JEQ Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 11 May 2005
Published in J Environ Qual 34:1055-1062 (2005)
DOI: 10.2134/jeq2004.0152
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Agricola
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Related Collections
Right arrow Soil Organic Matter
Right arrow Batch Studies
Right arrow Volatile Organic Compounds
Right arrow Geochemical Processes
Right arrow Organic Compounds

TECHNICAL REPORTS

Organic Compounds in the Environment

The Influence of Lipids on the Energetics of Uptake of Polycyclic Aromatic Hydrocarbons by Natural Organic Matter

Gabriela Chilom, Scott D. Kohl and James A. Rice*

Department of Chemistry and Biochemistry, South Dakota State University, Box 2202, Brookings, SD 57007-0896

* Corresponding author (james.rice{at}sdstate.edu)

Received for publication April 12, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Although most of the organic carbon in soils and sediments may be composed of humic substances, their interaction with other compounds, especially their sorption interactions, may be significantly affected by the presence of small amounts of the other components of natural organic matter (NOM). In this investigation, the influence of the lipid fraction of NOM on the sorption thermodynamics of fluorene, phenanthrene, and pyrene to several geosorbent samples was examined before and after extraction of lipids. Batch experiments were performed at the same concentration for all polycyclic aromatic hydrocarbons (PAHs) (0.025 x their solubility in water) at different temperatures (10, 20, 30, and 40°C), and the thermodynamic parameters were calculated. Removal of the lipids increases the sorption capacity of the samples as well as the exothermicity of the process. The free energy change was negative for all the samples and no significant differences were noticed on lipid removal. The entropy changes were small and positive for the whole geosorbent samples, but even smaller or more negative when the lipids were removed. This indicates that the interaction of PAHs with soils and sediments in the absence of extractable lipids is stronger and the mechanisms involved may be different, changing from a partitioning-like mechanism to specific adsorption. Because of the competition between lipids and PAHs for the same sorption sites, the lipids can be viewed as an "implicit sorbate."

Abbreviations: NMR, nuclear magnetic resonance • NOM, natural organic matter • PAH, polycyclic aromatic hydrocarbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE SORPTION OF PAHs to NOM is a critical control on the fate and transport of these contaminants in the environment (Karickhoff et al., 1979; Alexander, 1995; Luthy et al., 1997). Sorption processes depend strongly on the nature of solid phase as well as on the chemical structure of the contaminant. The complexity of these heterogeneous materials makes it difficult to develop a generalized sorption model.

For some time it has been assumed that the sorption of an organic contaminant to NOM is dominated by a partitioning mechanism. Chiou et al. (1979)(1983) suggested, "the uptake of neutral organic chemicals by soil is essentially a process of partitioning (dissolution) rather than physical adsorption." This model provided a means to estimate the contaminant soil–water distribution from aqueous solubility or solvent–water partition coefficients. The supporting data for the partitioning model were the linearity of the sorption isotherms for solute concentrations up to 90% of sorbate water solubility, lack of competition between multiple solutes, and low enthalpies of sorption. However, subsequent studies indicated nonlinear sorption over wide contaminant concentration ranges (McGinley et al., 1993; Young and Weber, 1995; Gustafsson et al., 1997; Xing and Pignatello, 1997; Kleineidam et al., 1999; Kohl and Rice, 1999; Xia and Ball, 2000), competitive sorption (Pignatello, 1991; McGinley et al., 1993; Xing and Pignatello, 1997), and sorption–desorption hysteresis (Kan et al., 1994; Cornelissen et al., 1998; Weber et al., 1998). Moreover, there are studies that show that variations in sorption behavior and the partition coefficient Koc between different soils and sediments may be due to the nature of the organic matter itself (Garbarini and Lion, 1986; Miller and Weber, 1986; Grathwohl, 1990; Lion et al., 1990). All these nonpartitioning-like sorption observations raise questions about the validity of partitioning as the sole sorption mechanism. To account for some of these observations Chiou and Kile (1998) considered other mechanisms. For example, adsorption onto mineral matter (for low organic carbon materials) and adsorption on high-surface-area carbonaceous materials are used to explain the nonlinearity and competitive sorption of nonpolar solutes at low concentrations. The presence of small amounts of high-surface-area carbonaceous materials, like charcoal, that exhibit nonlinear adsorption is also used by other conceptual models like the distributed reactivity model proposed by Weber et al. (1992). This model assumes that sorption occurs in two different domains present in NOM, a soft, expanded, rubbery domain and a rigid, condensed, glassy domain. It is a composite model that displays both linear partitioning into a "rubbery" domain within NOM and nonlinear uptake into a "glassy" domain. The dual-mode sorption model uses the existence of internal nanopores available for adsorption of solutes of different polarity (Pignatello, 1998). This mechanism assumes that the sorption process takes place by concurrent solid-phase dissolution and hole-filling mechanisms, similar to micropore filling within glassy polymers (Toscano et al., 1993; Xing and Pignatello, 1997).

Distinct from those models that consider NOM either as a bulk phase or as comprised of different carbon type pools is the "implicit-adsorbate" model proposed by Curl and Keoleian (1984). The implicit-adsorbate model is based on simple competitive adsorption between a sorbate under study and an "implicit" sorbate initially present on the sorbent. This model assumes that NOM has an "implicit adsorbate" that can desorb and affect the partition coefficient for new adsorbing species. The implicit adsorbate is assumed to have a two-site isotherm, consisting of Langmuir and linear-type components. It accommodates both partitioning and adsorption as being present depending on the concentration of the implicit adsorbate, and does so without making any assumptions about the structure of NOM. The implicit adsorbate present at high concentrations occupies both primary and secondary sites, so the sorption will be partitioning-like. At low concentrations of the implicit adsorbate, the sorption will resemble an adsorption-like process. Without identifying the possible implicit sorbate, or needing to identify it, this model accounts for observations like adsorption–desorption hysteresis, concentration effects, and the endothermicity of the sorption process.

Kohl and Rice (1999) have shown that removal of the lipids increases the nonlinear character of the sorption isotherms as well as the sorptive capacity of the mineral soils studied. They proposed that the lipids strongly compete for hydrophobic sites present in the natural organic matter. Lipids are operationally defined as organic geochemicals that are soluble in nonpolar, or weakly polar, organic solvents such as hexane, benzene, or dichloromethane (Bergman, 1963). Lipids include molecules ranging from simple fatty acids, alkanes, alkenes, and n-alkyl alcohols to more complex substances such as sterols, terpenes, polynuclear hydrocarbons, fats, waxes, and resins (Stevenson, 1994). The purpose of the present study is to investigate and further understand the role of the lipids in the sorption of hydrophobic organic contaminants on NOM. Specifically, this work tests the hypothesis that lipids compete with PAHs for high-energy sorption sites when they bind to NOM and by doing so, they behave as an implicit adsorbate.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sorbents
Experiments were performed with a marine sediment, three mineral soils, and a peat. The sediment was collected from San Diego Bay Harbor, California, and contains 1% (w/w) organic carbon (Kohl et al., unpublished data, 2003). The mineral soil samples were collected in eastern South Dakota. They are referred to as the Allivar soil [sandy, mixed, frigid Calcic Hapludolls; 2.2% (w/w) organic carbon], the Hetland soil (fine, smectitic, frigid Pachic Vertic Argiudolls; 2.2% (w/w) organic carbon], and the Poinsett silt loam (fine-silty, mixed, superactive, frigid Calcic Hapludolls; 2.9% (w/w) organic carbon] (Kohl and Rice, 1999). The peat, referred to as the Guanella Pass peat, was collected from a boggy soil in central Colorado and contains 26% (w/w) organic carbon (Rice, 1986). All the samples were air-dried and sieved to pass a 2-mm mesh before any treatment was applied to them.

When used with no further treatment the samples are referred to as "whole" samples. After removing the lipids by solvent extraction they are referred to as "extracted" samples. The lipid extraction procedure is described in detail elsewhere (Kohl and Rice, 1999). Briefly, the soil samples were extracted with a benzene–methanol azeotrope [3:1 (v/v)] for 72 h in a Soxhlet apparatus. The amount of extractable lipids was determined by weight after evaporation of the extraction solvent. The lipid content, together with mineralogical properties of the samples, are presented in Table 1.


View this table:
[in this window]
[in a new window]
  		Table 1. Lipid content and mineralogical properties of the geosorbents.

 
Sorbates
The three PAHs used, fluorene, phenanthrene, and pyrene, were chosen because they exhibit a range of solubility and hydrophobicity (Table 2). Fluorene (purity > 99%) and phenanthrene (purity > 99.5%) were obtained from Aldrich (St. Louis, MO). Fluorene-9-14C (purity > 98%) and phenanthrene-9-14C (purity > 98%) were purchased from Sigma (St. Louis, MO). Pyrene-4,5,9,10-14C (purity > 98%) was purchased from Amersham (Little Chalfont, UK). Solutions of 0.01 M CaCl2 were used as background electrolyte and HgCl2 (200 µg/mL) was added as bioinhibitor. The PAH solutions were prepared in the electrolyte solutions at 0.025 x the water solubility (Mackay et al., 1992) from stock solutions of PAH prepared in methanol. The amount of methanol added from the stock solutions was <0.1% (v/v) to avoid cosolvent effects on sorption (Rao et al., 1990).


View this table:
[in this window]
[in a new window]
  		Table 2. Selected properties of the polycyclic aromatic hydrocarbons.{dagger}

 
Sorption
Batch experiments were conducted in 8-mL glass culture tubes with PTFE-lined screw caps containing 25 mg of sample and filled to a minimum headspace with the PAH solution. They were placed in temperature-controlled chambers to maintain constant temperatures of 10, 20, 30, and 40°C. Blank vials containing just sorbate solution were prepared for each sorbate at each temperature to account for possible sorption on glass walls or the cap surface. At least three replicates of each suspension or blank solution were prepared and incubated in the dark with shaking three times a day. After 14 d, the tubes were centrifuged for 15 min at 800 x g and the supernatant collected and measured for 14C activity. The amount of PAH sorbed to the soil was determined by the difference in the activity between blank tubes and those containing the geosorbent.

Surface Area Determination
Surface areas of the geosorbents were obtained on an ASAP 2000 surface area analyzer (Micrometrics, Norcross, GA). The Brunauer–Emmett–Teller (BET) method of surface area determination was used with N2 as the sorbate at –196°C. Between 0.5 and 2.5 g of geosorbents samples were degassed at 105°C until the pressure was stable at <10 µm Hg. Surface areas were measured in triplicate or better for each sample and calculated using the instrument software (Table 3).


View this table:
[in this window]
[in a new window]
  		Table 3. Surface area values of the geosorbents.

 
Solid State Carbon-13 Nuclear Magnetic Resonance
The samples were characterized by quantitative 13C nuclear magnetic resonance (NMR), using the technique described by Mao et al. (2000). This procedure utilizes DPMAS at high rotation speeds (13 kHz), combined with a T1C correction obtained from a CP/T1–TOSS experiment. The recycle delays used for DPMAS were determined for each sample, and were between 6 and 15 s. The number of scans recorded ranged between 5000 and 35000. All the samples, except the whole and extracted peat, were treated with dilute solutions of hydrofluoric acid (Keeler and Maciel, 2003) before NMR analysis. The samples were packed in a 4-mm-diameter zirconia rotor with a Kel-F cap. Spectra were acquired at 75 MHz on a Bruker (Rheinstetten, Germany) ASX300 spectrometer. The 13C NMR spectra (Fig. 1) were integrated according to the following chemical shift regions: 0 to 50 ppm, aliphatic carbon; 50 to 108 ppm, carbohydrate carbon; 108 to 162 ppm, aromatic carbon; and 162 to 212 ppm, carboxyl carbon. The distribution among these four major carbon types (Fig. 2) was calculated by integration using software supplied with spectrometer operating system.



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 1. Carbon-13 nuclear magnetic resonance (NMR) spectra for San Diego sediment (SD), Poinsett soil (PS), Allivar soil (AS), Hetland soil (HS), and Guanella Pass peat (GP) before and after lipids extraction.

 


View larger version (26K):
[in this window]
[in a new window]
  		Fig. 2. The distribution of organic carbon for the whole geosorbents: San Diego sediment (SD), Poinsett soil (PS), Allivar soil (AS), Hetland soil (HS), and Guanella Pass peat (GP).

 
Data Analysis
The distribution coefficient (Kd) was calculated using:

[1]
where Cs is the sorbate concentration in the soil (solid phase) expressed as µg/g and Cl is the concentration of the sorbate in solution (liquid phase) expressed as µg/cm3. All sorption experiments had the same solid-to-solution ratio.

Thermodynamic parameters associated with the sorption process can be calculated from Kd (distribution coefficient) measured as a function of temperature. Using the following equations, the enthalpy ({Delta}H), entropy ({Delta}S), and free energy change ({Delta}G) associated with each sorption process were calculated:

[2]

[3]

[4]

If the enthalpy of sorption is independent of temperature then a plot of ln Kd versus (1/T) is linear according to the van't Hoff equation (Eq. [2]). The errors associated with the thermodynamic parameters were calculated from the errors in Cs and Cl using error propagation techniques (Skoog et al., 1988). A t test was used to compare the values for the whole and extracted samples.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The sorption of fluorene, phenanthrene, and pyrene was monitored initially for a period of 28 d at room temperature. Fluorene and phenanthrene had reached apparent equilibrium by 7 d and pyrene by 14 d. Thus, a 14-d time period was chosen to measure the influence of temperature on sorption equilibrium of the three PAHs on whole and extracted samples in all subsequent experiments.

Distribution Coefficients
The equilibrium distribution coefficients decreased as the temperature increased from 10 to 40°C (Fig. 3) for most of the samples. The exceptions were the values corresponding to 10°C for pyrene, and to a lesser extent for phenanthrene at the same temperature. These values are not attributed to experimental error because the pyrene experiments were repeated three times and the same trend was noticed. This same pattern was also noticed by Wauchope et al. (1983) for sorption of naphthalene on a loam soil [1% (w/w) organic carbon] and by Mills and Biggar (1969) for sorption of {gamma}-hexachlorocyclohexane on a clay soil. The Kd values for fluorene sorption on sediment, whole and extracted, were not determined because the changes in solution activity on sorption were within the range of experimental errors.



View larger version (14K):
[in this window]
[in a new window]
  		Fig. 3. The variation of distribution coefficients (Kd) with temperature for sorption of fluorene, phenanthrene, and pyrene on the geosorbents: {diamondsuit}, San Diego sediment (SD); {blacktriangleup}, Guanella Pass peat (GP); {blacksquare}, Allivar soil (AS); •, Poinsett soil (PS); and x, Hetland soil (HS). The open labels correspond to the extracted samples. The error bars represent the absolute standard deviations.

 
The Kd values obtained for pyrene were greater than phenanthrene and fluorene in all samples: between 5.7 and 4.7 times that of phenanthrene and between 22 and 16 times that of fluorene for whole and extracted sediment and mineral soils, respectively. The differences were four times less for peat samples, either whole or extracted.

The equilibrium distribution coefficients for extracted samples were up to approximately 1.7 times higher than for whole samples for the majority of the geosorbents and PAHs studied. In some cases, such as pyrene data, no significant difference were obtained between extracted and whole samples for three out of five geosorbents studied.

Organic Carbon–Normalized Distribution Coefficients
The equilibrium distribution coefficients for fluorene, phenanthrene, and pyrene were normalized to the organic carbon content of the soils . The temperature dependence of Kdoc is shown in Fig. 4 .



View larger version (15K):
[in this window]
[in a new window]
  		Fig. 4. The variation of distribution coefficients normalized to organic carbon content with temperature for sorption of fluorene, phenanthrene, and pyrene on the geosorbents: {diamondsuit}, San Diego sediment (SD); {blacktriangleup}, Guanella Pass peat (GP); {blacksquare}, Allivar soil (AS); •, Poinsett soil (PS); and x, Hetland soil (HS). The open labels correspond to the extracted samples. The error bars represent the absolute standard deviations.

 
The Kd values for both the whole and extracted geosorbents can be grouped into three categories based on organic matter content: sediment < mineral soils < peat (see Fig. 3). These groupings disappear when the Kd values are normalized for organic carbon content, with the resulting Kdoc values separating into two groups: whole < extracted samples.

Thermodynamic Parameters
Figure 5 presents the free energy change for fluorene, phenanthrene, and pyrene sorption. The values are calculated using Kd values and Eq. [3].



View larger version (15K):
[in this window]
[in a new window]
  		Fig. 5. The variation of free energy change ({Delta}G) with temperature for sorption of fluorene, phenanthrene, and pyrene on the geosorbents: {diamondsuit}, San Diego sediment (SD); {blacktriangleup}, Guanella Pass peat (GP); {blacksquare}, Allivar soil (AS); •, Poinsett soil (PS); and x, Hetland soil (HS). The open labels correspond to the extracted samples. The error bars represent the absolute standard deviations.

 
The sorption free energy change was negative for all samples indicating that all three PAHs prefer the geosorbent to the solution. The values become more negative in the order: fluorene, phenanthrene, and pyrene. The {Delta}G values for extracted samples were generally slightly more negative than for the whole samples.

The sorption enthalpy and entropy change values are presented in Table 4. By integrating the van't Hoff equation and assuming {Delta}H is independent of temperature, the plot of ln Kd vs. (1/T) will give a straight line with the slope equal to (–{Delta}H/R). The interval temperature where van't Hoff plots were linear was 10 to 40°C for all cases except pyrene where the 20 to 40°C temperature interval was considered. All {Delta}H values are negative, indicating the sorption processes for all three PAHs are exothermic. The low organic carbon content samples showed {Delta}H values in the same range but the peat, with the highest organic carbon content, showed values consistently more negative.


View this table:
[in this window]
[in a new window]
  		Table 4. Enthalpy ({Delta}H) and entropy ({Delta}S) change associated with sorption process.

 
The most important feature of the sorption enthalpy data is the difference between whole and extracted samples. There was a significant difference for 9 of the 14 samples and in those cases sorption onto extracted samples is between 1.3 and 2.5 times more exothermic than for the whole samples.

The entropy changes, calculated using Eq. [4], were temperature independent so the values presented in Table 4 represent the averaged value on the temperature interval. The entropy changes accompanying PAH sorption decreases for the extracted samples. The values either become smaller or more negative than the values calculated for the whole samples.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The Influence of Lipids
All the data indicate that the lipid fraction has an important role in the sorption of PAHs. The present results not only support the conclusions of Kohl and Rice (1999), but also demonstrate that the interaction of PAHs with soil and sediment organic matter in the absence of extractable lipids is stronger. Moreover the mechanisms involved may be different, changing from partitioning to a more specific sorption mechanism.

After extracting the lipids, the organic carbon content of the soils decreased by 2 to 9% (Table 1). Lipid extraction decreases the amount of aliphatic carbon present in the soils as shown by the decrease in the aliphatic carbon peak (0–50 ppm) in NMR spectra (Fig. 1). The extraction procedure does not leave any trace of benzene or methanol in the sample (Kohl and Rice, 1999) or change the surface area determined by Brunauer–Emmett–Teller adsorption isotherms (Table 3). Moreover, when the extraction procedure is applied to material that does not contain lipids, such as activated carbon, there is no change in the sorption behavior as shown by Ding and Rice (unpublished data, 2004) for sorption of naphthalene on activated carbon.

The Kd values presented in Fig. 3 indicate that removing the lipids increases the amount of PAH sorbed. Depending on the location of lipids in the NOM matrix their removal can either generate new sites for adsorption or open the path to partitioning of PAHs into hydrophobic pools with higher affinity that were previously inaccessible to the sorbate. The effect on Kd will be the same in each case because Kd is an apparent equilibrium constant that accounts for all processes occurring during sorption. However, the increase in the sorption exothermicity for extracted samples favors an assumption that competitive adsorption exists between PAH and lipids. The magnitude of the sorption enthalpy changes for both whole and extracted samples and is in the range of weak forces such as physical adsorption (Hasset and Banwart, 1989), van der Waals forces (0–9 kJ/mol), and H-bonding (16–22 kJ/mol). The difference in enthalpy changes between whole and extracted samples may be due to a more specific adsorption process. If it is assumed that the lipids were initially adsorbed onto the NOM surface and occupying sites that have different energies, then extraction of lipids will free the high-energy sites in addition to the low-energy ones. The high-energy sites will then become available for the incoming sorbate (the PAH) and the interaction will be stronger compared with the geosorbents where those sites are occupied by lipids. This sorption is also supported by the gain in sorption entropy for whole samples and the loss in sorption entropy for extracted ones. The positive entropy, typically observed in interactions resulting from the hydrophobic effect, is due to the loss of the structured water surrounding the sorbate (Tanford, 1980). The negative entropy is due to the rigid binding of the molecules adsorbed to the surface.

Possible Mechanism of Sorption
In spite of some similarities, a simple partitioning model cannot account for the differences between the whole and extracted samples. The Koc values can be predicted by empirical equations based on the octanol–water partition coefficients (Grathwohl and Kleineidam, 2000): log Koc = 0.807 log Kow + 0.068 (for log Kow between 2.4 and 7.4). The values obtained were 3.4, 3.7, and 4.2 for fluorene, phenanthrene, and pyrene. The log Koc values obtained from our data were between 3.8 and 4.4, between 4.4 and 5.0, and between 4.5 and 5.6 for fluorene, phenanthrene, and pyrene, respectively. The predicted values slightly underestimate the experimental distribution of the PAHs on the geosorbents studied, and the difference may be either the presence of another mechanism or simple prediction error (Piatt et al., 1996). However, according to the partition model, the sorptive uptake of a compound out of the aqueous phase positively correlates with the amount of organic matter in the sorbent (Chiou et al., 1998), which is the opposite of the behavior that we have observed with the extracted samples. The increase in exothermicity also cannot be explained by a simple partitioning theory that treats all the solid-phase organic matter as qualitatively equivalent with regard to its interactions with hydrophobic organic contaminants under hydrated conditions.

These observations, along with the data of Kohl and Rice (1999) that show an increase in nonlinearity of sorption isotherm for extracted samples, indicate that a partitioning mechanism alone cannot be considered as the mechanism of sorption.

The implicit-adsorbate model offers a better explanation of the possible role of lipids in the sorption of hydrophobic organic contaminants in natural systems. The extracted lipids can be the unspecified organic substances referred to as the "implicit adsorbate" by Curl and Keoleian (1984). The increase in both adsorptive capacity and sorption exothermicity as the lipids are removed is consistent with the implicit-adsorbate model. According to this model, the partition coefficient determined in the presence of the implicit adsorbate is always less than the true partition coefficient, which is determined in the absence of the implicit adsorbate. In our case this corresponds to the observation that the partition coefficient for the whole samples is less than for the extracted ones. According to the "implicit adsorbate" model, competitive sorption between the incoming and the implicit adsorbate will determine less exothermic or even endothermic processes. This is consistent with the increase in exothermicity we observed for the extracted samples compared with the whole ones.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study provides evidence that lipids present in NOM can compete with PAHs for specific sorption sites. The increase in sorption capacity, the increase in exothermicity, and a possible change in sorption mechanism suggest that the removal of lipids frees sites that possess relatively high energy. Those sorption sites would be accessible to an incoming PAH molecule. The result would be a more specific interaction between the PAHs and the geosorbent NOM.

The sorption model that best fits this competition between lipids and PAHs for the same sorption sites is referred to as the "implicit-adsorbate" model (Curl and Keoleian, 1984). Lipids may be identified as an implicit-adsorbate in geosorbent NOM based on the experimental data presented in this study.


   ACKNOWLEDGMENTS
 
This project has been supported by the Office of Naval Research Harbor Processes Program under Grant no. N00014-99-1-0587. It has not been subjected to the agency's peer and administrative review and therefore may not necessarily reflect the views of the agency. No official endorsement should be inferred.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 J. Environ. Qual.Home page
C. Lattao, J. Birdwell, J. J. Wang, and R. L. Cook
Studying Organic Matter Molecular Assemblage within a Whole Organic Soil by Nuclear Magnetic Resonance
J. Environ. Qual., June 23, 2008; 37(4): 1501 - 1509.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Agricola
Right arrow Articles by Chilom, G.
Right arrow Articles by Rice, J. A.
Related Collections
Right arrow Soil Organic Matter
Right arrow Batch Studies
Right arrow Volatile Organic Compounds
Right arrow Geochemical Processes
Right arrow Organic Compounds

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome