Spontaneous electrochemical uranium extraction from wastewater with net electrical energy production

Extracting uranium from uranium mine wastewater is highly important from both the environmental protection and the resource preservation perspectives. However, conventional adsorption methods and zero valent iron-induced reductive precipitation methods have intrinsic limitations. Here we propose a spontaneous electrochemical method that spatially decouples the uranium–adsorption–reduction reactions and the iron oxidation reaction, enabling stable and efficient uranium extraction with net electrical energy output. U(VI) species are firstly adsorbed on a carbonaceous electrode, and subsequently reduced by electrons derived from iron oxidation. Using simulated wastewater, the spontaneous electrochemical method achieves 12-fold higher uranium extraction efficiency in comparison with the adsorption method. Using real wastewater, the uranium extraction efficiency reaches 303 mg g−1 upon 60 h operation with simultaneous net electrical energy production of 0.65 Wh m−2 with an operating cost of only USD 3.94–6.94 per kg of U. This work can pave a new avenue for cost-effective uranium recovery from mine wastewater. Finding an efficient way to extract uranium from uranium mine wastewater is an essential environmental requirement. A spontaneous electrochemical method is now shown to enable stable and efficient uranium extraction with net electrical energy output.

Extracting uranium from uranium mine wastewater is highly important from both the environmental protection and the resource preservation perspectives.However, conventional adsorption methods and zero valent iron-induced reductive precipitation methods have intrinsic limitations.Here we propose a spontaneous electrochemical method that spatially decouples the uranium-adsorption-reduction reactions and the iron oxidation reaction, enabling stable and efficient uranium extraction with net electrical energy output.U(VI) species are firstly adsorbed on a carbonaceous electrode, and subsequently reduced by electrons derived from iron oxidation.Using simulated wastewater, the spontaneous electrochemical method achieves 12-fold higher uranium extraction efficiency in comparison with the adsorption method.Using real wastewater, the uranium extraction efficiency reaches 303 mg g −1 upon 60 h operation with simultaneous net electrical energy production of 0.65 Wh m −2 with an operating cost of only USD 3.94-6.94per kg of U.This work can pave a new avenue for cost-effective uranium recovery from mine wastewater.
As a reliable low-carbon energy source, nuclear power has avoided ~74 Gt of CO 2 emissions over the past 50 years, hence playing a crucial role in attaining carbon neutrality 1 .Nuclear power production is predicted to increase in the near future 2 .However, concerns about its sustainability have arisen.First, the security of nuclear fuel supply is questioned.Uranium, a vital nuclear fuel, is mainly supplied from terrestrial mines, but recent predictions have warned that terrestrial uranium reserves could be depleted within a century at the current consumption rate [3][4][5] .Another concern is the environmental impact of uranium mining activities.Mining and milling of uranium ores generate large quantities of wastewater containing highly mobile and toxic hexavalent uranium, that is, U(VI) (present as UO 2 2+ , or its complexes).If not properly treated, these uranium-bearing wastewaters will contaminate the adjacent environment and threaten the local ecosystem 6,7 .
Extracting uranium from mine wastewater is therefore highly interesting, because it simultaneously reduces the negative environmental footprint of the nuclear power industry and alleviates the depletion of conventional uranium resources.
One possible avenue is the use of physicochemical adsorption methods [8][9][10][11][12] .The uranium extraction capacity of a physicochemical adsorption process is constrained by the number of active sites available and accessible to U(VI) species.The availability of active adsorption sites would decrease as the adsorption proceeds, and the adsorbed U(VI) species would repulse other incoming U(VI) species due to Coulomb repulsion, lowering the overall accessibility of available active sites 3 .Once all accessible active sites are saturated, the adsorption of U(VI) stops.This is an intrinsic limitation of physicochemical adsorption methods.Recent studies have shown that the application Article https://doi.org/10.1038/s44221-023-00134-0 The Fe 2+ concentration and the U(VI) concentration are important factors affecting the OCV, that is, the driving force, of the SPEC system, according to the Nernst equation.In practical implementations, the concentration of Fe 2+ in the anode chamber will increase as the operation proceeds, and the U(VI) concentration can vary notably among different wastewaters and will decrease as the extraction proceeds.To further assess the theoretical feasibility of the SPEC method, we used the Nernst equation to model the anode potential E(Fe 2+ /Fe) as a function of the Fe 2+ concentration and the cathode potential E(U(VI)/ U(IV)) as a function of the U(VI) concentration.The results show that the anode potential increases with increasing Fe 2+ concentration as the operation proceeds, and the cathode potential decreases with decreasing U(VI) concentration (Fig. 1b,c).Nevertheless, even when high Fe 2+ concentration (1,000 mg l −1 ) and extremely low U(VI) concentration (0.00001 mg l −1 ) are combined, the SPEC system still has a theoretical OCV of ~0.512 V, representing sufficient driving force.Thus, the proposed SPEC uranium extraction method is feasible from the thermodynamics perspectives.

Proof of principle
Following the theoretical analysis described above, we carried out experiments using uranium-bearing simulated wastewater to evaluate the practical feasibility of the SPEC method.An H-type two-chamber (300 ml:300 ml) SPEC system consisting of an iron mesh anode and a chitosan-modified carbon felt (CCF; see Supplementary Fig. 1 for characterizations) cathode was constructed for the proof-ofprinciple tests.The CCF electrode had a surface area of 293.77 m 2 g −1 .
A CCF electrode is used here because of its known uranium extraction capacity 15 .Other electrodes, for example, amidoxime-functionalized carbon electrode 3 , may also be used as the working electrode, but the materials development lies beyond the scope of the present study.Diluted H 2 SO 4 solution was used as the anolyte, while uranium-bearing simulated wastewater was filled into the cathode chamber.H 2 SO 4 is the cheapest bulk acid available worldwide, and is widely used in uranium mines for acid leaching process 28 , so it will be easily accessible in uranium mines where uranium mine wastewater is generated.We first assessed the electrical energy recovery capacity of the SPEC system with varied anolyte pH using linear sweep voltammetry (LSV).The obtained polarization curves and the corresponding power curves are shown in Fig. 2a,b, and they suggest that low pH of the anolyte would benefit the electrical energy recovery.As the anolyte pH should not affect the redox potential of the Fe 2+ /Fe couple according to the Nernst equation, it might influence the energy recovery capacity by changing the conductivity of the anolyte.
We subsequently evaluated the uranium extraction performance of the SPEC method in comparison with physicochemical adsorption, using a series of simulated wastewaters with varied initial uranium concentration ranging from 5 to 100 mg l −1 .This concentration range covers most reported uranium concentrations in real mine wastewaters [29][30][31][32][33] .The physicochemical adsorption method showed a saturation uranium extraction efficiency of ~44 mg g −1 .In contrast, the SPEC method exhibited substantially higher uranium extraction efficiency (Supplementary Fig. 2).The uranium extraction capacity of the SPEC system increased drastically with increasing initial uranium concentration (Fig. 2c), and the difference in the uranium extraction performance between the two methods became greater at high initial uranium concentrations (Supplementary Fig. 2).With an initial concentration of 100 mg l −1 , the SPEC method achieved a uranium extraction efficiency of 635 mg g −1 after 23 h of operation, which was 12 times higher than that achieved by adsorption at identical conditions.The superior performance of the SPEC method can be explained by the fact that the extracted U(VI) species were constantly reduced by electrons, which would alleviate the Coulomb repulsion between extracted uranium species and aqueous U(VI) 3,[13][14][15] .These uranium species extracted by the SPEC method may also act as active sites for further uranium of a direct current can reduce the surface-adsorbed U(VI) species and avoid Coulomb repulsion, and therefore substantially promote uranium extraction 3,[13][14][15] , but these methods are rather energy-consuming.Meanwhile, as uranium in its tetravalent state, that is, U(IV), is sparingly soluble and much less toxic, reducing U(VI) to U(IV) is another viable approach for uranium extraction.Among different reductants, zero valent iron (ZVI) has attracted tremendous research interest because of its inexpensive and easily accessible nature, and the Fe/Fe 2+ redox couple has high reactivity and reduction power to drive reduction of U(VI) to U(IV) 7,16,17 .During ZVI-driven uranium extraction, soluble U(VI) species are firstly adsorbed onto the surface of ZVI, and subsequently reduced to U(IV) deposits 7,18 .Nanoscale ZVI (nZVI) materials are therefore favourable because of their large specific surface area [19][20][21][22] .However, in nZVI-based uranium extraction processes, aqueous U(VI) would adsorb onto the surface of nZVI nanoparticles and subsequently be reduced to insoluble U(IV) by Fe, and the extracted uranium would be deposited on the nZVI surface to form uranium-nZVI nanoscale particles 16 .Membrane filtration or sedimentation can be employed as a post-treatment process for recovering extracted uranium via solid-liquid separation, which would add to both the capital and the operating costs 19,23 .In addition, nZVI is prone to oxidation in air and surface passivation in aqueous solutions, and tends to form aggregates in complex water matrices, leading to substantial reactivity loss of nZVI over operating time [24][25][26] .If the oxidation of Fe to Fe 2+ and the reduction of U(VI) to U(IV) can be spatially decoupled, the use of nZVI and the associated drawbacks can be eliminated.The electrons derived from ZVI can be utilized to reduce the surface-adsorbed U(VI) to sparingly soluble U(IV), further promoting the uranium extraction capacity of a sorbent.
In this Article, inspired by these analyses, we present a spontaneous electrochemical (SPEC) method driven by ZVI oxidation for uranium extraction with simultaneous energy recovery.In the SPEC system, the U(VI) adsorption occurs on the surface of a porous carbonaceous cathode, while the oxidation of Fe 0 to Fe 2+ takes place in the anodic chamber, providing electrons that flow through an external circuit to the cathode to drive the reduction of U(VI) to U(IV).The SPEC method has achieved a uranium extraction capacity of 2,438 mg g −1 using simulated wastewater and 303 mg g −1 using real uranium mine wastewater without saturation, and the uranium extraction products are easily recoverable.The whole uranium extraction-recovery process requires no energy input, and net electrical energy production has been attained.This study potentially provides a new avenue for the development of energy-and cost-efficient uranium extraction technology.

Theoretical feasibility of the SPEC method
The concept of the proposed SPEC method for uranium extraction is illustrated in Fig. 1a.During operation, the anodic reaction is oxidation of Fe 0 to Fe 2+ oxidation (equation ( 1)) because it is more thermodynamically favourable than oxidation of Fe 0 to Fe 3+ (ref.27), while the desired cathodic reaction is the reduction of U(VI) to U(IV) (equation (2)).At standard conditions, the SPEC system can theoretically generate an open-cell voltage (OCV) of 0.767 V (according to the overall reaction shown in equation ( 3)) as a driving force for the electron flow that allows simultaneous electrical energy recovery.
Article https://doi.org/10.1038/s44221-023-00134-0extraction reactions (as discussed in detail in Extracted uranium and possible reactive pathways section).As a result, the saturation uranium extraction capacity of the SPEC method is much larger than that of the adsorption method, when identical material is applied.To test this hypothesis, we conducted an additional SPEC extraction experiment with an extremely high initial uranium concentration (1,000 mg l −1 ).
The SPEC method showed a high uranium extraction efficiency of 2,438 mg g −1 after 46 h of operation without showing a saturation trend (Supplementary Fig. 3).A series of SPEC uranium extraction experiments were carried out with varied external load (0-1,000 Ω), to evaluate the electrical energy recovery capacity of the SPEC method.The results show that the uranium extraction performance of the SPEC system did not vary noticeably as the external load was increased from 0 to 1,000 Ω (Fig. 2d), while the output electrical energy from the SPEC system increased with increasing external load (Fig. 2e).The results presented herein clearly evidence that the proposed SPEC method can simultaneously achieve uranium extraction and electrical energy recovery.

Extracted uranium and possible reactive pathways
The extracted uranium species were analysed to decipher the mechanisms of the SPEC uranium extraction method.First, the morphologies of the extracted uranium species after 23 h of extraction by both the SPEC method and the adsorption method were characterized and compared.A thick pale-yellow layer was directly visualized on the CCF electrode after SPEC extraction, but no noticeable morphological change of the CCF electrode was seen in the case of adsorption (Fig. 3a).Scanning electron microscopy (SEM) images confirmed the formation of micrometre-sized particles in the case of SPEC extraction, while the surface of the CCF electrode remained smooth without the formation of any precipitates in the case of adsorption (Fig. 3b and Supplementary Fig. 4).SEM images at higher magnification showed that the micrometre-sized particles observed in the case of SPEC extraction had similar, cuboid-like morphologies, and the energy-dispersive spectroscopy (EDS) results suggested they mainly consisted of U and O (Fig. 3c,d).These results reveal that uranium extraction proceeded via a phase-transformation pathway during the SPEC extraction.
The valence state of the uranium precipitates formed after SPEC extraction was studied through X-ray photoemission spectroscopy (XPS) U 4f analysis.The spectrum shows two major peaks separated from each other by ~10.8 eV, and a small peak on the higher binding energy side (Fig. 4a).The two major peaks located at ~380.0 eV and ~390.8 eV could be assigned to the primary peaks of U4 f 7/2 and U4 f 5/2 , respectively, while the small peak at ~397.4 eV is the satellite peak of U 4f 5/2 .The peak separation of 6.8 eV between the satellite peak and the U 4f 5/2 primary peak suggests the presence of U(IV) in SPEC-extracted uranium [34][35][36] , and the fitting results reveal the dominance of U(IV) content.In contrast, the uranium species extracted by adsorption mainly consisted of U(VI) (Supplementary Fig. 5).Through X-ray diffraction characterization, the uranium precipitates obtained from SPEC extraction were identified as (UO 2 )O 2 •2H 2 O, a U(VI) peroxide crystal also known as metastudtite (Fig. 4b).The structure of these metastudtite species was also confirmed by extended X-ray fine-structure spectroscopy (EXAFS) analysis (Fig. 4c,d and Supplementary Table 1).The lack of signals corresponding to U(IV)-containing phases in the X-ray diffraction spectrum reveals that the obtained U(IV) species were in amorphous form, considering that H 2 O 2 could be generated from dissolved oxygen (DO) by the negatively charged CCF electrode during the Fe-driven SPEC extraction via the two-electron reduction of DO 3,13,37 : The formation of (UO 2 )O 2 •2H 2 O was possibly owing to the oxidation of UO 2 by H 2 O 2 species 3,38,39 , or as a result of the reaction of UO 2+ with H 2 O 2 (ref.40).It is worth mentioning that, although the presence of DO has often been deemed detrimental in electrochemical uranium extraction processes 34 , we found a beneficial role of DO in the SPEC system studied herein.We compared the uranium extraction performance of the SPEC system in ambient air atmosphere and N 2 atmosphere.The results show that the uranium extraction performance of the SPEC system decreased remarkably when operating in N 2 atmosphere (Supplementary Fig. 6a).The recorded current also decreased sharply when operating in N 2 atmosphere (Supplementary Fig. 6b), because the current of DO reduction was absent.Meanwhile, no pale-yellow precipitates were formed and no (UO 2 )O 2 •2H 2 O was obtained when the SPEC system was operated in N 2 atmosphere (Supplementary Fig. 6c,e), suggesting that the DO-mediated (UO 2 )O 2 •2H 2 O-forming pathway played a vital role in the SPEC uranium extraction.
The CCF electrode surface was gradually covered by uranium precipitates (Fig. 4f and Supplementary Fig. 7 and Video 1), but it did not decelerate the uranium extraction reaction (Supplementary Fig. 2d).Previous studies on other electrochemical uranium extraction processes have reported similar findings 3,13 .These phenomena imply that the uranium precipitates played a vital role in the SPEC extraction.
Considering that interfacial electron transfer is a key process for the SPEC uranium extraction, we carried out an in situ electrochemical impedance spectroscopy (EIS) analysis to investigate how the development of the uranium precipitates layer would impact the electron transfer.Figure 4e shows the obtained EIS spectra (Nyquist plots).
The obtained EIS spectra consist of a semicircle in the high/middlefrequency region, a smaller semicircle in the middle/low-frequency region and an inclined line in the low-frequency region, corresponding to the interfacial resistance (R suf ), charge transfer resistance (R ct ) and Warburg impedance (Z W ), respectively [41][42][43][44] .The fitting results clearly show that both R surf and R ct decreased with the development of the uranium precipitates layer during the SPEC extraction (Fig. 4g and Supplementary Table 2).This evidences the beneficial role of the uranium precipitates layer, which acts as an important reaction interface that would greatly promote interfacial electron transfer.Previous studies suggest that either UO 2 or (UO 2 )O 2 •2H 2 O would facilitate electrochemical uranium extraction 3 .We further performed ex situ EIS analysis and compared the EIS spectra of the CCF electrode upon uranium extraction in different atmospheres (Supplementary Fig. 6d).The results confirmed that the presence of DO and the formation of (UO 2 )O 2 •2H 2 O are the keys to achieving efficient SPEC uranium extraction.
Based on these results, a probable working mechanism of the SPEC uranium extraction is proposed in Fig. 5a.First, aqueous U(VI) species (that is, UO 2 2+ ) are adsorbed onto the surface of the CCF electrode.Second, the adsorbed U(VI) species are reduced to amorphous UO 2 by receiving electrons from the CCF electrode.As shown in Fig. 5b, when DO is present, DO will be reduced to H 2 O 2 species.These H 2 O 2 species will subsequently oxidize part of the UO 2 to (UO 2 )O 2 •2H 2 O, or directly react with aqueous U(VI) to form (UO 2 )O 2 •2H 2 O.Both UO 2 and (UO 2 ) O 2 •2H 2 O are sparingly soluble and will precipitate on the surface of the CCF electrode, acting as active sites for further uranium extraction.These uranium precipitates can also reduce the interfacial resistance of the working electrode.When operating in ambient air atmosphere, the formed (UO 2 )O 2 •2H 2 O can further facilitate the charge transfer during SPEC uranium extraction.Third, as the electrode surface is covered by the uranium precipitates, the surfaces of precipitates become the

Recovery of extracted uranium
Upon SPEC extraction, the extracted uranium species must be recovered from the CCF electrode, for future applications.Because the SPEC uranium extraction proceeded via a reduction-precipitation mechanism, the extracted uranium species were fine particles deposited on the electrode.Hence, they can be partly recovered by simply peelingoff from the electrode (Fig. 6a).This method is easy to operate and pollution free, but because of the strong adhesion of the extracted uranium species on the CCF electrode and the fibrous nature of the CCF electrode, part of the extracted uranium species cannot be peeled off by this method (Supplementary Fig. 8).Interestingly, after peeling off, the reused CCF electrode exhibited better uranium extraction performance (Supplementary Fig. 9), probably due to the beneficial roles of the uranium precipitates.A similar phenomenon has been found in other electrochemical uranium extraction systems 34,45 .Alternatively, considering that the predominant extraction products are U(IV) species, the extracted uranium can also be recovered with simultaneous electrical energy production by an electrochemical method.The positive E 0 value (+0.327V versus SHE) of the reduction of U(VI) to U(IV) implies that part of the chemical energy from iron oxidation is stored in U(IV) species during SPEC extraction.By coupling the oxidation of U(IV) to U(VI) with a proper electron acceptor, this chemical energy can be recovered.Meanwhile, the rest of the U(VI) precipitates can be directly dissolved into the recovery solution.Here, we chose DO as the electron acceptor, because of its strong oxidation power and its high availability in aqueous solutions exposed to an air atmosphere.Figure 6b shows a schematic representation of the proposed method (see Methods section for further descriptions).The possible electrochemical reactions are shown in the equations below: Resistance (Ω) )  The recovery experiment results show that all of the SPECextracted uranium can be rapidly recovered within a short time frame (Fig. 6c), and ~830 mWh m −2 of electrical energy was generated (Fig. 6d).

Validation in real uranium mine wastewater
SPEC uranium extraction experiments were carried out in real uranium mine wastewater to validate its feasibility in realistic applications.Photographs of the SPEC modules are shown in Fig. 7a.When operated using real mine wastewater, a single SPEC module could generate an OCV of ~0.820 V and a short-circuit current (SCC) of 0.946 mA (Fig. 7b).Similar to the case using simulated wastewater, the electrical energy recovery capacity varied greatly when changing the pH of the H 2 SO 4 solution (Supplementary Fig. 10).Connecting multiple SPEC modules in series can linearly increase the OCV, while parallel connection can increase the SCC (Fig. 7b), implying good potential for scaling up the system.
We then tested the uranium extraction performance using two SPEC modules, operated in different connection modes (see Fig. 7c for the equivalent circuits).The results show that the uranium extraction performance of the SPEC system did not vary greatly between the different modes (Fig. 7d).Upon 60 h of continuous operation without changing the CCF cathode, the SPEC system operated in serial connection mode achieved a uranium extraction efficiency of 279 mg g −1 , while the extraction efficiency was 303 mg g −1 in parallel mode.The extraction kinetics remained stable throughout the operation (5.3 mg g −1 h −1 in parallel connection mode and 4.7 mg g −1 h −1 in serial connection mode).It is noteworthy that, although the uranium mine wastewater was rather complex with many co-existing metal ions, uranium was still the predominant metal species in the extraction products, as revealed by EDS analysis (Supplementary Fig. 11), implying high applicability of the SPEC method for complex water

Article
https://doi.org/10.1038/s44221-023-00134-0matrices.In addition, during uranium extraction, the SPEC system simultaneously achieved stable electrical energy output (Supplementary Video 2): upon 60 h of operation, 0.65 Wh m −2 and 0.60 Wh m −2 of electrical energy were recovered in serial and parallel connection mode, respectively (Fig. 7e).These results prove the uranium extraction and electrical energy generation capabilities of the SPEC method in a realistic application.

Implications and outlook
Uranium extraction from uranium mine wastewater is of great interest from both the environmental protection and the resource preservation perspectives.To tackle the likely shortage of uranium resources, uranium extraction from wastewater is an important, yet often underestimated, supplement to seawater uranium extraction.Uranium extraction from mine wastewater has also huge environmental benefits.Though uranium extraction from seawater is a hot topic, the importance of uranium extraction from wastewater should not be underestimated.In this study, we introduce an SPEC method powered by iron oxidation that can achieve efficient uranium extraction from real mine wastewater, with high stability.Unlike conventional electrochemical uranium extraction methods that consume electrical energy 3,[13][14][15] , the SPEC method introduced in the present work is fully independent of any external electricity supply and consumes no electrical energy.The formation of a uranium precipitates layer that provides reactive sites and conducts electron flows is the key to obtaining stable and efficient uranium extraction in electrochemical uranium extraction processes.We also reveal a vital role of DO in uranium extraction, which eliminates the need for the inert gas atmosphere during operation that was required by previous studies on other electrochemical uranium extraction methods 34 .Moreover, unlike conventional adsorption methods, the extraction products are ready to recover by peeling.
As the SPEC method requires no electricity input, the main operating cost depends on iron consumption.Iron is a very inexpensive resource, and is even available from waste products in many industries 27 .In our bench-scale SPEC uranium extraction experiments with real mine wastewater without process optimization, the operating cost for uranium extraction is calculated to be USD 3.94-6.94per kg of U (see Supplementary Note 1 and Supplementary Table 3 for details).As a comparison, the current uranium market price is ~USD 152.09 per kg of U (as of May 2022).Hence, using the proposed SPEC method for uranium extraction from real wastewater is practical.Although we used commercial fresh iron mesh in our experiments, it is also possible to use waste iron in practice.Waste iron is easily accessible as industrial waste from many industries 27 .The use of waste iron could further decrease the operating cost and reduce the carbon footprint of the system.
Further optimization and development of the SPEC method are needed to make it fully commercially applicable.For instance, our preliminary investigations found that the flow dynamics and the total area of the CCF working electrode exert a noteworthy influence on the uranium extraction kinetics (Supplementary Fig. 12).Future studies are encouraged to perform more systematic investigations into the effects of reactor design and operating parameters on the SPEC uranium extraction.Another research direction would be electrode design to broaden the applicability of the SPEC method to other application scenarios, for example, seawater uranium extraction.

Thermodynamics calculations
Equation (8) shows the general form of the Nernst equation, where E 0 (M n+ /M) is the potential of a given redox couple under standard conditions, E(M n+ /M) is the potential of the redox couple under specific conditions, R is the gas constant, T is the absolute temperature, n is the number of electrons transferred and F is the Faraday constant.
At room temperature (298 K), the Nernst equation can be simplified to equation (9).

Uranium extraction experiments
For uranium extraction experiments using simulated wastewater, simulated wastewater was prepared by dissolving UO 2 (NO 3 ) 2 •6H 2 O (analytical grade; Hubei Qifei Chemical, China) into natural seawater collected from Bohai Bay (China).The seawater used was filtered through a 0.24 μm filter to remove particles and microorganisms.
Seawater has high conductivity and contains various co-existing ions; thus it can better mimic the complexity of real wastewaters than ultrapure water.An H-type two-chamber (300 ml:300 ml) electrochemical cell was used as the reactor.An iron mesh (20.5 cm 2 ) was washed by using water and placed in the anode chamber, a CCF electrode (2.5 cm 2 , 0.02 g) was placed in the cathode chamber and an anion exchange membrane (AMI-7001; Membranes International, USA) was used to separate the two chambers.Diluted H 2 SO 4 solution (250 ml) with designated pH was filled into the anode chamber, and 250 ml of simulated wastewater was filled into the cathode chamber.Unless otherwise noted, the experiments were done in ambient air atmosphere and rigorous stirring was applied.
For uranium extraction experiments using real wastewater, uranium mine wastewater was collected from a granite-type uranium mine located in western China.The uranium concentration in the real uranium mine wastewater was measured to be ~7.7 mg l −1 after filtration through a 0.24 μm filter to remove suspended solids and microorganisms.The reactors used for real uranium mine wastewater experiments were two-chamber (50 ml:50 ml) electrochemical stack modules (Fig. 7a and Supplementary Fig. 13).An iron mesh (20.5 cm 2 ) was placed in the anode chamber, a CCF electrode (4 cm 2 , 0.032 g) was placed in the cathode chamber and an anion exchange membrane (AMI-7001; Membranes International, USA) was used to separate the two chambers.For each module, 1 l of diluted H 2 SO 4 solution and 1 l of wastewater were stored in storage tanks, respectively.Peristaltic pumps were used to circulate the H 2 SO 4 solution and the wastewater through the reaction chambers.The equivalent circuits of the assembled SPEC system using two modules are shown in Fig. 7c.During long-term operation, the iron mesh, the H 2 SO 4 solution and the wastewater were changed and refilled every 12 h, without changing the CCF electrode.
During experiments, the current was recorded using an EMK1080 current recorder (Emkia Technology, China).The accumulated output energy density was calculated following the equation below: where E out refers to the accumulated output energy density at time t, I refers to the current, R refers to the external load and A CCF refers to the area of the CCF electrode.
The residual uranium concentration in simulated wastewater was measured by the Arsenazo III spectrophotometric method 13 , and the extracted mass of uranium was calculated by comparing the difference between the remaining and the initial uranium concentration in the reaction solution, as instructed by the following equation: Article https://doi.org/10.1038/s44221-023-00134-0 where q t refers to the uranium extraction efficiency at time t, C 0 refers to the initial uranium concentration in the reaction solution, C t refers to the uranium concentration in the reaction solution at time t, m refers to the mass of the CCF electrode and V refers to the volume of the reaction solution.

Recovery of extracted uranium
For extracted uranium recovery experiments, an H-type two-chamber (300 ml:300 ml) electrochemical cell was used as the reactor.
The CCF electrode covered by the known amount of SPEC-extracted uranium (2.5 cm 2 ) was placed in the anode chamber, a carbon felt electrode (4.5 cm 2 ) was placed in the cathode chamber and an anion exchange membrane (AMI-7001; Membranes International, USA) was used to separate the two chambers.Air-saturated 0.1 M H 2 SO 4 solution (250 ml) was filled into both chambers.A 100 Ω resistor was connected between the two electrodes.Blank control experiments were also performed, replacing the uranium-covered CCF electrode with a fresh CCF electrode.
During the experiments, the current was recorded using an EMK1080 current recorder (Emkia Technology, China).The uranium concentration in the anolyte was measured by the Arsenazo III spectrophotometric method, and the recovery rate was calculated by comparing the difference between the mass of recovered uranium in the anolyte and the initial uranium mass on the CCF electrode, as instructed by the following equation: where R t refers to the uranium recovery rate at time t, C t refers to the uranium concentration in the reaction solution in the anode chamber at time t, M CCF refers to the initial uranium mass on the CCF electrode and V a refers to the volume of reaction solution in the anode chamber.

Characterizations
SEM and EDS were conducted using a Tescan Mira 4 FE-SEM equipped with an Xplore 30 EDS system.XPS characterization was carried out using a Thermo K-Alpha+ XPS with an Al Kα source.X-ray diffraction characterization was carried out using a Rigaku D-Max 2500PC with Cu Kα radiation.For EXAFS analysis, the CCF electrode deposited with SPEC uranium extraction products was analysed at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility at room temperature with a Zr foil serving as the reference for calibration.The obtained spectrum was processed by using the ATHENA and ARTEMIS programs 46 .The crystal structure of metastudtite was archived from ref. 47 and used as the starting model for EXAFS fitting, following the procedures described in ref. 48.
LSV and in situ EIS characterizations of the CCF electrode were carried out by using a CHI660E electrochemical workstation.
For the LSV analysis, the experimental set-up was exactly the same as that used for uranium extraction experiments: an iron mesh (20.5 cm 2 ) was placed in the anode chamber containing 250 ml diluted H 2 SO 4 solution (pH 1.2), and a CCF electrode (2.5 cm 2 , 0.02 g) was placed in the cathode chamber containing 250 ml simulated wastewater with an initial U(VI) concentration of 10 mg l −1 .A CHI660E electrochemical workstation was used for the analysis.The scan rate was 0.01 V s −1 .
For the in situ EIS analysis, the experimental set-up was exactly the same as that used for uranium extraction experiments: an iron mesh (20.5 cm 2 ) was placed in the anode chamber containing 250 ml diluted H 2 SO 4 solution (pH 1.2), and a CCF electrode (2.5 cm 2 , 0.02 g) was placed in the cathode chamber containing 250 ml simulated wastewater with an initial U(VI) concentration of 100 mg l −1 .The only difference is that a saturated calomel electrode was used as the reference electrode.A CHI660E electrochemical workstation was used for the analysis.The EIS analysis was carried out at open-circuit potentials at designated time, in the frequency range of 0.01-1×10 6 Hz, with an amplitude of 0.05 V.For the ex situ EIS analysis, before and after 23 h uranium extraction experiment, the CCF working electrode was taken out of the uranium extraction reactor and analysed in simulated wastewater with a U(VI) concentration of 100 mg l −1 .The same reactor configuration as for the uranium extraction experiments was used.In this case, a Pt foil (3 cm 2 ) was used as the counter electrode in the anode chamber containing 250 ml diluted H 2 SO 4 solution (pH 1.2), and a saturated calomel electrode reference electrode was used.The parameters for EIS analysis were identical to those used for in situ EIS analysis.

Fig. 1 |
Fig. 1 | A schematic representation of the SPEC method and the calculated potentials.a, A schematic representation of the SPEC uranium extraction method.b, The calculated anode potential as a function of Fe 2+ concentration.c, The calculated cathode potential as a function of U(VI) concentration.

Fig. 2 |
Fig.2| Uranium extraction from water by the SPEC method with simultaneous energy recovery.a, Polarization curves of the SPEC system containing H 2 SO 4 solution at varied pH as the anodic solution and simulated uranium-containing wastewater (10 mg l −1 ) as the cathodic solution.b, The power of the SPEC system calculated according to the polarization curves.c, The total uranium extraction efficiency of the SPEC method after 23 h of operation using simulated uranium-containing wastewater with varied initial uranium concentration ([U] 0 ).The data are presented as mean ± s.d.(n = 3), while the

Fig. 3 |
Fig. 3 | Morphological characterizations of the extracted uranium.a, Photographs of the fresh CCF electrode (left), the CCF electrode after 23 h of uranium extraction via the SPEC method (centre) and the CCF electrode after 23 h of uranium extraction via the adsorption method (right).b, SEM images of the fresh CCF electrode (left) and the CCF electrode after 23 h of uranium extraction using the SPEC method (centre) and the adsorption method (right).

Fig. 7 |
Fig. 7 | SPEC uranium extraction from real uranium mine wastewater.a, Photographs of the SPEC system (left) and cell (right) used for uranium extraction from real wastewater.AEM, anion exchange membrane.b, System OCV (connected in series) and SCC (connected in parallel) versus module