pH switching for the selective extraction of metal ions into supercritical CO2

Here we describe a unique extraction process utilizing pH switches for the selective removal of metal ions into supercritical CO2 using reversed micelles. Supercritical fluid extraction from an aqueous matrix has traditionally been limited by the inherent low pH of the aqueous phase of the system and the use of costly fluorinated chelating agents. The unique extraction process described herein utilizes a cheap, commercially available hydrocarbon surfactant as a complexing agent that forms reversed micelles in CO2. Subsequently, the pH of the biphasic system is controlled by the addition of buffers allowing facile manipulation of metal ion speciation. The control of the metal ion speciation is shown to have a quantitative effect on the efficiency at which metal ions are extracted from the aqueous phase. Furthermore, we demonstrate that pressure-induced pH switches of the aqueous phase can be utilized for the selective separation of chromium(III), cobalt(II), and gold(III) ions from an aqueous metallic broth.