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Evidence on the 2-nitrophenyl octyl ether (NPOE) facilitating Copper(II) transport through polymer inclusion membranes
Journal of Membrane Science 501 (2016) 228–235
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Evidence on the 2-nitrophenyl octyl ether (NPOE) facilitating Copper (II) transport through polymer inclusion membranes Duo Wang, Jiugang Hu n, Ya Li, Mingbo Fu, Dabiao Liu, Qiyuan Chen n College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 30 November 2015 Accepted 5 December 2015 Available online 10 December 2015
Polymer inclusion membranes (PIMs) containing 2-nitrophenyl octyl ether (NPOE) as plasticizer, LIX 84 I as carrier and poly (vinyl chloride) (PVC) as polymer were prepared to transport Cu (II) from ammoniacal solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency of PIMs was investigated and the interactions of membrane components were evaluated by attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The results show that the extraction and back-extraction efficiencies of Cu (II) in the PIMs could be significantly improved by addition of 30 wt% NPOE. The ATR-FTIR and XPS analyses indicate that NPOE molecules could bond with PVC chains, as a new peak located at 800 cm 1 appears in the spectra of PIMs with high content NPOE. Moreover, XPS spectra of the PIMs before and after transport experiments suggest that NPOE molecules could provide a solvent-like environment to transport the carriers and LIX 84 I–Cu (II) complexes in the membrane phase. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymer inclusion membrane NPOE Interaction Cu (II) transport
1. Introduction Polymer inclusion membrane (PIM) is a solid mixture of base membrane skeleton, plasticizer and carrier [1]. Compared with traditional liquid membranes, PIMs have been studied as a potential separation technology to extract and transport organic compounds [2,3] and heavy metals (e.g. Pb (II) [4,5], Zn (II) [6], Cr (VI) [7], Cs (II) [8], gold (III) [9] and Ur (VI) [10]) in solutions because of their specific advantages of negligible carrier loss [11], good selectivity [12], stability [13] and mechanical properties [14]. It is well known that membrane components have a significant influence on the transport properties and mechanical properties of PIMs [15,16]. The combinations of different polymers and carriers were studied by Pereira et al. and the lipophilicity, propensity, and hydrogen bonding interactions between polymers and carriers were considered as important factors to cast the successful PIMs [16]. However, the effect of plasticizer is especially prominent on the transport behavior of PIMs. Various plasticizers were added during the preparation of PIMs to enhance the transport efficiency of metal ions [7,17,18]. NPOE is used as a common plasticizer, since studies showed that some PIMs containing NPOE display better permeability compared to the membranes with other plasticizers (dibutyl phthalate (DBP) [17] and tris(2-ethylhexyl) phosphate (TEHP) [18]). Raut and co-workers [8] found that the permeability n
Corresponding authors. E-mail addresses: [email protected] (J. Hu), [email protected] (Q. Chen).
http://dx.doi.org/10.1016/j.memsci.2015.12.013 0376-7388/& 2015 Elsevier B.V. All rights reserved.
of Cesium ion in the PIMs can be improved from 1.6 10 5 to 5.9 10 5 cm s 1 by increasing the content of NPOE. Sabri et al. also reported that the permeability of Cr (VI) in the PIMs can be enhanced from 1.8 10 4 to 3.6 10 4 cm s 1 by NPOE [7]. Various studies have been carried out to illustrate the assisted transport phenomenon induced by plasticizers for the PIMs. Some authors suggested that the polarity and viscosity of plasticizers could be main factors [8,17]. Nevertheless, Kolev and co-workers [19,20] found that the current plasticizers have similar viscosity values. Kebiche-Senhadji et al. [17] investigated the effect of plasticizer dielectric constant on the transport efficiency of PIMs, utilizing three plasticizers (NPOE, 2-fluorophenyl 2-nitrophenyl ether (2-EP2-NPE) and DBP), with similar viscosity and different dielectric constant values (ɛr ¼ 24, 50 and 4, respectively). The PIMs with NPOE showed the best transport performance, thus the authors attributed the excellent transport efficiency to moderate dielectric constant of NPOE. By contrast, Scindia et al. [21] considered that the actual dielectric constant of PIMs also depends on the carrier and the base polymer. Besides, scanning electron microscopy (SEM) [22,23], contact angle [24] and X-ray diffraction measurements [18] were used to determine the surface and bulk features of PIMs. In essence, the transport enhancement of PIMs should result from the interactions between membrane components. De Gyves et al. [25] suggested that plasticizers could reduce intermolecular attractive forces between polymer chains. Therefore, the plasticization effect could facilitate PIMs as a better media for carrier–metal complexes transport. Sears and Darby also infered that plasticizer molecules may penetrate between polymer
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Table 1 Membrane compositions used for preparation of PIMs. No.
NPOE content (wt%)
PVC content (wt%)
LIX 84 I content (wt%)
1 2 3 4 5 6 7 8
0 15 20 25 30 30 20 25
70 55 50 45 40 70 40 40
30 30 30 30 30 0 40 35
chains or increase the distance of polymer molecules to weaken the interaction forces [26]. Unfortunately, the detailed structural information on the interaction between plasticizers and other membrane components is not so clear. In this paper, we wish to provide some evidence on the interaction between plasticizers and other membrane components. PIMs with LIX 84 I as carrier, PVC as polymer and NPOE as plasticizer were prepared for Cu (II) transport from ammoniacal solutions to sulfuric acid solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency was
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studied. The component interactions of PIMs were investigated by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), to illustrate the promotion role of NPOE on the Cu (II) transport in the PIMs.
2. Experimental 2.1. Materials High molecular weight poly (vinyl chloride) (PVC) (Aldrich), 2-nitrophenyl octyl ether (2-NPOE) (Aldrich), and LIX 84 I (Cognis Co.) were used as membrane components to prepare PIMs. Tetrahydrofuran (THF) (Aldrich) was used as the solvent. All reagents were used as received. Copper sulfate, ammonium sulfate, sodium hydroxide and sulfuric acid were purchased from Sinopharm, China. 1 mmol L 1 Cu (II) solution was prepared by dissolving copper sulfate and ammonium sulfate in ultrapure water as the feed solution. The pH was adjusted by sodium hydroxide and sulfuric acid. 1 mol L 1 sulfuric acid solution was prepared as stripping solution. Ultrapure water was used in the present work (Millipore MilliQ System, 18.2 MΩ cm).
Fig. 1. ATR-IR spectra of membrane components. (a) LIX84 Iþ NPOE; (b) LIX84 Iþ PVC; (c) NPOE þ PVC; (d) PIMs with various contents of NPOE.
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2.2. Membrane preparation and transport experiments The PIMs were prepared using solvent evaporation method [27]. The membrane compositions are listed in Table 1. Typically, 0.35 g membrane components were dissolved in 7 mL of THF. The solutions obtained were poured into a 7 cm diameter glass ring on a flat glass plate. THF was allowed to evaporate slowly overnight to yield a flexible, transparent and mechanically strong membrane. The obtained membrane was then carefully peeled off from the glass plate and used for the Cu(II) transport study. The membranes prepared with NPOE content beyond 30 wt% were sticky and mechanically too weak to be used for Cu (II) transport experiments. Transport experiments were performed using a 80 mL twocompartment cell as described elsewhere [25] at 25 °C. Both feed solution and striping solution were circulated by peristaltic pumps with a flow rate of 100 mL min 1. Membrane area exposed for Cu (II) transport was 7.1 10 4 m2. Aliquots of 1 mL were withdrawn separately from the feed and stripping solutions at certain time intervals for the analysis of Cu (II) concentration by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Perkin Elmer 5300DV America). The average value of the analyses for three times was taken in the present work.
groups from PVC (Fig. 1b and c). Fig. 1b suggests that there are only weak molecular interactions between the carrier and polymer without formation of covalent bonds [7,29]. However, a new peak at 800 cm 1 appears in the spectrum of the mixture containing NPOE and PVC (Fig. 1c), which indicates PVC molecules could bond with NPOE. As expected, this interaction can be also observed in the ATR-IR spectra of PIMs prepared with various concentrations of NPOE and LIX 84 I (Fig. 1d). The peak at 800 cm 1, attributed to the bonding between PVC and NPOE, strengthens abruptly when the content of NPOE is 30 wt%. Moreover, the intensity reduction of peaks at 1434 cm 1 and
2.3. Membrane characterization Atomic force microscopy (AFM) images were taken in air at room temperature on a SPM SOLVER Pro-M instrument at 255 kHz oscillation mean frequency. The 256 256 scan point size images were obtained in tapping mode with a scan velocity of 6 m s 1 at different scan areas (foursquare with sides of 30, 10, 5, 2 mm). RMS roughness values (Rq) were calculated on each 5 mm scan using routines written in Gwyddion-2.36.win32. Attenuated total reflectance Fourier transformed infrared (ATRFTIR) spectra were recorded on a Nicolet 6700 spectrophotometer equipped with a Ge IRE crystal. The spectra were collected with a 45° take-off angle. The IR spectra of 4000–700 cm 1 with a resolution of 4 cm 1 were determined by accumulating 64 scans. All original spectra were analyzed using Omnic 8 software. The chemical information on the surface of PIMs was studied by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher, USA). The X-ray radiation (hv ¼1486.7 eV, 200 W) was used in the experiment as the excitation source. Thermo avantage 5.52 software was used for data analysis.
3. Results and discussion 3.1. Characterization of PIMs In order to elucidate the specific interactions between the membrane components, PIMs were firstly characterized by ATR-IR, and the results are shown in Fig. 1. As seen from Fig. 1a, the strong peaks located around 2956–2855 cm 1 are attributed to phenol and aliphatic C-H groups for both NPOE and LIX 84 I. The sharp peak at 1525 cm 1 is attributed to C–NO2 asymmetric vibrations (NPOE), and the vibration bonds around 1164–960 cm 1 correspond to C–O–CH2 groups (NPOE). The characteristic peak located at 1390–1330 cm 1 is assigned to the phenol O-H group of LIX 84 I. It is noteworthy that no new peak can be observed in the IR spectrum of the mixture of NPOE and LIX 84 I. The absorption bands at 1068 cm 1 and 910 cm 1 correspond to C–O–C groups from THF (Fig. 1b and c), illustrating that a small amount of THF could retain in the individual PVC membrane [28], but it is negligible in the PIMs studied (Fig. 1d). The narrow peaks at 1434 cm 1 and 1426 cm 1 correspond to the halogenated C–H
Fig. 2. AFM images of PIMs with various NPOE contents: (a) 0 wt%, (b) 20 wt% and (c) 30 wt%.
Table 2 Roughness of PIMs with different NPOE contents. Error bars: mean7 SD, n¼ 3. NPOE content in PIMs (wt%) Root mean square roughness (Rq) (nm)
0 20 30 1.147 0.020 0.56 7 0.005 0.23 7 0.001
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1426 cm 1 results from the decrease of PVC content, while the stronger peaks around 1164–960 cm 1 are due to the increase of NPOE content in the PIMs. The morphologies of PIMs with various NPOE contents were studied by AFM. 3D topographic images in format of 5 mm 5 mm are shown in Fig. 2. There are numerous “haystacks” on the surface of the blank membrane without NPOE [21] (Fig. 2a), in which the distance from the lightest to the darkest point (Rmax) is about 23 nm. The Rmax of the PIM with 20 wt% NPOE is approximately 53 nm (Fig. 2b), whereas the value of Rmax increases to 94 nm in the PIM with 30 wt% NPOE (Fig. 2c). As shown in Table 2, the roughness of PIMs decreases significantly with the increase of NPOE. Namely, higher NPOE content in PIMs can soften the surface of PIMs, thus improving the formation of the more homogeneous PIMs.
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Generally, Cu (II) transport process can be divided into three stages: (1) Cu (II) reacts with LIX 84 I at the feed solution/interface of PIMs to form the metal–carrier complexes; (2) Metal–carrier complexes diffuse across PIMs towards the stripping solution; (3) At the interface of PIMs/stripping solution, the complexes are dissociated by hydrogen ions and Cu (II) is released into stripping solution. The extraction and back-extraction equilibria are described as Scheme 1 [31,32]. As shown in Fig. 3, the extraction rate is higher than back-extraction rate throughout the experimental time, indicating that diffusion process of metal-carrier complexes in the PIMs is the rate determining step for Cu (II) transport. Obviously, the transport of metal carrier complexes depends significantly on the carrier in the PIMs, since the membranes with only NPOE and PVC do not display the transport efficiency. Cussler et al [33] suggested that
3.2. Transport studies The effect of NPOE on Cu (II) transport efficiency of PIMs was studied by varying NPOE content and maintaining the concentration of LIX 84 I at 30 wt%. The extraction and back-extraction results are shown in Fig. 3. After 12 h, the PIM without NPOE has about 16 % Cu (II) extracted from the feed solution. Increasing of NPOE content leads to the slight increases of Cu (II) extraction efficiency. When adding 25 wt% NPOE in PIMs, the extraction efficiency is only 27%. Meanwhile, for the PIMs with lower than 25 wt% NPOE, the negligible Cu (II) back-extraction efficiency is observed. However, when NPOE content in the PIMs increases to 30 wt%, a great enhancement of Cu (II) transport efficiency can be found. The extraction efficiency of PIMs is about 80 % and the backextraction efficiency increases dramatically to 60% in 12 h. The sharp increase of both Cu (II) extraction and back-extraction efficiency could be explained by “percolation threshold” [30] in the Cu (II) transport process.
Scheme 1. The extraction and back-extraction equilibria at the PIM/feed solution and PIM/stripping solution interfaces.
Fig. 3. Effect of NPOE content on extraction (left) and back-extraction (right) efficiencies of Cu (II) in PIMs with 30 wt% LIX 84 I. Feed solution: 1 mmol L 1 Cu (II) and 50 mmol L 1 (NH4)2SO4, pH¼8.5; stripping solution: 1 mol L 1 H2SO4 solution. Error bar: mean 7 SEM, n¼ 3, (*) Po 0.05.
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carriers in solid membranes are bonding with the polymer, forming the chained carriers. Therefore, when the carrier concentration is lower than the critical value named percolation threshold, the chained carriers have no mobility, and the transport efficiency of Cu (II) is limited to almost zero. The specific transport mechanism would be discussed in the Section 3.3. Fig. 4 shows the Cu (II) transport behavior of PIMs with various NPOE content and constant PVC (40 wt%). It is worth noting that even though higher than 30 wt% LIX 84 I content is present in PIMs, the extraction and back-extraction percentages of Cu (II) decrease abruptly at the NPOE content of lower than 30 wt%. This result indicates that NPOE improves significantly Cu (II) transport. The similar phenomena were found in other studies. Fontas et al. [34] found that the PIMs with higher content of plasticizers have a better permeability for Pt (IV). Salazar-Alvarez et al. [35] also reported that the presence of NPOE can facilitate Pb (II) transport. 3.3. Transport mechanism XPS spectra of the PIMs with different NPOE contents were collected to further elucidate the effect of NPOE on the Cu (II) transport. Fig. 5 shows a comparison of C 1s XPS spectra of PIMs with various NPOE contents. Three clear peaks can be observed at binding energy of 284.5, 285.6 and 286.8 eV, which are attributed to C–C/C–H groups (aliphatic carbon), -CH2- groups and –CHCl– groups, respectively [36]. The interaction of membrane components with base polymer can be identified by the –CHCl– groups since they are specific for PVC. The perpendicular dotted line was used as marked line to illustrate the peak shifts of –CHCl- group. Compared with the individual PVC membrane, the binding energy of –CHCl– groups increases by 0.3 eV for the PIMs consisting of 70 wt% PVC and 30 wt% LIX 84I, indicating that electron density of carbon atom in –CHCl groups is weakened [37–39]. Namely, the molecular interactions between PVC and LIX 84I, such as hydrogen
bonding and Van der Waals forces, are established. After adding NPOE, the –CHCl peaks shift gradually to the higher binding energy region, suggesting that electron density of carbon atom in – CHCl groups decreases due to bonding to an electron withdrawing neighbor atom. For the PIMs containing 70 wt% PVC and 30 wt% NPOE, a strong interaction between NPOE and PVC can be illustrated since the C 1s XPS spectrum shows that the –CHCl peak can shift 0.7 eV to the higher binding energy region. The result is consistent with the ATR-IR analysis above (Fig. 1). According to the Gel Theory [39], the plasticized polymer could be neither solid nor liquid but an intermediate state, thus loosely held by a three-dimensional network of weak secondary bonding forces. These bonding forces are stronger than the intermolecular forces. As a consequence, it is presumed that PVC molecules could bond with NPOE rather than with LIX 84 I in the PIMs. Other characterizations of PIMs carried out by researchers are in concordance with our conclusion. Fontas et al. [34] found an apparently lower glass transition temperature in the DSC experiments and a perceptible change in the far IR spectra, when NPOE was added in the polymer CTA. These results suggest that LIX 84 I could be partially released from the chained interactions with PVC by addition of NPOE, decreasing the percolation threshold to a certain extent, therefore improving the efficiencies of Cu (II) extraction and transport. The interaction between targeted ions and surface active species of the PIMs firstly affects the ion transport. Therefore, the surface information of the PIMs was determined by XPS analysis before and after the transport experiments. The composition of the selected PIMs is 30 wt% LIX 84 I, 40 wt% PVC and 30 wt% NPOE. The O1s spectra (Fig. 6(a1-a3)) are resolved into four peaks. The peaks at binding energy of 531.7, 532.2, 532.7 and 533.8 eV are attributed to C–O–C and NO2 groups from NPOE, N–O–H and C–O– H (phenol) groups from LIX 84I, respectively. After extraction, the peak of N–O–H group shifts ca. 0.2 eV to the higher binding energy and the peak intensity decreases significantly. The possible
Fig. 4. Effect of NPOE content on extraction (left) and back-extraction (right) efficiencies of Cu (II) in PIMs with 40 wt% PVC. Feed solution: 1 mmol L 1 Cu (II) and 50 mmol L 1 (NH4)2SO4, pH ¼8.5; stripping solution: 1 mo L 1 H2SO4 solution. Error bar: mean 7 SEM, n¼ 3, (*) P o0.05.
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explanation is that copper ions bond with N–O–H groups during extraction, thus decreasing the electron density of oxygen atoms. After back-extraction, the binding energy of this characteristic peak returns to ca. 532.7 eV (Fig. 6(a-3)). As seen from Fig. 6(a-4), in contrast to the fresh PIM, the content of O atom decreases sharply after Cu (II) transport. These results suggest that O atom plays an important role in the Cu (II) transport process. From the N 1s spectra (Fig. 6(b)), it can be observed that the binding energy of N 1s increases by 0.8 eV after extraction compared with the fresh PIM. As shown in Scheme 1, the nitrogen atoms of LIX 84 I bond with Cu (II) through electron pair sharing. As a consequence, the electron density of the nitrogen atom increases. After back-extraction, the binding energy of N 1s returns again due to the release of Cu(II) from the carrier-metal complexes. The Cu (II) spectra (Fig. 6(c)) witness the main peak (934.89 eV) and the satellite peak (954.39 eV), which are attributed to Cu 2p 3/2 and Cu 2p 1/2, respectively. Apparently, copper species could be accumulated on the surface of the PIMs via extraction, though the exact species cannot be determined in the present work. As a contrast, the peak of Cu 2p is negligible after back-extraction. These results reveal that the Cu (II) transport through the PIMs can be dominated by the active carriers. An interesting phenomenon is observed from the Cl 2p spectra (Fig. 6(d)). There is negligible shift for the binding energy but sharp increase of peak intensity of Cl 2p after Cu (II) extraction, indicating an increase of the content of PVC molecules on the surface of the PIMs. After back-extraction, the little change of Cl 2p peak is observed compared with the fresh membrane. The results suggest that the plasticizer molecules are not steadily bound to the PVC molecules [37]. In other words, NPOE molecules could freely self-associate or associate with other molecules in the PIMs, since the bonding forces between NPOE and PVC could be weaker than chemical interactions but stronger than the intermolecular forces between PVC and LIX 84 I. If one NPOE molecule becomes a center or a site, it is easily replaced by another NPOE molecule. Therefore, NPOE provides a solvent-like environment for the carrier and carrier–metal complexes. The plasticizer allows these molecules to pass by each other in the three-dimensional network of PIMs, thus significantly improving the mobility of the extracted complexes in PIMs. As a result, the content of PVC increases on the surface of the PIMs after extraction. This assumption is consistent with similar phenomenon in other reports [34,38,40–42]. On the basis of the XPS and ATR-IR analyses, it could be concluded that PIMs prepared without NPOE plasticizer are rigid and hard network because of the PVC-LIX 84 I chains and PVC-PVC chains, thus LIX 84 I and LIX 84 I-Cu (II) molecules cannot move freely from one site to another in the rigid PVC chains. By addition of NPOE, intermolecular forces between PVC and LIX 84 I could be weakened because of the NPOE-PVC interactions. Increasing the concentration of plasticizer leads to the formation of solvent-like pathways for the carrier LIX 84 I and LIX 84 I-Cu (II) complexes between the two interfaces of PIMs, as the plasticizer NPOE allows complexes to pass by each other in the three-dimensional network of PIMs. This pathway can be confirmed by the dramatic enhancement of Cu (II) transport efficiency from nil to about 60% through the PIMs with 30 wt% NPOE. Thus, the assisted transport phenomenon induced by NPOE can be explained very well. These results could be used to further clarify the essence of the fixed sites in the solid membranes. Cussler et al. [33] proposed the theory of chained carriers and “percolation threshold” described that one chained carrier can reach the other fixed sites. The mechanism was revised with the addition of the plasticizers by Fontas et al [34]. In the case, plasticizers were thought of as the medium for carrier mobility through interaction with carriers. However, as mentioned above, we found that the NPOE could weaken the intermolecular forces between PVC and LIX84I by interaction with
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PVC, thus release more free carriers and provide a solvent-like environment for the metal–carrier species to diffuse through the PIMs.
4. Conclusions In the present work, PIMs were used for Cu (II) transport in ammoniacal solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency of PIMs was studied. It can be found that the extraction and back-extraction efficiencies of PIMs are significantly improved by NPOE. Compared with the PIMs without NPOE, the extraction efficiency of the PIMs with 30 wt% NPOE increases from 16% to about 80% and the backextraction efficiency increases from nil to 60% in 12 h. The ATR-IR spectral analysis indicates that a new peak located at 800 cm 1 appears for the PIMs with 30 wt% NPOE, suggesting that NPOE molecules could bond with PVC chains, which is also supported by XPS analysis. Moreover, XPS spectra of the PIMs before and after transport experiments suggest that NPOE molecules could not permanently bond with the PVC molecules, but provide a solventlike environment to transport the carriers and LIX 84 I-Cu (II) complexes in the membrane phase, thereby facilitating Cu (II) transport in the PIMs.
Fig. 5. C 1s XPS spectra of PIMs with various NPOE contents.
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Fig. 6. XPS spectra of PIMs before and after the transport experiment. (a) O 1s; (b) N 1s; (c) Cu 2p and (d) Cl 2p.
Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2014CB643401), National Natural Science Foundation of China (Nos. 51134007 and 51304244), China Postdoctoral Science Foundation (2014M552152) and Hunan Provincial Natural Science Foundation of China (2015JJ3154).
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Evidence on the 2-nitrophenyl octyl ether (NPOE) facilitating Copper (II) transport through polymer inclusion membranes Duo Wang, Jiugang Hu n, Ya Li, Mingbo Fu, Dabiao Liu, Qiyuan Chen n College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 30 November 2015 Accepted 5 December 2015 Available online 10 December 2015
Polymer inclusion membranes (PIMs) containing 2-nitrophenyl octyl ether (NPOE) as plasticizer, LIX 84 I as carrier and poly (vinyl chloride) (PVC) as polymer were prepared to transport Cu (II) from ammoniacal solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency of PIMs was investigated and the interactions of membrane components were evaluated by attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The results show that the extraction and back-extraction efficiencies of Cu (II) in the PIMs could be significantly improved by addition of 30 wt% NPOE. The ATR-FTIR and XPS analyses indicate that NPOE molecules could bond with PVC chains, as a new peak located at 800 cm 1 appears in the spectra of PIMs with high content NPOE. Moreover, XPS spectra of the PIMs before and after transport experiments suggest that NPOE molecules could provide a solvent-like environment to transport the carriers and LIX 84 I–Cu (II) complexes in the membrane phase. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymer inclusion membrane NPOE Interaction Cu (II) transport
1. Introduction Polymer inclusion membrane (PIM) is a solid mixture of base membrane skeleton, plasticizer and carrier [1]. Compared with traditional liquid membranes, PIMs have been studied as a potential separation technology to extract and transport organic compounds [2,3] and heavy metals (e.g. Pb (II) [4,5], Zn (II) [6], Cr (VI) [7], Cs (II) [8], gold (III) [9] and Ur (VI) [10]) in solutions because of their specific advantages of negligible carrier loss [11], good selectivity [12], stability [13] and mechanical properties [14]. It is well known that membrane components have a significant influence on the transport properties and mechanical properties of PIMs [15,16]. The combinations of different polymers and carriers were studied by Pereira et al. and the lipophilicity, propensity, and hydrogen bonding interactions between polymers and carriers were considered as important factors to cast the successful PIMs [16]. However, the effect of plasticizer is especially prominent on the transport behavior of PIMs. Various plasticizers were added during the preparation of PIMs to enhance the transport efficiency of metal ions [7,17,18]. NPOE is used as a common plasticizer, since studies showed that some PIMs containing NPOE display better permeability compared to the membranes with other plasticizers (dibutyl phthalate (DBP) [17] and tris(2-ethylhexyl) phosphate (TEHP) [18]). Raut and co-workers [8] found that the permeability n
Corresponding authors. E-mail addresses: [email protected] (J. Hu), [email protected] (Q. Chen).
http://dx.doi.org/10.1016/j.memsci.2015.12.013 0376-7388/& 2015 Elsevier B.V. All rights reserved.
of Cesium ion in the PIMs can be improved from 1.6 10 5 to 5.9 10 5 cm s 1 by increasing the content of NPOE. Sabri et al. also reported that the permeability of Cr (VI) in the PIMs can be enhanced from 1.8 10 4 to 3.6 10 4 cm s 1 by NPOE [7]. Various studies have been carried out to illustrate the assisted transport phenomenon induced by plasticizers for the PIMs. Some authors suggested that the polarity and viscosity of plasticizers could be main factors [8,17]. Nevertheless, Kolev and co-workers [19,20] found that the current plasticizers have similar viscosity values. Kebiche-Senhadji et al. [17] investigated the effect of plasticizer dielectric constant on the transport efficiency of PIMs, utilizing three plasticizers (NPOE, 2-fluorophenyl 2-nitrophenyl ether (2-EP2-NPE) and DBP), with similar viscosity and different dielectric constant values (ɛr ¼ 24, 50 and 4, respectively). The PIMs with NPOE showed the best transport performance, thus the authors attributed the excellent transport efficiency to moderate dielectric constant of NPOE. By contrast, Scindia et al. [21] considered that the actual dielectric constant of PIMs also depends on the carrier and the base polymer. Besides, scanning electron microscopy (SEM) [22,23], contact angle [24] and X-ray diffraction measurements [18] were used to determine the surface and bulk features of PIMs. In essence, the transport enhancement of PIMs should result from the interactions between membrane components. De Gyves et al. [25] suggested that plasticizers could reduce intermolecular attractive forces between polymer chains. Therefore, the plasticization effect could facilitate PIMs as a better media for carrier–metal complexes transport. Sears and Darby also infered that plasticizer molecules may penetrate between polymer
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Table 1 Membrane compositions used for preparation of PIMs. No.
NPOE content (wt%)
PVC content (wt%)
LIX 84 I content (wt%)
1 2 3 4 5 6 7 8
0 15 20 25 30 30 20 25
70 55 50 45 40 70 40 40
30 30 30 30 30 0 40 35
chains or increase the distance of polymer molecules to weaken the interaction forces [26]. Unfortunately, the detailed structural information on the interaction between plasticizers and other membrane components is not so clear. In this paper, we wish to provide some evidence on the interaction between plasticizers and other membrane components. PIMs with LIX 84 I as carrier, PVC as polymer and NPOE as plasticizer were prepared for Cu (II) transport from ammoniacal solutions to sulfuric acid solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency was
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studied. The component interactions of PIMs were investigated by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), to illustrate the promotion role of NPOE on the Cu (II) transport in the PIMs.
2. Experimental 2.1. Materials High molecular weight poly (vinyl chloride) (PVC) (Aldrich), 2-nitrophenyl octyl ether (2-NPOE) (Aldrich), and LIX 84 I (Cognis Co.) were used as membrane components to prepare PIMs. Tetrahydrofuran (THF) (Aldrich) was used as the solvent. All reagents were used as received. Copper sulfate, ammonium sulfate, sodium hydroxide and sulfuric acid were purchased from Sinopharm, China. 1 mmol L 1 Cu (II) solution was prepared by dissolving copper sulfate and ammonium sulfate in ultrapure water as the feed solution. The pH was adjusted by sodium hydroxide and sulfuric acid. 1 mol L 1 sulfuric acid solution was prepared as stripping solution. Ultrapure water was used in the present work (Millipore MilliQ System, 18.2 MΩ cm).
Fig. 1. ATR-IR spectra of membrane components. (a) LIX84 Iþ NPOE; (b) LIX84 Iþ PVC; (c) NPOE þ PVC; (d) PIMs with various contents of NPOE.
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2.2. Membrane preparation and transport experiments The PIMs were prepared using solvent evaporation method [27]. The membrane compositions are listed in Table 1. Typically, 0.35 g membrane components were dissolved in 7 mL of THF. The solutions obtained were poured into a 7 cm diameter glass ring on a flat glass plate. THF was allowed to evaporate slowly overnight to yield a flexible, transparent and mechanically strong membrane. The obtained membrane was then carefully peeled off from the glass plate and used for the Cu(II) transport study. The membranes prepared with NPOE content beyond 30 wt% were sticky and mechanically too weak to be used for Cu (II) transport experiments. Transport experiments were performed using a 80 mL twocompartment cell as described elsewhere [25] at 25 °C. Both feed solution and striping solution were circulated by peristaltic pumps with a flow rate of 100 mL min 1. Membrane area exposed for Cu (II) transport was 7.1 10 4 m2. Aliquots of 1 mL were withdrawn separately from the feed and stripping solutions at certain time intervals for the analysis of Cu (II) concentration by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Perkin Elmer 5300DV America). The average value of the analyses for three times was taken in the present work.
groups from PVC (Fig. 1b and c). Fig. 1b suggests that there are only weak molecular interactions between the carrier and polymer without formation of covalent bonds [7,29]. However, a new peak at 800 cm 1 appears in the spectrum of the mixture containing NPOE and PVC (Fig. 1c), which indicates PVC molecules could bond with NPOE. As expected, this interaction can be also observed in the ATR-IR spectra of PIMs prepared with various concentrations of NPOE and LIX 84 I (Fig. 1d). The peak at 800 cm 1, attributed to the bonding between PVC and NPOE, strengthens abruptly when the content of NPOE is 30 wt%. Moreover, the intensity reduction of peaks at 1434 cm 1 and
2.3. Membrane characterization Atomic force microscopy (AFM) images were taken in air at room temperature on a SPM SOLVER Pro-M instrument at 255 kHz oscillation mean frequency. The 256 256 scan point size images were obtained in tapping mode with a scan velocity of 6 m s 1 at different scan areas (foursquare with sides of 30, 10, 5, 2 mm). RMS roughness values (Rq) were calculated on each 5 mm scan using routines written in Gwyddion-2.36.win32. Attenuated total reflectance Fourier transformed infrared (ATRFTIR) spectra were recorded on a Nicolet 6700 spectrophotometer equipped with a Ge IRE crystal. The spectra were collected with a 45° take-off angle. The IR spectra of 4000–700 cm 1 with a resolution of 4 cm 1 were determined by accumulating 64 scans. All original spectra were analyzed using Omnic 8 software. The chemical information on the surface of PIMs was studied by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher, USA). The X-ray radiation (hv ¼1486.7 eV, 200 W) was used in the experiment as the excitation source. Thermo avantage 5.52 software was used for data analysis.
3. Results and discussion 3.1. Characterization of PIMs In order to elucidate the specific interactions between the membrane components, PIMs were firstly characterized by ATR-IR, and the results are shown in Fig. 1. As seen from Fig. 1a, the strong peaks located around 2956–2855 cm 1 are attributed to phenol and aliphatic C-H groups for both NPOE and LIX 84 I. The sharp peak at 1525 cm 1 is attributed to C–NO2 asymmetric vibrations (NPOE), and the vibration bonds around 1164–960 cm 1 correspond to C–O–CH2 groups (NPOE). The characteristic peak located at 1390–1330 cm 1 is assigned to the phenol O-H group of LIX 84 I. It is noteworthy that no new peak can be observed in the IR spectrum of the mixture of NPOE and LIX 84 I. The absorption bands at 1068 cm 1 and 910 cm 1 correspond to C–O–C groups from THF (Fig. 1b and c), illustrating that a small amount of THF could retain in the individual PVC membrane [28], but it is negligible in the PIMs studied (Fig. 1d). The narrow peaks at 1434 cm 1 and 1426 cm 1 correspond to the halogenated C–H
Fig. 2. AFM images of PIMs with various NPOE contents: (a) 0 wt%, (b) 20 wt% and (c) 30 wt%.
Table 2 Roughness of PIMs with different NPOE contents. Error bars: mean7 SD, n¼ 3. NPOE content in PIMs (wt%) Root mean square roughness (Rq) (nm)
0 20 30 1.147 0.020 0.56 7 0.005 0.23 7 0.001
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1426 cm 1 results from the decrease of PVC content, while the stronger peaks around 1164–960 cm 1 are due to the increase of NPOE content in the PIMs. The morphologies of PIMs with various NPOE contents were studied by AFM. 3D topographic images in format of 5 mm 5 mm are shown in Fig. 2. There are numerous “haystacks” on the surface of the blank membrane without NPOE [21] (Fig. 2a), in which the distance from the lightest to the darkest point (Rmax) is about 23 nm. The Rmax of the PIM with 20 wt% NPOE is approximately 53 nm (Fig. 2b), whereas the value of Rmax increases to 94 nm in the PIM with 30 wt% NPOE (Fig. 2c). As shown in Table 2, the roughness of PIMs decreases significantly with the increase of NPOE. Namely, higher NPOE content in PIMs can soften the surface of PIMs, thus improving the formation of the more homogeneous PIMs.
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Generally, Cu (II) transport process can be divided into three stages: (1) Cu (II) reacts with LIX 84 I at the feed solution/interface of PIMs to form the metal–carrier complexes; (2) Metal–carrier complexes diffuse across PIMs towards the stripping solution; (3) At the interface of PIMs/stripping solution, the complexes are dissociated by hydrogen ions and Cu (II) is released into stripping solution. The extraction and back-extraction equilibria are described as Scheme 1 [31,32]. As shown in Fig. 3, the extraction rate is higher than back-extraction rate throughout the experimental time, indicating that diffusion process of metal-carrier complexes in the PIMs is the rate determining step for Cu (II) transport. Obviously, the transport of metal carrier complexes depends significantly on the carrier in the PIMs, since the membranes with only NPOE and PVC do not display the transport efficiency. Cussler et al [33] suggested that
3.2. Transport studies The effect of NPOE on Cu (II) transport efficiency of PIMs was studied by varying NPOE content and maintaining the concentration of LIX 84 I at 30 wt%. The extraction and back-extraction results are shown in Fig. 3. After 12 h, the PIM without NPOE has about 16 % Cu (II) extracted from the feed solution. Increasing of NPOE content leads to the slight increases of Cu (II) extraction efficiency. When adding 25 wt% NPOE in PIMs, the extraction efficiency is only 27%. Meanwhile, for the PIMs with lower than 25 wt% NPOE, the negligible Cu (II) back-extraction efficiency is observed. However, when NPOE content in the PIMs increases to 30 wt%, a great enhancement of Cu (II) transport efficiency can be found. The extraction efficiency of PIMs is about 80 % and the backextraction efficiency increases dramatically to 60% in 12 h. The sharp increase of both Cu (II) extraction and back-extraction efficiency could be explained by “percolation threshold” [30] in the Cu (II) transport process.
Scheme 1. The extraction and back-extraction equilibria at the PIM/feed solution and PIM/stripping solution interfaces.
Fig. 3. Effect of NPOE content on extraction (left) and back-extraction (right) efficiencies of Cu (II) in PIMs with 30 wt% LIX 84 I. Feed solution: 1 mmol L 1 Cu (II) and 50 mmol L 1 (NH4)2SO4, pH¼8.5; stripping solution: 1 mol L 1 H2SO4 solution. Error bar: mean 7 SEM, n¼ 3, (*) Po 0.05.
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carriers in solid membranes are bonding with the polymer, forming the chained carriers. Therefore, when the carrier concentration is lower than the critical value named percolation threshold, the chained carriers have no mobility, and the transport efficiency of Cu (II) is limited to almost zero. The specific transport mechanism would be discussed in the Section 3.3. Fig. 4 shows the Cu (II) transport behavior of PIMs with various NPOE content and constant PVC (40 wt%). It is worth noting that even though higher than 30 wt% LIX 84 I content is present in PIMs, the extraction and back-extraction percentages of Cu (II) decrease abruptly at the NPOE content of lower than 30 wt%. This result indicates that NPOE improves significantly Cu (II) transport. The similar phenomena were found in other studies. Fontas et al. [34] found that the PIMs with higher content of plasticizers have a better permeability for Pt (IV). Salazar-Alvarez et al. [35] also reported that the presence of NPOE can facilitate Pb (II) transport. 3.3. Transport mechanism XPS spectra of the PIMs with different NPOE contents were collected to further elucidate the effect of NPOE on the Cu (II) transport. Fig. 5 shows a comparison of C 1s XPS spectra of PIMs with various NPOE contents. Three clear peaks can be observed at binding energy of 284.5, 285.6 and 286.8 eV, which are attributed to C–C/C–H groups (aliphatic carbon), -CH2- groups and –CHCl– groups, respectively [36]. The interaction of membrane components with base polymer can be identified by the –CHCl– groups since they are specific for PVC. The perpendicular dotted line was used as marked line to illustrate the peak shifts of –CHCl- group. Compared with the individual PVC membrane, the binding energy of –CHCl– groups increases by 0.3 eV for the PIMs consisting of 70 wt% PVC and 30 wt% LIX 84I, indicating that electron density of carbon atom in –CHCl groups is weakened [37–39]. Namely, the molecular interactions between PVC and LIX 84I, such as hydrogen
bonding and Van der Waals forces, are established. After adding NPOE, the –CHCl peaks shift gradually to the higher binding energy region, suggesting that electron density of carbon atom in – CHCl groups decreases due to bonding to an electron withdrawing neighbor atom. For the PIMs containing 70 wt% PVC and 30 wt% NPOE, a strong interaction between NPOE and PVC can be illustrated since the C 1s XPS spectrum shows that the –CHCl peak can shift 0.7 eV to the higher binding energy region. The result is consistent with the ATR-IR analysis above (Fig. 1). According to the Gel Theory [39], the plasticized polymer could be neither solid nor liquid but an intermediate state, thus loosely held by a three-dimensional network of weak secondary bonding forces. These bonding forces are stronger than the intermolecular forces. As a consequence, it is presumed that PVC molecules could bond with NPOE rather than with LIX 84 I in the PIMs. Other characterizations of PIMs carried out by researchers are in concordance with our conclusion. Fontas et al. [34] found an apparently lower glass transition temperature in the DSC experiments and a perceptible change in the far IR spectra, when NPOE was added in the polymer CTA. These results suggest that LIX 84 I could be partially released from the chained interactions with PVC by addition of NPOE, decreasing the percolation threshold to a certain extent, therefore improving the efficiencies of Cu (II) extraction and transport. The interaction between targeted ions and surface active species of the PIMs firstly affects the ion transport. Therefore, the surface information of the PIMs was determined by XPS analysis before and after the transport experiments. The composition of the selected PIMs is 30 wt% LIX 84 I, 40 wt% PVC and 30 wt% NPOE. The O1s spectra (Fig. 6(a1-a3)) are resolved into four peaks. The peaks at binding energy of 531.7, 532.2, 532.7 and 533.8 eV are attributed to C–O–C and NO2 groups from NPOE, N–O–H and C–O– H (phenol) groups from LIX 84I, respectively. After extraction, the peak of N–O–H group shifts ca. 0.2 eV to the higher binding energy and the peak intensity decreases significantly. The possible
Fig. 4. Effect of NPOE content on extraction (left) and back-extraction (right) efficiencies of Cu (II) in PIMs with 40 wt% PVC. Feed solution: 1 mmol L 1 Cu (II) and 50 mmol L 1 (NH4)2SO4, pH ¼8.5; stripping solution: 1 mo L 1 H2SO4 solution. Error bar: mean 7 SEM, n¼ 3, (*) P o0.05.
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explanation is that copper ions bond with N–O–H groups during extraction, thus decreasing the electron density of oxygen atoms. After back-extraction, the binding energy of this characteristic peak returns to ca. 532.7 eV (Fig. 6(a-3)). As seen from Fig. 6(a-4), in contrast to the fresh PIM, the content of O atom decreases sharply after Cu (II) transport. These results suggest that O atom plays an important role in the Cu (II) transport process. From the N 1s spectra (Fig. 6(b)), it can be observed that the binding energy of N 1s increases by 0.8 eV after extraction compared with the fresh PIM. As shown in Scheme 1, the nitrogen atoms of LIX 84 I bond with Cu (II) through electron pair sharing. As a consequence, the electron density of the nitrogen atom increases. After back-extraction, the binding energy of N 1s returns again due to the release of Cu(II) from the carrier-metal complexes. The Cu (II) spectra (Fig. 6(c)) witness the main peak (934.89 eV) and the satellite peak (954.39 eV), which are attributed to Cu 2p 3/2 and Cu 2p 1/2, respectively. Apparently, copper species could be accumulated on the surface of the PIMs via extraction, though the exact species cannot be determined in the present work. As a contrast, the peak of Cu 2p is negligible after back-extraction. These results reveal that the Cu (II) transport through the PIMs can be dominated by the active carriers. An interesting phenomenon is observed from the Cl 2p spectra (Fig. 6(d)). There is negligible shift for the binding energy but sharp increase of peak intensity of Cl 2p after Cu (II) extraction, indicating an increase of the content of PVC molecules on the surface of the PIMs. After back-extraction, the little change of Cl 2p peak is observed compared with the fresh membrane. The results suggest that the plasticizer molecules are not steadily bound to the PVC molecules [37]. In other words, NPOE molecules could freely self-associate or associate with other molecules in the PIMs, since the bonding forces between NPOE and PVC could be weaker than chemical interactions but stronger than the intermolecular forces between PVC and LIX 84 I. If one NPOE molecule becomes a center or a site, it is easily replaced by another NPOE molecule. Therefore, NPOE provides a solvent-like environment for the carrier and carrier–metal complexes. The plasticizer allows these molecules to pass by each other in the three-dimensional network of PIMs, thus significantly improving the mobility of the extracted complexes in PIMs. As a result, the content of PVC increases on the surface of the PIMs after extraction. This assumption is consistent with similar phenomenon in other reports [34,38,40–42]. On the basis of the XPS and ATR-IR analyses, it could be concluded that PIMs prepared without NPOE plasticizer are rigid and hard network because of the PVC-LIX 84 I chains and PVC-PVC chains, thus LIX 84 I and LIX 84 I-Cu (II) molecules cannot move freely from one site to another in the rigid PVC chains. By addition of NPOE, intermolecular forces between PVC and LIX 84 I could be weakened because of the NPOE-PVC interactions. Increasing the concentration of plasticizer leads to the formation of solvent-like pathways for the carrier LIX 84 I and LIX 84 I-Cu (II) complexes between the two interfaces of PIMs, as the plasticizer NPOE allows complexes to pass by each other in the three-dimensional network of PIMs. This pathway can be confirmed by the dramatic enhancement of Cu (II) transport efficiency from nil to about 60% through the PIMs with 30 wt% NPOE. Thus, the assisted transport phenomenon induced by NPOE can be explained very well. These results could be used to further clarify the essence of the fixed sites in the solid membranes. Cussler et al. [33] proposed the theory of chained carriers and “percolation threshold” described that one chained carrier can reach the other fixed sites. The mechanism was revised with the addition of the plasticizers by Fontas et al [34]. In the case, plasticizers were thought of as the medium for carrier mobility through interaction with carriers. However, as mentioned above, we found that the NPOE could weaken the intermolecular forces between PVC and LIX84I by interaction with
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PVC, thus release more free carriers and provide a solvent-like environment for the metal–carrier species to diffuse through the PIMs.
4. Conclusions In the present work, PIMs were used for Cu (II) transport in ammoniacal solutions. The influence of membrane components, especially NPOE, on Cu (II) transport efficiency of PIMs was studied. It can be found that the extraction and back-extraction efficiencies of PIMs are significantly improved by NPOE. Compared with the PIMs without NPOE, the extraction efficiency of the PIMs with 30 wt% NPOE increases from 16% to about 80% and the backextraction efficiency increases from nil to 60% in 12 h. The ATR-IR spectral analysis indicates that a new peak located at 800 cm 1 appears for the PIMs with 30 wt% NPOE, suggesting that NPOE molecules could bond with PVC chains, which is also supported by XPS analysis. Moreover, XPS spectra of the PIMs before and after transport experiments suggest that NPOE molecules could not permanently bond with the PVC molecules, but provide a solventlike environment to transport the carriers and LIX 84 I-Cu (II) complexes in the membrane phase, thereby facilitating Cu (II) transport in the PIMs.
Fig. 5. C 1s XPS spectra of PIMs with various NPOE contents.
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Fig. 6. XPS spectra of PIMs before and after the transport experiment. (a) O 1s; (b) N 1s; (c) Cu 2p and (d) Cl 2p.
Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2014CB643401), National Natural Science Foundation of China (Nos. 51134007 and 51304244), China Postdoctoral Science Foundation (2014M552152) and Hunan Provincial Natural Science Foundation of China (2015JJ3154).
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