Modeling the influence of physical and chemical heterogeneity on the performance of soil remediation for organic contaminants
The distributed mass-transfer coefficient approach is used to model the transport of pollutants where there is heterogeneous soil texture and mass-transfer limited partition kinetics. The experimental and simulation results indicate that in a length scale larger than that of a laboratory soil column the phenomenon of non-equilibrium transport or tailing is resulted mainly from the mass-transfer limited migration of the sorbate into the stagnant region inside the immobile phases. The problem of modeling each of these immobile phases due to the lack of geological information can be improved by using a distributed mass-transfer coefficient set, which is related to some of the easily obtained soil properties such as the length scale of the system of concerned, the moisture content and the heterogeneity of the soil texture profile. Also the soil video imaging system can be used to identify and locate the layers with high hydraulic conductivity and layers with low hydraulic conductivity, or say the heterogeneity of the soil column with quite low cost and in short time, which will be a promising tool to help on the characterizing, modeling and remediation of a contaminated site. Three VOCs were used to study the interaction between humin and organic contaminants. Higher sorbing capacity of humin for more polar VOCs and the C13-NMR data of humin indicate that humin was more hydrophilic than Aldrich humic acid. The apparent diffusivity of acetone, toluene, and hexane in the disks ranged from 10-8 to 10-10 cm2/s. The sorbed toluene in humin does not seem persistent to desorption; however, acetone and hexane, either a polar or a linear compound, show persistence against desorption. On the completion of the desorption experiments, there were approximately 35% and 20% sorbate residue for acetone and hexane, respectively. The sorption kinetics of toluene in dry and humid clay films was investigated by tracking the change of the IR absorbance. Under humid condition, similar toluene sorbed intensities are found on Ca -and Cu– montmorillonites. However, higher intensities of toluene sorbed were found on Cu-form under dry condition, which indicates stronger interaction occurring. On Ca- and Cu-montmorillonite, some portion of toluene is desorbed at an extremely slow rate under dry conditions. Either some original toluene peaks or some new peaks are persistent against desorption from montmorillonites, also suggesting the existence of irreversibly sorbed species. Theremay be some transformation of VOCs in clay systems. Although the persistence was not observed under high humidity conditions by spectroscopic method, the clay minerals could be a controlling factor of slow desorption in soil. The sorption and desorption of trichloroethylene (TCE) in humic acid and humin disks was investigated by microbalance. The apparent diffusivity of TCE in these two humic substances was in the 10-8 to 10-9 cm2/s magnitude. There are no residual sorbed TCE observed via a microbalance. The intrinsic sorption/desorption time scale of TCE on two cation exchanged montmorillonites was only few minutes by thin film/FTIR method. Molecular dynamic simulations were also used to study the sorption of organic contaminants in soil organic matter. The simulation results of the sorption kinetics and thermodynamics of toluene in humic acid are in good agreement with the experimental data. We believe that this technique will become a powerful tool not only to facilitate the solving of the problems of contaminated soil clean-up but also to be applied to a wider range of environmental problems. After studying soil chemical heterogeneity, we found the intrinsic sorption is fast for VOCs into humin, humic acid, and montmorillonite. So they do not contribute to the sequestration process in soils. The mass transfer of contaminants into soil plays the important role on the slow sorption/desorption in soils.
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