Magnetic TiO2-graphene composite as a high-performance and recyclable platform for efficient photocatalytic removal of herbicides from water

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Abstract

A new photocatalyst, magnetic TiO2-graphene, was designed and facilely produced by combining sol–gel and assembling processes. Taking advantages of graphene and TiO2, the catalyst exhibited strong light absorption in the visible region and high adsorption capacity to organic pollutants, resulting in almost 100% photocatalytic removal efficiency of typical herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) from water under simulated solar light irradiation, far higher than 33% on commercial P25. Toxicity assessment indicates the total decomposition of the original substrate. Furthermore, the catalyst can be rapidly recovered with highly stable photocatalytic performance. After 8 successive cycles, the removal efficiency of 2,4-D maintained 97.7%, and particularly, 99.1% 2,4-D removal efficiency came back at the ninth recycle when the catalyst was re-treated by ultrasonication. Moreover, even after being laid aside for one year the catalyst still kept the 2,4-D removal efficiency as high as 95.6%. For practical application, the photocatalytic also demonstrated high removal efficiencies of herbicide 2,4-D. The photocatalyst is a promising platform for removing herbicide pollutants from water.

Graphical abstract

A new magnetic TiO2-graphene hybrid photocatalyst is prepared and acts as a high-performance and recyclable platform for efficient photocatalytic removal of herbicides from water.

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Highlights

► A magnetically separable TiO2-graphene photocatalyst is carefully designed. ► The catalyst exhibited almost 100% photocatalytic removal of 2,4-D from water. ► The photocatalyst is easily recovered with excellent reusability. ► After being laid aside for one year the catalyst still kept 95.6% removal of 2,4-D. ► 2,4-D in real wastewater could be efficiently removed.

Introduction

Along with the expanding scale of urbanization, there are more and more farm crop bases in the outskirts of the cities in China. Many kinds of pesticides and herbicides have been used to inhibit the growth of weeds and protect crops from insect pests. The wastewater containing residual pesticides and herbicides is easily transported to the groundwater, then to rivers, and finally to drinking water system. 2,4-Dichlorophenoxyacetic acid (2,4-D) has been extensively used in many crops, and there are over 1500 pesticide and herbicide products that contain 2,4-D as the main ingredient [1]. It is a carcinogen and high toxic pollutant, injuring heart, liver and central nervous system. However, it is very difficult to decompose due to its chemical and biological stability [2], [3]. Therefore, how to remove this pollutant from water is urgently demanded.

In the past two decades, various techniques have been developed to remove residual pesticides and herbicides from contaminated media, such as biological and physical–chemical means [4], [5], either through degradation or retention [6], [7]. However, these techniques are usually not attractive due to low removal efficiency, high operation costs, secondary pollution, or time-consuming processes.

Photodegradation process of herbicide pollutants has attracted increasing attention during the past decades [8], [9], [10]. Among various strategies, TiO2-based materials have been the most promising candidates for photocatalytic decontamination. Nevertheless, the wide-spread technological use of TiO2 is impaired by its wide band gap which requires ultraviolet irradiation for photocatalytic activation, the high recombination rate of photogenerated electron–hole pairs that limits photodegradation efficiency, and its low adsorptivity to hydrophobic contaminants. Many strategies, including narrow-gap semiconductor coupling [11], [12] and metal-ion [13] and nonmetal doping [14], [15], have been proposed to extend the absorption of TiO2 to the visible spectrum region, but, to date, these materials have typically suffered from low doping concentration and/or low stability against photocorrosion [16], [17]. In any case, an extended light absorption may not necessarily guarantee significantly enhanced photocatalytic performance that is also determined by charge separation and transport [18]. Recently, graphene has drawn great attention in photocatalysis because graphene has perfect sp2-hybridized two-dimensional carbon structure with excellent conductivity and large surface area [19], [20]. Its remarkable electron capture-storage-transport properties enhanced the charge separation efficiency of TiO2 [21], [22], [23], [24], and meanwhile its large specific surface area increased adsorptivity [25], [26], which greatly promoted the photocatalytic removal efficiency for organic pollutants [27], [28], [29]. However, to date, most of the reported work focused on revealing potential excellent performances of the graphene-TiO2 photocatalysts, while little attention [30], [31], [32] has been paid to the recovery and reusability of the photocatalysts which is crucial and decides their further practical applications.

Herein, a new magnetic TiO2-graphene hybrid photoctalyst is carefully designed for use in removal of herbicides from water. This material was produced by a simple sol–gel method combined with self-assembly to obtain graphene supported core–shell magnetite@TiO2 particles with a thin SiO2 layer between the magnetic core and the TiO2 shell (Scheme 1), which will integrate the functions of: (1) TiO2 photocatalysis, (2) excellent electron-capture ability and high adsorptivity of graphene, (3) magnetic separation, and (4) high stability through suppressing photodissolution of the magnetite by SiO2 [33]. In this work, the photocatalytic performance, repetitive use, and lifetime of the photocatalysts for degradation of a typical herbicide 2,4-D in different water samples including real wastewaters were studied. The chemical oxygen demand (COD) and toxicity assessment of the treated water samples were also investigated. We confirmed the high photocatalytic activity of the designed photocatalyst under simulated solar light as well as easy recovery and long lifetime, indicating a high-performance and recyclable photocatalytic platform for potentially scalable removal of herbicides from water.

Section snippets

Preparation of photocatalysts

Five grams of Fe3O4 nanoparticles were ultrasonically dispersed in alcohol (50 mL) for 1 h, and then 100 mL of the mixture solution containing ammonia water (25%), deionized water and tetraethyl orthosilicate ((EtO)4Si) (Vammonia:Vwater:V(EtO)4Si = 54:45:1) was added followed by ultrasonication for 1 h, and then magnetically separated to get silica encapsulated Fe3O4 nanoparticles. The resulted particles were added into the alcohol solution of tetra-n-butyl titanate ((BuO)4Ti). The mixture was

Structure of the photocatalysts

MT@SiO2@TiO2 and MT@TiO2 composites were prepared through a surface sol–gel process (Fig. S2). The thickness of TiO2 layer can be controlled by the amount of (BuO)4Ti. The SEM and TEM images of MT@SiO2@TiO2 show that many particles in inhomogeneous size aggregated together (Fig. 1a) and magnetite particles were encapsulated in TiO2 shell (Fig. 1b). The EDS results further confirm the existence of Ti, Fe and Si elements (Fig. 1c). The XRD results show that after calcination, Fe3O4 partly changed

Conclusions

By carefully combining the benefits of TiO2, graphene, magnetite, and SiO2, the photocatalyst exhibits high removal efficiency for herbicide pollutants under simulated solar light, rapid recycling operation, and good stability. What deserves to be mentioned the most is that the photocatalyst laid aside for one year still kept high 2,4-D removal efficiency. In addition, 2,4-D in real waters were efficiently removed by photocatalytic degradation using the proposed photocatalyst. These results

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51178173, 51202065, 51078129), Innovation Research Team in University (IRT1238), Program for New Century Excellent Talents in University (11-0126), and the Key Program of National Natural Science Foundation of China (51238002).

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