Development of high-performance separation methods of inorganic compounds based on aqueous biphasic extraction systems

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Aqueous biphasic systems(ABS) are formed by mixing water, certain inorganic salts and water-soluble polymers and can be efficient for the separation of inorganic compounds as well as of organic compounds. Polyethylene glycols(PEG) are nontoxic, nonflammable and nonvolatile so that the PEG-based ABS can be regarded as environmentally friendly, in contrast to conventional solvent extraction systems utilizing water immiscible organic solvents. A number of investigators have considered that water molecules in ABS should play an important role in separation processes and therefore an understanding of the states of water in the polymer systems is of great importance in the elucidation of the mechanism of separation in ABS. We have thus studied, at first, the states of water in PEG gel as well as poly(vinyl alcohol)(PVA) gels by means of liquid chromatography and differential scanning calorimetry. It was found that the estimated amount of the stationary phase water is in good agreement with the sum of the amount of non-freezing water and that of freezable bound water for most of the PVA gels, while it agrees with the amount of only non-freezing water for the PEG gel. This result can be attributed to the difference in the structure of the gels ; the PVA gels containing PVA at relatively high concentrations have a homogeneous gel phase, whereas the PEG gel have a heterogeneous gel phase consisting hydrated polymer domains and relatively hydrophobic domains.Separation of inorganic anions and metal ions was investigated using high-speed countercurrent chromatography with aqueous biphasic systems consisting of PEG and sodium sulfate. The PEG-rich upper phase was used as the stationary phase and the salt-rich lower phase was used as the mobile phase. Separation of some inorganic anions and that of chromium(III) ion and chromate(VI) ion were easily accomplished by HSCCC with PEG#1000-Na_2SO_4 ABS.