Affinity extraction into CO2.: 2.: Extraction of heavy metals into CO2 low-pH aqueous solutions

Because it is an environmentally benign solvent, CO2 is increasingly being evaluated as a medium with which to perform liquid-liquid extraction from water (unlike conventional organic solvents, no contamination of the aqueous phase results). A number of researchers have explored the design of highly CO2-soluble chelating agents to allow extraction of metal ions, yet the low pH of water in contact with CO2 (2.8-3.0) can lead to poor extraction efficiencies with many common chelating agents. In this paper, we describe the generation of a CO2-soluble analogue to ammonium pyrrolidinedithiocarbamate, an agent which has been shown to effectively extract a variety of metal ions at pH's as low as 1.0. We have found that the CO2-soluble analogue can extract high fractions of copper, nickel, and cadmium, although, like conventional APDC, extraction of chromium was less successful.