Research paper
Enhanced visible-light photocatalysis of clofibric acid using graphitic carbon nitride modified by cerium oxide nanoparticles

https://doi.org/10.1016/j.jhazmat.2020.124204Get rights and content

Highlights

  • g-C3N4/CeO2 heterojunction exhibited superior activity over pure g-C3N4 and CeO2.

  • A nonradical mechanism dominated by photogenerated holes was revealed.

  • Possible pathway of clofibric acid oxidation were proposed.

  • The proposed system tends to destruct contaminants with electron withdrawing group.

Abstract

Recently, the emerging pharmaceutical micropollutants have become an environmental concern. Herein, we report an efficient elimination of clofibric acid (CA) using visible light-driven g-C3N4/CeO2 prepared by hydrothermal method. Among the catalysts with different compound ratios, g-C3N4/CeO2-3 (1.2 g g-C3N4 with 3 mmol Ce(NO3)3∙6H2O) exhibited the best photocatalytic performance. The effect of catalyst dosage was investigated and the optimal value was determined as 0.5 g L–1. The effect of initial pH (pH0) showed CA elimination decreased with increasing pH0. The underlying mechanism for CA oxidation was proposed based on synthetical analysis of photoluminescence emission spectra, transient photocurrent responses, electron paramagnetic resonance, chemical quenching experiments and band edge potential of g-C3N4 and CeO2. Photogenerated hole was primarily responsible for CA elimination while singlet oxygen played an auxiliary role. The products of CA oxidation were detected using liquid chromatography mass spectrometry (LC-MS) method and a possible pathway was put forward. Various organics were used as target contaminants to assess photocatalytic performance of g-C3N4/CeO2 heterojunction under acidic and alkaline pH conditions. The analysis of relationship between the oxidation peak potential (EOP) and the reaction rate constant indicated that photocatalysis using as prepared g-C3N4/CeO2-3 heterojunction is apt to oxidize contaminants with electron withdrawing group under acid condition.

Introduction

Over the last few decades, with the growth of the population and the improvement in quality of life, pharmaceuticals and personal care products (PPCPs) are becoming more and more widely used (Liu and Wong, 2013, Luo et al., 2014, Agerstrand et al., 2015, Al Balushi et al., 2018). PPCPs from industry or household use can be discharged into the environment through wastewater treatment plants, leading to the detection of various kinds of PPCPs in surface and ground water (Zhang et al., 2019a). This gives rise to an environmental concern since PPCPs are frequently occurred and cause risk to human beings and aquatic ecosystem (Zhang et al., 2019a, Evgenidou et al., 2015). Clofibric acid (CA), a bioactive metabolite of the blood lipid regulators clofibrate, etofibrate, and etofyllinclofibrate, is considered as a typical PPCP with potential risk to ecosystem and human health, and has been commonly detected in the aquatic environment (Emblidge and Delorenzo, 2006, Runnalls et al., 2007). Unfortunately, CA cannot be efficiently removed by conventional biological treatment due to its complex structure (Zhu et al., 2019). Therefore, quite a few physical and chemical treatment methods, such as adsorption (Hasan et al., 2012), ozone-based advanced oxidation processes (Wang et al., 2019a, Yao et al., 2018), electrochemical processes (Lin et al., 2014, Lin et al., 2018, Sirés et al., 2007) and photocatalysis (Doll and Frimmel, 2004, Chen et al., 2017, Gao et al., 2017) have been employed to deal with wastewater containing CA. Among the above methods, semiconductor photocatalysis is widely used owing to its low cost, high efficiency and environmental friendliness (Bora and Mewada, 2017, Matos et al., 2009).

Graphitic carbon nitride (g-C3N4), a typical metal-free photocatalyst, is particularly attractive thanks to its cheapness, favorable visible light response, non-toxicity and outstanding stability (Mamba and Mishra, 2016). Despite all the above advantages of g-C3N4, the photocatalysis performance is still insufficient due to less efficient response to visible light and higher recombination of the photogenerated electrons and holes (Liu et al., 2012, Chen et al., 2014a). Therefore, it is necessary to find ways to improve the photogenerated charge carrier separation of g-C3N4.

One of the attractive strategies for improving the photocatalysis performance of g-C3N4 is constructing heterojunctions by coupling g-C3N4 with other materials, which can broaden the photoresponsive range of g-C3N4 as well as facilitate charge separation and transfer (Xia et al., 2018). Furtherly, as a polymer, g-C3N4 has a flexible feature structure, which favors the formation of heterojunctions with close interconnection between g-C3N4 and various semiconductors (Cao et al., 2015). Consequently, a great number of semiconductors (e.g. TiO2, WO3, ZnS, CeO2, ZnFe2O4 and BiOCl) (Hu et al., 2020, Singh et al., 2019, Yan et al., 2019b, Humayun et al., 2019, Chen et al., 2014b, Wang et al., 2013) have been coupled with g-C3N4 to construct heterojunctions.

Among these semiconductors, cerium oxide (CeO2), an essential and inexpensive rare earth metal oxide, has been attracting intense attention in photocatalytic system owing to the great oxygen storage capacity (OSC) and valence change of Ce4+ to Ce3+ which could be beneficial to reduce the recombination of photogenerated charge carriers (Hu et al., 2011, Wu et al., 2019, Ren et al., 2018). CeO2 possesses a big band gap as its valence band (VB) is ~2.5 eV and conduction band (CB) around − 0.4 eV (Jourshabani et al., 2017), which is less negative than the CB of g-C3N4, indicating the coupling of g-C3N4 and CeO2 will provide a proper band alignments for accepting electrons of g-C3N4 and furtherly promote the charge transfer and separation through their heterojunctions (Humayun et al., 2019). So far, few studies on the heterojunction between g-C3N4 and CeO2 as photocatalyst for organics removal in water were reported. Humayun et al., (2019) employed g-C3N4/CeO2 to degrade 2,4-dichlorophenol (2,4-DCP) under visible light, and 57% of 10 mg L−1 2,4-DCP was removed during a 120 min photocatalytic reaction, and hydroxyl radical (OH) was the major oxidant involved in the degradation of 2,4-DCP. In the same photocatalytic system, Liu et al., (2020) reported that 95% removal of 500 μg L−1 sulfamethoxazole (SMX) could be achieved in a 30 min reaction, and superoxide radical (O2•–) was considered as the predominant reactive species responsible for SMX oxidation. It is known the initial pH value would change the surface charge of the catalyst and the species of organics, which might affect the contact of target contaminants with the catalyst and consequently the photocatalytic performance. Nevertheless, the effect of initial pH on the performance of photocatalytic system using C3N4/CeO2 heterojunction as catalyst has not been investigated in their studies (Humayun et al., 2019, Liu et al., 2020). More importantly, the abatement of various organic compounds with different functional groups has not been explored under acidic and alkaline pH conditions.

Herein, an efficient photocatalytic system for CA removal was reported using the visible light-driven g-C3N4/CeO2 heterojunction prepared by hydrothermal process. The composite particles were characterized using various techniques and the effects of important factors on CA elimination were studied. Furthermore, the reactive species in the photocatalytic process were verified by the electron paramagnetic resonance (EPR) detection and chemical quenching experiments. The products of CA oxidation were detected by liquid chromatography–mass spectrometry (LC–MS) method and a possible pathway was put forward. To investigate the photocatalytic performance of g-C3N4/CeO2 catalyst, the abatement of various organic contaminants was performed under acidic and alkaline pH conditions. Furtherly, the selective oxidation of organics with different functional groups by the Vis-g-C3N4/CeO2-3 process was elucidated based on the relationship between the oxidation peak potential (Eop) and the reaction rate constant (k).

Section snippets

Chemicals

Chemicals are described in Text S1 of Supplementary Information.

g-C3N4

Pure g-C3N4 samples was fabricated by directly calcining melamine. Typically, 5 g of melamine was put into a 50 mL ceramic crucible. The crucible was covered and placed in a muffle furnace, then heated to 550 °C at 5 °C min–1 of ramping rate and hold on for 4 h. The yellow g-C3N4 sample was obtained after natural cooling and crushing into fine powder (Zhang et al., 2017).

g-C3N4/CeO2 and CeO2

Hydrothermal method was used to prepare samples (Yang et al.,

Characterization

In Fig. 1a, the XRD results of g-C3N4 exhibit the typical diffraction peaks located at 13.1° and 27.6°, which certainly verify the presence of g-C3N4 phase (JCPDS No. 87-526) (Yan et al., 2019a). The XRD spectra of pure CeO2 and g-C3N4/CeO2-x composites display a typical XRD pattern of cubic CeO2 (JCPDS No. 78-0694) (Zang et al., 2017). Moreover, in g-C3N4/CeO2-x composites, weak peaks located on 27.6° indicate the existence of g-C3N4. Obviously, the intensity of g-C3N4 signal decreased

Conclusion

In this study, the g-C3N4/CeO2 heterojunction was successfully synthesized by hydrothermal method and its photocatalytic performance under visible light was evaluated via CA removal in water. A 98.5% of CA removal was achieved in the Vis-g-C3N4/CeO2-3 system during a 60 min reaction, while only 33.4% and 10.0% of CA was degraded in Vis-g-C3N4 and Vis-CeO2 systems, respectively. It indicates the visible-light photocatalysis performance of as prepared g-C3N4/CeO2 composite was considerably

CRediT authorship contribution statement

Heng Lin: Supervision, Writing - original draft, Resources, Writing - review & editing, Project administration, Funding acquisition. Xin Tang: Investigation, Methodology, Writing - original draft. Jing Wang: Investigation, Data curation. Qingyuan Zeng: Investigation, Data curation. Hanxiao Chen: Investigation, Data curation. Wei Ren: Investigation, Data curation. Jie Sun: Supervision, Resources. Hui Zhang: Conceptualization, Data curation, Funding acquisition, Project administration, Resources,

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

The financial support from National Natural Science Foundation of China (Grant No. 21806125), Postdoctoral Science Foundation of China (Grant No. 2016M602365) and the Fundamental Research Funds for the Central Universities of China (awarded at Wuhan University) is appreciated. FESEM and XPS analyses were partially supported by Large-Scale Instrument and Equipment Sharing Foundation of Wuhan University.

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