Nuclear energy is a green and clean energy supply with low consumption and no greenhouse gas emissions, which is the most effective alternative to fossil energy [1]. In recent years, with the booming of the nuclear industry, the exploitation of uranium ore has intensified to provide the raw materials of nuclear reactions [2,3]. However, this uranium mining process could generate large volumes of uranium-containing wastewater [4,5]. Uranium species in wastewater exist mainly in the soluble form of the uranyl ion (UO22+), which can easily penetrate underground aquifers and cause water pollution, bringing significant risks to the ecological environment and people's health [6]. In addition, since terrestrial deposits of uranium are not plentiful, increased over-exploitation of uranium has led to a strain on uranium resources, which could be depleted within a century [7]. Based on the above two aspects, the treatment and resource recovery of uranium-containing wastewater are both a requirement of environmental protection and a need for full utilization of uranium resources.

To effectively remove uranium from water, various technologies have been explored, including co-precipitation, ion exchange, adsorption, reduction, membrane separation, and others [[8], [9], [10]]. However, these methods still suffer from some disadvantages, such as low efficiency, high cost, secondary pollution, etc. Recently, capacitive deionization (CDI) has been developed for uranium extraction from aqueous solutions [11,12]. The principle of CDI operation is combining traditional physicochemical adsorption with electrochemistry. By applying a low voltage (0.6–2.0 V) to the CDI cell, the removal of charged ions from the solution is achieved as the charged ions move toward the electrodes of opposite charge [13,14]. The ions are electrostatically stored in the electric double layers (EDLs) on the surface of the porous electrodes, enabling the accumulation of target ions. Moreover, this process is also reversible, and target ions could be released by reversing the potential [15]. Compared with traditional adsorption, CDI possesses high efficiency and easy regeneration properties, rendering it a promising technique for uranium removal.

It is beyond doubt that electrode material is the key factor affecting the performance of capacitive deionization. The development and design of advanced electrode materials are crucial for uranium removal through CDI. A desired electrode material should exhibit the characteristics of good conductivity, high specific surface area, satisfying wettability, and low cost [16]. Commonly used electrode materials for CDI applications are carbon-based materials, including activated carbon, carbon nanotubes, carbon cloths, graphene, etc. [[17], [18], [19]]. However, these carbon materials need complicated calcination processes, and they also lack specific sites for uranium. Recently, conducting polymers, such as polypyrrole (PPy), polyaniline (PANI), etc., have been broadly investigated as electrode materials, owing to their good electron mobility, environmental stability, and ease of preparation. For instance, Ouyang's group deposited polyaniline on biocarbon as U(VI) electrosorption electrode, and a maximum U(VI) removal value of 282.2 mg g−1 was reached at pH 4.0 and 0.9 V [20]. The same group also prepared graphene oxide/polypyrrole composite electrodes, which achieved a maximum U(VI) electrosorption capacity of 246.5 mg g−1 at pH 4.0 and 0.9 V [21]. Nevertheless, the reported conducting polymer-based electrodes for uranium removal have an ordinary architecture and limited surface area, affecting their CDI performance. In our previous work, polypyrrole nanotubes with the one-dimensional hollow structure were prepared by the sacrificial template method. This nano-architecture could greatly facilitate mass transfer and electron transport [22]. Thus, hollow polypyrrole nanotubes are good candidates as CDI electrodes to replace traditional polypyrrole materials.

To further improve the affinity of electrodes toward uranium in complex wastewater environments, functionalization or modification is often adopted. Chitosan (CS) is the second most abundant natural polymer material in the world, and it shows a mass of amino groups and hydroxyl groups on its molecular chains [23,24]. Phytic acid (PA), a natural plant extract, has six phosphate groups in one its molecule [25]. They both have good biocompatibility, environmental friendliness, and low-cost properties. Moreover, according to Lewis acid-base theory, UO22+ is a hard Lewis base and has the tendency to form stable complexation with hard Lewis acids, e. g., hydroxyl, phosphate, carboxyl, and other oxygen donors [26]. As a result, chitosan and phytic acid with abundant oxygen-containing groups are good chelators for uranyl binding. Some studies have applied chitosan or phytic acid to adsorb UO22+ from water [[27], [28], [29]]. However, such composite materials used for uranium capture through capacitive deionization have not been reported to the best of our knowledge. Therefore, the introduction of chitosan and phytic acid on polypyrrole nanotubes is a feasible way to enhance the electrosorption performance of CDI electrodes.

To explore the validity of our hypothesis, herein, chitosan/phytic acid complexes were tightly loaded onto polypyrrole nanotubes (CS/PA-PPy) for efficient uranium capture from wastewater through capacitive deionization. PPy nanotubes provided a good electronic conductor and abundant space for UO22+ ion transfer. Chitosan/phytic acid complexes offered specific sites for UO22+ binding. As expected, CS/PA-PPy electrodes achieved rapid, high, and selective UO22+ ion removal via the CDI process, which was more efficient than physicochemical adsorption. Electrochemical tests, uranium removal performance, and mechanism analysis were investigated in detail. The results indicate that our work provides a novel and high-performance CDI electrode for the removal and recovery of uranium from wastewater.