Rapidly Enhanced Electro-Fenton Efficiency by in Situ

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Rapidly enhanced electro-Fenton efficiency by in situ electrochemistry-activated graphite cathode Shou Qiu, Lingling Yu, Diyong Tang, Wei Ren, Ke Chen, and Jie Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05380 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Industrial & Engineering Chemistry Research

Rapidly enhanced electro-Fenton efficiency by in situ electrochemistry-activated graphite cathode

Shou Qiu, Lingling Yu, Diyong Tang, Wei Ren, Ke Chen, Jie Sun* Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, College of Resource and Environmental Science, South-Central University for Nationalities, Wuhan 430074, P.R. China. * Corresponding author. Tel.: +86-27-67843698, Fax: +86-27-67843918 E-mail: [email protected] (J. Sun)

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Graphical abstract


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Abstract

For the development of a highly active and energy-effective electro-Fenton process, a simple and efficient electrochemical method was used to modify the nitrogen-doped graphite in situ. It showed that through ammonium nitrate electroactivation, this process could provide (I) a higher surface area, (II) more hydrophilicity, (III) much higher electrocatalytic activity toward the oxygen reduction reaction, and (IV) more nitrogen-containing functional groups. The reduction of oxygen to generate H2O2 was promoted, and the current efficiency of the electro-Fenton process was also upgraded from 57.96% to 83.76% over 15 min. The apparent rate constant for the dye Brilliant Red X3B by electrochemisty is 0.1781 min-1, much higher than that of the Raw (0.0731 min-1). The dimethyl phthalate (DMP) and the diethyl phthalate (DEP) had the same results. Hydroxyl (·OH) radicals were identified as the main reactive oxygen species, which clearly increased after electroactivation.

Keywords: graphite cathode; electrochemistry-activated; Electro-Fenton; hydrophily; nitrogen-doped .



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Introduction For the remediation of polluted wastewater, which includes refractory contaminants from biological processes, advanced oxidation processes (AOPs) are promising technologies for the discharge of aqueous effluents.1-3 Among AOPs, an Electro-Fenton (E-Fenton) process is a highly efficient technology for the in situ generation of H2O2 that has been a concern in recent years.4, 5 The E-Fenton process generates H2O2 (eq. 1) continuously in the solution through the 2-electron process of the oxygen reduction reaction (ORR), which can eliminate the risk of H2O2 storage and transportation.6 Meanwhile, the highly reactive and non-selective oxidant hydroxyl radicals (·OH, eq. 2), which can rapidly destroy most persistent organic pollutants from wastewater, were generated in the E-Fenton process. In addition, regenerating Fe2+ (eq. 3) circularly on the cathode is very helpful in decreasing the amount of iron sludge. O2+2H++2e-→H2O2

(1)

Fe2++H2O2+H+→Fe3++H2O+·OH

(2)

Fe3++e-→Fe2+

(3)

From the above reactions, H2O2 generation, via the oxygen reduction reaction (ORR), plays a crucial role in the EF process and is affected by the types and properties of cathode materials. Carbonaceous materials have many strengths as the cathode, such as no toxicity, low-cost, good stability, conductivity, low catalytic activity for H2O2 decomposition and so on. At present, there are many kinds of carbonaceous materials reported as cathodes used in the E-Fenton system, including


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graphite,7 carbon felt,8 graphite felt,9-12 carbon sponge,13 activated carbon fiber,14 carbon-PTFE,12,

15-17

OMC,18-21 Hierarchically Porous Carbon22,

23

and carbon

aerogel24. Graphite has been used due to its efficient reduction of iron ions, large electrocatalysis active surface, mechanical integrity and easy acquisition, which makes it a promising commercialization cathode material for the E-Fenton process. The ORR reaction activity is known to be quite sensitive to the surface properties of cathode materials. Therefore, various surface modification methods for cathodes composed of carbonaceous material have been investigated in order to improve the efficiency for ORR electrocatalysis, such as the hydrothermal method and application of heat treatment.18, 19 Anodic electrochemistry oxidation is an effective and low-cost route to change the performance, which is reported to improve catalytic performance, since the electrode worked in microbial fuel cells and improved sensitivity as a probe that detected heavy metals. There are few reports that it is the only easy method for in situ activation of the cathode via electrochemistry.25, 26 Compared with other dealing methods, this method does not need the sharp dealing conditions but only needs to give a constant and brief potential before the E-Fenton process proceeds. It not only changed the electrocatalytic performance but also transformed the hydrophily of the electrode surface, which plays a vital role in the diffusion of oxygen dissolved in water.27 Recently, a large amount of ammonia nitrogen wastewater was discharged into the water from the mine.28 With this acid mine drainage, activated in situ electrochemistry showed great superiority.29 In this study, ammonium nitrate liquor as



the electrolyte was first used to anodize the cathode in order to load nitrogen onto the surface of the cathode.30,31 It has been reported that nitrogen-containing carbon materials act as effective metal-free ORR electrocatalysts.32-36 The nitrogen atoms improve the catalytic performance of the electron-accepting ability, which creates a net positive charge on adjacent carbon atoms in the carbon plane to readily attract electrons from the anode to facilitate the ORR.37 Both the surface chemistry and the structure were studied, and the electrocatalytic activity for the ORR was investigated. This electrochemical method is an in situ and resourceful treatment and realizes the reuse of the pollutant ammonia nitrogen. Moreover, three model organic pollutants, dimethyl phthalate (DMP), diethyl phthalate (DEP), and the dye Brilliant Red X3B, were chosen to assess the performance of the modified graphite in the E-Fenton process for degrading environmental endocrine disrupters and dyes.

2. Experimental 2.1 Electrical activation of graphite plate All chemicals used in this study were analytical and used as received without further purification. The commercial graphite plate (G) was first polished and degreased with acetone in an ultrasonic bath for 15 min, then washed with deionized water 3 times to remove residual acetone, and finally dried at 60 °C for 2 h. All of the electrochemical performances were operated using an electrochemical workstation (CHI-650D, China), with a commercial graphite paper (3×4 cm) as the work electrode, platinum as the counter electrode and a saturated calomel electrode (SCE) as


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reference electrode. The distance between the working electrode and counter electrode was 4 cm. Three types of in situ modified anodes were obtained according to different electrolytes: ammonium nitrate (G-A), nitric acid (G-N), and sodium hydroxide (G-S).38,39 The supporting electrolyte was 1.0 M, and 1.75 V was used as supporting potential. After the electrochemical treatment for 10 min, the modified work electrode can be used immediately. 2.2 Characterization of materials The surface elemental composition of the as-prepared samples was determined by X-ray photoelectron spectroscopy (XPS, a Multilab 2000 XPS system with a monochromatic Mg Kα source and a charge neutralizer); all the binding energies were referenced to the C1s peak at 284.4 eV of the surface adventitious carbon. Scanning electron microscopy (SEM) images were used on a Hitachi SU 8010 (Japan) microscope operated at 20 KV. Contact angle measurements were carried out in a Digidrop goniometer under ambient conditions, and the contact angles of water drops were measured using the Digidrop software. 2.3 Electrochemical techniques Linear sweeping voltammetry (LSV) was applied to investigate and identify the potential for ORR under specific conditions. LSV used an electrochemical workstation, which was performed on a three-electrode system between 0 and -1.5 V at a scan rate of 5 mV·s-1 at room temperature. The LSV electrochemical test was performed in 0.1 M Na2SO4 (pH=3). Cyclic voltammetry (CV) was conducted in a solution of 10 mM K3[Fe(CN)6] and 1.0 M KCl using the Electrochemical



Workstation (CHI-65D, China) in a three-electrode cell including a working electrode (modified and unmodified graphite), a counter electrode (Pt foil), and a reference electrode (Saturated Calomel Electrode, SCE). Moreover, we could calculate the electroactive surface area according to the Randles-Sevcik equation40. 1

3

1

I p = 2.69 × 10 5 × AD 2 n 2 γ 2C where n is the number of electrons participating in the redox reaction, A is the area of the electrode (12 cm2), D is the diffusion coefficient of the molecule in solution (6.5×10-6 cm2s-1), C is the concentration of the probe molecule in the bulk solution (mol·cm-3), and γ is the scan rate of the potential perturbation (0.007 V·s-1). 2.4 E-Fenton degradation process E-Fenton degradation was carried out in a 100-mL cube reactor (5 cm×4 cm×5 cm) containing 50 mL of simulated wastewater under magnetic stirring at room temperature. The simulated wastewater consisted of 0.1 mM Brilliant Red X3B dye, 0.1 M Na2SO4 and 1.0 mM Fe2+ (pH=3.0). Before the electro-Fenton degradation, oxygen was bubbled into the solution at a flow-rate of 0.6 L·min-1 for 1 h. The concentration of X3B remaining in the filtrate was then determined using a UV1800 spectrometer (Shanghai, China) at 530 nm. The total organic carbon (TOC) analyzer (Vario TOC Select, Germany) was used to analyze the TOC. Dimethyl phthalate (DMP) at 50 ppm or diethyl phthalate (DEP) at 50 ppm, 0.1 M Na2SO4, and 1.0 mM Fe2+ (pH 3.0) were used in the simulated wastewater. The DMP concentration was detected by high-performance liquid chromatography (HPLC, Ultimate 3000, USA) equipped with a reversed-phase column (Phenomenex C18, 250 mm, 4.6 mm, 5 um)


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and a UV detector. A mixture of 50% acetonitrile and 50% H2O was used as the mobile phase, with a flow rate of 1.0 mL·min-1, and the detection wavelength was set at 276 nm. The detection method for DEP was similar to that of DMP, except the mobile phase was a mixed solution of 80% acetonitrile and 20% H2O. 2.5 Detection of the H2O2 formed and measurement of hydroxyl radicals The concentration of H2O2 during the electrogeneration process was monitored by UV-vis spectrophotometer (UV1800PC, Shanghai instrument analysis instrument) using the potassium titanium (IV) oxalate method. The current efficiency (CE) for H2O2 production was defined as follows:41,42

CE =

nFC H O V 2 2

t

∫ Idt

× 100%

0

where n is the number of electrons transferred for oxygen reduction to H2O2, F is the Faraday constant (96500 C·mol-1 ), CH2O2 is the concentration of H2O2 (mol·L-1), V is the bulk volume (L), I is the current (A), and t is the time (s). The ·OH radicals were tested by the photoluminescence method using coumarin as a molecular probe, which readily reacts with ·OH to produce a highly fluorescent product, 7-hydroxycoumarin. A 50-mL volume of a mixed solution containing coumarin

(0.5

mmol·L-1),

0.1

mol·L-1

Na2SO4

and

1.0

mmol·L-1

(NH4)2Fe(SO4)2·7H2O was adjusted to a pH of 3.0 with diluted H2SO4. The electrodes and the electrical current were the same as for the electro-Fenton degradation process. The filtrate was analyzed using a fluorescence spectrophotometer (Hitachi, F-7000, Japan) with an excitation wavelength of 332 nm. Electron paramagnetic resonance (EPR, EMX NANO, USA) were also used to test the ·OH radicals. A 10 mM



5,5-Dimethyl-1-Pyrroline-N-Oxide (DMPO) solution at pH 3.0 as a molecular probe to capture the ·OH radicals. 3．Results and discussion

3.1 XPS analysis The surface elements and functional groups of raw and modified graphite sheets were studied by XPS analysis. The results showed the presence of N in materials with an atomic ratio of ~ 1.64% (1.64 at.%). The peak by C1s spectra of the nitrogen-doped carbon at 284.6 eV, 285.0 eV and 285.8 eV correspond to the C=C, C-C and C-OH structures, respectively. The majority of the peaks at 284.6 eV and 285.0 eV are equal to sp2C and sp3C, respectively. So the sp2C and sp3C are the main structures of the materials. Moreover, the oxygen group of C-OH, which is the peak at 285.8 eV, still exists in the carbon anode. The hydroxyl radical is a hydrophilic group, which reveals the one reason why the hydrophilic property increased, and it is more beneficial for dissolved oxygen to be used for electron transport. There are only two kinds of nitrogen that have been doped into material samples from the N1s spectra in Figure 1. The peak at 401.1 eV corresponds to the graphitic N, and the peak at 406 eV can be ascribed to NO3- 43,44. By calculating the contents of two types of N, the results are 0.58 at.% and 1.06 at.% for graphitic N and NO3-, respectively. The N-doped samples are significant for the ORR process, and the graphitic N is more active in the two electron ORR reactions in particular45. According to previous studies on nitrogen-doped carbons, the nitrogen atom is more radical and electronegative than the carbon atom, so the carbon atoms around


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the nitrogen atoms possess higher positive charge density. This behavior can change the chemisorption mode of O2 on the catalyst and thus weaken the O-O bonding. According to previous studies on nitrogen-doping carbons, the graphitic N site doped on carbon gave a two-electron ORR with H2O2 as the main product, while the pyridinic N site gave the four-electron process of ORR and decreased the production of H2O2. The relative contents of these surface groups of the samples were obtained by Table 1. The contents of O1s at the surface of G-A, G-N, and G-S increased from 6.14% to 7.09%, 9.58% and 10.42%, respectively. It is worth noting that G-A and G-N nitrogen-containing functional groups will come out and that G-A will obtain more nitrogen-containing functional groups in the same condition.

(A) Elemental Name C O N

C1s

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at.% 91.26 7.09 1.64

O1s N1s

200

400

600

800

Binding energy (eV)


1000


Intensity(a.u.)

(B)

C-C C=C

C-OH

280

282

284

286

288

290

292

294

296

Binding energy (eV)

-

(C)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NO3

graphitic N

398

400

402

404

406

408

410

Binding energy (eV)

Figure 1. XPS spectra of modified material. (A) Survey scan; (B) XPS spectra of the C1S region; (C) XPS spectra of the N1S region.


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Table. 1. The contents of groups on the surfaces of G, G-S, G-N, and G-A.

3.2 SEM and contact angle The SEM images of the electrodes resulting from various electrical activated modifications are given in Figure 2. From this analysis, it can be observed that the surface after electrical activation was much rougher and had deeper grooves on the modified anodes compared with the relatively smooth surface of the unmodified anode. This increased roughness will be helpful for increasing the specific surface area of the electrode to a great extent, thus increasing the active surface area of the electrode and in this way promoting the ORR. Although the electrical activation of the

Elemental composition

G

G-S

G-N

G-A

C1s (at%)

93.86

89.58

89.52

91.26

O1s (at%)

6.14

10.42

9.58

7.09

N1s (at%)

-

-

0.8

1.64

C=C (%)

61.81

54.98

54.83

58.77

C-C (%)

28.27

29.65

26.83

24.75

C-O (%)

9.93

15.37

16.31

13.54

Graphitic N (%)

-

-

0.2

0.58

-

-

-

0.6

1.06

NO3 (%)

graphite sheet increased the specific surface area, even electrical activation by the sodium hydroxide electrolyte was better. Therefore, the material of the electrical activation increased the activity by both processes.



Figure 2. SEM images of samples (a) G, (b) G-S, (c) G-N, and (d) G-A. Scale bars = 50 µm

To obtain the wettability of the anode, the contact angles of water at the different dealing conditions were measured and are schematically shown in Figure 3. As can be seen, the respective initial contact angle of the materials is approximately θ=81.6°, and the dealing anode’s contact angles are θ=34.2°, θ=36.8° and θ=19.7°, confirming the improvement of the material’s electrical activation wettability. This could be the result of the introduced oxygen-containing functional groups on the surface as shown by XPS and the rougher surface shown by SEM. There is no doubt that the hydrophily of the anode is more beneficial to dissolved oxygen acting as the electron transfer in electron-Fenton. It is the same as the result of SEM; even electrical activation by the sodium hydroxide electrolyte is better. However, electrical activation by ammonium nitrate is the best in the electron-Fenton system, although N-doping also played a crucial role in this process.


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Figure 3. Contact angle measurements of G, G-A, G-N and G-S

3.3 Electrochemical measurements The surface chemistry of carbon-based electrodes had a strong response to the redox wave [Fe(CN)6]3-/[Fe(CN)6]4-, so it was used to evaluate the electrochemical activity of carbon cathodes. The ferrocyanide system by cyclic voltammetry (CV) is an effective and stable method to reflect the electroactive surface area of the electrode. Figure 4. shows the cyclic voltammograms of the different cathode electrodes in a potassium hexacyanoferrate solution. It can be clearly seen that the peak current density of electrical activation by ammonium nitrate was nearly 0.7 times higher than that of the substrate material, revealing that the electrical activation significantly increased the electroactive surface area (due to the large surface area of much rougher and deeper grooves). From the Randles-Sevcik equation, the electroactive surface areas of G-A, G-N, G-S, and G were 22.12 cm2, 21.49 cm2, 21.63 cm2, and 14.95 cm2, respectively.



G-N G G-A G-S

2

Current density(mA/cm )

1

0

-1 -0.4

0.0

0.4

0.8

Potential(V vs. SCE)

Figure 4. Comparison of the CV of G, G-N, G-A and G-S cathodes in a potassium hexacyanoferrate solution at a scan rate of 7 mV·s-1.

0

(A) N2

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1 O2

-2

-1.6

-1.2

-0.8

-0.4

0.0



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G G-S G-N G-A

0.0 2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

-0.8

-1.6

-2.4 -1.6

-1.2

-0.8

-0.4

0.0


Figure 5. LSV curves of G-A in deoxygenated and oxygen-saturated solution (A) and LSV curves of the different cathode composite materials in an oxygen-saturated solution (B)

Linear sweep voltammetry (LSV) is an electroanalytical technique dedicated to the study of electrochemical reaction kinetics on electrode materials and provides quantitative information on the reversible or quasi-reversible behavior of a redox couple, the number of electrons transferred in an oxidation or reduction process, rate constants, reaction mechanisms and diffusion coefficients. In Figure 5A, the deoxygenated and oxygen-saturated solutions are compared by the ORR activities of the carbon electrode. In the oxygen-saturated solution, there was evidence that the H2O2 formation was related to the oxygen reduction between 0 and -1.0 V/SCE by the appearance of a unique reduction pseudo-wave following eq. 1. A reduction peak at -0.6 V was observed for the cathode material, which was attributed to the ORR. Upon comparison of the currents of G-A, G-N, G-S and G in Figure 5B, the values were clearly observed as follows: -0.0299A > -0.0213A > -0.0157A > -0.01A. However, the cathode dealing with ammonium nitrate had the minimum potential and the highest



current density, indicating that nitrogen in the cathode reduced the overpotential of O2 reduction on the surface of the cathode material. The N-doping of the cathode is beneficial for O2 reduction. 3.4 H2O2 accumulation concentration and the current efficiency It is well-known that the efficiency of the E-Fenton system for removing persistent organic pollutants mainly depends on the generation of H2O2, because the highly oxidative hydroxyl radicals are produced from the decomposition of H2O245. Without doubt, the two-electron reduction of O2 on the cathode generated the H2O2, and the factors of electrical activation and potential influenced the amount of H2O2, as shown in Figure 6A. This is the same as the mineralization of the Brilliant Red X3B dye; the ammonium nitrate cathode material had the best formation rate of H2O2. Likewise, the H2O2 concentration increased as the cathode potential increased from -0.4 V to -0.7 V and reached a maximum value at -0.6 V, which is in accordance with the LSV results. The only reaction from eq. 1 for the current efficiency (CE) at the cathode is calculated. The result is shown in Figure 6D: the best potentials at -0.6 V have the highest CE of 83.76% at a duration of 15 min. As the time passed, the CE decayed due to the decomposition of H2O2.


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H2O2/(mg/L)

120

(A)

G-A G-N G-S G

0

40

80

40

0 80

120

160

200

Time(min)

80

CE(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G-A G-N G-S G

(B)

60

40

20 0

30

60

90

120

150

180

Time(min)



120

-0.6V -0.5V -0.7V -0.4V -0.9V

H2O2/(mg/L)

(C)

80

40

0 0

40

80

120

160

200

Time(min)

80

-0.6V -0.5V -0.7V -0.4V -0.9V

(D)

60

CE(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40

20

0

40

80

120

160

200

Time(min)

Figure 6. H2O2 concentration and current efficiency. (A) Influence of the modified cathode on H2O2 generation, (B) current efficiency calculation under different modified cathode, (C) Influence of the cathodic potential (vs SCE) on H2O2 generation and (D) current efficiency calculation under different applied potentials.

3.5 X3B degradation in the E-F system and mineralization To evaluate the effectiveness of the electrode, it was adopted to degrade the Brilliant Red X3B dye in the electro-Fenton system. The electro-Fenton degradation


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kinetics by different materials are shown in Figure 7. It can be seen clearly that the degradation profiles of Brilliant Red X3B obey pseudo-first-order reaction rate kinetics. Their corresponding apparent rate constants were G-A, G-N, G-S and G at 0.1781 min-1, 0.1300 min-1, 0.0868 min-1 and 0.0731 min-1, respectively.

1.0

4 G G-A G-N G-S

3

-ln(Ct/C0)

0.8 0.6

Ct/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

0

0.4

0

10

20

30

Time(min)

G G-A G-N G-S

0.2 0.0 0

10

20

30

40

50

60

Time(min)

Figure 7. Comparison with other cathodes for X3B degradation efficiency and kinetics: initial X3B concentration: 50 mg/L, initial pH=3.0, [Fe2+]=0.1 mmol-1, air flow-rate 0.6 L min-1, T=25 °C, applied cathode potential: -0.6 V.

To further confirm that electrical activation is beneficial for electro-Fenton reactivity, different dealing potentials are compared using ammonium nitrate. The control is only dipped in an ammonium nitrate solution. It is obvious that electrical activation favored the electro-Fenton system because electrical activation changed the hydrophily and the electrochemical properties. The loss of total organic carbon (TOC) was used to monitor the mineralization of the Brilliant Red X3B solution in the electro-Fenton process, as shown in Figure 8,



which shows the TOC removal rate for Brilliant Red X3B through the electro-Fenton reaction using different cathode materials. The effect on the mineralization of Brilliant Red X3B by the different cathode materials had the same trend as that of the degradation kinetics. The control showed the lowest mineralization of Brilliant Red X3B; it obtained 59.545% mineralization after 1 h by the electro-Fenton reaction with no N-doping or other electrical dealing. It has both the lowest degradation and lowest mineralization of Brilliant Red X3B. Dealing with ammonium nitrate cathode materials not only has the highest degradation kinetics but also had the highest mineralization ability as shown by the TOC removal rate, which showed that nitrogen and electrical activation played significant roles in the electro-Fenton degradation of Brilliant Red X3B when using carbon paper as the cathode. Moreover, nitrogen atoms created a net positive charge on adjacent carbon atoms in the carbon plane to readily attract electrons from the anode to facilitate the ORR. The recycling performance of the G-A was also investigated, the stability is a key performance of the electrode. Figure 9. shows the X3B removal after 20 min treatment in every cycle. After 10 cycles, 93.674% of X3B were still removed. This result indicates the promising usage of the material in the electro-Fenton.


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TOC Removal Rate(%)

80

G-A G-N G-S G

60

40

20

0 0

40

80

120

160

200

Time(min)

Figure 8. Mineralization of Brilliant Red X3B degradation in the electro-Fenton system using different samples. Conditions: pH 3.0, Na2SO4 0.1 M, initial X3B concentration: 50 mg/L, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C.

100

80

Ct/C0(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 2

4

6

8

10

Run Number

Figure 9. Recycle tests for electro-Fenton degradation of X3B using G-A as the cathode.

3.6 Degradation of different contaminants at -0.6 V To investigate the universality of the modified electrode, it was adopted to degrade other contaminants in the wastewater, such as DMP and DEP. Their



degradation percentages and the corresponding apparent rate constant (k) (pseudo first-order reaction) are given in Figure 10 and Figure 11. The corresponding apparent rate constants for DMP were 0.1040, 0.1402, 0.1575 and 0.1852 min-1 for G, G-S, G-N and G-A, respectively. DMP and DEP are the similar chemical structures. DEP degradation exhibited similar results to DMP, and G-A had the best degradation rate constants. DEP’s apparent rate constants were 0.10736, 0.14368, 0.15963 and 0.18947 min-1 for G, G-S, G-N and G-A, respectively. As a result, G-A performed the best at degrading organic contaminants in terms of a higher k value, which showed again that electrochemistry activated by ammonium nitrate is a promising path for the E-Fenton system.

6

G G-S G-N G-A

4

-ln ( C t /C 0 )

0.8

C t /C 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.4

2

0 0

G G-S G-N G-A

0.0 0

15

30

45

Time(min)

15

30

45

Time(min) Figure 10. The DMP degradation efficiency and kinetics of various contaminants. Conditions: pH = 3.0, Na2SO4 0.1 M, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C


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Figure 11. The DEP degradation efficiency and kinetics of various contaminants. Conditions: pH = 3.0, Na2SO4 0.1 M, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C

3.7 Determination of ·OH radicals To further confirm that equation 2 occurred in the E-Fenton system, the reactive oxygen species was tested. Coumarin was used as a probe to detect the formation of ·OH and the increase in photoluminescence (PL) intensity by the reaction time, and it obtained the highest peak at 451 nm (excited at 350 nm). In this way, it can prove that ·OH radicals have the primary impact during the E-Fenton process for degradation of the pollutants. Then, compared with different cathodes of the generated 7-hydroxycoumarin, the formation rates of H2O2 and ·OH radicals in the pristine sample control were much slower than the others in Figure 12. It is shown that the electrical activation by



ammonium nitrate can detect more ·OH radicals when the dealing cathodes were used as the cathode materials in the electro-Fenton system, showing that anodic oxidation played an influential role in the performance enhancement of the carbon cathode materials. The electro-induced generation of H2O2 and ·OH radicals directly estimated the degradation ability of environmental pollutions in an electro-Fenton system by eq. (1) and (2). The N-doping, the hydrophilic enhancement and the improved electrochemical performance are the reasons more ·OH radicals were generated. Figure 13 shows the EPR spectrum with the four characteristic bands of the ·OH-DMPO adduct that was detected when 10 mM DMPO solution at pH 3.0 was electrolyzed with the cathode at -0.6 V for 5 min. It means that ·OH radicals generate in this reaction and G-A without Fe can`t generate ·OH. The electro-induced generation of the ·OH need the cycle of iron by Eq (2) and (3). Moreover, based on the identical line shaes of the MOPO-adduct for different cathodes, the G-A was the higher, clearly indicating that the generation of ·OH was greatly improved.

G G-S G-N G-A

PL Intensity@451nm(a.u.)

3000

3000

2000 PL Intensity（ a.u.）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1000

8min

2500

6min 2000

4min 1500

2min 1000

0min 500 0

0

350

400

450

500

550

600

Wavelength（ nm）

0

2

4

6

Time（ min）


8

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Figure 12. PL spectral changes observed during the E-Fenton reaction of G-A and time dependence of the induced PL intensity at 451 nm of different cathodes.

G-A

G-N

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G-S G G-A without Fe 3400

3420

3440

3460

3480

Magnetic field(G)

Figure 13. Comparison of EPR-DMPO-·OH spectra of the G-A, G-N, G-S, G and

G-A without Fe. 4. Conclusion N-doped carbon materials modified by electrical activation were successfully prepared and characterized. This method can immensely improve the properties for the effective electro-Fenton degradation of Brilliant Red X3B dye. The electrical activation by ammonium nitrate is a very simple way to modify carbon materials and a very easy way to obtain a two-electron pathway, which is more beneficial for generating H2O2 and more beneficial to the electro-Fenton system. The enhanced reactivity of modified carbon materials is a result of the decreased hydrophilic nature and overpotential for O2 reduction, which encourages the diffusion and reduction of O2 . In this study, the electrical activation changed the surface roughness of graphite



paper, which increased the active area of the electro-Fenton reactivity. It also transformed the surface functional groups of the carbon cathode and the hydrophilus functional groups, such as the radicals, possibly because the oxygen in the water could easily contact the surface of graphite paper. Moreover, it is certainly due to the better electrical activation, which could achieve a better electrochemical performance, an expanded electro-active surface area, a faster oxygen diffusion rate and current efficiency of the system. Finally, it is a simple way to obtain nitrogen-containing functional groups, which are better for ORR activity. This study may provide new insights into the design and preparation of carbon materials with superior efficiency. Acknowledgments This work was financial supported by the National Natural Science Foundation of China (21477165), Technology Project of Wuhan (2017060201010190) and Team Foundation of SCUN (CZT18020).

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For Table of Contents Only Figure 1. XPS spectra of modified material. (A) Survey scan; (B) XPS spectra of the C1S region; (C) XPS spectra of the N1S region. Table. 1. The contents of groups on the surfaces of G, G-S, G-N, and G-A. Figure 2. SEM images of samples (a) G, (b) G-S, (c) G-N, and (d) G-A. Scale bars = 50 µm Figure 3. Contact angle measurements of G, G-A, G-N and G-S Figure 4. Comparison of the CV of G, G-N, G-A and G-S cathodes in a potassium hexacyanoferrate solution at a scan rate of 7 mV·s-1. Figure 5. LSV curves of G-A in deoxygenated and oxygen-saturated solution (A) and LSV curves of the different cathode composite materials in an oxygen-saturated solution (B) Figure 6. H2O2 concentration and current efficiency. (A) Influence of the modified cathode on H2O2 generation, (B) current efficiency calculation under different modified cathode, (C) Influence of the cathodic potential (vs SCE) on H2O2 generation and (D) current efficiency calculation under different applied potentials. Figure 7. Comparison with other cathodes for X3B degradation efficiency and kinetics: initial X3B concentration: 50 mg/L, initial pH=3.0, [Fe2+]=0.1 mmol-1, air flow-rate 0.6 L min-1, T=25 °C, applied cathode potential: -0.6 V. Figure 8. Mineralization of Brilliant Red X3B degradation in the electro-Fenton system using different samples. Conditions: pH 3.0, Na2SO4 0.1 M, initial X3B concentration: 50 mg/L, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C. Figure 9. Recycle tests for electro-Fenton degradation of X3B using G-A as the cathode.


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Figure 10. The DMP degradation efficiency and kinetics of various contaminants. Conditions: pH = 3.0, Na2SO4 0.1 M, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C Figure 11. The DEP degradation efficiency and kinetics of various contaminants. Conditions: pH = 3.0, Na2SO4 0.1 M, [Fe2+]=0.1 mmol-1, applied cathode potential: -0.6 V, air flow-rate 0.6 L min-1, T=25 °C Figure 12. PL spectral changes observed during the E-Fenton reaction of G-A and time dependence of the induced PL intensity at 451 nm of different cathodes. Figure 13. Comparison of EPR-DMPO-·OH spectra of the G-A, G-N, G-S, G and

G-A without Fe.


Rapidly Enhanced Electro-Fenton Efficiency by in Situ

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