Thermally enhanced vapor extraction for removing PAHs from lampblack-contaminated soil

This work summarizes the results of a feasibility study in support of a soi l venting-thermal desorption (SVTD) process for remediating lampblack-impac ted soil. Lampblack is the solid residue resulting from the gasification of crude oil. The SVTD process couples soil vapor extraction with in situ hea ting. The objective of this study is to determine the required temperature for desorbing polycyclic aromatic hydrocarbons (PAHs), the compounds of reg ulatory concern, from lampblack. Bench-scale results are reported for the t reatment of a soil-lampblack matrix containing 11 PAHs totaling about 4100 ppm (mg/kg) total PAH (TPAH). Solids characterization analyses suggested th at 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 wi th the surface of sand grains. SVTD testing for the sand-lampblack solids i ndicates that temperatures in excess of about 250 degreesC are sufficient t o mobilize most of the PAHs. Specifically, at temperatures between 250 degr eesC and 300 degreesC, the TPAH level in the soil-lampblack matrix was redu ced to less than 100 ppm in 10 days. The dynamics of PAH removal were captu red reasonably well by a mass balance accounting for the temperature depend ent 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 (r oughly 40 ppm TPAH) was observed to persist in the solids even at temperatu res in excess of 650 degreesC. Although the specific state of these persist ent PAHs is unknown, they may reside within an extremely nonvolatile residu e or be otherwise strongly sequestered in the solid matrix.