Leaching of BTEX from Aged Crude Oil Contaminated Model Soils: Experimental and Modeling Results

It is generally assumed that soil properties such as organic matter content, porosity, and mineral surface area have a significant effect on the bioavailability and leachability of aged petroleum hydrocarbons. In order to test this hypothesis, nine model soils or sorbents (i.e., fine and coarse quartz sand, montmorillonite and kaolinite clay, peat, 60Å and 150Å silica gel, a loam soil, and non-porous glass beads) were spiked with a crude oil, aged for 27 months in the laboratory, and transferred to glass columns for the performance of continuous flow leaching experiments. The column effluents were periodically sampled for 43 days and analyzed for BTEX. A one-dimensional flow model for predicting the dissolution and dispersion of individual hydrocarbons from a multi-component NAPL such as crude oil was used to fit the leaching data (i.e., the BTEX concentration versus time curves) by adjusting the equilibrium oil-leachate partitioning coefficient (K ^ol ) for each respective hydrocarbon. The Peclet number, which is a measure of dispersion and a required modeling parameter, was measured in separate chloride tracer experiments for each soil column.

Results demonstrate that soil properties did not significantly affect the leaching kinetics of BTEX from the columns. Instead, BTEX leaching curves could be successfully fitted with the one-dimensional NAPL dissolution flow model for all sorbents with the exception of montmorillonite clay. The fitting parameter K ^ol for each hydrocarbon was found to be similar to the K ^ol values that were independently measured for the same crude oil by Rixey et al. (Journal of Hazardous Materials B, 65: 137–156, 1999). In addition, the fitted K ^ol values were very similar for BTEX leaching from aged compared to freshly spiked loam soil. These findings indicate that leaching of BTEX in the aged soils that are contaminated with crude oil at the high concentrations commonly found in the environment (i.e., > 20,000 mg/kg) was not affected by soil properties or aging but rather was governed by the equilibrium dissolution of these hydrocarbons from the crude oil NAPL that is coating the soil particles.

Keywords:

• Crude oil
• BTEX
• leaching
• equilibrium dissolution model
• NAPL
• aged soils

This research was supported by the National Petroleum Technology Office of the U.S. Department of Energy. We would also like to thank Dr. Rixey for his assistance in designing the leaching apparatus.