The experimental conditions and the results of sorption experiments of kaolinite and humic acid are summarized in Table 9-2. It took about 4 hours for the sorption of toluene on kaolinite to reach steady state. The time for the sorption of toluene in humic acid to reach a steady state is longer than that of the sorption on kaolinite.
Sorption Kinetics on Clays. The sorption kinetics of toluene on kaolinite shows only
one step (Fig. 9-1). One step sorption for several organic sorbates on different clay minerals by the same gravimetric method was also observed by Morrissey and Grismer (1999).Although mineral surfaces as well as soil organic matter could contribute to slow sorption reported by Paviostathis and Mathavan (1992) and Pavlostathis and Jaglal (1991), the sorption of toluene on kaolinite took few hours to complete, which is faster than the sorption into humic acid. And the intrinsic sorption time of toluene on different cation-exchanged montmorillonite thin films was just a few minutes shown in Shih and Wu (2003).
Kaolinite, a common clay mineral in soils, provides large surface area for the sorbates to attach on. The surface adsorption on kaolinite reaches equilibrium more easily than the
Diffusion into Humic Substances. The sorption of toluene into humic acid powder took
about 15 hours, and showed a period of lag time at the beginning (Fig. 9-1). The penetration of VOCs is believed to be controlled by diffusion of VOC molecules in humic substances matrix (Chang et al., 1997; Shih and Wu, 200a2a and 2002b; Piatt and Brusseau, 1998). The average diffusivity of toluene into humin is 7.0×10-9 cm2/sec at 25 oC (Shih and Wu, 2002a).The value is on the same order for the diffusivity of toluene in humic acid (Chang et al., 1997;
Piatt and Brusseau, 1998). Diffusivities for toluene in humic and fulvic acids are in the order of 3.84×10-9 cm2/sec and 8.51×10-10 cm2/sec, respectively (Piatt and Brusseau, 1998).
By using the diffusivity of toluene in humic acid and the approximate diameter of humic acid powder, the time needed to diffusion into humic acid powder was around 13hours. This time period is very close to the diffusion time of toluene into humic acid after a lag period shown in Fig. 9-1. This result is coincident with the diffusion mechanism of VOCs into humic acid disks.
Sorption Process of Soil Samples. A two-stage sorption processes was observed in all
soil samples (Fig. 9-2 and 9-3). For soils with low contents of SOM, TC and CL soils, the first stage of the sorption process took up to one hour, and followed by a waiting time of 1.7 hours to continue onto the second stage. The second sorption process took longer time than the first stage. The similar sorption pattern was observed in TCD and CLD soils, the hydrogen peroxide treated TC and CL soils, respectively. The fractions of the sorbing capacity of the first stage of these soils are higher than 70%. It shows that the sorption on soil inorganic matter is the dominant mechanism in these two soils with low SOM content under dry conditions.Comparing to other soils, YM soil showed the least sorption percentage for the first stage.
After removing most of the SOM, the sorption percentage of the first stage increased significantly in YMD soil (Fig. 9-2). This enhancement could be the result of opening up the sites for surface adsorption after removing SOM.
The experimental conditions and the results of the sorption experiments were shown in Table 9-3. All experiments were performed under low toluene partial pressure. The relative saturation pressure (P/Po) of toluene ranges from 0.019 to 0.086 and the concentration ranges from 2.5 mg/L to 11.3 mg/L. In this concentration range, the distribution coefficient Kd (mg/g)/(mg/L-gas) between the solid phase and the gaseous phase is assumed constant and defined as
g e
d
C
K = q
(9-1)where q is the equilibrium sorbed amount (mg/g) and C is the toluene concentration
(mg/L).
Soil organic substances predominate the sorption capacity of soils under humid conditions. The relative abundance of SOM (represented by the organic carbon fraction) follows the order YM > KK > CL >TC. The distribution coefficients of these soils seem to follow the same trend; however, the Kd of CL, 0.67, is slightly higher than that of KK, 0.41.
The same pattern was observed in the hydrogen-peroxide-treated soils.
Sorption amount of organic contaminants in dry soils is controlled not only by the partition or sorption into SOM but also by the surface adsorption on the surface. The amount of organic contaminants adsorbed on the surface of soils can be estimated by the surface area of soils and the surface-based distribution constant.
By using two parameters, organic carbon fraction and surface area, distribution coefficient can be quantified via the following equation:
Kd = a foc + b SA (9-2) where foc (dimensionless) is the organic carbon fraction in soils, SA (m2/g) is the surface area of soils, and the a (L-gas/g-carbon) and b (L/ m2) are constants. These two constants were obtained by the multiple-variables regression of the distribution coefficients to the two soil properties, foc and SA, of these eight soils. So the a constant is 0.104 ± 0.0109 (L-gas/g-carbon) and b is 0.0320 ± 0.00221 (L/ m2). The good correlation was presented by the R-square, 0.96 and F-value, 60.3, respectively.
The comparison between the predicted distribution coefficients from equation (2) and the experimental values is shown in Fig. 9-4. It shows that we can estimate the distribution coefficients of neutral organic contaminants in soils by these two soil properties under a dry condition and a low pollutant concentration range. Two distribution parameters, the adsorption constant, Kds, and the partition coefficient, Kdc, can be extracted from Kd by this formula.
Two Sorption Processes. The surface adsorption rate of VOCs on the SIMs is faster
than that of the sorption of VOCs into humic substances (Fig. 9-1). Therefore, it is reasonable to suggest that the fast sorption process of soil samples is contributed mainly by the adsorption on mineral surfaces.The level of the first plateau between the two sorption stages is supposedly indicating as the first sorption fraction for the two soils with higher SOM (Fig. 9-2). And the second sorption fraction is the sorbed amount between the first plateau and the ultimate plateau.
The results are shown in Table 9-4. In soils with low SOM contents, the first sorption fraction dominates the total amount. The first sorption fraction is generally proportional to
the surface area while the second fraction increases with the increasing of SOM.
In the sorption equilibrium aspect, the first term in eq. (9-2), the contribution from surface adsorption, is denoted as Kds. The second term, the contribution from the soil organic matter, is deonoted as Kdc. These two fractions can be estimated from eq. (9-2) and are shown in Table 9-4. The surface adsorption fraction, Kds, is generally larger than the SOM sorption fraction, Kdc, except KK and YM soils due to their higher SOM content than others.