Aqueous biphasic extraction of particulates: Controlling the hydrophilic-hydrophobic balance for separations

It is possible to form a two-phase system in certain polymer/inorganic salt/water mixtures, such as those containing the polymer polyethylene glycol (PEG). Such aqueous biphase systems have many of the separation characteristics of conventional solvent extraction systems, but are environmentally friendlier. Just like the well-known organic solvent/water two-phase system, the top polymer-rich phase is more hydrophobic, and the bottom salt-rich phase more hydrophilic. The partition of dispersed particles is dependent on the interfacial interaction between the particles and the conjugate aqueous biphase solutions. In particular, the hydrophilic/hydrophobic properties of the surface determine the transfer of the dispersed particles into either the top (more hydrophilic) phase or the bottom (more hydrophobic) phase. Accordingly, differences in the surface chemistry of particles can be exploited to achieve selective separations. In this work, several approaches to separation control will be discussed, using pyrite (FeS₂), alumina (Al₂O₃), hematite (Fe₂O₃), silica (SiO₂), and titania (TiO₂) as model particulate materials. For metal sulfides, pH and redox potential can be used to alter the hydrophobicity/hydrophilicity of the particles. In the case of oxides, polymer adsorption and collector adsorption may be exploited to facilitate particle transfer.