Short-circuited Closed-cycle Operation of Flow-electrode CDI for

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Short-circuited Closed-cycle Operation of Flowelectrode CDI for Brackish Water Softening Calvin He, Jinxing Ma, Changyong Zhang, Jingke Song, and T. David Waite Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02807 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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Environmental Science & Technology

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Short-circuited Closed-cycle Operation of Flow-electrode CDI for Brackish

2

Water Softening

3

Calvin He†, Jinxing Ma†, Changyong Zhang, Jingke Song and T. David Waite*

4

UNSW Water Research Centre, School of Civil and Environmental Engineering, University

5

of New South Wales, Sydney, NSW 2052, Australia

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Email addresses: [email protected] (Calvin He); [email protected] (Jinxing

8

Ma);[email protected] (Changyong Zhang); [email protected] (Jingke

9

Song); [email protected] (T. David Waite)

10 11 12 13 14

Environmental Science and Technology

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Re-submitted July 2018

16 17 18 19 20

†

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*Corresponding author: E-mail [email protected]; Tel. +61-2-9385-5060

These authors contribute equally to this work.

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1 ACS Paragon Plus Environment


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Graphical abstract

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Synopsis

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The effectiveness of an emerging electrochemical technology for softening of brackish waters

28

is examined.

29 30



31

Abstract

32

While flow-electrode capacitive deionization (FCDI) is an emerging desalination

33

technology, reduction in water hardness using this technology has so far received minimal

34

attention. In this study, treatment of influents containing both monovalent and divalent

35

cations using FCDI was carried out with flow-electrodes operated in short-circuited

36

closed-cycle (SCC) configuration. Divalent Ca2+ cations were selectively removed compared

37

to monovalent Na+ with the selectivity becoming dominant when the FCDI unit was operated

38

at lower current densities and hydraulic retention times. Results showed that SCC FCDI

39

operation was much more energy-efficient for brackish water softening compared to

40

operation in isolated closed-cycle (ICC) mode, particularly with implementation of energy

41

recovery. This finding was largely ascribed to (i) charge neutralization of the flow-electrodes

42

in SCC configuration and (ii) regeneration of the active materials to maintain pseudo “infinite”

43

capacity during electrosorption. In addition, mixing of the flow-electrodes in SCC operation

44

significantly inhibited pH excursion in the flow-electrode with resultant alleviation of

45

calcium precipitation on the carbon surface.

46 47

Keywords: flow-electrode; capacitive deionization; hardness; short-circuited; low-energy

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1. Introduction

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Groundwater is a major water source and continues to be widely used to supply remote

51

villages, farmland and even cities. In regional Australia, for instance, groundwater may be the

52

only reliable source of water.1 However, the provision of potable groundwater is not without

53

its challenges as the composition of the groundwater is dependent on its hydrogeological

54

history. For aquifers in the vicinity of limestone formations, excessive salinity and hardness

55

results in water that is impaired for direct human consumption. High hardness is undesirable

56

in water due to scale formation in domestic and industrial applications. Additionally, studies

57

have indicated that consumption of water with extreme hardness may be linked to an

58

increased incidence of chronic kidney disease although some uncertainty remains.2, 3 While

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the World Health Organization (WHO) does not specify health-based limits on hardness,

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values between 100-200 mg/L (an equivalent mass concentration of CaCO3) are

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recommended to minimize scale formation and taste concerns whilst maintaining sufficient

62

buffering capacity to prevent corrosion of metallic components in water distribution systems.4

63

Capacitive deionization (CDI) is a technology capable of removing ions (including ions

64

contributing to hardness) from brackish influent via electrosorption of the ionic species onto

65

charged electrodes.5-7 Advantages of CDI over alternatives such as ion-exchange, chemical

66

precipitation and reverse osmosis include the generation of small amounts of waste stream,

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the limited use of chemical agents and, if designed and operated appropriately, low energy

68

consumption. Of the many exciting developments within the field of CDI, flow-electrode

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capacitive deionization (FCDI) involving the use of flowable carbon (and composite)

70

electrodes is increasing in popularity.8-10 The mobility of the flow-electrode allows for



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regeneration of the electrode material external to the apparatus, thus making continuous

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desalination a real possibility.

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A variety of approaches have been suggested for enhancing the performance of FCDI.

74

For example, it has been suggested that the carbon content in the flow-electrode could be

75

elevated for the purposes of (i) maximizing salt adsorption

76

availability of conduction pathways as a result of the formation of carbon clusters that bridge

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between the current collectors and the surface of the ion-exchange membranes (IEM).13

78

Porous carbon spheres

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percolation in the flow-electrode. With regard to the electrolyte, the use of high salt

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concentration 16 and the inclusion of redox-active species, such as p-phenylenediamine 17 and

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hydroquinone,10 are conducive to facilitating charge transfer, thus increasing the desalination

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efficiency. It has been reported that the integration of FCDI with other technologies (e.g.,

83

membrane stripping) leads to applications in harvesting valuable products from waste

84

streams.18

14

and carbon black

15

11, 12

and (ii) increasing the

have been demonstrated to facilitate electron

85

Among the broad range of publications in FCDI, there has been little investigation into

86

the softening of brackish water. A recent study has attempted to use FCDI using nanofiltration

87

membranes to separate monovalent and divalent ions, with permselectivity observed for

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anions although the membrane employed was minimally effective for cations.19 In contrast to

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monovalent sodium, calcium and magnesium are divalent species with distinct physical and

90

chemical properties. The operational parameters (e.g., current density and voltage) would

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likely have significant impacts on the selective removal of hardness ions from a brackish

92

influent. Furthermore, the particular ionic species present is dependent on pH.


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While non-Faradaic electrosorption is integral to FCDI, redox reactions are generally

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unavoidable due to the charge exchange at the electrode/electrolyte interface. Faradaic

95

reactions occur even if the applied voltage is less than the standard potential for water

96

splitting (1.23 V).7, 20 The concomitant production of OH− and H+ results in pH excursion in

97

the flow-electrodes

98

concentration of divalent cations in the cathode expected to exacerbate scale formation in this

99

chamber. Given that the effectiveness of FCDI as a water treatment technology is principally

100

related to the partitioning of ions in the electrical double layers (EDLs) of the electrodes, the

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precipitation of divalent cations is likely to occupy the adsorption sites and reduce the

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capacitance of the electrodes.23,

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compromised by the inability to release adsorbed ions, although this remains to be confirmed.

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In this study, brackish water softening using FCDI was carried out in short-circuited

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closed cycle (SCC) operation. SCC operation has been reported to be optimal due to

106

simultaneous discharging of the flow-electrodes, thereby enabling their adsorption capacity to

107

be maintained during electrosorption.9, 25,

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mode in which the positively and negatively charged flow-electrodes are individually

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recycled in their respective pipelines, the flow-electrodes in SCC configuration are mixed in a

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shared reservoir after leaving the channels of the FCDI apparatus resulting in charge

111

neutralization of the carbon particles. Electrically neutral carbon allows for the release of the

112

adsorbed ions, regenerating the surface of the carbon particles.25 The removal of hardness

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ions was investigated here over a range of different operating conditions (i.e., current density

114

and hydraulic retention time). Additionally, a comparison of the energy and salt removal

21, 22

with the increase in pH coupled with the presence of elevated

24

Electro-regeneration of the system may also be

26

Compared to the isolated closed-cycle (ICC)



115

performance between SCC and ICC modes of operation was undertaken. As the mixing of the

116

flow-electrodes may be expected to suppress pH excursion and the formation of precipitates,

117

we also evaluated the fate of cations following migration across the IEM in FCDI. We

118

conclude with recommendations on how to best regenerate FCDI electrodes and discuss

119

limitations of MCDI, especially for treating brackish water exhibiting elevated hardness.

120 121

2. Materials and methods

122

2.1. Reagents

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Aqueous solutions for the brackish stream and flow-electrode electrolyte were prepared

124

using 18.2 MΩ cm Milli-Q water (Millipore) and NaCl and CaCl2 solutes (>99.0%, ACS

125

grade). The carbon materials in the flow-electrodes consisted of carbon black, 100-mesh

126

DARCO® and Norit® activated charcoal. Nitric acid (70%, ACS grade) was used as the

127

reagent to acidify the flow-electrode samples. In the studies of flow-electrodes with buffering

128

capacity, PIPES buffer (protonated solid) was used with the addition of NaOH to titrate the

129

solution to pH 6.76. Chemicals were sourced from Sigma Aldrich unless stated otherwise.

130

2.2. Experimental setup

131

The FCDI cell used in all experiments is illustrated in Figure 1a.27 The spacer chamber

132

(~2.5 mL) consists of a nylon spacer sheet (100-mesh, 160 mm × 70 mm) residing within a

133

silicone gasket (500 µm thickness) sandwiched between anion- and cation-exchange

134

membranes (AEM-Type I/CEM-Type I, FUJIFILM Europe). The flow-anode and cathode

135

chambers are on the alternate sides of the AEM and CEM respectively, with the channels

136

fabricated by laser-cutting 1.5 mm thick acrylic sheets. Each serpentine channel was


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rectangular in profile (1.5 mm deep and 3 mm wide) with an effective contact area (Aeff) of

138

34.9 cm2 in contact with the current collector made of graphite paper.

139

2.3. Operating conditions and monitoring

140

The flow-electrodes consisted of suspensions of 100-mesh DARCO® activated carbon,

141

Norit® chemically activated charcoal and carbon black powders in a mass ratio of 8:1:1 in

142

2000 mg L−1 NaCl with an initial pH of around 7. The total carbon mass loading was 10% by

143

weight. To ensure the surface of the carbon was well wetted, the carbon suspensions were

144

continually mixed on a magnetic stirrer for at least 12 h prior to experiments. In this study,

145

FCDI experiments were conducted in single-pass mode with the brackish stream containing

146

2000 mg L−1 NaCl and 150 mg L−1 CaCl2 (equivalent to 135 mg L−1 CaCO3) running through

147

the spacer at different flow rates resulting in hydraulic retention times (HRT) of 2.94, 1.47

148

and 0.98 min respectively (Figure 1a). The electrical conductivity of the treated stream was

149

continuously monitored with the use of a conductivity meter (CON-BTA, Vernier) connected

150

to a data acquisition system (SensorDAQ, Vernier). After each experiment, the FCDI system

151

was rinsed with 1 mM HCl and then Milli-Q water to remove any precipitates on the

152

apparatus. Prior to the application of electric current to initiate FCDI operation, the influent

153

and flow-electrode flows were applied until the effluent reached a constant conductivity with

154

this procedure used in order to minimize the contribution of non-electrically driven ion

155

adsorption by the IEM.

156

In SCC configuration (Figure 1b), the stirred reservoir contained a 100 g flow-electrode

157

with the suspension pumped in parallel through the flow-electrode channels at a constant

158

flow rate of 50 mL min−1. In one operating step, electrosorption was carried out at a constant



159

current density (I = i/Aeff, from 0 to 40 A m−2) using a DC power supply (MP3094,

160

Powertech). Note that, in this study, the “anode” is termed the electrode and/or current

161

collector that was positively charged during the charging process while the “cathode” is the

162

electrode and/or current collector that was negatively charged during the charging process. To

163

evaluate the energy efficiency of FCDI for water softening applications, control experiments

164

were undertaken in ICC mode; i.e., the anode and cathode (50 g each) were separately

165

recirculated between the FCDI cell and two stirred reservoirs respectively (Figure 1c). In ICC

166

operation, electrosorption was carried out then followed immediately by reversed-current

167

desorption for electrode regeneration and energy recovery. The electrolyte component of the

168

flow-electrode samples was obtained through centrifugation (× 10000 g, 2 min) followed by

169

filtration (0.22-µm) to remove the carbon particles. In parallel, another set of flow-electrode

170

samples were first acidified (5 wt% nitric acid) prior to physical separation, with the intention

171

of solubilizing any calcium precipitates on the carbon surface.

172

2.4. Analytical methods and calculations

173

The current (i) through and voltage (V) across the FCDI electrodes were recorded using

174

Vernier current and voltage probes connected to a SensorDAQ. Measurements of the pH were

175

carried out using F-51 pH meters (Horiba, Japan). The concentrations of cationic species in

176

the effluent and flow-electrode solutions were determined using an ICP-OES spectrometer

177

(Agilent Varian vista pro 710) according to the protocols described elsewhere.28

178

Average salt adsorption rate (ASAR), energy-normalized adsorbed salt (ENAS) and

179

coulombic efficiency (CE) are commonly used indicators to evaluate CDI performance. In

180

FCDI, because ion removal is ascribed to both electrosorption and electrodialysis,27


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alternative terms, average salt removal rate (ASRR) and energy normalized removed salt

182

(ENRS), are more appropriate than ASAR and ENAS. In this study, ASRR (µg cm−2 s−1 or

183

µmol cm−2 min−1), ENRS (µmol J−1) and coulombic efficiency (%) are calculated as follows:

184

185

186

ASRR =

ENRS =

Sum of mass of ions removed ∑ = x Effective surface area × Time

Total moles of removed ions = Total energy supplied to electrodes

(C

0, x

− Ct , x ) ∫ Qdt (1)

Aeff t  C0, x − Ct , x  ∫ Qdt  M M ,x  ∫ iVdt

∑  x

 C − Ct , x  N Aex ∑  0, x  Qdt M M , x  ∫ x  Total charge of removed ions = ×100% CE = Total charge supplied to electrodes ∫ idt

(2)

(3)

187

where C0,x and Ct,x are the influent and effluent concentrations of x = Na+ or Ca2+ species at

188

time t respectively, Q is the influent flow rate, Aeff is the effective surface area of the FCDI

189

unit, ex is the electron charge, NA is Avogadro’s number, MM,x is the molar mass of the solute x

190

and i is the current through the FCDI apparatus. The selectivity of removal of the divalent

191

Ca2+ over the monovalent Na+ is also given by:

C0,Ca 2+ − Ct ,Ca 2+ 192

193

C0,Ca 2+  Ca 2 +  Proportion of Ca 2+ removed = =  + + C0,Na + − Ct ,Na +  Na  Proportion of Na removed C0,N a +

ρ

(4)

The Nernst equation can be used to estimate the potential difference generated by the

194

discrepancy in ion concentration on either side of the membrane

195

FCDI should be recognized to be limited to indicating relative ion potentials only in view of

196

the current applied (Eq.5):


though its application in


197

E=

RT  Cx,electrode  ln   zF  Cx ,influent 

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(5)

198

where R is the ideal gas constant, T is the temperature, F is Faraday’s constant, z is the charge

199

of the ion x, and Ci,electrode and Ci,influent respectively refer to the concentrations of the ion x in

200

the flow-electrode and influent bulk solutions on either side of the IEM.

201

The flow-electrode suspensions were circulated in a closed circuit. Assuming that water

202

transport and evaporation during the relatively short experiments are negligible, the

203

flow-electrodes should remain at constant mass. Therefore, ions removed from the brackish

204

steam reside within the corresponding flow-electrode, with mass balance calculations

205

applicable for predicting the fate of ions following migration across the membranes. The

206

theoretical cation (Ca2+ and Na+) concentrations (XCa,tot and XNa,tot) in the flow-electrode were

207

determined according to the change of Ca2+ and Na+ concentrations in the brackish stream

208

following desalination. The free Ca2+ and Na+ concentrations (XCa,free and XNa,free) in the

209

flow-electrode and the precipitate fractions (XCa,pre) on the electrode were measured

210

according to the procedures documented in Section 2.3. In SCC configuration, as the

211

flow-electrodes are expected to be discharged upon mixing within the reservoir, we can

212

assume that XCa,tot ≈ XCa,free + XCa,pre (and XNa,tot ≈ XNa,free). In ICC mode, considering that

213

the ions residing in the EDLs (XCa,ele and XNa,ele) should not be easily released by physical

214

separation of the carbon from solution, XCa,tot ≈ XCa,free + XCa,pre + XCa,ele (and XNa,tot ≈ XNa,free +

215

XNa,ele). It should be noted that XCa,pre could include parts of XCa,ele because the acidification

216

will likely result in the electrical replacement of Ca2+ by H+.

217

The energy consumed (and recovered) during a charging/discharging step in SCC and

218

ICC modes were calculated by integrating the current and potential difference over time. For 12 ACS Paragon Plus Environment

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219

ease of comparison, the values were normalized by the volume of brackish stream treated

220

(kWh m−3). Note that SCC does not require a distinct discharging step as charge

221

neutralization (i.e., short-circuited contact) of the flow-electrodes acts to regenerate the

222

carbon particles. Nonetheless, reverse current “discharging” of SCC flow-electrodes was

223

carried out (i) to confirm that no energy recovery is possible in SCC mode, and (ii) to allow

224

comparison of the cation transport into the effluent (waste) channel during discharging.

225 226

3. Results and discussion

227

3.1. SCC operation of the FCDI for ion removal

228

The performance of FCDI for the removal of salinity and hardness ions from brackish

229

water was initially evaluated in SCC configuration (Figure 2). Figures 2a and b graphically

230

show the change in effluent conductivity and voltage across the electrodes as a function of

231

elapsed time at a given current density (11.5 A m−2). At the beginning of the experiments,

232

because the treated brackish stream took time to pass through the entirety of the spacer

233

chamber before reaching the conductivity probe, varying signal lags were observed.

234

Afterwards, the system reached a steady state with the effluent exhibiting constant

235

conductivity (Figure 2a). The decrease in conductivity became more significant at higher

236

HRTs. Moreover, it can be seen from Figure 2b that the cell voltage required to maintain the

237

constant current is relatively stable in SCC configuration with the gradual decrease over time

238

largely ascribed to an increase in the ionic conductivity in the flow-electrodes.16 A decrease in

239

HRT (i.e., an increase in flow rate of the brackish stream) was expected to facilitate the ion

240

replenishment of the brackish stream in the spacer resulting in a higher conductivity and



241

therefore a lower charging voltage (Figure 2b). A minor improvement in desalting

242

performance was observed with a decrease in HRT; for example, at I = 11.5 A m−2, the ASRR

243

(including NaCl and CaCl2) were 0.676, 0.707 and 0.756 µg cm−2 s−1 and the corresponding

244

CE were all > 98 % for HRTs of 2.94, 1.47 and 0.98 min.

245

A summary of the steady-state effluent conductivity and average voltages during

246

deionization is provided in Figures 2c and d. As expected, a longer HRT resulted in a greater

247

change in ion concentration at a given current density. The desalting rates were proportional

248

to the current densities as evident from the linear relationship between the current density and

249

the effluent conductivity for a given HRT (Figure 2c). Figure 2d indicates that the voltage

250

required to sustain the constant current spikes dramatically beyond a certain limiting current

251

density with these limiting values inversely proportional to HRT. Specifically, the internal

252

resistance of the system varied from 10.7 to 12.5 Ω below the limiting current density at

253

different HRTs but this resistance increased to over 25 Ω when the polarization resistance

254

became dominant (SI Figure S1). It has been reported that the IEMs possess an affinity for

255

counter-ions, leading to localized ion concentrations far in excess of the bulk solution.6, 29 The

256

consequent gradient across the membranes induces back-diffusion against the electrical field

257

and inhibits further ion transport. At the influent/membrane interface, concentration

258

polarization further reduces the local conductivity compared to that of the desalted stream (
95~98%) indicate

484

limited loss from Faradaic processes. While the Faradaic current might increase due to the

485

abundance of oxygen in the electrode slurries and high operating potentials, the SCC

486

operation of the flow electrodes largely inhibits the acquisition of charge by competing

487

acceptors/donors (e.g., oxygen/carbon). Future studies of the long-term performance of FCDI

488

operated in SCC mode with attention given to low-cost, reliable methods of regenerating (and

489

recovering) the solution phase of the flow-electrodes are considered critical to the future

490

development of FCDI as an effective process for the treatment of brackish source waters.

491 492

Supporting information

493

The Supporting Information is available free of charge on the ACS Publications website.

494

Change of the internal resistance of the FCDI cell (SI Figure S1), comparison of the pH in the

495

SCC and ICC flow-electrode (SI Figure S2), PIPES buffering of the flow electrodes in ICC

496

configuration (SI Section S1), PIPES buffering of the flow electrodes in ICC mode at

497

constant-current charging (SI Figure S3) and constant-voltage charging (SI Figure S4),

498

desalting performance of FCDI cell for various carbon contents (SI Figure S5), recovery rates

499

of Na+ and Ca2+ (SI Figure S6), distribution of (a) sodium and (b) calcium in the flow

500

electrode (SI Figure S7) and long-term performance of SCC FCDI operation (SI Figure S8).

501 502

Author information

503

Corresponding author



504

*E-mail: [email protected] Tel: +61-2-9385-5060;

505 506

Acknowledgements

507

Mr Calvin He acknowledges the support of an Australian Government Research Training

508

Program Scholarship and the Petre Foundation Scholarship. Dr. Jinxing Ma acknowledges the

509

receipt of a UNSW Vice-Chancellor’s Postdoctoral Research Fellowship (RG152482). The

510

authors would also like to thank Dr Adele Jones and Mr. Jiayi Fu (UNSW) for their help in

511

the preparation of samples and determination of sodium and calcium ions in these samples.

512 513

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15. Liang, P.; Sun, X.; Bian, Y.; Zhang, H.; Yang, X.; Jiang, Y.; Liu, P.; Huang, X., Optimized

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desalination performance of high voltage flow-electrode capacitive deionization by adding carbon

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black in flow-electrode. Desalination 2017, 420, 63-69.

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16. Yang, S.; Choi, J.; Yeo, J.-g.; Jeon, S.-i.; Park, H.-r.; Kim, D. K., Flow-Electrode Capacitive

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Deionization Using an Aqueous Electrolyte with a High Salt Concentration. Environ. Sci. Technol.

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2016, 50, (11), 5892-5899.

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17. Hatzell, K. B.; Beidaghi, M.; Campos, J. W.; Dennison, C. R.; Kumbur, E. C.; Gogotsi, Y., A high

557

performance pseudocapacitive suspension electrode for the electrochemical flow capacitor.

558

Electrochim. Acta 2013, 111, 888-897.

559

18. Zhang, C.; Ma, J.; He, D.; Waite, T. D., Capacitive Membrane Stripping for Ammonia Recovery

560

(CapAmm) from Dilute Wastewaters. Environ. Sci. Technol. Lett. 2018, 5, (1), 43-49.

561

19. Nativ, P.; Lahav, O.; Gendel, Y., Separation of divalent and monovalent ions using flow-electrode

562

capacitive deionization with nanofiltration membranes. Desalination 2018, 425, 123-129.

563

20. Choi, J.-H., Determination of the electrode potential causing Faradaic reactions in membrane

564

capacitive deionization. Desalination 2014, 347, 224-229.

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21. Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A., Theory of pH changes in

566

water desalination by capacitive deionization. Water Res. 2017, 119, 178-186.

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22. He, D.; Wong, C. E.; Tang, W.; Kovalsky, P.; Waite, T. D., Faradaic Reactions in Water

568

Desalination by Batch-Mode Capacitive Deionization. Environ. Sci. Technol. Lett. 2016, 3, (5),

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222-226.


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23. Seo, S.-J.; Jeon, H.; Lee, J. K.; Kim, G.-Y.; Park, D.; Nojima, H.; Lee, J.; Moon, S.-H.,

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Investigation on removal of hardness ions by capacitive deionization (CDI) for water softening

572

applications. Water Res. 2010, 44, (7), 2267-2275.

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24. Choi, J.-H.; Kang, H.-S., Scale Formation by Electrode Reactions in Capacitive Deionization and

574

its Effects on Desalination Performance. Applied Chemistry for Engineering 2016, 27, (1), 74-79.

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25. Jeon, S. I.; Yeo, J. G.; Yang, S.; Choi, J.; Kim, D. K., Ion storage and energy recovery of a

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flow-electrode capacitive deionization process. J. Mater. Chem. A 2014, 2, (18), 6378-6383.

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26. Yang, S.; Kim, H.; Jeon, S.-i.; Choi, J.; Yeo, J.-g.; Park, H.-r.; Jin, J.; Kim, D. K., Analysis of the

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desalting performance of flow-electrode capacitive deionization under short-circuited closed cycle

579

operation. Desalination 2017, 424, 110-121.

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27. Ma, J.; He, C.; He, D.; Zhang, C.; Waite, T. D., Analysis of capacitive and electrodialytic

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contributions

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DOI:10.1016/j.watres.2018.07.049.

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28. Tsarev, S.; Waite, T. D.; Collins, R. N., Uranium reduction by Fe (II) in the presence of

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montmorillonite and nontronite. Environ. Sci. Technol. 2016, 50, (15), 8223-8230.

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29. Strathmann, H., Electrodialysis, a mature technology with a multitude of new applications.

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Desalination 2010, 264, (3), 268-288.

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30. Galama, A.; Post, J.; Stuart, M. C.; Biesheuvel, P., Validity of the Boltzmann equation to describe

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Donnan equilibrium at the membrane–solution interface. J. Membr. Sci. 2013, 442, 131-139.

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31. Hawks, S. A.; Knipe, J. M.; Campbell, P. G.; Loeb, C. K.; Hubert, M. A.; Santiago, J. G.;

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Stadermann, M., Quantifying the flow efficiency in constant-current capacitive deionization. Water

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Res. 2018, 129, 327-336.

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CDI.

Water

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32. Park, J.-S.; Song, J.-H.; Yeon, K.-H.; Moon, S.-H., Removal of hardness ions from tap water

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using electromembrane processes. Desalination 2007, 202, (1-3), 1-8.

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33. Yoon, H.; Lee, J.; Kim, S.-R.; Kang, J.; Kim, S.; Kim, C.; Yoon, J., Capacitive deionization with

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Ca-alginate coated-carbon electrode for hardness control. Desalination 2016, 392, 46-53.

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34. Suss, M.; Porada, S.; Sun, X.; Biesheuvel, P.; Yoon, J.; Presser, V., Water desalination via

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capacitive deionization: what is it and what can we expect from it? Energy Environ. Sci. 2015, 8, (8),

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2296-2319.

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35. Zhao, R.; Biesheuvel, P. M.; van der Wal, A., Energy consumption and constant current operation

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in membrane capacitive deionization. Energy Environ. Sci. 2012, 5, (11), 9520-9527.

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36. Jeon, S.-i.; Yeo, J.-g.; Yang, S.; Choi, J.; Kim, D. K., Ion storage and energy recovery of a

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flow-electrode capacitive deionization process. Journal of Materials Chemistry A 2014, 2, (18),

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6378-6383.

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37. Nativ, P.; Badash, Y.; Gendel, Y., New insights into the mechanism of flow-electrode capacitive

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deionization. Electrochem. Commun. 2017, 76, 24-28.

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38. Wang, M.; Hou, S.; Liu, Y.; Xu, X.; Lu, T.; Zhao, R.; Pan, L., Capacitive neutralization

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deionization with flow electrodes. Electrochim. Acta 2016, 216, 211-218.

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composite electrode to remove calcium ions by capacitive deionization. Desalination 2013, 326,

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109-114.

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40. Stumm, W.; Morgan, J. J., Aquatic chemistry: chemical equilibria and rates in natural waters.

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John Wiley & Sons: 2012; Vol. 126.

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materials. Physical Chemistry Chemical Physics 2013, 15, (28), 11740-11747.

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42. Zhang, W.; Mossad, M.; Zou, L., A study of the long-term operation of capacitive deionisation in

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inland brackish water desalination. Desalination 2013, 320, 80-85.

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249-260.

621



622

Figures

623 624

Figure 1. Schematic representations of (a) the structure of the FCDI cell and (b)

625

short-circuited closed-cycle (SCC) and (c) isolated closed-cycle (ICC) configuration of the

626

system.

627


Page 32 of 38

Page 33 of 38


628 629

Figure 2. SCC operation of the FCDI for brackish water softening. Time-course variation in

630

(a) effluent conductivity and (b) voltage across the electrodes at a constant current density =

631

11.5 A m−2 and different HRTs, and summary of (c) steady-state effluent conductivity and (d)

632

average voltages as a function of current densities and HRTs (lines serve to guide the eye).

633

All experiments were performed in single-pass mode at least in triplicate. In Figure 2d, the

634

average voltages were calculated according to the voltage profile (e.g., Figure 2b) from 100

635

to 1800 s.

636



637 638

Figure 3. (a) Energy comparison of SCC and ICC operation of FCDI for brackish water

639

softening and (b) ASRR vs ENRS in SCC mode. The initial brackish stream contains 2000

640

mg L−1 NaCl and 150 mg L−1 CaCl2. Note that the average energy consumption was

641

calculated for an 1800 s charging process and the specific energy consumption for

642

flow-electrode recirculation related to the treatment capacity of the FCDI system was

643

therefore not included. Lines serve to guide the eye.

644


Page 34 of 38

Page 35 of 38


645 646

Figure 4. Concentrations of (a) Na+ and (b) Ca2+ in the effluent, and (c) selectivity of

647

removal of Ca2+ over Na+ during SCC operation of FCDI at different current densities and

648

HRTs. The experiments were performed in single-pass mode at constant current (CC)

649

densities (n = 3).

650



651 652

Figure 5. Comparison between the short-circuited (SCC) (red) and isolated closed-cycle

653

(ICC) (blue) operation of FCDI with (a) effluent conductivity and (b) voltage as a function of

654

elapsed time. The duration of the constant-current charging step was 4800 s followed by a

655

reverse-current regeneration step for 2400 s. All experiments were performed in single-pass

656

mode with I = 22.9 A m−2 and HRT = 0.98 min. In Figure 5b, the dark shaded area indicates

657

the extra energy consumed in ICC mode compared to SCC while the light shaded area is

658

indicative of the extra energy recovered during electrode regeneration.

659


Page 36 of 38

Page 37 of 38


660 661

Figure 6. Comparison of the (a) Na+ and (b) Ca2+ concentrations in the effluent during FCDI

662

operation in short-circuited (SCC) (red) and isolated closed cycle (ICC) (blue) modes. The

663

unshaded region represents the 4800 s constant-current charging step followed by the shaded

664

region indicating the 2400 s reverse-current discharging step. All experiments were carried

665

out in single-pass mode with HRT = 0.98 min and I = 22.9 A m−2.

666



667 668

Figure 7. Increase in (a) sodium and (b) calcium concentrations in the flow-electrode in ICC

669

operation and (c) sodium and (d) calcium concentrations in the flow-electrode in SCC

670

operation. The grey plots were calculated according to the mass balance of ions in the

671

brackish stream with the blue plots (XNa,free and XCa,free) determined by the ion concentrations

672

in the solution phase of the flow-electrodes and the red plots the solutions after the addition

673

of 5% nitric acid. For sodium (Figures 7a and c), the difference between the grey and blue

674

plots (XNa,ele) indicated the electrosorption contribution to sodium removal while the

675

precipitation (XCa,pre) would be expected for calcium in the flow cathode. All experiments

676

were carried out in single-pass mode with HRT = 0.98 min and I = 22.9 A m−2.

677


Page 38 of 38

Short-circuited Closed-cycle Operation of Flow-electrode CDI for

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