Metal Ion Uptake Behaviour of Nafion in Presence of Organic Complexing Reagents

Membrane separation is a very well known technology having a large number of applications. Separation of metal ions by selective membrane transport is of great importance from both fundamental and practical viewpoints. According to IUPAC, a membrane is defined as a heterogeneous phase which acts as a barrier to the flow of molecules and ionic species in liquid or vapour phase. Membranes can be classified into solid and liquid membranes. Ionomers are a class of solid ion exchange membranes with ionic content as high as 10-15 mol%. The most common ionomer is Nafion 117. It is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer possessing excellent water sorption and metal ion transport properties [1]. Cellulose nitrate membrane was also used for sorption of metal ions but the solubility of the membrane in acid was used as an advantage in preconcentration techniques for the determination metal ions. A membrane filtration procedure applied for the preconcentration of nickel(II), cadmium(II), copper(II), cobalt(II) and lead(II) ions in which the metal ions were adsorbed on cellulose nitrate membrane filter as their ammonium pyrrolydinedithiocarbamate (APDC) complexes. Then membrane filter was dissolved by using nitric acid for quantification [2]. The metal-calmagite complexes on a soluble cellulose nitrate membrane filter were tried for the preconcentration of Cr(III), Co(II), Cu(II), Fe(III) and Pb(II) ions [3] The preconcentration/separation procedure is based on chelate formation of the metal ions like Cu(II), Fe(III), Pb(II), Co(II) and Cr(III) with benzopurpurine 4B and on retention of the chelates on cellulose nitrate membrane filter were studied for the determination of these metal ions by dissolving the membranes in acid [4]. The heavy metal ions like copper, iron, cobalt, and cadmium were preconcentrated by collected on cellulose nitrate membrane filter as their 8-hydroxyquinoline complexes and the membrane filter was dissolved by using small amounts of concentrated nitric acid for their determinations [5]. The main disadvantage of cellulose nitrate is its solubility in acidic conditions thus making it necessary to look for an alternate. Nafion with its high chemical stability appears as good alternate for metal ion uptake.