Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modified-PbO2 electrode

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JTICE-1013; No. of Pages 11 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

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Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modified-PbO2 electrode Xiaoyue Duan a,b,*, Yuanyuan Zhao a, Wei Liu a,b, Limin Chang a,*, Xin Li c a

Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Siping 136000, China School of Environmental Science and Engineering, Jilin Normal University, Siping 136000, China c State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 April 2014 Received in revised form 29 July 2014 Accepted 24 August 2014 Available online xxx

Modification of PbO2 electrode was carried out by carbon nanotube (CNT) and Ce co-deposition. The surface morphology and crystal structure of CNT and Ce modified PbO2 (CNT–Ce–PbO2) electrode was characterized by scanning electronic microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) specific surface area, respectively, and compared with pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes. The results indicate that the CNT and Ce were both doped into PbO2 film and the CNT–Ce–PbO2 electrode had smaller grain size and higher specific surface area than those of other three kinds of electrodes. The electrochemical properties of electrodes were tested by linear sweep voltammetry and cyclic voltammetry, and the results show that the CNT–Ce– PbO2 electrode had higher electro-catalytic activity than pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes. The accelerated lifetime tests demonstrate that the service life of CNT–Ce–PbO2 electrode was 4.76 times longer than that of pure PbO2 electrode. The CNT–Ce–PbO2 electrode was also used in electrochemical degradation of p-nitrophenol (p-NP) in aqueous solution. The degradation kinetics and mechanism of pNP were discussed. Besides, the variation of biodegradability of p-NP degradation solution was also investigated, and the results obtained revealed a feasible combination between electrochemical oxidation and conventional bio-treatment for the treatment of p-NP wastewater. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Electro-catalytic oxidation PbO2 electrode Carbon nanotube Ce p-Nitrophenol

1. Introduction p-Nitrophenol (p-NP), as one of the most common nitrophenolic compounds, has been heavily used in the production of explosives, herbicides, fungicides and dyes [1,2]. Due to its toxicity, skin irritation, and medicinal taste and odour, the discharge of p-NP wastewater from these industries may result in water pollutant, even cause serious effect to human health when these water is employed in domestic application [3]. So, p-NP is listed as one of the priority toxic pollutants by US Environmental Protection Agency [4]. However, due to its low biodegradability, it is difficult to be degraded by the conventional biological methods.

* Corresponding authors at: Jilin Normal University, School of Environment Science and Engineering, 1301 Haifeng Street, Siping, China. Tel.: +86 434 3290623; fax: +86 434 3292233. E-mail addresses: [email protected] (X. Duan), [email protected] (L. Chang).

Electrochemical oxidation has become a promising method for degrading refractory organic pollutants in water because of its strong oxidation performance, easy implementation, and environmental compatibility [5]. It is well known that the oxidation efficiency of organic pollutants during electrochemical oxidation process is strongly dependent on the material of anodes [6,7]. Typical anodes were graphite [7], noble metal (e.g., Pt) [8], dimensionally stable anodes (DSA) (e.g., PbO2, SnO2, RuO2, IrO2) [9– 12], and boron-doped diamond (BDD) [13] in literature, of which PbO2 electrode is particularly used because of its good electrical conductivity, favorable oxygen evolution overpotential, high chemical inertness and relatively low cost [14,15]. For all we know, the activity and stability of PbO2 electrode is largely affected by the modification of PbO2 film [16–19]. And some previous research has reported that the electro-catalytic oxidation activity and stability of PbO2 electrode can be significantly improved by the doping of foreign materials, such as fluoride [20], Fe3+ [21], Co2+ [22], Bi3+ [23], polytetrafluoroethylene [24], ZrO2 [25], chitosan [26], in the PbO2 films.

http://dx.doi.org/10.1016/j.jtice.2014.08.031 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031

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It is well known that rare earth oxides are powerful oxidants and widely used as catalysts for oxidation reaction. So, various rare earth oxides were used to modify PbO2 film to improve the electrochemical performance of electrode. Kong et al. reported that the doping of Er2O3, Gd2O3, La2O3 and Ce2O3 into the PbO2 film could enhance the degradation of 4-chlorophenol [19]. Ai et al. prepared Ce–PbO2 electrode and its catalytic activity was evidently greater than pure PbO2 electrode [27]. A cerium modified ternary SnO2 based oxides anode (Ce-Ru-SnO2) was prepared by Liu et al., and the Ce-Ru-SnO2 anode possessed smaller charge transfer resistance and longer service life than other modified SnO2 anodes in their study [28]. In addition, the recent discovery of carbon nanotube (CNT), has been attracted much attention in fields of electrochemical research because of its extraordinary electrochemical properties. Our previous studies have shown that the electro-catalytic activity and stability of PbO2 electrode could be improved by the modification of CNT [29,30]. However, to the best of our knowledge, there are no reports about the modification of PbO2 electrode by codoping of CNT and Ce in literature. Thus, in this study, we intend to improve the electro-catalytic activity and stability of PbO2 electrode by modification of CNT and cerium. The electro-catalytic activity and stability of the CNT and Ce modified PbO2 (CNT–Ce–PbO2) electrode were investigated and compared with those of pure PbO2, CNT modified PbO2 (CNT– PbO2), and Ce modified PbO2 (Ce–PbO2) electrodes. Then, the electrochemical degradation of p-NP was investigated using prepared CNT–Ce–PbO2 electrode as anode. The degradation kinetics and degradation pathway of p-NP on CNT–Ce–PbO2 electrode were analyzed. In addition, the variation of biodegradability of p-NP degradation solution was also investigated to evaluate the feasibility of combining electrochemical oxidation and biological technologies.

into the acid electrodeposition solution. Further details about the pretreatment and the preparation of SnO2–Sb2O3 oxide coating and a-PbO2 intermediate layer are provided in previous study [29]. 2.3. Electrode characterization Scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX) were carried out on a Hitachi S-570 model instrument. The Brunauer–Emmett–Teller (BET) specific surface area of electrodes was analyzed by N2 adsorption on a Micromeritics 3H-2000PS1 instrument. The transmission electron microscope (TEM) image was obtained from TECCNAI F20 model transmission electron microscope. X-ray diffraction (XRD) patterns of samples were obtained with an X-ray diffraction (Rigaku D-max/ 3C) using Cu Ka radiation (45 kV, 30 mA). Linear sweep voltammetry (LV) and cyclic voltammetry (CV) were executed in the PGSTAT302 electrochemical workstation. The LV and CV measurements were both performed with a conventional three-electrode system. The fabricated PbO2-based electrodes were used as working electrode whose apparent area was 1.0 cm2, a platinum sheet as auxiliary electrode, and a saturated calomel electrode as reference electrode. The stability tests (up to 20 h) were performed by the accelerated life test at 1 A cm2 in 2 M H2SO4 solution at 60 8C for PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 electrodes. These tests were conducted in a three-electrode system. The fabricated PbO2-based electrode was used as working electrode, a saturated Ag/AgCl electrode as reference electrode and a stainless steel sheet as counter electrode. The anode potential was monitored during tests, and the mass losses, surface morphologies and crystal structure of electrodes were checked after the stability tests. The Pb2+ concentrations of electrolytes after the accelerated life test were determined by the AA-680 atom spectrophotometer.

2. Experimental 2.4. Electro-catalytic oxidation 2.1. Materials The multi-wall CNT was purchased from Beijing Nachen Co. Ltd. (China) with an outer diameter of 10–20 nm, and length of 10– 30 mm. Ce(NO3)36H2O was purchased from Sinopharm Chemical Reagent Co. Ltd, China. All other chemicals were purchased from Huadong Medicine Co., Ltd., China. All chemicals were of analytical grade and were used without further purification. All solutions were prepared using deionized water. 2.2. Electrode preparation Ti satisfies the properties of good corrosion resistance, high mechanical strength, and excellent electrical conductivity that the substrate for the electrode should possess. In addition, Ti also has a thermal expansion rate close to the PbO2 coating, which can overcome the breaking-off of coating caused by temperature change. Thus, Ti plates (30 mm  50 mm  0.8 mm) were used as substrates. Before deposition, the Ti substrates underwent the pretreatment of sandblasting and ultrasonic cleaning. After pretreatment, a SnO2-Sb2O3 oxide coating of 2.5 mm thickness was prepared by thermal deposited on the Ti substrates. Then, the pretreated substrates were electrodeposited with an a-PbO2 intermediate layer of 19.8 mm thickness. Finally, pure or modified b-PbO2 film of 28.7 mm thickness was electrodeposited on above substrates in acid solution at 65 8C, applying a current density of 15 mA cm2. The acid solution composition consisted of 0.5 M Pb(NO3)2 plus 0.05 M NaF in 1 M HNO3. When the Ce-, CNT- or CNT–Ce-modified PbO2 electrodes were prepared, 5 g L1 CNT or/and 4 mM Ce(NO3)2, respectively, were added

The electro-catalytic oxidation experiments were carried out by batch processes and the apparatus was mainly consisted of a direct current power supply, a heat-gathering style magnetic stirrer, and a glass reactor. All the electro-catalytic oxidation tests were carried out in a three-electrode system. The anode (the fabricated PbO2-based electrode) and the cathode (a stainless steel sheet) were positioned vertically and parallel to each other with a distance of 1 cm. p-NP with a initial concentration of 50 mg L1 was selected as model pollutant, and the volume of solution was 200 mL. 0.05 M Na2SO4 was used as a supporting electrolyte. The reaction temperature was kept at 303 K during all the experimental runs. 2.5. Measurements The chemical oxygen demand (COD) and five days’ biochemical oxygen demand (BOD5) values of samples were determined by the dichromate reflux method and dilution inocula method, respectively. The determination of p-NP concentration and the identification of intermediates during the oxidation degradation of p-NP were both carried out on high-performance liquid chromatography (HPLC, Shimadzu, Japan). The mobile phase was a mixed solution containing 55% (volume fraction) methanol, and 45% water. The separation was performed using an Agilent TCC18 column (250 mm  4.6 mm, 5 mm) at the column temperature of 30 8C and at a flow rate of 1.0 mL min1. An UV detector was used with the wavelength set at 265 nm. Intermediates were characterized by comparing the retention time of the standard compounds.

Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031

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JTICE-1013; No. of Pages 11 X. Duan et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

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Fig. 1. SEM images and EDX analysis of PbO2 electrodes: (a) PbO2, (b) Ce–PbO2, (c) CNT–PbO2, (d) CNT–Ce–PbO2, (e) partial of c, (f) partial of d, (g) EDS of the zone in (a), (h) EDS of the zone in (b), (i) EDS of the zone in (c), (j) EDS of the zone in (d).

Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031

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3. Results and discussion 3.1. Surface morphology and crystal structure

Fig. 2. TEM image of CNT–Ce–PbO2 electrode.

40000

β-PbO2(301)

30000

Intensity / a.u.

The SEM images of pure PbO2 electrode and three modified PbO2 electrodes (Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2) are shown in Fig. 1, as well as the EDX elemental analysis of these electrodes. From the SEM images, it can be seen that the modification of CNT and Ce did not change the typical pyramid structure of PbO2 electrode. However, compared with pure PbO2 electrode, Ce–PbO2 and CNT–Ce–PbO2 electrodes were more compact and more uniform and had smaller particles, while CNT–PbO2 electrode had the similar particle size. The mean particle size of Ce–PbO2 electrode is about one half of that of pure PbO2 electrode, while that of CNT–Ce–PbO2 electrode was much smaller than those of Ce–PbO2 and CNT–Ce–PbO2 electrodes. Thus, we can conclude that the synergistic effect of Ce and CNT can significantly decrease the particle size of PbO2 electrode. This synergistic effect may be attributed to the competing adsorption of Pb2+ and Ce3+ ions onto CNT in electrodeposition solution. As we know, O–Pb bond was formed between Pb2+ ions and oxygen functional groups of CNT, and directly occupied the active adsorption sites of CNT. In addition, dense hydration nucleation was also formed around Pb2+ ions because of strong hydrate of Pb2+ ions, then another part of active adsorption sites of CNT were also covered. Therefore, the Pb2+ ions suppressed the adsorption of Ce3+ ions onto CNT, and molar ratio of Ce3+ to Pb2+ in solution was raised. The higher molar ratio of Ce3+ to Pb2+ in solution resulted in the forming of more CeO2 crystal cell, and then the grain growth of PbO2 crystals was inhibited [31,32]. It is well known that smaller grain size is favorable for the formation of large specific area. Therefore, it is can be deduced that the CNT–Ce–PbO2 electrode has a larger specific area than other three electrodes. To verify this result, the BET areas of different films were tested and they were 0.599, 0.625, 0.762, and 0.826 m2 g1 for pure PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 films, respectively. It’s worth mentioning that, from the SEM image of CNT–Ce– PbO2 electrode magnified to 10,000 times (Fig. 1f), some aggregated CNTs can be found on the surface of electrode. Besides, EDS spectra in Fig. 1g–j show that, there are Pb, O, Ce and C elements in CNT–Ce–PbO2 electrode, where only Pb, O, Ce and Pb, O, C in Ce–PbO2 and CNT–PbO2 electrodes, respectively, (Au peaks come from the gold coating for SEM). Therefore, Ce and CNT were both introduced into the CNT–Ce–PbO2 electrode. However, the CNTs were agglomerated in CNT–Ce–PbO2 film. This result was also proved by the TEM image of CNT–Ce–PbO2 electrode shown in Fig. 2. The TEM sample was prepared by grinding and sonicating of coating of CNT–Ce–PbO2 electrode in absolute ethanol for 1 h. A drop of the sonicated dispersion was put onto a carbon coated copper grid and allowed to dry for few minutes. As shown in Fig. 2, because the film is thick, the CNT is not clearly visible. However, the image still shows rope like shapes of CNT with the outer diameter of around 15  5 nm in zone marked in red line, and they are tangled together under electrostatic force. The XRD patterns of PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce– PbO2 films in Fig. 3 show that the crystal structure of PbO2 is pure b-PbO2 in all the electrodes, which is in good agreement with standard data of the JCPDS card (number: 760564). The diffraction peaks observed at 2u = 25.3698, 31.9818, 36.1838, 49.0538, and 62.4528 are assigned to the (1 1 0), (1 0 1), (2 0 0), (2 1 1), and (3 0 1) planes of b-PbO2, and the main crystal plane of b-PbO2 is (3 0 1) plane. In addition, as shown in Fig. 3, no diffraction peak related with CNT or cerium oxide was detected by XRD analysis, which should be due to that the CNT and cerium oxide amounts in film were below X-ray detection limit under our measurement condition. In order to identify the state of CNT and Ce doped in

CNT-Ce-PbO2 β-PbO2(101)

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Ce-PbO2

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0 10

20

30

40

50

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2θ / degree Fig. 3. XRD spectra of different PbO2 electrodes.

PbO2 film, the CNT–Ce–PbO2 film was prepared in electrodeposition solution containing a higher concentration CNT (20 mg L1) and Ce (40 mM) and tested by XRD. The result is presented in the Supporting Information (Fig. S1). Compared to the diffractogram of pure PbO2 electrode, it was found that the intensities of PbO2 diffraction peaks were weakened, and some new diffraction peaks appeared at 28.68, 33.18, and 59.38 in the pattern of CNT–Ce–PbO2 electrode, which are assigned to the (1 1 1), (2 0 0), and (3 2 2) planes of CeO2 according to the JCPDS card (number: 81-0792). However, the diffraction peak of CNT still wasn’t observed. Thus, we can conclude that Ce was easier doped into PbO2 film than CNT, and CeO2 crystal was formed. This result may be attributed to the similar ionic radius of Ce4+ (97 pm) and Pb4+ (84 pm), the Pb4+ was easily displaced by Ce4+, while only small amount of CNT was randomly scattered between PbO2 crystals during the forming of crystals, which was clearly observed in TEM image (Fig. 2). 3.2. Electrochemical measurements The oxygen evolution overpotential (OEP) of PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 electrodes was measured by means

Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031

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In order to ascertain the electron number (n) involved in p-NP oxidation, the n value was determined by the cyclic voltammogram at CNT–Ce–PbO2 electrode using the equation of an = 47.7/ (Ep  Ep/2) [33]. In the cyclic voltammogram at CNT–Ce–PbO2 electrode, Ep = 1011 mV, Ep/2 = 952 mV. Therefore, an was calculated to be 0.81 a (charge transfer coefficient) is assumed to be 0.5 in totally irreversible electrode process. So, n was calculated to be 2. 3.3. Electrode stability Service life is an important factor that limits practical application of an electrode [34].Thus, the accelerated life tests of different PbO2 electrodes were carried out in 2 M H2SO4 solution with an anodic current density of 1 A cm2 for 20 h. Fig. 6 shows the anode potential on different anodes during the electrolysis and the mass losses of electrodes after the accelerated life tests. In accordance with Fig. 6, CNT–Ce–PbO2 electrode exhibited better electrochemical stability than pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes, indicated by the more stable anode potential and less mass loss of CNT–Ce–PbO2 electrode. The mass loss of pure PbO2 electrode was 11.9 mg L1, which was significantly higher than 2.9, 5.0, and 2.5 mg L1 of Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 electrodes, respectively. It is well known that the main contributors to the mass losses of electrodes include detachment and dissolution of PbO2 films, and the mass losses are proportional to the service lifetimes of electrodes. Thus, it can be concluded that

0.0012

a PbO2

0.0008

Ce-PbO2 CNT-PbO2

I/A

of LV in 0.5 M Na2SO4 solution at a scan rate of 50 mV s1 and the potential corresponding to the inflexion point of polarization curve was defined as the OEP of electrode. Fig. 4 displays typical linear sweep voltammograms for all four kinds of electrodes. According to the polarization curves, the OEP for all electrodes was about 1.55 V (vs. SCE). So the modification of Ce and CNT could not increase the OEP of PbO2 electrode. However, it can be seen from Fig. 4 that the oxygen evolution current of CNT–Ce–PbO2 electrode was significantly higher than those of other three electrodes. The higher oxygen evolution current for CNT–Ce–PbO2 electrode verified that the electrochemical active surface area of CNT–Ce– PbO2 electrode is larger than those of other three PbO2-based electrodes, which is beneficial to the improvement of electrocatalytic performance. The electrochemical oxidation performance of different PbO2 electrodes was measured by cyclic voltammetry in 0.5 M Na2SO4 solution containing 200 mg L1 p-NP. The cyclic voltammetry was also performed in blank Na2SO4 solution for comparison. The results were shown in Fig. 5. No redox signals were observed at all electrodes in Na2SO4 solution without p-NP (Fig. 5a), suggesting that the PbO2 is an electrochemically inactive material in test situation. After p-NP was added into Na2SO4 solution (Fig. 5b), an obvious oxidation peak was obtained in the anodic branches at all electrodes. It is no doubt that the oxidation peak should be attributed to the oxidation of p-NP. However, no corresponding reduction peak was observed in the following reverse scan from 1.5 V to 0 V, implying that the oxidation of p-NP is a totally irreversible electrode process under the experimental conditions. It also can be seen from Fig. 5b that the potential of the oxidation peak of p-NP is 1.011 V vs. SCE at the CNT–Ce–PbO2 electrode while those potentials are 1.220, 1.079, and 1.069 V (vs. SCE) at pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes, respectively. In addition, the anodic peak current is 0.963 mA at the CNT–Ce– PbO2 electrode while at pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes those currents are 0.710, 0.816, and 0.708 mA, respectively. The oxidation peak potential obtained at CNT–Ce– PbO2 electrode was lower and the oxidation peak current was higher than those of other three electrodes, indicating CNT–Ce– PbO2 electrode had higher electrocatalytic activity for p-NP oxidation. The improvement of electrocatalytic activity, besides being caused by the increase of surface area of CNT–Ce–PbO2, may be attributed to the change of energy band structure of PbO2. The doping of CeO2 and CNT increased the donor level of PbO2, thereby the electron more easily jumped from donor level to conduction band. Thus, the conductivity of PbO2 was increased by the doping of CeO2 and CNT.

5

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E vs SCE / V

E vs SCE / V Fig. 4. LV curves of different electrodes measured in 0.5 M Na2SO4 at 25 8C, scan rate: 10 mV s1.

Fig. 5. Cyclic voltammograms of different PbO2 electrodes measured in (a) 0.5 mol L1 Na2SO4 solution and (b) 0.5 mol L1 Na2SO4 solution containing 200 mg L1 p-NP at 25 8C, pH = 6.5, scan rate: 50 mV s1.

Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031

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E vs Ag/AgCl / V

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α(020) α(111) α(002) α(200) TiO2(110)

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Fig. 6. Electrode stability tests: electrode potential vs. time for the electrolysis using PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 electrodes. Inset shows the mass losses of electrodes after accelerated life tests for 20 h.

Fig. 8. XRD spectrum of different PbO2 electrodes after accelerated life tests.

the service lifetime of CNT–Ce–PbO2 electrode was extended by 4.76 times greater than that of PbO2 electrode by modification of CNT and Ce. Actually, in the experiments, we found that film detachment occurred on the pure PbO2 electrode and not on other three modified PbO2 electrodes. To obtain additional information, the surface morphologies and crystal structure of PbO2, Ce–PbO2, CNT–PbO2 and CNT–Ce–PbO2 electrodes after accelerated life tests were also studied by SEM and XRD, and the SEM images and XRD spectrum are shown in Figs. 7 and 8, respectively. It can be seen from the SEM images that, after 20 h accelerated life tests, the rough and porous surface morphologies with locally separated zones on all electrodes appeared, the edges and apexes

of lead dioxide particles on the surface of as-prepared electrodes disappeared, many nano-sized and amorphous lead dioxide particles emerged on the surface of electrodes, and the average diameter of lead dioxide particles decreased and the size of gaps between the particles increased obviously. The cause of above phenomena has been given in our previous work [30]. Besides, there is remarkable difference in the surface morphology of different electrodes. The cauliflower-like and porous structure appeared on the surfaces of pure PbO2 electrode, covered with very fine granules (Fig. 7a). In contrast, the irregular blocks appeared on the surface of Ce–PbO2 electrode (Fig. 7b), but there are many holes between particles, while there are larger particles and less holes on CNT–PbO2 electrode (Fig. 7c). For the case shown in Fig. 7d, there

Fig. 7. SEM images of different PbO2 electrodes after accelerated life tests in 2 mol L1 H2SO4 solution: (a) PbO2; (b) Ce–PbO2; (c) CNT–PbO2; (d) CNT–Ce–PbO2.

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are significantly larger blocks under the fine particles formed on the surface. Thus, it can be concluded that the modification of CNT and Ce could effectively reduce the dissolution of PbO2 in H2SO4 solution at a high potential, which was also verified by the Pb2+ concentrations in H2SO4 solution tested by atomic absorption after accelerated life tests. The Pb2+ concentration of 4.9 mg L1 of CNT– Ce–PbO2 electrode was significantly lower than those of PbO2, Ce–PbO2 and CNT–PbO2 electrodes, which were 12.1, 5.9, and 6.8 mg L1, respectively. The phase composition of different PbO2 electrodes after accelerated life tests was studied by XRD, and the results are presented in Fig. 8. Compared with the diffractogram of fresh PbO2 electrodes (Fig. 3), it is clearly found that, after the accelerated life tests, the main diffractions of all electrodes was almost unchanged and was principally b-PbO2, while there were some new diffraction peaks appeared at all electrodes. According to the standard data of the JCPDS card (numbers: 82–1123, 45–1416 and 72–1147), the diffraction peaks observed at about 2u = 20.3888, and 45.9198 are assigned to the (1 1 0) and (1 3 1) planes of TiO2, the peaks at 2u = 23.2918, 28.6158, 29.9098, 32.8778, 36.1608, 45.0678 and 49.4968 are the (1 1 0), (1 1 1), (0 2 0), (0 0 2), (2 0 0), (0 2 2) and (1 3 0) planes of a-PbO2, and the peak at 2u = 26.6028 is the (1 1 0) planes of SnO2, indicating the oxide dissolution and detachment of films occurred during the accelerated life tests, and part of a-PbO2 intermediate layer, SnO2–Sb2O3 interlayer, and substrate oxidation was exposed. In addition, comparing the new peaks at all electrodes, it was can be seen that the intensity of pure PbO2 electrode was significantly higher than those of other three modified electrodes, indicating the serious oxide dissolution and detachment of active layer and substrate oxidation on pure PbO2 electrode. 3.4. Electrochemical degradation of p-NP The electrochemical degradation of p-NP was also compared at four kinds of electrodes, and the variation of p-NP removal ratio is presented in Fig. 9a. There were remarkable differences in p-NP degradation among different electrodes. p-NP was almost all converted on CNT–Ce–PbO2 electrode, with the highest removal of 98.22%, after the electrolysis time of 120 min. However, at the same time, when the pure PbO2, Ce–PbO2 or CNT–PbO2 electrode was used as anode, 88.90%, 96.23% or 95.09% p-NP removal can be achieved, respectively. The degradation processes were fitted by

a

pseudo-first-order model for all electrodes, and the fitting results are shown in Fig. 9b. Good linear plots for all electrodes show that the degradation of p-NP on these electrodes fitted to the pseudofirst-order reaction. The maximum rate of p-NP removal was observed on CNT–Ce–PbO2 electrode, the degradation rate with Ce–PbO2 and CNT–PbO2 electrodes were relatively slow, and the slowest on the PbO2 electrode. The rate constant of p-NP on the CNT–Ce–PbO2 electrode was 76% higher than that of pure PbO2 electrode. Obviously, above results indicate that the electrochemical oxidation ability of CNT–Ce–PbO2 electrode was much higher than those of pure PbO2, Ce–PbO2, and CNT–PbO2 electrodes. It may be attributed to the synergistic effect of CNT and Ce. At first, the doping of CNT is helpful for the adsorption of p-NP on anode and facilitates the mass transport of the reactants. Then, CeO2, as one kind of powerful oxidants, with high thermal stability, electrical conductivity and diffusivity [31], can catalytically decompose p-NP readily. The current density of the electrochemical system has a major role in anodic oxidation process, so the effect of current density on the electrochemical degradation of p-NP was studied under the different current densities from 10 to 50 mA cm2 using CNT–Ce– PbO2 electrode as anode in this study. Fig. 10a shows the influence of the current density on p-NP removal during the electrochemical oxidation. The results presented in Fig. 10a show that the p-NP removal percentages at different current densities all increased with prolonging the electrolysis time. The degradation rate of p-NP at early periods of electrolysis was higher that of later periods. The p-NP removal percentages at current densities 10, 20, 30, 40 and 50 mA cm2 at 60 min were 61.70%, 67.25%, 78.91%, 86.13%, and 89.48%, while at 120 min 83.47%, 89.26%, 97.54%, 98.91%, and 99.25%, respectively, demonstrating a higher current density caused a higher p-NP removal and there were minor differences in p-NP removal at the relatively higher current densities at the end of electrolysis. The dependence of ln(C0/C) on the electrolysis time at different current densities is shown in Fig. 10b. The linear relationship obtained by plotting ln(C0/C) versus electrolysis time t indicates that the initial degradation of p-NP on CNT–Ce–PbO2 anode followed the first-order kinetics under different current densities. Here, C0 is the initial concentration of p-NP, and C is the concentration of p-NP at t. The kinetics rate constants (k) were determined from the slopes of the straight lines of each current density. The k for p-NP degradation at current densities 10, 20, 30,

b

100

7

5 2

PbO2: y=-0.0689+0.0199x R =0.9892 2

Ce-PbO2: y=-0.0345+0.0281x R =0.9889 2

CNT-PbO2: y=-0.0489+0.0262x R =0.9949

80

2

CNT-Ce-PbO2: y=0.0469+0.0351x R =0.9918

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PbO2 Ce-PbO2

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Fig. 9. Variation of p-NP removal percentage with time during electrochemical oxidation (a) and pseudo first-order kinetics fitting curves (b) on different electrodes. Conditions: current density = 50 mA cm2; T = 303 K; [p-NP] = 50 mg L1; [Na2SO4] = 0.05 mol L1.

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a

b 6

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10 mA/cm : y=0.0238+0.0153x R =0.9979 2 2 20 mA/cm : y=-0.0073+0.0188x R =0.9996 2 2 30 mA/cm : y=-0.1618+0.0305x R =0.9921 2 2 40 mA/cm : y=-0.1745+0.0377x R =0.9950 2 2 50 mA/cm : y=0.1307+0.0404x R =0.9945

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3 2 1

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0

20

40

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Fig. 10. Variation of p-NP removal percentage with time during electrochemical oxidation (a) and pseudo first-order kinetics fitting curves (b) under different current densities. Conditions: T = 303 K; [p-NP] = 50 mg L1; [Na2SO4] = 0.05 mol L1.

40 and 50 mA cm2 was calculated to be 0.0153, 0.0188, 0.0305, 0.0377 and 0.0404 min1, respectively, which indicates that the higher current density results in higher reaction rate. This is mainly ascribed to the fact that more charge is passed into the system at a higher current density, which leads to the production of more OH radicals [35]. However, higher current density also causes higher energy consumption. So, the applied current density in electrolysis is not as high as possible, an appropriate current density should be chosen by considering both degradation rate and energy consumption. 3.5. Identification of intermediates and degradation pathway of p-NP UV–vis spectroscopy was firstly used to monitor the changes of intermediates during p-NP degradation process on CNT–Ce–PbO2 electrode. Fig. 11 shows the UV–vis absorbency spectrograms of p-NP in the wavelength range 200–500 nm at some electrolysis time. It can be seen that the UV–vis spectra varied very much after different periods of electrochemical oxidation. There are two defined absorption bands around 225 and 318 nm before the treatment and they can be attributed to the p ! p* transition in aromatic rings. The peak at 225 nm is weak and the peak at 318 nm is the maximum absorption peak. Two absorbance bands

3.5 3.0

0 min 5 min 10 min 20 min 30 min 40 min 60 min 90 min 120 min

Absorbance

2.5 2.0 1.5 1.0 0.5 0.0 200

250

300

350

400

450

500

Wavelength/nm Fig. 11. UV–vis absorption spectra of p-NP degradation solution at different electrolysis time.

continuously decreased with prolonging electrolysis time and disappeared after about 2 h of treatment, showing complete elimination of p-NP from the solution. In addition, a new absorption band around 245 nm appeared in the first 5 min, indicating that the benzene ring was oxidized to the intermediate of benzoquinone during the degradation of p-NP. The new peak rose in the initial period, after reaching its maximum intensity, it began to decrease and finally disappeared after 2 h, demonstrating that benzoquinone could also be degraded in the electrolysis process. This also can be confirmed by the color change of degradation solution. As the reaction proceeded, the solution changed from pale yellow to brown and then gradually disappeared. The brown compound should be the intermediate of benzoquinone. Total disappearance of three bands in electrochemical oxidation suggests that no more aromatic intermediates existed in the water, and the total degradation of p-NP and its aromatic intermediates was achieved by the electrochemical oxidation on CNT–Ce–PbO2 electrode. Further confirmation of intermediates during p-NP oxidation on CNT–Ce–PbO2 electrode was performed by HPLC analysis. The results of HPLC are presented in Fig. 12. As shown in Fig. 12a, identification of p-NP was shown by the chromatogram with the peak at retention time (tR) = 8.285 min and some new chromatogram peaks appearance with prolonging electrolysis time, indicating that the p-NP was degraded into other intermediates during the electrolysis process. The identification of some intermediates was obtained by comparison of retention time with those of the standard compounds. It can be seen that some aromatic compounds of p-aminophenol, hydroquinone, catechol and 4-benzoquinone (detected at 2.448, 2.899, 3.481, and 3.729 min, respectively, of retention time in our analysis conditions) and some organic acids of oxalic acid and maleic acid (detected at tR = 2.239 and 2.692 min, respectively) were formed during the whole degradation process. In addition, the intermediates detected at 3.7299 and 5.867 min were not detected because of lacking of standard compounds. In order to clarify the degradation pathway of p-NP, the concentration of various intermediates was estimated by areas of peaks as shown in Fig. 13. All the concentrations of intermediates increased first, and then decreased. After 180 min, only a small amount of oxalic acid left in degradation solution. Based on the above identification and concentration analysis of intermediates, the possible degradation pathway of p-NP was proposed and is shown in Fig. 14. It is well known that the phenolic –OH group on p-NP is electron-donating for the electrophilic

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Fig. 12. (a) HPLC chromatograms of p-NP degradation solutions at different time, (b) partial enlarged HPLC chromatograms of p-NP degradation solutions at 30, 60, 90 and 120 min.

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p-NP

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principal intermediates during the initial 60 min indicates that the–NO2 group of a small amount of p-NP was reduced into–NH2 on the cathode, and p-aminophenol was formed, which was also converted into hydroquinone. Subsequently, hydroquinone and catechol were oxidized into benzoquinone. During the further oxidation, the aromatic ring of benzoquinone was opened and oxalic and maleic acids were predominantly formed. Finally, these acids were oxidized into carbon dioxide and water. This is based on the chromatogram at 120 and 180 min which shows the decreasing of intensity in tR = 2.239 min without other peaks existence. 3.6. Variation of biodegradability of p-NP degradation solution

0 0

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180

Time/min Fig. 13. Changes of intermediates concentration of p-NP degradation.

aromatic substitution, while the –NO2 group is electron-withdrawing. The electron-donating substituent increases electron density at the ortho and para positions while the electronwithdrawing substituent is strongly deactivating and meta directing [36–39]. Thus, we proposed that the main initial reaction is that hydroxyl radicals attacked the ortho and para position of pNP in the first step of oxidation, and the hydroquinone and catechol were formed due to the–NO2 group was removed from the aromatic ring and the–OH group added in these position. In this denitration process, 4-nitrocatechol and 1,2,4-trihydroxybenzene might exist. Therefore, we proposed that the undetected intermediate (tR = 3.7299 and 5.867 min) might be 4-nitrocatechol, 1,2,4trihydroxybenzene or other compounds produced in degradation process. Perhaps these intermediates might all exist, but some of them was not be detected during HPLC test because of their very low amounts. In addition, the presence of p-aminophenol as one of

Above results of HPLC shows that p-NP can be completely mineralized by electrochemical oxidation technology. However, compared with biological method, electrochemical oxidation technology has a problem of high power cost, which leads to a high treatment cost. Thus, we consider combining these two technologies, that is, the electrochemical oxidation will be used as a pretreatment method to improve the biodegradability of the wastewater, and then the wastewater will be further degraded by the biological method to decrease the treatment cost. Thus, the biodegradability of p-NP wastewater was evaluated by the biodegradability index (BOD5/COD ratio) of p-NP degradation solution. The BOD5/COD ratio is commonly used as an indication of the biodegradability of organic contamination in wastewater. When the ratio is more than 0.3, the wastewater has a better biodegradability. Whereas the ratio is less than 0.3, the wastewater is difficult to be biodegraded [40,41]. The BOD5 and COD of p-NP degradation solution for different degradation time were tested and the COD removal percentage and BOD5/COD ratios are plotted in Fig. 15. It is observed that the initial BOD5/COD ratio was 0.069, meaning that the p-NP is not readily biodegradable. With the increase of the experiment duration, the COD removal percentages kept on rising, while the BOD5/COD ratio increased till 120 min, and then began to

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OH

OH

OH

OH

OH

O OH

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+

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C O

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maleic acid

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NH2

p-aminophenol

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oxalic acid

CO2 + H 2O

Fig. 14. Proposed pathways of p-NP degradation on CNT–Ce–PbO2 electrode.

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Fig. 15. Variation of COD removal efficiency and BOD5/COD value with electrolysis time.

decrease, which shows that the organic pollutants in solution was gradually mineralized, and the biodegradability of p-NP solution increased first, and then decreased. After 90 min of electrolysis, the COD removal and BOD5/COD ratio were 38.41% and 0.312, respectively, indicating that the obtained electrolyzed solution became biodegradable at this time since BOD5/COD ratio was greater than 0.3. When the electrolysis time was 120 min, the COD removal percentage was only 48.67%, but the BOD5/COD ratio reached the highest value of 0.484, representing an easily biodegradable solution. Subsequently, the COD removal continued to rise, but BOD5/COD ratio tended to decrease. The above biodegradability analysis highlights a feasible combination between electrochemical oxidation for 120 min and a subsequent bio-treatment for the remediation of p-NP wastewater under study.

4. Conclusions In this paper, the CNT–Ce–PbO2 electrode was successfully prepared on a titanium substrate by anodic co-deposition method. The morphology and component analysis of PbO2, Ce–PbO2, CNT– PbO2 and CNT–Ce–PbO2 electrodes showed that the CNT and Ce could be introduced into the PbO2 film. The crystal structure on the top layers of four PbO2-based electrodes was all pure b-PbO2. Due to the higher active surface area and synergistic effect of CNT and Ce doped in electrode, the CNT–Ce–PbO2 electrode exhibited higher electro-catalytic oxidation performance than PbO2, Ce– PbO2, and CNT–PbO2 electrodes in cyclic voltammetry and electrochemical degradation of p-NP tests. The results of accelerated life tests showed that the service life of CNT–Ce–PbO2 electrode was longer than those of other three kinds of electrodes

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under the same conditions, which was 4.76 times that of pure PbO2 electrode. The degradation kinetics and possible degradation pathway of p-NP were also studied in this study. The results revealed that the oxidation of p-NP can generally be described by simple pseudo first-order kinetics, the intermediates of aromatic compounds of p-aminophenol, hydroquinone, catechol and 4-benzoquinone and organic acids of oxalic and maleic acids were formed during the electrochemical degradation process of pNP. In addition, the variation of biodegradability of p-NP degradation solution demonstrated that, after 120 min electrolysis, the BOD5/COD ratio of p-NP solution increased to 0.484 from initial value of 0.069, which highlights a feasible combination between electrochemical oxidation for 120 min and a conventional bio-treatment for the treatment of p-NP wastewater.

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Please cite this article in press as: Duan X, et al. Electrochemical degradation of p-nitrophenol on carbon nanotube and Ce-modifiedPbO2 electrode. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.031