Engineering robust metal–phenolic network membranes for uranium extraction from seawater

Wei Luo ab, Gao Xiao cd, Fan Tian e, Joseph J. Richardson f, Yaping Wang ab, Jianfei Zhou ab, Junling Guo *abc, Xuepin Liao *ab and Bi Shi ab
aDepartment of Biomass and Leather Engineering, Sichuan University, Chengdu, Sichuan 610065, China. E-mail: xpliao@scu.edu.cn
bNational Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu, Sichuan 610065, China
cWyss Institute for Biologically Inspired Engineering, Harvard University, Boston, Massachusetts 02115, USA. E-mail: junling.guo@wyss.harvard.edu
dCollege of Environment and Resources, Fuzhou University, Fuzhou, Fujian 350108, China
eDepartment of Applied Mathematics and Statistics, Johns Hopkins University, Baltimore, Maryland 21287, USA
fDepartment of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia

Received 16th May 2018 , Accepted 16th October 2018

First published on 17th October 2018


Roughly 4 billion tons of uranium exists in the oceans, which equates to a nearly inexhaustible supply for nuclear power production. However, the extraction of uranium from seawater is highly challenging due the background high salinity and uranium's relatively low concentration (∼3 μg L−1). Current approaches are generally limited by either their selectivity, sustainability, or their economic competitiveness. Here we engineered a biomass-derived microporous membrane, based on the interfacial formation of robust metal–phenolic networks (MPNs), for uranium capture from seawater. These membranes displayed advantages in terms of selectivity, kinetics, capacity, and renewability in both laboratory settings and marine field-testing. The MPN-based membranes showed a greater than ninefold higher uranium extraction capacity (27.81 μg) than conventional methods during a long-term cycling extraction of 10 L of natural seawater from the East China Sea. These results, coupled with our techno-economic analysis, demonstrate that MPN-based membranes are promising economically viable and industrially scalable materials for real-world uranium extraction.



Broader context

Amongst the mature technologies for base-load power generation, nuclear energy is the only low-carbon option. With increasing global energy demand, nuclear power is expected to grow over the coming decades due to its low contribution to greenhouse gasses. Uranium is an important resource for nuclear reactors, and the oceans contain roughly 4 billion tons of uranium. However, the extraction of uranium from seawater is challenging due to its low concentration, the high salinity of seawater, and the abundance of competing ions. Cost analysis has revealed that none of the currently reported seawater extraction methods are competitive against mining uranium, which arises from the synthesis costs and reusability/lifetime of the sorbent materials. In this paper, natural biomass polyphenols were used to form robust supramolecular networks with uranium on commercial porous membranes. These materials display beneficial properties in terms of selectivity, kinetics, capacity, and renewability, for the separation of uranium from seawater. Our field-test from the East China Sea showed a total uranium adsorption mass of 27.81 μg after processing 10 L of seawater with a recovery efficiency of 84%. When integrated with tide-driven systems or desalination plants, uranium production based on our membrane was calculated to be ∼US $275 per kg, suggesting an economically viable method for state-of-the-art uranium extraction from seawater.

Nuclear power generation is capable of providing base-load electricity on a large scale without greenhouse gas emissions, and therefore accounts for approximately 75% of France's electricity generation, 32% of USA's, and 13% of the world's.1,2 Uranium is a key element for nuclear fuel, and the mining and recovery of uranium is of critical importance for the continued energy security of many nations.3 Ocean water contains ∼4.5 billion tons of uranium, which is renewed constantly and theoretically could be used to supply the world with nuclear energy for thousands of years. The key evaluation criteria for uranium extraction include selectivity, kinetics, capacity, and sustainability.4 The extraction of uranium from its high salinity background is extremely challenging because of the low uranium concentration in seawater, and because other cations such as sodium and calcium are abundant in seawater.5–8 Therefore, any viable uranium extraction technology should be capable of efficiently processing large volumes of seawater with fast removal kinetics at a practicable cost.

To increase the uranium adsorption capacity from seawater, elegant advances has been achieved in various extraction technologies,8 including synthetic organic polymers,9–15 protein-based sorbents,16 ionic liquids,17 organic–inorganic frameworks,18–23 carbon-based sorbents,24,25 and alternating current electrochemical methods.26 However, cost analysis has revealed that the performance and properties of these adsorbents, including selectivity and recyclability, are inadequate for economically viable uranium extraction in comparison with terrestrial mining.27 The cost of sorbent-based uranium production from seawater has recently been estimated to be as high as $400–1000 per kg with the main cost drivers being dependent upon the synthesis cost, sorbent materials, reusability, and sorbent lifetime.28 Therefore, it is highly desirable to develop a novel uranium extraction material with high selectivity, capacity, and regenerability that is economically viable and environmentally friendly.29

The conversion of renewable biomass into valuable functional materials for energy generation and environmental application has attracted increasing attention because of the demand to eliminate petrochemical derivatives and other hazardous precursors.30,31 The development of a sustainable route to prepare uranium extraction materials using abundant, low-cost precursors is desirable for commercialization and application.32–34 Polyphenols are the main constituent of biomass and can be obtained from all plants on earth, even those grown on unfertilized, marginal land. Metal–phenolic networks (MPNs), a versatile class of self-assembled supramolecular materials constructed from polyphenol building blocks and metal ions through reversible coordination bonds,35 have recently been introduced as promising materials for energy and environmental applications.36–39 The natural adherent properties of polyphenols enable MPN deposition onto various template materials, allowing for the rational design of functional films and particles.40–42 Additionally, the functionality of the MPN can be tuned through careful choice of the incorporated metal ions.35 As polyphenols themselves have been used to clean up contaminated wastewater,32–34 the ability to transform MPNs into robust porous materials capable of incorporating high concentrations of uranium ions could provide new opportunities for engineering high performance uranium extraction materials.

In this study, uranium–phenolic networks were formed in the presence of seawater on the surface of microporous polyamide membranes, a common used industrial substrate in membrane technology (Fig. 1a and b).43,44 This MPN-based membrane exhibited a number of material advantages for the processing of large volumes of seawater containing low concentrations of uranium, including excellent hydrophilicity, swelling capacity, and mechanical strength. The membrane was applied for uranium recovery from simulated seawater and natural seawater from the East China Sea. An adsorption efficiency of 84% could be maintained during the long-term cycling extraction of 10 L of water from the East China Sea (uranium adsorption mass of 27.81 μg). To the best of our knowledge, this research is the first report of field-testing uranium extraction from the seas surrounding China. The excellent performance coupled with our techno-economic analysis revealed that these MPN-based membrane materials are an attractive, and now proven, technique for uranium extraction that is scalable and amenable to easy integration with industrial-scale seawater processing systems.


image file: c8ee01438h-f1.tif
Fig. 1 Polyphenol functionalization and formation of the MPN-based microporous membrane. (a) SEM image of the microporous polyamide membrane and scheme highlighting uranium capture and release. Due to the unknown precise molecular structure of uranium–phenolic network, the structure presented in scheme is adapted based on the well-studied iron–phenolic complex. (b) Uranium exists in seawater containing a wide range of competitive ions. (c) FTIR spectra of tannins (polyphenols), and the original and polyphenol-functionalized membranes. (d) High resolution XPS O1s spectrum of the polyphenol-functionalized membrane. (e) Contact angles of membranes with different degrees of polyphenol functionalization.

Polyamide microporous membranes are commonly used in industrial membrane technologies as they exhibit excellent swelling capacity, chemical stability, and mechanical strength.45 Polyamide membranes have a highly porous structure (about 500–3000 nm in size) (Fig. S1a and b, ESI), which was retained after polyphenol functionalization through a robust glutaraldehyde-based covalent cross-linking strategy (Fig. 1c–e).46 The presence of polyphenols on the membrane was confirmed by Fourier-transform infrared spectroscopy (FTIR) spectra (Fig. 1c), where the spectrum of the polyphenol-functionalized membrane showed the additional stretching vibrations of the phenolic group ν(C–OH) at 1540, 1455, and 1370 cm−1.47 The surface chemical composition of the polyphenol-functionalized membrane was determined by X-ray photoelectron spectroscopy (XPS) (Fig. 1d and Fig. S2, ESI), as the high-resolution O1s spectra of the polyphenol-functionalized membrane split into two peaks, where the additional peak of C–O at 532.9 eV can be ascribed to the integration of polyphenols on the membrane structure.48 The durability of polyphenol functionalization was examined through long-term flowing experiment where 50 L of seawater was processed through the membrane, where the remaining peak at 532.9 eV suggested the presence of polyphenol moieties on the membrane (Fig. S3, ESI). The polyphenol-functionalized membrane showed higher flux rate than the original membrane and remained stable with the increase of flux volume, likely due to the increased hydrophilicity of the polyphenol-functionalized membranes (Fig. 1e). The good flowing properties of the polyphenol-functionalized membrane in combination with the affinity of polyphenols for binding metals, suggested potential high-performance operability in large-scale seawater processing for uranium extraction (Fig. S4a and b, ESI).

The uranium adsorption capacity of the polyphenol-functionalized membrane rapidly increased as the pH increased from 3.0 to 5.5 and slightly decreased at neural pH (Fig. 2a). Uranium might exists as UO22+, (UO2)2(OH)22+, (UO2)3(OH)2+, and (UO2)4(OH)7+ in different pH ranges (Fig. S5, ESI).8 Meanwhile, based on the distribution of uranium species, this natural pH value of seawater is favorable for the proton dissociation of phenolic hydroxyl groups, which allows for the formation of a robust uranium–phenolic network. In the following experiments examining the adsorption capacity, a pH value of 5.0 was chosen to avoid uranium precipitation and to study the theoretical maximum adsorption performance.8 Generally, various metal ions like Na+, Ca2+, Mg2+, Cu2+ and anions like Cl, NO3, HCO3 are simultaneously found in seawater,12 however these competitive ions only had minor effects on the uranium adsorption capacity even at 100 mmol L−1 (Fig. 2b–g). These data suggested that the MPN-based membranes exhibited a high selectivity for the adsorption of uranium in the pH range seen in coastal seas. Moreover, this membrane showed a considerable adsorption capacity for uranium in a range of 0.25 mmol g−1 to 0.4 mmol g−1 when the operation temperature increased from 293 K to 333 K. Though these values are modest compared with the state-of-art polymer-based adsorbents,15,49 a higher adsorption could be approached by increasing the density of the polyphenol adsorption sites.


image file: c8ee01438h-f2.tif
Fig. 2 Uranium adsorption properties of MPN-based membranes. (a) Effect of initial pH on uranium adsorption capacity. Initial uranium concentration: 1 mmol L−1, temperature 303 K. (b and c) Effect of competing ions on uranium adsorption capacity. Initial uranium concentration: 1 mmol L−1, pH 5.0, temperature 333 K. (d) Adsorption capacity of MPN-based membranes to co-existing metal ions individually. (e–g) Adsorption capacity of uranium and co-existing metals in bilateral mixed systems with different initial concentrations. (h) Uranium adsorption isotherms of MPN-based membranes at different temperatures. Initial uranium concentration: 0.1–2 mmol L−1, pH 5.0. (i) Uranium adsorption kinetics at different temperatures. Initial uranium concentration: 1 mmol L−1, pH 5.0. (j) Effect of initial uranium concentration on the breakthrough profiles for uranium adsorption in the triple layer membranes. qe (mmol g−1) is the equilibrium adsorption capacity; qt (mmol g−1) is the time-dependent adsorption capacity; Ce (mmol L−1) is the equilibrium concentration.

To gain insight into the adsorption process, adsorption isotherm data were further analyzed by the Langmuir and Freundlich models (Fig. 2h and Fig. S6, ESI).39 As summarized in Fig. S6 and Table S1 (ESI), the correlation coefficients obtained by the pseudo-second-order kinetic model fitting were closer to 1.0 than that of the pseudo-first-order model, suggesting that the chelating interaction between uranium and the polyphenol-functionalized membrane is the rate-limiting step of the adsorption process. The calculated theoretical adsorption capacity is close to those obtained from experiments, and consequently the Langmuir model is more suitable for describing the adsorption isotherms of uranium on polyphenol-functionalized membranes. The adsorption rate was relatively rapid and reached adsorption equilibrium within 200 min suggesting batch adsorption during the process in which convective mass transfer was generated on the surface of the microporous membrane with no diffusion mass transfer in the pore structure (Fig. 2i). Due to the temperature-dependence and considerable kinetics of adsorption, it is reasonable to conclude that the adsorption of uranium mainly occurs through the formation of supramolecular uranium–phenolic networks on the microstructure of the membrane. In addition, the breakthrough curves became sharper as the number of membrane layers increased, suggesting that the number of active binding sites on the membranes also increased (Fig. S7, ESI). The breakthrough points of double and triple layer membranes were greatly postponed in comparison to single layer membranes, and the triple layer membranes had a breakthrough point of 100 L m−2 of uranium solution when the initial uranium concentration was 0.05 mmol L−1 (Fig. 2j).

The ability to regenerate membranes and retain long-term performance while processing large-volumes of seawater is a key factor for the development of economically viable uranium extraction materials.50 We performed a filtration experiment over three cycles with a total operation time of 6 h (Fig. 3a). The permeate flux of the polyphenol-functionalized membrane reached 120 L m−2 h−1 even after three cycles, demonstrating a high adsorption efficiency. This MPN-based membrane had a high flux recovery ratio (FRR) value (97%), while the total flux decline ration (FDR) for all three cycles were less than 10% of their initial permeate flux after 2 h (Fig. 3b). Scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDS) element mapping revealed the surface morphology of the membrane after three uranium extraction cycles and confirmed the presence of uranium adsorbed onto the membranes (Fig. 3c). These results demonstrated that the micropores can be retained even after long-term operation (Fig. S8, ESI).


image file: c8ee01438h-f3.tif
Fig. 3 Recycling performance and uranium extraction efficiency of the polyphenol-functionalized membrane for stimulated seawater. (a) Time dependence of permeate flux variations. (b and c) Flux recovery ratio (FRR) and flux decline ration (FDR) performance of the membrane. (c) Desorption curves of the membrane after uranium adsorption. Feed rate: 20 mL min−1. (d) SEM and EDS mapping of the membrane after uranium adsorption. (e) Extraction rate of uranium by membranes with different numbers of layers. (f) Effect of hydrostatic pressure on uranium extraction rate. Initial uranium concentration: 0.05 mmol L−1.

The uranium-saturated membranes can be easily regenerated by using dilute acid to protonate the polyphenol moieties, and disrupt the uranium–phenolic complexation, without significant loss of adsorption capacity. As shown in Fig. 3d, the membrane showed a rapid elution process and about 97% of the uranium adsorbed on the membranes could be desorbed. The maximum uranium concentration recovered from the membrane was ∼3 mmol L−1 (714 mg L−1), which was higher than the original uranium solution by a factor of 60. In addition, the breakthrough curve of the regenerated membrane was almost the same as the initial breakthrough curve (Fig. S9, ESI). To obtain a higher extraction rate, the membrane filtration system was equipped with a multilayer membrane. As shown in Fig. 3e, the uranium extraction of the single layer membrane was inferior to the double and triple layer membrane, and when four membrane layers were used, the uranium extraction rate reached 90%. The extraction rate of each individual membrane in the three-layer membrane extraction system was also determined in detail (Table S2, ESI), and the first layer of the membrane had the highest extraction rate due to the gradient decrease in uranium concentration experienced by the subsequent layers.

The MPN-based membrane extraction system was tested with different hydrostatic pressures potentially used in industrial integrated systems. The extraction processes were carried out by extracting uranium from 1 L of artificial seawater with different numbers of membrane layers. When using single and double layered membranes, the extraction rate rapidly decreased as hydrostatic pressure increased from 1.5 to 5.0 Pa (Fig. 3f), while only slight decreases were observed in the four-layer membrane system. Increasing hydrostatic pressure enhanced the flow rate of seawater and thereby reduced the residence time of seawater, which resulted in a lower extraction rate (Fig. S10, ESI). The uranium extraction rate of the four-layer structure, could reach ∼85% with a high flow rate of 220 L h−1 m−2 and a hydrostatic pressure of 5 Pa.

In order to evaluate the potential practical applications of MPN-based membranes for uranium extraction from seawater, we carried out comprehensive extraction experiments in simulated seawater containing a variety of competing ions. The polyphenol-functionalized membrane exhibited stable uranium extraction even in the presence of competing ions at excess concentrations, including Cl, SO42−, Br, Na+, Mg2+, K+, Ca2+, B, F, Sr2+, Si4+, Cu2+, Zn2+, Ba2+, and Li+ (Fig. 4a). This highly selective extraction performance of polyphenols could possibly be due to the lower hindrance effects of polyphenol groups when coordinating with the large ionic radii of uranium species. Additionally, the special electronic configurations and high valency of uranium ions enable favorable interactions with the electron pairs offered by the hydroxyl groups of polyphenols. The titration curves of metal–phenolic complexes with different ions showed a significantly higher stability for uranium–phenolic complexes compared with other co-existing ions in seawater (Fig. S11, ESI). After this proof-of-concept, marine field tests were performed at the Yellow Sea portion (36°03′11.3′′N 120°25′23.7′′E) of the northern segment of the East China Sea (Fig. 4b and Fig. S12, ESI).51 The concentration of uranium ions in the seawater was detected to be 3.29 ± 0.134 μg L−1 through inductively coupled plasma optical emission spectrometry (ICP-OES). As the MPN-based membranes will potentially be operated with flowing seawater processes, we specifically compared our system with the current reported state-of-the-art technologies based on the half-wave rectified alternating current electrochemical method and other conventional methods focusing on flowing seawater. The MPN-based membrane achieved high-performance extraction of a total uranium mass of 27.81 μg with an extraction rate of 84% after processing 10 L of seawater (containing a total of ∼32.9 μg uranium) (Fig. 4c). This result highlights the remarkable performance of our materials among flowing seawater-based systems. The uranium extraction rate remained around 85% even when the seawater temperatures were varied from 293 to 323 K, which demonstrated the stable performance of MPN-based membranes (Fig. S13, ESI). Additionally, this membrane also exhibited a favorable adsorption affinity for uranium ions in the marine test. Though the molar-based extraction of uranium was modest among other ions, the extracted mass and efficiency of uranium was significantly higher than all of the other ions after processing 10 L of seawater (Fig. 4d and Fig. S14, ESI). The recycling capability of the polyphenol-functionalized membrane was also investigated in the marine test, where the seawater flux showed a slight decrease with all three cycles and the permeate flux could reach 137 L m−2 h−1 after 9 h, which indicated the efficiency of the polyphenol-functionalized membrane could be maintained during seawater extraction (Fig. 4d and Fig. S15, ESI). Antifouling and durability tests showed that the MPN-based materials did not have significant biocontamination or degradation after continuous seawater adsorption for one week, probably due to the antimicrobial properties of polyphenols (Fig. S16, ESI).49 This membrane showed a flux recovery ratio of 92%, and the permeate flux drop ratios for all three cycles were less than 20% of the initial permeate flux after 3 h (Fig. S17, ESI). These results indicated a highly efficient accumulation of uranium even in the high salinity and multi-ionic marine environment.


image file: c8ee01438h-f4.tif
Fig. 4 Marine field studies and techno-economic analysis of uranium extraction from seawater. (a) The effect of competing ions on uranium extraction. The effect of B and F on uranium extraction by adding a layer of Al2O3 padding. The addition of Al2O3 can reduce the influence of F on the uranium extraction rate. (b), Marine field study site location on the local map of East China Sea. Image was acquired from Google map. (c) Extraction rate and mass of uranium from seawater by three layers of MPN-based membranes. Half-wave rectified alternating current electrochemical (HW-ACE) method and conventional methods were used to compared with MPN-based membrane materials.26 (d), Extraction mass (column) and mole (purple square) of uranium and other ions after processing 10 L seawater. Inset shows the extraction rate of uranium depending on the number of cycles. (e) Scheme and material cost analysis of MPN-based membrane system for uranium extraction. (f) Schematic representations and combined cost analysis of three different possible real-world applications of MPN-based membranes for uranium extraction.

Our techno-economic analysis showed that the use of industrial polyphenols dramatically lowered the membrane synthesis cost and demonstrated a high reusability of the as-synthesized materials (Fig. 4e). The cost for uranium extraction based on our system was estimated by the previous studies.15,28,52 Specifically, Lindner and Schneider et al. detailed the basic system parameters used to prepare a cost estimate that primarily includes adsorption capacity, selectivity, operational cost, and adsorbent degradation rate. Based on this theory, we proposed that our system could potentially be applied through three different routes to achieve real-world application, namely electrically-driven pumping, tidal flowing, and integration with desalination plants (Fig. 4f). First, the membranes we used in our system are microporous with low mass transferring resistance. Therefore, the gravitational potential of one meter should be enough for the generation of seawater flow through the membrane. The operational cost for lifting 1 m3 seawater to 1 meter height is estimated to be 0.00272 kilowatt per hour. The extraction of 1 kg uranium requires processing 4 × 105 m3 seawater through our membrane, and therefore the cost of an electricity-driven system is 1088 kilowatt per hour. Based on the current electricity rate in China (RMB ¥0.8/kilowatts per hour), the cost of the electricity needed for extracting 1 kg uranium should be RMB ¥960 (US $148). Adding this to the cost of materials without considering the degradation rate and associated costs, the final cost on our MPN-based membrane using an electricity-driven process is US $428 per kg uranium.

Moreover, MPN-based membranes could be integrated with current natural tide-driven systems and in-line desalination systems being utilized throughout the world. Specifically, China has over 150 desalination plants in operation with more in development, and these require pumping seawater through membranes, and our membranes will not inhibit or contaminate the process. Moreover, the engineering of roll-to-roll or hollow structured MPN-membranes could enable a higher efficiency of uranium adsorption based on these integrated systems. Our cost analysis on natural tide-driven processes and desalination plant integration mainly relate to the material cost of our membrane, and therefore the projected cost was estimated to be approximately US $275 per kg uranium based on the assumed unchanged cost of materials without considering the degradation rate and any additional material cost. Note that though the uncertainty associated with this estimate remains considerable and future experimental data are needed to further reduce sources of uncertainty, the real-world marine test and estimated cost analysis suggested potential advantageous industrial and economic performance of these biomass-based materials for uranium extraction from seawater.

In summary, we have demonstrated that industrial polyphenol products (tannins) can be used to coat commercial membranes to form highly efficient and economically viable materials for the extraction of uranium from seawater. These polyphenol-functionalized membranes displayed advantageous properties in terms of selectivity, kinetics, capacity, and renewability, thus affording a promising platform for the separation and enrichment of uranium from seawater. The high-performance and economic feasibility of these materials arise from the polyphenol-based membrane functionalization, which allows for high specificity and efficient adsorption of uranium from complicated multi-ionic environments (real seawater). In our marine test, the polyphenol-functionalized membranes demonstrated a total uranium adsorption mass of 27.81 μg after a stable and long-term processing of 10 L seawater with a significant extraction rate of 84%. Moreover, 97% of the adsorbed uranium could be recovered through a rapid acidic desorption process without harming the subsequent use of the polyphenol-functionalized membranes. Coupled with our techno-economic analysis, we anticipate that this technology, and similar materials derived from natural products, has potential for efficiently extracting uranium from seawater at an economically viable scale and cost.

Conflicts of interest

The metal-phenolic network-based membranes used for uranium adsorption has been filed as a patent application CN108031450A submitted by Sichuan University.

Acknowledgements

This work was supported by the National Science and Technology Major Project (Grant No. 2017ZX07402004). National Natural Science Foundation of China (Grant No. 21506036). J. Guo is grateful for the Fellowships from Wyss Institute at Harvard University. J. J. R. acknowledges Japan Society for the Promotion of Science (JSPS) funding project (ID PE17019). We acknowledge Dr Blaise Tardy and Yana Klyachina for the helpful discussion.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: General materials details; synthesis and characterization; details regarding uranium adsorption kinetics; continuous extraction of uranium experimental details; anti-fouling testing; marine-field test of uranium extraction from seawater details. See DOI: 10.1039/c8ee01438h

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