Synthesis of a novel heterogeneous fenton catalyst and promote the degradation of methylene blue by fast regeneration of Fe2+

Accepted Manuscript Title: Synthesis of a novel heterogeneous Fenton catalyst and promote the degradation of methylene blue by fast regeneration of Fe2+ Authors: Zhan-fang Cao, Xin Wen, Pei Chen, Fan Yang, Xiao-li Ou, Shuai Wang, Hong Zhong PII: DOI: Reference:

S0927-7757(18)30279-6 https://doi.org/10.1016/j.colsurfa.2018.04.009 COLSUA 22407

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

10-3-2018 3-4-2018 6-4-2018

Please cite this article as: Cao Z-fang, Wen X, Chen P, Yang F, Ou X-li, Wang S, Zhong H, Synthesis of a novel heterogeneous Fenton catalyst and promote the degradation of methylene blue by fast regeneration of Fe2+ , Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010), https://doi.org/10.1016/j.colsurfa.2018.04.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of a novel heterogeneous Fenton catalyst and promote the degradation of methylene blue by fast regeneration of Fe2+

Zhan-fang Cao*1, 2, XinWen#1, 2, Pei Chen1, 2, Fan Yang1, 2, Xiao-li Ou1, 2, Shuai Wang1, 2, Hong Zhong*1, 2

(1.Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha

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410083, Hunan, China；2. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Corresponding authors. Tel.: +86 731 88879616

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E-mail addresses: [email protected] (Zhan-fang Cao)

Abstract

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Graphical abstract

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A novel heterogeneous catalyst (TEA/GO@Fe3O4) was synthesized by simple one-step

hydrothermal method. Surprisingly, TEA/GO@Fe3O4 showed an extremely fast decomposition

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rate for methylene blue (MB). According to the characterization results, the superior properties of the TEA/GO@Fe3O4 can be attributed to following factors: (1) the application of GO prevents the agglomeration of Fe3O4 nanoparticles, (2) the delocalized π electrons of GO and the lone pair electrons of triethanolamine and its oxidant products (TEA) promoted the regeneration of Fe2+, (3) the negative surface charge of TEA/GO@Fe3O4 and the coordination between Fe and S can 1

accelerate the diffusion rate of MB toward the surface of catalyst, (4) the prepared materials were superparamagnetic with a negligible coercivity and remanence, (5) the Fe3O4 nanoparticles were immobilized on the surface of GO and functionalized by the TEA. Finally, the presumption mentioned above was proved by TEM, FTIR, XRD, XPS, Zeta potential and the degradation

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experiments. After the degradation reaction, the catalyst can be quickly separated by external

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magnetic field. Therefore, the TEA/GO@Fe3O4 is a promising catalyst for the degradation of MB.

Key words: Fenton degradation; Fe3O4 composite; Triethanolamine; Graphene

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1. Introduction

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With the development of dye manufacturing industries, the waste water containing organic

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pollutants has become one of the main sources of water pollution [1]. Among these organic

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pollutants, MB is a cationic dyes, which is widely applied in textile, paper, plastics, leather, food

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and cosmetic industries. It may cause permanent injury to eyes even at low doses [2]. If ingested the water which containing MB, it will cause nausea, vomiting, profuse sweating, mental

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confusion and methemoglobinemia [3, 4]. Recently, iron oxide-based nanocomposites was widely used in catalyst considering its environmentally friendly processes, less expansive target products

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and superior recycling performance [5-8]. The Fe3O4 composite is usually applied as a Fenton-like degradation catalyst for the removal MB since it can catalyze the degradation of H2O2 and

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generate the hydroxyl radical (·OH). The generated ·OH is a green and non-selective oxidant which can react with the chromophoric groups of dye [9]. During the Fenton degradation process, the regeneration of Fe2+ is the rate-controlling step [10-12]. In recent year, the GO@Fe3O4 composite has become a research focus. Since the combination of GO and Fe3O4 not only can prevent Fe3O4 nanoparticles from agglomerating but 2

also increase the electron mobility of composite [13, 14]. Hence, GO@Fe3O4 composite is a superior catalyst for the degradation of MB in wastewater. The co-precipitation and hydrothermal method are the most widely used methods for the synthesis of GO@Fe3O4 composite [15]. Compared with co-precipitation, the hydrothermal synthesis has more uniform particle size and it

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can control the size of Fe3O4 nanoparticles more accurately by adjusting reaction conditions [16, 17]. Therefore, the hydrothermal method is the better choice for the synthesis of Fe3O4

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nanoparticles. However, the conventional hydrothermal synthesis method is to use ethylene glycol

as reductant which will cost lots of sodium acetate, ethylene glycol and polyethylene glycol [18,

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19].

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In this research, a simple one-step method for the synthesis of TEA/GO@Fe3O4 was reported.

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The triethanolamine was used as reductant which can convert FeCl3·6H2O to Fe3O4 nanoparticles.

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Not only that, the triethanolamine also acted as surfactant [20] and provides the pH environment

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required for the reaction during the synthesis process. Moreover, the excess triethanolamine and its oxidation products (Bicine and N-(2-Hydroxyethyl) iminodiacetic acid) can further interact

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with GO and Fe3O4 nanoparticles. It can improve the surface negative charge of GO/Fe3O4

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composite and enhance the diffusion rate of MB toward Fe3O4 nanoparticles. Besides, the delocalized π electrons of GO and the lone pair electrons of TEA promoted the reduction of Fe3+ to Fe2+ which is the rate-controlling step during the degradation reaction. To assess the structure

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properties and degradation mechanism of catalysts, the TEA/GO@Fe3O4 were characterized by TEM, FTIR, XRD, XPS and VSM.

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2. Experimental

2.1 Preparation of TEA/GO@Fe3O4 and Fe3O4 nanoparticle

Graphene oxide was prepared by modified Hummers method [21] and the concentration of

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synthesized graphene oxide suspension was 6 mg/L. Then, the TEA/GO@Fe3O4 was synthesized by a novel one-step hydrothermal method. Concretely, 20 mL distilled water was heated to 25 ℃.

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Then, 10 mL triethanolamine was poured into water and stirred for 30 min. Subsequently, 0.3 g FeCl3·6H2O was added into the mixture and continued to stir for 30 min. After that, 15 mL GO suspension was poured into the above mixture. 30 min later, the mixed homogeneous solution was

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transferred into Teflon stainless-steel autoclave and reacted at 200 ℃ for 6 h. Then, the obtained

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TEA/GO@Fe3O4 composite was thoroughly washed with ultrapure water until the pH of solution

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was neutral. Finally, the prepared materials were freeze-dried. The Fe3O4 nanoparticles were synthesized by similar method without the use of GO suspension. Besides, according to the five

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times synthesis results, the average weights of Fe3O4 nanoparticles and TEA/GO@Fe3O4

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composite were 82 and 165 mg, respectively. It indicated that the conversion rate of FeCl3·6H2O was 95.7% and the weight percent of Fe3O4 nanoparticles in TEA/GO@Fe3O4 composite was

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49.7%.

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2.2 Characterization

The synthesized composite materials was characterized by X-ray diffraction (XRD, D8

Advance, Bruker DaltonicsInc) with Cu Kα radiation (λ=1.5406 Å). The morphology of the composite was observed by transmission electron microscope (TEM, JEM-2100F, JEOL). The surface functional groups of composite were investigated by Fourier transform infrared (FTIR, 4

Nicolet 6700, Thermo Electron Scientific Instruments) spectroscopy. To further investigate the reaction mechanism, the GC-MS/MS (GCMS-QP2010ULTRA, Shimadzu) and LC-MS/MS (LC-20A, Shimadzu) were applied to analysis the chemical composition of residual reaction liquid. The vibrating sample magnetometer (VSM, Squid-VSM, Quantum Design) was applied to

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investigate the magnetic properties of the prepared materials.

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2.3 Degradation experiment and degradation kinetics of TEA/GO@Fe3O4 composite

The heterogeneous Fenton degradation experiment was carried out in 50 mL aqueous

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solution and the concentration of MB was 100 mg/L. A certain amount of TEA/GO@Fe3O4 and

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H2O2 were added into MB solution. Finally, the MB concentration of degraded solution was

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measured by UV-Vis spectrophotometer (WFZ UV-2100) at 664 nm. The effect of pH,

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temperature, concentration of H2O2 was investigated to obtain the optimization conditions.

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Besides, the pseudo-first-order kinetic equation was applied to evaluate the catalytic rate on the degradation of MB. Eq.(1) shows the pseudo-first order reaction kinetic:

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ln

Ct = −kt (1) C0

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Where C0 and Ct are the initial and instantaneous concentration of MB, respectively. The k

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is the rate constant and t is the reaction time.

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3. Results and discussion

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3.1 Characterization of synthesized materials

Fig.1. (a) The TEM image of TEA/GO@Fe3O4 at 0.5 μm; (b) The TEM image of TEA/GO@Fe3O4 at 0.2μm; (c) The particle size distribution of TEA/GO@Fe3O4 at 0.5 μm (d) The distance between two lattice fringes

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analyzed by Digital Micrograph (e) the green line in the lower right corner displayed the edge of TEA/GO@Fe3O4 (f) the enlarged view of the edge (g) the profile image analyzed by digital micrograph

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The morphology of TEA/GO@Fe3O4 was investigated by TEM. As shown in Fig.1 (a-d), the Fe3O4 nanoparticles show typical spherical shape and disperse on the surface of GO especially on

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the edge of GO. It can be attributed to the abundant oxygen-containing functional groups on the edge of GO and the rest Fe3O4 nanoparticles were immobilized by epoxy or hydroxyl groups on

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the plane of GO. As shown in Fig.1 (c), the average particle size of Fe3O4 nanoparticles is approximately 28.6 nm evaluated from the particle size distribution curve. The distance between two lattice fringes was analyzed by Digital Micrograph. The analysis results show that the distance between two lattice fringes is 0.288 nm corresponding to the (220) plane of Fe3O4 nanoparticles [22]. According to the enlarged view of edge, there are five layers graphene in the 6

edge of TEA/GO@Fe3O4. The thickness of each layer was analyzed by the profile image of digital micrograph and the distance between two troughs represents the thickness of each layer. It can be seen from Fig.1 (g) that the distance between two troughs is 0.351 nm. Therefore, the total thickness of TEA/GO@Fe3O4 is approximately 1.755 nm.

1718.3

1270.8 1080.3

1628.1

2920.8

2852.3

1452.5 1621.5

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3427.2

1063.9

3401.8

560.7

c

1647.1

1461.8

1743.1

2852.5

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b

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a

574.7

1089.6

2925.5

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1000

500

-1

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Wavenumber (cm )

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Fig.2. (a) The FTIR spectra of GO (b) The FTIR spectra of Fe3O4 nanoparticles (c) The FTIR spectra of TEA/GO@Fe3O4 composite

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Fig.2 shows the FTIR spectra of GO, Fe3O4 nanoparticles and TEA/GO@Fe3O4 composite.

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The peaks of GO located at 3427.25 cm-1, 1718.34 cm-1, 1270.8 cm-1 corresponding to -OH stretching vibration, C=O stretching vibrations of the –COOH groups and C-O-C of epoxy group,

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respectively [23]. The peaks at 1080.30 and 1398.16 cm-1 were attributed to stretching vibration of alkoxy C-O and carboxyl O=C-O, respectively. Moreover, the peak at 1628.09 cm-1 could be

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assigned to C=C skeletal vibration from unoxidized graphitic domains. These results indicated that GO contains varieties of oxygen-containing functional groups such as –COOH, -OH, C-O-C. The new peaks of Fe3O4 nanoparticles at 2920.8 and 2852.3 cm-1 were corresponding to the –CH2 asymmetric and symmetric stretch vibration of TEA [24]. The new peaks located at 560.7 and 1452.5 cm-1 can be assigned to the stretching vibration of Fe-O and C-N, respectively [25-27]. 7

Besides, the peak located at 1621.5 cm-1 represents the stretching vibration of C=O considering the possible structure of reacted triethanolamine. Compared with GO and Fe3O4 nanoparticles, the frequency of C=O, C-O, C=C, C-N and Fe-O on TEA/GO@Fe3O4 shifted to higher value. Therefore, the FTIR research results prove that the TEA has combined with GO and Fe3O4

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nanoparticles. 311

Intensity (a.u.)

400

511 422

400

511

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440 220

Fe3O4

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20

30

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422 50

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2 (degree)

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002

440

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220

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311

Fig.3. XRD patterns of Fe3O4 and TEA/GO@Fe3O4 composite

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The crystalline structure was investigated by XRD. Fig.3 showed the XRD patterns of the

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synthesized Fe3O4 nanoparticles and TEA/GO@Fe3O4 composite. The Fig.3 indicated that Fe3O4 nanoparticles had major well-defined peaks at 2θ=30.3, 35.6, 43.3, 53.7, 57.1 and 62.7°,

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corresponding to lattice plane (220), (311), (400), (422), (511) and (440), respectively. Therefore, the major crystal phase of Fe3O4 nanoparticles was in a good agreement with the JCPDS card No.

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88-0315 [28]. Compared with Fe3O4 nanoparticles, the major peaks of TEA/GO@Fe3O4 were basically similar to the Fe3O4 nanoparticles. Besides, TEA/GO@Fe3O4 composite showed new peak at 26.7° corresponding to the lattice plane (002) of graphite. The particle size of the Fe3O4 nanoparticles and TEA/GO@Fe3O4 composite were calculated by Debye-Scherrer equation. The equation showed in Eq.(2) where D (nm) is the average particle size, λ (nm) is the wave length of 8

Cu-Kα irradiation, β is the full width at half maximum intensity of the diffraction peak and θ is the diffraction angle for the each peak [29]. According to the calculated results, the average particle size of Fe3O4 nanoparticles is 28.6 nm and the average particle size of TEA/GO@Fe3O4 is 27.5 nm. It indicated the application of GO has little effect on the synthesis of Fe3O4 nanoparticles. (2)

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0.89λ βcosθ

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D=

Fig.4. (a) Mass spectra of residual reaction liquid obtained by LC-MS system, in positive ionization mode. (b)~ (d) Similarity search results of GC-MS/MS

To investigate the oxidation products of triethanolamine, the residual reaction solution was

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added to rotary evaporator to remove water. Then, the obtained organic components were dissolved in methanol solution for the further test. Fig.4 shows the results of LC-MS/MS and GC-MS/MS. As shown in Fig.4 (a), the m/z at 150, 172 and 188 are the TEA which combined with H+, Na+ and K+, respectively. The m/z at 164 and 216 can be attributed to the Bicine and

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N-(2-Hydroxyethyl) iminodiacetic acid which combined with H+ and K+, respectively. Moreover, GC-MS/MS show the similar results that the main chemistry compositions of residual reaction product are triethanolamine, Bicine and N-(2-Hydroxyethyl) iminodiacetic acid. Hence, considering the analysis results of LC-MS/MS and GC-MS/MS, the oxidation products of

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triethanolamine are Bicine and N-(2-Hydroxyethyl) iminodiacetic acid. Fig.5 shows the synthesis mechanisms of TEA/GO@Fe3O4 composite and Fe3O4 nanoparticles. For TEA/GO@Fe3O4, at first,

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the Fe3+ were adsorbed onto the surface oxygen-containing functional groups of GO. Then, the adsorbed Fe3+ could be reduced by triethanolamine and the reacted triethanolamine was converted

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to Bicine and N-(2-Hydroxyethyl) iminodiacetic acid. In the mean time, the excess TEA could

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further interact with Fe3O4 nanoparticles and GO. The synthesis mechanism of Fe3O4

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nanoparticles are similar to TEA/GO@Fe3O4 composite without the use of GO.

Fig.5. The synthesis mechanism of TEA/GO@Fe3O4 and Fe3O4 nanoparticles

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(b)

Intensity (a.u)

Intensity (a.u)

C 1s

(a)

O 1s Fe 2p N 1s

Fe 2p O 1s

C 1s N 1s 1000

800

600

400

200

0

1200

Binding energy (eV) (c)

1000

800

600

400

Binding energy (eV) (d)

Fe 2p3/2

C-C

730

725

720

715

710

705

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290

288

286

284

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Binding energy (eV)

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Binding energy (eV)

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π-π *

0

C=C C-N C-O

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Intensity (a.u)

Fe 2p1/2

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1200

Fig.6. (a) The survey scan XPS spectra of TEA/GO@Fe3O4 (b) The survey scan XPS spectra of TEA/Fe3O4 (c)

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The Fe 2p high-resolution spectra of TEA/GO@Fe3O4 (d) The C 1s high-resolution spectra of TEA/GO@Fe3O4

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The X-ray photoelectron spectroscopy (XPS) was applied to further confirm the synthesis of

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TEA/GO@Fe3O4 and TEA/Fe3O4. As shown in Fig.6 (a) and Fig.6 (b), the survey scan XPS spectra of TEA/GO@Fe3O4 and TEA/Fe3O4 showed that the synthesized composite materials all

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contain C 1s, N 1s, O 1s and Fe 2p peaks which means the TEA has combined with GO and Fe3O4

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nanoparticles. The lower Fe 2p intensity of TEA/GO@Fe3O4 can be explained by the fact that the GO and TEA partially covered on the surface of Fe3O4 nanoparticles. Fig.6 (c) showed two characteristic peaks at 710.3 and 723.9 eV which can be attributed to the Fe 2p3/2 and Fe 2p1/2

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spin-orbit peak of Fe3O4 since there are no obvious satellite peak between 710.3 and 723.9 eV [30]. Fig.6 (d) showed the C 1s high-resolution spectra of TEA/GO@Fe3O4. The C 1s can be deconvoluted into five peaks at 283.48, 284.56, 285.28, 286.38 and 288.91 eV. These peaks can be assigned to C-C, C=C, C-O, C-N and π-π* bonding of GO and TEA, respectively [31, 32].

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Fe3O4 TEA/GO@Fe3O4

40 20 0 -20 -40 -60 -80 -15000

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-5000

0

5000

10000

15000

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Magnetic Field (Oe)

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Magnetization (emu/g)

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Fig.7. Magnetic hysteresis of Fe3O4 and TEA/GO@Fe3O4 nanocomposite

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The VSM magnetization curves of Fe3O4 nanoparticles and TEA/GO@Fe3O4 nanocomposite

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were tested in a magnetic field of ±15 kOe at 25 ℃. As shown in Fig.7, saturation magnetization

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values (Ms) of Fe3O4 and TEA/GO@Fe3O4 nanocomposite were 66.6 and 35.5 emu/g,

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respectively. The lower saturation magnetization value of TEA/GO@Fe3O4 can be explained by

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the existence of non-magnetic GO. Moreover, since the size of Fe3O4 nanoparticles changed little after combining with GO. It indicated that the content of Fe3O4 nanoparticles on nanocomposite

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was approximately 53% which is basically consistent with the results of experiment. The S-shaped

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magnetic hysteresis loops suggested that the prepared materials were super paramagnetic with a negligible coercivity and remanence [33]. Therefore, the prepared Fe3O4 and TEA/GO@Fe3O4

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composite can be recovered by external magnetic field quickly.

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3.2 Optimization conditions of degradation experiment

3.2.1. Effect of pH 60

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0.0

30

20

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25

30

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(b) 2

3

t (min)

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1.0

Iron ion concentration (mg/L)

pH 2.0 pH 3.0 pH 4.0 pH 5.0 pH 6.0 pH 7.0 pH 8.0 pH 9.0

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5

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7

8

9

pH

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353 K, MB: 100 mg/L, TEA/GO@Fe3O4: 200 mg/L, H2O2: 200 mM)

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Fig.8. (a) The effect of pH on the degradation of MB (b) The iron concentration at pH varied from 2.0 to 9.0. (T:

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Fig.8 showed the effect of pH on the degradation of MB using synthesized TEA/GO@Fe3O4

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composite. The degradation experiment was conducted at pH varies from 2.0 to 9.0 and the pH of MB solution was adjusted by dilute HNO3 and NaOH. As shown in Fig.8 (a), the degradation rate

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of MB decreased with the increasing pH. As we know, the H2O2 can generate ·OH radical with the

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use of Fe2+ and the ·OH radical can degrade MB effectively. Therefore, lower pH is benefit to degradation reaction since Fe3O4 nanoparticles can generate more Fe2+ at lower pH. The low

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degradation rate at higher pH can be explained by the precipitation of Fe3+ and the adsorption of iron hydroxides onto TEA/GO@Fe3O4 composite, as reported in the previous literature [34, 35].

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Although, the lower pH is benefit to degradation, the results of Fig.8 (b) showed that large quantities of Fe3O4 nanoparticles will dissolve at pH lower than 4. It illustrated that lower pH reduce the reusability of TEA/GO@Fe3O4 composite and may cause secondary pollution. Hence, pH at 4.0 is the best degradation condition considering the effect of pH and reusability of TEA/GO@Fe3O4 composite. 13

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-120 0

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pH

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Zeta potential (mV)

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Fig.9. The zeta potential of different materials at pH varied from 1.0~9.0

To further understand the surface properties of synthesized materials, the zeta potential of

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materials at different pH was investigated by zeta potential analyzer (ZetaPLAS, Brookhaven

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Instruments). Among these materials, the Fe3O4 was synthesized by traditional co-precipitation

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method [36]. The TEA/Fe3O4 represented the Fe3O4 modified by TEA and TEA/GO@Fe3O4 was

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the synthesized catalyst. It can be seen from Fig.9 that the zeta potentials of these materials

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decreased with the rising pH. The zeta potential of TEA/GO is lower than Fe3O4 which can be attributed to the modification of TEA on the surface of Fe3O4 nanoparticles. Compared with

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TEA/Fe3O4, the TEA/GO@Fe3O4 showed much more negative zeta potential since the

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combination of GO. Since MB is a cationic dye and the negative zeta potential indicated that the prepared TEA/GO@Fe3O4 composite has a negative surface charge under experimental conditions. Therefore, the negative surface charge can enhance the diffusion rate of MB toward catalyst

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surface through electrostatic interaction.

3.2.2 Effect of temperature

The effect of temperature on the degradation of MB using TEA/GO@Fe3O4 composite was

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investigated and the temperature was in the range of 303 K-353 K. It can be seen from Fig.10 that the degradation efficiency and degradation rate increased dramatically with the rising temperature. The MB degrades approximately 90% at 353 K in 10 min. To further investigate the effect of temperature, the Arrhenius equation was applied to evaluate the activation energy of each

lnk = -

Ea RT

+ lnA0

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temperature. Eq. (3) shows the Arrhenius equation: (3)

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where Ea(kJ/mol) is the activation energy, R (8.314 J·mol-1·K-1) is ideal gas constant, T (K) is

the Kelvin temperature, A0 is a frequency factor and k is the measured first-order rate constant.

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The calculated Ea listed in Table.1. It can be seen from Table.1 that the rate constant increased

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rapidly with the increasing temperature and the value of Ea was 67.01 kJ/mol. These results

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TEA/GO@Fe3O4 composite.

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showed that the temperature is an important factor for the degradation of MB using

Table.1 The related parameters of Arrhenius equation

k

303 313 323 333 343 353

0.0027 0.0076 0.0090 0.0370 0.0621 0.1129

R2

0.917 0.989 0.931 0.988 0.986 0.922

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T (K)

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Ea (kJ/mol)

R2

67.01

0.965

1.0

0.8

0.4

30℃ 40℃ 50℃ 60℃ 70℃ 80℃

0.2

0.0 0

10

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30

40

50

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C/C0

0.6

60

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t (min)

Fig.10. The effect of temperature on the degradation of MB (pH: 4.0, MB: 100 mg/L, TEA/GO@Fe3O4: 200 mg/L, H2O2: 200 mM)

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3.2.3 Effect of H2O2 concentration

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In Fenton oxidation system, the oxidant concentration is an important factor that can

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significantly influence the degradation of MB. During the degradation process, the H2O2 can generate amounts of ·OH which can oxidize the MB into inorganic. The effect of H2O2

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concentration on the degradation of MB was evaluated in the range of 10-200 mM. 1.0

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0.8

0.6

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C/C0

10 mmol/L 20 mmol/L 30 mmol/L 40 mmol/L 200 mmol/L

0.4

0.2

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0.0

0

5

10

15

20

25

30

35

t (min) Fig.11. The effect of H2O2 concentration on the degradation of MB (T: 353 K, pH: 4.0, MB: 100 mg/L, TEA/GO@Fe3O4: 200 mg/L)

As shown in Fig.11, the degradation rate of MB decreased with the increase of H2O2

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concentration (10 to 200 mM). This phenomenon indicated that the excessive H2O2 act as radical scavengers giving rise to both inhibition of MB removal and self annihilation which can reduce the effective reaction between MB and ·OH [37]. The Eq.(4)-(6) show the effect of excessive H2O2 during the degradation process.

HO2·+·OH H2O+O2

(5) (6)

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2·OH H2O2

(4)

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H2O2+·OH HO2·+H2O

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3.3 A comparative study

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To further investigate the effect of different components during degradation process, the

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components (TEA/GO@Fe3O4, H2O2, Fe3O4/H2O2, and TEA/GO@Fe3O4/H2O2) were applied for

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the removal of MB (100 mg/L) at 353K, respectively. Fig.12 shows the effect of different

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components on the removal of MB. The TEA/GO@Fe3O4 showed the lowest removal efficiency since it only can remove MB through adsorption. The removal efficiency of H2O2 is much lower

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than Fe3O4 and TEA/GO@Fe3O4/H2O2 due to the lower generate rate of ·OH. Although, the

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removal efficiency of Fe3O4/H2O2 is more than 99%, its removal rate is much lower than TEA/GO@Fe3O4/H2O2. It can be explained by the fact that the agglomeration of Fe3O4 nanoparticles will decrease the contact area and the negative surface charge of TEA/GO@Fe3O4

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can enhance the diffusion rate of MB toward Fe3O4 nanoparticles. Besides, the delocalized π electrons of GO and the lone pair electrons of TEA can facilitate the reduction of Fe3+ by donating electrons [38]. Hence, the prepared TEA/GO@Fe3O4 composite not only can prevent the agglomeration of Fe3O4 nanoparticles but also accelerate the degradation rate of MB considering

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its superior properties. Moreover, the TEA/GO@Fe3O4 was compared with previous reported Fenton degradation catalyst. As shown in Table.2, the degradation rate of synthesized TEA/GO@Fe3O4 was much faster than other materials. Therefore, the comparative results indicated that the TEA/GO@Fe3O4 is a superior Fenton degradation catalyst for the removal of

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MB. 1.0

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0.8

C/C0

0.6

0.4

TEA/GO@Fe3O4 H2O2

U

Fe3O4/H2O2

0.2

N

TEA/GO@Fe3O4/H2O2

0

5

10

15

20

25

30

35

M

t (min)

A

0.0

Fig.12. The effect of different components on the removal of MB (T: 353K, pH: 4.0, MB: 100mg/L, Fe 3O4 and

ED

TEA/GO@Fe3O4: 200 mg/L, H2O2: 10 mM)

Table.2 Compared with other Fenton degradation catalyst

Time (min)

PT

Materials

CC E

rGO/CoFe2O4 Fe3O4/Fe/Fe3C@PCNF Fe0-Fe3O4-RGO RGO/Fe3O4 TEA/GO@Fe3O4

25 10 60 60 5

Degradation efficiency (%)

Reference

99 99 99 96 99

[2] [39] [40] [41] This work

A

3.4 The regeneration of Fe2+ and adsorption mechanism investigated by XPS 3.4.1 The ratio of Fe2+/Fe3+ calculated by XPS As we known, the reaction rate of Fe2+ is much faster than Fe3+ during the Fenton degradation process. Therefore, the ratio of Fe2+/Fe3+ on Fe3O4 nanoparticles will decrease inevitably after degradation reaction. In this research, the ratio of Fe2+/Fe3+ on TEA/GO@Fe3O4,

18

TEA/Fe3O4 and co-precipitation Fe3O4 was investigated to prove the effect of TEA and GO on the fast regeneration of Fe2+. As shown in Fig.13, the Fe 2p3/2 peak was deconvoluted into Fe2+ and Fe3+ peaks and the ratio of Fe2+/Fe3+ was calculated by the area ratio of Fe2+ and Fe3+ peak. Table.3 shows the ratio of Fe2+/Fe3+ on synthesized materials before and after degradation. The

IP T

ratio of Fe2+/Fe3+ on co-precipitation Fe3O4 reduced the most since there were no organic molecule modify on its surface. Compared with co-precipitation Fe3O4, the ratio of Fe2+/Fe3+ on

SC R

TEA/Fe3O4 reduced smaller which can be attributed to the lone pair electrons on TEA promote the regeneration of Fe2+. The ratio of Fe2+/Fe3+ on TEA/GO@Fe3O4 reduced only 2.2%. It can be

U

explained by the effect of delocalized π electron. Eq. (7) showed the effect of TEA/GO@Fe3O4

N

during the degradation process. Therefore, the application of TEA and GO can promote the

714

712

710

708

(b)

Intensity (a.u)

ED

(a)

(7)

M

Fe2+

PT

Intensity (a.u)

TEA/GO@Fe3O4(e-)＋ Fe3+

A

regeneration of Fe2+ which can accelerate the degradation rate of MB.

706

716

714

712

710

708

714

(e)

714

712

710

708

712

710

708

Binding energy (eV)

Binding energy (eV)

716

710

(d)

(f)

Intensity (a.u)

A

Intensity (a.u)

714

712

Binding energy (eV)

Intensity (a.u)

(c)

CC E

Intensity (a.u)

Binding energy (eV)

708

716

Binding energy (eV)

714

712

710

Binding energy (eV)

19

708

Fig.13. (a) The ratio of Fe2+/Fe3+ on TEA/GO@Fe3O4 before degradation (b) The ratio of Fe2+/Fe3+ on TEA/GO@Fe3O4 after degradation (c) The ratio of Fe2+/Fe3+ on TEA/Fe3O4 before degradation (d) The ratio of Fe2+/Fe3+ on TEA/Fe3O4 after degradation (e) The ratio of Fe2+/Fe3+ on co-precipitation Fe3O4 before degradation (f) The ratio of Fe2+/Fe3+ on co-precipitation Fe3O4 after degradation. Table. 3 The ratio of Fe2+/Fe3+ before and after degradation

Before degradation

After degradation

Change percent

TEA/GO@Fe3O4 TEA/Fe3O4 co-precipitation Fe3O4

0.44 0.45 0.56

0.43 0.43 0.52

2.3% 4.4% 7.1%

3.4.2 The adsorption mechanism analyzed by XPS 709.88

(a)

(b)

723.18

730

725

720

715

710

705

700

740

Binding energy (eV)

A Intensity (a.u)

M

Intensity (a.u)

(d)

PT

ED

165.81

167

166

165

735

164

730

725

720

715

710

705

700

Binding energy (eV)

164.63

(c)

U

735

N

740

709.68

SC R

Intensity (a.u)

Intensity (a.u)

723.68

IP T

Materials

163

162

Binding energy (eV)

163.30

164.84

166

164

162

160

Binding energy (eV)

CC E

Fig.14. (a) The Fe 2p spectra of TEA/GO@Fe3O4 before adsorption (b) The Fe 2p spectra of TEA/GO@Fe3O4 after adsorption (c) The S 2p spectra of MB before adsorption (d) The S 2p spectra of MB after adsorption.

The XPS analysis was applied to further investigate the adsorption mechanism of MB onto

A

TEA/GO@Fe3O4. As shown in Fig.14, the Fe 2p1/2 and Fe 2p3/2 peaks of TEA/GO@Fe3O4 changed from 723.68 to 723.18 eV, 709.88 eV to 709.68 eV, respectively. It suggested that the outer electron cloud density of Fe atom on TEA/GO@Fe3O4 increased after adsorption. In the mean time, the binding energy of S 2p1/2 and S 2p3/2 decreased from 165.81 to 164.84 eV, 164.63 to 163.30 eV, respectively. Therefore, it speculated that the MB can be adsorbed onto 20

TEA/GO@Fe3O4 through the coordination between Fe and S.

3.5 Recovery and reusability of catalyst

IP T

80

60

40

20

0 1

2

3

4

5

6

7

8

9

10

U

Cycle number

SC R

Degradation efficiency %

100

N

Fig.15. The degradation efficiency of TEA/GO@Fe3O4 at 15 min during ten times cycle (T: 353 K, pH: 4.0, MB: 100 mg/L, TEA/GO@Fe3O4: 200 mg/L, H2O2: 10 mM)

M

A

According to the previous studies, the reusability of metal oxide catalyst will be affected by catalyst lost during separation process and leaching of metal ions from nanoparticles which may

ED

reduce the catalytic activity of catalyst [42]. The reusability experiment of TEA/GO@Fe3O4 was

PT

carried out at 353K, pH 4.0, 10 mM H2O2 and the concentration of MB was 100 mg/L. After 35 min degradation, the degradation efficiency of catalyst basically can reach 99%. However, the

CC E

degradation rate of TEA/GO@Fe3O4 decreased slightly compared with first cycle. Fig.15 showed the degradation efficiency of TEA/GO@Fe3O4 at 15 min. The results indicated that the

A

degradation efficiency of TEA/GO@Fe3O4 decreased to 74.2% after ten times cycle. It may attributed to the loss of TEA/GO@Fe3O4 during recovery and the structure of TEA partially changed during degradation. All in all, the TEA/GO@ Fe3O4 can maintain superior properties since the coating of GO and lower H+ concentration.

21

4. Conclusions In this study, the TEA/GO@Fe3O4 composite materials were synthesized by a simple one-step hydrothermal method and the prepared materials were characterized by TEM, FT-IR, XRD, VSM, XPS and zeta potential techniques. The characterization results proved the synthesis

IP T

of the TEA/GO@Fe3O4. Besides, the results of LC-MS/MS, GC-MS/MS showed that the triethanolamine will be converted to Bicine and N-(2-Hydroxyethyl) iminodiacetic acid after

SC R

oxidation reaction. The zeta potential indicated TEA/GO@Fe3O4 can accelerate the diffusion rate

of MB toward catalyst surface through electrostatic interaction. The degradation experiments

U

showed temperature, pH and H2O2 concentration have significant influence on the degradation of

N

MB. The optimal conditions for the degradation of MB using TEA/GO@Fe3O4 were 353 K, pH

A

4.0, 10 mM H2O2. The comparative study and the change of Fe2+/Fe3+ calculated by XPS showed

M

that the lone pair electrons of TEA and delocalized π electrons of GO can induce the conversion of

ED

Fe3+ into Fe2+ which is the rate-controlling step of degradation process. The reusability experiments indicated TEA/GO@Fe3O4 has superior stability which is an important factor for the

PT

application of catalyst. Finally, this research proposed a novel method for the synthesis of

CC E

GO@Fe3O4 composites and the results of degradation experiment revealed that the prepared TEA/GO@Fe3O4 can be applied as an effective catalyst for the degradation of MB.

A

Acknowledgements This research was supported by National Natural Science Foundation of China (No.

21776320), the Hunan Provincial Science and Technology Plan Project (No.2016TP1007), the Open-End Fund for the valuable and precision Instruments of Central South University (CSUZC201827) and National Key Technology Support Program (No.2015BAB17B01).

22

Reference

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ED

M

A

N

U

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IP T

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