Novel Multilayer ACF@rGO@OMC Cathode Composite with

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A novel multilayer ACF@rGO@OMC cathode composite with enhanced activity for the electro-Fenton degradation of phthalic acid esters Wei Ren, Diyong Tang, Xiaoshuang Lu, Jie Sun, Mei Li, Shou Qiu, and Dingjin Fan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02896 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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Industrial & Engineering Chemistry Research

A novel multilayer ACF@rGO@OMC cathode composite with enhanced activity for the electro-Fenton degradation of phthalic acid esters Wei Ren a, Diyong Tang a, Xiaoshuang Lu a, Jie Sun a,*, Mei Li a, b, Shou Qiu a, Dingjin Fan a a

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs

Commission & Ministry of Education, Department of Resource and Environmental Science, South-Central University for Nationalities, Wuhan 430074, P.R. China. b

Centre for Organized Matter Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom * Corresponding author. Tel: +86-27-67843698, Fax: +86-27-67843918 E-mail: [email protected] (J. Sun)

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ACS Paragon Plus Environment


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Abstract

In this study, a novel multilayer cathode composite of ACF@rGO@OMC (ACF = activated carbon fibers, rGO = reduced graphene oxide, OMC = ordered mesoporous carbon), with high electrical conductivity were successfully prepared. Transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CVs) results show that the ACF@rGO@OMC composites possess excellent properties of ordered mesoporous structure, high specific surface area, high electroactive surface area and lower impedance. Appling this fabricated ACF@rGO@OMC composites electrode, the reduction of oxygen to generate hydrogen peroxide was promoted from 31 mg·L-1 to 85 mg·L-1, and current efficiency of electro-Fenton process was also improved from 25.0% to 40.0%. The yield of •OH had increased obviously. The ACF@rGO@OMC composites was applied for removal of phthalic acid esters (PAEs) using an electro-Fenton process, and it kept stable after 10 times reaction.

Keywords: Reduced graphene oxide (rGO); Ordered mesoporous carbon (OMC); Electro-Fenton; Phthalic acid esters (PAEs).

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1. Introduction Advanced oxidation processes (AOPs) are promising technologies for the remediation of polluted wastewater when the contaminants are recalcitrant by biological processes.1-3 Among AOPs, electro-Fenton (E-Fenton) can guarantee continuous in-situ generation of H2O2 in acidic medium (eq. 1) which can eliminate the risk of H2O2 storage and transportation.4,5 And electro-Fenton (E-Fenton) not only inherits the advantage of traditional Fenton generating hydroxyl radical (•OH) (eq. 2) which can degrade most organic pollutants into CO2 and H2O, but also has distinct advantages of cathodic regeneration of Fe2+ (eq. 3) and less sludge.6,7 O2 + 2H+ + 2e- → H2O2

0.582 V (vs. SCE)

(1)

Fe2+ + H2O2 + H+ → Fe3+ + H2O + •OH

(2)

Fe3+ + e- → Fe2+

(3)

From the reaction mechanism, H2O2 production from oxygen reduction reaction (ORR, eq. 1) plays a decisive role in the E-Fenton process on the cathode.8,9 But the ORR has another pathway, a four electrons transfer process, which can be expressed by eq.4.10 O2 + 4H+ + 4e- → 2H2O

1.229 V (vs. SCE)

(4)

Undoubtedly, the four electrons transfer of ORR should be avoided wherever possible from the current efficiency of E-Fenton point of view. Whether ORR follows two or four electrons process is affected by the cathode materials. Because of their high selectivity for two electrons process of ORR, nontoxicity, excellent conductivity, good stability, and low cost, carbonaceous materials have been intensively tested as cathode in the E-Fenton process, including carbon nanotube (CNT),11 graphite felt (GF),12,13 active carbon fiber (ACF),14,15 ordered mesoporous carbon (OMC),16

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reticulated vitreous carbon (RVC),17 carbon sponge (CS),18 porous carbon felt (PCF)19 and etc. Among these carbon materials, ordered mesoporous carbon (OMC) is an attractive material because of its high specific surface area, large pore volume and excellent thermal properties.20-22 However, despite the above advantages, indigent conductivity and arduous functionalization impeded their applications. Our previous work showed that the structure of OMC cathode materials can facilitate the diffusion and transformation of oxygen at the surface of cathode.23 While, the current efficiency of E-Fenton process is only 25%. The results showed that the conductivity of ACF@OMC was slightly destitute, the increased IR drop leading to depression of current efficiency. In order to further upgrade their properties and broaden the applications, nitrogen-atoms have been successfully doped in OMC.24 But the functionalized process performed with redundant nitrogen-atoms could destroy the ordered pore structure. Reduced graphene oxide (rGO), a novel two-dimensional (2D), one-atom thick sheet composed of sp2 carbon atoms arranged in a honeycomb structure carbon materials, has been received great attention for applications in nanocomposites, photochemistry, electrochemistry, and so on.25,26 Due to their admirable electric conductivity, rGO has been introduced to OMC to enhance the impoverished conductivity of the electrode material.27,28 However, the repressed electric conductivity resulted by addition of binder (PTFE) into the powdered rGO-OMC composite restricted its application in electrochemistry.29 Therefore, it is essential to promote the reduction of oxygen to generate hydrogen peroxide and improve the current efficiency of E-Fenton with low impedance cathode materials. Besides, the effects of conductivity on the performance of cathode materials in E-Fenton system need to be further studied. In this paper, the novel multilayer 4


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ACF@rGO@OMC cathode composites with lower impedance have been successfully prepared. Appling this cathode, higher concentration of hydrogen peroxide was detected, the E-Fenton system with ACF@rGO@OMC cathode exhibited superior talent for degrading refractory organic pollutants in wastewater, such as environmental endocrine disrupters (EEDs). Phthalic acid esters (PAEs) are used to produce plastic production for various consumer products, commodities, and construction materials. Some PAEs (such as dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), dioctyl phthalate (DOP)) possessing reproductive and developmental toxicities to animals has been verified.30-33 Therefore, DMP and DEP were chosen as the model target of EEDs to evaluate E-Fenton activity of the ACF@rGO@OMC composites. 2. Experimental Section 2.1. Materials and chemicals. The ACF was purchased from Xuesheng Technology Co. Ltd. All chemicals were of analytical purity and were used as received without further purification. A 0.1 M Na2SO4 was used as electrolyte. A 0.1 mM (NH4)2Fe(SO4)2·6H2O was used as Fenton catalyst. The solution pH was adjusted by sulfuric acid. 2.2 Preparation of ACF@rGO@OMC cathode composites. ACF (3.0 cm × 3.0 cm) was calcined at 800 °C under nitrogen atmosphere for 6 h before use. Graphite oxide (GO) was synthesized by graphite flakes through a modified Hummers method.27 Reduced graphene oxide (rGO) was prepared by heating GO under nitrogen atmosphere. OMC was prepared by a soft-template method using copolymer F127 as template in an ethanol solution, and ACF@rGO@OMC was prepared by two steps method of surface coating. Typically, the as prepared GO (0~90 mg) was dispersed in 20 mL water under

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sonication. Then, the GO suspension was dropwise added onto ACF and heated at 60 °C for 24 h. The obtained sample was denoted as ACF@GO. Subsequently, F127 (1.0 g) was dissolved in ethanol (20 mL). After that, 5.0 g of resol ethanol solution (20 wt.%) was added into the solution.20 After stirring for 30 min, the resulted homogeneous solution was poured into a porcelain boat which was filled with ACF@GO. After exposure in air for 8 h to evaporate the solvent, the boat was heated in oven at 100 °C for 24 h. The composite was then transferred into a crucible and calcined in a tubular furnace under N2 atmosphere, firstly at 350 °C for 2 h and then 800 °C for 4 h with a ramp rate of 1 °C·min-1. The prepared ACF@rGO@OMC sample was denoted as S30, where 30 is additive mass of 30 mg of GO. S0, S15, S60, S90 were prepared by the same way. Scheme 1 displays the schematic diagram of the preparation process for ACF@rGO@OMC composites.

Scheme 1. Schematic diagram of the preparation process for ACF@rGO@OMC. 2.3 Materials characterization. Scanning electron microscope (SEM) images were captured on a Hitachi SU8010 operated (Japan) at 20 kV. The pore structure was characterized by a transmission electron microscopy (TEM) (FEI Tecnai G20, USA) at 200 kV. N2 sorption isotherms were collected using a physisorption apparatus (Autosorb-1, USA) at 77 K. Prior to

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the measurement, the sample was degassed at 473K for 6 h. The Brunauer-EmmetTeller (BET) surface area was determined by a multipoint BET method using the adsorption data within the range of relative pressure (P/P0) from 0.05 to 0.3. The mesoporous pore size distributions were obtained by using Barrett-Joyner-Halenda (BJH) model. The crystallite size was calculated from the X-ray diffraction pattern using Scherrer's formula (eq. 5):19 D=

kλ β cos θ

(5)

Where D is the grain size (Å), k is a constant equal to 0.94, β is the full width at half maximum (FWHM) (radian) and λ (Å) is the wavelength of the X-rays. 2.4 Electrochemical measurements. The electrochemical performance of the electrode was investigated by Linear Sweep Voltammetry (LSV) using an Electrochemical Workstation (CHI-650D, China), which was performed in a three-electrode system between 0 and -1.5 V at a scan rate of 5 mV·s-1 at room temperature. The simulated solution consisted of 0.1 M Na2SO4. The single-compartment electrolytic cell including carbonaceous ACF@rGO@OMC electrode, platinum electrode and saturated calomel electrode (SCE) were used as working electrode, counter electrode and the reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) measurements were conducted on an Autolab 302N electrochemical workstation, the excitation AC signal with amplitude of 10 mV in a frequency domain from 1 Hz to 100 kHz was applied and the DC potential was controlled at the open circuit potential (OCP). The oxygen diffusion coefficient of the cathodes was calculated according to the Butler-Volmer formula (eq. 6):34

D=

R 2T 2 2 A 2 n 4 F 4 c 2σ 2

(6)

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Where D is the diffusion coefficient of the molecule in solution (cm2·s-1), R is the gas constant (8.314 J·mol-1·K-1), T is the temperature (K), A is the area of the electrode (cm2), n is the number of electrons participating in the redox reaction, F is the Faradaic constant (96500 C·mol-1), c is the concentration of oxygen in the solid phase (mol·cm-3), σ is the Warburg coefficient. Electrochemical characterization was investigated by cyclic voltammograms (CVs) using an Electrochemical Workstation (CHI-650D, China), which was performed in three-electrode system between -0.35 V and 0.80 V at a scan rate of 7 mV·s-1 at room temperature. Experiments were conducted in solution of 10 mM K3[Fe(CN)6] and 1.0 M KCl in a three-electrodes cell including a working electrode (ACF@OMC and ACF@rGO@OMC), a counter electrode (platinum electrode), and a reference electrode (SCE). The electroactive surface area of cathodes was calculated according to the Randles-Sevcik formula (eq. 7):19 1

3

1

I p = 2.69 × 105 × AD 2 n 2 γ 2 C

(7)

Where Ip is the peak current (A), n is the number of electrons participating in the redox reaction, A is the area of the electrode (cm2), D is the diffusion coefficient of the molecule in solution (cm2·s-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 (V·s-1). 2.5 E-Fenton degradation process. E-Fenton degradation was carried out in a 150 mL beaker containing 100 mL of simulated wastewater and equipped with three electrodes under magnetic stir at room temperature. The distance between cathode and anode was 3 cm. The working electrode was assembled by ACF@rGO@OMC carbon materials in size of 3.0 cm×3.0 cm on Ti plate, a platinum electrode was used as counter electrode. The simulated wastewater consisted of 50 ppm dimethyl phthalate (DMP) (or 50 ppm 8


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diethyl phthalate (DEP)), 0.1 M Na2SO4 and 1.0 mM Fe2+ (pH=3.0). Before E-Fenton degradation, oxygen was provide for the cathode surface at a flow rate of 0.3 L·min-1 for 1 h. During the electrolysis, optimal operative was maintained at -0.7 (vs. SCE) by CHI-650D electrochemical workstation. The DMP concentration was detected by HPLC (ULTIMATE 3000, USA) equipped with a reverse phase column (Phenomenex C18, 250 mm, 4.6 mm, 5 µm) and a UV detector. A mixture of 50% acetonitrile and 50% water, was used as mobile phase with 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 that the mobile phase was a mixed solution of 80% acetonitrile and 20% water. The total organic carbon (TOC) was determined by a TOC analyzer (Vario TOC Select, Germany). 2.6 Detection of H2O2 and measurement of hydroxyl radicals. The concentration of H2O2 formed during the E-Fenton process was detected by the potassium titanium (IV) oxalate method.35 The •OH was tested by a fluorometric method using coumarin as a probe molecule.7 And the current efficiency (CE) of E-Fenton reaction based on the production of H2O2 was defined as follows (eq. 8): 7 CE =

nFC H 2O2V t

∫ Idt

× 100% (8)

0

Where n is the number of electrons transferred for oxygen reduction to H2O2, F is the Faraday constant (96486 C·mol-1), CH2O2 is the concentration of H2O2 (mol·L-1), V is the solution volume (L), I is the externally imposed current (A), and t is the time (s).

3. Results and discussion. 3.1. Morphology observation. SEM images show that ACF@OMC has typical fiber filamentary structure (Figure 1a). After covered with a layer of rGO, the ACF@rGO@OMC exhibits three-layers

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cable like core-shell structure (Figure 1f for S90 as an example), indicating that rGO layer (exhibiting typical rippled silk-like texture in the middle layer) and OMC layer (about 1 µm in thickness at outmost layer) were successively grafted on the surface of ACF. According to the SEM images, higher amount of rGO may lead to aggregation in the system, which may hinder oxygen reduction of hydrogen peroxide production (Figure 1d-e).

Figure 1. SEM images of S0 (a), S15 (b), S30 (c), S60 (d) and S90 (e,f).

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The pore structure of the OMC layer was further observed by TEM (Figure 2). Figure 2a-e show highly ordered pore structures of OMC layers for S0, S15, S30, S60 and S90 samples, respectively. High resolution TEM image for OMC layer of S30 sample shows that these ordered mesopores are about 3.8 nm in diameter (Figure 2f). The formed ordered mesoporous channels would facilitate the diffusion and storage of oxygen to the rGO layer that would efficiently improve the reduction efficiency of oxygen to produce H2O2 on the cathode which is crucial to the electro-Fenton process. The rGO can improve the conductivity of composite cathode materials owing to its outstanding properties of excellent electron mobility and superior electrical performance. From the SEM and TEM images, it is confirmed that rGO and OMC were successfully grafted on the surface of ACF.

Figure 2. TEM images of S0 (a), S15 (b), S30 (c, f), S60 (d) and S90 (e).

3.2. Structural characterizations. The surface area and structure of the ACF@rGO@OMC materials were characterized 11



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by nitrogen adsorption measurements. As shown in the Figure 3A, all of the cathode materials have similar isotherms from the nitrogen adsorption-desorption isotherms. The S0 is consistent with typical type IV curve which has a clear hysteresis loop and a capillary condensation step in the P/P0 range from 0.6 to 1.0, implying the presence of mesopores (type A). The hysteresis loops of the cathode materials decrease in order of S0 > S30 > S90, hinting the cutback in mesopores, which is similar to the pore size distribution curves (Figure 3B). The BET surface area of the ACF@rGO@OMC materials decreased from 594.5 to 459.0 m2·g-1 and pore size slightly increased from 3.6 to 3.9 nm (Table 1) with an increase in the dose of GO from 0 to 90 mg. The aggregation is not beneficial for oxygen diffusion and subsequent reduction of oxygen on the ACF@rGO@OMC electrode, because of the decreased BET surface area resulting in the reduction of active sites. The average crystal size of the material was also improved with the GO addition. The crystallite size obtained from the X-ray diffraction pattern (Figure 3C) was calculated by Scherrer’s formula. For ACF@OMC (S0) and ACF@rGO@OMC (S30), the values are calculated to be 13.46 Å and 14.58 Å (Table 2) respectively. And the diffraction peaks C(002) and C(101) at 2θ=26.4° and 44.4° are attributed to the amorphous graphite structure of ACF@rGO@OMC. On the other hand, XRD of GO exhibited a reflection at approximately 10.0° (001) attesting of the high degree of oxidation of the obtained GO materials dispersed in the GO solution. Figure 3D presents transmittance FTIR spectra for the ACF@OMC, ACF@rGO@OMC and GO. Comparing to GO, intensity υ(C=O) transmittance peak of ACF@OMC and ACF@rGO@OMC at 1736 cm-1 decreased significantly, which confirms that the GO was reduced by the thermal treatment. More specifically, deconvolution of the C1s peak in the XPS spectrum in Figure 3E

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and F show the presence of four types of carbon bonds: C=C (284.6 eV), C-C (285.0 eV), C-O (286.8 eV), and HO-C=O (288.9 eV). Compared to ACF@OMC, HO-C=O peak of ACF@rGO@OMC became more obviously, which confirms that the oxygen functionalities of GO could not be reduced completely by the thermal treatment. There was 3.95% oxygen retaining in ACF@rGO@OMC (S30), because of 2.85% in ACF@OMC and only 1.10% from the residual hydroxyl, epoxy or carboxylic groups that still existed in rGO.

Table 1. BET, pore volume and average pore size of the cathode materials.

Sample

Dosage of GO (mg)

BET (m2·g-1)

Pore volume (cm3·g-1)

Average pore size(nm)

S0

0

594.5

0.40

3.6

S30

30

533.3

0.45

3.8

S90

90

459.0

0.56

3.9

(A)

3

Volume absorbed (cm STP/g)

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


S0 S30 S90 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

13


1.0


(B)

dV/dD (10-2cm3.g-1.nm-1)

S0 S30 S90

2

4

6

8 10 12 14 16 18 20 22 24 26 Pore size (nm)

2500

C(002)

C(001) 2000 Intensity (a.u.)

(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

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C(101)

1500

GO

1000 500

S90

0 10

20

30

40 50 60 2-Theta(Degree)

70

80

S60 S30 S15 S0

10

20

30

40

50

60

2-Theta(Degree)

14


70

80

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(D) Transmittance

ACF@OMC

ACF@rGO@OMC

1736

GO 4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

(E) Intensity (counts/s)

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


ACF@OMC C=C

C-C C-O

282

284

286

288

Binding Energy (eV)

15


290

292


(F) ACF@rGO@OMC 30

C=C

Intensity (counts/s)

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

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C-C

C-O

282

284

286

COOH

288

290

292

Binding Energy (eV) Figure 3. Nitrogen sorption isotherms (A) and pore size distribution curves (B) of S0, S30 and S90, X-ray diffraction patterns (C) of the cathode materials, FTIR spectra (D) of ACF@OMC, ACF@rGO@OMC and GO, X-ray photoelectron spectroscopy of ACF@OMC (E) and ACF@rGO@OMC 30 (F).

Table 2. Crystallite size, Electroactive surface area, Electron transfer resistance and oxygen diffusion coefficient of the cathode materials. Sample name Crystallite size (Å) Electroactive surface area (cm2·g-1) Electron transfer resistance (ohm) Oxygen diffusion coefficient (10-9·cm2·s-1)

S0 13.46

S15 14.35

S30 14.58

S60 14.01

S90 13.73

99.83

423.34

486.06

333.45

301.05

13.87

9.99

8.60

10.58

12.39

1.15

1.37

1.71

1.25

1.21

3.3 Electrochemical measurements. The redox wave [Fe(CN)6]3-/[Fe(CN)6]4- is sensitive to surface chemistry of 16


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carbon-based electrodes, so it was used to evaluate the electrochemical activity of carbon cathodes. Cyclic voltammograms of the different cathode composite electrodes in potassium hexacyanoferrate solution were shown in Figure 4. It could be seen that S30 could resulted in large current response toward [Fe(CN)6]3-/[Fe(CN)6]4- when compared to S0 in the same scan range between -0.35 V and

0.8 V vs. SCE.

Electroactive surface area was calculated from the Randles-Sevcik formula and was 486.06 cm2·g-1 for S30, 5 times higher than that of S0 (99.83 cm2·g-1) (Table 2). S30 has higher electroactive surface area indicating better kinetic properties compared to S0. 8

Current density (mA/cm2)

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


4 0

S0 S15 S30 S60 S90

-4 -8 -12 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Potential (V vs. SCE)

Figure 4. Cyclic voltammograms (CVs) of the cathode composite electrodes in potassium hexacyanoferrate solution at scan rate of 7 mV·s-1.

Electrochemical impedance spectroscopy (EIS) is an effective method for probing the interfacial properties of composite electrodes and the semicircle portion observed at high frequencies in the Nyquist diagrams corresponded to the electron transfer

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limiting process that could effectively evaluate the effect of rGO on the ACF@rGO@OMC composites. The interfacial electron transfer resistance (Ret) can be calculated from the diameter of Nyquist diagram for different composite electrodes. The equivalent circuit is based on assumptions that the electrode consists of a rough but continuous interconnected, porous solid of low bulk resistance. The faradic contribution is represented by Warburg impedance Zw, which includes a diffusion-controlled process in the solid. The electrolyte resistance is presented by Rs, and a constant phase element (CPE) in parallel with the Ret. The component values of the equivalent circuit shown in Figure 5 were obtained by fitting the ac impedance data using Solatronw impedance graphing and analysis software of Zview. It is clear to observe the following order in the Ret values: Ret (S30)CS60>CS90>CS0. The agreement of EIS with LSV results indicate that S30 (30 mg of rGO) is an optimized addition amount for the composite electrodes. 2H+ + 2e- → H2

(9)

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0

(A)

S30 N2

Current (mA)

-10 H2 evolution -20 reduction of O2

-30

S30 O2

-40 -50 -60

(I)

(II)

(III) -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential (V vs. SCE) 0 -10

(B)

-20

Current (mA)

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


-30

S0 S90 S60 S15 S30

-40 -50

S0 S15 S30 S60 S90

-60 -70 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential (V vs. SCE) Figure 6. LSV curves of S30 in deoxygenated and oxygen saturated solutions (A) and LSV curves of the different cathode composite materials in oxygen saturated solutions (B).

3.4 Formation of H2O2 and •OH radicals. The generation rate of H2O2 plays an important role in the EDCs degradation by electro-Fenton technology. Accumulation of H2O2 at different applied cathode 21



potentials was investigated and the results are presented in Figure 7A. As one can distinguish, H2O2 concentration increases as the cathode potentials increased from -0.5 V to -0.7 V and reaches the maximal value (85 mg·L-1) at -0.7 V. However, as the cathode potential further increased, H2O2 concentration decreases due to the occurrence of both hydrogen evolution reaction and reduction of H2O2 to water which are side reactions and restrain the ORR to generate H2O2. These results in agreement well with the LSV results (Figure 6A), indicating the optimal applied cathode potential for ACF@rGO@OMC cathode should be close to -0.7 V. The highest current efficiency of 40.4% was achieved at -0.7 V (Figure 7B). 100

(A)

-0.5V -0.6V -0.7V -0.8V -1.0V

80

[H2O2]/(mgL-1)

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 0

20

40

60

80 100 120 140 160 180

Time (min)

22


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(B)

40 35 30 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


-0.5V -0.6V -0.7V -0.8V -1.0V

25 20 15 10 5 20

40

60

80

100 120 140 160 180 Time (min)

Figure 7. Influence of cathodic potential (vs. SCE) on H2O2 generation (A) and current efficiency calculation under different applied potentials (B).

Likewise, the formation rate of H2O2 can reflect the relative activity of ACF@rGO@OMC cathode material during the E-Fenton process. In Figure 8A, a comparison of the concentration of accumulated H2O2 at different ACF@rGO@OMC cathodes is presented. The concentration of H2O2 increased with the dosage of rGO from 0 to 30 mg, but significantly decreased from 30 to 90 mg. The result is consistent with EIS (Figure 5A) and LSV results (Figure 6B). The current efficiency (CE) for H2O2 accumulation at a given electro-catalysis time is calculated assuming that eq. 1 is the only reaction occurs at the cathode. As depicted in Figure 8B, a highest CE of 40.4% was achieved for S30 in a duration time of 30 min. For S0, however, the value is only 25.0%. At longer electrolysis time, the current efficiency underwent a slow and similar decay due to the increased decomposition of H2O2.

23



100

(A) S0 S15 S30 S60 S90

-1

[H2O2]/(mgL )

80

60

40

20

0 0

30

60

90

120

150

180

Time (min)

40

S0 S15 S30 S60 S90

35

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

Page 24 of 43

30

(B)

25 20 15 30

60

90

120

150

180

Time (min) Figure 8. Influence of the cathode materials on H2O2 generation (A) and current efficiency calculation for different cathode materials (B).

To further confirm the reactive oxygen species in E-Fenton system (eq. 2), coumarin is used as a probe to detect the formation of •OH. It is observed that the PL intensity of the generated 7-hydroxycoumarin at 451 nm (excited at 332 nm) increases with the

24


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increase of reaction time (Figure 9A). It is clearly seen that the PL intensity at 451 nm does not increase linearly against the irradiation time, which has the same trend with that of the formation of H2O2. Therefore, it can be concluded that •OH plays the main roles during E-Fenton process for the degradation of the pollutants. Compared with S30 cathode material, the formation rate of H2O2 and •OH in S90 system were much slower (Figure 8A and Figure 9B). Little •OH can be detected when S0 was used as cathode material in E-Fenton system, reflecting the superiority of S30 in E-Fenton degradation.

350

(A)

8 min

300

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


6 min

250

4 min

200 150

2 min

100

0 min

50 0 350

400

450

500

550

Wavelength (nm)

25


600


350 PL Intensity@451nm (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

(B)

300 S0 S30 S90

250 200 150 100 50 0 0

2

4 Time (min)

6

8

Figure 9. Photoluminescence spectral changes observed during E-Fenton reaction of S30(A). Time dependence of the induced photoluminescence intensity @ 451 nm of the as-prepared cathode materials (B).

In summary, S30 has a preferable electro-catalytic activity for this electro-Fenton system. These might be explained as follows: a) rGO could improve the electron transport and current efficiency of ACF@OMC. b) The BET surface area of the ACF@rGO@OMC materials decreased from 594.5 to 495.0 m2·g-1 (Table 1) with an increased dose of GO from 0 to 90 mg, indicating that addition of rGO could lose sufficient active sites. c) rGO could provide a higher developed crystallization, an expanded electroactive surface area and a faster oxygen diffusion rate (Table 2). d) Besides that, too much dosage is easy to form nonuniformity on the ACF@OMC, the aggregation of rGO may hinder the transmission path of the ions in the electrolyte, leading to increase of electron transfer resistance. Therefore, an appropriate addition amount of GO is important for fabrication of highly efficient carbonaceous cathode 26


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Page 27 of 43

materials.

3.5. Electro-Fenton degradation and mineralization of PAEs. In order to investigate the effectiveness of ACF@rGO@OMC electrode, it was adopted to degrade PAEs contaminants in wastewater, such as dimethyl phthalate (DMP) and diethyl phthalate (DEP). The degradation percentage and corresponding apparent rate constant (pseudo first-order reaction) are given in Figure 10. S30 has a distinct superiority with the complete DMP and DEP degradation within 45 and 60 min. Their corresponding apparent rate constants for DMP degradation are 0.032, 0.047, 0.049, 0.045 and 0.038 min-1 at S0, S15, S30, S60 and S90, respectively. Similarly, S30 exhibits the most desirable DEP degradation efficiency among all the present investigated EF cathodes. Compared with the results, S30 possesses much better performance in terms of higher k value for degrading organic contaminants, demonstrating again that the S30 electrode is a promising EF cathode for wastewater treatment.

95

(A) 90.39

DMP DEP

92.51

91.88 90.99

90

Removal Rate/%

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


89.1

89.77

85

83.83 81.73 80.16

80 78.53

75

S0

S15

S30

S60

27


S90


2.5 Rate constant (10-2min -1)

ln([DMP]0/[DMP])

5

2.0 1.5

(B)

4.979

4.733

4.526 3.864

4 3.292 3 2 1 0 S0

S15

S30

S60

S90

1.0

S0 S30 S90

0.5

S15 S60

0.0 0

15

30

45

Time (min)

2.5 Rate constant (10-2 min-1)

4.126

2.0 ln([DEP]0/[DEP])

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

Page 28 of 43

1.5

(C)

4.308

4

3.574

3

2.705

2.461 2 1 0 S0

S15

S30

S60

S90

1.0 S0 S30 S90

0.5

S15 S60

0.0 0

15

30

45

60

Time (min) Figure 10. DMP and DEP removal rate of the cathode materials at 45 min and 60 min in E-Fenton system(A), the corresponding DMP degradation kinetics (B) and DEP degradation kinetics (C).

To acquire an effective and quick separation method for DMP, DEP and intermediates of DMP and DEP, a feasible program should be employed for separation of them. A 28


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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


series of typical chromatograms were obtained as shown in Figure 11A. The best separation method was that the mobile phase consisted of acetonitrile and water with volume ratio of 60:40 with a flowrate of 0.7 mL·min-1. In this way, the retention time of DMP-OH, DEP-OH, DMP and DEP were 5.013, 6.400, 6.960 and 10.560 min, respectively (Figure 11B). And the intermediate products were further verified by LC-MS. As Figure 11B displays, both of DMP and DEP can be effectively degraded by S30 within 45 min.

(A)

60% CH3CN + 0.5 mL/min

60% CH3CN + 0.7 mL/min

60% CH3CN + 1.0 mL/min 65% CH3CN + 1.0 mL/min 50% CH3CN + 1.0 mL/min

0

2

4

6

8

10

Ret. time

29


12


10

DMP

(B)

0 min 15 min 30 min 45 min

8

Peak height

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

6

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DEP-OH

DEP

DMP-OH

4 2 0 0

2

4

6

8

10

12

14

Ret. time

Figure 11. HPLC condition optimization chromatogram from the E-Fenton degradation of DMP-DEP (A), HPLC chromatogram of degradation products obtained from the E-Fenton degradation of the DMP-DEP (B).

To evaluate the efficiency of ACF@rGO@OMC cathode in mineralizing PAEs, the TOC of the solution was controlled during EF process. Compare to S0, S30 had a faster mineralization rate. After 90 min treatment, the removal rate of TOC reached 80.75%, 77.42% and 79.30% in DMP, DEP and DMP-DEP system, respectively (Figure 12). This result demonstrates that S30 could be accounted for a higher production of •OH in the system and faster degradation of PAEs intermediate products. This novel material could provide an effective solution to improve the performance of Electro-Fenton process.

30


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S0 S30 79.3

80.75 80

TOC Removal Rate (%)

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


77.42

71.91

70

67.64 63.15

60 DMP

DEP

DMP-DEP

Figure 12. Total organic carbon removal rate of DMP, DEP and DMP-DEP in E-Fenton system at 90 min.

3.6 Controlled experiment and stability of ACF@rGO@OMC For DMP degradation at the ACF@rGO@OMC cathode in the E-Fenton process, physical adsorption and electro-adsorption of DMP by ACF@rGO@OMC electrode itself would lead to direct oxidation of DMP by O2 and by the electro-generated H2O2, which probably contribute to the decrease of the DMP concentration. To investigate the above influences on DMP degradation, controlled experiments were carried out and the results are presented in Figure 13. It can be clearly seen that the DMP removal percentages of the tests without electricity, with H2O2, EF with tertiary butanol (TBA), EF without Fe and EF with N2 approaches 10%, 12%, 35%, 39% and 45%, respectively. It indicates that for physical adsorption, oxidation of DMP by O2 and by the electro-generated H2O2 has a slight contribution to the decrease of DMP concentration, and electro-adsorption also has a minor contribution. On the contrary, E-Fenton technology has a much higher DMP removal percentage and DMP can be completely removed within 45 min. Therefore, it is not difficult to conclude that 31



majority of DMP was removed through the E-Fenton process. 1.0 0.8 0.6

c/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

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0.4

S30-Without Electricity S30-H2O2 S30-EF/TBA S30-EF/Without Fe S30-EF/N2

0.2

S30-EF

0.0 0

15

30

45

Time (min)

Figure 13. Removal efficiency of DMP recorded under different reaction conditions at pH 3.0. For without electricity: the current circuit was turned off. For H2O2: the circuit was turned off, with addition of 85 mg·L-1 H2O2 but no ferric ions. For EF/TBA: EF with addition of 2.5 mmol·L-1 tertiary butanol as scavenger of •OH. For EF/without Fe: EF without addition of ferric ions. For EF/N2: EF in deoxygenated solutions.

The stability of the electrode is a key performance for practical application. It was found that 91.1% of the DMP was degraded within 45 min when fresh ACF@rGO@OMC was used as cathode. Even after running for 10 cycles, 85.5% of the pollutant can still be decomposed (Figure 14). This result infers the promising usage of this carbonous cathode material in practical application.

32


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100

80 [DMP]/[DMP]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


60

40

20

0 2

4

6

8

10

Run Number

Figure 14. Recycle tests for E-Fenton degradation of DMP using S30 as cathode materials.

3.7 PAEs degradation mechanism Comparing with the ACF@OMC cathode material, formation rates of H2O2 and current efficiency in ACF@rGO@OMC-30 system are much higher. Low impedance resistance can be detected when ACF@rGO@OMC-30 was used as a cathode material in the E-Fenton system, reflecting the importance of electrical properties of the reduced graphene oxide and structures of the carbonous cathode materials. The appropriate dosage of graphene oxide and uniform structures benefit the electrons that can be assembled from active carbon fibers to graphene oxide, and then the electrons can easily undertake electro-induced generation of H2O2 on ordered mesoporous carbon, therefore enhancing the current efficiency of H2O2 production. Meanwhile, ferric can be effectively reduced into ferrous at the cathode. To further understanding of E-Fenton degradation of PAEs, it is essential to test the

33



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

degradation pathways of the contaminants. It should be noted that •OH has three possible reaction pathways involving radical adduct formation (RAF), hydrogen atom transfer (HAT) and single electron transfer (SET).37,38 The first kind of approach was RAF: the •OH can easily react with aromatic molecules by adding to the benzene ring and producing OH-adducts. The second kind of approach was HAT: •OH could abstract a hydrogen atom to form methylbenzyl radicals. The third kind of approach was SET: single electron transfer with •OH. Base on HPLC-MS analysis of the products, the degradation mechanism of DMP in E-Fenton system was proposed in Figure 15. DMP can be hydroxylated by •OH addition reaction onto the aromatic ring, resulting in the formation of dimethyl 4-hydroxylphthalate (DMP-OH), and a hydrogen atom of DMP can also be abstracted by •OH addition reaction onto the methylene radical, resulting in the formation of phthalic acid1-hydroxymethyl ester 2-methyl ester. And phthalic acid 1-hydroxymethyl ester 2-methyl ester can be decomposed into monomethyl phthalate (MMP).39 The aromatic organics mentioned above would be further oxidized to organic acids by •OH until they are mineralized to CO2 completely. Therefore, in this research, we can conclude that the degradation of DMP involves two different pathways, RAF and HAT.

Figure 15. Proposed degradation mechanisms of DMP using ACF@rGO@OMC as cathode in E-Fenton system. 34


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4. Conclusions In this study, we have used ACF as substrate material, rGO as conducting layer and OMC as oxygen diffusion channel to prepare a novel multilayer ACF@rGO@OMC cathode composite with lower impedance. This favors H2O2 generation through oxygen reduction reaction, improving the current efficiency and thus accelerating the DMP degradation with the newly structured electrodes in E-Fenton system. With the optimized dosage of GO process, i.e. at 30 mg, S30 exhibited the lowest impedance. It is certainly due to the addition of rGO could provide a better electrochemical performance, a higher developed crystallization, an expanded electroactive surface area and a faster oxygen diffusion rate, improving the electron transport and current efficiency of the system. And the impedance has a close relationship between the uniformity (not obvious aggregation) of composite electrode materials. Moreover, S30 showed the highest DMP degradation performance at -0.7 V with an apparent rate constant of 0.049 min-1, about 1.5 times to that at ACF@OMC and desirable stability without significant performance decay was obtained after 10 times recycling with a reaction time of 45 min. Meanwhile, two different degradation pathways of DMP in electro-Fenton system were considered to be radical adduct formation (RAF) with formation of dimethyl 4-hydroxylphthalate (DMP-OH) and hydrogen atom transfer (HAT) with formation of monomethyl phthalate (MMP). Last but not the least, ACF@rGO@OMC has the potentiality in practical applications in many areas. This study may provide new insights into the design and preparation of carbonaceous materials with superior efficiency.

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Acknowledgments This work was financial supported by the National Natural Science Foundation of China (20807057&21477165), the New Century Excellent Talents Foundation of Ministry of Education of China (NCET-13-1047) and National Sci-Tech Support Plan (2015BAB01B04).

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mechanism of •OH mediated degradation of dimethyl phthalate in aqueous solution: experimental and theoretical studies, Environ.Sci.Technol. 2014, 48, 641-648.

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For Table of Contents Only Table 1. BET, pore volume and average pore size of the cathode materials. Table 2. Crystallite size, Electroactive surface area, Electron transfer resistance and oxygen diffusion coefficient of the cathode materials.

Figure 1. SEM images of S0 (a), S15 (b), S30 (c), S60 (d) and S90 (e,f). Figure 2. TEM images of S0 (a), S15 (b), S30 (c, f), S60 (d) and S90 (e). Figure 3. Nitrogen sorption isotherms (A) and pore size distribution curves (B) of S0, S30 and S90, X-ray diffraction patterns (C) of the cathode materials, FTIR spectra (D) of ACF@OMC, ACF@rGO@OMC and GO, X-ray photoelectron spectroscopy of ACF@OMC (E) and ACF@rGO@OMC 30 (F).

Figure 4. Cyclic voltammograms (CVs) of the cathode composite electrodes in potassium hexacyanoferrate solution at scan rate of 7 mV·s-1.

Figure 5. Nyquist plots (A) and the curves of Z’-ω-1/2 (B) for the cathode materials with different additive amount of GO.

Figure 6. LSV curves of S30 in deoxygenated and oxygen saturated solutions (A) and LSV curves of the different cathode composite materials in oxygen saturated solutions (B).

Figure 7. Influence of cathodic potential (vs. SCE) on H2O2 generation (A) and current efficiency calculation under different applied potentials (B).

Figure 8. Influence of the cathode materials on H2O2 generation (A) and current efficiency calculation for different cathode materials (B).

Figure 9. Photoluminescence spectral changes observed during E-Fenton reaction of S30(A). Time dependence of the induced photoluminescence intensity @ 451 nm of

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the as-prepared cathode materials (B).

Figure 10. DMP and DEP removal rate of the cathode materials at 45 min and 60 min in E-Fenton system(A), the corresponding DMP degradation kinetics (B) and DEP degradation kinetics (C).

Figure 11. HPLC condition optimization chromatogram from the E-Fenton degradation of DMP-DEP (A), HPLC chromatogram of degradation products obtained from the E-Fenton degradation of the DMP-DEP (B).

Figure 12. Total organic carbon removal rate of DMP, DEP and DMP-DEP in E-Fenton system at 90 min.

Figure 13. Removal efficiency of DMP recorded under different reaction conditions at pH 3.0. For without electricity: the current circuit was turned off. For H2O2: the circuit was turned off, with addition of 85 mg·L-1 H2O2 but no ferric ions. For EF/TBA: EF with addition of 2.5 mmol·L-1 tertiary butanol as scavenger of •OH. For EF/without Fe: EF without addition of ferric ions. For EF/N2: EF in deoxygenated solutions.

Figure 14. Recycle tests for E-Fenton degradation of DMP using S30 as cathode materials.

Figure 15. Proposed degradation mechanisms of DMP using ACF@rGO@OMC as cathode in E-Fenton system.

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For Table of Contents Only

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Novel Multilayer ACF@rGO@OMC Cathode Composite with

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