MOF-modified C₃N₄ for efficient photo-induced removal of uranium under air without sacrificial agents

Abstract

Photocatalytic reduction of uranium from U(VI) to U(IV) is an effective method to remove uranium from wastewater, while it often requires anaerobic conditions and/or the addition of sacrificial agents, which hinders its further application. Herein, a MOF-modified C₃N₄ composite material was prepared for uranium removal under air atmosphere without the addition of sacrificial agents, achieving a notable uranium removal capacity of 1355 mg g⁻¹. The introduction of the MOF enhanced the band structure and the photoelectric properties of C₃N₄, making it able to generate and separate electrons and holes efficiently. The electrons were applied to reduce O₂ to form H₂O₂ and the holes could oxidize H₂O to O₂. The generated H₂O₂ could react with UO₂²⁺ to form (UO₂)O₂·2H₂O to realize the solidification of uranium under both air and N₂ atmospheres. This work may give a new direction to the design of photocatalysts for highly efficient uranium removal under air atmosphere without sacrificial agents.

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1 Introduction

As the major nuclear fuel, uranium is a kind of heavy metal with chemical toxicity and radioactive properties. It makes economic and environmental sense to recover and remove uranium from the wastewater generated during its production and use.^1–3 Besides, enrichment of uranium from wastewater for recycling is also an effective way to relieve the shortage of uranium resources. In recent years, many methods have been proposed, including extraction, membrane separation, adsorption, precipitation and so on, for the remediation of uranium-containing wastewater.^4–8 However, these methods still possess some drawbacks, such as the low removal capacity and complex procedures making uranium enrichment a critical task and a huge challenge.^8,9

The photocatalytic method has been widely used to remove uranium due to its high efficiency, ease of operation and designable catalysts.^10–13 Normally, uranium could be reduced from soluble U(VI) to insoluble U(IV) with proper photocatalysts such as TiO₂, g-C₃N₄, inorganic oxides, COFs,^13–24etc. Although the removal properties of these materials for uranium reduction were remarkable, there are still some problems that restrict its development. Some photocatalytic experiments must be performed under anaerobic conditions, since photogenerated electrons (e⁻) as well as the newly generated U(IV) are easily reduced by oxygen in water. Moreover, due to the poor separation efficiency of photogenerated electron–hole (e⁻–h⁺) pairs on photocatalysts, most systems required the addition of sacrificial agents (methanol, ethanol, etc.) to capture h⁺ so that the remaining e⁻ can effectively reduce U(VI).¹⁸ As a result, maintaining an anaerobic environment and adding sacrificial agents will increase costs and may bring secondary pollution, restricting the further application of the photocatalytic method. Therefore, it is urgently necessary to develop new photocatalysts for the removal of uranium under air atmosphere without the addition of sacrificial agents.^11,12,25

In our previous work, we found that uranyl ions can react with photogenerated H₂O₂ to form metastudtite under air atmosphere, which can achieve efficient solidification and separation of uranyl ions.²⁶ Therefore, the synthesis of photocatalysts that can efficiently produce H₂O₂ is expected to be an effective way for the removal of uranium. Among various catalysts, carbon nitride (C₃N₄) is widely used in the field of photocatalysis due to its medium band gap (∼2.7 eV), easy preparation, non-toxicity and low cost.^24,25 However, the weak visible light absorption and the easy recombination rate of electrons and holes limit the application of C₃N₄.^27–29 To overcome these deficiencies, a number of methods for modifying C₃N₄ for the generation of H₂O₂ have been used, including element doping, structural engineering, and heterojunction structures.^30–33 Metal organic frameworks (MOFs) have excellent chemical stability, special electronic structures and suitable band structures to assist C₃N₄ in the generation of H₂O₂ driven by visible light.^34,35 Shi et al.³⁶ proposed a thermally induced strategy to inhibit the recombination of MOF photogenerated carriers, thereby improving the optoelectronic performance of MOFs. It confirmed the unlimited potential of MOFs in the field of photocatalysis. In addition, the modification with MOFs can not only improve the utilization rate of visible light but also provide additional reaction sites for U(VI). As a result, it is of great significance to study the activity of MOF-modified C₃N₄ in photocatalytic removal of U(VI).

In this paper, we select Ni₃ (HHTP)₂, which is a 2D conductive hexagonal MOF with strong light response and excellent electron transfer ability,³⁷ for the modification of C₃N₄ with a facile method. The morphology, structure and optical absorption properties of the C₃N₄–MOF composite material were systematically characterized. It was found that the composites could oxidize H₂O to generate H₂O₂ efficiently without the addition of any sacrificial agents under visible light. Then the photocatalytic performance for uranium removal was tested under different conditions and the mechanism was also discussed deeply. This work verifies that the photocatalyst for the generation of H₂O₂ could be applied in removal of uranium, broadening the way to select photocatalysts and deepening the understanding of photocatalytic uranium removal. It also provides an effective photocatalyst for uranium removal without sacrificial agents under air atmosphere, which presents great advantages for further application.

2 Experimental section

2.1 Chemicals and reagents

Urea (CO(NH₂)₂) and melamine (C₃N₃(NH₂)₃) were obtained from Beijing Chemical Works. Ni(OAc)₂·4H₂O (nickel(II) acetate tetrahydrate) and HHTP (2,3,6,7,10,11-hexahydroxytriphenylene) were purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Uranyl nitrate hydrate (UO₂(NO₃)₂·6H₂O) was bought from a commercial supplier. All reagents were used without further purification.

2.2 Synthetic method

2.2.1 Synthesis of C₃N₄. Urea (1.5 g), melamine (1.5 g), NaOH (0.2 g) and KCl (7.5 g) were fully mixed, and then the mixture was put into a tube furnace and heated up to 550 °C for 4 h with a heating rate of 2.5 °C min⁻¹. After the reaction, the sample was washed with deionized water and dried in an oven for further application.

2.2.2 Synthesis of the MOF. Ni(OAc)₂·4H₂O (80 mg, 0.320 mmol) and HHTP (56 mg, 0.168 mmol) were dispersed in 32 mL of ultrapure water. After uniform ultrasonic dispersion, the solution mixture was placed in an autoclave and kept at 85 °C for 12 h. After the reaction, the product was obtained by centrifugation and washed with deionized water.

2.2.3 Synthesis of C₃N₄–MOF. Ni(OAc)₂·4H₂O (80 mg, 0.320 mmol), HHTP (56 mg, 0.168 mmol) and C₃N₄ (0.24 g) were put in 32 mL of ultrapure water. After being fully ultrasound dispersed, the solution mixture was placed in an autoclave and kept at 85 °C for 12 h. After the reaction, the product was obtained by centrifugation and washed with deionized water three times.

2.3 Photocatalytic experiments

In the photochemical experiment, 50 mL of U(VI) solution with a concentration of 0.4 mM was mixed with 30 mg C₃N₄–MOF (C₃N₄ and MOF) in a 100 mL reactor. The pH of the mixture was adjusted in the range of 3.0 ± 0.1 to 6.0 ± 0.1 by adding negligible volumes of HNO₃ and NaOH solutions (∼0.5 M). The suspension was magnetically stirred in the dark for 1 h, then irradiated under visible light with a 300 W xenon lamp equipped with a UV-cut filter (λ > 420 nm), and the reactor was kept at a constant temperature of 25 °C with a circulation pump. At intervals of 20 min, 1 mL of the suspension was filtered through a 0.22 μm membrane filter after being pipetted through a syringe. The concentration of uranyl ions left in the solution was determined on a spectrophotometer.

3 Results and discussion

3.1 Characterization of the photocatalysts

The morphology and structure of C₃N₄, MOF and the C₃N₄–MOF composite material were tested by SEM (Fig. 1a–c). Fig. 1a shows that C₃N₄ had a typical block structure with rough and wrinkled surfaces, and the MOF in Fig. 1b presented a rod-like morphology with different lengths and a width of about 50 nm. Fig. 1c shows that in C₃N₄–MOF, the MOF crossed and tightly connected in the structure of C₃N₄, indicating that the MOF has been composited with C₃N₄ successfully.

Fig. 1 The SEM images of (a) C₃N₄, (b) MOF and (c) C₃N₄–MOF. (d) XRD spectra of C₃N₄, MOF and C₃N₄–MOF.

Then XRD was performed to evaluate the crystallinity of C₃N₄, MOF and C₃N₄–MOF (Fig. 1d). It was showed that the MOF has several diffraction peaks at 9.3°, 13.9°, 16.2°, 18.1°, 19.3° and 26.6°, corresponding to the (020), (121), (022), (022), (032) and (004) lattice planes, respectively.^37,38 The crystal structure of C₃N₄–MOF shown in Fig. 1d contains the characteristic peaks of both substances, which further confirms the successful combination of the C₃N₄ and MOF. The nitrogen adsorption–desorption isotherms of C₃N₄, MOF and C₃N₄–MOF are shown in Fig. S1† and the BET surface area of each sample was 66.2 m² g⁻¹, 302.6 m² g⁻¹ and 81.1 m² g⁻¹, respectively.

Furthermore, XPS was performed to analyze the surface chemical state and the elemental composition of different materials. The survey spectrum of materials in Fig. S2a–c† displayed the existence of C, N, O and Ni elements. The atomic ratios of different elements in C₃N₄, MOF and C₃N₄–MOF are listed in Table S1.†Fig. 2 present the element spectra of C₃N₄–MOF. The C 1s spectra (Fig. 2a) displayed three peaks at 288.5 eV, 285.2 eV and 284.4 eV, which can be attributed to the N–C=N, C–N and C–C bonds,^39,40 respectively. Two peaks centered at 873.9 eV and 856.1 eV in Ni 2p spectra can be attributed to Ni 2p_1/2 and Ni 2p_3/2.^38,41 The O 1s signal was fitted with two peaks at 532.9 eV and 530.73 eV and they were O–H and Ni–O bonds, respectively.^38,41 Furthermore, the peaks at 401.1 eV, 399.9 eV and 398.9 eV of N 1s spectra can be indexed to the free amino group (C–N–H), tertiary nitrogen N–(C)3 and the sp²-hybridized aromatic N bonded to carbon atoms (C=N–C),^38,42 respectively. In summary, the morphologies and the chemical structures based on SEM, XRD and XPS characterization indicated the successful synthesis of C₃N₄–MOF composite materials.

Fig. 2 XPS spectra of C₃N₄–MOF: (a) C 1s, (b) Ni 2p, (c) O 1s and (d) N 1s.

3.2 Optical, photoelectrochemical, and electrochemical properties

To evaluate the activity of different materials, the optical, photoelectrochemical and electrochemical properties were investigated. First, the optical properties were demonstrated with the UV-vis absorption spectrum in Fig. 3a. It can be seen that the MOF exhibited excellent light absorption capacity in the visible light range. C₃N₄ had stronger absorption capacity in the UV-light range than that in the range of 450–800 nm, indicating poor visible light harvesting capabilities.^42,43 However C₃N₄–MOF can capture sunlight in a wider range than C₃N₄ and the absorption of light greater than 450 nm was mainly contributed by the MOF. The bandgap energy of C₃N₄ and C₃N₄–MOF was calculated to be 2.67 eV and 2.28 eV from the UV-vis absorption spectrum and a narrower band gap was obtained after compositing with the MOF (Fig. S3a–c†). Therefore, compared with pure C₃N₄, C₃N₄–MOF can obtain more sunlight energy to generate more electron–hole pairs and also to accelerate the separation of electrons and holes.

Fig. 3 (a) UV-vis absorption spectra, (b) transient photocurrent responses (TPR), (c) the electrochemical impedance spectroscopy (EIS) and (d) energy band structure of different materials.

In order to investigate the interfacial electron transfer rate of the materials, the transient photocurrent responses and electrochemical impedance spectra were recorded. Fig. 3b shows the transient photocurrent response (TPR) curves of materials with on–off cycles of intermittent irradiation. When light illuminated the materials, the photocurrent value enhanced rapidly, indicating that much more efficient separation of charge carriers and interfacial charge transfer were obtained with the materials. Obviously, the C₃N₄–MOF composite material possessed a stronger photogenerated current than pure C₃N₄ and MOF, indicating that more surface photogenerated charge existed on it. The interfacial charge transfer of C₃N₄, MOF, and C₃N₄–MOF was studied by electrochemical impedance spectroscopy (EIS).^44,45 The arc radius represents the internal resistance of charge transfer. The EIS results (Fig. 3c) indicated that C₃N₄–MOF had much smaller semicircles compared with C₃N₄ and MOF. It illustrated that C₃N₄–MOF exhibited a lower resistance, and more efficient charge separation, which was consistent with photocurrent results.

To further investigate the energy band structure of the materials, Mott–Schottky experiments were performed as shown in Fig. S3d–f.† The conduction band (CB) potential of different materials was −0.59 V (C₃N₄), −0.56 V (MOF) and −0.74 V (C₃N₄–MOF), which were all more negative than the reduction potential of O₂/H₂O₂ (+0.68 V, vs. RHE). From the band structure in Fig. 3d, the CB energy of C₃N₄ was more negative than that of the MOF, so the photogenerated electrons induced by C₃N₄ could be transferred to the CB of the MOF. The reason was that the MOF made a connection between the block structures of C₃N₄, thereby boosting charge transfer between them. The PL spectra of C₃N₄ and C₃N₄–MOF with an excitation wavelength of 380 nm are exhibited in Fig. S4.† It can be seen that C₃N₄ exhibits strong fluorescence intensity at 425 nm while the fluorescence intensity decreased significantly when combined with the MOF, which further confirmed the rapid transfer of photoelectrons from C₃N₄ to the MOF. However, the valence band (VB) of MOF was only 1.19 V as calculated, which was not enough to oxidize water. Therefore, the water oxidation process mainly occurs on C₃N₄. The results above revealed that C₃N₄–MOF had a suitable band structure to initiate both the oxygen reduction reaction and water oxidation reaction under light irradiation.

3.3 Photocatalytic removal of uranium without sacrificial agents

To evaluate the performance of different materials, the photocatalytic removal properties of U(VI) were investigated with and without sacrificial agents in Fig. 4a and b, respectively. At the adsorption stage, the uranium solution mixed with materials was stirred in the dark for 1 h to reach adsorption equilibrium. And then the visible light was employed for the removal of uranium at the photocatalytic stage. As shown in Fig. 4a and b, about 60% of U(VI) was adsorbed by the MOF and C₃N₄, whereas the adsorption ratio was only 25% for C₃N₄–MOF at the adsorption stage. Combining the morphology observation, it was because pure MOF and C₃N₄ were well distributed that would provide more sites for U(VI) adsorption. When C₃N₄ was combined with the MOF, C₃N₄–MOF formed aggregates, reducing the exposure of adsorption sites and leading to a decreased adsorption of U(VI).⁴⁶ With methanol acting as a sacrificial agent in Fig. 4a, the removal efficiency of U(VI) with C₃N₄ and C₃N₄–MOF reached up to 95% after 20 minutes of light exposure. However, the concentration of U(VI) remained basically unchanged with the MOF in the photocatalytic stage, which demonstrated that the MOF possessed little photocatalytic properties for uranium. In the system without sacrificial agents in Fig. 4b, there was a visible difference in the photocatalytic stage and about 90% of U(VI) was removed with C₃N₄–MOF, but no significant decrease of U(VI) was observed with C₃N₄ and the MOF. The results above revealed that C₃N₄–MOF worked still well in the photocatalytic removal of uranium without sacrificial agents.

Fig. 4 Photocatalytic removal rate of U(VI) with different materials in the presence of methanol (a) and in the absence of methanol (b). (c) The production rate of H₂O₂ of materials in the presence or absence of methanol. Under different atmospheric conditions, the generation rate of H₂O₂ catalyzed by C₃N₄–MOF (d) photocatalytic removal rate of U(VI) catalyzed by C₃N₄–MOF (e) and the pseudo-first-order rate constant (k) of U(VI) removal by C₃N₄–MOF (f).

We have previously verified that H₂O₂ played an important role in the photo-induced removal of uranium and it could react with uranyl ions to form metastudtite to realize the solidification of uranium.²⁶ Herein the potential of different materials in photocatalytic production of H₂O₂ was demonstrated in Fig. 4c in the presence or absence of sacrificial agents. The concentration of H₂O₂ was determined by TiSO₄ absorbance.⁴⁷ Due to the rapid electron–hole recombination, C₃N₄ showed the H₂O₂ production of 0.096 mmol h⁻¹ g⁻¹ in the absence of methanol. And MOF only showed the H₂O₂ evolution of 0.022 mmol h⁻¹ g⁻¹. Especially, the production rate of H₂O₂ catalyzed by C₃N₄–MOF attained 1.02 mmol h⁻¹ g⁻¹, which was about 10 times that of C₃N₄. With the addition of methanol as a sacrificial agent, the H₂O₂ generation rate of all materials was increased, which was consistent with the trend without methanol. Theoretically, H₂O₂ can be generated from H₂O and O₂ by semiconductor photocatalysis. The photogenerated valence band holes (VB h⁺) oxidized H₂O to generate O₂ (eqn (1)), while conduction band electrons (CB e⁻) promoted the double-electron reduction of O₂ to generate H₂O₂ (eqn (3)).^48,49 The photocatalytic H₂O₂ generation via oxygen reduction can be improved by the use of methanol because the oxidation of methanol is thermodynamically more favorable than the direct oxidation of water.⁵⁰ In the presence of methanol, the holes were employed to oxidize H₂O (eqn (1)) and methanol (eqn (2)), and the liberated electrons can be used for oxygen reduction (eqn (3)) to generate H₂O₂.^51,52

2H₂O + 2h⁺ ⇌ O₂ + 4H⁺ + 2e⁻ (+1.23 V vs. RHE) (1)

CH₃OH + ·OH + 2h⁺ → CO₂ + 5H⁺ + 3e⁻ (2)

O₂ + 2H⁺ + 2e⁻ ⇌ H₂O₂ (+0.68 V vs. RHE) (3)

As mentioned above, H₂O₂ was obtained by the reduction of O₂ and a series of experiments were carried out to identify the role of oxygen in the photocatalytic process. Fig. 4d illustrates the H₂O₂ production rate of C₃N₄–MOF under different atmospheric conditions and it was visible that the production rate followed the sequence of O₂> air >N₂, indicating that O₂ was an important participant component in this catalytic process. However, in N₂-saturated systems, the production of H₂O₂ was reduced compared to that of air but still detected, which could be attributed to the oxidation of water. Then the photocatalytic removal activity of uranium is shown in Fig. 4e with C₃N₄–MOF under different atmospheric conditions. And the removal rate was coincident with the H₂O₂ production rate.

Then the pseudo-first-order rate constant (k) of UO₂²⁺ reduction of C₃N₄–MOF under different atmospheric conditions is shown in Fig. 4f; the photocatalytic reaction rate constant (k) was denoted by applying a pseudo-first-order reaction:²⁷

−ln(I/I₀) = kt (4)

The photocatalytic reaction rate constant (k) was fitted and k for the O₂ saturated system was 0.038 min⁻¹, which was 5 times higher than that for N₂ saturated test system (0.007 min⁻¹). The results confirmed the importance of oxygen in the generation of H₂O₂ and the photocatalytic removal of uranium and it also verified that C₃N₄–MOF can reduce O₂ to H₂O₂ by a two-electron reaction without adding any sacrificial agents.

3.4 Comprehensive research on the photocatalytic performance

To evaluate the uranium removal activity of C₃N₄–MOF, different amounts of C₃N₄–MOF were first used to remove U(VI) with a concentration of 0.4 mM and pH = 5. As shown in Fig. 5a, it was found that 20 mg of C₃N₄–MOF removed 85% of U(VI) solution effectively, and when the dosage of the catalyst increased, the efficiency increased significantly.⁵³ It was attributed to the increased likelihood of U(VI) coming in contact with the active site of C₃N₄–MOF. Under different acid-base conditions, the efficiency of C₃N₄–MOF removal of U(VI) was investigated, as shown in Fig. 5b. It was clear that after 60 min of irradiation at pH = 5, 95% of U(VI) was eliminated, and the removal performance was quicker in weak acid systems. However, the removal efficiency of U(VI) under strong acid conditions was extremely low, which was due to the adsorption competition between U(VI) and H⁺ on the surface of the catalyst, as well as the unstable photocatalytic products of uranium under acid conditions, which might have redissolved into the solution.⁵⁴ Besides, at pH 3–6, the U(VI) ions mainly existed as UO₂²⁺, UO₂OH⁺, (UO₂)₂(OH)₂²⁺ and (UO₂)₃(OH)₄²⁺ in the aqueous solution. From the zeta potential results (Fig. S5†), the surface of C₃N₄–MOF was positively charged at pH = 3, and the electrostatic repulsion between the catalyst surface and U(VI) resulted in the weak adsorption of U(VI). As the pH increased, the surface charge of the catalyst gradually became negative, which could trap electrons on the surface of the catalyst due to the electrostatic attraction, thus greatly improving the removal efficiency of U(VI).⁵⁰ To assess the application of the C₃N₄–MOF in uranium removal, the elimination capacity was studied and calculated to be about 1355 mg g⁻¹ without sacrificial agents. The stability and reusability of photocatalysts are important factors affecting their photocatalytic performance. Fig. 5c shows the availability of C₃N₄–MOF over five photocatalytic cycles. After each photocatalytic reaction cycle, the photocatalyst was separated and collected from the suspension by centrifugation and reacted with 1.0 M HNO₃ solutions successively to remove the U(VI) deposits on the surface of C₃N₄–MOF. The photocatalytic activity of C₃N₄–MOF did not decrease significantly and the removal rate remained above 80% after five repeated cycles, indicating the good stability and reusability of C₃N₄–MOF. In order to test the practical application of C₃N₄–MOF, the effects of Cs, La, Zr, Ce, Cu, Zn, and Ba on the extraction performance of U(VI) were further studied (Fig. 5d). The percentage of U(VI) extracted from solutions containing multiple interfering ions showed a slight decrease compared to the absence of interfering ions, but the selectivity for uranium extraction was still excellent. The above experimental data indicated that the catalyst was feasible for the actual removal of uranium.

Fig. 5 Comprehensive research of C₃N₄–MOF for photocatalytic reduction of U(VI): (a) effect of dosage of the catalyst, (b) effect of initial solution pH, (c) effect of cycle performance, and (d) effect of ion selection performance.

3.5 Photocatalytic mechanism

To gain an insight into the interaction mechanism and transformation process of uranium, C₃N₄–MOF after photoreaction under air and N₂ atmosphere was characterized. Under an air atmosphere, the products presented agglomerated short rod-like crystals as shown by the SEM image in Fig. 6a. In addition, the EDS presented the uniform distributions of O and U. In Fig. 6b, the XRD patterns confirmed that U(VI) was converted to (UO₂)O₂·2H₂O. The XPS spectra of C₃N₄–MOF before and after photocatalytic experiments (Fig. S2c and d†) were recorded to verify the presence of uranium on the surface of the catalyst. It was obvious that a new characteristic peak appeared in the range of 380 eV to 395 eV, which was attributed to U 4f. In Fig. 6c, two peaks at 392.57 eV and 381.72 eV were investigated which corresponded to U 4f_5/2 and U 4f_7/2 of U(VI),^51,55 respectively. Besides, the photoproducts under a N₂ atmosphere were also characterized with XRD in Fig. S6.† As mentioned in our previous work and other references,^26,56,57 U(VI) preferred to form UO₂ under a N₂ atmosphere after photoreaction with catalysts. However in this work, it was interesting that U(VI) was transformed into (UO₂)O₂·2H₂O other than UO₂ (ref. 58) under a N₂ atmosphere, which may be attributed to the photocatalytic generation of H₂O₂ with C₃N₄–MOF. In N₂-saturated systems, the valence band potential of C₃N₄–MOF was 1.54 V, which was more positive than the oxidation potential of H₂O/O₂ (+1.23 V, vs. RHE). Therefore, it can oxidize H₂O to generate O₂, and then the conduction band electrons can promote the double-electron reduction of O₂ to generate H₂O₂. Large amounts of H₂O₂ were generated along with the irradiation time, leading to the generation of (UO₂)O₂·2H₂O by the direct reaction of UO₂²⁺ and H₂O₂. This result also demonstrated that U(VI) could be solidified to (UO₂)O₂·2H₂O under both air and N₂ atmospheres due to the high generation of H₂O₂ with C₃N₄–MOF. Furthermore, the FTIR spectrum of C₃N₄–MOF before and after the reaction was characterized (Fig. 6d). There were no significant changes in functional groups and chemical configurations on the catalyst, indicating that the catalyst was stable before and after the reaction.

Fig. 6 (a) SEM images of C₃N₄–MOF after the photoreaction and elemental distribution of O and U. (b) XRD patterns of C₃N₄–MOF after the photocatalytic reaction under air. (c) U 4f XPS spectrum of C₃N₄–MOF after the reaction, (d) FTIR spectrum of C₃N₄–MOF before and after the photoreaction.

Based on the above discussion, we proposed a possible mechanism for the photocatalysis of U(VI) by C₃N₄–MOF. The introduction of MOF narrowed the band gap of C₃N₄ and broadened the visible light response area, thereby improving the utilization efficiency of solar energy. Under visible-light irradiation, C₃N₄–MOF was excited by visible light, and the photogenerated charge was rapidly separated at the surface. The transfer of photoelectrons from C₃N₄ to the MOF was accelerated by chemical bonding, thereby effectively inhibiting the recombination of photogenerated electron–hole pairs. Due to the influx of extraneous electrons, there would be more free electrons on the MOF that were available for further oxygen reduction reactions. At the same time, the electron–hole separation efficiency of C₃N₄ was improved and more holes were employed to oxidize water. H₂O₂ was also produced by C₃N₄–MOF through the conversion of the dissolving O₂ to O₂˙⁻ radicals under visible light irradiation (eqn (5)). Large amounts of H₂O₂ were generated with the further increase of irradiation time, leading to the generation of (UO₂)O₂·2H₂O by the direct reaction of UO₂²⁺ and H₂O₂ (eqn (6)).⁵⁵

O₂ + e⁻ → O₂˙⁻ → H₂O₂ (5)

UO₂²⁺ + 2H₂O + H₂O₂ → (UO₂)O₂·2H₂O (s) + 2H⁺ (6)

4 Conclusions

In conclusion, a novel C₃N₄–MOF composite material was synthesized by a hydrothermal reaction. Its morphology, structural and photoelectric properties were characterized and the results indicated the successful synthesis of C₃N₄–MOF. It has excellent optical adsorption capacity and a narrower band gap compared with C₃N₄, which was attributed to the introduction of the MOF. The electrons and holes on C₃N₄–MOF were generated and separated efficiently under visible light, where O₂ could be reduced to H₂O₂ with electrons and holes could oxidize H₂O to O₂. The production rate of H₂O₂ was 10 times higher than that of C₃N₄. In the uranium removal experiments, C₃N₄–MOF presented satisfactory photocatalytic properties without adding any sacrificial agents under air. In weakly acidic uranium solution, more catalyst and a higher concentration of uranium were beneficial for a faster removal rate. A high uranium elimination capacity of 1355 mg g⁻¹ was achieved with C₃N₄–MOF and it also displayed good stability in the recycle tests and excellent selectivity in the presence of other metal ions. The mechanism was studied deeply with SEM, XRD, XPS, and FT-IR methods and the results revealed that the product of uranium was (UO₂)O₂·2H₂O under both air and N₂ atmospheres, which was formed by the reaction of uranyl ions and H₂O₂. This work provided a novel photocatalyst for uranium removal without any sacrificial agents under an air atmosphere, promoting the further application of the photocatalytic method. It also proved that the photocatalysts that were used to produce H₂O₂ were also effective for uranium removal, broadening the selection of catalysts and also providing a new photocatalytic way to remove uranium.

Author contributions

Lingyu Zhang: conceptualization, methodology, data curation, writing original draft. Yuhao Yang: methodology, investigation, data curation. Nan Zhao: validation, methodology. Shuang Liu: writing review & editing. Zhe Wang: validation, methodology, funding acquisition. Xiangke Wang: resources, writing review & editing. Yuexiang Lu: resources, supervision.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 22376059 and 21976104), Young Elite Scientists Sponsorship Program (2021QNRC001) of the China Association for Science and Technology, and Brain Pool program funded by the Ministry of Science and ICT through the National Research Foundation of Korea (grant number: NRF-2022H1D3A2A02051548).

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Footnote

† Electronic supplementary information (ESI) available: Figures for nitrogen adsorption–desorption isotherms, XPS survey spectrum, and band gaps calculated from the UV-vis absorption spectrum of different materials. See DOI: https://doi.org/10.1039/d3ta07710a

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