Article pubs.acs.org/JAFC
Photodegradation of Imidacloprid in Aqueous Solution by the MetalFree Catalyst Graphitic Carbon Nitride using an Energy-Saving Lamp Xue Liu, Xiaoli Wu, Zhen Long, Che Zhang, Yongqiang Ma,* Xianghong Hao, Hongyan Zhang,* and Canping Pan Department of Applied Chemistry, College of Science, China Agricultural University, No. 2 Yuanmingyuanxi Road, Beijing 100193, People’s Republic of China S Supporting Information *
ABSTRACT: Imidacloprid has become a research hotspot, due to its high toxicity to bees and other nontarget organisms. Photodegradation is a common method for removing imidacloprid in an aquatic environment. Traditional methods of pesticide photodegradation have generally been confined by many factors, such as response to only high-energy ultraviolet light. Herein, the visible-light-driven photocatalyst graphitic carbon nitride (g-C3N4) was applied to the photodegradation of imidacloprid. Visible-light illumination (λ >400 nm) resulted in nearly 90% substrate transformation in 5 h. With the illumination of an energysaving lamp, imidacloprid has also been mostly removed. 1-((6-chloropyridin-3-yl)methylhydroxy)imidazolidin-2-ylidene nitramide) and 4,5-dihydro-N-nitro-1-(3-pyridinylmethyl)-1H-imidazol-2-amine were the main photoproducts identified by LC-MS analysis. The photocatalytic mechanism has also been discussed. This work could provide new perspective that g-C3N4, as a good visible-light photocatalyst could be applied to the cleanup of environmental pesticide pollution. KEYWORDS: imidacloprid, graphitic carbon nitride, metal-free, photodegradation, visible light, mechanism
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INTRODUCTION Under typical field application conditions, only a small portion of the applied pesticide reaches its biological target1 and the remaining contaminates the land, the air, and particularly the water.2 It is generally known that less than 1% of the world’s fresh water resources are readily available for human use. Worse, it was found in 2012 that 748 million people still rely on unimproved drinking water sources.3 Thus, there is a pressing need for methods to remove pesticides and their derivatives from wastewater. Imidacloprid has become the most widely used neonicotinoid insecticide for over 140 agricultural crops.4 In 2011, about 11.4 thousand tons of products containing imidacloprid was applied in China alone.5 Aside from its benefits to agricultural production and pest control,6 it has become a threat to agroecosystems and human health.7 There is evidence that imidacloprid can be toxic to nontarget soil organisms and aquatic arthropods.8−10 Imidacloprid also could reduce bumblebee colony growth and queen production, which result in further worsening global malnutrition.4,11 The European Council decided in April 2013 to impose a temporary ban of imidacloprid.12 In addition, the mobility of imidacloprid in the environment and its high solubility and persistent stability in water have placed a damper on its natural degradation.13,14 Various treatment processes have been studied to address the pollution of wastewater by pesticides. Different advanced oxidation processes (AOPs) have been proven to be some of the most commonly used techniques.15,16 Photocatalytic oxidation (TiO2/UV) and photo-Fenton or Fenton-like systems (H2O2/UV/Fe3+) are the most widely used AOPs for pesticide degradation.17 However, there are also many drawbacks to these traditional strategies, such as the formation of iron sludge, acidic pH requirements,18 and high operating costs.19 Most of © 2015 American Chemical Society
these processes use metal catalysts, which may lead to secondary pollution.20 In addition, it is worth noting that only UV light (4% of the solar spectrum) can be used in those processes.21 From the standpoint of solar energy availability, visible light (more than 43% of the solar spectrum) active photocatalysts have great prospects in this field.17 Graphitic carbon nitride (g-C3N4) has been a novel “metalfree”, visible-light-induced semiconductor.22,23 Constructed from tri-s-triazine units and connected by planar amino groups, graphitic carbon nitride has been regarded as the most stable allotrope of carbon nitrides under ambient conditions, both on heating and in solutions of pH 1−14.23 The ideal g-C3N4 consists solely of an assembly of C−N bonds without electron localization in the π state.24 With a band gap of about 2.70 eV, g-C3N4 was found to show an intrinsic absorption in the visible region. Recently it has attracted intense attention for hydrogen and oxygen evolution via water splitting,25 photosynthesis,26 biosensing,27 detection of metal ions,28 and photocatalytic contaminant degradation.29 Consisting of carbon and nitrogen, g-C3N4 is thus environmentally friendly and sustainable30 and can be easily obtained by direct heat treatment of several precursors, such as melamine, urea, dicyandiamide, etc.31−33 Herein, g-C3N4 was used as a visible-light catalyst for the degradation of imidacloprid. Synthesized from low-cost and abundant urea, g-C 3 N 4 showed an exceptionally high imidacloprid degradation ratio of nearly 90% under visible light over 5 h. Furthermore, optimized supported g-C3N4 showed an exceptionally high imidacloprid degradation ratio Received: Revised: Accepted: Published: 4754
March 2, 2015 April 26, 2015 April 29, 2015 April 29, 2015 DOI: 10.1021/acs.jafc.5b01105 J. Agric. Food Chem. 2015, 63, 4754−4760
Article
Journal of Agricultural and Food Chemistry of 83.4% under real indoor illumination using an energy-saving lamp, which could promote the practical environmental application of the g-C3N4 photocatalyst. As another commonly used precursor, g-C3N4 synthesized by melamine was also prepared under identical conditions. It was found that different precursors have a significant impact on the photocatalysis efficiency of g-C3N4. Further discussion was also made on the basis of this result. More importantly, the possible mechanism of imidacloprid degradation was deduced on the basis of the interaction between chemical scavengers and different active species.
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MATERIALS AND METHODS
Materials. All reagents for synthesis and analysis were commercially available and were used without further treatments. Deionized water was used throughout the experiments. Preparation of g-C3N4 Photocatalyst. g-C3N4 materials were prepared by polymerization of urea and melamine molecules under high temperature. Typically, 10 g of precursor powder was placed in a crucible with a cover and then calcined at 550 °C for 4 h in a muffle furnace, at a heating rate of 10 °C min−1. After it was naturally cooled to room temperature, the resulting product was washed with 0.1 M HNO3 and deionized water several times. Then the pure products were dried at 105 °C for 10 h. The resultant yellow product was collected and milled into powder for further use. The as-prepared g-C3N4 samples were labeled as U-g-C3N4 (from urea) and M-g-C3N4 (from melamine), respectively. Characterization. Powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded with a Bruker D/MAX 2500 X-ray diffractometer. The microstructure was observed using a scanning electron microscope (SEM; FEI, Sirion 200, 10 kV) and transmission electron microscopy (TEM; FEI Tecnai G2F20, 200 kV). Fourier transform infrared (FT-IR) spectra of products were obtained in the range of 400−4000 cm−1 (FTIR, PerkinElmer 2000). X-ray photoelectron spectroscopy (XPS) measurements were done using an Escalab 250Xi XPS system. The specific surface areas of the samples were measured by nitrogen sorption at 77 K on a Micromeritics ASAP 2020 analyzer and calculated by the Brunauer− Emmett−Teller (BET) method. For photodegradation, a 300 W xenon lamp (CEL-HFX300W, Beijing) with cutoff filter (λ >400 nm) was used. An Agilent 1100 series high-performance liquid chromatograph (HPLC) with a UV detector was used to determine the concentrations of imidacloprid. An Agilent 1100 LC/MSD ion trap was employed to identify byproducts. Imidacloprid Photocatalytic Degradation. The photocatalytic degradation of imidacloprid was performed under visiblelight (λ >400 nm) irradiation. In a typical experiment, the 0.05 g of the catalyst was suspended in an aqueous solution of imidacloprid (100 mL, 20 mg L−1). The indoor light photocatalytic study was performed using an 8 W energy-saving lamp (commercially available) as the light source. A 0.1 g portion of the catalyst was added to 100 mL of imidacloprid solution (5 mg L−1) with constant stirring. All tests were carried out in a 150 mL reactor fitted with a circulating water system to maintain a constant temperature. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to create an absorption−desorption equilibrium between the catalyst and imidacloprid. To investigate the possible mechanisms involved in the photocatalytic degradation process, a series of experiments were carried out by adding individual scavengers to the photocatalytic reaction system.
Figure 1. XRD patterns of (a) U-g-C3N4 and (b) M-g-C3N4.
the intensity of the 27.7° peak varies between urea and melamine samples. A more intense peak suggests a decreased polymerization in M-g-C3N4 in comparison to U-g-C3N4.34 Figure S1 in the Supporting Information displays the FT-IR spectra of U-g-C3N4 and M-g-C3N4 samples. From the spectra, it can be seen that all photocatalysts have absorption bands at 810 cm−1, corresponding to the breathing mode of the triazine units. The peaks observed at 1200−1650 cm−1 are representative of the stretching and bending modes of nitrogen-containing heterocycles. The broad band in the range 3000−3700 cm−1 is assigned to stretching modes of −NH− groups and surfaceadsorbed water molecules.21,34,35 The morphology and microstructure of samples were investigated by SEM, as shown in Figure 2. Figure 2a shows that U-g-C3N4 consists of smooth paper-thin sheets, while M-gC3N4 (Figure 2b) is composed of large particles with a layer. The morphological difference between U-g-C3N4 and M-g-C3N4 is distinctive. The morphological difference between U-g-C3N4 and M-gC3N4 was also consistent with TEM images (Figure 3). As can be seen from the images, U-g-C3N4 exhibits two-dimensional smooth and thin layers with typical porouslike morphology. M-g-C3N4 exhibits a dense and thick sheetlike morphology with irregular shape. This fact indicates that different heteroatomcontaining precursors can result in different morphologies and microstructures of the resultant g-C3N4. The specific bonding and structure of the samples were investigated by X-ray photoelectron spectroscopy (XPS). Figure 4 depicts the high-resolution XPS spectra for the N 1s peak. The N 1s peak has been resolved to three main peaks at 398.7, 399.7, and 400.9 eV correspond to sp2 C−N−C, sp3 H−N−[C]3 and C−NHx, respectively.36 The 404.4 eV peak was weak and was attributed to the π excitations.37 C 1s and O 1s XPS spectra of the two samples (U-g-C3N4 and M-g-C3N4) are shown in Figure S3 of the Supporting Information. A residual O 1s peak could be ascribed to the tiny amount of O2 adsorbed on the surface of the synthetic product during the calcination process. The C 1s spectra show adventitious carbon species, the tertiary carbon, and a trace amount of C−O bonding at 284.9, 288.2, and 289.2 eV, respectively.35 The ratio of sp2 C−N−C bonds to the sum of sp3 H−N−[C]3 and C−NHx bonds is 2.84 in U-g-C3N4 and only 1.83 in M-gC3N4 (Tables S1 and S2 in the Supporting Information), which means the former has a lower proton concentration. It might be the key factors affecting the degradation efficiency.
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RESULTS AND DISCUSSION Characterization of g-C3N4 Samples. The X-ray diffraction (XRD) patterns of U-g-C3N4 and M-g-C3N4 are presented in Figure 1. The patterns all exhibit peaks at 13.0° (d = 0.676 nm) and 27.7° (d = 0.322 nm), corresponding to the approximate dimension of the tri-s-triazine unit and the interlayer stacking peak of aromatic systems, respectively. It is worth noting that 4755
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Figure 2. SEM images of (a) U-g-C3N4 and (b) M-g-C3N4.
Figure 5 shows the nitrogen adsorption−desorption isotherms for U-g-C3N4 and M-g-C3N4. Type IV hysteresis loops can be observed, indicating the presence of mesopores. The specific surface area of g-C3N4 greatly increased after acid treatment.38 The BET surface areas and pore volumes of U-gC3N4 (393.8 m2g−1 and 0.62 cm3 g−1) are slightly higher than those of M-g-C3N4 (303.9 m2g−1 and 0.44 cm3 g−1), indicating that the surface area is not the main reason for the photocatalytic activity, in accordance with the results reported in previous papers.34 Photocatalytic Degradation. The photocatalytic degradation of imidacloprid in the presence of photocatalyst in the dark could be negligible under the consistent experimental conditions (Figure 6). The photocatalytic activities of U-gC3N4 and M-g-C3N4 under visible-light irradiation are shown in Figure 7. After 5 h of visible-light irradiation, the photocatalytic degradation rate of U-g-C3N4 for imidacloprid is nearly 90%. The photocatalytic efficiency of M-g-C3N4 is only 42.9%. The photodegradation process has been fitted to pseudofirst-order models, in which the value of rate constant k is equal to the corresponding slope of the fitting line. As indicated in Figure 8, the values for the apparent reaction rate constant k are 0.3941 and 0.1179 h−1, respectively, for U-g-C3N4 and M-gC3N4. Even under irradiation with visible light (λ >400 nm), the U-g-C3N4 degradation of imidacloprid is more than twice as fast as that of M-g-C3N4. On the basis of the results, it is clear that U-g-C3N4 shows excellent performance in the photocatalytic degradation of imidacloprid in aqueous solution. Figure 9 compares the indoor-light-induced photocatalytic degradation of imidacloprid over U-g-C3N4 and M-g-C3N4. The
Figure 3. TEM images of (a) U-g-C3N4 and (b) M-g-C3N4.
activity of the M-g-C3N4 sample was quite poor, with almost no degradation effect of imidacloprid even after 24 h of indoorlight irradiation. However, the photocatalytic degradation rate of U-g-C3N4 for imidacloprid actually reached 83.4%. This result could imply that only relatively low energy is needed for U-g-C3N4 to start its photoactivity. This characteristic of U-g-C3N4 could provide new perspectives for promoting its large-scale applications in real, natural environments for the degradation of pollutants. Proposed Mechanism of g-C3N4 for Degradation of Imidacloprid. •OH, h+, and •O2− have been reported as major reactive species for photocatalytic oxidation. When the asprepared samples are irradiated under visible light, g-C3N4 samples can easily absorb visible light and be excited to produce photogenerated electron−hole pairs. After capture by oxygen adsorbed on the surface of g-C3N4, the photogenerated electrons can generate •O2−. Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), ammonium oxalate (AO), and potassium iodide (KI) were used as the hole scavengers.39−41 tert-butyl alcohol (TBA) and isopropyl alcohol (IPA) were used as hydroxyl free radical (•OH) scavengers.42,43 Benzoquinone (BQ) was used to remove superoxide radical (•O2−).44 As shown in Figure 10, the photocatalytic efficiencies of imidacloprid are 85.4% and 42.9% without scavengers for 4756
DOI: 10.1021/acs.jafc.5b01105 J. Agric. Food Chem. 2015, 63, 4754−4760
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Figure 6. Photocatalytic activities of U-g-C3N4 and M-g-C3N4 in the degradation of imidacloprid in the absence of light.
Figure 4. XPS spectra from samples of (a) U-g-C3N4 and (b) M-g-C3N4.
Figure 7. Photocatalytic activities of U-g-C3N4 and M-g-C3N4 in the degradation of imidacloprid under visible-light irradiation (λ >400 nm).
Figure 5. N2 adsorption−desorption isotherms of U-g-C 3N4 and M-g-C 3N 4.
U-g-C3N4 and M-g-C3N4, respectively. The photocatalytic activity of g-C3N4 was largely inhibited by the addition of hole capture. However, •OH scavengers have almost no effect on the degradation process. Therefore, the hole is the main oxidative species for g-C3N4 samples, rather than •OH. Another point is that when benzoquinone (BQ) was used to remove superoxide radical (•O2−), the photocatalytic activity of g-C3N4 was slightly increased. This is because the consumption of superoxide radical (•O2−) was accompanied by the loss of electrons (e−), which enhanced the separation efficiency of photoexcited electron−hole pairs.
Figure 8. Kinetic analysis for the determination of apparent rate constants of the reaction between imidacloprid and photocatalysts.
On the basis of the above analysis, the photocatalytic degradation mechanism of g-C3N4 for imidacloprid is different from that of traditional techniques. Aqueous imidacloprid 4757
DOI: 10.1021/acs.jafc.5b01105 J. Agric. Food Chem. 2015, 63, 4754−4760
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(2)
e− + O2 → O2•−
(3)
h+(g‐C3N4) + imidacloprid → degradation product (compound 1)
(4)
•
OH + imidacloprid → degradation product (compound 2)
(5)
imidacloprid aqueous solution revealed two main products. The formation of 4,5-dihydro-N-nitro-1-(3-pyridinylmethyl)1H-imidazol-2-amine (compound 1) could be due to the mechanisms of holes (h+). Addition of •OH to the imidazolidine ring allows the formation of 1-((6-chloropyridin-3-yl)methylhydroxy)imidazolidin-2-ylidene nitramide (compound 2)45 (Figure 11).
Figure 9. Indoor-light-induced photocatalytic degradation of imidacloprid over various photocatalysts.
Figure 11. Proposed degradation pathway for imidacloprid using gC3N4 as a visible-light photocatalyst.
To the best of our knowledge, compound 1 has not been reported previously, as a type of imidacloprid degradation product. The stability of photocatalysts is important for their practical application. The renewable catalytic activity of U-g-C3N4 photocatalysts was further investigated. As shown in Figure 12, after three cycles, the high photodegradation rate of imidacloprid
Figure 10. Effects of a series of scavengers on the photocatalytic efficiency of samples (dosage of scavengers 6 × 10−3 mol L−1, illumination time t = 5 h): (a) hole scavengers; (b) radical scavengers.
degradation may be clarified by a mechanistic explanation using eqs 1−5.
g‐C3N4 → e− + h+
(1)
Possible Degradation Pathway and Main Degradation Products. In this work, LC/MS analysis of the irradiated
Figure 12. Cycling runs for the photocatalytic degradation of imidacloprid in the presence of U-g-C3N4 under visible light. 4758
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(9) Alves, P. R. L.; Cardoso, E. J. B. N.; Martines, A. M.; Sousa, J. P.; Pasini, A. Earthworm ecotoxicological assessments of pesticides used to treat seeds under tropical conditions. Chemosphere 2013, 90, 2674− 2682. (10) Song, M. Y.; Stark, J. D.; Brown, J. J. Comparative toxicity of four insecticides, including imidacloprid and tebufenozide, to four aquatic arthropods. Environ. Toxicol. Chem. 1997, 16, 2494−2500. (11) Chaplin-Kramer, R.; Dombeck, E.; Gerber, J.; Knuth, K. A.; Mueller, N. D.; Mueller, M.; Ziv, G.; Klein, A. M. Global malnutrition overlaps with pollinator-dependent micronutrient production. Proc. R. Soc. London, Ser. B 2014, 281, 1799−1805. (12) L 139/12EN Official Journal of the European Union, 25.5.2013. (13) Malato, S.; Caceres, J. Degradation of imidacloprid in water by photo-Fenton and TiO2 photocatalysis at a solar pilot plant: a comparative study. Environ. Sci. Technol. 2001, 35, 4359−4366. (14) Main, A. R.; Headle, J. V.; Peru, K. M.; Michel, N. L.; Cessna, A. J.; Morrissey, C. A. Widespread use and frequent detection of neonicotinoid insecticides in wetlands of Canada’s prairie pothole region. PLoS One 2014, DOI: 10.1371/journal.pone.0092821. (15) Gonzalez-Olmos, R.; Martin, M. J.; Georgi, A.; Kopinke, F.; Oller, I.; Malato, S. Fe-zeolites as heterogeneous catalysts in solar Fenton-like reactions at neutral pH. Appl. Catal., B 2012, 125, 51−58. (16) Xi, B. J.; Verma, L. K.; Li, J.; Bhatia, C. S.; Danner, A. J.; Yang, H.; Zeng, H. C. TiO2 thin films prepared via adsorptive self-assembly for self-cleaning applications. ACS Appl. Mater. Interfaces 2012, 4, 1093−1102. (17) Aragay, G.; Pino, F.; Merkoçi, A. Nanomaterials for sensing and destroying pesticides. Chem. Rev. 2012, 112, 5317−5338. (18) Zhao, H. Y.; Wang, Y. J.; Wang, Y. B.; Cao, T. C.; Zhao, G. H. Electro-Fenton oxidation of pesticides with a novel Fe3O4@Fe2O3/ activated carbon aerogel cathode: high activity, wide pH range and catalytic mechanism. Appl. Catal., B 2012, 125, 120−127. (19) Chu, W.; Lau, T. K.; Fung, S. C. Effects of combined and sequential addition of dual oxidants (H2O2/S2O82‑) on the aqueous carbofuran photodegradation. J. Agric. Food Chem. 2006, 54, 10047− 10052. (20) Fenoll, J.; Hellín, P.; Martínez, C. M.; Flores, P.; Navarro, S. Semiconductor-sensitized photodegradation of s-triazine and chloroacetanilide herbicides in leaching water using TiO2 and ZnO as catalyst under natural sunlight. J. Photochem. Photobiol., A 2012, 238, 81−87. (21) Li, H. P.; Liu, J. Y.; Hou, W. G.; Du, N.; Zhang, R. J.; Tao, X. T. Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity. Appl. Catal., B 2014, 160−161, 89−97. (22) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76−80. (23) Tian, J. Q.; Ning, R.; Liu, Q.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. P. Three-dimensional porous supramolecular architecture from ultrathin g-C3N4 nanosheets and reduced graphene oxide: solution self-assembly construction and application as a highly efficient metalfree electrocatalyst for oxygen reduction reaction. ACS Appl. Mater. Interfaces 2014, 6, 1011−1017. (24) Zhu, J. J.; Xiao, P.; Li, H. L.; Carabineiro, S. A. C. Graphitic carbon nitride: synthesis, properties, and applications in catalysis. ACS Appl. Mater. Interfaces 2014, 6, 16449−16465. (25) Su, F. Z.; Mathew, S. C.; Lipner, G.; Fu, X. Z.; Antonietti, M.; Blechert, S.; Wang, X. C. mpg-C3N4-catalyzed selective oxidation of alcohols using O2 and visible light. J. Am. Chem. Soc. 2010, 132, 16299−16301. (26) Xu, J.; Zhang, L. W.; Shi, R.; Zhu, Y. F. Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis. J. Mater. Chem. A 2013, 1, 14766−14772. (27) Tian, J. Q.; Liu, Q.; Ge, C. G.; Xing, Z. C.; Asiri, A. M.; AlYoubi, A. O.; Sun, X. P. Ultrathin graphitic carbon nitride nanosheets: a low-cost, green, and highly efficient electrocatalyst toward the
could be maintained. There was no significant loss of activity with the extension of reaction time, even on irradiation for 15 h. Thus, the present work could promote a new strategy dealing with the pollution of wastewater by pesticides. For the synergy between the reduced proton concentration and the increased polymerization, U-g-C3N4 possessed significant photodegradation activity for imidacloprid under both visible- and indoor-light irradiation. The main reactive species responsible for g-C3N4 photodegradation was a photogenerated hole. As it is economically feasible and sustainable, U-g-C3N4 has promising prospects for environmental applications.
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ASSOCIATED CONTENT
S Supporting Information *
FT-IR spectra of M-g-C3N4 and U-g-C3N4 samples (Figure S1), C1s and O1s XPS spectra of g-C3N4 synthesized from different precursors (Figure S2), ratios of bonds within the N1s core level peak in different samples and their comparisons to the photodegradation activity (Table S1), and percentage breakdown of different bonds within the N1s spectrum (Table S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.5b01105.
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AUTHOR INFORMATION
Corresponding Authors
*Y.M.: e-mail,
[email protected]; tel, +86-01062733620; fax, +86-010-62733620. *H.Z.: e-mail,
[email protected]. Funding
This work was supported by the National Science Foundation for Fostering Talents in Basic Research of China (no. J1210064) and the National Instrumentation Program of China (No. 2013YQ510391). Notes
The authors declare no competing financial interest.
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REFERENCES
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NOTE ADDED AFTER ISSUE PUBLICATION The incorrect Supporting Information was attached to this Article on May 12, 2015. The corrected version was published on the Web on September 16, 2015. An Addition and Correction was published in volume 63, issue 38.
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DOI: 10.1021/acs.jafc.5b01105 J. Agric. Food Chem. 2015, 63, 4754−4760
