Design of a Highly Efficient and Wide pH Electro-Fenton Oxidation

Article pubs.acs.org/est

Design of a Highly Efficient and Wide pH Electro-Fenton Oxidation System with Molecular Oxygen Activated by Ferrous− Tetrapolyphosphate Complex Li Wang, Menghua Cao, Zhihui Ai, and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental Chemistry, Central China Normal University, Wuhan 430079, P. R. China S Supporting Information *

ABSTRACT: In this study, a novel electro-Fenton (EF) system was developed with iron wire, activated carbon fiber, and sodium tetrapolyphosphate (Na6TPP) as the anode, cathode, and electrolyte, respectively. This Na6TPP−EF system could efficiently degrade atrazine in a wide pH range of 4.0−10.2. The utilization of Na6TPP instead of Na2SO4 as the electrolyte enhanced the atrazine degradation rate by 130 times at an initial pH of 8.0. This dramatic enhancement was attributed to the formation of ferrous− tetrapolyphosphate (Fe(II)−TPP) complex from the electrochemical corrosion (ECC) and chemical corrosion (CC) of iron electrode in the presence of Na6TPP. The Fe(II)−TPP complex could provide an additional molecular oxygen activation pathway to produce more H2O2 and •OH via a series single-electron transfer processes, producing the Fe(III)−TPP complex. The cycle of Fe(II)/Fe(III) was easily realized through the electrochemical reduction (ECR) process on the cathode. More interestingly, we found that the presence of Na6TPP could prevent the iron electrode from excessive corrosion via phosphorization in the later stage of the Na6TPP−EF process, avoiding the generation of iron sludge. Gas chromatograph-mass spectrometry, liquid chromatography-mass spectrometry, and ion chromatography were used to investigate the degradation intermediates to propose a possible atrazine oxidation pathway in the Na6TPP−EF system. These interesting findings provide some new insight on the development of a low-cost and highly efficient EF system for wastewater treatment in a wide pH range.

■

enhanced the organic pollutant degradation efficiency.15 Obviously, the high cost of Pd and the environmental risk of Ni restrict their further application for wastewater treatment. Therefore, it is still a challenge to develop low-cost and highly efficient EF oxidation systems. It is well known that the complexes of ferrous ions with suitable ligands can activate molecular oxygen to produce H2O2 and •OH.16−19 For example, Aust et al. systematically investigated the effect of ligands on the performance of Fe(II) to activate molecular oxygen.17 They found that all complexes of Fe(II) with ethylenediamine tetraacetate (EDTA), nitrilotriacetic acid, citric acid, and oxalic acid could significantly promote terephthalate hydroxylation during the iron autooxidation process, suggesting that more H2O2 and •OH were produced in the presence of these ligands. In our previous study, tetrapolyphosphate (TPP) was found to be a superior ligand to coordinate with Fe(II) to generate more H2O2 and • OH.20,21 Therefore, coupling of the molecular oxygen

INTRODUCTION The electro-Fenton (EF) process has been widely used to treat wastewater-containing dyes, herbicides, antibiotics, and landfill leachate because of its convenience and strong oxidation ability.1−4 In the EF process, Fe(II) is commonly obtained by the addition of ferrous salts, the reduction of Fe(III), or the oxidation of a sacrificial iron anode,5−9 while H2O2 is in situ generated via the electrochemical reduction of O2 on the cathode, which can avoid the long distance transportation of H2O2. The commonly used cathodes include graphite, activated carbon fiber (ACF), carbon sponge, and so on.1,7,10 However, these cathodes could only produce H2O2 by the two-electron reduction of molecular oxygen.11 This single H2O2 generation way would restrict the efficiency of the EF process. In recent years, the generation of H2O2 through the O2 reduction catalyzed by noble or transition metals offered a promising pathway to enhance the efficiency of EF process. For instance, Yuan et al. developed a new Pd-based EF process to produce H2O2 via the reaction of electrogenerated H2 and O2 on the Pd catalyst.12−14 Our group designed a 3D-EF system with foam Ni particle electrodes and found these Ni particles could generate more H2O2 through the sequential one-electron reduction processes (O2 → •O2−→ H2O2), which thus greatly © XXXX American Chemical Society

Received: December 9, 2014 Revised: January 26, 2015 Accepted: January 29, 2015

A

DOI: 10.1021/es505984y Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

the atrazine solution (10 mg L−1) overnight to reach the saturated adsorption for use. A 25 mL amount of atrazine solution (10 mg L−1) was added into the cell. The initial pH value of atrazine solution was 6.8. A 125 μL amount of Na6TPP stock solution was added to obtain an initial Na6TPP concentration of 0.5 mM as the supporting electrolyte. After the addition of Na6TPP, the pH value of the solution increased from 6.8 to 8.0. A constant current of 0.5 mA was applied with an initial cell potential of about 0.6 V. Air was fed to the cathode at a rate of 40 mL min−1 during the degradation. About 1 mL of aqueous solution was taken out at predetermined time intervals for the analysis of atrazine and dissolved iron concentrations. H2SO4 or NaOH (0.5 mol L−1) was used to adjust the pH of solutions to study the pH influence on the atrazine removal efficiency. For comparison, control experiments were carried out using 0.5 mM Na2SO4 as the electrolyte at the same initial pH values. A separated dual-cell electrochemical system was employed to evaluate the individual contribution of molecular oxygen activation by Fe(II)−TPP complex on the atrazine degradation (Na6TPP−EC system, Figure S1b, Supporting Information). Iron wire and ACF were used as the anode and cathode, respectively. The anodic cell was separated from the cathodic cell with an ion exchange membrane. A constant current of 0.5 mA was applied. The solution in the anodic compartment consisted of 10 mg·L−1 atrazine and 0.5 mmol·L−1 Na6TPP at an initial pH of 8.0, and the solution in the cathodic compartment consisted of 0.5 mmol·L−1 Na6TPP. Similar dual-cell electrochemical systems were also employed to evaluate the regeneration of Fe(II) by Fe(III) reduction on the ACF cathode and its contribution to the atrazine degradation (Figure S1c and S1d, Supporting Information). Pt and ACF were used as the anode and cathode, respectively. A constant working voltage of −0.7 V was applied. The solution in the cathodic compartment consisted of 10 mg L−1 atrazine, 0.3 mM Fe(III), and 0.5 mM Na6TPP with an initial pH of 8.0, and the solution in the anodic compartment consisted of 0.5 mM Na6TPP. Analytical Methods. The concentrations of Fe(II) and total dissolved iron were measured by using the 1,10phenanthroline colorimetric method with a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan).24 The hydrogen peroxide concentrations in the absence of iron electrode were determined by the triiodide method involving the oxidation of iodide to the triiodide anion (λmax = 352 nm) by peroxides catalyzed by ammonium molybdate,25 while the concentration of H2O2 in the presence of iron electrode was detected with the p-hydroxyphenylacetic acid (POHPAA) method.26 The detailed detection procedures were provided in the Supporting Information. Benzoic acid was used as a probe to quantify the generation of hydroxyl radical in the electrochemical systems.27 Atrazine and its degradation intermediates were detected by high-pressure liquid chromatography (HPLC, Ultimate 3000, Thermo) equipped with an Agilent TC-C18 column (150 mm × 4.6 mm, 5 μm). The mobile phase was a mixture of acetonitrile and water (50:50, v/v) with a flow speed of 1 mL min −1 . The detection wavelength was 220 nm. Gas chromatography-mass spectrometry (GC-MS, Trace 1300ISQ, Thermo) equipped with a capillary column (HP-5MS, 30 m × 0.25 mm × 0.25 μm) and liquid chromatography-mass spectrometry (LC-MS, TSQ Quantum Access MAX, Thermo) with a Hypersil ODS-C18 column (5 μm × 150 mm × 2.1 mm) were also used to identify the atrazine degradation

activation process induced by Fe(II)−TPP complex with the EF system might be a desirable way to develop a highly efficient EF system via in situ generating more H2O2. Besides the yield of H2O2, the efficiency of the EF system was also related to the regeneration of Fe(II). It was reported that the electroregeneration of Fe(II) was feasible below the pH value of 2.5.22 This was because the precipitation of Fe(III) at a pH higher than 2.5 would lower the dissolved Fe(III) concentration and also inhibit the Fe(II) regeneration by partially coating the electrode surface with iron hydroxide. Obviously, the low-pH operation requirement limits the practical application of the traditional EF oxidation system. It is known that the addition of TPP can prevent the precipitation of Fe(III) at a neutral or even weak basic pH range, broadening the application range of EF. Moreover, the utilization of TPP can avoid the undesirable reactive oxygen species consumption and molecular oxygen activation efficiency decline caused by the simultaneous decomposition of organic additives along with target pollutants.20,21,23 Nevertheless, the chelation of TPP with Fe(III) would decrease the redox potential of Fe(III)/Fe(II) and thus disfavor the electroregeneration of Fe(II) at neutral pH. Therefore, it is of great importance to investigate the possibility of TPP for the development of a highly efficient EF system working at a wide pH range, especially higher than 2.5. In this study, we design an EF system with iron wire, activated carbon fiber, and Na6TPP as the anode, cathode, and electrolyte, respectively. In this Na6TPP−EF system, ferrous ions are produced via the electrochemical and chemical corrosion of iron electrode. Subsequently, the complex of ferrous ions with TPP can activate molecular oxygen to produce H2O2 via the single-electron reduction pathway, besides the H2O2 generation by the two-electron reduction of molecular oxygen on the activated carbon fiber. We evaluate the performance of this novel EF system on the oxidative degradation of atrazine. Gas chromatography-mass spectrometry, liquid chromatography-mass spectrometry, and ion chromatography are used to detect the degradation intermediates for us to propose a possible atrazine oxidation pathway in the Na6TPP−EF system. We also systematically investigate the generation of hydrogen peroxide and hydroxyl radical as well as the regeneration of ferrous ions.

2. EXPERIMENTAL SECTION Materials. Atrazine, Na2SO4, tetrapolyphosphate acid, NaOH, and isopropanol were purchased from Sinopharm Chemical Reagent Co., Ltd. and all of analytical grade. Superoxide dismutase (SOD) and catalase (CAT) were bought from Shanghai Kayon Biological Technology Co., Ltd. All of these chemicals were used as received without further purification. Activated carbon fiber (ACF) was obtained from China Southern Chemical Import and Export Co., Ltd. Iron wire of 2 mm in diameter was purchased from Zhengyi Metal Co., Dongguan, China. Na6TPP stock solution with a concentration of 100 mM was prepared by dissolving tetrapolyphosphoric acid in water and then adjusted to pH 7.0 ± 0.1 with NaOH solution. Experimental Setup. The EF degradation of atrazine was carried out in a single electrolytic cell. An iron wire (8 cm) and a piece of ACF (1 cm × 2 cm) were used as the anode and cathode, respectively (Figure S1a, Supporting Information). Prior to the experiment, the iron wire was washed with 0.1 mol L−1 HCl and deionized water in sequence to remove the oxide on the surface. The ACF cathode was soaked under stirring in B


Article


Figure 1. Degradation of atrazine (a) and plots of ln(C/C0) versus time for the atrazine degradation (b) in different electrochemical systems with 0.5 mM Na6TPP or Na2SO4 as the electrolyte under a constant current of 0.5 mA. Initial concentration of atrazine was 10 mg L−1. Initial pH values were 8.0, and the final pH values of Na2SO4−EF, Na6TPP−EF, and Na6TPP−EF were 8.0, 7.0, and 7.0, respectively.

Supporting Information), but only 63% of atrazine was degraded at pH 3.0 with a degradation rate constant of 0.0165 min−1, which was slightly lower than that of Na6TPP− EF system at pH 3.0 but much lower than that of the Na6TPP− EF system at pH 8.0. These results suggested that there were other factors accounting for the enhanced atrazine removal efficiency of the Na6TPP−EF system besides iron precipitation prevention. As Fe(II)−TPP complex can activate molecular oxygen via a consecutive one-electron transfer process to produce hydroxyl radicals to oxidize organic pollutants;20 we therefore investigated the individual contribution of Fe(II)−TPP complexinduced molecular oxygen activation to the atrazine degradation in the Na6TPP−EF process by employing a separated dualcell electrochemical (Na6TPP−EC) system. Figure 1a reveals that 60% of atrazine degradation in the anodic compartment could be ascribed to the contribution of molecular oxygen activation by Fe(II)−TPP complex formed via the corrosion of iron anode. The pH value variations in the different systems were monitored during the reaction. The pH value of the Na6TPP−EF system remained unchanged thoroughly, but the pH values of Na2SO4−EF and Na6TPP−EC systems decreased slightly from 8.0 to 7.0 (Figure S6, Supporting Information). Moreover, we found that the sum of the atrazine degradation rate constants of Na6TPP−EC and Na2SO4−EF systems at pH 3.0 was still significantly lower than the rate constant (0.0645 min−1) of the Na6TPP−EF system at pH 8.0, further revealing the high efficiency of the Na6TPP−EF system coupled with Fe(II)−TPP-induced molecular oxygen activation process. Even in the absence of applied current (0 mA), 32% of atrazine was degraded, indicative of the chemical corrosion of iron electrode by TPP and the contribution of molecular oxygen activation induced by Fe(II)−TPP to the atrazine degradation (Figure S7, Supporting Information). In order to clarify the roles of Na6TPP on the atrazine degradation in the Na6TPP−EF system, we investigated the generation of reactive oxygen species (ROS) in the Na6TPP− EF system by employing excess scavengers (SOD for •O2−, CAT for H2O2, and isopropanol for •OH, Figure S8, Supporting Information). The atrazine degradation rates significantly decreased with adding SOD and CAT, revealing the involvement of • O 2 − and H 2 O 2 in the atrazine degradation.29 It is well known that the electrogenerated H2O2 on the cathode follows a two-electron dioxygen reduction pathway (O2 → H2O2),11 so the generation of •O2− in the Na6TPP−EF system was attributed to the one-electron

intermediates. The pretreatment process of the reaction mixture and the detailed information about the identification of degradation products by GC-MS and LC-MS are provided in the Supporting Information. The carboxylic acid and chlorine ions were detected by ion chromatography (Dionex ICS-900, Thermo). Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis were performed on a JEOL JSM6510 scanning electron microscope to characterize the microstructures and the composition of iron electrode. Highresolution X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos ASIS-HS X-ray photoelectron spectroscope to further identify the composition of iron electrode before and after use. The binding energies obtained in the XPS analysis were corrected by the C 1s peak at 284.6 eV of the surface adventitious carbon.

3. RESULTS AND DISCUSSION Figure 1a shows the degradation curves of atrazine by different electrochemical oxidation processes under the conditions of 0.5 mA, 10 mg L−1 atrazine, and pH 8.0. We found that 97% of atrazine was degraded in the EF system using 0.5 mM Na6TPP as the electrolyte (Na6TPP−EF) within 60 min, while the atrazine degradation rate significantly increased with improving the Na6TPP concentration from 0.5 to 2 mM (Figure S2, Supporting Information). However, atrazine was not degraded in the traditional EF system using iron wires as the anode, ACF as the cathode, and 0.5 mM Na2SO4 as the electrolyte (Na2SO4−EF) within 60 min. Even with prolonging the reaction time to 480 min, only 14% of atrazine was removed in the Na2SO4−EF system at pH 8.0 (Figure S3, Supporting Information). The atrazine degradation processes were found to obey pseudo-first-order kinetics. The degradation rate constant (0.0645 min−1) of the Na6TPP−EF process within 60 min was 130 times that (0.000497 min−1) of the Na2SO4− EF system within 480 min at pH 8.0 (Figure 1b and Figure S3, Supporting Information). These results suggested that the Na6TPP−EF system was of high efficiency because TPP served as the electrolyte and might also participate in the electrochemical reaction.28 Interestingly, iron sludge did not appear in the Na6TPP−EF system but formed in the Na2SO4−EF system (Figure S4, Supporting Information). This difference revealed that TPP could complex with Fe(II) to avoid the iron sludge formation. As for the Na2SO4−EF system, the initial pH value of 3.0 was required to prevent iron precipitation (Figure S5, C


Article

Environmental Science & Technology molecular oxygen activation route (O2 → •O2− → H2O2) induced by the Fe(II)−TPP complex. The inhibitory efficiency (η) of SOD on the degradation constant in the Na6TPP−EF system was calculated to be about 63%, suggesting that the single-electron reduction of molecular oxygen by Fe(II)−TPP complex mainly accounted for the atrazine degradation in the Na6TPP−EF system.30 We also found that the atrazine degradation was completely inhibited with the addition of isopropanol, suggesting that •OH was the predominant oxidant for the atrazine degradation.

Benzoic acid was then used to quantify the generated •OH (Figure 2). This was because benzoic acid could react with • OH to form o-, m-, and p-hydroxybenzoic acid in a ratio of 1.7:2.3:1.2. We detected p-hydroxybenzoic acid with HPLC and calculated its concentration. The total hydroxybenzoic acid concentration was then estimated from the ratio of phydroxybenzoic acid and regarded to be equal to the concentration of hydroxyl radicals. To ensure all •OH reacted with benzoic acid, benzoic acid of a high initial concentration (1000 mg L−1) was used. Within 60 min, the cumulative •OH in Na6TPP−EF, Na2SO4−EF, and Na6TPP−EC systems was, respectively, 0.40, 0.01, and 0.17 mM, in good agreement with their atrazine degradation trend. In view of the indispensable role of Fe(II) to the •OH generation in Fenton reactions, we monitored the variations of dissolved iron species in Na2SO4−EF, Na6TPP−EF, and Na6TPP−EC systems (Figure 3a). In the Na2SO4−EF system, the amount of dissolved iron ions was tiny (about 0.001 mM) owing to the fast transformation of Fe(II) to Fe(III) and the subsequent formation of ferric (oxy)hydroxide at pH 8.0. In contrast, the total dissolved iron concentrations gradually increased to 0.32 mM within 60 min in the Na6TPP−EF and Na6TPP−EC systems. Obviously, the presence of Na6TPP could efficiently prevent the precipitation of iron ions. Ferrous ions released from the iron electrode would first complex with TPP to produce Fe(II)−TPP complex, which could reduce molecular oxygen to generate • O 2 − or catalyze H 2 O 2 decomposition to form •OH, accompanying the rapid oxidation of Fe(II)−TPP into Fe(III)−TPP. The rapid oxidation rate of Fe(II)−TPP complex resulted in a much lower Fe(II) concentration than that of total dissolved iron. The

Figure 2. Generation of •OH reflected by the oxidation of 1000 mg L−1 benzoic acid in different electrochemical systems with 0.5 mM Na6TPP or Na2SO4 as the electrolyte under a constant current of 0.5 mA. Initial pH values were 8.0.

Figure 3. (a) Concentrations of the dissolved iron as a function of time in different electrochemical systems. (b) Accumulative concentration of electrogenerated H2O2 with different kinds of electrolyte in the absence of iron electrode. (c) Accumulative concentration of H2O2 in the different electrochemical systems in the presence of iron electrode. (d) Real-time concentration of H2O2 in the different electrochemical systems in the presence of iron electrode; 0.5 mM Na6TPP or Na2SO4 was used as the electrolyte. Constant current was 0.5 mA. Initial pH values were 8.0. D


Article


Figure 4. (a) Variations of the dissolved iron concentration in the cathodic cell under argon atmosphere with 0.5 mM Na6TPP as the electrolyte at a constant working voltage of −0.7 V. (b) Degradation of atrazine in the cathodic cell under aerobic condition with 0.5 mM Na6TPP as the electrolyte at a constant working voltage of −0.7 V. Initial concentration of atrazine was 10 mg L−1. Initial pH values were 8.0.

concentration (0.40 mM) of •OH at 60 min was about two times that (0.20 mM) of electrogenerated H2O2, 0.20 mM nonelectrogenerated H2O2 was generated via the Fe(II)−TPP complex induced molecular oxygen activation process, which would consume 0.40 mM Fe(II)−TPP complex. Meanwhile, the production of 0.40 mM •OH via the H2O2 decomposition would consume another 0.40 mM Fe(II)−TPP complex. Therefore, 0.80 mM Fe(II)−TPP complex was totally needed. This theoretical value was much more than the measured concentration (0.32 mM) of the cumulatively released Fe(II)− TPP complex. This difference might arise from the effective regeneration of Fe(II)−TPP complex during this Na6TPP−EF process at pH 8.0.

concentration of Fe(II) in the Na6TPP−EF system increased to a plateau at about 0.03 mM within 30 min and then decreased slightly. This variation trend of dissolved Fe(II) concentration suggested that the rate of Fe(II) released from the iron anode and regenerated on the cathode was faster than that of Fe(II) consumed by the molecular oxygen activation and the H2O2 decomposition in the early stage. Later the slower release of Fe(II) resulted in the slight Fe(II) concentration decrease. Besides Fe(II), the generation of H2O2 is also crucial for the generation of •OH to oxidize organic pollutants during the EF process. Therefore, we compared the generation of H2O2 in different systems. As it is difficult to quantify the real yield of hydrogen peroxide in the presence of Fe(II) because of the quick H2O2 decomposition catalyzed by Fe(II), the accumulative concentrations of electrogenerated H2O2 were measured by replacing the iron anode with a Pt plate electrode, which did not catalyze the direct decomposition of H2O2.14 It was found that the H2O2 concentration increased to approximately 200 μmol L−1 within 60 min for both the Na2SO4−EF and the Na6TPP−EF systems (Figure 3b), revealing that Na6TPP did not promote the electrogenerated H2O2 on the ACF cathode. The in situ POHPAA method was then used to detect the accumulative concentration of H2O2 in the presence of iron anode (Figure 3c). Interestingly, more H2O2 was detected in the Na6TPP−EF system than that of the Na2SO4−EF system even though more Fe(II) in the Na6TPP−EF system would result in the decomposition of more H2O2, confirming the great contribution of an additional H2O2 formation pathway via molecular oxygen activation by Fe(II)−TPP complex in the Na6TPP−EF system. As expected, the real-time concentrations of H2O2 in the presence of iron electrode approached zero in all electrochemical systems (Figure 3d), owing to the quick decomposition of H2O2 by Fe(II) released from the iron electrode no matter which kind of electrolyte was used. Subsequently, we compared the iron ions release and the generation of H2O2 and •OH in the Na6TPP−EF system. The total dissolved iron concentration was 0.32 mM at 60 min, which was slightly lower than the theoretical value (0.37 mM) calculated with Faraday’s law. This meant 0.32 mM Fe(II)− TPP complex was cumulatively released in the Na6TPP−EF system within 60 min. Theoretically, 2 mol of Fe(II)−TPP complex is required to generate 1 mol of H2O2 via the sequential one-electron reduction molecular oxygen activation process (eqs 1 and 2), and another mole of Fe(II)−TPP complex is consumed for the decomposition of 1 mol of H2O2 to produce 1 mol of •OH via Fenton reaction (eq 3). As the

Fe(II)−TPP + O2 → Fe(III)−TPP + •O2−

(1)

Fe(II)−TPP + •O2− + 2H 2O → Fe(III)−TPP + H 2O2 + 2OH−

(2)

Fe(II)−TPP + H 2O2 → Fe(III)−TPP + •OH + OH− (3)

To verify the regeneration of the Fe(II)−TPP complex, cyclic voltammetry was employed to determine the redox potential of Fe(III)/Fe(II) vs NHE in the presence of 0.5 mM Na6TPP as the electrolyte (Figure S9, Supporting Information). The redox potential of Fe(III)−TPP/Fe(II)−TPP was found to be −0.15 V, much more positive than the cathode potential (−0.7 V vs NHE), suggesting that Fe(III)−TPP complex could be reduced to Fe(II)−TPP on the cathode thermodynamically. The separated dual-cell electrochemical system was then employed to further evaluate the Fe(III)−TPP reduction on the ACF cathode under anaerobic conditions by bubbling ultrahigh pure argon gas at a constant working voltage of −0.7 V. As shown in Figure 4a, the concentration of Fe(III) decreased gradually along with the increase of Fe(II) concentration under anaerobic conditions, confirming the efficient reduction of Fe(III) to Fe(II) on the ACF cathode. The contribution of the Fe(III)−TPP complex regeneration to the atrazine degradation was then verified by the atrazine degradation in the cathodic compartment of the separated dualcell EF system with Fe(III)−TPP complex as both the iron source and the electrolyte under aerobic conditions. Although Fe(III)−TPP complex could not directly degrade atrazine under aerobic conditions without a bias voltage, the applied negative voltage of −0.7 V could efficiently degrade atrazine in the cathodic compartment, further verifying the regeneration of E


Article


Figure 5. Degradation of atrazine (a) and the atrazine degradation rate constant k (b) in the Na6TPP−EF system at different initial pH values with 0.5 mM Na6TPP as the electrolyte under a constant current of 0.5 mA. Initial concentration of atrazine was 10 mg L−1.

of the iron electrode were gradually filled in, accompanying the surface smoothing of iron electrode along with electrochemical reaction proceeding in the Na6TPP−EF system (Figure S13, Supporting Information). The phosphorization of iron electrode was confirmed by the appearance of P signal during the EDS and XPS analysis of the iron electrode used in the Na6TPP−EF system (Figures S13 and S14, Supporting Information). The survey XPS spectra also revealed that Fe0, FeII, and FeIII coexisted in the fresh iron electrode. The peaks at 710.4 and 724.4 eV are assigned to Fe 2p. A peak at a low binding energy of 706.9 eV could be ascribed to Fe0.29,35 After use in the Na6TPP−EF system, the peak of Fe0 disappeared, only FeII and FeIII could be detected on the surface of the used iron electrode, while the peak at around 132.5 eV was from phosphated iron compounds.36 Since pH strongly affects the oxidation performance of Fenton systems and the optimal pH of conventional EF process is below 3,11 we thus investigated the influence of initial pH values on the atrazine degradation in this Na6TPP−EF system and interestingly found that atrazine could be efficiently removed in a wide working pH value range of 4.0−10.2, although its removal efficiency still depended on the initial pH value of the solution (Figure 5). Along with increasing the pH value, the atrazine degradation constant increased from 0.0375 at pH 4.0 to 0.0645 min−1 at pH 8.0, then considerably decreased to 0.0241 min−1 at pH 10.2, and finally became negligible at pH 12.0. According to previous reports,37,38 the pH change did not significantly influence the H2O2 production, so hydroxyl radicals produced from the electrogenerated H2O2 and Fe(II) would not change obviously with the variation of pH after the addition of Na6TPP to prevent the iron species from precipitation in the Na6TPP−EF system. The oxidation efficiency of the Na6TPP-EF system was mainly dependent on the contribution of molecular oxygen activation induced by Fe(II)−TPP. As the optimal working pH value of the Fe(II)− TPP/Air system was 7.8,20 it was reasonable to observe the optimal efficiency of the Na6TPP-EF system at pH 8.0. As for the strong alkali condition, the oxidation capacity of •OH may decrease and Fe(III) would precipitate even in the presence of Na6TPP, which would then reduce the oxidation ability of the Na6TPP−EF system. During HPLC analysis, more peaks appeared for the sample degraded in the Na6TPP−EF system than those of the original atrazine solution, corresponding to the degradation intermediates. The comparison of the retention times and UV−vis spectra of these intermediates with standard compounds suggested the formation of desethylatrazine (CAIT) and

Fe(II) via the Fe(III) reduction on the cathode (Figure 4b). In the Na6TPP−EF system, the sacrificial iron anode was beneficial to the Fe(II) regeneration because the regeneration of Fe(II) not only depended on the Fe(III) reduction on the cathode but was also related to the Fe(II) oxidation on the anode.31 Obviously, Fe(II) would not be oxidized at the iron anode because of the preferential oxidation of Fe0. Therefore, we conclude that the regeneration of Fe(II) can be realized via the electrochemical reduction process on the cathode. In the Na6TPP−EF system, we interestingly found that the cell voltage increased suddenly at about 20 min and then keep steady even with prolonging the reaction time to 180 min (Figure S10a, Supporting Information). Therefore, we monitored the potentials of the anode and the cathode of the Na6TPP−EF system vs NHE before and after this cell voltage change. The anodic potential increased along with the cell voltage increasing, but the cathodic potential did not change throughout the electrochemical process. The anodic potential increase indicated that the corrosion of iron anode became difficult, which might be proven by Tafel analysis of the iron electrode in Na6TPP solution.32 Tafel curves revealed that the free corrosion potential of the iron electrode shifted positively by about 500 mV compared to that of pristine iron electrode (Figure S10b, Supporting Information), confirming that the corrosion resistance of the iron electrode significantly improved after its reaction with Na6TPP. In contrast, the cell voltage of Na2SO4−EF decreased slightly with increasing reaction time, and the corrosion potential of the iron electrode in the presence of Na2SO4 exhibited a slight negative shift, indicative of the acceleration of iron electrode corrosion with reaction time (Figures S8c and S8d, Supporting Information). Correspondingly, the iron electrode in the Na2SO4−EF system corroded seriously and plenty of iron sludge formed after 180 min. However, the corrosion of iron electrode in the Na6TPP− EF system was negligible without the appearance of iron sludge (Figure S11, Supporting Information). Meanwhile, the release of iron in the Na6TPP−EF system became slower after 30 min, and the concentration of dissolved iron remained relatively stable after 60 min (Figure S12, Supporting Information). On the basis of these results, we conclude that the excessive corrosion of iron electrode can be inhibited in the presence of Na6TPP. The iron electrode corrosion suppression in the Na6TPP−EF system might be attributed to the Na6TPP-induced phosphorization of iron electrode via the formation of phosphating membrane on iron metal surface,33,34 which was investigated by SEM, EDS, and XPS analyses. SEM analysis revealed the caves F



■

desisopropylatrazine (CEAT) (Figure S15a, Supporting Information). The concentration of CAIT and CEAT increased within 30 min and decreased gradually. Ion chromatography was used to detect and quantify carboxylic acids and chlorine ions generated during the oxidation of atrazine. Formic acid was gradually accumulated, up to 1.4 mg L−1 at 240 min, while chlorine ions began to appear at 30 min, and the dechlorination rate reached up to 92% at 240 min (Figure S15b, Supporting Information). These results revealed that the dealkylation of atrazine might be easier than its dechlorination−hydroxylation reaction. GC-MS and LC-MS analyses were employed to further probe the atrazine degradation intermediates (Table S1, Supporting Information), which included dealkylic, alkylic− oxidation, and dechlorination−hydroxylation products. On the basis of these intermediates, we proposed a possible degradation pathway of atrazine in the Na6TPP−EF system (Scheme S1, Supporting Information). Environmental Implications. The EF process is widely used for wastewater treatment because of its versatility and simpleness. However, the application of EF is limited by its low efficiency of H2O2 production and the requirement of acidic pH. In this study, a highly efficient and wide pH EF system was developed with molecular oxygen activation induced by Fe(II)−TPP complex. The utilization of Na6TPP as the electrolyte could widen the pH range of the EF system to 4.0−10.2 and also significantly enhanced the organic pollutant degradation efficiency. More importantly, the presence of Na6TPP in the EF system could prevent the iron anode from excessive corrosion via phosphorization and the generation of iron sludge, which is the major shortcoming of the EF system with sacrificial iron anode to supply Fe(II). Although the addition of TPP might cause phosphate contamination, we interestingly found both TPP and Fe(III)−TPP complex could be recovered from the solution with anion exchange resin (Table S2, Supporting Information). This study offers a promising EF technology with high efficiency and wide pH application range for organic pollutant control.

■

REFERENCES

(1) 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. (2) Moreira, F. C.; Garcia-Segura, S.; Boaventura, R. A.R.; Brillas, E.; Vilar, V. J. P. Degradation of the Antibiotic Trimethoprim by Electrochemical Advanced Oxidation Processes Using a Carbon-PTFE Air-Diffusion Cathode and a Boron-Doped Diamond or Platinum Anode. Appl. Catal. B: Environ. 2014, 160, 492−505. (3) Zhang, H.; Ran, X. N.; Wu, X. G. Electro-Fenton Treatment of Mature Landfill Leachate in a Continuous Flow Reactor. J. Hazard. Mater. 2012, 241, 259−266. (4) Borràs, N.; Oliver, R.; Arias, C.; Brillas, E. Degradation of Atrazine by Electrochemical Advanced Oxidation Processes Using a Boron-Doped Diamond Anode. J. Phys. Chem. A 2010, 114, 6613− 6621. (5) Yu, F. K.; Zhou, M. H.; Zhou, L.; Peng, R. D. A Novel ElectroFenton Process with H2O2 Generation in a Rotating Disk Reactor for Organic Pollutant Degradation. Environ. Sci. Technol. Lett. 2014, 1, 320−324. (6) Oturan, M. A.; Peiroten, J.; Chartrin, P.; Acher, A. J. Complete Destruction of p-Nitrophenol in Aqueous Medium by Electro-Fenton Method. Environ. Sci. Technol. 2000, 34, 3474−3479. (7) Li, J. P.; Ai, Z. H.; Zhang, L. Z. Design of a Neutral ElectroFenton System with Fe@Fe2O3/ACF Composite Cathode for Wastewater Treatment. J. Hazard. Mater. 2009, 164, 18−25. (8) Zhao, H. Y.; Wang, Y. J.; Wang, Y. B.; Cao, T. C.; Zhao, G. H. Electro-Fenton Oxidation of Pesticides with a Novel Fe3O4@Fe2O3/ Activated Carbon Aerogel Cathode: High Activity, Wide pH Range and Catalytic Mechanism. Appl. Catal. B: Environ. 2012, 125, 120− 127. (9) Boye, B.; Farnia, G.; Sandonà, G.; Buso, A.; Giomo, M. Removal of Vegetal Tannins from Wastewater by Electroprecipitation Combinedwith Electrogenerated Fenton Oxidation. J. Appl. Electrochem. 2005, 35, 369−374. (10) Ö zcan, A.; Sahin, Y.; Koparal, A. S.; Oturan, M. A. Carbon Sponge as a New Cathode Material for the Electro-Fenton Process: Comparison with Carbon Felt Cathode and Application to Degradation of Synthetic Dye Basic Blue 3 in Aqueous Medium. J. Electroanal. Chem. 2008, 616, 71−78. (11) Brillas, E.; Sirés, I.; Oturan, M. A. Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton’s Reaction Chemistry. Chem. Rev. 2009, 109, 6570−6631. (12) 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 3O 4 Nanoparticles: Simultaneous Production of H2O2 and Fe2+ for Efficient Electro-Fenton Degradation of Organic Contaminants. Water Res. 2014, 48, 190−9. (13) Liao, P.; Yuan, S. H.; Chen, M. J.; Tong, M.; Xie, W. J.; Zhang, P. Regulation of Electrochemically Generated Ferrous Ions from an Iron Cathode for Pd-Catalytic Transformation of MTBE in Groundwater. Environ. Sci. Technol. 2013, 47, 7918−26. (14) Yuan, S. H.; Fan, Y.; Zhang, Y. C.; Tong, M.; Liao, P. PdCatalytic in situ Generation of H2O2 from H2 and O2 Produced by Water Electrolysis for the Efficient Electro-Fenton Degradation of Rhodamine B. Environ. Sci. Technol. 2011, 45, 8514−20. (15) Liu, W.; Ai, Z. H.; Zhang, L. Z. Design of a Neutral ThreeDimensional Electro-Fenton System with Foam Nickel as Particle Electrodes for Wastewater Treatment. J. Hazard. Mater. 2012, 243, 257−264. (16) Biaglow, J. E.; Kachur, A. V. The Generation of Hydroxyl Radicals in the Reaction of Molecular Oxygen with Polyphosphate Complexes of Ferrous Ion. Radiat. Res. 1997, 148, 181−187. (17) Welch, K. D.; Davis, T. Z.; Aust, S. D. Iron Autoxidation and Free Radical Generation: Effects of Buffers, Ligands, and Chelators. Arch. Biochem. Biophys. 2002, 397, 360−9.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-27-6786 7535. E-mail: [email protected]. edu.cn. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Funds for Distinguished Young Scholars (Grant 21425728), National Science Foundation of China (Grants 21177048, 21407044, and 21477044), Key Project of Natural Science Foundation of Hubei Province (Grant 2013CFA114), China Postdoctoral Science Foundation (Grant 2014M560617), and Self-Determined Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grants CCNU14Z01001 and CCNU14Z01010). G


Article


(37) Ö zcan, A.; Sahin, Y.; Savas Koparal, A.; Oturan, M. A. Carbon Sponge as a New Cathode Material for the Electro-Fenton Process: Comparison with Carbon Felt Cathode and Application to Degradation of Synthetic Dye Basic Blue 3 in Aqueous Medium. J. Electroanal. Chem. 2008, 616, 71−78. (38) Yu, F. K.; Zhou, M. H.; Zhou, L.; Peng, R. D. A Novel ElectroFenton Process with H2O2 Generation in a Rotating Disk Reactor for Organic Pollutant Degradation. Environ. Sci. Technol. Lett. 2014, 1, 320−324.

(18) Keenan, C. R.; Sedlak, D. L. Ligand-Enhanced Reactive Oxidant Generation by Nanoparticulate Zero-Valent Iron and Oxygen. Environ. Sci. Technol. 2008, 42, 6936−6941. (19) Kachur, A. V.; Tuttle, S. W.; Biaglow, J. E. Autoxidation of Ferrous Ion Complexes: a Method for the Generation of Hydroxyl Radicals. Radiat. Res. 1998, 150, 475−82. (20) Wang, L.; Wang, F.; Li, P. N.; Zhang, L. Z. FerrousTetrapolyphosphate Complex Induced Dioxygen Activation for Toxic Organic Pollutants Degradation. Sep. Purif. Technol. 2013, 120, 148−155. (21) Wang, L.; Cao, M. H.; Ai, Z. H.; Zhang, L. Z. Dramatically Enhanced Aerobic Atrazine Degradation with Fe@Fe2O3 Core-Shell Nanowires by Tetrapolyphosphate. Environ. Sci. Technol. 2014, 48, 3354−3362. (22) Qiang, Z. M.; Chang, J. H.; Huang, C. P. Electrochemical Regeneration of Fe2+ in Fenton Oxidation Processes. Water Res. 2003, 37, 1308−1319. (23) Lee, C.; Keenan, C. R.; Sedlak, D. L. Polyoxometalate-Enhanced Oxidation of Organic Compounds by Nanoparticulate Zero-Valent Iron and Ferrous Ion in the Presence of Oxygen. Environ. Sci. Technol. 2008, 42, 4921−4926. (24) Harvey, A. E.; Smart, J. A.; Amis, E. S. Simultaneous Spectrophotometric Determination of Iron(II) and Total Iron with 1,10-Phenanthroline. Anal. Chem. 1955, 27, 26−29. (25) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Photocatalytic Production of H2O2 and Organic Peroxides in Aqueous Suspensions of TiO2, ZnO, and Desert Sand. Environ. Sci. Technol. 1988, 22, 798−806. (26) Lazrus, A. L.; Kok, G. L.; Gitlin, S. N.; Lind, J. A.; McLaren, S. E. Automated Fluorimetric Method for Hydrogen Peroxide in Atmospheric Precipitation. Anal. Chem. 1985, 57, 917−922. (27) Joo, S. H.; Feitz, A. J.; Sedlak, D. L.; Waite, T. D. Quantification of the Oxidizing Capacity of Nanoparticulate Zero-Valent Iron. Environ. Sci. Technol. 2004, 39, 1263−1268. (28) Daneshvar, N.; Aber, S.; Vatanpour, V.; Rasoulifard, M. H. Electro-Fenton Treatment of Dye Solution Containing Orange II: Influence of Operational Parameters. J. Electroanal. Chem. 2008, 615, 165−174. (29) Ai, Z. H.; Gao, Z. T.; Zhang, L. Z.; He, W. W.; Yin, J. J. CoreShell Structure Dependent Reactivity of Fe@Fe2O3 Nanowires on Aerobic Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2013, 47, 5344−5352. (30) Liu, W.; Ai, Z. H.; Cao, M. H.; Zhang, L. Z. Ferrous Ions Promoted Aerobic Simazine Degradation with Fe@Fe2O3 Core-Shell Nanowires. Appl. Catal. B: Environ. 2014, 150, 1−11. (31) Sirés, I.; Garrido, J. A.; Rodríguez, R. M.; Brillas, E.; Oturan, N.; Oturan, M. A. Catalytic Behavior of the Fe3+/Fe2+ System in the Electro-Fenton Degradation of the Antimicrobial Chlorophene. Appl. Catal. B: Environ. 2007, 72, 382−394. (32) Melitas, N.; Chuffe-Moscoso, O.; Farrell, J. Kinetics of Soluble Chromium Removal from Contaminated Water by Zerovalent Iron Media: Corrosion Inhibition and Passive Oxide Effects. Environ. Sci. Technol. 2001, 35, 3948−3953. (33) Zheng, J. W.; Chen, H. B.; Qiao, L.; Lin, M.; Jiang, L. Q.; Che, S. L.; Hu, Y. W. Double Coating Protection of Nd-Fe-B Magnets: Intergranular Phosphating Treatment and Copper Plating. J. Magn. Magn. Mater. 2014, 371, 1−4. (34) Li, R. X.; Yu, Q. M.; Yang, C. P.; Chen, H.; Xie, G. X.; Guo, J. Y. Innovative Cleaner Production for Steel Phosphorization Using ZnMn Phosphating Solution. J. Clean Prod. 2010, 18, 1040−1044. (35) Mills, P.; Sullivan, J. L. A Study of the Core Level Electrons in Iron and its Three Oxides by Means of X-ray Photoelectron Spectroscopy. J. Phys. D: Appl. Phys. 1983, 16, 723−732. (36) Ding, M.; De Jong, B. H. W. S.; Roosendaal, S. J.; Vredenberg, A. XPS Studies on the Electronic Structure of Bonding Between Solid and Solutes: Adsorption of Arsenate, Chromate, Phosphate, Pb2+, and Zn2+ Ions on Amorphous Black Ferric Oxyhydroxide. Geochim. Cosmochim. Acta 2000, 64, 1209−1219. H


Design of a Highly Efficient and Wide pH Electro-Fenton Oxidation

Recommend Documents