Accelerated Fenton reaction for antibiotic ofloxacin degradation in discharge plasma system based on graphene-Fe3O4 nanocomposites
Introduction
In decades, antibiotics have been widely used to treat various diseases of human and animals [1]. However, due to overuse, a large number of antibiotics and their metabolites can not been decomposed completely and entered the sewage system. Moreover, it is difficult to remove them by traditional water treatment technology because of the poor biodegradability, thus entering all kinds of water bodies and increasing the harm to the ecosystem and human health [2]. Ofloxacin (OFX) is the third generation of quinolones, which is mainly used in acute and chronic infections of respiratory tract, throat and tonsil caused by gram-negative bacteria [3]. However, excessive exposure to ofloxacin in water environment will cause renal dysfunction, allergic reaction and central symptoms [4]. Therefore, effective methods are needed to remove antibiotic OFX in water.
Due to advantages of fast, efficient and no secondary pollution, discharge plasma has been widely used in water treatment in recent years [5]. In the process of discharge, the high voltage applied between the electrodes induces gas ionization, which produces many physical and chemical effects [6,7]. The physical effects include high-energy electrons, light, ultrasound, electric field and shock wave. The chemical effects include some oxidizing substances: free radicals (·OH, ·O, ·HO2, etc.) and molecules (H2O2, O3, etc.). The oxidation potential of the relative oxidizing substances is provided in Table 1. It can be seen that compared to other oxidizers, ·OH has the highest oxidation potential, which can act on organic contaminants with non-selective [[8], [9], [10], [11]]. While the active substances with relative low oxidation potential (H2O2, O3, etc.) are difficult to react with organic compounds adequately, resulting in the low utilization rate of active substances [12]. In addition, ultraviolet light, ultrasonic wave and heat energy are emitted directly during the discharge process, which also causes energy loss. Therefore, how to make full use of the physical and chemical effects to further improve the ·OH production is an important development direction of this technology.
In order to solve this problem, iron ions were introduced to form Fenton reaction to decompose H2O2 and produce ·OH in discharge plasma system [13]. However, homogeneous Fenton reaction proceed smoothly only under acidic conditions (pH < 3) generally. Secondly, the addition of iron ions will lead to secondary pollution of water inevitably, which is not conducive to catalyst reuse. In recent years, research on heterogeneous Fenton reaction is gradually rising [14]. The process can be carried out in a wider range of pH value, and the catalyst can be recovered easily. Besides, UV assisted heterogeneous Fenton process could further improve the catalytic efficiency [15]. At the same time, heterogeneous photo-Fenton catalyst can significantly reduce the residual concentration of iron ion in solution. Owing to the high catalytic activity, magnetic Fe3O4 is used as catalysts to catalyze heterogeneous Fenton reaction [16,17]. However, Fe3O4 alone is easy to agglomerate in the preparation process and has poor dispersion in aqueous solution, which limits its wide application. Recently, graphene has attracted extensive attention as a carrier of metal oxides regards to its excellent electronic properties, chemical stability and large specific surface area [18,19]. The results show that the combination of graphene and Fe3O4 can prevent the agglomeration of Fe3O4 and improve its catalytic performance [20].
Herein, this study proposed to introduce graphene-Fe3O4 nanocomposites into discharge plasma system, which made full use of the light and H2O2 to form photo-Fenton reaction and then promote the formation of ·OH. Meanwhile, the magnetism endowed by catalyst is conducive to separation from aqueous solution and avoid secondary pollution. Firstly, graphene-Fe3O4 nanocomposites were prepared by hydrothermal synthesis method, and their structural morphology, chemical bond structure and optical absorption properties were characterized systematically. Then the catalytic effect of graphene-Fe3O4 nanocomposites in discharge plasma system was investigated. The effect of catalyst addition, applied voltage, initial concentration of solution and ·OH scavenger on the degradation efficiency was studied. Then the effect of catalyst on the formation of ·OH was analyzed. Finally, the degradation process of OFX was analyzed according to the variation of pH, conductivity and three-dimensional fluorescence.
Section snippets
Chemicals and reagents
Graphite, thick sulfuric acid, permanganate (KMnO4), sodium acetate anhydrous (CH3COONa), hydrochloric acid (HCl), ferric chloride (FeCl3·6H2O) and sodium nitrate (NaNO3) were purchased from Liaodong Chemical reagent Co. LTD. OFX was purchased from Aladdin. Titanium sulfate and salicylic acid were obtained by Tianjin Kemiou Chemical Reagent Co., Ltd. Deionized water was used for all experiments involved in water.
Experimental setup
The experimental setup is similar to our previous work [21], including pulse power,
Characterization
Fig. 2 depicts the XRD patterns of GO and rGO. It can be seen that GO presents a main characteristic peak at about 10.9°, representing the (002) plane. Compared with GO, rGO has no obvious characteristic peak near 10.9° but a new wide diffraction peak appears near 23.5°. Besides, the solution varies from brown yellow to black, indicating that GO has been successfully reduced to rGO. Fig. 3a shows SEM image of Fe3O4, exhibiting that Fe3O4 is in the state of nano particles. It can be seen from
Conclusion
In the paper, rGO-Fe3O4 were prepared to promote Fenton reaction for OFX degradation in discharge plasma system. The result showed that GO was successfully reduced to rGO, and the surface of rGO presented obvious phenomenon of graphite layer warping. Fe3O4 could be successfully loaded on rGO. The optical absorption range of rGO-Fe3O4 was much larger than that of Fe3O4. Compared to Fe3O4 alone, the degradation efficiency and kinetic constant of OFX were improved with rGO-Fe3O4. The degradation
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We greatly appreciate financial support from National Natural Science Foundation of China (No. 22006069; 21876070) and Natural Science Foundation of Jiangsu Province, China (No. BK20200801).
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