Design of highly CO 2 -soluble chelating agents for carbon dioxide extraction of heavy metals
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Carbon dioxide is an attractive organic solvent in today's chemical process environment, in that it is nonflammable, inexpensive, and exhibits low toxicity. Further, materials solubilized in carbon dioxide are easily and completely recovered/concentrated from solution via a simple pressure quench. Despite these favorable properties, CO2 is nonpolar and therefore is a very poor solvent for materials such as conventional metal chelating agents, thus blocking application of carbon dioxide in metal extraction/recovery. Consequently, we are exploring the molecular design of materials which are highly CO 2 phillic, that is, they exhibit solubilities in carbon dioxide which are significantly greater than alkanes with the same number of main-chain atoms. By functionalizing chelating moieties with CO2-phillic oligomers, we have generated materials that both effectively extract metals from solid matrices and that dissolve in carbon dioxide in significant quantities. The application of such chelating agents is not limited to soil cleaning operations. In fact, these chelates make the use of CO2 possible in many applications where precision cleanup/recovery of metal ions are required. For example, CO2 has been promoted as a replacement for CFC's in cleaning processes in the electronics industry. Use of these chelates would allow the removal of metals, along with other impurities in a CO 2 cleanup procedure. I. INTRODUCTION Carbon dioxide is one of the few solvents available that is not considered to be a VOC (Volatile Organic Compound) by the United States. Not only is CO 2 environmentally benign, but also nontoxic, nonflammable, and abundantly available from natural sources at low cost (less than $80 per ton). The versatility of CO 2 has made it the ideal supercritical solvent for such diverse applications as chromatography,1'2 biotechnology,3 polymerization,4 and extraction of thermally labile constituents from natural products.5 CO 2 drops essentially to zero under atmospheric conditions, depressurization of a CO2-based solution to 1 atm results in complete precipitation of any solutes or suspended material, substantially easing the downstream product recovery. These favorable properties present CO 2 as an attractive solvent for use in soil clean up procedures. In aqueous-based soil/cleaning systems, a large portion of the throughput (fines) cannot be separated, but is collected as sludge and must be buried in a landfill as hazardous waste. In addition, the wash solution requires significant treatment prior to reuse or disposal. Use of organic wash media eliminates the sludge by-product, but prompts concern owing to VOC regulations, as well as solvent toxicity and flamniability. The organic effluent also requires treatment prior to reuse. Substitution of CO 2 as the wash media eliminates the sludge byproduct, eliminates toxicity and flammability problems, and lessens the difficulty of the effluent treatment stage, 530
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J. Mater. Res., Vol. 10, No. 3, Mar 1995
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