1. IntroductionVarious dyes and additives enter the surface water body due to the inadequate treatment of printing and dyeing wastewater, causing harm to aquatic ecology and human health. Therefore, it is particularly important to develop efficient, simple, and low-consumption treatment methods. The traditional printing and dyeing wastewater treatment methods mainly include physical treatment, chemical treatment, and biological treatment, but they all have problems of varying degrees, such as complex operation, high cost, and secondary pollution. In recent years, the application of photocatalysis technology to the degradation of pollutants has received extensive attention and can effectively alleviate environmental pollution [1,2,3]. Photocatalytic technology uses the energy of sunlight through various semiconductor materials, and the electrons and holes generated by it have strong reducibility and oxidizability, respectively [4,5,6,7].Many photocatalysts with narrow band gaps have excellent photocatalytic activity under visible light irradiation, such as Bi2WO6 [8], BiOI [9], and g-C3N4 [10]. However, there are still some problems, such as the small response range of visible light and the high recombination rate of photogenerated carriers, which limit its application. Among these catalysts, Bi2WO6 (Eg = 2.7 eV) is the simplest perovskite sheet structure composed of a (Bi2O2)2+ layer and a (WO4)2− layer arranged alternately [11]. Bi2WO6 has a unique layered structure and a suitable band gap structure, making it a photocatalyst with a good response to visible light [12]. Bi2WO6 has excellent physical and chemical properties such as ferroelectric voltage, catalytic behavior, and nonlinear dielectric susceptibility that are non-toxic and harmless and can be used for organic matter degradation and oxygen release [13,14]. However, the fast recombination speed of photogenerated carriers inhibits their light energy conversion efficiency, so it is necessary to ameliorate them. As one of the most widely studied metal oxides, titanium dioxide (TiO2) has the properties of relatively high quantum yield, low cost, easy to obtain and low toxicity, and is relatively stable under light. The biggest disadvantage is that only about 5% of the ultraviolet light can be used, and the utilization of solar light is very low.Referring to the literature about Bi2WO6, there are two routes to improving the catalytic activity: expanding the light absorption range of the photocatalyst and improving the separation efficiency of photogenerated carriers. The specific methods are listed as follows: depositing precious metals on the surface of photocatalytic materials to change the surface properties and the electronic distribution [15]; doping ions to separate the electron hole pairs, to form the impurity energy levels, and more active centers [16,17,18,19]; compounding more than two different semiconductors to promote the separation of electron hole pairs [20]; controlling morphology and size to adjust the surface properties and the quantum size effect [21,22]. Therefore, in order to further improve the photocatalytic activity of Bi2WO6, it was coupled with the wide band gap semiconductor TiO2 photocatalyst to form a heterojunction nanocomposite [23,24]. The synergistic effect between TiO2 and Bi2WO6 can improve the light adsorption capacity and the separation efficiency of photogenerated electron hole pairs, prolonging the life of photogenerated carriers and enabling them to participate more in the photocatalytic reaction. In addition, the combination also effectively avoids the agglomeration of nano-titanium dioxide.However, the recovery and separation of photocatalyst became the problem of wastewater treatment by photocatalysis. At present, there is little research on magnetic Bi2WO6/TiO2 [25], and it still needs two sets of devices, namely, a photocatalysis reactor and a catalyst reclaimer. To establish a photocatalytic reactor with high mass transfer efficiency, simple operation, and no catalyst separation is significant for the application of photocatalysis in the field of wastewater treatment.

In this study, Bi2WO6/TiO2 was synthesized by a simple microwave solvothermal method. Then, Bi2WO6/TiO2 was magnetized by the ammonia-iron double-drop method. Furthermore, the photocatalytic activity of Bi2WO6/TiO2/Fe3O4 (mBT) composite photocatalyst was evaluated by the novel mBT-MPR visible light catalytic system with 5.5 W, which was proposed to treat 220 mL of RhB simulated wastewater. The mBT-MPR visible light photocatalysis system is a green and efficient treatment technology for organic pollutants in water with simple operation, low energy consumption, and no need for catalyst separation.

2. Materials and Methods 2.1. Reagents and Instruments

The main analytically pure reagents include: Bi(NO3)3·5H2O, Na2WO4·2H2O, HNO3, CH3COOH, FeCl3·6H2O, FeCl2·4H2O, NH3·H2O, absolute ethanol, RhB (C28H31ClN2O3), and etc. Nanometer titanium dioxide (P25, Degussa AG, average particle size 21 nm, BET surface area 50 ± 15 m2/g, anatase/rutile = 80:20) was purchased from Lijie Chemical Co., Ltd., Shaoxing, China. The annular focusing microwave synthesizer (Discover, CEM, Matthews, NC, USA) was used to prepare the photocatalysts by the microwave solvothermal method. The concentration of RhB was analyzed by an ultraviolet-visible intelligent multiparameter tester (LH-3BA, Beijing Lianhua Technology, Peking, China). The scanning electron microscope-energy spectrometer (Regulus 8100, Hitachi, Tokyo, Japan) and field emission transmission electron microscope (Tecnai G2 F20, FEI, Columbia, MD, USA) were employed to analyze photocatalyst morphology and element composition. An X-ray diffractometer (D8 ADVANCE, Bruker, Salbruken, Germany) was used to observe the crystal structure of mBT. Elements and valence states were detected by X-ray photoelectron spectroscopy (ESCALAB 250XI, Thermo, Waltham, MA, USA).

2.2. Preparation of mBT Photocatalyst

Bi2WO6, a certain amount of Bi (NO3)3·5H2O (5.4328 g) was dissolved into 10 mL of a 1 mol/L HNO3 or a 17.5 mol/L CH3COOH solution, and then stirred for 1 h until the white suspension became colorless, this was marked as solution A. Na2WO4·2H2O (1.8472 g) was dissolved in 5 mL of deionized water and marked as solution B. Then, solution B was added into solution A slowly by a rubber tip, stirred for 1 h and marked as solution C, putting into the microwave synthesizer at 160 °C for 1 h reaction. Finally, the white sediment was cleaned with deionized water, followed by absolute ethanol more than three times.

Bi2WO6/TiO2 (BT), the P25 (0.4473 g) was dispersed in 20 mL of an aqueous solution for 2 min by ultrasonic, and then added into solution C described above ultrasonic dispersed for 5 min, putting into the microwave synthesizer at 160 °C for 1 h. The cleaning process of white sediment was the same as mentioned above.

Bi2WO6/TiO2/Fe3O4 (mBT), the synthesis process of mBT is shown in Figure 1. A certain amount of BT was magnetized by the ammonia-iron double-drop method. FeCl2·4H2O (0.5572 g) and FeCl3·6H2O (1.5148 g) (molar ratio Fe2+:Fe3+ = 1:2) were dissolved in 2.3 mL of deionized water, heated at 70 °C and mechanically stirred for 20 min to obtain a 3.75 mol/L iron ion solution. Then 2.3mL of the iron ion solution and ammonia (2.3 mL 13.3 mol/L) were simultaneously dropped into the above uniformly dispersed 40 mL Bi2WO6/TiO2 aqueous solution by the peristaltic pump, and stirred for 30 min at 80 °C. Finally, the obtained mixed solution was heated and stirred for 30 min, and cross cleaned by deionized water and anhydrous ethanol three times. 2.3. mBT-MPR Visible Light Photocatalytic System ExperimentA tubular magnetic field-controlled photocatalytic reactor (MPR) was established, as referred to in the previous works [26]. The RhB simulated wastewater (220 mL) containing a certain amount of mBT photocatalyst was sent into the bottom of the MPR photoreactor by a peristaltic pump.

Firstly, a dark reaction lasting 20 min was carried out to demonstrate the adsorption equilibrium between the catalyst and RhB pollutant molecules. Then, the LED light belt (5.5 W) was turned on to perform photocatalytic degradation of Rhodamine B (RhB). Samples shall be taken every 10 min for 1 h degradation. Finally, the concentration of RhB in effluent was obtained by measuring the absorbance at 550 nm wavelength with an ultraviolet-visible spectrophotometer.

4. Conclusions

Magnetic composite visible light catalyst mBT was successfully prepared by the ammonia-iron double-drop and microwave solvothermal methods. The optimal molar ratio of Bi2WO6, TiO2, and Fe3O4 in the mBT photocatalyst was 1:1:0.5. The morphology results of mBT showed that TiO2 and Fe3O4 particles were loaded on the surface of Bi2WO6 sheets. Heterojunction structures of mBT are constructed by Fe3O4, TiO2, and Bi2WO6 in three parts, obviously improving the separation and transfer of photogenerated electron hole pairs at the heterojunction interface.

The novel mBT-MPR visible light catalytic system with 5.5 W was proposed to treat 220 mL of RhB simulated wastewater. The advantages of that system are simple operation, a good mass transfer effect, and the fact that no catalyst separation is required for the effluent. After 1 h of visible light irradiation, the average removal rate of RhB at an initial concentration of 10 mg/L for four times was 91.7%.

The removal effect of RhB by the mBT-MPR photocatalytic system was mainly due to the coupling effect of adsorption and photocatalysis by mBT. The dosage of mBT is correlated with the concentration of influent RhB. To obtain stable effluent, a reasonable mBT dosage should be chosen to match the corresponding influent pollutant concentration.