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Journal of Environmental Quality 31:970-978 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Vadose Zone Processes and Chemical Transport

Sorption Kinetics of Toluene in Humin under Two Different Levels of Relative Humidity

Yang-hsin Shih* and Shian-chee Wu

Graduate Institute of Environmental Engineering, National Taiwan University, No. 71, Chou Shan Road, Taipei 106, Taiwan R.O.C

* Corresponding author (d7541007{at}ms.cc.ntu.edu.tw)

Received for publication June 11, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULT AND DISCUSSION
 CONCLUSION
 REFERENCES
 
To identify any resistant fraction for desorption of toluene from humin and to quantify the sorption–desorption rates, the time courses of toluene sorption to compressed humin disks and to a thin humin film were investigated. The apparent diffusivity of toluene with humin disks ranges from 10-8 to 10-9 cm2/s and increases with temperature, based on the weight change of humin disks mounted on a microbalance and on the results simulated by use of a diffusion model. No detectable level of residual toluene was found after desorption, as revealed either by the gravimetric analysis or by the Fourier transform infrared (FTIR) spectrum obtained at either low or high humidity. The time scale for intrinsic vapor sorption without mass transfer hindrance is less than a few minutes with the thin film. All the results indicate that the sorption of toluene to humin is reversible and mainly diffusion controlled. This finding helps to better understand the sorption kinetics associated with humin and soil organic matter.

Abbreviations: FTIR, Fourier transform infrared • IR, infrared • NMR, nuclear magnetic resonance • SEM, scanning electron microscopy • SOM, soil organic matter • VOC, volatile organic compound


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULT AND DISCUSSION
 CONCLUSION
 REFERENCES
 
THE RATE OF sorption–desorption of volatile organic compounds (VOCs) plays an important role in pollutant fate modeling and contaminated soil remediation. Irreversible VOC sorption to or slow VOC desorption from soils in laboratory studies or field scales has been reported (Aochi and Farmer, 1995; Pignatello and Xing, 1996). It is found that a portion of VOCs desorbs very slowly from soil particles and a certain fraction of it is retained strongly by the particles (Steinberg et al., 1987; Pignatello, 1990). The retention of VOCs by soil components affects the fate of pollutants in the environment and the effectiveness of contaminated soil and aquifer remediation. Delineation of the sorption rate and of the reversibility of the sorption process with essential soil components is necessary to better predict the rates of pollutant migration and attenuation in soil.

Soil, a chemically heterogeneous matrix, contains various inorganic and organic components that each exhibit unique sorption behavior for pollutants (Chiou, 1998). The chemical heterogeneity complicates the prediction of the sorption or desorption rates of pollutants in the soil. Recent studies reveal the existence of a slow sorption–desorption of some organic compounds with soils (Luthy et al., 1997; Pignatello and Xing, 1996). This phenomenon has been attributed in part to a slow diffusion of the compounds through the micropores of soil particles (Farrell and Reinhard, 1994; Lin et al., 1994) and in part to their slow migration through soil organic matter (SOM) (Brusseau et al., 1991; Fu et al., 1994).

Sorption to humic substances is contended to contribute to the irreversible retention of xenobiotic compounds by soil (Cheshire, 1979). However, the sorption of toluene to humic acid, an integral member of a soil humic substance, is found to be reversible and diffusion controlled (Chang et al., 1997). Since humin represents a highly stable, recalcitrant, and high-molecular-weight fraction of SOM (Almendros et al., 1996; Hatcher et al., 1985), it may behave differently than the humic acid in the kinetics of sorption. Due to its cross-linked structure, humin may be capable of retaining VOCs for a significantly longer time and thus contribute to the slow or apparent irreversible sorption.

Although the sorption of nonpolar solutes by humin shows a slightly nonlinear effect, due to the existence of a small amount of high-surface-area carbonaceous material (Chiou et al., 2000), the soil organic matter, including humin, behaves by and large like a partition medium for VOCs (Boyd et al., 1988; Chiou et al., 1988, 1990; Chiou and Kile, 1994; Chiou, 1998). The amorphous humic structure provides a "solvent-like" medium that organic molecules can enter into or escape from according to the thermodynamic gradient.

Humin is defined as the portion of humic materials that is insoluble in an aqueous solution at any pH. To be separated from the inorganic minerals in soils, humin is obtained by extensive digestion of soils with a mixture of concentrated HF and HCl (Stevenson, 1982). The HF–HCl extraction method has been widely applied for humin preparation (Chefetz et al., 2000; Grasset and Ambles, 1998; Guthrie et al., 1999; Lichtfouse et al., 1998a,b), while the methyl isobutyl ketone (MIBK) method, as suggested by Rice and MacCarthy (1990), has also been applied for extraction of humin. In the MIBK method, humic substances are allowed to partition between water and MIBK as a function of the pH in the water phase. Humin is then isolated from fulvic acid and humic acid. However, this method is so selective that some humin component could not be retrieved by MIBK. To meet the purpose of our experiment on the sorption–desorption rate of toluene with near-natural humin, the solvent extraction was not adopted in order to prevent a significant alteration of the humic composition.

A spectroscopic approach has been adopted to address the interaction between VOCs and humic substances by Aochi and Farmer (1997). The authors investigated the sorption–desorption behavior of 1,2-dichloroethane on humic acid and fulvic acid under dry conditions. They found that an absorbance band increases continuously after days of desorption and the sorbed chemical was strongly retained. However, the sorption kinetics of VOCs on humin in a system resembling the natural level of humidity on a short time scale (say, minutes) has not been investigated. In this study, toluene, a model nonpolar, mononuclear hydrocarbon, was used as the sorbate to delineate the sorption behavior of VOCs with humin. Humin disks were prepared and used to study the rate of transport of toluene in a humin matrix with artificially exaggerated mass transfer distance. Thin humin films were also used to mimic natural humin in a near-natural soil environment.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULT AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Soil Sample
Approximately 10 kg of Yamingshan soil, classified as medial, thermic, Pachic Melanudands according to the definition of the USDA (USDA Natural Resources Conservation Service, 1993) were air-dried, freed of large plant debris, and screened through a 20-mesh (0.84 mm) sieve. Small plant debris was further removed by flotation using ethanol. Then, the soil sample was mixed well.

Humin
Humin was extracted according to the procedure developed by Rigol et al. (1998) and Russell et al. (1983), except that an ethanol–hexane mixture (1:1 v/v) was used to remove fats and waxes to avoid the interference from toluene, which is the target VOC to be studied. In short, soil was refluxed to remove fats and waxes and extracted by sodium hydroxide to remove humic acids and fulvic acids. The solid residue was separated by centrifugation, neutralized with 6 M HCl, washed with 0.1 M HCl and deionized water, and finally freeze-dried. The humin fraction was isolated from the solid residue by sequentially removing the mineral matter with a three-step digestion procedure: first, suspension in a 1:1 mixture of 0.2 M HF and 0.2 M HCl (20 mL/g) for 64 h; subsequently, digestion in a 1:1 HF (5.5 M) and HCl (1.1 M) mixture for 1 h three times; and, finally, digestion in 5.5 M HF four times for 16 h each time. After centrifuging and washing with 0.1 M HCl and water three times, the final residue designated as humin was freeze-dried and ready for use.

A previous investigation by IR spectroscopy (Rigol et al., 1998) suggested that the treatment of soils with HF decreases the structural mineral matter content with relatively little influence on the nature of humin. Despite the challenges presented by modification, humin obtained by the HF-extraction procedure was used for some sorption experiments (Chefetz et al., 2000; Guthrie et al., 1999; Rigol et al., 1998). The 13C nuclear magnetic resonance (NMR) spectrum (not presented) of the solid residue after removal of waxes, humic and fulvic acids, and treatment by weak acids before severe HF–HCl extraction, is the same as that of the humin product after the severe HF–HCl treatment. The severe treatment procedure removed most of the soil inorganic components while the humin components were preserved.

Sample Characterization
Element Analysis
Elements such as C, H, and N of humin were quantified in triplicate samples using a PerkinElmer (Wellesley, MA) CHN-2400 element analyzer. Inorganic carbon was removed according to Ball et al. (1990). To determine the contents of major elements such as Fe, Al, Si, and Ca, samples were pretreated by fusion with LiBO2 at 1000°C for 30 min. The product was dissolved in 0.9 M HNO3 and diluted to 0.3 M HNO3. The major elements in the solution were quantified in triplicates by inductively coupled plasma optical emission spectroscopy (ICP–OES) (Ingamells, 1970; Rigol et al., 1998).

Solid-State Carbon-13 Nuclear Magnetic Resonance Spectrometry
The cross polarization–magic angle spinning (CP–MAS) 13C spectra of samples were measured on a Bruker (Rheinstetten, Germany) DSX400WB NMR spectrometer with a 7-mm-diameter probe. The spinning rate was 7000 Hz. The acquisition parameters included contact time of 1 ms, pulse delay of 1 s, and pulse width of 4.2 µs.

The 13C NMR spectra were analyzed according to the chemical-shift assignments made by Perminova et al. (1999) and Chefetz et al. (2000): 5 to 50 ppm, aliphatic H- and C-substituted C atoms; 50 to 108 ppm, aliphatic O-substituted C atoms; 108 to 145 ppm, aromatic H- and C-substituted C atoms; 145 to 163 ppm, aromatic O-substituted atoms; 163 to 190 ppm, C atoms of carboxylic, esteric, and amide groups. The distribution of C in each structural group was calculated as the percentage to the total carbon. The region between 5 to 108 ppm was calculated as aliphatic C and 108 to 163 ppm as aromatic C. The total aromaticity was calculated by expressing the aromatic C as a percentage of the sum of aliphatic and aromatic C; the total aliphaticity was calculated as the percentage of aliphatic C to the sum of aliphatic and aromatic C (Hatcher et al., 1981, 1983). The regions 50 to 108 ppm and 145 to 190 ppm were calculated as O- or N-substituted C atoms. The polarity was assigned based on the percentage of the sum of O- and N- substituted C atoms.

Preparation of Humin Disks
Humin disks were prepared by pressing the humin powder under a pressure of 12.7 N/m2 for 1.5 min (Chang et al., 1997). Four disks, all being 12.45 mm in diameter, were 0.34, 0.44, 0.61, and 0.64 mm in thickness, and weighed 61.5, 72.8, 100.2, and 102.5 mg, respectively. Their bulk densities were 1.49, 1.36, 1.35, and 1.32 g/cm3, respectively. The disks were oven-dried (105°C) overnight and stored in a desiccator before use.

The scanning electron microscopy (SEM) photographs were taken with a Hitachi (Tokyo, Japan) S-800 SEM. Figure 1 shows the SEM photographs of one of the disks prepared by the above-mentioned procedure. The surface morphology (Fig. 1a) and the exposed inner surface of a broken edge with only a few pores and cracks display the homogeneity of the disk.



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  		Fig. 1. (a) Surface and (b) cross-section of a broken edge of a humin disk observed by scanning electron microscopy (SEM).

 
Sorption–Desorption Experiment
Gravimetric Method
The apparatus and procedure used for sorption have been described elsewhere (Chang et al., 1997). Briefly, the experimental apparatus was maintained in a thermostatic room at 15 ± 0.1°C, 25 ± 0.1°C, or 35 ± 0.1°C. The set temperature was closely monitored for at least one day to ensure its stability before initiation of the experiment. The disk was hung on the sample side of a Cahn (Madison, WI) 200 electric microbalance enclosed in a glass chamber. The toluene mass flux to the disk was significantly larger than the maximum toluene removal rate, which keeps the vapor concentrations inside the chamber at virtually fixed levels. The experiment was terminated when the change of weight could not be distinguished from the base noise of the microbalance, which was about 2 µg/5 h. The concentration of the toluene was determined with a Hewlett-Packard (Palo Alto, CA) 5890II gas chromatography–flame ionization detector (GC–FID).

Sorption–Desorption Experiments Traced by Fourier Transform Infrared
A drop of the humin suspension in water was placed on the inner surface of a ZnSe window of a gas cell and dried in a desiccator. The absorbance spectra of an IR beam passing through the gas cell windows were recorded on a Bio-Rad (Hercules, CA) FTS 40 IR spectrometer by averaging 16 scans at 2 cm-1 resolutions. The sample cell was purged with nitrogen gas carrying a constant toluene vapor concentration and relative humidity (RH below 1% for dry conditions and above 95% for humid conditions) during sorption experiments and purged with nitrogen gas without toluene during desorption experiments.

Estimating Diffusivity
The diffusion model and its incorporation into the gravimetric method has been described in detail by Chang et al. (1997). The model development is briefly summarized here. The one-dimensional mass conservation equation is:

[1]

[2]
where q (mg/cm3) is the sorbate concentration in the disk at a distance x from the center plane of the disk and at time t; D is the apparent diffusivity of the sorbate inside the disk; l is the half-thickness of the disk; M(t) is the total sorbed mass of sorbate in the disk; and S is the surface area of the disk.

Given the initial and boundary conditions:

[3]

[4]

[5]

The analytical solution for the fraction of equilibration is available in Crank and Park (1968)(Eq. [36] and [37] in Chapter 1) and can be expressed as:

[6]
where Mt is the sorbed mass and f(t) is the dimensionless solution that is zero at t = 0 and unity when t approaches infinity.

For the desorption process, the initial and boundary conditions are:

[7]

[8]
and Eq. 5. The analytical solution during desorption is:

[9]

The diffusivity can be estimated by the best fit of the experimental results using the least-squares method.


   RESULT AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULT AND DISCUSSION
CONCLUSION
 REFERENCES
 
Characteristics of Humin
The humin fraction contributes to 6.84% of the weight of the original soil and 15.9% of the weight of the total organic fraction (Table 1). The contents of the major elements, such as C, H, O, and N, in humin are shown in Table 2. The low atomic H to C ratio (1.08) indicates that a large fraction of the organic matter contains aromatic carbons. However, there is still a significant amount of aliphatic carbons according to the 13C NMR results (Fig. 2) .


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  		Table 1. Relative amount of soil components.

 

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  		Table 2. Relative content of the major elements in humin.

 


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  		Fig. 2. Solid-state 13C nuclear magnetic resonance (NMR) spectrum of humin.

 
Table 2 shows the amounts of Si, Al, and Fe in the humin fractions determined by inductively coupled plasma optical emission spectroscopy. No noticeable amount of crystalline minerals can be detected through X-ray diffraction (XRD) observation. A small amount of these metals must be in amorphous form. The low inorganic content of the humin plus its low sorbing power for toluene did not significantly affect the sorption experiments.

The 13C NMR spectrum of humin (Fig. 2) revealed the following composition: 50.2% aliphatic moieties (32 ppm), 32.5% carbohydrates (73 and 103 ppm), 14.4% nonpolar aromatic compounds (128 ppm), and 2.9% amide or carboxylic compounds (172 ppm), similar to the observation made by Chefetz et al. (2000). The total aromaticity of humin, 15.1%, close to 8.8% reported by Chefetz et al. (2000), is different from that of Aldrich humic acid (66.7%) (Perminova et al., 1999). The major peaks and distribution of C-containing contents were similar to those determined from the reported humin spectrum of Chefetz et al. (2000) but different from those for humic acid (Perminova et al., 1999).

Sorption Experiments
The experimental conditions and the results of seven sorption experiments, including two sets of duplicates of the same disks (A1 and A2 and B1 and B2, respectively), are shown in Table 3. The sorption of toluene on the humin disk took about 50 h to reach steady state under 15°C (Fig. 3a) . However, it took about twice that time for complete desorption (Fig. 3a). Similar slower desorption processes were observed at 25 and 35°C as well (Fig. 3b and 3c, respectively). The sorption process seemed reversible. There was no observable residual toluene remaining in the humin disks after desorption.


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  		Table 3. Conditions and results of sorption and desorption experiments by the gravimetric method.

 


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  		Fig. 3. Experimental results (squares and circles) and model best fittings (solid lines) of toluene sorption and desorption at different temperatures: (a) sorption and desorption at 15°C for Disk A, (b) sorption and desorption at 25°C for Disk B, and (c) sorption desorption at 35°C for Disk C. A1 and A2 and B1 and B2, respectively, are the results of duplicated experiments only with different volatile organic compound (VOC) concentrations.

 
We define the SOM–gas distribution coefficient [Kd, (mg/g)/(mg/L gas)] between the solid phase and the gaseous phase as:

[10]
where Cg is the VOC concentration (mg/L) in the gaseous phase and {rho} is the bulk density of the disk (g/cm3). The observed Kd decreases with the temperature in the partial pressure (P/P0) range from 0.029 to 0.054 (Table 3).

Whether the toluene sorption to humin is a physical or a chemical process can be elucidated via the measured enthalpy change (H). Applying the van't Hoff equation, the relationship between the change of enthalpy and Kd can be quantified as:

[11]
where is the ideal gas constant, T is the absolute temperature (K), and Kd is the equilibrium SOM–gas distribution coefficient.

The plot of ln Kd versus 1/T is shown in Fig. 4 . Assuming that H is constant over the studied temperature range, a relatively low negative value of enthalpy change, -9.0 kcal/mol, is obtained, which implies that the sorption process is a physical, exothermic process. The enthalpy change from the subcooled VOC to the sorbed VOC is calculated to be 1.5 kcal/mol by subtracting the enthalpy of condensation (-10.5 kcal/mol). This small and positive net enthalpy is illustrative of a partition process. There appears to be no chemical bond formed or broken during sorption and desorption.



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  		Fig. 4. (Top graph) Plot of ln Kd versus 1/T (K-1) for toluene sorption, and (bottom graph) plot of ln Ds and ln Dd versus 1/T (K-1) for toluene sorption and desorption at temperatures ranging from 15 to 35°C.

 
We may also estimate the value of Kd by using the following equation:

[12]

where Koc is the soil organic carbon–water partition coefficient, foc is the organic carbon fraction, and KH is the Henry's Law constant. Estimates of Koc were obtained from the relationships between log Koc and log Kow established by Karickhoff et al. (1979) and Chiou et al. (1983). The Kd values at other temperatures can be calculated by correcting the temperature effect according to Schwarzenbach et al. (1993). The experimental Kd values fall between those predicted by Karichoff's and Chiou's methods [i.e., 0.38 and 0.098 (mg/g)/(mg/L gas)] at 25°C, respectively, in all temperature ranges. The results indicate that the sorption of toluene in humin is a partition process and the distribution coefficient can be estimated from the Henry's Law constant and the organic carbon–water partition coefficient.

Diffusion inside Humin Disks
The rate of the penetration of VOC is quantified by the diffusivity in the humin matrix. Figure 3 shows the time courses of sorption and desorption and the simulation of the weight changes by using a diffusion model. The average of the three best-fitting diffusivities of toluene at 25°C is 7.0 x 10-9 cm2/s. The value is on the same order of the diffusivity of toluene in humic acid (Chang et al., 1997; Piatt and Brusseau, 1998) but lower than the diffusivity of the compound in water by a factor of 1000. According to the 13C NMR spectra, humin is not the same as humic acid. However, toluene molecules in the cross-linked network of humin have a diffusivity similar to that in humic acid. Diffusivity is 7.25 x 10-9 and 3.71 x 10-9 cm2/s for propane into two different olefinic matrix ionomers at 25°C (Del Nobile et al., 1995). However, the diffusivity is only 1.8 x 10-7 cm2/s for toluene in butyl rubber at 30°C (Schneider et al., 1994), and 5.2 x 10-7 and 1.7 x 10-7 cm2/s for benzene and o-xylene in natural rubber at 25°C (Guo et al., 1995). Thus, the value of the diffusion coefficient in the humin disks is closer to that of VOC in polymers rather than that of VOC in a liquid.

The value of the diffusivity in humin is far less than that in the air. Pore diffusion has been suggested by researchers to be the rate-controlling mechanism of sorption in soil aggregates. If pore diffusion is the major mechanism of the transport of VOC in humin, then the diffusivity of gases in porous medium will be reduced by sorption and matrix tortuosity. The expression of the effective intra-aggregate diffusivity in gaseous phase is similar to the effective diffusivity in water (Schwarzenbach et al., 1993), which is:

[13]
where {phi} is the porosity, f is tortuosity, {rho}s is true density of solid sorbent, Da is the diffusivity of the sorbate in air, R is the retardation factor, and Kd is the distribution coefficient of the sorbate in the sorbent. Based on the observation of SEM photos of the humin disk, we could not identify any porosity and therefore ignored the porosity term. The diffusion coefficient of toluene in air is 0.086 cm2/s (Gilliland, 1934) and the retardation factor [R = Kd(1 - {phi}){rho}s + {phi}] of humin is 420. The diffusivity of toluene molecules in this presumably porous organic matter is 2.07 x 10-4 cm2/s. The experimental diffusivity is lower than this estimate by five orders of magnitude. Therefore, diffusion in humin is more likely associated with a polymeric network than with a porous solid matrix. If a tortuosity factor were introduced to quantify the extra length that the sorbate molecules have to travel through, the tortuosity would be 3.38 x 10-5, which also indicates that the migration path of the sorbate is far less a straight path but is rather like narrow curving interstices through tightly woven fiber.

Thermodynamics of Diffusion
The sorption and desorption diffusivities of toluene in humin increase with temperature (Table 3). There were two sets of diffusivity results during desorption lost due to accidental power failure. A higher temperature raises the rate of diffusion because it provides more energy to facilitate the vibration of polymer segments and helps to mobilize VOC molecules. Hence, toluene molecules can surmount the activation energy barrier more easily when they are squeezing through the macromolecular matrix.

This effect of temperature on diffusivity can be described by the Arrhenius equation (Crank and Park, 1968, Chapter 2):

[14]
where D0 is the diffusivity of the reference state and E is the activation energy of diffusion. By plotting ln D versus 1/T (Fig. 4), the activation energy for sorption and desorption are found to be 19.4 and 22.2 kcal/mol, respectively. The values are slightly higher but at the same order of magnitude when compared with the activation energy of toluene for sorption (10.1 kcal/mol) and for desorption (15.7 kcal/mol) into humic acid at temperatures ranging from 25 to 45°C (Chang et al., 1997). The diffusion activation energy is 11.1 kcal/mol for propane into rubber at 23 to 45°C (Michaels and Bixler, 1991) and 4.8 kcal/mol for toluene into butyl rubber (Vahdat, 1991). A certain level of activation energy is needed to form an interstitial space or free volume for penetrants (Rogers, 1985) and involves intermolecular and intramolecular interactive forces, which depend on the dimensions of the penetrant and the structure of the polymer molecules (Crank and Park, 1968). This result indicates that humin has tighter cross-linkage or is more rigid than some polymers and humic acid.

Sorption in humin seems to be reversible. However, the desorption rate is slower than the sorption rate by a factor of 2. The activation energy of toluene desorption is slightly higher than that of sorption. The higher activation energy of desorption suggests that a new stable toluene–humin structure may have been formed during the sorption–equilibration period.

Sorption–Desorption Traced by Fourier Transform Infrared
During the triplicated thin-film sorption experiments, the thickness of the sorbent film was only roughly 5 µm. In fact, the film was composed of very small humin aggregates, whose median size was 12 µm, as measured with a Cilas (Marcoussis, France) Model 715 laser granulometer. The mass transfer distance through the sorbent layer was significantly reduced. The change in intensity of IR absorbance at 729 cm-1 showed that the sorption process for the thin-film system under dry conditions (relative humidity < 1%) had a time scale of a few minutes and was reversible (Fig. 5) . The observation of the reversibility was consistent with the aforementioned gravimetric experimental results. The 13C NMR spectra of Guthrie et al. (1999) suggested noncolvalent interaction between an aromatic compound (pyrene) and humin. These data support our hypothesis that there is no observable molecular bonding between toluene and humin.



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  		Fig. 5. Relative absorbance intensity of sorbed toluene on humin varying with time for sorption (solid symbols) and desorption (empty symbols) at 729 cm-1 by Fourier transform infrared (FTIR) under dry (diamond) and humid (triangle) conditions.

 
Sorption of toluene by thin humin film under humid conditions (relative humidity > 95%) showed similar results to those for dry humin. However, the sorption and desorption of toluene were slightly slower under humid conditions than dry conditions. The reason might be the hindrance of the hydrated humin network to the vapor movement. Saturating the humic acid with water is known to lower the partitioning capacity of the humic acid for nonpolar organic compounds, indicating that the overall polarity of the hydrated humic acid is increased (Chiou et al., 1988). The polarity of humin estimated by the integration of the characteristic response of 13C NMR is 35.4%. As a result of the humin hydration, the humin molecular network might become more cross-linked due to the hydrogen-bond formation between polar functional groups and water molecules. As a consequence, the migration of toluene within the hydrated humin film was retarded. The IR spectra of toluene sorbed to the thin humin film under either dry or humid conditions were similar to those of liquid toluene, suggesting that there is no specific binding formed for toluene with either dry or hydrated humin.

Long and Thompson (1954) studied water-induced acceleration of the diffusion of organic vapors in polymers. They found that the sorption rates for nonpolar compound in hydrophobic polymer are slower under humid conditions than dry conditions in a water–benzene–polystyrene system. Quite evidently, water has no appreciable effect on the diffusion rate of organic vapors in polyolefins (Crank and Park, 1968, Chapter 8). Chang (1982) studied the diffusion coefficients of toluene in polymer films under different humid conditions. The diffusion coefficients in humid conditions are less than in dry conditions. Our observation is consistent with this result.

Piatt and Brusseau (1998) have estimated that the intraorganic matter diffusion coefficient in the aqueous phase for toluene in a soil-predominated humic acid is 3.8 x 10-9 cm2/s. This value is also slightly lower than what Chang et al. (1997) have measured in a dry condition. This result for humic acid is consistent with our observation of toluene in humin by the FTIR method. They also concluded that diffusion is highly restricted within the SOM matrix and the mass transfer coefficients are not significantly different for different SOMs. The similarity of the diffusivities of toluene in humic acid and humin in our study agrees with their conclusion (Piatt and Brusseau, 1998).


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULT AND DISCUSSION
 CONCLUSION
REFERENCES
 
The results indicate that the sorption of toluene to humin could be a slow process (in a time scale of hours) because of constrained mass transfer through humin molecular structures only if the humin mass is as thick as about a half-millimeter. There seems to be no strong interaction between toluene and humin molecules according to the IR spectra of the sorbed toluene in humin. Partitioning is believed to be the major sorption mechanism. Based on the results above and results presented in other literature (Chang et al., 1997), it may be concluded that the sorption kinetics with a time scale of few minutes to days for toluene on natural humic substances is controlled primarily by mass transfer in polymeric humic structures.


   ACKNOWLEDGMENTS
 
The authors would like to thank Dr. Meei-ling Chang of the National Institute of Environmental Analysis, Environmental Protection Agency of Taiwan, Republic of China (R.O.C.) and Dr. Cary T. Chiou of the U.S. Geological Survey for providing much valuable discussion. The authors gratefully acknowledge the financial support of the National Science Council of Taiwan, R.O.C. (Contracts NSC 89-2211-E-002-009 and NSC 89-2211-E-002-080).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULT AND DISCUSSION
 CONCLUSION
 REFERENCES
 





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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