Direct recovery of high-purity uranium from fluoride-containing nuclear wastewater via extraction materials with ensemble Lewis sites and a tandem electrochemical device.

With the development of nuclear industry, large quantities of uranium-containing wastewater are generated from the nuclear fuel cycle, inducing resource loss and radioactive risk to the environment (Wei et al., 2024; Zhou et al., 2024). The efficient uranium extraction from the real nuclear wastewater is an efficient way to treat radioactive wastewater and recover the valuable uranium resource (Chen et al., 2022; Sun et al., 2024; Ye et al., 2022). In real situations, high concentrations of uranium and fluoride ions (F^–) commonly coexist due to the widespread use of UF₄ or UF₆ intermediates (Feng et al., 2022; Hlina et al., 2016, p. 1). In this mixture system, the U(VI) complex presents various existence forms (UO₂F_x), such as UO₂F⁺, UO₂F_2(aq), UO₂F₃^–, and UO₂F₄^2- (Busquim e Silva et al., 2010; Tu et al., 2023). The complex dominant uranium species and excess F^– seriously decrease the efficiency of traditional adsorption and ion exchange extraction methods, which inspires the exploration of new strategies for uranium extraction (Zhou et al., 2023).

As an alternative strategy, the conversion of U(VI) to insoluble crystalline products through electrochemical coordination and reduction brings new insights into enhancing the uranium extraction efficiency in a complex environment (Liu et al., 2019; Wang et al., 2021, 2024; Yanyong Wang et al., 2023; Xie et al., 2024). For example, the electrochemical method using amidoxime-functionalized carbon material was able to extract uranium from real seawater with a considerable capacity of 6.35 mg g^-1 d-¹, which proves the efficient extraction of the [UO₂(CO₃)₃]^4- complex (Liu et al., 2022). Additionally, the applied electric field drives the strongly competitive F^– to migrate toward the anode, thereby mitigating the interference of F^– with the cathodic reduction of UO₂²⁺. Recently, our previous work demonstrated the ion pair sites of PO₄^3--Ti^S+ efficiently extracted 99.6 % of uranium from real nuclear wastewater containing 8 g L-¹ of F^–, with 7.9 % impurities of alkali metals in the extracted product (Lin et al., 2024). However, the current progress hardly meets the actual requirements, because the F^– concentration in real nuclear wastewater from the factory can reach a peak value of 30 g L^-1, and the extraction product needs to have <3 % impurities of alkali metals for purification in a nuclear fuel element factory. As the main component of nuclear fuel elements, the high-purity UO_x as the extraction product from real nuclear wastewater is particularly valued. Such a challenging project motivates the further improvement of the electrochemical protocol.

Two parallel strategies are potentially effective for the improvement of electrochemical protocol, including the optimization of uranium extraction electrode materials and the design of the electrochemical device. For the electrode materials, the active sites require a universal enhanced binding for all UO₂F_x species, which increases the anti-interference ability against extremely high concentration of F^– (Chen et al., 2023; Lin et al., 2022; Yanjing Wang et al., 2023; Yang et al., 2021). The UO₂F_x complex contains positive charged target atom (U) and negative charged target atoms (O and F) for surface adsorption, which can respectively bind with other Lewis acid and Lewis base sites (Hu et al., 2024; Johnstone et al., 2018; Ng et al., 2023; Qin et al., 2024). Based on the ensemble effect, the neighboring ensemble Lewis acid-base pair sites (namely ensemble Lewis sites) enable the simultaneous effect on the both U target atom and negative charged target atoms (O or F) in the UO₂F_x complex. For the electrochemical device, the control of reaction period is a key factor, because the primarily UO_x products tend to be re-oxided and transform into alkali-containing crystals (Liu et al., 2024). Therefore, the rational design of ensemble Lewis sites in the electrode material and the integration of the electrochemical device are promising ways to achieve the efficient extraction of high-purity uranium under the interference of extremely high concentration of F^–.

Herein, we constructed ensemble Lewis sites of phosphate group-defective Bi ion (PO₄^3--Bi^?+, ?<3) on the surface of bismuth oxide nanosheets (Ca₅(PO₄)₃(OH)-Bi₂O_3-x) and integrated into a tandem electrochemical device for the recovery of uranium from real nuclear wastewater. Theoretical calculations and spectroscopic analyses indicated that the key to the resistance against extremely high F^– interference was the strengthening of the adsorption bonds of the ensemble Lewis sites for the dominant species of UO₂F_x, facilitating the initial formation of pentavalent uranium (U(V)). The evolution of uranium species revealed that the extracted crystals would readily transform from U₃O₈ to K₂U₂O₇ with the formation and dismutation of a key intermediate species of U(V). To prevent the waste of high-purity uranium resources caused by the re-oxidation of the uranium product, we developed a tandem electrochemical device to rationally control the reaction period for the generation of the U₃O₈ product. The electrolytic system achieved 99.9 % extraction of uranium from real nuclear wastewater for 3 h and recovered powdered uranium oxide with <2.2 % alkali metal impurities, superior to previously reported related work. This study aims to reveal an economically feasible and efficient strategy to resourcefully treat nuclear wastewater.