Electro-Fenton Process Catalyzed by Fe3O4 Magnetic Nanoparticles

Article pubs.acs.org/IECR

Electro-Fenton Process Catalyzed by Fe3O4 Magnetic Nanoparticles for Degradation of C.I. Reactive Blue 19 in Aqueous Solution: Operating Conditions, Influence, and Mechanism Zhiqiao He, Chao Gao, Mengqian Qian, Yuanqiao Shi, Jianmeng Chen, and Shuang Song* College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China ABSTRACT: Fe3O4 magnetic nanoparticles (MNPs) were employed for electro-Fenton (Fe3O4−electro-Fenton) degradation of C.I. Reactive Blue 19 (RB19) in an undivided electrochemical reactor with an activated carbon fiber felt cathode and a platinum anode. On the basis of physicochemical characterization of the Fe3O4 MNPs as well as quantitative measurements of iron leaching and H2O2 generation, it is concluded that the Fe3O4 MNPs facilitated the decomposition of H2O2 to generate hydroxyl radicals (•OH). Moreover, the cathodic electro-Fenton facilitated electro-regeneration of ferrous ion and maintained continuous supply of H2O2. The effect of several operational parameters such as pH, current density, amount of added Fe3O4 MNPs, initial RB19 concentration, and temperature on the removal of total organic carbon was investigated. It was found that the Fe3O4−electro-Fenton degradation of RB19 followed two-stage first-order kinetics with an induction period and a rapid degradation stage. Mineralization of RB19 proceeded rapidly only at pH 3.0. Increasing the current density and the dosage of Fe3O4 MNPs enhanced the rate of RB19 degradation. However, higher current densities and Fe3O4 dosages inhibited the reaction. The rate of RB19 degradation decreased with the increase in initial RB19 concentration and increased with the increase in temperature. The removal efficiency of total organic carbon reached 87.0% after 120 min of electrolysis at an initial pH of 3.0, current density of 3.0 mA/cm2, 1.0 g/L concentration of added Fe3O4 MNPs, 100 mg/L initial dye concentration, and 35 °C temperature. On the basis of the analytical results for the intermediate products and the assumption that •OH radicals are the major reactive species, we propose a possible pathway of RB19 degradation during the cathodic electro-Fenton process using Fe3O4 MNPs as iron source.

1. INTRODUCTION Many industries, such as those for textiles, leather, cosmetics, paper, printing, and plastics, use potentially harmful dyes to color their products.1 The dyeing process releases approximately 10−15% of the dyes into the environment, leading to serious concerns regarding the environment and health.2,3 Several methods, including biological, physical, and chemical treatments, have been proposed for the treatment and decolorization of effluent from the manufacturing of reactive dyes.4 However, because of the high chemical oxygen demand (COD) and low ratio of biochemical oxygen demand (BOD) to COD of the wastewater, traditional methods often fail to meet the increasingly stringent limits for its discharge to the environment.5 To date, Fenton-like technology using solid catalysts has received much attention as a relatively effective alternative for wastewater treatment. A variety of iron oxide minerals, namely, magnetite (Fe3O4), hematite (α-Fe2O3), and goethite (αFeOOH), have been used in Fenton-like reactions.6−8 Of these catalysts, inverse spinel magnetite is considered as a promising candidate because it offers many advantages, such as easy preparation, high stability, favorable reusability, and convenient separation from solution by an external magnetic field.6,9 Moreover, transfer of electrons between ferrous and ferric ions in the octahedral sites can prevent substantial loss of Fe species.9 Many recent works have focused on the preparation of Fe3O4 magnetic nanoparticles (MNPs). Its nanostructure usually has a large exposed surface area, which accelerates © 2014 American Chemical Society

heterogeneous and homogeneous reactions of Fenton-like systems utilizing Fe3O4 catalysts. Different from the conventional Fenton reaction in which the H2O2 concentration is gradually reduced with reaction time after batch chemical dosing, the electro-Fenton reaction process forms H2O2 in situ.10 Evidently, the cathodic electro-Fenton reaction can overcome the main drawbacks of its conventional counterpart, including the potential hazards of storing and transporting concentrated H2O2 solutions. The reaction rate obtained in the cathodic electro-Fenton process using solid catalysts is dependent on the operating conditions of the reaction. In previous work, the effects of many operating parameters have been studied, such as solution pH, current density, catalyst loading, initial concentration of target pollutant, and electrolysis temperature.11−15 Even so, simple analysis of the kinetics is still required to obtain insight into the influence of operating variables on the rate of pollutant degradation. Zeng et al. investigated the cathodic electroFenton removal of 4,6-dinitro-o-cresol (DNOC) by using nanoFe3O4 as the iron source. They obtained the pseudo-secondorder reaction rate constant for DNOC degradation in aqueous solution and proposed a kinetic model to describe the degradation mechanism at low pH conditions.16 However, Received: Revised: Accepted: Published: 3435

November 21, 2013 January 24, 2014 February 4, 2014 February 4, 2014 dx.doi.org/10.1021/ie403947b | Ind. Eng. Chem. Res. 2014, 53, 3435−3447

Industrial & Engineering Chemistry Research

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Figure 1. Characterization of Fe3O4 MNPs. (a) XRD patterns, (b) TEM micrographs, (c) XPS spectra and high-resolution scan of the Fe2p region (inset), (d) hysteresis loops and photograph of the sample exposed to a magnet (inset), and (e) pore size distribution and nitrogen adsorption/ desorption isotherms (inset).

the intermediates during the Fe3O4−electro-Fenton process. RB19 was chosen as a model target pollutant in our study because it is widely used by the textile, leather, pharmaceutical, printing, plastic, and cosmetic industries. The aromatic structure of anthraquinone is very stable toward chemical oxidation.

when magnetite is used as an iron source, both heterogeneous and homogeneous Fenton reactions occur. Fe species are continuously dissolved into the reaction solution as the reaction proceeds. Thus, pollutant degradation predominantly proceeds from a surface-catalyzed process to a Fenton-based reaction in bulk solution, yielding a sigmoidal profile of concentration versus time.17 In other words, an induction period for catalysis is always accompanied with the oxidation of organic substrates.17−19 However, few studies paid attention to the induction period associated with cathodic electro-Fenton processes that use solid catalysts partially because of its complexity, as its involves heterogeneous processes. In the present work, electro-Fenton processes that utilize Fe3O4 MNPs as catalyst (Fe3O4−electro-Fenton) were applied to the mineralization if C.I. Reactive Blue 19 (RB19) in a model wastewater. The effects of several operating parameters, such as the initial pH of the solution, current density, catalyst loading, initial concentration of target pollutant, and electrolysis temperature, were studied. The overall reaction period was subdivided into two phases with different values of kinetic constants representing different reaction rates. Finally, the mechanism of RB19 degradation was investigated by measuring

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. All chemicals used were of analytical grade. The target pollutant RB19 (99% purity) was obtained from Zhejiang Runtu, Co., Ltd. (Shaoxing, China). Ferric sulfate [Fe2(SO4)3, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China], ferrous sulfate (FeSO4·7H2O, Juhua Group Corp., Quzhou, China), sodium hydroxide (Hangzhou Xiaoshan Chemical Reagent Factory, Hangzhou, China), and sulfuric acid (Juhua Group Corp., Quzhou, China) used in the preparation of Fe3O4 MNPs were used as received. All other chemicals, such as hydrochloric acid, sodium sulfate, potassium iodide, sodium thiosulfate, hydroxylamine hydrochloride, 1,10phenanthroline, ferrous ammonium sulfate, ammonium chloride, Seignette salt, and mercuric iodide, were purchased from Huadong Medicine Co., Ltd. (Hangzhou, China). High-purity 3436

dx.doi.org/10.1021/ie403947b | Ind. Eng. Chem. Res. 2014, 53, 3435−3447


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Figure 2. Schematic diagram of the experimental setup.

the space group Fd3m ̅ (227) (JCPDS card no. 19-629). The average crystallite size estimated from the prominent (311) reflection by using the Scherrer equation was ∼5.9 nm. No other peaks are present in the curve, indicating that the spheres were pure crystalline Fe3O4. TEM images of the Fe3O4 MNPs can be seen in Figure 1b. The size distributions of the Fe3O4 particles fell within diameters in the range of 5−8 nm, which is in accordance with the XRD results calculated through the Scherrer equation. Figure 1c shows the XPS survey spectra of the Fe3O4 catalyst. The binding energies of Ti2p3, O1s, and C1s are found at 482.6, 530.6, and 284.6 eV, respectively. The photoelectron peak for C1s is probably attributed to contamination caused by specimen handling or by pumping oil from the XPS instrument. The inset in Figure 1c presents the Fe2p core peaks of the Fe3O4 sample, which exhibits two main Fe2p components due to spin−orbit coupling. Generally, Fe2p peaks at binding energies of 711.2 and 725.0 eV with a satellite signal at 719.0 eV are characteristic of FeIII, and those at binding energies of 709.9 and 723.4 eV with a satellite signal at 715.5 eV are characteristic of FeII.22,23 In addition, interaction between FeII and FeIII can lead to a shoulder at higher binding energy (around 713.4 eV). Deconvolution of Fe2p peaks showed that the synthesized sample consisted of mixed-valence compounds of FeII and FeIII. Figure 1d shows the magnetization hysteresis loop of the synthesized Fe3O4 MNPs obtained with an applied magnetic field sweep from −10 to 10 kOe at room temperature. The magnetic hysteresis loops are sigmoidal curves and the magnetic remanences of Fe3O4 MNPs are nearly zero (∼0.37 emu/g), indicating that the sample exhibited superparamagnetic behavior. In addition, magnetic measurements revealed a saturation magnetization of 27.3 emu/g, which is smaller than that of bulk Fe3O4 (92 emu/ g).24 This result is most likely attributed to the much smaller size of the prepared Fe3O4 primary nanocrystals, as well as surface-related effects such as surface disorder.24,25 The BET surface areas of the as-prepared catalyst were found to be 207.0 m2/g. N2 adsorption−desorption isotherms of the porous Fe3O4 MNPs are illustrated in Figure 1e. Fe3O4 MNPs produced type IV isotherms, indicating their mesoporous structure. The pore size distribution was determined from the

Ar gas was supplied by Hangzhou Jingong Special Gas Co., Ltd., Hangzhou, China. Deionized water was used throughout the experiments. The activated carbon fiber felt (ACFF) was obtained from Jiangsu Sutong Carbon Fiber Co., Ltd. (Rugao, China). The platinum electrode (99.9% in purity) was produced by Hangzhou Saiao Electrochemical Technology Co., Ltd. (Hangzhou, China). 2.2. Preparation and Characterization of Fe3O4 MNPs. Fe3O4 MNPs were prepared by coprecipitation of aqueous ferrous and ferric ions according to Massart’s method.20,21 In a typical run, an aqueous mixture of 100 mL of 0.01 M FeSO4· 7H2O and 0.02 M Fe2(SO4)3 with 0.2 mL of concentrated H2SO4 was added dropwise into 100 mL of 0.2 M NaOH solution that was vigorously stirred at 80 °C for 2 h under an Ar stream. The resulting precipitate was collected from the reaction medium by magnetic separation and washed with deionized water several times. Subsequently, the final products were dried at room temperature under vacuum. The morphology of the Fe3O4 sample was observed by transmission electron microscopy (TEM, Tecnai G2 F30 STwin microscope, 300 kV and 0.20 nm point resolution). Its Xray diffraction (XRD) pattern was obtained on a Thermal ARL X-ray diffractometer (Thermo, France) with Cu Kα radiation, 45 kV accelerating voltage, and 40 mA applied current. Through the Brunauer−Emmett−Teller (BET) method, surface areas of the catalysts were calculated from the nitrogen adsorption/desorption isotherm obtained at −196 °C on a Micromeritics ASAP 2010 analyzer. X-ray photoelectron spectroscopy (XPS) was performed on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg Kα radiation (1253.6 eV). The magnetic properties of the Fe3O4 prepared at room temperature were studied by using a vibrating sample magnetometer (Physical Property Measurement System, PPMS-9, Quantum Design). The crystalline phases of Fe3O4 MNPs are shown in Figure 1a. The diffraction peaks of the sample are sharp and intense, indicating the highly crystalline nature of Fe3O4. Diffraction peaks at 2θ values of 30.1°, 35.4°, 43.1°, 57.0°, and 62.5° can be indexed respectively to the (220), (311), (400), (511), and (440) planes of Fe3O4 with cubic spinel structure belonging to 3437



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was held at 80 °C for 1 min, and then a gradient temperature program at 15 °C/min between 80 and 250 °C was implemented. Finally, the column was heated at 250 °C for 15 min. Other experimental conditions were as follows: EI impact ionization, 70 eV; carrier gas, helium; injection temperature, 280 °C; and source temperature, 80 °C. Carboxylic acids and inorganic anions were identified by ion chromatography (IC) on a Dionex ICS 2000 (Sunnyvale, CA) equipped with a dual-piston pump (in series), an analytical column (4 mm × 250 mm IonPac AS19; Dionex), guard column (4 mm × 250 mm IonPac AG19; Dionex), and conductivity detector (DS6; Dionex). Suppression of the eluent was achieved by using an anion electrolytic suppressor (4 mm ASRS; Dionex) operated in the autosuppression external water mode. The ammonium concentration was measured at 420 nm through the ammonia-Nessler’s reagent colorimetric method by using a UV−visible spectrophotometer (T6, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) equipped with a photoelectric detector.26

desorption isotherm through the Barrett−Joyner−Halenda method. The average pore diameter was found to be ∼4.3 nm. 2.3. Electrochemical System. A diagram of the experimental setup is presented in Figure 2. Constant-current electrolyses were performed with direct-current (DC) power supply (SK 1760SL, Sanke Electrical CO., Ltd., Hangzhou, China) in an undivided cylindrical glass reactor of 6 cm diameter and 11 cm height. The cathode was ACFF (5 × 8 cm) and the anode was a Pt sheet (2 × 2.5 cm). The cylindrical reactor was placed on a magnetic stirrer (85-1, Hangzhou Instrument Motor Co., Ltd., Hangzhou, China) and thermostated to a predetermined temperature by circulating water from a constant-temperature bath (THD-2015, NingBo TianHeng Instrument Factory, Ningbo, China) through its jacket. The current between the cathode and the anode was measured with a multimeter (VC 890C, Shenzhen Vitor HiTech Co., Ltd., Shenzhen China). In a typical run, 200 mL solutions containing appropriate concentrations of RB19 (100, 200, and 300 mg/L), 0.05 M Na2SO4 as supporting electrolyte, and desired dosages of the catalyst Fe3O4 MNPs (0.5, 0.75, 1.0, and 1.5 g/L) were subjected to specific current densities (1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 mA/cm2) to initiate RB19 degradation. Each run was done at specified pH values (2.0, 3.0, 4.0, 5.0, and 7.0, attained by adjustments using H2SO4 or NaOH), and at various reaction temperatures (15, 25, 35, and 45 °C). At preset time intervals, samples were withdrawn and filtered immediately through a 0.22 μm membrane. The amount of sample collected was 2 mL for the first four time points and 3 mL for the other time points. Thus, the volume reduction of the electrolyte was no more than 10% after reaction. The sample was diluted with deionized water to the desired volume before analysis. It should be noted that the use of Na2SO4 as supporting electrolyte may lead to the passivation of the platinum electrode. However, the electrolytic voltage apparently did not increase in all runs, indicating that passivation had negligible effect on the Fe3O4−electro-Fenton process. Although NaCl could also be used to increase the solution conductivity by electrolysis, Na2SO4 was used instead as supporting electrolyte to avoid the effect of Cl2 and ClO− generated at the anode on total organic carbon (TOC) removal during the Fe3O4−electro-Fenton process. 2.4. Analytical Procedures. The TOC of the initial and electrolyzed samples was determined by a high-temperature combustion method (SM 5310B) using a Shimadzu TOC-VCPN analyzer. Concentrations of ferrous ion and total dissolved iron were determined by light absorbance measurements at 510 nm on a UV−visible spectrophotometer (T6, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) after complexation with 1,10-phenanthroline. The hydrogen peroxide concentration in the solution was measured by iodometric titration. The solution pH was measured on a Sartorius model PB-21 pH meter. The intermediates of RB19 degradation, namely, 1,4diamino-2-hydroxyanthra-9,10-quinone, 2-aminophenol, pyrocatechol, hydroquinone, 2-nitrosophenol, phthalic acid, salicylaldehyde, salicylic acid, and (2Z)-but-2-enedioic acid, were qualitatively analyzed by gas chromatography/mass spectrometry (GC/MS) (GC, Varian cp 3800 system; MS, Varian Saturn 2000 mass spectrometer). The gas chromatograph was equipped with a wall-coated, open, tubular, fused-silica series column (30 m × 0.25 mm, 0.25 μm film thickness) and was interfaced directly to the mass spectrometer. The GC column

3. RESULTS AND DISCUSSION 3.1. Comparison of Adsorption, Anodic Oxidation, Fe2+ Catalyzed Electro-Fenton and Fe3O4−Electro-

Figure 3. Removal of TOC in RB19 solution by electro-Fenton reaction in the presence of 1.0 g/L Fe3O4 MNPs, 1.0 g/L commercial Fe3O4, or 0.15 mM Fe2+, in the absence of catalysts, and by adsorption with ACFF. Experimental conditions: pH, 3.0; J, 3.0 mA/cm2; C0, 100 mg/L; T, 35 °C.

Fenton Reaction. Reduction of the normalized TOC concentration (%) as a function of time (Figure 3) was examined through various comparative experiments to determine the potential advantages of the Fe3O4−electroFenton process. RB19 was degraded through an electrochemical process in the presence of Fe3O4 MNPs and commercial Fe3O4, respectively, resulting in 87.0% and 33.7% reduction in TOC after 120 min of reaction. This apparent superiority in TOC removal of the Fe3O4 MNPs compared with commercial Fe3O4 powders may be attributed to the nanoscale particle diameter and large surface area, as determined through XRD, TEM, and BET surface area analyses. These properties produced large amounts of active sites for H2O2 decomposition.27 Moreover, 0.15 mM FeSO4 solution was used as catalyst instead of Fe3O4 MNPs to evaluate the capacity of traditional electro-Fenton and Fe3O4− electro-Fenton processes for degradation. As can be seen in 3438



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concentration of 100 mg/L was performed in the absence of Fe3O4 and at 3 mA/cm2 applied current density, pH 3.0, and 35 °C temperature. Within 180 min of reaction, the TOC diminished slowly, suggesting that H2O2 oxidation and anodic oxidation at the Pt electrode did not contribute markedly to RB19 degradation. The low oxidative ability of the anodic oxidation could be due to the fact that Pt electrode is a kind of active electrode, where chemically adsorbed “active oxygen” leads to the selective oxidation of organic compounds.29 In comparison, when Fe3O4 MNPs were added to the system, a significant increase in reaction rate was observed. The efficiency of RB19 degradation after 120 min reaction was increased to 3.4 times of that in the absence of catalyst. To further clarify whether H2O2 itself could directly degrade RB19, additional chemical reactions between RB19 and a 75fold molar concentration of H2O2 were carried out. Results show no reduction in the TOC after 180 min reaction. The aforementioned results indicate that RB19 is relatively stable and its degradation occurs mainly through the attack of •OH radicals in the reaction system, as discussed below. 3.2. Temporal Variation of Concentrations of H2O2 and Fe2+ in Aqueous Solution. During the Fe3O4−electroFenton process, a redox process involving FeII/FeIII that generates hydroxyl radicals occurs in the presence of H2O2 on the surface of Fe3O4, as shown in eqs 1−3:27 Fe II + H 2O2 → Fe II ·H 2O2 → Fe III + •OH + OH−

(1)

Fe III + H 2O2 → Fe III ·H 2O2 → Fe II + HOO• + H+ Fe III + HOO• → Fe II + O2 + H+

Figure 4. Variations of (a) H2O2 concentration and (b) iron dissolution in the Fe3O4−electro-Fenton process in the absence RB19. Experimental conditions: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; T, 35 °C.

(2) (3)

In addition, the homogeneous process may also take place in the bulk solution. This proceeds through Fenton reactions involving decomposition of H2O2 by dissolved iron produced by dissolution of iron oxides. The mechanism of the catalytic decomposition of H2O2 in the presence of Fe2+ can be described according to the classical Haber−Weiss mechanism:30,31

Figure 3, TOC removal was consistently higher with the Fe3O4−electro-Fenton process compared with that with the Fe2+-catalyzed electro-Fenton process. As demonstrated by other researchers,12,28 a higher concentration of Fe3+ is generated in electrolyte using conventional Fe2+ source in comparison with the concentration obtained by using Fe3O4 as catalyst in the electrolytic process. This higher concentration results in formation of more stable complexes with organic diacids and hinders the formation of hydroxyl radicals (•OH). In addition, adsorption of organics on the Fe3O4 surface facilitates RB19 degradation in the Fe3O4−electro-Fenton process. To estimate the influence of adsorption on the chemical degradation, adsorption of RB19 by the ACFF was done in an additional experiment. From Figure 3, RB19 adsorption almost reached equilibrium in 30 min and led to a limiting level of removal of less than 16.0%. To exclude the effect of adsorption on the degradation kinetics, all electrochemical experiments in this study were started after the ACFF was presaturated for 30 min. With the exception of adsorption, the other two processes, namely, anodic oxidation and H2O2 oxidation of pollutants, may also be involved in TOC removal from the solution when ACFF was used as cathode in the electro-Fenton process. In a separate experiment, degradation of RB19 with an initial RB19

Fe 2 + + H 2O2 → Fe3 + + •OH + OH−

(4)

Fe3 + + H 2O2 → Fe2 + + HOO• + H+

(5)

Fe3 + + HOO• → Fe 2 + + O2 + H+

(6)

In eqs 1, 2, 4, and 5, H2O2 is generated through an electrochemical reaction of oxygen, as expressed below:10 O2 + 2H+ + 2e− → H 2O2

E 0 = 0.69 V

(7)

When the anode is Pt, O2 needed for the reaction in eq 7 to generate H2O2 at the anode by oxidation of water:10 2H 2O → O2 + 4H+ + 4e−

E 0 = −1.23 V

(8)

To understand the reasons for the significant acceleration of the Fe3O4−electro-Fenton reaction, two main species involved in the reaction (Fe2+ and H2O2) were monitored quantitatively in the absence of RB19 and at 3 mA/cm2 applied current density, pH 3.0, and 35 °C temperature. It can be seen from Figure 4a that the concentration of accumulated H2O2 increased nonlinearly with time and reached a steady-state 3439



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Figure 5. Effect of process variables on the TOC removal of RB19 by the Fe3O4−electro-Fenton process. (a) Effect of initial pH: J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C. (b) Effect of current density: pH, 3.0; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C. (c) Effect of amount of added Fe3O4 MNPs: pH, 3.0; J, 3.0 mA/cm2; C0, 100 mg/L; T, 35 °C. (d) Effect of initial RB19 concentration: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; T, 35 °C. (e) Effect of temperature: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L.

value of approximately 1.575 mM after 60 min of electrolysis. Meanwhile, the current efficiency for H2O2 accumulation (CE, calculated from eq 9) decreased with reaction time32 because H2O2 could chemically decompose to O2 on the anode (heterogeneous process) or in the medium (homogeneous process) via reactions represented by eqs 10−1233 CE(%) = nFcV /(1000MQ ) × 100

In comparison with the case without Fe3O4 MNPs, the addition of the catalyst gave rise to a significant decrease in H2O2 concentration. The results also suggest that the catalytic action of Fe3O4 accelerated H2O2 decomposition via reactions in eqs 4, 5, and 13. H2O2 decomposition yields the hydroxyl radical, which has the highest oxidation potential in acidic medium (2.8 V vs standard hydrogen electrode).

(9)

H 2O2 + •OH → H 2O + HOO•

where n = 2 represents the stoichiometric number of electrons transferred in the reaction in eq 7, F is the Faraday constant (96 486 C/mol), c is the concentration of accumulated H2O2 (mg/L), V is the solution volume (L), 1000 is a conversion factor, M is the molecular weight of H2O2 (34 g/mol), and Q is the charge consumed during electrolysis. H 2O2 → H 2O +

1 O2 2

In addition to H2O2, a suitable concentration of dissolved ferrous ion was an important prerequisite in the electro-Fenton reaction.33 It can be seen from Figure 4b that the concentration of ferrous ions and total dissolved iron increased with reaction time, resulting in a gradual increase in the rate of H2O2 decomposition. Accordingly, the H2O2 concentration increased initially and then declined gradually. Different from results in another work,27 the decrease in ferrous ion concentration was not observed in this work, despite that remaining oxidants (such as •OH and H2O2) in the solution could oxidize ferrous ions into ferric ions. This is because Fe2+ can be regenerated through reactions in eqs 5 and

(10)

H 2O2 → HOO• + H+ + e−

(11)

HOO• → O2 + H+ + e−

(12)

(13)

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Figure 6. Pseudo-first-order degradation of RB19 versus time under various conditions in batch processes. (a) Effect of initial pH: J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C. (b) Effect of current density: pH, 3.0; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C. (c) Effect of amount of Fe3O4 MNPs added: pH, 3.0; J, 3.0 mA/cm2; C0, 100 mg/L; T, 35 °C. (d) Effect of initial RB19 concentration: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; T, 35 °C. (e) Effect of temperature: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L.

Figure 7. pH adjustment in the Fe3O4−electro-Fenton process. Experimental conditions: J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C.

Figure 8. Arrhenius plots for the Fe3O4-catalyzed electro-Fenton reaction. Experimental conditions: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L.

6. Rapid regeneration of Fe2+ is essential to the continuous production of •OH.34 In particular, Fe2+ can be electro3441



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the concentration of TOC in expressing the rate of RB19 degradation. 3.3.1. Effect of Initial pH. The pH value of the dye solution was one of the most important parameters for the electroFenton process. Thus, the effect of five different initial pH values (2.0, 3.0, 4.0, 5.0, and 7.0) on the Fe3O4−electro-Fenton system was studied. A notable effect of pH on RB19 degradation was found. No obvious induction period could be found at pH 2.0 because of the rapid dissolution of Fe3O4. Overall, the induction time shortened with decreasing initial solution pH. At pH 3.0, 4.0, 5.0, and 7.0, the kinetic constants of the induction period were 0.00321, 0.0037, 0.00324, and 0.003 min−1, respectively, and those of the second stage were 0.01615, 0.0125, 0.0117, and 0.0102 min−1, respectively. The highest extent of TOC removal by the Fe3O4−electro-Fenton process was around 89.7% at pH 3.0 within 180 min of reaction. A suitable pH facilitates H2O2 formation and iron dissolution and thus accelerates the homogeneous electro-Fenton reaction. At higher pH, the efficiency of the electro-Fenton process decreases rapidly, especially at pH >5.0 because of the instability of H2O2 toward rapid decomposition to H2O and O2 in basic solution.2 At lower pH, iron species form stable complexes with H2O2, leading to deactivation of catalysts and stabilization of hydrogen peroxide by formation of the oxonium ion.36

Figure 9. Variation of the concentration of related ions with time. Experimental conditions: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/ L; C0, 100 mg/L; T, 35 °C.

generated again by the reduction of Fe3+ in an electro-Fenton process:35 Fe3 + + e− → Fe2 +

E 0 = 0.77 V

(14)

Onsite electro-generation of H2O2 from dissolved oxygen and regeneration of Fe2+ by the electro-reduction of Fe3+ demonstrate the superiority of the Fe3O4−electro-Fenton process. 3.3. Investigation into the Kinetics of RB19 Degradation. A series of comparative experiments under different initial reaction conditions such as pH, current density, amount of added Fe3O4 MNPs, initial RB19 concentration, and temperature (Figures 5 and 6) were performed to investigate the kinetics of RB19 degradation. A model at pH 2.8 has been established by Zeng et al., based on the assumption that the heterogeneous Fenton reaction is negligible under acidic conditions.16 The model was deduced from a second-order kinetics equation. However, an induction period could be observed under certain operating conditions in our experiments, as shown in Figure 5. In particular, the induction period was greatly prolonged at higher pH and lower temperature. Consequently, our experimental system for RB19 degradation had a slow induction period followed by a rapid degradation stage, similar to the heterogeneous Fenton-like system.17−19 The small rate constant of the first stage could be ascribed to heterogeneous reactions that occur on the surface. As the concentration of dissolved iron(II) increases, homogeneous Fenton reaction in the bulk may dominate RB19 degradation in the second stage. These results indicate that the dissolution of Fe3O4 MNPs plays a key role in the Fe3O4− electro-Fenton degradation of RB19. In addition, according to the model by Zeng et al.,16 the rate of RB19 degradation may be expressed as −d[TOC]/dt = [TOC]t /(a + bt )

H 2O2 + H+ → H3O2+

(16)

Moreover, the scavenging effect of hydroxyl radicals by hydrogen ions becomes significant at very low pH.37 Low pH promotes a side reaction that evolves hydrogen (eq 17), reducing the number of active sites for generating hydrogen peroxide.2 H 2O2 + 2H+ + 2e− → 2H 2O

E 0 = 1.77 V

(17)

It is noteworthy that when the initial pH ranged from 3.0 to 7.0, the solution pH rapidly dropped to ∼3.0 after 15 min reaction (Figure 7). Even though the initial pH of the electrolyte was 2.0, the solution pH slowly approached 3.0. As concluded above, the best pH during the Fe3O4−electroFenton degradation of RB19 was determined to be 3.0. Accordingly, the Fe3O4−electro-Fenton system favored rapid change of the initial pH to the optimal value of 3.0 under the experimental conditions, regardless of the initial pH. This is because our experiments were done in an undivided reactor, which allowed H+ and OH− to counteract. The reaction maintained the pH at 2.0−4.0, thereby favoring the electroFenton reaction.10 The rapid self-adjustment of the solution pH to a suitable value may be one of the advantages of the Fe3O4− electro-Fenton system over other Fe3O4−Fenton-like reactions. 3.3.2. Effect of Current Density. In an electro-Fenton system, H2O2 is generated in situ by O2 reduction on the cathode, which is dependent on the applied current. The influence of current density on dye oxidation over 1.0 g/L Fe3O4 MNPs at pH 3.0 and 1.0−6.0 mA/cm2 was investigated (results are reported in Figure 6). The induction period was shortened from about 60 to 10 min and the rate constant of the second stage was increased from 0.00822 to 0.01615 min−1 when the applied current density was increased from 1.0 to 3.0 mA/cm2. Further increase in the applied current density to 6.0 mA/cm2 had no apparent effect on the induction period but

(15)

where [TOC] represents the TOC concentration (mg/L), t is the reaction time (min), and a and b are constants. According to eq 15, if the reaction time is long enough, then the model may be simplified to a rate equation for a first-order reaction. As •OH is a very reactive species and does not accumulate in solution, we can assume that the •OH concentration is at steady state during the reaction. To simplify the description of the reaction kinetics in all runs, it is reasonable to use a pseudo-first-order equation with respect to 3442



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Table 1. Intermediate Compounds Identified by GC/MS and IC: pH, 3.0; J, 3.0 mA/cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35°C

of RB19. As a result, the percentage of TOC removal dropped when the applied current density exceeded 3.0 mA/cm2. In addition, the undesirable side reaction of eq 17 is initiated at the cathode because a higher voltage should be supplied to the system to reach a larger current density.36 Furthermore, at high electrode potential, competitive electrode reactions occur, including discharge of oxygen at the anode (eq 8) and evolution of hydrogen at the cathode (eq 18), inhibiting main reactions such as those described in eqs 14 and 19.38 Hence, more electricity was wasted at applied current densities greater than 3.0 mA/cm2.

decreased the rate constant of the second stage from 0.01615 to 0.01142 min−1. The best current density for RB19 degradation was found to be 3.0 mA/cm2. In the cases wherein current densities less than 3.0 mA/cm2 were applied, higher oxidation rates at higher current could be observed. These could be ascribed to the increased rates of hydrogen peroxide production and of electroregeneration of ferrous ion from ferric ion (eq 14), which lead to the generation of higher amounts of •OH from Fenton chain reactions.36−38 However, excess H2O2 could also act as •OH scavenger, reacting with •OH to form hydroperoxyl radicals (HOO•) (eq 13).38 H2O2 could also be anodically oxidized to yield intermediate HOO• radicals (eq 11) and O2 (eq 12).33 Because HOO• has much lower oxidation potential than does •OH,39 this side reaction hinders the mineralization

2H+ + 2e− → H 2

E0 = 0 V

H 2O → •OH + H+ + e− 3443

(18) (19)



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FeOH+ + •OH → Fe3 + + 2OH−

3.3.4. Effect of Initial RB19 Concentration. The effect of varying initial RB19 concentration (100−300 mg/L) on TOC removal during Fe3O4−electro-Fenton oxidation was studied by using 1.0 g/L catalyst concentration and 3 mA/cm2 current density at pH 3.0 and 35 °C temperature. As seen in Figures 5 and 6, the percentage of RB19 removal gradually decreased in 180 min from 89.7 to 66.3% with the increase in RB19 concentration from 100 to 300 mg/L. Correspondingly, the induction time became slightly extended and the kinetic constants of the second stage decreased from 0.01615 to 0.006 min−1, as expected. Although the percentage removal of RB19 decreased with its initial concentration, the absolute amount of RB19 removed increased. For example, when the RB19 concentration was increased from 100 to 300 mg/L, the amount of TOC removed was increased from 24.5 to around 60.3 mg/L after 180 min of treatment. This behavior is one of the characteristics of advanced oxidation processes. Under the constant operating conditions of the electrolysis system, the yield of •OH radicals in the solution was constant. Therefore, the amount of hydroxyl radical was not enough to degrade the high concentration of pollutant and the removal efficiency thus decreased. In addition, the increased amount of pollutant might have occupied a greater number of iron active sites. In other words, less FeII/FeIII active sites were available for H2O2 decomposition at higher substrate concentrations, leading to a lower rate of •OH generation.18,41 3.3.5. Effect of Temperature. Temperature is one of the important parameters in the electro-Fenton reaction. In this study, the effect of temperature (15, 25, 35, and 45 °C) on the apparent rate constants for RB19 degradation was studied. As shown in Figure 6, the time to reach the second stage slightly decreased with the increase in temperature from 15 to 45 °C, suggesting that the dissolution of Fe3O4 MNPs in the first-stage was an endothermic process.17 Moreover, the mineralization rate constants of the second stage increased with temperature; the rate constants 0.0078, 0.00983, 0.01615, and 0.0216 min−1 were obtained at 15, 25, 35, and 45 °C, respectively. The relatively high temperature favors the Fe3O4−electroFenton reaction because higher reaction temperature can provide more energy for the reactant molecules to overcome the activation energy barrier.42 Accordingly, the kinetic constants for both radical production and Fe2+/Fe3+ regeneration exponentially increases with reaction temperature.43,44 For example, Millero and Sotolongo established a relationship between the rate constant of H2O2 decomposition and the reaction temperature. They demonstrated that an increase in temperature favors the decomposition of H2O2, thereby enhancing the generation of •OH.45 The apparent rate constants of the experiments performed at different temperatures allowed calculation of the apparent activation energy. Arrhenius plots46 of the two stages for the Fe3O4-catalyzed reactions are presented in Figure 8. From the slopes of the Arrhenius plots, the activation energies of the first and second stages were found to be 11.4 and 27.0 kJ/mol, respectively. Gordon and Marsh demonstrated that formation of hydroxyl radicals via surface-catalyzed hydrogen peroxide decomposition could contribute to the concomitant oxidation of 2-chlorophenol in the first stage of reaction.17 Therefore, we infer that the degradation of RB19 may be controlled either by the diffusion rate or by the chemical rate. In this investigation, the activation energy in the first stage was found to be 11.4 kJ/

Figure 10. The probable pathway of RB19 degradation by Fe3O4− electro-Fenton reaction. Experimental conditions: pH, 3.0; J, 3.0 mA/ cm2; catalyst dose, 1.0 g/L; C0, 100 mg/L; T, 35 °C.

3.3.3. Effect of Addition of Fe3O4 MNPs. The amount of Fe3O4 MNPs added is an important parameter in the study because of its main role in the Fenton reaction. Figure 6 displays the degradation kinetics of RB19 at different amounts of Fe3O4 MNPs added when the initial pH, current density, initial dye concentration, and temperature were 3.0, 3.0 mA/ cm2, 100 mg/L, and 35 °C, respectively. When the amount of catalyst used increased from 0.5 to 1.0 g/L, the induction time obviously shortened and the rate constants of the second stage increased from 0.0071 to 0.01615 min−1. This change is expected since the increase in the amount of Fe3O4 MNPs added increases the number of active sites on the catalyst surface, thereby accelerating H 2O 2 decomposition and increasing the amount of iron ions leaching into the solution, which enhances •OH production.40 However, upon further increase in the amount of catalyst from 1.0 to 1.5 g/L, the degree of degradation did not increase and instead slightly decreased with the decrease in second-stage rate constants from 0.01615 to 0.0154 min−1. The agglomeration of nanoparticles caused by the high surface energy and scavenging of hydroxyl radicals or other radicals due to the presence of excess iron species (eqs 6, 20−22) may be the reason for this decrease.27,39 Fe 2 + + •OH → Fe3 + + OH−

(20)

Fe 2 + + HOO• → Fe3 + + OOH−

(21)

(22)

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mol, which was within the range of 4−12 kJ/mol.47 This value reveals that RB19 degradation was a diffusion-controlled heterogeneous process under the selected conditions. The activation energy obtained for the second stage, 27.0 kJ/mol, is similar to the reported activation energy for a homogeneous Fenton process, which is in the range of 25.21−38.18 kJ/ mol.46,48−50 Therefore, the rate-limiting process of RB19 degradation in the Fe3O4−electro-Fenton system changed from a heterogeneous to a homogeneous reaction. 3.4. Variation in the Concentration of Related Ions and a Plausible Degradation Pathway. IC was employed to analyze quantitatively the main anions (oxalate, formate, acetate, nitrate, and sulfate) remaining in the electrolyte solution during 180 min of treatment. Unfortunately, oxalate, nitrate, and sulfate could not be determined accurately because Na2SO4 was used as supporting electrolyte. After 180 min of reaction, approximately 33.8% of the measured TOC was acetate, which accounted for the majority of the residual TOC in solution. Additionally, formate accounted for 7.0% of the residual TOC. Therefore, acetate was a relatively stable product that could not be efficiently oxidized. This finding agrees with our previous study wherein the azo dye C.I. Reactive Red 195 was degraded by anodic oxidation on Ti/SnO2−Sb/PbO2 electrodes.26 As shown in Figure 9, ammonium originating mainly from nitrogen in RB19 was detected in aqueous solution throughout the Fe3O4−electro-Fenton treatment of RB19. After a reaction time of 180 min, ∼27.0% of nitrogen was transformed to ammonia. The lower nitrogen content compared with that of the initial RB19 molecule may be ascribed to conversion of some of the nitrogen into NO2− and NO3−, which could not be monitored; furthermore, some of the nitrogen could have escaped gradually in the form of NH3, N2, NO, or NO2.51 The existence of some intermediates was further confirmed by GC/MS (Table 1). On the basis of our results and the assumption that •OH radicals are the major reactive species, a substitution reaction mechanism for the system was established. The postulated pathway for the degradation of RB19 during the Fe3O4−electro-Fenton process is presented in Figure 10. In the presence of •OH in aqueous solution, the C(9)−S and C(15)−N bonds are initially cleaved to form 1,4-diamino2-hydroxyanthra-9,10-quinone (D1) and resorcinol (S1). Cleavage of the C(9)−S and C(11)−N bonds seems to occur on the dye molecule concurrently with this degradation and leads to formation of 1-amino-2,4-dihydroxyanthra-9,10quinone (S2) and 3-aminophenol (S3). Pyrocatechol (D3) and hydroquinone (D4) are subsequently derived from the rearrangement of S1. D1 and S2 could undergo further oxidation by active radicals, yielding phthalic acid (D6), salicylaldehyde (D7), and salicylic acid (D8) by breakdown of the aromatic ring. Furthermore, rearrangement and further oxidation of S3 results in the generation of 2-aminophenol (D2) and 2-nitrosophenol (D5). Finally, The aromatic ring of D3, D4, D5, D6, and D8 may open to give carboxylic acids such as (2Z)-but-2-enedioic acid (D9), acetic acid (D10), formic acid (D11), and oxalic acid (S4) because of the nonselectivity of •OH.

Fe3O4 MNPs had higher degradation capacity than did the commercial Fe3O4 and Fe2+-catalyzed electro-Fenton process. Removal of the TOC of an aqueous solution of RB19 depended on the initial pH of solution, current density, catalyst loading, initial concentration of target pollutant, and electrolysis temperature. Optimal operating conditions in the investigated range were initial pH of 3.0, current density of 3.0 mA/cm2, 1.0 g/L concentration of added Fe3O4 MNPs, initial RB19 concentration of 100 mg/L, and reaction temperature of 35 °C. Under these conditions, >89.7% of TOC was removed after 180 min of treatment of an aqueous solution containing RB19. It was confirmed that RB19 degradation followed first-order kinetics, and that it could be divided into a slow induction period and a subsequent stage of rapid degradation. GC/MS and IC were employed to identify the oxidation byproducts of RB19 degradation by the Fe3O4−electro-Fenton process, and a probable mechanism for RB19 degradation by •OH was proposed. The results indicate that the cathodic electro-Fenton process using Fe3O4 MNPs as catalyst is a viable technique for treatment of dye-containing wastewater and has potential for commercialization in the future.

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AUTHOR INFORMATION

Corresponding Author

* Tel.: 86-571-88320726. Fax: 86-571-88320276. E-mail: ss@ zjut.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT13096), the National Natural Science Foundation of China (Grants 20977086, 21076196, 21177115 and 21207028), and the Zhejiang Provincial Natural Science Foundation of China (Grant 13B070002).

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REFERENCES

(1) Nidheesh, P. V.; Gandhimathi, R.; Ramesh, S. T. Degradation of dyes from aqueous solution by Fenton processes: A review. Environ. Sci. Pollut. Res. 2013, 20, 2099−2132. (2) Nidheesh, P. V.; Gandhimathi, R. Trends in electro-Fenton process for water and wastewater treatment: An overview. Desalination 2012, 299, 1−15. (3) He, F.; Hu, W. R.; Li, Y. Z. Biodegradation mechanisms and kinetics of azo dye 4BS by a microbial consortium. Chemosphere 2004, 57, 293−301. (4) Rodríguez, A.; Ovejero, G.; Sotelo, J. L.; Mestanza, M.; García, J. Heterogeneous Fenton catalyst supports screening for mono azo dye degradation in contaminated wastewaters. Ind. Eng. Chem. Res. 2010, 49, 498−505. (5) He, Y.; Li, G. M.; Jiang, Z. W.; Wang, H.; Zhao, J. F.; Su, H. X.; Huang, Q. Y. Diafiltration and concentration of Reactive Brilliant Blue KN-R solution by two-stage ultrafiltration process at pilot scale: Technical and economic feasibility. Desalination 2011, 279, 235−242. (6) Zhang, J. B.; Zhuang, J.; Gao, L. Z.; Zhang, Y.; Gu, N.; Feng, J.; Yang, D. L.; Zhu, J. D.; Yan, X. Y. Decomposing phenol by the hidden talent of ferromagnetic nanoparticles. Chemosphere 2008, 73, 1524− 1528. (7) Guo, L. Q.; Chen, F.; Fan, X. Q.; Cai, W. D.; Zhang, J. L. S-doped α-Fe2O3 as a highly active heterogeneous Fenton-like catalyst towards the degradation of acid orange 7 and phenol. Appl. Catal., B 2010, 96, 162−168. (8) Ortiz de la Plata, G. B.; Alfano, O. M.; Cassano, A. E. Decomposition of 2-chlorophenol employing goethite as Fenton

4. CONCLUSIONS This study focused on the degradation of RB19 dye in aqueous solution through the Fe3O4−electro-Fenton process using ACFF as cathode and platinum as anode. We have demonstrated that the electro-Fenton process catalyzed by 3445



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catalyst. I. Proposal of a feasible, combined reaction scheme of heterogeneous and homogeneous reactions. Appl. Catal., B 2010, 95, 1−13. (9) Sun, S. H.; Zeng, H. Size-controlled synthesis of magnetite nanoparticles. J. Am. Chem. Soc. 2002, 124, 8204−8205. (10) Liu, H.; Wang, C.; Li, X. Z.; Xuan, X. L.; Jiang, C. C.; Cui, H. N. A novel electro-Fenton process for water treatment: Reactioncontrolled pH adjustment and performance assessment. Environ. Sci. Technol. 2007, 41, 2937−2942. (11) Luo, M. S.; Yuan, S. H.; Tong, M.; Liao, P.; Xie, W. J.; Xu, X. F. An integrated catalyst of Pd supported on magnetic Fe 3 O 4 nanoparticles: Simultaneous production of H2O2 and Fe2+ for efficient electro-Fenton degradation of organic contaminants. Water Res. 2014, 48, 190−199. (12) Sanchez-Sanchez, C. M.; Exposito, E.; Casado, J.; Montiel, V. Goethite as a more effective iron dosage source for mineralization of organic pollutants by electro-Fenton process. Electrochem. Commun. 2007, 9, 19−24. (13) Wu, J. F.; Pu, W. H.; Yang, C. Z.; Zhang, M.; Zhang, J. D. Removal of benzotriazole by heterogeneous photoelectro-Fenton like process using ZnFe2O4 nanoparticles as catalyst. J. Environ. Sci. 2013, 25, 801−807. (14) Wang, C. T.; Hua, Y. J.; Tong, Y. X. A novel Electro-FentonLike system using PW11O39Fe(III)(H2O)4− as an electrocatalyst for wastewater treatment. Electrochim. Acta 2010, 55, 6755−6760. (15) Iglesias, O.; Gómez, J.; Pazos, M.; Sanromán, M.Á . ElectroFenton oxidation of imidacloprid by Fe alginate gel beads. Appl. Catal., B 2014, 144, 416−424. (16) Zeng, X.; Hanna, K.; Lemley, A. T. Cathodic Fenton degradation of 4,6-dinitro-o-cresol with nano-magnetite. J. Mol. Catal. A: Chem. 2011, 339, 1−7. (17) Gordon, T. R.; Marsh, A. L. Temperature dependence of the oxidation of 2-chlorophenol by hydrogen peroxide in the presence of goethite. Catal. Lett. 2009, 132, 349−354. (18) Zhou, T.; Li, Y. Z.; Ji, J.; Wong, F. S.; Lu, X. H. Oxidation of 4chlorophenol in a heterogeneous zero valent iron/H2O2 Fenton-like system: Kinetic, pathway and effect factors. Sep. Purif. Technol. 2008, 62, 551−558. (19) Luo, M. L.; Bowden, D.; Brimblecombe, P. Catalytic property of Fe-Al pillared clay for Fenton oxidation of phenol by H2O2. Appl. Catal., B 2009, 85, 201−206. (20) Massart, R. Preparation of aqueous magnetic liquids in alkaline and acidic media. IEEE Trans. Magn. 1981, 17, 1247−1248. (21) Recillas, S.; García, A.; González, E.; Casals, E.; Puntes, V.; Sánchez, A.; Font, X. Use of CeO2, TiO2 and Fe3O4 nanoparticles for the removal of lead from water Toxicity of nanoparticles and derived compounds. Desalination 2011, 277, 213−220. (22) Cappus, D.; Hassel, M.; Neuhaus, E.; Heber, M.; Rohr, F.; Freund, H. J. Polar surfaces of oxides: Reactivity and reconstruction. Surf. Sci. 1995, 337, 268−277. (23) Lu, L. R.; Ai, Z. H.; Li, J. P.; Zheng, Z.; Li, Q.; Zhang, L. Z. Synthesis and characterization of Fe-Fe2O3 core−shell nanowires and nanonecklaces. Cryst. Growth Des. 2007, 7, 459−464. (24) Xuan, S. H.; Wang, Y. X. J.; Yu, J. C.; Leung, K. C. F. Tuning the grain size and particle size of superparamagnetic Fe3O4 microparticles. Chem. Mater. 2009, 21, 5079−5087. (25) Ge, J. P.; Hu, Y. X.; Biasini, M.; Beyermann, W. P.; Yin, Y. D. Superparamagnetic magnetite colloidal nanocrystal clusters. Angew. Chem., Int. Ed. 2007, 46, 4342−4345. (26) Song, S.; Fan, J. Q.; He, Z. Q.; Zhan, L. Y.; Liu, Z. W.; Chen, J. M.; Xu, X. H. Electrochemical degradation of azo dye C.I. Reactive Red 195 by anodic oxidation on Ti/SnO2-Sb/PbO2 electrodes. Electrochim. Acta 2010, 55, 3606−3613. (27) Xu, L. J.; Wang, J. L. Fenton-like degradation of 2,4dichlorophenol using Fe3O4 magnetic nanoparticles. Appl. Catal., B 2012, 123−124, 117−126. (28) Expósito, E.; Sánchez-Sánchez, C. M.; Montiel, V. Mineral iron oxides as iron source in electro-Fenton and photoelectro-Fenton mineralization processes. J. Electrochem. Soc. 2007, 154, E116−E122.

(29) Comninellis, C. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Electrochim. Acta 1994, 39, 1857−1862. (30) Haber, F.; Weiss, J. The Catalytic Decomposition of hydrogen peroxide by iron salts. Proc. R. Soc. A-Math. Phys. Eng. Sci. 1934, 147, 332−351. (31) Lin, S. S.; Gurol, M. D. Catalytic decomposition of hydrogen peroxide on iron oxide: Kinetics, mechanism, and implications. Environ. Sci. Technol. 1998, 32, 1417−1423. (32) Chu, Y. Y.; Qian, Y.; Wang, W. J.; Deng, X. L. A dual-cathode electro-Fenton oxidation coupled with anodic oxidation system used for 4-nitrophenol degradation. J. Hazard. Mater. 2012, 199−200, 179− 185. (33) Wang, A. M.; Qu, J. H.; Ru, J.; Liu, H. J.; Ge, J. T. Mineralization of an azo dye Acid Red 14 by electro-Fenton’s reagent using an activated carbon fiber cathode. Dyes Pigm. 2005, 65, 227−233. (34) Qiang, Z. M.; Chang, J. H.; Huang, C. P. Electrochemical regeneration of Fe2+ in Fenton oxidation processes. Water Res. 2003, 37, 1308−1319. (35) Brillas, E.; Calpe, J. C.; Casado, J. Mineralization of 2,4-D by advanced electrochemical oxidation processes. Water Res. 2000, 34, 2253−2262. (36) Wang, C. T.; Chou, W. L.; Chung, M. H.; Kuo, Y. M. COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode. Desalination 2010, 253, 129− 134. (37) Masomboon, N.; Ratanatamskul, C.; Lu, M. C. Chemical oxidation of 2,6-dimethylaniline by electrochemically generated Fenton’s reagent. J. Hazard. Mater. 2010, 176, 92−98. (38) Zhang, H.; Fei, C. Z.; Zhang, D. B.; Tang, F. Degradation of 4nitrophenol in aqueous medium by electro-Fenton method. J. Hazard. Mater. 2007, 145, 227−232. (39) Ramirez, J. H.; Maldonado-Hódar, F. J.; Pérez-Cadenas, A. F.; Moreno-Castilla, C.; Costa, C. A.; Madeira, L. M. Azo-dye Orange II degradation by heterogeneous Fenton-like reaction using carbon−Fe catalysts. Appl. Catal., B 2007, 75, 312−323. (40) Hassan, H.; Hameed, B. H. Fe-clay as effective heterogeneous Fenton catalyst for the decolorization of Reactive Blue 4. Chem. Eng. J. 2011, 171, 912−918. (41) Nguyen, T. D.; Phan, N. H.; Do, M. H.; Ngo, K. T. Magnetic Fe2MO4 (M: Fe, Mn) activated carbons: Fabrication, characterization and heterogeneous Fenton oxidation of methyl orange. J. Hazard. Mater. 2011, 185, 653−661. (42) Xu, H. Y.; Prasad, M.; Liu, Y. Schorl: A novel catalyst in mineralcatalyzed Fenton-like system for dyeing wastewater discoloration. J. Hazard. Mater. 2009, 165, 1186−1192. (43) Liao, Q.; Sun, J.; Gao, L. Degradation of phenol by heterogeneous Fenton reaction using multi-walled carbon nanotube supported Fe2O3 catalysts. Colloid Surf. A 2009, 345, 95−100. (44) Sun, J. H.; Sun, S. P.; Wang, G. L.; Qiao, L. P. Degradation of azo dye Amido black 10B in aqueous solution by Fenton oxidation process. Dyes Pigm. 2007, 74, 647−652. (45) Millero, F. J.; Sotolongo, S. The oxidation of Fe(II) with H2O2 in seawater. Geochim. Cosmochim. Acta 1989, 53, 1867−1873. (46) Sun, S. P.; Li, C. J.; Sun, J. H.; Shi, S. H.; Fan, M. H.; Zhou, Q. Decolorization of an azo dye Orange G in aqueous solution by Fenton oxidation process: Effect of system parameters and kinetic study. J. Hazard. Mater. 2009, 161, 1052−1057. (47) Awe, S. A.; Samuelsson, C.; Sandström, Å. Dissolution kinetics of tetrahedrite mineral in alkaline sulphide media. Hydrometallurgy 2010, 103, 167−172. (48) Karthikeyan, S.; Titus, A.; Gnanamani, A.; Mandal, A. B.; Sekaran, G. Treatment of textile wastewater by homogeneous and heterogeneous Fenton oxidation processes. Desalination 2011, 281, 438−445. (49) Xu, H.; Zhang, D. X.; Xu, W. G. Monitoring of decolorization kinetics of Reactive Brilliant Blue X-BR by online spectrophotometric method in Fenton oxidation process. J. Hazard. Mater. 2008, 158, 445−453. 3446



Article

(50) Lucas, M. S.; Peres, J. A. Removal of COD from olive mill wastewater by Fenton’s reagent: Kinetic study. J. Hazard. Mater. 2009, 168, 1253−1259. (51) Hao, O. J.; Phull, K. K. Wet oxidation of nitrotoluenesulfonic acid: Some intermediates, reaction pathways, and byproduct toxicity. Environ. Sci. Technol. 1993, 27, 1650−1658.

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Electro-Fenton Process Catalyzed by Fe3O4 Magnetic Nanoparticles

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