J. Phys. Chem. C 2008, 112, 8957–8962
8957
Electrocatalytic Behavior of the Bare and the Anthraquinonedisulfonate/Polypyrrole Composite Film Modified Graphite Cathodes in the Electro-Fenton System Guoquan Zhang, Fenglin Yang,* Mingming Gao, and Lifen Liu Key Laboratory of Industrial Ecology and EnVironmental Engineering, MOE, School of EnVironmental and Biological Science and Technology, Dalian UniVersity of Technology, Dalian 116024, People’s Republic of China ReceiVed: January 25, 2008; ReVised Manuscript ReceiVed: April 3, 2008
Conducting polypyrrole (PPy) films with anthraquinonedisulfonate (AQDS) incorporated as dopants were prepared by electropolymerization of the pyrrole monomer in the presence of anthraquinone-2,6-disulfonic acid, disodium salt on a graphite electrode from aqueous solution. Cyclic voltammetry (CV), scanning electron microscope (SEM), and Fourier transfer infrared spectroscopy (FTIR) technologies were used to characterize the resulting AQDS/PPy composite film. The electrocatalytic activities of the bare graphite and the AQDS/ PPy/graphite cathodes toward oxygen reduction and Fe2+ regeneration were studied by using cyclic voltammetry and cathodic polarization technologies. In addition, the electron-Fenton degradation of amaranth azo dye was also studied with the potentiostatic electrolysis mode, using the bare graphite and the AQDS/PPy/graphite as cathode and Fe3+ as catalyst. The results show that (i) H2O2 generation and Fe2+ regeneration mainly depend on the cathode materials utilized, (ii) solution pH, cathodic potential, and oxygen flow rate influence H2O2 accumulation and current efficiency greatly, while the effect of AQDS doping concentration is insignificant, and (iii) Fe3+ concentration influences the electro-Fenton oxidation ability and efficiency; the main oxidizing species is hydroxyl radical (•OH) formed in the reaction solution from Fenton’s reagent electrogenerated concurrently at the cathode. 1. Introduction In recent years, the advanced oxidation process (AOP, which is based on the oxidative action of the hydroxyl radical (•OH)) has been widely used to treat organic pollutants, because the classical physicochemical processes such as precipitation, coagulation, filtration, adsorption, and biological oxidation were not always sufficient to completely remove all the pollutants. Among candidate AOPs, electrochemical technologies have been extensively studied for wastewater treatment due to their high efficiency, convenience, environmental compatibility, and cost effectiveness. In the direct anodic oxidation, the adsorbed hydroxyl radicals (•OH ads) are produced from the oxidation of a water molecule at high oxygen overvoltage anodes such as SnO2,1–3 PbO2,1,2,4 and a boron-doped diamond electrode4–11 through reaction 1. While in the indirect electrooxidation, •OH radicals are produced in the reaction solution from the electroFenton′s reaction 2 between Fe2+ in the medium and hydrogen peroxide (H2O2) formed by the two-electron reduction of oxygen (reaction 3) at the cathode.
H2O f •OHads + H + + e-
(1)
H2O2 + Fe2+ f Fe3+ + •OH + OH-
(2)
O2(g) + 2H + + 2e- f H2O2
(3)
The high oxidizing power of •OH allows it to oxidize most organic as well as many inorganic pollutants in the bulk solution.12–36 For the electro-Fenton system, most studies primarily focus on the electrogeneration of H2O2, using various cathode materi* Corresponding author. E-mail:
[email protected]. Phone: +86 411 84706328. Fax: +86 41184708083.
als such as carbon felt,12–15 activated carbon fiber,16 gas diffusion electrodes,17–26,37 graphite,37,38 reticulated vitreous carbon,39 and multiwalled carbon nanotube.40 However, the electroregeneration of Fe2+ (reaction 4) is largely neglected.
Fe3+ + e- f Fe2+
(4)
To promote the formation of •OH and eliminate the negative effect of iron sludge (Fe(OH)3(s)) generated in the Fenton oxidation process, the regeneration of Fe2+ was investigated by Qiang et al.41 with constant potential and constant current mode. In addition, the changes of Fe2+ and Fe3+ concentrations have been investigated in detail by Oturan et al.18,35 to test the transform ability of Fe3+ into Fe2+ and vice versa in undivided cells containing various anodes (platinum, boron-doped diamond) and cathodes (carbon felt, O2-diffusion cathode). Results showed that the conversion behavior of the Fe3+/Fe2+ system and the electrogeneration of H2O2 depended mainly on electrode materials. Conducting polymer-modified electrodes have many advantages such as catalytical activity, selectivity, sensitivity, and stability of the polymer film in electrochemical application. Conducting polymer-modified electrodes [such as polypyrrole (PPy), polyaniline, polythiophen, poly(3-methyl)thiophen, and poly(3,4-ethylenedioxythiophene)] have been studied for the oxygen reduction reaction (ORR) and showed excellent electrocatalytic activities.42 On the other hand, quinonoid compounds have been proved efficient as catalysts when immobilized onto the carbon electrode surface or incorporated into the PPy matrix for ORR.43–49 Results have shown a great improvement in electrocatalytic ability toward ORR and a shifting of the oxygen reduction potential to more positive values. Therefore, aimed at using advantageous conducting polymer and quinonoid compound, the anthraquinonedisulfonate/PPy
10.1021/jp800757v CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
8958 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Figure 1. Cyclic voltammograms recorded in 0.5 mol L-1 N2-saturated H2SO4 for the electropolymerization of Py (0.1 mol L-1) (a) in the absence of AQDS, (b) in the presence of AQDS (5 mmol L-1), and (c) for the AQDS/PPy/graphite electrode electropolymerized for 15 cycles. Potential range: -0.7 to 0.7 V. Scan rate: 10 mV s-1.
composite film modified graphite electrodes (AQDS/PPy/ graphite) were prepared and characterized in this study. Noting that the yield and current efficiency of H2O2 generation and Fe2+ regeneration are important factors for electro-Fenton oxidation ability and efficiency,18,35 the electrochemical behaviors of oxygen reduction in the absence and presence of Fe3+, and the effects of various operational parameters such as pH, cathodic potential, oxygen flow rate, and AQDS doping concentration on H2O2 accumulation and current efficiency were investigated systematically. In addition, the influence of Fe3+ concentration on the electro-Fenton degradation of amaranth azo dye was also studied with the potentiostatic electrolysis mode, using the bare and the composite film modified graphite cathodes. 2. Experimental Section 2.1. Reagents and Chemicals. Amaranth (99.5%) was purchased from Beijing Chemical Reagents Company and used as received without further purification. Pyrrole (Py, g99%, Aldrich) was used after two distillations. Anthraquinone-2,6disulfonic acid, disodium salt (Fluka) was used as received. Other chemicals such as Na2SO4, H2SO4, NaOH, NaH2PO4, Na2HPO4, TiSO4, (NH4)2SO4, and Fe2(SO4)3 · 9H2O were either reagent or analytic grade. High-purity O2 regulated by flowmeter (equipped with a needle valve) and N2 were used to maintain oxygen saturation and deoxygenation. The solution pH was adjusted to the desired values by adding 0.05 mol L-1 Na2HPO4/NaH2PO4 and 0.1 mol L-1 H2SO4 or NaOH and was measured with a pB-10 pH-meter (Sartorius AG, Germany). 2.2. Working Electrode. The commercial graphite with a geometrical area of 10.5 cm2 was polished to a mirror and subsequently washed and sonicated in doubly distilled-deionized water for 10 min. Then the AQDS/PPy composite films were prepared by incorporating the AQDS into the PPy matrix during the electropolymerization of Py from 0.5 mol L-1 H2SO4 solutions containing 0.1 mol L-1 Py and 5 mmol L-1 anthraquinone-2,6-disulfonic acid, disodium salt under the condition of N2 atmosphere. The amount of AQDS incorporated into the PPy matrix was controlled by the electropolymerization cycles. The AQDS/PPy/graphite electropolymerized for 15 cycles and the bare graphite was used as a cathode and washed by doubly distilled-deionized water before use. 2.3. Equipment and Apparatus. All electrochemical investigations were carried out with an EG&G Princeton Applied Research Model 263A potentiostat/galvanostat. The experimen-
Zhang et al. tal setup used in this study was a conventional three-electrode cell with a Pt foil (6 cm2) and a saturated calomel electrode (SCE) used as the counter and reference electrode, respectively. The electro-Fenton degradation experiments performed with the potentiostatic electrolysis mode were conducted at room temperature in an undivided glass cell of 400 mL and were controlled by REX DJS-292 potentiostat/galvanostat (Shanghai Cany Precision Instrument Co., Ltd.). A Philips XL-30 scanning electron microscope (SEM) was used to observe the morphology of the AQDS/PPy composite film. The chemical structure of the composite film was investigated with KBr-pellets, using a Fourier transfer infrared (FTIR) spectrometer (Thermo Nicolet Nexus, USA). 2.4. Analytical Methods. The concentration of H2O2 was analyzed by spectrophotometric (pharmaspec, UV-1700, Japan) analysis, using the titanium(IV) oxysulfonate-sulfuric acid complex (TiOSO4 · xH2SO4 · xH2O) method by measuring the absorbance at λ ) 408 nm (ε ) 1010 M-1 cm-1).50 The current efficiency (CE) for H2O2 formation is determined as:
CE )
nFc(H2O2)V × 100% Qt
(5)
where n (n ) 2) is the stoichiometric number of transferred electrons involved in ORR, F is the Faraday constant (96485 C mol-1), c(H2O2) represents the H2O2 concentration in the bulk solution (mol L-1), V is the total volume of electrolytic solution (L), and Qt is the cumulative charge applied to the system at any given time, obtained from the coulometer of the EG&G 263A electrochemical station. The volume of the amaranth solution was 200 mL (the dye concentration ) 80 mg L-1) containing 0.1 mol L-1 of Na2SO4 as supporting electrolyte. Samples were taken from the cell at regular time intervals. The disappearance of amaranth (decolorization) was analyzed spectrophotometrically (pharmaspec, UV-1700, Japan) by measuring the absorbance at the maximum wavelength λ ) 524 nm. The residual concentration of amaranth in the treated solution was calculated from the absorbance-concentration calibration curve. 3. Results and Discussion 3.1. Preparation and Characterization of the AQDS/PPy Composite Film. The cyclic voltammograms of graphite electrode in 0.5 mol L-1 N2-saturated H2SO4 are shown in Figure 1. The electropolymerization process of Py in the absence of AQDS (Figure 1a) indicates that the irreversible oxidation of Py starts at ca. 0.58 V and no other oxidation peak is observed. However, when Py is electropolymerized in the presence of AQDS, the voltammogram in Figure 1b is different from that observed in Figure 1a. The irreversible oxidation of Py also starts at 0.58 V, but an oxidation peak appears after two or three cycles. This oxidation peak is due to the inserted AQDS, because it does not exist when Py is polymerized in the absence of AQDS. The voltammogram of the resulting AQDS/PPy/graphite electrode electropolymerized for 15 cycles is shown in Figure 1c, there are single reduction and oxidation peaks, and the values of the peak potentials are equivalent to that obtained in Figure 1b. This phenomenon suggests the incorporation of AQDS into the PPy matrix. To further clarify this conclusion, the FTIR spectra of this AQDS/PPy composite film and the pure AQDS were measured and shown in Figure 2. According to the literature,51 PPy has three weak bands at around 1610, 1490, and 1410 cm-1 caused by ring stretching and bands at around 1050 cm-1 caused by N-H bending. In-plane and out-of-plane C-H bending is expected at 980 and 960 cm-1, respectively. The presence of
Electrocatalytic Behavior of Graphite Cathodes
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8959
Figure 2. The Fourier transfer infrared (FTIR) spectra of the AQDS/ PPy composite film electropolymerized for 15 cycles.
Figure 4. Cyclic voltammograms of oxygen reduction recorded at the bare graphite (a) and the AQDS/PPy/graphite (b) cathodes in various pH O2-saturated 0.1 mol L-1 Na2SO4 solutions. Scan rate: 10 mV s-1.
Figure 3. Scanning electronic micrograph (SEM) of the AQDS/PPy composite film electropolymerized for 15 cycles.
AQDS in the PPy matrix is well-demonstrated by the characteristic absorption band at 750, 1250, and 1669 cm-1.52,53 The strong absorption peak located at 1256 cm-1 in the spectrum of AQDS/PPy indicates the coupling of SdO with the stretching vibration of the Py ring.52 These results confirm that the AQDS is really incorporated into the PPy matrix. Figure 3 shows the SEM image of the AQDS/PPy composite film electropolymerized for 15 cycles. The surface of the modified electrode is homogeneous, thick, and compact. It is characterized by a cauliflower-like structure constituted by microspherical grains. This morphology indicates that the electrochemically prepared AQDS/PPy composite film has a three-dimensional reaction zone with a great surface area, which will enhance the energy efficiency of the electron transfer reaction and show excellent electrocatalytic activities as well as remarkable stability when used as an electrode material in the electrochemical reaction. 3.2. Reduction Behavior of Oxygen and Fe3+. It is wellknown that the cathode material and solution pH play important roles on ORR. To investigate the effects of these factors, cyclic voltammetry experiments of oxygen reduction were carried out with the bare graphite (Figure 4a) and the AQDS/PPy/graphite (Figure 4b) cathodes in various pH O2-saturated 0.1 mol L-1 Na2SO4 solutions, i.e., to determine the potential at which oxygen reduction to H2O2 was found to take place with high efficiency on the cathode surface. As can be seen, standard voltammograms with the characteristics of total irreversibility for ORR were obtained in all studied solutions on the bare and the composite film modified cathodes. The oxygen reduction potentials located at ca. -0.85, -0.82, and -0.77 V were observed on the bare graphite electrode for pH 3.0, 4.0, and
6.0, respectively, while at the modified cathode, higher currents were observed in the descending regions of the curves for all the investigated pH values. The oxygen reduction potentials shift to more positive values (-0.65, -0.60, and -0.52 V for pH 3.0, 4.0, and 6.0, respectively) compared to the bare cathode, which indicates the stronger electrocatalytic activity of the AQDS/PPy/graphite cathode toward ORR.45,46 In addition, the solution of pH 6.0 seems to be a more suitable medium for the ORR electrocatalyzed by the AQDS/PPy/graphite cathode, although this electrode exhibits good electrocatalytic activities toward ORR at all studied pH values. Besides electrode material and solution pH, the electrolyte was another important factor for ORR involved in the electroFenton process. The cathodic polarization curves corresponding to the influence of Fe3+ on oxygen reduction at the bare graphite and the AQDS/PPy/graphite cathodes in pH 3.0 O2-saturated 0.1 mol L-1 Na2SO4 solutions are depicted in Figure 5. As can be seen, the current-potential curves take similar shapes and nearly the same oxygen reduction initiating potential for each cathode, but show different H2 evolution potentials. The presence of Fe3+ results in reduction waves of reaction 4 at E1/2 ) -0.45 and -0.1 V for the bare and the composite film modified cathodes, respectively. The polarization curves of the AQDS/PPy/graphite cathode show a well-formed oxygenreduction wave at ca. E1/2 ) -0.55 V, while the value of E1/2 shifts toward the more negative potential direction and locates at ca. -0.80 V for the bare electrode. Although Fe3+ has the same concentration and diffusion coefficient in the reaction solutions, the mass transfer controlled limiting currents are quite different for the bare (-28.26 µA) and the modified (-8.41 µA) graphite cathodes because of the slow mass transfer of Fe3+ through the AQDS/PPy composite film. This result indicates that the reduction of Fe3+ to Fe2+ (reaction 4) at the AQDS/ PPy/graphite cathode is slower than that at the bare graphite
8960 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Zhang et al.
Figure 5. Cathodic polarization curves recorded at the bare graphite (a) and the AQDS/PPy/graphite (b) cathodes in pH 3.0 O2-saturated 0.1 mol L-1 Na2SO4 solutions containing 1.0 mmol L-1 Fe3+ or not. Scan rate: 10 mV s-1.
TABLE 1: Effects of Various Parameters on H2O2 Accumulation and Current Efficiency no.
O2 flow rate (m3 h-1)
cathodic potential (V)
pH
CH2O2 (mmol L-1)
CE (%)
1 2 3 4 5 6 7
0.01 0.015 0.02 0.02 0.02 0.02 0.02
-0.65 -0.65 -0.65 -0.5 -0.8 -0.65 -0.65
3 3 3 3 3 4 6
5.79 11.57 12.35 7.29 8.80 11.2 9.34
39.6 68.8 73.4 61.5 52.4 66.1 55.6
cathode. On the other hand, the oxygen reduction limiting currents are also different on the bare (-21.05 µA) and the modified graphite (-57.13 µA) cathodes in the absence of Fe3+, implying the stronger electrocatalytic activity of the AQDS/ PPy/graphite cathode toward ORR and that reaction 3 is very fast at this modified cathode. In addition, the different concentrations and diffusion coefficients of dissolved oxygen and Fe3+ result in their different limiting currents when reactions 3 and 4 take place simultaneously on the same cathode. These results indicate that ORR and Fe3+ reduction behaviors depend largely on the cathode material and the AQDS/PPy/graphite cathode demonstrated the characteristics of the O2-diffusion cathode reported in the literature.18 Moreover, a large amount of Fe(OH)3(s) was formed at higher pH. The formation of Fe(OH)3(s) not only decreases the concentration of dissolved Fe3+, but also inhibits Fe2+ regeneration by partially coating the electrode surface.41 In other words, the dissolved Fe3+ is the crucial factor for the regeneration of Fe2+. 3.3. Effect of Various Parameters on H2O2 Accumulation. Table 1 shows the H2O2 accumulation and current efficiency after 120 min of electrolysis with the AQDS/PPy/graphite cathode under different conditions. The oxygen flow rate is an important factor for dissolved oxygen concentration and H2O2
generation.24,38 As can be seen, both the concentration of H2O2 and current efficiency increased with the increase of oxygen flow rate. However, when the oxygen flow rate reaches 0.015 m3 h-1, the increase in H2O2 concentration and current efficiency is insignificant. The reason might be that higher oxygen flow rate could not influence the saturation of dissolved oxygen and would release gas with bigger bubbles, which was unfavorable for the oxygen adsorption onto the cathode. This result agrees well with the studies of Qiang et al.,38 who also indicated that further increase in oxygen flow rate after the optimal value did not enhance H2O2 production. As shown in Table 1, the cathodic potential at -0.65 V showed the best H2O2 generation and current efficiency. This result is in good accordance with the oxygen reduction behavior described in Figure 4b. In addition, it was observed from Figure 4b that a more positive cathodic potential resulted in a significant decrease in reduction current. Therefore, the current response for H2O2 generation was small (only 7.29 mM H2O2) at -0.5 V, although the current efficiency was still relatively high (61.5%). On the other hand, more electric charge is consumed for more negative cathodic potential (-0.8 V), a majority of which is wasted by side reactions such as H2O2 decomposition (reaction 6) and H2 evolution (reaction 7),24,38 thus decreasing the H2O2 production and current efficiency. In addition, the main side reaction of 4-electron reduction of oxygen to water (reaction 8) did not take place in this case (Figure 5b), which agrees well with the fact that quinonoid compound modified electrodes mainly catalyze 2-electron reduction of oxygen to H2O2.43,44,47–49
H2O2 + 2H + + 2e- f 2H2O
(6)
2H + + 2e- f H2
(7)
O2 + 4H + + 4e- f 2H2O
(8)
The solution pH has an important effect on ORR, and sequentially influences the generation of H2O2. As can be seen from Table 1, the H2O2 production and current efficiency have higher values in low pH solution, while the performance is reduced at high pH solution. This trend can be explained by the fact that HO2- will generate in a large extent at higher pH media, which catalyzes H2O2 decomposition (reaction 9) rapidly, resulting in a decrease in H2O2 production and current efficiency with the increase of the electrolysis time.
HO2- + H2O2 f H2O + O2 + OH-
(9)
The influence of the doping concentration of AQDS incorporated in the PPy matrix on H2O2 production was also investigated at Ecath ) -0.65 V in 0.1 mol L-1 Na2SO4 solution of pH 3.0. The doping concentration of AQDS was controlled by the electropolymerizing cycles (5, 15, and 25 cycles) during the preparation of the AQDS/PPy composite film. After 120 min of electrolysis, no significant difference in H2O2 generation efficiency was observed for the composite film doped by various amounts of AQDS. Because pure oxygen gas at a flow rate 0.02 m3 h-1 was continually bubbled into the cell, a constant dissolved oxygen concentration was maintained in the reaction solution. Therefore, the result can be attributed to H2O2 generation in aqueous solution being controlled either by the electron transfer between dissolved oxygen and cathode or by the mass transfer of an oxygen molecular across the bulk solution, the cathode-solution interface, and the AQDS/PPy composite film. 3.4. Electro-Fenton Degradation of Amaranth. It was reported that •OHads generated at the high oxygen overvoltage
Electrocatalytic Behavior of Graphite Cathodes
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8961 of Fe2+ at the cathode is slowed down causing the total decay of dye in longer time. Consequently, an optimal Fe3+ concentration in the starting solution is an important and crucial factor for the electro-Fenton oxidation ability in this system. Figure 6b shows the effect of Fe3+ concentration on amaranth removal at Ecath ) -0.65 V with the AQDS/PPy/graphite cathode. As can be seen, complete removal of dye is achieved for all cases within 120 min, and the degradation rate undergoes a gradual acceleration when Fe3+ concentration increases from 1.0 to 6.0 mmol L-1 (in about 100 and 30 min for 1.0 and 6.0 mmol L-1 Fe3+, respectively). In this system, greater amounts of •O2H are formed from reaction 12 when the Fe3+ concentration increases due to the much faster generation of H2O2 via reaction 3 at the modified cathode.
H2O2 + Fe3+ f Fe2+ + •O2H + H +
(12)
However, previous studies have proved that both H2O2 and are not strong oxidants and are unable to degrade organics efficiently.17,18,20,26,36 Thus, this faster removal of amaranth can be related to an increasing quantity of •OH formed from reaction 2 with the increase of the concentration of Fe2+ regenerated from reaction 4, when more Fe3+ is present in the solution. •O H 2
4. Conclusions
Figure 6. Effect of Fe3+ concentration on the electro-Fenton degradation of amaranth (80 mg L-1) using the bare graphite (a) and the AQDS/ PPy/graphite (b) cathodes with the potentiostatic electrolysis mode. Conditions: pH 3.0, oxygen flow rate 0.02 m3 h-1.
anode surface from reaction 1 and other weaker oxidizing reagents such as H2O2 and •O2H generated respectively from reaction 3 and 10 in reaction solution can also bring about the removal of organics.1–11,17,18,20,26,36
H2O2 f •O2H + H + + e-
(10)
But these oxidizing species generated by individual constituent such as anodic oxidation and anodic oxidation with electrogenerated H2O2 on Pt show poor efficiency on organics decay.17,18,20,26,36 The research groups of Oturan and Brillas points out that organics abatement in an undivided cell is mainly due to the •OH formed from the electro-Fenton reaction in the solution. A series of potentiostatic electrolysis reactions was carried out with 80 mg L-1 amaranth solutions of pH 3.0 to determine the effect of Fe3+ concentration on its decay using the bare graphite and the AQDS/PPy/graphite cathodes under the electroFenton conditions. As seen from Figure 6a, a fast and nearly complete removal of amaranth was observed in all cases. The quickest dye abatement in this system is achieved at 0.2 mmol L-1 Fe3+. However, the removal rate undergoes a gradual drop with the increase of Fe3+ concentration up to 2.0 mmol L-1, leading to an incomplete decay in 240 min electrolysis. As mentioned above, the bare graphite cathode has good electrocatalytic ability for reaction 4. Therefore, higher Fe3+ concentration leads to more regenerated Fe2+, which results in a progressive fall of •OH concentration in the medium through reaction 11. •
OH + Fe2+ f Fe3+ + OH-
(11)
Although the consumption of this radical is insignificant in the presence of lower Fe3+ concentration, the regeneration rate
The AQDS/PPy/graphite cathode exhibits better performance for H2O2 electrogeneration, but shows slower electrolysis for Fe2+ regeneration. With regard to the bare graphite cathode, the cases of H2O2 generation and Fe2+ regeneration are completely opposite. Solution pH, cathodic potential, and oxygen flow rate influence H2O2 generation rate and current efficiency greatly, while the effect of the amount of doping AQDS incorporated into the PPy matrix is insignificant. The AQDS/PPy/graphite cathode results in a large accumulation of electrogenerated H2O2; therefore, the electro-Fenton degradation of amaranth is enhanced by increasing the initial Fe3+ concentration because more Fe3+ leads to more regenerated Fe2+ at the cathode, giving rise to a higher quantity of •OH formed in the solution from Fenton′s reaction. In contrast, the regenerated Fe2+ is largely accumulated when the bare graphite is used as a cathode, and the lower Fe3+ concentration at 0.2 mmol L-1 in this electro-Fenton system is enough to achieve •OH generation at rate that is appropriate to the need of the quickest amaranth removal. Results indicate that the electro-Fenton technology is an environmental friendly and efficient method for the degradation of azo dye. References and Notes (1) Polcaro, A. M.; Palmas, S.; Renoldi, F.; Mascia, M. J. Appl. Electrochem. 1999, 29, 147. (2) Cossu, R.; Polcaro, A. M.; Lavagnolo, M. C.; Mascia, M.; Palmas, S.; Renoldi, F. Ind. Eng. Chem. Res. 1998, 32, 3570. (3) Chen, G.; Chen, X.; Yue, P. J. Phys. Chem. B 2002, 106, 4364. (4) Martinez-Huitle, C. A.; Quiroz, M. A.; Comninellis, C.; Ferro, S.; De Battisti, A. Electrochim. Acta 2004, 50, 949. (5) Saez, C.; Panizza, M.; Rodrigo, M. A.; Cerisola, G. J. Chem. Technol. Biotechnol. 2007, 82, 575. (6) Brillas, E.; Sire´s, I.; Arias, C.; Cabot, P. L.; Centellas, F.; Rodrı´guez, R. M.; Garrido, J. A. Chemosphere 2005, 58, 399. (7) Can˜izares, P.; Louhichi, B.; Gadri, A.; Nasr, B.; Paz, R.; Rodrigo, M. A.; Saez, C. J. Hazard. Mater. 2007, 146, 552. (8) Panizza, M.; Cerisola, G. Electrochim. Acta 2005, 51, 191. (9) Flox, C.; Garrido, J. A.; Rodı´guez, R. M.; Centellas, F.; Cabot, P. L.; Arias, C.; Brillas, E. Electrochim. Acta 2005, 50, 3685. (10) Nasr, B.; Abdellatif, G.; Can˜izares, P.; Saez, C.; Lobato, J.; Rodrigo, M. A. EnViron. Sci. Technol. 2005, 39, 7234. (11) Chen, X.; Chen, G. Sep. Pure. Technol. 2006, 48, 45. (12) Oturan, M. A.; Peiroten, J.; Chartrin, P.; Acher, A. J. EnViron. Sci. Technol. 2000, 34, 3474.
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