Structurally simple ligands that selectively bind to mercuric chloride could help remove this toxic salt from the environment, say scientists from India.
The tripodal ligands, developed by Parimal Bharadwaj and colleagues at The Indian Institute of Technology Kanpur, exploit the ready reactivity of mercuric chloride (MgCl2) with nitrogen and oxygen atoms. The ligand binds the salt, in 90-95 per cent yield, through a central nitrogen atom, and one or more of the three ethereal oxygen atoms that form the ligand's legs. The team claims their ligands selectively bind mercuric chloride in the presence of other metal ions.
Bharadwaj has also shown that the ligands can be recycled. Adding silver hexafluorophosphate to a solution of the mercury-ligand complex displaces the mercury, giving a complex where the ligands surround a hexafluorophosphate anion. Extracting an aqueous solution of this complex with organic solvent returns the free ligand, which can be reused to trap mercury.
The tripodal ligands can by recycled by forming complexes with hexafluorophosphate ions
'Mercuric chloride is extremely toxic and can easily enter biological cycles - to remove it from its place of formation is a challenging problem,' said Bharadwaj. The salt can be found in crude oil, and is formed during electrochemical processes, added Bharadwaj. 'We've shown that simple and cheap tripodal ligands can selectively bind mercuric chloride, and the entire process can be recycled many times.'
David Atwood, University of Kentucky, US, who has developed mercury ion-binding ligands, commented that the Kanpur team haven't shown that their tripodal ligands actually work in the environment. 'Their reactions aren't run in water, or in hydrocarbons for the oil application, but rather in methanol,' he said.
We are continuing to develop ligands that can selectively bind heavy metals ions, said Bharadwaj. 'Binding a particular metal ion in the presence of many others remains a challenging problem, although there are a few ground rules we can follow,' he said.
James Mitchell Crow