Environ. Sei. Technoi. 1992, 26, 1159-1 164
EPA/600/M-86/026; Office of Drinking Water: Washington, DC, 1987. Abelson, P. H. Science 1991, 253, 361. Roberts, L. Science 1991, 252, 911. Pruell, R. J.; Bowen, R. D.; Fluck, S. J.;LiVolsi, J. A,; Cobb, D. J.; Lake, J. L. USEPA. Environmental Assessment Group Report, Washington, DC, 1988. Tillitt, D. E.; Ankely, G. T.; Giesy, J. P.; Ludwig, J. P.; Kurita-Matsuba, H.; Weseloh, D. V.; Ross, P. S.; Bishop, C. A.; Sileo, L.; Stromborg, K. L.; Larson, J.; Kubiak, T. J. Environ. Toxicol. Chem., in press. Hong, C. S.; Bush, B. Chemosphere 1990, 21, 173-181. Kannan, N.; Tanabe, S.; Wakimoto, T.; Tatsukawa, R. J. Assoc. O f f . Anal. Chem. 1987, 70,451-454. Hong, C. S.; Bush, B.; Xiao, J. Ecotoxicol. Environ. Saf. 1992, 23, 118-131.
Received for review November 11, 1992. Revised manuscript received February 13, 1992. Accepted February 18, 1992.
Funding f o r this study was provided by the Ludington Area Charter Boat Association, the Michigan Agricultural Experiment Station, and the Michigan State University Department of Fisheries & Wildlife and Pesticide Research Center. Additionally, this publication is a result of work sponsored by the Michigan Sea Grant College Program, Project M-PM-3E, under Grant NA86AA-D-SG043 from the Office of Sea Grant, National Oceanic and Atmospheric Administration ( N O A A ) , U S . Department of Commerce, and funds from the State of Michigan. T h e U S . Government is authorized t o produce and distribute reprints for governmental purposes notwithstanding any copyright notation appearing hereon. Support for L.L.W. for a portion of this research was provided by the U S D A Cooperative State Research Service, Food and Agricultural Sciences National Needs Graduate Fellowships Program, under Grant 88-384203834. Portions of this study were presented at the 1991 meeting o f the International Association for Great Lakes Research in Buffalo, NY, and at the 1991 meeting of the American Fisheries Society i n S a n Antonio, T X .
Investigation of Adsorption Equilibria of Volatile Organics on Soil by Frontal Analysis Chromatography Catherine Thlbaud, Can Erkey, and Aydln Akgerman" Chemical Engineering Department, Texas A&M University, College Station, Texas 77843
IAdsorption isotherms of organic contaminants between
soil and nitrogen were determined by frontal analysis chromatography. The organic contaminants of interest were methylene chloride, carbon tetrachloride, tetrachloroethylene, chlorobenzene, n-hexane, and toluene. All the isotherms exhibit a BET type I1 behavior. The adsorption capacity on soil at saturation was determined for each contaminant and compared to the adsorption capacity corresponding to unimolecular layer adsorption. It was observed that the monolayer adsorption covers the whole surface uniformly rather than taking place on selective sites. The molar heats of adsorption were also calculated by analyzing the linear portion of the BET adsorption isotherms. Introduction Soil gas extraction (also known as vacuum stripping and stripping) is an in-situ remediation technique that mthdraws the gases from the vadose zone by pumping in forced air or by application of vacuum to a perforated or m n e d extraction well above the water level (sometimes Wmbined with in-situ agitation). In standard applications, vacuum is applied to extraction wells by means of fans: enters through the ground surface and flushes the Wntamhants out, Contaminants in the subsurface are in thermodynamic equilibrium between the soil solid comYnents and the soil gas. As the soil gas is vented, equih u m is shifted and the contaminants have to desorb from the solid phase. over the past few years, soil gas extraction systems have 'ucce8sfully been used in several remediation efforts. hclude removal of explosive landfill gases in urban a d developed areas (I, 2 ) ,remediation of soils contamiwith tetrachloroethylene at an industrial facility in midwest (3), removal of hydrocarbon vapors from a %he-contaminated waste site ( 4 ) , and remediation of contaminated with trichloroethylene, 1,l-dichloro*Ylene, l,1,1-trichloroethane, and Freon T F (5). How% mitial feasibility studies and design of these systems hampered by the lack of necessary engineering data.
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Desorption of contaminants from soil particles occurs through three consecutive mass-transport steps: intraparticle diffusion from the interior to the outer surface of the particle, mass transfer of the contaminant from the outer surface of the particle to the gas phase, and bulk transport of the contaminant in the gas phase. The adsorption isotherm, which relates the concentration of the contaminant in the gas phase to the concentration of the contaminant on the soil phase, defines the thermodynamic equilibrium and hence plays a crucial role in the desorption process. Therefore, in order to calculate engineering parameters such as the optimum flow rate, the pressure head, the well placement configuration, or the amount of adsorbent required to trap the organic vapors before they are released to the atmosphere, it is necessary to have good estimates of the mass-transfer coefficients, the effective diffusivities, the adsorption rates, and especially the adsorption isotherms. In addition, the adsorption isotherm is an important factor in the investigation and modeling of the transport and migration of contaminants in the subsurface environment, in the design and development of various waste treatment processes based on the desorption of contaminants from soil, in the assessment of soil contamination levels from soil gas analysis, and in the investigation of the nature of interactions between the contaminants and the soil. The adsorption isotherm depends on many factors such as the nature of the contaminant, the composition and pore structure of the soil particles, and the amount of moisture present in the soil. Chiou and Shoup (6) measured the sorption isotherms on dry Woodburn soil for benzene and polychlorinated benzenes by a dynamic-equilibrium sorption apparatus. In these studies, the presence of moisture reduced the uptake capacity of the soil considerably, which was attributed to adsorptive displacement of organics by water on the mineral components of the soil. Chiou et al. (7) determined the adsorption isotherms of a wide variety of organic compounds on soil humic acid using a static vapor sorption apparatus. Polar compounds exhibited greater sorption capacities and the isotherms were highly linear over a large concentration range, which was attrib-
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 6, 1992
1159
Dessicator
3-Way Valve IV
Metering Valve
To Hood
Soap Bubble Meter
U
Constant Temperature Bath
Nitrogen Cylinder
I 3-Way Valve V
Gas Chromatograph
Flgure 1. Schematic diagram of the experimental setup.
uted to the partitioning of organics in soil humic acid. In this article, we report the adsorption isotherms of various volatile organic compounds suitable to remediation by vacuum extraction, partitioned between soil and nitrogen, as a first step in our efforts t . ~model the desorption profiles of contaminants from soil. We employed a simple dynamic response technique to determine adsorption and desorption of organic contaminants from soil particles. The technique, commonly used in the industrial and scientific communities for investigation of gas and liquid adsorption on solid adsorbents, is frontal analysis chromatography. In this technique, a carrier gas containing the organic compound of interest at a certain concentration is introduced to the inlet of a packed bed of soil. The concentration boundary travels through the column and breaks through at the outlet of the column. An adsorption breakthrough curve is obtained by monitoring the effluent concentration as a function of time. Then, pure carrier gas is introduced at the inlet of the bed and the effluent concentration is monitored to obtain the desorption breakthrough curve. Integration of the adsorption breakthrough curves obtained at various inlet concentrations enables one to construct the adsorption isotherm through the whole concentration range. In this article, we report the experimental technique and our first series of results on adsorption equilibria of different common industrial solvents (chlorinated and nonchlorinated) on soil in the presence of nitrogen. The chlorinated contaminants were chlorobenzene, tetrachloroethylene, carbon tetrachloride, and methylene chloride and the nonchlorinated contaminants were nhexane and toluene. We chose nitrogen as the carrier gas instead of air because of accuracy limitations in our analytical equipment (physicochemicalproperties of nitrogen and air are equivalent). Experimental Section A schematic diagram of the experimental setup is presented in Figure 1. The nitrogen stream from the gas cylinder was passed through a desiccator and separated into two streams. One of the streams was saturated with the organic vapor by bubbling through the liquid in a glass container held at constant temperature and the other stream was maintained pure. Then the streams were mixed again. The flow rate of each stream was controlled by a fine metering valve (I and I1 in Figure 1) to allow precise control of the contaminant concentration in the resulting stream. An on-off valve (111)was placed at the exit of the contaminant saturator to ensure a perfect isolation of the saturator and avoid any contamination of the pure nitrogen stream during the desorption process. The bypass was used during the manipulation of the metering valves (I and 11) to attain the desired contaminant con1160 Environ. Sci. Technoi., Voi. 26, No. 6, 1992
centration in the stream. The soil column could also be isolated by the manipulation of the two three-way valves placed at its inlet and outlet (IV and V). Each experiment consisted of an adsorption phase and a desorption phase. Prior to the adsorption phase, the soil column was free of any contaminant and moisture. A stream containing the desired amount of organic vapor at the desired flow rate was passed through the bypass. To start the adsorption process, the stream was switched from the bypass to the soil column. The effluent concentration from the column was then monitored until it reached a constant value equal to the inlet concentration. This indicated the saturation of the soil bed by the organic contaminant and the end of the adsorption process. The column and the saturator were isolated from the
