Improving the extraction performance of polymer inclusion membranes by cross-linking their polymeric backbone

Highlights

• Cross-linked polymer inclusion membranes (PIMs) are prepared by UV-irradiation.

• They contain a base polymer, extractant, cross-linking polymer and photo-initiator.

• 3 of the most popular base polymers, cross-linking polymers and initiators are tested.

• Aliquat 336 or di(2-ethylhexyl) phosphoric acid act as Zn²⁺ or SCN⁻ carriers.

• Superior extracted amounts and rates compared to existing PIMs are achieved.

Abstract

This study aims at comprehensively investigating the possibility of fabricating cross-linked polymer inclusion membranes (PIMs) using the most common base polymers (i.e., poly(vinyl chloride), cellulose triacetate and poly(vinylidene fluoride-co-hexafluoropropylene)), cross-linking polymers or monomers (i.e., poly(ethylene glycol) dimethylacrylate, poly(ethylene glycol) divinylether and N-ethylmaleimide) and photo-initiators (i.e., 2,2-dimethoxy-2-phenyl acetophenone, triarylsulfonium hexafluorophosphate and triphenylphosphine oxide). The suitability of the photo-initiators for the different cross-linking polymers/monomers in poly(vinyl chloride)-based membranes (without extractants) was assessed for the first time, which was followed by optimizing the cross-linking conditions (i.e., membrane composition and duration of UV-irradiation) for all three base polymers studied. The optimum concentrations of photo-initiator and UV-treatment times were different for the different base polymers used, thus highlighting the importance of this study. Aliquat 336 and di-(2-ethylhexyl)phosphoric acid, as the most frequently used PIM extractants, were added to the compositions of the homogeneous cross-linked membranes produced under optimal conditions and the extraction performance of the newly developed cross-linked PIMs was compared with that of their non-cross-linked counterparts in the extraction of SCN⁻ and Zn²⁺, respectively. The results showed that all but one of the 13 homogeneous cross-linked PIMs obtained in this study could extract up to 45% more of the corresponding target species than their non-cross-linked counterparts and the remaining one performed similarly to the relevant non-cross-linked PIM. The initial fluxes for the cross-linked PIMs were up to 10 times higher than those of the relevant non-cross-linked membranes and in two cases the fluxes were similar in value. These results demonstrate the potential of cross-linking for enhancing the extraction performance of PIMs.

Introduction

The increasing interest in polymer inclusion membranes (PIMs) has established them as environmentally friendly materials for applications in chemical analysis [1] as well as in extraction-based separation processes [2]. The analytical applications of PIMs include their use as the sensing membranes in ion-selective electrodes and optodes [2,3]. Separation based on the use of PIMs has emerged as a less expensive, safer and environmentally friendlier alternative to conventional solvent extraction of both metallic or non-metallic species [2]. The separation of heavy metals is becoming more important nowadays due to their environmental effects associated with globalisation and industrialisation. Toxic metals are released into the environment in numerous ways. Fossil fuel combustion, sewage waste, vehicle emissions, discarded batteries, mining activities, tanneries, and metallurgy are examples of major sources of heavy metal pollution of aquatic systems [[4], [5], [6], [7]]. Several methods have been reported for the treatment of waters polluted with heavy metals, namely, ion exchange [8], co-precipitation [9], solvent extraction [10], and membrane-based separation [11].

PIMs are a type of liquid membranes composed of a base polymer and an extractant and in some cases they may also incorporate a plasticizer or a modifier. Poly(vinyl chloride) (PVC) [12] and cellulose triacetate (CTA) [13] are the most commonly used base polymers. However, there is a recent trend in using other base polymers, such as poly(vinylidene fluoride) (PVDF) [14] and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) [15,16]. The base polymer acts as the skeleton of the membrane and holds within its entangled chains the membrane liquid phase, consisting of the extractant (often referred to as carrier) and a plasticizer or a modifier (if used). Aliquat 336 (a mixture of quaternary alkylammonium chlorides with the dominant species being trioctylmethylammonium chloride), di-(2-ethylhexyl)phosphoric acid (D2EHPA), bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos® IL 104), and tetrahydroxycalix[4]arene (Calix[4]arene) are a few examples of PIM extractants [2,17,18]. The extractant is usually a complexing agent or an ion-exchanger which forms a complex or an ion-pair with the species of interest and the corresponding adduct is transported across the PIM which involves simultaneous extraction and back-extraction on the corresponding sides of the membrane. PIMs are characterized usually by lower rate of leaching of the membrane liquid phase into the adjacent aqueous phases compared to supported liquid membranes, which are the most frequently used liquid membranes at present. Notwithstanding this advantage, PIM extraction efficiency in terms of amount extracted and rate of extraction needs to be improved further in order to make them suitable for industrial applications [19,20]. Biofouling is also a major challenge in the membrane-based separation industry.

Different approaches for improving the PIM performance have been reported and they include modification of the membranes by the addition of reduced graphene oxide [21] or the incorporation of a cross-linking polymer in the membrane skeleton [16]. For instance, O'Bryan et al. [16] have demonstrated that a cross-linked PIM could transport thiocyanate across the membrane faster than its non-cross-linked counterpart.

The most efficient method for generating highly cross-linked polymers is based on UV irradiation of a multifunctional monomer in the presence of a suitable photo-initiator [22,23]. The advantages of photoinduced polymerisation over thermal cross-linking, such as solvent free processing and energy efficiency, make it suitable for the coating industry, photolithography, microelectronics, and the manufacturing of paints, composite materials, dental restorative formulations, and adhesives [[24], [25], [26]].

Different types of monomers including acrylate, vinylether, epoxide, and maleimide have been extensively used for UV-curing in a variety of applications [22,[27], [28], [29]]. Kara et al. have demonstrated the possibility of using poly(ethylene glycol) dimethacrylate (PEGDMA) beads in heavy metal removal. In their work, PEGDMA was copolymerised with vinyl imidazole to allow the removal of Cd(II), Hg(II) and Pb(II) [30]. Methacrylate based cryogels [31], monoliths [32] and microspheres [33] have also been reported for heavy metal removal. PEGDMA has also been used by us as a cross-linking polymer in PIMs for enhancing the extraction/transport efficiency of thiocyanate [16] and Zn(II) [34]. In both studies the cross-linked PIMs showed superior extraction/transport performance in comparison with the corresponding non-cross-linked PIMs in addition to providing better long-term stability. Apart from improving the rate of separation, cross-linking with poly(ethylene glycol) (PEG) units could improve the biofouling resistant activity of cross-linked PIMs as PEG is known to have bactericidal properties [35,36]. Waheed et al. [37] and Hassanien et al. [38] have demonstrated the anti-bacterial property of cross-linked polymer membranes made of PEG and cellulose acetate. Maleimide resins possess high tensile strength and modulus, excellent chemical and corrosion resistance, as well as good stability at elevated temperatures [39]. These advantages increase the applicability of maleimide resins in electronic and aerospace engineering. Vinyl ethers are highly reactive monomers and extensive curing can be achieved in a very short time [27]. The low odour and non-irritating nature of this type of monomers makes them suitable for different UV-curing applications, such as protective coating [40]. Low shrinkage, great impact strength and high adhesion of the UV-cured polymer are additional advantages which make vinyl ethers popular cross-linking polymer [27,41].

In our previous research we have demonstrated that the cross-linking polymer PEGDMA and the initiator DMPA can be used to prepare successful cross-linked PIMs composed of PVDF-HFP or CTA as the base polymer and Aliquat 336 [16] or D2EHPA [34] as the extractant. However, to the best of our knowledge, other cross-linking polymers or initiators have never been tested for the fabrication of cross-linked PIMs. Hence, in the present study we aimed to explore the applicability of a range of frequently used cross-linking polymers and photo-initiators for the preparation of cross-linked PIMs. The suitability of each photo-initiator studied was examined by fabricating cross-linked PVC-based membranes which contained PEGDMA, poly(ethylene glycol) divinyl ether (PEGDVE), or N-ethyl maleimide (NEM) as the cross-linking polymer and no extractant. This was followed by the optimization of the cross-linking conditions, for the fabrication not only of PVC-based membranes, but also of membranes containing CTA or PVDF-HFP as the base polymer. Cross-linked PIMs, containing either D2EHPA or Aliquat 336, were prepared under the optimal cross-linking conditions established earlier and their extraction performance was compared with that of their non-cross-linked counterparts by using Zn²⁺ or SCN⁻ as model target species, respectively.