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Selective separation of copper and nickel by membrane extraction using hydrophilic nanoporous ion-exchange barrier membranes
Accepted Manuscript Title: Selective Separation of Copper and Nickel by Membrane Extraction Using Hydrophilic Nanoporous Ion-exchange Barrier Membranes Authors: Jianfeng Song, Xuhong Niu, Xue-Mei Li, Tao He PII: DOI: Reference:
S0957-5820(17)30311-7 http://dx.doi.org/10.1016/j.psep.2017.09.008 PSEP 1185
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
11-7-2017 11-9-2017 12-9-2017
Please cite this article as: Song, Jianfeng, Niu, Xuhong, Li, Xue-Mei, He, Tao, Selective Separation of Copper and Nickel by Membrane Extraction Using Hydrophilic Nanoporous Ion-exchange Barrier Membranes.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2017.09.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective Separation of Copper and Nickel by Membrane Extraction Using Hydrophilic Nanoporous Ion-exchange Barrier Membranes
Jianfeng Song1, Xuhong Niu1,3 ,Xue-Mei Li*1,Tao He1,2*
1
Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai,201210, China
2
School of Physical Science and Technology, Shanghai Tech University, Shanghai, 201210, China
3
University of Chinese Academy of Sciences, Beijing, 100049, China
Corresponding authors: [email protected]; [email protected]
Highlights
Highly selective separation of Cu2+/Ni2+was achieved using membrane extraction
Hydrophilic nanoporous membrane (PES/SPPESK membrane) was used for Cu2+/Ni2+ separation
Higher copper flux was observedin sandwiched membrane extraction due to the coupling of stripping
The membrane showed high stability in the organic extractant
Abstract Hydrophilic nanoporous ion exchange barrier membrane, based on polyethersulfone (PES) /sulfonated polyphenylether sulfone ketone (SPPESK), was utilized for the pH dependent separation of copper and nickel ionswith a commercial extractant, LIX84-I. The extraction and stripping performance of the membrane was demonstrated in a single stage membrane contactor as well as in an integrated extraction/stripping system. By adjusting the feed pH to 2.9, a complete separation of copper and nickel was achieved using the membrane contactors, where a copper flux of 1.10*10-8 mol/cm2.s was observed. In an extraction/stripping integrated 1
sandwiched membrane extraction contactor, significantly higher copper flux was observed due to the coupling of copper stripping. In addition, the PES/SPPESK barrier membrane prevented the loss of the organic extractant, exhibiting high stability in 30 days dynamic test. The results indicated that the membrane extraction contactor system is of great potential for selective extraction of metal ions.
Keywords: membrane extraction, nanoporous membrane; ion exchange membrane; copper; nickel
1 Introduction In metallurgical industries,heavy metal ions contaminated water has been one of the most important environmental concerns (Cui et al., 2008; Barakat, 2011). The heavy metal ions in the wastewater are detrimental to soil, underground water, and surface water with destructive effects on ecology, health of animals and human beings, if not properly treated (Jamali-Zghal et al., 2015; Liu et al., 2015). On the other hand, purification and separation of the heavy metal ions for reuse is an extra resource for the sustainability of the industry (Fu et al., 2011). Conventional treatment technologies for copper and nickel removal, such as chemical precipitation, ion exchange, adsorption, and solution extraction(Monser et al., 2002; Kim, 2003; Santos Yabe et al., 2003; Fouad et al., 2009; Alguacil et al., 2015) suffered from low extraction efficiency, large footprint, possibly high energy consumption, and production of secondary liquid or solid wastes (Fu et al., 2011). The development of eco-friendly, highly energy effective separation process is of high importance. Supported liquid membrane (SLM) has been the research focus for the recovery of metal ions because of its potentially high separation efficiency, small footprint, and high energy efficiency. In SLM, membrane and solvent extraction are integrated, where the separation is achieved through affinity binding and/or complexation ofthe liquid membrane with a particular chemical species (Kemperman et al., 1998). The membrane functions as a porous support to hold the chemical extractant and the diluents. With a suitable selectivity, liquid membrane 2
extraction can achieve both the separation and recovery of metal ions from metallurgical wastewater (Duan et al, 2017; de los Ríos et al., 2013; Bhatluri et al., 2014; Yang et al., 2003). Besides, because SLM has a very large surface to volume ratio, much less extractant is needed (Pabby et al., 2013). LIX84-I (2-hydroxy-5-nonylbenzophenone) and similar extractant have been used for extraction, separation and recovery of copper and nickel by varying the pH of feed solution either by solvent extraction (Lee et al., 1999; Ramachandra et al., 2007), or in SLM (Valenzuela et al., 1999; Venkateswaran et al., 2007; Yang et al., 2007). Leakage of the liquid membrane and instability of the membrane materials have been the key issues (Neplenbroek et al., 1992; Kemperman et al., 1996; Kemperman et al., 1997; He et al., 2004；Kocherginsky et al., 2007). To avoid loss of the liquid membrane, many approaches have been tested, such as adding an extra coating layer on the support membrane (i.e. gel coating layer (Neplenbroek et al., 1992; Kemperman, Damink et al., 1997), interfacial polymerization coating (Kemperman, Rolevink et al., 1998), composite membranes with hydrophilic and hydrophobic layers(Sirkar, 1991; Song et al., 2014; He et al., 2008; Xin et al., 2016), ion exchange membranes (Kedem et al., 1993), immobilization of extraction molecules to the membrane matrix (Hayashi et al., 2003; Molinari et al., 2005; Vilt et al., 2009), addition of extractant into the strip solution (Ho et al., 2001), and optimization of the membrane module structure (Zhu et al., 1990). These methods could not completely block the passage of extractant into the aqueous phases and consequently the stability of SLM was still below satisfaction. The ion exchange layer, sulfonated poly ether ether ketone, SPEEK, was impermeable to the organic extractant, thereby effectively blocks the leakage of organic extractant after dip-coated onto a polysulfone or polypropylene porous support. This membrane has been applied as nanofiltration membrane for monovalent and multivalent ion separation (Song et al., 2013; Song et al.; 2015), concentration of oil emulsion (Jin, et al., 2017). When utilized in the membrane contactor process, the system, a stable operation for 75 days was reported (He et al., 2004). The disadvantage of such membrane was the close contact between the organic extractant and the porous support, which resulted in chemical degradation of the support membrane and consequently loss of mechanical strength (He, 2001). Although the SPEEK layer was stable, weakening in the support structure could not sustain the extraction. To resolve 3
this issue, a new nanoporous membrane based on SPPESK/PES hydrophilic ion-exchange membranes was reported recently for lithium ions separation from salt-lake brine in aliquid–liquid membrane extraction (Song et al., 2014). Unlike the composite SPEEK coated membranes, the organic extractant did not impregnate into the porous support, but the aqueous phase did; consequently, the membrane stability has been largely improved. However, the application potential of the SPPESK/PES blend membranes has been largely unexplored. In this paper, the nanoporous SPPESK/PES blend membrane was used for the separation of copper and nickel in membrane extraction contactor based LIX84-I. A synthetic wastewater containing copper and nickel was used as the feed solution. LIX84-I in dodecane (20vol.%) was used as extractant and a H2SO4 (2 mol/L) solution as the strip. Both membrane extraction and continuous extraction/striping processes were investigated. The extraction permeance and the membrane stability were examined. Potential applications and the pros-and-cons of membrane extraction are discussed.
2 Membrane extraction separation mechanism for copper and nickel According to the extraction characteristics of LIX84-I (Kumbasar, 2009), the separation of copper and nickel is largely dependent on the feed pH. LIX 84-I, a commercial extractant (Cognis), is based on 2-hydroxy-5-nonylbenzophenone (Fig. 1A). This extractant was used for selective removal of copper (Ali et al., 1996; Panigrahi et al., 2009), nickel (Parija et al., 1998) and zinc from acid leachate (Reddy et al., 2005). The separation of copper, nickel, zinc was achieved at pH of 4.0, 7.5, 9.0, respectively. Therefore, copper can be separated from nickel at pH < 4 (Fig.1B). It is a common practice to strip Cu2+ at pH = 2-2.9. Since Ni2+ is not stripped at this pH range, separation of Cu2+and Ni2+is realized.
In this work, the nanoporous membrane is cation permeable, and the diffusion in the membrane contactor are as follows (Fig.2): 1) Diffusion of Cu2+ from the feed bulk to the membrane/feed interface; 2) Diffusion of Cu2+ across the membrane from the feed/membrane interface to the membrane/organic phase interface; As demonstrated in our work (Song, et al., 2014), 4
the concentration gradient is the driving force; addition of a barrier layer to the organic phase added an extra step in membrane contactor system. 3) The Cu2+ reacts with LIX84-I, forming an extracting complex at the membrane /LIX84-I interface; 4) The extracting complex diffuses into the bulk of organic phase. In the stripping process, reversed steps could be easily identified (Fig.2). By integrating both extraction and stripping process, a continuous extraction-stripping membrane extraction system could be realized as show in Fig. 2C. This process is based on only diffusion and extraction reaction. Extra resistance caused by membrane and the concentration polarization could be largely reduced by optimizedof membrane, hydraulic pattern, as well as membrane module design (Yeh et al., 2001; Song et al., 2014).
3Experimental 3.1 Chemicals and materials Polyethersufone (PES Ultrason E6020P) was kindly supplied by BASF and was dried one week at 90oC before usage. Sulfonated polyphenyl ether sulfone ketone (SPPESK) was supplied by Dalian Polymer Co. Ltd (Dalian, China PNM P0403-029) (Wu et al., 2010). 2-Hydroxy-5-nonylbenzophenone (LIX84-I) was obtained from Cognis Chemical Industry (China) Co. Ltd. Dodecane (Aladdin-reagent Co. ltd, China) was used as diluent for organic extractant. Analytic grade dimethylacetamide (DMAc), CuSO4, sulfuric acid and NiSO4.6H2O were provided by Sinopharm Chemical Regent Co. Ltd. All chemicals were used as received without further purification. Deionized water was used throughout the experiments. 3.2 Liquid-liquid extraction LIX 84-I and dodecane was used as the extractant and diluent. Aqueous phase with copper and nickel was prepared by copper sulfate and nickel sulfate, the concentration of copper ion and nickel ion in aqueous was 0.147 mol/L and 0.041 mol/L respectively. The solution pH was 5
adjusted by adding stock H2SO4 solution of 2 mol/L. Extraction of copper and nickel was carried out at O/A phase ratio of 1:1 (50ml/50ml) at 25 oC under vigorous mixing for 5 min. The duration for organic and aqueous phase separation was 20 min. The concentration of copper and nickel in the aqueous phases was determined by inductively coupled plasma atomic emission spectroscopy (ICPE-9000,Shimadzu) after dilution by deionized water with 3 vol% HNO3 to the detection range (0.1-5 mg/L). The extraction rate (E %) was defined as (1)
Where Co.M (mol/L) and Cf.M (mol/L) are the ion concentration in the organic phase after extraction and in aqueous phase (feed) before extraction; Vo (L) and Vf (L) are the volume of organic phase and feed, respectively. 3.3 Preparation of PES/SPPESK blend membranes The detailed procedure for the preparation of PES/SPPESK blend membrane was reported elsewhere (Song, et al., 2014). Briefly, dried PES and SPPESK (IEC of 1.21 meq/g), at a weight ratio of 6/4, were dissolved in DMAc at 65 oC, and the total polymer concentration was 30 wt.%. The polymer solutions were filtered with a 40 m metal mesh and de-aerated before membrane casting. Flat nascent membranes were cast onto a clean dry glass plate at a thickness of 200 m, then immersed immediately in a water bath at 25.0 ± 0.2 oC for precipitation. After casting, membranes were rinsed in deionized water for 24 h to remove trace solvent in the membrane. 3.4 Scanning electron microscopy (SEM) Cross section of the membranes was examined using scanning electron microscopy (SEM, HITACH, TM1000) for low magnifications and the field emission scanning electron microscopy (FESEM, HITACH, S-4800) for high magnifications. Samples were made by fracturing the wet membranes in liquid nitrogen and then freeze-dried overnight. The samples were sputtered with a thin layer of gold before image acquisition. 6
3.5Membrane extraction Fig. 3 shows schematically the membrane extraction contactor (MEC) for the separation of Cu2+ and Ni2+. For the single-stage membrane extraction contactor, as shown in Fig. 3A, a flat membrane (3cm10cm, effective range of surface) was sealed between two Polytetrafluoroethylene (PTFE) half-cell test modules. The feed and extractant solutions (LIX84-I/dodecane, 20% vol/vol) were then fed into the module co-currently. To investigate the membrane performance in the stripping process, a Cu2+ pre-saturated organic extractant solution was used and 2 mol/L H2SO4 solution was used as the stripping solution. For the Sandwiched Membrane Extraction Contactors, as shown in Fig. 3B, two pieces of flat membranes (3 cm 10 cm) were mounted into a three-channel test cell. Three solutions (the feed, strip and the organic extractant) were circulated at a flow rate 1 L/min, 1 L/min and 0.5 L/min respectively. The copper ions in the feed diffused into the organic phase through the membrane. Subsequently, copper in organic phase was stripped by acid. The ion flux was calculated as
(2)
where V (L) and A (cm2) represent the feed volume in extraction process (or organic phase loaded Cu2+ in stripping process) and the effective membrane area, respectively. Detailed experimental parameters are listed in Table 1.
4 Results and Discussion 4.1 Membrane morphology Addition of SPPESK improves both the hydrophilicity and the ion exchange capacity of the PES membrane (Song, Li et al., 2014). Due to the high ion exchange capacity of the sulfonated polymer, low diffusion resistance for cations is realized. However, due to the strong 7
hydrophilicity, the sulfonated polymer tends to swell greatly in aqueous phases; high swelling results in low mechanical property and large deformation, thus is practically difficult for utilization. By introduction of PES, a comparably hydrophobic polymer (therefore less swollen in water), the swollen of the sulfonated polymer chains are constrained (Li et al., 2008). Because PES/SPPESK (6/4) membrane showed a moderate swelling and high ions permeation (Song, Li et al., 2014), it was selected for copper extraction in this work. As shown in Fig.4, the membrane has a macrovoid-free network-like structure. At higher magnification, nano-sized interconnected porous structure was observed. The formation of this particular morphology is originated from the demixing of PES and SPPESK, after liquid-liquid phase separation (Boom et al., 1992; Li et al., 2010; Xiao et al., 2015). The membrane showed an asymmetric structure with a compact skin. Fortunately, this compact skin layer is rather thin compared to the thickness of the whole membrane (about 1-2 μm for the flat membrane comparing to the membrane thickness of 110 μm). The resistance is expected to be low. 4.2 Effect of pH on selectivity of Cu2+ and Ni2+ in liquid-liquid extraction The feed pH has significant influence on the extraction selectivity for copper and nickel. As shown in Fig.1, the separation of copper and nickel can be achieved by adjusting the feed pH. Fig.5 shows the Cu2+and Ni2+ concentration in the organic extractant after one stage liquid-liquid extraction. No Ni2+ was measured in the extractant at the pH range of 2.1-3.8. The extraction rate for Cu2+ increased (66.7 % to 70.8 %) as the pH increased from 2.1 to 3.8. A high selectivity of Cu2+ and Ni2+was realized. Thus, a pH=2.9 was selected for the separation of Cu2+/Ni2+ in following experiment. 4.3Single-stage membrane extraction contactor Membrane extraction and stripping processes were investigated separately based on single-staged membrane extraction contactor. Fig. 6A shows the concentration profileof Cu2+ and Ni2+ in the feed solution as a function of time. A sharp decline of the copper concentration from 8.9 g/L to 7.0 g/L was observed for the first 4 hours. A concentration reduction about 30% was achieved at 660 min. After the organic extractant was gradually loaded with Cu2+, the 8
driving force for the diffusion of Cu2+ across then membrane decreased (Fig.2), the chemical reaction rate of Cu2+/LIX84-I decreased, leading to declined extraction rate. Fig. 6B shows the concentration profile ofCu2+ions in the stripping phase where a quick increase was observed up to 6 h, then leveled off . The Ni2+concentration in feed remained unchanged, because, at pH = 2.9, no extraction for Ni2+occurred. The average flux of Cu2+ in the membrane extraction was 4.20 10-9 mol/cm2.s. In the extraction process, visual color change was observed for the organic phase from light yellow to dark green, and finally black, which was ascribed to the increase of copper ions in the extractant (Figure 6C). The visual changes verified that the copper was extracted from the feed to the organic phase and stripped from the organic phase to the acid phase.
At the beginning of a stripping process, the reaction rate was high due to the high driving force. However, as the reaction proceeds, the driving force declines resulting in a slower stripping process. Thus, the increment of the Cu2+ concentration in the stripping phase declined. Fig.7 shows the Cu2+ flux with time in the membrane extraction/stripping process. It is obvious that the flux for both extraction and stripping declined against time. Copper flux in the membrane stripping process was higher than that in the membrane extraction. This was most probably due to the high H+ concentration in the stripping phase, consequently, higher H+ concentration at the interface between the membrane and the organic phase. The higher concentration resulted in higher reaction rate in the stripping process (Fig.2.A). It was showed that no volume change occurredin the extraction and stripping tank; this was due to the membrane barrier for the extractant, thus loss of organic extractant was prevented.
4.4 Separation of Cu2+/Ni2+ by sandwiched membrane extraction contactor The sandwiched membrane contactor for extraction Cu2+ is an integration of extraction and striping steps (Fig.3B). The Cu2+ ions were extracted from the feed to organic phase via the membranes and transferred to the stripping phase. Fig.8 shows the concentration profiles of the 9
Cu2+ in the feed and strip phases against time. The concentration of Cu2+ in feed decreased quickly at the beginning at a linear rate. At the late stage, the decline rate of the Cu2+ concentration slowed down, most probably due to the reduced Cu2+ concentration in the feed, thus lower Cu2+ flux (Fig.7). Correspondingly, the copper concentration in the stripping phase increased linearly at the beginning, then reached a plateau, due to decline in the Cu2+ flux in the extraction stage. As expected, the Ni2+concentration in the feed did not change (no Ni2+was detected in the stripping phase). A high selectivity was achieved between Cu2+and Ni2+ in membrane extraction process. Comparing the copper concentration values in Fig. 6 and Fig. 8, it was observed that the copper concentration decreases significantly faster in the sandwiched membrane extraction contactor than in the single stage membrane contactor, using the same membrane of the same area, In the sandwiched membrane extraction contactor, copper ions were stripped via the integrated stripping stage; therefore, the copper concentration in the organic extractant was balanced, the driving force for the copper transport was maintained; but in the single stage membrane contactor, the copper ions aggregated in the organic extractant, resulting in lower driving force. The average copper flux in the sandwiched membrane contactor system was 5.6 10-9 mol/cm2.s, much higher than that in the single stage membrane contactor. 4.5 Membrane stability The stability of the membrane materials in both the aqueous and organic solutions were tested by immersing in the corresponding liquid. No obvious visual change in the membrane was observed after 30 days this static test. In a dynamic test, the membranes were installed in a test cell filled with the organic extractant and the feed solution or strip solution for 30 days. Fig.9 shows the force at breakage and the stain at breakage of the membranes following the time. After immersion in organic/2 mol/L H2SO4 and organic /water, the force at breakage slightly decreased, but the stability was maintained. The PES/SPPESK barrier, swollen in aqueous phase, is impermeable to the extractant (LIX84-I), but permeable for cation. The contact between extractant and membrane exists at the membrane/feed or membrane/stripping interfaces. Deteriorating of membrane by organic 10
solvent may occur at the interfaces. However, due to the high swelling rate of the membrane in aqueous phase, a preferential water layer at the membrane surface may prevent itself from direct attack by the organic extractant. Consequently, the barrier membrane shows high stability. However, the strain declined as the membrane was in direct contact with acid and organic extractant. It seems that although the membranes are very hydrophilic in nature, slow attack by the solvent still occurred. Nevertheless, the membranes after 30 days dynamic stability test show rather good mechanical properties and are suitable for further test in the extraction process. It was proposed that further increase the hydrophilicity of the membrane material is the research direction to avoid the degradation of the membrane in the organic phase. Balance of the hydrophilicity and the mechanical properties have to be considered.
4.6 Outlook of membrane extraction This integrated extraction/stripping process is unique and highly efficient in extraction process. Because the organic extractant acts as a carrier to transport ions from feed side to the strip side, very small amount of organic extractant is necessary to realize the continuous extraction and stripping loop. Much less organic extractant is required; expensive and high selectivity extractant could be used; liquid volume in extraction system is largely reduced. Furthermore, no aqueous/organic phase interface exists in membrane extraction contactor, leakage of organic extractant is prevented; thus, saving in extractant is possible. In this report, a complete separation of copper and nickel was observed; the present membrane extraction contactor system could be utilized for the separation of other ion pairs. In case of aggressive organic extractant, an alternative solvent stable barrier membrane is needed (Xing et al., 2016); this would open a new area of research for universal solvent resistance and ion permeable membrane. One drawback of membrane extraction contactor is an extra mass transfer resistance caused by an extra barrier membrane. In fact, by blending the sulfonated polymer with PES, the membrane transfer resistance has been largely reduced (Geise et al., 2013). This extra resistance may be further optimized by improving the membrane morphology, or membrane 11
module design. Literature report concluded that a hollow fiber module has much better mass transfer (Prasad et al., 1990; Daiminger et al., 1996) than conventional mixing/settler or even column extraction equipment (Song et al., 2017). Electrospun nanofiber membranes has been reported for improve the mass transfer in other diffusion type of membrane, such as membrane distillation (An et al., 2016; Tijing et al., 2016). It is highly likely that this type of new membrane morphology could reduce the diffusion resistance of the ions in the membrane layer. It is thus one of the main targets for our ongoing research on membrane extraction contactor system. Benign and less chemically aggressive extractants are the other important research direction for membrane contactor for improved life time. Ionic liquids (ILs) are thermally stable green solvent, constituted byan organic cation and either an organic or an inorganicanion (Fortunato et al., 2004). However, the loss of ionic liquids was still an issue if used in the conventional SLM or membrane contactor system (Gruenauer et al., 2016; Zeh et al., 2016). A number of scientific research papers has identified the ionic liquids as the extractants for various targeted species (Depuydt et al., 2015; Yang et al., 2015; Alguacil et al., 2016; Gaillard et al., 2016; Rout et al., 2016; Wankowski et al., 2016; Depuydt et al., 2017; Merlet et al., 2017; Mohapatra, 2017), and other investigations reported using the ILs as the solvent to assistant the extraction of metal ions (Shi et al., 2016; Torrejos et al., 2016; Herce-Sesa et al., 2017; Mohapatra, 2017; Shi et al., 2017). Taking the advantages of the current hydrophilic ion exchange membranes, a new system combining the ILs and the novel membrane could result in a novel separation system for metal ions.
5 Conclusions Membrane extraction contactor was utilized for selective extraction of copper and nickel from wastewater. A barrier membrane composed of PES/SPPESK was used as the protection layer to prevent the loss of organic extraction, allowing free diffusion of copper and nickel ions. The barrier membrane shows nanoporous sponge-like morphology, and is resistant to the degradation of the organic chemicals. As a barrier, the membrane shows high stability in dynamic test. Based on the pH selectively of LIX84-I to Cu2+/Ni2+, membrane extraction results 12
realized the liquid-liquid extraction, at feed pH=2.9. The membrane stripping could be realized by using a 2 mol/L H2SO4 solution. By integrating extraction and stripping, a sandwiched membrane extraction contactor demonstrated a simultaneous extraction and stripping in one unit, a high selectively of two ions as well. The present selective MEC process could serve as a highly efficient separation platform for removal and separation of metal ions. Continuous improvement in the stability has been proposed in more hydrophilic membranes and possible new extraction systems as ionic liquids. The improvement in the mass transport could be achieved by the less resistant nanofiber support.
ACKNOWLEDGMENT The authors thank National Natural Science Foundation of China (U1507117, 21676290), TMSR from Chinese Academy ofSciences (XDA02020100), Key Research Fund (CAS2014Y424541211) for financial support.
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17
Fig. 1 The chemical structure of LIX84-I and its reaction mechanism with copper and nickel (A) ; the effect feed pH on cation concentration in extractant: LIX84-I (B) (Zhu et al., 1990; Reddy et al., 2005)
18
Fig.2 Schematic illustration of concentration profile of Cu2+ in the membrane extraction(A) and stripping steps(B). an integrated membrane extraction and stripping process(C). "CP": concentration polarization; F:Feed phase; Org: organic phase; S: stripping phase; m: membrane; b: bulk phase.
19
Fig.3 Schematics of a single-stage flat membrane contactor (A),asandwiched membrane extraction contactors(B).
20
Fig. 4 Cross-section images of PES/SPPESK flat membrane．A: Cross-section, B: Top skin layer, C: Bottom-section (Ratio of PES/SPPESK = 6/4, solvent: DMAc; solution temperature 65 oC; casting knife thickness: 200 μm; coagulation bath temperature: 25 oC).
21
100
Extraction (%)
80 60 40 20 0
2.1
2.8
3.6
pH value
Fig. 5 Effect of pH in feed on extraction of Cu2+ and Ni2+. Note that no Ni2+was detected in the organic phased( feed:0.147 mol/L CuSO4+ 0.041 mol/L NiSO4; organic phase : LIX84-I/dodecane, 20% vol/vol; O/A=1:1)
22
Fig. 6 Ion concentration in the feed (A) and the stripping phase (B) against time; the color of extractant. Membrane extraction process(C), feed:8.9 g/L Cu2+, 2.3 g/L Ni2+, pH = 2.9; Membrane stripping process, the extractant used was after membrane extraction process, H2SO4 (2 mol/L) was as stripping phase.
23
Cu2+ flux(10-8mol/cm2.s)
1.5 1.2 0.9
Feed Strip
0.6 0.3 0.0
0
120
240
360
480
Time (min) Fig.7 The flux of Cu2+in membrane extraction process (feed /extractant) and membrane stripping process (extractant/stripping phase) against time.(Membrane extraction process, feed: 8.9 g/L Cu2+, 2.3 g/L Ni2+, pH=2.9; Membrane stripping process, the extractant used was after membrane extraction process, H2SO4 (2 mol/L) was as stripping phase)
24
4 feed strip
8
3
2+
2
6 1 5 0
120
240 360 Time (min)
480
2+
7
Conc. Cu in strip (g/L)
Conc. Cu in feed (g/L)
9
0 600
Fig.8 Plot of the concentration of ion against time. (Feed:8.25 g/L Cu2+, 2.3 g/L Ni2+, pH=2.9; Stripping: H2SO4 2 mol/L)
25
Org.&2M Sulphuric Acid Org.&Water Org.&2M Sulphuric Acid Org.&Water
60 50
2.4
40 2.0
30 20
1.6
Elongation (%)
Load (N)
2.8
10
1.2 0
5
10
15
20
25
30
0
Time (Day)
Fig. 9 Mechanical properties of the membranes after in contact with the feed, extractant and strip solutions. The membranes were after dynamic stability test, with one side contact with water or 2 mol/L H2SO4 and the other side with the organic extractant.
26
Table1. Operating parameters for the membrane contactors Feed (g/L)
Cu2+：8.26 ; Ni2+：2.3
Feed pH
2.9
Feed Volume(ml）
100
Feed flow(L/min)
1
Volume of organic phase volume(ml）
85
Flow of organic phase (L/min)
0.5
Strip volume (ml）
100
Flow of stripping phase (L/min)
1
Area of membrane (cm2)
30
27
S0957-5820(17)30311-7 http://dx.doi.org/10.1016/j.psep.2017.09.008 PSEP 1185
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
11-7-2017 11-9-2017 12-9-2017
Please cite this article as: Song, Jianfeng, Niu, Xuhong, Li, Xue-Mei, He, Tao, Selective Separation of Copper and Nickel by Membrane Extraction Using Hydrophilic Nanoporous Ion-exchange Barrier Membranes.Process Safety and Environment Protection http://dx.doi.org/10.1016/j.psep.2017.09.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective Separation of Copper and Nickel by Membrane Extraction Using Hydrophilic Nanoporous Ion-exchange Barrier Membranes
Jianfeng Song1, Xuhong Niu1,3 ,Xue-Mei Li*1,Tao He1,2*
1
Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai,201210, China
2
School of Physical Science and Technology, Shanghai Tech University, Shanghai, 201210, China
3
University of Chinese Academy of Sciences, Beijing, 100049, China
Corresponding authors: [email protected]; [email protected]
Highlights
Highly selective separation of Cu2+/Ni2+was achieved using membrane extraction
Hydrophilic nanoporous membrane (PES/SPPESK membrane) was used for Cu2+/Ni2+ separation
Higher copper flux was observedin sandwiched membrane extraction due to the coupling of stripping
The membrane showed high stability in the organic extractant
Abstract Hydrophilic nanoporous ion exchange barrier membrane, based on polyethersulfone (PES) /sulfonated polyphenylether sulfone ketone (SPPESK), was utilized for the pH dependent separation of copper and nickel ionswith a commercial extractant, LIX84-I. The extraction and stripping performance of the membrane was demonstrated in a single stage membrane contactor as well as in an integrated extraction/stripping system. By adjusting the feed pH to 2.9, a complete separation of copper and nickel was achieved using the membrane contactors, where a copper flux of 1.10*10-8 mol/cm2.s was observed. In an extraction/stripping integrated 1
sandwiched membrane extraction contactor, significantly higher copper flux was observed due to the coupling of copper stripping. In addition, the PES/SPPESK barrier membrane prevented the loss of the organic extractant, exhibiting high stability in 30 days dynamic test. The results indicated that the membrane extraction contactor system is of great potential for selective extraction of metal ions.
Keywords: membrane extraction, nanoporous membrane; ion exchange membrane; copper; nickel
1 Introduction In metallurgical industries,heavy metal ions contaminated water has been one of the most important environmental concerns (Cui et al., 2008; Barakat, 2011). The heavy metal ions in the wastewater are detrimental to soil, underground water, and surface water with destructive effects on ecology, health of animals and human beings, if not properly treated (Jamali-Zghal et al., 2015; Liu et al., 2015). On the other hand, purification and separation of the heavy metal ions for reuse is an extra resource for the sustainability of the industry (Fu et al., 2011). Conventional treatment technologies for copper and nickel removal, such as chemical precipitation, ion exchange, adsorption, and solution extraction(Monser et al., 2002; Kim, 2003; Santos Yabe et al., 2003; Fouad et al., 2009; Alguacil et al., 2015) suffered from low extraction efficiency, large footprint, possibly high energy consumption, and production of secondary liquid or solid wastes (Fu et al., 2011). The development of eco-friendly, highly energy effective separation process is of high importance. Supported liquid membrane (SLM) has been the research focus for the recovery of metal ions because of its potentially high separation efficiency, small footprint, and high energy efficiency. In SLM, membrane and solvent extraction are integrated, where the separation is achieved through affinity binding and/or complexation ofthe liquid membrane with a particular chemical species (Kemperman et al., 1998). The membrane functions as a porous support to hold the chemical extractant and the diluents. With a suitable selectivity, liquid membrane 2
extraction can achieve both the separation and recovery of metal ions from metallurgical wastewater (Duan et al, 2017; de los Ríos et al., 2013; Bhatluri et al., 2014; Yang et al., 2003). Besides, because SLM has a very large surface to volume ratio, much less extractant is needed (Pabby et al., 2013). LIX84-I (2-hydroxy-5-nonylbenzophenone) and similar extractant have been used for extraction, separation and recovery of copper and nickel by varying the pH of feed solution either by solvent extraction (Lee et al., 1999; Ramachandra et al., 2007), or in SLM (Valenzuela et al., 1999; Venkateswaran et al., 2007; Yang et al., 2007). Leakage of the liquid membrane and instability of the membrane materials have been the key issues (Neplenbroek et al., 1992; Kemperman et al., 1996; Kemperman et al., 1997; He et al., 2004；Kocherginsky et al., 2007). To avoid loss of the liquid membrane, many approaches have been tested, such as adding an extra coating layer on the support membrane (i.e. gel coating layer (Neplenbroek et al., 1992; Kemperman, Damink et al., 1997), interfacial polymerization coating (Kemperman, Rolevink et al., 1998), composite membranes with hydrophilic and hydrophobic layers(Sirkar, 1991; Song et al., 2014; He et al., 2008; Xin et al., 2016), ion exchange membranes (Kedem et al., 1993), immobilization of extraction molecules to the membrane matrix (Hayashi et al., 2003; Molinari et al., 2005; Vilt et al., 2009), addition of extractant into the strip solution (Ho et al., 2001), and optimization of the membrane module structure (Zhu et al., 1990). These methods could not completely block the passage of extractant into the aqueous phases and consequently the stability of SLM was still below satisfaction. The ion exchange layer, sulfonated poly ether ether ketone, SPEEK, was impermeable to the organic extractant, thereby effectively blocks the leakage of organic extractant after dip-coated onto a polysulfone or polypropylene porous support. This membrane has been applied as nanofiltration membrane for monovalent and multivalent ion separation (Song et al., 2013; Song et al.; 2015), concentration of oil emulsion (Jin, et al., 2017). When utilized in the membrane contactor process, the system, a stable operation for 75 days was reported (He et al., 2004). The disadvantage of such membrane was the close contact between the organic extractant and the porous support, which resulted in chemical degradation of the support membrane and consequently loss of mechanical strength (He, 2001). Although the SPEEK layer was stable, weakening in the support structure could not sustain the extraction. To resolve 3
this issue, a new nanoporous membrane based on SPPESK/PES hydrophilic ion-exchange membranes was reported recently for lithium ions separation from salt-lake brine in aliquid–liquid membrane extraction (Song et al., 2014). Unlike the composite SPEEK coated membranes, the organic extractant did not impregnate into the porous support, but the aqueous phase did; consequently, the membrane stability has been largely improved. However, the application potential of the SPPESK/PES blend membranes has been largely unexplored. In this paper, the nanoporous SPPESK/PES blend membrane was used for the separation of copper and nickel in membrane extraction contactor based LIX84-I. A synthetic wastewater containing copper and nickel was used as the feed solution. LIX84-I in dodecane (20vol.%) was used as extractant and a H2SO4 (2 mol/L) solution as the strip. Both membrane extraction and continuous extraction/striping processes were investigated. The extraction permeance and the membrane stability were examined. Potential applications and the pros-and-cons of membrane extraction are discussed.
2 Membrane extraction separation mechanism for copper and nickel According to the extraction characteristics of LIX84-I (Kumbasar, 2009), the separation of copper and nickel is largely dependent on the feed pH. LIX 84-I, a commercial extractant (Cognis), is based on 2-hydroxy-5-nonylbenzophenone (Fig. 1A). This extractant was used for selective removal of copper (Ali et al., 1996; Panigrahi et al., 2009), nickel (Parija et al., 1998) and zinc from acid leachate (Reddy et al., 2005). The separation of copper, nickel, zinc was achieved at pH of 4.0, 7.5, 9.0, respectively. Therefore, copper can be separated from nickel at pH < 4 (Fig.1B). It is a common practice to strip Cu2+ at pH = 2-2.9. Since Ni2+ is not stripped at this pH range, separation of Cu2+and Ni2+is realized.
In this work, the nanoporous membrane is cation permeable, and the diffusion in the membrane contactor are as follows (Fig.2): 1) Diffusion of Cu2+ from the feed bulk to the membrane/feed interface; 2) Diffusion of Cu2+ across the membrane from the feed/membrane interface to the membrane/organic phase interface; As demonstrated in our work (Song, et al., 2014), 4
the concentration gradient is the driving force; addition of a barrier layer to the organic phase added an extra step in membrane contactor system. 3) The Cu2+ reacts with LIX84-I, forming an extracting complex at the membrane /LIX84-I interface; 4) The extracting complex diffuses into the bulk of organic phase. In the stripping process, reversed steps could be easily identified (Fig.2). By integrating both extraction and stripping process, a continuous extraction-stripping membrane extraction system could be realized as show in Fig. 2C. This process is based on only diffusion and extraction reaction. Extra resistance caused by membrane and the concentration polarization could be largely reduced by optimizedof membrane, hydraulic pattern, as well as membrane module design (Yeh et al., 2001; Song et al., 2014).
3Experimental 3.1 Chemicals and materials Polyethersufone (PES Ultrason E6020P) was kindly supplied by BASF and was dried one week at 90oC before usage. Sulfonated polyphenyl ether sulfone ketone (SPPESK) was supplied by Dalian Polymer Co. Ltd (Dalian, China PNM P0403-029) (Wu et al., 2010). 2-Hydroxy-5-nonylbenzophenone (LIX84-I) was obtained from Cognis Chemical Industry (China) Co. Ltd. Dodecane (Aladdin-reagent Co. ltd, China) was used as diluent for organic extractant. Analytic grade dimethylacetamide (DMAc), CuSO4, sulfuric acid and NiSO4.6H2O were provided by Sinopharm Chemical Regent Co. Ltd. All chemicals were used as received without further purification. Deionized water was used throughout the experiments. 3.2 Liquid-liquid extraction LIX 84-I and dodecane was used as the extractant and diluent. Aqueous phase with copper and nickel was prepared by copper sulfate and nickel sulfate, the concentration of copper ion and nickel ion in aqueous was 0.147 mol/L and 0.041 mol/L respectively. The solution pH was 5
adjusted by adding stock H2SO4 solution of 2 mol/L. Extraction of copper and nickel was carried out at O/A phase ratio of 1:1 (50ml/50ml) at 25 oC under vigorous mixing for 5 min. The duration for organic and aqueous phase separation was 20 min. The concentration of copper and nickel in the aqueous phases was determined by inductively coupled plasma atomic emission spectroscopy (ICPE-9000,Shimadzu) after dilution by deionized water with 3 vol% HNO3 to the detection range (0.1-5 mg/L). The extraction rate (E %) was defined as (1)
Where Co.M (mol/L) and Cf.M (mol/L) are the ion concentration in the organic phase after extraction and in aqueous phase (feed) before extraction; Vo (L) and Vf (L) are the volume of organic phase and feed, respectively. 3.3 Preparation of PES/SPPESK blend membranes The detailed procedure for the preparation of PES/SPPESK blend membrane was reported elsewhere (Song, et al., 2014). Briefly, dried PES and SPPESK (IEC of 1.21 meq/g), at a weight ratio of 6/4, were dissolved in DMAc at 65 oC, and the total polymer concentration was 30 wt.%. The polymer solutions were filtered with a 40 m metal mesh and de-aerated before membrane casting. Flat nascent membranes were cast onto a clean dry glass plate at a thickness of 200 m, then immersed immediately in a water bath at 25.0 ± 0.2 oC for precipitation. After casting, membranes were rinsed in deionized water for 24 h to remove trace solvent in the membrane. 3.4 Scanning electron microscopy (SEM) Cross section of the membranes was examined using scanning electron microscopy (SEM, HITACH, TM1000) for low magnifications and the field emission scanning electron microscopy (FESEM, HITACH, S-4800) for high magnifications. Samples were made by fracturing the wet membranes in liquid nitrogen and then freeze-dried overnight. The samples were sputtered with a thin layer of gold before image acquisition. 6
3.5Membrane extraction Fig. 3 shows schematically the membrane extraction contactor (MEC) for the separation of Cu2+ and Ni2+. For the single-stage membrane extraction contactor, as shown in Fig. 3A, a flat membrane (3cm10cm, effective range of surface) was sealed between two Polytetrafluoroethylene (PTFE) half-cell test modules. The feed and extractant solutions (LIX84-I/dodecane, 20% vol/vol) were then fed into the module co-currently. To investigate the membrane performance in the stripping process, a Cu2+ pre-saturated organic extractant solution was used and 2 mol/L H2SO4 solution was used as the stripping solution. For the Sandwiched Membrane Extraction Contactors, as shown in Fig. 3B, two pieces of flat membranes (3 cm 10 cm) were mounted into a three-channel test cell. Three solutions (the feed, strip and the organic extractant) were circulated at a flow rate 1 L/min, 1 L/min and 0.5 L/min respectively. The copper ions in the feed diffused into the organic phase through the membrane. Subsequently, copper in organic phase was stripped by acid. The ion flux was calculated as
(2)
where V (L) and A (cm2) represent the feed volume in extraction process (or organic phase loaded Cu2+ in stripping process) and the effective membrane area, respectively. Detailed experimental parameters are listed in Table 1.
4 Results and Discussion 4.1 Membrane morphology Addition of SPPESK improves both the hydrophilicity and the ion exchange capacity of the PES membrane (Song, Li et al., 2014). Due to the high ion exchange capacity of the sulfonated polymer, low diffusion resistance for cations is realized. However, due to the strong 7
hydrophilicity, the sulfonated polymer tends to swell greatly in aqueous phases; high swelling results in low mechanical property and large deformation, thus is practically difficult for utilization. By introduction of PES, a comparably hydrophobic polymer (therefore less swollen in water), the swollen of the sulfonated polymer chains are constrained (Li et al., 2008). Because PES/SPPESK (6/4) membrane showed a moderate swelling and high ions permeation (Song, Li et al., 2014), it was selected for copper extraction in this work. As shown in Fig.4, the membrane has a macrovoid-free network-like structure. At higher magnification, nano-sized interconnected porous structure was observed. The formation of this particular morphology is originated from the demixing of PES and SPPESK, after liquid-liquid phase separation (Boom et al., 1992; Li et al., 2010; Xiao et al., 2015). The membrane showed an asymmetric structure with a compact skin. Fortunately, this compact skin layer is rather thin compared to the thickness of the whole membrane (about 1-2 μm for the flat membrane comparing to the membrane thickness of 110 μm). The resistance is expected to be low. 4.2 Effect of pH on selectivity of Cu2+ and Ni2+ in liquid-liquid extraction The feed pH has significant influence on the extraction selectivity for copper and nickel. As shown in Fig.1, the separation of copper and nickel can be achieved by adjusting the feed pH. Fig.5 shows the Cu2+and Ni2+ concentration in the organic extractant after one stage liquid-liquid extraction. No Ni2+ was measured in the extractant at the pH range of 2.1-3.8. The extraction rate for Cu2+ increased (66.7 % to 70.8 %) as the pH increased from 2.1 to 3.8. A high selectivity of Cu2+ and Ni2+was realized. Thus, a pH=2.9 was selected for the separation of Cu2+/Ni2+ in following experiment. 4.3Single-stage membrane extraction contactor Membrane extraction and stripping processes were investigated separately based on single-staged membrane extraction contactor. Fig. 6A shows the concentration profileof Cu2+ and Ni2+ in the feed solution as a function of time. A sharp decline of the copper concentration from 8.9 g/L to 7.0 g/L was observed for the first 4 hours. A concentration reduction about 30% was achieved at 660 min. After the organic extractant was gradually loaded with Cu2+, the 8
driving force for the diffusion of Cu2+ across then membrane decreased (Fig.2), the chemical reaction rate of Cu2+/LIX84-I decreased, leading to declined extraction rate. Fig. 6B shows the concentration profile ofCu2+ions in the stripping phase where a quick increase was observed up to 6 h, then leveled off . The Ni2+concentration in feed remained unchanged, because, at pH = 2.9, no extraction for Ni2+occurred. The average flux of Cu2+ in the membrane extraction was 4.20 10-9 mol/cm2.s. In the extraction process, visual color change was observed for the organic phase from light yellow to dark green, and finally black, which was ascribed to the increase of copper ions in the extractant (Figure 6C). The visual changes verified that the copper was extracted from the feed to the organic phase and stripped from the organic phase to the acid phase.
At the beginning of a stripping process, the reaction rate was high due to the high driving force. However, as the reaction proceeds, the driving force declines resulting in a slower stripping process. Thus, the increment of the Cu2+ concentration in the stripping phase declined. Fig.7 shows the Cu2+ flux with time in the membrane extraction/stripping process. It is obvious that the flux for both extraction and stripping declined against time. Copper flux in the membrane stripping process was higher than that in the membrane extraction. This was most probably due to the high H+ concentration in the stripping phase, consequently, higher H+ concentration at the interface between the membrane and the organic phase. The higher concentration resulted in higher reaction rate in the stripping process (Fig.2.A). It was showed that no volume change occurredin the extraction and stripping tank; this was due to the membrane barrier for the extractant, thus loss of organic extractant was prevented.
4.4 Separation of Cu2+/Ni2+ by sandwiched membrane extraction contactor The sandwiched membrane contactor for extraction Cu2+ is an integration of extraction and striping steps (Fig.3B). The Cu2+ ions were extracted from the feed to organic phase via the membranes and transferred to the stripping phase. Fig.8 shows the concentration profiles of the 9
Cu2+ in the feed and strip phases against time. The concentration of Cu2+ in feed decreased quickly at the beginning at a linear rate. At the late stage, the decline rate of the Cu2+ concentration slowed down, most probably due to the reduced Cu2+ concentration in the feed, thus lower Cu2+ flux (Fig.7). Correspondingly, the copper concentration in the stripping phase increased linearly at the beginning, then reached a plateau, due to decline in the Cu2+ flux in the extraction stage. As expected, the Ni2+concentration in the feed did not change (no Ni2+was detected in the stripping phase). A high selectivity was achieved between Cu2+and Ni2+ in membrane extraction process. Comparing the copper concentration values in Fig. 6 and Fig. 8, it was observed that the copper concentration decreases significantly faster in the sandwiched membrane extraction contactor than in the single stage membrane contactor, using the same membrane of the same area, In the sandwiched membrane extraction contactor, copper ions were stripped via the integrated stripping stage; therefore, the copper concentration in the organic extractant was balanced, the driving force for the copper transport was maintained; but in the single stage membrane contactor, the copper ions aggregated in the organic extractant, resulting in lower driving force. The average copper flux in the sandwiched membrane contactor system was 5.6 10-9 mol/cm2.s, much higher than that in the single stage membrane contactor. 4.5 Membrane stability The stability of the membrane materials in both the aqueous and organic solutions were tested by immersing in the corresponding liquid. No obvious visual change in the membrane was observed after 30 days this static test. In a dynamic test, the membranes were installed in a test cell filled with the organic extractant and the feed solution or strip solution for 30 days. Fig.9 shows the force at breakage and the stain at breakage of the membranes following the time. After immersion in organic/2 mol/L H2SO4 and organic /water, the force at breakage slightly decreased, but the stability was maintained. The PES/SPPESK barrier, swollen in aqueous phase, is impermeable to the extractant (LIX84-I), but permeable for cation. The contact between extractant and membrane exists at the membrane/feed or membrane/stripping interfaces. Deteriorating of membrane by organic 10
solvent may occur at the interfaces. However, due to the high swelling rate of the membrane in aqueous phase, a preferential water layer at the membrane surface may prevent itself from direct attack by the organic extractant. Consequently, the barrier membrane shows high stability. However, the strain declined as the membrane was in direct contact with acid and organic extractant. It seems that although the membranes are very hydrophilic in nature, slow attack by the solvent still occurred. Nevertheless, the membranes after 30 days dynamic stability test show rather good mechanical properties and are suitable for further test in the extraction process. It was proposed that further increase the hydrophilicity of the membrane material is the research direction to avoid the degradation of the membrane in the organic phase. Balance of the hydrophilicity and the mechanical properties have to be considered.
4.6 Outlook of membrane extraction This integrated extraction/stripping process is unique and highly efficient in extraction process. Because the organic extractant acts as a carrier to transport ions from feed side to the strip side, very small amount of organic extractant is necessary to realize the continuous extraction and stripping loop. Much less organic extractant is required; expensive and high selectivity extractant could be used; liquid volume in extraction system is largely reduced. Furthermore, no aqueous/organic phase interface exists in membrane extraction contactor, leakage of organic extractant is prevented; thus, saving in extractant is possible. In this report, a complete separation of copper and nickel was observed; the present membrane extraction contactor system could be utilized for the separation of other ion pairs. In case of aggressive organic extractant, an alternative solvent stable barrier membrane is needed (Xing et al., 2016); this would open a new area of research for universal solvent resistance and ion permeable membrane. One drawback of membrane extraction contactor is an extra mass transfer resistance caused by an extra barrier membrane. In fact, by blending the sulfonated polymer with PES, the membrane transfer resistance has been largely reduced (Geise et al., 2013). This extra resistance may be further optimized by improving the membrane morphology, or membrane 11
module design. Literature report concluded that a hollow fiber module has much better mass transfer (Prasad et al., 1990; Daiminger et al., 1996) than conventional mixing/settler or even column extraction equipment (Song et al., 2017). Electrospun nanofiber membranes has been reported for improve the mass transfer in other diffusion type of membrane, such as membrane distillation (An et al., 2016; Tijing et al., 2016). It is highly likely that this type of new membrane morphology could reduce the diffusion resistance of the ions in the membrane layer. It is thus one of the main targets for our ongoing research on membrane extraction contactor system. Benign and less chemically aggressive extractants are the other important research direction for membrane contactor for improved life time. Ionic liquids (ILs) are thermally stable green solvent, constituted byan organic cation and either an organic or an inorganicanion (Fortunato et al., 2004). However, the loss of ionic liquids was still an issue if used in the conventional SLM or membrane contactor system (Gruenauer et al., 2016; Zeh et al., 2016). A number of scientific research papers has identified the ionic liquids as the extractants for various targeted species (Depuydt et al., 2015; Yang et al., 2015; Alguacil et al., 2016; Gaillard et al., 2016; Rout et al., 2016; Wankowski et al., 2016; Depuydt et al., 2017; Merlet et al., 2017; Mohapatra, 2017), and other investigations reported using the ILs as the solvent to assistant the extraction of metal ions (Shi et al., 2016; Torrejos et al., 2016; Herce-Sesa et al., 2017; Mohapatra, 2017; Shi et al., 2017). Taking the advantages of the current hydrophilic ion exchange membranes, a new system combining the ILs and the novel membrane could result in a novel separation system for metal ions.
5 Conclusions Membrane extraction contactor was utilized for selective extraction of copper and nickel from wastewater. A barrier membrane composed of PES/SPPESK was used as the protection layer to prevent the loss of organic extraction, allowing free diffusion of copper and nickel ions. The barrier membrane shows nanoporous sponge-like morphology, and is resistant to the degradation of the organic chemicals. As a barrier, the membrane shows high stability in dynamic test. Based on the pH selectively of LIX84-I to Cu2+/Ni2+, membrane extraction results 12
realized the liquid-liquid extraction, at feed pH=2.9. The membrane stripping could be realized by using a 2 mol/L H2SO4 solution. By integrating extraction and stripping, a sandwiched membrane extraction contactor demonstrated a simultaneous extraction and stripping in one unit, a high selectively of two ions as well. The present selective MEC process could serve as a highly efficient separation platform for removal and separation of metal ions. Continuous improvement in the stability has been proposed in more hydrophilic membranes and possible new extraction systems as ionic liquids. The improvement in the mass transport could be achieved by the less resistant nanofiber support.
ACKNOWLEDGMENT The authors thank National Natural Science Foundation of China (U1507117, 21676290), TMSR from Chinese Academy ofSciences (XDA02020100), Key Research Fund (CAS2014Y424541211) for financial support.
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Fig. 1 The chemical structure of LIX84-I and its reaction mechanism with copper and nickel (A) ; the effect feed pH on cation concentration in extractant: LIX84-I (B) (Zhu et al., 1990; Reddy et al., 2005)
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Fig.2 Schematic illustration of concentration profile of Cu2+ in the membrane extraction(A) and stripping steps(B). an integrated membrane extraction and stripping process(C). "CP": concentration polarization; F:Feed phase; Org: organic phase; S: stripping phase; m: membrane; b: bulk phase.
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Fig.3 Schematics of a single-stage flat membrane contactor (A),asandwiched membrane extraction contactors(B).
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Fig. 4 Cross-section images of PES/SPPESK flat membrane．A: Cross-section, B: Top skin layer, C: Bottom-section (Ratio of PES/SPPESK = 6/4, solvent: DMAc; solution temperature 65 oC; casting knife thickness: 200 μm; coagulation bath temperature: 25 oC).
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100
Extraction (%)
80 60 40 20 0
2.1
2.8
3.6
pH value
Fig. 5 Effect of pH in feed on extraction of Cu2+ and Ni2+. Note that no Ni2+was detected in the organic phased( feed:0.147 mol/L CuSO4+ 0.041 mol/L NiSO4; organic phase : LIX84-I/dodecane, 20% vol/vol; O/A=1:1)
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Fig. 6 Ion concentration in the feed (A) and the stripping phase (B) against time; the color of extractant. Membrane extraction process(C), feed:8.9 g/L Cu2+, 2.3 g/L Ni2+, pH = 2.9; Membrane stripping process, the extractant used was after membrane extraction process, H2SO4 (2 mol/L) was as stripping phase.
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Cu2+ flux(10-8mol/cm2.s)
1.5 1.2 0.9
Feed Strip
0.6 0.3 0.0
0
120
240
360
480
Time (min) Fig.7 The flux of Cu2+in membrane extraction process (feed /extractant) and membrane stripping process (extractant/stripping phase) against time.(Membrane extraction process, feed: 8.9 g/L Cu2+, 2.3 g/L Ni2+, pH=2.9; Membrane stripping process, the extractant used was after membrane extraction process, H2SO4 (2 mol/L) was as stripping phase)
24
4 feed strip
8
3
2+
2
6 1 5 0
120
240 360 Time (min)
480
2+
7
Conc. Cu in strip (g/L)
Conc. Cu in feed (g/L)
9
0 600
Fig.8 Plot of the concentration of ion against time. (Feed:8.25 g/L Cu2+, 2.3 g/L Ni2+, pH=2.9; Stripping: H2SO4 2 mol/L)
25
Org.&2M Sulphuric Acid Org.&Water Org.&2M Sulphuric Acid Org.&Water
60 50
2.4
40 2.0
30 20
1.6
Elongation (%)
Load (N)
2.8
10
1.2 0
5
10
15
20
25
30
0
Time (Day)
Fig. 9 Mechanical properties of the membranes after in contact with the feed, extractant and strip solutions. The membranes were after dynamic stability test, with one side contact with water or 2 mol/L H2SO4 and the other side with the organic extractant.
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Table1. Operating parameters for the membrane contactors Feed (g/L)
Cu2+：8.26 ; Ni2+：2.3
Feed pH
2.9
Feed Volume(ml）
100
Feed flow(L/min)
1
Volume of organic phase volume(ml）
85
Flow of organic phase (L/min)
0.5
Strip volume (ml）
100
Flow of stripping phase (L/min)
1
Area of membrane (cm2)
30
27