Research Paper
Highly efficient g-C3N4/TiO2/kaolinite composite with novel three-dimensional structure and enhanced visible light responding ability towards ciprofloxacin and S. aureus

https://doi.org/10.1016/j.apcatb.2017.08.044Get rights and content

Highlights

  • Novel 3D heterogeneous g-C3N4/TiO2/kaolinite was synthesized.

  • High efficiency was obtained through the construction of “sandwich” structure.

  • Both water-contained antibiotic and waterborne bacteria were degraded.

  • Mechanisms for enhanced activity of composite were investigated.

Abstract

A novel 3D heterogeneous g-C3N4/TiO2/kaolinite composite with enhanced visible light activity was fabricated via a mild sol-gel method associated with chemical stripping and self-assembly. Compared with bare photocatalysts, the g-C3N4/TiO2/kaolinite 3D structure exhibits enhanced adsorption-photocatalytic degradation ability for the removal of ciprofloxacin (CIP) under visible-light irradiation, and also facilitate the recyclability of the photocatalyst as demonstrated from the reusability test. The apparent rate constant of the composite is up to around 5.35 times, 6.35 times and 4.49 times that of bare TiO2, g-C3N4 and P25, respectively, and a possible degradation pathway was also proposed. On the other hand, the as-received composite also exhibited enriched disinfection ability towards S. aureus. It is indicated that the superoxide radical (radical dotO2) is the main active species in the degradation process, and the superior photocatalytic performance of composite should be mainly attributed to both the improvement of light harvesting as well as the enhanced separation and transfer efficiency. It is expected that this novel ternary visible-light responding composite would be a promising candidate material for the organic pollutants degradation and bacteria inactivation.

Introduction

With the rapid development of human beings’ industry and civilization, water pollution is gradually becoming more and more serious. Currently, both water-contained antibiotic and waterborne bacteria are regarded as the major groups of water contaminants due to their ubiquitous property and hazardous biological and ecotoxicological effects [1], [2]. Pharmaceuticals and personal care products (PPCPs) like ciprofloxacin are usually derived from industries, hospitals and domestic sites effluents. Sometimes, they are directly thrown into the surface and ground waters without any previous decontamination treatment, resulting in pollution and disturbing effects. Besides, this kind of pollutant cannot be completely removed or transformed through conventional wastewater treatment methods like adsorption, membrane filtration and chemical oxidation [3], [4]. Therefore, high-efficient removal and degradation of these pharmaceuticals has become a hot topic of environmental scientific research in recent years. On the other hand, waterborne bacteria (S. aureus) may pose significant health risks to human as well, which are generally difficult to be fully inactivated due to their small size and high resistance to harsh environmental conditions. Hence, efficient removal of bacteria is also urgently needed to guarantee the safety of drinking water [5]. In recent years, advanced oxidation processes (AOPs) especially for photocatalysis are well-accepted technologies to be used in the field of persistent organic pollutants degradation as well as bacteria inactivation [6], [7]. Photocatalysis technique has been considered to be a desired strategy for environmental remediation because of its low-cost, environmental benefit, nontoxicity and stability [8].

However, conventional semiconductor catalysts like TiO2 and g-C3N4 usually have a poor activity, narrow spectral response and limited electron transportation, which severely restricted their practical application. Hence, more and more effort has been focused on exploring the high-efficiency visible-light photocatalysts in recent decades [9]. According to previous reports [10], [11], heterojunctions like g-C3N4/TiO2 usually exhibit superior photoactivity than single photocatalyst through constructing two photo-reactive semiconductors with suitable electronic structures. The accelerated charge separation by the internal electric field in the heterojunctions is the main driving force of photocatalytic performance enhancement [12]. On the other hand, it is widely accepted that natural minerals is a good choice as the catalyst carriers to construct the macro-micro system considering their high adsorption ability, thermal and chemical stability, low cost, easy recycling and abundance [13], [14], [15]. In the past decades, the composites based on the combination between natural minerals and TiO2 or g-C3N4 has been widely studied. However, most of such composites like TiO2/montmorillonite or g-C3N4/montmorillonite are just responding to ultraviolet or limited visible light [16], [17], [18], [19]. Hence, it is meaningful and feasible to synthesize a ternary photocatalyst through the combination between g-C3N4/TiO2 heterojunction and natural minerals in order to construct a highly efficient adsorption and visible-light-driven photocatalytic degradation system.

In our present work, a kind of novel ternary g-C3N4/TiO2/kaolinite composite with a “sandwich” structure was first designed and constructed, which would be beneficial for the utilization of solar energy as well as regulating the path and velocity of the generated electron-hole pairs [20]. Ciprofloxacin (CIP) and S. aureus were taken as the target degradation pollutants in this study. A possible mechanism for the enhanced and stable photoactivity was also investigated and proposed based on the characterization results.

Section snippets

Materials

The purified kaolinite was obtained from Suzhou City, Jiangsu Province. Tetrabutyl titanate (C16H36O4Ti, TBOT) was bought from Maya Reagent Co. (Zhejiang, China). Ciprofloxacin was purchased from Aladdin Industrial Corporation. Dicyandiamide was gained from Tianjin Jinke Fine Chemical Reasearch Institute. Hydrochloric acid (HCl) and sulphuric acid (H2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). HPLC-grade formic acid, methanol and acetonitrile were achieved

Results and discussion

The general morphologies and microstructures of kaolinite, TiO2/kaolinite, TiO2, g-C3N4 and g-C3N4/TiO2/kaolinite were investigated by SEM analysis (Fig. 1(a-e)). It is indicated that kaolinite exhibited layered structures composed of many parallel nanosheets with 0.3–1 μm in width and 10–20 nm in thickness. The kaolinite has a smooth and regular surface without impurities, which could significantly promote the assembly of nano TiO2 particles. From Fig. 1(b), it is indicated that the TiO2

Conclusions

In conclusion, a novel 3D heterogeneous g-C3N4/TiO2/kaolinite composite was fabricated via a mild sol-gel method associated with chemical stripping and self-assembly in this study. The morphological analysis and other characterizations confirmed the formation of the novel “sandwich” structure, in which a better dispersion of nano TiO2 with smaller grain sizes, much more exposed g-C3N4 sheets, enlarged surface area as well as enhanced visible-light absorption ability were achieved. The

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51504263) and the Fundamental Research Funds for the Central Universities (2015QH01).

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