Three-Liquid-Phase Extraction Procedure.
Extraction experiments were performed in 50 mL graduated centrifugal
tubes by taking fixed volumes of organic (5 mL) and aqueous (20 mL) phases.
In all experiments, 30% (v/v) D2EHPA in n-hexane was used as the organic phase, whereas a mixture
containing 4 g of PEG (8 mL of PEG stock solution) and 4 g of (NH4)2SO4 [8 mL of (NH4)2SO4 stock solution] was used as the aqueous phases, unless otherwise stated. After mixing of the organic
and aqueous phases and pre-equilibration for a while,1 mL of Cr stock 1 was added,except in the study of the initial chromium amount, for which Cr stock 2 was used.
The system was made to volume (25 mL) with deionized water.
Note that,except for the investigation of the effects of the aqueous pH, the pH of the (NH4)2SO4-rich bottom phase of the pre-equilibratedTable 1.
Extraction of Cr(III) and Cr(VI) as a Function of pHini and pHeq in the Three-Liquid-Phase
SystemTLPS was adjust to the preferred value of pH 5.0, based on the results from the study of the effect of the aqueous pH.
Then, the tubes containing the solutions were shaken in a mechanical
shaker for a specified period followed by 10 min of centrifugation at a speed of 4000 rpm.
After phase separation, volumes of immiscible phases could be directly read from the scale of the
tube.
A sample of each separated phase was carefully pipetted and used for analysis.
The final equilibrium pH value (pHeq) of the (NH4)2SO4-rich bottom phase was also measured.