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Bromide ions specific removal and recovery by electrochemical desalination Izaak Cohen, Barak Shapira, Eran Avraham, Abraham Soffer, and Doron Anurbach Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00282 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018
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Bromide
ions
specific
removal
and
recovery
2
electrochemical desalination
3
Izaak Cohen*, Barak Shapira, Eran Avraham, Abraham Soffer, Doron Aurbach
4
Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel
5
* Corresponding author:
[email protected] by
6 7
Abstract
8
Removal and recovery of bromide ions by electro-oxidation and electro-reduction are
9
presented using hybrid physical adsorption and capacitive deionization cells, which
10
contain activated carbon cloth electrodes. This is a proof of concept research with results,
11
which indicate that when comparing the removal and recovery quantities of bromide and
12
chloride ions (starting with the same initial concentration of 0.05M for both salts), the
13
desalination capacity of the bromide ions is larger by almost two orders of magnitude
14
than that of the chloride ions; thus, we obtained specific desalination of bromide ions
15
from a solution containing chloride ions. Removal and recovery of 3.5 mmoles of
16
bromide ions were achieved by a working electrode with 1 gr of activated carbon cloth,
17
and the calculated energy consumption for the removal and recovery of 1 gr of bromide
18
ions was about 2.24 kJ/gr.
19
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1. Introduction
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The bromine industry encompasses a variety of applications, including fire
22
extinguishers, agriculture, and healthcare. The industrial waste of power generation,
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bromine factories and hydraulic fracturing1 contains a vast amount of bromide ions that
24
can be recovered and reused.
25
Removal and recovery of bromide ions was done in the past for environmental and
26
economic purposes1–4. There are problematic environmental aspects and concerns when
27
potable water contains traces of bromide ions, because their presence can lead to the
28
formation of trihalomethanes (THMs) 3,5 and haloacetic acids6 as by products due to
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chlorination, and bromate anions due to ozonation7,8. Such species are forbidden to
30
excide levels around 4X10-7, 3X10-7 and 7.8X10-8 M respectively by the United States
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Environmental Protection Agency due to detrimental effects of THMs on the liver,
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kidney or the central nervous system. Also, there is a serious risk that the above three
33
types of hazardous materials are carcinogenic. Therefore, researchers are trying to avoid
34
contamination of water reservoirs by bromine compounds3,4 by removal of bromide ions
35
from contaminated water sources1,2.
36
Regular desalination methods, such as reverse osmosis and direct distillation,
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require large amounts of energy to produce high pressures and temperatures, respectively,
38
while desalinating with no selectivity towards the removal of bromide9–11. Hence,
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previous research used methods such as electro-oxidation of bromide ions1–3 and ion-
40
exchange resin4 for specific removal of bromide ions from concentrated solutions and tap
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water. These developed working methods were not suitable for industrial processes due
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to the cost efficiency of the processes and the products. The electro-oxidation of bromide
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ions requires the addition of KI solution (that has a price) to change the bromine back to
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bromide. Producing I2 in turn, needs investment of further efforts.
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membrane and stable electrodes costs, and also a problem related to the poor regeneration
46
of the exchange resin. The new concept that we develop and describe herein is based on
47
the use of a special cell which includes simple activated carbon electrodes. By single
48
charge/discharge cycles this cell removes and recovers selectively bromide ions, from a
49
mixed solution of bromide and chloride ions. This concept may lead to the development
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of a new technology that might be implemented practically in industrial processes.
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Hence, this paper shows a working proof of concept. However, further experimental
52
work is required before the concept we show herein can be translated to fully practical
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separation processes.
There are the
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For selective electro-oxidation of bromide ions (Eq. 1), we need to consider the
55
electro-oxidation potentials of chloride ions and water (Eqs. 2 and 3), which are quite
56
close (the equations were taken from handbook of chemistry and physics12).
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2 ← → + 2 = 1.087 ,
(1)
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2 ← → + 2 = 1.358 ,
(2)
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! 2 ← → + 4 + 4 = 1.229 .
(3)
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Based on a previous study with graphite electrodes1, specific electro-oxidation of
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bromide to bromine was achieved using a solution containing different concentrations of
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chloride and bromide ions, without generating chlorine or oxygen. However, in that study
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a potassium iodide solution was used to capture the released bromine gas, and the use of
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graphite required a membrane to separate the anode and cathode, and the cathode
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(negative electrode) produced hydrogen.
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In this study, we use hybrid physical adsorption and capacitive deionization
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(HPA-CDI) technology, which is based on capacitive deionization (CDI), an energy-
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efficient water desalination technology13–15. CDI cells contain high-surface-area
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electrodes, which are usually activated carbon materials that when polarized below the
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water-splitting potential (Eq. 3) create an electrical double layer that adsorbs the counter-
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ions to the high surface area of the electrodes, thereby producing a diluted solution. When
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discharged, the solution becomes concentrated, and is therefore routed to a waste stream.
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Removal and recovery of bromide ions by electro-oxidation and electro-reduction
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were achieved using asymmetric CDI (A-CDI) cells, which contain activated carbon
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cloth (ACC) as electrodes. A-CDI cells were used previously to achieve better water
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desalination performance16–18. Here, we used A-CDI cells to enable electro-oxidation of
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bromide ions (applying a positive potential of around 1 V on the positive polarized
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electrodes, see Eq. 1), while mitigating the water electrolysis. A-CDI cells enable
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asymmetric polarization of the electrodes, whereby the applied potential is divided
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between the positive and negative electrodes asymmetrically, a higher potential falling on
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the positive electrode (with low surface area).
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The essentially non-polarized activated carbon exhibits a strong interaction with
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bromine molecules, and is thus commonly used in industry. A study that was conducted
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using carbon black with surface area of only 100 m2/gr showed a physical adsorption of
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bromine with a specific capacity capability of around 1.25 mmole/gr19. Upon electro-
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oxidation of the bromide ions, the bromine molecules are formed near the surface of the
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positive ACC electrodes. Thus, the bromine molecules are encouraged to physically
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adsorb to the surface of the ACC electrodes; hence, we call this technology hybrid
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physical adsorption and capacitive deionization (HPA-CDI). When discharging the
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electrodes, bromine molecules that are physically connected to the positive ACC
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electrodes are reduced back to bromide ions and go back to the solution.
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2. Materials and Methods
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2.1. CDI cell structures
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The flow-through HPA-CDI cell structure has a flange-type design (Figure 1).
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The solution is introduced through plates to ensure homogeneous flow through the entire
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circular cross-section of the cell. The electrodes were made of commercial activated
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carbon cloth (ACC-5092-15, Nippon Kynol, Japan) with high surface area (1440 m2/gr
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BET) originating from phenol-formaldehyde polymeric fibers that underwent
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carbonization and activation. The HPA-CDI cells contained 24 ACC disc electrodes – 4
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positively polarized working electrodes (WEs) and 20 negatively polarized counter
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electrodes (CEs). Sheets of porous polyethylene cloth served as separators between the
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electrodes and exhibited low resistance to the solution flow. Silicon glue was soaked into
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the rims of the separator discs at the perimeters, thereby forming soft and elastic gaskets.
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These separator sheets with gaskets at their perimeters provided the necessary mechanical
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and electrical separation between the electrodes (thereby preventing short circuits). Ring
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spacers made of poly-tetra-fluoro-ethylene (PTFE) were used for the electrode casings,
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with 0.5 and 2.5 mm deep grooves that held the positive (one ACC disc) and negative
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(five ACC discs) carbon electrodes, respectively. The current collectors were made of
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graphite paper sheets (Grafoil) that were attached to the electrodes in the cell; they were
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perforated to allow a smooth flow of solution. A reference electrode (RE) was placed at
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the middle of the cell: a silver mesh covered by AgCl (by anodization of the mesh in 0.1
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M HCl solution). When all HPA-CDI cell components – the electrodes in their plastic
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cases, the graphite sheet current collectors, and the separator sheets with polymeric
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gaskets at their perimeters – were pressed together, in the right order, they formed a
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hermetically sealed flow-through multi-electrode electrochemical cell that functioned as a
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three-electrode cell.
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2.2. System set-up
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A graphical description of the layout of the entire setup that was used here is
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presented in Figure 2. A 5-liter round-bottom flask was used as the solution reservoir for
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the experimental system. It contained 5 liters of 0.05 M NaCl (>99.5% pure, Sigma-
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Aldrich, USA) and 0.05 M NaBr (>99% pure, Strem Chemicals, USA), in highly purified
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water (18.2 MΩ). Before the polarization of the HPA-CDI cell, the solution was
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circulated for air evacuation in a closed system, which included the electrochemical cell,
125
the pumps, and the conductivity probe. The conductometer probe (Metrohm, 712 model)
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was connected to the outlet of the HPA-CDI cell to measure on-line the conductivity of
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the solution, which flows out of the cell. In order to control the solution flow, a
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parametric pump (Fluid Metering Inc.) was used, and the solution flow was calibrated to
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8.5 ml/min. The potentials were applied to the cell by a Potentiostat (Metrohm, Autolab,
130
PGSTAT302N). When polarizing the cell electrodes, the system setup, as described in
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Figure 2, was modified to an open system, where the solution exits the system for
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analytical sampling.
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2.3. Analytical tools
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The collected samples were measured for a quantitative evaluation of bromide, bromate
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and chloride ions solvated inside each sample solution. Ion chromatography was carried
136
out with 9X10-3 M of Na2CO3 (Dionex ICS-2100, Thermo Scientific) to measure the
137
weight of the different ions inside each sample. Additionally, the weights of the ions were
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measured by titration with AgNO3 at a concentration of 0.01 M (848 Titrino Plus, Page 7 of 25 ACS Paragon Plus Environment
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Metrohm) to confirm and evaluate the different ions, which were difficult to evaluate by
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ion chromatography.
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CDI technology is usually employed for its capacitive electrostatic properties. To use
144
it for electrochemical reactions with bromide ions and to avoid electrochemical reactions
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with chloride ions while using ACC electrodes, a preliminary study of the working
146
potential domains was conducted.
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3.1. Comparison between chloride/chlorine and bromide/bromine redox reactions
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by cyclic voltammetry
149
Two different solutions for cyclic voltammetry CV measurements were prepared:
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one contained 0.05 M of NaCl, and the second contained 0.05 M of NaBr. The electrodes
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used for the three-electrode cell were a saturated calomel electrode (SCE) as RE, and WE
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and CE, which were both made from ACC (ACC-5092-15, Nippon Kynol, Japan), in a
153
weight ratio of 1 to 10, respectively. Figure 3 shows the CV results, where the x axis is
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the potential that was measured between the RE and WE, and the y axis is the capacitance
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(in units of F/gr). The scan rate was 1 mV/sec, the lower vortex potential was -0.1 V and
156
was the same for all CVs, and the upper vortex potentials were applied in an arithmetic
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progression from 0.5 to 1 V, with a common increment of 0.05 V. Each CV was repeated
158
twice for every upper and lower vortex potential. It can be clearly seen from the results
159
that the chloride ions preserved almost the same electrostatic capacitive behavior, even
160
when the potential applied to the upper vortex was 1 V. In contrast, a redox reaction is
161
seen clearly in the CV plot, where the bromide ions were oxidized to bromine and
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reduced back to bromide ions, starting from a potential of 0.8 V, and reached a stronger
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bromide/bromine redox interaction when the applied upper vortex potential was higher.
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The high peak of the anodic oxidation indicates that there are also irreversible
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reactions like water splitting, which is also indicated by the NaCl high potential CVs.
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Nevertheless, the results are consistent for each voltage scan range in Figure 3 that shows
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reversible redox reactions, which confirms the reversibility of the bromide ions to
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bromine conversion and vice versa.
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3.2. Determination of the working potential domain of bromine/bromide by CV
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To determine the working potential domain, we used a mixed solution of NaCl and
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NaBr in concentrations of 0.05 and 0.005 M, respectively, where the sodium chloride salt
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was used as a supporting electrolyte. This was based on the knowledge acquired from the
173
previous results (Figure 3) that show CVs of chloride ions in capacitive behavior when
174
using potentials lower or equal to 1 V. The three-electrode cell was the same as the
175
previous one (Section 3.1), using pristine electrodes. Figure 4 shows the CV plots
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resulting from two different scan rates, 1 and 0.5 mV/sec. To focus on the redox
177
reactions, for which the beginning and termination are not clear in Figure 3, an electrolyte
178
with less bromide was required; to avoid the IR drop, we used a supporting electrolyte of
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NaCl. A scan rate of 1 mV/sec for the low concentration of NaBr was too fast for the
180
diffusion kinetics of the dilute bromide ions; therefore, the plot indicated a capacitive
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behavior. When using a slower scan rate of 0.5 mV/sec, the plot showed capacitive
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behavior between the potentials of -0.1 and 0.5 V and electrochemical redox behavior
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between 0.5 and 1 V.
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3.3. Specific removal of bromide by flow-through HPA-CDI cell
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After verifying the working potential domain (Section 3.2), specific desalination of
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HPA-CDI cell that enables to control the potential of the positive electrodes that were
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polarized between 0.5 to 1 V. The cell type was a flow-through HPA-CDI cell with
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pristine ACC electrodes in a weight ratio of 1 to 5 between the WE and CE respectively,
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and the solution contained bromide and chloride ions at concentrations of 0.05 M each.
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The HPA-CDI WEs were charged to 1 V and discharged to 0.5 V each cycle (when
192
polarized to 1 V the overall potential was measured to be around 1.4). An illustration of
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the HPA-CDI cell is shown by figure S2 A-D in order to clearly explain its operation
194
mechanism. Figure 5 shows three cycles of specific removal and recovery of bromide
195
ions. The samples were taken after two preliminary cycles that were carried out to
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achieve proper operation of the electrodes inside the HPA-CDI cell. The three cycles
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repeated themselves with a moderate rise in the removal and recovery capacity of the
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bromide ions, which can be explained by traces of air that were trapped inside the ACC
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micro-pores after the preliminary cycles20. While the removal and recovery of the
200
bromide ions were dominant, those of the chloride ions were so low that the change in
201
concentration was within the error of the analytical method; hence, we achieved a
202
specific removal of bromide ions. Figure S1 shows a plot of conductivity vs. time that
203
was obtained by a conductivity probe that was located next to the exit of the CDI cell.
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There is a significant correlation between the plot in figure 5, based on ex-situ ions
205
concentration measurements and the plot in figure S1 which show results of in-situ
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conductivity measurements. Furthermore, all measured samples showed that the
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concentration of bromate ions was less than 7.8X10-5 M, these measurements help to
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exclude the presence of bromine inside the water, which is hazardous to the environment.
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Figure 6 shows the current vs. time plot corresponding to Figure 5. The three cycles
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shown in Fig. 6 are consistent, which provides another indication for the reproducibility
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of the results, from the aspect of the electric charge. Based on the integral of the current,
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the overall charges that were used for the charge and discharge in each cycle were
213
calculated.
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The results from Figure 5 were calculated and organized into a column chart as
215
displayed in Figure 7, which compares the three cycles by the accumulated amount of
216
anions (bromide and chloride ions) that were adsorbed / desorbed by the HPA-CDI cell.
217
The y axis represents the amount of bromide and chloride ions that were removed and
218
recovered, in mmol units and normalized by the WE ACC electrode total weight, which
219
was 1.7 gr. The chart shows that the accumulated removal and recovery of bromide ions
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in each cycle were almost the same; hence, almost all of the bromide ions that were
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adsorbed and electro-oxidized into bromine inside the porous structure of the ACC
222
electrodes while polarizing to a potential of 1 V, were electrochemically reduced back
223
into the solution, as bromide ions, when the ACC electrodes were polarized back to a
224
lower potential of 0.5 V. Based on the results obtained when comparing the bromide and
225
chloride ion incremental removal and recovery, the bromide ion removal and recovery
226
were almost two orders of magnitude greater, thus we obtained specific desalination of
227
bromide ions from a solution that also contained chloride ions.
228
When working in capacitive mode, the capacitance of a 1 gr ACC electrode was
229
calculated to be about 100 F/gr (only per WE weight), which when translated to the
230
maximum salt removal (dividing by the Faraday constant) amounts to about 1 mmol. In
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this study, we multiplied the maximal theoretical desalination capacity based on electric Page 12 of 25 ACS Paragon Plus Environment
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double layer capacity by a factor of 3.5, to arrive at measurable bromide removal capacity
233
which is probably not the maximal removal capacity that can be reached. To better
234
understand how it is possible to obtain 3.5 mmol removal / recovery of bromide ions by 1
235
gr of ACC WE, we conceived a rational explanation. When the ACC electrodes are
236
polarized to 1 V, the bromide ions are electro-oxidized to bromine. Activated carbons
237
have a high physical connection with bromine molecules, and the bromine molecules are
238
created inside the micro-porous structure of the ACC, which impairs the molecule
239
movement back into the solution; meanwhile, other bromide ions, which are
240
electrostatically adsorbed into the pores and are electro-oxidized, interact with the
241
bromine molecules to give tribromide and pentabromide ions, as described by Eqs. 4–6: %&
242
+ '#($ ) ,
243
+ 2 '#($ + ,
244
) + '#($ + .
(4)
%*
(5)
%,
(6)
245
Bromide, tribromide and pentabromide equilibrium was well investigated21–23. Based on
246
this explanation, we can also understand the slow reduction of bromine to bromide ions
247
in Figure 4, which occurs due to the slow kinetics of the intermediate molecular
248
structures that finally leads to bromide ions.
249
To calculate the energy (J/gr) that was used for the removal and recovery of bromide
250
ions from a solution of NaCl and NaBr, both at a concentration of 0.05 M, we used Eq. 7:
251
012
= - ∙ / 3∙45
(7)
252
Where E is the energy used for the removal and recovery of 1 gr of bromide ions, V is the
253
overall potential used for the electro-oxidation of bromide ions to bromine, which was Page 13 of 25 ACS Paragon Plus Environment
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measured to be about 1.4 Volt, / 678 is the charge used for the removal of bromide ions,
255
n is the amount of bromide ions that were removed in moles, and Mw is the molar mass of
256
the bromide ions. The energy consumption for the removal and recovery of 1 gr of
257
bromide ions was calculated to be about 2.24 kJ/gr.
258
Nevertheless, when using ACC electrodes, there are problems arising from the oxidation
259
reactions of ACC electrodes that take place when the positive electrodes are polarized to
260
the potentials at which bromide ions are oxidized to bromine (eq. 8-9)24–26.
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C + O → CO + 2 ! + 2 = 0.518 V SHE,
262
CO + O → CO + 2 ! + 2 = 0.104 V SHE.
(8) (9)
263
The surface oxidation reactions cannot be seen in the CV plots of Figures 3 and 4 that
264
show mostly a capacitive behavior related to the bromide redox reactions and thus are
265
undetectable electrochemically. The study was focused upon a new concept of operation.
266
For measuring the effect of surface oxidation reactions on the desalination process, long
267
term experiments are required. We intend to continue studying the long term degradation
268
processes in CDI operation (as we also explored in the past27).Additionally, because of
269
the electro oxidation of bromide ions to bromine molecules inside of the nanoporous
270
structure of the ACC we need to consider the possible production of hypobromouse acid,
271
that might enhance the surface oxidation of the positively polarized electrodes.
272
The experiments in this paper give a starting point for research into optimization of the
273
HPA-CDI method by using different concentrations of electrolytes and a variety of
274
activated carbon electrodes with various flow regimes and flow rates.
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Addition to Stilbenes in Chloroform: Influence of the Bromide-Tribromide-Pentabromide
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Equilibrium in the Counteranion of the Ionic Intermediates. J. Org. Chem. 1992, 57 (24),
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Their Possible Involvement in the Product-Determining Step of Olefin Bromination. J .
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Chlorine Species. Inorg. Chem. 1994, 33 (25), 5872–5878.
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Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. The Effect of the Flow-
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Regime, Reversal of Polarization, and Oxygen on the Long Term Stability in Capacitive
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de-Ionization Processes. Electrochim. Acta 2015, 153, 106–114.
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Fig. 1.
350 Solution outlet
PVC upper cover Current collector 5 X Activated carbon cloth electrodes Spacer (polytetrafluoroethylene) Separator One activated carbon cloth electrode Ag/AgCl Reference electrode mesh Water dispenser PVC bottom cover
351 Solution inlet
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Fig. 1. Illustration of the flow-through HPA-CDI cell used in this study.
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Fig. 2.
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Fig. 2. Illustration of the HPA-CDI experimental setup used in this study.
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Fig. 3.
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Fig. 3. Cyclic voltammetry plots created by a three-electrode system, where the y axis
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represents the capacitance (F/gr) and the x axis represents the applied potential in
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reference to a saturated Hg/HgCl electrode (SCE). The scan rates and concentrations of
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both NaCl and NaBr solutions are 1 mV/sec and 0.05 M, respectively. The NaCl solution
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preserves its capacitive behavior even at high potential of 1 V, where a small rise in water
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splitting can be seen. On the other hand, the NaBr solution preserves capacitive behavior
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until the high vortex potential reaches 0.8 V, above which it displays electrochemical
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redox behavior.
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Fig. 4.
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Fig. 4. Cyclic voltammetry plot, where the y axis represents the capacitance (F/gr) and
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the x axis represents the applied potential in reference to saturated Hg/HgCl electrode
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(SCE). A mixed solution of NaCl and NaBr in concentrations of 0.05 and 0.005 M,
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respectively, was prepared and used in a three-electrode system, where the NaCl salt was
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used as a supporting electrolyte. Two different scan rates were used: the 1 mV/sec scan
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rate preserved capacitive behavior in the cathodic side, and at high potentials gave an
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oxidation peak in the anodic polarization; the 0.5 mV/sec scan rate preserved capacitive
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behavior between -0.1 and 0.5 V, with a working potential domain for the redox
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electrochemical reaction between 0.5 and 1 V.
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Fig. 5.
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Fig. 5. Plot of salinity of a mixed solution of NaCl and NaBr, in molar units vs. time,
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measured during the last three of five cycles of operation by an HPA-CDI cell. The
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starting concentrations of both NaCl and NaBr were 0.05M. Samples were taken at time
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intervals of five minutes from the solution outlet of the cell for ex-situ analysis to first
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separate between the chloride and bromide ions by ion chromatography and afterword the
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amount of ions were quantified by titration with silver nitrate. The plot shows a specific
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removal and recovery of bromide ions from a mixed solution that initially contained the
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same concentration of ions. The samples from the three cycles were taken after two
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preliminary cycles that were carried out in order to achieve proper steady-state operation
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of the electrodes.
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Fig. 6.
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Fig. 6. Plot of current against time for a mixed solution of NaCl and NaBr, where both
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concentrations were initially 0.05 M, measured during the last three of five cycles of
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operation by an HPA-CDI cell. The repetition in the three cycles gives another indication
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of the consistency of the results, from the aspect of the electric charge.
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Fig. 7.
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Fig. 7. Column chart of accumulated amounts of bromide and chloride ions, which were
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removed and recovered, in mmol/gr units, normalized by the total weight of the ACC
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working electrodes, during the last three of five cycles of operation by the HPA-CDI cell.
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The chart shows that the accumulated removal and recovery of bromide ions in each
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cycle were nearly the same, almost two orders of magnitude higher with respect to
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chloride ions.
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Abstract Art (TOC)
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