Solid phase extraction of zinc(II) using a PVC-based polymer inclusion membrane with di(2-ethylhexyl)phosphoric acid (D2EHPA) as the carrier
Introduction
Recently, polymer inclusion membranes (PIMs) have attracted considerable attention for use as solid phase absorbents in separation problems because they eliminate the need for the flammable diluents required in solvent extraction systems while retaining the selectivity associated with the extractant [1]. In addition, when PIMs are incorporated into a transport cell, they allow both extraction and back-extraction to proceed simultaneously at opposite sides of the membrane. This speeds up the separation process compared to conventional adsorption (e.g., ion-exchange resins) or solvent extraction-based separation.
PIMs consist of a carrier immobilized in a base-polymer and our papers using the commercial extraction reagent Aliquat 336 have demonstrated that PIMs can be successfully applied to a variety of separation problems involving metal ions [2], [3], [4], [5], [6], [7].
The extraction and separation of Zn(II) from aqueous solutions has achieved great importance because of the widespread industrial use of zinc and its classification as a dangerous substance, particularly in waste streams and saline waters [8]. Some studies on the application of PIMs for Zn(II) extraction and transport have been published [9], [10], [11].
Ulewicz and Walkowiak [9] studied the separation of Zn(II), Co(II), Ni(II), Cu(II) and Cd(II) from aqueous chloride solutions using a cellulose acetate (CTA)-based PIM containing di(2-ethylhexyl)phosphoric acid (D2EHPA) as carrier and 2-nitrophenyl pentyl ether as a plasticizer. In another study, Resina et al. [10] studied the transport of Zn(II), Cd(II) and Cu(II) from chloride solutions across CTA/sol–gel hybrid membranes with D2EHPA and di-(2-ethylhexyl)dithiophosphoric acid as carriers and 2-nitrophenyl pentyl ether or tris(2-butoxyethyl)phosphate as plasticizers.
There have also been reports of other solvent extraction reagents being used to extract Zn(II) with PIMs such as basic ion carriers like tri-n-octylamine immobilized in CTA [11]. However, to our knowledge, there have been no studies on the membrane extraction of Zn(II) that involve poly(vinyl chloride) (PVC)-based PIMs. PVC-based membranes are more resistant to acidic solutions than CTA-based membranes [1] and this is expected to offer advantages in terms of a wider working pH range and prolonged membrane lifetime.
D2EHPA is an acidic carrier used for the extraction of metal ions [12], which shows good selectivity for the extraction of Zn(II) in the presence of other metal ions (e.g., Co(II), Ni(II), Cu(II), Cd(II), Ca(II) and Mn(II)). The most seriously interfering ion is Fe(III) and this is usually eliminated in solvent extraction by its reduction to Fe(II).
In this paper we report on the use of a PVC/D2EHPA membrane for the solid phase extraction and transport of Zn(II) from acidic chloride solutions. We also demonstrate that the PVC-based membranes used in this study do not require the inclusion of an additional plasticizer in their composition as has been the case with CTA-based PIMs. This is due to the inherent plasticizing properties of D2EHPA itself towards PVC [1].
Section snippets
Chemical analysis and measurement of membrane thickness
The concentration of Zn(II) in the solutions used in the extraction and transport experiments was determined by atomic absorption spectrometry (AAS) (Shimadzu AA-3600, Japan). The experimental conditions were: slit width—0.7 nm; Zn hollow-cathode lamp (Photron, Australia); current—8 mA; working wavelength—213.9 nm and burner height—7 mm. The flow rates of acetylene and air were 2.0 and 15.0 L min−1, respectively.
The concentrations of Co(II), Ni(II), Cu(II), Cd(II), Fe(II), and Fe(III) in the
Effect of D2EHPA concentration
Membrane pieces (total mass 114 ± 2 mg) containing 15, 20, 25, 30, 40 and 45% (m/m) D2EHPA were studied for their Zn(II) extraction efficiency using an aqueous solution containing 22 mg L−1 Zn(II) at pH 3.0 and the results are shown in Fig. 2. These membranes were all homogeneous and transparent with an oil free surface and met the requirements of a stable PIM as described in our recent paper [13]. As expected, both the extraction efficiency, determined by the amount of metal extracted, and the rate
Conclusions
The results presented in this paper demonstrate that Zn(II) can be selectively and quantitatively extracted into a polymer inclusion membrane of composition 45% (m/m) D2EHPA and 55% (m/m) PVC from aqueous solutions at pH 3.0 which may contain other base metals ions, such as Cd(II), Co(II), Cu(II), Ni(II) and Fe(II). Potential interference from Fe(III) can be removed by precipitation with orthophosphate prior to the membrane extraction of Zn(II). The stoichiometric formula of the extracted
Acknowledgement
We are grateful to the Australian Research Council for financial support under its Discovery Project scheme.
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