Laboratory study evaluating thermal remediation of tetrachloroethylene impacted soil
Burghardt, Julie Marie
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A laboratory study was conducted to assess the relationship between degree of volatile organic compound (VOC) mass removal from soil and heating duration, initial dense non-aqueous phase liquid (DNAPL) saturation, and grain size. The relationship between post-remedy sampling temperature and VOC soil concentration was examined. Soil contained in glass jars was spiked with DNAPL phase tetrachloroethylene (PCE), saturated, and placed in an oven for a specified period of time. The soil temperature at the centre of each jar was monitored during heating. Upon removal from the oven, each jar was immediately capped with an air tight seal and placed into an ice bath until the soil temperature had cooled to the desired sampling temperature. The jar caps were subsequently removed and the soil was sampled using a coring tool and immersed into pre-weighed vials containing methanol. PCE in soil samples was quantified using purge-and-trap with gas chromatography/mass spectrometry. Soil temperature increased steadily from ambient until reaching a plateau at 89 ºC ± 4 ºC due to co-boiling of DNAPL phase PCE and water. A linear relationship was found between the length of the co-boiling plateau and the initial PCE saturation. Co-boiling continued until DNAPL phase PCE had been depleted, at which time the soil temperature increased to the boiling point of water and remained constant while remaining pore water was removed. PCE soil concentrations decreased rapidly in the early stages of heating, but leveled off between 9.0 and 19 ppb soon after the soil became dried out. Analysis of the sensitivity to initial PCE saturation data revealed that the concentration of PCE in post-remedy samples increased with increasing initial saturation. Results of the sensitivity to grain size tests showed a decreasing trend between PCE soil concentration and decreasing sand grain size while temperature at sampling was not found to affect the amount of PCE quantified post-thermal remedy. Soil temperature at the centre of each jar during cooling was measured and an analytical solution was fit to the recorded data. From this data, the thermal diffusivity of the soil was approximated and was found to range from 1.4 x 10-7 to 1.8 x 10-7 m2/s.