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Your Environment. Your Health.

Final Progress Reports: University of Arizona: Transport of Hydrophobic Organic Contaminants in the Vadose Zone

Superfund Research Program

Transport of Hydrophobic Organic Contaminants in the Vadose Zone

Project Leader: Roger Bales
Grant Number: P42ES004940
Funding Period: 1995 - 2000

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Final Progress Reports

Year:   1999 

In 1999/2000, researchers continued to elucidate the distribution and transport mechanisms of hydrophobic volatile organic compounds (HVOCs) in unsaturated porous media. Final analysis of water-advecting Mid-Size (30-cm) column experimental results indicates that vapor-phase diffusion and mass-transfer limitations become the dominant mechanisms for TCE transport in low-foc porous media at low water contents (< 50% saturation for the current sand studies). Overall consideration of the TCE breakthrough data suggests that multiple rate-limiting mechanisms affect the transport of HVOCs in unsaturated porous media. Consequently, a multi-phase model that accounts for multiple mass transfer processes is required to properly describe the movement of HVOCs in unsaturated porous media. Analysis of single-component adsorption studies (i.e., benzene, methylbenzene, 1,2-, 1,3-, 1,4-dimethylbenzene, and 1,3,5-trimethylbenzene) on water drops also have been completed. Maximum relative surface excess values were on the order of10-6 mol m-2, consistent with previous studies for aromatic and alkane adsorption. Adsorption isotherms show distinct separation as a function of degree of methyl substitution, with little difference between dimethylbenzene isomers. Standard thermodynamic parameters of adsorption were calculated from infinite dilution interpolations of the data. Derived free energy changes are linear with respect to degree of methyl substitution and temperature. Enthalpy changes on adsorption were all more negative (exothermic) than the corresponding heats of liquefaction and heats of vapor/aqueous phase solute transfer. Entropy calculations suggested additional restrictions of solutes to that approximated for the loss of one translational degree of freedom on adsorption. Similar conclusions are being drawn from continued analysis of mixed solute experimental results.

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