Influence of selected natural complexants on the mobilization and purging of copper from aqueous media into supercritical carbon dioxide

Abstract

The mobilization/purging of copper(II) or zinc(II) from aqueous solution by in situ complexometric extraction into supercritical carbon dioxide (SC-CO₂) is described. On line detection by using AAS has permitted the kinetics of the process to be monitored over the course of the extraction. A single step extraction procedure at moderate pressure and 50 °C resulted in virtually quantitative removal of 0.08–2.4 µmol ml^–1 Cu solution with an estimated half-life (t_1/2) of 3–5 min. By contrast, a small portion of the zinc in aqueous media was mobilized at an appreciably slower rate which was apparently the result of a non-equilibrium zinc carbonate precipitation. Recoveries of Cu^II from Cu–fulvic acid (FA) solutions or Cu–FA–sand suspensions were dependent on the concentrations of both Cu and FA in the sample. Whereas pseudo-first-order kinetics provided an adequate model for the extraction kinetics (mean t_1/2= 17.8 min) for a solution containing 1.2 µmol Cu^II+ 1.05 µmol ml^–1 FA, higher Cu^II loading caused an apparent coagulation of the FA which was accompanied by the release of a portion of the bound Cu (t_1/2= 2.5 min) but the immobilization of the remainder. For lower concentrations of Cu–FA (0.48 µmol Cu^II+ 0.10 µmol FA per millilitre), no coagulation was evident and the Cu^II analyte was purged (t_1/2= 16.7 min) virtually quantitatively. For extractions of Cu–FA–sand mixtures, recoveries were less than quantitative and changes in the AAS Cu response curve with time were better approximated by the sum of two Gaussian curves than by exponential decay curves. Cu^II was recovered quantitatively from 1%(m/m) oxalic acid but at a decreased rate of purging and this complexant increased Cu recovery from Cu–FA–sand suspensions only moderately.