Shapes Control of Bi₂WO₆ Nano-Structures as Photo-Fenton Catalysts for Pulping Wastewater Treatment

Abstract

Bi₂WO₆ assembled by flower-like microspheres and nanosheets were controllably synthesized through a one-step hydrothermal approach. Multiple technologies, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis diffuse reflectance spectrum (UV–Vis), were carried out to characterize the as-synthesized samples. The photocatalytic efficiency of Bi₂WO₆ synthesized with a series of temperature and pH values shows different morphologies and photocatalytic properties. The photocatalyst (Bi₂WO₆) 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 •O₂⁻ contribute negligible influence, while the addition of H₂O₂ significantly improves the photocatalytic activity of Bi₂WO₆. Biodegraded poplar preconditioning refiner chemical alkaline peroxide mechanical pulp wastewater (PPW) was treated over Bi₂WO₆ under UV light (Bi₂WO₆/UV/H₂O₂); chemical oxygen demand (COD_Cr) and color degradation rate were 85.8% and 92.0%, respectively. These results show that Bi₂WO₆ semiconductors can be introduced as an efficient and stable photocatalyst for industry wastewater treatment.

1. 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.

2. Results and Discussion

The crystalline structure of the synthesized samples were characterized by X-ray diffraction (XRD) technique. Figure 1 and Figure 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 Bi₂WO₆ (pH = 1 or 7) and Bi₁₄W₂O₂₇ (pH = 13), as seen in Figure 1 and Figure 2. The samples synthesized under the pH of 1 and 7 were well consistent with the standard card of orthorhombic Bi₂WO₆ according to the database of JCPDS NO.39-0256 [28]. The highest intensity peak at 28.54° belongs to (131) of Bi₂WO₆, the peaks positioned at 33.10°, 47.38°, 56.19°, and 58.69° in six patterns belong to (200), (202), (331), and (262) of the samples, respectively. However, the diffraction peaks from the samples obtained with pH of 13 were in good consistency with the standard card of orthorhombic Bi₁₄W₂O₂₇ according to the data base of JCPDS NO.39-0061 [29,32]. The diffraction peaks of the obtained samples were determined at 2θ = 27.56°, 31.80°, 45.78°, 54.17°, 54.3°, indexing to the (312), (004), (424), (315), and (552) crystallographic planes of Bi₁₄W₂O₂₇, respectively. In addition, the crystallinity of Bi₂WO₆ can be greatly adjusted by pH values, especially at 140 °C, where an obvious higher crystallinity can be obtained at pH = 7 compared to pH = 1. The obtained sharp diffraction peaks demonstrate high crystallinity of the as-prepared sample powders.

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. The surface area of Bi₂WO₆ and Bi₁₄W₂O₂₇.

Sample		BET Area (m2/g)
a	Bi2WO6-140 °C, pH = 1	9.73
b	Bi2WO6-140 °C, pH = 7	18.36
c	Bi14W2O27-140 °C, pH = 13	5.12
d	Bi2WO6-180 °C, pH = 1	11.88
e	Bi2WO6-180 °C, pH = 7	22.51
f	Bi14W2O27-180 °C, pH = 13	6.08
g	Bi2WO6-220 °C, pH = 1	14.67
h	Bi2WO6-220 °C, pH = 7	25.51
i	Bi14W2O27-220 °C, pH = 13	5.89

. 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 m²/g.

Figure 4 and Figure 5 show the UV–Vis diffused reflectance spectra of the as-synthesized samples under different temperature and pH values. As observed in Figure 4, Bi₂WO₆ displays the photo-absorption property limited to 460 nm. The band gaps of the samples were calculated based on the formula below:

αhν = A(hv − E_g)^n/2,

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 Bi₂WO₆ and Bi₁₄W₂O₂₇ was 1. A plot of (αhν)² ~hν according to the straight forward transition is shown as inset of Figure 4 and Figure 5, which were summarized in

Table 2. The band gaps of Bi₂WO₆ and Bi₁₄W₂O₂₇.

Sample		Hv (eV)
a	Bi2WO6-140 °C, pH = 1	2.85
b	Bi2WO6-140 °C, pH = 7	2.80
c	Bi2WO6-180 °C, pH = 1	2.81
d	Bi2WO6-180 °C, pH = 7	2.79
e	Bi2WO6-220 °C, pH = 1	2.83
f	Bi2WO6-220 °C, pH = 7	2.73
g	Bi14W2O27-140 °C, pH = 13	2.47
h	Bi14W2O27-180 °C, pH = 13	2.43
i	Bi14W2O27-220 °C, pH = 13	2.41

. 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 (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.

The photocatalytic degradation of MB with Bi₂WO₆ as the catalyst follows first-order kinetic model. The relevant kinetics equation is shown as follows:

$$\ln \frac{{\mathrm{C}}_{\mathrm{t}}}{C}=\mathrm{k}t,$$

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 Bi₂WO₆ catalyst were shown in Figure 6B, suggesting that the photocatalytic MB degradation reaction over Bi₂WO₆ catalyst is in accordance with the first-order kinetic reaction model. The achieved first-order kinetic constants (k) and corresponding coefficient (R²) were summarized in

Table 3. The first-order kinetic constants k (min⁻¹) and relative coefficient R² of Bi₂WO₆.

Sample		K (min−1)	R2
a	Bi2WO6-140 °C, pH = 1	0.020	0.966
b	Bi2WO6-140 °C, pH = 7	0.023	0.986
d	Bi2WO6-180 °C, pH = 1	0.032	0.984
e	Bi2WO6-180 °C, pH = 7	0.034	0.977
g	Bi2WO6-220 °C, pH = 1	0.035	0.980
h	Bi2WO6-220 °C, pH = 7	0.044	0.975

. As seen in Table 3, the rate constants of the MB degradation over Bi₂WO₆ (220 °C, pH = 7, Table 3h) is higher than that over the rest of the Bi₂WO₆ 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].

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 (•O₂⁻) scavenger, and disodium ethylenediamine tetraacetate (EDTA-Na₂ 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 •O₂⁻ 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-Na₂ and BZQ displays weaker effect on the MF degradation, which indicates that h⁺ and •O₂⁻ contribute little to the degradation reaction.

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 COD_Cr 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 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.

3. Experimental Section

3.1. 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 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.

3.2. 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.

3.3. 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. Characteristics of the PPW wastewater.

Sample	pH	CODCr/(mg/L)	Color/(Pt-Co)
PPW	7.16	216	745

. 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.

4. Conclusions

The 3D flower-like microspheres and nanosheets Bi₂WO₆ photocatalyst was prepared by a facile hydrothermal approach. The pH value and temperature play an important role on the synthetic progress of Bi₂WO₆, resulting in differences in morphology, crystallinity, optical properties, and electronic properties. Photocatalytic tests show that Bi₂WO₆ synthesized under 220 °C and the pH of 7 has great photocatalytic activities. Furthermore, Bi₂WO₆ 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 H₂O₂ 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.