Ag6Si2O7 under simulated visible light

Ag6Si2O7 under simulated visible light

    Photocatalytic degradation of methylene blue by magnetically recoverable Fe3 O4 /Ag6 Si2 O7 under simulated visible light Huiping Che...

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    Photocatalytic degradation of methylene blue by magnetically recoverable Fe3 O4 /Ag6 Si2 O7 under simulated visible light Huiping Chen, Nan Chen, Yu Gao, Chuanping Feng PII: DOI: Reference:

S0032-5910(17)30979-8 doi:10.1016/j.powtec.2017.12.029 PTEC 13018

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

10 May 2017 23 November 2017 4 December 2017

Please cite this article as: Huiping Chen, Nan Chen, Yu Gao, Chuanping Feng, Photocatalytic degradation of methylene blue by magnetically recoverable Fe3 O4 /Ag6 Si2 O7 under simulated visible light, Powder Technology (2017), doi:10.1016/j.powtec.2017.12.029

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ACCEPTED MANUSCRIPT Photocatalytic degradation of methylene blue by magnetically recoverable

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Fe3O4/Ag6Si2O7 under simulated visible light

School of Water Resources and Environment, China University of Geosciences

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a

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Huiping Chena, Nan Chena,b,*, Yu Gaoc, Chuanping Fenga,b

b

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(Beijing), Beijing, 100083, China;

Key Laboratory of Groundwater Cycle and Environment Evolution (China

College of Chemical and Environmental Engineering, Shandong University of

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c

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University of Geosciences (Beijing)), Ministry of Education, Beijing, 100083, China;

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Science and Technology, Qingdao 266590, China.

*Correspondence: Nan Chen, School of Water Resources and Environment, China

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University of Geosciences (Beijing), Beijing, 100083, China. Tel: +86 10 82322281 Fax: +86 10 82321081 E-mail: [email protected] (N. Chen)

ACCEPTED MANUSCRIPT Abstract

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Methylene blue (MB), a typical industrial dye, causes undesirable water pollution

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and is a serious concern in aquaculture industries. In this study, Fe3O4/Ag6Si2O7

The

Fe3O4/Ag6Si2O7

composites

were

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photocatalyst was prepared to remove MB from water under simulated visible light. fabricated

via

anchoring

Ag6Si2O7

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nanoparticles on the surface of spherical Fe3O4 by precipitation process. Various

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parameters of reaction were tested in batch experiments, including mass ratio of

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Fe3O4, catalyst dosage, initial MB concentration and solution pH. It can be observed

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that 98% of MB could be degraded under optimum conditions and the

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Fe3O4/Ag6Si2O7 composite performed commendably from pH 2.0 to 10.0. Reactive species trapping experiments revealed that the superoxide radical (·O2-) played

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a prominent part in the photo-degradation mechanism. Some intermediate products produced during the degradation process were also detected. The magnetic properties and high photocatalytic activity of Fe3O4/Ag6Si2O7 particles make it promising employed for practical application in water treatment.

Keywords: Methylene blue (MB); Magnetic separation; Photocatalyst; Simulated visible light; Ag6Si2O7; Water treatment

ACCEPTED MANUSCRIPT HIGHLIGHTS

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 A novel magnetically separable visible-light-driven Fe3O4/Ag6Si2O7 photocatalyst

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was prepared for the degradation of MB.

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 MB degradation was rapid and followed a pseudo-first-order reaction kinetics.

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 MB degradation by Fe3O4/Ag6Si2O7 was observed in a wide pH range.

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 Superoxide radical should be the predominant oxidant during the action.

ACCEPTED MANUSCRIPT 1. Introduction

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Water pollution has become an issue worldwide with the rapid development of

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population growth, industrialisation and urbanization [1-4]. Organic contaminants

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have become a significant threat to the environment, because they are recalcitrant and

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can persist for a long time in aqueous solution [5]. Many techniques like ozonation [6], chlorination [7], adsorption [8], biodegradation [9] and electrochemistry [10] have

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been employed to remove organic contaminants from wastewater. Although these

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technologies are effective in organics removal from wastewater, they still have lots of

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drawbacks in the application process. The operating cost could be high for ozonation

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and electrochemical method, the operation cycle is long for biodegradation, some undesirable products would be produced in the chlorination process, and pollutants

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cannot be degraded by adsorption [11]. Therefore, it is urgent to develop an efficient and rapid method for the removal of organic contaminants. Photocatalysis has been considered to be a “green” technology for treating organic contaminants with simulated visible light due to its high efficiency, non-selectively, total destruction ability and cost effectiveness [12, 13]. Among various semiconductor photocatalysts, TiO2 is one of the most extensively studied photocatalysts in view of degrading the organic contaminants [14-16]. However, limited by its wide band gap

ACCEPTED MANUSCRIPT (Eg = 3.2 eV), TiO2 can only absorb UV light which constitutes about 4% of the solar

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spectrum, seriously limiting the utilization of solar light [17-20]. Therefore, it is

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necessary to develop a photocatalyst material having improved visible light activity to

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more effectively use visible light, which accounts for about 43% of solar light. On the

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other hand, separation and recovery of single micro- and nano-scale photocatalyst from the treated solution is difficult and expensive [19, 21]. The remained

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photocatalysts could also generate secondary pollution [22]. Therefore, it is highly

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desirable to develop a photocatalyst with efficient visible light activity and easy

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solid-liquid separation for practical application in industrial processes.

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Silicate is easily available, abundant reserves on earth and chemically stable, so it was used in many fields [23, 24]. Recently, it has been reported that silicates have

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been used as photocatalyst for degradation of organic matter [25, 26]. Lou et al have synthesized a newly visible-light-driven photocatalyst (Ag6Si2O7), which was prepared by the hydrolysis and ion-exchange method [27]. It exhibited a strong photocatalytic activity nearly in the whole visible light region (< 740 nm) owing to its special electronic structure. In order to solve the problem of difficult separation and recovery of photocatalyst, many researchers have immobilized catalysts on supporting materials such as activated carbon [28], glass [29], and Al fiber [30].

ACCEPTED MANUSCRIPT Superparamagnetic Fe3O4 has a cubic inverse spinel structure and non-equivalent

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cations in two valence states, (Fe2+, Fe3+) leading to formation of a unique magnetic

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structure [31, 32]. It has been proved that the photocatalyst supported on Fe3O4 was an

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effective method to separate and recycle the photocatalyst from the treating solution

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[33-36].

In this study, novel magnetically separable Fe3O4/Ag6Si2O7 nanocomposites were

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synthesized for methyl blue (MB) removal under simulated visible light irradiation.

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The morphology, chemical composition, spectroscopic, and magnetic properties of

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as-prepared samples were studied by scanning electron microscope (SEM), X-ray

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photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectrum (DRS), and vibrating sample magnetometry (VSM). Influencing factors such as Fe3O4 ratio,

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catalyst dosage, initial MB concentration and pH value were also investigated in batch experiments. Photocatalytic degradation mechanism was explored by active species trapping experiment. Meanwhile, some intermediates produced during the reaction were also analyzed. 2. Experimental 2.1. Synthesis of Fe3O4/Ag6Si2O7 Fe3O4 nanoparticles were synthesized via a solvothermal method [33]. During the

ACCEPTED MANUSCRIPT preparation process of Fe3O4, 2.70 g of FeCl3·6H2O and 4.10 g of NaAC·3H2O were

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dissolved in 80 mL of ethylene glycol. Then, the above solution was sealed into a 100

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mL Teflon-lined stainless steel autoclave. The autoclave was slowly heated to 180 oC

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and maintained at this temperature for 24 h. After that, black precipitate was obtained

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at the bottom of the autoclave and separated by an external magnetic field. At last, the precipitate was washed three times with ethyl alcohol land distilled water, and dried at

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60 oC for 12 h.

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Fe3O4/Ag6Si2O7 was prepared by a simple precipitate route at room temperature (25

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± 1 oC). 0.4 g of Fe3O4 and 5.10 g of AgNO3 were dispersed into 60 mL of deionized

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water. Then, Na2SiO3 aqueous solution (20 mL, 0.5 M) was added dropwise to the above solution under continuous mechanical vibration. Then, the obtained products

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were separated by an external magnetic field and washed three times with distilled water. After that, the samples were dried at 60 oC in an oven for 12 h. The obtained Fe3O4/Ag6Si2O7 composite (the nominal mass ratio Fe3O4 to Ag6Si2O7 is 1:10) was noted as 10%Fe3O4/Ag6Si2O7. The x%Fe3O4/Ag6Si2O7 (x% is the nominal mass ratio Fe3O4 to Ag6Si2O7) with other mass ratio (x = 20, 40, 60 and 80, respectively) were synthesized by the same process except adding different amounts of Fe3O4. Pure Ag6Si2O7 particles were prepared under the same process without the presence of

ACCEPTED MANUSCRIPT Fe3O4 powders.

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2.2. Photocatalytic degradation experiments

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In the photocatalytic system, a 250 mL beaker was used as the reactor and was

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equipped with a magnetic stirrer. The photocatalytic activities of the samples were

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evaluated by degradation of aqueous MB under a Xe lamp (15 mW/cm2, CEL-HXF300, China), the distance between MB solution and the lamp was about 10

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cm. Photocatalysts and 100 mL of MB solution were added into the beaker. Prior to

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photocatalytic reaction, the suspension was magnetically stirred for 10 min in

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darkness to establish an adsorption-desorption equilibrium between photocatalysts

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and MB. Afterwards, the lamp was switched on and the photocatalytic reaction began. At a certain time interval, 2 mL of the liquid was taken out and filtered (0.22 µm

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drainage membrane) for the removal of catalyst particles. Finally, the supernatants were analyzed by measuring the absorbance with a UV-vis spectrophotometer (DR6000, HACH, USA) at 664 nm. Reaction conditions were summary as follows: different catalyst dosage with 0.02, 0.05, 0.10, 0.15 and 0.20 g, different initial MB concentration with 5, 10, 20, 30 and 50 mg/L, initial solution pH value within the range of 3.0-10.0. Either 0.1 M HCl or 0.1 M NaOH solution was used to achieve the desired pH value. Each test was conducted in triplicate and averaged results were

ACCEPTED MANUSCRIPT reported.

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2.3. Reactive species detection

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Reactive species trapping experiments were performed to detect the active species

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during the photocatalytic process. Superoxide radical (·O2-), hydroxyl radical (·OH),

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and holes (h+) can be consumed by adding 1 mM p-benzoquinone (BQ, a ·O2- radical scavenger), 1 mM iso-propanol (IPA, a ·OH radical scavenger), and 1 mM ethylene

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diamine tetraacetic acid disodium (EDTA-2Na, a h+ radical scavenger).

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2.4. Characterization

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The morphologies of the as-prepared samples were examined by SEM

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(JSM-7001F, Shimadzu, Japan). XPS was performed to determine the chemical composition and chemical states (ESCALAB 250Xi, Thermo Fisher, USA). UV-vis

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DRS was recorded within the 200-800 nm wavelength range using a UV-visible-near infrared spectrophotometer (Cary 5000, Varian, USA). The magnetic properties of the sample was measured by a vibrating sample magnetometer (7307, LakeShore, USA) with an external magnetic field ranging from -10000 to +10000 G. The zeta potential was measured at various pH with a micro electrophoresis instrument (JS94H, Shanghai, China). A gas chromatograph (Agilent 7890A, Agilent Technologies, USA) with an Agilent 5975C mass selective detector, was used to detect the degradation

ACCEPTED MANUSCRIPT products.

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3. Results and discussion

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3.1. Photocatalytic degradation

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3.1.1. Effect of Fe3O4 mass ratio

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In order to select the optimum photocatalyst, a series of catalysts with different materials ratio were prepared and their degradation efficiency was evaluated under the

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same conditions. The results are shown in Fig. 1(a). It can be seen that in the presence

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of 100%Fe3O4/Ag6Si2O7, the degradation efficiency of MB dyes could be hardly

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taken place. All other photocatalysts could degrade more than 97% of MB within 10

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min. To eliminate the influence of the dark reaction stage on the degradation rate of MB, kinetic plots over different samples were investigated in Fig. 1(b). The reaction

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of photocatalytic degradation of organic compounds followed a pseudo-first-order kinetic equation (Eq.1): ln (C0 / C) = kappt

(1)

where C0 is the initial concentration of MB (mg/L); C is the concentration of MB at time t (mg/L); t is the irradiation time (min) and kapp is the reaction rate constant (1/min). The k values of the samples (from 0%Fe3O4/Ag6Si2O7 to 100%Fe3O4/Ag6Si2O7)

ACCEPTED MANUSCRIPT were 0.38442, 0.38694, 0.40359, 0.49538, 0.48582, 0.46136 and 0.00335 1/min,

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respectively. It can be observed that with the increase of the amount of Fe3O4, k value

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increased gradually. This may be attributed to the combination of magnetite and

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silicate silver. Electron was captured by Fe3+, inhibiting the recombination of electron

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hole pairs, which accelerated the reaction rate. Burst phenomenon was occurred when the amplitude of the input signal increased. The content of silver silicate decreased

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with the increasing content of Fe3O4, resulting in the reducing of active sites.

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Therefore, the photocatalytic activity was inhibited with the increased Fe3O4,

combination

with

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In

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accompanied by the decreased k value. the

magnetic

properties

of

the

photocatalyst,

60%Fe3O4/Ag6Si2O7 could be separated completely under the magnetic field, and the

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40%Fe3O4/Ag6Si2O7

had

a

small

amount

of

residue.

Therefore,

the

60%Fe3O4/Ag6Si2O7 composite was selected as the most optimal photocatalyst, and was used for subsequent research. 3.1.2. Effect of catalyst dosage It has been reported that the catalyst had an optimal dosage to ensure more active sites and effective absorption of photons in the process of the degradation of organic compounds [37, 38]. To investigate the influence of catalyst dosage on MB removal, a

ACCEPTED MANUSCRIPT catalyst dosage series (from 0.02 to 0.20 g) was carried out and the results are

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presented in Fig. 2. It is clear that the photocatalytic efficiency increased as the

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dosage of 60%Fe3O4/Ag6Si2O7 increased to 0.10 g. When using 0.10 g catalyst,

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91.7% of MB was removed in the first 2 min, which is much higher than those

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situations (36.9% and 71.3%) with 0.02 and 0.05 g catalyst. The results indicated that, the active sites increased with the increase in catalyst dosage, which promoted the

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generation of electron-hole pairs and the formation of radicals to enhance the

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photocatalytic effect. However, the reaction rate did not dramatically increase when

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the adding amount of the catalyst was more than 0.10 g. It is expected that the

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concentration of 60%Fe3O4/Ag6Si2O7 was so high that the scattering of the incident light increased to the weak absorption of light [39]. Photons cannot be fully utilized,

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thereby reducing the catalytic effect. Taking degradation efficiency and catalyst dosage into account, 0.10 g was selected for the optimal dosage and used for the further studies. 3.1.3. Effect of initial concentration of MB The effect of initial MB concentration on the photocatalytic efficiency of 60%Fe3O4/Ag6Si2O7 is shown in Fig. 3. It can be seen that the photocatalytic efficiency decreased with the increase of MB concentration. When the initial

ACCEPTED MANUSCRIPT concentration of MB was 5 mg/L and 10 mg/L, the photocatalytic degradation

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efficiency was quite high. After 2 min of irradiation, the degradation efficiency of MB

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can reach about 98% and 84.8%, respectively. MB removal efficiency was

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significantly reduced with the increase concentration of MB. When the initial

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concentration of MB was 50 mg/L, only 27.7% of MB was removed in 10 min. Two reasons can be explained to this phenomenon. Firstly, more MB molecules

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were absorbed on the surface of 60%Fe3O4/Ag6Si2O7 particles with the increased MB

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concentration, while the active sites available were constant, inhibiting its degradation

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rate. Secondly, the higher concentration of MB solution made the light transmittance

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of the reaction system become worse. Photons cannot be effectively absorbed to generate the electron-hole pairs required for photo-degradation.

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3.1.4. Effect of solution pH The pH value of aqueous solution is one of the influencing factors of photocatalytic reaction, which can affect the surface charge, size and energy band structure of the catalysts. Therefore, photocatalytic effect under different pH conditions was studied. As noted in Fig. 4(a), MB degradation exhibits high efficiency (> 95%) at pH range from 3.0 to 10.0. Fotiou et al reported that the zero point charge (pHzpc) of catalyst could influence photocatalytic activity [37]. The positively charged surface of

ACCEPTED MANUSCRIPT catalysts presented electrostatic attractions toward negatively charged species when

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pH was less than pHzpc. The 60%Fe3O4/Ag6Si2O7 in water was negatively charged at

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pH range from 3.0 to 10.0 (Fig. 4(b)). Based on this, the reason why MB removal was

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not pH dependent can be explained. Cationic dyes, MB, were ionized in aqueous

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solution to form a positively charged colored group. Therefore, the positively charged colored groups were rapidly adsorbed on the surface of the catalyst to promote the

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rapid photocatalytic reaction.

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3.2. Characterization of as-prepared samples

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Fig. 5 displays the UV-Vis DRS spectra of as-prepared photocatalysts. The main

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absorption spectrum of the pure Ag6Si2O7 is shown in both ultraviolet and visible region (the absorption edge at about 780 nm). It was indicated that Ag6Si2O7 could act

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as a wide specture-responded photocatalyst. With regard to Ag6Si2O7, the absorption of visible light by Fe3O4/Ag6Si2O7 was highly enhanced in the visible region, which was mainly attributed to the Fe3O4 nanoparticles could act as a well-performing light harvesting material, similar phenomenon was reported by Wang et al [40]. This phenomenon meant that more visible light could be absorbed in the process of catalyst degradation of organic pollutants and the photocatalytic activity was also enhanced. This is in agreement with the results of photocatalytic degradation experiments. The

ACCEPTED MANUSCRIPT band gap of Fe3O4/Ag6Si2O7 could be calculated by the Kubelka-Munk expression

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(Eq. 2) [41, 42]:

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(hv)1/ n  A(hv  Eg )

(2)

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where α is the optical absorption coefficent; hv is the incident photonic energy(eV); A

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is the proportionality constant; Eg is the band gap energy (eV); and n is a factor that depend on the kind of optical transition induced by photon absorption (n = 1/2 for

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direct transition and n = 2 for indirect transition). As applying n = 1/2, the direct band

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gap energy of the samples (from 0%Fe3O4/Ag6Si2O7 to 100%Fe3O4/Ag6Si2O7) were

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estimated to be equal to 1.92, 1.00, 0.87,1.15, 1.18, 1.33eV, as can be seen in Fig. 5(b).

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The main reason for this change in band gap could be attributed to change in silicate ions in the samples [43].

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The morphology of Fe3O4, Ag6Si2O7 and 60%Fe3O4/Ag6Si2O7 composites were investigated using SEM analysis, which were shown in Fig. 6. Fig. 6(a) shows the SEM image of Fe3O4. It could be seen that Fe3O4 spheres had a relatively smooth surface. The pure Ag6Si2O7 (Fig. 6(b)) was made of nano-sized irregular particles. As shown in Fig. 6(c, d), large amount of irregular Ag6Si2O7 were coated on the surface of Fe3O4, which made the 60%Fe3O4/Ag6Si2O7 composite rougher than Fe3O4. In addition, the 60%Fe3O4/Ag6Si2O7 particles had a relatively uniform diameter of about

ACCEPTED MANUSCRIPT 400 nm. The small particles were favorable for the adsorption of MB, and further

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promoted the photocatalytic reaction.

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To investigate the structure of the as-prepared samples, the XRD spectra of the

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Fe3O4, Ag6Si2O7 and Fe3O4/Ag6Si2O7 were determined and are shown in Fig. 7. As

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shown in Fig. 7, XRD diffractogram of Fe3O4 showed the characteristic peaks at 18.46º, 30.17º, 35.56º, 37.20º, 43.10º, 53.60º, 56.57º, 62.79º, corresponding to (1 1 1),

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(2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), respectively. All the peaks

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were matched with the JCPDS file (PDF No.65-3107) of cubic iron oxide. The

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characteristic peaks of Fe3O4 were sharp and intense, which showed its highly

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crystalline nature. The characteristic peaks at about 34o of as-prepared Ag6Si2O7 could be indexed to (1 2 4) and (1 1 5) plans (JCPD File No. 85-281), indicating that

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monoclinic Ag6Si2O7 was prepared, though the diffraction peaks were broad due to its small particles [44]. In the XRD spectrum of the Fe3O4/Ag6Si2O7 composite, the mainly diffraction peaks of Fe3O4 and Ag6Si2O7 can be observed clearly. However, the diffraction intensity for Fe3O4/Ag6Si2O7 decreased, which indicated a decline in crystallinity [45]. The composition of the sample was characterized by XPS analysis. As shown in Fig. 8(a), the 60%Fe3O4/Ag6Si2O7 composite consists of Fe, Ag, Si, and O. The C 1s peak

ACCEPTED MANUSCRIPT at 284.8 eV was a reference for charge correction. The XPS spectra of Fe 2p is shown

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in Fig. 8(b). The peak position at 724.8 and 711.3 eV are Fe 2p1/2 and Fe 2p3/2,

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respectively. Satellite peak at 719.3 eV was attributed to the characteristic adsorption

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of Fe3+. Fig. 8(c) shows the two peaks located at 374.38 and 368.37 eV, which were

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indexed to Ag 3d3/2 and Ag 3d5/2, respectively. The Si 2p peak was deconvoluted into Si 2p1/2 and Si 2p3/2, which were located at 103.0 and 102.1 eV, respectively(Fig.

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8(d)).

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After the degradation reaction was completed, the 60%Fe3O4/Ag6Si2O7 composite

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photocatalysts can be rapidly separated under an applied magnetic field (in set graph

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in Fig. 9). The magnetic composite photocatalyst had a strong response to the magnetic field, and the magnetism was a desirable property for photocatalysts remove

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from the reaction system. The magnetic property had been quantified by VSM, which was shown in Fig. 9. The saturation magnetization (Ms) value of 60%Fe3O4/Ag6Si2O7 was 32.447 emu/g, and the coercivity and remanence were 16.824 G and 0.71874 emu/g, respectively. The value of Ms was lower than pure Fe3O4, which can be attributed to the presence of nonmagnetic Ag6Si2O7. Although the Ms value of 60%Fe3O4/Ag6Si2O7 was lower than that of pure Fe3O4, it is still high enough to magnetically separated from the solution after decontamination process in a few

ACCEPTED MANUSCRIPT seconds. Shekofteh-Gohari et al reported that the Ms value of the prepared

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ZnO/Ag3VO4/Fe3O4 was 4.9 emu/g [19]. The 60%Fe3O4/Ag6Si2O7 nanoparticles

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performed an excellent characteristic in the separation process due to its relatively

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higher saturation magnetization. In addition, the coercivity (16.824 G) demonstrated

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that the 60%Fe3O4/Ag6Si2O7 displayed a hard magnetic behavior. 3.3. Degradation Mechanism

the

photodegradation

process

of

MB,

using

60%Fe3O4/Ag6Si2O7

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during

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Active species trapping experiment was performed to investigate the mechanism

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photocatalysts. It can be seen that the degradation of MB was almost uninfluenced by

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adding IPA. However, with the addition of EDTA-2Na and BQ, the degradation of MB was decreased to be 98.5% and 73.7%, respectively. Thus it can be supposed that

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h+ and ·O2- were the main active species. Possible intermediate products, generating from the degradation process of MB, were analyzed to investigate the degradation pathway. As shown in Table 1, several intermediate products with matching greater than 80% were identified. It can be clearly observed that the macromolecules of MB were converted to smaller substances, indicating that the photocatalytic degradation process was not a simple decolorization.

ACCEPTED MANUSCRIPT Based on the results described above, the possible mechanism can be explained as

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follows:

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(1) The electron-holes pairs were separated by photons absorption of

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60%Fe3O4/Ag6Si2O7. As the strong absorption of 60%Fe3O4/Ag6Si2O7 composite in

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the visible region, photons can be absorbed efficiently. After the photons (hv > Eg) were absorbed by catalyst, the electrons at the valence band (VB) were excited to the

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conduction band (CB), corresponding h+ at the valence band. (3)

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60%Fe3O4/Ag6Si2O7 + hv → e-cb + h+vb

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(2) The photoinduced electron and hole migrated on the surface of Ag6Si2O7

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catalyst and took part in the oxidation-reduction reaction with H2O, O2 and so on. (3) The generation of active species during the degradation reaction process.

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Thereinto, ·O2- was generated by the reaction between O2 molecules on the surface and e- at the valence band. e-cb + (O2)ads → ·O2-

(4)

(4) MB was directly degraded by the active species. ·O2- + MB → Products

(5)

MB + h+ → Products

(6)

4. Conclusion

ACCEPTED MANUSCRIPT In this study, a novel magnetically recoverable 60%Fe3O4/Ag6Si2O7 composite

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photocatalyst was successfully synthesized for the degradation of MB, which causes

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undesirable water pollution. The photocatalytic efficiency of Fe3O4/Ag6Si2O7 was

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influenced by mass ratio of Fe3O4, catalyst dosage, initial MB concentration and

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solution pH. The results showed that the 60%Fe3O4/Ag6Si2O7 performed the best photocatalytic activity and degradation characteristic under the same conditions. The degradation

of

MB

increases

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photocatalytic

with

increasing

dosage

of

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Fe3O4/Ag6Si2O7, but it dropped beyond 0.10 g. The photocatalytic activity was

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inhibited by high concentration of MB and was not pH dependent in a wide range

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(from 3.0 to 10.0). It can be seen that MB could be quickly degraded by Fe3O4/Ag6Si2O7 under the optimum conditions. Active species trapping experiment

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showed that ·O2- ought to be the mainly activity component during the photocatalytic degradation process. Possible intermediate products were also analyzed to obtain the better understanding of the degradation pathway. Acknowledgements The authors acknowledge financial supports from the National Natural Science Foundation of China (NSFC) (No. 21407129) and the Fundamental Research Funds for the Central Universities (No. 2652017032).

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Fig. 1. (a) Photocatalytic degradation curves of MB under simulated visible light

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irradiation; (b) apparent rate constants for photocatalytic degradation of MB solution.

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(catalyst dosage = 1.0 g/L; C0 = 10 mg/L; pH = 6.7 ± 0.1)

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Fig. 2. Photocatalytic degradation of MB with different catalyst dosage.

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(catalysts = 60%Fe3O4/Ag6Si2O7; C0 = 10 mg/L; pH = 6.7 ± 0.1)

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Fig. 3. Photocatalytic degradation of MB with different initial MB concentrations.

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(catalysts = 60%Fe3O4/Ag6Si2O7; catalyst dosage = 1.0 g/L; pH = 6.5 ± 0.1)

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Fig. 4. (a) Effect of pH on the photocatalytic degradation of MB; (b) Zeta-potential of

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60%Fe3O4/Ag6Si2O7.

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(catalysts = 60%Fe3O4/Ag6Si2O7; catalyst dosage = 1.0 g/L; C0 = 10 mg/L)

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Fig. 5. UV−vis diffuse reflectance spectra (a) and band energy (b) of samples.

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Fig. 6. SEM images of (a) Fe3O4, (b) Ag6Si2O7, (c, d) 60%Fe3O4/Ag6Si2O7

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Fig. 7. XRD patterns of as-prepared samples.

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Fig. 8. The XPS spectra of 60%Fe3O4/Ag6Si2O7 (a); Fe 2p (b); Ag 3d (c) and Si 2p(d).

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Fig. 9. Magnetization curves of 60%Fe3O4/Ag6Si2O7. Inset of the figure is separation

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of the nanocomposite from the treated solution using an external magnetic field.

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Fig. 10. Photodegradation of MB on 60%Fe3O4/Ag6Si2O7 in the presence of different

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scavengers.

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The intermediates produced in the degradation process of MB by 60%Fe3O4/Ag6Si2O7

Name

a

2-Aminophenol-4-sulfonic acid

b

Phenol

c

1,2-Ethanedicarboxylic acid

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RT: Retention time

Molecular formula

RT (min)

C6H7NO4S

16.186

C6H6O

12.88

C6H4O4

3.199

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composite.

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Graphical abstract