UVM Theses and Dissertations
Format:
Print
Author:
Hackett, Emily Hazel
Dept./Program:
Civil and Environmental Engineering
Year:
2007
Degree:
MS
Abstract:
Nonaqueous phase liquids (NAPLs) have become trapped within porous media as a result of surface discharges of solvents and waste liquids to the subsurface, and act as long-term sources of groundwater contamination. Capillary forces influence the trapping and movement of NAPLs in the media and are affected by interfacial properties, such as interfacial tension and contact angle. Low temperature heating of low permeable subsurface media may enhance remediation of these contaminated zones either through enhanced volatilization of the NAPL, enhanced bioremediation of dissolved components or both. The effect of temperature on NAPL interfacial properties, however, is not clear.
This research focuses on investigating the effect of temperature changes on tetrachloroethylene (PCE) and trichloroethylene (TCE) NAPL behavior in porous media using sand, silt and clay micromodels. Micromodels consisting of media sandwiched between two microscope slides and sealed were water saturated, and dyed NAPL (either PCE or TCE) was used to displace water. The model was then flushed with water to displace the solvent and establish a residual NAPL saturation. Various sizes of residual NAPL blobs in different media were photographed and measured before, during and after heating.
In addition, the temperature relationship for interfacial tension and contact angles of PCE and TCE were investigated. Most notable changes of NAPL shape and behavior were observed for some larger NAPL ganglia. Increasing the temperature showed the movement of some of these blobs, resulting in more spherical shapes. However, not all ganglia moved. As temperature increased from 20°C to 30°C and 30°C to 40°C in the micromodels, 14% and 26% of the multi-pore ganglia decreased greater than 5% in measured area. The decrease in area of the complex ganglia appeared to be caused by the residual NAPL retreating into larger pores possibly in an attempt to minimizing the free energy of the system. As the temperature of the micromodels decreased from 40°C to 30°, approximately 40% of the multi-pore ganglia continued to decrease greater than 5% in measured area.
There were no multi-pore ganglia that increased in size greater than 5% duringheating and cooling of the rnicromodel. Single and double NAPL conformations showed little change of shape or size as a result of increasing temperature. The contact angle of the aqueous phase in a PCE/water system decreased with increasing temperature ( -0.44 °/°C for dyed PCE and -0.49 °/°C for undyed), but increased with increasing temperature for TCE (+0.38 °/°C for undyed). The decrease in aqueous phase contact angle in a PCEIwater system means that the PCE drop has become more spherical and less wetting. For TCE, the opposite occurred. This means that TCE has become more wetting with increasing temperature.
Literature values of interfacial tension indicate that PCE interfacial tension decreases with temperature, while TCE increases with increasing temperature. Low temperature heating has promise in helping enhance remediation technologies treatment efficiency in partially saturated lower permeable regions by increasing interfacial area of the NAPL exposed to the moving fluid (air). It appears based on the micromodel images and measured contact angle the interfacial area of both PCE and TCE are increasing as temperature increases in an unsaturated system promoting more favorable conditions for remediation.
This research focuses on investigating the effect of temperature changes on tetrachloroethylene (PCE) and trichloroethylene (TCE) NAPL behavior in porous media using sand, silt and clay micromodels. Micromodels consisting of media sandwiched between two microscope slides and sealed were water saturated, and dyed NAPL (either PCE or TCE) was used to displace water. The model was then flushed with water to displace the solvent and establish a residual NAPL saturation. Various sizes of residual NAPL blobs in different media were photographed and measured before, during and after heating.
In addition, the temperature relationship for interfacial tension and contact angles of PCE and TCE were investigated. Most notable changes of NAPL shape and behavior were observed for some larger NAPL ganglia. Increasing the temperature showed the movement of some of these blobs, resulting in more spherical shapes. However, not all ganglia moved. As temperature increased from 20°C to 30°C and 30°C to 40°C in the micromodels, 14% and 26% of the multi-pore ganglia decreased greater than 5% in measured area. The decrease in area of the complex ganglia appeared to be caused by the residual NAPL retreating into larger pores possibly in an attempt to minimizing the free energy of the system. As the temperature of the micromodels decreased from 40°C to 30°, approximately 40% of the multi-pore ganglia continued to decrease greater than 5% in measured area.
There were no multi-pore ganglia that increased in size greater than 5% duringheating and cooling of the rnicromodel. Single and double NAPL conformations showed little change of shape or size as a result of increasing temperature. The contact angle of the aqueous phase in a PCE/water system decreased with increasing temperature ( -0.44 °/°C for dyed PCE and -0.49 °/°C for undyed), but increased with increasing temperature for TCE (+0.38 °/°C for undyed). The decrease in aqueous phase contact angle in a PCEIwater system means that the PCE drop has become more spherical and less wetting. For TCE, the opposite occurred. This means that TCE has become more wetting with increasing temperature.
Literature values of interfacial tension indicate that PCE interfacial tension decreases with temperature, while TCE increases with increasing temperature. Low temperature heating has promise in helping enhance remediation technologies treatment efficiency in partially saturated lower permeable regions by increasing interfacial area of the NAPL exposed to the moving fluid (air). It appears based on the micromodel images and measured contact angle the interfacial area of both PCE and TCE are increasing as temperature increases in an unsaturated system promoting more favorable conditions for remediation.