RECOVERY OF METALS FROM AQUEOUS-MEDIA BY EXTRACTION WITH SUPERCRITICAL CARBON-DIOXIDE

Measurements of solubilities in supercritical CO2 corroborated the hypothesis that the nonpolar character of a series of tetraalkylammonium dialkyldithiocarbamate ion pairs was influenced (i) primarily by the chain length df the alkyl substituent on the carbamate nitrogen and (ii) to a lesser extent, by the chain length of the alkyl substituent(s) on the ammonium counterion. Similarly, the solubility of Zn-dithiocarbamate complexes decreased in the order dibutyl (DBDTC) > diethyl > pyrrolidine. An extraction protocol in which SC-CO2 was saturated with tetrabutylammonium DBDTC prior to dynamic complexation/mobilization of 12-20 mu g/mL Zn, Cd, or Pb analyte from aqueous solution resulted in >73% and >94% analyte removal within 5 and 15 min extraction, respectively, However, time-dependent declination curves of the residual content of analyte in the aqueous medium were distinctly biphasic for all three metals. The apparent pseudo-first-order rates of depuration were increased appreciably by acidifying the aqueous matrix prior to extraction with the result that >95% of the analytes was removed within 10 min. The influence of phosphate anionic species on the rates of metal analyte extraction suggests that by monitoring the changes in rates of mobilization/extraction with time it will be possible not only to quantify total levels of extractable analyte but also to distinguish between different analyte species based on their rates of transposition to an extractable form.