Spectroscopy, solubility, and modeling of cosolvent effects on metal chelate complexes in supercritical carbon dioxide solutions

Extraction of metals from aqueous solutions and solid matrixes with supercr itical CO2 is an attractive, environmentally benign alternative to organic solvent extraction to remove metal contaminants. Here, new measurements are presented for the solubility of iron tris(pentane-2,4-dionate) [Fe(acac)(3 )], a representative metal chelate complex, in pure supercritical CO2 and i n a mixture of supercritical CO2 and trichloromethane (chloroform), a typic al organic co-contaminant, as a function of temperature and pressure. Solub ilities ranged from 8.75 x 10(-6) mole fraction to 1.34 x 10(-3) mole fract ion with an average uncertainty of just 12%. It is shown that the presence of 3 mol % chloroform increases the solubility of Fe(acac)(3) by approximat ely a factor of 2. Thus, the co-extraction of organic contaminants with met als would be advantageous. In addition, spectroscopic measurements are pres ented of the local environment around the dissolved Fe(acac)(3) in the CO2/ chloroform mixture that show a cybotactic region that is enriched with chlo roform, especially at lower pressures. Finally, thermodynamic modeling resu lts are presented for this system using the Peng-Robinson equation of state . Using just one fit parameter, excellent agreement is obtained with the so lubility data in pure CO2. Without any new adjustable parameters the model underpredicts the solubility in the CO2/3 mol % chloroform mixture. Thus, t he solubility, spectroscopy, and modeling results suggest that the large in crease in solubility with the cosolvent (co-contaminant) present is not jus t attributable to the bulk density increase when chloroform is added, but e vidently is also due to the enrichment of the chloroform in the solvation s phere around the solute.