EXTRACTION OF METAL IONS BY COUPLED TRANSPORT MECHANISM USING SUPPORTED LIQUID MEMBRANES

Abstract

This work elucidates the transport study of Mn (II), Tl (III), Ag (I) and Pb (II) ions through supported liquid membranes. The triethanolamine (TEA) dissolved in cyclohexanone and TEA dissolved in xylene were used as carrier, incorporated in microporous polypropylene membranes, for transport of Mn (II) and Tl (III), respectively. The Pb (II) and Ag (I) transport was carried out through tri-n-octylamine (TOA)-xylene- polypropylene and tri-n-dodecylamine (TDDA)-cyclohexane-polypropylene supported liquid membrane, respectively. The purpose of this study is as follows: (I) Optimization of the various conditions (acid concentration in feed solution, carrier concentration in membrane phase, stripping phase composition and stirring speed, etc.) for transport of the given metal ions from feed solution into strip solution, (II) derivation of theoretical equations for transport of these metal ions and their use to investigate the stoichiometry and mechanism of transport of the complexes formed inside the organic membrane phase, (III) characterization of the supported liquid membranes in term of flux, permeability and diffusion coefficient and (IV) the investigation of the stability of supported liquid membranes to evaluate the chances of their applications in industry. The optimum conditions investigated for transport of Mn (II) are: 3.75 M TEA in cyclohexanone, 1.0 M H2SO4 in feed solution, 9.25  10-3 mol/dm3 FeSO4 in 0.5 M H2SO4 in strip solution and stirring speed of 1500 rpm, for Tl (III), 5.26 mol/dm3 of TEA in xylene in membrane phase, 1.0 mol/dm3 of HCl in feed solution and 1.0 mol/dm3 of NaOH in stripping phase, for transport of Pb (II), 0.2 mol/dm3 of HNO3 in feed solution, 0.872 M of TOA in xylene in membrane phase and 0.2 mol/dm3 of NaOH in stripping phase, and for transport of Ag (I), 0.75 mol/dm3 of HNO3 in feed solution, 0.788 M of TDDA in cyclohexane in membrane phase and 1.0 mol/dm3 of NH3(aq) in stripping phase. The species or complexes extracted into organic membrane phase have been investigated as: (C2H4OH)3NH.MnO4 for extraction of Mn (II) and [(C2H4OH)3NH]3.TlCl6 for transport of Tl (III). Furthermore it is also confirmed that addition of proton (H+) in these species target at the N site and not the –OH sites of TEA. The species Pb (NO3)4(HNR3)2 was found responsible for transport of Pb (II) where R stands for octyl group and (LH)Ag(NO3)2 for transport of Ag (I), where L stands for tri-n-dodecylamine. The stability and durability of TEA-cyclohexanone membrane was studied for Mn (II) extraction and found quite stable for 168 hours. The TEA-xylene SLM stability for transport of Tl (III) was investigated up to 192 hours and extraction found for each run was greater than 99%. The stability of tri-n-dodecylamine-cyclohexane and tri-n- octylamine-xylene SLMs for transport of Ag (I) and Pb (II) were investigated and found stable for 120 and 240 hours respectively. The stability studies indicate that these SLMs can be used comfortably for removal of these metal ions on industrial scale. To show practical utilization of these SLMs, these were used for extraction of aforementioned metal ions from industrial wastes, effluents and their other toxic sources. Almost all Mn (II) was extracted from a discharged zinc carbon dry battery cell solution using TEA-cyclohexanone SLM; similarly, up to 99% Tl (III) was recovered from coal ash leach liquors. The tri-n-dodecylamine-cyclohexane SLM was successfully used for extraction of Ag (I) from silver plating and photographic waste solution. Approximately all Pb (II) was transported from aqueous acidic leached solution of paint and industrial effluents, while using tri-n-octylamine-xylene SLM. It has also been confirmed in the case of transport of Ag (I), that flux of Ag (I) decreases as the thickness of membrane film increases. Furthermore, it was observed in Tl (III) transport that diffusion of the metal complex increased with increasing temperature.