Bifunctional BiOCl/TiO₂ decorated membrane for antibiotic photodegradation and oil-water emulsion separation

Highlights

• BiOCl/TiO₂ membrane was prepared by electrospinning and hydrothermal method.

• Microstructures formed by BiOCl can enhance the hydrophilicity of the membrane.

• The membrane can realize photodegradation and emulsion separation simultaneously.

Abstract

Membrane separation is a simple and efficient technology for waste water treatment. However, organic pollutants such as antibiotics in complex water environment are hard to remove by the physical separation process. The coupling of photocatalysis and membrane separation can effectively remove the oil pollutants, meanwhile degrade the antibiotics. In this work, a composite membrane (BiOCl/TiO₂@PAN) was prepared by the electrospinning technology and hydrothermal process. The membrane showed an excellent superhydrophilicity in air and an underwater superoleophobicity, thus it could separate a series of oil in water emulsions with a high permeability (231–613 L m^-2h⁻¹) without extra energy supply. Moreover, owing to the photoresponse ability of BiOCl and TiO₂, the membrane could effectively degrade a variety of antibiotics including the typical antibiotic tetracycline, chlortetracycline, oxytetracycline and demeclocycline within 30 min under full-spectrum and visible light irradiation. Eight-cycle repeating experiments showed that composite membrane had an excellent stability and an efficiency in photocatalytic degradation and oil–water emulsion separation. Photo-responsive composite membrane prepared in this work showed a broad application prospect in treatment of complex sewage where oily waste and antibiotics simultaneously exist.

Introduction

Oily waste water generated from pharmaceuticals, textiles and food industry has attracted widespread attentions [1], [2], [3], [4], [5], [6]. It can not only seriously threat the survival of animals and plants, but also cause water resource waste [7], [8]. As a simple and efficient separation technology, membrane separation has been widely used for oily water purification. In practice, the compositions of oily wastewater are commonly complex which may contain a large number of refractory pollutants, such as antibiotics, dyes, and other organic matters. Antibiotics including tetracycline (TC), oxytetracycline, and chloramphenicol, etc., can be easily absorbed by organisms and difficult to completely degrade from the conventional sewage treatment system. In most cases, membranes applied for oily water treatment can only separate the organic contaminations and purified water by their interception function, antibiotic pollutants which are far smaller than the most of membrane pore size are still needed for further treatment. Advanced oxidation processes (AOPs) involve the oxidation of compounds through their reaction with free radicals, such as hydroxyl and sulfate radicals, resulting in the degradation of the pollutants. Among the AOP methods, photocatalytic oxidation is one of the most used treatments for organic pollutant degradation. The light energy is converted to chemical energy by the photocatalysts, and the surrounding water molecules and oxygen are stimulated to produce active oxide species to realize the decomposition of organic pollutants [9], [10], [11], [12], [13].

At present, titanium oxide (TiO₂) has been usually used for photocatalytic oxidation owing to its low cost, non-toxic, chemically stable and highly photo-reactive [14], [15], [16], [17], [18], [19], [20]. However, TiO₂ has a wide band gap, low solar energy utilization and short photogenerated carrier life, and its quantum yield still needs to be further improved [21], [22]. Thus, it is important to enhance the solar energy utilization and the separation efficiency of photogenerated electrons. Bismuth oxychloride (BiOCl) is a highly anisotropic layered p-type semiconductor with a band gap of 3.44 eV, which can form a p-n type heterojunction with TiO₂. Nevertheless, most of the studies only focused on photocatalyst dispersion suspension systems, problems including the catalyst loss and potential ecological risks of these photocatalysts are still needed to be considered [23]. Therefore, the development of a material that takes into account both oil–water separation and photocatalytic degradation is of great importance in practical water treatment applications. For example, Zhou et al. [24] prepared a hierarchical porous oxygen-deficient TiO₂ (TiO_2-δ) fiber decorated with BiOCl nanosheets by centrifugal spinning sol–gel method. The results showed that the fibrous photocatalysts exhibited excellent photocatalytic performance and stability for the degradation of azo dye reactive brilliant red and colorless phenol in aqueous solution.

In recent years, the emergence of photocatalytic membrane has effectively solved the problems of degradation for organic pollutants and difficulties in photocatalyst recovery. That not only overcomes the shortcomings of membrane separation and photocatalysis technology, but also maintains the characteristics and capabilities of the two technologies. Photocatalytic membrane is a type of functional composite membrane that couples photocatalysis and membrane separation together, by blending photocatalyst with casting solution or loading on the membrane surface, which further shows high photocatalytic degradate rate and membrane separation efficiency, thereby completes the interception and degradation of organic pollutants, finally realizing effective treatment of organic wastewater [25]. For instance, EI Mrabate et al. [26] reported a novel bacterial cellulose-ZnO-MWCNT hybrid membrane, which showed excellent performance in MB photocatalytic degradation and antibacterial experiments with 92% degradation rate. Li et al. [27] successfully prepared RGO/PDA/gC₃N₄ composite membrane by vacuum filtration method. The composite membrane could achieve simultaneous degradation and separation of the mixed solution of dye and oil water emulsion. The test results found that the composite membrane had good water flux (20-30 L m^-2h⁻¹ bar⁻¹), excellent rejection rate (more than 99%), excellent photodegradation efficiency (97.5%) and recyclability. Even the photocatalytic membrane can effectivly remove pathogenic microorganisms and organic pollution in polluted water, there are still many problems such as possible damage of the support layer, low mass transfer rate, limited contact area, and catalyst deactivation and loss need to be solved urgently [28].

Electrospinning is a technology that can easily, quickly and massively obtain fibrous membranes. It shows the advantages of large specific surface area, easy modification, good structural adjustability, etc. [29] In response to the abovementioned problems, the BiOCl/TiO₂ heterojunction with photocatalytic function was introduced into electrospinning PAN nanofibers membrane by one step hydrothermal mehod in this work. The microstructures and the morphologies of the membranes were observed and the hydrophilicities of the membranes were investigated. Moreover, the photocatalytic activities of the membrane for the degradation of tetracycline antibiotics either under full-spectrum or visible light irradiation were evaluated. Additionally, an eight-cycle performance evaluation process was then conducted, the water flux and photocatalytic stability of the membranes were investigated.