Thermal desorption of hazardous and toxic organic compounds from soil matrices : dichloromethane, chloroform, benzene, toluene, 1-chloronaphthalene, 1, 2, 4-trichlorobenzene

This research project was undertaken to understand the process of desorption/removal of organic compounds from soil matrices under the influence of temperature and gas purging. The project was divided in two parts : A. Plug deposition experiments: Columns packed with soil matrices were purged with an inert gas under isothermal conditions. Organic compounds were then introduced as a plug on the front of the column by on-line injection. The rate of passage (due to sequential adsorption/desorption) of the plug through the various soil matrices was measured by a gas chromatograph equipped with a flame ionization detector (FID). The compounds studied were methylene chloride, chloroform, benzene, toluene and l1chloronaphthalene. The soil matrices used were sand, soil, gaschrom R and propak T. For a given combination of organic compound and a soil matrix the process was studied at different temperatures at a fixed flow of the inert gas. The plug deposition process was mathematically modeled and the model can be used to predict desorption of these compounds at any temperature in soil matrices under the same gas flow rate conditions. The sand column showed the weakest and the Poropak T column the strongest affinity for the pollutants. An increase in operating temperature also decreased the retention time of the compound. B. Desorption experiments with uniformly contaminated Soil: A desorption system was built to study a uniformly contaminated soil columns. It incorporated purge flow controls, an oven, valve switching system, temperature probes and a gas chromatograph (FID). The organic compounds studied were 1,2,4-trichlorobenzene (BP = 214°C) and 1-chloronaphthalene (BP = 263°C). Columns containing soil uniformly contaminated with a known concentrations of toxic organic compounds were placed into the oven at isothermal conditions. An inert gas (N2) at a constant flow of 30 ml/min was used to desorb the compounds from the soil. Vapors were directed by means of a 6-way switching valve either to an FID for analysis, or to activated carbon adsorbers for collection. Solvent extractions of the soil samples and adsorber columns provided a mass balance for the compound in the system. The desorption rate of the toxic organic compound was analyzed as a function of oven temperature. Mathematical models were developed and curves plotted that can be used to determine desorption-time for the removal of compounds from the soil at isothermal condition. At 200°C and at N2 flow of 30 cc/min the TCB concentration was 84 ppm after 69 minutes showing 92% removal. At 220°C and at N2 flow of 30 cc/min the CNAP concentration was 143 ppm after 70 minutes showing 84% removal. Based on an analysis of the results design suggestions are made for a pilot scale soil decontamination system