Effect of cross-linking on the performance of polymer inclusion membranes (PIMs) for the extraction, transport and separation of Zn(II)

This study demonstrated the advantages of cross-linking of polymer inclusion membranes (PIMs) in terms of improved stability, rate of extraction and extractive capacity. These advantages were illustrated with the new cross-linked PIM for the selective extraction of Zn(II), developed as part of this study and containing bis-(2-ethylhexyl)phosphoric acid (D2EHPA) as the carrier (extractant). Three different base-polymers were tested, namely poly(vinyl chloride) (PVC), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and cellulose triacetate (CTA), using poly(ethylene glycol) dimethacrylate (PEG-DMA) as the cross-linking polymer and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) as the initiator. Only PVDF-HFP- and CTA-based PIMs were found to be successful (i.e., homogenous, transparent and mechanically stable). CTA-based PIMs showed a higher extraction rate than the PVDF-HFP-based PIMs and non-cross-linked PIMs containing the same base-polymer, and thus CTA was chosen as the base-polymer for a more detailed study. The effect of the polymer ratio (CTA:PEGDMA), concentration of D2EHPA, and concentration of plasticizer (i.e., 2-nitrophenyloctyl ether (NPOE)) on the membrane performance was studied. Best performance in terms of stability, rate of extraction and extractive capacity, which were superior to the corresponding non-cross-linked PIM, was achieved with the cross-linked PIM composed of 40 wt% D2EHPA, 10 wt% NPOE, polymers ratio of 6:4 (CTA:PEG-DMA), and 0.6 wt% DMPA. The PIM surface morphology, examined by atomic force microscopy (AFM), correlated well with the rate of Zn (II) extraction. It was also demonstrated that the cross-linked PIM could selectively extract Zn(II) from aqueous solutions at pH 3.0 in the presence of other common base metal ions such as Cd(II), Co(II), Cu(II), and Ni(II) in concentrations 5 times higher than that of Zn(II). Interference from Fe(III) was removed by precipitation with phosphate. The newly developed PIM transported practically all Zn(II) from a feed solution at pH 3 to a 1.0 M HCl receiving solution as a result of the simultaneous extraction and back-extraction of Zn(II) at the corresponding PIM/solution interfaces, and the facilitated transport of Zn(II) across the membrane.