Supercritical carbon dioxide extraction of metals from aqueous solutions: a review

Organometallic chemistry, chemistry of compounds containing metal-carbon bo nds or compounds in which an organic molecule (sometimes with a net negativ e charge) is bonded to a metal atom through an oxygen or nitrogen atom, is one of the most rapidly growing areas of chemical research. Organometallic compounds are being extensively utilized as reagents in preparation and pro cessing of advanced inorganic materials, as catalysts in production of a wi de variety of chemicals and as chemotherapy drugs. Supercritical fluid (SCF ) science and technology is another rapidly growing field due to the intere sting and desirable properties of SCFs as solvents. The combination of orga nometallic chemistry and SCFs is a relatively new research area with signif icant potential. Some applications include (1) use of organotransition meta l complexes as homogeneous catalysts for reactions in SCFs [D.A. Morgenster n, R.M. LeLacheur, D.K. Morita, S.L. Borkowsky, S. Feng, G.H. Brown, L. Lua n, M.F. Gross, M.J. Burk, W. Tumas, Supercritical carbon dioxide as a subst itute solvent for chemical synthesis and catalysis, in: P.T. Anastas, T.C. Williamson (Eds.), Green Chemistry: Designing Chemistry for the Environment , American Chemical Society, Washington, DC, 1996, p. 132 and G.P. Jessop, T. Ikariya, R. Noyori, Homogeneous catalysis in supercritical fluids, Scien ce 269 (1995) 1065], (2) impregnation of polymers with various organometall ic complexes from SCF solutions for property enhancement or for subsequent in-situ chemical transformations within such matrices [J.J. Watkins, T.J. M cCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2, Chem. Mater. 7 (1995) 1991, and A.I. Cooper, S.G. Kazarian, M. Poliakoff, Supercr itical fluid impregnation of polyethylene films, a new approach to studying equilibria in matrices; the hydrogen bonding of fluoroalcohols to (eta(5)- C5Me5)Ir(CO) and the effect on C-H activation. Chem. Phys. Lett. 206 (1993) 175], (3) decomposition of organometallic complexes in SCFs for formation of inorganic powders with controlled size distribution [M. Barj, J.F. Bocqu et, K. Chhor, C. Pommier, Submicronic MgAl2O4 powder synthesis in supercrit ical ethanol, J. Mater. Sci. 27 (1992) 2187], (4) SCF extraction of heavy m etals from various matrices by formation of organometallic complexes [K.E. Laintz, C.M. Wai, C.R. Yonker, R.D. Smith, Extraction of metal ions from li quid and solid materials by supercritical carbon dioxide, Anal. Chem. 64 (1 992) 2875]. At the University of Connecticut, our research efforts are conc entrated on evaluation of technical and economical feasibility of some of t hese applications. The three primary research thrusts in our group have bee n the utilization of supercritical carbon dioxide (scCO(2)) as a solvent in rhodium catalyzed homogeneous hydroformylation reactions [D.R. Pale, C. Er key, Homogeneous catalytic hydroformylation of 1-octene in supercrit ical c arbon dioxide using a novel rhodium catalyst with fluorinated aryl phosphin e ligands, Ind. Eng. Chem. Res. 37 (1998) 4203], impregnation of polyuretha ne foams with organometallic oxidants from scCO(2) solutions and subsequent vapor phase polymerization in these foams for production of electrically c onductive composite foams [Y. Fu, D.R. Pale, C. Erkey, R.A. Weiss, Synthesi s of conductive polypyrrole/polyurethane foams via a supercritical fluid pr ocess, Macromolecules 30 (1997) 7611], and investigation of extraction of h eavy metals from aqueous solutions by compound formation using scCO(2) [J. Murphy, C. Erkey, Copper(II) removal from aqueous solutions by chelation in supercriti cal carbon dioxide using fluorinated beta-diketones, Ind. Eng. Chem. Res. 3 6 (1997) 5371]. Advances in these areas greatly depend on our understanding the interactions of SCFs and organometallic complexes and how these intera ctions affect a particular application. The subject matter of this review i s extraction of heavy metals from aqueous solutions in the presence of SCFs . Since solvent extraction of heavy metals is utilized on a commercial scal e, the replacement of organic solvents by SCFs has been the major driving f orce behind our research efforts. Therefore, this review was prepared to hi ghlight the areas important for commercial scale application of the technol ogy. In Section 1, an introduction to solvent extraction of metals is given . A brief introduction to the possible advantages of using SCFs is also pre sented in the same section. The fundamentals of extraction with different t ypes of extractants (cation exchangers, solvating extractants and ion-pair extractants) are given in Sections 2, 3 and 4, together with the studies in the literature on metal extraction using SCFs for each type of extractant. Thermodynamics of extraction is particularly emphasized due to its governi ng role in the economical feasibility of a large scale process. The experim ental methods that are utilized in evaluation of thermodynamic behavior of such systems are provided in Section 5. The current methods to recycle the extractants are presented in Section 6. The kinetics of extraction is descr ibed in Section 7 where no studies using SCFs have been reported to date an d a brief conclusion is provided in Section 8. (C) 2000 Elsevier Science B. V. All rights reserved.