Thermally Enhanced Vapor Extraction for Removing PAHs from Lampblack-Contaminated Soil

Abstract

This work summarizes the results of a feasibility study in support of a soil venting-thermal desorption (SVTD) process for remediating lampblack-impacted soil. Lampblack is the solid residue resulting from the gasification of crude oil. The SVTD process couples soil vapor extraction with in situ heating. The objective of this study is to determine the required temperature for desorbing polycyclic aromatic hydrocarbons (PAHs), the compounds of regulatory concern, from lampblack. Bench-scale results are reported for the treatment of a soil-lampblack matrix containing 11 PAHs totaling about 4100 ppm (mg/kg) total PAH (TPAH). Solids characterization analyses suggested that these PAHs constitute about 60% of the organic residue on a solid matrix dominated by fine-grained sand and carbon-based lampblack particles. Thin section imagery supports the conceptual model of hydrocarbons associated with the surface of sand grains. SVTD testing for the sand-lampblack solids indicates that temperatures in excess of about 250°C are sufficient to mobilize most of the PAHs. Specifically, at temperatures between 250°C and 300°C, the TPAH level in the soil-lampblack matrix was reduced to less than 100 ppm in 10 days. The dynamics of PAH removal were captured reasonably well by a mass balance accounting for the temperature dependent volatilization of an ideal PAH mixture. Both simulated and experimental results support the finding that the vast majority of the PAH removal from this sand-lampblack matrix was controlled by thermodynamic considerations (as opposed to mass transfer resistances). A small residual PAH fraction (roughly 40 ppm TPAH) was observed to persist in the solids even at temperatures in excess of 650°C. Although the specific state of these persistent PAHs is unknown, they may reside within an extremely nonvolatile residue or be otherwise strongly sequestered in the solid matrix.