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Effects of divalent cations on electrical membrane resistance in reverse electrodialysis for salinity power generation Yoontaek Oh, Yejin Jeong, Soo-Jin Han, Chan Soo Kim, Hanki Kim, JiHyung Han, Kyo-Sik Hwang, Namjo Jeong, Jin-Soo Park, and Soryong Chae Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03513 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018
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Effects of divalent cations on electrical membrane resistance in reverse
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electrodialysis for salinity power generation
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Yoontaek Oh1,†, Yejin Jeong2,†, Soo-Jin Han2, Chan-Soo Kim3, Han-Ki Kim3, Ji-Hyung Han3,
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Kyo-Sik Hwang3, Nam-Jo Jeong3, Jin-Soo Park2,*, and Soryong Chae1,*
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1 Department
of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio, U.S.A.
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2 Department
of Green Chemical Engineering, College of Engineering, Sangmyung University, 31 Sangmyungdae-
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gil, Dongnam-gu, Cheonan-si, Chungnam Province 31066, Republic of Korea
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3
Jeju Global Research Center, Korea Institute of Energy Research, Jeju -si, Republic of Korea.
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* Corresponding authors:
[email protected] (Dr. Soryong Chae) and
[email protected] (Dr. Jin-Soo. Park).
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† Equally
contributed.
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Abstract
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Reverse electrodialysis (RED) is an emerging technology that can generate electricity from the
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mixing of two water streams (i.e., the concentrated and the diluted streams) with salinity gradient.
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In RED, the higher salinity gradient between water streams yields the higher power production.
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Therefore, water sources containing high concentration of salts such as reverse osmosis brine,
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hypersaline lakes, and produced water from hydraulic fracturing could be considered as feed
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streams for enhancing energy production in RED. However, these water sources contain not only
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NaCl but also various multivalent ions, which are likely to increase electrical resistance of ion
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exchange membranes (IEMs) and potentially decrease power generation. In this study, we
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investigated the effects of divalent cations in the concentrated stream, including magnesium,
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calcium, and barium ions on electrical resistance of IEMs in static mode. The electrical resistance
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of IEMs in static mode was found to be correlated to power production in a bench-scale RED
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process during continuous operation. As a result, it was found that divalent cation with the smaller
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hydrated radius showed the higher electrical resistance in the static mode and the increased
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electrical resistance of cation exchange membrane (CEM) resulted in power reduction during the
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continuous operation of the bench-scale RED process.
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Keywords: Reverse electrodialysis; Ion Exchange Membranes; Divalent cations; Electrical
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resistance; Hydrated radius; Power generation.
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1. INTRODUCTION
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Reverse electrodialysis (RED) technology is an emerging technology that extracts energy from the
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mixing of two water streams of different salt concentrations. In RED, a stream with relatively
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higher salt concentration (i.e., the concentrated stream) and a stream with relatively lower salt
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concentration (i.e., the diluted stream) flow through alternately stacked cation exchange
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membranes (CEMs) and anion exchange membranes (AEMs). The salinity gradient between the
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two streams generates chemical potential difference over the ion exchange membranes (IEMs).
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Consequently, the chemical potential difference initiates transport of cations to a cathode through
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the CEMs, while anions are transported to an anode through the AEMs. The transport of ions is
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counterbalanced by electrons, which are supplied from the anode to the cathode via an external
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circuit generating electrical current.1–3
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Gibbs free energy of mixing is increased when the salinity gradient between the concentrated
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and the diluted streams is increased, and therefore, more energy can be harnessed. For example,
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natural seawater (Total dissolved solids, TDS = ~ 35,000 mg/L) and fresh water (TDS = ~ 1,000
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mg/L) are considered as common sources for RED applications. Through the mixing of natural
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seawater and fresh water, theoretically approximately 2 TW of energy could be generated.4,5
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To increase power generation from RED, highly saline water sources including reverse
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osmosis brine (TDS ~ 70 g/L), hypersaline lakes (TDS ~ 340 g/L), and produced water from
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hydraulic fracturing (TDS ~ 261 g/L), could be considered as the high concentration solution
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instead of natural seawater. Natural seawater and those hypersaline water sources include not only
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sodium but also various divalent cations such as magnesium and calcium, which may affect electric
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resistance of IEMs and performance of RED.1,6–9
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However, most of RED studies were performed using model seawater and fresh water mostly
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containing NaCl alone.10–21 Recent studies reported that the presence of multivalent ions
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(particularly Mg2+ and Ca2+) deteriorated the performance of RED. For instance, Post et al.22
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reported that the presence of MgSO4 in the diluted stream decreased the stack voltage as compared
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to NaCl alone in the feed due to the back-transport of multivalent ions against the activity gradient.
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Vermaas et al.
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using natural feed waters, and demonstrated that the uphill transport of multivalent ions, which is
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the same with the back-transport, hampered voltage of the RED stack resulted in the lower power
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density.24 Avic et al.25 revealed that CEM resistance was critically affected by Mg2+ concentration
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through electrical impedance spectroscopy (EIS) analysis. Hong et al.26 found that the presence of
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Mg2+ and SO42- in the seawater feed resulted in a decrease of maximum power density (MPD) by
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15-43 % as compared to NaCl alone. In a pilot-scale RED process, it was observed that the use of
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natural feed solutions containing other ions (especially Mg2+) caused approximately 50% decrease
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in the performance.27,28
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also discovered a negative effect of multivalent ions on RED performance by
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In addition to Mg2+ and Ca2+, produced water from hydraulic fracturing also contains Ba2+ with
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higher concentrations (4,700 mg/L) than natural seawater, which may adversely affect the
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electrical resistance of IEMs and power density of RED. However, there is a critical knowledge
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gap in understanding of the effects of Ba2+ on performance of IEMs and energy generation in RED.
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Recently, Zhu et al.29 investigated the effects of solution composition using 15 different single-
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salt solutions on the resistance of IEMs through EIS analysis, however, the experiments were
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limited in a batch mode at equilibrium and no further data are available regarding the effect of
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multivalent ions on performance of a RED process under dynamic operating conditions.
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The main objective of this study is to broaden our understanding of the effects of divalent
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cations (particularly Ba2+) on electrical resistance of IEMs and RED performance. We
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accomplished this by investigating changes in the electrical resistance of IEMs on batch mode with
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solutions containing divalent cations such as Mg2+, Ca2+, or Ba2+, and comparing the results with
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the performance of a bench-scale RED process during continuous operations under dynamic
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condition. As we were interested in divalent cations that were transported solely by the salinity
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gradient, we excluded the effect of the uphill transport in the current study.
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2. MATERIALS AND METHODS
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2.1. IEMs Preparation. Both CEMs (CEM-Type I, thickness = 125 µm) and AEMs (AEM-
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Type I, thickness = 124 µm) were obtained from Fujifilm (Fujifilm Manufacturing Europe B.V.).
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All membranes were preconditioned in 0.513 M NaCl solution that represents average salt
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concentration of sea water for 24 h before use.
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2.2. Electrical Resistance of IEMs at Equilibrium. Electrical resistance of IEMs was
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determined by measuring impedance in a clip-cell (two electrodes) system as described in the
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previous study.3 The impedance was measured using a potentiostat (SP-150, Bio-Logic Science
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Instruments) with alternating current (AC) in scan range of 1 MHz to 1 Hz with signal amplitude
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of 10 mV for one minute at 20±1 °C (Figure 1). The electrical resistance was obtained at zero
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phase angle. Firstly, the electrical resistance of a salt solution with 0.513 M NaCl (Rs) was
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measured in the absence of IEMs. Secondly, the preconditioned CEM or AEM (2 cm × 2 cm) was
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placed between the two electrodes filled with the salt solution to measure a combined electrical
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resistance of the solution and the membrane (Rs+m). Finally, the electrical resistance of the
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membrane (Rm) calculated using Equation (1).
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𝑅𝑚 = (𝑅𝑠 + 𝑚 ― 𝑅𝑠) × 𝐴
(1)
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where Rm is the electrical area resistance of the membrane (‧cm2), Rs+m is the combined electrical
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resistance of the salt solution with the membrane (), Rs is the electrical resistance of the salt
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solution (), and A is the effective area of the membrane, which was 0.785 cm2.
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To measure the electrical resistance of IEMs with divalent cations, the preconditioned IEMs
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were immersed in mixed solutions for up to 7 days with 0.4617 M NaCl + 0.02565 M MeCl2 (Me
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is Mg2+, Ca2+, or Ba2+). Then the IEMs were moved to the clip cell system filled with the mixed
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solution in which the IEMs were immersed. The electrical resistance of IEMs at Day zero indicates
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that the IEMs were only immersed in 0.513 M NaCl solution. It is instructive to note that the
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concentration of the mixed solution has the identical normality to the pure NaCl solution implying
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that the salinity gradient was maintained.
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Figure 1. Experimental set-up for the measurement of electrical resistance of IEMs.
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2.3. A Bench-scale RED System. As shown in Figure 2, a bench-scale RED stack
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consisting of three CEMs and two AEMs has been used for continuous energy generation from
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salinity gradient. The total effective area of IEMs was 78.5 cm2. Polytetrafluoroethylene (PTFE)
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gaskets of 0.2 mm thickness were inserted between the IEMs and mesh-type spacers of 0.2 mm
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thickness were also placed between the IEMs. The electrodes are made of 50 mm diameter titanium
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mesh with platinum coating (Sung Wing Technology Co., Hong Kong, China).
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In this study, two different types of electrode rinse solution (ERS) were used. The first ERS
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was composed of 0.05 M K3[Fe(CN)6] (Junsei Chemical Co.), 0.05 M K4[Fe(CN)6] (Junsei
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Chemical Co.), and 1 M Na2SO4 (Junsei Chemical Co.). The second ERS was composed of 0.05
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M K3[Fe(CN)6], 0.05 M K4[Fe(CN)6], and 1 M NaCl. The initial volume of ERS was 100 mL.
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Figure 2. Schematic diagram of a bench-scale RED system.
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2.4. Continuous Operation of the Bench-scale RED System. Two synthetic feed
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streams (i.e., fresh water with 0.017 M NaCl and seawater with either 0.513 M NaCl or 0.4617 M
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NaCl + 0.02565 M MeCl2) were fed into the RED system at 5 mL/min. The ERS was circulated
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through the RED system at 50 mL/min in all experiments (Figure 3). The produced chemical
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potential was measured using the potentiostat connected to the RED stack.
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The protocol for all RED experiments comprised these sequential steps: 1) The RED system
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was stabilized and equilibrated with 0.017 M NaCl fresh water and 0.513 M NaCl seawater for at
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least one hour while open circuit voltage (OCV) was measured for one minute followed by the
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power density measurement using a linear sweep voltammetry (LSV) with a sweep rate of 40 mV/s.
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These were repeated every five minutes throughout all experiments. 2) When stable OCV and
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power density were attained, the RED experiment was initiated by supplying a fresh ERS to the
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system. 3) After 20 minutes, seawater with 0.513 M NaCl was switched to seawater with 0.4617
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M NaCl + 0.02565 MeCl2, and the run continued for an additional 50 minutes.
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OCV was measured every three seconds for one minute and the average value was determined.
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A maximum power was calculated from the multiplication of voltage and current obtained from
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LSV and the MPD is the maximum power divided by the total effective membrane area. A linear
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relationship between current and voltage during the LSV measurement is shown in Figure S1
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(Supporting Information). The results showed that the linear relationship was maintained in the
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presence of multivalent ions, which demonstrated that the calculation for the MPD was rational.
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The electrical resistance of the RED stack (Rstack) can be derived from the OCV and MPD using
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the Equation (2)24. 𝑂𝐶𝑉2
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𝑃𝑚𝑎𝑥 = 4𝑁𝑚𝑅𝑠𝑡𝑎𝑐𝑘
(2)
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where Pmax is the maximum power density (W/m2), OCV is the open circuit voltage (V), Nm is the
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number of membranes (-), and Rstack is the electrical area resistance of the RED stack (·m2 stack
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cross-sectional area).
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Figure 3. Schematic diagram of a bench-scale RED system.
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2.5. Characterization of Fouling of IEMs. At the end of the experiment, the IEMs were
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removed from the RED system and gently rinsed with deionized (DI) water followed by overnight
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drying at 40 °C. Then foulants on the membrane surface were characterized using a scanning
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electron microscope (SEM) coupled with energy dispersive x-ray spectroscopy (EDX) (Scios
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DualBeam, Thermo fisher scientific).
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3. Results and Discussion
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3.1. Effects of Various Divalent Cations on Electrical Resistance of the CEM at
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Equilibrium. Figure 4(a) shows the electrical resistance of CEM and AEM that were immersed
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in different salt solutions for 7 days. The change in the electrical resistance of AEM was negligible
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but the electrical resistance of the CEM significantly increased in the presence of Mg2+ or Ca2+. As
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shown in Table 1, the CEM’s electrical resistance in the presence of Mg2+ increased to 146% after
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7 days compared to the initial electrical resistance. Due to double coulombic force between the
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fixed functional groups such as sulfonic group and divalent cations in the CEM, the affinity
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between Mg2+ and the ion exchanger in the CEM decreased the mobility of Mg2+ through the CEM
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and ultimately increased the electrical resistance. However, the AEM was not significantly
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influenced by the presence of Mg2+, as the AEM possibly only interacts with the anion while
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positively charged functional groups in the membrane repel the cation.
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Figure 4. Electrical resistance of CEM and AEM in 0.4617 M NaCl + 0.02565 M MgCl2
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solution (a) or 0.4617 M NaCl + 0.02565 M CaCl2 solution (b).
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The identical procedure was repeated to study the effects of Ca2+ on the electrical resistance of
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IEMs using a mixture of 0.4617 M NaCl + 0.02565 M CaCl2. Figure 4(b) shows that the electrical
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resistance of the CEM significantly increased when Ca2+ was added in the solution. The electrical
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resistance of the CEM was increased to 170% in the presence of Ca2+ after 7 days (Table 1). This
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is possibly because the ion exchanger in the CEM tends to prefer 1) the counter ion of higher
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valence, 2) the counter ion with the smaller (solvated) equivalent volume, 3) the counter ion with
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the greater polarizability, 4) the counter ion that interacts more strongly with the fixed ionic groups
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or with the matrix, and 5) the counter ion which participates least in complex formation with the
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co-ion.30
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Table 1. Electrical Resistance of the CEM in the Presence of Mg2+ or Ca2+
Time
0.4617 M NaCl + 0.02565 M MgCl2
0.4617 M NaCl + 0.02565 M CaCl2
(day)
Resistance (‧cm2)
Rt/R0*
Resistance (‧cm2)
Rt/R0*
0
1.90
100%
1.93
100%
1
2.66
140%
3.25
168%
2
2.83
149%
3.42
177%
4
2.66
140%
3.36
174%
7
2.77
146%
3.29
170%
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*Rt denotes electrical resistance at time = t, and R0 denotes electrical resistance at time zero.
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For example, if two ions have the identical conditions except for the solvated equivalent
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volume, the one with the smaller solvated volume will be more selectively attracted than the other
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by the CEM. As shown in Table 2, the hydrated radius of Mg2+ is larger than that of Ca2+ although
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the ionic radius of Mg2+ is smaller than that of Ca2+. In other words, Ca2+ has a smaller solvated
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volume than Mg2+.30,31 Therefore, the fixed functional groups in the CEM have a stronger affinity
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with Ca2+ than Mg2+ at the identical conditions, such as pH, ionic strength and temperature. To
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verify the hypothesis, further experiments have been done using Ba2+ that has smaller solvated
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volume than Mg2+ and Ca2+. Along with magnesium and calcium, barium is categorized as alkaline
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earth metals. Ba2+ has the same charge valence but different hydrated radius (Table 2). Thus, this
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ion is a good candidate to verify the relationship between the hydrated radius of divalent cations
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and the electrical resistance of the CEM.
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Table 2. Characteristics of Various Cations Used in This Study Ion
Molar conductivity* (mS m2 mol-1)
Ionic radius* (Å)
Hydration number**
Radius of hydrated ion*** (Å)
Na+
5.011
0.98
1.5
3.58
Mg2+
10.6
0.66
7.0
4.28
Ca2+
11.90
0.99
5.2
4.12
Ba2+ 12.72 1.35 2.0 4.04 * Molar conductivity and ionic radius were converted into the given unit using information adopted from Oxtoby et al. (2016)32.
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** Hydration number was adopted from Glueckauf and Kitt (1955)31. *** Radius of hydrated ion was adopted from Dove et al. (1997)33.
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Another mixed solution with the identical ionic strength was prepared (i.e., 0.4617 M NaCl +
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0.02565 M BaCl2) and the identical experimental procedure was repeated to measure the electrical
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resistance of the CEM as described above. The electrical resistances of the CEM with different
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alkaline earth metals after 7 days are shown in Figure 5(a). As shown in this figure, Ba2+ resulted
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in the highest increase of the electrical resistance of the CEM followed by Ca2+ and Mg2+. Based
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on these results, the relationship between the hydrated radius of divalent cations and the electrical
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resistance of the CEM was plotted in Figure 5(b). This figure shows a monotonic inverse
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relationship between the electrical resistance of the CEM and the hydrated radius of the divalent
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cations. As the hydrated radius of the divalent cations increased, the electrical resistance of the
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CEM decreased. This trend is in good agreement with the previous study on affinity of ion
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exchange resin for various cations, showing the higher affinity of resin for Ba2+ compared to
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Ca2+.34 We also performed swelling degree test and found that the swelling degrees with different
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salt solutions were almost the same (Figure S2). The maximum difference among the swelling
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degrees was less than 3% implying that physical effect was insignificant.
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Figure 5. Electrical resistance of the CEM with various divalent cations after 7 days (a), and the
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relationship between electrical resistance of the CEM and hydrated radius of various divalent
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cations (b). All measurements were performed in triplicate and standard deviations are included.
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3.2. Effects of Divalent Cations on RED Performance during Continuous
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Operation. The increase in electrical resistance of IEMs in the presence of divalent cations was
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demonstrated under static condition. However, it is necessary to investigate the effects divalent
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cations on the RED process under dynamic operating conditions because feed solutions were
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continuously fed into the system and accordingly ions transported through IEMs consecutively.
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Moreover, it is also important to explore how much the electrical resistance of IEMs influences on
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the Rstack.
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Figure 6 shows the normalized OCV, MPD, and Rstack in the bench-scale RED system in the
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presence of divalent cations such as Mg2+, Ca2+, or Ba2+. Note that the OCV, MPD, and Rstack are
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normalized by the last value that is measured right before switching seawater from the pure NaCl
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solution to the mixed solution. By doing this, the effects of divalent cations on the RED
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performance were solely considered.
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OCV indicates maximum potential throughout the RED process and one of the basic
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parameters to determine gross power density. Figure 6(a) exhibits that the normalized OCV
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fluctuated in the presence of divalent cations. Initially, the CEMs were exposed to 0.513 M NaCl
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solution for at least one hour for stabilization in a bench-scale RED process. Then the 0.513 M
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NaCl solution was switched to the mixed solutions containing one of cations as described in the
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section 2.4. The increase in the normalized OCV within the first 10 minutes might be attributed to
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temporal increase in salinity gradient between seawater and fresh water streams before equilibrium.
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We found that this effect was being negligible after 10 minutes. After 50-minutes operation, the
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overall normalized OCV decreased by approximately 1% in the presence of Mg2+ and Ca2+,
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whereas the decrease in the normalized OCV by Ba2+ was more than 4%.
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As shown in Figure 6(b), it is clearer that the presence of Mg2+ and Ca2+ reduced the normalized
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MPD by 8% and 12%, respectively (This trend between Mg2+ and Ca2+ was in good agreement
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with the results from the section 3.1), whereas the normalized MPD decreased by 79% in the
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presence of Ba2+, which showed the abnormal behavior more apparently. The effects of divalent
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cations on the normalized MPD was more influential than the normalized OCV.
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Figure 6. RED performance during continuous operation with various divalent cations. The ERS
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containing Na2SO4 as an electrolyte was used. (a) The normalized OCV, (b) the normalized
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MPD, and (c) the normalized Rstack. Fresh water contains 0.017 M NaCl and seawater contains
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0.4617 M NaCl + 0.02565 M MeCl2 (Me: Mg2+, Ca2+, or Ba2+). Re-scaled plot of (b) for 0 < the
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normalized power density < 1.1 and (c) for 0 < the normalized Rstack < 12 are shown in the inset.
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Note that OCV, MPD, and Rstack are normalized by the last value that is measured right before
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switching seawater. All measurements were performed in triplicate and standard deviations are
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included.
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3.3. Inorganic Fouling on CEMs during the RED Operation. As shown in Figure 6(c),
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the normalized Rstack was higher in the presence of Ca2+ than in the presence of Mg2+. When the
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concentrated stream containing Ba2+ was introduced into the RED process, the normalized OCV
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and the normalized MPD were significantly decreased, and the normalized Rstack was increased to
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800% whereas the normalized Rstack with seawater containing the other divalent cations was
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increased to maximum 111% compared to NaCl alone in the feed (Data shown in the small box
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inside Figure 6(c)). Moreover, the large standard deviation implies that the test results were
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inconsistent resulted from uncontrollable factors. There was no significant change in temperature,
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pH, and conductivity through all experiments. The Nernst equation for the OCV calculation cannot
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explain this phenomenon. This abrupt drop in performance rather resembles fouling phenomena
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in membrane processes.
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After the experiments, it was found that precipitates were formed on both outer CEMs surface
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that met the first type ERS particularly in the presence of Ba2+. Electrodes and spacers that were
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placed between the outer membranes and the electrodes were also coated with the precipitates.
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However, the CEM in the middle and two AEMs were visually clean. In addition, in the presence
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of Mg2+ or Ca2+, no perceivable precipitate was observed with all IEMs, spacers, and electrodes.
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The SEM images in Figure 7 confirmed that precipitates were formed only on the outer CEMs
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surface in the presence of Ba2+. It seems that the divalent cations transported to the ERS through
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the outer CEM, then contact with SO42-, and formed precipitates on membrane surface (Equation
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(3)).35 The EDX spectrum clearly showed intensive peaks for barium indicating that the
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precipitates were crystals of BaSO4 (Figure 8). However, the water solubility of BaSO4 (0.00031
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g/100 g H2O at 20oC) is much lower than MgSO4 (25.1 g/100 g H2O at 20oC) and CaSO4 (0.201
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g/100 g H2O at 20oC).36 As a result, relatively large amounts of the Ba2+-associated precipitates
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could be accumulated on the membrane surface precipitate compared to the other divalent
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precipitates.
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𝐵𝑎𝐶𝑙2(𝑎𝑞) + 𝑁𝑎2𝑆𝑂4(𝑎𝑞)→𝐵𝑎𝑆𝑂4(𝑠) + 2𝑁𝑎𝐶𝑙(𝑎𝑞)
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Figure 7. SEM images of CEMs surface that met the ERS containing Na2SO4 as an electrolyte.
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Seawater containing Mg2+ (a), Ca2+ (b), and Ba2+ (c).
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Figure 8. EDX analysis of CEMs surface that met the ERS containing Na2SO4 as an electrolyte.
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Seawater containing Mg2+ (a), Ca2+ (b), and Ba2+ (c).
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3.4. Effects of Various Divalent Cations on RED Performance without the
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Inorganic Fouling. To eliminate membrane fouling by the Ba2+-associated precipitates, the
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identical experiments were repeated with the second type of ERS that contained 1 M NaCl as an
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electrolyte instead of 1 M Na2SO4. It is instructive to note that the fouling above mentioned could
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be avoided by using NaCl, but toxic gas, such as chlorine gas, could be generated, and moreover,
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ClO-, ClO3-, and ClO4- could be formed during the RED operation, which was the main reason we
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selected Na2SO4 initially.37
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Figure 9 shows that the normalized OCV and the normalized MPD were decreased in the
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presence of divalent cations. There was no abrupt change in the normalized OCV and the
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normalized MPD implying that no fouling took place during the operation. Ba2+ in the concentrated
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feed stream resulted in the most significant reduction in the normalized OCV and the normalized
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MPD followed by Ca2+, and Mg2+. These results also demonstrate that the multivalent ion effects
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are more critical impacts on the normalized MPD, as the normalized OCV decreased by only 2.5%
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for Ba2+ but the normalized MPD decreased by 17% for the same ion. This means that an actual
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RED performance can be varied with the ion composition of feed solutions even though they give
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practically the same OCV. The overall RED performances are slightly abated probably due to the
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low normality of 1 M NaCl compared to 1 M Na2SO4. In summary, this change is correlated with
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the results as described in Section 3.1. It suggests that the statically obtained electrical resistance
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of IEMs is practically a good indicator to estimate the RED performance, specifically MPD and
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Rstack.
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Figure 9. RED performance during continuous operation with various divalent cations. The ERS
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containing NaCl as an electrolyte was used. (a) The normalized OCV and (b) the normalized
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MPD Fresh water contains 0.017 M NaCl and seawater contains either 0.513 M NaCl or 0.4617
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M NaCl + 0.02565 M MeCl2 (Me: Mg2+, Ca2+, or Ba2+). Note that OCV and MPD are
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normalized by the last value that is measured right before switching seawater. All measurements
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were performed in triplicate and standard deviations are included.
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333
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4. CONCLUSIONS In this study, we have focused on the effects of divalent cations, such as Mg2+, Ca2+, and Ba2+
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on the electrical resistance of IEMs and performance of a bench-scale RED process. At equilibrium,
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it was revealed that the electrical resistance of the CEM was significantly influenced by the
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presence of divalent cations in the concentrated feed solution, whereas the electrical resistance of
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the AEM was not significantly affected by the divalent cations. It was also found that the electrical
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resistance of the CEM increased with decreasing the hydrated radius of divalent cations (i.e., Mg2+
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> Ca2+ > Ba2+) due to the affinity between the fixed functional groups in the CEM and the counter-
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ions passing through the CEM. The effects of divalent cations on the performance of RED were
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also confirmed during continuous operation of a bench-scale RED process. The presence of
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divalent cations in the feed solution reduced the normalized OCV and the normalized MPD, and
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increased the normalized Rstack. The change in the RED performance is correlated with the results
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obtained from batch mode suggesting that the statically obtained electrical resistance of IEMs can
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be used as a predictor to estimate the RED performance, specifically MPD and Rstack. The presence
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of Ba2+ in the feed stream along with SO42- in ERS formed Ba2+-associated precipitates on the
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CEM surface resulted in significant drops in the normalized OCV and the normalized MPD. By
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replacing Na2SO4 in ERS with NaCl, the fouling was successfully mitigated.
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■ ASSOCIATED CONTENT
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Supporting Information
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Current-Voltage curve during LSV, Swelling degree of IEMs.
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■ NOTE
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The authors declare no competing financial interest.
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■ ACKNOWLEDGEMENTS
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This work was conducted under the framework of the Research and Development Program of the
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Korean Institute of Energy Research (KIER) (Project Number: B8-2441).
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■ REFERENCES
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Ba2+ in the feed solution formed precipitates on the surface of the cation exchange membrane resulted in significant drops in performance of a reverse electrodialysis process.
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