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 supercritical 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 in a mixture of supercritical CO2 and trichloromethane (chloroform), a typical organic co-contaminant, as a function of temperature and pressure. Solubilities ranged from 8.75 x 10(-6) mole fraction to 1.34 x 10(-3) mole fraction 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 approximately a factor of 2. Thus, the co-extraction of organic contaminants with metals would be advantageous. In addition, spectroscopic measurements are presented of the local environment around the dissolved Fe(acac)(3) in the CO2/chloroform mixture that show a cybotactic region that is enriched with chloroform, especially at lower pressures. Finally, thermodynamic modeling results are presented for this system using the Peng-Robinson equation of state. Using just one fit parameter, excellent agreement is obtained with the solubility data in pure CO2. Without any new adjustable parameters the model underpredicts the solubility in the CO2/3 mol % chloroform mixture. Thus, the solubility, spectroscopy, and modeling results suggest that the large increase in solubility with the cosolvent (co-contaminant) present is not just attributable to the bulk density increase when chloroform is added, but evidently is also due to the enrichment of the chloroform in the solvation sphere around the solute.