Ni(OH)2 decorated rutile TiO2 for efficient removal of tetracycline from wastewater
Graphical abstract
Introduction
Antibiotics have been widely selected in prophylaxis and therapy of human and animal diseases and as promoters for animal growth. Tetracycline, as one among the cheapest classes of antibiotics [1] has been used to treat many kinds of bacterial infections, for instance urinary tract infections, acne and gonorrhea. Additionally, tetracycline is commonly adopted in agricultural industry to prevent livestock diseases. However, like many other antibiotics, tetracycline can only be partially metabolized by humans and animals, causing the residue to be released into wastewater [1]. Because of the absence of biodegradability, the removal of tetracycline during biological process in conventional wastewater treatment plants is mainly achieved by biosorption instead of biodegradation [2], [3]. This always leads to a difficulty in completely and efficiently eliminating tetracycline before effluent is discharged and it inevitably reaches natural water bodies; it has accelerated growth of antibiotic-resistant bacteria and genes in environment [4]. Among various other technologies investigated so far, photocatalytic oxidation reaction appears to be a cost- effective, high-performance and environmentally sustainable alternative to treat tetracycline wastewater.
Titanium dioxide (TiO2) is a promising semiconductor, which has attracted great interest in a variety of applications, including water splitting and pollutant degradation [5], [6], [7], [8]. The photocatalytic activity of TiO2 has been shown to strongly rely on its phase structure, crystallite size, morphological characteristic, surface area and pore structure. In general, rutile TiO2 is less often investigated as compared to its anatase phase due to its lower photocatalytic activity arising from higher positive conduction band edge potential and faster recombination rate of photoelectron and holes [9], [10]. Nevertheless, in addition to higher light scattering efficiency, better chemical stability and lower production cost, rutile TiO2 has a smaller bandgap energy of only 3.00 eV than anatase TiO2 (3.20 eV); thus it is fundamentally interesting for developing visible light driven photocatalytic applications [11].
It is well known that the high-performance photocatalysts should possess several features, including high crystallinity and large surface area/porosity. The traditional synthetic methods utilized the phase transformation of either amorphous titania or anatase via high temperature calcination (e.g. 600 °C) to produce rutile TiO2 [12], [13]. However, this might unavoidably lead to agglomeration and growth of nanocrystalline particles, thus forming large particle sizes and small surface areas. Furthermore, the rutile phase prepared at low temperature exhibited better photocatalytic activity as compared with that produced at high temperature [14]. A number of prior studies reported the preparation of rutile TiO2 at relatively low temperature; but it usually required introduction of acids, solvents or other chemicals, and in turn increased synthesis complexity and cost [15], [16]. On the other side, in spite of advantageously large external surfaces, small sizes of nanoparticles make them very difficult to be separated from solution after photocatalytic reaction. From a practical application perspective, forming mesocrystals, as a class of micro/nanostructured materials, which are composed of nanocrystals arranged in a regular fashion, could be an alternative solution. Three-dimensional hierarchical structure of mesocrystals offers favorable surface area and light scattering for efficient photon harvesting compared to nanocrystallites [17].
To further enhance photocatalytic activity of TiO2, several strategies have been investigated so far, including doping with metals [18], [19] and non-metals [20], [21], [22] to narrow its bandgap energy. Some composites consisting of two oxide or hydroxide particles with different energy levels have been reported with better photocatalytic activities because of more efficient charge separation [23]. For instance, the Ni(OH)2/ZnO composite could potentially inhibit rapid recombination of photogenerated electrons and holes, thus enhancing photocatalytic oxidation of organic azo dyes as compared with either pure ZnO or Ni(OH)2 [23]. Some previous investigations have also indicated transition-metal hydroxide Ni(OH)2 is an economical and good co-catalyst when modifying TiO2 for producing H2 from water splitting [24], [25]. Up to date, there has been no study published on applying Ni(OH)2 on TiO2 as a photocatalyst to degrade antibiotics from water under visible light illumination.
In this paper we report a simple and environmentally friendly method for synthesizing rutile TiO2 mesocrystals, via hydrolysis of TiCl4 in water in the absence of other chemicals at room temperature and subsequent crystallization at 180 °C. We have further modified the TiO2 mesocrystals with Ni(OH)2 clusters via precipitation. Their structural properties as well as adsorption and photodegradation of tetracycline were examined in detail, in comparison with the commercial TiO2 product, P25.
Section snippets
Chemicals
Titanium chloride (TiCl4, ≥99.0%), nickel chloride (NiCl2, 98%) and tetracycline (≥98.1%) were purchased from Sigma-Aldrich, Australia. Sodium hydroxide (NaOH, 32%) and Aeroxide® TiO2 P25 were obtained from Merck, Germany and Evonik Industries, Germany, respectively. All chemicals were used as received without further treatment. Double deionized (DI) water was used for all experiments.
Preparation of TiO2 and Ni(OH)2-TiO2 catalysts
Typically, TiO2 mesocrystals were prepared by hydrolyzing 20 mL of 0.1 M TiCl4 aqueous solution at 25 °C for 24 h
Results and discussion
Fig. 1 shows the low magnification and high magnification SEM images of the samples T1 (a and b), T2 (c and d) and P25 (e and f). As shown in Fig. 1e and f, the commercial TiO2 P25 consisted of irregular particles with sizes ranging from 20 nm to 30 nm. The sample T1, synthesized from the hydrolysis of 0.1 M TiCl4 and subsequent crystallization, exhibits characteristic nanorod-aggregated flower-like bundles (Fig. 1a and b). As the concentration of TiCl4 in the synthetic solution was increased to
Conclusions
In conclusion, a simple and effective method was developed to synthesize flower-like and coral-like nanorod-aggregated rutile TiO2 (T1 and T2). Thanks to large surface area, small average pore size and reduced bandgap energy, both rutile TiO2 catalysts exhibited improved properties in terms of adsorption and photocatalytic degradation of tetracycline under visible light. The modification of the as-prepared rutile TiO2 mesocrystals with Ni(OH)2 resulted in further enhancement in the removal of
Acknowledgements
This work was supported by the Australian Research Council. H.W. thanks the Australian Research Council for a Future Fellowship (Project no. FT100100192). The authors acknowledge use of the facilities at the Monash Centre for Electron Microscopy.
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