Figure 1 –Moisture and Vapor Pressure 

 
     The problem is that the vapor pressures of low concentrations of chemicals are reduced by adsorption on soil and desorption of water.  As Figure 1 shows, the vapor pressure of a chemical in soil decreases several orders of magnitude when the water at the surface of a soil particle evaporates.

     What happens to cause this phenomenon is that the less volatile, less polar chemical begins to be adsorbed to replace water that had been hydrogen-bonded to the surface.  This adsorption is generally reversible, i.e., adsorbed chemical is released from dried soil when a polar solvent such as water or methanol is available.

     Competitive adsorption of chemicals can be modeled with a isotherm where the vapor pressure is proportional to the function

Y = Xw/(1- Xw – XA).

 
     Where Xw and XA are the fractions of the surface occupied by water and chemical.  This function is plotted in Figure 2.  The figure shows that if the soil contains enough moisture to cover 5% of the surface, the relative pressure is less than 0.5% of the expected vapor pressure for a moisture content of 95% of a monolayer.  Moisture equivalent to 95% of a monolayer corresponds to approximately 5% soil moisture.
 

Figure 2 –Coverage and Vapor Pressure 

 
     Thus, the HAVE process must either operate with a controlled humidity or at a high-temperature like that used in a mobile thermal desorption process.   Computer simulators have been used since the 60’s to model vapor-liquid equilibrium and adsorption relationships and are the ideal tool for determining optimum operating conditions for HAVE and other thermal projects.

     STARS, a very well-proven oil-field thermal simulator, was used to find the solution to this problem by modeling an element of symmetry from the soil pile.  This element of symmetry represented a volume between injection holes in the hot-air pipes.  A three-component model of the adsorbed SVOC was used.  In this model, the SVOC could have a vapor pressure as high as its textbook value or it could be reduced as much as a factor of several million.   The adsorption model included a strong dependence on the water saturation in the pile.  Desorption of the chemical from the piles remediated with dry air was modeled, than the model was used to predict the effect of water addition on the rate of SVOC desorption.

     Figure 3 compares the dew-point saturation of water in air at several temperatures and the residual SVOC in the HAVE soil pile.  The line in the figure labeled “humidity” shows that the soil dries out if the air is not almost saturated with water.  The line labeled “residual” presents results of simulations in which the air was nearly saturated with water. The residual concentration of SVOC decreases steadily as the wet-soil temperature increases, and the soil reaches the desired cleanup criteria if it can be held above 140°F for one week.
 

Figure 3 – Humidity of Air and Prediction of Residual SVOC after Seven Days 

 
     If the soil was not kept wet it would have to be heated to over 320°F to be cleaned in 7 days.  Then, cooling the soil back to 140°F after it was cleaned would take almost as long as it took to clean the soil pile at 320°F.

     This model was used to help select day-to-day operating conditions in the early stages of an active sediment remediation project.

     Summary - Simulations like this example are invaluable in designing remediation projects.  Not only do they assist in selection of the operating parameters, but they show how long the process takes, the energy required and how best to destroy contaminants. If you would like to discuss planning or optimizing thermal or chemical remediation projects please contact MK Tech Solutions.

     Acknowledgement – MK Tech Solutions, thanks the United States Army Corps of Engineers for permission to publish this summary and Professors Thibodeaux and Valsaraj of Louisiana State University for their research which supports this important phenomena.