Investigation on the effect of organic dye molecules ...

Electrochimica Acta 279 (2018) 24e33

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Investigation on the effect of organic dye molecules on capacitive deionization of sodium sulfate salt solution using activated carbon cloth electrodes Annadurai Thamilselvan a, b, *, Kadarkarai Govindan c, A. Samson Nesaraj d, **, Subramanian Uma Maheswari e, Yoram Oren f, Michael Noel a, E.J. James a a

Water Research Laboratory, Water Institute, Karunya Institute of Technology and Sciences (Deemed-to-be University), Karunya Nagar, Coimbatore, 641 114, Tamil Nadu, India Electro-Organic Division, CSIR-Central Electrochemical Research Institute, Karaikudi, 630 003, India c Centre of Excellence in Advanced Materials and Green Technologies, Department of Chemical Engineering and Materials Sciences, Amrita of School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore, 641 112, Tamil Nadu, India d Department of Chemistry, Karunya Institute of Technology and Sciences (Deemed-to-be University), Karunya Nagar, Coimbatore, 641 114, Tamil Nadu, India e Department of Physics, Mother Teresa Women's University, Kodaikanal, Tamil Nadu 624 101, India f Department for Desalination and Water Treatment, Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva, 84106, Israel b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2018 Received in revised form 1 May 2018 Accepted 5 May 2018

Capacitive deionization (CDI) is an emerging electrochemical desalination technique for the energyefficient removal of dissolved ions from aqueous solution. This is a first research attempt which describes the influence of dye molecules on capacitive deionization of salt solution. In this regard, a CDI flow cell has been fabricated and tested in order to scrutinize the electrosorptive removal of three different dye molecules such as amido black 10B (AB) (acidic dye), eosin yellow (EY) (neutral dye) and methyl violet (MV) (basic dye) from synthetic aqueous solutions. The electrosorption capacitance was analyzed by cyclic voltammetry cell and CDI flow cell using activated carbon cloth (ACC) electrodes with 1 cm2 and 24 cm2 surface areas respectively. The capacitance values of 106 and 99 F/g correspondingly were obtained for a steady-state CV and CDI flow cell with 50 mM Na2SO4 electrolyte solution. In addition to this, the dye removal efficiency was also examined by a CDI flow cell for the solution containing 10 ppm of dye and 500 ppm of Na2SO4. The experimental results substantiate that EY exhibits strong adsorption during charging and strong desorption during discharge cycle when compared with other two dye molecules (AB & MV). Conclusively, electrosorption of dye molecules at the carbon cloth electrodes surface was found in the following order: EY > AB > MV. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Capacitive deionization Electrosorption Specific capacitance Charge recovery Dye molecules

1. Introduction In the last few decades, a wide variety of carbon materials with high surface area, excellent absorptive and ion exchange capacity has been developed for water and wastewater treatment [1].

* Corresponding author. Water Research Laboratory, Water Institute, Karunya Institute of Technology and Sciences (Deemed-to-be University), Karunya Nagar, Coimbatore, 641 114, Tamil Nadu, India. ** Corresponding author. E-mail addresses: [email protected] (A. Thamilselvan), drsamson@ karunya.edu (A.S. Nesaraj). https://doi.org/10.1016/j.electacta.2018.05.053 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

Preferentially, the activated carbon and modified activated carbon composite electrodes have been employed in electrosorption process. However, a major shortcoming is poor recovery and recycling of these adsorbents. Therefore, the activated carbon mediated electrosorption process requires further optimization [2,3]. To overcome this drawback; a high surface area carbon aerogel based electrode has been extensively demonstrated in electrosorption process. These electrodes have greater surface area and an increased micropore which in turn facilitates the electrosorption process efficiently [4e6]. Another viable and cost-effective electrode material is activated carbon cloth (ACC). This material has received more attention in the recent times as adsorbent and

A. Thamilselvan et al. / Electrochimica Acta 279 (2018) 24e33

electrode material [7]. Generally, the carbon cloth surface has activated through chemical pathway [8,9] and electrochemical route [10]. Such modifications have shown to significantly improve the adsorptive as well as electrochemical properties. All forms of high surface area carbon electrodes were mainly developed for double layer capacitor related applications [2]. ACC electrodes have been initially evaluated as electrical double layer capacitor [11]. In addition to chemical modifications [8,9], the influence of various additives to ACC cloth and their electrode performances have also been examined and evaluated. Importantly, the performance of the electrochemical capacitor was rationalized using different ACC modified electrodes. ACC electrode is altered with fluoride ion [12] oxygen doping [13] reduced grapheme oxide [14] and porous grapheme particles [15]. Polyaniline modified ACC electrode is used in an asymmetric hybrid capacitor in combination with activated carbon electrodes [16]. The introduction of high surface area aerogel electrodes into CDI has appreciably improved the electrode performance and system integration for commercial utilization of CDI techniques in water treatment devices [4e6]. These efforts are consistently rationalized in recent times [17e20]. Likewise, the cost-effectiveness of electrode fabrication using ACC and other high surface area carbon powders and their mixtures were systematically assessed [21,22]. Especially nowadays, activated carbon powder and its composites are effectively used as electrode materials in CDI flow cell [19e22]. Carbon-based porous electrode and carbon-based composite electrode materials for CDI process were comprehensively reviewed and presented in our earlier publication [23]. A few studies have already established the suitability and efficiency of ACC electrodes [24,25]. The activation of carbon cloth by the chemical method has facilitated greater electrode efficiency in CDI process [26]. It is shown that modification of ACC with titania [27], zinc oxide [28e30] and grafting with carbon nanofibers [31] has escalated capacitive deionization efficiency. Monopolar and bipolar electrode configuration confers good deionization efficiency [32]. Besides these efficiencies, selective adsorption of divalent cations using these electrodes in water softening has also been reported [33]. Commercial wastewater treatment units based on CDI technology were introduced recently [4e6]. Technology up gradation involving bipolar operation and energy recovery during discharge stage has been researched and examined [17e20]. Despite these developments, reverse osmosis based units dominate the watertreatment market till date. It appears that the long-term operational viability of activated carbon-based CDI units requires serious investigation. In particular, the major consequences in CDI processes are organic contaminants like humic acid, surfactants and dye molecules in water can irreversibly adsorb on the electrode surface and reduce the adsorption/desorption efficiency [34]. In this regard, the organic molecule electrosorption capacity during polarization of carbon electrode in CDI processes was consistently investigated [35e40]. The available literature is sparse in describing sufficient understanding in organic molecules adsorption/desorption phenomenon in CDI process. In specific to author's knowledge, no research attempts have yet demonstrated the influence of organic molecules on deionization process. Therefore, the detailed examination on organic decontamination by CDI is utmost important to reinforce this claim. Hence, the present investigation aims to examine the interference of electrosorption by dye molecules on CDI process. In this prospect, the effect of different dye molecules and its concentration on adsorption capacity is evaluated using different activated carbon electrodes. Furthermore, the deionization efficiency loss during the CDI process in stationary as well as flow cell setup is also investigated.

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2. Materials and methods Three dye molecules namely, Amidoblock (AB), Eosin yellow (EY) and methyl violet (MV), (99.8% Loba, India) were used in the present study as representative dye molecules. Carbon cloth with a surface area 1000 m2/g and pore volume (0.3e0.4 cc/g) [HEG Limited, India], graphite sheet [Chemapol Industries, India] and graphite conductive adhesive [Electron Microscopy Sciences, India] were used as materials for fabrication of CDI electrodes. Sodium sulfate anhydrous (Na2SO4 99%) and concentrated nitric acid (HNO3 70%) were purchased from Himedia, India. The molecular formula, molecular weight and molecular structure of these dye molecules are summarized in Table 1. The initial and final concentration of dye molecules were determined by UVevisible spectroscopy using their specific absorbance wavelength. The absorbance data obtained on the dye molecules are indicated in Table 1. Typical UV spectra obtained with the above three dye molecules is presented in Fig. S1. Flow cell based CDI experiments were conducted using 50 ml synthetic effluent containing different concentration (5, 10, 15 mM) of dye molecules in the presence of 50 mM Na2SO4. All the experiments were carried out at 25 ± 1 C. The effluent pH was adjusted to pH 7 by the addition of NaOH/HCl. 2.1. Activation of carbon cloth The activation of carbon cloth was carried out in 8 M nitric acid (HNO3) for 9 h as per the optimized procedure indicated in our previous research work [41]. After this process, the treated carbon cloth was used for all the subsequent studies. The untreated and chemically treated carbon cloth electrode capacitance was calculated by cyclic voltammetry and chronocoulometry method using following equations (1) and (2):

Specific capacitance ðCÞ ¼

ia ic 2ðdv=dtÞ m

(1)

where, C is specific capacitance (F/g), ia and ic are the anodic and cathodic current, dv/dt is scan rate, m is mass of carbon electrode. Where, Q is coulombs (charge), V is the potential change during discharge excluding the portion of IR drop and m is mass of the carbon electrode.

Capacitance ðCÞ ¼ Q =DVm

(2)

2.2. Cyclic voltammetric cell setup A 25 ml capacity electrochemical cell was utilized using three electrode system (Fig. 1). The activated carbon cloth electrode with a surface area of 1 cm2 was employed as working and counter electrode. The activated carbon cloth was fixed on a graphite sheet using graphite conductive adhesive to ensure good electrical contact. Two of these identical electrodes (working and counter electrodes) were connected parallel to each other for the electrochemical measurements in an electrochemical workstation. Ag/Ag2SO4 electrode was used as reference electrode in this entire study. In addition, all the electrochemical experiments were carried out at N2 atmosphere in room temperature. 2.3. CDI flow cell setup The schematic diagram of the flow cell used in this experimental study is represented in Fig. 2. The precise dimension of flow cell is given in Fig. S2. The flow cell was fabricated by using Plexiglas sheet

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Table 1 Characteristics of the dye molecules used in this study (with measured absorbance values by UV measurements). S. No

Name of dye molecule

Molecular formula/ molecular weight (g/mol)

Solubility at pH 7 (g/L)

Absorbance (lmax) nm

1

Amido Black 10B (AB)

(C22H14N6O9S2)2- 2Naþ (570.49)

10

619

2

Eosin Yellow (EY)

(C20H6Br4Na2O5)-.2Naþ (645.88)

280

517

3

Methyl Violet (MV)

C25H30N3Cl (407.98)

10

585

Molecular structure

Fig. 1. Schematic representation of cyclic voltammetric (CV) cell.

(20 mm thickness) with a nylon mesh separator (0.3 mm). The solution inlet and outlet were provided at the bottom and top of this cell on the opposite compartments. Two activated carbon cloths (ACC) with an exposed area of 24 cm2 (8 3 cm) pasted on the graphite sheets in contact with graphite rods were used as the electrodes in the CDI flow cell. The inter-electrode distance was maintained at 5 mm including the nylon spacer. Platinum wire was used as the quasi-reference electrode. In the above flow cell experiments were performed in the presence of salt solution with and

without the dye molecules. A peristaltic pump (RHP100L) was connected as per the diagram mentioned in Fig. 2. The CDI electrodes and Pt quasi-reference electrodes were connected to the electrochemical workstation respectively (CHI6038D, CH Instruments, USA). The conductivity of the solutions were noted at regular intervals of treatment by connectivity meter (EUTECH CON 2700) and the charge recovery (CR) was calculated using following equation (3).


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Fig. 2. Flow diagram of actual CDI cell unit.

CR ¼

Q f Charge Q r Discharge 100 Q f Charge

3.2. Effect of concentration of amidoblock dye

(3)

where CR is the charge recovery, Qf is the forwarded charge in coulombs and Qr is the reversed discharge in coulombs respectively. The dye removal efficiency was calculated by measuring the initial absorbance (A0) and the absorbance after time‘t’ (At) using the following equation (4).

DR ¼

A0 At 100 A0

(4)

3. Results and discussion 3.1. Reproducibility of the electrode performance The performance of the ACC electrode and its reproducibility was studied by chronocoulometry (CC) (Fig. 3(a)) and cyclic voltammetry (CV) (Fig. 3(b)) in 50 mM Na2SO4 salt solution under same experimental conditions. The calculated specific capacitance values from CV and charging/discharging capacitance and charge recovery from CC study is given in Table 2. The results indicate that the ACC electrode exhibits good reproducibility behavior. Moreover, it is interesting to note that the specific capacitance values were found uniform (100 F/g) for all the five cycles. Further, it is noticed that the charge recovery (CR) value is almost same with ±2.0% variation for the first five cycles. Hence, these results ensure that the electrochemical characteristic of the ACC electrode doesn't change during the test cycles.

Typical chronocoulometric curves and cyclic voltammetric curves were obtained from solution containing Na2SO4 with different dye concentrations ([AB] ¼ 5, 10 and 15 mM). The observed chronocoulometric and CV curves are presented in Fig. 4(a) and Fig. 4(b) respectively. The corresponding estimated capacitance values are given in Table 2. The above experimental observations reveals significant variations in capacitance, charging/ discharging and charge recovery values with different dye concentrations. The overall CV charging capacitance obtained during the first CV cycle was found to be around 103 ± 3 F/g for all the four experiments (Table 3). However, under chronocoulometric conditions at a constant charging potential of 0.25 V, the charging capacitance fluctuates over a wide range. The discharging capacitance values were also found to decrease correspondingly with increasing dye concentrations. The charge recovery ratio decreases from 99.5% in pure Na2SO4 solution to as low as 80% in the presence of 15 mM AB. These results substantiate the strong irreversible organic adsorption on carbon cloth electrodes which fails to get discharged at lower potentials.

3.3. Effect of dye molecules The impact of different dye molecules on the electrochemical performance of ACC electrode were rationalized using solution containing 5 mM dye in the presence of 50 mM Na2SO4. In this regard, dye molecules such as amido black 10 B (AB), eosin yellow (EY) and methyl violet (MV) were taken as model dye molecules. The obtained CC profiles and CV trends are exhibited in Fig. 5(a) and Fig. 5(b) respectively. The capacitance values obtained during the

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Fig. 3. The reproducibility of activated carbon cloth electrode studied in [Na2SO4] ¼ 50 mM. (a) Chronocoulometry curves (with charging potential of 0.25 V and discharging potential of 0 V) for five cycles. (b) Cyclic voltammetry curves (at a scan rate of 1 mV/s).

Fig. 4. Effect of concentration of (i) [AB] ¼ 0 mM, (ii) [AB] ¼ 5 mM, (iii) [AB] ¼ 10 mM, (iv) [AB] ¼ 15 mM dye molecule with presence of [Na2SO4] ¼ 50 mM. (a) Chronocoulometry curves (with charging potential of 0.25 V and discharging potential of 0 V) for five cycles. (b) Cyclic voltammetry curves (at a scan rate of 1 mV/s).

3.4. Effect of concentration and nature of dye molecules first cycle are found to significantly differ based on the organic molecules (Table 4). Quiet similar trend is observed for the charging capacity values gained from chronocoulometric data (Table 4). Eosin yellow dye is a highly water-soluble dye molecule which facilitated strong adsorption during charging process along with inorganic salt ions leading to a high capacity of 120 F/g. However, the discharging capacity was observed very low suggesting irreversible nature of the adsorption process. The charge recovery is also found to be lower compared to the other two dyes.

The effect of different dye concentrations (5, 10 and 15 mM) and nature of dye molecules (AB, EY, and MV) on the first chargedischarge cycle was investigated in 50 mM Na2SO4 solution under identical experimental conditions. The observed charging capacity values are presented as bar charts in Fig. 6(a). The result indicates that the charging capacity increases with increasing dye concentration. In the case of EY, the charging capacitance is found to be greater than the pure Na2SO4 solution. This confirms the electrosorption of dye molecules on the electrode surface. Organic molecules can thus exhibit salt removal through the adsorption of organic molecules as well as electrosorption of charge dependent

Table 2 Reproducibility data obtained with activated carbon cloth electrodes: Specific capacitance, charging/discharging capacitance and charge recovery measured by cyclic voltammetry and chronocoulometry analysis. Cycle No

Specific capacitance (F/g)

Charging capacitance (F/g)

Discharging capacitance (F/g)

Charge recovery (%)

1 2 3 4 5.

100 100 100 100 100

106.45 103.92 101.37 103.67 103.05

105 102.09 98.69 98.69 99.93

99.46 98.23 97.35 96.61 96.27


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Table 3 Effect of concentration of AB (with 50 mM Na2SO4 solution)in measuring the specific capacitance, charging/discharging capacitance and charge recovery by cyclic voltammetry and chronocoulometry analysis. S.No

[Dye] (mM)

CV Capacitance (F/g)

Charging Capacitance (F/g)

Discharging Capacitance (F/g)

Charge Recovery (%)

1 2 3 4

0 5 10 15

100 105 106 106

106.45 86.67 103.07 106.85

105 69.76 87.58 83.46

99.46 90.48 83.81 79.85

Fig. 5. Effect of three different dye molecules ([AB] ¼ [EY] ¼ [MV] ¼ 5 Mm) with presence of [Na2SO4] ¼ 50 mM. (a) Chronocoulometry curves (with charging potential of 0.25 V and discharging potential of 0 V) for five cycles. (b) Cyclic voltammetry curves (at a scan rate of 1 mV/s).

Fig. 6. Effect of concentration of three dye molecules ([AB] ¼ [EY] ¼ [MV] ¼ 0, 5, 10 and 15 mM) in presence of [Na2SO4] ¼ 50 mM. (a) Charging capacitance, (b) charge recovery.

increasing EY concentration (Fig. 6(b)). ions. The charge recovery data is compared in Fig. 6(b) and it is observed that charge recovery decreases with increasing dye concentration. Higher adsorption process during charging step would lead to lower charge or salt recovery during discharge step. By this, EY exhibits a very strong decrease in charge recovery value with

3.5. Effect of initial electrode dipping time in dye solutions Organic molecules can get adsorbed on the highly porous

Table 4 Effect of three dye molecules, viz., AB, EY, and MV with a dye concentration of 5 mM in presence of 50 mM Na2SO4solution: Specific capacitance, charging/discharging capacitance measured by cyclic voltammetry and chronocoulometry analysis. Name of dye molecules

CV Capacitance (F/g)

Charging Capacitance (F/g)

Dis charging Capacitance (F/g)

Charge Recovery (%)

Na2SO4 AB þ Na2SO4 EY þ Na2SO4 MV þ Na2SO4

100 105 120 78

106.45 86.67 124.3 74

105 69.76 59.4 63

99.46 90.48 60.43 88

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carbon surfaces even under open circuit conditions. To evaluate this effect, the ACC electrode was dipped for 0, 1, 2 and 3 h in 5 mM dye solution of AB, EY and MV in the presence of 50 mM Na2SO4 before initiating chronocoulometric analysis. The effect of such dipping time on the overall charge/discharge responses are summarized in Fig. 7. AB dye appears to be least adsorbed on the electrode surface under open-circuit conditions. The slight decrease in the capacitance values is noticed for this molecule (Fig. 7(a)). Similarly, the charge recovery also decreases slowly with dipping time. The charging capacitance, as well as charge recovery decreases gradually for MV. Certainly, the electrochemical performance of ACC electrode exhibits notable differences with E ¼ Y containing system. The charging capacitance decreases quite sharply with dipping time (Fig. 7(a)). The percentage of charge recovery, however, considerably rises with increasing dipping time (Fig. 7(b)). 3.6. Influence of fluid flow The aforementioned experimental analysis is discussed based on dye adsorption under steady-state or non-electrolyte flow conditions. Nonetheless, the charging and discharging as well as dye adsorption behavior may also depend significantly on solution flow rate. Since the performance CDI process preferentially relates to fluid circulation (flow rate). In this prospective experiments were carried out to evaluate the dye adsorption effects under flow

Fig. 7. Effect of time of three dye molecules with different dipping times (0, 1, 2 and 3 h) and ([AB] ¼ [EY] ¼ [MV] ¼ 5 Mm) in presence of [Na2SO4] ¼ 50 mM. (a) Charging capacitance, (b) charge recovery.

conditions. The obtained chronocoulometric curves under steady state cell and flow cell are depicted in Fig. 8(a) and (b). The analysis divulged that charge and discharge curves are more reproducible during first five cycles under zero flow conditions in 50 mM Na2SO4 solution. Particularly, the charge-discharge capacities decrease continually and then reach a steady state under the flow rate of 2 ml/min (compare Fig. 8(a) and (b)). The calculated capacitance values under flow conditions are also substantially higher. This is obviously due to the dependence of competitive adsorption/ desorption processes on fluid flow rate. To further confirm the effect of flow rate, the charging capacitance values were also rationalized using cyclic voltammetric cell (1 cm2) and flow cell (24 cm2electrode surface area). These experiments were performed under non-flow conditions, and the obtained capacitance values at a flow rate of 2 ml/min were compared. The charging capacitance data for 1 gm of ACC material obtained from these studies for 50 mM Na2SO4solution (first column in Fig. 9(a)) shows that the charging capacitances per gram of ACC electrode under zero flow conditions are quite similar. The charging capacitance decreases slightly with electrode surface area, but substantially with flow rate. The charge recovery, however, improves remarkably under flow conditions. As anticipated fluid flow decreases the adsorption/electrosorption rate, but it also enhances desorption and removal rate. These similar trends were observed in the case of 5 mM concentration of all three dye molecules under identical experimental flow conditions (Fig. 9(a) and b).

Fig. 8. Effect of five cycles in [Na2SO4] ¼ 50 mM electrolyte solution (a) steady state cell, (b) flow cell in chronocolumetry in charging potential 0.25 V and discharging potential 0 V.


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Fig. 10. Effect of three dye molecules ([AB] ¼ [EY] ¼ [MV] ¼ 10 ppm) with presence [Na2SO4] ¼ 500 ppm, during adsorption/desorption cycles (a) conductivity of salt solution and (b) concentration of dye solution in CDI process. Fig. 9. Comparison of 1 cm2 cyclic voltammetry cell and 24 cm2 CDI flow cell without circulation and with solution circulation condition with flow rate 2 ml/min in three different dye molecules ([AB] ¼ [EY] ¼ [MV] ¼ 5 mM) and presence of [Na2SO4] ¼ 50 Mm, (a) charging capacitance, (b) charge recovery on Chronocoulometry process (with charging potential of 0.25 V and discharging potential of 0 V).

The charge/discharge cycles were repeated five times in the presence of 5 mM of dye molecules AB, EY, and MV with50 mM Na2SO4. Under flow conditions, the observed charging capacitance and charge recovery values are presented in Fig. S3(a). The charging capacity increases with charge/discharge cycling (Fig S3a) for dye molecules AB and MV. The charge recovery values, on the other hand, increases with increasing cycle for these two molecules (Fig S3b). The highly water-soluble EY molecule exhibits the opposite behavior. In this case, the charging capacity decreases with cycle numbers (Fig S3a middle set) while charge recovery increases with cycle number. As noted earlier (section 3.5) the response of this molecule on increasing dipping time of the electrode was also similar (compare Fig. S3 and Fig. 7). 3.7. Conductivity and colour changes during charge/discharge cycles In addition to the chronocoulometric and cyclic voltammetric studies reported so far, the actual conductivity measurements were also carried out during the flow cell experiments. The conductivity values of the salt solutions are found to decreases during the charging process (due to continuous electrosorption) and increases during the discharging process Fig. 10(a). This is found to be true for

500 ppm Na2SO4 solutions in the presence of 10 ppm dye molecules and also in their absence. However, there are slight variations in the minimum and maximum conductivity values achieved as shown in Fig. 10(a). These results specify the smooth electrosorption and desorption behavior of cations and anions, even when different types of organic dye molecules are present in the feed solution. Organic molecules tend to vary the surface area available for electrosorption of ionic species without influencing their electrosorption behavior. The concentration of dye molecules in the salt solution was also monitored using UVevisible spectroscopy at the end of each charging (C1 to C5) and discharging (D1 to D5) stage. Interesting results obtained from these studies are summarized in Fig. 10(b). The residual dye concentration decreases significantly in the aqueous medium for less soluble dye molecules AB and MV and but it increases slightly during the discharge step (Fig. 10(b)). In the case of high water-soluble EY, the compound dissolves back into the solution along with inorganic salts during the discharge cycle.

3.8. Electrode washing and recovery After charge/discharge cycles, the ACC electrodes may be contaminated with organic dyes as well as inorganic salts. To recover the electrodes from the contaminants, they are washed with distilled water in the CDI flow cell. The conductivity and the concentration of dye in outlet solution were measured at regular intervals. The observed outlet solution conductivity and dye concentrations with different washing time are presented in Fig. 11. It

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the inlet solution containing [Na2SO4] ¼ 500 ppm with AB, EY, and MV respectively. However, the contaminants present in the electrodes were removed by simple treatments like water washing. Further investigation with dilute and concentrate recirculation with ACC electrodes would indeed be worthwhile. The present study demonstrates that the CDI technology could be an alternative substitute technique for saline water desalination, especially in cases of presence of small amount of organic concentrations. Acknowledgments The authors would like to thank the Water Technology Initiative program, Department of Science and Technology (DST), Government of India, New Delhi for awarding financial grant (Ref. No. DST/ TM/WTI/2k11/213(c)) and Karunya Institute of Technology and Sciences for providing the lab facilities to carry out this present research work. Dr. Annadurai Thamilselvan thanks the Science and Engineering Research Board, Department of Science and Technology, Government of India for the award of National Postdoctoral Fellowship (PDF/2017/000564). We would also like to thank Mr. R. Jayakumar and Mr. A. Rubanfor his support in fabricating the CDI flow cell system. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.05.053. References

Fig. 11. Effect of washing time for the outlet solution ([AB þ Na2SO4] ¼ [EY þ Na2SO4] ¼ [MV þ Na2SO4] (a) conductivity of salt solution, (b) concentration of dyes.

can be seen that the notable conductivity is noticed in the wash water during initial stage even in the absence of dye molecule. Similarly, the study inferred that around 45 min of washing time is required to reduce the salty nature of the outlet solution. Eventually, the result of this experiment corroborates that by increasing washing time the conductivity and concentration of dye decreases noticeably in all the three dye molecules.

4. Conclusions The present investigation describes that activated carbon cloth electrodes are chemically activated well in nitric acid and it can be used efficiently for the capacitive deionization process. In the presence of organic dye molecules, the deionization process confers with lower efficiency due to the overall blocking effect of the dye molecules. The nature and property of the dye molecule also influences the deionization process. Good water-soluble dye molecule eosin yellow dye is easily dissolved into the concentrate solution during discharge step. Less water-soluble molecules like amido black and methyl violet are strongly held on the electrode surface. Over the long period use, the activated carbon cloth electrode could contaminate by inorganic salts as well as dye molecules. The concentration of dye molecules in the outlet solution is found in the following order: EY > AB > MV. The organic dye molecule removal efficiencies 46%, 60% and 43% are achieved for

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