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Supercritical fluids, especially carbon dioxide and water, are viable replacement solvents for a variety of hazardous organic solvents. By changing the temperature and pressure and, perhaps, adding small amounts of cosolvents, these two environmentally benign compounds can span a remarkably wide range of solvating power. In fact, supercritical CO2 is used commercially for the decaffeination of coffee and tea and the extraction of a wide variety of natural oils, spices, flavors and fragrances. we are particularly interested in the use of these two supercritical fluids as solvents for reactions and for some specialty extractions, such as the removal of metals from soils, sludges and aqueous waste streams.
Using spectroscopic techniques such as laser flash photolysis, pulse radiolysis and time-resolved fluorescence spectroscopy, we have measured the effect of temperature and pressure on a wide variety of reactions in supercritical CO2 and supercritical water. We have found that many different types of reactions, such as hydrogen abstractions and esterifications, follow similar mechanisms as in liquids and occur at reasonable rates in CO2. Diffusion controlled reactions occur at the expected diffusion controlled limit in both supercritical CO2 and water, but kinetically controlled reactions can be enhanced or hindered by the thermodynamic pressure effect on the reaction rates and/or by local compositions that differ from bulk concentrations. Nonetheless, both CO2 and water at supercritical or slightly subcritical conditions appear to be good solvents to support a variety of reactions. Currently, we are investigation energy and electron transfer reactions, as well as reactions that involve ionic species. Also, we plan to focus our attention on reaction chemistry in supercritical water, which shows great promise for the partial or complete oxidation of aqueous waste streams, chemical weapons and munitions.
While the ability to support a variety of reaction chemistry is very important, a key in the economic viability of supercritical CO2 and water will be the solubilities of reactants and products in the supercritical fluid phase. Thus, we are supporting our work on reaction kinetics in supercritical fluids with phase equilibrium measurements. These include solid/fluid equilibria, high pressure vapor/liquid equilibria and other multiphase (e.g., solid/liquid/vapor, liquid/liquid/vapor) equilibria measurements where the major component in the system is carbon dioxide. These measurements are complemented with the appropriate modeling and computation, which is discussed below.
Also, we are interested in some specialty applications of supercritical CO2 for extractions. Recently, it has been shown that supercritical CO2 can be used with chelating agents to replace organic solvents for 1) the extraction of uranium from aqueous solutions produced in the processing of the ore and 2) the removal of heavy metals from soils and sludges. Our research plans in this area focus on the use of spectroscopic techniques to measure the solubility and stability of metal chelates in supercritical CO2 solutions. Instructors can contact Professor Brennecke for this information.
