By Aqueous Biphasic Extraction Chromatography ABEC

In classical solvent extraction, it is often inconvenient for the extracted species to remain in the solvent phase and it is usual to back-strip into a fresh aqueous phase. This may often be achieved in some simple fashion, such as by reduction or, more commonly, by change in pH to produce a charged moiety that is now more soluble in the aqueous phase. In such cases, albeit after suitable clean-up, spent solvent may usually be recycled and reused, if necessary, after distillation. For ABS, this is much more difficult to achieve. Back-stripping steps may be more difficult to design—especially where the forward extraction has been most effective. Recycle of the extracting PEG-rich phase may be difficult and expensive to achieve because it necessitates the removal of water.

Covalent attachment and immobilization of PEG onto a solid support appears to overcome the limitations of ABS, because the biphasic conditions necessary for the forward extraction of a given species may be created or destroyed at will simply by addition of the appropriate solvent, i.e., salt solution or water, without any loss of the phase-forming polymer (11,32). In the simplest case, extraction to the immobilized PEG phase may be made under appropriate conditions of salt, pH, and so forth, and then recovery effected by the simple expedient of adding water overcoming in large part many of the potential drawbacks of adopting aqueous biphasic extraction technology. All those applications outlined in Subheading 1.1. for ABS thus may be simply and directly transferred to the ABEC mode of operation, with considerable advantage to the overall efficiency of the process.

Figure 3 shows the distribution ratios for ABEC and ABS of the pertechnetate anion for a variety of different salts. The distribution ratio (D) referred to in the figure is simply the ratio of the concentration of the solute between the two phases (PEG-rich/salt-rich). An analogous distribution ratio may be defined for partition to a solid phase (DW, see Subheading 3.). The similarity in the effect of changing salt type and/or concentration in ABS and ABEC may be judged from Fig. 3.

Again, for illustrative purposes, and because of its high practical potential, the example given here concerns the extraction of technetium in the form of the pertechnetate anion onto an ABEC resin. The two procedures may thus be directly compared. Figure 3 shows such a comparison for the distribution of pertechnetate in ABS and ABEC with increasing concentration of ammonium sulfate. As in the case of the ABS procedure, the technique may easily be adapted to the examination of other conditions and ions. Additionally, the ABEC procedure as outlined may be adapted to packed bed or column mode of operation, thus permitting the examination of performance criteria pertinent to industrial scales of operation (32). A simple procedure for the operation of ABEC adsorbents in chromatographic mode is also presented here.

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