Photodegradation of pollutant pesticide by oxidized graphitic carbon nitride catalysts

doi:10.1016/j.jphotochem.2020.112955

Journal of Photochemistry and Photobiology A: Chemistry

Volume 404, 1 January 2021, 112955

https://doi.org/10.1016/j.jphotochem.2020.112955 Get rights and content

Highlights

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Graphitic carbon nitride (g-CN) and oxygen-doped g-CN (Og-CN) were synthesized.
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2,4-dichlorophenoxyacetic acid were photocatalytically degraded by catalysts.
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The degradation efficiency of Og-CN was higher than that of g-CN.
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Og-CN exerted the high total organic carbon removal (88 % after 120 min).
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The active species-trapping experiment confirmed the reactivity order of $O_{2}^{• -} > {}^{•}O H + h^{+}$ .

Abstract

Oxygen-doped graphitic carbon nitride (Og-CN) was synthesized by H₂O₂-treating of graphitic carbon nitride (g-CN), which was prepared by the condensation-polymerization of melamine and the exfoliation. The successful oxidation was confirmed by the decrease of photoluminescence intensity, the detection of oxygen-including bond and the slight increase of surface area. Og-CN was applied to the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid in the aqueous phase under the UV and visible light irradiation and compared with the catalytic behavior of g-CN. The degradation efficiency of Og-CN was higher than that of g-CN, the UV light irradiation was more effective than the visible light irradiation, and Og-CN was stable in three photocatalytic cycles with only 4% decrease. The kinetics analysis indicated that the photoreactions of both Og-CN and g-CN follow the zeroth-order reaction rather than the first-order reaction. On the photoreaction mechanism study, the active species-trapping experiment, which was performed using 2,2,6,6-tetramethyl-1-piperdinyloxy free radical, t-BuOH and KI for scavenging $O_{2}^{° -}$ , $O ° H$ and h⁺, respectively, confirmed the reactivity order of $O_{2}^{• -} > {}^{•}O H > h^{+}$ . Then the present research refers that Og-CN is a promising photocatalyst because it has extremely high total organic carbon removal (88 % of degraded 2,4-D after 120 min).

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 $O_{2}^{• -} >^{•} O H + h^{+}$ .

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 ( $O_{2}^{• -}$ ) 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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Photodegradation of pollutant pesticide by oxidized graphitic carbon nitride catalysts

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents and instruments

Characterization of photocatalysts

Conclusions

Declaration of Competing Interest

Acknowledgments

J. Hazard. Mater.

Sci. Total Environ.

Environ. Int.

J. Colloid Interface Sci.

Arab. J. Chem.

J. Colloid Interface Sci.

Sep. Purif. Technol.

Appl.Catal B: Environ.

J. Photochem. Photobiol. C Photochem. Rev.

Powder Technol.

J. Alloys Compd.

Appl. Catal. B: Environ.

Carbon

Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

Carbon

J. Colloid Interface Sci.

Appl. Catal. B: Environ.

J. Hazard. Mater.

Mater. Chem. Phys.

Chem. Eng. J.

J. Sauidi. Chem. Soc.

J. Mol. Catal. A.-Chem.

Powder Technol.

Chemical and toxicologic assessment of organic contaminants in surface water using passive samplers

J. Environ. Qual.

Agricultural insecticides threaten surface waters at the global scale

Proc. Natl. Acad. Sci. U. S. A.

2, 4-D toxicity: cause, effect and control

Aquatic. Environ. Toxicol.