Shapes Control of Bi 2 WO 6 Nano-Structures as Photo-Fenton Catalysts for Pulping Wastewater Treatment

: Bi 2 WO 6 assembled by ﬂower-like microspheres and nanosheets were controllably synthesized through a one-step hydrothermal approach. Multiple technologies, including X-ray powder di ﬀ raction (XRD), scanning electron microscopy (SEM), and UV–Vis di ﬀ use reﬂectance spectrum (UV–Vis), were carried out to characterize the as-synthesized samples. The photocatalytic e ﬃ ciency of Bi 2 WO 6 synthesized with a series of temperature and pH values shows di ﬀ erent morphologies and photocatalytic properties. The photocatalyst (Bi 2 WO 6 ) synthesized at 220 ◦ C and pH of 7 exhibited the best photocatalytic performance, with the methylene blue (MB) degradation approaching 91.6% after reaction time of 60 min. Free radical capture experiments indicate that • OH is the primary reactive species in the methylene blue (MB) degradation reaction, h + and • O 2 − contribute negligible inﬂuence, while the addition of H 2 O 2 signiﬁcantly improves the photocatalytic activity of Bi 2 WO 6 . Biodegraded poplar preconditioning reﬁner chemical alkaline peroxide mechanical pulp wastewater (PPW) was treated over Bi 2 WO 6 under UV light (Bi 2 WO 6 / UV / H 2 O 2 ); chemical oxygen demand (COD Cr ) and color degradation rate were 85.8% and 92.0%, respectively. These results show that Bi 2 WO 6 semiconductors can be introduced as an e ﬃ cient and stable photocatalyst for industry wastewater treatment.


Introduction
In recent years, the pulp and paper industry has experienced rapid development, and its economic benefits have contributed to it becoming one of the most paramount industrial components in the world [1,2]. However, pulp and paper mills produce a massive amount of wastewater, which bring serious threats to the environment [3,4]. The as-obtained effluents derived from raw materials and production processes inevitably have a high chemical oxygen demand (COD) and low biodegradability properties, which usually consists of >200 organic compounds and total organic and inorganic species of approximately 700 [5,6]. Among those pollutants, dioxins, chlorate, chlorinated hydrocarbons, chloroform, phenols, and furans were observed, which are seriously harmful to human [7,8]. Pulp wastewater is usually treated by three processes, primary physicochemical treatment, secondary biochemical treatment, and tertiary advanced treatment [9][10][11]. The primary physicochemical strategy was employed for removing colloidal particles, suspended solids, floating matters, toxic compounds, and colors from wastewaters [12,13]. The main steps include screening, sedimentation, flotation, flocculation, and coagulation [14]. Secondary biochemical treatment is the combination of anaerobic and aerobic treatment processes, which is efficient and economical for the treatment of soluble biodegradable organic components [15]. After two-stage processing, toxicity in the discharged effluents could be remarkably eliminated [16,17]. However, the effluents still contain parts of toxic organics, high color, and (suspended solids) SS, which need further treatment [4,6,18]. Coagulation and Fenton techniques were considered as the efficient and advanced treatment approaches, which have been applied in the wastewater treatment field [19]. During the reported processes, large amounts of sludge, high cost, and corrosion were generated, resulting in secondary pollution [20]. Therefore, developing an environmentally friendly and cost-efficient technique to deal with industrial wastewater is highly desirable, yet a challenge.
Photocatalytic degradation of organic pollutants over semiconductor catalysts have been studied and are regarded as an efficient means for environmental purification [21]. As a typical Aurivillius oxide photocatalyst, Bi2WO6 has been widely applied for mineralization of organic pollutants with UV light irradiation [22][23][24]. Thanks to its layered structure, Bi2WO6 possesses outstanding intrinsic physical and chemical properties [25][26][27][28]. Up to date, the facile hydrothermal route is still the dominant method for the preparation of Bi2WO6 photocatalyst, which affords a high crystallinity and size-controllable particles under mild conditions [27,[29][30][31]. Herein, two kinds of bismuth tungsten oxides, including Bi2WO6 and Bi14W2O27, were synthesized in this work under different pH and temperature conditions, and evaluated for the pulping wastewater treatment. The influences of pH and temperatures on the morphology transformation, photocatalysts activities of Bi2WO6, and its catalytic performance were then discussed systematically.

Results and Discussion
The crystalline structure of the synthesized samples were characterized by X-ray diffraction (XRD) technique. Figures 1 and 2 show the XRD patterns of temperature series samples corresponding with different pH values.  It is worth noting that different pH values produced different crystallized products, such as Bi 2 WO 6 (pH = 1 or 7) and Bi 14 W 2 O 27 (pH = 13), as seen in Figures 1 and 2   For comparing the morphology and surface area of as-synthesized samples under different temperature and pH values, scanning electron microscopy (SEM) and BET were performed and are shown in Figure 3 and Table 1. It can be seen from Figure 3, the morphologies of products were significantly influenced by the pH value. The Bi 2 WO 6 powders synthesized hydrothermally at pH of 1 exhibits 3D flower-like microspheres, which consist of the irregular nanosheets (Figure 3a,d,g). As the pH is increased up to 7, the morphology of Bi 2 WO 6 consists of a large quantity of nanosheets ( Figure 3b,e,h). In addition, both the morphology and composition of the obtained samples changed simultaneously as the pH value further increased to 13. Bi 14 W 2 O 27 was formed instead of Bi 2 WO 6 and irregular granular particles are formed at the cost of the 3D flower-like microspheres and nanosheets. As seen in Table 1, the BET surface areas were significantly influenced by both pH and temperature. BET surface of Bi 2 WO 6 was higher than Bi 14 W 2 O 27 ; Bi 2 WO 6 was synthesized at 220 • C and pH of 1 (Table 1h) with the highest surface area 25.89 m 2 /g.  Figure 4, Bi 2 WO 6 displays the photo-absorption property limited to 460 nm. The band gaps of the samples were calculated based on the formula below: where α, h, ν, A, E g , and n are the absorption coefficient, Planck constant, light frequency, gap band energy, and a constant, respectively. Based on this equation, the value of n for both Bi 2 WO 6 and Bi 14 W 2 O 27 was 1. A plot of (αhν) 2~h ν according to the straight forward transition is shown as inset of  Table 2. The result shows that the band gaps of a, b, c, d, e, f were different, the values were 2.85, 2.80, 2.81, 2.79, 2.83, and 2.73 eV, respectively. As observed in Table 2, the band gaps of g, h, and i were 2.47, 2.43, and 2.41 eV, respectively. In general, Bi 2 WO 6 has a better light absorption strength when the synthesized pH value is 7 and a high synthesis temperature can reduce the band gap of the catalysts.
For comparing the morphology and surface area of as-synthesized samples under different temperature and pH values, scanning electron microscopy (SEM) and BET were performed and are shown in Figure 3 and Table 1. It can be seen from Figure 3, the morphologies of products were significantly influenced by the pH value. The Bi2WO6 powders synthesized hydrothermally at pH of 1 exhibits 3D flower-like microspheres, which consist of the irregular nanosheets (Figure 3a,d,g). As the pH is increased up to 7, the morphology of Bi2WO6 consists of a large quantity of nanosheets ( Figure 3b,e,h). In addition, both the morphology and composition of the obtained samples changed simultaneously as the pH value further increased to 13. Bi14W2O27 was formed instead of Bi2WO6 and irregular granular particles are formed at the cost of the 3D flower-like microspheres and nanosheets. As seen in Table 1, the BET surface areas were significantly influenced by both pH and temperature. BET surface of Bi2WO6 was higher than Bi14W2O27; Bi2WO6 was synthesized at 220 °C and pH of 1 (Table 1h)with the highest surface area 25.89 m 2 /g.    Table 2. The result shows that the band gaps of a, b, c, d, e, f were different, the values were 2.85, 2.80, 2.81, 2.79, 2.83, and 2.73 eV, respectively. As observed in Table 2, the band gaps of g, h, and i were 2.47, 2.43, and 2.41 eV, respectively. In general, Bi2WO6 has a better light absorption strength when the synthesized pH value is 7 and a high synthesis temperature can reduce the band gap of the catalysts.    The photocatalytic activities of the as-prepared catalysts under different temperature and pH values were comparatively evaluated by simultaneous degradation of MB under UV light irradiation. Figure 6A displays the degradation of MB within 60 min over different catalysts. It can be concluded that Bi2WO6 shows outstanding photocatalytic activities under UV light irradiation, while Bi14W2O27 has almost no photocatalytic activities. In addition, Bi2WO6 catalysts synthesized by different pH values and temperatures show different catalytic performance. Clearly, after 60 min of reaction, the MB degradation efficiency approaches 91.6% (Figure 6h), higher than those achieved by using Figure  6g  In general, when Bi2WO6 is synthesized at the same temperature, pH = 7 shows better photocatalytic activities. When the catalyst is synthesized at the same pH, the higher synthetic temperature contributes to better catalytic activity.  The photocatalytic activities of the as-prepared catalysts under different temperature and pH values were comparatively evaluated by simultaneous degradation of MB under UV light irradiation. Figure 6A displays the degradation of MB within 60 min over different catalysts. It can be concluded that Bi 2 WO 6 shows outstanding photocatalytic activities under UV light irradiation, while Bi 14 W 2 O 27 has almost no photocatalytic activities. In addition, Bi 2 WO 6 catalysts synthesized by different pH values and temperatures show different catalytic performance. Clearly, after 60 min of reaction, the MB degradation efficiency approaches 91.6% (Figure 6h), higher than those achieved by using Figure 6g In general, when Bi 2 WO 6 is synthesized at the same temperature, pH = 7 shows better photocatalytic activities. When the catalyst is synthesized at the same pH, the higher synthetic temperature contributes to better catalytic activity.
The photocatalytic degradation of MB with Bi 2 WO 6 as the catalyst follows first-order kinetic model. The relevant kinetics equation is shown as follows: where C is the added concentration of MB prior to reaction, C t is the concentration of MB at different reaction time (t), k is an apparent rate constant, and t is the reaction time under UV irradiation condition. The kinetic plots for the MB degradation over Bi 2 WO 6 catalyst were shown in Figure 6B, suggesting that the photocatalytic MB degradation reaction over Bi 2 WO 6 catalyst is in accordance with the first-order kinetic reaction model. The achieved first-order kinetic constants (k) and corresponding coefficient (R 2 ) were summarized in Table 3. As seen in Table 3, the rate constants of the MB degradation over Bi 2 WO 6 (220 • C, pH = 7, Table 3h) is higher than that over the rest of the Bi 2 WO 6 samples (Table 3a,b,d,e,g). The activities of the synthesized samples are mainly related to the morphology and crystallinity. The flower-like catalysts show an inferior catalytic activity, which is mainly ascribed to the fact that it is difficult for macromolecules MB to enter into the pores in flower-like catalysts [33]. Figure 6A displays the degradation of MB within 60 min over different catalysts. It can be concluded that Bi2WO6 shows outstanding photocatalytic activities under UV light irradiation, while Bi14W2O27 has almost no photocatalytic activities. In addition, Bi2WO6 catalysts synthesized by different pH values and temperatures show different catalytic performance. Clearly, after 60 min of reaction, the MB degradation efficiency approaches 91.6% (Figure 6h), higher than those achieved by using Figure  6g (84.5%), Figure 6e (83.3%), Figure 6d (77.3%), Figure 6b (72.5%), and Figure 6a (67.6%). In general, when Bi2WO6 is synthesized at the same temperature, pH = 7 shows better photocatalytic activities. When the catalyst is synthesized at the same pH, the higher synthetic temperature contributes to better catalytic activity.  In order to further identify the reactive species during this reaction, we conducted the free radical capture experiments. Benzoquinone (BQ, 1 mM) as hydroxyl radical (•OH) scavenger, benzoquinone (BZQ 1 mM) as superoxide radical (•O 2 − ) scavenger, and disodium ethylenediamine tetraacetate (EDTA-Na 2 1mM) as hole (h + ) scavenger were added, respectively, in the above-mentioned reaction. It can be illustrated from Figure 7 that three radicals including •OH, h + and •O 2 − showed an improved photocatalytic performance, while the MB removal yield significantly decreased to 48.0%, from 91.6%, in the presence of IPA, suggesting that •OH should be the dominant photoreaction radical species, and the addition of EDTA-Na 2 and BZQ displays weaker effect on the MF degradation, which indicates that h + and •O 2 − contribute little to the degradation reaction.
In order to solve the serious wastewater pollution, Bi 2 WO 6 which synthesized under 220 • C and the pH value of 7 were used to treat PPW, which were collected after secondary biochemical treatment, and the results are seen in Figure 8.
It can be clearly summarized that after 3 h of treatment, the COD Cr and color degradation rate of PPW over Bi 2 WO 6 under UV light (Bi 2 WO 6 /UV) were 75.1% and 87.3%, respectively. When H 2 O 2 (10 mM) were added in the solution (Bi 2 WO 6 /UV/H 2 O 2 ), the photo-Fenton-like reaction was produced, and the removals of COD Cr and color of PPW were improved, 85.8% and 92.0% were degraded, respectively. The COD Cr of treated PPW were 53.8 and 30.67 mg/L, satisfying the discharge standard of waste pollutants for the pulp and paper industry (GB3544-2008) in China. In order to solve the serious wastewater pollution, Bi2WO6 which synthesized under 220 °C and the pH value of 7 were used to treat PPW, which were collected after secondary biochemical treatment, and the results are seen in Figure 8. It can be clearly summarized that after 3 h of treatment, the CODCr and color degradation rate of PPW over Bi2WO6 under UV light (Bi2WO6/UV) were 75.1% and 87.3%, respectively. When H2O2 (10 mM) were added in the solution (Bi2WO6/UV/H2O2), the photo-Fenton-like reaction was produced, and the removals of CODCr and color of PPW were improved, 85.8% and 92.0% were degraded, respectively. The CODCr of treated PPW were 53.8 and 30.67 mg/L, satisfying the discharge standard of waste pollutants for the pulp and paper industry (GB3544-2008) in China.

Synthesis of the Samples
All chemicals were analytical grade and were used directly with no further purification. The Bi2WO6 nanoplates were prepared through one-step hydrothermal process [31]. The experimental procedures were as follows: First, 2.5 mmol of Na2WO4 was dissolved in 20 mL deionized water, 5.0 mmol of Bi(NO3)3·5H2O was dissolved in 40 mL deionized water and was ultrasonic treated for 15 min to disperse evenly, then 20 mL of sodium tungstate solution was dropped into Bi (NO3)3 solution slowly under vigorous stirring. The pH value of the mixture solution was adjusted by adding 2.5 mol/L NaOH to 1, 7, and 13. After being stirred for 2 h, the suspension was then transferred into a 100 mL Teflon-lined autoclave and filled with deionized water up to 80%. The samples achieved from In order to solve the serious wastewater pollution, Bi2WO6 which synthesized under 220 °C and the pH value of 7 were used to treat PPW, which were collected after secondary biochemical treatment, and the results are seen in Figure 8. It can be clearly summarized that after 3 h of treatment, the CODCr and color degradation rate of PPW over Bi2WO6 under UV light (Bi2WO6/UV) were 75.1% and 87.3%, respectively. When H2O2 (10 mM) were added in the solution (Bi2WO6/UV/H2O2), the photo-Fenton-like reaction was produced, and the removals of CODCr and color of PPW were improved, 85.8% and 92.0% were degraded, respectively. The CODCr of treated PPW were 53.8 and 30.67 mg/L, satisfying the discharge standard of waste pollutants for the pulp and paper industry (GB3544-2008) in China.

Synthesis of the Samples
All chemicals were analytical grade and were used directly with no further purification. The Bi2WO6 nanoplates were prepared through one-step hydrothermal process [31]. The experimental procedures were as follows: First, 2.5 mmol of Na2WO4 was dissolved in 20 mL deionized water, 5.0 mmol of Bi(NO3)3·5H2O was dissolved in 40 mL deionized water and was ultrasonic treated for 15 min to disperse evenly, then 20 mL of sodium tungstate solution was dropped into Bi (NO3)3 solution slowly under vigorous stirring. The pH value of the mixture solution was adjusted by adding 2.5 mol/L NaOH to 1, 7, and 13. After being stirred for 2 h, the suspension was then transferred into a 100 mL Teflon-lined autoclave and filled with deionized water up to 80%. The samples achieved from

Synthesis of the Samples
All chemicals were analytical grade and were used directly with no further purification. The Bi 2 WO 6 nanoplates were prepared through one-step hydrothermal process [31]. The experimental procedures were as follows: First, 2.5 mmol of Na 2 WO 4 was dissolved in 20 mL deionized water, 5.0 mmol of Bi(NO 3 ) 3 ·5H 2 O was dissolved in 40 mL deionized water and was ultrasonic treated for 15 min to disperse evenly, then 20 mL of sodium tungstate solution was dropped into Bi (NO 3 ) 3 solution slowly under vigorous stirring. The pH value of the mixture solution was adjusted by adding 2.5 mol/L NaOH to 1, 7, and 13. After being stirred for 2 h, the suspension was then transferred into a 100 mL Teflon-lined autoclave and filled with deionized water up to 80%. The samples achieved from different pH values were placed under hydrothermal reaction for 12 h at 140, 180, and 220 • C, respectively. The resulting samples were collected and washed with deionized water and ethanol, and the obtained sample was then dried at 60 • C for 12 h.

Characterization
X-ray diffraction (XRD) spectra were carried out on a Rigaku Rotaflex diffractometer equipped with a rotating anode with Cu KR radiation with 2θ from 10 • to 80 • . Scanning electron microscopy (JSM-7001F, JEOL, Japan) was employed to clarify the morphologies of different samples. The surface area of the samples were obtained by V-Sorb 2800. UV-Vis absorption spectra were carried out on a Shimadzu UV-2450 spectrometer.

Photocatalytic Tests
The photodegradation of MB were performed under UV irradiation condition with a 100 W mercury lamp in a cylindrical quartz reactor, with magnetic stirring apparatus and water circulation facility. Before illumination, the suspensions were vigorously stirred in the dark for 30 min to ensure the establishment of an adsorption equilibrium between the photocatalyst powder and MB. After that, the solution was exposed to UV light irradiation. For each experiment, 0.2 g catalyst was dispersed to 250 mL of MB solution (30 mg/L). After the reaction, 10 mL aliquots were collected every fifteen minutes and then centrifuged to separate the photocatalyst powders. The concentrations of MB were determined by analyzing the absorbance at 664 nm with a UV-2550 spectrophotometer (Shimadzu, Japan).
Poplar preconditioning refiner chemical alkaline peroxide mechanical pulp wastewater (PPW) was collected after biological treatment from a secondary sedimentation tank, and the characteristics of the wastewater are shown in Table 4. The wastewater was treated as the above process for 3 h with photodegradation. COD was measured according to Standard Methods (1998). The HACH-6000 spectrophotometer was used for the color measurements in the APHA Pt-Co (cobalt) unit.

Conclusions
The 3D flower-like microspheres and nanosheets Bi 2 WO 6 photocatalyst was prepared by a facile hydrothermal approach. The pH value and temperature play an important role on the synthetic progress of Bi 2 WO 6 , resulting in differences in morphology, crystallinity, optical properties, and electronic properties. Photocatalytic tests show that Bi 2 WO 6 synthesized under 220 • C and the pH of 7 has great photocatalytic activities. Furthermore, Bi 2 WO 6 was also employed to treat the industrial PPW, which has a high removal of COD Cr and color. Photo-Fenton-like reaction was produced with H 2 O 2 addition and significantly increased the photocatalytic activities. From the results, it can be proposed that semiconductors may provide a promising platform for high-performance photocatalytic applications in industrial wastewater treatment.