 2005. Phenanthrene sorption to sequentially extracted soil humic acid and humins. Environ. Sci. Technol. 39:134–140.[Medline]
• Karickhoff, S., D. Brown, and T. Scott. 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res. 13:241–248.[CrossRef][Web of Science]
• Kohl, S.D., P.J. Toscano, W. Hou, and J.A. Rice. 2000. Solid-state ¹⁹F NMR investigation of hexafluorobenzene sorption to soil organic matter. Environ. Sci. Technol. 34:204–210.[CrossRef]
• Kramer, R.W., E.B. Kujawinski, and P.G. Hatcher. 2004. Identification of black carbon derived structures in a volcanic ash soil humic acid by fourier transform ion cyclotron resonance mass spectrometry. Environ. Sci. Technol. 38:3387–3395.[Medline]
• Kujawinski, E.B., P.G. Hatcher, and M.A. Freitas. 2002. High-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) of humic and fulvic acids: Improvements and comparisons. Anal. Chem. 74:413–419.[Medline]
• Lambert, S.M. 1967. Functional relationship between sorption in soil and chemical structure. J. Agric. Food Chem. 15:572–576.[CrossRef]
• Lambert, S.M., P.E. Porter, and R.H. Schieferstein. 1965. Movement and sorption of chemicals applied to soil. Weeds 13:185–190.
• Luthy, R.G., G.R. Aiken, M.L. Brusseau, S.D. Cunningham, P.M. Gschwend, J.J. Pignatello, M. Reinhard, S. Traina, W.J. Weber, Jr., and J.C. Westall. 1997. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 31:3341–3347.[CrossRef]
• Mao, J.-D., L.S. Hundal, M.L. Thompson, and K. Scmidt-Rohr. 2002. Correlation of poly(methylene)-rich amorphous aliphatic domains in humic substances with sorption of a nonpolar organic contaminant, phenanthrene. Environ. Sci. Technol. 36:929–936.[Medline]
• Mingelgrin, U., and Z. Gerstl. 1983. Reevaluation of partitioning as a mechanism of nonionic chemicals adsorption in soils. J. Environ. Qual. 12:1–11.[Web of Science]
• Murphy, E.M., J.M. Zachara, S.C. Smith, J.L. Phillips, and T.W. Wietsma. 1994. Interaction of hydrophobic organic compounds with mineral-bound humic substances. Environ. Sci. Technol. 28:1291–1299.
• Nanny, M.A., J.M. Bortiatynski, and P.G. Hatcher. 1997. Noncovalent interactions between acenaphthenone and dissolved fulvic acid as determined by ¹³C NMR T₁ relaxation measurements. Environ. Sci. Technol. 31:530–534.[CrossRef]
• Nanny, M.A., and J.P. Maza. 2001. Noncovalent interactions between monoaromatic compounds and dissolved humic acids: A deuterium NMR T₁ relaxation study. Environ. Sci. Technol. 35:379–384.[Medline]
• Nearpass, D.C. 1976. Absorption of picloram by humic acids and humin. Soil Sci. 121:272–277.
• Pignatello, J., and B. Xing. 1996. Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30:1–11.
• Preston, C. 1996. Applications of NMR to soil organic matter analysis: History and perspectives. Soil Sci. 161:144–166.[CrossRef]
• Preston, C.M. 2001. Carbon-13 solid-state NMR of soil organic matter- using the technique effectively. Can. J. Soil Sci. 81:255–270.
• Rice, J.A. 2001. Humin. Soil Sci. 166:848–857.[CrossRef]
• Rice, J.A., and P. MacCarthy. 1990. A model of humin. Environ. Sci. Technol. 24:1875–1877.[CrossRef]
• Sachleben, J.R., B. Chefetz, A.P. Deshmukh, and P.G. Hatcher. 2004. Solid-state NMR characterization of pyrene-cuticular matter interactions. Environ. Sci. Technol. 38:4369–4376.[Medline]
• Salloum, M.J., B. Chefetz, and P.G. Hatcher. 2002. Phenanthrene sorption to aliphatic-rich natural organic matter. Environ. Sci. Technol. 36:1953–1958.[Medline]
• Salloum, M.J., M.J. Dudas, and W.B. McGill. 2001a. Sorption of 1-naphthol to soil and geologic samples with varying diagenetic properties. Chemosphere 44:779–787.[Medline]
• Salloum, M.J., M.J. Dudas, and W.B. McGill. 2001b. Variation of 1-naphthol sorption with organic matter fractionation: The role of physical conformation. Org. Geochem. 32:709–719.[CrossRef]
• Schmidt, M., H. Knicker, P. Hatcher, and I. Kogel-Knabner. 1997. Improvement of ¹³C and ¹⁵N CPMAS NMR spectra of bulk soils, particle size fractions and organic material by treatment with 10% hydrofluoric acid. Eur. J. Soil Sci. 48:319–328.[CrossRef]
• Simpson, A. 2001. Multidimensional solution state NMR of humic substances: A practical guide and review. Soil Sci. 166:795–809.[CrossRef]
• Simpson, M.J., B. Chefetz, and P.G. Hatcher. 2003. Phenanthrene sorption to structurally modified humic acids. J. Environ. Qual. 32:1750–1758.[Abstract/Free Full Text]
• Simpson, M.J., and P.C.E. Johnson. 2006. Identification of mobile aliphatic sorptive domains in soil humin by solid-state ¹³C nuclear magnetic resonance. Environ. Toxicol. Chem. 25:52–57.[CrossRef][Web of Science][Medline]
• Simpson, A.J., W.L. Kingery, D.R. Shaw, M. Spraul, E. Humpfer, and P. Dvortsak. 2001. The application of ¹H HR-MAS NMR spectroscopy for the study of structures and associations of organic components at the solid-aqueous interface of a whole soil. Environ. Sci. Technol. 35:3321–3325.[Medline]
• Simpson, M.J., A.J. Simpson, and P.G. Hatcher. 2004. Noncovalent interactions between aromatic compounds and dissolved humic acid examined by nuclear magnetic resonance spectroscopy. Environ. Toxicol. Chem. 23:355–362.[CrossRef][Web of Science][Medline]
• Thorn, K.A., and K.R. Kennedy. 2002. ¹⁵N NMR investigation of the covalent binding of reduced TNT amines to soil humic acid, model compounds and lignocellulose. Environ. Sci. Technol. 36:3787–3796.[Medline]
• Xing, B. 1997. The effect of the quality of soil organic matter on sorption of naphthalene. Chemosphere 35:633–642.[CrossRef]
• Xing, B. 2001. Sorption of naphthalene and phenanthrene by soil humic acids. Environ. Pollut. 111:303–309.[CrossRef][Medline]
• Xing, B., W. McGill, and M. Dudas. 1994a. Cross-correlation of polarity curves to predict partition coefficients of nonionic organic contaminants. Environ. Sci. Technol. 28:1929–1933.[CrossRef]
• Xing, B., W. McGill, and M. Dudas. 1994b. Sorption of benzene, toluene, and o-xylene by collagen compared with non-protein organic sorbents. Can. J. Soil Sci. 74:465–469.

This article has been cited by other articles:

				D. C. Olk and E. G. Gregorich Overview of the Symposium Proceedings, "Meaningful Pools in Determining Soil Carbon and Nitrogen Dynamics" Soil Sci. Soc. Am. J., April 19, 2006; 70(3): 967 - 974. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Simpson, M. J. Search for Related Content PubMed Articles by Simpson, M. J. Agricola Articles by Simpson, M. J. Related Collections Humic Substances Organic Compounds Soil Pollution Soil Chemistry