Soil vapor extraction properties

Poulsen, T. G. et al., 1999, Predicting Soil-Water and Soil-Air Transport Properties and Their Effects on Soil-Vapor Extraction Efficiency Ground Water Monitoring and Remediation, Vol. 119, No. 3, pp. 61-70. 

Terra-Vac, Inc., developed the In-Situ Vacuum Extraction process, which removes volatile organic constituents (VOCs) from the unsaturated zone of soils through extraction wells. The extracted gases and water proceed to a vapor-liquid separator and an emission control system. The process has been employed at over 60 sites. Four case studies, including three Superfund sites, have been documented. The process represents a viable technology to fully remediate sites contaminated with volatile organics. Considerations for use include contaminent volatility, site-specific cleanup level, and soil properties. The process works well with most soil types. The air-filled porosity of a soil is an important criterion to indicate whether vacuum extraction will work. Soils with low permeability but with... 

In an SVE system, the primary mechanism for contaminant removal from the soil to the vadose zone is the volatilization of contaminants present in the pure or adsorbed phase onto soil into the vapor phase, as the vapor phase is continually extracted. The property that shows the extent to which this transfer can take place during SVE is vapor pressure, which provides an indication of the extent to which each contaminant will partition between the liquid phase and the vapor state at equilibrium conditions. Generally, a contaminant with a greater vapor pressure more readily volatilizes than one with a lesser vapor pressure. 

Of all the materials available for use as a supercritical fluid, CO2 has become the material of choice because of its chemical properties. Instruments have been developed to utilize the principles described to effect extractions of compounds from a variety of sample matrices including asphalt, plant material, and soils (Figure 25.1). The supercritical fluid is pumped through the sample, through a filter or column to a trap where the fluid vaporizes and solvent is added to transfer the analyses to a vial for analysis. More recent instruments combine the supercritical fluid extraction system with a variety of columns and detectors to acquire data from complex samples. 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

The properties of SFs make them ideal for extracting analytes from solid matrices such as soils, agricultural products, foods, and solid sorbents. Supercritical fluids have the ability to maximize the extraction selectivity by controlling the temperature and pressure of the supercritical fluid (Figure 11.27) (85). Initially, the solubility of an analyte in a supercritical gas is dependent on solute vapor pressure thus the solubility of the analyte in the gas first decreases with a rise in pressure reaching a point of minimum solubility. As the gas is compressed into the critical phase, there is a rapid increase in analyte solubihty, which ends at a maximum pressure that is determined by the extraction temperature. Any additional increase in pressure will only slightly increase analyte solubility. Also, in... 

---