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Removal of phenols from the aqueous solutions based on their electrochemical polymerization on the polyaniline electrode
Electrochimica Acta 55 (2010) 7219–7224
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Removal of phenols from the aqueous solutions based on their electrochemical polymerization on the polyaniline electrode Ya Zhang, Qin Li, Hao Cui, Jianping Zhai ∗ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 22 Hankou Road, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2010 Received in revised form 30 June 2010 Accepted 1 July 2010 Available online 8 July 2010 Keywords: Removal of phenol and 3-nitrophenol Electrochemical polymerization Polyaniline as a working electrode SEM images IR spectra
a b s t r a c t Phenol and 3-nitrophenol in a aqueous solution of NaCl with pH 4.0 can be polymerized on the polyaniline electrode to form polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites, respectively, using potential cycling between −0.20 and 1.00 V (vs. SCE). Polyaniline played an important role in lowering passivation of the electrode, which made the consecutive electrochemical polymerization of phenol and 3-nitrophenol become possible. The growth of the polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) films in the electrolytic process was proved by the increasing area of the cyclic voltammograms as the electrolysis proceeded. The SEM images and IR spectra of polyaniline and phenolic polymers demonstrated the formation of the phenolic polymers on the polyaniline electrode. Therefore, the removal of both phenolic compounds is based on the formation of their polymers on the polyaniline film. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Phenols are toxic to humans and aquatic lives [1,2]. Phenolic compounds are released in the surface water by a number of industries, mainly by pharmaceutical plants, oil refineries, coke plants, pulp, resin and food-processing industries [3–5]. These industries are distributed in wide area, which result in serious environmental problems because of their toxicity, poor biodegradability and accumulation potential in plants and tissues, such as fish live in the lake and river contaminated with phenolic compounds have a queer taste and bad odour. Therefore the wastewater containing phenolic compounds must be treated before their discharge into water streams. Various techniques have been employed for the removal of phenolic compounds from aqueous media including adsorption [6–9], oxidation [10–12], membrane filtration [13], ion exchange [14], solvent extraction [15], photocatalytic degradation [16], and biological degradation [17]. Some of them have progressed quickly in the removal of phenolic compounds [18,19], while adsorption using activated carbons is successful to remove efficiently phenolic compounds from the contaminated water due to large surface area, high adsorption capacity, chemical stability and fast adsorption rate [20,21]. However, activated carbons are related to the high initial cost and a costly regeneration. Therefore, creating new
∗ Corresponding author. Tel.: +86 25 8359 2903; fax: +86 25 8359 2903. E-mail address: [email protected] (J. Zhai). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.002
technique and looking for new material for the treatment of phenols in the wastewater are still full of chances and challenges to researchers. Phenol and its derivatives in aqueous solutions with a large pH range and non-aqueous solutions can be polymerized by the electrochemical oxidation to form very tight polymer film on the metal electrodes [21–24], which are used for protecting metal corrosion [25,26]. The phenolic polymer is generally a very thin passive layer (5 nm–15 m) [25–27], which inhibits the film growth during the electrolytic process due to the film tightness and very low conductivity. In this case, the electrochemical polymerization of phenols cannot be used to remove phenols in the contaminated water. A strategy for overcoming the formation of the passive layer is to use a conducting polymer as a working electrode. The conducting polymers can be reduced and oxidized reversibly, accompanied with ion exchange between the conducting polymer and the solution, which therefore has been used to remove ClO4 − [28] and Cr(VI) [29] in the aqueous solutions. Among conducting polymers, polyaniline is one of the most promising conducting polymers due to its high conductivity, good redox reversibility and good stability. Furthermore, polyaniline can catalyze polymerization of aniline itself and its derivatives because it carries free radicals to initiate their oxidation [30–32]. Therefore, polyaniline is used here to study the electrochemical polymerization of phenols. The objective of the current study is to investigate the polymerization of phenols and their film growth on the polyaniline electrode and remove phenols from the aqueous solution.
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Fig. 1. Cyclic voltammograms of a glassy carbon electrode in a solution containing 0.10 M NaCl and (A) 5 mM phenol; (B) 5 mM 3-nitrophenol. Curves: (1) first cycle; (2) second cycle; (3) third cycle and (4) fourth cycle. pH 4.0, at a scan rate of 60 mV s−1 .
2. Experimental Chemicals were of analytical reagent grade obtained from Shanghai Chemical Company. Aniline was distilled under reduced pressure prior to use. Doubly distilled water was used to prepare all aqueous solutions. The pH values of the solutions were determined with a PXD-12 pH meter. An electrolytic cell for the synthesis of polyaniline consisted of a glassy carbon (GC) disk working electrode (3 mm diameter), a platinum foil counter electrode and a saturated calomel reference electrode (SCE). The GC disk electrode was polished with alumina slurry of 0.5 m on polishing cloth with water, and then was washed with water and sonicated in a doubly distilled water bath for 10 min before use. All electrochemical experiments were performed on a model CHI 407 electrochemical workstation. The polymerization of aniline was carried out in a solution containing 0.20 M aniline and 1.0 M HCl at 0.75 V. The amount of polyaniline film polymerized on a GC electrode was controlled by passing charge during the electrolytic process, which was 1.8 × 10−2 C. After synthesis, polyaniline film polymerized on a GC electrode was washed with 0.05 M HCl solution and then cycled in 0.20 M HCl solution for several cycles in a potential range between −0.20 and 0.80 V to remove further aniline remaining in the polyaniline film. The FTIR spectra of the polymer samples were measured on a pressed pellet with KBr using a Bruker IFS 66/s spectrometer. The morphology of the polymer samples was observed with a Hitachi S-4800 II scanning electron microscope (SEM).
3. Results and discussion
increasing number of cycles. This indicates that the electrochemical polymerization of phenol or 3-nitrophenol would not continue to take place on the bare GC electrode. Therefore, the electrochemical oxidation behavior of phenol and 3-nitrophenol shown in Fig. 1A and B is very similar to that of earlier studies [22,24], indicating that a thin phenolic layer with low conductivity was formed on a GC electrode. Even though the cyclic voltammograms in Fig. 1B are very similar in shape to those in Fig. 1A, however, the peak current density of curve 1 in Fig. 1B is a little smaller than that of curve 1 in Fig. 1A; and especially, the peak width of curve 1 in Fig. 1B is evidently smaller than that of curve 1 in Fig. 1A, indicating that the total charges under curve 1 in Fig. 1B are smaller than those of curve 1 in Fig. 1A. This difference is caused by different chemical properties between phenol and 3-nitrophenol. 3.2. Electrochemical polymerization characteristic of phenols on a polyaniline electrode Fig. 2 shows the cyclic voltammograms of the polyaniline electrode in 0.10 M NaCl solution with pH 4.0. As can be seen in Fig. 2, the area of the cyclic voltammograms decreases with an increasing number of cycles. This result is caused by a decrease in the electrochemical activity of polyaniline at pH 4.0. However, the polyaniline electrode approximately reached a stable state after the ninth cycle because the change in the area of cyclic voltammograms is a little from the ninth cycle to the 10th cycle. Therefore, before the electrolysis of phenols using a polyaniline electrode, the polyaniline
3.1. Electrochemical polymerization of phenols on a bare GC electrode A solution of 0.10 M NaCl with pH 4.0 was used in the following experiments. The reason for choosing NaCl as an electrolyte is to avoid the second pollutant discharge; and considering the fact that polyaniline lost its redox activity at pH >4.0, thus, the pH value of the NaCl solution was controlled at pH 4.0 in this work. Fig. 1A and B show the electrochemical oxidation of phenol and 3-nitrophenol on a bare GC electrode, respectively, in an electrolytic solution consisting of 0.10 M NaCl and 5 mM phenol or 5 mM 3nitrophenol with pH 4.0. An oxidation peak at 0.98 V occurs in Fig. 1A for the first cycle, which is caused by phenol oxidation; an oxidation peak at 0.96 V occurs in Fig. 1B, which is attributed to 3-nitrophenol oxidation. The feature that phenol oxidation and 3-nitrophenol oxidation have in common is that the currents of their oxidation peaks decrease dramatically from the first cycle to the second cycle, followed by the continuous decrease with an
Fig. 2. Cyclic voltammograms of polyaniline in 0.10 M NaCl solution with pH 4.0, (1) first cycle; (2) second cycle; (3) eighth cycle; (4) ninth cycle and (5) 10th cycle, at a scan rate of 60 mV s−1 .
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Fig. 3. Cyclic voltammograms for the electrochemical polymerization of phenol and 3-nitrophenol on a polyaniline electrode, the electrolytic solutions consisted of 0.10 M NaCl and (A) 5 mM phenol; (B) 5 mM 3-nitrophenol, curves: (1) first cycle; (2) second cycle; (3) third cycle; (4) 10th cycle and (5) 20th cycle, at a scan rate of 60 mV s−1 , pH 4.0.
electrode must be cycled for 10 cycles in 0.10 M NaCl solution with pH 4.0. Fig. 3A shows the cyclic voltammograms for the oxidation of phenol on a polyaniline electrode in a solution consisting of 0.10 M NaCl and 5 mM phenol with pH 4.0. There are two oxidation peaks on the first cycle (curve 1) to the third cycle (curve 3). Therefore, their I–E curve shapes are similar to those on curves 1 and 2 in Fig. 2 on the forward scan; however, their wave shapes on the reverse scan are different from those in Fig. 2. This difference indicates the formation of the phenol polymer on the polyaniline electrode during the oxidation process of phenol. Even though the current densities of the oxidation peaks at 0.40 and 0.60 V in Fig. 3A decreases from the first cycle to the second cycle, its decrement is much smaller than that of phenol oxidation on the bare GC electrode in compared with Fig. 1A. Furthermore, the area of the cyclic voltammograms in Fig. 3A decreases slowly with an increasing number of cycles from the second cycle to the twentieth cycle (curve 5), which is much different from that in Fig. 1A. This indicates that the electrolysis of phenol was able to continue on the polyaniline electrode. A decrease in the area of the cyclic voltammograms demonstrates the formation of phenolic polymer because the conductivity of polyphenol is low [25–27]. Fig. 3B shows the cyclic voltammograms for the oxidation of 3-nitrophenol on a polyaniline electrode. The cyclic voltammograms of the first and the second cycles in Fig. 3B are different in shape from those in Fig. 2, which is caused by the oxidation
of 3-nitrophenol on the polyaniline electrode. In comparison with Fig. 1B, it is clear that the decrement of the current density of the oxidation peak from the first cycle to the second cycle in Fig. 3B is also smaller than that in Fig. 1B, and the area of the cyclic voltammograms in Fig. 3B decreases slowly with an increasing number of cycles from the second cycle to twentieth cycle. These experimental phenomena indicate that the electrolysis of 3-nitrophenol was able to continue on the polyaniline electrode as well as the electrolysis of phenol on the polyaniline electrode. It is noteworthy that there are two oxidation peaks on curve 1 in Fig. 3A; however, only one oxidation peak appears on curve 1 in Fig. 3B; and especially, the current density of both oxidation peaks on curve 1 in Fig. 3A are much higher than that of the oxidation peak on curve 1 in Fig. 3B. This difference is also caused by the different chemical properties between phenol and 3-nitrophenol. As can be seen in Figs. 1A and 3A, the electrochemical oxidation activity of phenol on the polyaniline electrode is much larger than that on the bare GC electrode. This indicates that polyaniline can initiate the electrochemical oxidation activity of phenol because of the free radicals contained in polyaniline. Marking a comparison between Figs. 1B and 3B, polyaniline can also initiate the electrochemical oxidation activity of 3-nitrophenol, but this influence is less than the effect of polyaniline on phenol. The results from Figs. 1 and 3 demonstrate that phenol is more easily to be oxidized compared with 3-nitrophenol, because there is an electro-withdrawing group, –NO2 , in the aromatic ring of phenol, which results in a decrease in reactivity of 3-nitrophenol.
Fig. 4. Curve 1 is the cyclic voltammogram of the 10th cycle for polyaniline in 0.10 M NaCl solution with pH 4.0. Curves 2–4 are the cyclic voltammograms of (A) polyaniline/polyphenol and (B) polyaniline/poly(3-nitrophenol) in 0.10 M NaCl solution with pH 4.0. They were synthesized using potential cycling (shown in Fig. 3) under different cycles, curves: (2) 20; (3) 40; (4) 60. At a scan rate of 60 mV s−1 .
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Fig. 5. SEM images: (A) polyaniline film; (B) polyaniline/polyphenol film and (C) polyaniline/poly(3-nitrophenol) film.
It is well known that the oxidation processes of both phenol and 3-nitrophenol are accompanied with the release of protons that result in a decrease in the pH value of the solution. In this case, the redox activity of polyaniline would increase as the electrolysis proceeds. However, the results from Fig. 3 show a decrease in the redox activity of polyaniline. This is due to the formation of the phenolic polymers with low conductivities on the polyaniline electrode, which predominates over the effect of pH on the redox activity of polyaniline. 3.3. Evidence for the film growth for the electrolysis of phenols on a polyaniline electrode The results from cyclic voltammograms in Fig. 3 indicate that the electrochemical polymerization of phenol and 3-nitrophenol took place on the polyaniline electrode. However, we cannot directly observe film growth in Fig. 3 as the number of cycles increases, because the area of the cyclic voltammograms decreases as the electrolysis proceeded. To prove the growth of the polyaniline film in the electrolytic process of phenol or 3-nitrophenol, the experiment must be carried out by step-electrolysis procedure. Therefore, after 20 cycles shown in Fig. 3, the polyaniline/polyphenol or polyaniline/poly(3-nitrophenol) film was washed with 0.05 M HCl solution, then put in 0.20 M HCl solution to scan five cycles between −0.20 and 0.80 V to remove phenol or 3-nitrophenol in the polyaniline film and finally the experiment shown in Fig. 2 was repeated. After that, the polyaniline/polyphenol electrode or polyaniline/poly(3-nitrophenol) electrode was again put in the solution containing 5 mM phenol or 5 mM 3-nitrophenol, respectively, to do potential cycling for another 20 cycles
as shown in Fig. 3. The resulting polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) electrodes obtained from 20, 40 and 60 cycles in the solution containing phenol and 3-nitrophenol, respectively, were used in following experiments. Curve 1 in Fig. 4A shows the 10th cyclic voltammogram of the polyaniline electrode in 0.10 M NaCl solution of pH 4.0 as shown in Fig. 2. Curves 2–4 in Fig. 4A show the cyclic voltammograms of polyaniline/polyphenol in 0.10 M NaCl solution with pH 4.0, which were obtained from 20, 40 and 60 cycles using potential cycling shown in Fig. 3A, respectively. The area of curve 2 is larger than that of curve 1, indicating that phenol was polymerized on the polyaniline electrode. This is because the plot of I–E on the cyclic voltammogram represents the quantity of electricity. It is clear that the quantity of electricity of curve 2 is larger than that of curve 1, which is caused by only the formation of polyphenol on the original polyaniline electrode because the area of the cyclic voltammogram for the polyaniline electrode itself decreases slowly during the consecutive potential cycles in 0.10 M NaCl solution of pH 4.0. Fig. 4A shows that the oxidation peak potential shifts towards the positive potentials as the potential cycle for synthesis of polyphenol increases, which is mainly attributed to IR drop of the polyaniline/polyphenol film due to low conductivity of polyphenol. An increase in the area of cyclic voltammograms and a shift in the oxidation peak potential towards the positive potential direction are typical of the growth of the polymer film. Curve 1 in Fig. 4B is the 10th cyclic voltammogram of the polyaniline electrode in 0.10 M NaCl solution of pH 4.0 as shown in Fig. 2. Curves 2–4 in Fig. 4B show the cyclic voltammograms of polyaniline/poly(3-nitrophenol) in 0.10 M NaCl solution with pH 4.0, which were obtained from 20, 40 and 60 cycles using poten-
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tial cycling shown in Fig. 3B, respectively. The results in Fig. 4B are similar to those in Fig. 4A. A difference between Fig. 4A and B is that a shift in the oxidation peak potential in Fig. 4B is smaller than that in Fig. 4A, which is caused by different properties of two phenolic polymers. The results from Fig. 4 demonstrate that phenol and 3-nitrophenol in the aqueous solution can be removed by their electrochemical polymerization on the polyaniline electrode. Here, polyaniline played an important role in lowering passivation of the electrode caused by phenolic polymers. 3.4. Morphology of polymers Fig. 5A shows the SEM image of the polyaniline film that polymerized on a GC electrode surface. The image shows that the film consisted of interwoven nanofibers with an average diameter of about 55 nm with lengths varying from 240 to 310 nm. The formation of nanostructures is due to the fact that the electrochemical polymerization rate of aniline in 1.0 M HCl solution is fast enough at 0.75 V. The fast polymerization rate is favorable for the formation of a large amount of polyaniline nuclei, which leads to the slow growth of the polyaniline nuclei to form nanostructures [33]. Fig. 5B and C show the SEM images of the polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) films, which were prepared using potential cycling between −0.20 and 1.0 V for 40 cycles in the solutions containing 5 mM phenol and 5 mM 3-nitrophenol as described previously. Their SEM images are similar to that shown in Fig. 5A, but the nanostructure of the polyaniline/polyphenol film is a little more compact than that of the polyaniline film, which is mainly caused by the formation of many thinner nanofibers between the predominant nanofibers; in addition, the average diameter of the predominant nanofibers in Fig. 5B is a little large than that in Fig. 5A. The nanostructure of the polyaniline/poly(3-nitrophenol) film is also a little more compact than that of polyaniline film, which is mainly attributed to that the average diameter (about 70 nm) of the nanofibers in Fig. 5C is larger than that of the polyaniline nanofibers; in addition, a small amounts of the thinner nanofibers are also observed in Fig. 5C. The above results indicate that the phenolic polymers were formed on the polyaniline nanofibers and the neighboring polyaniline nanofibers, indicating that the presence of polyaniline nanofibers is advantageous to the formation of the phenolic polymers on polyaniline. This is because the catalytic activity of polyaniline is mainly attributed to the free radicals existing in polyaniline. The nanostructures of polyaniline provide a large specific surface area that increases the impact possibility between the free radicals in polyaniline and reactants, phenolic compounds. As a result the nanostructures of polyaniline play an important role in enhancing the electrochemical catalytic polymerization rates of phenolic monomers. 3.5. IR spectra of polymers Curve 1 in Fig. 6 shows the IR spectrum of polyaniline, in which a absorption peak at 3424 cm−1 is attributed to the stretching vibrations of N–H; this peak also occurs on the IR spectra of polyaniline/polyphenol (curve 2) and polyaniline/poly(3-nitrophenol) (curve 3) composites. The peak at 1635 cm−1 on curve 1 is assigned to the C C stretching vibrations of benzene rings. However, two strong peaks at 1569 and 1482 cm−1 occur on curves 2 and 3, which are assigned to the C C stretching vibrations of quinone ring and benzene ring, respectively. But, they were not observed on curve 1. A strong peak at 1386 cm−1 occurs on each curve in Fig. 6, which is attributed to the deformation vibrations of C–O–H because the deformation vibrations of C–O–H are in the range of 1390–1310 cm−1 [34]. A peak at 1237 cm−1 occurs on each curve in Fig. 6, which is attributed to C–N stretching vibrations because the stretching vibrations of C–N in aromatic amines are in the range of
Fig. 6. IR spectra: (1) polyaniline; polyaniline/poly(3-nitrophenol).
(2)
polyaniline/polyphenol
and
(3)
1280–1180 cm−1 [34]. A peak at 1292 cm−1 on curves 2 and 3, but is not detected on curve 1. This peak would be assigned to the C–O–C stretching modes because the stretching vibrations of C–O–C in ethers are in the range of 1300–1000 cm−1 [34]. This indicates that the polymerization of phenol or 3-nitrophenol was carried out via the formation of the ether bond between benzene rings. A peak at 1122 cm−1 occurs on each curve in Fig. 6, which is attributed to the C–H in-plane bending vibrations. However, this peak is very weak in the IR spectrum of polyaniline compared to the IR spectra of polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites. In summary, the IR spectra of polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites are different from that of polyaniline, which is caused by the phenolic polymers polymerized on the polyaniline electrode. 4. Conclusions Phenol and 3-nitrophenol in a solution of NaCl with pH 4.0 can be polymerized on the polyaniline electrode using potential cycling. Polyaniline played an important role in lowering passivation of the electrode, which made the consecutive electrochemical polymerization of phenol and 3-nitrophenol become possible. Therefore, phenol and 3-nitrophenol in the aqueous solution can be removed via the formation of the phenolic polymers. On the basis of this method, aniline and its derivatives in the aqueous solution could be also removed because they can be polymerized on the polyaniline electrode. In addition, toxic ions in the aqueous solutions were removed using conducting polymers based on the doping and de-doping principle of the conducting polymers. Therefore, the conducting polymers are a promising material for the electrochemical treatment of contaminated water. Acknowledgments This work was supported by the Foundation of The State Key Laboratory of Pollution Control and Resource Reuse of China and the Scientific Research Foundation of Graduate School of Nanjing University. We are grateful to the anonymous reviewers for their valuable suggestions. References [1] A. Dabrowski, P. Podkoscielny, M. Hubicki, M. Barczak, Chemosphere 58 (2005) 1049. [2] Z. Aksu, J. Yener, Waste Manage. 21 (2001) 695. [3] F.J. Rivas, F.J. Beltrán, O. Gimeno, P. Alvarez, J. Hazard. Mater. 96 (2003) 259.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Removal of phenols from the aqueous solutions based on their electrochemical polymerization on the polyaniline electrode Ya Zhang, Qin Li, Hao Cui, Jianping Zhai ∗ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 22 Hankou Road, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2010 Received in revised form 30 June 2010 Accepted 1 July 2010 Available online 8 July 2010 Keywords: Removal of phenol and 3-nitrophenol Electrochemical polymerization Polyaniline as a working electrode SEM images IR spectra
a b s t r a c t Phenol and 3-nitrophenol in a aqueous solution of NaCl with pH 4.0 can be polymerized on the polyaniline electrode to form polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites, respectively, using potential cycling between −0.20 and 1.00 V (vs. SCE). Polyaniline played an important role in lowering passivation of the electrode, which made the consecutive electrochemical polymerization of phenol and 3-nitrophenol become possible. The growth of the polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) films in the electrolytic process was proved by the increasing area of the cyclic voltammograms as the electrolysis proceeded. The SEM images and IR spectra of polyaniline and phenolic polymers demonstrated the formation of the phenolic polymers on the polyaniline electrode. Therefore, the removal of both phenolic compounds is based on the formation of their polymers on the polyaniline film. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Phenols are toxic to humans and aquatic lives [1,2]. Phenolic compounds are released in the surface water by a number of industries, mainly by pharmaceutical plants, oil refineries, coke plants, pulp, resin and food-processing industries [3–5]. These industries are distributed in wide area, which result in serious environmental problems because of their toxicity, poor biodegradability and accumulation potential in plants and tissues, such as fish live in the lake and river contaminated with phenolic compounds have a queer taste and bad odour. Therefore the wastewater containing phenolic compounds must be treated before their discharge into water streams. Various techniques have been employed for the removal of phenolic compounds from aqueous media including adsorption [6–9], oxidation [10–12], membrane filtration [13], ion exchange [14], solvent extraction [15], photocatalytic degradation [16], and biological degradation [17]. Some of them have progressed quickly in the removal of phenolic compounds [18,19], while adsorption using activated carbons is successful to remove efficiently phenolic compounds from the contaminated water due to large surface area, high adsorption capacity, chemical stability and fast adsorption rate [20,21]. However, activated carbons are related to the high initial cost and a costly regeneration. Therefore, creating new
∗ Corresponding author. Tel.: +86 25 8359 2903; fax: +86 25 8359 2903. E-mail address: [email protected] (J. Zhai). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.002
technique and looking for new material for the treatment of phenols in the wastewater are still full of chances and challenges to researchers. Phenol and its derivatives in aqueous solutions with a large pH range and non-aqueous solutions can be polymerized by the electrochemical oxidation to form very tight polymer film on the metal electrodes [21–24], which are used for protecting metal corrosion [25,26]. The phenolic polymer is generally a very thin passive layer (5 nm–15 m) [25–27], which inhibits the film growth during the electrolytic process due to the film tightness and very low conductivity. In this case, the electrochemical polymerization of phenols cannot be used to remove phenols in the contaminated water. A strategy for overcoming the formation of the passive layer is to use a conducting polymer as a working electrode. The conducting polymers can be reduced and oxidized reversibly, accompanied with ion exchange between the conducting polymer and the solution, which therefore has been used to remove ClO4 − [28] and Cr(VI) [29] in the aqueous solutions. Among conducting polymers, polyaniline is one of the most promising conducting polymers due to its high conductivity, good redox reversibility and good stability. Furthermore, polyaniline can catalyze polymerization of aniline itself and its derivatives because it carries free radicals to initiate their oxidation [30–32]. Therefore, polyaniline is used here to study the electrochemical polymerization of phenols. The objective of the current study is to investigate the polymerization of phenols and their film growth on the polyaniline electrode and remove phenols from the aqueous solution.
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Fig. 1. Cyclic voltammograms of a glassy carbon electrode in a solution containing 0.10 M NaCl and (A) 5 mM phenol; (B) 5 mM 3-nitrophenol. Curves: (1) first cycle; (2) second cycle; (3) third cycle and (4) fourth cycle. pH 4.0, at a scan rate of 60 mV s−1 .
2. Experimental Chemicals were of analytical reagent grade obtained from Shanghai Chemical Company. Aniline was distilled under reduced pressure prior to use. Doubly distilled water was used to prepare all aqueous solutions. The pH values of the solutions were determined with a PXD-12 pH meter. An electrolytic cell for the synthesis of polyaniline consisted of a glassy carbon (GC) disk working electrode (3 mm diameter), a platinum foil counter electrode and a saturated calomel reference electrode (SCE). The GC disk electrode was polished with alumina slurry of 0.5 m on polishing cloth with water, and then was washed with water and sonicated in a doubly distilled water bath for 10 min before use. All electrochemical experiments were performed on a model CHI 407 electrochemical workstation. The polymerization of aniline was carried out in a solution containing 0.20 M aniline and 1.0 M HCl at 0.75 V. The amount of polyaniline film polymerized on a GC electrode was controlled by passing charge during the electrolytic process, which was 1.8 × 10−2 C. After synthesis, polyaniline film polymerized on a GC electrode was washed with 0.05 M HCl solution and then cycled in 0.20 M HCl solution for several cycles in a potential range between −0.20 and 0.80 V to remove further aniline remaining in the polyaniline film. The FTIR spectra of the polymer samples were measured on a pressed pellet with KBr using a Bruker IFS 66/s spectrometer. The morphology of the polymer samples was observed with a Hitachi S-4800 II scanning electron microscope (SEM).
3. Results and discussion
increasing number of cycles. This indicates that the electrochemical polymerization of phenol or 3-nitrophenol would not continue to take place on the bare GC electrode. Therefore, the electrochemical oxidation behavior of phenol and 3-nitrophenol shown in Fig. 1A and B is very similar to that of earlier studies [22,24], indicating that a thin phenolic layer with low conductivity was formed on a GC electrode. Even though the cyclic voltammograms in Fig. 1B are very similar in shape to those in Fig. 1A, however, the peak current density of curve 1 in Fig. 1B is a little smaller than that of curve 1 in Fig. 1A; and especially, the peak width of curve 1 in Fig. 1B is evidently smaller than that of curve 1 in Fig. 1A, indicating that the total charges under curve 1 in Fig. 1B are smaller than those of curve 1 in Fig. 1A. This difference is caused by different chemical properties between phenol and 3-nitrophenol. 3.2. Electrochemical polymerization characteristic of phenols on a polyaniline electrode Fig. 2 shows the cyclic voltammograms of the polyaniline electrode in 0.10 M NaCl solution with pH 4.0. As can be seen in Fig. 2, the area of the cyclic voltammograms decreases with an increasing number of cycles. This result is caused by a decrease in the electrochemical activity of polyaniline at pH 4.0. However, the polyaniline electrode approximately reached a stable state after the ninth cycle because the change in the area of cyclic voltammograms is a little from the ninth cycle to the 10th cycle. Therefore, before the electrolysis of phenols using a polyaniline electrode, the polyaniline
3.1. Electrochemical polymerization of phenols on a bare GC electrode A solution of 0.10 M NaCl with pH 4.0 was used in the following experiments. The reason for choosing NaCl as an electrolyte is to avoid the second pollutant discharge; and considering the fact that polyaniline lost its redox activity at pH >4.0, thus, the pH value of the NaCl solution was controlled at pH 4.0 in this work. Fig. 1A and B show the electrochemical oxidation of phenol and 3-nitrophenol on a bare GC electrode, respectively, in an electrolytic solution consisting of 0.10 M NaCl and 5 mM phenol or 5 mM 3nitrophenol with pH 4.0. An oxidation peak at 0.98 V occurs in Fig. 1A for the first cycle, which is caused by phenol oxidation; an oxidation peak at 0.96 V occurs in Fig. 1B, which is attributed to 3-nitrophenol oxidation. The feature that phenol oxidation and 3-nitrophenol oxidation have in common is that the currents of their oxidation peaks decrease dramatically from the first cycle to the second cycle, followed by the continuous decrease with an
Fig. 2. Cyclic voltammograms of polyaniline in 0.10 M NaCl solution with pH 4.0, (1) first cycle; (2) second cycle; (3) eighth cycle; (4) ninth cycle and (5) 10th cycle, at a scan rate of 60 mV s−1 .
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Fig. 3. Cyclic voltammograms for the electrochemical polymerization of phenol and 3-nitrophenol on a polyaniline electrode, the electrolytic solutions consisted of 0.10 M NaCl and (A) 5 mM phenol; (B) 5 mM 3-nitrophenol, curves: (1) first cycle; (2) second cycle; (3) third cycle; (4) 10th cycle and (5) 20th cycle, at a scan rate of 60 mV s−1 , pH 4.0.
electrode must be cycled for 10 cycles in 0.10 M NaCl solution with pH 4.0. Fig. 3A shows the cyclic voltammograms for the oxidation of phenol on a polyaniline electrode in a solution consisting of 0.10 M NaCl and 5 mM phenol with pH 4.0. There are two oxidation peaks on the first cycle (curve 1) to the third cycle (curve 3). Therefore, their I–E curve shapes are similar to those on curves 1 and 2 in Fig. 2 on the forward scan; however, their wave shapes on the reverse scan are different from those in Fig. 2. This difference indicates the formation of the phenol polymer on the polyaniline electrode during the oxidation process of phenol. Even though the current densities of the oxidation peaks at 0.40 and 0.60 V in Fig. 3A decreases from the first cycle to the second cycle, its decrement is much smaller than that of phenol oxidation on the bare GC electrode in compared with Fig. 1A. Furthermore, the area of the cyclic voltammograms in Fig. 3A decreases slowly with an increasing number of cycles from the second cycle to the twentieth cycle (curve 5), which is much different from that in Fig. 1A. This indicates that the electrolysis of phenol was able to continue on the polyaniline electrode. A decrease in the area of the cyclic voltammograms demonstrates the formation of phenolic polymer because the conductivity of polyphenol is low [25–27]. Fig. 3B shows the cyclic voltammograms for the oxidation of 3-nitrophenol on a polyaniline electrode. The cyclic voltammograms of the first and the second cycles in Fig. 3B are different in shape from those in Fig. 2, which is caused by the oxidation
of 3-nitrophenol on the polyaniline electrode. In comparison with Fig. 1B, it is clear that the decrement of the current density of the oxidation peak from the first cycle to the second cycle in Fig. 3B is also smaller than that in Fig. 1B, and the area of the cyclic voltammograms in Fig. 3B decreases slowly with an increasing number of cycles from the second cycle to twentieth cycle. These experimental phenomena indicate that the electrolysis of 3-nitrophenol was able to continue on the polyaniline electrode as well as the electrolysis of phenol on the polyaniline electrode. It is noteworthy that there are two oxidation peaks on curve 1 in Fig. 3A; however, only one oxidation peak appears on curve 1 in Fig. 3B; and especially, the current density of both oxidation peaks on curve 1 in Fig. 3A are much higher than that of the oxidation peak on curve 1 in Fig. 3B. This difference is also caused by the different chemical properties between phenol and 3-nitrophenol. As can be seen in Figs. 1A and 3A, the electrochemical oxidation activity of phenol on the polyaniline electrode is much larger than that on the bare GC electrode. This indicates that polyaniline can initiate the electrochemical oxidation activity of phenol because of the free radicals contained in polyaniline. Marking a comparison between Figs. 1B and 3B, polyaniline can also initiate the electrochemical oxidation activity of 3-nitrophenol, but this influence is less than the effect of polyaniline on phenol. The results from Figs. 1 and 3 demonstrate that phenol is more easily to be oxidized compared with 3-nitrophenol, because there is an electro-withdrawing group, –NO2 , in the aromatic ring of phenol, which results in a decrease in reactivity of 3-nitrophenol.
Fig. 4. Curve 1 is the cyclic voltammogram of the 10th cycle for polyaniline in 0.10 M NaCl solution with pH 4.0. Curves 2–4 are the cyclic voltammograms of (A) polyaniline/polyphenol and (B) polyaniline/poly(3-nitrophenol) in 0.10 M NaCl solution with pH 4.0. They were synthesized using potential cycling (shown in Fig. 3) under different cycles, curves: (2) 20; (3) 40; (4) 60. At a scan rate of 60 mV s−1 .
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Fig. 5. SEM images: (A) polyaniline film; (B) polyaniline/polyphenol film and (C) polyaniline/poly(3-nitrophenol) film.
It is well known that the oxidation processes of both phenol and 3-nitrophenol are accompanied with the release of protons that result in a decrease in the pH value of the solution. In this case, the redox activity of polyaniline would increase as the electrolysis proceeds. However, the results from Fig. 3 show a decrease in the redox activity of polyaniline. This is due to the formation of the phenolic polymers with low conductivities on the polyaniline electrode, which predominates over the effect of pH on the redox activity of polyaniline. 3.3. Evidence for the film growth for the electrolysis of phenols on a polyaniline electrode The results from cyclic voltammograms in Fig. 3 indicate that the electrochemical polymerization of phenol and 3-nitrophenol took place on the polyaniline electrode. However, we cannot directly observe film growth in Fig. 3 as the number of cycles increases, because the area of the cyclic voltammograms decreases as the electrolysis proceeded. To prove the growth of the polyaniline film in the electrolytic process of phenol or 3-nitrophenol, the experiment must be carried out by step-electrolysis procedure. Therefore, after 20 cycles shown in Fig. 3, the polyaniline/polyphenol or polyaniline/poly(3-nitrophenol) film was washed with 0.05 M HCl solution, then put in 0.20 M HCl solution to scan five cycles between −0.20 and 0.80 V to remove phenol or 3-nitrophenol in the polyaniline film and finally the experiment shown in Fig. 2 was repeated. After that, the polyaniline/polyphenol electrode or polyaniline/poly(3-nitrophenol) electrode was again put in the solution containing 5 mM phenol or 5 mM 3-nitrophenol, respectively, to do potential cycling for another 20 cycles
as shown in Fig. 3. The resulting polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) electrodes obtained from 20, 40 and 60 cycles in the solution containing phenol and 3-nitrophenol, respectively, were used in following experiments. Curve 1 in Fig. 4A shows the 10th cyclic voltammogram of the polyaniline electrode in 0.10 M NaCl solution of pH 4.0 as shown in Fig. 2. Curves 2–4 in Fig. 4A show the cyclic voltammograms of polyaniline/polyphenol in 0.10 M NaCl solution with pH 4.0, which were obtained from 20, 40 and 60 cycles using potential cycling shown in Fig. 3A, respectively. The area of curve 2 is larger than that of curve 1, indicating that phenol was polymerized on the polyaniline electrode. This is because the plot of I–E on the cyclic voltammogram represents the quantity of electricity. It is clear that the quantity of electricity of curve 2 is larger than that of curve 1, which is caused by only the formation of polyphenol on the original polyaniline electrode because the area of the cyclic voltammogram for the polyaniline electrode itself decreases slowly during the consecutive potential cycles in 0.10 M NaCl solution of pH 4.0. Fig. 4A shows that the oxidation peak potential shifts towards the positive potentials as the potential cycle for synthesis of polyphenol increases, which is mainly attributed to IR drop of the polyaniline/polyphenol film due to low conductivity of polyphenol. An increase in the area of cyclic voltammograms and a shift in the oxidation peak potential towards the positive potential direction are typical of the growth of the polymer film. Curve 1 in Fig. 4B is the 10th cyclic voltammogram of the polyaniline electrode in 0.10 M NaCl solution of pH 4.0 as shown in Fig. 2. Curves 2–4 in Fig. 4B show the cyclic voltammograms of polyaniline/poly(3-nitrophenol) in 0.10 M NaCl solution with pH 4.0, which were obtained from 20, 40 and 60 cycles using poten-
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tial cycling shown in Fig. 3B, respectively. The results in Fig. 4B are similar to those in Fig. 4A. A difference between Fig. 4A and B is that a shift in the oxidation peak potential in Fig. 4B is smaller than that in Fig. 4A, which is caused by different properties of two phenolic polymers. The results from Fig. 4 demonstrate that phenol and 3-nitrophenol in the aqueous solution can be removed by their electrochemical polymerization on the polyaniline electrode. Here, polyaniline played an important role in lowering passivation of the electrode caused by phenolic polymers. 3.4. Morphology of polymers Fig. 5A shows the SEM image of the polyaniline film that polymerized on a GC electrode surface. The image shows that the film consisted of interwoven nanofibers with an average diameter of about 55 nm with lengths varying from 240 to 310 nm. The formation of nanostructures is due to the fact that the electrochemical polymerization rate of aniline in 1.0 M HCl solution is fast enough at 0.75 V. The fast polymerization rate is favorable for the formation of a large amount of polyaniline nuclei, which leads to the slow growth of the polyaniline nuclei to form nanostructures [33]. Fig. 5B and C show the SEM images of the polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) films, which were prepared using potential cycling between −0.20 and 1.0 V for 40 cycles in the solutions containing 5 mM phenol and 5 mM 3-nitrophenol as described previously. Their SEM images are similar to that shown in Fig. 5A, but the nanostructure of the polyaniline/polyphenol film is a little more compact than that of the polyaniline film, which is mainly caused by the formation of many thinner nanofibers between the predominant nanofibers; in addition, the average diameter of the predominant nanofibers in Fig. 5B is a little large than that in Fig. 5A. The nanostructure of the polyaniline/poly(3-nitrophenol) film is also a little more compact than that of polyaniline film, which is mainly attributed to that the average diameter (about 70 nm) of the nanofibers in Fig. 5C is larger than that of the polyaniline nanofibers; in addition, a small amounts of the thinner nanofibers are also observed in Fig. 5C. The above results indicate that the phenolic polymers were formed on the polyaniline nanofibers and the neighboring polyaniline nanofibers, indicating that the presence of polyaniline nanofibers is advantageous to the formation of the phenolic polymers on polyaniline. This is because the catalytic activity of polyaniline is mainly attributed to the free radicals existing in polyaniline. The nanostructures of polyaniline provide a large specific surface area that increases the impact possibility between the free radicals in polyaniline and reactants, phenolic compounds. As a result the nanostructures of polyaniline play an important role in enhancing the electrochemical catalytic polymerization rates of phenolic monomers. 3.5. IR spectra of polymers Curve 1 in Fig. 6 shows the IR spectrum of polyaniline, in which a absorption peak at 3424 cm−1 is attributed to the stretching vibrations of N–H; this peak also occurs on the IR spectra of polyaniline/polyphenol (curve 2) and polyaniline/poly(3-nitrophenol) (curve 3) composites. The peak at 1635 cm−1 on curve 1 is assigned to the C C stretching vibrations of benzene rings. However, two strong peaks at 1569 and 1482 cm−1 occur on curves 2 and 3, which are assigned to the C C stretching vibrations of quinone ring and benzene ring, respectively. But, they were not observed on curve 1. A strong peak at 1386 cm−1 occurs on each curve in Fig. 6, which is attributed to the deformation vibrations of C–O–H because the deformation vibrations of C–O–H are in the range of 1390–1310 cm−1 [34]. A peak at 1237 cm−1 occurs on each curve in Fig. 6, which is attributed to C–N stretching vibrations because the stretching vibrations of C–N in aromatic amines are in the range of
Fig. 6. IR spectra: (1) polyaniline; polyaniline/poly(3-nitrophenol).
(2)
polyaniline/polyphenol
and
(3)
1280–1180 cm−1 [34]. A peak at 1292 cm−1 on curves 2 and 3, but is not detected on curve 1. This peak would be assigned to the C–O–C stretching modes because the stretching vibrations of C–O–C in ethers are in the range of 1300–1000 cm−1 [34]. This indicates that the polymerization of phenol or 3-nitrophenol was carried out via the formation of the ether bond between benzene rings. A peak at 1122 cm−1 occurs on each curve in Fig. 6, which is attributed to the C–H in-plane bending vibrations. However, this peak is very weak in the IR spectrum of polyaniline compared to the IR spectra of polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites. In summary, the IR spectra of polyaniline/polyphenol and polyaniline/poly(3-nitrophenol) composites are different from that of polyaniline, which is caused by the phenolic polymers polymerized on the polyaniline electrode. 4. Conclusions Phenol and 3-nitrophenol in a solution of NaCl with pH 4.0 can be polymerized on the polyaniline electrode using potential cycling. Polyaniline played an important role in lowering passivation of the electrode, which made the consecutive electrochemical polymerization of phenol and 3-nitrophenol become possible. Therefore, phenol and 3-nitrophenol in the aqueous solution can be removed via the formation of the phenolic polymers. On the basis of this method, aniline and its derivatives in the aqueous solution could be also removed because they can be polymerized on the polyaniline electrode. In addition, toxic ions in the aqueous solutions were removed using conducting polymers based on the doping and de-doping principle of the conducting polymers. Therefore, the conducting polymers are a promising material for the electrochemical treatment of contaminated water. Acknowledgments This work was supported by the Foundation of The State Key Laboratory of Pollution Control and Resource Reuse of China and the Scientific Research Foundation of Graduate School of Nanjing University. We are grateful to the anonymous reviewers for their valuable suggestions. References [1] A. Dabrowski, P. Podkoscielny, M. Hubicki, M. Barczak, Chemosphere 58 (2005) 1049. [2] Z. Aksu, J. Yener, Waste Manage. 21 (2001) 695. [3] F.J. Rivas, F.J. Beltrán, O. Gimeno, P. Alvarez, J. Hazard. Mater. 96 (2003) 259.
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