The Effects of Halide Anions on the Partitioning Behavior of Pertechnetate in Polyethylene Glycolbased Aqueous Biphasic Systems

Abstract

The pertechnetate anion quantitatively partitions to the upper polyethylene glycol (PEG)-rich phase in aqueous biphasic systems (ABS) formed by the addition of aqueous high-molecular-weight PEG solutions to aqueous water-structuring salt solutions. Most metal cations partition to the salt-rich phase in these systems. Other matrix ions, such as those that might be present in real-world waste streams, affect the distribution ratios observed for TcO₄ ⁻, however, the effects are not well understood. The halide ions F⁻, Cl⁻, Br⁻, I⁻ represent a wide range of water-structuring to chaotropic ions. The heavier halide ions have distribution ratios above one and are observed to follow the order D_I > D_Br > D_Cl. These ions depress D_Tc. Fluoride saltsout PEG and thus partitions primarily to the salt-rich phase and fluoride increases D_Tc. These trends can be correlated with the ions' Gibbs free energies of hydration (ΔG_hyd), a measure of their interactions with water. Based upon these results, it is possible to predict which ions will depress, elevate, or not affect D_Tc for a given PEG-ABS based upon the matrix ion's Δ G_hyd.