Effects of divalent cations on electrical membrane resistance in

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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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Industrial & Engineering Chemistry Research

1

Effects of divalent cations on electrical membrane resistance in reverse

2

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.

8

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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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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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.

20

Therefore, water sources containing high concentration of salts such as reverse osmosis brine,

21

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

23

NaCl but also various multivalent ions, which are likely to increase electrical resistance of ion

24

exchange membranes (IEMs) and potentially decrease power generation. In this study, we

25

investigated the effects of divalent cations in the concentrated stream, including magnesium,

26

calcium, and barium ions on electrical resistance of IEMs in static mode. The electrical resistance

27

of IEMs in static mode was found to be correlated to power production in a bench-scale RED

28

process during continuous operation. As a result, it was found that divalent cation with the smaller

29

hydrated radius showed the higher electrical resistance in the static mode and the increased

30

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

45

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

49

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



57

However, most of RED studies were performed using model seawater and fresh water mostly

58

containing NaCl alone.10–21 Recent studies reported that the presence of multivalent ions

59

(particularly Mg2+ and Ca2+) deteriorated the performance of RED. For instance, Post et al.22

60

reported that the presence of MgSO4 in the diluted stream decreased the stack voltage as compared

61

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

66

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

68

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

70

in the performance.27,28

23

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

72

higher concentrations (4,700 mg/L) than natural seawater, which may adversely affect the

73

electrical resistance of IEMs and power density of RED. However, there is a critical knowledge

74

gap in understanding of the effects of Ba2+ on performance of IEMs and energy generation in RED.

75

Recently, Zhu et al.29 investigated the effects of solution composition using 15 different single-

76

salt solutions on the resistance of IEMs through EIS analysis, however, the experiments were

77

limited in a batch mode at equilibrium and no further data are available regarding the effect of

78

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

80

cations (particularly Ba2+) on electrical resistance of IEMs and RED performance. We

81

accomplished this by investigating changes in the electrical resistance of IEMs on batch mode with

82

solutions containing divalent cations such as Mg2+, Ca2+, or Ba2+, and comparing the results with

83

the performance of a bench-scale RED process during continuous operations under dynamic

84

condition. As we were interested in divalent cations that were transported solely by the salinity

85

gradient, we excluded the effect of the uphill transport in the current study.

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2. MATERIALS AND METHODS

88

2.1. IEMs Preparation. Both CEMs (CEM-Type I, thickness = 125 µm) and AEMs (AEM-

89

Type I, thickness = 124 µm) were obtained from Fujifilm (Fujifilm Manufacturing Europe B.V.).

90

All membranes were preconditioned in 0.513 M NaCl solution that represents average salt

91

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

95

previous study.3 The impedance was measured using a potentiostat (SP-150, Bio-Logic Science

96

Instruments) with alternating current (AC) in scan range of 1 MHz to 1 Hz with signal amplitude

97

of 10 mV for one minute at 20±1 °C (Figure 1). The electrical resistance was obtained at zero

98

phase angle. Firstly, the electrical resistance of a salt solution with 0.513 M NaCl (Rs) was



99

measured in the absence of IEMs. Secondly, the preconditioned CEM or AEM (2 cm × 2 cm) was

100

placed between the two electrodes filled with the salt solution to measure a combined electrical

101

resistance of the solution and the membrane (Rs+m). Finally, the electrical resistance of the

102

membrane (Rm) calculated using Equation (1).

103

𝑅𝑚 = (𝑅𝑠 + 𝑚 ― 𝑅𝑠) × 𝐴

(1)

104

where Rm is the electrical area resistance of the membrane (‧cm2), Rs+m is the combined electrical

105

resistance of the salt solution with the membrane (), Rs is the electrical resistance of the salt

106

solution (), and A is the effective area of the membrane, which was 0.785 cm2.

107

To measure the electrical resistance of IEMs with divalent cations, the preconditioned IEMs

108

were immersed in mixed solutions for up to 7 days with 0.4617 M NaCl + 0.02565 M MeCl2 (Me

109

is Mg2+, Ca2+, or Ba2+). Then the IEMs were moved to the clip cell system filled with the mixed

110

solution in which the IEMs were immersed. The electrical resistance of IEMs at Day zero indicates

111

that the IEMs were only immersed in 0.513 M NaCl solution. It is instructive to note that the

112

concentration of the mixed solution has the identical normality to the pure NaCl solution implying

113

that the salinity gradient was maintained.


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Figure 1. Experimental set-up for the measurement of electrical resistance of IEMs.

116

117

2.3. A Bench-scale RED System. As shown in Figure 2, a bench-scale RED stack

118

consisting of three CEMs and two AEMs has been used for continuous energy generation from

119

salinity gradient. The total effective area of IEMs was 78.5 cm2. Polytetrafluoroethylene (PTFE)

120

gaskets of 0.2 mm thickness were inserted between the IEMs and mesh-type spacers of 0.2 mm

121

thickness were also placed between the IEMs. The electrodes are made of 50 mm diameter titanium

122

mesh with platinum coating (Sung Wing Technology Co., Hong Kong, China).

123

In this study, two different types of electrode rinse solution (ERS) were used. The first ERS

124

was composed of 0.05 M K3[Fe(CN)6] (Junsei Chemical Co.), 0.05 M K4[Fe(CN)6] (Junsei



125

Chemical Co.), and 1 M Na2SO4 (Junsei Chemical Co.). The second ERS was composed of 0.05

126

M K3[Fe(CN)6], 0.05 M K4[Fe(CN)6], and 1 M NaCl. The initial volume of ERS was 100 mL.

127

128 129

Figure 2. Schematic diagram of a bench-scale RED system.

130

131

2.4. Continuous Operation of the Bench-scale RED System. Two synthetic feed

132

streams (i.e., fresh water with 0.017 M NaCl and seawater with either 0.513 M NaCl or 0.4617 M

133

NaCl + 0.02565 M MeCl2) were fed into the RED system at 5 mL/min. The ERS was circulated

134

through the RED system at 50 mL/min in all experiments (Figure 3). The produced chemical

135

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

137

was stabilized and equilibrated with 0.017 M NaCl fresh water and 0.513 M NaCl seawater for at

138

least one hour while open circuit voltage (OCV) was measured for one minute followed by the

139

power density measurement using a linear sweep voltammetry (LSV) with a sweep rate of 40 mV/s.

140

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

143

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.

145

A maximum power was calculated from the multiplication of voltage and current obtained from

146

LSV and the MPD is the maximum power divided by the total effective membrane area. A linear

147

relationship between current and voltage during the LSV measurement is shown in Figure S1

148

(Supporting Information). The results showed that the linear relationship was maintained in the

149

presence of multivalent ions, which demonstrated that the calculation for the MPD was rational.

150

The electrical resistance of the RED stack (Rstack) can be derived from the OCV and MPD using

151

the Equation (2)24. 𝑂𝐶𝑉2

152

𝑃𝑚𝑎𝑥 = 4𝑁𝑚𝑅𝑠𝑡𝑎𝑐𝑘

(2)

153

where Pmax is the maximum power density (W/m2), OCV is the open circuit voltage (V), Nm is the

154

number of membranes (-), and Rstack is the electrical area resistance of the RED stack (·m2 stack

155

cross-sectional area).



156 157

Figure 3. Schematic diagram of a bench-scale RED system.

158

159

2.5. Characterization of Fouling of IEMs. At the end of the experiment, the IEMs were

160

removed from the RED system and gently rinsed with deionized (DI) water followed by overnight

161

drying at 40 °C. Then foulants on the membrane surface were characterized using a scanning

162

electron microscope (SEM) coupled with energy dispersive x-ray spectroscopy (EDX) (Scios

163

DualBeam, Thermo fisher scientific).

164

165

3. Results and Discussion

166

3.1. Effects of Various Divalent Cations on Electrical Resistance of the CEM at

167

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

169

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

171

7 days compared to the initial electrical resistance. Due to double coulombic force between the

172

fixed functional groups such as sulfonic group and divalent cations in the CEM, the affinity

173

between Mg2+ and the ion exchanger in the CEM decreased the mobility of Mg2+ through the CEM

174

and ultimately increased the electrical resistance. However, the AEM was not significantly

175

influenced by the presence of Mg2+, as the AEM possibly only interacts with the anion while

176

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).

182

183

The identical procedure was repeated to study the effects of Ca2+ on the electrical resistance of

184

IEMs using a mixture of 0.4617 M NaCl + 0.02565 M CaCl2. Figure 4(b) shows that the electrical

185

resistance of the CEM significantly increased when Ca2+ was added in the solution. The electrical

186

resistance of the CEM was increased to 170% in the presence of Ca2+ after 7 days (Table 1). This

187

is possibly because the ion exchanger in the CEM tends to prefer 1) the counter ion of higher

188

valence, 2) the counter ion with the smaller (solvated) equivalent volume, 3) the counter ion with

189

the greater polarizability, 4) the counter ion that interacts more strongly with the fixed ionic groups

190

or with the matrix, and 5) the counter ion which participates least in complex formation with the

191

co-ion.30

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193

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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194


*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

197

volume, the one with the smaller solvated volume will be more selectively attracted than the other

198

by the CEM. As shown in Table 2, the hydrated radius of Mg2+ is larger than that of Ca2+ although

199

the ionic radius of Mg2+ is smaller than that of Ca2+. In other words, Ca2+ has a smaller solvated

200

volume than Mg2+.30,31 Therefore, the fixed functional groups in the CEM have a stronger affinity

201

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

203

volume than Mg2+ and Ca2+. Along with magnesium and calcium, barium is categorized as alkaline

204

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

206

and the electrical resistance of the CEM.

207 208

209 210

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.



211 212

** Hydration number was adopted from Glueckauf and Kitt (1955)31. *** Radius of hydrated ion was adopted from Dove et al. (1997)33.

213

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Another mixed solution with the identical ionic strength was prepared (i.e., 0.4617 M NaCl +

215

0.02565 M BaCl2) and the identical experimental procedure was repeated to measure the electrical

216

resistance of the CEM as described above. The electrical resistances of the CEM with different

217

alkaline earth metals after 7 days are shown in Figure 5(a). As shown in this figure, Ba2+ resulted

218

in the highest increase of the electrical resistance of the CEM followed by Ca2+ and Mg2+. Based

219

on these results, the relationship between the hydrated radius of divalent cations and the electrical

220

resistance of the CEM was plotted in Figure 5(b). This figure shows a monotonic inverse

221

relationship between the electrical resistance of the CEM and the hydrated radius of the divalent

222

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

224

exchange resin for various cations, showing the higher affinity of resin for Ba2+ compared to

225

Ca2+.34 We also performed swelling degree test and found that the swelling degrees with different

226

salt solutions were almost the same (Figure S2). The maximum difference among the swelling

227

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

231

relationship between electrical resistance of the CEM and hydrated radius of various divalent

232

cations (b). All measurements were performed in triplicate and standard deviations are included.

233

234

3.2. Effects of Divalent Cations on RED Performance during Continuous

235

Operation. The increase in electrical resistance of IEMs in the presence of divalent cations was

236

demonstrated under static condition. However, it is necessary to investigate the effects divalent

237

cations on the RED process under dynamic operating conditions because feed solutions were

238

continuously fed into the system and accordingly ions transported through IEMs consecutively.

239

Moreover, it is also important to explore how much the electrical resistance of IEMs influences on

240

the Rstack.

241

Figure 6 shows the normalized OCV, MPD, and Rstack in the bench-scale RED system in the

242

presence of divalent cations such as Mg2+, Ca2+, or Ba2+. Note that the OCV, MPD, and Rstack are

243

normalized by the last value that is measured right before switching seawater from the pure NaCl



244

solution to the mixed solution. By doing this, the effects of divalent cations on the RED

245

performance were solely considered.

246

OCV indicates maximum potential throughout the RED process and one of the basic

247

parameters to determine gross power density. Figure 6(a) exhibits that the normalized OCV

248

fluctuated in the presence of divalent cations. Initially, the CEMs were exposed to 0.513 M NaCl

249

solution for at least one hour for stabilization in a bench-scale RED process. Then the 0.513 M

250

NaCl solution was switched to the mixed solutions containing one of cations as described in the

251

section 2.4. The increase in the normalized OCV within the first 10 minutes might be attributed to

252

temporal increase in salinity gradient between seawater and fresh water streams before equilibrium.

253

We found that this effect was being negligible after 10 minutes. After 50-minutes operation, the

254

overall normalized OCV decreased by approximately 1% in the presence of Mg2+ and Ca2+,

255

whereas the decrease in the normalized OCV by Ba2+ was more than 4%.

256

As shown in Figure 6(b), it is clearer that the presence of Mg2+ and Ca2+ reduced the normalized

257

MPD by 8% and 12%, respectively (This trend between Mg2+ and Ca2+ was in good agreement

258

with the results from the section 3.1), whereas the normalized MPD decreased by 79% in the

259

presence of Ba2+, which showed the abnormal behavior more apparently. The effects of divalent

260

cations on the normalized MPD was more influential than the normalized OCV.


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261 262

Figure 6. RED performance during continuous operation with various divalent cations. The ERS

263

containing Na2SO4 as an electrolyte was used. (a) The normalized OCV, (b) the normalized

264

MPD, and (c) the normalized Rstack. Fresh water contains 0.017 M NaCl and seawater contains

265

0.4617 M NaCl + 0.02565 M MeCl2 (Me: Mg2+, Ca2+, or Ba2+). Re-scaled plot of (b) for 0 < the



266

normalized power density < 1.1 and (c) for 0 < the normalized Rstack < 12 are shown in the inset.

267

Note that OCV, MPD, and Rstack are normalized by the last value that is measured right before

268

switching seawater. All measurements were performed in triplicate and standard deviations are

269

included.

270

271

3.3. Inorganic Fouling on CEMs during the RED Operation. As shown in Figure 6(c),

272

the normalized Rstack was higher in the presence of Ca2+ than in the presence of Mg2+. When the

273

concentrated stream containing Ba2+ was introduced into the RED process, the normalized OCV

274

and the normalized MPD were significantly decreased, and the normalized Rstack was increased to

275

800% whereas the normalized Rstack with seawater containing the other divalent cations was

276

increased to maximum 111% compared to NaCl alone in the feed (Data shown in the small box

277

inside Figure 6(c)). Moreover, the large standard deviation implies that the test results were

278

inconsistent resulted from uncontrollable factors. There was no significant change in temperature,

279

pH, and conductivity through all experiments. The Nernst equation for the OCV calculation cannot

280

explain this phenomenon. This abrupt drop in performance rather resembles fouling phenomena

281

in membrane processes.

282

After the experiments, it was found that precipitates were formed on both outer CEMs surface

283

that met the first type ERS particularly in the presence of Ba2+. Electrodes and spacers that were

284

placed between the outer membranes and the electrodes were also coated with the precipitates.

285

However, the CEM in the middle and two AEMs were visually clean. In addition, in the presence

286

of Mg2+ or Ca2+, no perceivable precipitate was observed with all IEMs, spacers, and electrodes.

287

The SEM images in Figure 7 confirmed that precipitates were formed only on the outer CEMs


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288

surface in the presence of Ba2+. It seems that the divalent cations transported to the ERS through

289

the outer CEM, then contact with SO42-, and formed precipitates on membrane surface (Equation

290

(3)).35 The EDX spectrum clearly showed intensive peaks for barium indicating that the

291

precipitates were crystals of BaSO4 (Figure 8). However, the water solubility of BaSO4 (0.00031

292

g/100 g H2O at 20oC) is much lower than MgSO4 (25.1 g/100 g H2O at 20oC) and CaSO4 (0.201

293

g/100 g H2O at 20oC).36 As a result, relatively large amounts of the Ba2+-associated precipitates

294

could be accumulated on the membrane surface precipitate compared to the other divalent

295

precipitates.

296

𝐵𝑎𝐶𝑙2(𝑎𝑞) + 𝑁𝑎2𝑆𝑂4(𝑎𝑞)→𝐵𝑎𝑆𝑂4(𝑠) + 2𝑁𝑎𝐶𝑙(𝑎𝑞)


(3)


297 298

Figure 7. SEM images of CEMs surface that met the ERS containing Na2SO4 as an electrolyte.

299

Seawater containing Mg2+ (a), Ca2+ (b), and Ba2+ (c).


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300 301

Figure 8. EDX analysis of CEMs surface that met the ERS containing Na2SO4 as an electrolyte.

302

Seawater containing Mg2+ (a), Ca2+ (b), and Ba2+ (c).



303

304

3.4. Effects of Various Divalent Cations on RED Performance without the

305

Inorganic Fouling. To eliminate membrane fouling by the Ba2+-associated precipitates, the

306

identical experiments were repeated with the second type of ERS that contained 1 M NaCl as an

307

electrolyte instead of 1 M Na2SO4. It is instructive to note that the fouling above mentioned could

308

be avoided by using NaCl, but toxic gas, such as chlorine gas, could be generated, and moreover,

309

ClO-, ClO3-, and ClO4- could be formed during the RED operation, which was the main reason we

310

selected Na2SO4 initially.37

311

Figure 9 shows that the normalized OCV and the normalized MPD were decreased in the

312

presence of divalent cations. There was no abrupt change in the normalized OCV and the

313

normalized MPD implying that no fouling took place during the operation. Ba2+ in the concentrated

314

feed stream resulted in the most significant reduction in the normalized OCV and the normalized

315

MPD followed by Ca2+, and Mg2+. These results also demonstrate that the multivalent ion effects

316

are more critical impacts on the normalized MPD, as the normalized OCV decreased by only 2.5%

317

for Ba2+ but the normalized MPD decreased by 17% for the same ion. This means that an actual

318

RED performance can be varied with the ion composition of feed solutions even though they give

319

practically the same OCV. The overall RED performances are slightly abated probably due to the

320

low normality of 1 M NaCl compared to 1 M Na2SO4. In summary, this change is correlated with

321

the results as described in Section 3.1. It suggests that the statically obtained electrical resistance

322

of IEMs is practically a good indicator to estimate the RED performance, specifically MPD and

323

Rstack.

324


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325 326

Figure 9. RED performance during continuous operation with various divalent cations. The ERS

327

containing NaCl as an electrolyte was used. (a) The normalized OCV and (b) the normalized

328

MPD Fresh water contains 0.017 M NaCl and seawater contains either 0.513 M NaCl or 0.4617

329

M NaCl + 0.02565 M MeCl2 (Me: Mg2+, Ca2+, or Ba2+). Note that OCV and MPD are

330

normalized by the last value that is measured right before switching seawater. All measurements

331

were performed in triplicate and standard deviations are included.

332

333

334

4. CONCLUSIONS In this study, we have focused on the effects of divalent cations, such as Mg2+, Ca2+, and Ba2+

335

on the electrical resistance of IEMs and performance of a bench-scale RED process. At equilibrium,

336

it was revealed that the electrical resistance of the CEM was significantly influenced by the

337

presence of divalent cations in the concentrated feed solution, whereas the electrical resistance of

338

the AEM was not significantly affected by the divalent cations. It was also found that the electrical

339

resistance of the CEM increased with decreasing the hydrated radius of divalent cations (i.e., Mg2+



340

> Ca2+ > Ba2+) due to the affinity between the fixed functional groups in the CEM and the counter-

341

ions passing through the CEM. The effects of divalent cations on the performance of RED were

342

also confirmed during continuous operation of a bench-scale RED process. The presence of

343

divalent cations in the feed solution reduced the normalized OCV and the normalized MPD, and

344

increased the normalized Rstack. The change in the RED performance is correlated with the results

345

obtained from batch mode suggesting that the statically obtained electrical resistance of IEMs can

346

be used as a predictor to estimate the RED performance, specifically MPD and Rstack. The presence

347

of Ba2+ in the feed stream along with SO42- in ERS formed Ba2+-associated precipitates on the

348

CEM surface resulted in significant drops in the normalized OCV and the normalized MPD. By

349

replacing Na2SO4 in ERS with NaCl, the fouling was successfully mitigated.

350

351

■ ASSOCIATED CONTENT

352

Supporting Information

353

Current-Voltage curve during LSV, Swelling degree of IEMs.

354

355

■ NOTE

356

The authors declare no competing financial interest.

357


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358

■ ACKNOWLEDGEMENTS

359

This work was conducted under the framework of the Research and Development Program of the

360

Korean Institute of Energy Research (KIER) (Project Number: B8-2441).

361

362

■ 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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Effects of divalent cations on electrical membrane resistance in

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