The Mechanism for Remediation of Low-Permeable Soil Contaminated with Petroleum Hydrocarbons by Thermally-Enhanced Soil Vapor Extraction Combined with Sand Mixing

Thermally-enhanced soil vapor extraction (T-SVE) combined with sand mixing is an alternative technology for remediating low-permeable soil polluted by organic contaminants. In this study, a T-SVE apparatus with a large heating cylinder was constructed for exploring removal mechanisms of typical petroleum hydrocarbons of n-C₁₁ and C₁₃₋₁₆ alkanes, and dynamics of heat propagation within soils during T-SVE operation was simulated by CMG-STARS software. After 6 days of T-SVE, most of the soil concentration-gradient curves of n-alkanes almost coincided with their isothermal contours, suggesting the crucial role of heat conductivity on T-SVE remediation efficiency. The instantaneous concentration of n-alkanes in extracted gas showed great fluctuations, and higher than 90% of spiked n-alkanes was removed from soils. n-alkanes were more easily eliminated from soil with lower organic matter due to the less retention ratio despite the trivial impact of organic matter on heat transfer. By contrast, the higher soil moisture adopted in the study retarded heat transfer in the initial 2 days due to thermal consumption induced by water evaporation. During the last 4 days, however, the average temperature of more humid soil was obviously elevated, especially in the zones receiving less energy input. It was attributed to the fact that soil temperature was mainly determined by heat conduction, and soil pores still occupied by water were more beneficial for heat conduction, leading to the promoted evaporation of n-alkane and therefore eventual remediation efficiency. Suitable original soil moisture is important for T-SVE remediating low-permeable soil combined with sand mixing.