A comparison of the use of commercial and diluent free LIX84I in poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based polymer inclusion membranes for the extraction and transport of Cu(II)
Introduction
LIX84I is a commercially available extractant in the class of reagents commonly known as hydroxyoximes [1], which was originally produced by the Henkel Corporation in 1986 as a Cu(II) extractant. However, in order to improve the selectivity and extraction rate for Cu(II) this extractant was blended with other extractants such as LIX 860 [2]. The high selectivity of these LIX blends for Cu(II) over other metallic cations together with other advantages, such as chemical stability, low aqueous solubility and simple phase separation [3], have resulted in the successful application of these blends in industrial solvent extraction (SX). A number of recent studies based on SX have optimized the conditions of Cu(II) extraction depending on the two most significant factors in this process, namely solution pH and extractant concentration [4], [5], [6], [7], [8], [9]. Even though, SX is a highly successful hydrometallurgical process, it employs large amounts of often toxic, flammable and volatile diluents [10]. This is one reason why other separation techniques have been of interest, one of which involves liquid membranes [11].
Liquid membranes (LMs) do not require the use of large amounts of diluents and the separation process is simplified by carrying out extraction and back-extraction in a single step. Different types of LMs have been applied to the separation of Cu(II) using LIX84I as the extractant, namely, emulsion liquid membranes [11], supported liquid membranes (SLMs) [3], [12], hollow fibre LMs [13], [14], [15], [16], [17], [18], and polymer inclusion membranes (PIMs) [19], [20], [21], [22], [23]. Among them, PIMs appear to be more attractive due to their high stability in comparison with the other types of liquid membranes (e.g., SLMs) [10], [24]. PIMs are produced by casting from a solution of a volatile solvent (e.g., tetrahydrofuran, dichloromethane) containing a base-polymer and the components of the membrane liquid phase, i.e., an extractant (often referred to as carrier) and in some cases a plasticizer or modifier [10], [24]. After the evaporation of the solvent, the membrane liquid phase will be localized between the entangled chains of the base-polymer which explains the better stability of PIMs compared to SLMs. Despite this fact, further improvements in PIM stability and permeability will be required before these membranes can be adopted in large scale industrial separation processes. However, it should be pointed out that PIMs have been already successfully applied in chemical analysis [25], e.g., passive sampling [26], [27], online separation in flow analysis techniques [28], [29], [30], [31], [32] and analytical paper-based microfluidics [33].
De Gyves et al. [20] have optimized the efficiency factors (e.g., permeability, selectivity and stability for Cu(II) transport) using a LIX84I-based PIM with tris(2-butoxyethyl)phosphate (TBEP) as the plasticizer and cellulose triacetate (CTA) as the base-polymer. In recent studies Wang et al. used commercial LIX84I to prepare PIMs with poly(vinyl chloride) (PVC) [21], [22] or poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) [23] as the base-polymer and 2-nitrophenyloctyl ether (NPOE) as the plasticizer for Cu(II) separation from ammonium sulfate solutions in which factors influencing the transport efficiency of the PIMs and their selectivity for Cu(II) were also studied.
The active component of commercial LIX84I, 2-hydroxy-5-nonylacetophenone (HNAPO), is dissolved in a high flash point hydrocarbon diluent as reported by the supplier. Tanaka et al. [34] determined the concentration of HNAPO in commercial LIX84I as 1.72 mmol g−1 by fully loading the organic phase with Cu(II). Narita et al. [35] also quoted a similar HNAPO content of 44 wt% (1.60 mmol g−1). To the best of our knowledge, only Warren et al. [36] have reported on the use of diluent free LIX84I in studying the Ni(II)/HNAPO SX system. An increase in the concentration of LIX84I in SX is usually carried out by simply increasing its concentration in the organic phase. However, this approach when applied to the fabrication of PIMs is limited, since PIMs, depending on their composition, can accommodate only a certain amount of liquid phase. Furthermore, it is unknown whether the diluent or any other additives could potentially inhibit the PIM transport performance for Cu(II). Thus, it is of interest to study the extraction and transport efficiencies for Cu(II) and the stability of PIMs using diluent free LIX84I.
This paper reports on a comparison between the Cu(II) extraction and transport performance of commercial and diluent free LIX84I-based PIMs, and on their respective stability in terms of transport performance and mass loss in consecutive transport experiments. The PIM stability is of particular interest since the presence of diluent in commercial LIX84I might be expected to lead to some loss of the membrane liquid phase to the aqueous solution. A previously optimized commercial LIX84I-based PIM composition was chosen for the present study, and thus PVDF-HFP and NPOE were used as the base-polymer and the plasticizer, respectively [23]. Ammonium sulfate solutions of Cu(II) at pH 8.5 were used to mimic leach liquors derived from ammonium sulfate/ammonia leaching of low grade copper ores, and Cu(II) is thus extracted according to Eq. (1) [23].where RH represents HNAPO and subscripts aq and PIM refer to the aqueous and membrane phases, respectively.
Section snippets
Reagents and solutions
LIX84I (Cognis Co.), sodium hydroxide (BDH), acetone (Chem-Supply), chloroform (Chem-Supply), anhydrous sodium sulfate (BDH), and sulfuric acid (Chem-Supply) were used in the purification process of commercial LIX84I. Commercial and purified LIX84I, PVDF-HFP (Arkema), NPOE (Sigma-Aldrich), and tetrahydrofuran (THF) (Chem-Supply) were used to prepare membranes. CuSO4⋅5H2O, ZnSO4⋅7H2O, MgSO4⋅7H2O, NiSO4⋅6H2O, CoSO4⋅7H2O, and (NH4)2SO4 were obtained from Chem-Supply (Australia) and used to prepare
GC–MS analysis
Gas chromatograms in the full scan mode of the commercial and diluent free LIX84I are shown in Fig. 1. As expected, the gas chromatogram of diluent free LIX84I had a lower number of peaks than that of the commercial sample. The peaks at retention times (RTs) shorter than 5 min were considered to be associated with hexane which was used for dissolving the LIX84I samples. However, there are several regions of particular interest in the gas chromatograms and these are discussed below in terms of
Conclusions
The main aim of this research was to remove the diluent components from commercial LIX84I and to determine if the use of the diluent free extractant in PIMs would result in those membranes having superior properties in terms of stability on one hand and improved extraction and transport rates of Cu(II) on the other. The use of diluent free extractant would also potentially allow the casting of PIMs with higher extractant concentrations. The purification of commercial LIX84I (49 wt% HNAPO)
Acknowledgements
The authors dedicate this paper to the memory of Yuzo Baba from the Department of Applied Chemistry of Tokushima University who passed away unexpectedly on February 10 2017 at the age of 29. He provided valuable suggestions for this research. Duo Wang is grateful for the financial support provided by the National Basic Research Program of China (No. 2014CB643401), National Natural Science Foundation of China (No. 51134007) and China Scholarship Council (No. 201506370136).
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