Extraction and recovery of metals using a supercritical fluid with chelating agents

Abstract

The supercritical fluid extraction (SFE) of metal ions was assessed in terms of the ability of the process to extract, solvate, transport and collect metal species. The extraction efficiency (per cent removed from the sample matrix), collection efficiency (per cent recovered in the collection solvent) and SFE residue (unextracted metal complex or metal ion recovered from the apparatus by flushing with an organic or acidic solvent, respectively) were determined to ascertain whether a mass balance for the metal could be achieved. Using this approach, it was demonstrated that the amount of metal extracted from the sample matrix did not necessarily correspond to the concentration of metal recovered in the collection solvent, as the chelated metal complex could partially degrade and/or dissociate in the supercritical fluid prior to collection. Typically, a discrepancy of 1 to 15% could be obtained. The non-fluorinated metal β-diketones and dithiocarbamates had a greater tendency to dissociate and/or degrade in a supercritical fluid (i.e., ca. 14 and 5%, respectively) than the corresponding fluorinated metal complex (i.e., ca. 6 and 1%, respectively). As for the collection solvents, a range of suitable fluids were found to be capable of obtaining quantitative metal complex recoveries, including methanol, chloroform, isobutyl methyl ketone or 10% nitric acid. A radioactive isotope, ¹⁷⁷Lu, was used to trace the location of the metal residue in the SFE apparatus.