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petroleum stream could be employed to create salable products, meaning less is simply burned. Aqueous waste from electroplating operations gen- erally contains substantial amounts of dissolved met- als in a low pH (2.0 and below) medium. Chelating agents dissolved in carbon dioxide can be used to extract many of the relevant metals from such low pH media [284], provided that the agents are designed to operate under such conditions. Generally, the strategy by which chelating agents are rendered CO2-soluble involves the attachment of ‘CO2 -philic’ functional groups to a moiety known to bind certain metals, and as such there are in theory no restrictions as to the type of chelating agent employed, so long as the functionalization chemistry can be performed. The competing technologies for CO2 extraction include the use of precipitants, compounds that react with dissolved metals to form insoluble species, as well as chelating agent-functional ion exchange resins (solid sorbents). Precipitants are inexpensive, yet they pro- duce a sludge that must be collected and disposed. Ion exchange resins (following back extraction) pro- duce instead a concentrated (ideally) solution of the metals, which must be subsequently treated to recover the metal. The most problematic application to analyze is that where CO2 plus a chelating agent is being used to remove metals from a matrix to accomplish remedi- ation. Indeed, the primary focus of green chemistry is the elimination of waste production, rather than the clean up of existing problems, yet the use of CO2 to remediate metal contamination may be considered green processing in some circumstances. First, it has been shown by various research groups that one can extract a variety of metals from solid matrices (in- cluding soil [285]) using chelating agents dissolved in carbon dioxide. If CO2 was to be used to replace either an organic solvent or water in the washing of contaminated soil, this could be considered green processing, provided that the energy required for the process was equal to or less than that employed for the conventional route. A large amount of sludge (as much as 15% of soil throughput, created from sus- pended fine particles) is produced, for example, when soil is washed with water. Because carbon dioxide is a low density, low viscosity, low interfacial tension fluid, it is likely that sludge production would be greatly reduced if CO2 were used to wash soil. On the other hand, because soil washing typically involves excavation of the contaminated material, remediation strategies that eliminate the problem without excava- tion (in-situ remediation) should be preferred. Such strategies range from the use of green plants to absorb and concentrate metals, to the addition of agents to the oil that stabilize the metals, preventing their transport. 4.7.2. Inorganic chemistry: industrial activity Materials Technology Limited has obtained several patents [286] describing the use of high pressure CO2 to enhance the rate of curing of concrete, where the CO2 actually dissolves in the concrete mixture and re- acts with the matrix. While one might consider this as sequestration of CO2 and hence green chemistry, it should be remembered that the preparation of the concrete precursor involves the calcining of the raw material, where CO2 is driven off while injecting sig- nificant energy. Thus, more CO2 is probably produced during this sequence than is sequestered. Both Texas Instruments [287] and Micron Technol- ogy [288] have patented inventions where inorganic chemistry is performed in CO2 to support clean- ing/processing of silicone wafers. The Micron patent describes the use of mixtures of CO2 and etching chemicals to pattern inorganic substrates, while the Texas Instrument patent describes a process where inorganic contamination on wafers is first derivatized, then dissolved in CO2 and removed. Note that in these patents, the use of CO2 is designed to replace the use of water. In many parts of the world, significant water usage by industry is not sustainable and hence, there is a need to find replacement technologies for large-scale water usage. 4.8. Reactions at interfaces and/or multi-phase mixtures Reactions at interfaces (or transport across inter- faces to facilitate reaction) in CO2-based systems have been proposed as a useful means by which to sup- port green chemistry in carbon dioxide while easing separation problems post-reaction. Indeed, if one can effectively segregate catalyst, reactants and products in various phases in the reactor, downstream separa- tion is certainly easier. However, one is now also faced with thermodynamic (phase behavior) and transport limitations to reaction. A key proviso in attempting E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 175PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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