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DIVISION S-1 - SOIL PHYSICS

Diffusion of Sorbing Organic Chemicals in the Liquid and Gaseous Phases of Repacked Soil

^a Dept. of Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b Dep. of Social and Environmental Engineering, Graduate School of Engineering, Hiroshima Univ., 1-4-1 Kagamiyama, Higashi-Hiroshima, 739, Japan
^c Soils and Biogeochemistry, Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616

* Corresponding Author (i5pm{at}civil.auc.dk)

Transport models for sorbing organic chemicals in soil require accurate predictions of the diffusion and sorption processes in both the liquid and gaseous phases. In this study, the ability of recently-presented diffusivity models in combination with equilibrium sorption models to predict the effective (i.e., including sorption effects) diffusion coefficient, D_eff, as a function of soil-water content, , is tested for different sorbing organic chemicals in different soils. The water-induced linear reduction (WLR) gas diffusivity model, combined with a two-component (hydrophobic and vapor) equilibrium sorption model, well described short-term (<24 h) D_eff() of two volatile organic compounds, toluene and trichloroethylene (TCE), in Yolo loam (fine-silty, thermic Typic Xerorthent). Further extended with the constant slope impedance factor (CSIF) solute diffusivity model, the resulting two-phase diffusion and two-component equilibrium sorption (DATES) model accurately predicted short-term (48 h) D_eff() of a semivolatile pesticide, lindane (C₆H₆Cl₆), in Gila silt loam (coarse-loamy, thermic Typic Torrifluvents). To test the DATES model for longer-term data, D_eff() of naphthalene (C₁₀H₈) in Lerbjerg sandy clay (L5) was measured at different soil-water contents and incubation times in air-tight diffusion cells. Because of sorption kinetics, both the apparent hydrophobic sorption coefficient, K_D, derived from the column diffusion experiments and K_D derived from batch desorption experiments increased with naphthalene-soil contact time for t > 700 h. Consequently, an increasing K_D value with time was required in the DATES model to obtain successful D_eff() predictions. Since chemical-soil contact time and adsorption-desorption history typically vary in each compartment of a soil profile, transport models, including sorption kinetics, will become very complex and DATES with a time-dependent K_D relation may represent a useful model alternative.

Abbreviations: CSIF, Constant slope impedance factor • D_eff, effective diffusion coefficient • DATES, Diffusion and two-component equilibrium sorption • L5, Lerbjerg sandy clay • TCE, Trichloroethylene • WLR, Water-induced linear reduction

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