Thermodynamics of extraction of copper(II) from aqueous solutions by chelation in supercritical carbon dioxide

The primary goal of this research was to investigate the feasibility of removing heavy metals from wastewater by chelation in supercritical carbon dioxide. Copper was used as the model contaminant. Extraction efficiency was determined as a function of temperature, pressure, initial moles of chelating agent, and initial moles of copper in a 300-cm(3) autoclave. Results indicated that 60% removal of copper can be achieved in a single equilibrium stage with an initial copper concentration of 100 ppm and hexafluoroacetylacetone (HFA), present in excess, as the chelating agent. Experimental extraction efficiencies ranged from 14 to 60% and increased with increasing initial chelate concentration and CO2 density and with decreasing initial copper concentration. A thermodynamic model based on combined reaction and phase equilibria was developed for the prediction of extraction efficiencies. Five aqueous-phase reactions and four phase equilibrium relations were considered in the development of the model. Equilibrium constants and phase distribution coefficients we re either drawn from the available literature or measured. The distribution coefficient of the copper-chelate complex was estimated as the ratio of the solubility of the complex in the supercritical phase to its solubility in the aqueous phase. The model successfully predicted the effects of temperature, pressure, and initial amounts of chelating agent and metal contaminant on extraction efficiencies without the use of any adjustable parameters.