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Highlights in Chemical Technology

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Zwitterionic metal salt extractants


18 January 2010

UK scientists have developed zwitterionic salicyclaldoxime ligands that could allow industrially important metal ions to be selectively recovered from mixtures. The molecules bind both ions of a metal salt independently, which should improve the efficiency of extraction processes. 

Around 25 per cent of the world's copper is recovered using conventional phenolic oxime extractants, which only bind the copper cation, not its companion anions. When such extractants transfer copper cations from an aqueous solution to an organic phase, there is a build up of acid in the aqueous phase, which must then be neutralised, complicating the extraction process. Using metal salt extractants eliminates this acid build up and hence does not require any adjustment of pH says Peter Tasker from the University of Edinburgh. The technology could eventually find commercial applications in the recovery of a variety of base metals. 

Tasker and colleagues made dialkylaminomethyl substituted salicyclaldoximes that are capable of binding a metal cation and attendant anions in separated sites of a zwitterionic form of the extractant making a charge-neutral assembly. Tasker explains that 'these new reagents extract metal salts, which is in contrast to the conventional oxime reagents which transport metal cations' and as a result there is no net exchange of cations or anions, which makes them better suited to treating waste streams, he adds. As a consequence of the trans structure formed by bis-salicyclaldoximato complexes of Cu(II), the complexes are tritopic (they have three binding sites) meaning that divalent metals can be transported with both their accompanying mono-anions, leading to increased efficiency. 

bis-salicyclaldoximato complexes of Cu(II)

The trans structure formed by bis-salicyclaldoximato complexes of Cu(II) have three binding sites

'Core principles of supramolecular chemistry have been used to solve an extremely important industrial problem' says Jonathan Steed, an expert in supramolecular chemistry from the University of Durham, UK. 'As sources of industrially important metals become increasingly rare, techniques such as this will allow the extraction of important metal from lower grade sources, at lower concentration or from more complex structures,' he adds. 

Tasker points out that one of the major challenges associated with the commercial exploitation of these new reagents is to obtain high selectivity of both cation and anion transport. This work has shown that co-operative cation and anion binding may solve this problem. He adds that the group are hoping to 'extend this approach to the recovery of precious metals, lanthanides and actinides'. 

Nicola Wise 

 

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Link to journal article

Cation and anion selectivity of zwitterionic salicylaldoxime metal salt extractants
Ross S. Forgan, James E. Davidson, Francesca P. A. Fabbiani, Stuart G. Galbraith, David K. Henderson, Stephen A. Moggach, Simon Parsons, Peter A. Tasker and Fraser J. White, Dalton Trans., 2010, 39, 1763
DOI: 10.1039/b916877j

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