Elsevier

Journal of Membrane Science

Volume 545, 1 January 2018, Pages 57-65
Journal of Membrane Science

Selective extraction of vanadium(V) from sulfate solutions into a polymer inclusion membrane composed of poly(vinylidenefluoride-co-hexafluoropropylene) and Cyphos® IL 101

https://doi.org/10.1016/j.memsci.2017.09.058Get rights and content

Highlights

  • Polymer inclusion membrane (PIM) for the selective extraction of V(V) was developed.

  • Poly(vinylidene fluoride-co-hexafluoropropylene) (55%) was used as the base polymer.

  • Cyphos® IL 101 (35%) and 2-nitrophenyloctyl ether were the carrier and plasticizer.

  • 100% back-extraction of V(V) was obtained using 6 mol L-1 sulfuric acid solution.

  • Mo(VI) could be separated from V(V) using a two-step separation procedure.

Abstract

A poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP)-based PIM containing trihexyltetradecylphosphonium chloride (Cyphos® IL 101) as its carrier was developed for the selective extraction of V(V) from sulfate solutions. Various plasticizers/modifiers were tested and best results in terms of rate of extraction and amount extracted were obtained with 2-nitrophenyloctyl ether (NPOE). The optimal PIM composition, with respect to the same criteria was determined to be 35 wt% Cyphos® IL 101, 55 wt% PVDF-HFP and 10 wt% NPOE. Back-extraction of V(V) was achieved using 6 mol L−1 H2SO4. An anion exchange mechanism is proposed for the extraction of V(V) involving the formation of the complex anion VO2SO4-. The selectivity of the PIM towards V(V) was tested in the presence of Mo(VI), Al(III), Co(II), Cu(II), Fe(III), Mn(II), and Ni(II). Only Mo(VI) was co-extracted with V(V) at pH 2.3. The Mo(VI) interference was eliminated by a two-step separation procedure in which Mo(VI) was extracted at pH 1.1 which was followed by the selective extraction of V(V) at pH 2.3. The PIM was found to be stable over five extraction/back-extraction cycles.

Introduction

Although vanadium is present at trace levels in biological systems [1], it is hazardous to living organisms at high concentration levels [2]. Moreover, the specific properties of vanadium, such as its hardness, fatigue resistance, tensile strength, good corrosion resistance at low temperature and high melting point, define this metal as a strategic industrial material [3]. It is commonly used in the preparation of various catalysts for the desulfurization process in the petroleum industry [4], as well as in the manufacturing of numerous goods such as batteries, military equipment, fuel cells, nuclear reactors, cars, and ceramic products [5].

Vanadium is mainly obtained through hydrometallurgical processes from vanadium-containing ores, chromium-bearing vanadium slag, stone coal, fly ash, and spent industrial catalysts [6], [7], [8]. The scarcity and gradual depletion of vanadium containing ore reserves on one hand, and the increased consumption of this metal in various industries on the other, have promoted research towards its recycling from secondary sources. For instance, spent hydrodesulfurisation catalyst is a significant secondary vanadium source [9], which also contains molybdenum, cobalt and nickel on an alumina carrier [4], [6]. A common strategy for the recovery of vanadium from such materials includes a two-step solvent extraction procedure in which molybdenum is quantitatively extracted from the feed solution, by using a mixture of Alamine 336 and tri-n-butylphosphate and this is followed by the extraction of vanadium from the raffinate under different operational conditions [6].

Although the hydrometallurgical recovery of vanadium is generally based on traditional solvent extraction techniques, the application of liquid membranes has also been reported. Separation based on the use of liquid membranes minimises and even can eliminate the use of diluents and allows extraction and back-extraction to occur simultaneously at the corresponding membrane/solution interfaces. Palet et al. introduced a supported liquid membrane (SLM) containing Aliquat 336 dissolved in cumene and dodecane for the transport of V(V) from acetate buffer solutions [10]. Lozano et al. have proposed the transport of vanadium oxyanions from sulfate media through SLMs containing an Alamine 336/Cyanex 923 mixture and demonstrated the potential of this technique for the recovery of vanadium from diluted effluents [11]. Melita and Gumrah examined activated composite membranes incorporating Aliquat 336 as carrier in cyclohexane and dodecane for the transport of V(V) and Ni(II) [12]. It has also been shown that the addition of the ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)] imide to an SLM containing tri-n-octyl methyl ammonium chloride enhances the transport of V(IV) from a sulfate into an ammonia solution [13]. A transport study of V(V) through SLMs based on tri-n-octyl amine as the carrier in cyclohexane has been reported by Chadury et al. [14]. The application of bis(2-ethylhexyl)phosphoric acid and tri-n-octylphosphine oxide as phase transfer reagents in SLMs for the transport of VO2+ has been studied by Hor et al. [15].

Even though SLM-based separation is an attractive alternative to conventional solvent extraction, these membranes suffer from poor stability due to leaching of the organic phase into the adjacent liquid phases. However, another type of liquid membranes called polymer inclusion membranes (PIMs) are known to be more stable than SLMs because the membrane liquid phase (i.e. extractant and plasticizer/modifier) is entangled within the polymer chains instead of being attached to a porous support by capillary forces as in SLMs [16]. Moreover, PIMs are a greener alternative to solvent extraction as they virtually use no toxic and flammable organic diluents and require a very small amount of extractant. These membranes have been used extensively for the extraction/transport of various chemical species [17], [18]. Nevertheless, to the best of our knowledge, there is no published work on the application of PIMs for the extraction/separation of vanadium ions. Hence, in this paper we report the development of a PIM for the extraction, separation and recovery of vanadium(V) from sulfate media containing other metal ions (e.g. molybdenum(VI)). The sulfate media was selected because the leaching from raw materials and the secondary sources containing vanadium are widely performed with sulfuric acid [7], [8].

Ionic liquids have been attracting considerable interest in recent years in a wide range of research areas such as analytical chemistry, separation processes and techniques [19], [20], organic synthesis [21], and food and bioproducts industries [22], due to their unique properties as non-molecular solvents, very low vapour pressure, high thermal stability, and tuneable viscosity and miscibility with water and organic solvents [23]. For example, Aliquat 336 which is a well-known ionic liquid [24], has been extensively used in solvent extraction and in the preparation of PIMs [25], [26], [27]. Phosphonium-based ionic liquids belonging to the Cyphos® family of ionic liquids have been gaining considerable attention in PIM research [28], [29], [30], [31], [32]. Among them trihexyltetradecylphosphonium chloride (Cyphos® IL 101) (Fig. 1) has been used for the solvent extraction of vanadium(V) and molybdenum(V) [8] and has been selected as the PIM extractant/carrier in the present study.

Although PVC and CTA are widely used in the preparation of PIMs, in this study, we have not included CTA because of its instability in acidic media. PVDF-HFP was selected because of its high hydrophobicity, good thermal and mechanical properties, high stability in strong acids, and high solubility in tetrahydrofuran [31].

Section snippets

Reagents

Cyphos® IL 101 (≥ 95.0%, Aldrich, Germany) (Fig. 1a), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) (Aldrich, USA) (Fig. 1b), high molecular weight poly(vinyl chloride) (PVC) (Aldrich, Netherlands), HPLC grade tetrahydrofuran (THF) (99.70%, VWR, Australia), 2-nitrophenyloctyl ether (NPOE) (> 99.0%, Fluka, Switzerland), tributyl phosphate (TBP) (97%, Aldrich, USA), dioctylphthalate (DOP) (99%, Aldrich, USA), tris(2-ethylhexyl)phosphate (TEHP) (97%, Aldrich, France), 1-dodecanol

Selection of the base-polymer

PVC and cellulose triacetate (CTA) are the most commonly used base-polymers in the preparation of PIMs, however, PVDF-HFP offers some advantages over the other polymers, such as high hydrophobicity, excellent thermal and mechanical properties, high resistance towards acids, and excellent solubility in THF. These advantages have prompted the introduction of this polymer in the manufacturing of PIMs used for the extraction of metal ions [30], [31]. In the present study, CTA was not tested,

Conclusions

This research demonstrated that a PVDF-HFP-based PIM containing Cyphos® IL 101 as its carrier could be successfully applied to the selective extraction of V(V) from its sulfate solutions. PIMs prepared with PVDF-HFP as their base-polymer showed to be superior to PVC-based PIMs in terms of extraction rate and amount extracted. A study of various PIM plasticizers/modifiers (i.e. NPOE, 1-dodecanol, 1-tetradecanol, DOP, TEHP, and TBP) demonstrated that NPOE provided the highest extraction

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      At further H2SO4 concentration increase, V(V) transport efficiency decreased. Without an electric field, 6.0 M H2SO4 was used as a stripping solution for V(V) stripping from PVDF-HFP-based PIMs containing Cyphos IL 101 (Yaftian et al., 2018). The incompletely dissociated HSO4- replaced the V(V) ion group at the interface between the PIM and stripping solution, thus completing the V(V) stripping process.

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