Dissolution and transport of metal species and polar compounds in supercritical carbon dioxide and potential applications

Supercritical fluid CO2 (SC-CO2) is effective for dissolving non-polar and slightly polar organic compounds. Several methods have been developed in recent years for dissolving polar or ionic substances in SC-CO2. One method is the in situ chelation method for dissolving metal species as CO2-soluble metal chelates in SC-CO2. Another method is utilizing a water-in-CO2 microemulsion for transporting ionic species and polar compounds in SC-CO 2.; A synergistic effect of using two selected chelating agents can significantly enhance the efficiency of removing metal ions from solid materials in supercritical CO2. A mixed-ligand approach described in this study indicates that effective supercritical fluid extraction (SFE) processes can be developed using two chelating agents, one contributes to enhanced solubility and the other enhanced stability of the resulting metal chelates in SC-CO2.; Water-in-CO2 microemulsions have been shown to be an effective medium for transporting metal species in SC-CO2. Relatively high concentrations of metal ions can be dissolved in the water core of a microemulsion and dispersed in the CO2 phase. Because of their small size and dynamic nature, the microemulsions show great promise as transporting media for removing metal species from solid matrices into SC-CO2. Compared with a conventional washing or acid leaching process, the CO2 microemulsion technique could minimize liquid waste generation.; Dissolution of zero-valent transition metals particularly precious metals in SC-CO2 has potential applications in semiconductor device processing and in recovery of valuable metals from abandoned electronics. The CO 2-soluble tri-n-butylphosphate (TBP)-nitric acid complex provides a suitable oxidizing agent for metal dissolution. TBP serves as a carrier for introducing the acid into the SC-CO2 phase for chemical reactions. Methods for dissolving precious metals in SC-CO2 using a TBP-nitric acid complex as an oxidizing agent followed by complexation with a beta-diketone as a chelating agent have been developed.; Extraction of organic acids into SC-CO2 is important for recovering organic acids produced in fermentation processes. Many organic acids are not soluble in SC-CO2. TBP is known to form complexes with a number of inorganic and organic acids by hydrogen bonding through the P=O group forming CO2-soluble Lewis acid-base complexes of the general form TBP(acid) x(H2O)y. This approach provides a method for effective recovery of organic acids in SC-CO2.