Bi2MoO6 microspheres for the degradation of orange II by heterogeneous activation of persulfate under visible light

Materials Letters 261 (2020) 127099

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Bi2MoO6 microspheres for the degradation of orange II by heterogeneous activation of persulfate under visible light Boyuan Zhu a,b, Hao Cheng b, Jianfeng Ma a,b,⇑, Yong Qin c, Yong Kong c, Sridhar Komarneni d,⇑ a

School of Environmental and Safety Engineering, Changzhou University, Jiangsu 213164, China Guangxi Key Laboratory of Green Processing of Sugar Resources, College of Biological and Chemical Engineering, Guangxi University of Science and Technology, Guangxi 545006, China c School of Chemical Engineering, Changzhou University, Jiangsu 213164, China d Department of Ecosystem Science and Management and Materials Research Institute, 204 Energy and Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b

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Article history: Received 28 August 2019 Received in revised form 16 November 2019 Accepted 1 December 2019 Available online 2 December 2019 Keywords: Bi2MoO6 catalyst K2S2O8 Visible light Orange II degradation

a b s t r a c t The 3D microspheres of Bi2MoO6 samples were synthesized hydrothermally using different Bi:Mo molar ratios and evaluated for the removal of orange II under visible light by activating with K2S2O8. The Bi-Mo2 sample (Bi:Mo molar ratio of 2:1, i.e., stoichiometric composition of Bi2MoO6) showed the strongest catalytic activity by removing 99.43% orange II within 120 min among the several different Bi-Mo materials. The surface of the microspheres was composed of irregular nanosheets with exposed reactive sites, which are beneficial for degrading orange II. Moreover, the Bi2MoO6 catalyst demonstrated good cycling stability and could be reused for at least five runs with only a 6.99% decrease in degradation. The proposed mechanism of photocatalytic activity could be explained by the generation of superoxide radical O2 along with SO4 and OH radicals, which were found to be responsible for the orange II degradation. Our research led to an efficient and stable photochemical catalyst for orange II dye degradation in wastewater. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction The problem of organic contaminants in water has been the focus of environmental attention due to the rapid industrialization. Sulfate radical-based SO4 advanced oxidation processes have become a promising alternative to the Fenton reaction [1]. SO4 could react effectively with organic compounds over a wide pH range of 2–8 [2]. Persulfate activation is an emerging oxidizing technology, which can produce SO4 radicals by using ultra-violet light irradiation, heat, or metals [3]. Bi2MoO6 has been reported as a promising photocatalyst because of its non-toxicity, high quantum yield, narrow band gap (2.5–2.8 eV) [4]. However, the application of Bi2MoO6 alone is limited due to the low carrier mobility and rapid electron-hole recombination rate. Many researchers combined semiconductor catalysts with persulfate additive to solve the above deficiency with good success. Liu et al. synthesized Bi2WO6/Fe3O4 and showed great decomposition efficiency of 17b-estradiol with persulfate [5].

⇑ Corresponding authors at: School of Environmental and Safety Engineering, Changzhou University, Jiangsu 213164, China (J. Ma). E-mail addresses: [email protected] (J. Ma), [email protected] (S. Komarneni). https://doi.org/10.1016/j.matlet.2019.127099 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

Tang et al. found that novel Cu0.84Bi2.08O4 could be activated by peroxydisulfate for decomposing ciprofloxacin under visible light [6]. These studies with persulfate illustrate that SO4 plays a very critical role in the process of organic degradation. Therefore, the purpose of this article is to combine Bi2MoO6 with persulfate for rapid degradation of organics in wastewater. Bi2MoO6 nanoparticles with different Bi and Mo contents were prepared by simple hydrothermal method. The performance of Bi2MoO6 was evaluated by visible light and persulfate in the degradation of orange II (OII) dye. The mechanism of persulfate activation of Bi-Mo compound to degrade OII was found to be due to the generation of SO4 and OH radicals. 2. Experimental 2.1. Synthesis of hierarchically structured Bi-Mo Chemicals of Bi(NO3)3
5H2O (0.422 g, 0.844 g, 1.687 g, 2.532 g or 3.376 g) and 0.421 g Na2MoO4
2H2O were mixed in 5 mL ethylene glycol according to different molar ratios of Bi:Mo (0.5:1, 1:1, 2:1, 3:1, 4:1) molar ratios, respectively. The above solution was mixed with 20 mL ethanol. This mixture was stirred for 0.5 h and

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transferred to a 35 mL Teflon-lined stainless-steel autoclave, which was heated at 160 for 12 h. The products were separated, washed and dried at 60 °C. The prepared samples were denoted as Bi-Mo-0.5, Bi-Mo-1, Bi-Mo-2, Bi-Mo-3 and Bi-Mo-4.

absorbance of OII dye was monitored at a fixed wavelength of 484 nm with UV–vis spectrophotometer.

3. Results and discussion 2.2. Characterization The XRD patterns of samples were collected by a Rigaku D/Max-2500 PC X-ray diffractometer (Rigaku Co., Japan) with Cu Ka radiation (40 kV, 100 mA) of 0.154 nm wavelength. The morphology of the synthesized samples was characterized by a scanning electron microscope (SEM, Hitachi S4800). The optical spectra of the prepared samples were measured by using an Ultraviolet-vis (UV–vis) spectrometer (UV-2450, Shimadzu). 2.3. Measurement of photocatalytic reaction Catalytic performance was evaluated by the degradation of OII (80 mg/L) in the presence of 0.04 g Bi-Mo catalysts and 0.1 g K2S2O8 under visible light (a 120 W LED lamp). During the reaction, 2 mL solution was collected and then filtered at every 15 min. The

The XRD patterns of Bi-Mo samples are presented in Fig. 1a. All the diffraction peaks of Bi-Mo-2, Bi-Mo-3, Bi-Mo-4 can be indexed to Bi2MoO6 standard card (PDF#21–0102). However, the peaks of Bi-Mo-2 are sharper than those of Bi-Mo-3 and Bi-Mo-4, the latter showed broad peaks suggesting smaller crystal/particle size. When the molar ratios of Bi:Mo were 1:1 and 0.5:1, some other unknown phases can be seen. Bi-Mo-2 (Bi2MoO6) showed the highest degree of crystallization. Fig. 1b displays the UV–vis DRS of Bi-Mo-2 suggesting that this material can absorb both ultraviolet and visible light below 485 nm. The band gap energy (Eg) was calculated to be about 2.8 eV by Tauc equation, indicating that electron-hole pairs could be easily generated owing to the narrow band gap. The morphology and particle size of Bi-Mo-2 were examined by SEM as shown in Fig. 1c. The Bi-Mo-2 material crystallized as 3D microspheres in the diameter range of about 2 to 3 lm. It can be clearly observed from the surface magnification picture (Fig. 1d)

Fig. 1. (a) XRD patterns of all Bi-Mo samples, (b) UV–vis light absorption spectrum and energy band gap of Bi-Mo-2 and (c), (d) SEM photographs of Bi-Mo-2.

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Table 1 Comparison of the effectiveness of our catalyst with related catalysts in the literature. Similar system

Pollutant

Reaction Removal time

Bi-Mo-2, K2S2O8 + v. light (This work) b-Bi2O3/BiOI + visible light[7] In(OH)3/Bi2M oO6 + visible light[8] Bi2O3/MoO3 + visible light[9] CuBi2O4 + K2S2O8[10]

orange II methyl orange rhodamine B methylene blue 1H-benzotriazole

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99.4% 95% 86% 86.6% 99%

that the surface of Bi-Mo-2 sample is composed of self-assembled irregular nanosheets with a thickness of ~25 nm. Fig. 2a displays the results of the degradation of OII by Bi-Mo in the presence of K2S2O8 and visible light. Bi-Mo-0.5 and Bi-Mo-1 have little degradation effect, which may be due to the presence of unknown phase. The degradation efficiency of stoichiometric Bi-Mo-2 (99.43%) material was the highest, which was 18.47% and 8.73% higher than those of Bi-Mo-3 and Bi-Mo-4, respectively. This is probably due to the smaller amounts of Bi2MoO6 phase that was crystallized because of the deficiency in Bi but with excess Mo in the starting compositions.

Little or no degradation of OII occurred by itself under visible light in Fig. 2b. When the reaction was conducted with only K2S2O8, less than 4% OII was detected. However, the degradation rate was slightly improved to 12.52% with K2S2O8 and visible light. When Bi-Mo-2 and K2S2O8 were added together in the dark, 11.31% OII was observed. 21.43% OII was degraded by Bi-Mo-2 under visible light indicating that Bi-Mo-2 played a photo-catalytic role in degradation. However, with simultaneous presence of Bi-Mo-2, K2S2O8 and visible light, 99.43% OII was degraded in 120 min. The great improvement may be due to the formation of a heterogeneous Fenton-like system with Bi-Mo-2/K2S2O8/visible light. Table 1 lists the previous results with Bi and Mo, and the comparison results show that Bi-Mo-2 prepared in this study has better degradation effect. The cyclic degradation efficiency of Bi-Mo-2 (Bi2MoO6) is depicted in Fig. 2c. No significant decrease (6.99%) was observed after five cycles. Moreover, no obvious changes could be discerned by XRD (Fig. 2d) before and after repeated use of Bi-Mo-2 for five times, demonstrating that the Bi2MoO6 is highly stable as a catalyst for potential application in OII removal. As shown in Fig. 3a, the removal rate of OII decreased by 32.48% by adding tert-butyl alcohol (TBA) as OH scavenger. When excess

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Fig. 3. (a) Effects of different scavengers and (b) reaction mechanism on the degradation of OII.

ethanol (EtOH), which is a OH and SO4 scavenger was added to the reaction solution, the removal decreased to 49.65%. The results showed that OH and SO4 radicals were produced. When benzoquinone (BQ), which is a scavenger for surface-bound free radicals was added to the system, the degradation was suppressed by 80.39%. This suggests that the free radicals on the surface also play a leading role in the reaction. The possible mechanism is hypothesized in Fig. 3b. The e- in the conduction band (CB) moves to Bi-Mo-2 surface to generate O2 . The h+ in the valence band (VB) could react with H2O/OH– to form OH because its Evb equal to 3.21 eV is higher than that of the E (OH/H2O), the latter is equal to 2.38 eV [11]. The Ecb equal to 0.41 eV is lower than that of the  2E (S2O28 /SO4 ), the latter is equal to 2.06 eV. Therefore, the S2O8 can be transformed to SOwith high oxidation ability by combin4 2ing with e- on CB [12]. Many scholars found that E (SO4 /SO4 ) of   2.5–3.1 eV is higher than E (OH /H2O) indicating that OH can also be produced through oxidation of H2O by SO4 [13]. In summary, OII was completely degraded by the generation of O2 , SO4 and OH. 4. Conclusions In summary, 3D microspheres of Bi2MoO6 were successfully prepared with Bi and Mo molar ratio of 2:1, which showed excellent degradation of OII dye among all Bi-Mo samples in the presence of K2S2O8 and visible light. The remarkable degradation effect is due to the existence of active O2 , OH and SO4 radicals. The Bi2MoO6 exhibited great stability and reusability as determined by five cycles. The presently developed photocatalyst materials serve as alternatives to TiO2 or even ZnO based modified photocatalysts in treatment of waste water containing OII dye. 5. Authors’ contributions The conceptual idea is from Jianfeng Ma. Boyuan Zhu and Hao Cheng performed the research. All authors, Boyuan Zhu, Hao

Cheng, Jianfeng Ma, Yong Qin, Yong Kong, and Sridhar Komarneni contributed in writing the manuscript. Declaration of Competing Interest 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. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21477009), Natural Science Foundation of Jiangsu Province (SBK2016021419), ‘‘333 project” of Jiangsu Province and the Opening Project of Guangxi Key Laboratory of Green Processing of Sugar Resources (No. GXTZY201803). One of us (SK) was supported by the College of Agricultural Sciences under Station Research Project No. PEN04566. References [1] A. Rastogi, S.R. Al-Abed, D.D. Dionysiou, Appl. Catal. B. Environ. 85 (2009) 171– 179. [2] P. Hu, M. Long, Appl. Catal. B. Environ. 181 (2016) 103–117. [3] B. Xie, X. Li, X. Huang, W. Zhang, B. Pan, J. Environ. Sci. 54 (2017) 231–238. [4] Y. Hao, X. Dong, X. Wang, S. Zhai, H. Ma, X. Zhang, J. Mater. Chem A. 4 (2016) 8298–8307. [5] L. Yang, H. Guo, Y. Zhang, W. Tang, Rsc Adv. 6 (2016) 79910–79919. [6] T. Han, D. Zan, X. Xiande, W. Zhipan, C. Rong, Chem. Eng. J. 256 (2019) 472– 482. [7] S. Han, L. Jia, K. Yang, J. Lin, Chin. J. Catal. 36 (2015) 2119–2126. [8] T. Hu, H. Li, Z. Liang, N. Du, W. Hou, J. Colloid Interf. Sci. 545 (2019) 301–310. [9] Z. Hu, J. Zhou, Y. Zhang, W. Liu, J. Zhou, W. Cai, Chem. Phys. Lett. 706 (2018) 208–214. [10] W.-D. Oha, S. Lua, Z. Dong, Nanoscale. 7 (2015) 8149–8158. [11] G. Zhou, H. Sun, S. Wang, H.M. Ang, M.O. Tadé, Sep. Purif. Technol. 80 (2011) 626–634. [12] S. Yan, Y. Shi, Y. Tao, H. Zhang, Chem. Eng. J. 359 (2019) 933–943. [13] D. Wu, P. Ye, M. Wang, Y. Wei, X. Li, A. Xu, J. Hazard. Mater. 352 (2018) 148.