diatomite photocatalysts for the degradation of organic dyes under visible light irradiation

Accepted Manuscript Structure and properties of Ag3PO4/diatomite photocatalysts for the degradation of organic dyes under visible light irradiation

Pengfei Zhu, Yanjun Chen, Ming Duan, Mei Liu, Ping Zou PII: DOI: Reference:

S0032-5910(18)30430-3 doi:10.1016/j.powtec.2018.05.060 PTEC 13433

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

12 December 2017 29 May 2018 30 May 2018

Please cite this article as: Pengfei Zhu, Yanjun Chen, Ming Duan, Mei Liu, Ping Zou , Structure and properties of Ag3PO4/diatomite photocatalysts for the degradation of organic dyes under visible light irradiation. Ptec (2017), doi:10.1016/ j.powtec.2018.05.060

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ACCEPTED MANUSCRIPT Structure and properties of Ag3PO4/diatomite photocatalysts for the degradation of organic dyes under visible light irradiation Pengfei Zhu1,2 , Yanjun Chen1 , Ming Duan1,2 , Mei Liu1 , Ping Zou1 1. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China

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2. Oil and Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest

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Petroleum University, Chengdu, 610500, P. R. China

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Corresponding author. Tel.: +86 28 83037319; fax: +86 28 83037305.

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E-mail address: [email protected] (M. Duan), [email protected] (P. Zhu)

ACCEPTED MANUSCRIPT Abstract A novel visible- light-responsive Ag3 PO4 /diatomite composite is successfully prepared by ultrasound assisted precipitation method. The structure and properties of Ag3 PO 4 /diatomite composite are characterized by X-ray diffraction (XRD), fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM),

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energy dispersive spectroscopy (EDS), high resolution transmission electron microscope (HRTEM), N2 adsorption/desorption, X-ray photoelectron spectroscopy

The

obtained

Ag3 PO4 /diatomite

composites

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(XPS), UV-vis diffuse reflectance spectra (UV-vis DRS) and photoluminescence (PL). show

enhanced

photocatalytic

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performance for the degradation of acid brilliant scarlet dye compared with pure Ag3 PO 4 under visible light irradiation, and the 10%-Ag3 PO4 /diatomite composite

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exhibits the optimum photocatalytic degradation efficiency when the initial concentration of acid brilliant scarlet dye is 50 mg/L with the pH 6 and the catalyst

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dosage is 1 g/L, which is approximately 3.6 times higher than that of pure Ag3 PO 4 . Meanwhile, it shows the better stability after three cycles. The enhanced performance

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can be attributed to the uniform dispersion of Ag3 PO4 on the surface of diatomite, the stable structure, the enlarged specific surface area and the high-efficiency separation

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of photogenerated electron-hole pairs. Furthermore, the free radical trapping experiments indicate that h+ and •O 2 − are the major active species in the photodegradation process of acid brilliant scarlet dye. This work has the potential to

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provide a high-efficiency and cost-effective photocatalyst for the degradation of dye pollutants.

Keywords:

Ag3 PO 4 ; Diatomite; Visible- light-driven; Photocatalysis degradation; Organic dyes

ACCEPTED MANUSCRIPT 1. Introduction Semiconductor-based photocatalysis has received extensive attention and been widely applied in the removal and degradation of organic pollutants from industrial effluents, due to its environmental friendliness, no secondary pollution and the effective utilization of abundant solar energy [1-4]. However, most conventionally

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used semiconductor photocatalysts (such as TiO2 , ZnO, CdS and Cu2 O) have poor photocatalytic activity or inefficient photogenerated electron-holes separation ability

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under the visible light irradiation (accounting for 43% of solar spectrum), which greatly restrict their pratical applications [5-7]. Therefore, it is necessary to explore

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novel and more efficient visible-light-responsive photocatalysts. Rencently, Ye and co-workers have been reported that Ag3 PO4 shows extremely high photocatalytic

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efficiency for oranic dyes degradation in aqueous solution as well as for O 2 generation from water splitting under visible light irradiation [8-11]. Nevertheless, there are still

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some problems for the pratical applications of Ag3 PO 4 photocatalyst. Firstly, Ag3 PO 4 photocatalyst with a relatively higher Ksp value (1.6 × 10−16 ) can slightly dissolve in

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aqueous solution and then the Ag+ is inevitablely converted into Ag0 during the photocatalytic process, resulting in the photocorrosion of Ag3 PO4 , which will destroy

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the structure of Ag3 PO4 and reduce its light absorption efficiency, thus the photocatalytic activity and stability of Ag3 PO4 are decreased. Secondly, severe agglomeration of Ag3 PO4 leads to the relatively large particle size. And thirdly, the

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high cost restricts its wide applications [12-15]. Currently, many researches have shown that immobilizing Ag3 PO 4 on some support materials such as Ag3 PO 4 /CNT [6], Ag3 PO 4 /TiO 2 [16], Ag3 PO4 /graphene [17], Ag3 PO4 / bentonite [18] and Ag3 PO4 /SiO 2 [19] can solve the above problems. For example, Chen et al. [20] prepared an Ag3 PO 4 /GO visible- light-driven photocatalyst by electrostatically driven method, which exhibited superior photocatalytic activity and stability than pure Ag3 PO4 . Chai et al. [21] reported the high-efficiency and easily recyclable Ag3 PO 4 /SBA-15 composites with different Ag3 PO 4 contents for the degradation of RhB and indicated that 20% Ag3 PO 4 /SBA-15 composite had the highest photocatalytic efficiency. Ma et al. [22] deposited Ag3 PO4 particles on the surface of exfoliated bentonite (EB) to

ACCEPTED MANUSCRIPT prepare the EB-Ag3 PO4 photocatalyst which showed improved photocatalytic activity. In recent years, diatomite has been widely used as photocatalyst support due to its high porosity, large specific surface area, thermal resistance, low density, chemical stability, low cost and abundance. Diatomite is originated from the deposition of single-celled aquatic algae and primarily made up of amorphous SiO 2 [23-27]. Zhang

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et al. [28] synthesized the TiO 2 /diatomite composite and the catalyst exhibited better photocatalytic activity as well as outstanding reusability than pure TiO 2 . Wang et al.

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[29] obtained V-doped TiO 2 /diatomite photocatalysts by a modified sol- gel method and found that TiO 2 nanoparticles uniformly dispersed on the surface of diatomite

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which inhibited its agglomeration. Zhu et al. [30] immobilized Cu2 O and ZnO on natural diatomite and indicated that the Cu2 O-ZnO/diatomite composite showed an

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excellent photocatalytic activity for the degradation of red water under visible light irradiation. These results prove that diatomite used as the carrier of catalyst can not

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only improve the degradation rate of pollutants, but also reduce the preparation cost of catalysts.

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In this study, a series of Ag3 PO4 /diatomite composites with different Ag3 PO 4 contents are successfully prepared by ultrasound assisted precipitation method. The

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compositions, morphologies, structures and optical properties of the as-prepared samples are characterized. The photocatalytic performance and stability of Ag3 PO 4 /diatomite composites are evaluated by the degradation of acid brilliant scarlet

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dye aqueous solution under visible light irradiation, and the effects of Ag3 PO 4 contents on the photocatalytic activity of composites are also investigated. Addtionally, the predominant reactive species during the photocatalytic process of Ag3 PO 4 /diatomite composite are investigated by the free radicals trapping experiments. 2.

Experimental

2.1 preparation of photocatalysts All the chemicals used in the experiments were analytical reagent without further purification. The x-Ag3 PO4 /diatomite (x=2, 6, 10, 20, 30 wt% of Ag3 PO 4 ) composites with different mass ratios were prepared by ultrasound assisted precipitation method.

ACCEPTED MANUSCRIPT Firstly, a certain amount of AgNO 3 was dissolved in the distilled water, and 1mol/L NH3 ·H2 O was added until the solution became colorless transparent. Then, an appropriate amount of diatomite was dispersed in the above solution by ultrasonic treatment for 30 min. Subsequently, 0.1 mol/L KH2 PO 4 solution was added to the obtained solution and maintained with stirring for 2 h. After that, the mixture was centrifuged, collected and washed several times with distilled water and then dried in

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vacuum at 80◦ C for 6 h to obtain the x-Ag3 PO4 /diatomite composites. Pristine Ag3 PO 4

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nanoparticle was prepared by using the same method without the adding of diatomite.

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2.2 characterization of photocatalysts

XRD patterns were recorded on a PANalytical B.V. powder diffractometer using

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nickel- filtered Cu-Kα radiation (λ =1.54060 Å) in the range of 5 ◦ ≤2 ≤80◦ with a scanning rate of 1.2˚/min operated at 40 kV/25 mA. FT-IR were recorded on a

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WQF-520 spectrometer in the range of wave numbers from 400 to 4000 cm-1 with a resolution of 2 cm-1 and using KBr as the beam splitter. The surface morphologies of the as-prepared samples were observed by using SEM (Carl Zeiss AG, ZEISS EV0

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MA15) at an operating voltage of 20Kv and HRTEM images were recorded using a FEI model (Tecnai G2 F20) field emission transmission electron microscope. The

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elemental compositions and distributions were analyzed by EDS and EDS mapping, respectively. The UV-vis DRS spectra were measured on a PerkinElmer Lambda 850

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UV-vis spectrophotometer equipped with an integrated sphere attachment using BaSO 4 as reference standard, and the spectra were recorded in the range of 190~800 nm under the diffuse reflectance mode. XPS spectra of the sample were carried out on a Thermo ESCALAB 250Xi X-ray photoelectron spectrometer with an exciting source of Al-Kα and the binding energies were corrected by referencing the C 1s peak to 284.6 eV. N 2 adsorption/desorption measurements at 77 K performed with a ST-MP-9 analyzer. The PLspectra of the samples were conducted by PerkinElmer LS55 spectrofluorometer with the excitation wavelength at 340 nm. 2.3 photocatalytic activity evaluation The photocatalytic activities of the as-prepared samples were evaluated by the

ACCEPTED MANUSCRIPT degradation of acid brilliant scarlet dye in a photocatalytic reactor (BL-GHX-V, Shanghai) with a 65 W visible lamp irradiation. In each experiment, 50 mg photocatalyst was added into 50 mL acid brilliant scarlet dye aqueous solution (50 mg/L). Prior to irradiation, the suspension was magnetically stirred for 30 min in the dark to obtain good dispersion and reach adsorption-desorption equilibrium between

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the photocatalyst and acid brilliant scarlet dye. Then the suspension was exposed to the visible- light irradiation under magnetic stirring, at certain time intervals, 5 mL

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solution was sampled and centrifuged to separate the photocatalyst. The concentration of acid brilliant scarlet dye was determined by a UV–visible spectrophotometer

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(UV-1800, Shimadzu, Japan) at maximum absorption wavelength (λ=505nm). The photocatalytic degradation rate 𝜂 can be calculated by the following equation: 𝐶0 −𝐶 𝐶0

) × 100% = (

𝐴0 −𝐴 𝐴0

) × 100% (1)

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𝜂=(

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where C0 was the initial concentration and C was the corresponding concentration of acid brilliant scarlet dye after irradiation at a certain time interval; A0 and A were the corresponding absorbance values. Results and discussion

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3.

3.1 Structure characterization of photocatalysts

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The XRD patterns of diatomite, pure Ag3 PO4 and Ag3 PO4 /diatomite composites with different Ag3 PO4 contents are shown in Fig.1 (A). It can be seen that the

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characteristic peak at around 21.98° in the XRD pattern of raw diatomite is assigned to amorphous SiO 2 , which is consistent with the main phase of non-crystalline opal-A [23, 29]. As for the pure Ag3 PO 4 , the diffraction peaks at 21.0°, 29.8°, 33.5 °, 36.7°, 42.6°, 47.8°, 52.7°, 54.9°, 57.3°, 61.7°, 69.9° and 71.8° respectively correspond to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (420) and (421) planes of Ag3 PO 4 [31-32]. All of the diffraction peaks could be well in agreement with the body-centered cubic phase of Ag3 PO4 [33]. In the XRD pattern of Ag3 PO 4 /diatomite composites, the characteristic diffraction peaks of diatomite and Ag3 PO 4 are both observed, suggesting that Ag3 PO 4 /diatomite composites are successfully prepared and the crystalline phases of Ag3 PO 4 are not affected by the

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of diatomite.

Meanwhile,

the

characteristic

peak

of diatomite

in

Ag3 PO 4 /diatomite composites with low Ag3 PO4 contents (≤10%) is all at around 21.98°, which speculates that the reactions maybe mainly occur on the surface of diatomite during the prepared process and the crystalline structure of diatomite is not destroyed [34]. However, when the Ag3 PO4 content is more than 10% in the

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composite, this peak of the samples exhibits a slight shifting towards the lower angle in comparison to that of diatomite (Fig.1 (B)), due to a part of Ag3 PO 4 enter into the

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interlayer of diatomite and then lead to an increase of interlayer spacing. Furthermore, the intensity of diffraction peaks for Ag3 PO4 in all the as-prepared composites

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gradually increases with the increase of Ag3 PO4 contents, indicating that more Ag3 PO 4 nanaoparticles have been attached on the surface of diatomite [19]. In

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addition, no other impurity phases are observed, which confirm the high purity of all samples.

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The FT-IR spectra of diatomite, pure Ag3 PO4 and Ag3 PO4 /diatomite composites are displayed in Fig. 2. The broad absorption peaks at around 3221~3418 cm-1 and the

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band at 1651 cm-1 could be assigned to the O-H stretching vibration and H-O-H bending vibration of surface adsorbed water molecules in the sample, respectively [5,

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20, 35]. For the spectrum of diatomite, a strong band at 1087 cm-1 and two bands at 787 and 472 cm-1 correspond to the asymmetric stretching vibration, symmetric stretching and bending vibration of Si-O-Si bonds, respectively, which are

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characteristic peaks of diatomite [24, 36]. And all these characteristic peaks indicate that diatomite is mainly made up of SiO 2 . There are two bands at 1002 and 556 cm-1 observed in the pure Ag3 PO4 spectrum, which could be assigned to the characteristic stretching vibration and bending vibration of P-O bond [9, 37-38]. In the FT-IR spectra of Ag3 PO 4 /diatomite composites, all the characteristic absorption peaks of diatomite are displayed and do not shift at low Ag3 PO4 contents (≤10%). However, for the Ag3 PO 4 /diatomite composites with high Ag3 PO4 contents (>10%), the absorption peak of Si-O-Si at 1087 cm-1 shifts to 1079 cm-1 because a portion of Ag3 PO4 enter into the interlayer of diatomite, which results in a growth of Si-O-Si bond as well as implys an interaction between diatomite and Ag3 PO4 . And this result is in agreement

ACCEPTED MANUSCRIPT with the characterization of XRD. Moreover, the intensity of absorption peaks decrease as the increase of Ag3 PO 4 contents, combined with the XRD results, which indicates that Ag3 PO4 nanoparticles have been successfully immobilized on the surface of diatomite and there is no chemical reaction between them. Additionally, no other impurity absorption bands are detected in all samples.

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The surface morphologies of pure diatomite, Ag3 PO 4 and Ag3 PO 4 /diatomite composite are observed by SEM, and the results are shown in Fig.3. The SEM image

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(Fig.3 (a, b) indicate that diatomite exhibits a disk- like shape with a great number of the regular and clear pore structures on its surface, which is beneficial to the

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adsorption of effluents [39]. Seen from Fig.3 (c, d), the pristine Ag3 PO 4 particle exhibits a sphere or polyhedral morphology and has an obvious agglomeration due to

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its nanometer effect [28]. Moreover, as shown in Fig.3 (e, f), it can be observed that the Ag3 PO 4 nanoparticles are dispersed homogeneously on the surface of diatomite

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which decreases the agglomeration effect for the Ag3 PO 4 /diatomite composite as well as provides more surface site for the adsorption and degradation of pollutants. In

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addition, the surface of diatomite is rougher after the introduction of Ag3 PO 4 , but the disk-like morphology is still maintained and pore structure is still evident, which

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indicate that diatomite is an ideal photocatalyst support. The surface compositions of Ag3 PO 4 /diatomite composite are identified by EDS. And the results illustrate that the elements oxygen (O), aluminum (Al), silicon (Si), phosphorus (P) and silver (Ag) are

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coexisting.

TEM images of Ag3 PO4 /diatomite composite with different magnifications are shown in Fig.4 (a)-(b). It can be clearly seen that Ag3 PO 4 nanoparticles have been successfully immobilized on the surface of diatomite, which is in accordance with the SEM result. The lattice fringe spacing of 0.291, 0.263 and 0.248 nm correspond the (2 0 0), (2 1 0) and (2 1 1) planes of Ag3 PO4 particles, respectively (Fig.4 (c)) [20, 40-41]. The element mappings are shown in Fig.4 (d)-(g), and it can be seen that diatomite mainly consists of SiO 2 and the elements Ag and P are relatively uniform distributed in the Ag3 PO4 /diatomite composite. The specific surface area and pore structure parameters of diatomite, pure Ag3 PO 4

ACCEPTED MANUSCRIPT and Ag3 PO4 /diatomite composite are determined by N 2 adsorption–desorption isotherms. The BET surface area of samples are summarized in Table 1. Compared with pure Ag3 PO 4 (0.435 m2 /g), the lager surface area of Ag3 PO4 /diatomite composite (2.662 m2 /g) is ascribed to the addition of diatomite and the relatively uniform dispersion of Ag3 PO4 that can be proved by SEM and TEM. The higher specific

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surface area for phototcatalysts can provide more reaction active sites and be benifical to the improvement of photocatalytic activity.

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The elemental chemical state of pure Ag3 PO4 and Ag3 PO4 /diatomite composite are analyzed by XPS, and the results are shown in Fig.5. For pure Ag3 PO4 , the peaks

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at about 367.63 and 373.65 eV belong to Ag 3d5/2 and Ag 3d3/2 binding energies (Fig.5 (a)), respectively, indicating that the main valence of silver is Ag+ in the Ag3 PO 4

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particles [6, 21]. And the binding energy of P 2p for pure Ag3 PO4 observed from Fig.5 (b) is at about 132.48 eV, confirming that the valence state of P is +5 [31, 42]. The O

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1s spectrum of Ag3 PO4 is shown in Fig.5 (c), the peak at about 532.01 eV can be assigned to the surface hydroxyl oxygen [13, 43]. Compared to pure Ag3 PO4 , the

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characteristic peaks of Ag 3d, P 2p and O 1s for the Ag3 PO4 /diatomite composite exhibit a slight shift, which further imply an interaction between diatomite and

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Ag3 PO 4 , and this result is in accordance with the characterization of XRD and FT-IR. 3.2 Optical absorption properties The

optical

absorption

properties

of

diatomite,

pure

Ag3 PO4

and

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Ag3 PO 4 /diatomite composites are investigated by UV- vis DRS. It can be observed in Fig.6 (a) that diatomite shows few adsorption in the visible light region but a large adsorption in the ultraviolet region. However, it can not be excited to produce photogenerated electrons and holes after absorption of light. Thus, diatomite has no photocatalytic activity and is usually used as photocatalyst carrier. The pure Ag3 PO 4 exhibits a strong visible light absorption ability with a wavelength shorter than 530 nm. Compared with diatomite, the Ag3 PO4 /diatomite composites show enhanced visible light response with the absorption edge at about 513 nm. Moreover, the absorption intensity of composites gradually increase as the increase of Ag3 PO 4 contents, but weaker than that of pure Ag3 PO4 due to the load of diatomite, indicating

ACCEPTED MANUSCRIPT that more Ag3 PO 4 particles have been coated on the surface of diatomite which is in accordance with the characterization result of XRD. The band gap energy (Eg ) of the photoctalyst can be calculated by the following equation [6, 44]: (𝐴ℎ𝑣)2 = ℎ𝑣 − 𝐸𝑔

(2)

where A, h, v and Eg are the absorbance, the Planck constant, light frequency and band

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gap energy, respectively. In Fig.6 (b), according to extrapolate the straight linear portion of the (Ahv)2 versus hv on the x-axis, the band gaps (Eg ) of pure Ag3 PO4 and

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the 10%-Ag3 PO 4 /diatomite composite are estimated to be 2.42 and 2.46 eV,

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respectively. 3.3 PL spectra

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The PL emission spectra are carried out to investigate the recombination efficiency of photogenerated electron- hole pairs in semiconductors. Generally, a

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weaker PL intensity implies a lower recombination rate of photogenerated charge carriers [15, 45]. Fig.7 shows the PL spectra of pure Ag3 PO4 and Ag3 PO 4 /diatomite

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composites. It can be seen that the PL emission intensity of Ag3 PO4 /diatomites are obvious weaker than that of pure Ag3 PO4 , and the order of emission intensity of them

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is 10%-Ag3 PO4 /diatomite < 6%-Ag3 PO 4 /diatomite < 20%-Ag3 PO 4 /diatomite < Ag3 PO 4 , which indicates that there is the synergistic effect between Ag3 PO4 and diatomite. The improved separation efficiency of photogenerated electron- hole pairs

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in the composites can be attributed to the electrostatic repulsion between the negatively charged diatomite and the negatively charged electron, as well as electrostatic attraction between the negatively charged diatomite and the positively charged hole [22], and then the photocatalytic performance is also improved. 3.4 Photocatalytic performance of catalysts The photocatalytic performance of diatomite, pure Ag3 PO4 and Ag3 PO 4 /diatomite composites with different Ag3 PO4 contents were evaluated by the degradation of acid brilliant scarlet dye aqueous solution (50 mg/L) under visible light irradiation. Prior to irradiation, the mixture was magnetically stirred for 30 min in darkness until the adsorption-desorption balance between the photocatalyst and dye had been

ACCEPTED MANUSCRIPT established. As shown in Fig.8 (a), the degradation rate of acid brilliant scarlet dye for all samples are less than 5% in dark. After the visible light irradiation for 120 min, the degradation rate of acid brilliant scarlet dye over the diatomite and pure Ag3 PO4 are 9.8% and 53.6%, respectively. Moreover, the Ag3 PO 4 /diatomite composites present enhanced photocatalytic activities and the photocatalytic performance of the

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composites are gradually increase with the increase of Ag3 PO4 contents. The 10%-Ag3 PO4 /diatomite composite with a degradation rate of 92.5% exhibits the

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highest photocatalytic activity and the corresponding UV- vis absorption spectra are shown in Fig.8 (b). Furthermore, when the Ag3 PO4 content increases from 20 wt% to

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30 wt%, the photocatalytic degradation efficiency is slight decrease. Combined with the XRD, FT-IR and PL results, on the one hand, the decrease can be attributed to that

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the excess Ag3 PO 4 particles on the surface of diatomite lead to agglomeration of Ag3 PO 4 or a few Ag3 PO4 enter into the interlayer of diatomite, which make some of

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them can not be excited by the light and decreae the reactive surface. On the other hand, The excess Ag3 PO4 particles may become the recombination center of

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photogenerated electrons and holes, thus reducing the photocatalytic activity. Fig.8 (c) illustrates that the photodegradation kinetics curves of acid brilliant scarlet dye in the

[46]:

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precence of as-prepared samples follow the pseudo- first-order reaction kinetic model

−ln(𝐶⁄𝐶0 ) = 𝑘𝑎𝑝𝑝 𝑡 (3)

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where C0 and C are the concentrations of acid brilliant scarlet dye at irradiation times 0 and t (mg/L), respectively, and k app is the apparent reaction rate constant (min-1 ). The results are shown in Fig.8 (d). It is clearly seen that the rate constant value of the 10%-Ag3 PO4 /diatomite composite is 0.0238 min-1 , which is close to 3.6 times higher than that of pure Ag3 PO4 . In addition, for a practical photocatalyst, the stability is an important factor. In this study, the recycle experiments for the photocatalytic degradation of acid brilliant scarlet dye under visible light irradiation were carried out in the presence of pure Ag3 PO 4 and 10%-Ag3 PO4 /diatomite composite. At the end of each cycle experiment, the suspension was centrifuged and then photocatalyst was washed several times with

ACCEPTED MANUSCRIPT distilled water. As shown in Fig.8 (e), the photocatalytic degradation efficiency of pure

Ag3 PO4

decreases

by

31.06%

after

three

cycles,

while

the

10%-Ag3 PO4 /diatomite composite maintains an efficient and stable photocatalytic activity except for a decrease of 5.14% after three recycling runs. Combined with the above characterization results, the enhanced stability can be attributed to the reduction

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of decomposition and photocorrosion in Ag3 PO4 as well as the improved mechanical property of the composite after the introduction of diatomite [39]. Moreover, the XPS

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spectrum of 10%-Ag3 PO4 /diatomite after 3 reuse is presented in Fig.8 (f). The peaks loated at 368.34 (Ag 3d5/2 ) and 374.48 (Ag 3d3/2 ) eV correspond to Ag0 [10 , 38].

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Some reseaches have been reported that a few metallic Ag could act as an electron capturer to promote the photocatalytic activity of composite catalyst [7, 47]. In

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addition, the slight decrease of photocatalytic activity for10%-Ag3 PO 4 /diatomite composite can be explained by two factors. Firstly, some intermediates generated in

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the photodegradation process can be adsorbed on the catalyst surface which leads to deactivation of active sites on the photocatalyst. Secondly, the decrease is probably

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caused by the slight loss of photocatalyst during filtering and washing process [23, 29, 48-49].

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Based on the above analysis, the efficient photocatalytic activity of the 10%-Ag3 PO4 /diatomite composite is mainly related to the following factors. Firstly, the uniform dispersion of Ag3 PO4 on the surface of diatomite can effectively protect it

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from dissolution in the aqueous solution and then the structural stability of composite is greatly improved [12]. Secondly, compared with pure Ag3 PO4 , the larger specific surface area of the Ag3 PO4 /diatomite composite can provide more surface active sites for the degradation of organic dyes [4, 40]. Thirdly, the separation of photogenerated electron-hole pairs is effectively promoted after the introduction of diatomite. In addition, the diatomite support can not only enhance the visible light photocatalytic activity and stability of Ag3 PO 4 but also reduce its cost because the silver content of the whole catalyst is decreased in the preparation process, which indicate that the Ag3 PO 4 /diatomite composite can be regarded as a promising photocatalyst for the degradation of pollutants in the pratical application.

ACCEPTED MANUSCRIPT 3.5 Effects of operational parameters on acid brilliant scarlet dye degradation 3.5.1 Effect of Ag3 PO4 /diatomite catalyst dosage The effect of Ag3 PO4 /diatomite photocatalyst dosage that varied from 0.25 to 2.5 g/L on the degradation rate of acid brilliant scarlet dye (50mg/L) was studied under the visible light irradiation. The results are shown in Fig.10 (a). It can be seen that the

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degradation rate of acid brilliant scarlet dye significantly increases with an increasing catalyst dosage (< 1 g/L). And then the growth of degradation rate tends to be smooth

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when the catalyst dosage increases from 1 to 2 g/L. After that, the degradation rate

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decreases when the catalyst dosage increases further, which can be attributed to the increasing turbidity of the suspension, resulting in a shielding effect and the reduced

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light transmittance. Thus, the effective light absorption of photocatalyst is limited and then the photocatalytic activity decreases. Considering the photocatalytic degradation

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efficiency and cost, the optimum Ag3 PO 4 /diatomite catalyst dosage is 1 g/L for further experiments.

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3.5.2 Effect of pH

The effect of pH on the photocatalytic degradation of acid brilliant scarlet dye

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was studied in the pH range of 1~12, with an initial concentration of 50mg/L and the Ag3 PO 4 /diatomite catalyst dosage of 1g/L. The results are shown in Fig.10 (b). It can be observed that Ag3 PO4 /diatomite composite shows the highest photocatalytic

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activity for the degradation of acid brilliant scarlet dye when the pH of the solution is 6. However, when the solution is a strong acid or alkali, the photocatalytic efficiency of the catalyst is poor. This could be caused by changes in the electrostatic attraction or repulsion between the dye molecules and the catalyst. In an acidic solution, the surface of Ag3 PO 4 is protonated, that is, positively charged. Meanwhile, acid brilliant scarlet dye is an anionic dye due to –SO 3 Na group, and its surface is negatively charged after ionizing in solution. A large number of acid brilliant scarlet dye molecules can be adsorbed on the surface of Ag3 PO4 /diatomite composite photocatalyst because of the electrostatic attraction, which increases the chance of contact between dye molecules and the active centers of catalyst, promoting the dyes

ACCEPTED MANUSCRIPT degradation. However, in an alkaline solution, the surface of Ag3 PO 4 is negatively charged, the adsorption of acid brilliant scarlet dye molecules on the catalyst surface is reduced due to the coulombic repulsion, as well as the photocatalytic efficiency of the catalyst decreases. Additionaly, in strong acid or alkali solution, the H+ or OH− has an influence on the the production of the reactive species (•OH, h+ and •O 2 −) that play

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the important roles in the photocatalytic process[50-51]. Therefore, the optimal pH in this experiment is 6, which is the initial pH of the dye solution.

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3.5.3 Effect of kinds of dye

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In order to evaluate the extensive adaptability for water treatment, the photodegradation of malachite green (MG), sunset yellow (SY), direct fast bordeaux

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(DB) and lemon yellow (LY) dyes that the initial concentrations of them are all 50 mg/L are also investigated by using 10%-Ag3 PO4 /diatomite composite under the

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same degradation condition with acid brilliant scarlet dye (AS). It can be seen form Fig.10 (c) that 10%-Ag3 PO4 /diatomite composite is also active for MG, SY, DB and LY dyes, indiacating that Ag3 PO 4 /diatomite composite displays relatively wide

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applications in the dyes degradation under visible light.

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3.6 Photocatalytic mechanism

It has been reported that hydroxyl radicals (•OH), photogenerated holes (h+) and superoxide radicals (•O 2 −) are possible reactive species in the photocatalytic process

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[52]. In this study, to investigate the major reactive species during the photocatalytic degradation of acid brilliant scarlet dye by 10%-Ag3 PO 4 /diatomite composite, the trapping

experiments

are

conducted.

And

tert-butyl

alcohol

(TBA),

ethylenediaminetetraacetic acid (EDTA) and potassium oxalate monohydrate (POM) act as the scavengers of •OH, h+ and •O 2 −, respectively [42, 53]. As shown in Fig. 9, the degradation rates of acid brilliant scarlet dye decline from 92.5% to 9.45%, 25.17% and 80.47% after the addition of EDTA-2Na, POM and IPA, respectively. Hence, •OH plays a subordinate role in the acid brilliant scarlet dye degradation, and h+ and •O 2 − are the primary active species in the photodegradation pro cess of acid brilliant scarlet dye. Moreover, the potentials of valence band(VB) edge and conduction band (CB)

ACCEPTED MANUSCRIPT edge can be calculated by the following equations [3]: 𝐸𝐶𝐵 = Χ − 𝐸 𝐶 − 0.5𝐸𝑔

(4)

𝐸𝑉𝐵 = 𝐸𝐶𝐵 + 𝐸𝑔

(5)

where Eg and EC are the band gap energy of the semiconductor and the energy of free electrons on the hydrogen scale (about 4.5 eV), respectively. X is the absolute

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electronegativity of the semiconductor which is 5.96 eV for Ag3 PO4 . Consequently, the ECB and EVB of Ag3 PO4 are respectively calculated to be 0.25 and 2.67eV. Due to

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the CB potential of Ag3 PO 4 is more positive than E(O 2 /•O2 − ) (−0.33 eV), the

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electrons in the CB of Ag3 PO 4 cannot reduce O 2 to •O2 − [4]. However, some reseaches have been reported that the transfer of photogenerated electrons from Ag3 PO4 to

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metallic Ag can yield •O2 − when there are a few metallic Ag in the composite system [7, 47, 54]. According to the XPS spectra of Ag3 PO4 /diatomite composite after reuse,

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it can be kown that metallic Ag particles exist in the photocatalytic process. Thus, a possible photocatalytic mechanism of Ag3 PO4 /diatomite composite is proposed. As illustrated in Fig.11, Ag3 PO4 can be be excited under the visible light irradiation and

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produce the electrons and holes. Then the photogenerated electrons in the CB of Ag3 PO 4 could react with itself to form metallic Ag. The metallic Ag particles will

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accept electrons from CB of Ag3 PO4 and react with dissolved oxygen molecules to produce •O2 − radicals [7]. Combined with the results of trapping experiments, these

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•O2 − radicals and photogenerated holes can directly oxidize organic dyes into water, carbon dioxide and mineral salts. In addition, according to the results of PL spectra, the recombination rate of photogenerated electron and hole pairs in the Ag3 PO 4 /diatomite can be effectively reduced due to the electrostatic repulsion and attraction between diatomite and Ag3 PO4 , which is further promte the photocatalytic activity of composite. 4.

Conclusions In summary, a series of visible-light-driven Ag3 PO 4 /diatomite composites with

different Ag3 PO4 contents are prepared by ultrasound assisted precipitation method. The characterization results of XRD, FT-IR, SEM and TEM show that Ag3 PO 4

ACCEPTED MANUSCRIPT nanoparticles are successfully immobilized and homogeneously dispersed on the surface of diatomite. The Ag3 PO4 /diatomite composites exhibit an improved photocatalytic performance for the degradation of acid brilliant scarlet dye than pure Ag3 PO 4 under visible light irradiation. The composite with 10% Ag3 PO 4 content presents the highest photocatalytic activity that the degradation rate reaches up to 92.5%

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when the initial concentration of acid brilliant scarlet dye is 50 mg/L, the pH value of the solution is 6 and the catalyst dosage is 1 g/L, and it has a better stability than pure

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Ag3 PO 4 . at the same time, it shows a good photodegradation efficiency on the other four dyes under visible light irradiation. The PL results reveal that photoinduceed

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electron-hole pairs of composites are effectively separated, which is benefical to the enhanced photocatalytic activity. According to the radical trapping experiments, it is

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found that h+ and •O2 − as active species play primary roles in the photocatalytic process. In addition, the Ag3 PO 4 /diatomite composite is a low-cost photocatalyst due

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to the decreased silver content of the whole catalyst after the addition of diatomite. This study proves that the Ag3 PO 4 /diatomite composite is a promising photocatalyst

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for environmental remediation and water treatment.

ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge the financial supports from the Ministry of Natural Science Foundation of China (No.21406184) and the Foundation of Youth Science and Technology Innovation Team of Sichuan Province (grant no.

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2015TD0007).

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ACCEPTED MANUSCRIPT Table 1 BET surface area of diatomite, Ag3 PO 4 and 10%-Ag3 PO4 /diatomite composite. BET surface area (m2 /g)

Diatomite 10%-Ag3 PO 4 /diatomite Ag3 PO4

2.784 2.662 0.435

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Sample

ACCEPTED MANUSCRIPT Fig.1 (A) XRD patterns of (a) Diatomite; (b) 2% Ag3 PO4 /diatomite; (c) 6% Ag3 PO 4 /diatomite; (d) 10% Ag3 PO 4 /diatomite; (e) 20% Ag3 PO4 /diatomite; (f) 30% Ag3 PO 4 /diatomite; (g) Ag3 PO 4 and (B) corresponding magnified peaks in the range of 21~23° for the samples. Fig.2 FT-IR spectra of (a) Ag3 PO 4 ; (b) Diatomite; (c) 2% Ag3 PO4 /diatomite; (d) 6% Ag3 PO 4 /diatomite; (e) 10% Ag3 PO 4 /diatomite; (f) 20% Ag3 PO4 /diatomite and (g) 30% Ag3 PO 4 /diatomite.

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Fig. 3 SEM images of (A, a) diatomite; (B, b) Ag3 PO4 ; (C, c) 10% Ag3 PO 4 /diatomite and (d) the corresponding EDS result. Fig.4 (a)-(b) TEM images with different magnifications; (c) lattice fringe image of 10% Ag3 PO 4 /diatomite and (d)-(g) element mapping of O, Si, Ag, P. Fig. 5 XPS spectra of Ag3 PO4 and 10% Ag3 PO4 /diatomite: (a) Ag 3d, (b) P 2p and (c) O 1s.

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Fig.6 (a) UV-Vis-DRS spectra of diatomite, Ag3 PO 4 and Ag3 PO 4 /diatomite composites; (b) Plot of (Ahv)2 versus energy (hv).

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Fig.7 PL emission spectra of pure Ag3 PO4 and Ag3 PO4 /diatomite composites. Fig.8 (a) Photocatalytic degradation curves of acid brilliant scarlet dye with different

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catalysts under visible light irradiation; (b) the UV-Vis absorption spectra of acid brilliant scarlet dye in the presence of 10%-Ag3 PO 4 /diatomite composite; (c) the

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kinetics plots over the as-prepared samples and (d) corresponding the apparent rate constant kapp ; (e) cycling runs of Ag3 PO4 and 10%-Ag3 PO4 /diatomite for the of

acid

brilliant

scarlet

dye

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(f)

XPS

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10%-Ag3 PO4 /diatomite after 3 reuse. Fig.9 The photocatalytic efficiency of 10%-Ag3 PO4 /diatomite composite with various

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catalyst dosages (a); at various pH (b) and for other dyes (c). Fig.10 Photocatalytic activity of the 10%-Ag3 PO4 /diatomite composite for the degradation of acid brilliant scarlet dye in the presence of different scavengers. Fig.11 The proposed photocatalytic mechanism of Ag3 PO 4 /diatomite composite.

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Graphical abstract:

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Highlights: A novel Ag 3 PO4 /diatomite composite is prepared by ultrasound assisted ion-exchange

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method. Ag 3 PO4 /diatomite composite shows a good photodegradation efficiency for five different

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dyes under visible light irradiation. Ag 3 PO4 /diatomite exhibits improved photocatalytic activity and stability than pure Ag3 PO4 .

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The cost of the treatment for organic dye will be significantly reduced.

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Graphics Abstract

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