Selective removal of chloride ions by bismuth electrode in capacitive deionization

Selective removal of chloride ions by bismuth electrode in capacitive deionization

Journal Pre-proofs Selective removal of chloride ions by bismuth electrode in capacitive deionization Junjun Chang, Yuping Li, Feng Duan, Chunlei Su, ...

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Journal Pre-proofs Selective removal of chloride ions by bismuth electrode in capacitive deionization Junjun Chang, Yuping Li, Feng Duan, Chunlei Su, Yujiao Li, Hongbin Cao PII: DOI: Reference:

S1383-5866(19)35328-6 https://doi.org/10.1016/j.seppur.2020.116600 SEPPUR 116600

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

19 November 2019 6 January 2020 19 January 2020

Please cite this article as: J. Chang, Y. Li, F. Duan, C. Su, Y. Li, H. Cao, Selective removal of chloride ions by bismuth electrode in capacitive deionization, Separation and Purification Technology (2020), doi: https://doi.org/ 10.1016/j.seppur.2020.116600

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© 2020 Published by Elsevier B.V.

Selective removal of chloride ions by bismuth electrode in capacitive deionization Junjun Chang a,b,c, Yuping Li a,b,c, Feng Duan a,b, Chunlei Su a,b,c, Yujiao Li a,b,c, Hongbin Cao a,b* a

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, China b

Beijing Engineering Research Center of Process Pollution Control, Institute of Process Engineering,

Chinese Academy of Sciences, Beijing 100190, China c University

of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author. Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected] (Hongbin Cao)

1

Abstract Common methods in capacitive deionization (CDI) for selective removal are derived from membrane separation mechanism or size-based intercalation mechanism. In this study, based on the electrochemical activity of bismuth (Bi) material to chloride ions (Cl−) and inertness to sulfate ions (SO42−), Bi electrode is used as anode in CDI to selectively remove Cl− ions from mixed NaCl and Na2SO4 solution. Through the reaction with Bi, Cl− ions are stored in bismuth oxychloride (BiOCl). Results show that Bi electrode exhibits selectivity for Cl− over SO42− with a selectivity coefficient of more than 1.5 in solutions with mole ratio of Cl− to SO42− larger than 1. The selectivity coefficient is greatly dependent on the mole ratio of Cl− to SO42− and increases when the ratio rises. In solution with a fixed Cl−/ SO42− mole ratio, higher applied voltage and prolonged time result in higher Cl− ion removal capacity, but not necessarily better selectivity. The highest selectivity coefficient of 4.5 is achieved in 8.5 mM NaCl and 1.1 mM Na2SO4 mixed solution at voltage of 1.6 and 2.0 V after charging for 1 h. These results demonstrate that Bi electrode can selectively remove Cl− ions from mixed solution with a relatively lower concentration of SO42− ions, which provides new insights into ion separation and selective removal by CDI. Keywords: Bismuth electrode, selective removal, chloride ions, capacitive deionization.

2

1 Introduction Chloride ions (Cl−) extensively exist in seawater and wastewater discharged from tannery, paper making, chemical industry, landfill and seafood canning [1]. High concentration of Cl− lead to corrosion of steel pipelines [2, 3], agricultural wreck of crops and even contamination of groundwater. Chemical precipitation, adsorption or ion exchange [4-7], oxidation [8] or electrochemical method [9], electrodialysis [10, 11] and membrane separation technologies [12] are common technologies to remove Cl− ions. However, addition of chemicals must be coupled with a removal process. The cost of membranes in electrolysis and membrane separation technologies are high and the membranes require regular maintenance. Capacitive deionization (CDI) is commonly considered as an energy-saving desalination technology based on two electrodes to respectively store charged anions and cations [13-15]. It also provides a method for Cl− ions removal. In most practical cases, Cl− ions are not the only anions and they coexist with other competing anionic species, sulfate ions (SO42−) are often the cases. Under these conditions, only one or several species need to be removed depending on the desirable final water composition. For example, in production of sodium chloride (NaCl) through electrodialytic concentration of seawater, Cl− ions are expected to be separated from SO42− and other anions. What’s more, the separation of Cl− ions in the wastewater is necessary when the wastewater is reused, for example, for supplement of circulating cooling water, because Cl− ions may make the corrosion of stainless steel pipes and equipments more serious. Therefore, Cl− ions are expected to be preferentially removed. In membrane separation technologies, selective removal of Cl− to SO42− has been widely studied using nanofiltration (NF) [16, 17] and electrodialysis (ED) [18-20] technologies. The selectivity for monovalent Cl− ion over 3

divalent SO42− ion benefits from the selective separation of NF membrane and anion exchange membrane (AEM), respectively. Monovalent Cl− ions pass through the membrane while divalent SO42− ions are trapped. Based on their permselectivity to monovalent ions, the NF membrane is integrated into CDI to selectively remove Cl− ions from mixed solution [21]. However, the application of NF membrane and ion exchange membrane greatly increases the cost of CDI. Coating a thin film of ion exchange resin onto the corresponding electrode may reduce the cost [22, 23], but the electrode fabrication is troublesome. Since CDI works by storing ions in the electrode materials, many studies have been devoted to develop novel electrode materials for selective separation, in addition of using membranes or resins [21-27]. In this case, the selectivity mechanism is mainly based on the size-selective intercalation. In most of the publications, the cations are extensively studied. Target ions possessing smaller hydrated radius than the size of the lattices are preferentially intercalated into the lattices of the material. For example, NH4+ ions possessing a smaller hydrated radius than Na+ ions were preferentially removed by copper hexacyanoferrate [28]. Smaller K+ ions were effectively removed compared with bigger Na+ ions by Berlin green nanoparticles [29]. In addition, γ-MnO2 is applied to recovery Li+ from brines containing various cations (Na+, K+, Mg2+, Ca2+) [30]. And Nickel hexacyanoferrate was used to selectively capture K+ ions [31]. Compared with cations, selective removal of anions using non-carbon material is rarely reported. Recently, bismuth (Bi) [32, 33] and silver (Ag) [34] materials have been applied in desalination field to store Cl− ions respectively in bismuth oxychloride (BiOCl) and silver chloride (AgCl) through chemical reaction to form new materials. Inspired by this storage mechanism, if the material can only electrochemically react with Cl− ions 4

while cannot react with other anions, then selective removal of Cl− ions could be achieved. Interestingly, the selective removal of Cl− ions by conversion-type mechanism has not been reported yet. In this study, different from the membrane separation mechanism and size-based selectivity mechanism, we report the selective removal of Cl− ions from mixed NaCl and Na2SO4 solution using Bi electrode in CDI system based on its electrochemical activity to Cl− ions and electrochemical inertness to SO42− ions. The selective removal performance is studied at different applied voltage, time and the initial mole ratio of Cl− to SO42−. We demonstrate that Bi electrode is suitable for selective removal of Cl− ions from mixed solution with a mole ratio of Cl−/ SO42− greater than 1. It may also be applied to capture Cl− ions from solutions containing lower concentration of other anions that cannot electrochemically react with Bi material. 2 Material and methods 2.1 Materials Bismuth (Bi) and activated carbon (AC) powder were obtained from Aladdin (China) and Sigma-Aldrich (USA), respectively. Poly (vinylidene fluoride) (PVDF) and carbon black (CB) were both from Alfa Aesar (USA). N, N-Dimethylformamide (DMF) solvent, sodium chloride (NaCl) and sodium sulfate (Na2SO4) powders were purchased from Beijing Chemical Works (China). Detailed pore structure information of Bi, AC and CB powder is given in Fig. S1 and Table S1 in Supporting Information. 2.2 Electrode preparation Coating method was adopted to prepare Bi and AC electrodes, followed by heattreatment to remove the organic solvent DMF. Typically, AC or Bi powder, PVDF and CB were added into DMF. The mass ratio of AC, PVDF and CB in AC electrode 5

was 8:1:1, while the mass ratio of Bi, PVDF and CB in Bi electrode was 8:0.5:0.1. After stirring overnight, a certain amount of slurry was coated onto a graphite paper (100 × 120 mm2, its weight “w0” was recorded before coating) to form an effective region of 40 × 40 mm2. Then the graphite paper was heated at 80℃ for 24 h. Its weight “wf” was recorded again and the mass loading of Bi and AC was obtained by “wf − w0”. According to our previous work exploring the effect of electrode weight on chloride removal [35], both Bi and AC electrodes with an average mass of ~200 mg were prepared. 2.3 Selective removal tests The selective removal test was performed in a batch-mode operation condition (Fig. S2). The devices and instruments used here include a CDI cell packed with BiAC electrode pair, a DC power supply (M8800, Maynuo DC Source Meter), a peristaltic pump (BT-300L, Lead Fluid) and a liquid storage tank equipped with a stirrer. 80 mL of the feed solution was cycled between the CDI cell and the tank at a flow rate of 40 mL min−1. During charging, 0.1 mL of the treated solution was sampled and diluted periodically. Cl− and/or SO42− concentration were/was analyzed by an ions chromatograph. In controlling experiment, single NaCl or Na2SO4 (8.5 mM) solution was fed. In selectivity tests, four mixed NaCl and Na2SO4 solutions were used as feed solutions. The concentration of NaCl was fixed at 8.5 mM (~500 mg L−1), while Na2SO4 concentration varied at 8.5, 4.3, 2.1 and 1.1 mM. Correspondingly, the mole ratio (MR) of Cl− to SO42− was 1, 2, 4 and 8, respectively. Bi electrode was respectively denoted as Bi-MRa-bV and Bi-NaCl-bV or Bi-Na2SO4bV for mixed and single solution, where a was the MR of Cl− to SO42− ions, b was the applied voltage. Besides, ion removal tests by AC-AC electrode pair packed in CDI were also examined. 6



Cl Selectivity coefficient [21, 25] ( SSO 2  , calculated by Equation 1) was used to 4

compare the removal percentage of Cl− and SO42− ions. Current efficiency (CE) was also calculated (Equation 2).





Cl f 0  0  /  C 0 2  C f 2 / C 0 2  SSO 2   C   C   / C  Cl Cl   SO4 SO4 SO4   Cl 4





0 f CE  3F   CCl0   CClf    V  2F  CSO  V  /  I (t)  dt 2  C SO24  4  

Where

CCl0 

and

(Equation 1) (Equation 2)

CClf  (mM) are the initial and final Cl− concentration,

0 f 2− concentration, respectively. CSO and CSO 2 2  (mM) are the initial and final SO4 4

4

respectively. F (96485 C mol−1) is the Faraday's constant, V (0.08 L) is the volume of the fed solution, and I(t) (A) is the measured current at time ‘t’. 2.4 Characterization The cyclic voltammetry (CV) performance of the Bi electrode (10 × 10 mm2) in single NaCl and Na2SO4 solution was tested on an Autolab electrochemical workstation (PGSTAT302N, Metrohm). A platinum sheet (20 × 20 mm2) was used as the counter electrode and a silver/silver chloride electrode (Ag/AgCl, 3.5 M KCl) as the reference electrode. The morphology of the Bi electrode before and after tests was observed by a field emission scanning electron microscopy (SEM, SU8020, HITACHI). Its crystal structure was measured by an X-ray diffractometer (XRD, Empyrean, PANalytical B.V.). During ion removal process, the concentration of Cl− and SO42− in the samples were analyzed using an ions chromatograph (ICS-5000+, ThermoFisher Scientific).

7

3 Results and Discussion 3.1 CV performance of Bi electrode in NaCl and Na2SO4 solution CV performance of the Bi electrode in single NaCl and Na2SO4 electrolyte was separately measured in 1 M NaCl and Na2SO4 electrolyte (Fig. 1). A pair of peaks centers at 0.28 V and −0.85 V vs. Ag/AgCl when measured in NaCl solution[35]. The two peaks respectively correspond to the oxidation reaction of Bi to bismuth oxychloride (BiOCl) and the reduction reaction of BiOCl to Bi [33]. While measured in Na2SO4 electrolyte, no peaks assigned to oxidation reaction are observed. The very small reduction peak may ascribe to the impurity in Bi electrode. It evinces that Bi can react with chloride ions (Cl−) but cannot react with sulfate ions (SO42−). The electrochemical activity of Bi to Cl− ions and inertness to SO42− ions provide a theoretical basis for removal of Cl− ions by Bi electrode from mixture containing Cl− and SO42− ions. 4

Current / A g

-1

3

1 M NaCl 1 M Na2SO4

2 1 0 -1 -2 1 mV s

-3 -2.0 -1.5 -1.0 -0.5 0.0

0.5

1.0

-1

1.5

Voltage / V (vs. Ag/AgCl)

Fig. 1. Cyclic voltammetry (CV) curves of Bi electrode respectively measured in 1 M NaCl and Na2SO4 electrolyte.

3.2 Removal of Cl− or SO42− ions in single NaCl or Na2SO4 solution Based on CV analysis, the more effective removal performance of Bi electrode to Cl− ions than SO42− ions was firstly confirmed. During ion-removal tests, 8.5 mM NaCl or Na2SO4 single solution was respectively treated in CDI. The typical ion 8

concentration versus time curve is shown in Fig. S3. After applying voltage for 2 h, Cl− concentration decreases from 8.5 to ~7.1, 6.2, 5.1 and 4.2 mM at 0.8, 1.2, 1.6 and 2.0 V, respectively. However, SO42− concentration only drops to ~ 8.3 mM even at 2.0 V. The calculated Cl−-ion removal capacity (~0.6, 0.9, 1.4 and 1.7 mmol g−1) at 0.8, 1.2, 1.6 and 2.0 V is 10.4, 10.8, 13.5 and 14.7 times of SO42−-ion removal capacity (0.05, 0.09, 0.10 and 0.12 mmol g−1). Since the initial concentration of Cl− ions equals to that of SO42− ions, the multiple is also the “selectivity coefficient”. After tests, peaks assigned to BiOCl in XRD patterns are observed for Bi-NaCl-1.2 V electrode (Fig. 2C). However, no new peaks corresponding to new matters show up in BiNa2SO4-1.2 V electrode. In addition, some particles in Bi-NaCl-1.2 V grow larger and bud-shaped structures appear after Cl− storage (Fig. 2D). While Bi-Na2SO4-1.2 V electrode exhibits the same morphology as the pristine Bi electrode. Therefore, it is concluded that Cl− ions are stored in BiOCl through oxidation reaction with Bi particles ( Bi + Cl  + H 2O  3e   BiOCl + 2H + ) [33]. While the slight removal of SO42− ions is not due to the storage in a new substance. The specific surface area of Bi and CB powder is 5.8 and 84 m2 g−1 (Fig. S1 and Table S1), which is only a few percent of the specific surface area for commonly used porous carbon material. SO42− ions are stored in the electric double layers (EDLs) formed on the limited surface of Bi electrode.

9

8

Initial concentration: 8.5 mmol L

7

-1

9

A

SO4 concentration / mM

-

8.5 mM NaCl

6 5 4 3 2

2-

Cl concentration / mM

9

1 0

1.2

0.8

1.6

Bi-NaCl-1.2 V Bi-Na2SO4-1.2 V Bi-Pristine

Intensity / A.U.

6 5 4 3 2 1

8.5 mM Na2SO4

0.8

1.2

1.6

2.0

Applied voltage / V

Applied voltage / V

C

7

0

2.0

B

8

Bi-Pristine

D

50 μm BiOCl (JCPDS No.06-0249)

Bi (JCPDS No.85-1329)

Bi-Na2SO4-1.2 V

Bi-NaCl-1.2 V

10 20 30 40 50 60 70 80 90

2 Theta / Degree

Fig. 2. Concentration of (A) Cl− and (B) SO42− ions in single solution after charging at 0.8−2.0 V for 2 h. (C) XRD analysis and (D) SEM images of Bi electrode before (Bi-pristine) and after treated with single solution.

3.3 Removal of Cl− ions in mixed NaCl and Na2SO4 solution 3.3.1 Impact of mole ratio of Cl−/ SO42− on Cl-removal On the basis of the superiority in ion removal capacity for Cl− ions over SO42− ions in single solution, then it is predicted that Bi electrode can selectively capture Cl− ions from mixed solution containing Cl− and SO42− ions.

10

0.8 0.6

Cl

-

t=1h t=2h

-1

2-

SO4

Ion removal capacity / mmol g

1.9

Bi 0.8 V

A

Mixed Single solution solution

0.4 0.2

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

NaCl Na2SO4 C0=8.5 mM

Bi 1.6 V

8 2 1 4 2Mole ratio of Cl to SO4 Cl

-

-1

0.0

2-

SO4

Ion removal capacity / mmol g

-1

Ion removal capacity / mmol g -1

Ion removal capacity / mmol g

2.0

C

t=1h t=2h

Mixed Single solution solution

NaCl Na2SO4 C0=8.5 mM

8 1 2 4 2Mole ratio of Cl to SO4

2.0

Bi 1.2 V

1.9

Cl

-

t=1h

2-

SO4

B

t=2h

0.8 Mixed Single solution solution

0.6 0.4 0.2 0.0 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

8 1 2 4 2Mole ratio of Cl to SO4

NaCl Na2SO4 C0=8.5 mM Bi 2.0 V

Cl t=1h t=2h

-

2-

SO4

D

Single Mixed solution solution

NaCl Na2SO4 C0=8.5 mM

8 1 2 4 2Mole ratio of Cl to SO4

Fig. 3. Ion removal capacity of Cl− and SO42− ions by Bi electrode in mixed NaCl and Na2SO4 solution with different mole ratio (MR) of Cl−/ SO42− at voltage of 0.8−2.0 V. Single Na2SO4

Charge efficiency / %

100 80 60

solution Single NaCl solution

Mixed solution

0.8 V 1.2 V 1.6 V 2.0 V

40 20 0

1

2

4

8

2-

Mole ratio of Cl - to SO4

Fig. 4. Charge efficiency in single and mixed NaCl and Na2SO4 solution at 0.8−2.0 V with Bi as anode and AC as cathode.

Fig. 3 shows the ion removal capacity for Cl− and SO42− ions in mixed solutions charging at the voltage range of 0.8−2.0 V. Unexpectedly, the ability of Bi electrode to capture Cl− ions is suppressed by SO42− ions, especially for mixed solution 11

containing the same concentration of Cl− and SO42− ions (MR of 1), with Cl−-ion removal capacity decreased by ~90%. Further decreasing in SO42− concentration (increasing in MR) results in increase of Cl−-ion removal capacity, but the value is still smaller than that in single NaCl solution. Besides, the current also changes with the variation of SO42− concentration (Fig. S4). Compared with single NaCl solution, current of mixed solution in MR of 1 is very small and almost coincides with the current of single Na2SO4 solution. And the current increases as the concentration of SO42− decreases, indicating that more and more charges and ions transfer between Bi and AC electrodes. At a given voltage, charge efficiency also rises when MR changes from 1 to 8 (Fig. 4). Even the presence of SO42− ions decreases Cl− removal performance, Bi electrode still shows selectivity for Cl− over SO42− in solution containing lower concentration of SO42− ions, with selectivity coefficient larger than 1 (Fig. 5). Moreover, selectivity coefficient rises as MR increases. The highest

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

A

Bi

0.8 V 1.2 V 1.6 V 2.0 V

1

Selectivity coefficient

Selectivity coefficient

selectivity coefficient of ~4.5 is achieved at 1.6 and 2.0 V in solution with MR of 8.

2

-

4

2-

8

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

B

AC

0.8 V 1.2 V 1.6 V 2.0 V

1

2

4

-

2-

8

Mole ratio of Cl to SO4

Mole ratio of Cl to SO4

Fig. 5. Selectivity coefficient of (A) Bi and (B) AC electrode at different voltages after charging for 1 h.

In solution containing the same concentration of Cl− and SO42− ions, the preferential occupation of SO42− ions on Bi anode surface prevents the contact of Cl− ions with Bi particles, thereby limits their reaction for Cl− storage. It is further due to 12

the higher flux of SO42− than it for Cl− ions calculated by Nernst-Planck equation [24, 27, 36] (Table S2). As SO42− concentration decreases (MR changes from 2 to 8), the flux of Cl− ions exceeds the flux of SO42− ions. More Cl− ions have access to contact and react with Bi particles, resulting in higher Cl−-ion removal capacity and higher selectivity coefficient. One solution to minimize the limitation effect of SO42− on Cl− removal may be to use Bi material as flowable electrode, where Cl− ions are more likely to contact with Bi particles. Morphology and structure characterization of the Bi electrode further provides evidence for Cl− removal in mixed solution with lower SO42− concentration. Typically, when charged at 1.2 V, some Bi particles in Bi-MR2-1.2 V, Bi-MR4-1.2 V and Bi-MR8-1.2 V electrodes involve in volume expansion and bud-shaped structures appear (Fig. 6A and Fig. S5). Besides, BiOCl peaks are also observed in XRD patterns for these electrodes (Fig. 6B) and others charged at 0.8, 1.6 and 2.0 V in solution with MR greater than 1 (Fig. S7). As MR rises from 2 to 8, the signal intensity for BiOCl also increases. These morphology and structure changes suggest the storage of Cl− ions in Bi electrode. However, for Bi-MR1-1.2 V electrode, budshaped structures and BiOCl peaks are both not detected by SEM and XRD, indicating that almost no BiOCl is generated. Indeed, after Cl− storage in mixed solutions, Bi electrodes turn whiter when MR increases (Fig. S6).

13

A

B Bi-MR8-1.2 V

Bi-MR1-1.2 V

50 μm

Intensity / A.U.

Bi-MR4-1.2 V

Bi-MR2-1.2 V

Bi-MR2-1.2 V Bi-MR1-1.2 V BiOCl (JCPDS No.06-0249) Bi (JCPDS No.85-1329)

Bi-MR4-1.2 V

10 20 30 40 50 60 70 80 90

Bi-MR8-1.2 V

2 Theta / Degree

Fig. 6. (A) SEM images and (B) XRD analysis of Bi electrode treated by mixed solution in different mole ratio (MR) of Cl−/ SO42−. The applied voltage is 1.2 V.

In order to make comparison, ion removal tests by AC electrode in CDI is also conducted. Ion removal capacity of AC after charging for 1 h is displayed in Fig. S8 and the calculated selectivity coefficient is shown in Fig. 5B. Compared with Bi electrode, AC can store more SO42− ions. Similarly, as SO42− concentration decreases, Cl−-ion removal capacity increases and SO42−-ion removal capacity decreases too. However, the selectivity coefficient of AC fluctuates around 1, indicating that AC shows no particular selectivity for Cl− or SO42− ions. It is concluded that Bi electrode is suitable for removal of Cl− ions from mixed solution with a relative low concentration of SO42− ions. It may also be applied to remove Cl− ions from mixed solution containing other anions that cannot electrochemically react with Bi, such as carbonate ions (CO32−) or hydrogen carbonate ions (HCO3−) and hydrogen phosphate ions (HPO42−, Fig. S9). 3.3.2 Impact of applied voltage on selectivity coefficient In solution with a fixed MR, high voltage results in high Cl−-ion removal capacity. However, high voltage does not necessarily correspond to high selectivity coefficient. Because high voltage contributes to both Cl− removal and SO42− storage, 14

and the ratio of the percent removed Cl− to that of SO42− ions varies. For example, in solution with MR of 2, the highest and lowest selectivity coefficient locates at 1.6 and 2.0 V, respectively. In solution with MR of 4, the voltage corresponding to the maximum and minimum selectivity coefficient is 1.6 V and 0.8 V, respectively. While in solution with MR of 8, the selectivity coefficient at 1.6 V almost equals to that at 2.0 V. What’s more, side reactions become severer as voltage rises, demonstrating by the decrease in charge efficiency (Fig. 4). Therefore, optimized voltage should be applied to balance the Cl−-ion removal capacity and selectivity in different solutions. In addition, the concentration of the dissolved oxygen and the pH of the solution vary at different voltages (Fig.7). Compared with dissolved oxygen in controlling experiment with the feed solution flowed but without voltage applied to CDI cell, dissolved oxygen decreases rapidly in the early stage of charging (Fig.7A). The decay rate rises with an increase in applied voltage. The decreased oxygen mainly undergoes reduction reaction onto the AC cathode. During the first 60 min of charging, the protons

(H+)

generated

from

Cl−

storage

process

( Bi + Cl  + H 2 O  3e   BiOCl + 2H + ) can cover the hydroxyl ions (OH−) mainly formed from oxygen reduction, therefore, the pH decreases (Fig.7B). While in the later 60 min of charging, the concentration of dissolved oxygen is almost constant. Because the voltage is applied to the CDI cell, oxygen is still involved in reduction reaction. There should be a supply source of oxygen to balance the consumed oxygen. Water oxidation reaction may be a possible source. The level-off trend in the later stage of charging is the result of the dynamic balance between the oxygen produced by water oxidation and the oxygen consumed by reduction reaction. And the reduction of H+ at low voltage or the generated OH− from water reduction at high voltage lead to the increase in pH value. 15

8.5

8.5 mM NaCl and 2.1 mM Na2SO4 solution

10

A

8.0

2.0 V

8

Without voltage

7

7.5 7.0

0.8 V 1.2 V

6.5

1.6 V 2.0 V

6.0 5.5

B

9

pH

-1

Dissolved oxygen / mg L

9.0

0

20

40

60

80

100 120

1.6 V

6 5

1.2 V

4

0.8 V

3 2

0

20

Time / min

40

60

80

100 120

Time / min

Fig.7. The variations of (A) the dissolved oxygen concentration and (B) the pH of the treated solution at different applied voltages. 8.5 mM NaCl and 2.1 mM Na2SO4 mixed solution is fed.

Selectivity coefficient

3.3.3 Impact of time on selectivity coefficient 5.0 2h 4.5 1 h 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1

0.8 V 1.2 V 1.6 V 2.0 V

4 2 2Mole ratio of Cl to SO4

8

Fig. 8. Summary of selectivity coefficient of Bi electrode at time of 1 h and 2 h.

Fig. 8 summarizes the selectivity coefficient of Bi electrode at time of 1 h and 2 h. The impact of the prolonged time on selectivity is both related to the MR of Cl− to SO42− and the applied voltage. For example, in solution with MR of 4, the prolonged time favors selectivity, but in solution with MR of 8, the selectivity coefficient decreases when time increases. In solution with MR of 4, the applied voltage still contributes more to Cl− removal after charging for 1 h. In solution with MR of 8, more Bi particles react with Cl− ions and more particles is exposed, which leads to an 16

increase in surface area. The increased surface area may provide more sites for SO42− ions to store. After charging for 1 h, the Cl− removal rate slows down, and more SO42− ions are stored. The percent of the removed SO42− increases and the selectivity coefficient decreases. Even the selectivity coefficient reduces, it is still larger than 3. In all cases, with prolong of time, more Cl− ions are removed. Therefore, charging time can be varied to meet different requirements for Cl− removal and selectivity. 3.3.4 Long-time stability A

8

Selectivity coefficient

Concentration / mM

9

7 Cl

2.20 2.15

-

2-

SO4

Storage stage (1.2 V) Release stage (-0.4 V)

2.10 2.05 2.00

0

100 200 300 400 500 600 700

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

B

1

2

3

pH

12 11 10 9 8 7 6 5 4 3 2

4

5

6

7

8

9 10

Cycle number

Time / min

C

1.2 V-storage -0.4 V-release

0

100 200 300 400 500 600 700

Time / min

Fig. 9. Cycle performance of Bi electrode when treated by mixed solution in MR of 4: (A) concentration variations of Cl− and SO42− ions, (B) selectivity coefficient during 10 storage-release cycles and (C) the variation of the pH value.

Multiple storage-release cycles (10 cycles) were run to evaluate the stability of Bi electrode and selectivity performance. A forward voltage of 1.2 V and a reversed voltage of −0.4 V were applied for 60 and 10 min, respectively. Mixed solution with MR of 4 was fed. 17

Fig. 9A and Fig. 9B shows the changes of ions concentration and selectivity coefficient in 10 cycles. The concentration difference during storage stage decreases as increasing in cycle number, both for Cl− and SO42− ions. In addition, the stored Cl− ions are not all released when the voltage is reversed. One reason may be ascribed to the pH change (Fig. 9C). In release process, pH of the solution rises and the solution shows

alkaline

condition,

where

water

severs

as

the

oxygen

acceptor

( BiOCl + H 2O + 3e   Bi + Cl  + 2OH  ). It is confirmed that alkaline condition does not favor Cl− release and the kinetics of BiOCl reduction is limited in alkaline condition compared with acidic condition [33]. In addition, if the solution contains calcium and magnesium ions, they should be removed previously by ion exchange or nanofiltration to avoid precipitation on electrode in such an alkaline condition. Another reason for the incomplete release may be the poor charge transfer between BiOCl and current collector or carbon black. In the pristine Bi electrode (Fig. S5G), Bi particles are in contact with CB particles by PVDF binder. After Cl− storage, Bi particles undergo volume expansion. Some CB particles are not in contact with BiOCl particles (Fig. S5H) anymore. Charges cannot transfer between these CB and BiOCl particles. Even Cl−-ion storage capacity degrades, Bi electrode shows better stability for selective removal of Cl− ions, with selectivity coefficient around ~3.2 (Fig. 9B).

Conclusions

In this study, derived from the electrochemical activity and inertness of bismuth (Bi) material on chloride ions (Cl−) and sulfate ions (SO42−) respectively, Bi electrode is used to selectively remove Cl− ions from mixed NaCl and Na2SO4 solution in capacitive deionization. It provides a new reaction-active mechanism for selectivity removal. Even SO42− ions decreases Cl−-ion storage capacity compared with that in 18

single NaCl solution, Bi shows selectivity for Cl− over SO42− ions in mixed solution containing a lower SO42− concentration. Moreover, Bi electrode may also be used to remove Cl− ions from mixed solution containing other anions such as carbonate (CO32−) and hydrogen phosphate ions (HPO42−). Although Cl− storage performance attenuates as cycle increases, Bi electrode exhibits a more stable selectivity to Cl− ions. The cycle performance may be further developed by applying Bi materials as flowable electrode.

Acknowledgments

This work was supported by National Natural Science Foundation of China (NSFC) [grant number 21976180, 51878645].

References

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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CRediT author statement Junjun Chang: Conceptualization, Methodology, Writing - Original Draft Yuping Li: Conceptualization, Resources, Funding acquisition Feng Duan: Investigation, Writing - Review & Editing Chunlei Su: Investigation, Methodology Yujiao Li: Formal analysis, Writing - Review & Editing Hongbin Cao: Conceptualization, Resources, Supervision

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Graphical abstract

+ SO42− Cl− Cl− Cl−

SO42−

Cl− SO42− Cl− Cl− SO42− − SO42− Cl − Cl− Cl Cl− Cl− Cl− Cl−SO 2− SO42− 4Cl−

Cl− − Cl Cl− Cl− Cl− Cl− Cl− − Cl−Cl− Cl

Generation of BiOCl

Bi

Cl−

BiOCl

SO42−

SO42−

SO42− Cl−

SO42− Cl



SO42−

Bi

Inflow

+

Bi 2−

SO4

Adsorption

26

Cl−SO

2− 4

SO42−

Outflow

Highlights 

Bismuth (Bi) material is electrochemically active to Cl− and inert to SO42− ions.



SO42− ions decreases chloride storage capacity in Bi electrode.



Bi can selectively remove Cl− from mixed solution with lower SO42− concentration.

27