Electrically Switched Ion Exchange Based on Polypyrrole and Carbon

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Electrically Switched Ion Exchange Based on Polypyrrole and Carbon Nanotubes Nanocomposite for the Removal of Chromium(VI) from Aqueous Solution Jianyu Xing, Chengzhou Zhu, Indranil Chowdhury, Yuhao Tian, Dan Du, and Yuehe Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03520 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Industrial & Engineering Chemistry Research

Electrically Switched Ion Exchange Based on Polypyrrole and Carbon Nanotubes Nanocomposite for the Removal of Chromium(VI) from Aqueous Solution †‡

†

§

§

†

Jianyu Xing , , Chengzhou Zhu , Indranil Chowdhury , Yuhao Tian , Dan Du† and Yuehe Lin * †

School of Mechanical and Material Engineering, Washington State University, Pullman, WA 99164, United States ‡

School of Environmental Science and Engineering, Chang’an University, Xi’an, Shaanxi 710054, China

§

Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington 99164, United States

*

Yuehe Lin: [email protected].

KEYWORDS: electrically switched ion exchange, conducting polymer, carbon nanotubes, nanocomposites, chromium(VI),

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Adsorption is one of the most commonly used methods for the remediation of heavy metal. Generally, the spent adsorbent is regenerated using chemicals. Although chemical regeneration is efficient, it often leads to secondary wastes. To overcome such a problem, a novel remediation protocol for Cr(VI) featured with high capacity adsorption and electrochemical regeneration was carried out in this study. Experimental results showed that multi-walled carbon nanotubes (MWCNTs) modified carbon cloth (CC) can be used as an excellent carrier for electrodepositing polypyrrole (PPy) film and the resultant nanocomposite termed as CCMWCNTs-PPy could be used as an adsorbent with high adsorption capacity and stability. Moreover, CC-MWCNTs-PPy could be electrically regenerated to reduce secondary wastes. Desorption of Cr(VI) can be enhanced greatly by applying reduction potential on spent CCMWCNTs-PPy. It was also observed that the Cr(VI) removal efficiency of CC-MWCNTs-PPy almost remain unchanged after at least 7 cycles.

1. INTRODUCTION Water pollution is a worldwide problem. Heavy metal pollution undoubtedly occupied large proportion in wastewater. Among all kind of heavy metal pollutants, chromium is considered one of the most hazardous heavy metals found in different industrial wastewaters.1 According to the guidelines of World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA), the maximum permissible concentration of chromium in drinking water is 0.05 mg/L and that in industrial wastewater should be less than 0.1 mg/L.2,3 Chromium is mostly found in industrial areas of developing countries because of its versatile uses in leather tanning, electroplating and metal finishing.4,5 Generally, there are two common oxidation states of chromium in nature, trivalent chromium (Cr(III)) and


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hexavalent chromium (Cr(VI)). Cr(VI) is found to be carcinogenic and is a toxic substance when compared with its reduced form. It is reported that Cr(VI) compounds appear to be 10 to 100 times more toxic than their Cr(III) counterparts by the oral route.6 Therefore, removal of Cr(VI) or reduction of Cr(VI) to Cr(III) is the main remediation way for chromium pollution. In recent years, adsorbent based on conductive polymer has attracted more attention. Of all conductive polymers, polypyrrole (PPy)-based adsorbents exhibit more potential for the removal of various heavy metal ions, such as Pb(II), Cr(VI), Cd(II), Cu(II), Zn(II), Hg(II),4,7,8,9,10,11 due to its high chemical stability and non-toxicity. Excellent adsorption prospect of PPy is mainly attributed to the presence of nitrogen atoms in the polymer chains. Due to its redox property, PPy especially suitable for the remediation processing of Cr(VI). The reduced form of PPy has been already used for the reduction of Cr(VI) and proved to be a good redox material.12 Adsorption of the Cr(VI) on the surface of PPybased material was due to the anion exchange by replacing the doped anion of PPy with Cr(VI) and reduction of Cr(VI) to Cr(III) was due to the -NH+ group in pyrrole ring.13,14 Present investigation suggests that Cr(VI) adsorption mainly occurs on the surface of PPy.13 In order to improve the adsorption efficiency and adsorption capacity, PPy was often polymerized on materials with the high specific surface or high conductivity, such as sawdust, rice husk ash, chitin and carbon nanotubes.10,8,15,16 Despite the high adsorption capacity that achieved, extensive research deems necessary to increase the efficiency of desorption and shorten adsorption-desorption cycle time.17 In addition to good adsorption property, the ion exchange and redox properties enable PPy to be a suitable matrix for electrically switched ion exchange (ESIX).18,19,20,21 In


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ESIX, PPy layer is deposited onto a conducting carrier to be an ion-exchanger which can be controlled directly by the potential modulation for ion uptake and elution. During this process, electrical energy can be used efficiently and eluted solution can be used repeatedly. Thereby, secondary chemical wastes can be avoided and operation costs can be minimized.18 Although ESIX exhibits great potential in wastewater remediation, its adsorption capacity is limited because there is only one positive charge per three or four pyrrole units can be produced by oxidation.22 It seems that the high capacity batch adsorption of PPy-based adsorbents and electrochemical regeneration of ESIX can be combined with each other perfectly to realize high adsorption capacity and electrochemical regeneration. Inspired by above analysis and based on our previous work of carbon nanotubes (CNTs),18,19,20 in this study, nanostructured composite with PPy and multi-walled carbon nanotubes (MWCNTs) on carbon cloth (CC) were synthesized, characterized, and evaluated as an adsorbent for removing Cr(VI) from aqueous solution by batch adsorption and electrochemical regeneration. 2. EXPERIMENTAL SECTION 2.1. Materials All reagents and starting materials were obtained commercially and were used as received without any other process, except pyrrole, which need to be distilled under reduced pressure before use. CC was purchased from Fuel Cell Store (USA). Multiwalled carbon nanotubes (95% purity, diameter 20-30 nm, length 10-30 µm) were purchased from Nanostructured & Amorphous Materials, Inc. (Houston, TX). All other chemicals and reagents are of analytic grade and prepared by ultrapure water (18.3 MΩ·cm).


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2.2. Synthesis of the CC-MWCNTs-PPy nanocomposites The pre-treatment of CC consisted of washing in warm acetone, ethanol and ultra-pure water and each for 30 min respectively, followed by drying at 80 °C for 30 min to eliminate all humidity. MWCNTs were dispersed in N, N-dimethylformamide (DMF) with the aid of ultrasonic treatment to form a stable, dispersed solution of MWCNTs ink. Subsequently, CC (1×2 cm2) was sunk into the MWCNTs ink for 10 min and then baked at 80 °C for 1 h to eliminate residual solvent. This resulted in a thick, black coating of MWCNTs on the surface of CC. The electrochemical polymerization of pyrrole was performed in a three-electrode cell which contains an aqueous solution of 0.5 M pyrrole monomer and 0.2 M NaCl. PPy film was directly deposited on the surface of CC by applying a constant potential of 0.7 V vs saturated calomel electrode (SCE) for a period of 10 min. 2.3. Characterization of CC-MWCNTs-PPy nanocomposites FEI Sirion field emission scanning electron microscope (FESEM) was used for imaging and energy-dispersive X-ray analysis (EDX). 2.4. Batch adsorption experimental procedures The adsorption experiments were conducted by batch mode and were all performed at solution pH of 2.0. Cr(VI) solutions of appropriate concentrations were prepared by diluting the stock solution (1000 mg/L). In a typical experiment, the adsorption experiments were carried out with one piece of CC-MWCNTs-PPy (1×2 cm2) in 5 mL of Cr(VI) solution for a certain time. Then the used CC-MWCNTs-PPy was withdrawn and rinsed with water for desorption process. Measurement of unabsorbed Cr(VI) ion in solution was carried out by 1, 5-diphenyl carbazide method. 1, 5-diphenyl carbazide can


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makes a highly colored violet complex with Cr(VI) in 0.1-0.2 M H2SO4. Cr(VI) ion removal percentage (%) was calculated using the difference of initial and equilibrium concentrations: %removal =

C0 − Ce × 100 C0

where C0 and Ce are the initial and final concentration of Cr(VI). 2.5. Thickness change analysis during adsorption and desorption process Thickness change of PPy was measured by Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) (Qsense E4, Biolin Scientific, Sweden). QCM-D system has been equipped with electrochemistry module coupled with Gamry Reference 600+. The PPy films were formed on gold-coated quartz QCM sensors (Q-sense Inc.) under 0.7 V potential for about 140 seconds. In the adsorption process, as the coated sensors were exposed to a Cr(VI) solution, changes in the fundamental frequencies of the sensors were measured. The Sauerbrey Equation was used to calculated changes in thickness and mass.23,24 The calculation based on the changes of fifth overtone of the fundamental frequency due to Cr(VI) association with positive charged sites in the PPy film. 2.6. Desorption and regeneration experiment Desorption and regeneration experiments are directly controlled by the potential modulation of CC-MWCNTs-PPy in a three-electrode cell, with SCE and Pt as the reference electrode and counter electrode, respectively. Desorption of Cr(VI) was performed by apply negative potential on CC-MWCNTs-PPy in 10 mL of 0.02 M NaHCO3 solution. Thereafter, regeneration of the sorption sites was carried out by


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applying potential 0.4 V on CC-MWCNTs-PPy in 10 mL of 0.02 M HCl to change it to oxidation status, resulting in desorption of reduced Cr(III). 3. RESULTS AND DISCUSSION 3.1. Characterization of CC-MWCNTs-PPy nanocomposites

0.035

0.030

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.025

0.020

CC-MWCNTs-PPy CC-PPy

0.015

0.010 0

100

200

300

400

500

600

Time (s)

Figure 1. Amperometric i-t curves for electrodeposit of PPy films on the CC and CC-MWCNTs. The amperometric i-t curves during the electrochemical deposition of PPy on the pristine CC and the MWCNTs modified CC (CC-MWCNTs) are shown in Figure 1. The current produced by electrochemical deposition of PPy on the CC-MWCNTs is much higher than that of the pristine CC. This result demonstrated that MWCNTs have higher electronic conductivity and can generate a larger number of nucleation sites compared to the pristine CC. The SEM images in Figure 2A and Figure 2C shows the pristine CC and CC-MWCNTs (Figure 2B, and Figure 2D were enlarge pattern respectively). It can be seen that MWCNTs modified on the surface of CC give 3D porous nanostructure for the polymerization of pyrrole. As shown in Figure 2E and Figure 2F, PPy film on CC is tight


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coupling and lacks nanoporosity. However, the PPy film on CC-MWCNTs (Figure 2G and Figure 2H) is loose coupling and contains porous nanostructure which has larger surface area and will be highly beneficial for ion diffusion and adsorption.


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Figure 2. SEM images of the CC (A, B), CC-MWCNTs (C, D), electrochemically deposited PPy on CC (E, F), and electrochemically deposited PPy on CC-MWCNTs (G, H). 3.2. Adsorption process of CC-MWCNTs-PPy

100 90 80

Removal Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 CC CC-PPy CC-CNT-PPy

60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

Time (min) Figure 3. Removal of Cr(VI) by the CC, CC-PPy, and CC-MWCNTs-PPy.

In our previous work, CNTs were proved to be favourable substrate for depositing PPy film because of its high surface area and good conductivity.18 Moreover, CNTs can provide a stable frame for PPy and prevent PPy from structural destruction. Therefore, it is expected that deposition of PPy on MWCNTs can not only increase adsorption capacity, but also enhance the stability of PPy. As a result, CC-MWCNTs-PPy shows more Cr(VI) removal efficiency than CC-PPy when they were used to treat Cr(VI) solution. As shown in Figure 3, CC-MWCNTs-PPy shows more rapid removal rate and


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higher removal efficiency than CC-PPy while CC-MWCNTs-PPy and CC-PPy with the same size (1×2 cm2) was placed in 5 mL 100 mg/L Cr(VI) solution (pH 2.0), respectively. CC-MWCNTs-PPy can remove 80% of Cr(VI) in 80 min while CC-PPy removes 40% of Cr(VI) and CC almost shows no removal efficiency of Cr(VI). The presence of Cr(VI) on CC-MWCNTs-PPy after adsorption was identified by using a SEM-EDX technique as shown in Figure 4. It can be observed two apparent peaks appeared at 5.4 keV and 5.9 keV in EDX spectra that were corresponding to Cr(VI). The amount of Cr(VI) on CCMWCNTs-PPy was determined by EDX analysis. The composition analysis was calculated from the peak areas of different elements, and the results are shown in the inset of Figure 4. Specifically, N and Cl come from PPy and HCl respectively, and Cr and O are mainly from HCrO4-. All the data clearly shows that CC-MWCNTs-PPy contains a high amount of Cr(VI) after adsorption. The total content of Cr(VI) on CC-MWCNTsPPy was found to be 15.03 wt% after adsorption.


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Figure 4. EDX spectrum and SEM image of CC-MWCNTs-PPy after adsorption. 3.3. Analysis of adsorption process of CC-MWCNTs-PPy


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65 60 55


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50 45 40 35 30 25 20 2

4

6

8

10

12

pH

Figure 5. Effect of pH on the removal of Cr(VI) by the CC-MWCNTs-PPy.

Scheme 1. Schematic illustration for adsorption of Cr(VI) in acid condition.


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To explore the mechanism involved in the removal of Cr(VI), batch adsorption experiment was carried out by placing CC-MWCNTs-PPy in 50 mg/L Cr(VI) solution with different pH values. In general, Cr(VI) exists as HCrO4- (80%) and Cr2O72- (20%) between pH 2-6, while CrO42- is the major species above pH 6.0.13 The existing research results show that the Cr(VI) adsorption process occurred via replacing the doped Cl- with HCrO4- by the ion exchange property of PPy.13 In order to investigate pH effect on Cr(VI) adsorption process, one piece of CC-MWCNTs-PPy (1×2 cm2) was divided into 5 pieces, and dipped into 2 mL 50 mg/L Cr(VI) solution with different pH values. As shown in Figure 5, a decline in Cr(VI) removal efficiency is observed while the solution pH increased from 2.0 to 10.4. It can be seen that CC-MWCNTs-PPy was most effective at pH 2.0 with a removal efficiency of 63%. However, its effectiveness decreased at higher pH as evidenced by its removal efficiency of 27% and 25% at pH 7.0 and 10.4, respectively. In ESIX process, there is only one positive charge per three or four pyrrole units can be produced.22 However, as shown in Scheme 1, in the adsorption process under acid condition, each pyrrole ring have a chance to be positively charged by H+ because of the lone pair on the nitrogen atom in the pyrrole ring. Therefore, the number of adsorption sites for Cr(VI) was increased dramatically while adsorption capacity was increased too. The mechanism involved in this adsorption process is due to the electrostatic attraction between the negatively charged HCrO4- or Cr2O72- and the positively charged sorption sites of the PPy. On the other hand, higher pH values mean more OH- in solution, which can compete with Cr(VI) for adsorption sites and lead to decreasing adsorption efficiency.


13


0.08

Elunt before oxidaiton Elunt after oxidaiton

0.07

Adsorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.06 0.05 0.04 0.03 0.02 400

450

500

550

600

650

Wavelength (nm)

Figure 6. Elute solution before (A) and after (B) oxidation with 5% KMnO4. Investigation of chemical states change during adsorption process was carried out by analysis of adsorption solution. Firstly, adsorption of Cr(VI) on CC-MWCNTs-PPy was performed by immersing a piece of CC-MWCNTs-PPy (1×2 cm2) in 100 mg/L Cr(VI) solution (pH 2.0) for 24 h. As shown in Figure 6, there was no Cr(VI) can be detected in the residual solution. After oxidation by KMnO4, there are some Cr(VI) appeared. This result means that adsorption process is associated with a partial reduction of adsorbed Cr(VI) to Cr(III) species by the electron-rich PPy moieties (As shown in Scheme 1).13 Furthermore, pH increase was observed after adsorption (pH 2.0 to pH 2.2) indicating the consumption of H+ in adsorption process which can be represented by the equation as following: HCrO

4

+ 7H

+

+ 3 e − ←→ Cr 3 + + 4 H 2 O


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Cr 2 O 7- + 14 H

+

+ 6 e − ← → 2 Cr

3+

+ 7 H 2O

.

The reaction formula means that Cr(VI) is easy to be reduced to Cr(III) in acid condition. After reduction of Cr(VI), Cr(III) are partially released into solution by electrostatic repulsive force because of its positive charge is identical to the whole positive charged CC-MWCNTs-PPy and the acidic environment around. Moreover, some of reduced Cr(III) may be still adsorbed on CC-MWCNTs-PPy. 3.4. Electrochemical assistant desorption and regeneration process

42600 4.38E-007 42400

Thickness Mass

4.36E-007

42200

4.34E-007 42000 4.32E-007 41800 4.30E-007 41600

Mass (ng)

Thickness (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.28E-007 41400 4.26E-007 41200 4.24E-007 41000 4.22E-007 -0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (V) Figure 7. Thickness and mass changes of PPy layer in redox process. In traditional desorption process, a higher concentration of eluent always needed for the exchange of Cr(VI).25,26 Adsorbent plays a passive role in this process. Based on the redox properties of CC-MWCNTs-PPy, it can play an active role in promoting desorption


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process by switching its chemical states from oxidation to reduction. In this study, we propose a cost-effective protocol for desorption of Cr(VI) from spent CC-MWCNTs-PPy. As measured by EQCM-D and shown in Figure 7, in one redox cycle, both thickness and mass of PPy layer was increased when applied potential changes from -0.5 V to +0.4 V and decreased when applied potential changes from +0.4 V to -0.5 V. It means that when spent CC-MWCNTs-PPy changed its chemical state from oxidation state (positively charged) to reduction state (electrically neutral), thickness of PPy layer will be decreased and hydrophobicity of PPy layer will be increased.27 As shown in Scheme 2, those physical changes of PPy layer accompanied by the absence of positive charge will force doped Cr(VI) out of the film and promote the desorption of Cr(VI).

Scheme 2. Scheme of reduction of PPy and its function in desorption. During adsorption and desorption processes, part of Cr(VI) can be reduced to Cr(III) and existed in PPy layer. In order to get rid of those reduced Cr(III), CC-MWCNTs-PPy was applied with an oxidation potential 0.4 V in 0.02 M HCl and therefore carry a certain amount of positive charge which is similar to Cr(III). Based on electric repulsion, Cr(III) will be forced into the eluted solution and CC-MWCNTs-PPy can be regenerated for the next Cr(VI) remediation cycle.


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3.5. Effect of reduction potential on desorption of Cr(VI)

0.085 0V -1.0V -1.5V -2.0V

0.080 0.075 0.070

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.065 0.060 0.055 0.050 0.045 0.040 0.035 0.030 0.025 400

450

500

550

600

650

wavelength (nm)

Figure 8. Effect of reduction potential on desorption of Cr(VI). As a most basic prerequisite, the applied potential on PPy to change its chemical status will play an important role in the Cr(VI) release process. In order to investigate the effect of reduction potential on desorption of Cr(VI), low concentration of the eluted solution was used for desorption process. As shown in Figure 8, without reduction potential (0 V), the competing anion level in low concentration electrolyte was not high enough to achieve any significant anion exchange with the adsorbed Cr(VI). When potentials were applied to Cr(VI)-laden composite, the amount of released Cr(VI) increased with reduction potential. When reduction potential reached -2.0 V, amount of Cr(VI) in eluent started to decrease. This may be due to the partial reduction of Cr(VI) to Cr(III) under potential -2.0 V. 3.6. Reusability of CC-MWCNTs-PPy


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40

30

20

10

0 1

2

3

4 5 Adsorption cycle

6

7

Figure 9. The Cr(VI) removal efficiency of CC-MWCNTs-PPy after different cycles. Reusability of CC-MWCNTs-PPy (1×2 cm2) was investigated in excessive amounts of Cr(VI) solution (200 mg/L) by electrochemical assisted regeneration. One regeneration cycle contains two steps, reduction under -1.5 V in 0.02 M NaHCO3 for 10min, oxidation under 0.4 V in 0.02 M HCl for 10min. As shown in Figure 9, removal efficiency of fresh prepared CC-MWCNTs-PPy can reach 31.5% and restore its adsorption ability after one regeneration cycle for about 26.7%. Then, CC-MWCNTs-PPy was used by this procedure repeatedly. It was observed that the removal efficiencies almost remain unchanged after at least 7 cycles. The long lifetime of adsorbent maybe contributes to the low concentration of eluent. Positively charged site in the PPy oxidation state is easy to be attacked by the nucleophilic reagent and lead to overoxidation of PPy. The overoxidation was directly


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related to the concentration of nucleophile reagent.

28

As a result, elution with a high

concentration of alkaline will greatly affect the lifetime of PPy-based adsorbent.4,25 Compared with traditional regeneration method, electrochemical regeneration with low concentration of eluent can prolong lifetime of CC-MWCNTs-PPy and reduce second waste greatly. 4. CONCLUSIONS Polypyrrole was successfully deposited on MWCNTs modified CC and used as an adsorbent for the remediation of Cr(VI). A novel method contains high adsorption capacity and electrochemical regeneration has been developed. Such a novel and green protocol show promising removal efficiency of Cr(VI) in wastewater, while minimizing the production of secondary wastes. The adsorption process for the removal of Cr(VI) by CC-MWCNTs-PPy was highly pH dependent. In order to achieve more higher removal efficiency, pH 2.0 was chosen as optimum pH. Analysis of adsorption process shows that adsorption involves reduction of Cr(VI) to Cr(III). Reduction of CC-MWCNTs-PPy by applying -1.5 V potential enhanced Cr(VI) desorption greatly. Electrochemical regeneration can decrease the dependence on eluent concentration, while MWCNTs can improve the stability of PPy. CC-MWCNTs-PPy can be used for at least 7 adsorptiondesorption cycles. The results show that conducting polymer-based adsorbent can be electrochemically regenerated to shorten adsorption-desorption cycle time, reduce the production of secondary wastes, and prolong the lifetime of adsorbent. ACKNOWLEDGMENT This work was supported by start-up funds from Washington State University and Fundamental Research Funds for the Central Universities of China (NO. 310829161015). We


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thank the Franceschi Microscopy & Image Center at Washington State University for TEM and SEM measurement. Jianyu Xing thanks the China Scholarship Council for the financial support. REFERENCES (1) Alves, M. M.; Beça, C. G. G.; Carvalho, R. G. D.; Castanheira, J. M.; Pereira, M. C. S.; Vasconcelos, L. A. T. Chromium removal in tannery wastewaters “polishing” by Pinus sylvestris bark. Water Res. 1993, 27, 1333. (2) World Health Organization. Guidelines for drinking-water quality: recommendations.

World Health Organization, 2004. (3) Chiha, M.; Samar, M. H.; Hamdaoui, O. Extraction of chromium (VI) from sulphuric acid aqueous solutions by a liquid surfactant membrane (LSM). Desalination 2006, 194, 69. (4) Bhaumik, M.; Maity, A.; Srinivasu, V. V.; Onyango, M. S. Removal of hexavalent chromium from aqueous solution using polypyrrole-polyaniline nanofibers. Chem. Eng. J.

2012, 181, 323. (5) Yusof, A. M.; Nik Ahmad, N. N. M. Removal of Cr (VI) and As (V) from aqueous solutions by HDTMA-modified zeolite Y. J. Hazard. Mater. 2009, 162, 1019. (6) Wampler, W. A.; Basak, S.; Rajeshwar, K. Composites of polypyrrole and carbon black: 4. Use in environmental pollution abatement of hexavalent chromium. Carbon 1996, 34, 747. (7) Karthik, R.; Meenakshi, S. Chemical modification of chitin with polypyrrole for the uptake of Pb (II) and Cd (II) ions. Int. J. Biol. Macromol. 2015,78, 157


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(8) Ghorbani, M.; Eisazadeh, H. Removal of COD, color, anions and heavy metals from cotton textile wastewater by using polyaniline and polypyrrole nanocomposites coated on rice husk ash. Compos. Part B Eng. 2013, 45, 1. (9) Chávez-Guajardo, A. E.; Medina-Llamas, J. C.; Maqueira, L.; Andrade, C. A. S.; Alves, K. G. B.; Melo, C. P. D. Efficient removal of Cr (VI) and Cu (Ⅱ) ions from aqueous media

by

use

of

polypyrrole/maghemite

and

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Electrically Switched Ion Exchange Based on Polypyrrole and Carbon

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