Effect of cross-linking on the performance of polymer inclusion membranes (PIMs) for the extraction, transport and separation of Zn(II)
Introduction
Emerging as a potential alternative to traditional solvent extraction, separation based on polymer inclusion membranes (PIMs), consisting of a base-polymer, carrier (extractant) and plasticizer or modifier in some cases, has been attracting increased attention in recent years [1]. The main reason behind this trend is based on the better stability of PIMs than supported liquid membranes (SLMs) which are the most frequently used liquid membranes at present [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. Despite the better stability of PIMs [1,11], their robustness is still considered insufficient for applications on an industrial scale. Hence, recent research on PIMs has also been focused on further improving their stability together with other performance characteristics such as rate of extraction and extractive capacity.
The limited stability of PIMs is mainly caused by the loss of membrane liquid phase, composed of the PIM carrier and plasticizer or modifier (if used), to the aqueous phase(s) (i.e., feed and receiving solutions) in contact with the membrane [[12], [13], [14]]. The factors, responsible to a great extent for the leaching of the membrane liquid phase, include (1) problems associated with the miscibility of the membrane components [15], (2) solubility of the membrane liquid phase in the aqueous phase(s) [16,17], and (3) the composition of the aqueous phase(s) in contact with the membrane [10,13]. Different approaches have been proposed with the aim to eliminate or minimise the effect of these three factors. Recently, Kaya et al. have demonstrated improvement of the PIM mechanical stability by doping the membrane with reduced graphene oxide (rGO) without compromising its permeability [18]. According to the authors, the addition of rGO to the membrane composition improved its structure, increased its roughness and also prevented the physical aging of the membrane. Salazar-Alvarez et al. reported on the use of ethanol as a modifier, which was added to the casting solution and resulted in better miscibility of the membrane components (i.e., bis-(2-ethylhexyl)phosphoric acid (D2EHPA), tris-(2-butoxyethyl)phosphate and cellulose triacetate (CTA)) [15]. Matsuoka et al. reduced the loss of the carrier tributyl phosphate (TBP) from the corresponding PIMs by saturating the feed solution with TBP [17]. It was also demonstrated by Cho et al. that the membrane stability could be improved by incorporating in the membrane composition a modifier with very low water solubility [16]. The ionic strength of the aqueous feed or receiving solutions also plays a determining role in the loss of membrane liquid phase, and it has been reported by Zhang et al. that the membrane mass loss during extraction could be minimized by conditioning the PIM in an aqueous solution with the same background composition as the feed solution for a certain period of time [13]. Recently a new approach has been developed by us for improving the stability as well as the permeability of PIMs which is based on introducing an additional polymer, capable of creating a cross-linked polymer network in the presence of UV-light [19].
Cross-linking polymers are extensively used engineering materials because of their excellent mechanical and thermal stability [20]. There are two common approaches used to induce the cross-linking, i.e., thermal polymerisation or photo-initiated radical polymerisation. Radiation induced polymerisation has some advantages over heat induced polymerisation, which include high reaction rate at ambient temperature, low energy consumption and solvent free formulation [21]. There are several polymers that can be used as cross-linking agents, namely poly(ethylene glycol) dimethacrylate (PEG-DMA) [22,23], 4-hydroxy-butyl vinyl ether (HBVE), bismaleimide (Q-bond) [24], and oxetane-acrylate [21]. PEG-DMA is among the most frequently used cross-linking polymers with numerous applications in biochemistry and medicinal chemistry [[25], [26], [27]]. Its capability in improving mechanical resistance under high vacuum conditions or under chemical stress is well known [28]. Engineered mechanical properties and biodegradability of this polymer make it suitable for orthopaedic tissue engineering [29].
Hence, this cross-linking polymer was chosen for the present study which focused on utilising the cross-linking polymerisation concept in the development of a more stable PIM for the extraction, transport and separation of Zn(II) with improved extraction rate and capacity. The compatibility of different base-polymers with the cross-linking polymer PEG-DMA was investigated using D2EHPA as the carrier. The long-term stability and surface morphology of the PIMs studied were also examined.
Section snippets
Reagents and solutions
D2EHPA (97%, Aldrich, USA) as the carrier (extractant) was used as received in the preparation of all membranes. Three different polymers, i.e., poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP, Aldrich, USA), CTA (Acros Organics, USA) and poly(vinyl chloride) (PVC, Aldrich, USA), were tested as base-polymers. PEG-DMA (Aldrich, USA) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA, Aldrich, Italy) were used as the cross-linking polymer and the initiator, respectively. 2-Nitrophenyloctyl
Selection of the base-polymer
Although the base-polymer is not the active constituent of a PIM, it plays an important role in determining its properties as it provides the back-bone of the membrane which holds the carrier within its structure. The PIM's performance depends on the compatibility between the base-polymer, the membrane liquid phase and, in the case of cross-linked PIMs, also on the nature of the cross-linking polymer. Hence, three different base-polymers, namely PVC, CTA and PVDF-HFP, were used to cast
Conclusions
On the basis of the results obtained it was concluded that depending on the nature of the base polymer and cross-linking polymer, cross-linked PIMs can exhibit superior stability, extraction rate and extractive capacity compared to their non-cross-linked counterparts.
A screening of the base-polymers PVC, CTA and PVDF-HFP for their suitability in preparing cross-linked PIMs was performed. Of these, PVC showed to be unsuitable for cross-linking with PEG-DMA due to polymer incompatibility.
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