Divalent cation removal by Donnan dialysis for improved reverse

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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

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ACS Sustainable Chemistry & Engineering

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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

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Nijmeijer4,*

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

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

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

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8911 MA Leeuwarden, The Netherlands

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

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

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

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

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The Netherlands

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

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Abstract

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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electrodialysis, hardness removal

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Introduction

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

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

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

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

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

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

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

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efficiencies for ED processes.

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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species.11-14

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

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

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

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

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

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range comparable to RED.16

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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for two monovalent cations), which decreases resistance.

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

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

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

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

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

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

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DD pretreatment.

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

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

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divalent cations for monovalent Na+ from seawater is achieved.

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Theory

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

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

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

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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

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

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

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

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

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

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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

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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


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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

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

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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

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

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

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

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

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

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

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

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

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

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

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potentials of both ions are equal.

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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]

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Time

0

Position

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

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

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

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

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

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

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

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

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RED model

8


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

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

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

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

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

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

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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

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

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

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

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

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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

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

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

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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

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R

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

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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

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

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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

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

176

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


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

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

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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

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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


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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

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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

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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



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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


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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


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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

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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.

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26


Divalent cation removal by Donnan dialysis for improved reverse

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