Divalent Cation Removal by Donnan Dialysis for ... - ACS Publications

Subscriber access provided by UNIV OF DURHAM

Divalent cation removal by Donnan dialysis for improved reverse electrodialysis Timon Rijnaarts, Nathnael Shenkute, Jeffery A. Wood, Wiebe M. de Vos, and Kitty Nijmeijer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00879 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

ACS Sustainable Chemistry & Engineering

1

Divalent cation removal by Donnan dialysis for improved reverse electrodialysis

2

Timon Rijnaarts1,2, Nathnael T. Shenkute1, Jeffery A. Wood3, Wiebe M. de Vos1, Kitty

3

Nijmeijer4,*

4

1) Membrane Science & Technology, University of Twente, MESA+ Institute for

5

Nanotechnology, Drienerlolaan 5, P.O. Box 217, 7500 AE Enschede, The Netherlands

6

2) Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9,

7

8911 MA Leeuwarden, The Netherlands

8

3) Soft Matter, Fluidics and Interfaces, University of Twente, MESA+ Institute for

9

Nanotechnology, Drienerlolaan 5, P.O. Box 217, 7500 AE Enschede, The Netherlands

10

4) Membrane Materials and Processes, Department of Chemical Engineering and Chemistry,

11

Eindhoven University of Technology, Het Kranenveld 14, P.O. Box 513, 5600 MB Eindhoven,

12

The Netherlands

13

*corresponding author: Kitty Nijmeijer ([email protected])

14

Abstract

15

Divalent cations in feed water can cause significant decreases in efficiencies for membrane

16

processes, such as reverse electrodialysis (RED). In RED, power is harvested from the mixing of

17

river and seawater, and the obtainable voltage is reduced and the resistance is increased if

18

divalent cations are present. The power density of the RED process can be improved by

19

removing divalent cations from the fresh water. Here, we study divalent cation removal from

20

fresh water using seawater as draw solution in a Donnan dialysis (DD) process. In this way, a 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

21

membrane system with neither chemicals nor electrodes but only natural salinity gradients can be

22

used to exchange divalent cations. For DD, the permselectivity of the cation exchange membrane

23

is found to be crucial as it determines the ability to block salt leakage (also referred to as co-ion

24

transport). Operating DD using a membrane stack achieved a 76% reduction in the divalent

25

cation content in natural fresh water with residence times of just a few seconds. DD pretreated

26

fresh water was then used in a RED process, which showed improved gross and net power

27

densities of 9.0 and 6.3% respectively. This improvement is caused by a lower fresh water

28

resistance (at similar open circuit voltages), due to exchange of divalent for monovalent cations.

29

Keywords: Calcium, Magnesium, cation exchange membrane, Donnan dialysis, reverse

30

electrodialysis, hardness removal

31

Introduction

32

In the treatment of water there is an increasing interest in pretreatment technologies to enhance

33

process efficiencies. For reverse electrodialysis (RED), where the controlled mixing of fresh

34

water and seawater is used to harvest power, divalent cations in river water can significantly

35

decrease RED performance.1-3 In membrane-based processes that use natural waters, scaling can

36

lead to lower efficiencies. Typical scaling consists of the divalent cations Ca2+ and Mg2+ with the

37

anions SO42- and CO32-, which form salts with low solubilities leading to precipitation.4-7 These

38

precipitates cause membrane fouling, which decreases water recoveries for filtration or current

39

efficiencies for ED processes.

40

It is therefore of interest to remove divalent cations prior to these hardness-sensitive processes.

41

Divalent cations can be precipitated by chemical precipitation or exchanged by processes such as 2


Page 2 of 26

Page 3 of 26 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


42

ion exchange (IEX) or Donnan dialysis (DD).8 In a typical IEX process, a column loaded with

43

ion exchange resin is preconditioned with monovalent ions such as Na+ or H+, which can

44

exchange for undesired divalent cations. The total charge of the ions in solution (concentration

45

multiplied by valence of the ions) does not change; however, divalent cations are exchanged for

46

monovalent cations in the resin. The driving force of this exchange is entropic gain: release of

47

two monovalent cations by capturing a single divalent cation. After using IEX to exchange

48

undesired divalent cations, the IEX resin has to be regenerated with a highly concentrated

49

monovalent cation (or acid) containing stream. Typically, two IEX columns are required so one

50

can exchange divalent cations while the other is regenerated. This doubles capital expenses and

51

in addition requires chemicals (brine or acid) to regenerate the IEX resin.8

52

In contrast, DD is a technique that can exchange divalent cations continuously without the need

53

for regeneration.9 The same mechanism is found in biological cell membranes, where Na+/Ca2+

54

exchangers use a gradient of Na+ to transport Ca2+ against its concentration gradient.10 This

55

exchange process relies on ion exchange membranes, instead of ion exchange resins as used for

56

IEX. The solution with undesired divalent cations is brought into contact with a cation exchange

57

membrane (CEM) and on the other side of the CEM, there is a draw solution containing a high

58

concentration of monovalent cations. This configuration allows for exchange of the divalent

59

cations with monovalent cations in the draw solution. The driving force behind this process is a

60

similar entropic gain as found in IEX, resulting in high Donnan potentials for monovalent

61

species.11-14

3



Page 4 of 26

62

Recent studies on divalent cation exchange pretreatments have been performed by Vanoppen et

63

al. for reverse-osmosis (RO), using IEX as well as DD.8, 15 In their work they found IEX to be

64

substantially more cost-effective due to the high price of ion exchange membranes (300 €/m²)

65

required for the DD process. However, they calculated that if the price of ion exchange

66

membranes decreases to ~10 €/m² the process becomes economically interesting, which is a price

67

range comparable to RED.16

68

Previous work on RED has shown that divalent cations are a major challenge in this process and

69

limit the obtainable power densities due to both membrane resistance increase and uphill

70

transport.1, 2 The use of new membranes that do not suffer from significant membrane resistance

71

increase, overcomes the first limitation.17 However, uphill transport remains a problem due to the

72

presence of divalent cations in the river water. Uphill transport is the exchange of divalent

73

cations in the river water against their concentration gradient with monovalent cations in the

74

seawater, which effectively lowers the obtainable voltage. Monovalent-selective CEMs could

75

counter uphill transport, however, recent work showed that these monovalent selective CEMs

76

have an increase in membrane resistance over time while multivalent permeable CEMs have

77

stable resistances for longer times.18 Besides uphill transport, the low conductivity of the river

78

water also limits power output. For a typical RED stack, the dominant resistance is that of the

79

river water.19-22 There are two reasons for this low conductivity: first, the ionic conductivity of

80

the river water is low and therefore this compartment resistance in the RED stack is high. Recent

81

studies have focused on decreasing this resistance either using decreased spacer thickness19 or by

82

introducing dynamic spacer to lower the river water compartment thickness.22 As such, it was

83

possible to reduce the resistance of this compartment, as its resistance scales linearly with 4


Page 5 of 26 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


84

compartment thickness. Secondly, the low river water concentration combined with a large

85

concentration gradient between the seawater and the river water on either side of the membrane

86

induces an increased membrane resistance in both AEMs and CEMs.20, 21 Both of these effects

87

stress the importance of decreasing the river water resistance to decrease the overall stack

88

resistance. DD can therefore be an attractive process to alleviate these resistances by exchanging

89

divalent cations for monovalent cations with a higher activity (one divalent cation is exchanged

90

for two monovalent cations), which decreases resistance.

91

In this work, we study the applicability of DD to exchange divalent cations from river water by

92

using seawater as a draw solution, as shown in Fig. 1. Studies using artificial natural waters are

93

performed with different cation exchange membranes to show the proof-of-principle and to

94

investigate the influence of membrane properties on DD performance. The effect of DD

95

pretreatment on RED power harvesting is then assessed with natural river and seawater and

96

compared to theoretical models to determine the maximum possible enhancement for RED with

97

DD pretreatment.

98

5



Page 6 of 26

99

Figure 1. Schematic concept of DD as pretreatment, to exchange divalent cations from river

100

water and subsequent use in combination with RED. In the DD pretreatment step, exchange of

101

divalent cations for monovalent Na+ from seawater is achieved.

102

Theory

103

In this section, a detailed background of DD is given to understand the principles and to predict

104

divalent cation exchange. The influence of river water ion concentration on RED performance is

105

predicted to determine the theoretical benefits of using DD pretreatment as well.

106

DD

107

The driving force of DD is an entropic gain by exchanging a single divalent cation for two

108

monovalent cations at the low concentration side. This entropic gain can be analyzed by Donnan

109

potentials of the different ionic species. For each ionic species ‘i’, Donnan potentials can be

110

calculated by their respective concentrations on each side of the membrane (Eq. 1).8,

111

Assumptions in this equation are that there is negligible convection through the membrane, that

112

electro-neutrality is conserved and that the membrane is perfectly permselective (no co-ion

113

transport through the membrane). The system will strive towards equilibrium, driven by entropy,

114

and as such the Donnan potentials of each ionic species will equilibrate. This does not imply that

115

their concentrations are equal, only the ratio of the ionic species to the power one over their

116

respective valence will equilibrate. In the case of monovalent sodium (Na+, z = 1) and a divalent

117

cation (M2+, z = 2) the following equilibrium will establish (Eq. 2):

9, 13

1

118

EDon =

ai,c zi ln ൬ ൰ F ai,d

RT

(1) 6


Page 7 of 26 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


ቆa

aNa+ ,c

119

Na+ ,d

1

ቇ

1

a 2+ 2 = ቆaM ,c ቇ 2+ M

(2)

,d

120

where EDon is the Donnan potential (V), R is the gas constant (8.314 J/K·mol), T is the

121

temperature (K), F is the Faraday constant (96485 C/mol), z is the valence of the ion (-) and a is

122

the activity of ion i (M) in the concentrate (c) or diluate (d) compartment.

123

Donnan potentials depend on the respective activities of each ion across the membrane (Eq. 2).

124

Therefore, when the ion activities in both streams are known, one can predict the direction of

125

transport for each ion. For typical DD processes, the activity in the concentrate should be (at

126

least) ten times larger compared to that in the diluate to ensure sufficient driving force.13 For this

127

application, when using (artificial) river and seawater, the activity ratio of ions in seawater and

128

river water is ~90. Donnan equilibria and concentrations over time are given in Fig. 2 for Na+

129

and Mg2+. For each mole of divalent Mg2+ exchanged, double the quantity of moles of Na+ is

130

exchanged until equilibrium is reached. This equilibrium can be determined from Donnan

131

potentials for Na+ and Mg2+ (see Fig. 2c.) during the exchange. Initially, the Donnan potential of

132

Mg2+ is lower than Na+. This means that Na+ will move with its concentration gradient (from

133

seawater to river water), whereas Mg2+ will move against its concentration gradient (from river

134

water to seawater), to maintain electro-neutrality. This exchange will continue until the Donnan

135

potentials of both ions are equal.

7



a

River water

b

Seawater [Na+]

3.5

0.5 0

Equilibrium

100

Equilibrium

[Mg2+]

42

[Mg2+]

0 Time

[Na+]

c

[Mg2+] Equilibrium

50

Time

d

Sea- CEM water

500

River water

[Na+] [Mg2+]

0

136

[Na+]

Concentration [mM]

4.5

Concentration [mM]

Concentration [mM]

420

Donnan potential [mV]

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

Page 8 of 26

Time

0

Position

137

Figure 2. Theoretical batch DD process of river water with seawater as draw solution. Top

138

figures show the concentrations over time in the river water (a) and seawater (b) respectively,

139

where initial concentration are for natural waters. Figure 2c shows Donnan potentials of Na+ and

140

Mg2+ over time. The large potential of Na+ (122 mV) compared to Mg2+ (57 mV) drives the

141

exchange of Mg2+ uphill. Finally, at equilibrium, the Donnan potential of each cation is equal (in

142

this case around 115 mV). Figure 2d shows the initial cation concentration profiles in the

143

seawater, CEM and river water respectively calculated from the initial Donnan equilibrium.

144

Straight arrows indicate transport direction for each cation and dashed arrows show equilibrium.

145

RED model

8


Page 9 of 26 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


146

DD treated water can subsequently be used as input for RED to harvest power from the salinity

147

gradient. For RED, there is a trade-off between open circuit voltage (OCV), which increases at a

148

lower river water salt concentration, and the resistance (Rstack), which decreases at a higher river

149

water salt concentration. To predict the maximum obtainable power density in RED after

150

complete exchange of divalent for monovalent cations in the river water using DD, a previously

151

developed model by Veerman et al. is adapted.23 The aim of this model is to understand the

152

influence of the concentration of salt in the river water. Specific details on the model are given in

153

the Supporting information (SI 1).

154

The dependences of the open circuit voltage (OCV), stack resistance (Rstack) and the power

155

density (PD) are calculated as function of the river water NaCl concentration. In the model, the

156

effect of divalent cations is excluded, as the aim of the model is to establish the effect of

157

complete divalent cation exchange by DD for RED. For natural river water, the NaCl

158

concentration is 0.0035 M, which is well below the NaCl concentration with highest predicted

159

power densities (between 0.01 - 0.02 M for this system). This trade-off between open circuit

160

voltage and resistance in the stack is shown in Fig. 3. The maximum power density at a

161

concentration of 0.014 M NaCl enables a 13% increase in gross power density compared to

162

natural river water (0.0035 M NaCl). In the results section, the experimental results obtained

163

with DD pretreated water will be compared with this predictive model.

9



PD

60

0.9

0.6 30 0.3

OCV

0.0 0.00

164

max at 0.014 M

Cell area resistance [Ω·cm²]

1.2

Power density [w/m²] Cell open circuit voltage [V]

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

Page 10 of 26

R

0 0.01 0.02 0.03 0.04 0.05 River water concentration [M]

165

Figure 3. Modeled trade-off between RED stack performance and concentration of NaCl in river

166

water in M. The graph shows the optimum between open circuit voltage (OCV) and cell area

167

resistance (R). The optimum for gross power density (PD) is between 0.01 - 0.02 M with a

168

maximum at 0.014 M. Seawater concentration (not shown) is at natural concentrations for the

169

Waddensea (NL) (0.40 M).

170

Materials and Methods

171

Membranes and chemicals

172

CEMs – Neosepta standard-grade CMX (Eurodia, France), multivalent-permeable T1 and

173

standard-grade Type II CEMs (FUJIFILM, The Netherlands) were used for DD experiments,

174

only T1 membranes are used for RED experiments. Anion exchange membranes (AEMs) Type I

175

were used for the RED experiments (FUJFILM, The Netherlands). Potassium chloride,

176

magnesium chloride hexahydrate, anhydrous calcium chloride; potassium hexacyanoferrate (III) 10


Page 11 of 26 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


177

and potassium hexacyanoferrate (II) trihydrate, which act as a redox couple for RED, were

178

purchased from Sigma-Aldrich. Sodium chloride (pharmaceutical grade) was kindly supplied by

179

AkzoNobel (Hengelo, The Netherlands).

180

DD in diffusion cells and stacks

181

For lab-scale DD, diffusion cells with compartment volumes of 65 mL (with magnetic stirrers)

182

and an active membrane area of 11.3 cm² were used. Three different CEMs were evaluated.

183

These CEMs were soaked in the same draw solutions as used for DD for 24 h prior to use.

184

Samples of river and seawater compartments were taken over time to monitor the cation

185

concentrations. To scale-up and operate DD continuously, a crossflow RED stack (REDstack

186

BV, The Netherlands) with a 10×10 cm² active area was used. Both set-ups are illustrated in Fig.

187

4. Four CMX membranes were used to create two channels to feed fresh water and three

188

channels to feed seawater. In this manner both CEMs in contact with the fresh water channel

189

were used for ion exchange.

190

11



191

Figure 4. Diffusion cell (a) and stack (b) configurations for DD used in this study. Cation

192

exchange membranes are shown in dark green, river water is shown in dark blue and seawater in

193

light blue; spacers and stir bars are shown in grey.

194

Polyamide woven spacers, to separate the membranes in DD and RED (Deukum GmbH,

195

Germany), had a thickness of 260 µm and a free void fraction (volume) and free surface fraction

196

(projected surface) of 0.726 and 0.476 respectively.

197

Ion chromatography (IC) was used to analyze the cation concentrations (Metrosep C6 – 150/4.0

198

column in a Metrohm 850 Professional IC with eluent of 1.7 mM HNO3 and dipicolinic acid).

199

Cation samples were diluted with 2 mM HNO3 prior to analysis.

200

RED stack performance evaluation

201

Reverse electrodialysis experiments were performed with 10 cell pairs of T1 CEM and Type I

202

AEMs. These membranes were soaked for 24 h in 0.5 M NaCl prior to assembly in the stack.

203

The T1 CEM was selected because of its superior RED performance in waters with divalent

204

cations, however, it still suffers from uphill transport.17

205

A cross-flow RED stack (REDstack BV, The Netherlands) with an active area of 6.5×6.5 cm²

206

was equipped with Pt-coated Ti mesh electrodes (Magneto Special Anodes BV, The

207

Netherlands). The water composition obtained from DD was used as feed in the RED stack. The

208

temperature was controlled (20 ± 1 oC) by a Julabo F12-ED thermostat and pulsation dampeners

209

(in-house built) were used to suppress pulsations caused by the peristaltic feed water pumps

210

(Cole Parmer). An in-house built flow meter (McMillan Co. 101 flo-sen) was used to measure

211

the flow rate. For the electrode rinse, a solution of 0.25 M NaCl with 0.1 M K4Fe(CN)6 and 0.1 12


Page 12 of 26

Page 13 of 26 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


212

M K3Fe(CN)6 was used. An overpressure of 0.1 bar on the electrolyte was applied to ensure

213

membrane and spacer packing.

214

Electrochemical analysis of the stack was performed as described in previous work.17 First,

215

membranes were equilibrated with the feed solution for 20 minutes under current (20 A/m²).

216

Next, the OCV and resistance by alternating current (AC) were measured. For the AC resistance,

217

three measurements were performed at 10, 5 and 1 kHz in that order, at an amplitude of 0.01 A

218

(2.4 A/m²) to measure the ohmic resistance of the stack. Finally, for the direct current (DC)

219

resistance (including non-ohmic resistance) and the power densities, ten current steps from 0 to

220

50 A/m² and ten current steps back to 0 A/m² were applied to measure the total resistance and

221

calculate the stack power densities. Each current density was applied for 30 seconds before the

222

voltage was measured to allow for build up of boundary layers. Membranes were characterized

223

on membrane resistance in 0.5 M NaCl using DC and permselectivity in 0.1/0.5M NaCl.17

224

Results and discussion

225

In this study, we investigate the exchange of divalent cations by DD, as a pretreatment for RED.

226

First, results on DD are reported using lab-scale diffusion cells with three different CEMs. Based

227

on the lab-scale study, the best membrane is selected for continuous operation in a membrane

228

stack. The scale-up of DD in a membrane stack is performed, followed by using the pretreated

229

water in a RED process to assess its effect.

230

DD in lab-scale diffusion cells

231

Figure 5 shows the cation concentrations in the river water during DD. The divalent cation

232

concentrations decrease over time while the Na+ concentrations increase. After 180 minutes, the 13



233

majority of divalent cations is removed (above the experimental detection limit of 0.1 mM).

234

Correspondingly, in the seawater a small decrease in Na+ concentration and a small increase in

235

divalent cation concentrations is observed. Since the absolute concentrations in the seawater are

236

much higher than those in the river water, the relative change in these concentrations is small and

237

therefore the composition of the seawater is hardly changed. These results demonstrate that the

238

exchange of divalent for monovalent cations indeed occurs. Moreover, the reached equilibrium

239

concentrations are equivalent to those predicted by the Donnan potential calculations, shown in

240

the Supporting information (SI 2). 0.002

0.01

0.001

0.00

0.000 0

241

100 200 Time (min)

Seawater

300

Concentration M²⁺ [M]

[Na⁺] [Ca²⁺] [Mg²⁺]

Concentration Na⁺ [M]

River water

Concentration M²⁺ [M]

0.02 Concentration Na⁺ [M]

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

Page 14 of 26

0.40

0.04

[Na⁺] [Ca²⁺] [Mg²⁺]

0.20

0.02

0.00

0.00 0

100 200 Time (min)

300

242

Figure 5. Concentrations of cations in the DD process with natural river (left) and seawater

243

(right) across CMX in a small scale diffusion cell over time. Lines are shown to guide the eye.

244

Measurements are triplicates and errors are typically below 1%.

245

These measurements are performed for three different CEMs and their performances are

246

compared in Fig. 6. The exchange rate should be as high as possible to allow for short residence

247

times. It is expected that the membrane resistance is a suitable predictor for exchange rates, as it

248

is a measure for ionic transport rate at a certain driving voltage. These driven voltages are 14


Page 15 of 26

249

defined by the concentrations in the solutions and are therefore equal for all CEMs. However,

250

there is no clear correlation between the exchange rate and the electrical resistance. This

251

difference, we believe, is caused by the significant leakage for the T1 membrane, which happens

252

simultaneously with the exchange (see Fig. 6a). The leakage is defined here as the total

253

equivalent concentration (ion concentration times charge) in the river water at time t normalized

254

to the starting concentration. Therefore, its effective exchange rates are lower than would be

255

expected from its low resistance. For the best CEM, CMX, 98% divalent cation removal after

256

three hours with 40 mmol/(m2·h) is achieved.

257 200 T1

150

Type II 1.9 Ω·cm²

100

7.4 Ω·cm²

50 0

4

T1 Type II CMX

3

90% 98%

2

99% 1

5

258

c

CMX

3.0 Ω·cm²

Normalized leakage (-)

b Divalent cation exchange rate (mmol/m²h)

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


50 500 Time (min)

5000

5

50 500 Time (min)

5000

15



Page 16 of 26

259

Figure 6. Schematic drawing of the exchange and leakage processes occurring in DD (a).

260

Divalent cation exchange rates (b) and normalized leakage (c) of CMX, Type II and T1 CEMs

261

are shown as a function of log time. Resistances shown in the exchange rate graph are membrane

262

area resistances (b) and percentages shown in leakage graph are the permselectivities of each

263

membrane (c). In the graph with exchange rates, logarithmic trend lines and in the figure with

264

leakage, linear trend lines are plotted for visual aid.

265

Next to the exchange, there is the (unwanted) leakage of salts (due to co-ion (anion) diffusion)

266

from the seawater through the CEM to the river water. The purpose of DD in this case is to

267

exchange the divalent cations from the river water without a major increase of the total salt

268

content in the river water, as this could lower the RED power density due to a lower driving

269

force (salinity gradient). The model in the theory section, predicted an optimum NaCl

270

concentration between 0.01 - 0.02 M. and in Fig. 5, DD reaches the lower limit of this range

271

(0.010 M). The normalized leakage depends on permselectivity, for 100% permselective

272

membranes the normalized leakage should remain close to 1 (the initial concentration). In Fig. 6,

273

it is shown that the lowest permselective membrane T1, with a permselectivity of 90%, leaks

274

more salt over time compared to Type II and CMX with permselectivities of 98% and 99%

275

respectively, as was expected. It appears that there is a trade-off for the divalent cation exchange

276

rate between permselectivity and resistances, and a high (98%) permselectivity is desired to

277

prevent significant leakage. For these reasons (high permselectivity and low leakage), the CMX

278

membrane was selected for scale-up.

279

Scale-up of DD in stacks 16


Page 17 of 26 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


280

These lab-scale experiments show the potential of DD using cation exchange membranes with

281

seawater as draw solution to exchange divalent cations from river water. Residence times or

282

surface-to-volume ratios (~17 m²/m³) are limiting factors in these cells, so to decrease residence

283

times from hours to seconds, one must improve the membrane surface to treated volume ratio to

284

obtain viable residence times. These high surface-to-volume ratios are required for a continuous

285

and high-throughput process without the need for large vessels. For this reason, membrane stacks

286

are investigated to improve surface-to-volume ratios compared with diffusion cells. Crossflow

287

RED stacks have a very high surface-to-volume ratio (~7700 m²/m³ in this study), as the active

288

area is 10×10 cm² with a channel thickness of only 260 µm.

289

Scale-up of DD was performed using CMX membranes, as this membrane showed the lowest

290

leakage and the highest exchange rate in diffusion cells. In Fig. 7, the results for DD with CMX

291

in a stack are shown. At residence times of only several seconds, there is already a clear decrease

292

in Ca2+ in the river water through exchange for Na+, indicating the applicability of larger-scale

293

operation of these systems. Similar divalent cation exchange rates are observed as in the

294

diffusion cells (44 mmol/(m2·h) at 11.3 s). At residence times larger than 7 seconds, the

295

exchange appears to level off. Initially, the diluate compartment consists of nearly 40 mol% of

296

divalent cations, however, after DD the divalent cation content is reduced to between 13 to 4

297

mol% (divalent cation removal of 45 to 76%) at an exchange rate of 100 – 40 mmol/(m2·h)

298

depending on residence time (from 2.7 to 11.3 s), shown in Supporting Information (SI 2).

299

Similar removal results have been obtained in recent work on DD by Vanoppen et al., who

300

achieved divalent cation removal of 70 – 80% at a rate of 20 mmol/(m2·h) using Fuji Type II

301

CEMs in a stack.8 17



0.002

DD-S

DD-L Concentration M²⁺ [M]

0.010

Concentration Na⁺ [M]

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

Page 18 of 26

[Na⁺]

0.001

[Ca²⁺] [Mg²⁺]

0.005

0.000

0.000 0

302

5 10 Residence time (s)

15

303

Figure 7. Concentrations as function of residence time for the scale-up DD in the river water

304

compartment. Short (DD-S) and long (DD-L) residence times for DD are indicated by grey bars,

305

both are used for RED pretreatment. Relative errors of the cation concentrations are below 1%,

306

for clarity error bars are omitted.

307

The results shown here, demonstrate that DD is scalable, and that divalent cations can be

308

removed with residence times of just a few seconds. Detailed comparison between DD in the cell

309

and in the stack is given in the Supporting information (SI 2).

310

RED stack performance

311

Finally, the effect of using DD pretreated river water for RED is investigated. Initially, RED

312

experiments with benchmark concentrations (0.5 M vs 0.017 M) of NaCl are done for

313

comparison with the model calculations and literature.2, 17 The model river and seawaters, as well

314

as treated water by DD at a short (DD-S: 2.7 s) and long DD residence time (DD-L: 11.3 s), are

18


Page 19 of 26

315

used to evaluate the obtainable energy through RED. Water compositions for all streams are

316

provided in the Supporting information (SI 3).

317

90

60 0.1

DD-S DD-L natural

100% NaCl

30

Cell area resistance [Ω·cm²]

0.2 Cell open circuit voltage [V]

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


0.0 0 0.00 0.01 0.02 River water concentration [M]

318

Figure 8. Cell OCV and cell area resistance versus river water NaCl concentration. Data (♦ =

319

OCV, ● = area resistance) and model predictions (lines) are shown. Natural is untreated river

320

water and DD-S and DD-L denote short and long DD pretreatment. Measurements are performed

321

in triplicates and error bars are standard deviations.

322

In Fig. 8, the measured OCV and cell area resistances are plotted together with the calculated

323

values from the model. The two extremes of DD are shown in the graph. Longer DD residence

324

times allow for longer exchange times for divalent cations with Na+, resulting in lower divalent

325

cation concentrations. Effectively, the OCV in RED remains very similar before and after DD.

326

This was unexpected as the DD pretreatment decreases divalent cation concentrations and thus

327

should decrease the effect of uphill transport. This expected increase in OCV is shown in the

328

model predictions (see Fig. 8). However, we also exchange these divalent cations for Na+ and 19



Page 20 of 26

329

therefore the concentration gradient in RED decreases as well. In other words, the overall result

330

is that the mitigation of divalent cation uphill transport and the decrease in Na+ gradient

331

counterbalance each other and the OCV is minimally affected.

332

However, the cell resistance is just as important for determining the obtainable power density. In

333

benchmark river water (0.017 M NaCl), this resistance is low due to the relatively high

334

conductivity of the river water compartment. In contrast, natural river water contains divalent

335

cations and a lower overall salt concentration, which result in a higher resistance. The river water

336

treated by DD has a higher concentration of Na+, which has higher activity compared to divalent

337

cations, and therefore the resistance of the river water compartment decreases. The predicted

338

resistances are not in perfect agreement with the measured values, since divalent cations in water

339

and membranes as well as gradient-induced resistances are not included in the predictions. These

340

divalent cations decrease river water conductivity, but increase membrane resistance, so the

341

overall effect is not straightforward to predict. Qualitatively, the predictions do match the data

342

showing that more exchanged Na+ for divalent cations decreases the cell resistance.

343

One could argue that simply increasing the river water conductivity has the same effect. To test

344

this, seawater is mixed with river water to a conductivity equal to the one reached by DD-L, but

345

with different composition (high divalent cation concentrations). RED stack resistances for this

346

water are close to DD treated river water, but the OCV drops significantly due to both a high

347

divalent cation concentration (uphill transport) and a lower overall gradient. This causes a

348

decrease in power density of 21% compared to untreated water (Supporting Information (SI 4)).

20


Page 21 of 26 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


349

In the end, power density is the main performance parameter for RED as it takes both resistance

350

and OCV effects into account. The OCV hardly changes while resistance decreases upon DD

351

pretreatment. Gross power densities - the power output per m² of membrane cell pair area – with

352

DD can be improved by 1.4 to 9.0% compared to the case without DD as determined by

353

experiments. By using a model, a 13% improvement in gross power density is predicted for DD-

354

L at 0.010 M NaCl. With the DD pretreatments in this study, not all divalent cations are

355

exchanged, hence in the RED experiments there are still ~25% divalent cations present, which

356

explains the lower improvements as compared to the predictions. These improvements in power

357

density are achieved without any addition of chemicals (such as for IEX) or electrical energy

358

(such as ED), by utilizing only a concentrated stream that is already present. The cost of DD,

359

however, is an extra pretreatment step, which does require pumping energy. In Figure 9, the net

360

power density after subtracting pumping losses for DD and RED is shown. At high flow

361

velocities (DD-S) there is a large pressure drop and thus loss in net power density, whereas at

362

low flow velocities (DD-L) this pressure drop is smaller. DD-L gives a net power density gain of

363

6.3% over the case without DD pretreatment.

21



0.8

Net Power Density (W/m²)

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

Page 22 of 26

0.6

0.4

0.2

0.0

364

100% NaCl

natural

DD-S

DD-L

365

Figure 9. Net power densities with 100% NaCl, natural waters and DD (short and long residence

366

times) pretreated river water. Measurements are performed in triplicates and error bars are

367

standard deviations.

368

This work aimed to provide a proof of principle of using seawater to exchange divalent cations in

369

the river water as pretreatment for RED. In this research, there is still room for optimization of

370

the DD stack. Higher surface-to-volume ratios and enhanced mixing without drastic increases in

371

pressure drops would allow further improvements in removal of divalent cations in DD. This

372

could be achieved by using mixing promotors, such as profiled membranes.24 In that way, more

373

divalent cations can be exchanged, allowing the experimental power density improvements to

374

approach the calculated 13% power density improvement. Furthermore, less scaling is another

375

benefit of this pretreatment.

376

Acknowledgements

22


Page 23 of 26 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


377

This work was performed in the cooperation framework of Wetsus, European Centre of

378

Excellence for Sustainable Water Technology (www.wetsus.eu). Wetsus is co-funded by the

379

Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the

380

Province of Fryslân, and the Northern Netherlands Provinces. The authors like to sincerely thank

381

the participants of the research theme “Blue Energy” for fruitful discussions and financial

382

support. The authors like to thank Dr. Sylwin Pawlowski from Universidade Nova de Lisboa for

383

the many fruitful discussions.

384

Supporting Information. Additional information on the RED model, Donnan potentials and

385

surface contact and residence times is given. Scale up comparison is given between diffusion

386

cells and stacks. Ionic compositions for all streams are given as well as RED results for pre-

387

mixing some seawater with river water.

388

References

389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407

1. Post, J. W.; Hamelers, H. V. M.; Buisman, C. J. N., Influence of multivalent ions on power production from mixing salt and fresh water with a reverse electrodialysis system. Journal of Membrane Science 2009, 330, (1–2), 65-72, DOI 10.1016/j.memsci.2008.12.042. 2. Vermaas, D. A.; Veerman, J.; Saakes, M.; Nijmeijer, K., Influence of multivalent ions on renewable energy generation in reverse electrodialysis. Energy & Environmental Science 2014, 7, (4), 1434-1445, DOI 10.1039/C3EE43501F. 3. Avci, A. H.; Sarkar, P.; Tufa, R. A.; Messana, D.; Argurio, P.; Fontananova, E.; Di Profio, G.; Curcio, E., Effect of Mg2+ ions on energy generation by Reverse Electrodialysis. Journal of Membrane Science 2016, 520, 499-506, DOI 10.1016/j.memsci.2016.08.007. 4. Turek, M.; Dydo, P., Electrodialysis reversal of calcium sulphate and calcium carbonate supersaturated solution. Desalination 2003, 158, (1), 91-94, DOI 10.1016/S00119164(03)00438-7. 5. Bazinet, L.; Araya-Farias, M., Effect of calcium and carbonate concentrations on cationic membrane fouling during electrodialysis. Journal of Colloid and Interface Science 2005, 281, (1), 188-196, DOI 10.1016/j.jcis.2004.08.040. 6. Casademont, C.; Pourcelly, G.; Bazinet, L., Effect of magnesium/calcium ratio in solutions subjected to electrodialysis: Characterization of cation-exchange membrane fouling. Journal of Colloid and Interface Science 2007, 315, (2), 544-554, DOI 10.1016/j.jcis.2007.06.056. 23



408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450

Page 24 of 26

7. Rózańska, A.; Wiśniewski, J., Modification of brackish water composition by means of Donnan dialysis as pretreatment before desalination. Desalination 2009, 240, (1), 326-332, DOI 10.1016/j.desal.2008.01.054. 8. Vanoppen, M.; Stoffels, G.; Demuytere, C.; Bleyaert, W.; Verliefde, A. R. D., Increasing RO efficiency by chemical-free ion-exchange and Donnan dialysis: Principles and practical implications. Water Research 2015, 80, 59-70, DOI 10.1016/j.watres.2015.04.030. 9. Higa, M.; Tanioka, A.; Miyasaka, K., Simulation of the transport of ions against their concentration gradient across charged membranes. Journal of Membrane Science 1988, 37, (3), 251-266, DOI 10.1016/S0376-7388(00)82432-1. 10. Liao, J.; Li, H.; Zeng, W.; Sauer, D. B.; Belmares, R.; Jiang, Y., Structural Insight into the Ion-Exchange Mechanism of the Sodium/Calcium Exchanger. Science 2012, 335, (6069), 686-690, 10.1126/science.1215759. 11. Miyoshi, H., Diffusion coefficients of ions through ion exchange membrane in Donnan dialysis using ions of different valence. Journal of Membrane Science 1998, 141, (1), 101-110, DOI 10.1016/S0376-7388(97)00297-4. 12. Nouri, S.; Dammak, L.; Bulvestre, G.; Auclair, B., Studies of the crossed ionic fluxes through a cation-exchange membrane in the case of Donnan dialysis. Desalination 2002, 148, (1), 383-388, DOI 10.1016/S0011-9164(02)00734-8. 13. Velizarov, S.; Reis, M. A.; Crespo, J. G., Removal of trace mono-valent inorganic pollutants in an ion exchange membrane bioreactor: analysis of transport rate in a denitrification process. Journal of Membrane Science 2003, 217, (1), 269-284, DOI 10.1016/S03767388(03)00142-X. 14. Rozanska, A.; Wisniewski, J., Brackish water desalination with the combination of Donnan dialysis and electrodialysis. Desalination 2006, 200, (1), 615-617, DOI 10.1016/j.desal.2006.03.490. 15. Vanoppen, M.; Stoffels, G.; Buffel, J.; De Gusseme, B.; Verliefde, A. R. D., A hybrid IEX-RO process with brine recycling for increased RO recovery without chemical addition: A pilot-scale study. Desalination 2016, 394, 185-194, DOI 10.1016/j.desal.2016.05.003. 16. Post, J. W.; Goeting, C. H.; Valk, J.; Goinga, S.; Veerman, J.; Hamelers, H. V. M.; Hack, P. J. F. M., Towards implementation of reverse electrodialysis for power generation from salinity gradients. Desalination and Water Treatment 2010, 16, (1-3), 182-193, DOI 10.5004/dwt.2010.1093. 17. Rijnaarts, T.; Huerta, E.; van Baak, W.; Nijmeijer, K., Effect of divalent cations on RED performance and cation exchange membrane selection to enhance power densities. Environmental Science & Technology 2017, DOI 10.1021/acs.est.7b03858. 18. Moreno, J.; Díez, V.; Saakes, M.; Nijmeijer, K., Mitigation of the effects of multivalent ion transport in reverse electrodialysis. Journal of Membrane Science 2018, 550, 155-162, DOI 10.1016/j.memsci.2017.12.069. 19. Vermaas, D. A.; Saakes, M.; Nijmeijer, K., Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance. Environmental Science & Technology 2011, 45, (16), 7089-7095, DOI 10.1021/es2012758. 20. Galama, A. H.; Vermaas, D. A.; Veerman, J.; Saakes, M.; Rijnaarts, H. H. M.; Post, J. W.; Nijmeijer, K., Membrane resistance: The effect of salinity gradients over a cation exchange 24


Page 25 of 26 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

451 452 453 454 455 456 457 458 459 460 461 462 463 464 465


membrane. Journal of Membrane Science 2014, 467, 279-291, DOI 10.1016/j.memsci.2014.05.046. 21. Geise, G. M.; Curtis, A. J.; Hatzell, M. C.; Hickner, M. A.; Logan, B. E., Salt Concentration Differences Alter Membrane Resistance in Reverse Electrodialysis Stacks. Environmental Science & Technology Letters 2014, 1, (1), 36-39, DOI 10.1021/ez4000719. 22. Moreno, J.; Slouwerhof, E.; Vermaas, D. A.; Saakes, M.; Nijmeijer, K., The Breathing Cell: Cyclic Intermembrane Distance Variation in Reverse Electrodialysis. Environmental Science & Technology 2016, 50, (20), 11386-11393, DOI 10.1021/acs.est.6b02668. 23. Veerman, J.; Saakes, M.; Metz, S. J.; Harmsen, G. J., Reverse electrodialysis: A validated process model for design and optimization. Chemical Engineering Journal 2011, 166, (1), 256268, DOI 10.1016/j.cej.2010.10.071. 24. Pawlowski, S.; Rijnaarts, T.; Saakes, M.; Nijmeijer, K.; Crespo, J. G.; Velizarov, S., Improved fluid mixing and power density in reverse electrodialysis stacks with chevron-profiled membranes. Journal of Membrane Science 2017, 531, 111-121, DOI 10.1016/j.memsci.2017.03.003.

466 467

25



468

Page 26 of 26

For Table of Concents Use Only

469 470

TOC art

471 472 473

Synopsis: This paper described a sustainable method of divalent cation exchange that avoids

474

chemical and power usage to enhance renewable energy harvesting from natural salinity

475

gradients.

476

26


Divalent Cation Removal by Donnan Dialysis for ... - ACS Publications

Recommend Documents