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Electrochemical removal of fluoride from water by PAOA-modified carbon felt electrodes in a continuous flow reactor
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Electrochemical removal of fluoride from water by PAOAmodified carbon felt electrodes in a continuous flow reactor Hao Cui, Yan Qian, Hao An, Chencheng Sun, Jianping Zhai, Qin Li* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China
article info
abstract
Article history:
A novel poly(aniline-co-o-aminophenol) (PAOA) modified carbon felt electrode reactor was
Received 9 November 2011
designed and investigated for fluoride removal from aqueous solutions. This reactor design
Received in revised form
is innovative because it operates under a wider pH range because of coating with
21 April 2012
a copolymer PAOA ion exchange film. In addition, contaminant mass transfer from bulk
Accepted 23 April 2012
solution to the electrode surface is enhanced by the porous carbon felt as an electron-
Available online 2 May 2012
conducting carrier material compared to other reactors. The electrically controlled anion exchange mechanism was investigated by X-ray photoelectron spectroscopy and cyclic
Keywords:
voltammetry. The applicability of the reactor in the field was tested through a series of
Fluoride
continuous flow experiments. When the flow rate and initial fluoride concentration were
Poly(aniline-co-o-aminophenol)
increased, the breakthrough curve became sharper, which lead to a decrease in the
(PAOA)
breakthrough time and the defluoridation capacity of the reactor. The terminal potential
Carbon felt
values largely influenced fluoride removal by the reactor and the optimal defluoridation
Electrically switched ion exchange
efficiency was observed at around 1.2 V. The breakthrough capacities were all >10 mg/g
Water treatment
over a wide pH range (pH 5e9) with an initial fluoride concentration of 10 mg/L. Consecutive treatmenteregeneration studies over a week (once each day) revealed that the PAOAmodified carbon felt electrode could be effectively regenerated for reuse. The PAOAmodified carbon felt electrode reactor is a promising system that could be made commercially available for fluoride removal from aqueous solutions in field applications. ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Drinking water containing fluoride concentrations above the World Health Organization recommended level of 1.5 mg/L is a major threat to the health of large numbers of people in remote and emerging regions of the world (WHO, 2004). Current methods used to remove fluoride from water includes precipitation (Saha, 1993), membrane techniques (Ndiayea et al., 2005; Diawara, 2008), adsorption (Maliyekkal et al., 2006; Teng et al., 2009) and electrocoagulation (Zhao et al., 2009, 2010). In recent years, it has become more important to find simple, inexpensive, and environmentally friendly technologies to remove fluoride from water supplies.
Conducting polymers and conducting polymer-based composites have shown potential for use as sustainable water treatment media for fluoride tainted drinking water. To date, most attention has focused on the adsorption behaviors of different conducting polymer-based composites for fluoride in batch tests (Karthikeyan et al., 2009a, b, c, 2011; Subramanian and Ramalakshmi, 2010; Bhaumik et al., 2011). Polyaniline (PANI) and polypyrrole (PPy) are the most commonly investigated conducting polymers, because of their high electrical conductivity, ease of preparation, and environmental stability (Chandrasekhar and Naishadham, 1999). Lin et al. (2006) first employed electrically switched ion exchange (ESIX) for perchlorate removal using polypyrrole
* Corresponding author. Tel./fax: þ86 2583592903. E-mail addresses: [email protected], [email protected] (Q. Li). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.04.039
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deposited on high surface area carbon nanotubes. In a previous study, we reported the removal of fluoride via ESIX by a polyaniline modified electrode reactor (Cui et al., 2011). It was observed that that the uptake and elution of fluoride wee effectively controlled by modulating the potential of the polyaniline (PANI) film. This indicates that the ESIX technique with a conducting polymer modified electrode is a good method for fluoride removal. However, some problems have still not yet been resolved: (1) in previous work, the PANImodified electrode reactor performed well only at acidic pH, which would limit its application in drinking water treatment; and (2) the service time of the reactor was short because of the limited surface area of the electrode. In order to improve the physicochemical and electrochemistry properties of the conducting polymers, many new copolymers were prepared and investigated (Huang et al., 2011; Li et al., 2010, 2011). The pH dependence of the conductivity and redox activity of aniline copolymers that contain functional groups, such as eSO3H, eOH, and eCOOH groups, could be much better than for polyaniline itself, because these functional groups can adjust the pH (Mu, 2004). Among these aniline copolymers, poly(aniline-co-o-aminophenol) (PAOA) has good redox activity and is stable over a wide pH range (<1 to 10) (Zhang et al., 2006, 2007; Shah and Holze, 2006, 2008). These results suggest that highly effective defluoridation of water under neutral and basic conditions could be achieved by combining the PAOA copolymer with the ESIX system. In this study, two measures were adopted to improve the applicability of the ESIX system for fluoride removal: (1) PAOAmodified electrode was used for electrically controlled anion exchange; (2) porous carbon felt was employed as an electronconducting carrier material to enhance the contaminant mass transfer from bulk solution to the electrode surface. Mechanism of electrically controlled fluoride removal with PAOA was investigated by X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was used to study the electrochemical reactions of the fluoride ion on the PAOA-modified carbon felt surface as a function of the terminal potential. To optimize the operational parameters of the electrochemistry defluoridation reactor, a series of continuous flow experiments were conducted to examine the effects of the initial fluoride concentration, flow rate, solution pH, and terminal potential on the defluoridation capability of the electrode. Regeneration of the PAOA-modified carbon felt electrode was also studied.
2.
Experimental section
2.1.
Chemicals
Fluoride (NaF, >99%) was purchased from Fluka (a subsidiary of SigmaeAldrich, St. Louis, MO). Other chemicals like aniline, o-aminophenol, HCl, NaCl, NaNO3, Na2CO3, NaH2PO4$2H2O, Na2SO4, Ca(NO3)2$4H2O, Mg(NO3)2$6H2O, and KNO3 (all analytical grade) were obtained from Nanjing Chemical Reagent Co., Ltd (Nanjing, China). Aniline monomer was purified by distillation under reduced pressure before polymerization. Ultrapure water (18.25 MU cm) was used to prepare
all the solutions in this study, and the pH of each solution was adjusted by addition of HCl and NaOH. The water quality data for the tap water used in the flow experiment is shown in Table S-1 (Supporting Information).
2.2.
Preparation of the anion exchange electrodes
Before use, carbon felt pieces were soaked in a 0.10 mol/L HNO3 for 24 h to facilitate wetting of the hydrophobic carbon fibers by the aqueous solutions Electrochemical synthesis of the PAOA film on the surface of carbon felt was carried out in HCl (200 mL, 2.0 mol/L) containing 0.3 mol/L aniline and 5.5 mmol/L o-aminophenol. A titanium electrode was used as the cathode, and one piece of carbon felt was used as the anode. PAOA was deposited onto one piece of circular carbon felt (1.0 cm thick, B 5 cm) at 0.8 V for 30 min. After synthesis, the PAOA-modified carbon felt pieces were washed with a 0.1 mol/L HCl solution to remove unreacted aniline or oaminophenol, then rinsed with ultrapure water, and finally dried at room temperature. Twenty PAOA-modified carbon felt pieces were prepared, and the mass loading of the PAOA coating was determined by measuring the mass of each sample before and after modification. The average mass of the PAOA coating on the carbon felt was 1.170.1 g.
2.3.
Electrode reactor and experimental procedure
Fluoride-spiked tap water was used in a continuous flow experiments to investigate the defluoridation capability of the PAOA-modified carbon felt electrode. During the continuous flow experiments, some key parameters including the pH, external voltage, and flow rate were evaluated. Fig. 1 shows the experimental setup for the electrochemical defluoridation experiments and the structure of the electrochemical defluoridation flow cell. As show in Fig. 1A, the up-flow column experiments were conducted using a syringe pump (Baoding Longer Precision Pump Co., Ltd., Baoding, China). A DC voltage-stabilized power supply (Shanghai Liyou Electrification Co., Ltd., Shanghai, China) was connected to the anode and cathode to control the potential. Effluent samples were collected by an automatic collector (Shanghai Huxi Analytical Instrument Co., Ltd., Shanghai, China) at different intervals to ensure the volume of each effluent sample was 20 mL. The external and internal structures of the electrochemical defluoridation flow cell are shown in Fig. 1B. The flow cell contained four major modules, which were made of acrylic plastic glazing. These modules were fixed together by six flange bolts from bottom to top, with a fluorubber gasket inserted between every two modules. The four modules consisted of two electrode modules in the middle and two connector modules on each end. One piece of circular PAOA-modified carbon felt (1.0 cm thick, B 5 cm) was used as the anode, while circular titanium plate (0.05 cm thick, B 5 cm) with holes for distribution of the solutions was used as the cathode. The distance between the anode and cathode was 0.5 cm.
2.4.
Regeneration of the anion exchange electrodes
Preliminary tests showed that the optimal terminal potential for desorption of fluoride from PAOA-modified carbon felt
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 9 4 3 e3 9 5 0
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Fig. 1 e Experimental setup for the electrochemical defluoridation experiments (A), and images of both the external and internal structure of the electrochemical defluoridation flow cell (B).
electrode was a negative voltage of 0.4 V. Therefore, after treatment, the used PAOA-modified carbon felt electrode module was moved out of the reactor, and regenerated at 0.4 V in 200 mL of HCl solution (0.10 mol/L) for 20 min. The regenerated PAOA-modified carbon felt pieces were rinsed with ultrapure water before reuse.
2.5.
Characterization and analysis
To investigate the electrically controlled anion exchange mechanism, XPS measurements of raw PAOA, PAOA after treatment, and regenerated PAOA were conducted on a Thermo ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA) spectrometer with Al Ka X-rayn (1486.6 eV). All the binding energies were referenced to the C1s neutral carbon peak at 284.6 eV. The morphology of the carbon felt pieces before and after modification with PAOA was observed with a S-3400 NII (Hitachi, Japan) scanning electron spectroscope (SEM). Electrochemical characterization of the electrodes was
performed by CV in an electrolytic cell consisting of the working electrode (PAOA-modified carbon felt), a platinum foil counter electrode (1.5 1.5 cm2), and a saturated calomel reference electrode (Zhang et al., 2009). The electrolytic solution consisted of 0.10 mol/L NaCl and required concentration of fluoride. The fluoride concentrations were evaluated by ion chromatography using a Dionex ICS-2000 (Sunnyvale, CA) ion chromatograph (Cui et al., 2011).
3.
Results and discussion
3.1. Characterization of the PAOA-modified carbon felt electrode The SEM images of the carbon felt before and after electrochemical modification with PAOA are shown in Fig. S1 (Supporting Information). The carbon felt had a fibrous structure and an average fiber diameter of about 20 mm (Fig. S1-A1). The
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uncoated carbon fiber shows a smooth surface with some small particles (Fig. S1-A2). SEM images of the carbon felt after modification indicated that the surface of the carbon fibers was coated uniformly in PAOA sheets (Fig. S1-B1 and S1-B2). Carbon felt electrodes have an extended surface area which provides an additional advantage for contaminant to react (Yang et al., 2009). Because PAOA film was coated on the surfaces of the porous carbon felt, it was more accessible to the contaminant, thereby facilitating the contaminant mass transfer from bulk solution to the electrode surface.
Electrically controlled anion exchange mechanism
To confirm the uptake and elution of fluoride ions in the PAOA films, XPS was used to characterize the samples. Survey and high-resolution XPS spectra for the key elements on the PAOA surfaces are shown in Fig. 2. The presence of a F1s peak in curve b (Fig. 2A) confirmed fluoride uptake by the PAOAmodified carbon felt after it was added with an anodic voltage of 1.2 V in a 50 mg/L fluoride solution. After regeneration of the fluoride-doped PAOA with 0.1 mol/L HCl, the F1s peak disappeared in curve c, which was similar to the original PAOA. This indicated that fluoride ions were removed from PAOA. Similar to PANI, conformational changes of nitrogen atoms at a molecular level on the PAOA chains could further explain
N
O
Cl
Relative intensity, c/s
F
C
C
A
(c) PAOA-R F
(b) PAOA-F
(c) PAOA-R
Relative intensity, c/s
3.2.
the anion exchange mechanism (Lin et al., 2006). Peak fitting illustrated that N1s spectra of PAOA (Fig. 2B) could be deconvoluted into three Gaussian peaks at about 398, 399, and >400.00 eV, which corresponded to nitrogen atoms in nitride (]Ne), amine (eNHe), and doped imine (eNHþ e), respectively (Chen et al., 2002). The nitride/imine ratio of the original PAOA film increased significant after fluoride removal, which suggested that PAOA was less protonated (Kang et al., 1998). After regeneration, the nitride/imine ratio decreased again, indicating that the PAOA film was regenerated well and could be used repeatedly. The F1s spectra in Fig. 2C and Cl2p spectra in Fig. 2D clearly showed that the anion exchange occurred between F and Cl. In Fig. 2C, the F content increased in the PAOA-F samples (curve b) and little F could be seen in the PAOA-R samples (curve c). This demonstrates that the PAOA could achieve both effective uptake and complete elution of fluoride ions, which was controlled by modulating its terminal potential. The Cl2p spectra (Fig. 2D) also showed substantial changes after different treatments of the PAOA films. The Cl2p spectra could be divided into three spineorbit split doublets (Cl2p3/2 and Cl2p1/2) for Cl, Cl*, and CeCl, which represent the chloride anion, anionic chloride resulting from charge transfer between chlorine and the PAOA polymer chain, and the covalently bonded chlorine, respectively. The Cl2p3/2 were situated at binding energies of 196.3 0.1 (Cl), 197.7 0.1
(b) PAOA-F
(a) PAOA 0
200
400
600
800
(a) PAOA 1000
680
682
684
Binding Energy, eV -NH-
N
-NH -
D
(c) PAOA-R
Relative intensity, c/s
690
692
(b) PAOA-F
Cl
Cl
Relative intensity, c/s
-N=
688
694
696
698
Binding Energy, eV
·
B
686
C-Cl
Cl
(c) PAOA-R
(b) PAOA-F
(a) PAOA (a) PAOA 396
397
398
399 400 401 402 Binding Energy, eV
403
404
405
194
195
196
197 198 199 200 Binding Energy, eV
201
202
203
Fig. 2 e Survey scan (A) and high-resolution XPS spectra of N1s (B), F1s (C) and Cl2p (D) for different samples: (a) original PAOA, (b) PAOA-F, the PAOA film after electrochemical fluoride removal, and (c) PAOA-R, the PAOA film after regeneration.
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 9 4 3 e3 9 5 0
(Cl*), and 199.9 0.1 (CeCl) eV (Kang et al., 1998). As shown in Fig. 2D, a considerably lower Cl content and considerably higher CeCl content were observed at the surface of the PAOA-F samples (curve b). This indicates that loss of the dopant in the surface region of the emeraldine HCl doped PAOA film occurs during its exposure to fluoride solution at an anodic voltage.
3.3. Effect of fluoride concentration and pH on the cyclic voltammograms For reproducible anion exchange of the electrode, the electrochemical properties of the PAOA-modified carbon felt should be stable. The stabilities of the electrode materials were assessed using CV, and the results showed that bare carbon felt had good electrical conductivity (Fig. S2-A, Supporting Information). CV of the PAOA film coated on the carbon felt was consistent over 10 cycles (Fig. S2-B, Supporting Information). This indicates that the processes of oxidation and reduction are chemically reversible, and that the electrode potential can be modulated to control loading and unloading of anions (Ding et al., 2010). For qualitative analysis of the electrochemical reaction of the fluoride ion on the PAOA-modified carbon felt surface as a function of the terminal potential, a series of CV experiments were performed. Fig. 3 shows the CV of PAOA in 0.10 mol/L NaCl solutions without (curve a) and with different concentrations of fluoride (50e200 mg/L) (curve bed) at pH 5.0. Our previous study on fluoride removal by the PANI-modified electrode revealed that at a suitable anodic voltage (about 1.5 V), the state of the nitrogen atoms on PANI chains changed. This caused secondary doping of PANI by fluoride ions and loss of electrons from PANI chains (Cui et al., 2011). In this study, as can be clearly seen in Fig. 3, the oxidation peak potentials on curves bed shifted toward positive potentials and the oxidation peak currents increased as the
c
0.02
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concentration of fluoride increased. This indicates that the oxidation peaks on curves bed are caused by fluoride uptake on the PAOA-modified carbon felt. It is interesting to note that the peak potentials were slightly lower that those recorded in our previous study, which ranged from about 0.8 to 1.1 V at fluoride concentrations of 50e200 mg/L. This is possibly because of the difference in electrode material. The results indicate that this reactor system will be energy saving compared to our earlier system. CV was also used to investigate the effects of the pH on fluoride removal by the PAOA-modified carbon felt. Fig. 4 shows the voltammograms obtained for the PAOA-modified carbon felt in 0.10 mol/L NaCl at different pH values for solutions containing 100 mg/L F at a scan rate of 20 mV/s. Generally, the trends for the voltammograms were similar at the different pH values. A decrease in the redox activity of PAOA was observed from the first cycle to the tenth cycle in all of the figures, possibly because of the pH difference between the bulk solution and the inner polymer matrix. It was observed that pH had no significant effect on the redox activity of PAOA as solution pH increased from 5 to 9. This result was similar to that reported by other researchers (Mu, 2004; Zhang et al., 2006; Shah and Holze, 2008). In this study, it was found that PAOA has good defluoridation capability in a wide pH range under both acidic and alkaline conditions.
3.4. Optimization of operational parameters in continuous flow mode Treatment of fluoride-contaminated tap water in continuous flow mode was conducted to study the effect of various process parameters, such as the terminal potential, solution
d
b a
I/A
0.01
0.00
(a ) (b ) (c ) (d )
-0.01
0 m g /L 5 0 m g /L 1 0 0 m g /L 2 0 0 m g /L
-0.02 -0.4
0.0
0.4
0.8
1.2
1.6
E / V (vs SCE) Fig. 3 e Cyclic voltammograms of the PAOA-modified carbon felt electrode in a solution containing 0.10 mol/L NaCl at pH 7 with different fluoride concentrations. (a) Without fluoride, (b) with 50 mg/L fluoride, (c) with 100 mg/ L fluoride, (d) with 200 mg/L fluoride. Voltammograms were recorded from L0.40 V to 1.6 V at a scan rate of 20 mV/s.
Fig. 4 e Effect of pH on the cyclic voltammograms of the PAOA-modified carbon felt electrode in a solution containing 0.10 mol/L NaCl and 100 mg/L fluoride. Voltammograms were recorded from L0.40 V to 1.6 V at a scan rate of 20 mV/s.
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Table 1 e Effect of terminal potential and solution pH on fluoride removal.a Initial pH
Terminal potential (V)
Breakthrough volumeb (mL)
Exhaust volumec (mL)
Breakthrough capability (mg/g)
0.8 1 1.2 1.4 1.2 1.2 1.2 1.2 1.2
720 1080 1620 900 1660 1680 1620 1640 1620
1260 1640 2720 1460 2780 2800 2720 2740 2740
6.5 8.2 10.5 7.5 11.2 10.9 10.5 10.6 10.6
7.2 7.2 7.2 7.2 5.3 6.1 7.2 8.1 9.3
a Flow rate, 5 mL/min; initial fluoride concentration, 10 mg/L. b The volume of the effluent with fluoride concentration below 1.5 mg/L. c The volume of the effluent at Ct/C0 ¼ 1.
pH, inlet flow rate, and initial fluoride concentration. Tap water contaminated with fluoride was prepared by adding NaF to authentic tap water to obtain final F concentrations of 5, 10 and 20 mg/L. Table 1 shows the effect of the solution pH and terminal potential on the defluoridation efficiency by the PAOAmodified carbon felt electrode in the continuous flow reactor. It should be noted that the pH value had a negligible effect on fluoride uptake by the PAOA-modified carbon felt electrode. The breakthrough volume, effective treatment volume, and removal capacity of the reactor remained almost constant over the natural water pH range (5e9). This is possibly due to the fact that o-aminophenol has a functional group eOH according to the views of Mu (2004) in a previous study. Oxidation of eOH group occurs accompanied with proton exchange between the copolymer and the solution. The eOH group can be oxidized to quinone, and quinone can be reduced. The redox reaction of the eOH group plays an important role in adjusting the pH around the copolymer electrode. Further studies are still needed to better understand
the mechanism behind the pH adjustment effect of the functional group eOH. Potential is always used first to optimize electrochemical reactor operation conditions because it directly determines the contaminant removal and side reactions. As shown in Table 1, from 0.8 to 1.2 V, the fluoride removal increased as the terminal potential increased, exhibiting a positive correlation. Optimal removal was observed at around 1.2 V during the continuous flow study, and 10.5 mg/g of fluoride was removal at pH 7.2. A higher potential means higher current density, which leads to higher fluoride removal capacity. However, terminal potentials above 1.4 V result in decreased fluoride removal efficiency because of overoxidation of the PAOA films. The effect of the flow rate of the feed fluoride solution was studied with a fixed initial fluoride concentration of 10 mg/L at pH 7.2 under a potential of 1.2 V. The typical breakthrough curves (BTCs) are shown in Fig. 5. The values of Ct/C0 were very low and the BTCs were flat at the beginning. During this time, the fluoride concentration in the outflow was below the World Health Organization recommended level of 1.5 mg/L. As more
1.0 1.0
0.9 0.8 0.7
5 mg/L 10 mg/L 20 mg/L
0.8 0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
C /C
Ct/C0
0.9
2 ml/min 5 ml/min 10 ml/min
0.3
0.2
0.2
0.1
0.1
0.0 0
250
500
750
1000 1250 1500 1750 2000
water yield, mL
Fig. 5 e Fluoride breakthrough in continuous flow experiments at different inlet flow rates (pH, 7.2; C0, 10 mg/ L; terminal potential, 1.2 V). The dotted line indicates the World Health Organizations recommended level (Ct [ 1.5 mg/L), and the arrows indicate the breakthrough points.
water yield, mL
0.0 0
1000
2000
3000
4000
5000
water yield, mL
Fig. 6 e Fluoride breakthrough in continuous flow experiments with different initial fluoride concentrations (pH, 7.2; flow rate, 5 mL/min; terminal potential, 1.2 V). The arrows indicate the breakthrough points. The inset shows the details during the first 1250 min.
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Table 2 e FL removal capability in consecutive treatmenteregeneration cycles over one week. Datea
Test Ac b
Mon. Tue. Wed. Thu. Fri. Sta. Sun. a b c d
Test Bc d
b
Test Cc d
b
Mean value d
b
Qe (mg/g)
BV (mL)
Qe (mg/g)
BV (mL)
Qe (mg/g)
BV (mL)
Qe (mg/g)
BVd (mL)
10.5 10.4 10.2 9.7 9.5 9.6 9.3
1620 1600 1620 1580 1560 1560 1520
11.3 11.1 10.5 10.6 10.3 10.0 9.7
1660 1660 1640 1620 1600 1620 1600
10.7 10.3 9.7 9.4 9.6 9.6 9.5
1620 1600 1580 1560 1560 1550 1520
10.8 0.4 10.6 0.4 10.2 0.4 9.9 0.6 9.8 0.5 9.7 0.3 9.5 0.2
1633 23 1620 35 1613 31 1587 31 1573 23 1577 38 1546 47
The regenerationetreatment cycle was carried out once a day. Qe, breakthrough capability. The test was carried out in three replicate runs. BV, breakthrough volume.
solution passed through the anion exchange layer, the BTC became steeper, and quickly reached the breakthrough point. The effective treatment volume decreased as the sample flow rate increased from 2.0, to 5.0, to 10.0 mL/min, and the breakthrough capacity decreased from 11.8, to 10.5, to 3.2 mg/ g. This was possibly because of the shorter residence time of fluoride ions in column at higher inlet flow rates. It could be expected that a lower flow rate would lead to efficient adsorption during the initial step of the process because of adequate interaction time, and this would cause the longer breakthrough time. Fig. 6 shows the effect of the initial F concentration on the BTCs. As shown in Fig. 6, the breakthrough curves were smooth in the beginning. As more fluoride-spiked tap water passed through the flow cell, the BTC showed steep slopes, rapidly reaching the breakthrough point. This is possibly because the ESIX process between F and Cl may proceed with high rate, leading to a quick saturation of the PAOA film. A steeper BTC was obtained with higher concentrations of F in the feed solution. This was probably because earlier saturation of the PAOA-modified carbon felt electrode would occur with higher initial F concentrations than lower concentrations. The inset of Fig. 6 illustrates the F concentration in the outflow for different initial fluoride concentrations at the beginning stage of the BTCs. The results showed that F retention by the PAOA-modified carbon felt electrode would result in an obvious decrease in the fluoride concentration to below 0.1 mg/L. At initial F concentrations of 5, 10, and 20 mg/L, the breakthrough capacity was 10.0, 10.5, and 9.9 mg/ g, respectively.
3.5.
Regeneration of anion exchange electrodes
To investigate the feasibility of application of the PAOAmodified carbon felt electrode, consecutive regenerationtreatment runs were conducted over 1 week. Three parallel experiments were performed, and the results are illustrated in Table 2. In addition, according to the results of ion chromatography analysis, it was found that most of the fluoride (>95% in each cycle) could be eluted into the solution, which indicates that there was some accumulation of residual F in the felt. The fluoride removal capacity of the fresh electrode was slightly higher than that of the regenerated electrode. However, the
defluoridation efficiency of the regenerated electrode reached 85% on the seventh cycle, which suggests that the PAOAmodified electrode can be regenerated and reused.
4.
Conclusions
In this study, detailed results are presented for fluoride removal by PAOA-modified carbon felt electrodes in a continuous flow reactor. The performance of the PAOA-modified carbon felt electrode reactor was evaluated for fluoride removal in continuous mode. Fluoride retention increased as the terminal potential increased from 0.8 to 1.2 V, and decreased as inlet flow rate and initial fluoride concentration increased. The pH had a negligible effect on the defluoridation capacity of the reactor. Optimal removal was observed at around 1.2 V, and the total amount of fluoride removal reached 10.5 mg/g at pH 7.2 with an initial fluoride concentration of 10 mg/L. Consecutive regeneration-treatment runs showed that the reactor could be reused for at least seven cycles. The results show that the PAOA-modified carbon felt electrode reactor is a promising system for removal of fluoride from contaminated waters.
Acknowledgements This work was funded by the Natural Science Foundation of China (Grant No. 51008154), the Jiangsu Natural Science Foundation (Grant No. SBK201022682), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20090091120007), the Fundamental Research Funds for the Central Universities (Grant No. 1112021101), and the Scientific Research Foundation of Graduate School of Nanjing University (Grant No. 2010CL07).
Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.watres.2012. 04.039
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references
Bhaumik, M., Leswifi, T.Y., Maity, A., Srinivasu, V.V., Onyango, M.S., 2011. Removal of fluoride fromaqueous solution by polypyrrole/ Fe3O4 magnetic nanocomposite. Journal of Hazardous Materials 186, 150e159. Chandrasekhar, P., Naishadham, K., 1999. Broadband microwave absorption and shielding properties of a poly(aniline). Synthetic Metals 105, 115e120. Chen, Y.J., Kang, E.T., Neoh, K.G., 2002. Electroless polymerization of aniline on platinum and palladium surfaces. Applied Surface Science 185, 267e276. Cui, H., Li, Q., Qian, Y., Tang, R., An, H., Zhai, J.P., 2011. Defluoridation of water via electrically controlled anion exchange by polyaniline modified electrode reactor. Water Research 45, 5736e5744. Diawara, C.K., 2008. Nanofiltration process efficiency in water desalination. Separation and Purification Reviews 37, 303e325. Ding, L.A., Li, Q., Cui, H., Tang, R., Xu, H., Xie, X.C., Zhai, J.P., 2010. Electrocatalytic reduction of bromate ion using a polyanilinemodified electrode: an efficient and green technology for the removal of BrO 3 in aqueous solutions. Electrochimica Acta 55, 8471e8475. Huang, M.R., Rao, X.W., Li, X.G., Ding, Y.B., 2011. Lead ionselective electrodes based on polyphenylenediamine as unique solid ionophores. Talanta 85, 1575e1584. Kang, E.T., Neoh, K.G., Tan, K.L., 1998. Polyaniline: a polymer with many interesting intrinsic redox states. Progress in Polymer Science 23, 277e324. Karthikeyan, M., Kumar, K.K.S., Elango, K.P., 2011. Batch sorption studies on the removal of fluoride ions from water using ecofriendly conducting polymer/bio-polymer composites. Desalination 267, 49e56. Karthikeyan, M., Satheeshkumar, K.K., Elango, K.R., 2009a. Defluoridation of water via doping of polyanilines. Journal of Hazardous Materials 163, 1026e1032. Karthikeyan, M., Satheeshkumar, K.K., Elango, K.P., 2009b. Removal of fluoride ions from aqueous solution by conducting polypyrrole. Journal of Hazardous Materials 167, 300e305. Karthikeyan, M., Satheeshkumar, K.K., Elango, K.P., 2009c. Conducting polymer/alumina composites as viable adsorbent for the removal of fluoride ions from aqueous solution. Journal of Fluorine Chemistry 130, 894e901. Li, X.G., Li, A., Huang, M.R., Liao, Y.Z., Lu, Y.G., 2010. Efficient and scalable synthesis of pure polypyrrole nanoparticles applicable for advanced nanocomposites and carbon nanoparticles. Journal of Physical Chemistry C 114, 19244e19255. Li, X.G., Li, H., Huang, M.R., Moloney, M.G., 2011. Synthesis and multifunctionality of self-stabilized poly(aminoanthraquinone) nanofibrils. Journal of Physical Chemistry C 115, 9486e9497.
Lin, Y.H., Cui, X.L., Bontha, J., 2006. Electrically controlled anion exchange based on polypyrrole and carbon nanotubes nanocomposite for perchlorate removal. Environmental Science & Technology 40, 4004e4009. Maliyekkal, S.M., Sharma, A.K., Philip, L., 2006. Manganese-oxidecoated alumina: a promising sorbent for defluoridation of water. Water Research 40, 3497e3506. Mu, S.L., 2004. Electrochemical copolymerization of aniline and o-aminophenol. Synthetic Metals 143, 259e268. Ndiayea, P.I., Moulin, P., Dominguez, L., Millet, J.C., Charbit, F., 2005. Removal of fluoride from electronic industrial effluent by RO membrane separation. Desalination 173, 25e32. Saha, S., 1993. Treatment of aqueous effluent for fluoride removal. Water Research 27, 1347e1350. Shah, A.A., Holze, R., 2006. Spectroelectrochemistry of aniline-oaminophenol copolymers. Electrochimica Acta 52, 1374e1382. Shah, A.A., Holze, R., 2008. Spectroelectrochemistry of twolayered composites of polyaniline and poly (o-aminophenol). Electrochimica Acta 53, 4642e4653. Subramanian, E., Ramalakshmi, R.D., 2010. Pristine, purified and polyaniline-coated tamarind seed (Tamarindus indica) biomaterial powders for defluoridation: synergism and enhancement in fluoride-adsorption by polyaniline coating. Journal of Scientific & Industrial Resarch 69, 621e628. Teng, S.X., Wang, S.G., Gong, W.X., Liu, X.W., Gao, B.Y., 2009. Removal of fluoride by hydrous manganese oxide-coated alumina: performance and mechanism. Journal of Hazardous Materials 168, 1004e1011. World Health Organization (WHO), 2004. In: Guidelines for drinkingwater quality, third ed., vol. 1. WHO, Geneva. Yang, J., Wang, J., Jia, J.P., 2009. Improvement of electrochemical wastewater treatment through mass transfer in a seepage carbon nanotube electrode reactor. Environmental Science & Technology 43, 3796e3802. Zhang, J., Shan, D., Mu, S.L., 2007. Chemical synthesis and electric properties of the conducting copolymer of aniline and oaminophenol. Journal of Polymer Science Part A: Polymer Chemistry 45, 5573e5582. Zhang, J., Shan, D., Mu, S.L., 2006. Electrochemical copolymerization of aniline with m-aminophenol and novel electrical properties of the copolymer in the wide pH range. Electrochimica Acta 51, 4262e4270. Zhang, Y., Li, Q., Tang, R., Hu, Q.C., Sun, L., Zhai, J.P., 2009. Electrocatalytic reduction of chromium by poly(aniline-co-oaminophenol): an efficient and recyclable way to remove Cr(VI) in wastewater. Applied Catalysis B e Environmental 92, 351e356. Zhao, H.Z., Yang, W., Zhu, J., Ni, J.R., 2009. Defluoridation of drinking water by combined electrocoagulation: effects of the molar ratio of alkalinity and fluoride to Al(III). Chemosphere 74, 1391e1395. Zhao, H.Z., Zhao, B., Yang, W., Li, T.H., 2010. Effects of Ca2þ and Mg2þ on defluoridation in the electrocoagulation process. Environmental Sciences & Ecology 44, 9112e9116.
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Electrochemical removal of fluoride from water by PAOAmodified carbon felt electrodes in a continuous flow reactor Hao Cui, Yan Qian, Hao An, Chencheng Sun, Jianping Zhai, Qin Li* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China
article info
abstract
Article history:
A novel poly(aniline-co-o-aminophenol) (PAOA) modified carbon felt electrode reactor was
Received 9 November 2011
designed and investigated for fluoride removal from aqueous solutions. This reactor design
Received in revised form
is innovative because it operates under a wider pH range because of coating with
21 April 2012
a copolymer PAOA ion exchange film. In addition, contaminant mass transfer from bulk
Accepted 23 April 2012
solution to the electrode surface is enhanced by the porous carbon felt as an electron-
Available online 2 May 2012
conducting carrier material compared to other reactors. The electrically controlled anion exchange mechanism was investigated by X-ray photoelectron spectroscopy and cyclic
Keywords:
voltammetry. The applicability of the reactor in the field was tested through a series of
Fluoride
continuous flow experiments. When the flow rate and initial fluoride concentration were
Poly(aniline-co-o-aminophenol)
increased, the breakthrough curve became sharper, which lead to a decrease in the
(PAOA)
breakthrough time and the defluoridation capacity of the reactor. The terminal potential
Carbon felt
values largely influenced fluoride removal by the reactor and the optimal defluoridation
Electrically switched ion exchange
efficiency was observed at around 1.2 V. The breakthrough capacities were all >10 mg/g
Water treatment
over a wide pH range (pH 5e9) with an initial fluoride concentration of 10 mg/L. Consecutive treatmenteregeneration studies over a week (once each day) revealed that the PAOAmodified carbon felt electrode could be effectively regenerated for reuse. The PAOAmodified carbon felt electrode reactor is a promising system that could be made commercially available for fluoride removal from aqueous solutions in field applications. ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Drinking water containing fluoride concentrations above the World Health Organization recommended level of 1.5 mg/L is a major threat to the health of large numbers of people in remote and emerging regions of the world (WHO, 2004). Current methods used to remove fluoride from water includes precipitation (Saha, 1993), membrane techniques (Ndiayea et al., 2005; Diawara, 2008), adsorption (Maliyekkal et al., 2006; Teng et al., 2009) and electrocoagulation (Zhao et al., 2009, 2010). In recent years, it has become more important to find simple, inexpensive, and environmentally friendly technologies to remove fluoride from water supplies.
Conducting polymers and conducting polymer-based composites have shown potential for use as sustainable water treatment media for fluoride tainted drinking water. To date, most attention has focused on the adsorption behaviors of different conducting polymer-based composites for fluoride in batch tests (Karthikeyan et al., 2009a, b, c, 2011; Subramanian and Ramalakshmi, 2010; Bhaumik et al., 2011). Polyaniline (PANI) and polypyrrole (PPy) are the most commonly investigated conducting polymers, because of their high electrical conductivity, ease of preparation, and environmental stability (Chandrasekhar and Naishadham, 1999). Lin et al. (2006) first employed electrically switched ion exchange (ESIX) for perchlorate removal using polypyrrole
* Corresponding author. Tel./fax: þ86 2583592903. E-mail addresses: [email protected], [email protected] (Q. Li). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.04.039
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deposited on high surface area carbon nanotubes. In a previous study, we reported the removal of fluoride via ESIX by a polyaniline modified electrode reactor (Cui et al., 2011). It was observed that that the uptake and elution of fluoride wee effectively controlled by modulating the potential of the polyaniline (PANI) film. This indicates that the ESIX technique with a conducting polymer modified electrode is a good method for fluoride removal. However, some problems have still not yet been resolved: (1) in previous work, the PANImodified electrode reactor performed well only at acidic pH, which would limit its application in drinking water treatment; and (2) the service time of the reactor was short because of the limited surface area of the electrode. In order to improve the physicochemical and electrochemistry properties of the conducting polymers, many new copolymers were prepared and investigated (Huang et al., 2011; Li et al., 2010, 2011). The pH dependence of the conductivity and redox activity of aniline copolymers that contain functional groups, such as eSO3H, eOH, and eCOOH groups, could be much better than for polyaniline itself, because these functional groups can adjust the pH (Mu, 2004). Among these aniline copolymers, poly(aniline-co-o-aminophenol) (PAOA) has good redox activity and is stable over a wide pH range (<1 to 10) (Zhang et al., 2006, 2007; Shah and Holze, 2006, 2008). These results suggest that highly effective defluoridation of water under neutral and basic conditions could be achieved by combining the PAOA copolymer with the ESIX system. In this study, two measures were adopted to improve the applicability of the ESIX system for fluoride removal: (1) PAOAmodified electrode was used for electrically controlled anion exchange; (2) porous carbon felt was employed as an electronconducting carrier material to enhance the contaminant mass transfer from bulk solution to the electrode surface. Mechanism of electrically controlled fluoride removal with PAOA was investigated by X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was used to study the electrochemical reactions of the fluoride ion on the PAOA-modified carbon felt surface as a function of the terminal potential. To optimize the operational parameters of the electrochemistry defluoridation reactor, a series of continuous flow experiments were conducted to examine the effects of the initial fluoride concentration, flow rate, solution pH, and terminal potential on the defluoridation capability of the electrode. Regeneration of the PAOA-modified carbon felt electrode was also studied.
2.
Experimental section
2.1.
Chemicals
Fluoride (NaF, >99%) was purchased from Fluka (a subsidiary of SigmaeAldrich, St. Louis, MO). Other chemicals like aniline, o-aminophenol, HCl, NaCl, NaNO3, Na2CO3, NaH2PO4$2H2O, Na2SO4, Ca(NO3)2$4H2O, Mg(NO3)2$6H2O, and KNO3 (all analytical grade) were obtained from Nanjing Chemical Reagent Co., Ltd (Nanjing, China). Aniline monomer was purified by distillation under reduced pressure before polymerization. Ultrapure water (18.25 MU cm) was used to prepare
all the solutions in this study, and the pH of each solution was adjusted by addition of HCl and NaOH. The water quality data for the tap water used in the flow experiment is shown in Table S-1 (Supporting Information).
2.2.
Preparation of the anion exchange electrodes
Before use, carbon felt pieces were soaked in a 0.10 mol/L HNO3 for 24 h to facilitate wetting of the hydrophobic carbon fibers by the aqueous solutions Electrochemical synthesis of the PAOA film on the surface of carbon felt was carried out in HCl (200 mL, 2.0 mol/L) containing 0.3 mol/L aniline and 5.5 mmol/L o-aminophenol. A titanium electrode was used as the cathode, and one piece of carbon felt was used as the anode. PAOA was deposited onto one piece of circular carbon felt (1.0 cm thick, B 5 cm) at 0.8 V for 30 min. After synthesis, the PAOA-modified carbon felt pieces were washed with a 0.1 mol/L HCl solution to remove unreacted aniline or oaminophenol, then rinsed with ultrapure water, and finally dried at room temperature. Twenty PAOA-modified carbon felt pieces were prepared, and the mass loading of the PAOA coating was determined by measuring the mass of each sample before and after modification. The average mass of the PAOA coating on the carbon felt was 1.170.1 g.
2.3.
Electrode reactor and experimental procedure
Fluoride-spiked tap water was used in a continuous flow experiments to investigate the defluoridation capability of the PAOA-modified carbon felt electrode. During the continuous flow experiments, some key parameters including the pH, external voltage, and flow rate were evaluated. Fig. 1 shows the experimental setup for the electrochemical defluoridation experiments and the structure of the electrochemical defluoridation flow cell. As show in Fig. 1A, the up-flow column experiments were conducted using a syringe pump (Baoding Longer Precision Pump Co., Ltd., Baoding, China). A DC voltage-stabilized power supply (Shanghai Liyou Electrification Co., Ltd., Shanghai, China) was connected to the anode and cathode to control the potential. Effluent samples were collected by an automatic collector (Shanghai Huxi Analytical Instrument Co., Ltd., Shanghai, China) at different intervals to ensure the volume of each effluent sample was 20 mL. The external and internal structures of the electrochemical defluoridation flow cell are shown in Fig. 1B. The flow cell contained four major modules, which were made of acrylic plastic glazing. These modules were fixed together by six flange bolts from bottom to top, with a fluorubber gasket inserted between every two modules. The four modules consisted of two electrode modules in the middle and two connector modules on each end. One piece of circular PAOA-modified carbon felt (1.0 cm thick, B 5 cm) was used as the anode, while circular titanium plate (0.05 cm thick, B 5 cm) with holes for distribution of the solutions was used as the cathode. The distance between the anode and cathode was 0.5 cm.
2.4.
Regeneration of the anion exchange electrodes
Preliminary tests showed that the optimal terminal potential for desorption of fluoride from PAOA-modified carbon felt
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 9 4 3 e3 9 5 0
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Fig. 1 e Experimental setup for the electrochemical defluoridation experiments (A), and images of both the external and internal structure of the electrochemical defluoridation flow cell (B).
electrode was a negative voltage of 0.4 V. Therefore, after treatment, the used PAOA-modified carbon felt electrode module was moved out of the reactor, and regenerated at 0.4 V in 200 mL of HCl solution (0.10 mol/L) for 20 min. The regenerated PAOA-modified carbon felt pieces were rinsed with ultrapure water before reuse.
2.5.
Characterization and analysis
To investigate the electrically controlled anion exchange mechanism, XPS measurements of raw PAOA, PAOA after treatment, and regenerated PAOA were conducted on a Thermo ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA) spectrometer with Al Ka X-rayn (1486.6 eV). All the binding energies were referenced to the C1s neutral carbon peak at 284.6 eV. The morphology of the carbon felt pieces before and after modification with PAOA was observed with a S-3400 NII (Hitachi, Japan) scanning electron spectroscope (SEM). Electrochemical characterization of the electrodes was
performed by CV in an electrolytic cell consisting of the working electrode (PAOA-modified carbon felt), a platinum foil counter electrode (1.5 1.5 cm2), and a saturated calomel reference electrode (Zhang et al., 2009). The electrolytic solution consisted of 0.10 mol/L NaCl and required concentration of fluoride. The fluoride concentrations were evaluated by ion chromatography using a Dionex ICS-2000 (Sunnyvale, CA) ion chromatograph (Cui et al., 2011).
3.
Results and discussion
3.1. Characterization of the PAOA-modified carbon felt electrode The SEM images of the carbon felt before and after electrochemical modification with PAOA are shown in Fig. S1 (Supporting Information). The carbon felt had a fibrous structure and an average fiber diameter of about 20 mm (Fig. S1-A1). The
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uncoated carbon fiber shows a smooth surface with some small particles (Fig. S1-A2). SEM images of the carbon felt after modification indicated that the surface of the carbon fibers was coated uniformly in PAOA sheets (Fig. S1-B1 and S1-B2). Carbon felt electrodes have an extended surface area which provides an additional advantage for contaminant to react (Yang et al., 2009). Because PAOA film was coated on the surfaces of the porous carbon felt, it was more accessible to the contaminant, thereby facilitating the contaminant mass transfer from bulk solution to the electrode surface.
Electrically controlled anion exchange mechanism
To confirm the uptake and elution of fluoride ions in the PAOA films, XPS was used to characterize the samples. Survey and high-resolution XPS spectra for the key elements on the PAOA surfaces are shown in Fig. 2. The presence of a F1s peak in curve b (Fig. 2A) confirmed fluoride uptake by the PAOAmodified carbon felt after it was added with an anodic voltage of 1.2 V in a 50 mg/L fluoride solution. After regeneration of the fluoride-doped PAOA with 0.1 mol/L HCl, the F1s peak disappeared in curve c, which was similar to the original PAOA. This indicated that fluoride ions were removed from PAOA. Similar to PANI, conformational changes of nitrogen atoms at a molecular level on the PAOA chains could further explain
N
O
Cl
Relative intensity, c/s
F
C
C
A
(c) PAOA-R F
(b) PAOA-F
(c) PAOA-R
Relative intensity, c/s
3.2.
the anion exchange mechanism (Lin et al., 2006). Peak fitting illustrated that N1s spectra of PAOA (Fig. 2B) could be deconvoluted into three Gaussian peaks at about 398, 399, and >400.00 eV, which corresponded to nitrogen atoms in nitride (]Ne), amine (eNHe), and doped imine (eNHþ e), respectively (Chen et al., 2002). The nitride/imine ratio of the original PAOA film increased significant after fluoride removal, which suggested that PAOA was less protonated (Kang et al., 1998). After regeneration, the nitride/imine ratio decreased again, indicating that the PAOA film was regenerated well and could be used repeatedly. The F1s spectra in Fig. 2C and Cl2p spectra in Fig. 2D clearly showed that the anion exchange occurred between F and Cl. In Fig. 2C, the F content increased in the PAOA-F samples (curve b) and little F could be seen in the PAOA-R samples (curve c). This demonstrates that the PAOA could achieve both effective uptake and complete elution of fluoride ions, which was controlled by modulating its terminal potential. The Cl2p spectra (Fig. 2D) also showed substantial changes after different treatments of the PAOA films. The Cl2p spectra could be divided into three spineorbit split doublets (Cl2p3/2 and Cl2p1/2) for Cl, Cl*, and CeCl, which represent the chloride anion, anionic chloride resulting from charge transfer between chlorine and the PAOA polymer chain, and the covalently bonded chlorine, respectively. The Cl2p3/2 were situated at binding energies of 196.3 0.1 (Cl), 197.7 0.1
(b) PAOA-F
(a) PAOA 0
200
400
600
800
(a) PAOA 1000
680
682
684
Binding Energy, eV -NH-
N
-NH -
D
(c) PAOA-R
Relative intensity, c/s
690
692
(b) PAOA-F
Cl
Cl
Relative intensity, c/s
-N=
688
694
696
698
Binding Energy, eV
·
B
686
C-Cl
Cl
(c) PAOA-R
(b) PAOA-F
(a) PAOA (a) PAOA 396
397
398
399 400 401 402 Binding Energy, eV
403
404
405
194
195
196
197 198 199 200 Binding Energy, eV
201
202
203
Fig. 2 e Survey scan (A) and high-resolution XPS spectra of N1s (B), F1s (C) and Cl2p (D) for different samples: (a) original PAOA, (b) PAOA-F, the PAOA film after electrochemical fluoride removal, and (c) PAOA-R, the PAOA film after regeneration.
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 9 4 3 e3 9 5 0
(Cl*), and 199.9 0.1 (CeCl) eV (Kang et al., 1998). As shown in Fig. 2D, a considerably lower Cl content and considerably higher CeCl content were observed at the surface of the PAOA-F samples (curve b). This indicates that loss of the dopant in the surface region of the emeraldine HCl doped PAOA film occurs during its exposure to fluoride solution at an anodic voltage.
3.3. Effect of fluoride concentration and pH on the cyclic voltammograms For reproducible anion exchange of the electrode, the electrochemical properties of the PAOA-modified carbon felt should be stable. The stabilities of the electrode materials were assessed using CV, and the results showed that bare carbon felt had good electrical conductivity (Fig. S2-A, Supporting Information). CV of the PAOA film coated on the carbon felt was consistent over 10 cycles (Fig. S2-B, Supporting Information). This indicates that the processes of oxidation and reduction are chemically reversible, and that the electrode potential can be modulated to control loading and unloading of anions (Ding et al., 2010). For qualitative analysis of the electrochemical reaction of the fluoride ion on the PAOA-modified carbon felt surface as a function of the terminal potential, a series of CV experiments were performed. Fig. 3 shows the CV of PAOA in 0.10 mol/L NaCl solutions without (curve a) and with different concentrations of fluoride (50e200 mg/L) (curve bed) at pH 5.0. Our previous study on fluoride removal by the PANI-modified electrode revealed that at a suitable anodic voltage (about 1.5 V), the state of the nitrogen atoms on PANI chains changed. This caused secondary doping of PANI by fluoride ions and loss of electrons from PANI chains (Cui et al., 2011). In this study, as can be clearly seen in Fig. 3, the oxidation peak potentials on curves bed shifted toward positive potentials and the oxidation peak currents increased as the
c
0.02
3947
concentration of fluoride increased. This indicates that the oxidation peaks on curves bed are caused by fluoride uptake on the PAOA-modified carbon felt. It is interesting to note that the peak potentials were slightly lower that those recorded in our previous study, which ranged from about 0.8 to 1.1 V at fluoride concentrations of 50e200 mg/L. This is possibly because of the difference in electrode material. The results indicate that this reactor system will be energy saving compared to our earlier system. CV was also used to investigate the effects of the pH on fluoride removal by the PAOA-modified carbon felt. Fig. 4 shows the voltammograms obtained for the PAOA-modified carbon felt in 0.10 mol/L NaCl at different pH values for solutions containing 100 mg/L F at a scan rate of 20 mV/s. Generally, the trends for the voltammograms were similar at the different pH values. A decrease in the redox activity of PAOA was observed from the first cycle to the tenth cycle in all of the figures, possibly because of the pH difference between the bulk solution and the inner polymer matrix. It was observed that pH had no significant effect on the redox activity of PAOA as solution pH increased from 5 to 9. This result was similar to that reported by other researchers (Mu, 2004; Zhang et al., 2006; Shah and Holze, 2008). In this study, it was found that PAOA has good defluoridation capability in a wide pH range under both acidic and alkaline conditions.
3.4. Optimization of operational parameters in continuous flow mode Treatment of fluoride-contaminated tap water in continuous flow mode was conducted to study the effect of various process parameters, such as the terminal potential, solution
d
b a
I/A
0.01
0.00
(a ) (b ) (c ) (d )
-0.01
0 m g /L 5 0 m g /L 1 0 0 m g /L 2 0 0 m g /L
-0.02 -0.4
0.0
0.4
0.8
1.2
1.6
E / V (vs SCE) Fig. 3 e Cyclic voltammograms of the PAOA-modified carbon felt electrode in a solution containing 0.10 mol/L NaCl at pH 7 with different fluoride concentrations. (a) Without fluoride, (b) with 50 mg/L fluoride, (c) with 100 mg/ L fluoride, (d) with 200 mg/L fluoride. Voltammograms were recorded from L0.40 V to 1.6 V at a scan rate of 20 mV/s.
Fig. 4 e Effect of pH on the cyclic voltammograms of the PAOA-modified carbon felt electrode in a solution containing 0.10 mol/L NaCl and 100 mg/L fluoride. Voltammograms were recorded from L0.40 V to 1.6 V at a scan rate of 20 mV/s.
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Table 1 e Effect of terminal potential and solution pH on fluoride removal.a Initial pH
Terminal potential (V)
Breakthrough volumeb (mL)
Exhaust volumec (mL)
Breakthrough capability (mg/g)
0.8 1 1.2 1.4 1.2 1.2 1.2 1.2 1.2
720 1080 1620 900 1660 1680 1620 1640 1620
1260 1640 2720 1460 2780 2800 2720 2740 2740
6.5 8.2 10.5 7.5 11.2 10.9 10.5 10.6 10.6
7.2 7.2 7.2 7.2 5.3 6.1 7.2 8.1 9.3
a Flow rate, 5 mL/min; initial fluoride concentration, 10 mg/L. b The volume of the effluent with fluoride concentration below 1.5 mg/L. c The volume of the effluent at Ct/C0 ¼ 1.
pH, inlet flow rate, and initial fluoride concentration. Tap water contaminated with fluoride was prepared by adding NaF to authentic tap water to obtain final F concentrations of 5, 10 and 20 mg/L. Table 1 shows the effect of the solution pH and terminal potential on the defluoridation efficiency by the PAOAmodified carbon felt electrode in the continuous flow reactor. It should be noted that the pH value had a negligible effect on fluoride uptake by the PAOA-modified carbon felt electrode. The breakthrough volume, effective treatment volume, and removal capacity of the reactor remained almost constant over the natural water pH range (5e9). This is possibly due to the fact that o-aminophenol has a functional group eOH according to the views of Mu (2004) in a previous study. Oxidation of eOH group occurs accompanied with proton exchange between the copolymer and the solution. The eOH group can be oxidized to quinone, and quinone can be reduced. The redox reaction of the eOH group plays an important role in adjusting the pH around the copolymer electrode. Further studies are still needed to better understand
the mechanism behind the pH adjustment effect of the functional group eOH. Potential is always used first to optimize electrochemical reactor operation conditions because it directly determines the contaminant removal and side reactions. As shown in Table 1, from 0.8 to 1.2 V, the fluoride removal increased as the terminal potential increased, exhibiting a positive correlation. Optimal removal was observed at around 1.2 V during the continuous flow study, and 10.5 mg/g of fluoride was removal at pH 7.2. A higher potential means higher current density, which leads to higher fluoride removal capacity. However, terminal potentials above 1.4 V result in decreased fluoride removal efficiency because of overoxidation of the PAOA films. The effect of the flow rate of the feed fluoride solution was studied with a fixed initial fluoride concentration of 10 mg/L at pH 7.2 under a potential of 1.2 V. The typical breakthrough curves (BTCs) are shown in Fig. 5. The values of Ct/C0 were very low and the BTCs were flat at the beginning. During this time, the fluoride concentration in the outflow was below the World Health Organization recommended level of 1.5 mg/L. As more
1.0 1.0
0.9 0.8 0.7
5 mg/L 10 mg/L 20 mg/L
0.8 0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
C /C
Ct/C0
0.9
2 ml/min 5 ml/min 10 ml/min
0.3
0.2
0.2
0.1
0.1
0.0 0
250
500
750
1000 1250 1500 1750 2000
water yield, mL
Fig. 5 e Fluoride breakthrough in continuous flow experiments at different inlet flow rates (pH, 7.2; C0, 10 mg/ L; terminal potential, 1.2 V). The dotted line indicates the World Health Organizations recommended level (Ct [ 1.5 mg/L), and the arrows indicate the breakthrough points.
water yield, mL
0.0 0
1000
2000
3000
4000
5000
water yield, mL
Fig. 6 e Fluoride breakthrough in continuous flow experiments with different initial fluoride concentrations (pH, 7.2; flow rate, 5 mL/min; terminal potential, 1.2 V). The arrows indicate the breakthrough points. The inset shows the details during the first 1250 min.
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Table 2 e FL removal capability in consecutive treatmenteregeneration cycles over one week. Datea
Test Ac b
Mon. Tue. Wed. Thu. Fri. Sta. Sun. a b c d
Test Bc d
b
Test Cc d
b
Mean value d
b
Qe (mg/g)
BV (mL)
Qe (mg/g)
BV (mL)
Qe (mg/g)
BV (mL)
Qe (mg/g)
BVd (mL)
10.5 10.4 10.2 9.7 9.5 9.6 9.3
1620 1600 1620 1580 1560 1560 1520
11.3 11.1 10.5 10.6 10.3 10.0 9.7
1660 1660 1640 1620 1600 1620 1600
10.7 10.3 9.7 9.4 9.6 9.6 9.5
1620 1600 1580 1560 1560 1550 1520
10.8 0.4 10.6 0.4 10.2 0.4 9.9 0.6 9.8 0.5 9.7 0.3 9.5 0.2
1633 23 1620 35 1613 31 1587 31 1573 23 1577 38 1546 47
The regenerationetreatment cycle was carried out once a day. Qe, breakthrough capability. The test was carried out in three replicate runs. BV, breakthrough volume.
solution passed through the anion exchange layer, the BTC became steeper, and quickly reached the breakthrough point. The effective treatment volume decreased as the sample flow rate increased from 2.0, to 5.0, to 10.0 mL/min, and the breakthrough capacity decreased from 11.8, to 10.5, to 3.2 mg/ g. This was possibly because of the shorter residence time of fluoride ions in column at higher inlet flow rates. It could be expected that a lower flow rate would lead to efficient adsorption during the initial step of the process because of adequate interaction time, and this would cause the longer breakthrough time. Fig. 6 shows the effect of the initial F concentration on the BTCs. As shown in Fig. 6, the breakthrough curves were smooth in the beginning. As more fluoride-spiked tap water passed through the flow cell, the BTC showed steep slopes, rapidly reaching the breakthrough point. This is possibly because the ESIX process between F and Cl may proceed with high rate, leading to a quick saturation of the PAOA film. A steeper BTC was obtained with higher concentrations of F in the feed solution. This was probably because earlier saturation of the PAOA-modified carbon felt electrode would occur with higher initial F concentrations than lower concentrations. The inset of Fig. 6 illustrates the F concentration in the outflow for different initial fluoride concentrations at the beginning stage of the BTCs. The results showed that F retention by the PAOA-modified carbon felt electrode would result in an obvious decrease in the fluoride concentration to below 0.1 mg/L. At initial F concentrations of 5, 10, and 20 mg/L, the breakthrough capacity was 10.0, 10.5, and 9.9 mg/ g, respectively.
3.5.
Regeneration of anion exchange electrodes
To investigate the feasibility of application of the PAOAmodified carbon felt electrode, consecutive regenerationtreatment runs were conducted over 1 week. Three parallel experiments were performed, and the results are illustrated in Table 2. In addition, according to the results of ion chromatography analysis, it was found that most of the fluoride (>95% in each cycle) could be eluted into the solution, which indicates that there was some accumulation of residual F in the felt. The fluoride removal capacity of the fresh electrode was slightly higher than that of the regenerated electrode. However, the
defluoridation efficiency of the regenerated electrode reached 85% on the seventh cycle, which suggests that the PAOAmodified electrode can be regenerated and reused.
4.
Conclusions
In this study, detailed results are presented for fluoride removal by PAOA-modified carbon felt electrodes in a continuous flow reactor. The performance of the PAOA-modified carbon felt electrode reactor was evaluated for fluoride removal in continuous mode. Fluoride retention increased as the terminal potential increased from 0.8 to 1.2 V, and decreased as inlet flow rate and initial fluoride concentration increased. The pH had a negligible effect on the defluoridation capacity of the reactor. Optimal removal was observed at around 1.2 V, and the total amount of fluoride removal reached 10.5 mg/g at pH 7.2 with an initial fluoride concentration of 10 mg/L. Consecutive regeneration-treatment runs showed that the reactor could be reused for at least seven cycles. The results show that the PAOA-modified carbon felt electrode reactor is a promising system for removal of fluoride from contaminated waters.
Acknowledgements This work was funded by the Natural Science Foundation of China (Grant No. 51008154), the Jiangsu Natural Science Foundation (Grant No. SBK201022682), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20090091120007), the Fundamental Research Funds for the Central Universities (Grant No. 1112021101), and the Scientific Research Foundation of Graduate School of Nanjing University (Grant No. 2010CL07).
Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.watres.2012. 04.039
3950
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 9 4 3 e3 9 5 0
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