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PSS membrane for selective separation of dilute copper ions from wastewater
Chemical Engineering Journal 328 (2017) 293–303
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An electrochemically-switched BPEI-CQD/PPy/PSS membrane for selective separation of dilute copper ions from wastewater Fengfeng Gao a, Xiao Du a, Xiaogang Hao a,⇑, Shasha Li a, Xiaowei An a, Mimi Liu a, Nianchen Han a, Tonghua Wang b, Guoqing Guan c,⇑ a b c
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China State Key Laboratory of Fine Chemicals, Carbon Research Laboratory, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0812, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel BPEI-CQD/PPy/PSS membrane
was successfully synthesised and used for the removal of Cu2+. Sorption-diffusion model was proposed for the ESIP system. 3D BPEI-CQD/PPy/PSS membrane exhibited excellent permselectivity for Cu2+. The current efficiency of the ESIP system reached 39.9%.
a r t i c l e
i n f o
Article history: Received 25 April 2017 Received in revised form 27 June 2017 Accepted 29 June 2017 Available online 1 July 2017 Keywords: Electrochemically switched ion permselectivity BPEI-CQD/PPy/PSS membrane Cu2+ separation Sorption-diffusion
a b s t r a c t The continuous separation of dilute Cu2+ from wastewater was performed by using a branched poly (ethylenimine) (BPEI)-functionalized carbon quantum dots (BPEI-CQD)/polypyrrole (PPy)/ polystyrenesulfonate (PSS) membrane in an electrochemically switched ion permselective (ESIP) process. The membrane with a high selectivity for Cu2+ separation was prepared by pressure filtering BPEI-CQD/PPy/PSS solution through a polytetrafluoroethylene (PTFE) membrane. In this ESIP system, the directional uptake/release of Cu2+ was realized by modulating the redox state of the membrane coupling with an external electric field. The effects of the operating parameters on the flux of Cu2+ and membrane permselectivity were investigated. The BPEI-CQD/PPy/PSS composite electrode showed excellent permselectivity for Cu2+ with a flux of 0.108 mgcm2h1, a current efficiency of 39.9% and an excellent cycling stability. The Cu2+ concentration in the solution was reduced from 30 to 0.82 ppm with a removal efficiency of 97.2%. Furthermore, the effects of BPEI-CQD content and the membrane thickness on the separation properties of the membrane from a mixed nitrate solution containing Cu2+, Ni2+ and Cd2+ were investigated. It is expected to understand the sorption-diffusion mechanism of such a membrane and provide more information on the design of it for a practical metal ion separation process. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal pollution has become an environmental issue of global concern [1]. It requires to recover heavy metals as precious ⇑ Corresponding authors. E-mail addresses: [email protected] (X. Hao), [email protected] (G. Guan). http://dx.doi.org/10.1016/j.cej.2017.06.177 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
resources for sustainable development in the rapid industrializing process. Thus, how to separate heavy metal ions and recycle them play a significant role nowadays. Copper ions (Cu2+) is an essential element for living organisms, but the excessive ingestion of copper causes serious health problems, such as vomiting, cramps, convulsions, or even death [2]. Therefore it is of great importance to separate Cu2+ from wastewater. The membrane-based separation
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techniques for Cu2+ ions mainly include ultrafiltration (UF) [3], nanofiltration (NF) [4], reverse osmosis (RO) [5], electrodialysis (ED) [6], electrodeionization (EDI) [7], hollow-fiber supported liquid membrane (HFSLM) separation [8], polymer inclusion membrane (PIM) separation [9] and electrochemical treatment (ECT) [10], and so on. Among them, RO and EDI are considered as the most widely-used two ways. However, the low selectivity for the target ions limits their large-scale application. Recently, electrochemically switched ion exchange (ESIX) method [11–14] has been attracted great attention as a novel environmentally benign ion separation technique since it can eliminate the secondary wastes. In the process of ESIX, the rapidly reversible uptake and release of target ions even at very low concentrations can be achieved by changing the redox state of the ESIX film. However, as reported in our previous studies [11–14], the regeneration of the film has to be performed separately. Weidlich et al. designed a special electrochemical cell by reversing the polarity of the ESIX film periodically and exchanging the solutions in the two compartments simultaneously, achieving a ‘‘semi-continuous operation process” [15,16]. To realize the continuous operation, a series of ion transport systems based on conducting polymer membranes were developed by Wallace and Bobacka [17–19]. Based on these ion transport systems, we proposed an in situ potentialenhanced, electrochemically switched ion permselective (ESIP) system, which realized a continuous ESIX separation of Ca2+ and Mg2+ for water softening [20], and designed a potentialcontrolled ion pump based on three-dimensional (3D) PPy@GO or MWCNT/PTFE membrane for separating dilute lead ions from wastewater [21,22]. In the ESIP system, the development of ESIP membrane with high selectivity as well as high flux for the target heavy metal ions is the key issue for the industrial application. Polypyrrole (PPy), as a widely used conducting polymer, is a particularly promising electroactive membrane material due to its easy synthesis, good electrochemical activity and low cost [23]. However, the practical application of PPy is usually limited by its poor cycle-life, resulting from the volumetric change, active material loss and over-oxidative degradation in the doping/de-doping process. To overcome these disadvantages, combining it with carbon-based nanomaterial to obtain a 3D composite was studied. Cao et al. prepared 3D graphene oxide (GO)/PPy composite electrode by a direct electrochemical method for supercapacitors [24]. Lin’s group demonstrated that 3D nitrogen-doped graphene with PPy can be used as electrocatalyst for oxygen reduction reaction [25]. Jian et al. fabricated carbon quantum dots (CQDs) reinforced PPy nanowire via electrostatic self-assembly strategy for high-performance supercapacitors [26]. Lin’s group also investigated the electrochemically controlled ion-exchange properties of MCWNT/PPy composite in various electrolyte solutions [27]. However, to the best of our knowledge, no such a 3D carbon-based composite membrane was used for the selective separation of Cu2+ in an ESIP system. CQDs, a novel zero-dimensional (0D) carbon nanomaterial with high surface area, good electrochemical conductivity and well dispersion property in various solvents, has been attracted extensive attentions [28,29]. It can be easily grafted by functional groups because of abundant oxygen-containing functional groups on its surface. Dong et al. prepared a new kind of polyamine functionalized CQDs by capping with branched poly-(ethylenimine) (BPEI) [30,31], which can be used for selective and sensitive detection of Cu2+. Chen’s group successfully fabricated CQDs functionalized gold nanoparticles for sensitively electrochemical detection of heavy metal ions [32]. Thus, it should be very interesting to combine BPEI-functionalized CQDs (i.e., BPEI-CQDs) with 3D PSS-doped PPy, which could not only have a cation exchange property, but also create the room for volume swelling and shrinkage.
Especially, the BPEI-CQDs can be served as the recognition component for Cu2+. In addition, combining BPEI-CQDs with 3D PSS-doped PPy could (i) provide extra mechanical support, (ii) decrease the charge transfer resistance and (iii) promote the access of target ions to the active sites of the polymer. These are extremely significant for the preparation of organic-inorganic ESIP membranes including BPEI-CQDs and PSS-doped PPy for the continuous separation of Cu2+ from wastewater. Based on these considerations, in this study, a 3D BPEI-CQD/ PPy/PSS composite membrane was fabricated on polytetrafluoroethylene (PTFE) membrane by pressure filtering, and used for the effective and selective removal of Cu2+ from wastewater by ESIP system, in which 2 two-electrode subsystems were included (Fig. 1). Herein, a cell voltage is applied on a pair of platinum plate electrodes to form a constant external electric field, and a pulse potential is applied on the ESIP membrane to control the uptake/ release of target ions. As shown in Fig. 1, when the pulse potential is switched to the reduced potential, the target cations in the source solution will be incorporated on the membrane. Once a positive potential is applied on the membrane, by coupling with an external electric field, the incorporated ions will be immediately expelled into the receiver solution. The synergistic effect of the cell voltage and the pulse potential was described in details in our early work [21,22]. Since the amino groups on the surface of the CQDs can bind the Cu2+ to form cupric amine, it is expected that the combination of it with BPEI could promote the selectivity of the membrane towards Cu2+ while the unique 3D interconnected structure ensure the fast diffusion of electrolyte ions through the membrane. In this study, the effects of BPEI-CQD content and the membrane thickness on the separation properties of the membrane for a mixed nitrate solution containing Cu2+, Ni2+ and Cd2+ were investigated. It is expected to understand the sorption-diffusion mechanism of such a membrane and provide more information on the design of it for a practical metal ion separation process. 2. Experimental 2.1. Reagents and instruments Citric acid (CA) was obtained from Alfa Aesar. BPEI with a molecular weight of 1800 Da was bought from Aladdin (Shanghai, China). Pyrrole (Py, A.R.), ammonium persulphate (APS, A.R.) and hexadecyl trimethyl ammonium bromide (CTAB, A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Pyrrole was distilled and tightly sealed and maintained in the dark below 0 °C prior to use. Polyvinyl alcohol (PVA, Mw = 100 000) was obtained from Fischer Scientific, China. Hydrochloric acid (36–38%) and succinic acid (99.5%) were bought from Tianjin Guangfu Fine Chemical Research Institute, China. PTFE support (pore size: 0.22 mm) was purchased from Sartorius Inc, China. Hydrochloric acid (36–38%) was bought from Tianjin Guangfu Fine Chemical Research Institute, China. Copper, cadmium and nickel nitrates (99%) were employed for the ion separation experiments. All reagents were of analytical grade, and all solutions were prepared using ultrapure water (Millipore 18.2 MX cm). 2.2. Preparation of BPEI-CQD/PPy/PSS membrane BPEI-CQD/PPy/PSS ESIP membrane was prepared via the following steps. Firstly, the BPEI-CQDs was synthesized following the reported method [30,31]. In brief, 1.0 g of CA and 0.5 g of BPEI were added to 10 mL hot water and then heated up (<200 °C) using a heating mantle until most of the water was evaporated and a
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Fig. 1. Schematic illustration of the continuous separation of Cu2+ across the BPEI-CQD/PPy/PSS system. ESIPM: ESIP membrane.
uniform pale-yellow gel was formed. 1 mL of water was added to prevent the gel scorched. Repeating this procedure for 10 times, and finally the color of the gel became orange, proving the formation of BPEI-CQDs. Then, 10 mL BPEI-CQDs solution obtained by adding double-distilled water was purified with a 0.01 mol/L HCl solution by silica gel column chromatography. Finally, it was freeze-dried at 45 °C for 10 h with a Scientz-12N (SCIENTZ, China). Then, to synthesize PPy/PSS, 0.2 g of CTAB as soft template, 0.2 mL of pyrrole monomer and 0.02 g of PSS were dissolved in 1.0 mol/L HCl solution and stirred for 30 min at room temperature. Thereafter, 1 mol/L HCl (5 mL) solution containing 0.68 g of APS was added as an oxidant, and stirred constantly in an ice bath for 12 h. After the polymerization, the black PPy/PSS precipitate was obtained by filtration and rinsed with DI water and ethanol for several times. Finally, the 3D BPEI-CQD/PPy/PSS composite membrane was fabricated via the following strategy. Firstly, 30 mg of PPy/PSS was added to 40 mL of BPEI-CQDs solution (1.5 mg/mL) under ultrasonication for 2 h. Then, 60 mL of 1 w/% PVA solution containing 0.1% succinic acid which was directly involved into the crosslinking reaction and 2 mol/L HCl which was acted as a catalyst to the cross-linking reaction were introduced as the cross-linking agent. Thereafter, the mixture solution was sonicated with a microtip for 10 min and filtrated through a ɸ = 70 mm PTFE support membrane held in the glass vacuum filtration flask. As a result, a black BPEI-CQD/PPy/PSS membrane was formed on the surface of PTFE support. Here, to collect all the CQDs, the filtration process was repeated for several times. Finally, the obtained membrane was dried in oven at 25 °C for 8 h. For comparison, the PPy/PSS membrane was also prepared using the same procedure except the addition of BPEI-CQDs.
2.3. Characterization of BPEI-CQDs and BPEI-CQD/PPy/PSS Transmission electron microscopy (TEM) images of BPEI-CQDs, PPy/PSS and BPEI-CQD/PPy/PSS were taken with a Tecnai G220 (FEI, USA) operating at 200 kV. Scanning electron microscope (SEM) images were characterized with a JSM-7001 F (JEOL, Japan). The membrane thickness was determined based on the crosssection image of the freshly prepared composite membrane. The chemical structure of BPEI-CQD/PPy/PSS composite was analyzed by a Fourier transform infrared spectroscopy (FTIR-Prestige-21, Shimadzu Corporation, Japan) using a standard KBr pellet method. The composite crystalline phase was determined by X-ray diffraction (XRD, Rigaku SmartLab, Japan). Cyclic voltammetry (CV) was performed using a VMP3 Potentiostat (Princeton, USA) controlled with EC-Lab software at room temperature. The cation concentration in aqueous solution was detected with atomic absorption spectrometry (AAS-990). The masterflex L/S (07519–05) was bought from Cole-Parmer, USA. 2.4. Continuous separation test Before the continuous separation test, the washed membrane was assembled in the membrane module. 200 mL of mixed metal nitrate solution and nitric acid solution (pH = 3) were poured into two tanks as the source and receiver solutions, respectively. After Masterflex L/S (07519–05) peristaltic pump was turned on, a continuous running system was formed. Here, a pulse potential between 0 V and 2.0 V was applied on the membrane (the duration time was equal for the low and high pulses), and a 2.0 V cell voltage was applied on the platinum plates shown in Fig. 1. During the
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test, aliquots of 1 mL were sampled from the source or receiver chambers to detect the metal ion concentration by AAS-990. The flux of each metal ion across the composite membrane (F) was calculated using Eq. (1):
F¼
ðC1 C2 ÞV tA
ð1Þ
where C1 and C2 (mg/L) are the initial and final concentrations of the metal ion (mg/L), respectively; V (L) is the volume of the metal ion solution; t (h) is the duration of the separation; and A (cm2) is the area of the membrane exposed to the receiver solution. The membrane separation factor between two different metal ions M and N was calculated by the following equation:
x0 =x0 Separation factor ðasep: M=NÞ ¼ M N xM =xN
ð2Þ
where x0M and x0N , xM and xN are the mass fractions of M and N components in the permeate and feed, respectively. The charge stored in the ESIP system was easily calculated using the electrochemical workstation. In addition, the current efficiency (g) is defined as follows:
Cu2þ removal amouts Z F g ð%Þ ¼ 100 QH þ QL
ð3Þ
where QH and QL are the quantities of charge stored in the ESIP system when the high and low potentials are applied to the membrane, respectively. F is the Faraday constant 96,500 C/mol, and z is the valence state of Cu2+. 2.5. Adsorption and diffusion properties of the BPEI-CQD/PPy/PSS membrane The adsorption properties were tested using a two-electrode system in 100 mL mixture solutions of 30 mg/L Cu(NO3)2 and Cd (NO3)2 and Ni(NO3)2. The PPy/PSS and BPEI-CQD/PPy/PSS membranes were the working electrodes. A 0.1 V constant potential was applied on the membrane for 120 min.
The selectivity factor of the composite membrane for metal ions is defined as follows:
Sorption selectivity ðasorp: M=NÞ ¼
y0M =y0N yM =yN
ð4Þ
where the numerator is the ratio of the cation mass fraction within the composite membrane, and the denominator is the mass fraction ratio in the bulk solution that is in contact with the composite membrane. The diffusion selectivity can be calculated from the separation factor and the sorption selectivity by the following equation [33,34]:
Diffusion selectivity ðadif: M=NÞ ¼
asep: M=N asorp: M=N
ð5Þ
3. Results and discussion 3.1. Morphological characterization Fig. 2 shows SEM morphologies of PTFE and BPEI-CQD/PPy/PSS membranes. As shown in Fig. 2A, PTFE support contained an irregular network structure. Fig. 2B, C and D show the typical surface and cross-sectional morphologies of the BPEI-CQD/PPy/PSS composite membrane deposited on PTFE support. From Fig. 2B, the interpenetrating 3D network structure was composed of chainlike connected nanospheres in random orientations, which could supply a large amount of active sites and ionic conducting channels for the uptake/release of Cu2+. Fig. 2C shows the cross-sectional morphology of the membrane, which indicates that the membrane thickness was approximately 11.8 lm (membrane area: 4.0 cm2). Here, it should be noted that the thickness can be precisely tuned by adjusting the BPEI-CQD/PPy/PSS content in the solution used for fabrication. Magnified cross-sectional view of the BPEI-CQD/PPy/ PSS membrane (Fig. 2D) further proved the existence of highly interpenetrated porous structure, which could provide fast ionic transfer channels.
Fig. 2. SEM images of the surfaces of (A) PTFE and (B) BPEI-PPy/PSS membranes. Insets: photographs of the corresponding membranes. (C) and (D) are cross-sectional SEM images of the BPEI-PPy/PSS membrane under different magnifications.
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Fig. 3. TEM images of (A) BPEI-CQDs, (B) PPy/PSS and (C) BPEI-CQD/PPy/PSS.
The morphologies of BPEI-CQDs, PPy/PSS and BPEI-CQD/PPy/PSS composites were further characterized by TEM. Fig. 3(A) shows that resultant BPEI-CQDs were monodisperse nanocrystals with near spherical morphology and a relatively narrow lateral size in the range from 3 to 8 nm. The representative high-resolution TEM (HRTEM) image of the individual BPEI-CQDs showed highly ordered structure with an interlayer spacing of 0.212 nm (see the inset of Fig. 3(A)) [31]. A thin layer network of PPy/PSS with chain-like structure is demonstrated in Fig. 3(B). Moreover, as shown in Fig. 3(C), BPEI-CQDs appeared on the surface of PPy/PSS which should be interacted with PPy through strong p–p interactions.
assigned to CAH deformation vibrations and the CAN stretching vibration, respectively. In addition, the peaks at 1170 and 901 cm1 are due to the doping state of PPy and the band at 783 cm1 verifies the presence of polymerized pyrrole. The bands observed at 2920 and 2850 cm1 are attributed to the asymmetrical and symmetrical stretching of CH2 group. In addition, an extra peak was found at 1706 cm1 in the FTIR spectrum of BPEI-CQD/ PPy/PSS, which is associated with the amide linkage (ACONHA), indicating that BPEI-CQDs were successfully doped on the surface of PPy/PSS.
3.2. FTIR spectroscopy characterization
Fig. 5(A) demonstrates CV curves obtained with the PPy/PSS and BPEI-CQD/PPy/PSS membrane electrodes in 0.1 M H2SO4 solution at a scan rate of 50 mV/s. Obviously, a higher current density was observed in the CV curve of BPEI-CQD/PPy/PSS, indicating that it had higher electroactivity than PPy/PSS with the same mass in the same potential range (0.1 –0.8 V). This should be attributed to the good conductivity of BPEI-CQDs and its additional chemical interactions with the electrolyte, which could reduce the charge transfer resistance and increase the rate of electrochemical reaction in the membrane. From Fig. 5(B), XRD measurements were further used to characterize the crystal structures of BPEI-CQD/ PPy/PSS and PPy/PSS. As presented in Fig. 5(B), XRD patterns of both pure PPy/PSS and BPEI-CQD/PPy/PSS composite generally present two broad peaks at 2h = 25.2°, indicating that the amorphous states of PPy/PSS and BPEI-CQD/PPy/PSS in nature. The bright blue emission of the BPEI-CQDs was observed obviously under the excitation of 365 nm (the inset of Fig. 5(C)). The UV–vis absorption result reveals that the BPEI-CQDs solution had two absorption bands at 245 and 355 nm, respectively, which further confirmed that BPEI-CQD solution was successfully prepared.
FTIR spectrums were used to characterize the chemical structures of BPEI–CQDs, PPy/PSS and BPEI-CQD/PPy/PSS. As shown in Fig. 4, for BPEI–CQDs, many characteristic absorption bands of BPEI (NH of 3423 and 1590 cm1, CH2 of 2965 and 2817 cm1, CN of 1120 cm1) were observed [30,31]. A sharp peak at 1706 cm1, which is corresponding to amide linkage (ACONHA), indicates that BPEI was successfully grafted onto the surface of the CQDs by the amide linkages. In the PPy/PSS spectrum, a weak absorption peak at 1633 cm1 corresponded to the hydrated water molecules appeared, indicating that PPy and PSS were combined via hydrogen bonding [35]. The peaks observed at approximately 1548 and 1454 cm1 are the symmetric and asymmetric pyrrole ring stretching modes while the strong NAH stretching and bending vibration at 3423 cm1, the bands located at 1040 and 1308 cm1 are
3.3. Electrochemical and inner structures characterization
3.4. Operation parameters
Fig. 4. FTIR spectra of PPy, PPy/PSS, BPEI-CQD/PPy/PSS and BPEI-CQDs.
3.4.1. Effect of BPEI-CQD content on permeation performance Prior to ion separation test, the effect of BPEI-CQD content in BPEI-CQD/PPy/PSS membrane on permeation performance was investigated. Equi-mass solutions (30 mg/L) of Cu(NO3)2, Ni (NO3)2 and Cd(NO3)2 were used as the source solution and the receiver solution was nitric acid solution (pH = 3). Fig. 6 demonstrates the transport profiles of Cu2+, Ni2+ and Cd2+ across BPEICQD/PPy/PSS membrane with various BPEI-CQD contents (15, 30, 60, 120 mg), and the final concentration vs. BPEI-CQD content histograms of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes are shown in Fig. S1 of Supporting Information. One can see that there was a gradual increase in the Cu2+ transport rate but obvious decreases in Ni2+ and Cd2+ transport rates with the increase in BPEI-CQD content, and the permeability sequence
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Fig. 5. (A) CV responses of PPy/PSS (black) and BPEI-CQD/PPy/PSS (red) membranes in 0.1 M H2SO4 solution at a scan rate of 50 mV/s, (B) XRD spectra of PPy/PSS (black) and BPEI-CQD/PPy/PSS (red), (C) UV absorption spectra of BPEI-CQD solution. The inset shows the photo of BPEI-CQD solution illuminated by UV light of 365 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
was in the order of Cu2+ > Ni2+ > Cd2+. It can be concluded that the BPEI-CQD/PPy/PSS membrane had a higher permselectivity towards Cu2+ than Ni2+ and Cd2+, and its permselectivity increased with the increase in BPEI-CQD content. Chi et al. reported that the
Fig. 6. Transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes of variable BPEI-CQD contents: (A) 15 mg, (B) 30 mg, (C) 60 mg, (D) 120 mg and invariable PPy/PSS content: 50 mg.
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amino groups on the surface of the CQDs can bind Cu2+ to form cupric amine, and verified that the BPEI-CQDs had a high selectivity to Cu2+ [30,31]. Herein, the BPEI-CQDs played a key role in the increase of the permselectivity towards Cu2+. Goon et al. reported that BPEI can selectively capture trace-level free Cu2+ [36]. It is because that the nitrogen of amino groups has a lone pair of electrons, which can complex heavy metal ions with the sharing electrons. In this study, the higher selectivity of BPEI-CQDs towards Cu2+ should be also attributed to the higher binding force between amino groups and Cu2+. However, there were no significant increases in the Cu2+ transport rate and permselectivity from Fig. 6C to D. Thus, considering the energy-saving and costeffectivity, the optimum BPEI-CQD content should be 60 mg, in which the PPy/PSS content was 50 mg. Based on the above assumption, the quantization calculations of the binding energy between amino groups and Cu2+, Ni2+ and Cd2+ were carried out. Herein, the density functional theory (DFT) method in the DMOL3 module included in the Accelrys Material Studio 6.0 software package and the widely used generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function were adopted, and the valence electron functions were expanded into a set of numerical atomic orbitals by a double-numerical basis with polarization functions (DNP). The calculated binding energy of amino groups with Cu2+, Ni2+ and Cd2+ are shown in Table S1. One can see that the binding energy is consistent with the expected, and the binding energy sequence is in the order of Cu2+ > Ni2+ > Cd2+. This can be easily understood from the fact that the attachment of BPEI-CQDs towards Cu2+ was the highest in the three heavy metal ions. In order to study the adsorption selectivity of BPEI-CQD/PPy/PSS membrane, the adsorption capacities of Cu2+, Cd2+ and Ni2+ with various BPEI-CQD contents were tested in 100 mL of 30 mg/L Cu (NO3)2, Ni(NO3)2 and Cd(NO3)2 solutions, respectively. Here, 0.1 V constant negative potential of two-electrode system was applied on the membrane for 120 min. Fig. 7 shows the adsorption capacities of the BPEI-CQD/PPy/PSS membranes with various BPEICQD contents. It can be seen that the adsorption capacity for Cu2+ increased but that for Ni2+ or Cd2+ decreased with the increase in BPEI-CQD content. This further confirms that the BPEI-CQDs had higher adsorption selectivity for Cu2+. In this study, the sorption-diffusion model was proposed for the ESIP system, in which the process of ion transport mainly contains three steps: (i) the heavy metal ions are selectively incorporated into the membrane from the source solution; (ii) the heavy metal
Fig. 7. Adsorption capacities of Cu2+, Cd2+ and Ni2+ into the BPEI-CQD/PPy/PSS membrane with variable BPEI-CQDs content in 100 mL of 30 mg/L Cu(NO3)2 and Cd (NO3)2 and Ni(NO3)2 solutions for each.
Table 1 Flux and selectivity factor of variable BPEI-CQD content: (A) 15 mg, (B) 30 mg, (C) 60 mg, (D) 120 mg. Flux (mgcm2h1)
A B C D
Selectivity factor Permselectivity
Adsorptive selectivity
Diffusion selectivity
Cu2+
Cu2+/ Ni2+
Cu2+/ Cd2+
Cu2+/ Ni2+
Cu2+/ Cd2+
Cu2+/ Ni2+
Cu2+/ Cd2+
0.163 0.176 0.204 0.210
1.150 4.224 5.360 5.956
2.698 16.616 29.970 38.572
2.510 5.561 6.572 7.158
2.521 5.978 8.653 10.044
0.458 0.759 0.815 0.832
1.070 2.779 3.464 3.840
ions diffuse in the membrane and (iii) the heavy metal ions are released from the membrane to the receiver solution. In the ESIP system, the permeation is assumed to occur via potentialdependent diffusion. Thus, the sorption-diffusion model proposed here cannot be interpreted using Fick’s Law. According to the ion transport and adsorption tests, the flux, permselectivity and adsorption selectivities of the BPEI-CQD/PPy/ PSS membrane with various BPEI-CQD contents towards Cu2+, Ni2+ and Cd2+ were calculated. Also, the diffusion selectivity was calculated by Eq. (5). Table 1 lists the flux, permselectivity, adsorption selectivity and diffusion selectivity. One can see that the flux of Cu2+ increased but that of Ni2+ and Cd2+ decreased with the increase in BPEI-CQD content (A: 15 mg, B: 30 mg, C: 60 mg, D: 120 mg); meanwhile, the permselectivity, adsorption selectivity and diffusion selectivity increased. These should be resulted from the high electrical conductivity of BPEI-CQDs and its high selectivity towards Cu2+, which can reduce the charge transfer resistance and increase the electrochemical reaction rate of the membrane with electrolyte. From the diffusion selectivity, it can be seen that the selectivities of Cu2+/Ni2+ were all less than 1 but the selectivity of Cu2+/Cd2+ was more than 1. This should be due to that the hydrated ionic radii of metal ions are in the order of Cd2+ > Cu2+ > Ni2+ [37]. As such, the diffusion selectivity was closely associated with the hydrated ionic radius of metal ion. Furthermore, it can be seen that the adsorption selectivity was higher than the diffusion selectivity. Thus, it is believed that the permselectivity of ESIP membrane was mainly depended on the adsorption selectivity. 3.4.2. Effect of membrane thickness Fig. 8 shows the transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes with various membrane thicknesses (6.27 mm, 8.93 mm, 11.75 mm, 13.63 mm), where the ration of BPEI-CQD and PPy/PSS was 1.2: 1. One can see that the transport rates of three metal ions all decreased with the increase in membrane thickness. However, it is obvious that the permselectivity was enhanced. Here, a high BPEI-CQD/PPy/PSS content could provide more active sites for the binding of Cu2+, which is beneficial to the transport of Cu2+. Meanwhile, the diffusion resistance also increased with the increase in membrane thickness, resulting in the decrease of flux. The adsorption selectivity of BPEI-CQD/PPy/PSS membrane was also investigated. Fig. 9 demonstrates the absorbance-time profiles of the BPEI-CQD/PPy/PSS membrane for Cu2+, Ni2+ and Cd2+. One can see that the membrane had a high adsorption capacity for Cu2+. Here, the adsorption selectivity was calculated based on Eq. (4). The obtained flux, permselectivity, adsorption selectivity and diffusion selectivity are listed in Table 2. It can be seen that there was a decrease in the flux and an increase in the permselectivity as well as diffusion selectivity with the increase in membrane thickness. It is also found that the selectivities of Cu2+/Ni2+ were all less than 1 while the selectivities of Cu2+/Cd2+ were all higher
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Fig. 9. Time dependence of adsorption capacities of Cu2+, Cd2+ and Ni2+ into the BPEI-CQD/PPy/PSS membrane in 100 mL of 30 mg/L Cu(NO3)2, Cd(NO3)2 and Ni (NO3)2 solutions for each.
than 1. This also confirms that the diffusion selectivity was associated with the hydrated ionic radius of metal ion. 3.4.3. Permeation properties of BPEI-CQD/PPy/PSS and PPy/PSS membranes Fig. 10A and B show the transport profiles of Cu2+, Ni2+ and Cd2+ across the PPy/PSS and BPEI-CQD/PPy/PSS membranes in the receiver solution, respectively. In region b, when a CV-PP coupling circuit was applied, the concentrations of Cu2+, Ni2+ and Cd2+ in the receiver solution increased. However, in region a and c, no ions were transported across the membrane since no CV-PP coupling circuit was applied to the system. According to the two experiments, it can be proved that the transport of ions across BPEICQD/PPy/PSS and PPy/PSS membranes was controlled by the coupling circuit. From the Fig. 10A (section b), the concentration of Ni2+ in the receiver solution was higher than Cu2+ and Cd2+, which indicates that the PPy/PSS membrane had a higher permselectivity toward Ni2+. It should be noted that the ionic radius of Ni2+ is the smallest among the three metal ions, resulting in the highest diffusion selectivity of the PPy/PSS membrane toward Ni2+. Since the PPy/PSS membrane had no adsorption selectivity for heavy metal ions, the permselectivity mainly depended on the diffusion selectivity. Interestingly, when BPEI-CQDs were added to the PPy/PSS membrane, the more copper ions were found to be transported to the receiver solution (see Fig. 10B (section b)). This result identifies that BPEI-CQDs had a high adsorption selectivity toward Cu2+.
Fig. 8. Transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes of variable membrane thicknesses: (A) 6.27 mm, (B) 8.93 mm, (C) 11.75 mm, (D) 13.63 mm.
3.4.4. Ion transport study The profiles of Cu2+, Ni2+ and Cd2+ concentration versus time in the source and receiver solutions are shown in Fig. 11. In Fig. 11A, the concentrations of three metal ions in the source solution decreased with the time, and the removal percentage of Cu2+ achieved as high as 97.2%. Especially, the Cu2+ concentration in the source solution decreased from 30 to 0.82 ppm, and the permselectivity of Cu2+/Ni2+ reached as high as 7.865. From Fig. 11B, the concentrations of three metal ions in the receiver solution increased with the time, and the flux of Cu2+ achieved at 0.108 mg cm2 h1. It was also observed that the increments of Cu2+, Ni2+ and Cd2+ in the receiver solution were less than the reduction in the source solution, indicating that the Cu2+, Ni2+
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F. Gao et al. / Chemical Engineering Journal 328 (2017) 293–303 Table 2 Flux and selectivity factor of variable membrane thickness: (A) 6.27 mm, (B) 8.93 mm, (C) 11.75 mm, (D) 13.63 mm. Flux (mgcm2h1)
Selectivity factor Adsorptive selectivity
Permselectivity
A B C D
Diffusion selectivity
Cu2+
Cu2+/Ni2+
Cu2+/Cd2+
Cu2+/Ni2+
Cu2+/Cd2+
Cu2+/Ni2+
Cu2+/Cd2+
0.228 0.204 0.141 0.108
2.785 5.360 6.251 6.457
21.734 29.970 40.482 42.892
6.572 6.572 6.572 6.572
8.653 8.653 8.653 8.653
0.424 0.815 0.722 0.982
2.512 3.464 4.678 4.956
Fig. 10. Transport profiles of Cu2+, Cd2+ and Ni2+ across the PPy/PSS (A) and BPEI-CQD/PPy/PSS (B) membranes.
Fig. 11. Concentration/time profiles during the separation process in the (A) feed solution and (B) receiver solution (HNO3, pH = 3).
Table 3 Comparison of Cu2+ flux, selectivity factor and current efficiency of membrane system with various separation methods. Methods
Metal
Feed concentration (mgL1)
Flux (mgcm2 h1)
Selectivity factor
Current efficiency (℅)
Ref.
PIMs HFSLM ED EDI ECT ESIP
Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+
64 0.64 100 25 6400 30
0.0208 0.15 102 1.152 1.201 1.055 0.108
0.155(Cu2+/Zn2+) – – – – 7.865(Cu2+/Ni2+)
– – 41.00 40.02 – 39.90
[38] [39] [40] [7] [10] This work
and Cd2+ were not fully released to the receiver solution from the membrane. That is to say, some Cu2+, Ni2+ and Cd2+ still maintained in the membrane during the transport process. According to the ion transport study, the current efficiency of the ESIP system was calculated by the Eq. (3). Using the experimental parameters of QH = 0.177 C, QL = 0.885 C, and the Cu2+ removal amount of 2.638 106 mol, the current efficiency was calculated as g = 39.9%. Compared with the traditional EDI technology (active carbon electrode) [7,40], the method proposed in this study showed a similar current efficiency but a high selectivity factor (Table 3). In particular, the permselectivity of Cu2+/Ni2+ reached as high as 7.865, which was higher than the reported data
[10,38–40]. Therefore, the present method should be an effective technique for the treatment of dilute wastewater in industrial applications. 3.5. Stability In order to test the reusability of the BPEI-CQD/PPy/PSS membrane in the ion transport process, 200 mL of Cu(NO3)2, Ni(NO3)2 and Cd(NO3)2 solutions with an equal concentration of 30 mg/L were respectively added to the source tank, and 200 mL of nitric acid solution (pH = 3) to the receiver tank. Here, the membrane and membrane module were rinsed with DI water before the fresh
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References
Fig. 12. Removal percentages by BPEI-CQD/PPy/PSS membrane during reuse.
solutions were added to cells prior to each cycle. Fig. 12 shows the removal percentages of the BPEI-CQD/PPy/PSS membrane in the stability test. It can be seen that the removal percentages of three heavy metal ions remained almost unchanged with the number of recycle using times. It should be because that the ions uptaking into the membrane can be timely released into the receiver solution owing to the application of pulse potential to the membrane. In addition, in the BPEI-CQD/PPy/PSS membrane, the unique 3D interconnected structure not only provided the room for the vigorous volume changes during the charging–discharging process, but also enhanced the fast diffusion of electrolyte ions across the membrane. And the cycle-life comparison of BPEI-CQD/PPy/PSS and PPy membranes is shown in the Fig. S2. As a result, an outstanding stability performance was obtained in an ESIP process. 4. Conclusions The novel 3D BPEI-CQD/PPy/PSS membrane was successfully fabricated by pressure filtering of BPEI-CQD/PPy/PSS on the surface of porous PTFE support. Due to the addition of BPEI-CQDs to the PPy/PSS, Cu2+ was selectively separated from dilute wastewater. Especially, a continuous separation was realized in an ESIP process with a flux of 0.108 mg cm2 h1 when the membrane thickness was 13.63 mm (BPEI-CQDs: PPy/PSS = 1.2: 1), where the Cu2+ concentration of the source solution was reduced from 30 to 0.82 ppm with a removal efficiency of 97.2%, and the permselectivity of Cu2+/Ni2+ reached as high as 7.865. Furthermore, the sorption-diffusion property of the BPEI-CQD/PPy/PSS membranes was investigated, and it is found that the sorption selectivity was the dominant factor for the selective separation of Cu2+. It is expected that such an electroactive membrane can be applied in the ESIP process for continuous separation of Cu2+ from the industrial wastewater. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21476156) and JSPS KAKENHI Grant 15K06532, Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.06.177.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
An electrochemically-switched BPEI-CQD/PPy/PSS membrane for selective separation of dilute copper ions from wastewater Fengfeng Gao a, Xiao Du a, Xiaogang Hao a,⇑, Shasha Li a, Xiaowei An a, Mimi Liu a, Nianchen Han a, Tonghua Wang b, Guoqing Guan c,⇑ a b c
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China State Key Laboratory of Fine Chemicals, Carbon Research Laboratory, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0812, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel BPEI-CQD/PPy/PSS membrane
was successfully synthesised and used for the removal of Cu2+. Sorption-diffusion model was proposed for the ESIP system. 3D BPEI-CQD/PPy/PSS membrane exhibited excellent permselectivity for Cu2+. The current efficiency of the ESIP system reached 39.9%.
a r t i c l e
i n f o
Article history: Received 25 April 2017 Received in revised form 27 June 2017 Accepted 29 June 2017 Available online 1 July 2017 Keywords: Electrochemically switched ion permselectivity BPEI-CQD/PPy/PSS membrane Cu2+ separation Sorption-diffusion
a b s t r a c t The continuous separation of dilute Cu2+ from wastewater was performed by using a branched poly (ethylenimine) (BPEI)-functionalized carbon quantum dots (BPEI-CQD)/polypyrrole (PPy)/ polystyrenesulfonate (PSS) membrane in an electrochemically switched ion permselective (ESIP) process. The membrane with a high selectivity for Cu2+ separation was prepared by pressure filtering BPEI-CQD/PPy/PSS solution through a polytetrafluoroethylene (PTFE) membrane. In this ESIP system, the directional uptake/release of Cu2+ was realized by modulating the redox state of the membrane coupling with an external electric field. The effects of the operating parameters on the flux of Cu2+ and membrane permselectivity were investigated. The BPEI-CQD/PPy/PSS composite electrode showed excellent permselectivity for Cu2+ with a flux of 0.108 mgcm2h1, a current efficiency of 39.9% and an excellent cycling stability. The Cu2+ concentration in the solution was reduced from 30 to 0.82 ppm with a removal efficiency of 97.2%. Furthermore, the effects of BPEI-CQD content and the membrane thickness on the separation properties of the membrane from a mixed nitrate solution containing Cu2+, Ni2+ and Cd2+ were investigated. It is expected to understand the sorption-diffusion mechanism of such a membrane and provide more information on the design of it for a practical metal ion separation process. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal pollution has become an environmental issue of global concern [1]. It requires to recover heavy metals as precious ⇑ Corresponding authors. E-mail addresses: [email protected] (X. Hao), [email protected] (G. Guan). http://dx.doi.org/10.1016/j.cej.2017.06.177 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
resources for sustainable development in the rapid industrializing process. Thus, how to separate heavy metal ions and recycle them play a significant role nowadays. Copper ions (Cu2+) is an essential element for living organisms, but the excessive ingestion of copper causes serious health problems, such as vomiting, cramps, convulsions, or even death [2]. Therefore it is of great importance to separate Cu2+ from wastewater. The membrane-based separation
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techniques for Cu2+ ions mainly include ultrafiltration (UF) [3], nanofiltration (NF) [4], reverse osmosis (RO) [5], electrodialysis (ED) [6], electrodeionization (EDI) [7], hollow-fiber supported liquid membrane (HFSLM) separation [8], polymer inclusion membrane (PIM) separation [9] and electrochemical treatment (ECT) [10], and so on. Among them, RO and EDI are considered as the most widely-used two ways. However, the low selectivity for the target ions limits their large-scale application. Recently, electrochemically switched ion exchange (ESIX) method [11–14] has been attracted great attention as a novel environmentally benign ion separation technique since it can eliminate the secondary wastes. In the process of ESIX, the rapidly reversible uptake and release of target ions even at very low concentrations can be achieved by changing the redox state of the ESIX film. However, as reported in our previous studies [11–14], the regeneration of the film has to be performed separately. Weidlich et al. designed a special electrochemical cell by reversing the polarity of the ESIX film periodically and exchanging the solutions in the two compartments simultaneously, achieving a ‘‘semi-continuous operation process” [15,16]. To realize the continuous operation, a series of ion transport systems based on conducting polymer membranes were developed by Wallace and Bobacka [17–19]. Based on these ion transport systems, we proposed an in situ potentialenhanced, electrochemically switched ion permselective (ESIP) system, which realized a continuous ESIX separation of Ca2+ and Mg2+ for water softening [20], and designed a potentialcontrolled ion pump based on three-dimensional (3D) PPy@GO or MWCNT/PTFE membrane for separating dilute lead ions from wastewater [21,22]. In the ESIP system, the development of ESIP membrane with high selectivity as well as high flux for the target heavy metal ions is the key issue for the industrial application. Polypyrrole (PPy), as a widely used conducting polymer, is a particularly promising electroactive membrane material due to its easy synthesis, good electrochemical activity and low cost [23]. However, the practical application of PPy is usually limited by its poor cycle-life, resulting from the volumetric change, active material loss and over-oxidative degradation in the doping/de-doping process. To overcome these disadvantages, combining it with carbon-based nanomaterial to obtain a 3D composite was studied. Cao et al. prepared 3D graphene oxide (GO)/PPy composite electrode by a direct electrochemical method for supercapacitors [24]. Lin’s group demonstrated that 3D nitrogen-doped graphene with PPy can be used as electrocatalyst for oxygen reduction reaction [25]. Jian et al. fabricated carbon quantum dots (CQDs) reinforced PPy nanowire via electrostatic self-assembly strategy for high-performance supercapacitors [26]. Lin’s group also investigated the electrochemically controlled ion-exchange properties of MCWNT/PPy composite in various electrolyte solutions [27]. However, to the best of our knowledge, no such a 3D carbon-based composite membrane was used for the selective separation of Cu2+ in an ESIP system. CQDs, a novel zero-dimensional (0D) carbon nanomaterial with high surface area, good electrochemical conductivity and well dispersion property in various solvents, has been attracted extensive attentions [28,29]. It can be easily grafted by functional groups because of abundant oxygen-containing functional groups on its surface. Dong et al. prepared a new kind of polyamine functionalized CQDs by capping with branched poly-(ethylenimine) (BPEI) [30,31], which can be used for selective and sensitive detection of Cu2+. Chen’s group successfully fabricated CQDs functionalized gold nanoparticles for sensitively electrochemical detection of heavy metal ions [32]. Thus, it should be very interesting to combine BPEI-functionalized CQDs (i.e., BPEI-CQDs) with 3D PSS-doped PPy, which could not only have a cation exchange property, but also create the room for volume swelling and shrinkage.
Especially, the BPEI-CQDs can be served as the recognition component for Cu2+. In addition, combining BPEI-CQDs with 3D PSS-doped PPy could (i) provide extra mechanical support, (ii) decrease the charge transfer resistance and (iii) promote the access of target ions to the active sites of the polymer. These are extremely significant for the preparation of organic-inorganic ESIP membranes including BPEI-CQDs and PSS-doped PPy for the continuous separation of Cu2+ from wastewater. Based on these considerations, in this study, a 3D BPEI-CQD/ PPy/PSS composite membrane was fabricated on polytetrafluoroethylene (PTFE) membrane by pressure filtering, and used for the effective and selective removal of Cu2+ from wastewater by ESIP system, in which 2 two-electrode subsystems were included (Fig. 1). Herein, a cell voltage is applied on a pair of platinum plate electrodes to form a constant external electric field, and a pulse potential is applied on the ESIP membrane to control the uptake/ release of target ions. As shown in Fig. 1, when the pulse potential is switched to the reduced potential, the target cations in the source solution will be incorporated on the membrane. Once a positive potential is applied on the membrane, by coupling with an external electric field, the incorporated ions will be immediately expelled into the receiver solution. The synergistic effect of the cell voltage and the pulse potential was described in details in our early work [21,22]. Since the amino groups on the surface of the CQDs can bind the Cu2+ to form cupric amine, it is expected that the combination of it with BPEI could promote the selectivity of the membrane towards Cu2+ while the unique 3D interconnected structure ensure the fast diffusion of electrolyte ions through the membrane. In this study, the effects of BPEI-CQD content and the membrane thickness on the separation properties of the membrane for a mixed nitrate solution containing Cu2+, Ni2+ and Cd2+ were investigated. It is expected to understand the sorption-diffusion mechanism of such a membrane and provide more information on the design of it for a practical metal ion separation process. 2. Experimental 2.1. Reagents and instruments Citric acid (CA) was obtained from Alfa Aesar. BPEI with a molecular weight of 1800 Da was bought from Aladdin (Shanghai, China). Pyrrole (Py, A.R.), ammonium persulphate (APS, A.R.) and hexadecyl trimethyl ammonium bromide (CTAB, A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Pyrrole was distilled and tightly sealed and maintained in the dark below 0 °C prior to use. Polyvinyl alcohol (PVA, Mw = 100 000) was obtained from Fischer Scientific, China. Hydrochloric acid (36–38%) and succinic acid (99.5%) were bought from Tianjin Guangfu Fine Chemical Research Institute, China. PTFE support (pore size: 0.22 mm) was purchased from Sartorius Inc, China. Hydrochloric acid (36–38%) was bought from Tianjin Guangfu Fine Chemical Research Institute, China. Copper, cadmium and nickel nitrates (99%) were employed for the ion separation experiments. All reagents were of analytical grade, and all solutions were prepared using ultrapure water (Millipore 18.2 MX cm). 2.2. Preparation of BPEI-CQD/PPy/PSS membrane BPEI-CQD/PPy/PSS ESIP membrane was prepared via the following steps. Firstly, the BPEI-CQDs was synthesized following the reported method [30,31]. In brief, 1.0 g of CA and 0.5 g of BPEI were added to 10 mL hot water and then heated up (<200 °C) using a heating mantle until most of the water was evaporated and a
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Fig. 1. Schematic illustration of the continuous separation of Cu2+ across the BPEI-CQD/PPy/PSS system. ESIPM: ESIP membrane.
uniform pale-yellow gel was formed. 1 mL of water was added to prevent the gel scorched. Repeating this procedure for 10 times, and finally the color of the gel became orange, proving the formation of BPEI-CQDs. Then, 10 mL BPEI-CQDs solution obtained by adding double-distilled water was purified with a 0.01 mol/L HCl solution by silica gel column chromatography. Finally, it was freeze-dried at 45 °C for 10 h with a Scientz-12N (SCIENTZ, China). Then, to synthesize PPy/PSS, 0.2 g of CTAB as soft template, 0.2 mL of pyrrole monomer and 0.02 g of PSS were dissolved in 1.0 mol/L HCl solution and stirred for 30 min at room temperature. Thereafter, 1 mol/L HCl (5 mL) solution containing 0.68 g of APS was added as an oxidant, and stirred constantly in an ice bath for 12 h. After the polymerization, the black PPy/PSS precipitate was obtained by filtration and rinsed with DI water and ethanol for several times. Finally, the 3D BPEI-CQD/PPy/PSS composite membrane was fabricated via the following strategy. Firstly, 30 mg of PPy/PSS was added to 40 mL of BPEI-CQDs solution (1.5 mg/mL) under ultrasonication for 2 h. Then, 60 mL of 1 w/% PVA solution containing 0.1% succinic acid which was directly involved into the crosslinking reaction and 2 mol/L HCl which was acted as a catalyst to the cross-linking reaction were introduced as the cross-linking agent. Thereafter, the mixture solution was sonicated with a microtip for 10 min and filtrated through a ɸ = 70 mm PTFE support membrane held in the glass vacuum filtration flask. As a result, a black BPEI-CQD/PPy/PSS membrane was formed on the surface of PTFE support. Here, to collect all the CQDs, the filtration process was repeated for several times. Finally, the obtained membrane was dried in oven at 25 °C for 8 h. For comparison, the PPy/PSS membrane was also prepared using the same procedure except the addition of BPEI-CQDs.
2.3. Characterization of BPEI-CQDs and BPEI-CQD/PPy/PSS Transmission electron microscopy (TEM) images of BPEI-CQDs, PPy/PSS and BPEI-CQD/PPy/PSS were taken with a Tecnai G220 (FEI, USA) operating at 200 kV. Scanning electron microscope (SEM) images were characterized with a JSM-7001 F (JEOL, Japan). The membrane thickness was determined based on the crosssection image of the freshly prepared composite membrane. The chemical structure of BPEI-CQD/PPy/PSS composite was analyzed by a Fourier transform infrared spectroscopy (FTIR-Prestige-21, Shimadzu Corporation, Japan) using a standard KBr pellet method. The composite crystalline phase was determined by X-ray diffraction (XRD, Rigaku SmartLab, Japan). Cyclic voltammetry (CV) was performed using a VMP3 Potentiostat (Princeton, USA) controlled with EC-Lab software at room temperature. The cation concentration in aqueous solution was detected with atomic absorption spectrometry (AAS-990). The masterflex L/S (07519–05) was bought from Cole-Parmer, USA. 2.4. Continuous separation test Before the continuous separation test, the washed membrane was assembled in the membrane module. 200 mL of mixed metal nitrate solution and nitric acid solution (pH = 3) were poured into two tanks as the source and receiver solutions, respectively. After Masterflex L/S (07519–05) peristaltic pump was turned on, a continuous running system was formed. Here, a pulse potential between 0 V and 2.0 V was applied on the membrane (the duration time was equal for the low and high pulses), and a 2.0 V cell voltage was applied on the platinum plates shown in Fig. 1. During the
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test, aliquots of 1 mL were sampled from the source or receiver chambers to detect the metal ion concentration by AAS-990. The flux of each metal ion across the composite membrane (F) was calculated using Eq. (1):
F¼
ðC1 C2 ÞV tA
ð1Þ
where C1 and C2 (mg/L) are the initial and final concentrations of the metal ion (mg/L), respectively; V (L) is the volume of the metal ion solution; t (h) is the duration of the separation; and A (cm2) is the area of the membrane exposed to the receiver solution. The membrane separation factor between two different metal ions M and N was calculated by the following equation:
x0 =x0 Separation factor ðasep: M=NÞ ¼ M N xM =xN
ð2Þ
where x0M and x0N , xM and xN are the mass fractions of M and N components in the permeate and feed, respectively. The charge stored in the ESIP system was easily calculated using the electrochemical workstation. In addition, the current efficiency (g) is defined as follows:
Cu2þ removal amouts Z F g ð%Þ ¼ 100 QH þ QL
ð3Þ
where QH and QL are the quantities of charge stored in the ESIP system when the high and low potentials are applied to the membrane, respectively. F is the Faraday constant 96,500 C/mol, and z is the valence state of Cu2+. 2.5. Adsorption and diffusion properties of the BPEI-CQD/PPy/PSS membrane The adsorption properties were tested using a two-electrode system in 100 mL mixture solutions of 30 mg/L Cu(NO3)2 and Cd (NO3)2 and Ni(NO3)2. The PPy/PSS and BPEI-CQD/PPy/PSS membranes were the working electrodes. A 0.1 V constant potential was applied on the membrane for 120 min.
The selectivity factor of the composite membrane for metal ions is defined as follows:
Sorption selectivity ðasorp: M=NÞ ¼
y0M =y0N yM =yN
ð4Þ
where the numerator is the ratio of the cation mass fraction within the composite membrane, and the denominator is the mass fraction ratio in the bulk solution that is in contact with the composite membrane. The diffusion selectivity can be calculated from the separation factor and the sorption selectivity by the following equation [33,34]:
Diffusion selectivity ðadif: M=NÞ ¼
asep: M=N asorp: M=N
ð5Þ
3. Results and discussion 3.1. Morphological characterization Fig. 2 shows SEM morphologies of PTFE and BPEI-CQD/PPy/PSS membranes. As shown in Fig. 2A, PTFE support contained an irregular network structure. Fig. 2B, C and D show the typical surface and cross-sectional morphologies of the BPEI-CQD/PPy/PSS composite membrane deposited on PTFE support. From Fig. 2B, the interpenetrating 3D network structure was composed of chainlike connected nanospheres in random orientations, which could supply a large amount of active sites and ionic conducting channels for the uptake/release of Cu2+. Fig. 2C shows the cross-sectional morphology of the membrane, which indicates that the membrane thickness was approximately 11.8 lm (membrane area: 4.0 cm2). Here, it should be noted that the thickness can be precisely tuned by adjusting the BPEI-CQD/PPy/PSS content in the solution used for fabrication. Magnified cross-sectional view of the BPEI-CQD/PPy/ PSS membrane (Fig. 2D) further proved the existence of highly interpenetrated porous structure, which could provide fast ionic transfer channels.
Fig. 2. SEM images of the surfaces of (A) PTFE and (B) BPEI-PPy/PSS membranes. Insets: photographs of the corresponding membranes. (C) and (D) are cross-sectional SEM images of the BPEI-PPy/PSS membrane under different magnifications.
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Fig. 3. TEM images of (A) BPEI-CQDs, (B) PPy/PSS and (C) BPEI-CQD/PPy/PSS.
The morphologies of BPEI-CQDs, PPy/PSS and BPEI-CQD/PPy/PSS composites were further characterized by TEM. Fig. 3(A) shows that resultant BPEI-CQDs were monodisperse nanocrystals with near spherical morphology and a relatively narrow lateral size in the range from 3 to 8 nm. The representative high-resolution TEM (HRTEM) image of the individual BPEI-CQDs showed highly ordered structure with an interlayer spacing of 0.212 nm (see the inset of Fig. 3(A)) [31]. A thin layer network of PPy/PSS with chain-like structure is demonstrated in Fig. 3(B). Moreover, as shown in Fig. 3(C), BPEI-CQDs appeared on the surface of PPy/PSS which should be interacted with PPy through strong p–p interactions.
assigned to CAH deformation vibrations and the CAN stretching vibration, respectively. In addition, the peaks at 1170 and 901 cm1 are due to the doping state of PPy and the band at 783 cm1 verifies the presence of polymerized pyrrole. The bands observed at 2920 and 2850 cm1 are attributed to the asymmetrical and symmetrical stretching of CH2 group. In addition, an extra peak was found at 1706 cm1 in the FTIR spectrum of BPEI-CQD/ PPy/PSS, which is associated with the amide linkage (ACONHA), indicating that BPEI-CQDs were successfully doped on the surface of PPy/PSS.
3.2. FTIR spectroscopy characterization
Fig. 5(A) demonstrates CV curves obtained with the PPy/PSS and BPEI-CQD/PPy/PSS membrane electrodes in 0.1 M H2SO4 solution at a scan rate of 50 mV/s. Obviously, a higher current density was observed in the CV curve of BPEI-CQD/PPy/PSS, indicating that it had higher electroactivity than PPy/PSS with the same mass in the same potential range (0.1 –0.8 V). This should be attributed to the good conductivity of BPEI-CQDs and its additional chemical interactions with the electrolyte, which could reduce the charge transfer resistance and increase the rate of electrochemical reaction in the membrane. From Fig. 5(B), XRD measurements were further used to characterize the crystal structures of BPEI-CQD/ PPy/PSS and PPy/PSS. As presented in Fig. 5(B), XRD patterns of both pure PPy/PSS and BPEI-CQD/PPy/PSS composite generally present two broad peaks at 2h = 25.2°, indicating that the amorphous states of PPy/PSS and BPEI-CQD/PPy/PSS in nature. The bright blue emission of the BPEI-CQDs was observed obviously under the excitation of 365 nm (the inset of Fig. 5(C)). The UV–vis absorption result reveals that the BPEI-CQDs solution had two absorption bands at 245 and 355 nm, respectively, which further confirmed that BPEI-CQD solution was successfully prepared.
FTIR spectrums were used to characterize the chemical structures of BPEI–CQDs, PPy/PSS and BPEI-CQD/PPy/PSS. As shown in Fig. 4, for BPEI–CQDs, many characteristic absorption bands of BPEI (NH of 3423 and 1590 cm1, CH2 of 2965 and 2817 cm1, CN of 1120 cm1) were observed [30,31]. A sharp peak at 1706 cm1, which is corresponding to amide linkage (ACONHA), indicates that BPEI was successfully grafted onto the surface of the CQDs by the amide linkages. In the PPy/PSS spectrum, a weak absorption peak at 1633 cm1 corresponded to the hydrated water molecules appeared, indicating that PPy and PSS were combined via hydrogen bonding [35]. The peaks observed at approximately 1548 and 1454 cm1 are the symmetric and asymmetric pyrrole ring stretching modes while the strong NAH stretching and bending vibration at 3423 cm1, the bands located at 1040 and 1308 cm1 are
3.3. Electrochemical and inner structures characterization
3.4. Operation parameters
Fig. 4. FTIR spectra of PPy, PPy/PSS, BPEI-CQD/PPy/PSS and BPEI-CQDs.
3.4.1. Effect of BPEI-CQD content on permeation performance Prior to ion separation test, the effect of BPEI-CQD content in BPEI-CQD/PPy/PSS membrane on permeation performance was investigated. Equi-mass solutions (30 mg/L) of Cu(NO3)2, Ni (NO3)2 and Cd(NO3)2 were used as the source solution and the receiver solution was nitric acid solution (pH = 3). Fig. 6 demonstrates the transport profiles of Cu2+, Ni2+ and Cd2+ across BPEICQD/PPy/PSS membrane with various BPEI-CQD contents (15, 30, 60, 120 mg), and the final concentration vs. BPEI-CQD content histograms of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes are shown in Fig. S1 of Supporting Information. One can see that there was a gradual increase in the Cu2+ transport rate but obvious decreases in Ni2+ and Cd2+ transport rates with the increase in BPEI-CQD content, and the permeability sequence
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Fig. 5. (A) CV responses of PPy/PSS (black) and BPEI-CQD/PPy/PSS (red) membranes in 0.1 M H2SO4 solution at a scan rate of 50 mV/s, (B) XRD spectra of PPy/PSS (black) and BPEI-CQD/PPy/PSS (red), (C) UV absorption spectra of BPEI-CQD solution. The inset shows the photo of BPEI-CQD solution illuminated by UV light of 365 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
was in the order of Cu2+ > Ni2+ > Cd2+. It can be concluded that the BPEI-CQD/PPy/PSS membrane had a higher permselectivity towards Cu2+ than Ni2+ and Cd2+, and its permselectivity increased with the increase in BPEI-CQD content. Chi et al. reported that the
Fig. 6. Transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes of variable BPEI-CQD contents: (A) 15 mg, (B) 30 mg, (C) 60 mg, (D) 120 mg and invariable PPy/PSS content: 50 mg.
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amino groups on the surface of the CQDs can bind Cu2+ to form cupric amine, and verified that the BPEI-CQDs had a high selectivity to Cu2+ [30,31]. Herein, the BPEI-CQDs played a key role in the increase of the permselectivity towards Cu2+. Goon et al. reported that BPEI can selectively capture trace-level free Cu2+ [36]. It is because that the nitrogen of amino groups has a lone pair of electrons, which can complex heavy metal ions with the sharing electrons. In this study, the higher selectivity of BPEI-CQDs towards Cu2+ should be also attributed to the higher binding force between amino groups and Cu2+. However, there were no significant increases in the Cu2+ transport rate and permselectivity from Fig. 6C to D. Thus, considering the energy-saving and costeffectivity, the optimum BPEI-CQD content should be 60 mg, in which the PPy/PSS content was 50 mg. Based on the above assumption, the quantization calculations of the binding energy between amino groups and Cu2+, Ni2+ and Cd2+ were carried out. Herein, the density functional theory (DFT) method in the DMOL3 module included in the Accelrys Material Studio 6.0 software package and the widely used generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function were adopted, and the valence electron functions were expanded into a set of numerical atomic orbitals by a double-numerical basis with polarization functions (DNP). The calculated binding energy of amino groups with Cu2+, Ni2+ and Cd2+ are shown in Table S1. One can see that the binding energy is consistent with the expected, and the binding energy sequence is in the order of Cu2+ > Ni2+ > Cd2+. This can be easily understood from the fact that the attachment of BPEI-CQDs towards Cu2+ was the highest in the three heavy metal ions. In order to study the adsorption selectivity of BPEI-CQD/PPy/PSS membrane, the adsorption capacities of Cu2+, Cd2+ and Ni2+ with various BPEI-CQD contents were tested in 100 mL of 30 mg/L Cu (NO3)2, Ni(NO3)2 and Cd(NO3)2 solutions, respectively. Here, 0.1 V constant negative potential of two-electrode system was applied on the membrane for 120 min. Fig. 7 shows the adsorption capacities of the BPEI-CQD/PPy/PSS membranes with various BPEICQD contents. It can be seen that the adsorption capacity for Cu2+ increased but that for Ni2+ or Cd2+ decreased with the increase in BPEI-CQD content. This further confirms that the BPEI-CQDs had higher adsorption selectivity for Cu2+. In this study, the sorption-diffusion model was proposed for the ESIP system, in which the process of ion transport mainly contains three steps: (i) the heavy metal ions are selectively incorporated into the membrane from the source solution; (ii) the heavy metal
Fig. 7. Adsorption capacities of Cu2+, Cd2+ and Ni2+ into the BPEI-CQD/PPy/PSS membrane with variable BPEI-CQDs content in 100 mL of 30 mg/L Cu(NO3)2 and Cd (NO3)2 and Ni(NO3)2 solutions for each.
Table 1 Flux and selectivity factor of variable BPEI-CQD content: (A) 15 mg, (B) 30 mg, (C) 60 mg, (D) 120 mg. Flux (mgcm2h1)
A B C D
Selectivity factor Permselectivity
Adsorptive selectivity
Diffusion selectivity
Cu2+
Cu2+/ Ni2+
Cu2+/ Cd2+
Cu2+/ Ni2+
Cu2+/ Cd2+
Cu2+/ Ni2+
Cu2+/ Cd2+
0.163 0.176 0.204 0.210
1.150 4.224 5.360 5.956
2.698 16.616 29.970 38.572
2.510 5.561 6.572 7.158
2.521 5.978 8.653 10.044
0.458 0.759 0.815 0.832
1.070 2.779 3.464 3.840
ions diffuse in the membrane and (iii) the heavy metal ions are released from the membrane to the receiver solution. In the ESIP system, the permeation is assumed to occur via potentialdependent diffusion. Thus, the sorption-diffusion model proposed here cannot be interpreted using Fick’s Law. According to the ion transport and adsorption tests, the flux, permselectivity and adsorption selectivities of the BPEI-CQD/PPy/ PSS membrane with various BPEI-CQD contents towards Cu2+, Ni2+ and Cd2+ were calculated. Also, the diffusion selectivity was calculated by Eq. (5). Table 1 lists the flux, permselectivity, adsorption selectivity and diffusion selectivity. One can see that the flux of Cu2+ increased but that of Ni2+ and Cd2+ decreased with the increase in BPEI-CQD content (A: 15 mg, B: 30 mg, C: 60 mg, D: 120 mg); meanwhile, the permselectivity, adsorption selectivity and diffusion selectivity increased. These should be resulted from the high electrical conductivity of BPEI-CQDs and its high selectivity towards Cu2+, which can reduce the charge transfer resistance and increase the electrochemical reaction rate of the membrane with electrolyte. From the diffusion selectivity, it can be seen that the selectivities of Cu2+/Ni2+ were all less than 1 but the selectivity of Cu2+/Cd2+ was more than 1. This should be due to that the hydrated ionic radii of metal ions are in the order of Cd2+ > Cu2+ > Ni2+ [37]. As such, the diffusion selectivity was closely associated with the hydrated ionic radius of metal ion. Furthermore, it can be seen that the adsorption selectivity was higher than the diffusion selectivity. Thus, it is believed that the permselectivity of ESIP membrane was mainly depended on the adsorption selectivity. 3.4.2. Effect of membrane thickness Fig. 8 shows the transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes with various membrane thicknesses (6.27 mm, 8.93 mm, 11.75 mm, 13.63 mm), where the ration of BPEI-CQD and PPy/PSS was 1.2: 1. One can see that the transport rates of three metal ions all decreased with the increase in membrane thickness. However, it is obvious that the permselectivity was enhanced. Here, a high BPEI-CQD/PPy/PSS content could provide more active sites for the binding of Cu2+, which is beneficial to the transport of Cu2+. Meanwhile, the diffusion resistance also increased with the increase in membrane thickness, resulting in the decrease of flux. The adsorption selectivity of BPEI-CQD/PPy/PSS membrane was also investigated. Fig. 9 demonstrates the absorbance-time profiles of the BPEI-CQD/PPy/PSS membrane for Cu2+, Ni2+ and Cd2+. One can see that the membrane had a high adsorption capacity for Cu2+. Here, the adsorption selectivity was calculated based on Eq. (4). The obtained flux, permselectivity, adsorption selectivity and diffusion selectivity are listed in Table 2. It can be seen that there was a decrease in the flux and an increase in the permselectivity as well as diffusion selectivity with the increase in membrane thickness. It is also found that the selectivities of Cu2+/Ni2+ were all less than 1 while the selectivities of Cu2+/Cd2+ were all higher
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Fig. 9. Time dependence of adsorption capacities of Cu2+, Cd2+ and Ni2+ into the BPEI-CQD/PPy/PSS membrane in 100 mL of 30 mg/L Cu(NO3)2, Cd(NO3)2 and Ni (NO3)2 solutions for each.
than 1. This also confirms that the diffusion selectivity was associated with the hydrated ionic radius of metal ion. 3.4.3. Permeation properties of BPEI-CQD/PPy/PSS and PPy/PSS membranes Fig. 10A and B show the transport profiles of Cu2+, Ni2+ and Cd2+ across the PPy/PSS and BPEI-CQD/PPy/PSS membranes in the receiver solution, respectively. In region b, when a CV-PP coupling circuit was applied, the concentrations of Cu2+, Ni2+ and Cd2+ in the receiver solution increased. However, in region a and c, no ions were transported across the membrane since no CV-PP coupling circuit was applied to the system. According to the two experiments, it can be proved that the transport of ions across BPEICQD/PPy/PSS and PPy/PSS membranes was controlled by the coupling circuit. From the Fig. 10A (section b), the concentration of Ni2+ in the receiver solution was higher than Cu2+ and Cd2+, which indicates that the PPy/PSS membrane had a higher permselectivity toward Ni2+. It should be noted that the ionic radius of Ni2+ is the smallest among the three metal ions, resulting in the highest diffusion selectivity of the PPy/PSS membrane toward Ni2+. Since the PPy/PSS membrane had no adsorption selectivity for heavy metal ions, the permselectivity mainly depended on the diffusion selectivity. Interestingly, when BPEI-CQDs were added to the PPy/PSS membrane, the more copper ions were found to be transported to the receiver solution (see Fig. 10B (section b)). This result identifies that BPEI-CQDs had a high adsorption selectivity toward Cu2+.
Fig. 8. Transport profiles of Cu2+, Cd2+ and Ni2+ across the BPEI-CQD/PPy/PSS membranes of variable membrane thicknesses: (A) 6.27 mm, (B) 8.93 mm, (C) 11.75 mm, (D) 13.63 mm.
3.4.4. Ion transport study The profiles of Cu2+, Ni2+ and Cd2+ concentration versus time in the source and receiver solutions are shown in Fig. 11. In Fig. 11A, the concentrations of three metal ions in the source solution decreased with the time, and the removal percentage of Cu2+ achieved as high as 97.2%. Especially, the Cu2+ concentration in the source solution decreased from 30 to 0.82 ppm, and the permselectivity of Cu2+/Ni2+ reached as high as 7.865. From Fig. 11B, the concentrations of three metal ions in the receiver solution increased with the time, and the flux of Cu2+ achieved at 0.108 mg cm2 h1. It was also observed that the increments of Cu2+, Ni2+ and Cd2+ in the receiver solution were less than the reduction in the source solution, indicating that the Cu2+, Ni2+
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F. Gao et al. / Chemical Engineering Journal 328 (2017) 293–303 Table 2 Flux and selectivity factor of variable membrane thickness: (A) 6.27 mm, (B) 8.93 mm, (C) 11.75 mm, (D) 13.63 mm. Flux (mgcm2h1)
Selectivity factor Adsorptive selectivity
Permselectivity
A B C D
Diffusion selectivity
Cu2+
Cu2+/Ni2+
Cu2+/Cd2+
Cu2+/Ni2+
Cu2+/Cd2+
Cu2+/Ni2+
Cu2+/Cd2+
0.228 0.204 0.141 0.108
2.785 5.360 6.251 6.457
21.734 29.970 40.482 42.892
6.572 6.572 6.572 6.572
8.653 8.653 8.653 8.653
0.424 0.815 0.722 0.982
2.512 3.464 4.678 4.956
Fig. 10. Transport profiles of Cu2+, Cd2+ and Ni2+ across the PPy/PSS (A) and BPEI-CQD/PPy/PSS (B) membranes.
Fig. 11. Concentration/time profiles during the separation process in the (A) feed solution and (B) receiver solution (HNO3, pH = 3).
Table 3 Comparison of Cu2+ flux, selectivity factor and current efficiency of membrane system with various separation methods. Methods
Metal
Feed concentration (mgL1)
Flux (mgcm2 h1)
Selectivity factor
Current efficiency (℅)
Ref.
PIMs HFSLM ED EDI ECT ESIP
Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+
64 0.64 100 25 6400 30
0.0208 0.15 102 1.152 1.201 1.055 0.108
0.155(Cu2+/Zn2+) – – – – 7.865(Cu2+/Ni2+)
– – 41.00 40.02 – 39.90
[38] [39] [40] [7] [10] This work
and Cd2+ were not fully released to the receiver solution from the membrane. That is to say, some Cu2+, Ni2+ and Cd2+ still maintained in the membrane during the transport process. According to the ion transport study, the current efficiency of the ESIP system was calculated by the Eq. (3). Using the experimental parameters of QH = 0.177 C, QL = 0.885 C, and the Cu2+ removal amount of 2.638 106 mol, the current efficiency was calculated as g = 39.9%. Compared with the traditional EDI technology (active carbon electrode) [7,40], the method proposed in this study showed a similar current efficiency but a high selectivity factor (Table 3). In particular, the permselectivity of Cu2+/Ni2+ reached as high as 7.865, which was higher than the reported data
[10,38–40]. Therefore, the present method should be an effective technique for the treatment of dilute wastewater in industrial applications. 3.5. Stability In order to test the reusability of the BPEI-CQD/PPy/PSS membrane in the ion transport process, 200 mL of Cu(NO3)2, Ni(NO3)2 and Cd(NO3)2 solutions with an equal concentration of 30 mg/L were respectively added to the source tank, and 200 mL of nitric acid solution (pH = 3) to the receiver tank. Here, the membrane and membrane module were rinsed with DI water before the fresh
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References
Fig. 12. Removal percentages by BPEI-CQD/PPy/PSS membrane during reuse.
solutions were added to cells prior to each cycle. Fig. 12 shows the removal percentages of the BPEI-CQD/PPy/PSS membrane in the stability test. It can be seen that the removal percentages of three heavy metal ions remained almost unchanged with the number of recycle using times. It should be because that the ions uptaking into the membrane can be timely released into the receiver solution owing to the application of pulse potential to the membrane. In addition, in the BPEI-CQD/PPy/PSS membrane, the unique 3D interconnected structure not only provided the room for the vigorous volume changes during the charging–discharging process, but also enhanced the fast diffusion of electrolyte ions across the membrane. And the cycle-life comparison of BPEI-CQD/PPy/PSS and PPy membranes is shown in the Fig. S2. As a result, an outstanding stability performance was obtained in an ESIP process. 4. Conclusions The novel 3D BPEI-CQD/PPy/PSS membrane was successfully fabricated by pressure filtering of BPEI-CQD/PPy/PSS on the surface of porous PTFE support. Due to the addition of BPEI-CQDs to the PPy/PSS, Cu2+ was selectively separated from dilute wastewater. Especially, a continuous separation was realized in an ESIP process with a flux of 0.108 mg cm2 h1 when the membrane thickness was 13.63 mm (BPEI-CQDs: PPy/PSS = 1.2: 1), where the Cu2+ concentration of the source solution was reduced from 30 to 0.82 ppm with a removal efficiency of 97.2%, and the permselectivity of Cu2+/Ni2+ reached as high as 7.865. Furthermore, the sorption-diffusion property of the BPEI-CQD/PPy/PSS membranes was investigated, and it is found that the sorption selectivity was the dominant factor for the selective separation of Cu2+. It is expected that such an electroactive membrane can be applied in the ESIP process for continuous separation of Cu2+ from the industrial wastewater. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21476156) and JSPS KAKENHI Grant 15K06532, Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.06.177.
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