BiPO4 composites as a heterogeneous catalyst for photo-Fenton treatment of textile and pulping wastewater

Journal Pre-proofs ZnAl2O4/BiPO4 composites as a heterogeneous catalyst for photo-Fenton treatment of textile and pulping wastewater Qingwen Tian, Miao Ran, Guigan Fang, Laibao Ding, Aixiang Pan, Kuizhong Shen, Yongjun Deng PII: DOI: Reference:

S1383-5866(19)34471-5 https://doi.org/10.1016/j.seppur.2020.116574 SEPPUR 116574

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

30 September 2019 9 January 2020 14 January 2020

Please cite this article as: Q. Tian, M. Ran, G. Fang, L. Ding, A. Pan, K. Shen, Y. Deng, ZnAl2O4/BiPO4 composites as a heterogeneous catalyst for photo-Fenton treatment of textile and pulping wastewater, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116574

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ZnAl2O4/BiPO4 composites as a heterogeneous catalyst for photo-Fenton treatment of textile and pulping wastewater Qingwen Tian,† Miao Ran,† Guigan Fang,* Laibao Ding,* Aixiang Pan, Kuizhong Shen, Yongjun Deng

Key Lab. of Biomass Energy and Material, Jiangsu Province; Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; Co-Innovation Center of Efficient Processing and Utilization of Forest Resource, Jiangsu Province; Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; National Engineering Lab. For Biomass Chemical Utilization, Nanjing, 210042, P.R. China

† These authors contributed equally to this work. *Corresponding author. Tel: +86 02585482542; fax: +86 025 85413445 E-mail: [email protected]

1

Abstract ZnAl2O4/BiPO4 heterostructures were successfully obtained via co-precipitation and hydrothermal method. The photocatalytic efficiency of ZnAl2O4/BiPO4 composites were improved and seriously influenced by the loading content of ZnAl2O4 under UV light. Photocatalytic tests showed that the kinetic constant of 1 wt% ZnAl2O4/BiPO4 composites for methylene blue degradation under UV irradiation were 0.959 h-1, which is 2.47 and 18.10 times as large as that of pure ZnAl2O4 and BiPO4, respectively.

The

enhanced

photocatalytic

performance

of

ZnAl2O4/BiPO4

heterojunctions was ascribed to effective separation of photo-generated electron–hole pairs. The photogenerated holes (h+) and •OH were the primary reactive species in the MB degradation over ZnAl2O4/BiPO4 composites. Photo-Fenton-like reaction was initiated with H2O2 addition, which significantly improved the photocatalytic activity of ZnAl2O4/BiPO4. The CODCr and color removals of textile wastewater over ZnAl2O4/BiPO4/H2O2 under UV irradiation were 76.74% and 83.22%, and the CODCr and color removals of eucalyptus chemimechanical pulp wastewater (ECMPW) over ZnAl2O4/BiPO4/H2O2 under UV irradiation were 73.98% and 84.54%, respectively. This work provides a new insight for constructing efficient flexible heterogeneous photocatalysts for wastewater purification. Keywords:

ZnAl2O4/BiPO4;

heterojunction;

textile

wastewater;

chemimechanical pulp wastewater; photo-Fenton-like reaction

2

eucalyptus

1. Introduction In recent years, the pulping and textile industry have experienced rapidly development, which simultaneously have brought serious threats to the environment with massive discharge of wastewater [1-5]. The pulping and textile wastewater contain high chemical oxygen demand (COD), suspend solid (SS), color, salts and low ratio of BOD/COD, which was considered as refractory organic wastewater with high concentration [6-8]. Heavy metal ions such as arsenic, lead, mercury, cadmium, nickel, cobalt and zinc, and dye molecular containing aromatic and azoic compounds were produced in the textile wastewater [9], while resin acid, chloroform, dioxins, chlorate, chlorinated hydrocarbons, phenols and furans were generated in the pulping wastewater [8, 10]. These compounds were seriously harmful to human, and they can enter human body through lung, gastrointestinal tract or skin, causing cancer, neurological disorders and even genetic diseases [11]. The treatment of pulp and textile wastewater was extensively and intensively investigated, and various technologies, such as, physical, chemical and biological treatment, have been developed as a response to the emerging contaminants [5, 12]. Three processes, i.e., primary physicochemical treatment, secondary biochemical treatment and tertiary advanced treatment, were generally used to treat the textile and pulp wastewater [8, 13-15]. After processing the first two stages, most of pollutants, such as, COD, biological oxygen demand (BOD) and SS were removed from the wastewater. However, these treated biologically effluents still contain toxic organics, high color and SS, which need further treatment [13, 16]. Advanced oxidation processes (AOPs) 3

were considered as efficient and competitive technology for removing refractory pollutant from the wastewater [17-20]. Coagulation and Fenton techniques have been successfully applied to the industrial wastewater treatment [18]. However, large chemical consumption, large sludge production, high operating cost and severe equipment corrosion were introduced in the process of wastewater treatment [17], bringing huge pressure to the factory and society. Therefore, it is imperative to develop environmentally friendly and economically sound techniques with high efficiency to deal with industrial wastewater. Heterogeneous photocatalysis, a process of AOPs, have exhibited advantages on the biochemical treatment wastewater [21-24]. Under illumination, the electrons and holes in the semiconductors can be efficiently separated and participate in redox reactions to oxidize the organic contaminants to CO2 and H2O [24-26]. No sludge, corrosion and poisonous products were generated in the photocatalytic process. BiPO4 with a typical perovskite structure has been firstly revealed by Zhu et al. as an excellent photocatalyst under UV irradiation [25, 27, 28]. The degradation rate of methylene blue (MB) over BiPO4 is twice larger than that of P25 (Degussa) under UV light [27]. However, its poor adsorptive performance, large size and large band gap energy seriously limit the photocatalytic application of BiPO4[29]. Many efforts have been taken to improve the performance by constructing heterostructures with another semiconductor,

such

as,

Ag/BiPO4[30],

SrTiO3/BiPO4[31],

C3N4/BiPO4[32],

Bi2SiO5/BiPO4[33], BiOI/BiPO4[34] and Ag3PO4/BiPO4[35] composites. Our group has synthesized SrTiO3/BiPO4 heterostructures and applied it to the MB degradation 4

under UV light, and found the MB degradation increased by 17.16% after coupling SrTiO3 on the surface of BiPO4[31]. ZnAl2O4, a typical spinel structure and naturally gahnite mineral, has been considered as suitable material for photocatalytic application due to wide band gap, high chemical and thermal stability, low surface acidity and high mechanical resistance [36-40]. Foletto et al. [40] synthesized ZnAl2O4 nanoparticles with three different methods, i.e., co-precipitation, hydrothermal, and microwave-hydrothermal, and results showed that the RR141 dye degradation rate were 0.0047, 0.0012 and 0.0026 mg/(L•min), respectively, which was strongly influenced by the pore size of the particles. Unfortunately, the poor adsorption performance seriously affect the photocatalytic application of ZnAl2O4 [41]. To solve the issue, it is promising to couple it with semiconductors. ZnO/ZnAl2O4 composites have been fabricated in many literatures, and found that the photocatalytic efficiency was greatly improved [36, 41, 42]. To the best of our knowledge, ZnAl2O4/BiPO4 heterostructures have not been reported and used for the textile and pulping wastewater purification. In this study, ZnAl2O4/BiPO4 heterostructures were synthesized via co-precipitation and hydrothermal methods, and MB dyeing solution was used as model compound to estimate the photocatalytic activity under UV light. These prepared catalysts were characterized by XRD, SEM, TEM, XPS, PL and UV-vis spectra. Studies of kinetics, active species for MB oxidation and recyclability and stability over ZnAl2O4/BiPO4 composites were discussed in details, and the photocatalytic mechanism was also proposed. Particularly, the composites were also used to treat industrial textile and 5

pulping wastewater.

2. Experimental section 2.1 Materials Bismuth nitrate (Bi(NO3)3•5H2O), sodium phosphate(Na3PO4), sodium hydroxide (NaOH), zinc sulfate (ZnSO4•7H2O), aluminum nitrate [Al(NO3)3•9H2O], ammonia aqueous (NH3•H2O), methylene blue (MB), hydrogen peroxide aqueous solution [H2O2 (27.5wt%)] Benzoquinone, ethylenediamine tetraacetic acid disodium salt dihydrate (EDTA-2Na), and 2-propanol were purchased from Shang-hai Chemical Reagent Co. Ltd in a analytic grade and used as received without any purification. Ferrous sulfate (FeSO4), hydrogen peroxide (27.5 wt% H2O2), poly aluminum chloride (PAC) and polyacrylamide (PAM) were supplied by a pulping plant in Jiansu Province, China. 2.2

Preparation of ZnAl2O4 crystal

The ZnAl2O4 crystal was prepared according to the previous report with some modification [40]. Typically, 2 mmol of ZnSO4•7H2O and 4 mmol of Al(NO3)3•9H2O were dissolved in 10 mL deionized water under magnetic stirring, respectively. Then, the ZnSO4 solution was slowly added to the Al(NO3)3 solution with vigorous stirring. And appropriate amount of NH3•H2O were added to the above mixture to adjust the pH of 9.0. After the white precipitate completely produced, the mixture was filtered, washed with deionized water, and then dried in an over at 60 oC. Finally, the white powers were calcined at 700 oC for 4 h in muffle furnace to obtain the ZnAl2O4 6

nanocrystals. 2.3 Preparation of ZnAl2O4/BiPO4 composites The ZnAl2O4/BiPO4 heterostructures were synthesized via simple hydrothermal method [31]. Generally, 3 mmol of Bi(NO3)3·6H2O and 3 mmol of Na3PO4 were put in a 40 mL deionized water under magnetic stirring, and then appropriate amount of ZnAl2O4 were added in the above mixture. 30wt% HNO3 was added dropwise into the solution with vigorous stirring to adjust the pH to 1. The solution was stirred and sonicated for 30 min to make a homogeneous dispersion system. The resulting suspension was sealed in a stainless steel autoclave and maintained at 180 oC for 24 h. Finally, the precipitate was centrifuged, washed with deionized water for several times and dry in an oven at 60 oC for 24 h. The pure BiPO4 were prepared with the above method without the addition of ZnAl2O4. The designations of prepared composites were shown in Table.1. Table 1 Designations of prepared samples and their compositions

m(ZnAl2O4):m(BiPO4)/ %

0

0.2

0.5

Designation

BiPO4

ZB1

ZB2

1

2

5

10

100

ZB3 ZB4 ZB5 ZB6 ZnAl2O4

2.4 Characterization X-ray powder diffraction (XRD) analysis was carried out using a Bruker D8 diffractometer with Cu Kα radiation. The morphology and structure of prepared sample were characterized by SEM (JSM-7001F, JEOL, Japan) and TEM (H-600-II, 7

Hitachi, Japan). The chemical composition and the chemical valence of ZnAl2O4/BiPO4 nanocrystals were determined by X-ray energy dispersion spectrum (EDS) and X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250), respectively. UV-vis diffuse reflectance spectra (DRS) of the samples were measured by using UV-vis spectrophotometer (Shimadzu UV-2450). The photoluminescence (PL) spectra were measured by fluorescence spectrophotometer (FP-6500, Japan) with an excitation wavelength of 325 nm. 2.5 Photocatalytic treatment of MB and pulping wastewater The photocatalytic activities of prepared samples were estimated by MB removal under UV irradiation, and the temperature of all experiments were controlled at 25±1 oC

in a water bath. A 100 W mercury lamp was used as the source of UV light. In a

typical experiment, 0.2 g photocatalysts were put in a Pyrex reactor containing 250 mL 30 mg/L MB solution. Before irradiation, the suspensions were stirred for 30 min in dark to keep an adsorption-desorption equilibrium between the photocatalysts. During the reaction, 5 mL suspension at 30 min intervals was sampled and centrifuged at 4000 rmp for 5 min to separate photocatalyst. The concentration of MB in the filtrates was analyzed by a UV-vis spectrophotometer at 664 nm using deionized water as a reference. The photocatalytic degradation efficiency of MB was calculated by C/C0 ×100% (C is the concentration of the MB solution at reaction time, C0 is the initial concentration of MB). At the same time, the seperated photocatalysts were washed with distilled water and dried in the oven at 60 oC after each experiment. The collected samples were used to degrade the MB solution with the same method to 8

test the recycling and stability of ZnAl2O4/BiPO4 hetero-catalysts. In addition, the trapping experiments of active species were carried out in the photocatalytic reaction to clarify photocatalytic mechanism. Benzoquinone (BQ, 1 mM), EDTA-2Na (1 mM), and 2-propanol (IPA, 1 mM) were added as · O2-, h+ and·OH scavengers in the reaction, respectively. The textile wastewater (TW) and eucalyptus chemimechanical pulp wastewater (ECMPW) were collected in a textile and pulping plant after biological treatment from secondary sedimentation tank, and the characteristic of wastewater were shown in Table 2. The wastewater was treated as the above process in a Pyrex reactor. The chemical oxygen demand (COD) of wastewater were titrated by potassium dichromate, and the color were measured by HACH-Dr-B model spectrophotometer in APHA Pt–Co (cobalt) unit. Table 2 Characteristics of the TW and ECMPW.

pH

CODCr/ (mg/L)

Color/ ( Pt-Co)

TW

7.32

258

824

ECMPW

6.84

173

456

9

3. Results and Discussion

Fig.1 XRD patterns of pure BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 samples

The structure and crystallinity of prepared pure BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 samples were characterized by XRD measurement and shown in Fig.1. The peaks of pure ZnAl2O4 and BiPO4 matched well with the crystal planes of monoclinic BiPO4 phase (JCPDS No.80-0209) [31] and gahnite ZnAl2O4 phase (JCPDS No. 05-0669) [40], respectively, and no other peaks corresponding to impurity were detected. Meanwhile, the peaks of ZnAl2O4/BiPO4 composites were analogous with that of pure BiPO4, and ZnAl2O4 peaks were not observed in ZB samples, which is due to a low content, high distribution and weak crystallization in the composites [31, 43-45].

10

Fig. 2 SEM images of pure BiPO4(a), ZnAl2O4(b) and 1wt% ZnAl2O4/BiPO4(c-d),

EDS spectra (e) of 1wt% ZnAl2O4/BiPO4

Fig. 2 shows the morphologies of BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 composites and EDS pattern of 1wt% ZnAl2O4/BiPO4 detected by SEM. As illustrated in Fig.2(a), the pure BiPO4 consists of some smooth nanorods and irregular shapes with the size of nanorods ranging from 100 nm to 600 nm[28]. It can be clearly seen from Fig.2(b) that the pure ZnAl2O4 was composed by some irregular nanosheets. From Fig.2(c-d), 11

the shapes of ZnAl2O4/BiPO4 (ZB3) composites were similar to that of pure BiPO4, and the introduced ZnAl2O4 covers on the surface of BiPO4, making a rough surface of nanorods. The element composites of 1wt% ZnAl2O4/BiPO4 were also characterized by EDS spectrum and shown in Fig.2(e), and the peaks of Bi, P, O, Zn and Al are also observed, confirming the coexistence of ZnAl2O4 and BiPO4 in the heterostructures.

Fig. 3 TEM and HRTEM pattern (a, c), EDS in different areas (b, d) and elemental mapping (e-j) of 1wt% ZnAl2O4/BiPO4

The detailed structure of 1wt% ZnAl2O4/BiPO4 (ZB3) was also characterized by 12

TEM, HRTEM, EDS and elemental mapping. It can be seen from Fig.3(a,c) that small ZnAl2O4 nanosheets were deposited on the surface of BiPO4 nanorods, and the interface between BiPO4 and ZnAl2O4 can also be observed. The lattice fringe of 0.466 nm was indexed to (011) plane of BiPO4 (JCPDS NO.80-0209), and the fringe with an interval of 0.248 nm can be ascribed to the (311) plane of cubic ZnAl2O4 phase. EDS spectra in different area were also detected and shown in Fig.3(b,d). The Bi, P and O elements were found in the black-circled area, and the Bi, P, O, Zn and Al elements were detected in the red-circled region, further confirming that both BiPO4 and ZnAl2O4 particles were present in the ZB3 sample. Additionally, the elemental mapping of ZB3 were also carried out and shown in Fig.3(e-j). The Bi, P and O elements were uniformly dispersed in the selected region with high density, and the Zn and Al elements were shown in the selected area with low density. Interestingly, the O, Zn and Al elements were dispersed around BiPO4 nanorods without the distribution of Bi and P elements, demonstrating the successfully synthesis and uniformly distribution of ZnAl2O4 in the composites.

13

Fig.4 XPS survey spectrum of BiPO4 and 1wt% ZnAl2O4/BiPO4 (a), and its high-

resolution XPS spectra: (b) Bi 4f, (c) P 2p, (d) O 1s, (e) Zn 2p and (f) Al 2p

In order to analyze the composition and elemental valence in the BiPO4 and ZnAl2O4/BiPO4, XPS spectra were carried out and shown in Fig.4. The binding energies of the elements were calibrated with the peaks for C 1s at 284.8 eV. As shown in Fig.4a, the peaks of Bi, P and O were found in BiPO4 and ZnAl2O4/BiPO4, whereas, the peaks of Zn and Al were also detected in ZnAl2O4/BiPO4 composites. As 14

shown in Fig. 4b, two signals in ZnAl2O4/BiPO4 located at 164.98 eV and 159.68eV were corresponding to Bi 4f5/2 and Bi 4f7/2, illustrating the presence of Bi3+[31, 45-48], which is slightly higher than that of BiPO4 (164.48 eV and 159.15 eV). The binding energy (Fig.4c) of P 2p in ZnAl2O4/BiPO4 was found at 132.98 eV, demonstrating that the valence of P was +5 [47], a slight shift than that of pure BiPO4. The binding energy of O 1s (Fig.4d) in ZnAl2O4/BiPO4 can be detected at 529.68eV [31, 44], showing a positive shift compared with that of pure BiPO4 (529.54 eV). In a word, the shifted binding energies of Bi 4f, P 2p and O 1s in the ZnAl2O4/BiPO4 composites were ascribed to the strong interaction between ZnAl2O4 and BiPO4 with different electronegativity, resulting in the shifted valence electron and the changing electric screening [49-51]. And the existence of electron transfer and chemical bonds between ZnAl2O4 and BiPO4 in the heterostructures can be estimated from the XPS spectra. It can be clearly seen from Fig.4e that binding energy of Zn 2p1/2 and Zn 2p3/2 in ZnAl2O4/BiPO4 were 1044.18 eV and 1021.41 eV, respectively. The peak (Fig.4f) in ZnAl2O4/BiPO4 at 73.91 eV was attributed to Al 2p [42, 52]. Notably, the peaks of Zn 2p and Al 2p were rough with larger fluctuation due to a low content of ZnAl2O4. Therefore, this XPS result further reveals the co-existence of ZnAl2O4 and BiPO4 in the 1wt% ZnAl2O4/BiPO4 heterostructures.

15

Fig. 5 Photoluminescence (PL) spectra of BiPO4, 1wt% ZnAl2O4/ BiPO4 (ZB3) and ZnAl2O4 (λex=325 nm)

The separation and recombination processes of photogenerated charge carriers in the semiconductor could be verified by photoluminescence (PL) spectra [53, 54]. Generally, a strong PL intensity presents a fast recombination of photogenerated holes and electrons, resulting in a lower photocatalytic efficiency. Fig.5 shows the PL spectra of pure BiPO4, ZnAl2O4 and 1wt% ZnAl2O4/BiPO4 samples with the excitation wavelength of 325 nm. Emission peaks located at 443 nm were observed in BiPO4 and 1wt% ZnAl2O4/BiPO4. Clearly, the intensity of emission peak over 1wt% ZnAl2O4/BiPO4 was weaker than that of pure BiPO4 and ZnAl2O4, demonstrating that the coupling ZnAl2O4 with BiPO4 can effectively accelerated electrons and holes pairs separation and thus benefited to improve the photocatalytic performance of pure BiPO4 and ZnAl2O4.

16

Fig. 6 UV-vis diffuse reflectance spectra (a) and the plots of transformed Kubelka-Munk functions versus the light energy (b) of pure BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 composites

The electronic states and optical properties of BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 composites have a great influence on the photocatalytic activities, which can be determined by UV-vis diffuse reflection spectra (DRS). It can be clearly seen from Fig.6a that the absorption edge of all the prepared catalyst were located at UV light region, and an apparent red-shift of the absorption edge with ZnAl2O4/BiPO4 samples were observed compared to that of BiPO4. The band gap energy of a semiconductor can be calculated by Kubelk-Munk theory [55], which were shown below:

 hv  A(hv  Eg ) n / 2 Where α, ν, Eg and A are absorption coefficient, light frequency, band gap energy, and a constant, respectively. And the value of n was determined by characteristics of transition in a semiconductor. According to the theory, a plot of (αhν)2 versus photon energy were calculated and described in Fig.6b and an enlarged curve of ZnAl2O4 were also inserted in the figure. The band energy of BiPO4 and ZnAl2O4 were 4.15 eV and 2.75 eV, respectively. 17

The conduction band edge (ECB) and valence band edge (ECB) of a semiconductor is vital to study the photocatalytic mechanism, which can be deduced from the following formula [24, 33]: EVB = X - Ee + 0.5 Eg ECB = EVB - Eg Where EVB, ECB, X, Ee and Eg are the valence band (VB) edge potential, conduction band (CB) edge potential, electronegativity, energy of free electrons on the hydrogen scale (about 4.5 eV) and the band gap energy of the semiconductor, respectively. According to the literature [27, 29], the electronegativity (X) of BiPO4 and ZnAl2O4 are 6.85 eV and 5.49 eV, respectively. As illustrated by the UV-vis diffuse reflection spectra, the band gap energy of BiPO4 and ZnAl2O4 are 4.15 eV and 2.75 eV. The conduction band (CB) of BiPO4 and ZnAl2O4 were estimated to be 0.27 eV and -0.38 eV, and the valence band (VB) of BiPO4 and ZnAl2O4 were calculated to be 4.43 eV and 2.37 eV, respectively.

Fig. 7 Concentration variation of MB (a) and kinetic fits for the degradation of MB (b) with commercial TiO2 (P25), BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 composites

In order to investigate the photocatalytic performance of ZnAl2O4/BiPO4 18

heterostructures, a series of photocatalytic degradation experiments using methylene blue (MB) as a model compound were conducted under Hg light irradiation and the results were depicted in Fig. 7a. Meanwhile, the MB adsorption experiments have been carried out, and the result was described in the Fig. S1. According to the Fig.S1, the MB adsorption efficiency over the prepared samples were less than 5% at 30min, thus the MB adsorption were neglected in our research [27, 56]. Clearly seen from Fig. 7a, the photocatalytic degradation of MB over ZnAl2O4/BiPO4 is greatly superior to that of pure BiPO4 and ZnAl2O4, and seriously affected by the loading content of ZnAl2O4. The highest MB degradation (92.56%) was obtained at 3 h via 1wt% ZnAl2O4/BiPO4. With introducing a small amount of ZnAl2O4, the photocatalytic activities of ZnAl2O4/BiPO4 composites were obviously enhanced, which mainly due to that appropriate ZnAl2O4 content could accelerate the separation between photogenerated holes and electrons, resulting in a higher MB degradation. When the amount of ZnAl2O4 continues to increase, photocatalytic efficiency of composites decreased. This is partially because that the intensity of light through the depth of the reaction solution decreased which shielded the BiPO4 from absorbing light with excessive ZnAl2O4. Moreover, the active sites of samples might be covered with increasing percentage of ZnAl2O4. These cases promote the recombination of the photo-generated electron–hole pairs, resulting in a lower photocatalytic effciency [31, 57].

Table 3 The first-order kinetic constants (k) and relative coefficient (R2) of prepared samples. 19

sample

k(h-1)

R2

BiPO4

0.388

0.980

ZB1

0.690

0.974

ZB2

0.727

0.977

ZB3

0.959

0.977

ZB4

0.607

0.984

ZB5

0.526

0.982

ZB6

0.421

0.985

ZnAl2O4

0.053

0.922

P25

0.290

0.972

A pseudo-first-order kinetic model by the linear transforms fit experimental data well for the photocatalytic process to study the photodegradation rate of MB [21], ln

C0  kt Ct

Where Co is the initial concentration of MB, Ct is MB concentration at t times irradiation, t is the UV irradiation time, and k is the apparent rate constant. The kinetic plots for the MB degradation over BiPO4, ZnAl2O4 and ZnAl2O4/BiPO4 heterojunctions were shown in Fig.7b, and the corresponding first-order kinetic constants (k) and relative coefficient (R2) were described in Table 3. It can be clearly observed that the kinetic constant of 1 wt% ZnAl2O4/BiPO4 composites is greater than that of BiPO4 and ZnAl2O4, and the kinetic constant of 1wt% ZnAl2O4/BiPO4 is 2.47

20

and 18.10 times as large as that of pure BiPO4 and ZnAl2O4, respectively.

Fig. 8 (a) Recyclability for photocatalytic degradation of MB by 1 wt% ZnAl2O4/BiPO4 heterojunction, and (b) XRD patterns of fresh and reused 1 wt% ZnAl2O4/BiPO4 catalyst

Concerning the practical application, cycling tests to evaluate the stability and reusability of a catalyst were studied and shown in Fig. 8a. It can be found that no obviously decrease was observed for the MB degradation over 1 wt% ZnAl2O4/BiPO4 after four cycling running, implying that the ZnAl2O4/BiPO4 heterostructures are stable in the photocatalysis. Fig. 8b shows the XRD curves of the fresh and used ZnAl2O4/BiPO4 catalyst. The peaks of used ZnAl2O4/BiPO4 catalyst were similar to that of fresh catalyst, and no significant deactivation occurs during the reaction. Hence, the composites have promising and broad prospects in the industrial wastewater purification.

21

Fig. 9 Effects of different radical scavengers on the MB degradation over BiPO4, 1wt% ZnAl2O4/BiPO4 and ZnAl2O4 under UV light at 3 h

It is believed that the active species generated in the photocatalytic process were responsible for MB degradation. To understand the photocatalytic mechanism of BiPO4, ZnAl2O4/BiPO4 and ZnAl2O4, scavenger experiments were carried out to detect the active species, and the result was depicted in Fig.9. Benzoquinone (BQ, •O2− scavenger), EDTA-2Na (h+ scavenger), and 2-propanol (IPA, •OH scavenger) were put in the MB solution [33]. Effects of different radical scavengers on the pH of MB solution over BiPO4, 1wt% ZnAl2O4/BiPO4 and ZnAl2O4 were detected and shown in Fig. S2. It can be seen that the pH of MB solution with different scavengers were ranged from 6.64 to 6.85, which could be ignored according to the previous report [45, 48, 56]. From Fig. 9, The degradation of MB over pure BiPO4 reduced from 65.58 % to 60.83%, 53.27% and 22.35% in the presence of BQ, EDTA-2Na and IPA, respectively, suggesting that • OH is the key radicals for the MB degradation which is consistent with previous report [30]. For pure ZnAl2O4, the addition of BQ and IPA apparently suppressed the degradation of MB, indicating that •OH and •O2− 22

are the main dominant radicals in the photocatalytic reaction [37]. However, the MB degradation in the ZnAl2O4/BiPO4 system was not affected by the addition of BQ, implying that the •O2− contributes little to MB photocatalytic decomposition. While, the MB degradation over ZnAl2O4/BiPO4 decreased by 41.33% and 51.72%, respectively, when EDTA-2Na and IPA were added in the reaction, illustrating that the hole and •OH were the main reactive species in the MB degradation reaction.

Fig. 10 Possible photocatalytic mechanism over ZnAl2O4/BiPO4 heterostructures under UV light irradiation

According to the above experiments and calculation, a possible mechanism of MB degradation over ZnAl2O4/BiPO4 heterostructures under UV light irradiation was proposed and shown in Fig. 10. Under UV irradiation, both BiPO4 and ZnAl2O4 can be easily excited from valence band (VB) to conduction band (CB) to produce holes and electrons. The holes (h+) can not only degrade the pollutant in the solution, but also oxidize the H2O molecules into •OH possessing a strong oxidation property. The photogenerated holes of BiPO4 in VB can transfer to that of ZnAl2O4, while electrons 23

on CB of ZnAl2O4 can easily migrated to the CB of BiPO4. As a result, the separation between generated electron and holes pair was accelerated, which could improve the photocatalytic efficiency of ZnAl2O4/BiPO4 heterojunction.

Fig.11 The CODCr and color removal of TW and ECMPW over ZB3 and ZB3/H2O2 at 25 oC under UV light for 3 h (c(27.5wt%H2O2):1g/L)

The final purpose of our research was to solve the serious wastewater crisis, hence 1wt% ZnAl2O4/BiPO4 composites were used to treat TW and ECMPW collected after the biological treatment, and the results were shown in Fig. 11. The original CODCr and color of TW were 258 mg/L and 824 Pt-Co, and the original CODCr and color of TW were 173 mg/L and 456 Pt-Co, respectively. It can be clearly seen that the CODCr and color of TW over 1wt% ZnAl2O4/BiPO4 under UV light (UV/ZB3) were 64.34% and 58.28%, and the removal of CODCr and color of ECMPW over UV/ZB3 were 59.23% and 52.47%, respectively. Fenton and PAC reagents were also applied to the

24

wastewater treatment to compare photocatalytic efficiency, and the result were shown in Table 4. As illustrated from Table 4, the CODCr removal of TW and ECMPW over ZB3 under UV light were analogous to that with 0.5g/L H2O2 (27.5%) of Fenton reagent. With 1 g/L H2O2 (27.5 wt%) adding in the solution, the photo-Fenton-like reaction (UV/ZB3/H2O2) was produced, and the removals of CODCr and color of TW and ECMPW were significantly enhanced, which were similar to that with 1 g/L H2O2 (27.5 wt%) of Fenton reagent and higher than that with 2g/L 30wt% PAC. The CODCr of treated TW and ECMPW over UV/ZB3/H2O2 system were 60 mg/L and 45 mg/L, respectively, satisfying the discharge standard of waster pollutants for pulp and paper industry (GB3544-2008) and dyeing and dishing of textile industry (GB4287-2012) in China. Table 4 Eucalyptus CMP wastewater treated by different method

TW 　

UV/ZB3a UV/ZB3/H2O2b

Fentonc

PACd

ECMPW

dosage

CODCr

Color

CODCr

Color

(g/L)

removal

removal

removal

removal

(%)

(%)

(%)

(%)

0.8

64.34

58.28

59.23

52.47

0.8/1

76.74

83.22

73.98

84.54

0.5

66.32

68.75

64.73

65.24

1

80.75

88.52

76.24

87.12

1.5

88.23

91.05

81.52

90.22

1

50.43

50.12

52.22

45.22

2

63.12

63.87

60.23

60.27

a. dosage of ZB3 in the solution was 0.8g/L, b. dosage of ZB3 and 15 wt% H2O2 in the solution and were 0.8 and 1 g/L, c. dosage of 27.5 wt% H2O2 in the solution were 1g/L, d. the 25

concentration of PAC solution of 30 wt%.

Conclusions A novel ZnAl2O4/BiPO4 heterojunctions were successfully synthesized by a facile two-step method. The photocatalytic efficiency of ZnAl2O4/BiPO4 composites was enhanced under UV light irradiation, and the highest degradation rate of MB over 1 wt% ZnAl2O4/BiPO4 were 92.55% at 3h under UV light irradiation, which is 1.41 and 6.46 times than that of pure BiPO4 and ZnAl2O4, respectively. Such excellent photocatalytic performances were due to the effective separation between photoinduced electrons and holes. The ZnAl2O4/BiPO4 shows excellent stability and reusability in the photocatalytic reaction. Furthermore, the composites were also used to treat the industrial TW and ECMPW with a high removal of CODCr and color, and the photocatalytic activity can significantly increase with H2O2 addition, which might provide a promising platform for high performance photocatalytic applications in industrial wastewater treatment.

Conflicts of interest The authors declare no conflict of interest.

Acknowledgments This work was supported by Jiangsu Key Laboratory for Biomass Energy and Material (JSBEM-S-201809), the National Key Research and Development Program of China (2017YFD0601005).

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[57] H. Xu, Y. Xu, H. Li, J. Xia, J. Xiong, S. Yin, C. Huang, H. Wan, Synthesis, characterization

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35

of

AgBr/BiPO4

heterojunction

Qingwen Tian: Conceptualization, Methodology, Investigation, Writing - Original Draft

Miao Ran: Methodology, Investigation, Resources, Writing - Original Draft

Guigan Fang: Conceptualization, Writing - Review & Editing, Funding acquisition

Laibao Ding: Conceptualization, Writing - Review & Editing

Aixiang Pan: Formal analysis, Resources

Kuizhong Shen: Writing - Review & Editing

Yongjun Deng: Writing - Review & Editing

36

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

37

Highlights  A novel ZnAl2O4/BiPO4 heterostructures have been successfully prepared.  ZnAl2O4/BiPO4 heterostructures exhibits excellent activity in dye decomposition.  Photo-Fenton-like process was applied to treat industrial textile and pulping wastewater.  The photocatalytic mechanism of catalysts were proposed.

38