Photodegradation of pollutant pesticide by oxidized graphitic carbon nitride catalysts
Graphical abstract
The degradation efficiency of pesticide (2,4-D) by Og-CN was higher than by g-CN, and the UV light irradiation was more effective than the visible light irradiation. The photoreactions of both Og-CN and g-CN follow the zeroth-order reaction rather than the first-order reaction. The active species-trapping experiment confirmed the reactivity in order of .
Introduction
Environmental pollutions by organic and organometallic compounds are drastically rising and becoming a serious problem from time to time all around the world [[1], [2], [3]]. This rapidly increased issue may be attributed to industrialization, urbanization and agricultural expansion. Specifically, the recent attention is that along with the intense agricultural demand, the amount of pesticides and/or herbicides administrated in agricultural works remarkably increases and it is a strong risk to human health [4,5]. Among organic chlorine pesticides, the most common one is 2,4-dichlorophenoxyacetic acid (2,4-D), which is used in the whole world, specifically, in the developing countries [6,7]. It is a non-biodegradable aromatic herbicide utilized to damage broad leaves from weeds, and thus its residue may admix in soil-borne and water bodies and becomes the environmental hazard. The World Health Organization classifies 2,4-D as a mildly toxic chemical and the maximum tolerable amount in drinking water is 100 ppb [[8], [9], [10]]. Thus, the development of appropriate treatments is necessary to maintain this condition. Among many treatments, filtration, adsorption, coagulation, and ion exchange methods have been explored to remove 2,4-D from water systems [11,12]. However, those treatments should be forced the resumption of 2,4-D and/or provide excess secondary chemical pollutants that can affect the environments.
Photocatalytic degradation is a more promising technique, because it actively uses renewable solar energy to change harmful pollutants to harmless products without using other oxidative chemicals [13,14]. In these cases, metal oxide or metal semiconductor catalysts have generally been used, although they are less abundant and toxic in some cases [15,16]. Graphitic carbon nitride (g-CN) is a metal-free material consisting of the most abundant elements (carbon and nitrogen) and easily synthesized by the condensation polymerization of nitrogen-rich compounds such as thiourea, urea, cyanamide, dicyandiamide and melamine [[17], [18], [19]]. It is one of the most promising photocatalysts because of its stability, non-toxicity, tunability, reusability, flexible layer formability and small bandgap (about 2.7 eV) [20,21]. However, it suffers the recombination of charge carriers, the low visible light-harvesting ability and hence the low photocatalytic efficiency [22]. Recently it has been reported that when g-CN is hydrothermally introduced oxygen, it makes the product (oxygen-doped g-CN: Og-CN), which is expected to be derived more amount of pores, increased the electron mobility, decreased the charge recombination and enhanced the visible light responsibility [[23], [24], [25], [26]].
In this work, g-CN was synthesized by the thermal condensation polymerization of melamine and followed by the hydrothermal-treating with H2O2. The produced oxygen-doped catalyst (Og-CN) was compared to g-CN on their characteristics. The photocatalytic degradation of 2,4-D on the Og-CN catalyst utilizing ultraviolet (UV) and visible light sources was evaluated under the spectrophotometric analysis and compared with that on the g-CN catalyst. The kinetics analysis of the 2,4-D degradation was also performed, and the active species of the reaction was assessed by the trapping experiments. This research will clarify the effective photoreaction activity on Og-CN superior to g-CN and the degradation mechanism of 2,4-D.
Section snippets
Reagents and instruments
All chemicals were of analytical grade reagents and used without further purification. Rhodamine 6 G (Rh 6 G), 2,4-dichlorophenoxyacetic acid (2,4-D, 99+ %), melamine powder (99 %), tert-butyl alcohol (99.5 %), potassium iodide, hydrogen peroxide solution (35 wt%) and 2,2,6,6-tetramethyl-1-piperdinyloxy free radical (TEMPO) (98+ %) were purchased from Across Organics (New Jersey, USA). Ultrapure water (resistivity: 18.2 MΩ cm−1) was used throughout this work for the preparation of solutions.
Characterization of photocatalysts
To assess the photocatalytic activities, the photocatalysts (g-CN and Og-CNs) were synthesized and characterized. As indicated in the TEM images (Fig. 1), both catalysts (g-CN and 2-Og-CN) represented the morphology where the sheet-like textures with amorphous shape are agglomerated. Similar TEM images of g-CN and 2-Og-CN have been reported [31]. The surface morphologies of pristine g-CN and oxidized 2-Og-CN were also visualized via FE-SEM. As depicted in Fig. 1B(a,b), the sheet like structures
Conclusions
In this work, the photocatalytic degradation performance of Og-CN on 2,4-D pesticide with UV and visible light sources was reported and compared with that of g-CN. Because some amounts of oxygen were displaced the nitrogen atom in the g-CN, the surface area and PL properties of Og-CN varied from those of g-CN, and the photocatalytic degradation efficiency of 2,4-D increased compared to the g-CN catalysts. The superoxide ion () species, which dominates in photocatalytic degradation of 2,4-D,
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This investigation was partly supported by the Ministry of Science and Technology, Taiwan (MOST 107-2221-E-011-067-). S.Y.E. fully acknowledges and appreciates the National Taiwan University of Science and Technology, Taiwan, for the opportunity of studying and the financial support of PhD student scholarship. We give thanks to Prof. Lai and Prof. Hu in the National Taiwan University of Science and Technology, Taiwan, for their kind measuring of TOC.
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