CLD soil KKD soil
Chapter 10 Summary 10.1 Summary of the results
The effects of soil heterogeneity on the transport of pollutants in the groundwater can be described by a set of distributed mass-transfer coefficients. A numerical model was developed in which the groundwater aquifer was divided into a mobile phase and several immobile, or stagnant, phases. The movement of pollutants is governed by advection and dispersion in the mobile phase, and by mass transfer between mobile and immobile phases.
The distributed mass-transfer coefficient approach is able to model the transport of pollutants where there is heterogeneous soil texture and mass-transfer limited partition kinetics. The experimental and simulation results also 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.
Each component of the chemically heterogeneous soil exhibits a unique sorption behavior toward organic sorbates; and therefore influences greatly the fate and the remediation efficiency of organic pollutants. Humin, instead of composite soil organic matter or mineral complex, was used to investigate the possibility of irreversible sorption under both dry and humid conditions. The gravimetric and spectroscopic methods were applied to observe the sorption behaviors of organic compounds to humin and clay mineral via compressed either a disk form or a thin film. A FTIR experimental method allowed us to observe the sorption kinetics of VOCs on these soil components in either a system near the natural humidity or a dehydrated system on a short time scale. Furthermore, a new approach to use the molecular simulation method
to predict the sorption kinetics results, i.e. the diffusivities, was validated in this research. Overall, the experimental and theoretical findings of this research indicate that sorption processes of VOCs in humin and clays are diffusion-control and micropore-diffusion-control, respectively.
The sorption kinetics of toluene in pressed humin disks was investigated by tracking the weight change of the disks with a microbalance. For the apparent diffusivity of toluene in the disks, it ranged from 10-8 to 10-9 cm2/s and increased with temperature. And desorption diffusivity was about one-half that of sorption. No significant permanent residue of sorbed molecules was observed with the FTIR method on a thin humin film under either low or high-humidity conditions. These results are consistent with the gravimetric method, indicating that the sorption of toluene in humin is a reversible process and mainly diffusion-controlled.
Comparing the sorption kinetics of two hydrocarbons with different polarity, hexane and acetone, in humin, it was found that the apparent diffusivity of acetone and hexane in the disks ranged from 10-8 to 10-10 cm2/s. The lowest diffusivity of acetone may result from the high affinity of acetone with humin. From the differences in distribution coefficients for these given solutes, it reflects the higher hydrophilicity in humin. Moreover, higher sorbing capacity for more polar VOCs as well as C13-NMR data indicates that humin was more hydrophilic than Aldrich humic acid. The retention of acetone and hexane in humin after thorough purging with VOC-free nitrogen gas was observed. On the completion of the desorption experiments, there were approximately 35% and 20% sorbate residue for acetone and hexane, respectively. Humin could be a controlling soil component for the sequestration process of organic contaminants except the monoaromatic compounds, i.e. toluene. Based on the above results and results presented in other literature (Chang et al., 1997; Piatt and Brusseau, 1998.), it may be concluded that the sorption kinetics for VOCs on natural humic substances with a time scale between a few minutes to days is controlled primarily by mass transfer in polymeric humic structures.
For soil inorganic matter, one expanding clay mineral, montmorillonite, which exhibits a unique sorption behavior toward organic sorbates was investigated by tracking the intensity change with a FTIR/thin film method to study the sorption kinetics of toluene in dry and humid conditions. Two cation-exchanged forms of montmorillonite were used to identify the sorption mechanism of toluene in
Ca-montmorillonite. For the sorption under humid conditions, similar toluene sorbed intensities were found on Ca -and Cu– montmorillonites; however, higher intensity of toluene was showed on Cu-form under dry conditions than under humid condition. The sorption capacity of Ca-form is lower than those of Cu-form under a dehydrated system.
This result indicate that the sorption process of toluene to Ca-montmorillonite could be different from that of toluene/Cu-from system. Among the first-order- rate constants of toluene on these two montmorillonites, the sorption and desorption rates of toluene on Cu-form under dry conditions are the lowest. Furthermore, the peak shift in the spectrum and the color change of Cu-montmorillonite may be resulted from the chemisorption of toluene on clay. There is no similar observation for Ca-montmorillonite. Consequently, there seems to be no chemical interactions between the toluene molecules and Ca-montmorillonite according to the IR spectra.
There may be micropore diffusion control in these two montmorillonites; in addition, the single electron transfer chemisorption occurred in Cu-montmorillonite.
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 the thin film/FTIR method. So these two humic substances, humic acid and humin, and montmorillonite do not contribute to the sequestration process of TCE in soils.
Molecular dynamic simulation was used to solve the problem of sorption kinetics of organic pollutants in humic substances. Toluene, a main contaminant in soil/groundwater pollutant sites, and humic acid, a main soil organic matter, are selected to validate this method by comparing to real experimental data. The computer simulated results of the sorption kinetics and thermodynamics of toluene in humic acid are in good agreement with the experimental data. This method helps us to get useful information without the risk of applying the toxic contaminants and producing a lot of contaminants after the experiments. Moreover, our studies have shown that the molecular dynamics simulation of volatile organic compounds in humic substance is able to yield meaningful result.
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.
References
Achtnich, C., E. Fernandes, J. Bollag, H. Knackmuss, and H. Lenke. 1999. Covalent binding of reduced metabolites of [15N3]TNT to soil organic matter during a bioremediation process analyzed by 15N NMR spectroscopy. Environ Sci Technol 33:4448-4456.
Almendros, G., M. E. Guadalix, F. J. Gonzalez-vila, and F. Martin. 1996. Preservation of aliphatic macromolecules in soil humins. Org. Geochem. 24:651-659.
Aochi, Y. O., and W. J. Farmer. 1995. Spectroscopic evidence for the rate-limited accumulation of a persistent fraction of 1,2-dichloroethane sorbed onto clay minerals. Environ. Sci. Technol. 29:1760-1765.
Aochi, Y. O., and W. J. Farmer. 1997. Role of microstructural properties in the time-dependent sorption/desorption behavior of 1,2-dicholoroethane on humic substances. Environ. Sci. Technol. 31:2520-2526.
Aochi, Y. O., W. J. Farmer, and B. L. Sawhney. 1992. In situ investigation of 1,2-dibromoethane sorption/desorption processes on clay mineral surfaces by diffuse reflectance infrared spectroscopy. Environ. Sci. Technol. 26: 329-335.
Archa, M., A. P. Jackman, and B. J. Mccoy. 1996. Adsorption kinetics of toluene on soil agglomerates: Soil as a biporous sorbent. Environ. Sci. Technol. 30, 1500-1507.
Ball, W. P., and P. V. Roberts. 1991. Long-term sorption of halogenated organic-chemicals by aquifer material. 2. intraparticle diffusion. Environ. Sci.
Technol. 25,1237.
Ball, W. P., C. H. Buehler, T. C. Harmon, D. M. Mackay, and R. V. Roberts. 1990.
Characterization of sandy aquifer material at the grain scale. J. Contam. Hydrol.
5:253-295.
Bedient, P. B., and H. S. Rifai. 1994. Groundwater contamination: transport and
remediation. Englewood Cliffs, N. J. :PTR Prentice Hall.
Boyd, S. A., M. M. Mortland, and C. T. Chiou. 1988. Sorption characteristics of organic compounds on hexadecyltrimethylammonium-smetite. Soil Sci. Soc. Am. J.
52:652-657.
Brusseau, M. L., and P. S. C. Rao. 1989. The influence of sorbate-organic matter interactions on sorption nonequilibrium. Chemosphere. 18, 1691-1706.
Brusseau, M. L., R. E. Jessup, and P. S. C. Rao. 1991. Nonequilibrium sorption of organic chemicals: elucidation of rate-limiting processes. Environ. Sci. Technol.
25:134-142.
Chang, G. P. 1982. The study on solvent diffusion coefficient in the drying of coated
film of polymer solution. Thesis, National Taiwan University, Taipei, Taiwan.
Chang, M. 1998. Sorption and desorption kinetics of volatile organic vapor in soil, Dissertation, National Taiwan University, Taipei, Taiwan.
Chang, M., S. Wu, and C. Chen. 1997. Diffusion of volatile organic compounds in pressed humic acid disks. Environ. Sci. Technol. 31:2307-2312.
Chefetz, B., A., Deshmukh, P. G. Hatcher, and E. A. Guthrie. 2000. Pyrene sorption by natural organic matter. Environ. Sci. Technol. 34:2925-2930.
Cheng, P. 1999. The mechanisms of slow sorption of volatile organic compounds on
clay minerals. Thesis, National Taiwan University, Taipei, Taiwan.
Cheshire, M. V. 1979. Nature and origin of carbohydrates in soils. Academic Press, New York.
Chiou, C. T. 1998. Soil sorption of organic pollutants and pesticides. p. 4517-4554. In R. A. Meyers (ed.) Encyclopedia of environmental analysis and remediation. John Wiley & Sons. New York.
Chiou, C. T., and D. E. Kile. 1994. Effects of polar and nonpolar groups on the solubility of organic compounds in soil organic matter. Environ. Sci. Technol.
28:1139-1144.
Chiou, C. T., and D. E. Kile. 1998. Deviations from sorption linearity on soils of polar and nonpolar organic compounds at low relative concentrations. Environ. Sci.
Technol. 32: 338-343.
Chiou, C. T., D. E. Kile, and R. L. Malcolm. 1988. Sorption of vapors of some organic liquids on soil humic acid and its relation to partitioning of organic compounds in soil organic matter. Environ. Sci. Technol. 22:298-303.
Chiou, C. T., D. E. Kile, D.W. Rutherford, G. Sheng, and S. A. Boyd. 2000. Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fractions: potential sources of the sorption nonlinearity. Environ. Sci.
Technol. 34:1254-1258.
Chiou, C. T., J. Lee, and S. A. Boyd. 1990. The surface area of soil organic matter.
Environ. Sci. Technol. 24:1164-1166.
Chiou, C. T., P. E. Porter, and D. W. Schmedding. 1983. Partition equilibria of nonionic organic compounds between soil organic matter and water. Environ Sci
Technol 17:227-231.
Chiou, C. T., S. E. Mcgroddy, and D. E. Kile. 1998. Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments. Environ Sci Technol 32:264-269.
Crank J, Park GS. 1968. Diffusion in Polymers. Academic Press, London, UK.