Photo‐enhanced uranium recovery from spent fuel reprocessing wastewater via S‐scheme 2D/0D C3N5/Fe2O3 heterojunctions

Re‐extracting environmentally transportable hexavalent uranium from wastewater produced by spent fuel reprocessing using the photocatalytic technology is a crucial strategy to avoid uranium pollution and recover nuclear fuel strategic resources. Here, we have designed S‐scheme 2D/0D C3N5/Fe2O3 heterojunction photocatalysts based on the built‐in electric field and the energy band bending theory, and have further revealed the immobilization process of hexavalent uranium conversion into relatively insoluble tetravalent uranium in terms of thermodynamics and kinetics. According to the results, the hexavalent uranium removal and recovery ratios in wastewater are as high as 93.38% and 83.58%, respectively. Besides, C3N5/Fe2O3 heterojunctions also exhibit satisfactory catalytic activity and selectivity even in the presence of excessive impurity cations (including Na+, K+, Ca2+, Mg2+, Sr2+, and Eu3+) or various organics (such as xylene, tributylphosphate, pyridine, tannic acid, citric acid, and oxalic acid). It is believed that this work can provide a potential opportunity for S‐scheme heterojunction photocatalysts to re‐enrich uranium from spent fuel wastewater.


INTRODUCTION
5][16][17][18] Based on the previous work, 19 introducing additional nitrogen-rich moieties into C 3 N 4 can further enhance the visible light absorption efficiency due to the synergistic effect between the azo structure (-N = N-) and the π-conjugated systems.In addition, the photo-reductant with a more extended network structure can better resist the radiation damages caused by nuclear decay in wastewater environments. 20,21][24] Therefore, C 3 N 5 with a high N-ratio is more suitable for efficient U(VI) recovery due to the moderate bandgap and the higher E CB position.
Nevertheless, the inherent carrier re-combination for carbonitrides seriously limits the U(VI) recycling efficiency.C 3 N 5 -based photocatalysts with type-II heterostructure can guarantee the smooth transfer of photogenerated electrons, 25 but this configuration sacrifices the inherent strong redox ability. 26,27][30] The internal electric field, band bending, and Coulomb attraction are the collaborative driving forces that make the electrons generated by photo-oxidant inject the holes generated by photo-reductant, thus ensuring the stronger redox activity.According to the previous work, 31,32 the 2D/2D photocatalysts obtained by in situ coupling of 2D carbonitride with 2D Fe 2 O 3 acting as a photo-oxidant can significantly enhance the photocatalytic kinetics, which is determined by the short carrier migration path. 33,34owever, the complex environment of nuclear wastewater seems to be unfriendly to 2D/2D nanohybrids due to the radiation penetrability.If Fe 2 O 3 with a thicker scale can hybridize with 2D C 3 N 5 , covering the photo-reductant surface to ensure its reducing activity, it may be an effective strategy for recovering uranium species from spent fuel reprocessing wastewater.
Herein, we have synthesized a 2D/0D C 3 N 5 /Fe 2 O 3 photocatalyst and further constructed an S-scheme heterostructure.The carrier thermodynamics indicates that the photogenerated electrons excited by Fe 2 O 3 are forced to be injected into the valence band (VB) of C 3 N 5 continuously to consume its photogenerated holes, thus retaining the strong reduction ability of C 3 N 5 .Furthermore, the carrier dynamics of the C 3 N 5 /Fe 2 O 3 heterostructure are further analyzed by optimizing the heterojunction interface.According to the results, the removal and recovery ratios of U(VI) in wastewater can reach 93.38% and 83.58% in 90 min, respectively.Besides, owing to the photogenerated electrons of Fe 2 O 3 being transferred by the collaborative driving forces, the photogenerated holes accumulated on its E VB can better degrade the organics to purify wastewater.Therefore, although the impurity ions are excessive or various organics have existed, the C 3 N 5 /Fe 2 O 3 heterojunctions still have excellent photoreduction activity and durability.It is expected that this work could provide an important basis for the effective purification of nuclear wastewater and the recovery of strategic resources via designing S-scheme photocatalysts.

RESULTS AND DISCUSSION
The preparation process of S-scheme 2D/0D C 3 N 5 /Fe 2 O 3 photocatalysts is exhibited in Figure 1A.First, the C 3 N 5 orange powders were obtained by a continuous pyrolysishydrothermal-pyrolysis process.Then, the C 3 N 5 /Prussian blue precursors were prepared under ultrasound conditions.Finally, C 3 N 5 /Fe 2 O 3 photocatalysts were synthesized via the annealing treatment in the air (Figure S1).As suggested in Figure 1B 35 The C 1s (Figure 1G) has two peaks at 288.1 and 284.8 eV, which is caused by the sp 3 and sp 2 hybridized carbon. 36The sp 3 peak is attributed to the adventitious carbon in C 3 N 5 polymers and the sp 2 is related to N = C-N aromatic carbon, thus building a basic skeleton for C 3 N 5 .A shift corresponding to the binding energy of N = C-N is observed in the C 1s spectrum of the C 3 N 5 /Fe 2 O 3 hybrids.Similarly, the shift of binding energy is also found in the HRXPS spectrum of Fe 2p, as shown in Figure 1H.Compared with the pure Fe 2 O 3 , the Fe 2p 3/2 and Fe 2p 1/2 peaks of C 3 N 5 /Fe 2 O 3 shift about 0.7 and 0.41 eV to the lower binding energy, respectively, proving the formation of heterojunction interface after the hybridization.
We have directly characterized the direction of electron transfer in 2D/0D C 3 N 5 /Fe 2 O 3 heterojunctions by employing the in situ micro-scale Kelvin Probe Force Microscope (KPFM).In the AFM image (Figure 1I), the planar and raised area are considered as C 3 N 5 and Fe 2 O 3 , respectively, and the overall thickness of the sample is about 30 nm.By comparing the in situ KPFM results of the sample under dark and light conditions, the potential at The more heterojunction information of the C 3 N 5 /Fe 2 O 3 hybrids is further revealed by the density functional theory (DFT).The electron is aggregation on the C 3 N 5 monolayer and the deletion occurs at the Fe 2 O 3 side, which causes an electrostatic field in the out-of-plane directions (Figure 2A).Moreover, it is found that a total of 0.75 electrons are transferred from the Fe 2 O 3 side to the C 3 N 5 monolayer via Bader topological analysis based on the charge density distribution (Figure 2B).This internal electric field caused by electron transfer can effectively promote the photogenerated electron to migrate from Fe 2 O 3 to C 3 N 5 .Therefore, the vertical interface of the C 3 N 5 /Fe 2 O 3 heterostructure serves as an auxiliary booster to break up the recombination of charge carriers, thus enhancing the catalytic activity.As shown in Figure 2C, the work function is carefully investigated with the help of electrostatic potential calculations in the vertical direction of the heterostructure.And the obtained work function is 5.532 eV, indicating the significant surface charge polarization effect.On the other hand, the calculated electronic structure of the C 3 N 5 is a semiconductor with an HSE06 estimated band gap of 2.16 eV (Figure S6a).
The electronic structure shows an indirect band gap with 2.24 eV for Fe 2 O 3 , and the bottom of the CB and the top of the VB are located at the Γ point and the X point between the high symmetry points of T and L points, respectively (Figure S6b).The bandgap results of C 3 N 5 and Fe 2 O 3 are consistent with Tauc plots (Figure S6c).Also, the VB-XPS results suggest the E VB of C 3 N 5 and Fe 2 O 3 are 0.67 and 2.25 eV, respectively (Figure S7).Accordingly, we quantitatively provide the band coordinates of C 3 N 5 and Fe 2 O 3 before band contact, relative to NHE (Figure 2D).Due to the built-in electric field, the electron potential energy in C 3 N 5 increases gradually after band contact, forcing the band edge to bend upward, whereas Fe 2 O 3 does the opposite.As a result, the S-scheme heterostructure is built in the C 3 N 5 /Fe 2 O 3 hybrids with a stronger U(VI) photoreduction ability than that of type-II.
The electron spin response (ESR) spectra of active species captured by 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) are further studied under light illumination (Figure 2E,F).Herein, only the ⋅O 2 − signal is detected in C 3 N 5 since its E VB is not positive enough to oxidize H 2 O/-OH to ⋅OH, while Fe 2 O 3 has the opposite result.However, four peaks with a relative intensity of 1:1:1:1 (DMPO-⋅O 2 − ) and four characteristic peaks with a relative intensity of 1:2:2:1 (DMPO-⋅OH) have emerged in the C 3 N 5 /Fe 2 O 3 hybrids.These experimental results further confirm that the prepared C 3 N 5 /Fe 2 O 3 is the S-scheme electronic configuration rather than the type-II.In addition, the higher signal strength of C 3 N 5 /Fe 2 O 3 means that more electrons and holes are gathered on the CB of C 3 N 5 and the VB of Fe 2 O 3 , respectively, which is also consistent with the enhanced carrier separation efficiency.
During the photoreaction process, the separation kinetics of carriers can directly affect the U(VI) immobilization performance.Qualitatively, C 3 N 5 /Fe 2 O 3 -15 has the highest photocurrent response (Figure 3A) and the smallest electrochemical impedance (Figure 3B hole pairs is most effectively suppressed.Moreover, the τ 2 and corresponding percentage of C 3 N 5 /Fe 2 O 3 -15 are both the highest, revealing a high probability and priority of participation in the photocatalytic activity. 37The average lifetimes (τ A ) of the samples obtained from the final calculation are 2.253, 2.912, and 2.426 ns, respectively, indicating the optimal carrier separation kinetics for the C 3 N 5 /Fe 2 O 3 -15.
A 300 W Xe light source is used to measure the U(VI) recovery efficiency in uranium-containing wastewater with a concentration of 10 ppm.The C 3 N 5 /Fe 2 O 3 -15 has the highest U(VI) removal ratio of 93.38% (Figure S8), which is consistent with the above-mentioned kinetic characterization.Figure S9 indicates that the C 3 N 5 /Fe 2 O 3 -15 (still marked as C 3 N 5 /Fe 2 O 3 thereafter) heterojunction also exhibits an excellent removal ratio at a wide range of pH values.Besides, the U(VI) removal ratio is the highest when pH = 5, and the performance changes at different pH may be attributed to the forms of uranium species and the surface potential of the sample (Figure S10).Under dark conditions (Figure 4A), for Fe 2 O 3 , C 3 N 5, and C 3 N 5 /Fe 2 O 3 heterojunction, the U(VI) removal ratios calculated by Equation (S1) are 40.74%,50.12%, and 56.44%, respectively.When simulated sunlight is introduced (Figure 4B), the photocatalytic activity of Fe 2 O 3 , C 3 N 5, and C 3 N 5 /Fe 2 O 3 heterojunction is all improved: the removal ratios of uranium species reach 46.86%, 58.76%, and 93.38%, respectively (kinetic fitting can be found in Figure S11 and Table S1).SEM-EDX mapping exhibits that the uranium species had been immobilized on C 3 N 5 /Fe 2 O 3 (Figure S12).As revealed in Figure 4C, the U(IV) holds only ∼5.4% of the total in the dark process according to the integral area of HRXPS, indicating that the U(VI) removal in solution is mainly attributed to the adsorption reaction.However, the U(IV) contents on the sample increase sharply to ∼89.51% in 90 min, when the visible light is introduced.In other words, C 3 N 5 /Fe 2 O 3 photocatalysts can achieve a recycling target of over 83.5% (Equation (S2)).Moreover, the photocatalytic activity of the sample also remains satisfactory after five regenerations (Figure S13).Remarkably, these results are carried out without any sacrificial agents, suggesting that our elaborately designed S-scheme C 3 N 5 /Fe 2 O 3 heterojunction can capture uranium species more efficiently (Table S2).
Due to the complexity of the spent fuel reprocessing wastewater environment, we have fully considered the radiation characteristics, coexisting cations, and possible coexisting organics.As shown in Figure 4D, the removal ratio is still > 88% when the γ irradiation dose is increased from 0 to 100 kGy.In the presence of 10 times excess of competing cations including monovalent (Na + , K + ), divalent (Ca 2+ , Mg 2+ , and Sr 2+ ), approximately 9 ppm of U(VI) can be removed (Figure 4E).And even in the presence of a high concentration of Eu 3+ , the removal ratio toward U(VI) is 78.2%.Moreover, the reprocessing wastewater usually contains various organics, such as xylene, tributylphosphate, pyridine, tannic acid, etc., which can complicate the uranium environments, 38 thereby increasing the difficulty of uranium recovery.As suggested in Figure 4F, benefiting from the well-designed C 3 N 5 /Fe 2 O 3 heterostructure, the U(IV) removal ratio is still higher than 85%.As shown in Figure S14, tannic acid (TA) is a typical organic matter in nuclear wastewater, the degradation mechanism is also analyzed by UV-visible (UV-vis) spectroscopy and liquid chromatograph-mass spectrometer (LC-MS).The of small molecular organics with a mass-charge ratio (m/z) less than 100 (Table S3) indicates that C 3 N 5 /Fe 2 O 3 can successfully degrade TA into H 2 O and CO 2 .This result demonstrates that the strong oxidizing ability of the S-scheme C 3 N 5 /Fe 2 O 3 heterostructure has also been preserved, making it a promising candidate for the nuclear wastewater treatment.
Figure 5A shows the XRD pattern of  5C), indicating that the uranyl is preferentially considered to bind to C = N-C, which is also consistent with the shift of the deconvolution peak (N = C-N) in C 1s HRXPS (Figure S16).As demonstrated in Figure 5D, the Fe 2p characteristic peaks without difference after reaction prove that the organics in nuclear wastewater tend to con-sume the holes generated by Fe 2 O 3 rather than adsorb on the surface sites directly, thereby not polluting the sample or causing catalyst poisoning.
Based on the above analysis, here we have proposed a possible mechanism of photo-enhanced U(VI) recovery via the S-scheme C 3 N 5 /Fe 2 O 3 heterostructure (Figure 5E).When the sample is added to the wastewater system, the U(VI) is preferentially captured by the secondary C ═ N-C in C 3 N 5 (Equation ( 1)).Under the excitation of light, C 3 N 5 has a more extended π-conjugated system due to azo bonds, resulting in higher carrier concentration (Equation ( 2)), while the photogenerated electrons generated by Fe 2 O 3 combine with the photogenerated holes of C 3 N 5 via the internal electric field and Coulomb attraction (Equation ( 3)), thereby inhibiting the recombination of carriers in C 3 N 5 polymers (Equation ( 4)).Among them, the holes of Fe 2 O 3 are directly (or indirectly by the generation of ⋅OH) to degrade the organics (Equation ( 5)), while the electrons of C 3 N 5 bind to the adsorbed U(VI) on the surface, resulting in the uranium species immobilization (Equation ( 6)).Therefore, the carefully designed S-scheme heterojunction can effectively achieve uranium recovery from spent fuel wastewater and further purify nuclear wastewater (Equation ( 7)).

CONCLUSIONS
In summary, the 2D/0D S-scheme C 3 N 5 /Fe 2 O 3 photocatalysts used for U(VI) recovery from spent fuel wastewater are synthesized successfully.The charge transfer pathway of the C 3 N 5 /Fe 2 O 3 heterostructure not only realizes the spatial separation of carriers but also still retains its oxidative capacity.The U(VI) removal and recovery ratios by optimized C 3 N 5 /Fe 2 O 3 hybrids achieve as high as 93.38% and 83.58%, respectively, even under a low initial concentration of 10 ppm.In addition, the catalytic activity is hardly unaffected in complex environments (including ,C, C 3 N 5 presents 2D nanosheet shapes, while Fe 2 O 3 particles (0D) are octahedral.Besides, F I G U R E 1 (A) Schematic of the preparation of 2D/0D C 3 N 5 /Fe 2 O 3 hybrids.(B) Scanning electron microscopy (SEM), (C) transmission electron microscopy (TEM), (D) high-resolution TEM (HRTEM), and (E) TEM-energy dispersive X-ray spectrum (TEM-EDX) mapping of the sample.High-resolution X-ray photoelectron spectra (HRXPS) of C 3 N 5 /Fe 2 O 3 hybrids: (F) N 1s, (G) C 1s, and (H) Fe 2p.(I) Atomic force microscope (AFM) image and the thickness of C 3 N 5 /Fe 2 O 3 .(J) Surface photovoltage 3D distributions and (K) surface photovoltage curves of the sample under dark and visible light conditions.thesample has a rich mesoporous structure and a specific surface area of 29.82 m 2 ⋅g −1 (FigureS2), which provides enough space for U(VI) immobilization.HRTEM images of C 3 N 5 /Fe 2 O 3 exhibit a lattice spacing value of ∼0.24 nm (Figures1D and S3), corresponding to the (1 1 0) face of Fe 2 O 3 .As shown in Figure1E, TEM-EDX mapping proves the presence of C, N, Fe, and O elements, and more information about element contents can be found in FigureS4.Moreover, X-ray diffraction (XRD) patterns and Fourier transform infrared spectra (FTIR) can further confirm the successful fabrication of the C 3 N 5 /Fe 2 O 3 photocatalysts (Figure S5).To demonstrate the strong electronic coupling in the C 3 N 5 /Fe 2 O 3 heterostructure, the chemical states of each element are investigated by HRXPS.As shown in Figure 1F, the N 1s characteristic signal is deconvolved into two peaks at binding energies of 398.6 and 400.2 eV, corresponding to the tertiary N -(C) 3 /secondary C = N-C and the residual -NH 2 /bridging C-N = N-C.
Fe 2 O 3 (position-B) decreases by ∼5.4 mV, while the potential at C 3 N 5 (position-A) increases by ∼4.2 mV after light irradiation, which demonstrates that the electrons generated after photoexcitation are enriched on the surface of C 3 N 5 .The continuous directional electron migration path indicates the successful construction of heterojunction and the effective reduction of the carrier recombination.

C 3 N 5 /
Fe 2 O 3 photocatalysts after experiments.Compared with the initial, the (002) diffraction peak attributed to C 3 N 5 and all the (104), (110), and (116) diffraction peaks attributed to Fe 2 O 3 can be found in C 3 N 5 /Fe 2 O 3 heterojunctions after photoreaction, indicating that the structure has unchanged.In addition, the sample exhibits characteristic diffraction peaks corresponding to UO 2 species (PDF#23-7175), further suggesting that uranium species in the liquids have been successfully recovered.The N 1s characteristic signal of the C 3 N 5 /Fe 2 O 3 after photoreaction can also be deconvolved into two peaks (Figure 5B), corresponding to the tertiary N-(C) 3 /secondary C = N-C and the residual -NH 2 /bridging C-N = N-C.Among them, the azo (-N = N-) structure does not appear any difference, but the binding energy corresponding to the tertiary N-(C) 3 /secondary C = N-C has a deviation of about 0.2 eV.This phenomenon is caused by the in situ immobilization of the U(VI) species on the C 3 N 5 structures related to the heptazine aromatic ring, rather than on the azo structure.The evidence is given theoretically by DFT (Figure S15).The adsorption energies of U(VI) on the N-(C) 3 , -N = N-, and C = N-C sites of C 3 N 5 are −4.012eV, −4.991 eV, and −5.962 eV, respectively (Figure

F I G U R E 5
(A) XRD pattern of C 3 N 5 /Fe 2 O 3 after the reaction.HRXPS spectrum of C 3 N 5 /Fe 2 O 3 after the reaction: (B) N 1s and (D) Fe 2p.(C) The adsorption energy at different sites.(E) The mechanism schematic of photo-enhanced U(VI) recovery via the S-scheme C 3 N 5 /Fe 2 O 3 heterostructure.thehigh concentrations of interfering ions, various organics, and the high-dose γ irradiation).Therefore, this work could provide a feasible solution for the uranium pollution treatment of nuclear radioactive wastewater and further suggest a potential opportunity for heterojunction photocatalysts to re-enrich U(VI) from spent fuel reprocessing wastewater.A C K N O W L E D G M E N T SThis work is by the National Natural Science Foundation of China (21976148, 11705152), the National Key Research and Development Project of China (2016YFC1402500), the Long Shan Talent Project (18LZX304, 18LZXT04), the Project of State Key Laboratory of Environment-friendly Energy Materials (18zxhk04), and