Self-cleaning isotype g-C3N4 heterojunction for efficient photocatalytic reduction of hexavalent uranium under visible light

Journal Pre-proof Self-cleaning isotype g-C3N4 heterojunction for efficient photocatalytic reduction of hexavalent uranium under visible light Zhanggao Le, Chuanbao Xiong, Junyuan Gong, Xi Wu, Tao Pan, Zhongsheng Chen, Zongbo Xie PII:

S0269-7491(19)33585-7

DOI:

https://doi.org/10.1016/j.envpol.2020.114070

Reference:

ENPO 114070

To appear in:

Environmental Pollution

Received Date: 3 July 2019 Revised Date:

19 January 2020

Accepted Date: 23 January 2020

Please cite this article as: Le, Z., Xiong, C., Gong, J., Wu, X., Pan, T., Chen, Z., Xie, Z., Self-cleaning isotype g-C3N4 heterojunction for efficient photocatalytic reduction of hexavalent uranium under visible light, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114070. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Self-cleaning Isotype g-C3N4 Heterojunction for Efficient Photocatalytic Reduction of

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Hexavalent Uranium under Visible Light

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Zhanggao Le,1,3 Chuanbao Xiong,2,3 Junyuan Gong,3 Xi Wu,1 Tao Pan,3 Zhongsheng Chen,1

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Zongbo Xie,1*

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Technology, No. 418, Guanglan avenue, Nanchang 330013, P. R. China

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2

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P.R. China

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3

State Key Laboratory of Nuclear Resources and Environment, East China University of

Anhui Nuclear Exploration Technology Central Institute, No. 8, Zhanghe road, Wuhu 241000,

School of nuclear science and engineering, East China University of Technology, No. 418,

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Guanglan avenue, Nanchang 330013, P. R. China

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*Corresponding author: [email protected] (Zongbo Xie)

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Abstract: Photocatalysis is a promising method to eliminate hexavalent uranium (U( )) and

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recycle it from wastewater. However, most of researched photocatalysts are metal-contained,

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inactive in visible light, and inconvenient to recycle, which unfortunately impedes the further

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utilization of photocatalytic technology in U( ) pollution treatment. Herein, g-C3N4 isotype

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heterojunction with interpenetrated tri-s-triazine structure (ipCN) was prepared by inserting urea

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into the interlayer of tri-s-triazine planes of thiourea-derived g-C3N4 and in-site thermal treating.

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The synthesized nanocomposites were used to convert soluble U( ) ions into U( ) sediment

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under visible light. Experimental and characterization results reveal that ipCN possess larger

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BET surface area, more negative-charged surface, higher U( ) adsorption capability, and more

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efficient mass diffusion and charges transfer properties. With these excellent characteristics,

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nearly 98% U( ) could be removed within 20 min over ipCN5:1 and 92% photoreduction

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efficiency could also be kept after 7 cycle uses, which were equal to or even superior than most

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reported metal-based photocatalysts. It is also proven that the configuration of U( ) and

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photogenerated ·O2- play a significant role in the photocatalytic U( ) reduction process, with

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(UO2)x(OH)y2x-y are more prone to be adsorbed and the photoinduced process of ·O2- will steal

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electrons from photocatalysts. Furthermore, with the self-generated ·O2- and H2O2, a green and

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facile regeneration process of photocatalysts was proposed

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scheme to extract U( ) from the perspectives of photocatalysts exploitation, photocatalytic

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reduction, and photocatalysts regeneration, which is meaningful for the sustainable U( )

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resource recovery and U( ) pollution purification.

This work provides a promising

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Keywords: g-C3N4; self-cleaning photocatalyst; photocatalytic U( ) reduction; facile and green

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recycle

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A metal-free photocatalyst exhibits excellent U( ) removal efficiency under visible light is

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developed and a green recycle technology for photocatalysts recovering with the aiding of O2 is

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proposed.

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1. Introduction

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Hexavalent uranium (U( )) is a typical poisonous and radioactive ions that exist in

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radioactive effluents with high solubility, which has been widely researched from both the

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perspective of resource recovery and environmental protection[1-3]. Among various proposed

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methods, photocatalysis has been deemed as the most eco-friendly and energy-saving

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technologies. Many photocatalysts, such as TiO2[S1-S2], Sn-doped In2S3[S3], Fe2O3/GO hybrids[S4],

2

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niobate/titanate heterojunctions[S5], or TiO2/Fe3O4/graphene composites[S6], etc., have shown

48

exciting photocatalytic activity in the field of photocatalytic U(VI) reduction. However,

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considering the inferior transfer ability of photo-generated charges, the insufficient response to

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visible light, and the instability and toxicity of many metal-based photocatalysts, further efforts

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are needed to construct metal-free photocatalysts with high efficiency. Furthermore, as many

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reported researches were focused on photocatalysts regeneration by acid or alkali washing or

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thermal treating[4-5], it is advisable to exploit a facile, energy-saving, and eco-friendly

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reactivation method for the avoiding of toxic chemicals, simplify of the complicated regeneration

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process, and protection of photocatalysts structure[6].

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Polymer graphitic carbon nitride (g-C3N4) is a metal-free, visible light responsive (up to 455

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nm) semiconductor with appropriate energy band potentials (CB≈-1.4 V; VB≈1.3 V) and

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excellent stability. It is reported that the conduction band potential of g-C3N4 is

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thermodynamically satisfied the requirements for photocatalytic U( ) reduction[7-8]. Our

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previous studies have proven that S, P, and B doped g-C3N4 with modified electronic structures

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and improved charge transfer ability can greatly enhance the photocatalytic reduction

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performance of U( ) [7, S7-S8]. However, as the traditional elements doping is preceded by direct

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thermal treating, many defects are inevitably introduced. These defects can become not only the

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catalytic activity centers but also the electron-hole recombination centers, thus, excessive defects

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will impose an adverse influence to the photocatalytic activity. From the perspective of material

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preparation, it is hard to keep the numbers of defects in an ideal constant, which limits the

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development of doping in photocatalysis.

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deteriorate the reduction ability[9-10]. The construction of heterojunctions by hybridizing g-C3N4

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with other catalytically reactive species such as metal oxides, metal halides, metal sulfides, and

Besides, the uncontrollable doping site also seriously

3

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various carbon-based materials is another effective method to enhance photocatalytic

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activities[S9-S12]. Nevertheless, the contact intensity of heterojunctions are usually a key parameter

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to evaluate the availability of photocatalysts, as it will influence the interfacial charge transfer

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and separation ability[11-13]. It is known that g-C3N4, prepared by calcining different precursors

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under various situations, will have disparate band gap structures which may result from the

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diversities in crystal phases, layer thickness, structural defects, or element doping[S13-S16].

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Therefore, it is a reasonable proposal to couple two types of g-C3N4 with well-matched band

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structures and similar physicochemical properties to construct close contact g-C3N4/g-C3N4

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isotype heterojunction. Wang et al.[14] took the first steps in this concept and successfully

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constructed the first isotype heterojunction with enhanced H2 yield, based on the slight band

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offsets between g-C3N4 and S doped g-C3N4. Dong et al.[15] followed this concept and prepared a

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newly isotype heterojunction via directly thermal polymerizing the mixture of urea and thiourea.

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This isotype heterojunction exhibits higher photocatalytic performance due to the efficient

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charge separation and transfer, resulting from the band potential differences between the two g-

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C3N4 components. Wu et al.[16] further found that the photocatalytic reaction rate is higher for the

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heterojunction synthesized by the two-step calcination method, comparing to that prepared by

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the one step calcination method. Zeng et al.[17] subsequently proposed that during the second

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thermal treatment, not only is the type II junction formed, but also the surface amino and cyano

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defects can be repaired. Recently, Li et al.[18] also identified that a built-in electric field can be

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formed in the interface between homophasic TiO2 with different particle dimensions, which

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provides an example for that the origination of the enhanced photocatalytic performance of g-

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C3N4 isotype heterojunction may not only results from the different band structure over different

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components, but also may influences by the various stacked thickness or particle size. However,

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none of researches has established the mutual relation between the synthesis process of isotype

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heterojunction and the photocatalytic reduction performance of U(VI).

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Inspired by the prior works, in this work, an interpenetrated g-C3N4 heterojunction (ipCN) was

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fabricated. This ipCN is composed of interpenetrated interfaces with close contact, similar to

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previously reported van der Waals heterojunctions. And the photocatalytic U(VI) reduction tests

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show that the remove degree of U( ) in water under visible light can reach 98%, and the

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efficiency is nearly unchanged even after five cycle uses with O2 serve as harmless regenerating

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reagent. To investigate the relationship between photocatalytic performance and the

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physicochemical properties of photocatalysts, the physical morphology, chemical composition,

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band structure, and charge utilization ability of ipCN were analyzed by using numerous

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characterization methods. Furthermore, the underlying mechanisms of U(VI) photoreduction

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over ipCN were revealed and the unexpected role of O2 in the photoreduction reaction and

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photocatalyst recovery were proposed.

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2. Materials and methods Experimental and characterization details are described at Appendix A. Supplementary data.

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3. Results and discussion

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3.1. Crystal phase and structural

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The XRD, FT-IR and XPS patterns of the uCN, tCN and ipCN nanocomposites are presented

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in Figure S2, S3, and S4, respectively. All these results hint that the uCN, tCN and ipCN

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preserve the typical tri-s-triazine structure of g-C3N4. Furthermore, the interaction between uCN

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and tCN over ipCN was also confirmed by the shift of XRD diffraction peaks position, the

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variation in infrared absorption peaks, and the changes among XPS binding energy, which is a

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favorable proof for the successful construction of isotype heterojunction. More detailed

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explanations of XRD, FT-IR and XPS can be found in Supplementary Data.

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The SEM micrographs of uCN, tCN and ipCN are seen in Figure S5. After inserting urea into

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the interlayers of tCN, the newly in-site thermal polymerized ipCN has more larger and smoother

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2D graphite-like plane. TEM reveals that ipCN is compound of porous core and surface

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surrounded 2D uCN layers (Figure 1A-B), which is completely different from the TEM results of

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typical g-C3N4, where uCN is more porous and tCN is more compact (Figure S6) . It is known

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that methanol is an excellent solvent with low surface tension (σmethanol: 20.14 mN·m-1),

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therefore, the methanol molecules can penetrate into the interlayers space of g-C3N4 planes

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during the ultrasonic treating and grinding[S23-S24]. Aided by the hydrogen bond between the

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methanol and urea, urea molecules can insert into the overlapped layer structure of tCN, which

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further react to produce uCN. On the one hand, the penetrating urea is beneficial to the

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fabrication of isotype heterojunction with interpenetrated interface for efficient charge transfer

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(Figure 1C-E); on the other hand, the released gases produced in the thermal polymerization

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process, such as CO2, NH3, or H2O, will significantly enlarge the interlayer spacing and facilitate

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the thermal oxidation reaction of tCN, resulting the formation of porous core. Furthermore, the

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adsorbed urea surrounded over tCN is favorable to form g-C3N4 nanolayer at the edges. This

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unique structure is beneficial for mass transfer and charges migration, which consequently

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promote the photocatalytic removal of U( ) over ipCN.

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In order to understand the microstructure and surface physicochemical properties of uCN, tCN

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and ipCN, N2 adsorption-desorption isotherm and BJH pore distribution are obtained and

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illustrated in Figure 1F-1G. All the obtained curves behave as representative type-IV isotherm

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plots with H3-hysteresis loops, suggesting the presence of slit-like mesopores in the

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photocatalysts. BET specific surface areas are 31 (ipCN1:5), 38 (ipCN1:1) and 67 m2·g-1 (ipCN5:1),

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which is 2.58, 3.17, and 5.58 times that of tCN (12 m2·g-1), respectively. The pore size calculated

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by the BJH method distributes at 17.98, 6.09, 19.14, 20.34 and 30.99 nm according to uCN, tCN,

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ipCN1:5, ipCN1:1 and ipCN5:1. The elaborate data of BET specific surface areas, pore volume, and

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average pore sizes for the other samples are listed in Table S1 of SI. Despite the fact that some of

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the concrete data suggest that the coupled photocatalysts are inferior to uCN, the ipCN samples

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still present increased surface area and enlarged open-pore sizes in comparison with individual

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uCN or tCN, which further promote U( ) adsorption, light capture, and photocatalytic reaction.

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3.2. Optical and electrical properties

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Figure 2A illustrates the UV-vis absorption spectra of samples. Apparently, the light-

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absorption edge of tCN is 464.7 nm, while the edge of uCN has an obvious blueshift (438.5 nm).

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The XPS findings have proven that no S was existed in tCN, therefore, the blueshift originates

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from the decreased layer stack and the resulting small size effect[S24]. Additionally, more O

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defects in the tri-s-triazine lattice of tCN also bring strengthened diffuse reflection intensities at

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the wavelengths between 325 and 650 nm[19-20]. For ipCN, the light adsorption intensity and

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wavelength coverage are elevated to approximate uCN, suggesting that the relationship between

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the small size effect and light adsorption abilities of photocatalyst were regulated. Therefore, the

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photocatalytic reduction efficiency was boosted. On the basis of the Kubelka-Munk function, the

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band gap energies (Eg) are calculated at 2.48 eV and 2.70 eV (Figure 2B). The tangential slopes

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of the Mott-Schottky plots (Figure 2C) further reveal that the conduction band potentials are -

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1.18 V and -1.42 V for uCN and tCN (vs. NHE, pH=7), respectively. As a result, the VB

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potentials can be deduced as 1.30 V and 1.28 V, corresponding to uCN and tCN, which is also in

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accordence of the results of XPS valence band spectra (Figure 2D). As a result, the matched band

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structure between uCN and tCN is conducive to construct type

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wherein the photogenerated electrons from the CB of tCN can transfer to the CB of uCN via the

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built-in electric field, and the holes migrate in the opposite direction (Figure 2E). Accordingly,

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the life of electrons is prolonged and prone to react with the adsorbed U( ) species.

charge transfer interface,

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EPR signals of uCN, tCN and ipCN are detected in Fig 2F, the g values of all the materials are

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situated at 2.0000, revealing that the N lone electrons over the π-conjugated plane of g-C3N4.

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Among these samples, ipCN5:1 exhibits the highest peak, suggesting the surface electron

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delocalization effect over the coupled heterojunction. Those unpaired electrons will enhance the

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surface electronegativity of ipCN and benefit the formation of hydrogen bonds between ipCN

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and (UO2)x(OH)y2x-y[21]. Interpenetrated structure between uCN and tCN can greatly promote

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charge transfer over the abundant interfaces, therefore the PL emissions are tested to show the

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recombination possibility of the photo-generated electrons and holes (Figure 2G). The emission

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peaks of uCN, tCN and ipCN are centered at 438 nm. Clearly, the radioluminescence intensity of

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ipCN is suppressed as a result of the promoted separation efficiency in the interpenetrated

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interface. Note that the tCN possesses lower PL intensity than uCN, which is associated with the

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O defects in the lattice structure, as evidenced by XPS. These defects will capture

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photogenerated charges and reduce their availability. Meanwhile, the abundant existing defects

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undesirably hamper charge transfer in the tri-s-triazine plane[22-23]. As Figure 2H shows, the tCN

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have larger electrical resistance in comparison with uCN, as reflected by a smaller semicircle in

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the EIS Nyquist patterns. This also proven that the O defects and bulked structure of tCN is not

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good for its charge transfer efficiency. Meanwhile, for ipCN, with a perfectly contacted interface

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and type

charge transfer mechanism, the radius of the circle pattern is further decreased. The

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enhanced charge separation and transfer properties are also revealed by the I-t curves (Figure 2I).

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Notably, the anodic photocurrent intensity over ipCN5:1 far exceeds that of all other samples,

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which proves the efficient utilization of photo-induced charges over ipCN5:1. The physical

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morphologies, surface chemical states, and electron structure characterizations confirm that this

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synthesized interpenetrated g-C3N4 heterojunction will be excellent for mass transfer, U( )

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adsorption, and the utilization of photogenerated charges.

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3.3. Photocatlytic reduction of U( )

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The photoreduction efficiency of U(VI) over ipCN was tested as presented in Figure 3A. It is

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apparent that the concentration of U(VI) remains almost constant under simulated light

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irradiation without adding photocatalysts, suggesting that the U(VI) is stable under visible light

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so that any further elimination of U(VI) stems from the influence of the photocatalysts. In the

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dark adsorption period, U(VI) was captured by the surface groups or surface electronegative sites

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over the photocatalysts, inducing U(VI) concentration decrease and gradually reach a constant.

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After reacting at dark about 120 min, the adsorption-desorption equilibrium is achieved over

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photocatalysts with 12.5% U(VI) be absorbed over ipCN5:1, which far exceeds tCN (3.5%) and

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uCN (6.5%). The apparent reaction rate constant (k) were calculated on the basis of pseudo first

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order kinetics, the k value of ipCN sample is 0.154 min-1, which is 1.73 and 22 times for that of

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uCN (0.089 min-1) and tCN (0.007 min-1) (Figure 3B). In order to find the underlying reasons of

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the improved adsorption capability, the surface charged states of the prepared materials were

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determined by zeta potential (Figure 3C). Accordingly, the isoelectric points (IEP) of tCN, uCN

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and ipCN are 3.87, 4.03 and 4.36, while the IEP for U(VI) in the solution is 5.62. It has been

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demonstrated that g-C3N4 featured by the Lewis base surface as a result of the N lone electrons

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in the conjugated tri-s-triazine and the surface -NH2 in accordance with EPR results[48]. As the

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BET areas over ipCN5:1 are increased, more alkaline conjugated surfaces and -NH2 will be

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exposed, resulting in the IEP of ipCN shifting to lower pH which is favorable for the surface

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adsorption of U(VI). More importantly, the Lewis base feature of g-C3N4 also leads to the

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desirability of photo-induced electrons from the exposed tri-s-triazine plane transfer to the

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adsorbed U(VI) species, due to the electron-donating characteristics of the Lewis base, which

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thus favors U(VI) reduction. Consequently, the photoreduction efficiency of U(VI) over ipCN5:1

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can reach 98% within 30 min, compared to uCN (80%) and tCN (13%). The photocatalytic

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activity of ipCN is superior than most investigated photocatalysts, such as TiO2, Fe2O3/GO,

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TiO2/Fe3O4/graphene, Nb/TiNFs, ZnO/rectorite[S25], Sn-doped In2S3 and S-doped g-C3N4 (Table

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S2).

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The photoreduction reaction of U(VI) over ipCN5:1 was confirmed by XPS. The chemical state

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of U(VI) on the surface of fresh ipCN, adsorbed ipCN, and photocatalytic used ipCN is shown in

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Figure 3E. After the dark adsorption, the obtained photocatalyst exhibit two representative XPS

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peaks of U 4f distributed in the regions of 378-383 eV and 389-394 eV; the former refers to U

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4f7/2 while the latter is its satellite peak. The U 4f7/2 peaks were further subdivided into two

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components centered at 380.4 and 381.6 eV, which correspond to U(VI) and reduced U( )[25]. It

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is apparent that there is only U(VI) present on the surface of adsorbed ipCN. However, the U( )

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peak appears after the photoreduction reaction. Combined with the observations of the

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contrastive photographs that the color of the suspension changes from ivory to gray after

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irradiating for 30 min (Figure 3D and Figure S7), it can be deduced that the soluble U(VI) can be

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rapidly converted into insoluble U( ) over the ipCN during photocatalytic reaction. Therefore,

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the elimination of the U(VI) pollution is achieved.

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The influence of experimental conditions such as atmosphere, pH and sacrificial agent were

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systematically investigated. In Figure 4A, the photocatalytic reduction of U( ) over ipCN under

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ambient air and N2 were compared. It is obvious that the concentration of U(VI) decreases

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sharply in the N2 environment while the elimination efficiency of U(VI) becomes insufficient

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under ambient indoor air, indicating that O2 will impede the photoreduction reaction. Notably,

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the initial photocatalytic removal rate of U( ) under the ambient air is sluggish, which is

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ascribed to the inevitably dissolved O2. Our previous researches have revealed that O2 can

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capture electrons from g-C3N4 to evolve reactive oxygen species (·O2-) under visible light, and

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the lowest reduction potential that is thermodynamically required for O2→·O2- transformation is

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-0.33 V[26-27], which is approximate to that for U(Ⅵ)→U(Ⅳ) (-0.264 V). Because the slight

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difference of reaction potential between ·O2- yield and U(Ⅵ) reduction, the dissolved O2 will

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compete with U(Ⅵ) to accept electrons from the surface of the photocatalysts. Therefore, for

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achieving the highest U(Ⅵ) removal efficiency, it is advisable to operate the photocatalytic

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system under degassing environment.

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The pH of solution can exerts great influence on photocatalytic reaction, which may attribute

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to the altered surface charge state or the completely different U(Ⅵ) forms under various pH as a

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result of the hydration and chelation interaction[28]. In this research, U(Ⅵ) elimination

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performances over samples under selected pH were tested (Figure 4B). The highest reduction

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rate over ipCN5:1 is achieved at pH=5.03, where 99% of U(Ⅵ) can be eliminated within 20 min.

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Adjusting the pH of the reaction system, the U(Ⅵ) removal efficiency will be significantly

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suppressed. Especially, while the pH decreases to 4.04, the photoreduction reaction of U(Ⅵ) over

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ipCN is thoroughly prohibited. To unearth the relationship between pH and photocatalytic

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performance, the remained U(VI) species at different pH were calculated by the standard

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thermodynamic database in Visual MINTEQ 3.0 (Figure S8). According to the simulation

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results, the prime U(VI) species can be divided into three classes: (i) pH<4: UO22+ is the main

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uranium species; (ii) 4
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solution. (iii) pH>7: (UO2)x(CO3)y2x-2y are the dominant ions. Considering that the photocatalytic

257

performance over ipCN is optimal at pH=4-6, the summary percentage of (UO2)x(OH)y2x-y is

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obtained (Figure S8B). It is apparent that the photoreduction performance is consistent with the

259

fluctuation of the total ratio of (UO2)x(OH)y2x-y, which grows to a peak value and gradual

260

declines, indicating that (UO2)x(OH)y2x-y is prone to accept electrons from the photocatalyst.

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UO22+ is found to occur with liner configuration that similar to CO2. Consequently, it is hard to

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adsorb UO22+ onto the negative charged surface of g-C3N4. While (UO2)x(CO3)y2x-2y behaves as

263

chelated state, it is also hard for (UO2)x(CO3)y2x-2y to bond with the Lewis base sites over the tri-

264

s-triazine plane, leading to the inferior photoreduction efficiency.

265

Kinetically, the transferal of photo-induced electrons (e-) to the adsorbed O2 undergoes a slow

266

reaction process (µs), nevertheless, the hole (h+) oxidation of the sacrificial agents is rapid

267

(ns)[S54-S55]. Therefore, adding suitable organic or inorganic salts to react with h+ can greatly

268

promote the life of the photogenerated e-. Considering this, triethanolamine was firstly selected

269

as sacrificial agent to capture h+. However, residual TEOA will compete with arsenazo III to

270

chelate with U( ), resulting the determination of U( ) concentration become difficult.

271

Therefore, methanol is chosen as the sacrificial agent for the system. In Figure 4C, the optimal

272

additive amount of methanol was considered. Obviously, without adding of methanol, little U( )

273

can be eliminated. After methanol is introduced, the photocatalytic reduction of U( ) over ipCN

274

is realized. Further increasing the adding amount of methanol to 1 mL, the highest photocatalytic

275

efficiency is achieved. In ordering to prove that CH3OH or its decomposed products will not

12

276

directly react with U( ), a series of control experiments was introduced as Figure S9A stated. It

277

is obvious that pure CH3OH or photocatalyst can not eliminate U( ), and the adding order of

278

CH3OH also shows inconspicuous effects on the removal efficiency, which firmly support the

279

fact that CH3OH is just play a role in the oxidation reaction. The theoretical proofs that reveal the

280

positive roles of CH3OH were provided by the I-t curves (Figure S9B), and this is a visual

281

method to unveil the role of additives in photocatalytic. As the anodic photocurrent intensities

282

directly reflect the charges separation and transfer properties, it is clearly that the addition of

283

CH3OH can greatly enhance the charges intensity over electrode by consuming the holes over

284

photocatalyst. These control experiments prove that the addition of CH3OH just accelerate the

285

relative photocatalytic oxidation process and will no exert direct influence on the photocatalytic

286

reduction process. In order to systematical investigate the photoreduction mechanism, 2.5 mL

287

CH3OH was considered as the optimal additive amount in following experiments.

288

Interpenetrated g-C3N4 (ipCN), physically mixed g-C3N4 (usCN), and chemically mixed g-

289

C3N4 were fabricated with an identical uCN/tCN ratio. And the photocatalytic removal

290

efficiencies of U(VI) over g-C3N4 that synthesizes by different strategies were tested (Figure

291

4D). After irradiating for 30 min, the removal ratio of U( ) over ipCN can reach 98%, while the

292

reduction ratio for usCN and UTCN are 87% and 65%, respectively. It is known that g-C3N4,

293

characterized by the tri-s-triazine lattice planes with periodic “crown-ether-like” vacancy, is

294

thermally stable at 600 , which provides a desirable platform to in-site construct interpenetrated

295

tri-s-triazine heterojunction. Facilitated by the methanol, urea molecules can penetrate into the

296

interlayer spaces of tCN or bound with the surface group. Therefore, after thermal treating, the

297

adsorbed urea in the interspaces of the tCN will in-site polymerize to produce uCN. Benefited

298

with the similar physicochemical properties, intimate contact between uCN and tCN can be

13

299

constructed, thus favoring the interfacial charge transfer and dissociation of photogenerated

300

electron-hole pairs. As for usCN, the negatively charged surface and the inefficient exfoliated

301

layered structure of both uCN and tCN in solution make it is very hard for uCN and tCN to form

302

close contact in the ultrasonic process. Furthermore, for UTCN, urea and thiourea may develop

303

to a homogeneous tri-s-triazine phase with unexpected physical morphology and electronic

304

structure. Accordingly, ipCN, with its interpenetrated interface and intimate contact, exhibits

305

superior photocatalytic performance.

306

From Figure 4A, it is concluded that the presence of O2 will exert negative effect to the

307

photoreduction reaction. To investigate the detailed role of O2, a group of control experiments

308

are implemented. As Figure 4E presenting, the four tests were initially reached the equilibrium of

309

photocatalytic reduction where little U( ) was left. Sequentially, N2 or O2 was introduced into

310

the system, with the gas flow rate was set at 1000 sccm to minimize the duration of the

311

experiment. According to the concentration of U( ), it can be inferred that U( ) is stable in both

312

the darkness and under irradiation with N2 protection. When O2 is bubbled into the system, U( )

313

can be detected in the supernatant. It is apparent that 96% U( ) can be transformed to U( ) in

314

the presence of O2 and light, while the concentration of U( ) decreases slightly under the O2

315

ambient without irradiation. In ordering to unearth the imprehensive role of O2, the oxygen-

316

trapping EPR spectra arise from the signals of DMPO-·OH and DMPO-·O2 were detected. As

317

Figure 5 and Figure S10 displayed, EPR signals of DMPO-·O2 with the characteristic peaks of

318

1:1:1:1 over samples were observed, and the characteristic peaks of DMPO-·O2 are increased

319

over combined ipCN. However, no distinct peak of DMPO-·OH with a peak intensity ratio of

320

1:2:2:1 can be found. Combined with the fact that the conduction band potential of ipCN is more

321

negative than the reduction potential of O2→·O2-, it can be concluded that the influence of O2 is

−

−

−

14

322

derived from the photogenerated reactive oxygen (·O2-) over ipCN. Consequently, ·O2- react with

323

H+ and electrons to generate H2O2 as proposed in our former research[S26-S27], which will further

324

re-oxidize the insoluble U( ) to form migratory U( ), which is directly due to its highly active

325

and oxidizing ability[S28-S29]. The evolution of H2O2 over ipCN was also proven by the designed

326

titration expriment where 0.2 mmol·L-1 KMnO4 can be discolored (Figure S11). Inspired by the

327

result that the evolution of ·O2- can oxidize the deposited U( ), a facile and environmentally

328

friendly scheme for reusing photocatalysts is proposed. As the scheme contained in Figure 5C

329

illustrated, the poisoned photocatalysts can be regenerated by a cycle photocatalytic reaction

330

process under ambient conditions. This finding suggests a simple, energy-saving, and eco-

331

friendly way to reactivate used photocatalysts without using poisonous ingredients (acid, alkali,

332

inorganic salt) or strict conditions (ultrasonic, thermal treatment), which is similar to a self-

333

cleaning process and could also be employed in other reactive oxygen species photocatalytic

334

systems.

335

The long-term stability of the photocatalyst is crucial for practical applications. In Figure 4F,

336

the recycling usability of ipCN5:1 was evaluated with aforementioned regeneration scheme. The

337

specific operation steps are as follows: Firstly, after each reaction, the used photocatalyst was

338

collected by vacuum filtration; Secondly, the collected photocatalyst was dispersed in water and

339

bubbled with O2 (100 sccm) and illumination for 1 h to remove the deposited U(IV). Finally, the

340

reactivated photocatalyst was washed by deionized water and dried in vacuum at 60

341

ready for another photocatalytic reaction (Figure 5C). Benefiting from the close coupling

342

between uCN and tCN, this metal-free photocatalyst has both high activity and excellent stability

343

in the photocatalytic reduction removal of U(VI). After 7 such recycled tests, 93%

344

photoreduction efficiency can still be achieved.

for 12 h to

15

345

The physical structure and chemical state of ipCN5:1 after 7 cycle use were also analyzed with

346

XRD, XPS, SEM and EDS (Figure S12, S13, and S14) to confirm the stability of the

347

photocatalyst. It is clear that the physical and chemical structure of ipCN5:1 are similar to the

348

initial state without any significant change. The little residual uranium existed on the surface of

349

ipCN5:1, as proved by the XPS and EDS results, suggests that the O2 oxidation treatment can

350

efficiently reactivate the photocatalysts without destroying the physicochemical structure of

351

photocatalyst. Additionally, the photocatalyst was stored in sealed centrifuge tubes or opened

352

containers for 0, 1, 4, 15 and 90 days to confirm the stability under different storage conditions

353

and time. It can be seen from Figure S15 that the photocatalytic removal efficiency of U( ) over

354

ipCN5:1 remains almost unchanged, which demonstrates that the ipCN is very stable in air. All

355

these results indicate the potential of ipCN performs as a promising photocatalyst for the

356

practical elimination of U( ).

357 358

4. Conclusion

359

In summary, metal-free isotype heterojunction ipCN with interpenetrated tri-s-triazine

360

networks was synthesized. Benefited from the intimate interface, enlarged BET area, and the

361

negatively charged surface, ipCN exhibits excellent photocatalytic reduction performance for

362

U( ), with 98% U( ) can be eliminated within 20 min; furthermore, ipCN presents superior

363

stability. The influences of experimental conditions are investigated: (i) The solution pH will

364

change the surface charged state of the photocatalyst and the existed U( ) species, therefore,

365

will greatly influence the adsorption and activation process of U( ) over ipCN. (ii) Sacrificial

366

agent is crucial for the photoreduction reaction as it can efficiently boost the dissociation of the

367

photogenerated charges. (iii)·O2- evolution reaction over photocatalyst serves as a double-edged

16

368

sword. On one hand, it hinders the uranium reduction by decreasing the utilization of the

369

photogenerated electrons. On the other hand, the photo-generated H2O2 yielded from O2 can be

370

employed as a green, cheap, and highly active oxidant to remove the deposited U( ) over the

371

used ipCN, realizing the self-clean process of the photocatalyst. This work provides a promising

372

scheme to achieve the removal of U( ) from wastewater, offering a possible solution for future

373

sustainable U( ) resource recovery and U( ) pollution purification.

374 375 376

Competing financial interest declaration The authors declare that they have no competing financial interests.

377 378 379

Acknowledgments This work was supported by the National Natural Science Foundation of China

380

(Nos.11765002,

21966003)

and

National

Natural

Science

Foundation

of

Jiangxi

381

(No.20181BAB203019). We also acknowledge the experimental testing provided by other

382

research groups in East China University of Technology.

383 384 385 386

Appendix A. Supplementary data All of the Supplementary data are available via the website of ScienceDirect: http://www.sciencedirect.com.

387 388

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Figure 1. (A-B) TEM images of ipCN5:1, and the EDS results of (C) C, (D) N, (E) O. (F) N2

478

adsorption-desorption isotherms, and (G) pore distribution curves of samples

479

22

480 481

Figure 2. (A) UV-vis diffuse reflectance spectra; (B) plots of (αhυ)1/2 vs. photon energy (hυ) of

482

uCN, tCN and ipCN. (C) electrochemical Mott-Schottky potentials, and (D) XPS valence band

483

potentials of uCN and tCN; (E) illustration of the band structure over ipCN. (F) EPR responses,

484

(G) PL emission peaks, (H) electrochemical impedance spectra (EIS), and (I) the corresponding

485

transient photocurrent response of samples

486

23

487 488

Figure 3. Photocatalytic reduction performance of U( ) over ipCN: (A) The variation of U( )

489

concentration vs. reaction time; (B) the fitted first-order reaction rate constant (k). MCat=1 g·L-1,

490

VCH3OH=2.5 mL, pH=5.32, N2 protect; (C) Zeta potential curves of uCN, tCN, ipCN5:1 and U( )

491

under different pH; (D) the color changes of the suspension during the photoreduction reaction

492

process and (E) XPS results of U 4f on the surface of ipCN5:1.

493

24

494 495

Figure 4. Studies of the photocatalytic U(VI) removal efficiency over ipCN5:1: (A) influence of

496

atmosphere, VCH3OH=2.5 mL, pH=5.03; (B) the effect of pH, VCH3OH=2.5 mL, N2 protect; (C) the

497

adding amount of CH3OH, pH=5.16, N2 protect; (D) photoreduction efficiency over

498

nanocomposites fabricated by different strategies, VCH3OH=2.5 mL, pH=5.28, N2 protect; (E) the

499

significant role of O2 in reactivating the used photocatalyst, VCH3OH=2.5 mL, pH=5.22; (F)

500

cycling test of ipCN5:1, VCH3OH = 2.5 mL, pH=5.30, N2 protect

501

502 503

Figure 5. EPR spectra of (A) DMPO-·O2- and (B) DMPO-·OH under irradiation for 8 min in

504

deionized water and CH3OH solution; (C) The proposed reactivating process and mechanism

25

Highlights:


Similar physicochemical properties benefit the tightness of interfacial contact.




The Lewis base surface promotes U( ) adsorption and reduction over ipCN.




pH, O2 and sacrificial agent will significant influence the photocatalytic process.




O2 can be used to realize photocatalyst reactivation and U( ) recovery.

All of the listed authors are builder of this work, and the finish of this paper is benefited from energy and time spent by all the participator. The main contributions of individuals are briefly listed as follows: Zhanggao Le: Ideas; formulation or evolution of overarching research goals and aims; Supervision; Oversight and leadership responsibility for the research activity planning and execution. Chuanbao Xiong: Development or design of methodology; Original draft preparation; Preparation of photocatalysts; Conducting a research and investigation process. Junyuang Gong: Maintain research data; Preparation of photocatalysts; Search for references；data collection. Xi Wu: XPS, XRD, SEM Characterization. Tao Pan: Testing the photocatalytic performance; data collection. Zhongsheng Chen: providing useful research methods; Check for grammatical errors. Zongbo Xie: Ideas; formulation or evolution of overarching research goals and aims; , Supervision; Management and coordination responsibility for the research activity planning and execution.

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in the manuscript submitted.