Mining Critical Metals and Elements from Seawater: Opportunities and

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Mining Critical Metals and Elements from Seawater: Opportunities and Challenges Mamadou S. Diallo,*,†,‡ Madhusudhana Rao Kotte,† and Manki Cho† †

Graduate School of EEWS (Energy, Environment, Water and Sustainability), Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon, South Korea ‡ Environmental Science and Engineering, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States metals and elements that are currently utilized in industrial manufacturing and energy generation, conversion and storage are produced through the mining, extraction and processing of mineral ores.5 Because there is a significant lag time between the discovery of new virgin ores and the commissioning of new mines,5 current and future shortages of critical metals and elements cannot be addressed by just opening new mines and mineral/metal extraction and processing facilities. Moreover, mining has a heavy environmental footprint, that is, it requires significant amounts of land, energy, and water and generates a lot of wastes.5 During the last two decades, advances in industrial ecology (e.g., material flow analysis), water purification (e.g., desalination) and resource recovery have established that seawater, brines and industrial wastewater are important and largely untapped sources of critical metals and elements.6−12 This feature article discusses the opportunities and challenges of mining critical metals and valuable elements from seawater. Following the “Introduction”, we discuss the potential of oceans as sources of critical metals and elements. The availability and sustainable supply of technology metals We then review recent work on the development of high and valuable elements is critical to the global economy. There is capacity chelating ligands and separation materials for the a growing realization that the development and deployment of selective extraction of critical and valuable metal ions from the clean energy technologies and sustainable products and aqueous solutions and seawater. Finally, we discuss the manufacturing industries of the 21st century will require large integration of metal mining systems into desalination plants amounts of critical metals and valuable elements including rareand conclude this article by providing an outlook of the future earth elements (REEs), platinum group metals (PGMs), of seawater metal mining and resource recovery. lithium, copper, cobalt, silver, and gold. Advances in industrial ecology, water purification, and resource recovery have established that seawater is an important and largely untapped OCEANS AS SOURCES OF CRITICAL METALS AND source of technology metals and valuable elements. This feature ELEMENTS article discusses the opportunities and challenges of mining Oceans cover 71% of Planet Earth13 and store approximately critical metals and elements from seawater. We highlight recent 97% of its water.14 They contain large deposits of fossil fuels15 advances and provide an outlook of the future of metal mining (e.g., oil and methane hydrates) and huge amounts of and resource recovery from seawater. embedded renewable energy.16,17 Mixing of ocean water and INTRODUCTION freshwater produces a salinity gradient that can be exploited to generate electricity using pressure retarded osmosis (PRO)16 or Recent stresses in the global market of rare-earth elements reverse electrodialysis (RED).16 In contrast, the process of (REEs) have brought the availability and supply of technology ocean thermal energy conversion (OTEC) utilizes the metals to the forefront of the sustainability debate and research temperature difference between warm ocean surface water 1−4 agenda. Metals are used to fabricate the critical components and cold deep ocean water to generate electricity.17 Oceans also of numerous products and finished goods, including airplanes, contain vast mineral deposits18 that are rich in critical metals automobiles, smart phones, and biomedical devices.5 There is a and valuable elements including ferromanganese (Fe−Mn) growing realization that the development and deployment of crusts,19 polymetallic nodules19 and seabed massive sulfides.20 the clean energy technologies and sustainable products, Fe−Mn crusts consist of mineral deposits (∼1−260 mm in processes and manufacturing industries of the 21st century will also require large amounts of critical metals and valuable Special Issue: Critical Materials Recovery from Solutions and Wastes elements including REEs, platinum group metals (PGMs), lithium, copper, cobalt, silver, and gold. Most of the critical

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© XXXX American Chemical Society

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DOI: 10.1021/acs.est.5b00463 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Table 1. Estimated Amounts of Selected Critical Metals and Valuable Elements in Terrestrial Reserves and Oceans a

element

terrestrial reserves (×106 tonnes)

Cu Ni V Mo Li Co Nb Ag Au U

690 74 14 11 13 7.2 4.3 0.52 0.054 g 5.9025

a

2013 world production (×106 tonnes) 17.9 2.49 0.076 0.27 0.035 0.12 0.051 0.026 0.00277 h 0.05937

2013 reserve supply ratio (years)

CCZ nodules (×106 tonnes)

d seawater concentration (mg/L)

38 30 184 41 371 60 84 20 19 99

226 274 9.4 12 2.8 44 0.46 f NA f NA f NA

0.0009 0.0066 0.0019 0.010 0.178 0.00039 0.000015 0.00028 0.000011 0.0033

b

c

e

dissolved metals in seawater (×106 tonnes) 1170 8580 2470 13 000 231 400 507 1.3 364 14.3 4290

a Data compiled from the 2014 USGS mineral commodity summaries.21 bReserve supply ratio (years) = TR/WP; where TR (x 106 tonnes) and WP (×106 tonnes/year) are equal to the terrestrial reserves and 2013 annual world production, respectively. cPolymetallic Fe−Mn nodules from the Clarion-Clipperton Zone (CCZ) located in the Pacific Ocean midway between Hawaii and Mexico.19 The CCZ has some of the largest and better characterized deposits of polymetallic Fe−Mn nodules.19 dData taken from Anthony.6 eFollowing Bardi,7 the total amount of dissolved in seawater was calculated assuming a total ocean volume of 1.3 × 109 Km3. fNA: Not available. gData taken from the World Nuclear Association.22 hData taken from the World Nuclear Association.23

Figure 1. Seawater mining: extraction of dissolved ions of critical metals and elements from seawater using conventional and emerging separation processes. Adapted from Fromer and Diallo.2

nucleus.18 They are predominantly found in the seabed sediments of ocean abyssal plains at water depths of 4000− 6500 m.18 Seabed massive sulfides (SMS) are sulfur-based mineral deposits that are found at extinct hydrothermal vents.20 They are formed by precipitation when hot mineral-rich fluids (∼400 °C) come into contact with ambient cooler seawater at hydrothermal vent sites.18,20 In contrast to Fe−Mn crusts and polymetallic nodules, metal-rich SMS can be found at shallower water depths of 1000−3500 m.18 Most SMS are rich in Cu and Zn with some deposits containing significant amounts of silver (up to 1200 ppm) and gold (up to 20 ppm).20 In addition to

thickness) that are formed by the sorption and precipitation of MnO2 and FeOOH colloids/particles onto the surfaces of the substrate rocks of seamounts, ridges and plateaus typically found at depths of 6000−7400 m.18 Because MnO2 and FeOOH colloids/particles serve as sinks for dissolved ions in seawater, Fe−Mn crusts contain appreciable amounts of critical metals and valuable elements (e.g., Co, Mo, Pt, and W).18 In contrast to Fe−Mn crusts, polymetallic nodules are richer in Ni, Cu, Li, and REEs.18 Polymetallic nodules (∼1−12 cm in diameter) are formed by the deposition and accumulation of MnO2 and FeOOH colloids/particles onto a core (seed) B


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Environmental Science & Technology

Figure 2. Chelating agents and extractants for the selective recovery of critical and valuable metal ions from aqueous solutions. (A) Ligand chemistry and architecture and (B) Cu(II) coordination to selected monodentate, bidentate, and macrocyclic ligands with nitrogen donors.

and recovery of critical metals and valuable elements from seawater (Figure 1) has the potential to increase their reservesupply ratios with little or no significant long-term adverse impact on marine ecosystems. However, metal mining from seawater is not economical if seawater is pumped through a metal recovery system without the cogeneration of desalinated water.7 Below, we examine one of the major challenges in seawater metal mining; that is, the availability of high capacity and selective chelating ligands that can be processed and configured into low-energy metal extraction materials and modules that can be integrated into desalination plants to leverage the amounts of energy spent in pumping and pretreatment.

Fe−Mn crusts, polymetallic nodules and SMS, oceans contain large amounts of dissolved ions (∼30−45 g/L) including hydrated ions of critical metals/elements such as Li, Mo, Ni, Zn, V, and Au.7 Table 1 lists estimates of the amounts of selected critical metals and valuable elements in terrestrial reserves, ocean deposits and seawater including Cu, Ni, V, Mo, Li, Co, Nb, Ag, Au and U. Estimates of the total reserves and 2013 annual productions of all metals/elements but uranium were taken from the 2014 USGS mineral commodity summaries.21 For uranium, estimates of its total reserves and 2013 annual production were provided by the World Nuclear Association.22,23 It is worth mentioning that seven of the critical metals/elements listed in Table 1 have limited and tighter reserve-supply ratios; that is ∼20 years for Au and Ag, 30 years for Ni, ∼40 years for Cu and Mo, and ∼60 years for Co. Although the mining of metal-rich ocean mineral deposits (e.g., Fe−Mn nodules from the Clarion-Clipperton Zone (CCZ)24 and SMS from the Solwara 1 deposit in the territorial waters of Papua New Guinea25) has the potential to increase the reservesupply ratios of Cu, Ni, Ag, and Au, the potential environmental impacts of deep-sea mining (e.g., habitat removal/ disturbance and chemical pollution) need more-in-depth studies and assessments.18,26 In contrast, the direct extraction

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HIGH CAPACITY LIGANDS FOR SELECTIVE EXTRACTION OF METAL IONS FROM AQUEOUS SOLUTIONS In aqueous solutions and seawater, technology metals and valuable elements can exist as cationic or anionic species.27−29 Critical metals/elements that are often present as cationic species in aqueous solutions include copper, nickel, cobalt, lithium and REEs. In contrast, uranium, platinum, molybdenum, and vanadium tend to form anionic species in aqueous solutions.29 Chelating agents are the most effective ligands for C


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Figure 3. Postulated Cu(II) complexation sites of a G4-NH2 PAMAM dendrimer in aqueous solution. Although the proposed Cu2+ complexation sites were derived based on the results of our previous study37 and published literature,32,35,41−43 the full multisite model has not been validated by independent experiments and/or atomistic simulations.

metal ion species in solutions.27 Metal ion complexation is a ligand exchange reaction that depends on several parameters including (i) ion size and acidity, (ii) ligand basicity and molecular architecture and (iii) solution physical-chemical properties.27−29 Four milestones in coordination chemistry were the discoveries of the Hard and Sof t Acids and Bases (HSAB) principle, the chelate ef fect, the macrocyclic effect, and the cryptate ef fect.27 Figure 2A shows a broad range of chelating agents with different chemistries and architectures including (i) unidentate ligands, (ii) bidendate ligands, (iii) macrocycles, and (iv) cryptands. The HSAB principle provides “rules of thumb” for selecting an effective ligand (i.e., Lewis base) for a given metal ion (i.e., Lewis acid).27−29 The OH− ligand is representative of ligands with negatively charged hard O donors such as oxalate and catecholate (Figure 2A). Conversely, NH3 and imidazole (Figure 2A) are representative of ligands with saturated/hard N donors (e.g., ammonia and ethylene diamine (EDA)) and unsaturated/soft N donors, respectively. In contrast, the mercaptoethanol group (HOCH2CH2S−) (Figure 2A) is representative of ligands

with soft donors (e.g., thiols). Consistent with the HSAB principle, soft metal ions (e.g., Ag+ and Au+) tend to form more stable complexes with soft ligands containing S donors (e.g., mercaptoethanol) (Figure 2A).27 In contrast, hard metal ions (e.g., UO22+ and VO2+) tend to prefer hard ligands with negatively charged O donors (e.g., oxalate and catecholate) (Figure 2A); whereas metal ions of borderline hardness/ softness (e.g., Cu2+, Ni2+ and Co2+) bind with soft/hard ligands containing nitrogen, oxygen and sulfur donors (e.g., EDA, oxalate and mercaptoethanol) (Figure 2A) depending on their specific affinity toward the ligands.27 It is worth mentioning that alkaline-earth metal ions such as Na+ and Li+ preferentially bind to macrocycles (e.g., crown ethers) and macropolycyclic ligands (e.g., cryptands) with neutral oxygen donors.27 Although the macrocyle cyclam and bidentate EDA ligands (Figure 2B) have very large stability/binding constants for metal ions such as Cu2+ with log K1 respectively equal to 24.4 and 10.5,27 their limited binding capacity (i.e., 1:1 complex) and low molecular weight/size limit their utilization as high D


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Figure 4. Preparation of high capacity chelating resins and membrane absorbers using branched polyethylenimine (PEI) macromolecules as precursors. (A) PEI beads (prepared by inverse suspension polymerization) were reacted with glucono-1,5-D-lactone to afford a resin (BSR-2) with high density of vicinal diol groups.66 (B) Polyvinylidene fluoride (PVDF) membrane absorbers with in situ synthesized PEI particles were prepared by non solvent-induced phase separation (NIPS) using epichlrohydrin (ECH) as cross-linker and deoinized water as non solvent.67

high capacity and selective macroligands with well-defined molecular composition, size and shape.31−38 Dendrimers are highly branched 3-D macromolecules with controlled composition and architecture consisting of three components: a core,

capacity chelating agents for industrial and environmental separations such as metal extraction from seawater. Advances in macromolecular chemistry such as the invention of dendrimers30 are providing new opportunities to develop E


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Environmental Science & Technology interior branch cells, and terminal branch cells.30 Poly(amidoamine) (PAMAM) dendrimers were the first class of dendrimers to be commercialized. Figure 3 highlights the exceptional metal complexing ability of a generation 4 PAMAM dendrimer (G4-NH2) as a macroligand for Cu2+ in aqueous solutions. The G4-NH2 PAMAM is a globular macromolecule with a molar mass of 14215 Da and a hydrodynamic diameter of ∼5 nm.39 This dendrimer has 64 terminal primary amine groups, 62 tertiary amine groups and 124 amide groups. Molecular dynamics simulations by Maiti et al.40 have shown that a G4-NH2 PAMAM can encapsulate ∼201 water molecules at pH 7.0. During the last two decades, a broad array of experimental studies have shown that a G4-NH2 PAMAM dendrimer behaves as a macroligand with large binding capacity and selectivity for transition metal ions and actinides in aqueous solutions including Cu2+, Co2+, Pt2+, Pd2+, Ag+, Au+ and UO22+).32−38 Figure 3 illustrates of our postulated Cu2+ binding sites in a G4-NH2 PAMAM in aqueous solutions. Based on the results of our previous study 37 and published literature,32,35,41−43 we hypothesize that three classes of Cu2+ coordination and inclusion complexes (Figure 3) could be formed depending on metal ion loading and solution pH including (i) complexes of Cu2+ with four nitrogen donors (Complexes A1, A2, and A3), (ii) complexes of Cu2+ with two nitrogen donors and two oxygen donors (Complexes B1 and B2) and (iii) complex of Cu2+ with six water molecules (Complex C). However, more in-depth investigations will be required to fully validate the multisite model highlighted in Figure 3. Although dendrimers have shown great potential as high capacity, selective and recyclable chelating agents,37,38,44,45 they are expensive due to the multiple steps required for their synthesis and purification.30 Branched polyethylenimine (PEI) macromolecules (Figure 4) have emerged as low-cost alternatives to dendrimers with amine groups such PAMAM for a broad range of industrial applications.46−51 Two very attractive features of branched PEI macromolecules are (i) their high contents of primary, secondary and tertiary amine groups (18−20 mol/kg) and (ii) the availability of industrial scale quantities of PEI macromolecules with molecular weights (Mw) ranging from about 300 to several millions Dalton. In addition, branched PEI macromolecules can be reacted with various functional reagents to prepare high capacity and ion-selective polymeric ligands.49−51 Thus, they have great potential to serve as low-cost alternatives to dendrimers for the synthesis of high capacity and selective separation media and membranes (Figure 4) for metal ion extraction from aqueous solutions and seawater.

been devoted to the development of separation materials (e.g., sorbents and membranes) for the selective extraction of critical metals and valuable elements from seawater.52−64 One of the earliest and most comprehensive seawater mining R&D program was carried out by the Japanese Atomic Energy Agency (JAEA).52,53 Key accomplishments by JAEA researchers include (i) the synthesis of selective sorbents for uranium extraction from seawater via the covalent grafting of amidoxime onto macroporous fibrous polymeric adsorbents and (ii) the design and implementation of field tests to validate the uranium extraction capacity of amidoxime-based sorbents estimated to be equal to 1.5 mg of U/g of sorbent after 30 days of contact with seawater. In addition to the JAEA investigators,52,53 several research groups have reported the synthesis and evaluation of high capacity and selective sorbents for uranium seawater mining including (i) metal−organic frameworks with phosphorylurea groups,8 (ii) amidoxime-functionalized graphene oxide,54 and (iv) amidoxime-functionalized copolymer monoliths.55 More recently, Kim et al.56 combined field tests with process modeling to evaluate the uptake of uranium from seawater by high-surface-area polyethylene fibers that were functionalized with amidoxime. The authors reported a uranium uptake of 3.3 mg of U/g of sorbent after 8 weeks of contact with seawater. A broad range of uranium-selective inorganic sorbents,57 biosorbents58 and liquid extractants have also been reported including (i) amidoxime-functionalized magnetic mesoporous silica particles,59 (ii) layered metal sulfides,60 (iii) amidoximefunctionalized ionic liquids58 and (iv) amidoxime functionalized chitin fibers.61 Significant progress has also been made in the development of separation materials for the selective extraction of lithium (Li) from seawater including (i) manganese oxide (MnO2) based sorbents and electrode materials,9,62 (ii) mixed matrix poly(vinyl chloride) (PVC) membranes embedded with MnO2 sorbents,63 (iii) ionic liquid membranes for Li recovery by electrodialysis64 and (iv) superconductor ionic ceramic membranes for Li recovery by dialysis.65 Branched PEI macromolecules are also providing new opportunities to develop high capacity chelating resins and membrane absorbers (Figure 4) for the selective recovery of critical metals and valuable elements from aqueous solutions and seawater.66,67 Mishra et al.66 reported the reaction of cross-linked PEI beads (prepared by inverse suspension polymerization) with glucono1,5-D-lactone to afford a resin (BSR-2) with high density of vicinal diol groups (Figure 4A) that could be utilized to extract anionic species of critical metals and valuable elements such as boron, molybdenum and vanadium66,68 from aqueous solutions and saline water. Kotte et al.67 reported the preparation of a new family of mixed matrix polyvinylidene fluoride (PVDF) membrane absorbers with in situ synthesized PEI particles (Figure 4B). Because of the well documented affinity of PEIbased ligands and resins for copper and uranium,69,70 these new membranes have great potential as high capacity and recyclable sorbents for the selective recovery of dissolved Cu2+ and UO22+ from seawater.

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HIGH CAPACITY SEPARATION MATERIALS FOR THE SELECTIVE EXTRACTION OF CRITICAL METALS AND VALUABLE ELEMENTS FROM AQUEOUS SOLUTIONS AND SEAWATER Although the chelating ligands (e.g., EDA and amidoxime), macrocycles (e.g., crown ethers and cyclams) and cryptands shown in Figure 2A have very high binding affinity for aqueous ionic species of some of the critical/valuable metals listed in Table 1, these compounds need to be processed into separation materials with the appropriate physicochemical properties (e.g., molecular weight and solubility) and form factors (e.g., soluble extractants, particles, fibers and membranes) to enable their efficient integration into seawater metal mining systems (Figure 1). During the last decades, significant research efforts have

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INTEGRATION OF METAL EXTRACTION SYSTEMS INTO DESALINATION PLANTS During the last decades, there has a significant increase in the use of seawater desalination to meet the needs of water-stressed regions of the world.14,71,72 By 2030, it is estimated that the global production of desalinated water could reach 155 to 345 million cubic meters (m3) per day.73 Reverse osmosis has F


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Figure 5. A model seawater factory of the future: Integration of water production with energy generation from salinity gradients, resource recovery (e.g., metal mining) and agriculture..

valuable metals and elements.73−76 A broad range of technologies are being evaluated to treat SWRO brines prior to their discharge including (i) membrane distillation, (ii) liquid−liquid extraction, (iii) forward osmosis, and (iv) crystallization.74 The ultimate goal of SWRO brine management is to achieve zero liquid discharge (ZLD) while recovering water and valuable resources including salts and critical/valuable metals and elements. Jeppensen et al.76 reported that the “adoption” of ZLD desalination systems could (i) reduce the environmental impact of desalination and (ii) provide more viable platforms for metal extraction and resource recovery from SWRO desalination plants. Hollins73 recently evaluated the “prospects for obtaining minerals from desalination brines”. The author focused on the extraction of Li from desalination brines as benchmark. Using a “conservative estimate” of the projected global desalination capacity and assuming a recovery efficiency of 80%, Hollins73 reported that 23 000 tons of Li could be recovered from desalination brines by 2030 compared to the current global production of 37 000 tons. Although Hollins’s analysis showed that the integration of metal recovery systems into SWRO brine treatment systems could be a viable strategy for mining critical/valuable metals

emerged as the best available technology for commercial seawater desalination during the last two decades.71,72 A typical seawater reverse osmosis (SWRO) desalination plant consists of five to six components:71 (1) intake unit, (2) pretreatment unit, (3) RO membrane unit, (4) post treatment unit, (5) energy recovery unit, and (6) brine processing unit. It is worth mentioning that the concentrations of Mg2+ (1290 mg/L) and Ca2+ (411 mg/L) in seawater are relatively large compared to those of most critical/valuable metal ions.6 Because current SWRO membranes can achieve >99% rejection for divalent ions, acids and scale inhibitors are added into the pretreated feedwater of desalination plants to prevent scaling, that is, the precipitation of divalent cations on the surface of SWRO membranes.71 Thus, the extraction of Mg2+ and Ca2+ from pretreated seawater could help improve the water recovery of SWRO plants while recovering valuable elements. It is worth mentioning that some of the separation processes highlighted in Figure 1 could be utilized to selectively extract Mg2+ and Ca2+ from pretreated seawater including (i) nanofiltration, (ii) ion exchange, and (iii) membrane absorption. SWRO brine management provides another opportunity to improve the sustainability of desalination plants while extracting critical/ G



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such as Li from seawater,62−65 the availability of high capacity and selective separation materials remains a major and unresolved challenge. By exploiting the reactivity of branched PEI macromolecules toward epoxides,66,67 alkyl halides,77 polyols,66 and acyl chlorides,48 we anticipate that a variety of base polymers and functional reagents could be utilized as precursors to prepare new families of ultrafiltration (UF) membranes with in situ synthesized supramolecular hosts for metal ions. Such new UF membrane absorbers could be configured into low-pressure and high capacity regenerable modules for the selective extraction of cationic/anionic species of critical metals and valuable elements from the pretreated feedwater and brines of SWRO desalination plants.

REFERENCES

(1) Diallo, M. S.; Fromer, N.; Jhon, M. Nanotechnology for sustainable development: Retrospective and outlook. J. Nanopart. Res. 2013, 15, 2044. (2) Fromer, N.; Diallo, M. S. Nanotechnology and clean energy: Sustainable utilization and supply of critical materials. J. Nanopart. Res. 2013, 15, 2011. (3) Diallo, M. S. et al. Implications: Convergence of knowledge and technology for a sustainable society. In Convergence of Knowledge, Technology and Society: Beyond Convergence of Nano-Bio-Info-Cognitive Technologies, Science policy reports; Roco, M. C., Bainbridge, W. S., Tonn, B., Whitesides, G., Eds.; Springer: Dordrecht, 2013; pp 311− 356. (4) Diallo, M. S.; Brinker, J. C. Nanotechnology for sustainability: Environment, water, food, minerals and climate. In Nanotechnology research directions for societal needs in 2020: Retrospective and outlook, Science Policy Reports; Roco, M. C., Mirkin, C., Hersham, M., Eds.; Springer, 2010; pp 221−259. (5) Minerals, Critical Minerals, and the U.S. Economy; National Research Council (NRC), 2008; ISBN 0-309-11283. (6) Anthoni, J. Oceanic abundance of elements. www.seafriends.org.nz/ oceano/seawater.htm. (7) Bardi, U. Extracting minerals from seawater: An energy analysis. Sustainability 2010, 2, 980−992. (8) Carboni, M.; Abney, C. W.; Liu, S.; Lin, W. Highly porous and stable metal-organic frameworks for uranium extraction. Chem. Sci. 2013, 4, 2396−2402. (9) Lee, J.; Yu, S.-H.; Kim, C.; Sung, Y.-E.; Yoon, J. Highly selective lithium recovery from brine using a λ-MnO2-Ag battery. Phys. Chem. Chem. Phys. 2013, 15, 7690−7695. (10) Nakazawa, N.; Tamada, M.; Ooi, K.; Akagawa, S. Experimental studies on rare metal collection from seawater. In Proceedings of the Ninth (2011) ISOPE Ocean Mining Symposium, Maui, Hawai (USA), June 19−24, 2011, ISBN 978-1-880653-95-1, pp 184−189. (11) Gilbert, O.; Valderrama, C.; Peterkóva, M.; Cortina, J. L. Evaluation of selective sorbents for the extraction of valuable metal ions (Cs, Rb, Li, U) from reverse osmosis rejected brine. Solvent Extr. Ion Exch. 2010, 28, 543−562. (12) Allen, D. T.; Shonnard, D. R. Sustainable Engineering: Concepts, Design and Case Studies, 2011; ISBN-13: 978-0-13-275654-9. (13) Pidwirny, M. Introduction to the oceans. In Fundamentals of Physical Geography, 2nd ed., 2006; http://www.physicalgeography.net/ fundamentals/8o.html. (14) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J.; Marinas, B. J.; Mayes, A. Science and technology for water purification in the coming decades. Nature 2008, 54, 301−310. (15) Hoffmann, R. Old gas, new gas. Am. Sci. 2006, 94, 16−18. (16) Logan, B. E.; Elimelech, M. Membrane-based processes for sustainable power generation using water. Nature 2013, 488, 313−319. (17) Fujita, R.; Markham, A. C.; Diaz, J. E. D.; Garcia, J. R. M.; Scarborough, C.; Greenfield, P.; Black, P.; Aguilera, S. E. Revisiting ocean thermal energy conversion. Mar. Policy 2012, 36, 463−465. (18) UNEP (United Nations Environment Programme) Website. Wealth in the oceans: Deep sea mining on the horizon? Thematic focus: Environmental governance, resource efficiency. May 2014; www. unep.org/pdf/GEAS_May2014_DeepSeaMining.pdf. (19) Hein, J. R.; Mizell, K.; Koschinsky, A.; Conrad, T. A. Deepocean mineral deposits as sources of critical metals for high-and greentechnology applications: Comparison with land-based resources. Ore Geol. Rev. 2013, 51, 1−14. (20) Hoagland, P.; Beaulieu, S.; Tivey, M. A.; Eggert, R. G.; German, C.; Glowka, L.; Lin, J. Deep-sea mining of seafloor massive sulfides. Mar. Policy 2010, 34, 728−732. (21) United States Geological Survey (USGS). Mineral commodity summaries 2014. www.minerals.usgs.gov/minerals/pubs/mcs/2014/ mcs2014.pdf. (22) World Nuclear Association. //www.world-nuclear.org/info/ Nuclear-Fuel-Cycle/Uranium-Resources/Supply-of-Uranium/.

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OUTLOOK Oceans store 97% of Planet Earth’s water. Seawater is a promising resource for achieving a sustainable water-energymaterials-food nexus. It contains large amounts of dissolved ions including cationic/anionic species of critical metals and valuable elements such as Li, Mo, Ni, Zn, V, Au and U. Mixing of seawater with an aqueous solution of lower salinity (e.g., secondary wastewater) produces a salinity gradient that can be exploited to generate electricity using PRO or RED. Partially desalinated seawater can also be utilized in the cultivation of microalgae or salt-tolerant crops to produce biofuels, nitrogen (N) and phosphorus (P) rich fertilizer substitutes and food supplements. Figure 5 highlights a seawater factory concept of the future designed to integrate the production of clean water for potable uses and agriculture with energy generation and resource recovery (e.g., metal mining). The implementation of this seawater factory concept will require transformative advances including the development of (i) low-energy desalination materials and systems with high water recovery and tunable salt rejection, (ii) high performance PRO/RED membrane materials and systems for energy generation from salinity gradients and (iii) efficient and cost-effective separation materials and systems that can selectively extract critical metals and valuable elements (e.g., Li, Ag, Au and U) from seawater and brines with high recovery. In the next decades, we anticipate that the convergence between (i) supramolecular chemistry, (ii) separation science, (iii) materials chemistry, (iv) nanobiotechnology, (v) process engineering and (vi) scalable manufacturing will lead to the transformative advances needed to build, optimize and operate the model seawater factory of the future highlighted in Figure 5.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 011 1 626 578 0311; fax: 011 626 585 0918; e-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mamadou Diallo thanks the KAIST EEWS Initiative (Grant No. NT080607C0209721), the National Research Foundation of Korea (NRF) (MEST grant No. 2012M1A2A2026588), the US National Science Foundation (NSF) (CBET grants 0948485 and 0506951) and the Caltech Dow Resnick Bridge Program for funding his research programs on sustainable chemistry, engineering and materials (SusCHEM). H


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