An in Situ Potential-Enhanced Ion Transport System Based on FeHCF

Article pubs.acs.org/IECR

An in Situ Potential-Enhanced Ion Transport System Based on FeHCF−PPy/PSS Membrane for the Removal of Ca2+ and Mg2+ from Dilute Aqueous Solution Pengle Zhang,† Junlan Zheng,† Zhongde Wang,† Xiao Du,† Fengfeng Gao,† Xiaogang Hao,*,† Guoqing Guan,‡ Chuncheng Li,† and Shibin Liu† †

Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan

‡

S Supporting Information *

ABSTRACT: An in situ potential-enhanced ion transport system based on the electrochemically switched ion permselectivity (ESIP) membrane was developed for the effective removal of Ca2+ and Mg2+ from dilute aqueous solution. In this system, uptake/release of the target ions can be realized by modulating the redox states of the ESIP membrane, and continuously permselective separation of the target ions through the ESIP membrane can be achieved by tactfully applying a pulse potential on the membrane and combining with an external electric field. In this study, iron hexacyanoferrate (FeHCF)−polypyrrole/polystyrenesulfonate (PPy/PSS) ESIP membrane with high conductivity and high flux was prepared by using stainless steel wire mesh (SSWM) as conductive substrate. The driving force for the ion transport was analyzed in detail by the equivalent circuit of the system. It is found that the FeHCF interlayer between the SSWM substrate and the PPy/PSS membrane played an important role in removing Ca2+ and Mg2+ from aqueous solutions, and markedly enhanced the separation performance of the membrane due to the improvement of the electroactivity as well as the change of the surface morphology. Influences of the applied cell voltage of the external electric field and the pulse (constant) potential across the membrane on the separation of Ca2+ and Mg2+ were investigated. It is demonstrated that the pulse potential was more beneficial for improving the removal efficiency than the constant potential applied on the membrane. The hardness of the treated water was reduced to 50 ppm (CaCO3) by applying a pulse potential of ±2.0 V and an cell voltage of 5.0 V when the initial concentration of Ca2+ was 10 mM (1000 ppm (CaCO3)). It is expected that the in situ potential-enhanced ion transport system based on the FeHCF−PPy/PSS membrane could be used as a novel water softening technology. redox state of ESIX film. Simultaneously the film can be regenerated; thus no secondary waste will be produced.11 In our previous work, we have prepared various ESIX films and used for separations of dilute monovalent ions such as Cs+12,13 and I−,13 divalent ions such as Ni2+,14,15 Pb2+,16 and Cu2+,17 and even trivalent metal ions such as Y3+18 in aqueous solutions. Conductive polymers (CPs), such as polypyrrole (PPy), can be tailored as anion or cation exchangers by entrapping or doping of different counterions for the removal of F−,19 ClO4−,20 and Ca2+21 from aqueous solutions. The development of an ESIX process for water softening requires the preparation of CP polypyrrole doped with the larger polystyrenesulfonate (PSS) as immobile counterion (PPy/PSS),22,23 and related studies have been investigated on planar21 or porous electrodes.24,25

1. INTRODUCTION Water softening is a basic requirement for several technological and domestic applications. Ca2+ and Mg2+ ions are directly responsible for unwanted precipitate formation as well as for the deterioration of the efficiency of detergents (e.g., soap).1 Scale can plug pipelines, reduce the heat transfer efficiency in cooling or heating systems, and may lead to furnace or boiler blasting.2 Various water softening processes, such as ion exchange,3,4 nanofiltration,5,6 and polymer-assisted ultrafiltration7,8 have been applied to remove minerals. Among them, ion exchange is the most broadly used process due to its ease of operation and high removal efficiency of minerals.2 However, other exchanged cations can be also released into water during the ion exchange process and periodic regeneration of resin is required when ion exchange capacity is saturated.9 Electrochemically switched ion exchange (ESIX) is an environmentally benign ion separation technique and has gained great attention in recent years.10 The reversible uptake and release of the target ions can be realized by modulating the © 2016 American Chemical Society

Received: Revised: Accepted: Published: 6194

February 13, 2016 April 19, 2016 May 7, 2016 May 9, 2016 DOI: 10.1021/acs.iecr.6b00597 Ind. Eng. Chem. Res. 2016, 55, 6194−6203

Article

Industrial & Engineering Chemistry Research

mesh (SSWM) was used as the substrate and iron hexacyanoferrate (FeHCF) film was deposited on it, followed by deposition a layer of PPy/PSS on the FeHCF modified SSWM.

However, the uptake and release of the cations are generally performed separately for the traditional ESIX process with intermittent operation. To realize continuous operation of the ESIX process, a special arrangement of two identical cation exchanger electrodes divided by an anion exchanger membrane (AEM) in an electrochemical cell has been designed.12,13,26 For water softening, one ion exchanger electrode is reduced for the uptaking Ca2+ ions from the solution while the other is oxidized for the release of Ca2+ ions corresponding to its regeneration.26 However, the process needs to reverse the electrodes periodically and exchange the fluids of the two compartments. Such an operation is only a “semicontinuous operation process”. Wallace and co-workers reported a series of metal ion transport systems based on CP membranes for the separation of ions in aqueous solutions to realize continuous operation.27−29 They developed stand-alone conducting polymer membranes for this purpose at first.28,29 However, due to the limited strength and porosity of these stand-alone membranes, they developed other new composite membranes by using PVDF/Pt as a conducting substrate.30−33 However, since no cell voltage was applied on the ion transport system, no constant ion transfer driving force can be formed between the source solution and the receiving solution.34 Also, the PVDF/ Pt conducting substrate had relatively weak conductivity (slow potential response) and high preparation cost. In this study, to solve the above problems, the concept of an electrochemically switched ion permselectivity (ESIP) membrane was proposed, and an in situ potential-enhanced ion transport system based on the ESIP membrane was developed. As shown in Figure 1, the ion transport system consisted of one

2. EQUIVALENT CIRCUIT ANALYSIS OF THE IN SITU POTENTIAL-ENHANCED ION TRANSPORT SYSTEM To analyze the ion transport driving force of the in situ potential-enhanced ion transport system, the equivalent circuit of the transport system was illustrated. As shown in Figure 2,

Figure 2. Equivalent circuit of the in situ potential-enhanced ion transport system. MP = membrane potential. ES and ER are the electric fields of the source solution and receiving solution.

Ucell is the cell voltage applied by the two-electrode system, UR and US are the potential differences of the receiving cell and the source cell, and RS and RR are the resistances of the source solution and the receiving solution, respectively. Membrane potential (MP) is the potential applied to the ESIP membrane by the three-electrode system. When cell voltage and pulse potential are applied simultaneously on the ion transport system, the driving forces for the ion transport should be like the following: when the low potential of the pulse potential is applied on the membrane, cations in the source solution are incorporated into the membrane under the assistance of the cell voltage, US is responsible for the migration rate of cation from the bulk source solution to the membrane surface and MP is responsible for the cation adsorption rate of the membrane, and the rate of cations incorporated into the membrane from the source solution increases as MP decreases (becomes negative) and/or US increases; when the high pulse potential is applied to the membrane, cations are ejected from the membrane into the receiving solution with the cell voltage, where MP is responsible for the diffusion rate of cation from inside to the surface of the membrane and UR is responsible for the migration rate of cation from the membrane surface to the bulk receiving solution, and the ejection rate of cation from the membrane into the receiving solution increases as MP increases (becomes positive) and/or UR increases. In contrast, when cell voltage and constant potential are applied to the ion transport system, MP determines the charge density of the membrane and Ucell provides the constant external field driving force that makes the cations migrate directionally from the source solution to the receiving solution. In addition, the transport rate of cations across the membrane increases as MP decreases (negative charge density of the membrane increases)33 and/or Ucell increases.

Figure 1. Schematic illustration of the in situ potential-enhanced ion transport system. WE = working electrode; RE = reference electrode; CE = counter electrode.

two-electrode system and one three-electrode system. In this system, the pulse potential provided by the three-electrode system was applied on the ESIP membrane working electrode so that the uptake/release of the target ions was realized by modulating the redox states of the membrane. Meanwhile, the cell voltage provided by the two-electrode system resulted in a constant external electric field driving force, which made the target ions directionally migrate from the source solution to the receiving solution. As such, continuously permselective separation of the target ions through the ESIP membrane is expected to be achieved by tactfully applying a pulse potential on the membrane combining with the constant cell voltage. In addition, to improve the conductivity and reduce the preparation cost of the ESIP membrane, stainless steel wire 6195

DOI: 10.1021/acs.iecr.6b00597 Ind. Eng. Chem. Res. 2016, 55, 6194−6203

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Figure 3. SEM images of the cross sections of (c, e) FeHCF−PPy/PSS and (d, f) PPy/PSS membranes and surfaces of (a, b) FeHCF film, (g, i) FeHCF−PPy/PSS membrane, and (h, j) PPy/PSS membrane.

3. EXPERIMENTAL SECTION

other reagents were used as received without further treatment. Type 304 SSWM (400 mesh) was used as the substrate to prepare the ESIP membrane. Electrochemical testing with the three-electrode system was performed using a VMP2 potentiostat (Princeton USA) controlled with EC-Lab software. The surface and cross-section morphologies of the FeHCF−PPy/PSS and PPy/PSS membranes were investigated by scanning electron microscopy

3.1. Reagents and Instruments. Chemicals were obtained from National Medicine Group Chemical Reagent Co., Ltd., PRC. All reagents were of analytical grade, and all solutions were prepared using ultrapure water (Millipore 18.2 MΩ·cm). Pyrrole was distilled before use. The distilled pyrrole was sealed tightly and kept in the dark below 0 °C until further use. All 6196


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Industrial & Engineering Chemistry Research (SEM; JSM-6700F, Japan). The concentrations of Ca2+, Mg2+, Na+, K+, and NO3− in the solutions were analyzed by a DX-600 (DIONEX USA) ion chromatograph. The concentration of Fe3+ was measured by atomic absorption spectrometry (TAS990). The pH value of the solution was measured by an acidometer (PHS-4CT). 3.2. ESIP Membrane Preparation. The SSWM (400 mesh, 4 cm × 4 cm) was immersed in absolute ethyl alcohol for 1 h, followed by washing with ultrapure water to remove the organics on its surface, and then immersed in 0.1 M sulfuric acid solution for another 10 min, and finally washed with ultrapure water and dried in an air stream. The FeHCF−PPy/PSS ESIP membrane was prepared via the following steps. 1. FeHCF film electrode: FeHCF was first electrodeposited on the treated SSWM from a freshly prepared solution containing 40 mM FeCl3, 40 mM K3[Fe(CN)6], and 0.1 M Na2SO4 via a pulse potential between 0.3 and 0.9 V (versus Hg/Hg2Cl2) at a pulse width of 1 s for 1 h in a three-electrode cell. 2. FeHCF−PPy/PSS ESIP membrane: PPy/PSS was further electrodeposited on the prepared FeHCF film electrode in a three-electrode cell containing an aqueous solution of 0.1 M PSS and 0.2 M pyrrole via potentiostatic method with a constant potential of 0.8 V for 1 h. For comparison, the PPy/PSS membrane was also prepared on the treated SSWM electrode only by the second step using a constant potential of 0.8 V for 1 h. Finally, all the prepared membranes were rinsed thoroughly with ultrapure water before use for the removal and adsorption experiments. The PPy/PSS and FeHCF−PPy/PSS ESIP membranes were also prepared by electrodeposition (PPy/PSS) only for 10 min, respectively, and used for the determination of current− potential response in a three-electrode cell. 3.3. Continuous Removal of Ca2+ and Mg2+. The continuous separation of Ca2+ and Mg2+ from aqueous solution was performed in the in situ potential-enhanced ion transport system with FeHCF−PPy/PSS and PPy/PSS as the ESIP membranes (Figure 1). The transport cell consisted of two compartments separated by the ESIP membrane. One compartment is termed the source cell and the other is the receiving cell. Two stainless steel electrodes were respectively placed in the two compartments, and a constant cell voltage was applied on these two electrodes by a dc power supply. The ESIP membrane, a saturated calomel electrode (SCE), and the stainless steel electrode in the receiving cell served as the working electrode, the reference electrode, and the counter electrode of the three-electrode system in conjunction with a VMP2 potentiostat, respectively. The thickness of each compartment was 1 cm, and the effective membrane area was 2 cm × 2 cm. A 200 mL volume of 1 mM Ca(NO3)2 or Mg(NO3)2 or 10 mM Ca(NO3)2 solution was respectively pumped into the source cell while 200 mL of deionized water was pumped into the receiving cell. In this study, the effects of the cell voltage applied to the ion transport system and the pulse (constant) potential across the FeHCF−PPy/PSS membrane on the removal efficiency of Ca2+ and Mg2+ were investigated. The interference experiments measurements were carried out with the binary solutions of Ca(NO3)2/NaNO3, Ca(NO 3) 2 /KNO 3, Mg(NO 3) 2 /NaNO3 , and Mg(NO 3) 2 / KNO3. A 200 mL volume of 1 mM Ca(NO3)2 and 1 mM NaNO3, Ca(NO3)2 and KNO3, Mg(NO3)2 and NaNO3, and Mg(NO3)2 and KNO3 mixed solutions were pumped into the

source cell, respectively. Simultaneously, 200 mL of deionized water was pumped into the receiving cell. The pulse potential (−1 to 1 V, pulse width 60 s) and 5 V cell voltage were applied to the ion transport system for 5 h. 3.4. Adsorption Properties of PPy/PSS and FeHCF− PPy/PSS Membranes. The adsorption properties were tested using a three-electrode system in 200 mL of 1 mM Ca(NO3)2 and Mg(NO3)2 solutions. The PPy/PSS and FeHCF−PPy/PSS membranes were the working electrodes. A −1 V constant potential versus Hg/Hg2Cl2 was respectively applied on the membrane for 100 min.

4. RESULTS AND DISCUSSION 4.1. Morphological Characterization. Figure 3 shows morphologies of the FeHCF film (a, b), FeHCF−PPy/PSS (c, e, g, i), and PPy/PSS (d, f, h, j) membranes prepared. One can see that the FeHCF film increased the surface roughness of the SSWM conductive substrate. When PPy/PSS was deposited on the FeHCF film, the FeHCF and the PPy/PSS could embed with each other on the interface, which would enhance the interfacial bonding between the conductive substrate and the PPy/PSS. Since FeHCF is a good electroactive material, the charge transport between the SSWM substrate and the PPy/ PSS layer could be enhanced with this good interfacial adherence due to the existence of FeHCF interlayer. This will be further proved by the latter current−potential response tests. Figure 3c−f shows SEM images of the cross sections of the FeHCF−PPy/PSS (c, e) and PPy/PSS (d, f) membranes. It is found that the addition of FeHCF obviously improved the interfacial bonding degree between the PPy/PSS layer and SSWM substrate, and a closer bonding was observed for the FeHCF−PPy/PSS membrane than for the PPy/PSS one. It is well-known that the adsorbing performance of conductive polymer membrane is strongly dependent on the surface morphology. 24 High surface roughness of the membrane is beneficial to improving the adsorption performance.24,25 Here, comparing with the morphology of PPy/PSS membrane with a dense and compact structure (Figure 3h,j), the FeHCF−PPy/PSS membrane had a more porous structure with typical “cauliflower-like” nodules (Figure 3g,i). It is considered that the different charge transfer ways and the surface roughness between the bare SSWM and the SSWM modified with FeHCF should contribute to the surface morphology difference between the FeHCF−PPy/PSS and PPy/PSS membranes. The porous structure of the FeHCF− PPy/PSS membrane should increase the contact area between the membrane and the solution and enhances ion incorporation into the membrane. 4.2. Current−Potential Response of the Membrane. Current−potential responses of the FeHCF−PPy/PSS and PPy/PSS membranes were tested in 0.1 M KNO3 solution. The pulse potential between −1 and 1 V with a pulse width of 250 s was applied to the membranes. If the membranes used here are too thick compared with the membranes used in the other paragraphs (electrodeposited PPy/PSS for 1 h), the difference of the response currents per unit mass between the FeHCF− PPy/PSS and PPy/PSS membranes is not obvious as shown in Figure S1 in the Supporting Information because of the strong ion diffusion resistance of the thick membranes. Therefore, we decreased the thickness of the membranes by shortening the electrodeposited time (electrodeposited PPy/PSS for 10 min) here. Figure 4 shows the current−potential response profiles of 6197


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Figure 4. Current−time curves of FeHCF−PPy/PSS and PPy/PSS membranes between −1 V reduced state and 1 V oxidized state in 0.1 M KNO3.

both membranes. One can see that there was a higher responsive current per unit mass for the FeHCF−PPy/PSS membrane than that for the PPy/PSS membrane when ±1 V step potential was applied to the membranes. It is demonstrated that the FeHCF−PPy/PSS membrane had higher electroactivity than the PPy/PSS one. The higher electroactivity of the FeHCF−PPy/PSS membrane should enhance the uptake/release of the cations and the continuously permselective separation by combining with an external electric field. 4.3. Effect of FeHCF Interlayer on the Removal Efficiencies of Ca2+ and Mg2+. To investigate the effect of FeHCF interlayer on the removal efficiency of the ESIP membrane for Ca2+ and Mg2+, the FeHCF−PPy/PSS and PPy/ PSS membranes with similar thicknesses were respectively assembled in the ion transport system. When the membrane was assembled, 200 mL of 1 mM Ca(NO3)2 and 1 mM Mg(NO3)2 solutions were respectively pumped into the source cell while 200 mL of deionized water was pumped into the receiving cell. The experiments were carried out using pulse potential (−2 to 2 V, pulse width 60 s) applied on the membrane and 5 V constant cell voltage applied to the ion transport system. Figure 5A shows the removal percentages of Ca2+ and Mg2+ with the FeHCF−PPy/PSS and PPy/PSS membranes for 5 h. Compared with the PPy/PSS membrane, the removal percentages of both Ca2+ and Mg2+ increased nearly 2 times by using the FeHCF−PPy/PSS membrane. This is due to the fact that the FeHCF−PPy/PSS membrane had a rougher surface than the PPy/PSS membrane (see the SEM images) so that it had a larger contact area with the solution, which facilitated the Ca2+ and Mg2+ incorporation into or ejection from the FeHCF−PPy/PSS membrane by modulating the redox states of the membrane. In addition, the good interfacial adherence of the FeHCF−PPy/PSS membrane profited from the FeHCF interlayer that improved the electroactivity of the membrane and could also increase the ion transfer rate across it by combining with the external electric field. Figure 5B illustrates the removal percentage/time profiles observed for Ca2+ with FeHCF−PPy/PSS and PPy/PSS membranes. As shown in Figure 5B, the removal percentages of Ca2+ with both membranes increased with the increase in removal time, and a linear relationship between the removal percentage and time was found within almost the whole Ca2+ removal process. Here, the FeHCF−PPy/PSS membrane showed higher removal efficiency than the PPy/PSS membrane. At the end of the tests, the removal percentage of Ca2+ with the FeHCF−PPy/PSS membrane reached 98.97% while that with

Figure 5. (A) Removal percentages of PPy/PSS and FeHCF−PPy/ PSS membranes for Ca2+ and Mg2+ with 5 V cell voltage and pulse potential (−2 to 2 V; pulse width 60 s) applied on the membranes for 5 h and (B) removal percentage/time profiles of FeHCF−PPy/PSS and PPy/PSS membranes for Ca2+ with 5 V cell voltage and pulse potential (−2 to 2 V; pulse width 60 s) applied on the membranes for 16 h.

the PPy/PSS membrane was only 69.17%. It is also demonstrated that the FeHCF−PPy/PSS membrane is more beneficial for removing hardness ions than the PPy/PSS membrane with this ion transport system. 4.4. Adsorption Properties of FeHCF−PPy/PSS and PPy/PSS Membranes for Ca2+ and Mg2+. The adsorption rate and capacity of the FeHCF−PPy/PSS and PPy/PSS membranes were tested in 200 mL of 1 mM Ca(NO3)2 and 1 mM Mg(NO3)2 solutions. A −1 V constant negative potential verse Hg/Hg2Cl2 was applied on the membranes for 100 min. Figure 6 shows the absorbance−time profiles of the FeHCF−

Figure 6. Time dependence of adsorption capacities of Ca2+ and Mg2+ into the FeHCF−PPy/PSS and PPy/PSS membranes in 200 mL of 1 mM Ca(NO3)2 and Mg(NO3)2 solutions. The membrane areas are the same.

PPy/PSS and PPy/PSS membranes for Ca2+ and Mg2+. One can see that the absorbance of both membranes increased with the increase of time, but the adsorption capacity and adsorption rate of the FeHCF−PPy/PSS membrane were about 2 times higher than those of the PPy/PSS membrane. Here, FeHCF in the FeHCF−PPy/PSS membrane only occupied 4.76% of the whole membrane mass. Thus, PPy/PSS should be the main 6198


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Figure 7. (A) Removal percentages of Ca2+ and Mg2+ with open circuit potential (OCP) and pulse potentials (−0.5 to 0.5 V, −1 to 1 V, and −2 to 2 V; pulse width 60 s) applied on the membrane and 5 V cell voltage for 5 h. (B) Potential differences of the source cell (US) when the low potentials were applied on the membrane and the receiving cell (UR) when the high potentials were applied with the different applied potentials.

Figure 8. (A) Removal percentages of Ca2+ and Mg2+ with 1, 3, and 5 V cell voltages and pulse potential (−1 to 1 V; pulse width 60 s) applied on the membrane for 5 h. (B) Potential differences of the source cell (US) when −1 V reduction potential was applied on the membrane and the receiving cell (UR) when 1 V oxidation potential was applied with the different cell voltages.

component for the adsorption of Ca2+ and Mg2+ in the membrane. The results also demonstrate that the PPy/PSS in the FeHCF−PPy/PSS membrane used for the adsorption of Ca2+ and Mg2+ had a higher utilization rate than that of the pure PPy/PSS membrane. This should be due to the high electroactivity and the porous structure of the FeHCF−PPy/ PSS membrane. 4.5. Effect of Pulse Potential on the Removal of Ca2+ and Mg2+. Here, the FeHCF−PPy/PSS membrane was used. A 5 V cell voltage was applied to the ion transport system, and open circuit potential (OCP) and pulse potentials (−0.5 to 0.5 V, −1 to 1 V, −2 to 2 V, pulse width 60 s) were respectively applied on the membrane for 5 h to investigate the effect of pulse potential on the removal percentages of Ca2+ and Mg2+. Figure 7A shows the removal percentages of Ca2+ and Mg2+ with different operating potentials applied to the membrane. As shown in Figure 7A, when the pulse potentials were applied to the membrane, the removal percentages of Ca2+ and Mg2+ increased significantly. This is because once the pulse potential was applied to the membrane, it triggered the electrochemically switched ion transport in the ESIP membrane, which enhanced Ca2+ and Mg2+ incorporation into the membrane from the source solution and ejection from the membrane into the receiving solution. In addition, with the increase of the applied pulse potential, the removal percentages of Ca2+ and Mg2+ increased accordingly. The incorporation rates of Ca2+ and Mg2+ increased as the low potential (reduced potential) of the applied pulse potential decreased. On the other hand, the ejection rate increased as the high potential (oxidized potential) increased. In addition, as shown in Figure 7B, when the low potential was applied to the membrane, with the decrease in the applied low potential, US (potential difference of the source

cell) increased and thus the migration rates of Ca2+ and Mg2+ from the bulk solution to the membrane surface (source cell) were increased; when the high potential was applied to the membrane, with the increase in the applied high potential, UR (potential difference of the receiving cell) increased and, as a result, the migration rates of Ca2+ and Mg2+ from the membrane surface to the bulk solution (receiving cell) were also increased. These results demonstrate that the increase of the pulse range of the potential applied on the membrane improved the removal efficiencies of Ca2+ and Mg2+. However, the ultrahigh oxidized potential applied to the CP membrane would also result in overoxidation of the membrane and make it lose electroactivity. 4.6. Effect of the External Electric Field on the Removal of Ca2+ and Mg2+. The tests with cell voltages of 1, 3, and 5 V applied to the ion transport system for 5 h were carried out to investigate the effect of the external electric field on the removal of Ca2+ and Mg2+. Here, a fixed pulse potential (−1 to 1 V, pulse width 60 s) was applied on the FeHCF−PPy/ PSS membrane. Figure 8A shows the removal percentages of Ca2+ and Mg2+ with different cell voltages. As shown in Figure 8A, with the increase of the cell voltage, the removal percentages of Ca2+ and Mg2+ increased accordingly. This is because, as shown in Figure 8B, when −1 V reduced potential was applied to the membrane, US increased with the increase of the cell voltages, and the migration rates of Ca2+ and Mg2+ from the bulk solution to the membrane surface (source cell) were enhanced; when 1 V oxidized potential was applied to the membrane, with the increase in the applied cell voltage, UR increased and the migration rates of Ca2+ and Mg2+ from the membrane surface to the bulk solution (receiving cell) were also increased. However, with the increase in cell voltage, water 6199


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Industrial & Engineering Chemistry Research splitting reaction could occur on the stainless steel electrodes in the two compartments, resulting in the decrease in the utilization ratio of electroenergy for providing the external electric field. Therefore, the appropriate cell voltage value should be obtained from the actual production and economic accounting. 4.7. Effect of Constant Potential on the Removal of Ca2+ and Mg2+. To investigate the effects of the constant potential applied across the ESIP membrane on the removal efficiencies of Ca2+ and Mg2+, the ion transport experiments were performed with the FeHCF−PPy/PSS membrane. Here, the cell voltage of 5 V was applied to the ion transport system and the constant potentials of 2, 1, 0, −1, and −2 V were respectively applied on the membrane for 5 h. Figure 9 shows

Figure 10. Ca2+ removal percentage/time profiles with pulse potential (−2 to 2 V; pulse width 60 s), constant negative potential (−2 V) applied on the membrane, and 5 V cell voltage for 60 h. The initial concentration of Ca2+ was 10 mmol·L−1.

increasing trend of removal rates slowed down when −2 V constant potential was applied; after 20 h test, only about 40% removal percentage was reached, and there was almost no significant increase in the removal rate of Ca2+. However, when the pulse potential was used, the removal percentage of Ca2+ continuously increased until 95.67% at the end of the experiment; the hardness of the treated water was decreased below 0.5 mM (50 ppm (CaCO3)). For the constant negative potential operation, the long-term negative potential applied to the membrane could result in the following reactions:1,35

Figure 9. Removal percentages of Ca2+ and Mg2+ with −2, −1, 0, 1, and 2 V constant potentials applied on the membrane and 5 V cell voltage for 5 h.

2H 2O + 2e− → H 2 + 2OH− 2OH− + Ca 2 + + CO2 → CaCO3 (scaling) + H 2O

the removal percentages of Ca2+ and Mg2+ with the different applied constant potentials. As shown in Figure 9, when the constant potential applied on the membrane became more negative, the removal percentages of Ca2+ and Mg2+ increased. It is obvious that the more negative potential was beneficial for increasing the transfer rates of Ca2+ and Mg2+ across the membrane. When the negative potential was applied to the membrane, the polymer membrane was reduced. As a result, Ca2+ or Mg2+ ions present in the source solution were incorporated into the membrane to maintain electrical charge balance. After a certain period of time, the membrane was loaded with Ca2+ or Mg2+ ions which compensated the negative charges present on the PSS dopant.33 Then, the high concentration of cations in the membrane was released to the receiving solution with the constant external electric field driving force. However, when the applied potential became more negative, the hydrogen evolution reaction could occur on the membrane surface, which will result in the pH value increasing on the membrane surface and membrane scaling. 4.8. Comparison of the Effects of Applied Pulse and Constant Potentials. The effects of pulse and constant potential applied on the ESIP membrane on the performance of the system were also compared using the FeHCF−PPy/PSS membrane. Here, 5 V cell voltage was applied to the ion transport system and pulse potential (−2 to 2 V, pulse width 60 s) and constant potential (−2 V) were respectively applied on the membrane for 60 h, and 200 mL of 10 mM Ca(NO3)2 solution was pumped into the source cell while 200 mL of deionized water was pumped into the receiving cell. Figure 10 shows the removal percentage/time profiles for the Ca2+ separation. As shown in Figure 10, at the beginning of the tests, both of the removal percentages of Ca2+ under the pulse and constant potential increased rapidly. After 5 h test, the

Therefore, the membrane scaling caused by the above reactions would hinder the ion transport across the membrane. However, the pulse potential applied on the membrane was expected to slow down the reactions by making the membrane oscillate periodically between oxidized and reduced states. Therefore, it is considered that the pulse potential applied on the membrane is more beneficial for the removal of Ca2+ and Mg2+ from hard water than the constant potential. With the pulse potential (−1 to 1 V, pulse width 60 s) and 5 V cell voltage applied to the ion transport system, the interference of Na+/K+ on the removal rates of Ca2+/Mg2+ in the water softening was investigated. The measurements were carried out with the binary solutions of Ca(NO3)2/NaNO3, Ca(NO3 ) 2 /KNO3 , Mg(NO3 ) 2/NaNO3, and Mg(NO3 ) 2 / KNO3. The Ca2+/Mg2+ ion removal percentages with the different binary mixed solutions are shown in Figure S2 in the Supporting Information. As shown in Figure 7 and Figure S2, there is no obvious decrease in the removal percentages of Ca2+ (33.95%) and Mg2+ (34.97%) in pure nitrate solution and of Ca2+ (30.96% to Na+, 31.67% to K+) and Mg2+ (29.97% to Na+, 30.50% to K+) in binary mixed solutions by using the same membrane and same ion concentration of Ca2+ and Mg2+ ions with the same operation potential and voltage. Thus, it is considered that the FeHCF−PPy/PSS ESIP membrane could be used for water softening process even in the mixed solution containing the K+/Na+ ions. Even so, the poor selectivity of this membrane (shown in Figure S2) is still an obvious limitation of the ESIP technique for water softening process, because the extra energy should be consumed for the transport of Na+ and K+ ions in the mixed solution. Some research shows that the selectivity of the membranes for Ca2+/Mg2+ ions can be effectively improved by grafting the other functional groups36 or adjusting their structural parameters.31 Hence, the main 6200


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Industrial & Engineering Chemistry Research research in our future work is to improve the selectivity of this membrane by using the proposed method and increase the energy efficiency of this technology for water softening. 4.9. Mechanism of Macroscopic Charge Balance. To understand how the anions or cations in the source and receiving solutions are compensated for the electroneutrality, the concentrations of Ca2+, NO3−, and Fe3+ and the pH values in the solutions were measured at the end of the removal experiment. Here, the FeHCF−PPy/PSS membrane was used for the removal experiment, 5 V cell voltage was applied to the ion transport system, pulse potential (−1 to 1 V, pulse width 60 s) was applied on the membrane for 5 h, and 200 mL of 1 mM Ca(NO3)2 solution was pumped into the source cell, while 200 mL of deionized water was pumped into the receiving cell. Table 1 provides a summary of the measured pHs and the

Figure 11. Removal percentage of FeHCF−PPy/PSS membrane of different repeating use times for Ca2+ with 5 V cell voltage and pulse potential (−2 to 2 V; pulse width 60 s) applied on the membranes for 5 h.

membrane be released into the receiving solution in time. In other words, the pulse potential applied on the membrane could relieve the membrane scaling in the removal process (illustrated in section 4.8). 4.11. Current Efficiency. The current efficiency of the above ion transport system was determined based on the transport experiment: pulse potential (−1 to 1 V, pulse width 60 s) was applied on the membrane and 5 V cell voltage was applied to the transport system for 5 h. A 200 mL volume of 1 mM Ca(NO3)2 solution was pumped into the source cell, while 200 mL of deionized water was pumped into the receiving cell. As shown in Figure 12, the ion transport system was divided

Table 1. Concentrations of Ca2+, Fe3+, and NO3− and pH in Both Solutions for the Removal Experiment Performed for 5 h source solution receiving solution

Ca2+/mM

Fe3+/mM

NO3−/mM

pH

0.6519 −

− −

0.9814 −

3.21 9.20

concentrations (mM) of the ions in the solutions. As shown in Table 1, the residual concentrations of Ca2+ and NO3− in the source solution were 0.6519 and 0.9814 mM, respectively. This demonstrated that the in situ potential-enhanced ion transport system based on the FeHCF−PPy/PSS membrane had good permselectivity. This could be due to the good cation exchange performance of the FeHCF−PPy/PSS membrane and the constant cell voltage of the external electric field. There were almost no Fe3+ ions were measured in the source solution. However, it is found that the source solution became acidic (pH 3.21). It is considered that almost no iron ions were released into the solution in the above removal experiment. Thus, the main electrode reaction occurring on the stainless steel anode should be

Figure 12. Schematic graphs of the reduction circuit and oxidation circuit of the ion transport system.

2H 2O → 4H+ + O2 + 4e−

The cations (H+) produced from the above reaction were released into the source solution and served to maintain the electroneutrality (the theoretical value of pH is 3.46). Moreover, almost no NO3− ions were measured in the receiving solution but the pH value of the receiving solution was measured to be 9.20. It is considered that the following electrode reaction occurred on the stainless steel cathode:

into reduction and oxidation circuits. Figure 13A shows the current changes of the reduction circuit when −1 V reduction potential was applied on the membrane. As shown in Figure 13A, current Icell ≈ −Iw; thus, it is considered that there was almost no current through the oxidation circuit when the membrane was reduced. Figure 13B shows the current changes of the oxidation circuit and the reduction potential when 1 V oxidation potential was applied on the membrane. As shown in Figure 13B, Icell ≈ 0 mA; thus, it is considered that there was almost no current through the reduction circuit when the membrane was oxidized. Here, the current efficiency (η) is defined as follows:

2H 2O + 2e− → 2OH− + H 2

The anions (OH−) produced from the reaction could serve to balance the Ca2+ released into the receiving solution. 4.10. Stability. To investigate the reusability of the FeHCF−PPy/PSS membrane in the above ion transport system, pulse potential (−2 to 2 V, pulse width 60 s) was applied on the FeHCF−PPy/PSS membrane and 5 V cell voltage was applied to the ion transport system for 5 h. A 200 mL volume of 1 mM Ca(NO3)2 solution was pumped into the source cell, while 200 mL of deionized water was pumped into the receiving cell. Figure 11 shows the removal percentage of FeHCF−PPy/PSS membrane of different repeating use times for Ca2+. With the increase of the repeating use time, the removal percentage of Ca2+ was not decreased obviously. Here, the pulse potential could help the Ca2+ adsorbed into the

η=

Ca 2 + removal amounts × z × F × 100% QR + Q O

QR and QO are the quantities of electricity through the reduction circuit and oxidation circuit, respectively. F is the Faraday constant of 96 500 C/mol, and z is the valence state of Ca2+. By calculations, QR = 38.07 C, QO = 5.49 C, and the Ca2+ removal amount was 0.68 × 10−4 mol. Therefore, the current efficiency η = 30.13%. Capacitive deionization (CDI) is an electrochemically controlled method for removing salt from 6201



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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00597. Current−time curves of FeHCF−PPy/PSS and PPy/PSS membranes (electrodeposited PPy/PSS for 1 h) between −1 V reduced state and 1 V oxidized state in 0.1 M KNO3; removal percentages of Ca2+, Mg2+, Na+, and K+ of the binary solutions of Ca(NO3)2/NaNO3, Ca(NO3)2/KNO3, Mg(NO3)2/NaNO3, and Mg(NO3)2/ KNO3 with pulse potential (−1 to 1 V, pulse width 60 s) and 5 V cell voltage for 5 h (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21276173, 21476156, 21306123) and JSPS KAKENHI Grant 15K06532, the Salt Science Research Foundation (No. 1403), Japan.

Figure 13. Current changes through (A) the reduction circuit when the membrane was reduced and (B) the reduction and oxidation circuits when the membrane was oxidized.

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aqueous solutions by taking advantage of the excess ions adsorbed in the electrical double layer region at an electrode− solution interface when the electrode was electrically charged by an external power supply.37 Compared with the traditional CDI technology (active carbon electrode),38−40 the method proposed in this study showed a similar current efficiency but it had a higher removal rate. In addition, from the calculations, QR ≫ QO; therefore, it is considered that electricity consumption of the reduction circuit (QR) resulted in the low current efficiency. It is possible that the oxygen produced from the stainless steel anode increased the dissolved oxygen concentration in the source solution, which resulted in the high current consumption in the reduction circuit when the membrane was reduced.22,25 Therefore, the current efficiency of the above ion transport system could be improved by decreasing the dissolved oxygen concentration of the source solution in the removal process.

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5. CONCLUSIONS An in situ potential-enhanced ion transport system coupling the ESIP membrane with an external electric field was designed as a water softening apparatus to remove Ca2+ and Mg2+ from aqueous solutions effectively. The FeHCF−PPy/PSS electroactive membranes were prepared via a two-step electrodeposition method and used as the ESIP membrane. The FeHCF interlayer improved the interfacial adherence, changed the surface morphology of the membrane, and increased the electroactivity of the membrane and its ion flux. The removal percentage of Ca2+ reached 95.67% by applying the pulse potential of ±2.0 V across the ESIP membrane under the assistance of the cell voltage of 5.0 V when the initial concentration of Ca2+ was 10 mM. The proposed ion transport system could also be used for the continuous separation of other target ions from dilute aqueous solution or industrial wastewater streams by selecting suitable ESIP membranes, which is of significant interest from a water treatment or environmental perspective. 6202


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An in Situ Potential-Enhanced Ion Transport System Based on FeHCF

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