Heterogeneous photocatalytic treatment of wastewater in ultraviolet light irradiation—photocatalyst Bi₂WO₆ microsphere with high repeatability

Abstract

The treatment of wastewater that includes toxic organic pollutants such as dyes, phenoaniline, phenols and their derivatives is still a challenge due to their biorecalcitrant and acute toxicity to the widespread acceptance of water recycling. Three-dimensional (3D) Bi₂WO₆ microsphere was synthesized by the hydrothermal method using Bi(NO₃)₃ and Na₂WO₄ as raw materials. This structure exhibits high photocatalytic activity for the dyes, toxic organic compounds. The degradation of methlyene blue is 100% in 30 min, 4-nitrylphenol is 95% in 60 min and p-nitrylphenol is 95% in 75 min in ultraviolet (UV) light irradiation. 3D Bi₂WO₆ microsphere is also a good photocatalyst to treat the printing and dyeing sewage, and exhibits high repeatability. After being used the 20th time, Bi₂WO₆ still has high activity to degrade the printing and dyeing sewage, which is very important for a photocatalyst to be used in industry. This study will pave a new way to treat industry wastewater.

Keywords

photocatalyst / semiconductors / wastewater treatment

Introduction

The treatment of wastewater effluent that includes toxic organic pollutants such as dyes, phenoaniline, phenols and their derivatives is still a challenge due to their biorecalcitrant and acute toxicity to the widespread acceptance of water recycling [1,2]. Despite their low concentration, these contaminants are a major health concern because of their extremely high endocrine disrupting potency and genotoxicity [3]. In response, considerable efforts have been devoted to effectively remove the persistent organic pollutants from wastewater effluent to minimize the risk of pollution problems from such toxic chemicals and to enable its reuse. Conventional waste water purification systems, such as activated carbon adsorption, membrane filtration, chemical coagulation, ion exchange on synthetic adsorbent resins, etc., also generate wastes during the treatment of contaminated water, which requires additional steps and costs.

Photocatalysis has received enormous attention in recent years because of its application in organic synthesis [4,5], and in the abatement of pollutants in water or in air [6,7], since the reaction can be driven by sunlight at near ambient temperatures and pressure using molecular oxygen as an oxidant [8,9]. TiO₂ has been extensively investigated for its application in several fields and in particular for its activity in the oxidative photodegradation of a range of organic substrates [10,11]. There have been many reports on the mineralization of organic pollutants by heterogeneous photocatalytic oxidation with TiO₂ [12,13] or multimetal oxide materials such as Bi₂WO₆ [14].

Bi₂WO₆ and Bi₂MoO₆ are the member of cation-deficient Aurivillius phases, which have the potential for water splitting photocatalytic oxidative decomposition of organic pollutants under visible light irradiation [14–20]. The crystal of Bi₂WO₆ is composed of accumulated layers of corner-sharing WO₆ octahedral sheets and bismuth oxide sheets. The solution phase condition is proved to be fit for anisotropic crystal growth of Bi₂WO₆ and formation of rectangle-shaped nanoplates [15,20]. The Bi₂WO₆ microstructures with a flake-ball shape or rectangular-plate shape have exhibited high photocatalytic activity to induce photocatalytic decolorization of organic dies, such as rhodamine B, under visible-light irradiation. However, there have been few reports of other photocatlytic reactions, such as oxidative decomposition of toxic organic compounds, and photocatalytic degradation of the wastewater under ultraviolet light irradiation over hydrothermally prepared Bi₂WO₆ [21].

In this article, we report the synthesis of the three-dimensional (3D) Bi₂WO₆ microspheres and their high photocatalytic activity for the dyes, toxic organic compounds and the printing and dyeing sewage. The Bi₂WO₆ has the best photocatalytic activity for the methylene blue (MB) dye, which is completely degraded only for 30 min. 3D Bi₂WO₆ microsphere is a very good photocatalyst to treat the printing and dyeing sewage in the ultraviolet (UV) light irradiation, and exhibits high stability and repeatability. After being used the 20th time, Bi₂WO₆ still has high activity to degrade the printing and dyeing sewage, and make its chemical oxygen demand (COD) value decrease to 78.4531 mg/L.

Experimental

Synthesis of Bi₂WO₆ microstructures

In a typical procedure, Bi(NO₃)₃·5H₂O (1.212 g, 2.5 mmol) was added to 40 mL of deionized water. Then, Na₂WO₄·2H₂O (0.412 g, 1.25 mmol) was added to the solution, and the mixture was vigorously stirred for 30 min to ensure that all reagents were dispersed homogeneously. The mixture was translated into a 50 mL Teflonlined autoclave, maintained at 180°C for 24 h, and cooled to room temperature naturally. The precipitate was collected and rinsed several times with distilled water and absolute ethanol, respectively. Then, the sample was dried in a vacuum at 60°C for 6 h.

Characterization

The obtained product was characterized by X-ray powder diffraction (XRD) using a Rigaku (Japan) D/max-γA X-ray diffractometer with Cu–K radiation (λ = 0.154178 nm). Field-emission scanning electron microscopy (FESEM) was captured on a Hitachi model S-4800 instrument operating at 15 kV. Room temperature UV-Vis absorption was recorded on a UV-2550 spectrophotometer in the wavelength range of 200–800 nm.

Photocatalytic activity measurements

Photocatalytic activity of as-prepared Bi₂WO₆ samples were tested by decomposing different dyes, toxic organisms and the printing and dyeing sewage under UV light irradiation of a 300 W UV lamp (main wavelength 365 nm). In a typical experiment, as-prepared Bi₂WO₆ (40 mg) was added into 100 mL of the printing and dyeing sewage, or 100 mL 10 mg/L of dye (or toxic organism) to obtain suspension for degradation reaction at room temperature under air. Prior to UV light irradiation, the suspensions were stirred in the dark for 30 min to ensure an adsorption/desorption equilibrium of dye on surfaces of Bi₂WO₆ particles. After a given irradiation time, the suspensions were withdrawn, and centrifuged to remove the particles. The degradation process as a function of irradiation time was monitored by measuring the absorption of dyes in the solution with UV–Vis absorption spectra. The COD value of the treated printing and dyeing sewage is measured with K₂Cr₂O₇ method.

Results and discussion

Structure and morphology of Bi₂WO₆ sample

The morphology of the as-synthesized Bi₂WO₆ was investigated by FESEM. Figure 1(a) shows the morphology of the product in large scale. It reveals that the product is composed of a large quantity of nearly monodispersed flower-like spheres with an average diameter of about 3.0 μm. No other morphologies can be detected, indicating a high yield of these 3D microspheres. The as-prepared Bi₂WO₆ sample possesses a superstructure of a flower-like appearance (Fig. 1(b)). The hierarchical flower-like Bi₂WO₆ structures are built by a large quantity of two-dimensional nanoplates with a thickness of about 15.0 nm. The aggregation and/or assembly of the nanoplates may produce abundant hierarchical pores on nanoscale.

Figure 1(c) shows the typical XRD pattern of the as-prepared hierarchical flower-like Bi₂WO₆. All peaks for this sample could be indexed to the orthorhombic phase of Bi₂WO₆ (JCPDS card no. 73-1126), No characteristic peaks of the other impurities were observed.

Photocatalytic degradation of dyes and organic compounds

Methyl red (MR), Rhodamine-B (RhB) and methylene blue (MB), a widely used dyes, the widespread occurrence of toxic organisms phenols, phenoaniline in wastewater were selected as the model pollutants to evaluate the photocatalytic activity of the Bi₂WO₆. Figure 2 displayed the photodegradation curve of these pollutions by the Bi₂WO₆ sample under UV light illumination. The MR solution gradually was decolored in the 75 min upon UV light irradiation in the presence of photocatalyst. However, the dyes MB and RhB were diminished quickly and 100% photodegration efficiencies in 30 and 45 min. The results showed that photocatalyst Bi₂WO₆ has an elective ability to degraded the dyes MB and RhB. The degradation rate of the toxic organic compounds is relatively slow, o-nitrylaniline needed 90 min to degrade completely, and p-nitrylphenol needed 75 min. A comparison experiment showed that these pollutions did not degrade in the dark with the presence of Bi₂WO₆ photocatalysts. On the other hand, the blank test revealed that the degradation of these pollutions were very slow when illuminated by UV light in the absence of photocatalysts. These results confirmed that the catalyst and UV light are indispensable to the photodegradation of these pollutions.

For a photocatalyst to be useful, it should be stable and has high repeatability under the photocatalytic application. To test the repeatability of MB dye degrading on the Bi₂WO₆, we repeated ten times under the same conditions. As shown in Fig. 3, the MB dye was still quickly degraded in the tenth experiment. It is noticeable that, the photodegradation rates of MB dyes almost remain unchanged in the degradation processes of all 20 experiments, indicating that the Bi₂WO₆ photocatalyst was stable under repeated application.

Photocatalytic treatment of printing and dyeing sewage

Printing and dyeing sewage is the wastewater discharged from printing and dyeing plants in processing of cotton, bast fiber, synthetic fiber and their blended product. This sewage contains dye, sizing material, auxiliaries, oiling agent, acid, alkali, fiber impurities, sand, inorganic salt, etc. It is very difficult to treat this kind of sewage due to the large quantity, high colority and poor biochemical purification ability of the sewage. Here, the Bi₂WO₆ sample was used to treat the printing and dyeing sewage (without being any treatment) through photocatalytical reaction. After 60 min of irradiation, the suspensions were centrifuged and the COD values were obtained via K₂Cr₂O₇ method. As shown in Table 1, before the treatment, the COD value of the sewage is 386.0000 mg/L, which reduces to 8.2368 mg/L after 60 min of UV light irradiation in the presence of Bi₂WO₆ sample. After reused 20 times, the photocatalyst Bi₂WO₆ still exhibits high activity for the sewage, with the COD value 78.4531 mg/L. The colority of water is very low, and no suspended matter from the treated sewage is found at the bottom of the bottle after one week.

Table 2 gives out the COD value of prior to treatment and post-treatment and light irradiation time for the printing and dyeing sewages come from different wastewater treatment factory using Bi₂WO₆ as photocatalyst. It could be found that the Bi₂WO₆ was a good photocatalyst for the wastewater treatment.

Photocatalytic mechanism of Bi₂WO₆ sample

In the photocatalytic oxidation process, organic pollutants are destroyed in the presence of semiconductor photocatalyst Bi₂WO₆, an energetic light source, and an oxidizing agent such as oxygen or air. Figure 4 illustrates the mechanism of Bi₂WO₆ photocatalysis. The illumination of the photocatalytic surface with sufficient energy leads to the formation of a positive hole (h⁺) in the valence band and an electron (e^-) in the conduction band (CB). The positive hole oxidizes either pollutant directly or water to produce ·OH radicals, whereas the electron in the conduction band reduces the oxygen adsorbed on the catalyst Bi₂WO₆.

The activation of Bi₂WO₆ by UV light can be represented by the following steps

:

\mathrm{B}{\mathrm{i}}_{\mathrm{2}}\mathrm{W}{\mathrm{O}}_{\mathrm{6}}+h\nu \to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}

{\mathrm{e}}^{-}+{\mathrm{O}}_{\mathrm{2}}\to {\mathrm{O}}_{\mathrm{2}}^{-}

In this reaction, h⁺ and e^- are powerful oxidizing and reductive agents respectively. The oxidative and reductive reaction steps are expressed as

Oxidative reaction:

{\mathrm{h}}^{+}+\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\to \mathrm{C}{\mathrm{O}}_{\mathrm{2}}

{\mathrm{h}}^{+}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to ·\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}

Reductive reaction:

·\mathrm{O}\mathrm{H}+\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\to \mathrm{C}{\mathrm{O}}_{\mathrm{2}}

Hydroxyl radical generation by the photocatalytic oxidation process is shown in the above steps. In the degradation of organic pollutants, the hydroxyl radical, which is generated from the oxidation of adsorbed water where it is adsorbed as OH^-, is the primary oxidant; and the presence of oxygen prevents the recombination of an electron-hole pair.

Conclusions

3D Bi₂WO₆ microsphere, which was synthesized by the hydrothermal method, exhibits high photocatalytic activity for the dyes, toxic organic compounds, and the printing and dyeing sewage in UV light irradiation. Furthermore, in the degradation processes, the Bi₂WO₆ sample also showed high stability and repeatability, which is very important for a photocatalyst to be used in industry.

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