An asymmetric photoelectrocatalytic system, its preparation method and application in uranium removal

Abstract

The present invention discloses an asymmetric photoelectrocatalytic system, its preparation method, and its application in uranium removal. The asymmetric photoelectrocatalytic system comprises a photoanode and a cathode connected by an external circuit, wherein the anode material in the photoanode is composed of nano-titanium dioxide supported on graphene nanosheets, and the cathode material in the cathode is composed of reduced graphite oxide aerogel and conductive carbon black bonded by a xanthan gum-polyacrylic acid composite binder. The asymmetric photoelectrocatalytic system has enhanced photoelectrocatalytic activity, rapid electron transfer capability, and abundant binding active groups, enabling it to integrate multiple synergistic mechanisms such as electrical adsorption, micro-electric field attraction, coordination, and catalytic reduction to capture U(VI). It has the characteristics of high U(VI) removal rate, fast kinetic rate, high U(VI) reduction efficiency, and reusability.

Description

Technical Field

[0001] The present invention relates to an asymmetric photoelectrocatalytic system, in particular to an asymmetric photoelectrocatalytic system consisting of a 1DTiO2 / 2D graphene photoanode and a functionalized graphene cathode, and also relates to a preparation method thereof, and the application of the asymmetric photoelectrocatalytic system in removing U(VI) in a solution system, belonging to the technical field of uranium resource recovery. Background Art

[0002] The fundamental evaluation criteria for uranium capture methods are adsorption capacity, kinetics, selectivity, and reusability. Over the past two decades, various methods have emerged, including adsorption, solvent extraction, membrane separation, and ion exchange. Adsorption is a classic and considered one of the most viable methods for U(VI) capture. Recently, a variety of novel adsorbents have been reported, including graphene-based materials, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs). Unfortunately, for adsorption, the diffusion of uranyl ions, driven primarily by concentration differences, is always relatively slow, resulting in sluggish kinetics. Furthermore, due to the limited number of active sites, the first adsorbed uranyl ions exert Coulombic repulsion on the subsequently adsorbed uranyl ions, known as the co-ion repulsion effect, which hinders further adsorption and leads to relatively low adsorption capacity. Furthermore, the highly mobile adsorbed U(VI) may re-enter the solution, causing secondary contamination.

[0003] Electrochemical methods, primarily electrosorption and electrodeposition, are emerging as effective methods for uranium extraction. Driven by an electric field, charged ions can rapidly migrate to the electrode surface and be separated from the solution. Clearly, electrochemical methods offer significant advantages over traditional adsorption methods in terms of both kinetic rate and capacity. In recent years, significant research progress has been made using electrochemical methods. The inventors' early research demonstrated that by rationally designing the microstructure of graphene-based electrodes, electrosorption methods can achieve significantly higher uranium removal rates than physical and chemical adsorption methods, as reported in Reference 1 (Liao, Y., et al., Electrosorption of uranium(VI) by highly porous phosphate-functionalized graphene hydrogel. Applied Surface Science, 2019. 484: pp. 83-96.) and Reference 2 (Liao, Y., et al., Asymmetric polysaccharide-bound graphene electrode configuration with enhanced electrosorption performance for uranium (VI)ions. Chemical Engineering Journal, 2021. 424.). In addition, reference 3 (Liu, C., et al., A half-wave rectified alternating current electrochemical method for uranium extraction from seawater. Nature Energy, 2017. 2(4)) reported that by inducing electrodeposition of uranium compounds, an extraction capacity 9 times higher than that of traditional adsorption methods, kinetics 4 times faster, and better selectivity for uranium were achieved. However, in studies using electrosorption, the common ion exclusion effect and sufficient selectivity for uranium remain challenging tasks; in studies using electrodeposition, although the selectivity for uranium can be effectively improved by reducing mobile U(VI) to immobile tetravalent uranium (U(IV)) solid, the electrodeposition of uranium-based substances usually requires a high potential of 5V to tens of volts, which inevitably leads to water electrolysis, resulting in energy waste; at the same time, the electrodeposition efficiency in low-concentration uranium-containing solutions is always limited by the performance of the electrode material.

[0004] Compared with electrochemical methods, photocatalytic reduction can utilize abundant solar energy but does not consume expensive electricity to remove uranium. Under light, photoelectrons excited by photocatalysts can reduce mobile U(VI) ions to immobile U(IV), thereby enabling the selective separation of uranyl species from the solution. In the past decade, photocatalysts such as TiO2 and g-C3N4 have been widely studied. However, research using these nanoparticle photocatalysts is not practical because it is particularly challenging to separate the suspended photocatalyst from the aqueous suspension and the U(IV) deposits from the catalyst solid. In addition, photoexcited electrons and holes can easily recombine, which greatly affects the removal efficiency of uranium.

[0005] In general, the aforementioned electrical / optical methods have their unique advantages, but they cannot fully meet the high demands of uranium capture. In 2015, Reference 4 (Kim, YK, et al., Solar conversion of seawater uranium (VI) using TiO2 electrodes. Applied Catalysis B: Environmental, 2015. 163: pp. 584-590.) reported a photoelectrochemical method for water splitting that can be effectively applied to uranium capture. Under the action of an electric field and sunlight, uranium (VI) can be selectively photoelectrocatalytically reduced on a TiO2 electrode. More importantly, the presence of an electric field drives photoexcited electrons to the counter electrode through an external circuit, thus avoiding the recombination of photoexcited electrons and holes. Furthermore, coating the TiO2 photocatalyst on an FTO (fluorine-doped tin oxide) glass support, rather than in the form of a nanoparticle suspension, facilitates its reusability. Inspired by Kim's work, several studies have been conducted on uranium capture using photoelectrochemical methods. For example, reference 5 (Lee, S., et al., Homogeneous photoconversion of seawateruranium using copper and iron mixed-oxide semiconductor electrodes. AppliedCatalysis B: Environmental, 2017.207: p. 35-41.) synthesized photocatalysts such as CuO2 / CuFeO and successfully reduced U(VI) to a lower oxidation state; reference 6 (Dai, Z., et al., Fabrication of g-C3N4 / Sn3O4 / Ni electrode for highly efficient photoelectrocatalytic reduction of U(VI). Chemical Engineering Journal, 2022. 433.) used g-C3N4 / Sn3O4 / Ni electrode as a photoanode and developed a method for photoelectrochemical reduction of U(VI), which proved that its removal rate of U(VI) was better than that of photocatalytic and electrochemical methods. Although photoelectrochemical methods show advantages over photocatalytic and electrochemical methods in uranium capture, researchers currently focus on the design of various photoanode materials, while simply using platinum or titanium plates as cathodes makes it difficult to further improve uranium recovery efficiency. Summary of the Invention

[0006] In response to the defects of the existing technology, the first object of the present invention is to provide an asymmetric photoelectrocatalytic system (MMCD). MMCD has enhanced photoelectrocatalytic activity, rapid electron transfer ability and rich binding active groups, enabling it to integrate multiple synergistic mechanisms such as electrical adsorption, micro-electric field attraction, coordination and catalytic reduction to capture U(VI). It has the characteristics of high U(VI) removal rate, fast kinetic rate, high U(VI) reduction efficiency and reusability.

[0007] A second object of the present invention is to provide a method for preparing MMCD, which is simple, highly reproducible, low-cost, and conducive to industrial production.

[0008] A third object of the present invention is to provide an application of MMCD for the photoelectrochemical extraction of U(VI) from a solution system. MMCD has multiple synergistic U(VI) capture mechanisms, including electrical adsorption, micro-electric field attraction, coordination, and catalytic reduction. The MMCD has the advantages of high removal rate, fast kinetic rate, high U(VI) reduction efficiency, and reusability, and can be widely promoted and applied.

[0009] In order to achieve the above technical objectives, the present invention provides an asymmetric photoelectrocatalytic system, which includes a photoanode and a cathode connected by an external circuit; the anode material in the photoanode is composed of nano-titanium dioxide loaded on graphene nanosheets; the cathode material in the cathode is composed of reduced graphite oxide aerogel and conductive carbon black bonded by a xanthan gum-polyacrylic acid composite binder.

[0010] The asymmetric photoelectrocatalytic system (MMCD) of the present invention comprises a photoanode composed of a one-dimensional TiO2 / two-dimensional graphene nanosheet composite (1D / 2DGTR) as the active electrode material, and a cathode composed of a three-dimensional network of xanthan gum-polyacrylic acid bonded with graphene aerogel (RGX). The two electrodes are coupled together via an external circuit. The two electrodes utilize multiple synergistic mechanisms, including electroadsorption, micro-electric field attraction, coordination, and catalytic reduction, to capture U(VI), significantly improving the U(VI) removal rate and kinetic rate, while also achieving high U(VI) reduction efficiency. The GTR photoanode, which contains a one-dimensional TiO2 structure with a high specific surface area and high photocatalytic activity, is supported by highly conductive two-dimensional graphene, which forms a charge transfer highway, helping to improve carrier mobility and energy level matching between the two. As for the RGX cathode, its three-dimensional graphene aerogel has a porous interconnected structure, inherent high conductivity, large specific area and chemical stability. The three-dimensional graphene aerogel uses a hydrophilic three-dimensional network bonded by xanthan gum (XG) and polyacrylic acid (PAA) cross-linking to form a stable three-dimensional structure. The covalent and non-covalent cross-linking between XG and PAA not only gives the RGX cathode strong mechanical strength, but also through the rich polar groups in the cross-linked network structure of the XG-PAA adhesive, especially the negatively charged carboxyl groups, it can play a dual role. It can act as an ion exchange membrane to alleviate the common ion expulsion effect, and can also serve as an active site to bind U (VI) ions through coordination. The MMCD constructed based on the above-mentioned 1D / 2D GTR photoanode and RGX cathode has the following advantages in the process of uranium capture: (1) It can combine multiple synergistic mechanisms, including electrosorption, micro-electric field attraction, coordination and photoelectrocatalytic reduction to capture uranium; (2) The electric field effect and the electrostatic interaction between the negatively charged carboxyl group and the positively charged uranium (VI) ion drive the fast kinetic rate; (3) Good selectivity is ensured by the coordination between uranium (VI) and the functional group and its further photoelectrocatalytic reduction to an insoluble uranium (IV) precipitate; (4) It can utilize solar energy to reduce the use and waste of electricity.

[0011] As a preferred solution, the mass ratio of nano-titanium dioxide to graphene nanosheets in the anode material is (0.1-1):1. Composite nano-titanium dioxide semiconductors with highly conductive two-dimensional graphene nanosheets can form Ti-OC bonds between the nano-titanium dioxide and graphene nanosheets, reducing the resistance to electron conversion. This allows the two-dimensional graphene to form a charge transfer highway, helping to increase carrier mobility and improve energy level matching between the nano-titanium dioxide semiconductor and the two-dimensional graphene nanosheets. With the addition of more graphene, the transient photocurrent of the GTR composite material first increases and then decreases. Therefore, within the preferred range of mass control of the nano-titanium dioxide and graphene nanosheets, better photoelectrocatalytic activity can be achieved.

[0012] As a preferred solution, the nano-titanium dioxide is titanium dioxide nanorods. Nanorod-shaped titanium dioxide has a high length-to-diameter ratio, which is beneficial for the rapid directional transmission of electrons and the improvement of light collection and scattering performance.

[0013] As a preferred embodiment, the cathode material comprises reduced graphite oxide aerogel, conductive carbon black, and xanthan gum-polyacrylic acid composite binder in a mass ratio of 60%-80%: 10%-20%: 10%-20%. A too low ratio of the xanthan gum-polyacrylic acid composite binder (XG-PAA) makes it difficult to form a strong cross-linked network, resulting in poor stability and poor modification of the reduced graphite oxide aerogel, making it difficult to achieve high U(VI) adsorption efficiency. A too high ratio of the xanthan gum-polyacrylic acid composite binder reduces the conductivity of the reduced graphite oxide aerogel, increasing electrode resistance and lowering current efficiency. The conductive carbon black primarily fills the interlayers of the wrinkled graphene nanosheets, forming an interconnected conductive network. However, a too low amount of the conductive carbon black makes it difficult to establish a conductive network. A too high ratio of the conductive carbon black reduces the porosity of the electrode structure, hindering the penetration of U(VI) ions.

[0014] As a preferred solution, the mass ratio of xanthan gum to polyacrylic acid in the xanthan gum-polyacrylic acid composite binder is (0.5~2):1. Xanthan gum (XG) and polyacrylic acid (PAA) can form covalent crosslinks through polyester reaction, or form non-covalent crosslinks through intramolecular / intermolecular hydrogen bonds. By controlling the ratio of the two within a suitable range, better bonding performance can be obtained to ensure that the reduced graphene oxide aerogel is bonded to form a strong three-dimensional interconnected conductive structure, thereby enhancing the electrochemical performance of the composite electrode and maintaining its mechanical stability and recyclability. At the same time, by controlling the amount of polyacrylic acid introduced, the carboxyl content introduced into the crosslinking system is regulated, and the negatively charged COO - The group can attract UO2 in advance 2+ ions, thereby weakening the CO - The repulsive effect of ions and the introduction of an appropriate amount of carboxyl groups can improve the wettability of the reduced graphene oxide electrode and improve the contact performance between U(VI) ions and the electrode in aqueous solution.

[0015] The present invention also provides a method for preparing an asymmetric photoelectrocatalytic system, which comprises the following steps:

[0016] 1) Graphene oxide is dispersed in water to obtain a graphene suspension, an acid catalyst and a titanate are added to the graphene suspension for hydrolysis, the resulting reaction solution is transferred to a hydrothermal reactor, and an FTO substrate is added to carry out a hydrothermal reaction to obtain a photoanode;

[0017] 2) dispersing graphene oxide in water to obtain a graphene suspension, adding a reducing agent to the graphene suspension for a reduction reaction to obtain a reduced graphene oxide hydrogel, and freeze-drying the reduced graphene oxide hydrogel to obtain a reduced graphene oxide aerogel;

[0018] 3) The reduced graphene oxide aerogel is mixed with a conductive agent and a xanthan gum-polyacrylic acid composite binder by a liquid phase method, coated on a graphite plate, and dried and solidified to obtain a cathode.

[0019] The present invention constructs an asymmetric system consisting of a 1D / 2D GTR photoanode and an RGX cathode. To prepare the GTR photoanode, a GO suspension is first uniformly mixed with hydrochloric acid and a nano-titanium dioxide precursor. The mixture is then poured into a Teflon-lined autoclave with a tilted FTO glass. After hydrothermal treatment, a 1D / 2D GTR nanocomposite is in situ grown onto a conductive FTO substrate, thereby producing a 1D / 2D GTR photoanode. Compared to traditional coating methods, the hydrothermal method employed in the present invention is highly convenient, cost-effective, and easy to control. Furthermore, the 1D / 2D GTR composite material directly grown in situ on the conductive FTO substrate facilitates rapid interfacial charge transfer between TiO2 and FTO, and between TiO2 and graphene, thereby improving the photoelectric properties of the GTR anode. To prepare the RGX cathode, GO is first reduced with a reducing agent to produce an RGO hydrogel. This is then freeze-dried to yield a three-dimensional porous conductive RGO aerogel. The RGO active material and conductive agent are then bonded to the graphite current plate using a cross-linked XG-PAA adhesive. XG-PAA binder has a triple role: as a binder, it can form a three-dimensional interconnected tough conductive structure; as an ion exchange membrane, it can attract UO2 in advance through COO- groups. 2+ ions, thereby alleviating the common ion repulsion effect; as a modifier, it provides abundant O-containing active sites to coordinate U(VI) ions, while also improving the hydrophilicity of the RGX cathode. The conductive carbon fills the interlayers of the RGO aerogel to form a conductive network. By rationally designing the asymmetric system of the GTR photoanode and RGX cathode, MMCD can achieve the synergistic capture of U(VI) through electroadsorption, micro-electric field attraction, coordination, and photoelectrocatalytic reduction.

[0020] As a preferred solution, the hydrothermal reaction is carried out at a temperature of 130-180° C. for 3-12 hours. Optimal hydrothermal conditions are conducive to obtaining nanorod-shaped titanium dioxide.

[0021] As a preferred embodiment, the reduction reaction conditions are: stirring at room temperature for 20-40 minutes, followed by standing at 35-45°C for 6-24 hours. Ascorbic acid is used as a reducing agent in the reduction reaction, with a mass ratio of ascorbic acid to graphene oxide of (1-3):1. The graphene oxide is reduced to form a reduced graphene oxide hydrogel with a three-dimensional structure.

[0022] The present invention also provides an application of an asymmetric photoelectrocatalytic system for the photoelectrochemical extraction of U(VI) from a solution system. The MMCD of the present invention integrates multiple synergistic mechanisms, including electroadsorption, micro-electric field attraction, coordination, and catalytic reduction, to capture U(VI). The system exhibits high U(VI) removal rates, fast kinetic rates, high U(VI) reduction efficiency, and reusability.

[0023] As a preferred embodiment, the conditions for the photoelectrochemical extraction of U(VI) from a solution system are: applying light and voltage, with the voltage ranging from -0.4V to -1.2V vs. SCE, the solution system contains Na2SO4 and methanol, and the pH of the solution system is controlled between 3 and 8. The removal rate of U(VI) by MMCD steadily increases with increasing external voltage. However, excessively high input voltage may reduce the electrochemical performance of the electrode, so the most preferred voltage is -0.7V to -1.0V vs. SCE. The most preferred pH of the solution system is controlled between 4 and 6.

[0024] Compared with the existing technology, the technical solution of the present invention brings the following beneficial technical effects:

[0025] The MMCD of the present invention is used to capture uranium and has the following advantages: (1) The MMCD is constructed based on a 1D / 2D GTR photoanode and an RGX cathode. It can combine multiple synergistic mechanisms including electrosorption, micro-electric field attraction, coordination, and photoelectrocatalytic reduction to achieve uranium capture, and has the characteristics of high U(VI) removal rate, fast kinetic rate, high U(VI) reduction efficiency, and reusability. (2) The electric field effect and the electrostatic interaction between the negatively charged carboxyl group and the positively charged uranium (VI) ion drive the fast kinetic rate. (3) Good selectivity is ensured by the coordination between uranium (VI) and the functional group and its further photoelectrocatalytic reduction to an insoluble uranium (IV) precipitate. (4) Solar energy can be utilized to reduce the use and waste of electrical energy. Based on these advantages, the MMCD composed of a 1D / 2D GTR photoanode and an RGX cathode exhibits excellent uranium capture performance. For example, the MMCD system achieves a U(VI) removal rate of 90.5% at a voltage of 1.0 V, which is 1.7 times and 4.5 times that of the EC and PC methods. Its kinetic rate is also 116% and 403% faster than that of the EC and PC methods, respectively, showing good application prospects.

[0026] The MMCD construction method of the present invention is simple, highly repeatable, low-cost, and is conducive to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 .(a~b) are scanning electron microscope images of pure TR at different magnifications; (c~d) are scanning electron microscope images of GTR composite materials at different magnifications; (e) is the SEM image of the side of the GTR composite material; (f) is the TEM image of the GTR composite material; (g) is the elemental map of the GTR composite material; (h) is the HRTEM image of the GTR composite material.

[0028] Figure 2 . (a) XRD patterns of pure TR and GTR-2; (b) full XPS spectra of TR and GTR-2; (c) high-resolution C1s spectrum of RGO; (d) high-resolution C1s spectrum of GTR-2; (e) high-resolution O1s spectrum of GTR-2; (f) high-resolution Ti2p spectra of pure TR and GTR-2.

[0029] Figure 3 . (a) is the UV-visible spectrum of pure TR and GTR composites; (b) is the Tauc's plot of pure TR and GTR composites; (c) is the transient photocurrent density curve of pure TR and GTR composites; (d) is the Nyquist curve of pure TR and GTR composites.

[0030] Figure 4 .(a) is a photo of RGO hydrogel, (b) is a photo of aerogel; (c~d) are scanning electron microscope images of RGO aerogel at different magnifications; (e~f) are scanning electron microscope images of RGX at different magnifications; (g) is the element distribution map of RGX composite material.

[0031] Figure 5 (a) is the Nyquist curve of RGX electrode; (b) is the Nyquist curve of RGX electrode at 5mVs -1 CV curves at different scanning speeds; (c~e) are the CV curves of RGX electrode at different scanning speeds; (f) is the specific capacitance of RGX electrode at different scanning speeds.

[0032] Figure 6 (a) CV curves in Na2SO4 or Na2SO4+U (VI) (without irradiation); (b) CV curves at different scan rates in a Na2SO4+U (VI) mixed solution (without irradiation); (c) CV curves at a scan rate of 5 mV s under light and dark conditions. -1(d) is the CV curve of GTR-2 electrode in Na2SO4+U(VI) solution (no irradiation) and Na2SO4+methanol+U(VI) solution (irradiation, pH=5, E bias = -1.0 V vs. SCE); (e) is the removal of U(VI) over time under different conditions using the PC method (with irradiation and no bias), the EC method (without irradiation and with bias), and the MMCD method (with irradiation and with bias). -1 , pH = 5.0, solution: Na2SO4 + methanol + U (VI)); (f) is the instantaneous U (VI) adsorption rate of PC, EC and MMCD methods (C0 = 50 mg L -1 , pH=5.0, E bias = -1.0 V vs. SCE, solution: Na2SO4 + methanol or Na2SO4 + methanol + U(VI)); (g) Effect of pH value on U(VI) removal efficiency in PC, EC, and MMCD methods (for EC and MMCD methods, C0 = 50 mg L -1 :E bias = -1.0Vvs.SCE, solution: Na2SO4+methanol+U (VI)); (h) Effect of initial concentration of U(VI) on removal rate by PC method, EC method and MMCD method (EC method and MMCD method, pH=5, E bias = -1.0V vs. SCE, solution: Na2SO4+methanol+U (VI)). (k) is the number of electrode cycles (C0=50mg L -1 , MMCD method: pH=5, E bias = -1.0V vs.SCE, solution: Na2SO4+methanol+U(VI)). DETAILED DESCRIPTION

[0033] The following specific examples are intended to further illustrate the present invention, rather than to limit the scope of protection of the claims.

[0034] The raw materials involved in the following examples are all conventional commercially available raw materials.

[0035] Characterization methods used in the following examples: The pore structure, morphology, and elemental distribution of the cathode and anode samples were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) at an accelerating voltage of 20 kV. The morphology and crystallographic characteristics of the samples were determined using high-resolution transmission electron microscopy (HRTEM, Talos F200X G2). The crystal structure of the GTR sample was characterized using Cu Kα X-ray diffractometer (XRD, Bruker D8 Advance). Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS50) was used to investigate changes in the functional groups of the RGX cathode. The optical absorption properties of the samples were measured using a UV-visible spectrophotometer (Hitachi, U-3900). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge (GCD), transient photocurrent density (it curve) and Mott-Schottky curve were measured in 0.1M Na2SO4 solution with or without uranium using a three-electrode system on a CHI660D electrochemical station. Saturated calomel electrode, GTR and RGX were used as reference electrode, counter electrode and working electrode, respectively. Methanol was used as a sacrificial agent to remove the photogenerated holes generated by the GTR anode. The photocurrent density was 5mVs in the range of 0.01Hz-100KHz. -1 The EIS test was carried out at a rate of 1000 Hz in the potential range of -0.6 V to 0.2 V. The Mott-Schottky curve was tested at a frequency of 1000 Hz. -1 The GCD curve was obtained in the potential range of -0.5 V to 0.5 V. The concentration of uranium in the solution was measured using a UV-visible spectrophotometer (Hitachi, U-3900) at a wavelength of 652 nm.

[0036] Example 1

[0037] Preparation of the GTR anode: First, a FTO substrate (F:SnO2, 15U) was ultrasonically cleaned in a mixture of 2-propanol, anhydrous ethanol, and acetone (1:1:1 by volume) for at least 60 minutes and then dried in vacuo at 60°C. GO powders at various mass ratios (GO powder to tetrabutyl titanate: 0%, 2%, 5%, 7%, and 9%) were ultrasonically treated in 7.5 mL of deionized water for 60 minutes to form a homogeneous suspension. 7.5 mL of concentrated hydrochloric acid and 0.25 mL of tetrabutyl titanate (TBT) were added to the suspension and stirred for 10 minutes. The mixture was then transferred to a 25 mL Teflon-lined reactor, with the conductive side of the FTO glass (approximately 5 cm2) placed against the inner wall of the Teflon-lined high-pressure reactor. The reactor was maintained at 150°C for 12 hours and then cooled naturally to room temperature. After the reaction, the FTO glass was removed, washed several times with anhydrous ethanol and deionized water, and dried overnight in a vacuum drying oven at 60°C. The samples were labeled TR, GTR-1, GTR-2, GTR-3, and GTR-4 based on the GO mass ratio.

[0038] Preparation of RGX cathode:

[0039] Preparation of RGO: Graphene oxide (GO) was synthesized using expanded graphite powder using a modified Hummer method. GO was ultrasonically treated in aqueous solution for 60 minutes to form a uniform suspension (4 mg / mL). Ascorbic acid (L-AA) (L-AA:GO mass ratio of 3:1) was added, and the mixture was magnetically stirred for 30 minutes. The mixture was then allowed to stand at 40°C for 24 hours and cooled naturally to room temperature to obtain an RGO hydrogel. Excess ascorbic acid was then repeatedly washed with deionized water to remove the excess. Finally, the hydrogel was frozen in a -50°C ultra-low temperature freezer for 48 hours and then freeze-dried in a -50°C freezer for 48 hours to obtain an RGO aerogel.

[0040] Preparation of the RGX electrode: Reduced graphite oxide (RGO), conductive carbon black (CB), xanthan gum (XG), and polyacrylic acid (PAA) were dissolved in deionized water at a mass ratio of 8:1:0.5:0.5. The mixture was then magnetically stirred for 48 hours to obtain a uniform RGX electrode slurry. Reduced graphite oxide (RGO) served as the active material, conductive carbon black (CB) as the conductive additive, and xanthan gum (XG) and polyacrylic acid (PAA) as the composite binder. The resulting slurry was then evenly coated onto a 5 mm tall graphite plate using a coater with a 200 μm gap height. The slurry was then dried under vacuum at 80°C for 12 hours and further at 155°C for 4 hours to obtain the RGX-1 electrode.

[0041] By changing the mass ratio of reduced graphite oxide (RGO), conductive carbon black and composite binder (as shown in Figure 1b), RGX-2 (7:2:0.5:0.5) and RGX-3 (6:2:1:1) electrodes were prepared using the same method.

[0042] The MMCD device was constructed with GTR-2 and RGX-1 as the photoanode and cathode, respectively. The uranium capture experimental results (for specific adsorption conditions, see the application example, in the case of an initial concentration of 50 mg L-1 of U(VI) electrolyte) were obtained. -1 , pH = 5, applied voltage of 1.0 V): The uranium removal rate was 90.5% within 180 min, and the uranium removal synergy coefficient was 1.43.

[0043] Example 2

[0044] Preparation of the GTR anode: First, a FTO substrate (F:SnO2, 15U) was ultrasonically cleaned in a mixture of 2-propanol, anhydrous ethanol, and acetone (volume ratio 1:1:1) for at least 60 minutes and then dried in vacuum at 60°C. GO powder (GO powder mass was 7% of the mass of tetrabutyl titanate) was ultrasonically treated in 7.5 mL of deionized water for 60 minutes to form a homogeneous suspension. 7.5 mL of concentrated hydrochloric acid and 0.25 mL of tetrabutyl titanate (TBT) were added to the suspension and stirred for 10 minutes. The mixture was then transferred to a 25 mL Teflon-lined reactor, with the conductive side of the FTO glass (approximately 5 cm2) leaning against the inner wall of the Teflon-lined high-pressure reactor. The reactor was maintained at 160°C for 8 hours and then cooled naturally to room temperature. After the reaction, the FTO glass was removed, rinsed several times with anhydrous ethanol and deionized water, and dried in a vacuum oven at 60°C overnight. According to the mass ratio of GO, the samples were labeled as GTR.

[0045] Preparation of RGX cathode:

[0046] Preparation of RGO: Graphene oxide (GO) was synthesized using expanded graphite powder using a modified Hummer method. GO was ultrasonically treated in aqueous solution for 60 minutes to form a uniform suspension (4 mg / mL). Ascorbic acid (L-AA) (L-AA:GO mass ratio of 2:1) was added, and the mixture was magnetically stirred for 30 minutes. The mixture was then allowed to stand at 45°C for 18 hours and cooled naturally to room temperature to obtain an RGO hydrogel. Excess ascorbic acid was then repeatedly washed with deionized water to remove the excess. Finally, the hydrogel was frozen in a -50°C freezer for 48 hours and then freeze-dried in a -50°C freezer for 48 hours to obtain an RGO aerogel.

[0047] Preparation of the RGX electrode: Reduced graphite oxide (RGO), conductive carbon black (CB), xanthan gum (XG), and polyacrylic acid (PAA) were dissolved in deionized water at a mass ratio of 7:2:0.65:0.35. The mixture was then magnetically stirred for 48 hours to obtain a uniform RGX electrode slurry. The slurry consisted of reduced graphite oxide (RGO) as the active material, conductive carbon black (CB) as the conductive additive, and xanthan gum (XG) and polyacrylic acid (PAA) as the composite binder. The resulting slurry was then evenly coated onto a 5 mm tall graphite plate using a coater with a gap height of 200 μm. The slurry was then dried under vacuum at 80°C for 12 hours and further at 155°C for 4 hours to obtain the RGX electrode.

[0048] The MMCD device was constructed with GTR and RGX as the photoanode and cathode, respectively. The uranium capture experimental results (for specific adsorption conditions, see the application example, in the case of an initial concentration of 50 mg L-1 of U(VI) electrolyte) were obtained. -1 , pH = 5, applied voltage of 1.0 V): The uranium removal rate was 90.3% within 180 min, and the uranium removal synergy coefficient was 1.46.

[0049] Application Examples

[0050] In 4×4×4cm 3 The MMCD device mainly consists of four parts: light source (xenon lamp, PLS-SXE300, AM1.5, 100mWcm -2 ), electrochemical workstation, magnetic stirrer, and transparent quartz cell. Specifically, the GTR anode and RGX cathode were assembled in quartz cells, and a xenon lamp was installed on the back of the GTR electrode as a simulated light source. The effective active area of ​​the anode and cathode was 1.5 × 3.5 cm 2 . The electrochemical workstation is used to adjust the voltage and reaction parameters. In order to reduce the effect of water evaporation on the removal of uranium ions in the reaction tank, the top of the quartz reaction tank was sealed before the MMCD experiment. In the MMCD experiment, 40 ml of a mixed solution of UO2(NO3)2⋅6H2O and 0.1MNa2SO4 was placed in the quartz cell as an electrolyte solution. Magnetic stirring was continuously performed during the experiment to ensure that the uranium ions could fully contact the anode and cathode surfaces. The pH value of the solution in the reaction cell was adjusted by 0.1M NaOH and 0.1M HNO3.

[0051] At the same time, as a control, the removal rate of U(VI) in EC (bias, no irradiation) and PC experiments (irradiation, no bias) was also studied under the same electrode as the MMCD method. The initial concentration of uranium (VI) ranged from 20 to 400 mg / L. During the experiment, samples were taken every 30 minutes, and the concentration of uranium solution was determined by UV-visible spectrophotometry at a wavelength of 652 nm using azoarsine III as a chelating indicator. After each adsorption experiment, the electrode was immersed in 0.1 M HNO3 solution to achieve the desorption effect. The uranium removal rate (%) and synergistic effect were calculated using formulas (1) and (2):

[0052]

[0053]

[0054] Among them, C0 (mg / L) and C t (mg / L) represents the initial uranium concentration at time 0 and at t (minutes), respectively. and represent the U(IV) removal rates from the solution by the MMCD and EC methods, respectively. represents the uranium removal rate by the GTR anode under irradiation and no-voltage conditions. In the MMCD experiments, 2 mL of methanol was added as a sacrificial agent to reduce U(IV) reoxidation caused by holes generated by the GTR anode during irradiation.

[0055] Material characterization:

[0056] Morphology, structure and performance of GTR anode: The morphology of TR and GTR composites was analyzed by scanning electron microscopy images. Figure 1 As can be clearly observed in Figures 1a and 1b, the TiO2 nanorods exhibit a neatly arranged one-dimensional rod-like morphology with relatively uniform size. The cross-section of these TiO2 nanorods is a quasi-square with a length and width of 200-300 nm. For the GTR composite, a small amount of graphene does not significantly alter the morphology of the TiO2 nanorods. Furthermore, after the hydrothermal reaction, the wrinkled layered GO forms a tight bond with the TiO2 nanorods by capping or entanglement ( Figure 1 c and 1d). From the side view of GTR-2 ( Figure 1 e) It can be seen that the TiO2 nanorods grow uniformly, with a relatively smooth surface, densely distributed perpendicular to the FTO glass substrate, and have an average length of approximately 2.7μm. TEM further characterized the morphology and crystal structure of the TR and GTR composites. For the 1D / 2D GTR composite, rod-shaped TiO2 ( Figure 1 f). Figure 1g shows the distribution of O, Ti, and C elements, confirming the presence and relatively uniform distribution of graphene in 1D / 2D GTR-2. Zooming in on a specific area, lattice fringes can be clearly observed, with plane spacings of ~0.24nm and ~0.32nm, respectively, corresponding to the (101) and (110) planes of rutile TiO2 crystals ( Figure 1 h). All these phenomena verify the successful preparation of 1D / 2D GTR composites and their close integration, which is beneficial to the rapid transport and separation of photogenerated electrons and holes.

[0057] The crystal structure of the photoanode samples was characterized by XRD. Similar anatase crystal peaks can be clearly observed in the XRD patterns of GTR-2 and pure TR, indicating that the introduction of graphene did not destroy the crystal structure of TiO2 nanorods ( Figure 2 a). The diffraction peaks at 26.6°, 36.13°, 41.37°, 54.53°, 62.86°, 65.65°, and 69.88° correspond to the (110), (101), (111), (211), (002), (221), and (112) crystal planes of anatase TiO2, respectively (JCPDS NO. 21-1276).

[0058] XPS further analyzed the interaction between two-dimensional graphene and one-dimensional nanorods. Figure 2 As shown in Figure b, compared with pure TR, in addition to the Ti and O peaks, a more obvious C1s peak appears in GTR-2, which confirms the presence of graphene. For RGO, the high-resolution XPS peaks of C1 can be divided into CC, C-OH, COC, C=O and OC=O peaks ( Figure 2 c). However, in the high-resolution C1s spectrum of GTR-2 ( Figure 2 d), a new peak representing the Ti-OC bond appears at 287.9 ​​eV; in the O1s spectrum of GTR-2 ( Figure 2 In Figure e), in addition to the Ti-O-Ti peak and the CO peak, a Ti-OC signal can also be observed. These phenomena indicate that during the in situ growth process, a chemical interaction in the form of Ti-OC bonds occurred between the TiO2 nanorods and graphene. In addition, for pure TR, the binding energy representing the Ti2p peak is located at 458.1eV, while for GTR-2, the binding energy shifts to 458.9eV, which further indicates that the chemical environments of the TR and GTR composites are different ( Figure 2 f). It is worth mentioning that the bridging structure of the Ti-OC bond is conducive to the formation of an electron transfer channel between TiO2 nanorods and graphene, thereby greatly promoting the rapid transmission of interfacial electrons.

[0059] The optical properties of pure TR and GTR composites were recorded using UV-visible diffuse reflectance spectroscopy. Figure 3 As can be seen, the absorption edges of the GTR composites exhibit varying degrees of redshift toward the visible light region compared to pure TR. Among them, GTR-2 exhibits the broadest adsorption range in the visible light region, indicating its enhanced light-harvesting capability. These phenomena are frequently observed in doped semiconductors and indicate electron transfer between graphene and TiO2 nanorods, concomitantly narrowing the GTR band gap. Figure 3 The Tauc's plot is shown in b, and the optical band gap of each sample was estimated using the Tauc's / David-Mott model (Eq. (3)).

[0060]

[0061] Among them, represents the absorption index, represents Planck constant, represents frequency, is related to the semiconductor type, and is the band gap value of the semiconductor.

[0062] Based on linear extrapolation of the Tauc's plot, the band gaps of pure TR, RGO, GTR-1, GTR-2, GTR-3, and GTR-4 are 2.98 eV, 2.20 eV, 2.89 eV, 2.84 eV, 2.87 eV, and 2.93 eV, respectively. Clearly, the reduced band gap of the GTR composite compared to pure TR indicates that the introduction of graphene has a positive effect on improving photocatalytic activity. This reduction in the band gap is likely due to the formation of Ti-OC bonds between the one-dimensional TiO2 nanorods and the two-dimensional graphene, which significantly reduces the resistance to electron conversion. The enhanced light-harvesting capacity and reduced band gap of the GTR composite mean that light can be more efficiently utilized, generating charge carriers with less energy, thereby enhancing the photocatalytic performance of the GTR photoanode.

[0063] The transient photocurrent density curves of pure TR and GTR composites are shown in Figure 3 c. It can be seen that with the addition of graphene, the transient photocurrent of all samples increases. Among them, the transient photocurrent of the GTR-2 composite material is the highest. These phenomena indicate that when the graphene content is 5wt%, a large number of photoelectrons can be generated in the GTR-2 photocatalyst and effectively separated under light. In addition, the electrochemical impedance spectroscopy (EIS) of pure TR and GTR composite materials was also studied. Figure 3 As can be clearly seen in Figure d, the arc diameter of GTR-2 is the smallest, while that of pure TR is the largest. This result indicates that the introduction of graphene helps reduce carrier transport resistance, minimizes the interfacial transport resistance between GTR-2 and the solution, and maximizes photoelectron transport efficiency. Together, these results confirm that the two-dimensional graphene in the GTR-2 composite structure can serve as a photoelectron transport channel, thereby enhancing its photoelectrocatalytic activity.

[0064] Morphology, structure and properties of RGX cathode: L-AA was used to reduce GO suspension to form porous RGO hydrogel ( Figure 4 a). After freeze-drying, RGO aerogel was used as the active material to make RGX cathode ( Figure 4 b). Figure 4 (c-d) show the surface morphology of RGO aerogel. It can be seen that there are a large number of interconnected macropores in the aerogel, with pore sizes ranging from tens of microns to hundreds of microns, and the pore walls are relatively coarse. The surface morphology of RGX electrode is also as shown. Figure 4 (e-f) As shown. The surface of the RGX electrode is rough and uniform. Granular conductive carbon black fills the interlayers of the wrinkled graphene nanosheets, forming an interconnected conductive network. At the same time, the structure of the RGX electrode maintains a large number of pores, providing a very favorable environment for the penetration of U (VI) ions. Figure 4 The elemental map of the RGX electrode in g shows an obvious O map, which mainly comes from the XG-PAA binder. The O map is consistent with the C elemental map, indicating that the O-containing groups are uniformly dispersed on the surface of the RGX electrode.

[0065] The electrochemical properties of RGX were studied by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Figure 5 Figure a shows the Nyquist plot for an RGX cathode. The curve consists of a nearly 45-degree straight line at low frequencies, a Warburg zone at medium frequencies, and an incomplete semi-arc at high frequencies. The x-intercept represents the electrode volume resistance, the arc diameter represents the charge transfer resistance between the electrode and the electrolyte, and the slope of the line is related to ion diffusion. Figure 5 The inset in (a) shows that the x-intercept and arc diameter of the RGX-1 electrode are smaller than those of RGX-2 and RGX-3, while the slope of the low-frequency line is larger than that of RGX-2 and RGX-3. These evidences indicate that the RGX-1 electrode has excellent electrical conductivity, charge transfer ability, and ion diffusion capacity.

[0066] Figure 5 b shows the potential window of -0.5V~0.5V with a speed of 5mVs -1 The CV curves of each electrode were scanned at a scan rate of 100 nm. All RGX electrodes showed quasi-rectangular symmetrical curves, which indicated that the RGX electrodes had good EDL capacitance characteristics. The pair of weak broad peaks appearing in the CV curves may be a small contribution from the pseudocapacitance of the RGX containing O groups. Among the three electrodes, the RGX-1 electrode had the largest CV curve area. In addition, at a current density of 0.8 A g -1 The charge and discharge time of RGX-1 sample is the longest when RGX-1 has the largest specific capacitance. Figure 5c~e describe the CV curves of RGX-1, RGX-2 and RGX-3 electrodes at different scan rates. -1 Increased to 40 mVs -1 All RGX electrodes maintained relatively complete rectangular curves. As the scan speed increased, the CV curves gradually shifted from rectangular to elliptical, forming a leaf shape. This phenomenon may be due to the fact that as the scan speed increased, ions did not have enough time to penetrate the pore structure of the RGX electrode to form a double layer. Figure 5 f calculated the specific capacitance of RGX electrode at different scan speeds. Specific capacitance usually plays a crucial role in evaluating the capacitive behavior of materials. -1 At a scan rate of 1.5 GHz, the specific capacitance of the RGX-1 electrode is 180.8 F g -1 , while the specific capacitances of RGX-2 and RGX-3 electrodes are 168.3 Fg -1 and 159.3 Fg -1 As the scan rate increases, the specific capacitance of all RGX electrodes continues to decrease due to the influence of ion diffusion rate. However, even when the scan rate is increased to 60 mV s -1 , the specific capacitance of RGX-1 is still higher than that of RGX-2 (60.6 Fg -1 ) and RGX-3 (60.5 Fg -1 ). According to the results of EIS, CV, and GCD curves, the RGX-1 electrode shows excellent conductivity and specific capacitance characteristics, which are crucial for the electrosorption of U(VI) ions.

[0067] MMCD capture of uranium (VI): According to the above analysis, the GTR-2 electrode has the best photoelectrocatalytic performance, while the RGX-1 electrode has the best electrochemical performance. Therefore, GTR-2 and RGX-1 were selected as the photoanode and cathode to construct the MMCD device for capturing U (VI). First, the CV method was used to study the change in the valence state of uranium ions during the removal process. Figure 6As shown in a, no redox peaks were detected in Na2SO4 solution; however, in Na2SO4+U(VI) solution, reduction peaks appeared between -0.71V vs.SCE and -0.26V vs.SCE, respectively. At the same time, a corresponding oxidation peak appeared at 0.26V vs. SCE. Considering that no redox peaks were observed in Na2SO4 solution, it can be inferred that these redox peaks in Na2SO4+U(VI) solution are related to the redox state changes of uranium species. Previous reports indicated that the reduction peak near -0.26V vs.SCE was from U(VI) to U(V), while the peak near -0.71V vs.SCE was from U(V) to further reduction to U(IV). In addition, the CV curves in Na2SO4+U(VI) solution at different scan rates were also studied. As shown in Figure 6b, as the scan rate increased from 5mV s -1 Increase to 100mVs -1 , the reduction peak shifts to more negative potentials, indicating that more electrons are required to reduce U(VI) at higher scan rates, and the electrochemical reduction is related to the diffusion of U(VI) ions. In addition, the peak currents of the oxidation and reduction peaks are proportional to the square root of the scan rate, further indicating that the electrochemical redox between U(VI) and U(IV) is regulated by diffusion processes. Figure 6 c) CV curves under dark and light conditions were further investigated. In both cases, two reduction peaks and one oxidation peak were observed. The redox current density was significantly higher under light conditions compared to the dark, indicating that in addition to electrons from the applied electric field, photogenerated electrons from the GTR-2 anode also participated in the redox reaction, with illumination favoring the electrochemical reduction of U(VI) to U(IV). Furthermore, the intensity of the oxidation peak was significantly greater than that of the two reduction peaks, indicating that U(VI) exhibits quasi-reversible redox behavior. That is, once generated, U(V) is converted disproportionately into U(VI) and U(IV). These results suggest that the MMCD system, combining an applied electric field with photocatalysis, can promote the electrochemical reduction of U(VI), thereby improving its removal efficiency.

[0068] Considering that the photoelectrons generated by the GTR photoanode can be used to reduce U(VI), and the holes generated will accumulate on the GTR surface and affect the photocatalytic efficiency, methanol is considered to be used as a hole scavenger. Figure 6As can be clearly seen in Figure d, the photocurrent generated by GTR-2 in a mixed solution of Na2SO4 + methanol + U(VI) is higher than that in a Na2SO4 + U(VI) solution. The effect of methanol as a scavenger on U(VI) removal during the MMCD process was further investigated. In a mixed solution of Na2SO4 + U(VI), the U(VI) removal efficiency was only 52% and approached saturation. However, in the presence of methanol, the U(VI) removal efficiency exceeded 90.5%, nearly 1.7 times that of the former. This result indicates that methanol does play a positive role in U(VI) removal. Therefore, although photoelectrons can be transferred to the RGX cathode and separated from holes under an applied bias in the MMCD method, the addition of methanol can still help further improve the photoelectrocatalytic efficiency.

[0069] Figure 6 eComparison of the removal of U(VI) over time by PC, EC, and MMCD. The removal of U(VI) varied over time for all methods. PC only removed approximately 10 mg L in 3 hours. -1 The removal rate of U(VI) increased rapidly with the increase of voltage. When the applied potential was -1.0 V vs. SC, the adsorption capacity of U(VI) reached about 25.7 mg L -1 The MMCD method achieved a removal efficiency of 51.5%. Its capture capacity for U(VI) surpassed that of the PC and EC methods throughout the entire U(VI) removal process. This phenomenon demonstrates the MMCD method's superiority over PC and EC methods, attributed to its ability to combine multiple mechanisms for U(VI) capture. Furthermore, for the MMCD method, the applied bias also positively impacted U(VI) removal. As the external voltage increased from -0.4 V, -0.7 V, to -1.0 V vs. SCE, the U(VI) removal efficiency steadily increased, ranging from 53.3%, 71.3%, to 90.5%, respectively. However, excessively high input voltages may degrade the electrode's electrochemical performance. When the voltage was below 1.0 V, the intensity of the reduction peak increased significantly with increasing voltage, indicating that the bias had a constructive effect on U(VI) removal. When the voltage reached -1.2 V, the CV curve began to polarize. Excessively high potentials may also induce water splitting and increase energy consumption. Therefore, the maximum voltage used in the present invention is -1.0 V vs. SCE.

[0070] The synergy factor was further calculated to estimate the contributions of electrosorption and photocatalysis. Across all voltage ranges, the synergy factor was greater than 1.00, indicating that the MMCD method captured more U(VI) ions than the PC and EC methods combined. Furthermore, the synergy factor varied slightly with the external bias voltage, reaching a maximum of 1.45 at -0.7 V, which should be attributed to the good match between electrosorption and photocatalysis at this bias voltage. Nevertheless, in this system, the maximum removal rate of the MMCD method occurred at -1.0 V, which was 1.7 times and 4.5 times that of the EC and PC methods, respectively, indicating that the MMCD method has a clear advantage in terms of removal rate, i.e., adsorption capacity.

[0071] To further clarify the effect of different methods on the kinetic rate, the instantaneous removal rate was calculated, which is the removal rate divided by time. Figure 6 As shown in Figure 5, the kinetic rates of the three methods increased rapidly within the first 30 minutes, then gradually decreased and reached an equilibrium value. During the entire U(VI) removal process, the kinetic rate of the MMCD method was the fastest compared to the PC method and the EC method. By calculation, the average kinetic rate of the MMCD method was 116% and 403% faster than that of the EC method and the PC method, respectively. Therefore, the superiority of the MMCD method lies not only in the removal rate but also in the kinetic rate. It is concluded that the external bias plays two important roles in the MMCD treatment: (1) the external negative bias promotes the electrochemical adsorption of uranium species; (2) the external bias drives the photogenerated electrons on the GTR anode to migrate to the RGX-1 cathode, reducing and solidifying the adsorbed uranium species.

[0072] Figure 6 g shows the effect of pH on the removal rate of U(VI) by PC, EC and MMCD methods. It can be clearly seen that in the pH range of 3.0~8.0, the removal rate of U(VI) by MMCD method is higher than that by PC and EC methods. When the pH value is less than 5.0, the removal rates of MMCD and EC methods continue to rise and reach a maximum value at pH=5. When the pH value is higher than 5.0, the removal rates of U(VI) ions by MMCD and EC methods begin to decline. These phenomena may be the result of the combined action of uranyl species and the functional groups on the surface of RGX cathode. At lower pH values, uranyl in the solution is converted into positively charged UO2 2+ 、(UO2)2(OH) 2+ At the same time, as the pH value increases, the carboxyl group gradually deprotonates and becomes electronegative, thereby forming an electrostatic attraction with the positively charged uranyl species. Therefore, the removal rate of U(VI) gradually increases and reaches a maximum value at a pH of 5.0. On the contrary, when the pH value rises above 5.0, the uranyl species in the solution begin to form negatively charged (UO2)3(OH)7 - and UO2(OH)3- The removal efficiency of uranium (VI) is reduced by 1.5% after 24 h. The removal efficiency of uranium (VI) is basically unaffected by pH value. This is probably due to the fact that the GTR-2 anode has fewer electronegative functional groups.

[0073] The effect of initial uranium (VI) concentration on the removal efficiency was further studied ( Figure 6 h). It can be seen that when the initial uranium concentration is 25 mg / L -1 Increased to 100 mg / L -1 When the U(VI) concentration is further increased to 200 mg L, the removal efficiency of PC and EC methods for uranium (VI) decreases significantly, while the removal efficiency of MMCD method remains above 85%. -1 , the removal rate of U(VI) will decrease, which may be due to the fact that it takes more time to capture U(VI) ions.

[0074] The reusability of the electrode is an important aspect of its practical application and economic feasibility. To regenerate the electrode, it is necessary to disconnect the power supply and soak it in 0.1M HNO3 for 10 minutes. Figure 6 It can be clearly seen in Figure k that for the MMCD method, the removal efficiency still remains at 87% after four adsorption-desorption cycles, with only a slight decrease of ~3% compared to the first cycle. In summary, the asymmetric electrode pair has significant reusability and shows great potential in treating U(VI)-containing wastewater.

Claims

1. An asymmetric photoelectrocatalytic system, characterized in that: comprising a photoanode and a cathode connected by an external circuit; The anode material in the photoanode is composed of nano-titanium dioxide loaded on graphene nanosheets; The cathode material in the cathode is formed by bonding reduced graphite oxide aerogel and conductive carbon black via a xanthan gum-polyacrylic acid composite binder; the mass ratio of xanthan gum to polyacrylic acid in the xanthan gum-polyacrylic acid composite binder is (0.5-2):

1.

2. The asymmetric photoelectrocatalytic system according to claim 1, characterized in that: The mass ratio of nano-titanium dioxide to graphene nanosheets in the anode material is (0.1-1):

1.

3. An asymmetric photoelectrocatalytic system according to claim 1 or 2, characterized in that: The nano titanium dioxide is titanium dioxide nanorods.

4. The asymmetric photoelectrocatalytic system according to claim 1, wherein: The mass percentage composition of the reduced graphite oxide aerogel, the conductive carbon black and the xanthan gum-polyacrylic acid composite binder in the cathode material is 60%-80%:10%-20%:10%-20%.

5. The method for preparing an asymmetric photoelectrocatalytic system according to any one of claims 1 to 4, characterized in that: The following steps are involved: 1) Graphene oxide is dispersed in water to obtain a graphene suspension, an acid catalyst and a titanate are added to the graphene suspension for hydrolysis reaction, the resulting reaction solution is transferred to a hydrothermal reactor, and an FTO substrate is added to carry out a hydrothermal reaction to obtain a photoanode; 2) dispersing graphene oxide in water to obtain a graphene suspension, adding a reducing agent to the graphene suspension for a reduction reaction to obtain a reduced graphene oxide hydrogel, and freeze-drying the reduced graphene oxide hydrogel to obtain a reduced graphene oxide aerogel; 3) The reduced graphene oxide aerogel is mixed with conductive carbon black and a xanthan gum-polyacrylic acid composite binder by a liquid phase method, coated on a graphite plate, and dried and solidified to obtain a cathode.

6. The method for preparing an asymmetric photoelectrocatalytic system according to claim 5, characterized in that: The hydrothermal reaction conditions are: at a temperature of 130-180° C. and keeping warm for 3-12 hours.

7. The method for preparing an asymmetric photoelectrocatalytic system according to claim 5, characterized in that: The reduction reaction conditions are as follows: first, stirring the reaction at room temperature for 20 to 40 minutes, and then standing the reaction at a temperature of 35 to 45° C. for 6 to 24 hours; the reduction reaction uses ascorbic acid as a reducing agent, and the mass ratio of the reducing agent to graphene oxide is (1 to 3):

1.

8. Use of an asymmetric photoelectrocatalytic system according to any one of claims 1 to 4, characterized in that: Applied to the photoelectrochemical method to extract U(VI) from solution system.

9. The use of an asymmetric photoelectrocatalytic system according to claim 8, characterized in that: The conditions for extracting U(VI) from the solution system using the photoelectrochemical method are as follows: applying light and voltage, the voltage range is -0.4V~-1.2Vvs.SCE, the solution system contains Na2SO4 and methanol, and the pH of the solution system is controlled at 3~8.

Images