Naphthol- and resorcinol-based azo dyes as metal ion complexants in aqueous biphasic systems

Aqueous biphasic systems (ABS) are comprised of both a polymer-rich phase (e.g., polyethylene glycol, PEG) and a salt-rich phase [e.g., (NH4)2SO4] such that both phases are 80% water on a molar basis. ABS have demonstrated applications as environmentally-friendly methods to separate relatively hydrophobic anionic species, such as pertechnetate and mercury halide anionic complexes, from high ionic strength solutions although partitioning of hydrated metal ions, such as Fe3+ and actinides, to the PEG-rich phase is negligible without the addition of a metal ion complexant to the system. Four naphthol- or resorcinol-based dyes; 1-(2-pyridylazo)-2-naphthol (PAN), 1-(thiazolylazo)-2-naphthol (TAN), 4-(2-pyridylazo)-resorcinol (PAR) and 4-(2-thiazolylazo)-resorcinol (TAR), each incorporating a naphthol or resorcinol with an ortho azo functional group, have been studied as metal ion extractants in ABS as a function of pH. In the PEG-2000/(NH4)2SO4 ABS, the distribution ratios of Fe3+, Co2+ and Ni2+ were enhanced by several orders of magnitude at high pH in contrast to the behavior of Cs+, Cd2+ and Eu3+ whose partitioning behavior was largely unaffected by the presence of these extractants at any pH. The three extracted metal ions, Fe3+, Co2+ and Ni2+, could be stripped by contact with a fresh salt phase at low pH.