Characterization of Mineral Oil, Coal Tar and Soil Properties and Investigation of Mechanisms That Affect Coal Tar Entrapment in and Removal from Porous Media
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Mineral oils and coal tars are complex nonaqueous phase liquids (NAPLs), which can serve as long-term sources of ground water contamination. Very limited data are available on mineral oil and coal tar entrapment in and removal from porous media. Thus, the objectives of this research were to evaluate the behavior of these NAPLs in porous media, and investigate the mechanisms governing NAPL entrapment in and recovery from porous media. Quantification of properties of three commercial mineral oils and six MGP coal tars reveals that mineral oils are slightly viscous LNAPLs (density: ~0.88 g/cm3; viscosity: 10-20 cP), whereas coal tars are highly viscous DNAPLs (density: 1.052-1.104 g/cm3; viscosity: 32-425 cP). Measured oil (tar)-water interfacial tensions (IFT) were lower than that of pure NAPLs. Properties of 16 field soil samples (soil particle size distribution, specific surface area, total carbon content, cationic exchange capacity and soil moisture release curves) were characterized. Correlations between residual NAPL saturation and NAPL and soil properties were developed, and show that the entrapment of NAPL dependent upon soil particle size distribution, total carbon content, NAPL viscosity and NAPL-water IFT. Aqueous pH and ionic strength were found to influence the interfacial properties in tar-water-silica systems. At pHs greater than 7.0, observed reduction in contact angle were attributed to the repulsive electrostatic force between coal tar and solid surface. When pH less than 4, hydration forces played a role on the contact angle decrease. The IFT reduction was resulted from the accumulation of surface-active molecules at the tar-water interface. The effect of ionic strength on interfacial properties was not significant below 0.5 M. The effects of temperature and surfactant or surfactant/polymer addition on coal tar removal was investigated by conducting coal tar displacement experiments at three different temperatures (22, 35, and 50??with sequential flushing of water, surfactant and surfactant/polymer. Coal tar removal from porous media was enhanced by elevating temperature and surfactant flushing due to the viscosity and IFT reduction, respectively. Xanthan gum was used as the polymer to increase the viscosity of the displacing fluid. In summary, these results provide tools for the prediction of NAPL entrapment in porous media, and for the selection of remediation strategies for coal tar contaminated source zone.