Bacterial Cellulose Hybrid Membranes as

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Functional Nanostructured Materials (including low-D carbon)

Alpha Fe2O3 Nanodisk/Bacterial Cellulose Hybrid Membranes as High-Performance Sulfate-Radical Based VisibleLight Photocatalysts Under Stirring/Flowing States Zhong-Shuai Zhu, Jin Qu, Shu-Meng Hao, Shuang Han, Kun-Le Jia, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10128 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Applied Materials & Interfaces

Alpha-Fe2O3 Nanodisk/Bacterial Cellulose Hybrid Membranes as High-Performance Sulfate-Radical Based Visible-Light Photocatalysts Under Stirring/Flowing States Zhong-Shuai Zhua,b, Jin Qua*, Shu-Meng Haoa, Shuang Hanb, Kun-Le Jiab and Zhong-Zhen Yua,b* a

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China. b

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China. E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) KEYWORDS: visible light photodegradation; peroxymonosulfate; bacterial cellulose; α-Fe2O3; advanced oxidation process ABSTRACT: High activity and long-term stability are particular important for peroxymonosulfate (PMS) based degradation processes in wastewater treatment, especially under a flowing state. However, if the high active nanomaterials are in powder form, they could disperse well in water but would not be convenient for application under varied flow rates. A metal oxide/bacterial cellulose hybrid membrane fixed in a flowing bed is expected to solve these problems. Herein, α-Fe2O3 nanodisk/bacterial cellulose hybrid membranes as high-performance sulfate-radical based visible light photocatalysts are synthesized for the first time. The bacterial cellulose with excellent mechanical stability and film-forming feature not only benefits to form a stable membrane to avoid the separation and recycling problems, but also helps disperse and accommodate α-Fe2O3 nanodisks and thus enhances the visible light absorption performances, leading to an excellent PMS-based visible light degradation 1

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efficiency under both stirring and flowing states. Particularly, the optimized hybrid membrane photocatalyzes both cationic and anionic organic dyes under a flowing bed state for at least 84 hours with the catalytic efficiency up to 100 % and can be easily separated after the reaction, confirming its remarkable catalytic performance and long-term stability. Even if under varied flow rates during the continuous process, it efficiently degrades RhB and OII from 3 to 16 mL h-1. When the flow rate goes back from high to low, the hybrid membrane quickly recovers to its original performance, demonstrating the high activity and stability of the α-Fe2O3/bacterial cellulose membrane. 1. INTRODUCTION Water pollution has become a thorny environmental problem for governments and scientists, because the wastewater produced by industries usually includes toxic components and dyes that are not easily degraded by the nature.1-3 Advanced oxidation process (AOP) can completely mineralize or oxidize these organic compounds, and thus arouses international attention in the past years.4 In addition to the most common ·OH radical (1.8-2.7 V vs. NHE), SO4·- radical generated by peroxymonosulfate (PMS) has a higher redox potential of 2.5-3.1 V vs. NHE and a longer half-life period (30-40 µs vs. 20 ns of ·OH). Therefore, PMS-based system exhibits more efficient degradation performances.5 Transition metal oxide-based catalyst is highly attractive because of its high catalytic activity for PMS activation.6-11 For example, PrBaCo2O5+δ9 and α-Fe2O310,11 have been widely reported due to the high efficiency, environmental friendliness and low cost. However, the separation after degradation and the long-term recycling performance are still not satisfactory. Powdery catalysts are difficult to be recycled and might cause second pollution. A post-processing system like high-speed 2


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centrifugation or filtration is still required to separate the catalysts for reuse.12 Such a problem can be solved by putting the catalysts into membranes. A flexible and large membrane would make the operation convenient by constructing a flowing bed for sequent degradation process without separation and recycling problems.13 Moreover, visible light can be used to excite α-Fe2O3 and activate PMS effectively. It is expected that both α-Fe2O3 and visible light could bring about better photodegradation performances. To construct a highly efficient α-Fe2O3 based membrane, the substrate should well disperse and accommodate α-Fe2O3 nanodisks and not hinder the visible light absorption of α-Fe2O3. Moreover, high hydrophilicity and adsorption feature of the substrate could further enrich toxic components and dyes around α-Fe2O3 to enhance the degradation performances.14 In addition, settling the separation and recycling problems of photocatalysts is important and many carbon substrates have been used.15-17 Bacterial cellulose is a fantastic substrate owing to its special porous structure, high aspect ratio, high specific surface area, and unique mechanical properties.18-20 Its high transparency benefits the light transmission to excite α-Fe2O3, while its satisfactory formability makes it form membranes or gels easily.21 The high crystallinity of bacterial cellulose provides it with good physical and chemical stabilities,22 and its hydrophilicity resulted from its large number of hydroxyl groups makes it suitable as the catalyst supporter in water.23-25 Besides, low cost and favorable biocompatibility are also advantages for water purification applications.26 As a result of these merits, bacterial cellulose is well suitable as the substrate to compound with other inorganic materials, and used in many areas including energy storage27-29, pollutant adsorption30 and degradation.31,

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there are few reports about α-Fe2O3 based membrane to activate PMS in an advanced oxidation process so far. Herein, we fabricate α-Fe2O3 nanodisk/bacterial cellulose (BFO) photocatalysts by a pre-adsorption and hydrothermal approach for the first time, and investigate their photodegradation performances towards different dyes under stirring and flowing bed states. The bacterial cellulose helps construct a freestanding photocatalytic membrane owing to its excellent film-forming feature and mechanical performances, while its rich hydroxyl groups benefit to accommodate and disperse α-Fe2O3 nanodisks. Moreover, the rich hydroxyl groups help enrich the dyes to the surface of α-Fe2O3 to boost the degradation performances. Thus, the

optimized

hybrid

membrane

exhibits

excellent

PMS-based

photodegradation

performances towards both cationic and anionic organic dyes with the assistance of visible light, not only under common stirring state but also under the flowing bed state. Especially under the flowing bed state, the membrane still retains an excellent catalytic efficiency, indicating its long-term stability towards varied flow rates. 2. EXPERIMENTAL SECTION 2.1. Materials. Bacterial cellulose (BC) hydrogels were purchased from Hainan Yide (China). Ferric chloride hexahydrate (FeCl3·6H2O), sodium silicate (Na2SiO3·9H2O), 1,4-benzoquinone (BQ), ethylenediamine tetraacetic acid disodium salt (EDTA-2Na), methanol (MeOH), ethanol (EtOH) and tert-Butanol (TBA) were bought from Beijing Chemical Factory (China). Potassium peroxymonosulfate (2KHSO5·3KHSO4·K2SO4, PMS), Rhodamine B (RhB), methylene blue (MB), crystal violet (CV), methyl orange (MO), orange

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ΙΙ (OII), and malachite green (MG) were provided by Aladdin Bio-Chem Technol. (China). All reagents were used without further purification. 2.2. Preparation of α-Fe2O3/BC Nanocomposites. Pristine BC hydrogel was washed by deionized water and the neutral BC was cut into small pieces and smashed into suspension by ultrasonication to obtain a BC suspension (2 mg L-1). To prepare the α-Fe2O3/BC nanocomposite, FeCl3·6H2O (0.75 mmol) and the BC suspension (7.5 mL) were mixed and stirred to obtain suspension A. Na2SiO3·9H2O (0.375 mmol) was dissolved in 20 mL of deionized water as solution B, which was then added dropwise into the suspension A. The resulting mixture was stirred for 1 h and transferred into a Teflon-lined autoclave for 12 h at 140oC. The resultant was centrifuged and washed with water and ethanol for three times, and freeze-dried at -54 oC in a FD-1C-50 freeze dryer for 2 days. BFO nanocomposites were fabricated using different BC amounts of 10, 20, 30 and 40 mg, and designated as BFO-x, where x is the initial BC dosage in mg. For comparison, α-Fe2O3 was also synthesized in the same way in the absence of BC component. 2.3 Fabrication of α-Fe2O3/BC Membranes. 20 mg of BFO nanocomposite was dispersed in 100 mL ethanol with the assistances of sonication and stirring. The homogeneous suspension was filtered by vacuum filtration with a PTFE membrane filter (0.22 mm pore size, 47 mm in diameter), and after drying at room temperature for 24 h the BFO membrane was peeled off from the filter. 2.4 Characterization. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/Max 2500 X-ray diffractometer with Cu Ka radiation at a generator voltage of 40 kV and a generator current of 40 mA. Microstructures were observed with a Hitachi S4700 5



field-emission scanning electron microscope (SEM) and a JEOL JEM-3010 transmission electron microscope (TEM). The nanocomposites were characterized with a Thermo VG RSCAKAB 250X high resolution X-ray photoelectron spectroscopy (XPS) and a Nicolet Nexus 670 Fourier-transform infrared (FT-IR) spectroscopy. Thermogravimetric analysis (TGA) data were collected in an air atmosphere at a heating rate of 10 °C/min using a Perkin-Elmer diamond thermogravimetric analyzer. UV-vis diffuse reflectance spectra were obtained with a Shimadzu UV-3600 UV-vis spectrophotometer. 2.5 Photocatalysis Measurements under Stirring and Flowing Bed States. Photocatalytic experiment system includes a photocatalytic reaction chamber with a CEL-HXUV300 xenon lamp and a 400 nm cutoff filter. The photocatalytic oxidation was conducted as follows: 10 mg of BFO photocatalyst was added into 50 mL of RhB solution (10 ppm) in a 100 mL glass beaker, and the mixture was ultrasonicated for 5 min and then stirred for 55 min in the dark to reach the absorption equilibrium. Afterwards, PMS (0.2 g L-1) was added into the mixture quickly. At a given interval of irradiation, 3 mL of the solution was taken out and filtered with a 0.22 µm filter, and the filtrate was measured with the UV-vis spectrophotometer. Besides, different dyes of MO, MB, CV, OII and MG were also photodegraded using the same procedure. Flowing bed photocatalytic reaction was also carried out: After BFO-30 membrane (~7.5 mg, 3.8 x 1.6 cm2) was put at the bottom of the Teflon reactor, RhB solution (20 ppm) and PMS solution (0.8 g L-1) were pumped through the reactor at a flow rate of 3, 6 or 9 mL h-1. The diagram of the experiment device is shown in Scheme 1f. For a given interval, 3 mL of the solution was taken out and measured with the

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UV-vis spectrophotometer. In addition, OII solution was used to evaluate the photocatalytic performances under varied flow rates of 8, 12 and 16 mL h-1. 3. RESULTS AND DISCUSSION The fabrication processes of BFO nanocomposite and BFO membrane are schematically illustrated in Scheme 1. Firstly, the pristine BC hydrogel (Scheme 1a) with a unique 3D interconnected fibrous network was smashed into BC suspension (Scheme 1b). The rich -OH groups make BC easily adsorb Fe3+, and the 1 h stirring treatment ensures the uniform distribution of the iron source on the BC nanofibers (Scheme 1c). By the silicate ions assisted hydrothermal approach,33 α-Fe2O3 nanodisks were uniformly grown on BC nanofibers, forming an α-Fe2O3/BC (BFO) nanocomposite (Scheme 1d). The BFO membrane was easily formed by vacuum filtration of the ethanol suspension of BFO powder (Scheme 1e).

Scheme 1. (a-e) Schematic illustrating the fabrication process of BFO nanocomposite and BFO membrane; and (f) the flowing bed device.

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Figure 1. TEM image of (a) BC; SEM images of (b) α-Fe2O3, (c) BFO-10, (d) BFO-20, (e) BFO-30, and (f) BFO-40; HRTEM images of (g) BFO-30 (inset: SAED pattern) and (h) the enlarged white square area in the image (g). Figure 1a shows the randomly arranged fibrous structure of white BC nanofibers with diameters around 20-50 nm after freeze-drying. Neat α-Fe2O3 nanodisks are approximately 50 nm in thickness and about 150-200 nm in diameters (Figure 1b). Although the presence of BC does not change the shape and size of these nanodisks (Figure 1c-f), the dosage of BC obviously affects the dispersion quality of α-Fe2O3 nanodisks. At the low BC content of 10 mg, lots of α-Fe2O3 nanodisks are not anchored on the nanofibers (Figure 1c). However, when the BC dosage increases to 20-30 mg, the nanodisks are well dispersed and all grown on BC nanofibers, confirming the successful fabrication of α-Fe2O3/BC nanocomposites. Further increase the BC content to 40 mg, less amount of α-Fe2O3 nanodisks is observed under the same area of the α-Fe2O3/BC nanocomposite (Figure 1f). TEM images of BFO-30 depict the subtler morphology (Figure S1a, b). The tight combination of BC with α-Fe2O3 is confirmed by high-resolution HRTEM images (Figure 1g, h). The selected-area electron diffraction 8


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(SAED) pattern reveals the single crystalline characteristic of α-Fe2O3 nanodisks (Figure 1g), and the clear lattice fringes with lattice spacing of 0.25 nm correspond to the (1 1 0) plane of α-Fe2O3 (Figure 1h). The growth and dispersion of α-Fe2O3 nanodisks on BC would affect the membrane formation and its photocatalytic performances. The crystalline structures of BC, α-Fe2O3 and BFOs are evaluated with their X-ray diffraction patterns (Figure 2a). The diffraction peaks of BC are at 15.0o, 16.5o and 22.8o, corresponding to typical cellulose I pattern (PDF No. 50-2241), while the characteristic peaks of α-Fe2O3 nanodisks at 24.1o, 33.2o, 35.6o, 40.8o, 49.5o, 54.1o, 62.4o and 64.0o correspond to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) crystalline planes of α-Fe2O3 (PDF No. 33-0664). All the peaks of α-Fe2O3 are still obvious in the BFOs with different BC dosages. Differently, because BC has a much lower crystallinity than α-Fe2O3, it is hard to identify the peaks of BC in BFO-10, but the diffraction peak of BC at ~23o becomes clearer with increasing the BC dosage. FT-IR curves are used to investigate the chemical bonds and composition changes of as-prepared nanocomposites (Figure 2b). All curves have the peaks around 3346 and 1630 cm-1, which are indexed to hydrogen-bonded hydroxyl groups and physically adsorbed water molecules, respectively.34 Moreover, all the peaks in the range of 1000-1500 cm-1 are ascribed to BC, while the peak at 528 cm-1 is related to Fe-O bond, indicating the existence of α-Fe2O3 in the nanocomposite. After the thermal treatment in air atmosphere, the contents of α-Fe2O3 in BFO-10, BFO-20, BFO-30 and BFO-40 are 85.8, 76.9, 61.4 and 49.5 wt%, respectively (Figure 2c). XPS spectra illustrate the chemical compositions of BFOs. Figure 2d distinctly reveals that the nanocomposites combined with α-Fe2O3 and BC are composed of Fe, O, C elements. The peaks at 531.6 and 528.9 eV are 9



ascribed to C-O bond of BC and Fe-O bond of α-Fe2O3, respectively. The new peak at 530.8 eV is related to Fe-O-C bond (Figure 2e), indicating the successful combination of BC and α-Fe2O3.35 Moreover, the peaks of Fe 2p3/2, Fe 2p1/2 and the satellite peak of BFO-30 are at the same location as neat α-Fe2O3, confirming that the valence state of Fe element has not been changed in the presence of BC (Figure 2f).

Figure 2. (a) XRD patterns, (b) FT-IR spectra, (b) TGA curves, (d) XPS spectra of α-Fe2O3 and BFOs; (e) O 1s of BFO-30 and BC; (f) Fe 2p of BFO-30 and α-Fe2O3. Because of the good film-forming feature of BC, BC membranes are readily prepared by vacuum filtering of ethanol suspensions of BC powders. Figure 3a shows a freestanding BFO-30 membrane with smooth surface, which can be easily peeled off from the PTFE membrane filter. However, at low dosages of BC (10 or 20 mg), it is hard to form a complete membrane that can be easily detach from the filter, and the presence of more BC makes the membrane wrinkle spontaneously (Figure S2). At the dosage of 30 mg, the resultant BFO-30 10


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membrane is able to recover its original shape after simply bending and folding without structural damage, implying its superior flexibility (Figure 3b-d). More importantly, the membrane has good hydrophilicity and wettability. As displayed in Figure 3e, its water contact angle is as low as 29.4o, indicating the water solution can quickly infiltrate into the membrane. Even if the membrane is immersed in water for 14 days, the morphology does not change, revealing its excellent mechanical stability in water (Figure S3). The BFO membrane is thus suitable for the dynamic catalytic process.

Figure 3. Digital photographs of (a) BFO-30 membrane peeled off from substrate; (b, c) BFO-30 membrane during and after bending; (d) BFO-30 membrane before, during and after folding; (e) static contact angle of water on BFO-30 membrane. The photocatalytic performances of powdery BFO nanocomposites are investigated with RhB as a model dye at 20 oC. As shown in Figure 4a, RhB solution itself is irradiated by visible-light for 60 min, the removal percentage of RhB is ~3 % only. After adding 0.2 g L-1 11



of PMS, the photodegradation efficiency increases to 20 % and 35 % corresponding to dark and visible-light conditions, respectively, indicating that PMS can degrade the dyes and the visible light can accelerate the rate. It is noted that, in the cases of α-Fe2O3/PMS or α-Fe2O3/visible light, the degradation of RhB is inefficient. α-Fe2O3 alone cannot efficiently activate PMS, while the visible light excited α-Fe2O3 cannot effectively degrade dyes in the absence of PMS. Interestingly, in the case of α-Fe2O3/PMS/visible light, the photodegradation activity is remarkably boosted. The visible light excited α-Fe2O3 efficiently activates PMS to generate active radicals to photodegrade RhB. As listed in the reactions 2-4, the high oxidation and reduction features of the photo-generated e- and h+ benefit the formation of high active radicals of SO4·- and ·OH. Thus, only 15 % and 3 % of RhB remain after 40 and 60 min photodegradation, respectively, which is attributed to the synergistic effect of PMS, α-Fe2O3 and visible light.

Figure 4. Relative concentration changes (Ct/C0) of (a) RhB (10 ppm) under different conditions with or without 0.2 g L-1 PMS, (b) RhB (10 ppm) using different catalysts with 0.2 12


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g L-1 PMS, (c) RhB (10 ppm) using BFO-30 and different amounts of PMS, (d) different concentrations of RhB with BFO-30 and 0.4 g L-1 PMS, (e) RhB (10 ppm) using BFO-30 with the same amount of PMS and H2O2, and (f) different dyes (10 ppm) with BFO-30 and 0.4 g L-1 PMS. The photodegradation of RhB follows the pseudo-first-order kinetics as ln(Ct/C0) = -kt, where k, t, C0 and Ct are respectively the rate constant, reaction time, initial dye concentration, and the dye concentration at t min. The rate constant of α-Fe2O3/PMS/visible light is over 500% higher than the others, further proving their synergistic effect (Figure S4). However, it is noted that BFO-40 exhibits a reduced photodegradation performance (Figure 4b), because the less amount of α-Fe2O3 nanodisks per unit area indeed decreases the catalytic efficiency, although more BC nanofibers serve as the substrate to disperse α-Fe2O3 well and benefit the membrane forming. Based on the UV-vis diffuse reflectance spectra (Figure S5) of BC, α-Fe2O3 and BFO-30, it is clear that the light-absorption ability of BFO is improved significantly to a great extent, resulting in the better photodegradation performances. Figure S6 shows the degradation rate constants (k) of RhB and the k value decreases as follows: BFO-30 (6.6×10-2 min-1) > BFO-20 (5.1×10-2 min-1) > α-Fe2O3 (4.2×10-2 min-1) > BFO-40 (4.0×10-2 min-1) > BFO-10 (3.4×10-2 min-1). Obviously, BFO-30 exhibits the best performance, 92% of RhB is removed in 40 min and the removal efficiency reaches 99% in 50 min. To study the influence of PMS dosages, Figure 4c shows the relative concentration changes of RhB (10 ppm) in the presence of BFO-30 and different amounts of PMS. 90 % of RhB is degraded in 60 min with 0.1 g L-1 of PMS. When the PMS dosage increases to 0.3 g 13



L-1, 90% and 95 % of RhB is removed in 30 and 40 min, respectively. With 0.4 g L-1 of PMS, only 3 % of pollution remains in 30 min and no residual is detected in 40 min. After 40 min degradation, the mineralization degree of RhB achieves to 54.2 %. There is no further increase in the degradation efficiency with 0.5 g L-1 of PMS. At a given dosage of BFO-30, the amount of generated active radicals from activating PMS per min is certain. When the dosage of PMS increases beyond the ability of BFO-30, the degradation efficiency could not increase further. For the same reason, the degradation efficiency would decrease with increasing the concentration of RhB (Figure 4d). As shown in Figure S7, the maximum rate constant of 9.9×10-2 min-1 is achieved at the PMS concentration of 0.4 g L-1. The UV-vis spectra show that RhB with a characteristic peak at 554 cm-1 can be completely degraded in 40 min (Figure S8). As previously reported,36-38 α-Fe2O3 combined with H2O2 is a famous and effective Fenton system to degrade organic dyes under visible light. To compare the effect of PMS and H2O2, Figure 4e shows the relative concentration changes of RhB (10 ppm) in the presence of BFO-30 with the same amount of PMS and H2O2. It is seen that only 40% of RhB is photodegraded in the presence of H2O2 after 40 min, much smaller than that of the PMS-containing system. The removal rate of RhB in the BFO/PMS system is 947 % higher than that of the BFO/H2O2 system (Figure S9), suggesting the superior photodegradation efficiency of PMS over H2O2. To explore the universality of BFO/PMS system, two types of organic compounds are selected based on their distinct charges, including cationic dyes (RhB, MB, CV and MG) and anionic dyes (MO and OII). The specific structures of these dyes are given in Figure S10. Figure 4f shows the adsorption rates and photocatalysis efficiencies of different organic 14


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pollutant models in the BFO/PMS system. The adsorption capability of BFO-30 is in the order of MG＞ CV＞RhB ＞MB ＞OII ＞ MO. Evidently, the adsorption capabilities of BFO-30 on cationic dyes are much higher than those on anionic dyes, because of the opposite charges between the photocatalyst and the cationic dyes. In contrast, the electrostatic repulsion between the negative charged photocatalyst and the anionic dyes causes the poor adsorption of MO and OII. However, the BFO/PMS system can handle both cationic and anionic dyes, and exhibits excellent photocatalytic performances. As shown in Figure S11, the photodegradation rate constants are in a fast-to-slow order: MO (57.2×10-2 min-1) ＞ CV (23.8×10-2 min-1) ＞ MG (17.3×10-2 min-1) ＞ OII (15.8×10-2 min-1) ＞ RhB (9.9×10-2 min-1) ＞ MB (9.0×10-2 min-1). Excellent photocatalysts not only require a superior photocatalytic performance, but also need an outstanding structural stability and reusability. BFO-30 has been used to photocatalyze RhB for five times, and its degradation curves are similar and the degradation efficiency of RhB for the fifth time still reaches 97% within 40 min (Figure 5a), proving its superior photocatalytic efficiency and stability. After the fifth cycle, the corresponding characteristic peaks of cellulose and α-Fe2O3 have no changes (Figure 5b) and the morphology of BFO-30 is nearly the same as its initial counterpart (Figure 5c), certifying its cyclic stability. Such exceptional efficiency and long-term stability make the BFO-30 membrane suitable for long-time flowing bed application.

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Figure 5. (a) Reusability tests of BFO-30 for RhB degradation; (b) XRD patterns of BFO-30 before and after using for 5 times; (c) SEM image of BFO-30 after using for 5 times. Based on the excellent photocatalytic efficiency and reusability of powdery BFO-30, a flowing bed device (Scheme 1f) is established to measure the photocatalytic behavior of BFO-30 membrane with the same concentrations of cationic and anionic dyes, using RhB and OII as the model dyes. For flowing bed photocatalytic application, a long-time stability under various flow rates is required. At the flow rate of 3 mL h-1, 100 % degradation degree of RhB remains for as long as 84 h, exhibiting a remarkable photocatalytic activity of the BFO membrane (Figure 6a). When the flow rate increases to 6 mL h-1, the removal efficiency still achieves approximately 100 % in the initial 6 h. As time goes on, the breakthrough time occurs at 7 h, and the RhB degradation degree gradually decreases and eventually stabilizes at about 93 % until 62 h. Further increasing the flow rate to 9 mL h-1, the removal degree of RhB drops within 6 h, but retains at 87 % from 6 to 42 h. The degradation data of different flow rates are listed in Table S1. For a chemical reaction, dyes should transport to the membrane surface, and then diffuse into the inside of the membrane, where the generated active radicals around α-Fe2O3 effectively degrade the dyes. However, the residual dye that is adsorbed but not degraded would cause the agglomeration and coalescence between the 16


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nanodisks and nanofibers after several dozens of hours (Figures 6b, d). These dyes fill in the pores of the membrane and increase the mass transfer resistance, thus reducing the photocatalytic activity. Additionally, the high flow rate cannot allow the dyes and PMS stay for enough time to get effective contact with α-Fe2O3, which also decreases the degradation efficiency. To adapt the high flow rate, multi-layer membrane would be a simple and effective approach. Here, the flow rate of 3 mL h-1 is optimal for single-layer BFO membrane with 100 % degradation degree of RhB.

Figure 6. Fractions of (a) RhB (10 ppm) and (c) OII (10 ppm) photocatalyzed by BFO-30 with 0.4 g L-1 PMS (flowing bed state) at different flow rates. Top-view SEM images of BFO-30 membrane (b) before and (d) after photocatalysis at the flow rate of 6 mL h-1 for 62 h.

17



In most practical cases, the flow rate is not stable and would vary all the time. Here, varying of flow rate with time is designed to illustrate the advantages of the BFO membrane. Three different flow rates are selected with steps from a low flow rate to a high flow rate, the period at each rate is 8 h, and then the flow rate is back to the initial rate (Figure 6c). During the initial 8 h, the conversion of OII remains above 98%, and the change of flow rate to 12 mL h-1 decreases the degradation efficiency to 83%. Further increase of the flow rate (16 mL h-1), the removal effect reduces to 65%. As expected, when the flow rate is kept to 8 mL h-1, the removal efficiency of OII raises up to 94 % again, a slight lower than the initial removal efficiency, suggesting that the BFO membrane has an excellent stability and photocatalytic properties in the long-time reaction under varied flow rates. What is more, not only BFO powders but also the BFO membrane efficiently photodegrade both cationic and anionic pollutants. As an active oxidant, PMS can generate SO4·- and ·OH to decompose organic dyes to small molecules like CO2 and H2O. Tert-butanol (TBA) without α-H reacts with SO4·- and ·OH with different rates (k·OH = 3.8-7.6×108 L mol-1s-1, kSO4·- = 4-9.1×105 L mol-1s-1). The reaction rate constant of TBA and ·OH is 417-1900 times higher than that of SO4·-, implying that TBA would preferentially combine with ·OH and be used to capture ·OH.39 Differently, ethanol (EtOH) with α-H could suppress both SO4·- and ·OH (k·OH = 1.9×109 L mol-1s-1, kSO4·- = 1.6×107 L mol-1s-1) with comparable reaction activities.12, 40-41 Thus, a dramatic decrease in activity is observed after adding TBA or ethanol (Figure 7a, b). Compared with the degradation efficiency of no scavenger, the removal of RhB with the addition of TBA decreases to 57% and 71% within 40 min and 60 min, implying that ·OH captured by TBA 18


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plays an important role in the BFO/PMS system. Similarly, by adding EtOH, more than 56% of dye remains in 40 min and 45% of residues is detected in 60 min. Owing to the quenching capability of EtOH towards both two types of radicals, the more obvious decline in the photocatalytic efficiency proves the existence of SO4·-.

Figure 7. (a) Relative concentration (Ct/C0) changes and (b) the degradation rate constants (kapp) of RhB using BFO-30 with TBA and EtOH (TBA or EtOH/PMS=1800:1; RhB: 50 mL 10 ppm; PMS: 0.4 g L-1). (c) Proposed mechanism of PMS activation on BFO photocatalyst. Besides, BQ and EDTA-2Na are used to quench ·O2- and photo-generated holes, respectively. The effect of active species of ·O2- and photo-generated holes on the whole catalytic process is shown in Figure S12, evidently demonstrating that both of the active species work on the photocatalytic system. However, the efficiency of the two is less than 19



·OH and SO4·-. Hence, both ·OH and SO4·- as two active substances play vital roles in the RhB degradation. Based on these results, the degradation mechanisms of organic dyes using BFO as the catalyst to decompose PMS under visible light are as follows (Figure 7c). When PMS (HSO5as the active ingredient) is exposed to the visible light, a small amount of decomposition appears and SO4·-/·OH are generated (Equation 1).42 More importantly, the photo-excited electrons in BFO could be excited from the VB to the CB under the visible light irradiation (Equation 2). Obviously, electrons (e-) on CB can react with HSO5- to form SO4·- (Equation 3) and combine with O2 to generate ·O2-(Equation 4), which are confirmed to remove targeted dyes effectively.43 Additionally, ·O2- could interact with H2O to produce ·OH (Equation 5). At the same time, holes (h+) and H2O combine to form ·OH (Equation 6), and the highly active SO4·- could also generate ·OH (Equation 7). The processes of equations 2-7 are the major reason for the excellent photodegradation performance. In addition, α-Fe2O3 as a common transition metal oxide has a certain catalytic property towards PMS. The transition metal with high valence (Fe3+) can be reduced to the oxidation state metal (Fe2+) by PMS through the single electron reduction process and SO5·- is also generated (Equation 8).44 Although SO5·- does not like SO4·- that has a high catalytic activity, it must exist in order to complete the valence cycle between Fe3+ and Fe2+. Then, Fe2+ could combine with HSO5- to transform to Fe3+ and SO4·- (Equation 9), and the initial catalyst can be obtained through this process, ensuring the excellent long-term stability of BFO (Figure 5a, 6a, c). Finally, SO4·-, ·OH and ·O2- could decompose the dyes to intermediate products because of the break of chains and further destroy the intermediates to inorganic compounds (Equation 10). 20


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HSO5- + hv → SO4·- + ·OH

(1)

α-Fe2O3 + hv → α-Fe2O3 (e- + h+)

(2)

α-Fe2O3 (e-) + HSO5- → SO4·- + OH-

(3)

α-Fe2O3 (e-) + O2 → ·O2-

(4)

·O2- + H2O → ·OH + OH-

(5)

α-Fe2O3 (h+) + H2O → ·OH + H+

(6)

SO4·- + OH-→ SO42- + ·OH

(7)

≡Fe3+ + HSO5- → ≡Fe2+ + SO5·- + H+

(8)

≡Fe2+ + HSO5- → ≡Fe3+ + SO4·- + OH-

(9)

SO4·- + ·OH + ·O2- + pollutant → intermediate → CO2 + H2O+ SO42-

(10)

4. CONCLUSION Novel α‑Fe2O3 nanodisk/bacterial cellulose hybrids are designed and fabricated with a pre-adsorption and hydrothermal method for the first time. The bacterial cellulose benefits to form a stable membrane to avoid the separation and recycling problems, but also helps disperse and accommodate α-Fe2O3 nanodisks, while the well dispersed and tightly anchored α-Fe2O3 nanodisks benefit the enhancement of visible light absorption efficiency, endowing the hybrid membrane excellent PMS based visible light degradation performances towards both cationic dyes (RhB, MB, CV and MG) and anionic dyes (MO and OII) in a continuous flowing bed device, demonstrating a good photocatalytic activity, long-term stability and easy separation after the reaction. The mechanism indicates that both SO4·- and ·OH participate in the degradation and the synergy of visible light and α-Fe2O3 largely leads to the enhanced photodegradation efficiency. RhB could be photodegraded to 100%, 93% and 87% for at least 21



84 h (3 mL h-1), 62 h (6 mL h-1) and 42 h (9 mL h-1), respectively. Neither the flow rates increase from low to high nor decrease from high to low, OII could be effectively photodegraded in the range of 8-16 mL h-1, and the catalytic efficiency quickly recovers to its original value, indicating the α-Fe2O3/bacterial cellulose membrane is highly competitive for wastewater treatment. ASSOCIATED CONTENT Supporting Information TEM images of BFO-30; Photographs of BFO membranes; The degradation rate constants of different dyes; UV-vis spectra of RhB; Molecular structures of organic dyes; Photodegradation performance of BFO-30 membrane with different flow rates. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author: E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51402012, 51533001, 51521062), the National Key Research and Development Program of China (2016YFC0801302), the Fundamental Research Funds for the Central Universities (JD1820), and State Key Laboratory of Organic-Inorganic Composites (OIC-201801002) is gratefully acknowledged. REFERENCES 1. Akpan, U. G.; Hameed, B. H. Parameters Affecting the Photocatalytic Degradation of 22


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2017, 219, 216-230.

29



TOC

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Bacterial Cellulose Hybrid Membranes as

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