Preparation of visible light-responsive photocatalytic paper containing BiVO₄@diatomite/MCC/PVBCFs for degradation of organic pollutants

Highlights

• Adsorption and photocatalysis synergy for degrading MB and HCHO.

• High degradation rates of MB and HCHO under visible light.

• More potential application fields for paper.

Abstract

Combining adsorption and photocatalysis is an effective strategy for degrading organic pollutants. Here, BiVO₄@diatomite composite photocatalyst (BiVO₄@diatomite CP) was prepared by hydrothermal synthesis from Bi(NO₃)₃·5H₂O glycerin solution, NH₄VO₃ solution and diatomite. BiVO₄@diatomite/microcrystalline cellulose/PVB composite fibers (BiVO₄@diatomite/MCC/PVBCFs) were prepared from BiVO₄@diatomite CPs, microcrystalline cellulose (MCC) and PVB ethanol solution using the electrospinning method. BiVO₄@diatomite/MCC/PVBCFs were then mixed with pulp fibers to prepare the visible light-responsive photocatalytic paper. BiVO₄@diatomite CP with a BiVO₄/diatomite ratio of 6:4 had good interface states and displayed good photocatalytic activity with 64.32% degradation of methylene blue (MB) after 4 h. A PVB ethanol solution (6%) was formulated with BiVO₄@diatomite CP and MCC to provide an ethanol spinning solution (12% solid) to prepare BiVO₄@diatomite/MCC/PVBCFs (3:3:4). The resulting fibers had smooth surfaces, compact structures and exhibited good photocatalytic activity (66.80% and 56.80% degradation of MB and formaldehyde (HCHO), respectively, after 4 h). Photocatalytic paper containing 18% BiVO₄@diatomite/MCC/PVBCFs had good photocatalytic activity with 50.20% degradation of HCHO after 4 h. This paper also had good physical properties and has the potential to be used for the photocatalytic degradation of indoor air pollutants, such as HCHO.

Introduction

Since people now typically spend more than 80% of their daily time indoors, the quality of indoor air has a large effect on the human health (Tang et al., 2009). The human carcinogen (HCHO) is a typical example of a volatile organic compounds (VOC) founds indoors, and the detection and removal of HCHO are thus attracting much interest (Wu et al., 2015; Salthammer et al., 2010). Photocatalytic oxidation, which is one of the more promising technologies for the degradation of hazardous organic chemicals to water, carbon dioxide, and simple mineral acids, has the advantages of being simple to carry out, requiring low energy consumption and not generating secondary pollution (Chen et al., 2010; Yin et al., 2010; Karagoz et al., 2019; Meenakshi and Sivasamy, 2016). Semiconductors, which are the most frequently used photocatalysts, have a discontinuous band structure, which is composed of a low energy valence band (VB), a high energy conduction band (CB) and a forbidden band (FB). The forbidden bandwidth (Eg) determines the degree of difficulty of electron transition from the VB to the CB, the smaller the value of Eg is, the easier it is for electron transition to the CB (Chen et al., 2010; Shen et al., 2017; Ren et al., 2004).

Bismuth vanadate (BiVO₄), a non-toxic, ecofriendly, visible light-responsive photocatalytic semiconductor, is currently attarcting widerspread interest because of its narrow band gap (2.4 eV) and suitable band position (Wang et al., 2017). Photocatalytic degradation of organic pollutants using BiVO₄ has been described previously (Deebasree et al., 2018; Dong et al., 2016). Sayama et al. (2006) used the metal-organic decomposition method to synthesize a porous BiVO₄ film that was used as an electrode for the photochemical splitting of water. Chala et al. (2014) synthesized Fe-loaded BiVO₄ using hydrothermal method and found that its ability to degrade MB under visible light irradiation (82% after 30 min) was superior to that of BiVO₄ itself. Zhang and Zhang, (2010) prepared a Eu/BiVO₄ composite photocatalyst by the hydrothermal method, and using decolorization of methyl orange as the test, showed that Eu/BiVO₄ was a highly efficient photocatalyst (93.6% of methyl orange degraded after 180 min under visible light irradiation). Li et al. (2013) used a simple two step hydrothermal method to synthesize F-doped BiVO₄ spheres and showed that they had higher photocatalytic activity than undoped BiVO₄ (99% degradation of MB after 180 min under visible light irradiation).

BiVO₄ is a typical ternary semiconductor that has a layered structure with three crystallographic forms, i.e., monoclinic scheelite-like (s-m), tetragonal scheelite-like (s-t) and tetragonal zircon-like (z-t), where phase transitions may take place between these crystallographic forms under different thermal conditions (Wang et al., 2017; Liu et al., 2015). Tokunaga et al. (2001) found that the monoclinic scheelite structure was the most thermodynamically stable crystal form and also had the best photocatalytic activity.

Combining BiVO₄ with porous materials that have a high adsorption capacity can improve the ability to degrade organic pollution by increasing exposure of the pollutant to the photocatalyst. Naturally abundant porous minerals, with high porosity, large specific surface area and good stability, are excellent carriers for photocatalysts (Xu et al., 2007). There are abundant reserves of the porous mineral diatomite, which is easy to obtain, low in cost, and light in weight, with good in corrosion resistance and strong in adsorption properties (He et al., 2019). David et al. (2015) used Al₂O₃ and natural mineral materials to prepare composite nano-photocatalysts and achieved 80% degradation of organic pollutants. It was reported that the adsorption properties of natural diatomite can be markedly improved by treatments with acid washing and burning. Wang and Zheng, (2011) pretreated diatomite by aerobic roasting method to increase the specific surface area and pore size, and found that this method also removed the impurities from the surface of the diatomite.

Electrospinning is a process in which nanofibers are produced by subjecting a jet of polymer solution or melt to a strong electrical field (Huan et al., 2016). Electrospinning is a green, convenient, simple and versatile method for generating nanofibers. Membranes composed of randomly oriented, as-spun nanofibers can be produced from a wide variety of materials, including ceramics, polymers and even metals (Park and Kim, 2015; Sun et al., 2014). Incorporation of a composite photocatalyst into electrospun fibers can promote separation between photoelectrons and holes in the photocatalyst and also increase its retention. The photocatalyst was compounded with MCC, which was easy to form hydrogen bonding between cellulose so as to improve the physical strength of photocatalytic paper. This method can also help to prevent the photocatalyst particles from aggregating. The structure of fiber was easy to fix and disperse the photocatalyst well. Moreover, it could provide more active sites for the photocatalytic reaction. When the photocatalyst was under visible light irradiation, the photoelectrons (e⁻) were motivated to migrate and generate holes (h⁺). And electrospun fiber could transmit the e⁻, which inhibited the recombination of photoelectrons and holes. (Bao et al., 2019). Yang et al. (2016) prepared g-C₃N₄/C nanofibers by electrospinning and, using these nanofibers, was able to achieve 85.6% degradation of rhodamine B after 3 h.

Photocatalytic paper was first prepared by the Japanese researcher Matsubara in 1995 (Matsubara et al., 1995). Over recent years, photocatalytic paper has become the subject of much research interest. Baruah et al. (2010) prepared photocatalytic paper loaded with ZnO nanorods, and achieved 93% degradation of MB. Abdel Rehim et al. (2016) mixed pulp fibers with TiO₂/superoxide dismutase (SOD) to prepare modified photocatalytic paper that inhibited microbial growth and had good prospects for application in environmental and health fields. Dai et al. (2017) prepared AgCl/Ag hybrid-cellulose paper with good antibacterial and photocatalytic activity, using green in-situ synthesis technology. Fujiwara et al. (2017) complexed TiO₂ and cellulose fibers to produce a photocatalytic paper which degraded 90% of VOCs under UV light irradiation.

In this paper, we describe the preparation of photocatalytic paper from photocatalytic composite fibers and pulp fibers. The photocatalytic paper not only provided a good carrier for the photocatalyst, but also retained the original three-dimensional porous network structure of the paper sheet (Qin and Liu, 2016). The retained network structure increased the contact area of the photocatalyst with pollutants and improved the performance of the paper. The new photocatalytic paper has the potential to be used for the photocatalytic degradation of indoor air pollutants, such as HCHO.