Temperature Effect on Desorption Kinetics of Benzene on Various Soils

Abstract

Since the advent of diesel fuel use, insufficient storage and inadequate transport and disposal practices have resulted in widespread contamination of the subsurface environment. Beginning in the 1970's, the United States EPA has established a number of regulations controlling current and future storage, transport, and disposal efforts and address the need to remediate existing contaminated sites. However, regulations provide only the desired goal, not a roadmap of how to accomplish remediation. It falls then, to individuals in research and industry, to devise techniques effective in reducing / eliminating contamination levels at sites of concern. Existing remediation techniques of pump and treat and air sparing / soil vapor extraction are effective, but often take many years to accomplish remediation to regulatory levels. Thermal treatments such as steam stripping and electrical heating of soils, are much less time consuming, but much more costly endeavors. Low temperature thermal desorption (at temperatures less than 80 degrees C) holds promise by incorporating the benefits of higher temperatures, such as the increase in vapor pressure and the decrease in viscosity, without the extreme cost usually associated with thermal treatments. In order to test this hypothesis, a research testing program focused on batch testing in the laboratory setting was developed to assess the viability of increased temperatures on the desorption efficiency of benzene on various soils. Testing consisted of three soils, a poorly graded Ottowa sand, kaolinite, and a natural silty sand soil from the Lockbourne Air Force Base experimental testing site. The contaminant of concern is benzene, a carcinogenic and mutagenic compound that is one of the four major components of BTEX, a constituent of most diesel fuels. Benzene was chosen because of its presence at the LAFB testing site at contamination levels 164 times groundwater regulation limit of 5 ppb. Laboratory testing was conducted at initial benzene solution concentrations of 10, 100 and 1000 mg/L. Four temperatures; 20, 40, 60, and 80 degrees C, were used in the batch testing program. Results from testing support the theory that increased temperatures result in higher desorption efficiency. For lower concentrations of 10 and 100 ppm, temperatures as low as 40 degrees C correlated to increases in desorption levels from 40 percent (at 20 degrees C) to over 80 percent for the kaolinite and natural soil. Sand also experienced a doubling in desorption efficiency (from roughly 30 percent to roughly 70 percent) at the 60 degrees C temperature and 10 and 100 ppm concentrations. The 1000 ppm testing concentration resulted in more modest, but still increasing removal efficiencies at increased temperatures. Remediation at moderately increased temperatures appears to be a promising technique, but further research needs to be performed on soils that have experienced long term exposure to contamination to assess whether or not increased desorption efficiency trends are maintained.