Scaling behavior of iron in capacitive deionization (CDI) system

Journal Pre-proof Scaling behavior of iron in capacitive deionization (CDI) system Tianyu Wang, Changyong Zhang, Langming Bai, Binghan Xie, Zhendong Gan, Jiajian Xing, Guibai Li, Heng Liang PII:

S0043-1354(19)31144-3

DOI:

https://doi.org/10.1016/j.watres.2019.115370

Reference:

WR 115370

To appear in:

Water Research

Received Date: 23 June 2019 Revised Date:

22 November 2019

Accepted Date: 2 December 2019

Please cite this article as: Wang, T., Zhang, C., Bai, L., Xie, B., Gan, Z., Xing, J., Li, G., Liang, H., Scaling behavior of iron in capacitive deionization (CDI) system, Water Research (2020), doi: https:// doi.org/10.1016/j.watres.2019.115370. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical Abstract

1

Revised Manuscript for Water Research

2

Date: November-21th-2019

3 4

Scaling behavior of iron in capacitive deionization

5

(CDI) system

6

Tianyu Wanga, Changyong Zhangb, Langming Baia, Binghan Xiea, Zhendong Gana, Jiajian

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Xinga, Guibai Lia, Heng Lianga*

8 9 a

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State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, PR China

b

UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW, 2052, Australia

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E-mail: [email protected] (T. Wang); [email protected] (C. Zhang);

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[email protected] (L. Bai); [email protected] (B. Xie); [email protected] (Z.

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Gan); [email protected] (J. Xing); [email protected] (G. Li); [email protected]

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(H. Liang).

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* Corresponding author.

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E-mail address: [email protected] (H. Liang). 1

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Abstract:

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This study investigated the fouling and scaling behaviors in a capacitive deionization (CDI)

25

system in the presence of iron and natural organic matter (NOM). It was found that the salt

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adsorption capacity (SAC) significantly decreased when treating Fe-containing brackish water,

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with higher Fe concentrations leading to severer SAC reduction. Raman spectroscopy, X-ray

28

photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis demonstrated that Fe2O3

29

appeared to be the predominant foulant attached on the electrode surface, which was difficult to be

30

removed via backwashing or polarity reversal, indicating the irreversible property of the foulant.

31

Further characterizations (e.g., N2 sorption-desorption isotherms, electrochemical impedance

32

spectroscopy and cyclic voltammetry) revealed that the CDI electrodes suffered from obvious

33

deterioration such as specific surface area loss, resistance increase and capacitance decline with

34

the occurrence of Fe scaling. While the presence of NOM alleviated the Fe scaling through

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NOM-Fe complexing effects, NOM itself was found to have negative impacts on CDI desalination

36

performance due to their strong interactions with the carbon electrodes.

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Keywords: Capacitive deionization; Iron scaling; Natural organic matter; Fouling mechanisms

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1. Introduction

45

Nowadays, the scarcity of potable water resources is becoming one of the key challenges

46

worldwide (Elimelech and Phillip 2011, Schwarzenbach et al. 2010). Therefore, many countries

47

are actively developing technologies to produce fresh drinking water from unconventional water

48

resources such as brackish water and seawater (Ghaffour et al. 2013, Sheikholeslami 2009,

49

Subramani and Jacangelo 2015). Capacitive deionization (CDI) has emerged as one of the most

50

promising tools capable of low/medium salinity water desalination, with advantages of low energy

51

consumption, low environmental impact and convenient operation over other competitors such as

52

reverse osmosis, thermal distillation and electrodialysis (AlMarzooqi et al. 2014, Porada et al.

53

2013b, Suss et al. 2015, Wang and Lin 2018b, Zhang et al. 2018). Upon application of a constant

54

voltage/current, ions in the feedwater will be driven by the electrostatic force and stored in the

55

electric double layers (EDLs) formed between the porous electrodes and electrolyte, resulting in

56

the production of freshwater (charging/electroadsorption stage); once the electrodes reach

57

saturation, the adsorbed ions can be released back into the bulk solution by short-circuiting or

58

reversing the polarity of the two electrodes, leading to generation of a brine stream and restoration

59

of the electrodes (discharging/electrodesorption stage) (Porada et al. 2013a). The operating voltage

60

of the CDI is generally less than 1.5 V to prevent the electrolysis of water molecules, minimizing

61

the energy consumption (Garcia-Quismondo et al. 2016, Wang and Lin 2018a, Wu et al. 2017).

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Meanwhile, when appropriately controlling the charging/discharging procedure, a high water

63

recovery rate (~90%) can be achieved, with a high-concentration and low-volume brine stream

64

generated (Bian et al. 2015). 3

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Although the past two decades have seen tremendous advances and innovations in the CDI

66

field including cell architectures, electrode and membrane designs, experimental methods,

67

application broadening and fundamental processes (Faradaic and non-Faradaic reactions), less

68

effort has been paid to investigate fouling (Chen et al. 2018, Hassanvand et al. 2019, Liu et al.

69

2018, Wang et al. 2018) and scaling issues (Zhang et al. 2013), the study of which is highly

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required before the wide application of CDI. It is unavoidable that groundwater may contain

71

excess compounds such as calcium, magnesium, iron and natural organic matter. Previous works

72

demonstrated that calcium and magnesium could cause serious scaling in reverse osmosis or

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nanofiltration, leading to membrane flux reduction, permeate quality decline and membrane

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system lifetime shortening (Cao et al. 2018, Goh et al. 2018, Warsinger et al. 2015). The ionic

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concentrations near the membrane surface may exceed the critical solubility limit of soluble salts

76

due to the retention effect of nanofiltration (NO)/reverse osmosis (RO) membranes, causing the

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in-situ crystallization (Benecke et al. 2018, Thompson et al. 2012). The formed crystal is deposited

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on the membrane surface and results in the scaling and fouling of the NO/RO membrane. However,

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the presence of calcium and magnesium in feedwater has a negligible influence on CDI

80

performance. It was reported that the adsorption and desorption of calcium and magnesium were

81

reversible (<5% retained on the electrode), with less than a 2% reduction in TDS removal

82

efficiency throughout a 30-hour continuous operation (Mossad and Zou 2013).

83

Iron, however, could be another potential threat for scaling of the CDI system. It is widely

84

known that iron is the fourth most abundant element (by mass), with its distribution on earth only

85

second to oxygen, silicon and aluminum (Khatri et al. 2017). Iron is mainly present in the divalent

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form in groundwater due to the reducing environment of groundwater (Doggaz et al. 2018, Ellis et 4

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al. 2000, Hamdouni et al. 2016, Michalakos et al. 1997). However, ferrous iron is liable to be

88

oxidized and converted into ferric iron when groundwater is in contacts with the air (Knocke et al.

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1992, van Halem et al. 2012, Vries et al. 2017). Previous studies showed that when treating a

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solution containing ferrous/ferric ions using CDI cells, nearly 30% of iron was observed to

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accumulate on the carbon electrodes every operation cycle, resulting in significant deterioration

92

(~10%) in CDI treatment efficiency (Mossad and Zou 2013). However, the authors did not

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investigate the electrolyte composition and fundamental mechanisms associated with the ferric ion

94

fouling and scaling phenomenon. Meanwhile, researches on the migration of iron ions in CDI, the

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main components and formation mechanism of iron scale and the mechanism of CDI performance

96

degradation caused by iron scaling could provide theoretical guidance and research directions for

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how to alleviate iron scaling in the CDI system.

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In this study, we evaluated the effect of iron in brackish water on the desalination

99

performances of CDI system. The transformation of iron species, iron fouling and scaling

100

behaviors, the major component of iron foulants and the underlying scaling mechanisms were

101

thoroughly investigated via a wide range of tests and characterizations. In addition,

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representative natural organic matter (NOM) such as humic acid (HA) and bovine serum

103

albumin (BSA) were used to further illustrate their synergistic effects with iron on the

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desalination and fouling behavior.

105 106

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2. Materials and methods

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2.1. Reagents

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Analytical reagent grade chemicals (NaCl and FeCl3) were obtained from Aladdin Chemical

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(China). HA and BSA were purchased from Sigma-Aldrich (USA) as representatives of humic

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substances and proteins, respectively. Artificial groundwater was prepared by dissolving 500 mg/L

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NaCl in ultrapure water (18.2 MΩ/cm) produced by a Milli-Q purification system (Millipore,

113

U.S.A). To investigate fouling and scaling on CDI system, 1~10 mg/L FeCl3 and/or 1~10 mg/L

114

HA and BSA were added into the artificial solution.

115

2.2. Experimental setup

116

Activated carbon (YP-50F, Kuraray Chemical, USA), carbon black (BP-2000, Cabot, USA)

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and PVDF (HSV900, Arkema, France) were mixed in an 8:1:1 ratio to prepare the electrodes. First,

118

activated carbon, carbon black, and PVDF were dissolved in N,N-dimethylacetamide under

119

ultrasonic conditions and then stirred in an 80 °C water bath to form a uniform coating slurry.

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Finally, the slurry was applied to a titanium plate, which was placed in a 65 °C oven for 12 hours.

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The experimental device consisted of a CDI system, a computer, a DC power supply

122

(HCP01-5B2, Yangzhou Huatai Electronics, China), a multimeter (VC 8246A, Victor, China) and

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a conductivity meter (DDSJ-308F, Rex Electric Chemical, China), as presented in Fig. S1. The

124

CDI system is composed of two activated carbon electrodes (90×90 mm), a pair of glass support

125

plates (150×150×50 mm), a pair of titanium plates (200×150×1 mm), an insulating mesh (0.2 mm)

126

and silicone pads (0.5 mm). 6

127

The effect of Fe scaling on the CDI system were evaluated using a batch model experiment,

128

which included two processing stages, adsorption and desorption. In the adsorption stage, 100 ml

129

of NaCl solution containing different concentrations of FeCl3 was circulated in the CDI system for

130

20 min at 1.4 V DC. In the desorption stage, deionized water was used to clean the CDI system

131

under short circuit. The reaction solution was reconfigured after each cycle of the experiment. The

132

effect of NOM on the Fe scaling was illustrated by adding different concentrations of HA and BSA

133

to the reaction system. The concentration of ions was determined by inductively coupled plasma

134

optical emission spectrometry (ICP-OES, Perkin Elmer Optima 5300DV, USA).

135

The salt adsorption capacity (SAC) was calculated based on the following equation: =

136 137

(

)×

（1）

where C0 is the initial concentration of NaCl (mg/L), Cf is the final concentration of NaCl

138

(mg/L), V is the volume of solution (L) and m is the total mass of electrodes (g).

139

The charge efficiency (Λ) was calculated from following the equation:

140

=

×

（2）

×

141

where SAC is the salt adsorption capacity, F is the Faraday constant (96485 C/mol), M is the molar

142

mass of NaCl (58.5 g/mol) and

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2.3. Analytical methods

is the current.

144

The Fe scale on the electrode was characterized by Raman spectroscopy (HORIBA HR

145

Evolution, France), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA),

146

X-ray diffraction (XRD, Bruker D8 Advance, Germany), scanning electron microscopy (SEM,

147

FEI Quanta650, USA). The N2 sorption-desorption isotherms were used to investigate the pore 7

148

structure of electrode by a Micromeritics Brunauer–Emmett–Teller (BET) analyzer (ASAP 2020).

149

The specific surface area and microspore surface of electrode were analyzed by the BET and

150

T-plot methods, respectively. The pore size distribution was evaluated by quenched-solid density

151

functional theory. The resistance characteristics were illustrated by electrochemical impedance

152

spectroscopy (EIS), which were performed multiple times by a Metrohm Autolab potentiostat

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(PARSTAT302N) in 1 M NaCl at room temperature. The frequency of the EIS measurement was

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from 100 kHz to 0.01 Hz and the applied voltage was 5 mV open-circuit voltage (OCV). The EIS

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data was fitted by Zview from Scribner Associates Inc. The capacitance characteristics of the CDI

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were evaluated by cyclic voltammetry (CV). CV tests were performed in a two-electrode mode

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(one electrode is 0.5 g), with the potential ranging from -1.4 to 1.4 V at a scan rate of 10 mV s−1.

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The electrolyte was 1 M NaCl, whereas the temperature was controlled at 25 ℃. The specific

159

capacitance (Cs) was calculated according to the following equation:

160

=

∆



161

where

162

electrode and ∆U (V) is the window voltage.

163

3. Results and discussion

164

3.1. Effects of iron on CDI desalination performance

（3）

(A) is the response current, v (m/s) is the potential scan rate, m (g) is the mass of

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The effects of Fe concentrations on CDI desalination performance were tested, and the results

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are shown in Fig. 1. The salt adsorption capacity (SAC) of the CDI decreased throughout the

167

operating cycle. Meanwhile, higher Fe concentrations led to faster SAC deterioration. For instance, 8

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when 1 mg/L Fe was presented in the influent groundwater, the SAC of CDI decreased from ~97%

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to ~87% after a 5-cycle continuous operation, while when a higher Fe concentration (10 mg/L)

170

was used, the SAC showed a sharp decreasing from ~85% to ~70%. These results are similar to

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previous studies, in which it was reported that CDI adsorption capacity dropped to 81% after 30

172

hours operation (Mossad and Zou 2013). In addition, the trend of charge efficiency was also

173

similar to that of SAC, decreasing form ~45% to ~35% as the Fe concentration increased from 0 to

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10 mg/L (Fig. 1b). The formation of iron scale on the electrode might be responsible for the

175

decrease in the desalination performance of the CDI system.

176

Figure 1

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3.2. Effects of iron and NOM combinations on CDI

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desalination performance

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To determine the combined effects of Fe and NOM on the CDI desalination performance,

180

brackish waters containing 1 mg/L Fe and different amounts and types of NOM (HA, BSA and

181

HA/BSA) were treated. As illustrated in Fig. 2, a synergistic negative effect was observed when

182

combining iron with various organic matter, with the SAC decreased with the increasing of initial

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NOM concentration. More importantly, the ternary complex (i.e., Fe/HA/BSA) resulted in a more

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severe decline in salt removal performance compared to that of the binary complex (i.e., Fe/HA

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and Fe/BSA). For instance, the SAC decreased to 54% after five-cycle operation treating saline

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water containing 1 mg/L Fe, 5 mg/L HA and 5 mg/L BSA, while lower reductions were found

187

when treating saline waters containing binary compounds (20.9% for Fe/HA and 41.6% for

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Fe/BSA). 9

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Regarding charge efficiency, it presented a decreasing trend as the number of cycles

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increased. Meanwhile, a high concentration of NOM caused a more severe decline in charge

191

efficiency.

192

Figure 2

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3.3. Fate and distribution of iron

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3.3.1. Mass balance analysis

195

To clarify the cause of the decline in desalination performance of CDI induced by Fe, Fe

196

migration in the CDI system was investigated by measuring the Fe concentration in the effluent

197

and backwash. As shown in Fig. 3a, the iron that was removed from the electrode by backwashing

198

was scarce. The amount of desorption only accounted for a small portion of the adsorption amount,

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and the proportion varied between 5% and 18%. The results indicated that the adsorbed Fe could

200

not be completely removed, and most of the adsorbed Fe accumulated on the electrode. This

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process caused irreversible inorganic fouling of the CDI electrode, inducing a continuous decrease

202

in the desalination performance of the CDI system. This effect was consistent with the above

203

results of the desalination performance decreasing over time. Therefore, incomplete desorption of

204

iron was the main cause of CDI fouling caused by Fe. Moreover, the amount of iron desorption

205

decreased gradually with increasing numbers of operating cycles. This was mainly because that

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the amount of iron adsorbed on the electrode per cycle decreased as the running time increased.

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The deposition of Fe on the electrode in each cycle exhibited similar tendencies in the

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concentration range investigated and decreased as the running time increased. At low initial

10

209

concentrations, the downward trend was gentle, whereas at high initial concentrations, the

210

downward trend became steep. The results were attributed to the Fe that had deposited on the

211

electrode hindered the adsorption of Fe in the solution on the electrode. In addition, as the initial

212

concentration of Fe increased, the proportion of iron that could be adsorbed by the electrode

213

decreased. Limited adsorption sites on the electrode were responsible for the phenomena.

214

The accumulated deposition amount exhibited an increasing trend as the Fe concentration

215

increased, as shown in Fig. 3b. Meanwhile, the accumulated deposition amount increased linearly

216

with time. The effluent of the CDI system was filtered by 0.45 µm membrane, and it formed a

217

yellow-brown deposit on the membrane. In addition, there was a significant change in the iron

218

concentration of the effluent before and after filtration. At 10 mg/L Fe3+, the iron concentration in

219

the effluent without membrane filtration was 5.933 mg/L, while the iron concentration after

220

membrane filtration was only 0.172 mg/L. The results showed that after CDI treatment, the iron in

221

the solution was almost completely in the form of insoluble iron compounds.

222

Figure 3

223

3.3.2. Correlation between iron scaling and desalination performance

224

decline

225

Based on the analysis in the previous section, it can be found that the presence of Fe in

226

groundwater caused irreversible fouling of the CDI electrode. The correlation between the amount

227

of Fe deposited on the CDI electrode and the desalination performance is shown in Fig. 4.

228

Obviously, more Fe precipitation led to more severe desalination deterioration. When the

229

precipitated amount of Fe increased from 0 to 1.2 mg/g, SAC decreased from 100% to ~66.5%. 11

230

Meanwhile, the specific SAC began to decrease rapidly as the deposition amount of Fe scaling

231

increased, and then, the downward trend became flat.

232

233

Figure 4

3.3.3. Effects of NOM on iron distribution

234

The effects of organic matter on Fe fate and distribution were investigated and are shown in

235

Figs S2-4. Interestingly, increasing the organic matter concentration in saline waters led to a

236

decline in the Fe precipitation on the carbon electrodes. Ferric chloride can be used as a coagulant,

237

and the following reactions may be occurred in the presence of HA: (1) Complexation reaction

238

between Fe3+ and dissolved HA; (2) Charge neutralization and precipitation of Fe-HA; (3)

239

Adsorption of iron oxyhydroxide by colloidal HA; and (4) Adsorption of HA by precipitation of

240

iron oxyhydroxide. Previous study also reported similar NOM and Fe removal mechanisms when

241

NOM and Fe coexisted in the feed streams (Davis and Edwards 2017). Under the abovementioned

242

series of reactions, an insoluble complex of Fe-HA was finally formed. Obvious brown-yellow

243

flocs were observed after the CDI effluent was settled for a while. These flocs contained a large

244

amount of Fe. Hence, the Fe scaling on the electrode was prevented by the interaction between Fe

245

and HA, resulting in a decrease in the precipitation of Fe on the electrode. Additionally, the

246

interaction became stronger as the HA concentration increased, further reducing the adsorption of

247

Fe on the electrode. However, the HA precipitation on the electrode exacerbated the irreversible

248

fouling of the CDI.

249

As a protein substance in NOM, BSA also reacted with ferric iron in the four reactions

250

mentioned. This process could induce aggregation of BSA and formation of Fe-BSA colloid 12

251

complexes, resulting in a reduction in the Fe precipitation. This result was consistent with

252

previous studies that demonstrated the aggregation of BSA and formation of the Fe-BSA complex

253

by dynamic light scattering. However, it can be found that the coexistence of Fe and BSA could

254

not produce any obvious precipitate while the coexistence of Fe and HA caused brown-yellow

255

flocs. This difference was attributed to the narrow molecular weight of BSA, leading to the

256

formation of smaller floc (Cheng et al. 2017, Hao et al. 2013). Compared with that in the

257

combination of Fe and HA, the micro-flocs in the combination of Fe and BSA reduced the capture

258

of Fe. Moreover, the adsorption of Fe by BSA was weaker than that by HA (Mizuno et al. 2005).

259

Hence, the Fe adsorption on the electrode in the presence of Fe and BSA was greater than that in

260

the presence of Fe and HA.

261

The precipitation of Fe in the presence of Fe, HA and BSA was greater than that in the

262

presence of Fe and BSA, while it was less than that in the presence of Fe and HA. The strong

263

interaction between HA and Fe could further hinder the adsorption of Fe on the electrode

264

compared with that for Fe and BSA. However, BSA was encapsulated by HA under electrostatic

265

force (Tan et al. 2009), resulting in a decrease in the capture of Fe.

266

3.4. Fouling and Scaling Characterization

267

3.4.1. Form of iron scaling

268

After treatment of the iron-containing salt solution, the surface of the cathode was covered

269

with a layer of yellow substance (Fig. S5a) while the anode remained unchanged. This

270

phenomenon was mainly due to Fe3+ moving toward the cathode in the electric field. At the same

13

271

time, a yellow-brown layer remained on the 0.45 µm membrane after filtering the CDI effluent

272

(Fig. S5b). To further identify the phases of Fe scale on the electrode and precipitation on the

273

microfiltration membrane, the Raman spectral analysis was performed. As shown in Fig. S6a,

274

sharp peaks appeared at 219.9, 286.4, 403.4, 488.1 and 605.1 cm-1, demonstrating that Fe2O3 was

275

formed on the CDI cathode (Binitha et al. 2013, Jian et al. 2014, Lu et al. 2014). However, for the

276

residue on the filtration membrane, FeOOH was shown to exist due to the appearance of peaks

277

such as those of 382 and 477 cm-1 (Hanesch 2009, Spray and Choi 2009), and Fe2O3 was also

278

generated based on the peaks at 211, 274 and 1277 cm-1. Previous study also reported similar

279

findings that the FeOOH could be converted into Fe2O3 under the action within the electrostatic

280

force (Meng et al. 2016). Therefore, the transformation of Fe in the CDI system can be described

281

as follows (Meng et al. 2016, Song et al. 2018), while the carbon oxidation reactions are on-going

282

at the anode (He et al. 2016, Zhang et al. 2018, 2019):

283

Cathode:

284

4Fe3+ + 3O2 + 12e- → 2Fe2O3

285

2H+ + O2 + 2e- → H2O2

(5)

286

H2O2 + 2H+ + 2e- → 2H2O

(6)

(4)

287 288

Anode:

289

C + H2O → C=O + 2H+ + 2e-

(7)

290

C + H2O → C-OH + H+ + e-

(8)

291

C + 2H2O → CO2 + 4H+ + 4e-

(9)

292

Solution: 14

293

Fe3+ + 3OH- → FeOOH + H2O

(10)

294

2FeOOH → Fe2O3 + H2O

(11)

295

XPS was then carried out to study the elemental compositions and valences of the Fe species

296

(Fig. S7). The XPS survey spectrum of both samples confirmed the existence of Fe, and O

297

elements (Fig. S7a). In the Fe 2p core level XPS spectra of Fe scale, Fe 2p3/2 and 2p1/2 peaks were

298

located at 711.2 and 724.7 eV (Fig. S7b), respectively, which was characteristic of Fe3+ (Lu et al.

299

2014, Quan et al. 2016). Meanwhile, a satellite peak centered at 719.9 eV further confirmed the

300

existence of Fe3+. The precipitate revealed similar XPS results (Wei et al. 2017). In addition, three

301

peaks were observed in the O 1s spectrum of the precipitation sample (Fig. S7c). The peaks

302

located at 529.6 eV corresponded to Fe-O-Fe, while the peak centered at 531.2 eV was attributed

303

to Fe-O-H, which indicated the presence of FeOOH. Previous research reported similar results

304

when analysis the electrodeposition of Fe2O3 on electrodes (Song et al. 2018). The last peak at

305

533.3 eV was associated with adsorbed water (Chen et al. 2016). Additionally, the XRD patterns

306

of the samples were recorded to characterize the crystal structures. As shown in Fig. S8, no

307

obvious peaks were observed in either samples, indicating that the samples were amorphous (Chen

308

et al. 2016, Song et al. 2018), similar to previous results which shown a hump in the XRD patterns

309

of the amorphous FeOOH.

310

The morphology of the Fe scale on the electrode was analyzed by SEM. Three samples of

311

complete scaling, partial scaling and raw electrode were selected, with SEM images shown in Fig.

312

5. Compared with the surface of the raw electrode, it can be clearly seen that the fouled electrode

313

surfaces were covered by a layer of iron scale. The precipitation of iron scale hindered the access

314

of ion for transport and reduced the adsorption sites on the electrode, resulting in a decrease in 15

315 316

317

desalination performance. Figure 5

3.4.2 Variations in electrode pore structure after fouling

318

To investigate the effect of Fe scaling on the specific surface area and pore distribution of

319

the electrode, N2 sorption-desorption isotherms were analyzed. As shown in Fig. 6a, N2 uptake

320

mainly occurred in the relatively low-pressure range, indicated that all electrodes had abundant

321

micropores with a typical type I isotherm for the N2 sorption-desorption curve (Tang et al. 2017).

322

The surface areas that were calculated according to the N2 sorption-desorption isotherms are

323

presented in Fig. 6b. Compared with those of the raw electrode, the decline in specific surface area

324

and micropore surface area of the Fe-scaled electrode were 104.76 and 88.84 m2/g, respectively,

325

with the decrease in micropore surface area contributing to 84.8% of the total decrease. These

326

results further revealed that the decrease of electrode performance was mainly due to the blocking

327

of micropores and reduction in accessible adsorption sites after Fe fouling.

328

After addition of organic matter to the influent, the specific surface area of the electrode

329

further decreased. The reduction in the specific surface area of Fe-HA, Fe-BSA, and Fe-HA-BSA

330

was 14.3, 14.6 and 24.53% higher than that of Fe alone, respectively. This result indicated that not

331

only Fe scale but also organic matter was present on the electrode. However, there was almost no

332

difference in the micropore surface area of the fouled electrode under different conditions. This

333

phenomenon may be attributed to pore size limitation and most of the micropores were not

334

suitable for electrode foulants to enter. A similar conclusion was obtained by analyzing the pore

335

size distribution of the electrode (Fig. S9). All fouled electrodes presented an analogous pore size 16

336

distribution, and the majority of micropores were not blocked by electrode foulants.

337

Figure 6

338

3.4.3 Electrochemical characterization

339

Electrochemical impedance spectroscopy (EIS). The impact of Fe scaling on the CDI resistance

340

was investigated by EIS analysis, and the results are presented in Fig. 7. The curves of all samples

341

contained a high-frequency semicircle loop and a low-frequency sloped line, and the results was

342

consistent with the study of Luo (Luo et al. 2018). The intercept on the x-axis of the curves

343

represented the series resistance (Rs), which included the intrinsic resistance of the salt solution,

344

the current collector, the electrodes and the any wires(Qu et al. 2015). The semicircle loop in the

345

high frequency was attributed to ion accumulation on the interface of electrode and electrolyte,

346

which was the parallel connection of the charge transfer resistance (Rct) and the electric double

347

layer capacitor (C1), and the diameter of the semicircle loop indicated value of Rct

348

(Mendoza-Hernandez et al. 2014). The sloped line in the low frequency was a Warburg

349

impendence, indicating the diffusion of ions in the pores of the electrode. The ion diffusion

350

property was assessed by Warburg coefficient, which was equal to the slope of the Randle plot in

351

the low frequency region (Min et al. 2018).

352

The component of the internal resistance was obtained by fitting the EIS plot according to the

353

equivalent circuit, and Fig. 7b presents the resistance of all samples. The Rs of the raw electrode

354

was 0.072 Ω, considerably smaller than that of the electrode fouled by Fe (0.091 Ω). The results

355

indicated that Fe scaling induced an increase in the intrinsic resistance of the electrode, resulting

356

in a decline in the conductivity of the electrode. Compared with the value of raw electrode (0.074 17

357

Ω), the Rct of the electrode fouled by Fe was increased by 145%. The intrinsic resistivity of the

358

electrode and the contact area between the electrode and the reaction solution are two factors that

359

affect the charge transfer resistance of the CDI (Cheng et al. 2011). Fe scaling reduced the specific

360

surface area of the electrode by blocking pores, resulting in a decline in contact area between the

361

electrode and the reaction solution. On the other hand, the intrinsic resistivity of the electrode was

362

improved by Fe scaling. The Warburg coefficient of the electrode fouled by Fe increased from

363

0.0167 (raw electrode) to 0.0271 Ω/s-1/2, demonstrating that Fe scaling hindered the migration of

364

ions in the electrode. The addition of NOM resulted in an increase in the internal resistance of CDI

365

compared to that with a single Fe scaling. The Rs and Rct of the CDI fouled by Fe-HA, Fe-BSA

366

and Fe-HA-BSA increased by 48.7%, 59.5%, 97.5% and 267%, 339%, 639%, respectively. In

367

addition, NOM further decreased the rate of ion diffusion. The Warburg coefficient of the

368

electrode fouled by Fe-HA, Fe-BSA and Fe-HA-BSA increased from 0.0271 to 0.0363, 0.0509

369

and 0.0858 Ω/s-1/2, respectively.

370

Figure 7

371

Cyclic voltammetry (CV). CV measurements can be used to evaluate the capacity of ion

372

accumulation at the electrode/electrolyte interface. As shown in Fig. 8, the CV curves exhibited a

373

nearly rectangular shape without obvious redox peaks, demonstrating the ideal capacitive behavior

374

of CDI electrodes(Tang et al. 2017). The area of the closed curves decreased when fouling and

375

scaling formed on the electrodes, with the synergetic combination of Fe, HA and BSA fouling

376

resulting in the smallest area. Fig. 8(b) shows the specific capacitance of the electrode calculated

377

according to Eq (3). Compared with that of the raw electrode, the specific capacitance of the

378

Fe-fouled electrode was reduced by 15%. The capacitance of the CDI was affected by the specific 18

379

surface area of the electrode and the conductivity of the electrode(Wang et al. 2014). Fe scaling

380

decreased the specific surface area and caused a reduction in the contact area between the

381

electrode and the solution, resulting in the loss of capacitance. Moreover, the conductivity of the

382

CDI was decreased by Fe scaling, thereby hindering ion accumulation on the electrode. When HA

383

and/or BSA were presented in the electrolyte, the specific capacitance of the CDI fouled by Fe-HA,

384

Fe-BSA and Fe-HA-BSA decreased by 30%, 36% and 55% compared to that of the CDI fouled by

385

Fe only. The precipitation of NOM reduced the ion accumulation. These results, however, are not

386

surprising, as we demonstrated in previous section that Fe/NOM related species would block the

387

carbon surface and micropores, leading to a lower electrode capacitance.

388

389

Figure 8

4. Conclusions

390

This study investigated the fouling and scaling behavior in a CDI module when treating

391

brackish water containing Fe. The causes and main component of Fe scaling were revealed, which

392

could provide a theoretical guidance for relieving Fe scaling in CDI systems. The major

393

conclusions in the study are:

394

(i) The presence of Fe caused a significant decrease in CDI performances, with salt adsorption

395

capacity decreased 31% and charge efficiency reduced 25%.

396

(ii) The deteriorated performances were mainly attributed to Fe scaling of the carbon electrodes.

397

Fe2O3 precipitation was proved to be the predominant foulant. which blocked the micropores of

398

the carbon electrodes and prevented the ion transportation into the microspores, resulting in lower

19

399

surface areas and capacitance, as well as elevated electric resistances of the carbon electrodes.

400

(iii) While the presence of NOMs alleviated the Fe scaling through NOM-Fe complexing effects,

401

NOMs themselves were found to have negative impacts on CDI desalination performance due to

402

their strong interactions with the carbon electrodes.

403

(iv) These foulants were irreversible, and cannot be removed by backwash once formed on the

404

CDI electrodes.

405

Acknowledgements

406

This research was jointly supported by the National Key R&D Program of China

407

(2018YFC0408001), the National Natural Science Foundation of China (51778170), the State Key

408

Laboratory of Urban Water Resource and Environment (2019DX01) and Fundamental Research

409

Funds for the Central Universities.

410

References

411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

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559

24

Salt adsorption capacity reduction (%)

a

105 100 95 90 85 80 0mg/L Fe 1mg/L Fe 2.5mg/L Fe 5mg/L Fe 10mg/L Fe

75 70 65 1

2

3

4

5

Cycle number

b

50 0 mg/L Fe 5 mg/L Fe

Charge efficiency (%)

48

1 mg/L Fe 10 mg/L Fe

2.5 mg/L Fe

46 44 42 40 38 36 34 32 30 1

2

3

4

5

Cycle number Fig. 1. Effect of Fe on CDI performance: (a) salt adsorption capacity, (b) charge efficiency.

1

100

b

1mg/L Fe+1mg/L HA 1mg/L Fe+2.5mg/L HA 1mg/L Fe+5mg/L HA 1mg/L Fe+10mg/L HA

95

46 1 mg/L Fe+1 mg/L HA 1 mg/L Fe+5 mg/L HA

44

90

Charge efficiency (%)

Salt adsorption capacity reduction (%)

a

85 80 75 70

1 mg/L Fe+2.5 mg/L HA 1 mg/L Fe+10 mg/L HA

42 40 38 36 34 32

65

30

60

1

1

2

3

4

2

5

c

4

5

d

100

1mg/L Fe+1mg/L BSA 1mg/L Fe+2.5mg/L BSA 1mg/L Fe+5mg/L BSA 1mg/L Fe+10mg/L BSA

95 90

1 mg/L Fe+1 mg/L BSA 1 mg/L Fe+2.5 mg/L BSA

44

85 80 75 70

40 38 36 34 32

65

30

60

28 1

55 1

2

3

4

1 mg/L Fe+2.5 mg/L BSA 1 mg/L Fe+10 mg/L BSA

42

Charge efficiency (%)

Salt adsorption capacity reduction (%)

3

Cycle number

Cycle number

2

3

4

5

Cycle number

5

Cycle number

e

f 1mg/L Fe+1.25mg/L HA+1.25mg/L BSA 1mg/L Fe+2.5mg/L HA+2.5mg/L BSA 1mg/L Fe+5mg/L HA+5mg/L BSA

90

42 1 mg/L Fe+1.25 mg/L HA+1.25 mg/L BSA 1 mg/L Fe+2.5 mg/L HA+2.5 mg/L BSA 1 mg/L Fe+5 mg/L HA+5 mg/L BSA

40

Charge efficiency (%)

Salt adsorption capacity reduction (%)

100

80

70

60

50

38 36 34 32 30 28 26 24

40

1

1

2

3

4

5

2

3

4

5

Cycle number

Cycle number

Fig. 2. Effect of Fe-NOM combination on the salt adsorption capacity and charge efficiency of CDI: (a-b) Fe-HA, (c-d) Fe-BSA, (e-f) Fe-HA-BSA.

2

a Fe distribution (%)

100 1mg/L residue 2.5mg/L residue 5mg/L residue 10mg/L residue

80

60

1mg/L desorption 2.5mg/L desorption 5mg/L desorption 10mg/L desorption

40 1mg/L precipitation 2.5mg/L precipitation 5mg/L precipitation 10mg/L precipitation

20

0 1

2

3

4

5

Cycle number

Accumulated precipitation amount (mg/g)

b 1.2

1mg/L Fe 2.5mg/L Fe 5mg/L Fe 10mg/L Fe

1.0

1mg/L Fe 2.5mg/L Fe 5mg/L Fe 10mg/L Fe

0.8 0.6 0.4 0.2 0.0 1

2

3

4

5

Cycle number Fig. 3. Mass balance of Fe in single Fe3+ solution: (a) mass distribution of iron in effluent, backwash fluid and electrodes, (b) accumulated precipitation amount of Fe in electrode.

3

Specific salt adsorption capacity

1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fe deposited on electrode (mg/g)

Fig. 4. The effect of Fe scaling on the desalination performance of the CDI

Fig. 5. SEM images of Fe scale on the electrode: (a-c) full coverage, (d-f) partial coverage, (g-i) raw electrode. 4

a 550 Raw electrode 1mg/L Fe 1mg/L Fe+2.5mg/L HA 1mg/L Fe+2.5mg/L BSA 1mg/L Fe+1.25mg/L HA+1.25mg/L BSA

Volumn absorbed @ STP (cm3/g)

500 450 400 350 300 250 200 150 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

b

1430 1425 1420 1320

Specific surface area Micropore surface area External surface area

Surface area (m2/g)

1315 1310 1305 1300 1295 1290 1000 800 600 400 200 0 Raw electrode

Fe

Fe-HA

Fe-BSA

Fe-HA-BSA

Fig. 6. Pore structure of electrode: (a) N2 sorption-desorption isotherm, (b) Surface area. 5

a 1mg/L Fe+1.25mg/L HA+1.25mg/L BSA 1.1 1mg/L Fe+2.5mg/L BSA 1.0 1mg/L Fe+2.5mg/L HA 0.9 0.8 1mg/L Fe 0.7 1.0M NaCl 0.6

0.0858

Zreal （ Ω）

1.4

1.2

0.0509 0.0363

0.5

-Zim （ Ω）

1.0

0.0271

0.4 0.3

Slope=0.0167

0.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

ω-1/2 (s-1/2)

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

b

1.0

1.2

1.4

0.0040 Raw electrode Fe Fe-HA Fe-BSA Fe-HA-BSA

0.5

e

0.0035

a

0.0030 0.0025

0.4

d

0.2

0.1

0.0020

c

0.3

a b

c c

b

b

d

0.0015

bc cd

a

d

Capacitance (F)

0.6

Resistance (Ω)

0.8

Zreal （ Ω）

0.0010 0.0005

0.0

0.0000 Rs

Rct

C1

Fig. 7. Electrochemical impedance spectroscopy of CDI: (a) Nyquist plot; (b) electrochemical parameters. Lowercase letters represent the significant difference within the group and these significant differences were analyzed by post hoc Tukey's test (n=5) (p<0.05).

6

a 0.8 1mg Fe+1.25mg/L HA+1.25mg/L BSA 1mg/L Fe+2.5mg/L BSA 1mg/L Fe+2.5mg/L HA 1mg/L Fe 1.0M NaCl

0.6 0.4

Current (A)

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

Potential applied (V)

Specific capacitiance (F/g)

b

80

60

40

20 Raw electrode

Fe

Fe-HA

Fe-BSA

Fe-HA-BSA

Fig. 8. Capacitance characteristic of electrode: (a) Cyclic voltammetry curves, (b) specific capacitance before and after fouling.

7

Highlights

Iron scaling caused a significant decrease in desalination performances of CDI. Fe2O3 was the predominant component of iron scale. CDI fouled suffered from resistance increasing and capacitance losing. NOM reduced precipitation of Fe on electrodes due to coagulation effect.

Declaration of interests ☒The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: