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Journal of Environmental Quality 32:240-246 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Sorption and Desorption of Naphthalene by Soil Organic Matter

Importance of Aromatic and Aliphatic Components

Amrith S. Gunasekara and Baoshan Xing*

Department of Plant and Soil Sciences, Univ. of Massachusetts, Stockbridge Hall, Amherst, MA 01003-7245

* Corresponding author (bx{at}pssci.umass.edu)

Received for publication May 10, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nonlinear isotherm behavior has been reported for the sorption of hydrophobic organic compounds (HOCs) in soil organic matter (SOM), but the exact mechanisms are unknown. Our objective was to provide insight into the sorption mechanism of HOCs in SOM by studying the sorption–desorption processes of naphthalene in a mineral soil, its humic fractions, and lignin. Additionally, humin and lignin were used for studying the effects of temperature and cosolvent on HOC sorption. All isotherms were nonlinear. The humin and lignin isotherms became more linear at elevated temperatures and with the addition of methanol indicating a condensed to expanded structural phase transition. Isotherm nonlinearity and hysteresis increased in the following order: soil humic acid (HA) < soil < soil humin. Of the samples, aliphatic-rich humin exhibited the largest degree of nonlinearity and had the highest sorption capacity for naphthalene. High nonlinearity and hysteresis in humin were most likely caused by its condensed structure. A novel aliphatic, amorphous condensed conformation is proposed. This conformation can account for both high sorption capacities and increased nonlinearity observed for aliphatic-rich samples and can explain many sorption disparities discussed in the literature. This study clearly illustrates the importance of both aliphatic and aromatic moieties for HOC sorption in SOM.

Abbreviations: FA, fulvic acid • HA, humic acid • HOC, hydrophobic organic compound • KF, sorption capacity • K'FOC, organic carbon–normalized sorption capacity • SOM, soil organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOILS PLAY AN IMPORTANT ROLE in the environment by controlling the fate and availability of organic compounds because of their sorption capability. The sorption and desorption of these compounds in soil are primarily regulated by soil organic matter (SOM) (Huang and Weber, 1997; Chiou et al., 1998). Therefore, knowledge of sorption behavior by SOM is important for the removal of organic chemicals from soils.

Currently, the solid-phase dissolution (partition) model is widely used for predicting the sorption of organic chemicals in soils. This model employs the gel polymer concept, where SOM is characterized as a three-dimensional network structure. Sorption of hydrophobic organic compounds (HOCs) into this network is uniform, concentration-independent, and yields linear isotherms. Also, competition between two sorbates is not expected (Chiou, 1989). However, nonlinear sorption isotherms (Xing et al., 1996; Huang and Weber, 1997; Xia and Pignatello, 2001) and competitive sorption between sorbates (McGinley et al., 1993; White and Pignatello, 1999) have been observed, suggesting that the partition model does not adequately address the sorption mechanisms of HOCs to SOM.

Further studies on the interactions between HOCs and SOM fractions led to the proposal that SOM is analogous to synthetic polymers; sorption behavior follows multimechanistic dual-mode sorption (DMSM) or dual–reactive domain (DRDM) models (Xing and Pignatello, 1997; White and Pignatello, 1999; Hu et al., 2000; LeBoeuf and Weber, 2000a; Yuan and Xing, 2001 and references therein).

According to these models, which consider organic matter in soil as a heterogeneous substance, SOM consists of two types of amorphous domains. The domains are characterized as expanded and condensed, analogous to rubbery and glassy synthetic polymers, respectively. Sorption of HOCs to the expanded "rubbery-like" domain generates linear isotherms due to partitioning while nonlinear isotherms are observed for the condensed "glassy-like" domain due to adsorption in Langmuir-type sorption sites. Although the dual mode sorption models have been numerously cited in the sorption literatures, structural compositions of these domains (expanded or condensed) in SOM are not well understood (Cuypers et al., 2002).

Recent sorption (Chin et al., 1997; Perminova et al., 1999; Perminova et al., 2001) and spectroscopic studies (Xing and Chen, 1999; Johnson et al., 2001) have shown that the condensed domain is mainly attributed to aromatic moieties. For example, with sorption studies using model HOCs and 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, Xing (2001a) found a positive correlation between sample aromaticity and isotherm nonlinearity. Such studies conclude that HOC sorption in soil is strongly influenced by aromatic moieties of SOM. However, other sorption studies are not in agreement with such a conclusion.

Mao et al. (2002) and Salloum et al. (2002) have shown that the sorption of a model HOC, phenanthrene, by aliphatic-rich (aromatic deficient) organic samples renders nonlinear isotherms. The samples also had high sorption capacities. In addition, Chefetz et al. (2000) showed that HOC sorption by aliphatic-rich plant cuticle, a precursor to SOM formation and diagenesis, was nonlinear and had a strong affinity for HOCs as observed by the high pyrene sorption capacity. The conclusion reached by these studies was that aliphatic components in SOM contribute substantially to HOC sorption processes. Thus, as one may see, there are clearly some contradictory theories for sorption of HOCs in SOM.

The contrasting findings as described above are a good example of the difficulty involved with determining contaminant sorption by SOM. Moreover, the contrasting conclusions provide significant disparities in understanding the sorption mechanism of HOCs to SOM. This study attempts to clarify these disparities.

To date, most isotherm studies have been based on the less complex, easily extractable organic fractions of soil; humic and fulvic acids (FA) or highly organic peat soils. In this study we used a whole mineral soil, its organic fractions (humic acid and humin), and a biopolymer, lignin (a precursor to soil humic substances), as sorbents. The objective of this work was to determine the sorption mechanism(s) of naphthalene by SOM, particularly in the humin fraction. New paraffinic carbon configurations for HOC sorption in humin are proposed and the importance of aromatic and aliphatic components in SOM is evaluated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sorbates and Sorbents
Naphthalene was used as a model HOC for all sorption and desorption experiments. Ring-UL-14C and unlabeled naphthalene (>98% purity) was purchased from Aldrich Chemical (Milwaukee, WI). Naphthalene was chosen as the sorbate because it is a common environmental pollutant and studies found that it can lead to DNA damage in cultured macrophage cells (Bagchi et al., 1998). Further, it is listed as a priority pollutant by the USEPA (USEPA, 1992).

Four sorbents were used in this study. A mineral soil was sampled from the South Deerfield Experiment Farm in Massachusetts. The soil was classified as a coarse well-mixed sandy loam (mesic Fluventic Dystrudept) with low organic carbon content (1.08%) and is a major agricultural soil in the region. The soil sample was air-dried and passed through a 2-mm sieve. Humic acid (HA) and humin were extracted from the mineral soil with procedures outlined by Schnitzer (1982) with slight modifications. Briefly, HA and humin extraction involved mixing 50 g of soil with 500 mL of 0.01 M Na4P2O7 in a 1-L bottle. The air in the bottle was replaced with N2 gas and the system was shaken for 24 h. The insoluble residue (humin) was separated from the dissolved solution by centrifugation at 1100 x g. The humin fraction was washed with deionized water, dried at 70°C, ground, and de-ashed with HF (3.5% v/v). The HA was precipitated by acidifying the solution to pH 1 with 6 M HCl for 24 h. The precipitated HA was centrifuged, de-ashed, washed with deionized water, and freeze-dried. The details of the extraction procedures were reported in our previous work (Ding et al., 2002a,b). Lignin (Organosolv), purchased from Aldrich Chemical, was the forth sorbent and was used without further treatment. The percent carbon content of HA, humin, and lignin was 54.2, 1.98, and 69.6, respectively. Relative structural carbon percentages (RSCP) were determined by 13C NMR; paraffinic (0–50 ppm), O-alkyl (50–110 ppm), aromatic (110–160 ppm), and carboxyl + carbonyl (160–215 ppm). The RSCP, in the order of paraffinic, O-alkyl, aromatic, and carboxyl + carbonyl, was 26, 30, 28, and 16 for HA (Ding et al., 2000); 40, 27, 10, and 23 for humin (Ding et al., 2001); and 20, 33, 43, and 4 for lignin (Salloum et al., 2002).

Sorption Experiments
All sorption isotherms were run according to the batch equilibration technique (Xing et al., 1996) at 20 ± 1°C in 8-mL screw cap vials with Teflon-lined septa. The background solution was 0.01 M CaCl2 in deionized distilled water with 200 mg/L HgCl2 as a biocide. Mercury chloride has been effectively used for preventing microbial growth in sorption and desorption studies (Xing, 1997, 2001b). Naphthalene concentrations ranged from 0.008 to 15 mg/L. Due to the low solubility, naphthalene was mixed at high concentrations in methanol before being added (did not exceed 0.1% by volume) to the background solution. Based on initial kinetic studies (not shown), naphthalene was mixed with all sorbents for 3 d except lignin (8 d) to reach apparent equilibrium. Mixing for an additional week beyond these equilibration periods did not yield any significant increase in sorption. The quantity of sorbent was adjusted to maintain sorption of naphthalene between 30 and 70%; the amount of lignin, HA, humin, and soil in the vials was approximately 0.008, 0.07, 0.2, and 0.7 g, respectively. All samples, including the blanks, were run in duplicate. After mixing, vials were centrifuged at 1000 x g for 30 min, followed by a 1-mL removal of supernatant. The supernatant was added to a Scintiverse cocktail (12 mL), purchased from Fisher Scientific (Pittsburgh, PA), for scintillation counting (Beckman [Fullerton, CA] LS 3801). Sorbed naphthalene concentration by the sorbent was determined by mass balance calculations because sorption to the vials was insignificant (Yuan and Xing, 2001).

Variable temperature isotherm experiments followed the same sorption procedures as described above. Sorbate–sorbent mixing during equilibration was maintained on hemotology mixers placed in an oven (VWR Scientific 1350FSM; Sheldon MFG, Cornelius, OR). The temperature in the oven was maintained at 60 or 85 ± 1°C. Cosolvent experiments at 20 ± 1°C were conducted with methanol (5 or 20% by volume) in the background solution. Sorbate–sorbent separation and isotope analysis followed the procedures as discussed above.

Desorption Experiments
Desorption was conducted by sequential decant-refill steps after the completion of the sorption experiments (Chen et al., 2000). After the 1-mL aliquot was withdrawn, as part of the sorption procedure, about 6 mL of the remaining supernatant was discarded and replaced by the fresh background solution (dilution). Dilution factors for the different concentrations were determined by weight. Following dilution, the vials were mixed for the same equilibration time as used in the sorption experiments; 3 d for all samples except lignin (8 d). After mixing, the vials were centrifuged and an aliquot (approximately 1 mL) of the supernatant was taken out for scintillation analysis. The remaining supernatant was replaced by the fresh background solution and the above process was repeated for two more cycles. Mass balance calculations were conducted to determine the amount of naphthalene desorbed in each cycle.

A significant concern in the naphthalene desorption experiments was the loss of sorbent during supernatant decanting for each cycle. Therefore, to eliminate this variable of experimental error, the dry weights of all sorbents, at selected concentrations, were determined before and after the sorption and desorption experiments. No significant change in the sorbent dry weights was observed. The minimum loss of sorbent during decanting has been attributed to flocculation by 0.01 M CaCl2 (electrolyte) and is consistent with previous findings (Xing et al., 1996; Yuan and Xing, 2001). In addition, total naphthalene recovery rates were determined for selected concentration points. The rates ranged from 92 to 103%.

Data Analysis
All sorption and desorption data were fitted to the logarithmic form of the Freundlich equation:

[1]
where S is the total sorbed concentration (mg/kg), C is the solution phase concentration (mg/L), and KF (mg/kg) (mg/L)-N and N are constants with N often <1 for sorption of HOCs to soil and its organic fractions. Isotherms were plotted by log S vs. log C, and log KF and N were obtained from the fitting. Because the units of KF depend on the N value for a given sample (Bowman, 1981; Chen et al., 1999), KF values cannot be compared between different isotherms. However, the KF values from different isotherms can be compared with each other after normalizing C by the super-cooled liquid state solubility (Sscl) of the sorbate; naphthalene = 106.6 mg/L (Carmo et al., 2000). This method of normalization generates the modified Freundlich parameter, K'F, which allowed us to compare KF values from different isotherms. Further modifications to K'F involved normalizing it by the organic carbon content of each sample (K'FOC). For the variable temperature and solvent studies, K'F was not used because Sscl of naphthalene at elevated temperatures and in mixed solvent systems was unknown.

The ratio of Freundlich exponents for desorption and sorption, ND/NS (subscripts D and S refer to desorption and sorption, respectively), was calculated and used as the hysteresis index. Lower hysteresis index values indicate that sorbate molecules experience increased difficulty to desorb from the sorbent matrix. The same methods have been successfully applied by other investigators to explain chemical desorption behavior (Barriuso et al., 1994; Yuan and Xing, 2001).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sorption
All sorbents exhibited nonlinear isotherms (Table 1 and 2); sorption affinity of the mineral soil and its organic fractions decreased as the naphthalene concentration increased. Nonlinearity decreased (N approached 1) in all variable temperature and cosolvent experiments (Table 2). Humin, containing high aliphatic carbon content (67%) and low aromaticity (10%), had the highest degree of nonlinearity while more linear isotherms were observed for HA with higher aromatic content (28%). Isotherm nonlinearity increased in the following order: soil HA < soil < soil humin. These results are consistent with previous findings. For example, Xing (2001b) observed the same nonlinear isotherm trend after studying naphthalene and 1,2-dichlorobenzene sorption to Florida Pahokee peat and its organic fractions: peat HA < peat < peat humin. This order of nonlinearity is in line with the degree of condensed structures or glassy character expected from organic matter extraction procedures (Cuypers et al., 2002). Expanded organic matter (lower molecular weight components with high polar functional groups) would be easily extracted, HA and FA, as compared with insoluble condensed organic matter fractions (humin) (Xing and Pignatello, 1997).


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  		Table 1. Isotherm parameters for naphthalene sorption by humic acid (HA), soil, humin, and lignin at 20 ± 1°C.

 

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  		Table 2. Isotherm parameters for naphthalene sorption by lignin and humin at different temperatures without cosolvent and different methanol concentrations at 20 ± 1°C.

 
In addition to isotherm nonlinearity, the K'FOC for naphthalene by the soil and its organic fractions provided insight into structural conformations of SOM. The K'FOC for humin (110 900 mg/kg OC) was about three times greater than soil (31 810 mg/kg OC) and the HA (39 960 mg/kg OC). These findings are consistent with previous studies. A threefold increase in 1-naphthol sorption capacity for humin, as compared with HA, was observed by Salloum et al. (2001) using soil organic fractions from diverse geologic environments. High HOC sorption to humin, as compared with the whole soil, is believed to be related to its extraction procedure. Humin has been defined as the organic fraction in soil associated with minerals (Rice, 2001). We speculate that in soil, the humin complex may be surrounded by the HA and FA fractions and deeply embedded in the SOM matrix, making HOC sorption less accessible to it. In contrast, greater HOC sorption to humin is expected after its extraction and separation because the isolation of humin exposes additional binding sites on its structure, which would be otherwise occupied by the HA and FA. A similar notion was used by Salloum et al. (2001) to explain the high sorption capacity of 1-napthol in soil humins. In addition, low K'FOC of the whole soil may be in part due to very slow sorption kinetics, that is, naphthalene molecules did not diffuse into some sorption sites in the embedded humin. A relatively low K'FOC was observed for HA, which might be also associated with SOM extraction procedures because loosely bound, polar fractions (HA and FA) would be preferentially removed. Sorbents with high content of polar functional groups sorb lower amounts of HOCs (Xing et al., 1994; Xing, 1997).

Many studies show that aromatic moieties are responsible for high sorption of HOCs (Xing and Chen, 1999; Ahmad et al., 2001; Johnson et al., 2001; Salloum et al., 2001). The sorption results of "aromatic-rich" lignin used in this study provide further evidence for this observation (Table 1). Lignin (43% aromatic carbon) is a glassy polymer with a glass transition temperature at about 70°C (LeBoeuf and Weber, 2000b). However, several other studies have shown that HOC sorption is primarily dominated by aliphatic components of SOM (Chefetz et al., 2000; Mao et al., 2002; Salloum et al., 2002). Thus, there is a significant disparity in understanding the relative contribution of individual structural units of SOM to HOC sorption.

Recently, Mao et al. (2002) found that the amorphous aliphatic poly(methylene) components of SOM play a dominant role in the sorption of HOCs. Similar results were observed in this study (Table 1); the humin sample had a significantly larger percentage of aliphatic components (67%) than aromatic groups (10%). Mao et al. (2002) suggested that partitioning of HOCs into these amorphous aliphatic domains would be the controlling sorption process, which was supported by a linear isotherm of aliphatic-rich Amherst humin. However, our results also show increased nonlinearity for humin (N = 0.848), the most nonlinear isotherm of the three samples (Table 1). Similarly, Florida humin in the study by Mao et al. (2002) had a strongly nonlinear isotherm (N = 0.69). Chefetz et al. (2000) also showed that aliphatic-rich plant cuticles and humin had high K'FOC, but exhibited highly nonlinear isotherms for pyrene sorption. Aliphatic carbon percentages of the plant cuticle and humin were 90 and 83%, respectively. The N values for both plant cuticle and humin were 0.65.

On careful examination of the Amherst peat humin isotherm by Mao et al. (2002), we observed that the linear, partition type sorption model does not quite fit the data. The isotherm is nonlinear except for the last two data points. The deviation of the last two data points from the nonlinear isotherm might result from matrix swelling due to the high sorbate concentration. Soil organic matter matrix swelling can be explained by the multimechanistic extended dual-mode model (EDMM). This model is based on sorption and dilation (swelling) isotherms for gases and hydrocarbons in synthetic polymers (Kamiya et al., 1992). Recently, Xia and Pignatello (2001) employed the EDMM to explain their sigmoidal isotherms for HOC sorption to a peat soil. According to the model, HOC sorption to a condensed "glassy" polymer is initially concave-down to the solute concentration axis. Such a concave trend is observed for the Amherst humin isotherm except the last two points as presented by Mao et al. (2002). As the sorbate concentration increases, the molecules will penetrate the polymer matrix and swell its structure by weakening the attractive forces between macromolecular chains. In the case of SOM, this process is the "melting-away" of nanometer-size holes in the condensed domain, which leads to an expanded "rubbery-like" matrix. The sorbate concentration at the condensed to expanded phase transition point is named Sg, the isothermal glass transition concentration (Xia and Pignatello, 2001). Below Sg, sorption of HOCs to SOM can be explained with the dual-mode sorption model but above Sg, sorption of HOCs follows EDMM behavior (Xia and Pignatello, 2001). Most isotherms by Mao et al. (2002), including aliphatic-rich humins, tend to be sigmoidal and can be explained with the EDMM. Therefore, the nonlinear results by Chefetz et al. (2000), Xing (2001b), Mao et al. (2002), and our data indicate that aliphatic structures in SOM, as observed for humin, must contribute to both isotherm nonlinearity and large sorption capacity.

We propose that both aliphatic and aromatic moieties contribute significantly to HOC sorption. For lignin, isotherm nonlinearity is mainly dictated by aromatic moieties (43%) that swell on heating and cosolvent initiation (discussed below). Humin, rich in aliphatic moieties, has a similar nonlinear sorption process to lignin. To explain the contribution of humin to increased sorption capacity and nonlinearity, as a function of high aliphatic constituents, a new substructural conformation is proposed (Fig. 1) . According to Painter and Coleman (1997), semicrystalline polymer chains exist in a crystalline–amorphous domain as folded polymer crystals (switchboard model). We (Hu et al., 2000) and others (Mao et al., 2002) observed crystalline regions (to which HOC sorption is negligible) adjacent to amorphous regions in SOM. A modified schematic conformation of semicrystalline polymer interphase is shown in Fig. 1a. We suggest that on interaction of the crystalline–amorphous complex with soil mineral surfaces, the first several molecular layers of the amorphous region may rearrange to take a more condensed form (Fig. 1b). This condensed region can cause enhanced nonlinear sorption and both condensed and expanded amorphous aliphatic regions will contribute to high HOC sorption. Murphy et al. (1994) observed increased isotherm nonlinearity and competitive sorption of three organic compounds by HA adsorbed on minerals as compared with unsorbed HA. They suggest that additional adsorption sites were created by the interactions between HA and minerals, probably due to the formation of more rigid (condensed) HA structures on the mineral surface. This and many other sorption anomalies found in the literature, particularly for aliphatic-rich humins, can be explained with the proposed substructural conformation. Experiments are currently underway to further test this concept.



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  		Fig. 1. (a) Schematic diagram showing the distribution of crystalline (-{square}-) and amorphous (-{circ}-) aliphatic organic matter. (b) Interaction of the aliphatic components with a clay mineral surface (shaded area) rearranging part of the amorphous domain into a condensed configuration (-{triangleup}-) having nanometer-size holes.

 
Desorption
Sorption parameters N and K'F provide important information regarding HOC binding to SOM. Desorption behavior provides further insight into possible sorptive mechanisms and structural comprehension for SOM. Given that sorption processes are reversible, partition-type sorption of HOCs by SOM would yield no sorption–desorption hysteresis assuming solute molecules did not alter the sorbent matrix (Yuan and Xing, 2001). However, with a heterogeneous sorption site distribution in SOM, desorption can deviate from the sorption isotherm. Fig. 2 presents the sorption (NS) and desorption (ND) isotherm data for HA (Fig. 2a), whole soil (Fig. 2b), and humin (Fig. 2c). All desorption isotherms showed hysteresis, consistent with the dual mode sorption theory. A trend common to all desorption isotherms was observed; desorption increased from low to high naphthalene solution concentration and then decreased at the highest concentration point as shown by the ND values (Fig. 2). At low naphthalene concentrations, the ratio of high energy sites to naphthalene molecules was larger than at high concentrations. Thus, a large percentage of naphthalene molecules would bind to these energetic sites, resulting in decreased desorption. At high concentrations, the ratio of high energy binding sites to sorbate molecules was lower because the limited number of energetic sites in the SOM matrix were saturated with naphthalene molecules. Therefore, a large percentage of naphthalene molecules at high concentrations occupied low energy binding sites and they would be readily desorbed. At the highest concentration, naphthalene molecules overwhelmed the matrix system and caused a condensed to expanded phase transformation by swelling. Structural swelling of SOM led to additional binding sites and reduced ND on desorption. Such desorption behavior was clearly observed for soil and its organic fractions in Fig. 2. In one of our previous studies (Ding et al., 2002b), similar results were observed for metolachlor desorption in a mineral soil from South Carolina and its humic fractions. Thus, desorption data are supportive of the extended dual-mode model and lend credence to the presence of both condensed and expanded domains within SOM.



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  		Fig. 2. Sorption ({blacksquare}) and desorption ({circ}) isotherms of naphthalene by (a) humic acid (HA), (b) whole soil, and (c) humin.

 
Hysteresis index values of the mineral soil and its humic fractions at different initial concentrations of naphthalene are shown in Fig. 3 . The HA had the highest hysteresis index values at all initial concentrations indicating that naphthalene desorption from its matrix was comparatively easy. The mineral soil and humin fraction had similar hysteresis index values but lower than HA, reflecting that naphthalene desorption from their matrices was relatively difficult. The degree of hysteresis followed the order of soil HA < soil < soil humin. This trend was the same as the order of isotherm nonlinearity. Low hysteresis index and sorption N values for humin, as compared with HA, indicate more condensed domains as part of its structural conformation. Condensed to expanded phase transition is supported by the variable temperature and cosolute experiments as discussed below.



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  		Fig. 3. Distribution of hysteresis index for humic acid (HA) ({circ}), whole soil ({square}), and humin ({triangleup}).

 
Variable Temperature and Cosolvent
The results of the variable temperature and cosolvent studies are presented in Table 2. Humin and lignin were the only samples subject to these experiments because they differ considerably in molecular structure. Elevated temperatures resulted in increased isotherm linearity for both lignin and humin. The N of lignin increased from 0.931 at 20°C to 0.991 at 85°C and N of humin increased from 0.848 at 20°C to 0.955 at 85°C. Similarly, the cosolvent studies showed a positive trend between increased methanol concentration and isotherm linearity (Table 2). These experiments address the issue of SOM phase transformation. LeBoeuf and Weber (1997) have shown that condensed domains in SOM and synthetic polymers become expanded (with a gradual elimination of holes) above their glass transition temperatures (Tg). Subsequent studies by Young and LeBoeuf (2000) found that peat HA and Suwannee River FA had Tg values between 43 and 62 and 36 and 49°C, respectively. The Tg of lignin was determined to be about 70°C (LeBoeuf and Weber, 2000b). These findings suggest that for variable temperature isotherms below Tg = 70°C (20 and 60°C isotherms), amorphous condensed and expanded domains could be found in lignin structure and is supported by nonlinear sorption. However, at an elevated temperature (85°C) the condensed domain became expanded (or rubbery) as shown by the nearly linear isotherm (Table 2). Phase transition behavior is also evident after introducing lignin to varying concentrations of methanol, a solvent capable of expanding the SOM structure (Lyons and Rhodes, 1993). The humin sample followed the same trend as lignin for the variable temperature and cosolvent sorption isotherms. These studies clearly demonstrate that although lignin and humin have very different structural components, their sorption behavior was similar in response to temperature and cosolvents.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study emphasizes the importance of both aromatic and aliphatic components in SOM for HOC sorption. Both components can be present in the form of condensed or expanded domains depending on the origin, nature, and diagenic age of SOM samples. It has been reported that for diagenetically old SOM, condensed domains are enriched in aromatic carbons while for relatively young SOM, condensed domains can be enriched in aliphatic carbons (Cuypers et al., 2002). Therefore, both aromatic and aliphatic components of SOM can contribute to nonlinear sorption and high sorption capacity. Furthermore, interaction between minerals and SOM may render the structural configuration of SOM to a more condensed state, resulting in highly nonlinear isotherms and competitive sorption as observed for humins. Additionally, sorption processes may be strongly affected by physical conformation of and accessibility to SOM as demonstrated by a higher K'FOC of humin than that of the whole soil. Thus, it is important to use the sorption kinetics, mechanism, and capacity determined from whole soil/sediment samples for HOC fate and bioavailability predictions.


   ACKNOWLEDGMENTS
 
The authors thank Ms. Shilpa Pujari and Dr. Guangwei Ding for their assistance during the sorption and desorption isotherm experiments. This work was supported by the USDA National Research Initiative Competitive Grants Program (98-35107-6319 and 2002-35107-12544) and the Federal Hatch Program (Project no. MAS00860).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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