Elsevier

Chemosphere

Volume 300, August 2022, 134511
Chemosphere

Efficient organics heterogeneous degradation by spinel CuFe2O4 supported porous carbon nitride catalyst: Multiple electron transfer pathways for reactive oxygen species generation

https://doi.org/10.1016/j.chemosphere.2022.134511Get rights and content

Highlights

  • Spinel CuFe2O4 was successfully anchored on porous carbon nitride support.

  • Massive .•OH, SO4 and 1O2 were generated during PMS activation.

  • Three electron transfer pathways were responsible for ROS generation.

  • CuFe2O4@O–CN/PMS system presents strong oxidation ability for diverse organics.

  • CuFe2O4@O–CN catalyst exhibits good stability and reusability.

Abstract

Facilitating reactive oxygen species (ROS) generation is an effective way to promote the heterogeneous catalytic efficiency for organics removal. However, the metal leaching in metal-based catalysts and the low activity of non-metallic materials restrict ROS production. In this work, the purpose was achieved by loading a small amount of spinel CuFe2O4 onto porous carbon nitride substrate. The synthesized CuFe2O4@O–CN composite first to activate peroxymonosulfate (PMS), which produce a plenty of ROS (•OH, SO4 and 1O2) for organics removal, leading to highly oxidation for diverse organics. Through the comparative analysis of the surface composition before and after reaction, we found that the interface multi-electron transfer routs, including surface Cu(II)/Cu(I), Fe(III)/Fe(II) and their cross interaction, participated in the redox cycle, giving rise to the rapid and massive production of ROS, so that DMPO and TEMP were instantly oxidized in electron paramagnetic resonance (ESR) detection. Importantly, the carrier of porous O–CN, which acted as the electron transfer mediator, not only favors PMS adsorption via surface –OH, but also facilitates the conversion between different metal species. As a result, the CuFe2O4@O–CN/PMS system can remove 99.1% BPA and achieve 52.6% mineralization under optimized conditions. Thus, this study not only sheds light on the tailored design of heterogeneous catalyst for organics removal and elucidates the interfacial catalytic mechanisms for PMS activation.

Introduction

Peroxymonosulfate (PMS) activation has been exploited to achieve prospective application for organic contaminants removal (Shang et al., 2021). The asymmetric structure (HO–O–SO3-) of PMS is believed to be easily dissociated compared to symmetric peroxydisulfate (PDS) and H2O2 (Guo et al., 2021). That, the generated radicals (OH• and SO4) or non-radicals (1O2) after PMS cleavage can highly elaborate reactivity for organics degradation (Li et al., 2018; Xu et al., 2020). Notably, the most important of PMS activation is to find an ultra-efficient heterogeneous catalyst, which could be stable enough to consecutively produce reactive oxygen species (ROS) and then to attack pollutants. Normally, the more reactive species generated, the high catalytic efficiency could be achieved (Li et al., 2016). Thus, improvement of ROS generation in activating PMS is an effective way to facilitate the heterogeneous catalysis for organics removal.

Transition-metal-based catalysts have been verified to exhibit superior catalysis in PMS activation for ROS generation (Yu et al., 2018). Especially, spinel CuFe2O4 has attracted considerable attention recently owing to its excellent heterogenous catalytic performance and certain stability (Guan et al., 2013; Zhang et al., 2013). For example, Guan et al. found synthesized magnetic porous CuFe2O4 presented a notable PMS activation, resulting in over 98% of atrazine was degraded within 15 min as 1 mM PMS and 0.1 g/L CuFe2O4 addition (Guan et al., 2013). Zhang et al. prepared magnetically separable CuFe2O4 spinel to activate PMS, which showed an excellent activity for iopromide removal (Zhang et al., 2013). Nevertheless, some effort needs to be required for CuFe2O4-involved catalytic improvement. For one thing, the agglomeration of prepared CuFe2O4 blocked some active sites, and the sluggish reduction rate of surface metal ions restrained the catalysis (Zhang et al., 2016a). For another, the phenomenon of reactive metal ions leaching still occurred during activation. That not only limited the practical prospect and resulted in ambiguous PMS activation mechanism due to both heterogeneous and homogeneous catalytic contribution.

Against this problem, anchoring CuFe2O4 onto a certain non-metallic support is expected to reduce the dissolved metal ions (Lei et al., 2019; Wang et al., 2021a). In the process, PMS decomposition generally occurred on catalyst surface. The carrier could adsorb PMS first to form PMS-catalyst complex, and then the redox loop between PMS and metal sites contribute to ROS generation. However, the number of produced ROS was restrained due to the reduction of active sites when loaded on non-metallic substrate. Therefore, the selection of effective support becomes particularly crucial. It not only requires the carrier can anchor the spinel CuFe2O4 well, meanwhile itself can act as effective mediator for redox cycle.

Among various supporting materials, carbon nitride (g-C3N4) benefits from the superior 2D sheet structure and surface delocalized electrons, which is capable to be used as ideal substrate (Cao et al., 2015). Besides, recent studies proved that the oxygen-doped g-C3N4 (O–CN) could enhance PMS activation for organic pollutants degradation (Gao et al., 2018). The introduction of active oxygen-containing functional groups can enhance the PMS adsorption, and the modification of local electronic structures further promote redox circulation (Qiu et al., 2017). Moreover, the doped oxygen normally leads to pores generation, which can maximize the exposed active sites to some extent (Zhang et al., 2021). Our previous work verified the doped oxygen could act as a linkage to connect g-C3N4 and metal sites for metal-oxo-bridge construction on the surface (Chen et al., 2021b). Based on above consideration, it is acceptable to assume that anchoring CuFe2O4 on O–CN carrier can reduce the aggregation of CuFe2O4 and increase the stability of the catalyst. Importantly, large amounts of ROS might be generated theoretically during PMS activation, ascribe to the regulation of interfacial electrons for both CuFe2O4 and O–CN contribution.

To this end, this study designed a strategy to incorporate small amount of spinel CuFe2O4 on O–CN surface via a one-pot approach. The obtained CuFe2O4@O–CN composite was firstly employed to activate PMS for organics degradation. The morphology and structure of prepared CuFe2O4@O–CN were thoroughly investigated. The catalytic performance of CuFe2O4@O–CN/PMS system was assessed by bisphenol A (BPA) degradation. Several crucial operating conditions (such as catalyst dosage, PMS concentration and initial pH) were explored to optimize reaction conditions. Electron paramagnetic resonance (ESR) technique and quenching tests were conducted to identify the produced ROS. The changes of CuFe2O4@O–CN surface composition before and after reaction were particularly analyzed to reveal the interface mechanism. As a result, we found the developed CuFe2O4@O–CN/PMS system exhibits strong oxidation capability due to large amounts of ROS generation. Three electron transfer routes on CuFe2O4@O–CN surface were figured out to explain the excellent activity. This study provides a reference for metallic oxides-carbon catalyst fabrication, not only significantly improves the heterogeneous catalysis, but also opens a new insight for interfacial electron transfer exploration.

Section snippets

Chemicals

PMS (Oxone, 2KHSO5·KHSO4·K2SO4) purchased from Sigma-Aldrich (St. Louis, USA). Urea (AR), oxalic acid dihydrate (AR), Cu(NO3)2·3H2O (AR), Fe(NO3)3·9H2O (AR), Na2S2O3 (AR), HNO3 (AR) and NaOH (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd (China). l-histidine (≥99.5%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%), 2,2,6,6-tetramethyl-4-piperidinol (TEMP, >98%), and bisphenol A (BPA, >99%) were purchased from Aladdin Industrial Corporation (China). D2O was supplied by ANPEL

Morphology of CuFe2O4@O–CN composite

The surface morphology and microstructural details of the prepared CuFe2O4@O–CN was observed by SEM, TEM and HR-TEM investigations. As depicted in Fig. 1b, the SEM image shows that CuFe2O4@O–CN exhibits lamellar structure accompanied with intensive reticular intertwined, and no evident metal oxide particles are accumulated on the surface. This could be originated from the relatively less amounts of raw metal salts during calcination process. Meanwhile, the TEM image (Fig. 1c) presents the

Conclusions

In summary, this study successfully prepared a CuFe2O4@O–CN catalyst by one-pot calcination route, which can act as effective PMS activation material to degrade diverse organics in water. The results found that the developed CuFe2O4@O–CN/PMS system exhibits strong oxidation ability, which was ascribed to the efficient multi-electrons transfer routes on CuFe2O4@O–CN surface. The surface Cu(II)/Cu(I), Fe(III)/Fe(II) and their synergistic effect are responsible for the facilitated redox cycle. The

Author contribution statement

Ting Chen: Conceptualization, Formal analysis, Methodology, Visualization, Writing – original draft. Zhiliang Zhu: Project administration, Resources, Supervision, Writing – review & editing. Yue Wang: Methodology, Data curation. Hua Zhang: Characterization. Yanling Qiu: Methodology, Investigation. Daqiang Yin: Conceptualization, 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

This work was supported by the National Key Research and Development Project of China (No. 2021YFC3200805) and the Chinese Scholarship Council (Grant No. 202006260273). The authors thank the facilities and scientific technical assistance from the State Key Laboratory of Pollution Control and Resource Reuse in Tongji University and FACTS Lab in Nanyang Technological University.

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