Design of a polymer inclusion membrane having proton-ionizable polyether carriers and their separation function for lead ion

Abstract

The selective proton-driven transport of Pb2+ ion by novel polymer inclusion membranes (PIMs), which were composed of cellulose triacetate (CTA) as a membrane support, o-nitrophenyl octyl ether (NPOE) as a membrane plasticizer, and proton-ionizable polyethers 1-6 bearing different alkyl chain lengths from heptyl to hexadecyl as a metal ion carrier, were carried out. Due to a low solubility in NPOE, the PIMs with carriers 5 and 6 having dodecyl and hexadecyl chains exhibited no transportability for both Pb2+ and Cu2+. On the other hand, the selective transport of Pb2+ over Cu2+ was noted for PIMs with carriers 1-4. To elucidate the transport mechanism, the contribution of several factors, such as the membrane thickness, source-phase pH, carrier concentration in the membrane, and source solution concentration to the Pb2+ and Cu2+ ion flux across the PIM were examined. These factors were found to be well described by an analysis based on a carrier-mediated transport mechanism. The observed diffusion coefficient of a Pb2+ complex with 2 was 7 × 10 -12 m2 s-1, which was comparable with that of metal complexes with crown ether carriers in a NPOE-based liquid membrane. The transport selectivity of the PIM with 2 was Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+, which was consistent with the extraction selectivity of 2. The superior durability of CTA-based PIM compared with poly(vinyl chloride)-based PIM was confirmed by replicate experiments. The durability of CTA-based PIM was also supported by a scanning electron microscope analysis.