Visible light degradation of textile effluent using novel catalyst ZnO/γ-Mn2O3

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Highlights

  • Facile thermal decomposition method.

  • Visible light photocatalyst.

  • Outstanding degradation of methylene blue.

  • Nanorod.

Abstract

The novel ZnO/γ-Mn2O3 (various weight percentages) nanocomposite catalysts were prepared by thermal decomposition method and their size, shape, and surface area were characterized by various techniques. Further, the prepared samples were used to degrade methylene blue (MB) and methyl orange (MO) in aqueous medium under visible light irradiation. Finally, the best catalyst was employed to degrade phenol and a textile effluent. The recycling ability and their efficiency of catalyst are discussed in detail.

Introduction

Water is the most important resource for every living organism, as the existence of life in the world is not possible without water. The quality of water is an essential factor for micro-organisms, eutrophication, and human beings. Worldwide 1.5 million children die per year because of unsafe water, poor sanitation, and inadequate hygiene. Thus, the quality of water directly affects the normal life of human beings and animals. The main water contamination sources are from industrial discharge of chemicals, agricultural activities, and other environmental changes [1], [2]. It has already been reported that the major organic compounds that constitute the increasing environmental danger are the industrial dyes and textile dyes. Wastewater treatment and recycling is an important concern and the researchers are looking forward to develop inexpensive and suitable technology. The wastewater has been treated by different physical, chemical, and biological processes. Advanced oxidation process (AOPs) is the most attractive technologies for waste water treatment. Among various AOPs, heterogeneous photocatalysis is a promising method, which can be used for the degradation of various organic pollutants in wastewater [3].

During the last two decades, significant attention has been paid to the reactions that take place on the illuminated surface of semiconductor metal oxides and sulphides. TiO2, ZnO, CuO, CdS, ZnS, WO3 are the semiconductors which possess a moderate band-gap energy of 1.7–3.32 eV between their valence and conduction bands. The two predominant photocatalytic materials found in the literature are TiO2 and ZnO [4], [5]. Unfortunately, TiO2 and ZnO which are the benchmarks of UV photocatalysis are inactive under visible light due to their wide band gaps. Worldwide efforts are underway (including our laboratories) to make use of sunlight for environmental protection and water purification. In our previous studies, silver nanoparticles incorporated in zinc oxide nanorods were used as excellent photocatalysts for the degradation of textile effluent under visible light [6]. Many researchers have focused to synthesize manganese oxides because of its magnetic, electrical, and catalytic properties [7], [8], [9], [10], [11], [12], [13]. Among the oxides of manganese (MnO, MnO2, Mn2O3, and Mn3O4), Mn2O3 has higher activity for the decomposition of N2O and NO, because of its stability [9]. Mn2O3 has a lot of advantages including cost effectiveness and eco-friendly nature for the decomposition of nitrogen oxide, carbon monoxide, and organic pollutants. It also acts as an electrode for lithium batteries. Recently, successful preparation of Mn2O3/TiO2 and its application in the photocatalytic degradation of various dyes under visible light has been reported [14]. However, to the best of our knowledge, no research reports were published on the photocatalytic activity of ZnO/γ-Mn2O3 nanocomposites. In the present work, different weight percentages of ZnO/γ-Mn2O3 nanocomposite samples were prepared by thermal decomposition method. The prepared samples were characterized by XRD, FE-SEM, TEM, BET, XPS, and UV–vis absorption. Finally, the catalysts were used for the photo degradation of organic dyes under visible light.

Section snippets

Materials

Zinc acetate dihydrate (Rankem) and manganese (II) acetate tetrahydrate (Aldrich) used in the present study were of analytical reagent grade. Methylene blue (MB) and methyl orange (MO) were purchased from Aldrich chemicals. All the chemicals were used as received without further purification. All aqueous solutions were prepared using double distilled water.

Synthesis of ZnO/Mn2O3 composites

ZnO was synthesized by thermal decomposition method which is already reported in our previous work [15]. 3.0 g of zinc acetate dihydrate was

Structure analysis

The powder X-ray diffraction pattern of the synthesized ZnO, Mn2O3, and ZnO/Mn2O3 (99:1, 97:3, 95:5, 90:10, 80:20, 70:30, 60:40, and 50:50) nanocomposites are presented in Fig. 1.

All the diffraction peaks of ZnO are indexed and it exhibits a hexagonal structure which coincides with JCPDS No: 79-0208. The X-ray diffraction pattern of Mn2O3 is shown in Fig. 1(b), and the characteristic peaks (1 1 1), (2 0 2), (2 2 1), (2 0 3), and (2 1 3) were observed and indexed. This confirms the tetragonal structure of

Conclusion

The novel catalysts ZnO/γ-Mn2O3 were successfully prepared by thermal decomposition method, and their characterization confirms the formation of nanosized composite materials without any impurities. TOC analysis was carried out to confirm the mineralization of the intermediates. The ZnO/γ-Mn2O3(90:10) sample shows higher degradation efficiency compared with other compositions. This is due to the synergetic coupling effect between ZnO and Mn2O3, and the high specific surface area of the

Acknowledgement

We acknowledge National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai, India for XPS and TEM characterizations.

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