English

• The environment and energy are two key issues related to human life and daily production [1,2]. The expanded use of nuclear energy has reduced dependence on fossil fuels and greenhouse gas emissions [3]. However, the mining and utilization of uranium (U) generate massive quantities of U-containing wastewater, which may cause serious pollution if discharged without treatment [4,5]. And once U-containing wastewater is discharged into the aquatic environment without proper treatment, the released U may eventually be stored in the food chain [6]. U toxicity impairs the basic functions of the lungs and kidneys, which may lead to death [7]. Therefore, further research is needed to successfully eliminate this contaminant from wastewater. In addition, the widespread use of pesticides, antibiotics and pharmaceuticals, and the illegal discharge of pollutants have resulted in water bodies containing many organic pollutants, while uranyl ion (UO₂²⁺, a typical species of U in water) tends to complex/chelate with organic pollutants in the water bodies, making it even more difficult to remove U containing wastewater completely [8,9]. However, organics act as electron carriers and have considerable chemical energy potentials [10]. Therefore, it is worthwhile to explore rational systems for simultaneous U recovery and organic removal while utilizing energy and electrons.

  Current U remediation strategies, including chemical precipitation [11], ion exchange [12], biological treatment [13], adsorption [14,15] and photocatalysis [16], have showed great potential in remediating radioactive wastewater. However, some inherent limitations make effective U removal still difficult due to inefficiencies, potential secondary contamination and increased operating costs. In recent years, photoelectrochemical (PEC) reduction has attracted attentions as a promising method for metal recovery because of its highly efficient charge transfer property and spatially separated oxidation/reduction reactions [17-20]. In PEC system, photogenerated holes exhibits significant efficacy in oxidative degradation of organic matter [17]. Meanwhile, positively charged ions may be directed to the cathode under the influence of an electric field for effective reduction. For example, Wang et al. [21] demonstrated the efficient removal and recovery of Hg²⁺ during PEC processes by constructing WO₃/Ag Schottky heterojunctions as electrodes, facilitating electric field enhancement and hot electron transfer. Li et al. [22] achieved 100% reduction and oxidation efficiency of simulated wastewater containing Cr(Ⅵ) and organic pollutants using a photocathode consisting of SnS₂ atomic layers grown on TiO₂ nanowire arrays to construct a PEC system. However, the use of PEC for the environmentally friendly treatment of complex wastewater containing radioactivity is rarely reported.

  The key to PEC reduction is to find suitable electrode materials and develop electrocatalytic electrodes with high electron transfer efficiency and sufficient active sites to break through the concentration limit of U complexes. Meanwhile, the electrocatalytic electrode can overcome the reaction overpotential and further reduce the energy consumption for UO₂²⁺ reduction. In this study, we developed a self-driven PEC (SD-PEC) method using TiO₂ nanorod array (TNR) backed with silicon cells (SSC) as photoanode, and carboxylated carbon nanotube modified carbon felt (CCNT/CF) as cathode, in which the photoanode generates high-energy holes/electrons pairs, extracts electrons from the organics, allows oxidative degradation of the organics, and facilitates electron transfer from photoanode to cathode in the presence of sunlight. The cathode is the most critical part in the U recovery process as U is deposited on the cathode in the presence of an electric field. Carbon nanotubes (CNTs) are nanomaterials with excellent electrical, thermal, chemical, mechanical and biological properties [18,23]. The carboxyl groups in CNTs affect their surface energy and increase their hydrophilicity and chemical reactivity. Therefore, CCNT/CF was used as cathode for efficient recovery of UO₂²⁺. The synthesis and characterization of TNR and CCNT/CF, assembly and evaluation of the SD-PEC system, reagents used in the experiments, and sample presentation are shown in Fig. 1a, Texts S1-S4 and Fig. S1 (Supporting information).

  Figure 1

  

  Figure 1.  (a) Illustration of the synthesis of CCNT/CF. (b) Contact angle of the CF and CCNT/CF surface. (c) Transmission electron microscopy (TEM) image of CCNT. (d) FTIR spectrum, and (e) O 1s spectrum of CCNT/CF cathodes. (f) EIS curves and (g) CV curves of CF and CCNT/CF cathodes.

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  The prepared TNR with a thickness of ~2.76 µm grown directly on fluorine-doped tin oxide (FTO) glass (Fig. S2 in Supporting information) and showed the characteristic peak of rutile TiO₂ (002) facet (Fig. S3 in Supporting information), indicating that the TiO₂ nanorods are single crystalline and have a highly oriented morphology with preferred orientations [24]. X-ray photoelectron spectroscopy (XPS) analysis further revealed that the product on the FTO was rutile TiO₂ (Figs. S4a–c in Supporting information), and the UV–vis absorbance spectra confirmed the TNR with a good adsorption capacity in the UV region (< 400 nm) (Fig. S5a in Supporting information). Under AM 1.5 (simulated sunlight) illumination, the TNR exhibited a satisfactory photocurrent response with a stable photocurrent of ~2.00 mA/cm² at 0.8 V vs. Pt (Figs. S5b and c in Supporting information). Additionally, the SSC has excellent power generation performance (Fig. S5d in Supporting information). These results indicate that the photoanode was successfully prepared and possessed excellent PEC properties.

  Unlike the smooth surface of CF (Figs. S6a and b in Supporting information), the CCNT/CF had a rough surface with dispersed CCNT deposited on the fiber, suggesting that the CF provided a suitable support for CCNT (Figs. S6c and d in Supporting information). In addition, compared to CF, CCNT/CF has a significantly higher hydrophilicity (contact angle drops rapidly to 0 within 20 ms, Fig. 1b), and the structure of the attached CCNT was not damaged and preserved its nanotubular structure and crystalline shape (Fig. 1c). The Fourier transform infrared spectroscopy (FTIR) showed a broad and strong stretching peak observed at 3432 cm^-1 that could be attributed to the O–H stretching vibration (Fig. 1d), indicating that a significant amount of O–H remains on CCNT/CF [25]. Two peaks were observed at 1585 and 1104 cm^-1, corresponding to the C═O and C–O-C stretching vibrations, respectively, which further confirmed by the XPS results (Figs. S7a and b in Supporting information) [25]. Similarly, O 1s showed the existence of C═O/C–O and O–H, further indicating that CCNT/CF contains many carboxyl functional groups (Fig. 1e) [26]. Furthermore, the Raman spectrum revealed a high R-value of 1.38 for CCNT/CF (Fig. S8 in Supporting information), which could be attributed to the presence of surface functional groups and defects in the CCNT [27].

  The electrochemical impedance spectroscopy (EIS) of CCNT/CF exhibited a reduced semicircle (Fig. 1f), indicating a decreased charge transfer resistance at the cathode-electrolyte interface compared to CF [28]. Besides, the cyclic voltammetry (CV) curves displayed symmetric shuttle shapes without discernible oxidation/reduction peaks (Fig. 1g), implying that the incorporation of CCNT effectively enhances the electrocatalytic kinetics and induces double-layer capacitive behavior in CCNT/CF [29]. Specially, the signal becomes sharp at −1.2 V, which should be ascribed to the reason that the electrode undergoes water splitting at this potential [30].

  To evaluate the feasibility of the SD-PEC||CCNT/CF system for UO₂²⁺ removal, we firstly performed the degradation experiments with a solution containing 10 mg/L of UO₂²⁺. The SD-PEC||CCNT/CF system remarkably enhanced the removal rate (~100%) of UO₂²⁺compared with the SD-PEC||CF system, with an increasing degradation rate as the loading amount increased. A maximum value (k_obs 0.248 min^-1) was reached at 0.03 g CCNT loading amount (area 8 cm², CCNT/CF-3, abbreviation CCNT/CF for convenience, Fig. S9 in Supporting information), indicating the positive effect of CCNT in UO₂²⁺ reduction.

  As U contaminated water bodies typically contain organic matters [29,31], complex model effluents containing both UO₂²⁺ and organic compounds (e.g., tetracycline (TC)) were used to evaluate the performance of SD-PEC||CCNT/CF system. Under dark conditions, pure CF adsorbed only 15% and 23% of UO₂²⁺ and TC, respectively (Fig. 2a). However, when CCNT/CF was used, these values increased significantly to 36% and 30%, respectively, indicating that the CCNT enhanced the adsorption affinity of CF. The direct degradation of UO₂²⁺ and TC was almost negligible under AM 1.5 light. In the presence of TNR as photocatalyst, only 14% of UO₂²⁺ and 22% of TC were removed, whereas the construction of the SD-PEC||CCNT/CF system resulted in removal efficiencies of ~100% (60 min) and ~95% (80 min) for UO₂²⁺ and TC, respectively, corresponding to k_obs values of 0.078 and 0.028 min^-1. These values were significantly higher than those of the SD-PEC||CF system (0.008 and 0.016 min^-1), suggesting that CCNT/CF has a specific activity in improving the overall performance of SD-PEC||CCNT/CF system. Of interest is the fact that there is a greater improvement in the rate of TC degradation in the SD-PEC||CF system compared to that of CF adsorption, suggesting that the degradation of TC may be more influenced by the anode, which could be due to the highly oxidizing species (e.g., h⁺-e^- pairs, ^•OH) produced by the anode under light conditions (Fig. S10 in Supporting information). Meanwhile, the SD-PEC||CCNT/CF system exhibited a good power output that is enough to light up a LED lamp (Fig. S11 in Supporting information), with a short-circuit current density (J_sc) of 1.74 mA/cm², an open-circuit voltage (V_oc) of 2.23 V, and a maximum power density (P_max) of 1.13 mW/cm² (Fig. 2b). Furthermore, the SD-PEC||CCNT/CF system achieved ~100% and ~95% UO₂²⁺ removal as well as ~66% and ~40% TOC mineralization efficiencies within 60 min in UO₂²⁺-ethylene diamine tetra acetic acid (EDTA) and UO₂²⁺-tannic acid (TA) solutions, respectively, which are two typical strong complexing agents in U containing wastewater (Fig. S12 in Supporting information). These results demonstrate the great potential of SD-PEC||CCNT/CF system for treating complex radioactive wastewaters while simultaneously power generation.

  Figure 2

  

  Figure 2.  (a) Simultaneous removal of UO₂²⁺ and TC by different reaction systems. (b) Power generation performance of SD-PEC with different cathode materials in treating complex wastewater. UO₂²⁺ and TC removal in (c) real sunlight and (d) corresponding I-t, V-t curves. (e) Repeated 50 times for UO₂²⁺and TC removal by SD-PEC||CCNT/CF system (after each cycle, the recovered U on CCNT/CF was dissolved with 0.1 mol/L sodium NaHCO₃). Conditions: [C₀(UO₂²⁺)] = 10 mg/L, [C₀(TC)] = 20 mg/L, [Na₂SO₄] = 0.1 mol/L, [complexing agents] = 20 mg/L, the initial pH = 4.3, T = 25 ℃, AM 1.5 illumination.

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  Real-world application scenarios are crucial for the economic viability and sustainable access to U in the nuclear power industry [32,33]. We assessed the efficacy of SD-PEC||CCNT/CF in removing UO₂²⁺ and organic compounds under actual sunlight conditions, while the results showed that complete removal of UO₂²⁺ and 94% TC removal within 7 h (Fig. 2c). Meanwhile, exceptional power output was observed (Fig. 2d), highlighting the significant potential of the SD-PEC||CCNT/CF system for treating U-contaminated wastewater using solar irradiation. Moreover, the SD-PEC||CCNT/CF system completely removed UO₂²⁺ and TC from the simulated seawater solution, with k values of ~0.081 and ~0.079 min^-1, respectively (Fig. S13 in Supporting information). These results demonstrate the excellent potential of SD-PEC||CCNT/CF system in the real world.

  To clarify the possible mechanism of the rapid removal of UO₂²⁺in the SD-PEC||CCNT/CF system, factors affecting the degradation properties were studied. The details were presented in Figs. S14–S19 (Supporting information). Firstly, the effects of initial TC concentration (10–40 mg/L) and initial UO₂²⁺concentration (5–40 mg/L) were evaluated. The results showed that both UO₂²⁺ and TC were efficiently removed at different concentration, with removal ratios of ~100% (except when UO₂²⁺ reached 40 mg/L) and ~90%, respectively (Fig. S14 in Supporting information). Secondly, the pH of the solution significantly affects several aspects, including the UO₂²⁺ species, cathode surface properties, UO₂²⁺ adsorption, and subsequent UO₂²⁺ reduction (Fig. S15 in Supporting information) [34]. Under lower pH conditions (pH 3.0), the UO₂²⁺ reduction on the CCNT/CF electrode was reduced due to the increased solubility of U species and competition of H⁺ (Fig. S16 in Supporting information) [34]. In contrast, the removal of TC was greater than 90% in the pH ranges from 4.3 to 7.2, indicating that the SD-PEC||CCNT/CF system has good degradation kinetics for UO₂²⁺ over a wide pH range. The concentration of the supporting electrolyte plays a pivotal role in determining the charge transfer dynamics in the SD-PEC||CCNT/CF system and increasing the sodium sulphate concentration significantly improved the removal rates (Fig. S17 in Supporting information). Radioactive effluents inevitably contain inorganic ions that can potentially affect the formation and migration of specific target metal ions [35]. Because of the presence of ions forming complexes with UO₂²⁺ (HCO₃^-, H₂PO₄^-), elution of UO₂²⁺ species (HCO₃^-), trapping of e^--h⁺pairs (Cl^-, H₂PO₄^-, NO₃^-, HCO₃^-), competition for electrons and active sites (Ni²⁺, Zn²⁺, Cu²⁺, Cd²⁺), and hydrolysis to produce precipitation (Fe³⁺), inorganic ions have diverse effects on UO₂²⁺ removal (Fig. S18 in Supporting information). Furthermore, the performance of the SD-PEC||CCNT/CF system was further evaluated for the elimination of UO₂²⁺ and various organic contaminants, including pesticides, antibiotics, pharmaceuticals, and endocrine disrupting chemicals (Fig. S19a in Supporting information). The removal efficiencies of UO₂²⁺ and organic matter were found to exceed 98% (within 60 min) and 93% (within 80 min), respectively, while the mineralization efficiency of organic compounds reached a minimum of 43% (Fig. S19b in Supporting information). In conclusion, these results highlight the ability of the SD-PEC||CCNT/CF system to effectively treat complex wastewaters which contain UO₂²⁺ and different categories of organic contaminants and has a good interference immunity.

  Reusability is an excellent indicator for evaluating the feasibility and economy of materials [36]. Even after 50 cycles, the SD-PEC||CCNT/CF system showed a UO₂²⁺ removal rate of over 95% and a TC removal rate of 90% (Fig. 2e). This demonstrated the excellent sustainability and recoverability of the SD-PEC||CCNT/CF system, which makes it a promising candidate for use in practical applications.

  The CCNT/CF showed significant deposits on the surface after U recovery, indicating that U compounds were deposited on the cathode (Figs. 3a and b), which was further demonstrated by the energy dispersive spectrometer (EDS) mapping (Fig. 3c and Fig. S20 in Supporting information). The spectrum of U 4f can be seen on the CCNT/CF-used (Fig. 3d), and the U species on the CCNT/CF was U(Ⅳ) (Fig. 3e), indicating that U(Ⅵ) in the solution was electro-reduced to UO₂ and deposited on the surface of CCNT/CF [37]. Comparably, direct adsorption by CCNT/CF could not enable UO₂²⁺reduction (Fig. S21 in Supporting information). In addition, the XPS spectrum of O 1s shows a new characteristic U-O peak at 530.5 eV (Fig. 3f), providing evidence for the presence of uranium oxide species [37]. These results indicate that U(Ⅵ) in solution was effectively captured and reduced to insoluble U(Ⅳ) species by CCNT/CF, providing valuable insights into the SD-PEC||CCNT/CF system for U deposition and recovery.

  Figure 3

  

  Figure 3.  (a, b) The scanning electron microscope (SEM) images and (c) EDS mapping of the CCNT/CF after UO₂²⁺ removal. (d) XPS spectra, (e) U 4f, (f) O 1s before and after CCNT/CF deposition of UO₂²⁺. Conditions: [C₀(UO₂²⁺)] = 10 mg/L, [C₀(TC)] = 20 mg/L, [Na₂SO₄] = 0.1 mol/L, the initial pH = 4.3, T = 25 ℃, AM 1.5 illumination.

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  Based on the above experimental results, a mechanism for treating complex wastewater by SD-PEC||CCNT/CF was proposed. As shown in Fig. 4, under sunlight irradiation, TNR can be excited by high-energy photons with a wavelength lower than 400 nm (Fig. S5a in Supporting information), generating e^--h⁺ pairs on the surface of TNR, which can oxidize adsorbed water to produce hydroxyl radicals, or oxidize the organic pollutants directly to produce H₂O and CO₂ [38,39]. Oxidation of organic matters can release UO₂²⁺ from organic matter-uranyl complexes. It migrates to the cathode and is deposited on the cathode in the presence of an electric field. At the same time, light transmitted through the TNR can excite the SSC, generating a bias voltage that accelerates the transfer of electrons from the TNR to the CCNT/CF, thereby generating electricity in the external circuit. The continuous influx of electrons allows the negatively charged CCNT/CF to attract more UO₂²⁺. In addition, due to an ongoing electron transfer mechanism, the adsorbed UO₂²⁺ undergoes reduction to UO₂ by electrons. After operation, the deposited UO₂ on CCNT/CF can be easily recovered using sodium bicarbonate solution [40]. In this way, the SD-PEC||CCNT/CF system achieves synergistic U recovery and organic matter decomposition with simultaneous power generation.

  Figure 4

  

  Figure 4.  The mechanism of SD-PEC||CCNT/CF system for simultaneous reduction of UO₂²⁺, organic pollutant oxidation and power generation.

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  In summary, we describe a new SD-PEC strategy for the simultaneous recovery of U(Ⅵ) and removal of organic matter from complex U-containing wastewater with concomitant power generation using only sunlight. Due to the abundant CCNT on the CF, the current density of the SD-PEC system is greatly enhanced, which enables the UO₂²⁺ in the wastewater to be completely deposited on CCNT/CF in 60 min for the purpose of removal and recovery. The SD-PEC||CCNT/CF system is remarkably stable and resistant to interferences, which can be effectively regenerated. Under real sunlight, UO₂²⁺ was completely removed within 7 h, and the removal rate of TC reached 95% without the need of other energy supply (e.g., electrical energy), indicating the potential application of the SD-PEC||CCNT/CF system. Our work not only develops an effective system with efficient U recovery and organic removal accompanied by power generation, but also provides a strategy for U recovery from complex wastewater.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 52170083, 51808143), the Science and Technology Innovation Program of Hunan Province (No. 2022RC1125), the Hunan Provincial Natural Science Foundation of China (No. 2021JJ20007). The comments and advice from editor and anonymous reviewers are highly appreciated, which help greatly enhance the overall quality of the paper.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110673.

  
  
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  Figure 1  (a) Illustration of the synthesis of CCNT/CF. (b) Contact angle of the CF and CCNT/CF surface. (c) Transmission electron microscopy (TEM) image of CCNT. (d) FTIR spectrum, and (e) O 1s spectrum of CCNT/CF cathodes. (f) EIS curves and (g) CV curves of CF and CCNT/CF cathodes.

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  Figure 2  (a) Simultaneous removal of UO₂²⁺ and TC by different reaction systems. (b) Power generation performance of SD-PEC with different cathode materials in treating complex wastewater. UO₂²⁺ and TC removal in (c) real sunlight and (d) corresponding I-t, V-t curves. (e) Repeated 50 times for UO₂²⁺and TC removal by SD-PEC||CCNT/CF system (after each cycle, the recovered U on CCNT/CF was dissolved with 0.1 mol/L sodium NaHCO₃). Conditions: [C₀(UO₂²⁺)] = 10 mg/L, [C₀(TC)] = 20 mg/L, [Na₂SO₄] = 0.1 mol/L, [complexing agents] = 20 mg/L, the initial pH = 4.3, T = 25 ℃, AM 1.5 illumination.

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  Figure 3  (a, b) The scanning electron microscope (SEM) images and (c) EDS mapping of the CCNT/CF after UO₂²⁺ removal. (d) XPS spectra, (e) U 4f, (f) O 1s before and after CCNT/CF deposition of UO₂²⁺. Conditions: [C₀(UO₂²⁺)] = 10 mg/L, [C₀(TC)] = 20 mg/L, [Na₂SO₄] = 0.1 mol/L, the initial pH = 4.3, T = 25 ℃, AM 1.5 illumination.

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  Figure 4  The mechanism of SD-PEC||CCNT/CF system for simultaneous reduction of UO₂²⁺, organic pollutant oxidation and power generation.

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