Effects of divalent cations on electrical membrane resistance in

Kyo-Sik Hwang3, Nam-Jo Jeong3, Jin-Soo Park2,*, and Soryong Chae1,* ... [email protected] (Dr. Soryong Chae) and [email protected] (Dr. Jin-Soo...
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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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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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