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Uranium and lithium extraction from seawater:
challenges and opportunities for a sustainable
energy future
Yu Jie Lim, †
a
Kunli Goh, †
a
Atsushi Goto,
b
Yanli Zhao
b
and Rong Wang *
ac
Amid the global call for decarbonization efforts, uranium and lithium are two important metal resources
critical for securing a sustainable energy future. Extraction of uranium and lithium from seawater has
gained broad interest in recent years due to the thousand-fold higher quantity available as compared to
land-based reserves, but the challenge lies in the ability to extract them at ultralow concentrations. Over
the past two decades, the rise of nanotechnology has brought together an abundance of adsorptive
materials that are poised to incentivize technologies capable of achieving high extraction performances.
The objective of this review is to consolidate recent advances in uranium and lithium extraction from the
standpoint of adsorptive materials and technologies for application in seawater. First, adsorptive
materials for uranium extraction are reviewed, before we discuss the technology platforms into which
they can be deployed (e.g., membrane-based adsorption). Second, a comprehensive review of lithium
extraction technologies is presented by examining the materials and platforms capable of achieving high
extraction performances. Since the scope of this review is geared towards application in seawater and
desalination brines (in particular, seawater reverse osmosis (SWRO) brine), we highlight the main
challenges to date –selectivity required against competing ions and long-term stability against marine
Cite this: J. Mater. Chem. A,2023,11,
22551
Received 25th August 2023
Accepted 20th September 2023
DOI: 10.1039/d3ta05099h
rsc.li/materials-a
Yu Jie Lim received his BEng
(First Class Honours) in Chem-
ical Engineering from the
National University of Singapore
(2018). He was awarded the
Water Graduate Scholarship by
Singapore's National Research
Foundation (administered by
PUB, Singapore's National
Water Agency) to develop
biomimetic desalination
membranes. He completed his
PhD at Nanyang Technological
University (2022), and his PhD thesis was selected as the winner
(rst-place recipient) of the Research Excellence Award granted by
the Interdisciplinary Graduate Programme of Graduate College.
He is currently a Research Fellow working under the supervision of
Prof. Rong Wang with research interests in adsorptive membranes
for resource recovery from seawater.
Kunli Goh received his BSc
(Hons) in Chemistry from the
National University of Singapore
in 2004. He served as a Chem-
istry Subject Head in a high
school in Singapore before
returning to pursue his PhD
degree in the Sustainable Earth
Program at the Interdisciplinary
Graduate School of Nanyang
Technological University in
2016. He is currently a Senior
Research Fellow in Prof. Rong
Wang's research group at the Singapore Membrane Technology
Centre. His research interests focus on the synthesis, functionali-
zation, and nanoarchitectonics of advanced functional materials
for developing next-generation gas separation and water treatment
membranes.
a
Singapore Membrane Technology Center, Nanyang Environment and Water
Research Institute, Nanyang Technological University, 637141, Singapore. E-mail:
rwang@ntu.edu.sg
b
School of Chemistry, Chemical Engineering and Biotechnology, Nanyang
Technological University, 637371, Singapore
c
School of Civil and Environmental Engineering, Nanyang Technological University,
639798, Singapore
†These authors contributed equally to this work.
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biofouling. Then, we put together an outlook, featuring our perspectives on next-generation materials and
techno-economic analysis. Since the properties of desalination brines are unique from those of seawater,
we also distinguish the traits of next-generation materials to be used for SWRO brines to provide insights for
advancing new tailored materials and technologies for application in the latter. Overall, this review sums up
state-of-the-art technologies for uranium and lithium extraction, putting into perspective various
technology platforms to realize high extraction performances that can address our future demands for
uranium and lithium at the water-energy nexus.
1. Introduction
Mitigating climate change is one of the key global challenges of
our time. At the core of this challenge is the question of energy –
more precisely, the ability to provide the growing global
economy and population with clean, economical and reliable
energy.
1–3
Nuclear ssion energy, as a mature and low carbon
technology, can satisfy the rapidly burgeoning demand for
power while lessening greenhouse emissions and maximising
sustainability.
4–7
In early 2020, nuclear power accounted for
11% of the electricity generated in the world (∼2500 TW h).
8
Uranium is the main fuel for nuclear reactors, and it exists
naturally on land as uranium ore deposits largely concentrated
in countries such as Australia, Canada, and Kazakhstan. Adding
up to about 7.9 million tons, uranium reserves on land are
forecasted to be adequate for continuous power generation at
the present rates of consumption for the next 130 years
(Fig. 1A).
9
However, to match the tremendous increase in global
energy requirements in the next 20–30 years, nuclear power
generation is projected to increase two-fold to 5200 TW h
(requiring ∼100 kilotons U/year; see Fig. 1B),
8
making uranium
availability an issue of energy security.
Amid the call for green energy and deep decarbonization of
electricity production, there is a huge thrust to develop energy
storage technologies because of their ability to complement
clean intermittent energy resources. For example, battery
energy storage can cumulate surplus renewable energy
produced by solar and wind and deliver it when needed to
power electric vehicle (EV) charging stations.
16,17
This can help
strengthen renewable energy utilization, reduce dependence
on fossil fuels, and decrease greenhouse gas emissions to
improve the all-inclusive sustainability of the transportation
division. Renewable energy, energy storage, and clean energy
production are keys to accomplishing global net zero targets.
Atsushi Goto is a Full Professor in
the School of Chemistry, Chem-
ical Engineering and Biotech-
nology at Nanyang Technological
University, Singapore. He has
been a Nippon Shokubai
Professor in Chemistry since
2021. He received his Bachelor’s
degree (1996), Master’s degree
(1998), and PhD degree (2001)
from the Department of Polymer
Chemistry, School of Engi-
neering, Kyoto University, Japan.
He was subsequently appointed as an instructor (2001), an assis-
tant professor (2002–2010), and an associate professor (2010–
2015) at the Institute for Chemical Research, Kyoto University,
Japan. His research interests include polymer chemistry and poly-
mer materials, particularly controlled syntheses of polymers.
Yanli Zhao currently holds the
Lee Soo Ying Professorship at
the School of Chemistry, Chem-
ical Engineering and Biotech-
nology at Nanyang
Technological University. He
received his BSc degree in
Chemistry from Nankai Univer-
sity and his PhD degree in
Physical Chemistry there under
the supervision of Prof. Yu Liu.
He was a postdoctoral scholar
with Prof. Sir Fraser Stoddart at
the University of California, Los Angeles, and subsequently at
Northwestern University. He also conducted postdoctoral research
with Prof. Jeffrey Zink at the University of California, Los Angeles.
He specializes in the development of integrated systems for
theranostics, energy storage and catalysis.
Rong Wang is a Full Professor
and President's Chair in Civil
and Environmental Engineering
at Nanyang Technological
University, Singapore. She
received her PhD in Chemical
Engineering from the Chinese
Academy of Sciences in 1992.
She has been the Director of the
Singapore Membrane Tech-
nology Centre since 2012 and an
elected Fellow of the Academy of
Engineering Singapore. She
specializes in novel membrane development for water desalination
and gas separation with a particular focus on the use of nano-
materials to advance the performance of current state-of-the-art
membranes. She is an Editor-in-Chief of the Journal of
Membrane Science –the agship journal of the membrane
community.
22552 |J. Mater. Chem. A,2023,11, 22551–22589 This journal is © The Royal Society of Chemistry 2023
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Lithium (Li), as the lightest metal on earth with a density of
0.53 g cm
−3
, is a key raw material in batteries due to its high
energy-to-weight ratio. Besides its widespread use in EVs,
lithium has emerged as a vital commodity in the 21st century
due to its diverse industrial applications and ubiquitous role
in our everyday lives (e.g., heat-resistant ceramics and Li-ion
rechargeable batteries in consumer electronics,
respectively).
18–20
To produce Li-ion batteries, high purity
production of lithium carbonate (Li
2
CO
3
) and lithium
hydroxide (LiOH) is necessary. Thus far, about 98 million tons
of lithium have been identied on land, but only 25% of it is
economically viable to mine as reserves (Fig. 1A). 60% of land-
based lithium is amassed in the lithosphere with the remain-
ing 40% stored in salt-lake brines in the hydrosphere, where its
Li concentration is ∼100–1000 ppm.
21,22
Representative exam-
ples of the former and latter are mineral ores in Southwest
China and South America's lithium triangle of Chile, Argen-
tina, and Bolivia, respectively. As energy storage becomes
a cornerstone of the transition to clean energy, the demand for
lithium has drastically increased over the last 10 years
(Fig. 1C), with its global consumption projected to reach 1000
kilotons by 2030.
18
Fig. 1 (A) The amount of uranium and lithium on land and in seawater. The trend of global (B) uranium requirements and (C) lithium consumption
(statistics were extracted from ref. 9–12). (D) The concentration and price of metal elements in seawater (data points were obtained from ref. 13–15).
(E) The number of publications related to uranium and lithium extraction from seawater in the past 20 years. The publications are further categorized
into more specific outputs in the pie charts. (F) The statistics were obtained from the ScienceDirect database on June 20, 2023.
This journal is © The Royal Society of Chemistry 2023 J. Mater. Chem. A,2023,11, 22551–22589 | 22553
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1.1 Uranium and lithium in seawater
Whilst land-based reserves are sufficient to address current
market needs, they might struggle to meet surging demands in
the future. To alleviate concerns about energy and resource
security in countries that do not possess land-based resources,
some researchers have proposed the recovery of critical metals
from oceans as a more sustainable option than land-based
mining.
23,24
As a possibly inexhaustible and geographically
independent source, seawater contains ∼1000 times more
uranium and ∼5000 times more lithium than what is available
on land (see Fig. 1A). Even considering the tremendous increase
in nuclear power generation, this quantity of uranium could
sustain power generation for thousands of years.
25
On the other
hand, lithium extraction from seawater would enable mankind
to secure an adequate supply of lithium to support the extensive
deployment of EVs and satisfy the forecasted demand for Li in
the coming decades. More importantly, uranium and lithium
extraction from seawater could potentially reduce the negative
environmental impacts of land-based mining. In light of the
aforementioned propositions, there are a number of impetuses
to establish cost-effective methods to recover uranium and
lithium from a wider set of resources.
1.2 Fundamental challenges of uranium and lithium
extraction from seawater
Natural seawater is an intricate biogeochemical system with an
ultralow concentration of uranium (3.3 ppb) and lithium (0.1–0.2
ppm), supplemented by a signicant amount of competing metal
ions, high salinity, specic pH, and a substantial propensity for
marine biofouling.
26–28
Amongst the well-established separation
techniques, adsorption is the most cost-efficient method for the
extraction of uranium and lithium from seawater because of its
ease of operation, high efficiency, and low cost.
29
In general,
uranium and lithium extraction from seawater is challenging
because of their ultralow concentration in seawater, and the
presence of competing ions makes it difficult to selectively
capture target ions (i.e., uranyl or lithium) while other ions are
leuntouched. In addition, the presence of marine organisms
and bacteria brings about biofouling, which compromises the
long-term performance of adsorptive functionalities. For
instance, amidoxime-functionalized polymers, state-of-the-art
adsorbents for uranium extraction, are known to capture note-
worthy amounts of vanadium, iron, and copper in addition to
uranium. Likewise, lithium-ion sieve (LIS) adsorbents are
susceptible to competing adsorption from other ions in seawater,
with studies pinpointing the main competing ions to be sodium
and magnesium. Moreover, the hydrated radiii of alkali metal
ions in seawater are quite similar, rendering it difficult to
selectively extract Li
+
ions amongst other competing ions.
30
1.3 Scope and outline of the current review
During our survey of the literature, a few similar reviews came to
our attention. These reviews mainly discussed uranium
29,31
and
lithium extraction separately,
32–34
with the focus placed more
broadly on a variety of aqueous sources beyond seawater, such
as wastewater, geothermal water, and salt-lake brines. To the
best of our knowledge, there has been no holistic review that
has discussed both uranium and lithium extraction from
seawater, which would have helped readers paint a more
complete picture from an energy perspective. We have also
included studies looking at desalination brines (in particular,
seawater reverse osmosis (SWRO) brines) as a source of metal
elements. Furthermore, most of the previous reviews focused on
narrower scope, either from a materials science
31
or technology
point of view.
33,34
Our proposed review is distinctly different as it
encompasses both the materials and technologies to bridge the
translation gap between fundamental research and technology
realization for practical applications. This review also provides
readers with a wider breadth of knowledge to make data-driven
decisions when selecting materials and platforms that suit their
needs for maximizing extraction performances when coupled
with appropriate technologies.
In this review, we rst lay the background for uranium
extraction by focusing on the key physicochemical properties of
a comprehensive number of adsorptive materials, before dis-
cussing the methods to architect them into various technology
platforms. In the second part of this review, we outline different
technologies for lithium extraction by focussing on the mate-
rials chemistry and associated platforms that feature high
extraction performances. Different examples are explored to
provide a comprehensive understanding of the pathways and
challenges to capture viable amounts of uranium and lithium at
ultralow concentrations. Our analysis of the current literature
also shows that advances in extractive technologies oen lie at
the intersection between molecular simulation, nanotech-
nology and materials science, electrochemistry, and membrane
engineering. It is proposed that with a strong interdisciplinary
approach we can continue to make headway in achieving high-
performance uranium and lithium extraction at the water-
energy nexus.
2. Recent advances in uranium
extraction from seawater and SWRO
brines
As with every new technology that has gained traction for
adoption, innovations in the design of new materials are crucial
for their advancement initially. Since uranium extraction tech-
nologies are all based on the overarching process of adsorption,
we will begin the discussion by introducing the basic chemistry
and structural characteristics of the adsorptive materials
(Section 2.1), prior to examining the technologies into which
these materials can be deployed (Section 2.2).
2.1 Materials for uranium extraction
In this sub-section, the discussion will rst revolve around
inorganic and polymeric materials with a critical discussion of
their inherent strengths and weaknesses. Thereaer, we will
examine the newer class of nanostructured materials and bio-
logical entities that have evolved in the past 10 years to over-
come the limitations of polymeric materials. It is noted that
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a large volume of research papers currently exists in the litera-
ture (Fig. 1E). To provide a more succinct account of the cutting-
edge materials, we will only highlight the milestone studies and
latest developments in this paper, rather than a detailed
description of all the materials explored thus far (readers are
referred to ref. 35 for a comprehensive review of all types of
materials).
2.1.1 Inorganic adsorbents. The inorganic adsorbents
elaborated in this review can be categorized into layered metal
suldes or polysulde-intercalated layered hydroxides, in which
the functional groups lining the internal mesopores of their
hierarchical structures act as reaction sites for complexation
with uranyl ions (refer to Table 1).
36–38
Manos et al. rst
demonstrated the potential of a layered sulde-functionalized
ion-exchanger (K
2
MnSn
2
S
6
; KMS-1) to recover uranium from
seawater in the Gulf of Mexico.
36
At a mass to volume (m/V) ratio
of 10 g L
−1
, KMS-1 could extract 84% of uranium from seawater
within 12 h in static adsorption tests (Table 1). Based on the
reduction in interlayer spacing aer adsorption tests (from
0.85 nm to 0.74 nm), as well as the diameters of UO
22+
and K
+
,
they deduced that the dehydrated uranyl cation intercalates
parallel to the KMS-1 planes (Fig. 2A). In another study, Ma et al.
studied the use of polysulde layered double hydroxide mate-
rials to recover uranium from seawater near Tianjin, China.
37
The adsorptive material demonstrated fast kinetics (78% U
extraction within 1 d) at a m/Vratio of 10 g L
−1
. However, whilst
these materials demonstrate fast kinetics in adsorbing uranyl
ions, they were tested in relatively high m/Vratios which are not
representative of practical deployment conditions (e.g., poly-
meric adsorbents are typically deployed at a lower m/Vratio of
∼0.015 g L
−1
).
31
A recent study has also highlighted the diffi-
culty of eluting adsorbed uranyl ions for some types of layered
double hydroxides due to the stable complexation formed with
uranyl ions.
39
Hence, further validation of the performance of
layered inorganic materials is needed in terms of the ease of
adsorbent regeneration.
2.1.2 Synthetic polymers and derivatives. Of all the mate-
rials studied in the past 40 years, polymeric adsorbents are the
closest to practical deployment for uranium extraction due to
their ease of handling. In particular, the amidoxime func-
tionality has been identied as the state-of-the-art ligand for
uranium extraction due to its high affinity for uranyl ions in
a chelating coordination fashion and the ability to synthesize
this moiety in high yields through fast and efficient tech-
niques.
86
Thus far, the most studied amidoxime-based poly-
mers are synthesized by graing acrylonitrile monomers onto
polyolensupportswithsturdybackbones,suchaspoly-
ethylene (PE) and polypropylene (PP) trunk bers via radiation-
induced grapolymerization (RIGP). Thereaer, amidox-
imation is performed to convert the polyacrylonitrile (PAN)
bers to polyamidoxime (PAO). This is commonly achieved by
treatment with hydroxylamine at temperatures ranging from
60 to 80 °C, and in typical solvents such as water and alcohols
for durations varying from a few hours to a few days.
41–44
However, the strong nucleophilic nature of hydroxylamine can
result in the occurrence of several side reactions, such as the
cyclization of adjacent amidoximes to form a cyclic imide
dioxime. An earlier study outlined that the polyimide dioxime
(PIDO) functionality could enhance uranium adsorption
because of the electron donation ability of the imine N that
bindsuranylionsinseawater.
52
The degree of graing of acrylonitrile groups onto the trunk
bers is controlled by irradiation parameters and graing
reaction conditions (e.g., the dose rate and temperature,
respectively) during the RIGP process. Although RIGP is
currently the most technically mature technique, other
methods such as suspension polymerization, anionic polymer-
ization, and atom transfer radical polymerization (ATRP) have
been explored (readers are referred to ref. 31 for a comprehen-
sive review). Overall, the versatility of these methods could
endow the polymer matrices with a diverse range of functional
groups besides amidoxime, such as phosphoric acid, sulfonic
acid, and other types of amines and molecules. For instance,
two promising amidoxime material formulations, the AF and AI
series (made with itaconic acid and vinylphosphonic acid as co-
monomers, respectively) have shown promise for uranium
extraction from seawater and SWRO brine (see Table 1).
42–44
Both adsorbents were made from PE bers with various ami-
doxime to acid co-monomer molar ratios through RIGP. By
optimizing the co-monomer ratios, the materials with the best
formulations displayed adsorption capacities of 3.35–3.95 mg
g
−1
(refer to Table 1).
To maximize the utilization of adsorption sites, thinner
bers are oen desired to maximize the specic surface area
available in these adsorbents. However, the mechanical
strength of the bers could be compromised if they are too thin
(typical diameter of trunk bers is ∼20 mm). Alternatively, some
researchers have proposed the modication of trunk bers to
preserve their mechanical strength (breaking strengths of
hundreds of MPa) but the challenge lies in the ability to
enhance uranyl ion diffusion into the bers. There are currently
three approaches to maximize the utilization of adsorption sites
in the trunk bers, of which the rst is a top-down method of
creating plentiful nanochannels via co-graing and ammox-
imation of the amorphous parts of the ber. As shown in Fig. 2B
and C, the formation of an interconnected open-pore architec-
ture increased the specic surface area of the ber, endowing it
with a superb adsorption capacity of 15.42 mg g
−1
in seawater.
45
Notably, the open-pore bers possessed a breaking strength of
352 MPa and showed no performance degradation in 30 cycles
of adsorption–desorption tests.
The second approach of graing polymers onto the trunk
ber backbone seeks to dramatically increase the surface area
available for adsorption.
46,87
Xu et al. outlined the feasibility of
a two-step grapolymerization method to fabricate a three-
dimensional (3D) hierarchical structure on a commercial PE-
coated PP ber via the self-assembly of axial graing chains
(Fig. 2D–F).
47
The eventual amidoxime-treated ber could ach-
ieve an adsorption capacity of 11.5 mg g
−1
in natural seawater
(Table 1). They attributed the impressive performance to the
ordered morphology of the bers that accelerated the diffusion
of uranyl ions as well as the expanded surface area available for
adsorption due to the graed chains. The third approach
utilizes a three-step process of controlled cyclization and cyano
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Table 1 Summary of representative studies on the materials for uranium extraction from seawater and SWRO brine. The adsorption perfor-
mances are reported in terms of the adsorption capacity or (%) extracted after a time period
Type Material
U extraction efficiency/
adsorption capacity Feed solution (U conc.) Ref., year
Inorganic materials Layered sulde ion
exchanger K
2
MnSn
2
S
6
84% aer 12 h Natural seawater (3.8 ppb) 36, 2012
Polysulde layered double
hydroxides
78% aer 1 d Natural seawater (9 ppb) 37, 2015
Layered double hydroxides
derived from ZIF-67
>98% aer 1 d Synthetic seawater (3 ppb) 38, 2017
Layered double hydroxide
with benzamidoxime
87% aer 1 d Natural seawater (3.93 ppb) 39, 2021
Layered organic–inorganic
hybrid thiostannate
9% aer 1 d Contaminated seawater (33
ppb)
40, 2016
Synthetic polymers and
derivatives
Polyamidoxime (PAO) 3.1 mg g
−1
aer 42 d Natural seawater (2.84 ppb) 41, 2016
PAO-AF series 3.83 mg g
−1
aer 56 d Natural seawater (3.3 ppb) 42, 2016
PAO-AF1 3.95 mg g
−1
aer 84 d SWRO brine (6.66 ppb) 43, 2018
PAO-AI series 3.35 mg g
−1
aer 56 d Natural seawater (3.3 ppb) 44, 2016
PAO open-pore bers 15.42 mg g
−1
aer 90 d Natural seawater 45, 2020
PAO with a guanidine
polymer
3.19 mg g
−1
aer 30 d Natural seawater 46, 2021
PAO with a 3D structure 11.5 mg g
−1
aer 90 d Natural seawater (3.1 ppb) 47, 2019
PAO with an antimicrobial
polymer
1.144 mg g
−1
aer 40 d Natural seawater 48, 2020
Cyclized PAN amidoxime 5.2 mg g
−1
aer 30 d Natural seawater 49, 2022
Zn
2+
-preloaded PAO 0.29 mg g
−1
aer 30 d Natural seawater 50, 2019
AO-phon-DETA 0.789 mg g
−1
aer 21 d Natural seawater 51, 2016
Polyimide dioxime (PIDO) 4.48 mg g
−1
aer 56 d Natural seawater (3 ppb) 52, 2016
Silica Nanostructured silica 99–100% aer 2 h U-spiked seawater (59 ppb) 53, 2016
MOF UiO-66-AO 2.68 mg g
−1
aer 3 d Natural seawater 54, 2017
UiO-66-NH-(AO) 5.2 mg g
−1
aer 8 d Natural seawater 55, 2021
UiO-66-3C4N 6.85 mg g
−1
aer 28 d Natural seawater 56, 2020
Uranyl-imprinted MOF
derived from UiO-66
7.35 mg g
−1
aer 16 d Natural seawater (3.3 ppb) 57, 2022
Neomycin crosslinked UiO-
66
4.62 mg g
−1
aer 30 d Natural seawater 58, 2019
AO-functionalized MIL-101 4.6 mg g
−1
aer 5 d Natural seawater 59, 2019
MIL-101-AO 96% aer 12 h Simulated seawater (3.3
ppb)
60, 2020
PCN-222-AO 0.29 mg g
−1
aer 7 d Natural seawater (3.3 ppb) 61, 2023
Acylamide- and carboxyl-
functionalized MOF
0.53 mg g
−1
aer 1 min Simulated seawater (6 ppb) 62, 2015
Cationic MOF (Co-SLUG-
35)
1.05 mg g
−1
Natural seawater (5.35 ppb) 63, 2017
Activated cerium-based
MOF
>99% extraction Simulated seawater (0.31
ppm)
64, 2022
Ionic macroporous i-
MZIF90(50)
28.2 mg g
−1
aer 25 d Natural seawater (3.4 ppb) 65, 2022
COF COF-TpDb-AO 127 mg g
−1
aer 1.5 h U-spiked seawater (20 ppm) 66, 2018
[NH
4
]
+
[COF-SO
3
−
] 17.8 mg g
−1
aer 7 d Seawater (10 ppb) 67, 2019
Polyarylether-based (COF-
HHTF-AO)
5.12 mg g
−1
aer 25 d Natural seawater 68, 2021
COF-R
5
11.3 mg g
−1
aer 15 d Natural seawater 69, 2023
Naphthalene-based sp
2
-
carbon COF (NDA-TN-AO)
6.07 mg g
−1
aer 27 d Natural seawater 70, 2020
Benzoxazole-based COF
(Tp-DBD)
10.31 mg g
−1
aer 8 d Natural seawater 71, 2021
Olen-linked COF (PT-BN-
AO)
5.78 mg g
−1
aer 27 d Natural seawater 72, 2021
COF-4 20.6 mg g
−1
aer 3 d Natural seawater 73, 2023
COF 2-Ru-AO 7.36 mg g
−1
aer 3 d Natural seawater 74, 2023
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hydrolysis to transform PAN bers into cyclized PAN amidoxime
(CPAO) bers (Fig. 2G).
49
In a 30-day test conducted in seawater
offthe coast of Dalian (China), CPAO bers demonstrated
a three-fold higher adsorption capacity (5.2 mg g
−1
) than the
control PAO bers. The enhanced performance was attributed
to the highly extended pconjugated system of CPAO that
improved its adsorption and photocatalytic activity. Specically,
the photothermal effect enhanced the interaction between the
amidoxime groups of CPAO and uranyl ions, whereas the
photoelectric effect produced positively charged photo-
generated holes on the ber that is electrostatically attracted
to [UO
2
(CO
3
)
3
]
4−
in seawater.
Nevertheless, one critical limitation of amidoxime polymers is
their susceptibility to competitive adsorption from other ions in
seawater (e.g., vanadium and iron), with studies indicating an
increasing rate of vanadium adsorption over time.
44,88
In parallel to
the design of amidoxime polymers with higher adsorption
capacities, researchers have also explored novel material formu-
lations to enhance their elution efficiencies and long-term stability
in seawater.
50,51,89
However, given the vast amount of studies in this
area and that it has no direct relevance to uranium adsorption, we
would not make an extended discussion here (readers are directed
totheliteratureforcomprehensivediscussion
31,86
).
2.1.3 Nanostructured materials. Besides classical amidox-
ime polymers, many researchers have proposed the use of
nanostructured materials as a framework for uranium capture
in recent years (see Table 1 and Fig. 3). The rationale for using
nanostructured materials lies in their exible molecular design,
tunable pore sizes and functionalities, broad chemical and
topological variety, as well as ultrahigh porosities and surface
areas. These traits furnish excellent opportunities to include
a high density of chelating groups to foster the interaction
between the ligands and uranyl ions. Early studies on nano-
structured materials primarily focussed on nanostructured
silica,
53
but the landscape in the past six years has evolved
towards newer types of materials such as metal- and covalent-
organic frameworks (MOFs and COFs).
Chen et al. demonstrated the rst MOF adsorbent for uranium
extraction from seawater comprising amidoxime ligands appen-
dedontotheframeworkofhydrolyticallystableUiO-66.
54
The
adsorbent was synthesized via a microwave-assisted cyanation
reaction prior to amidoximation treatment. The resultant MOF
(UiO-66-AO) exhibited a high Brunauer–Emmett–Teller (BET)
surface area of 711 m
2
g
−1
and a uranium extraction efficiency of
95% in seawater collected from the Bohai Sea within 2 h. As
shown in Table 1, various types of MOF adsorbents constructed
using different frameworks and ligands have been explored for
uranium extraction.
59,60,63
From a practical perspective, on top of
displaying rapid adsorption kinetics and high adsorption
capacity, the adsorptive materials should possess high desorption
efficiencies (>90%) as well as structural stability for recyclability
(e.g., tolerance to common acids and bases).
61,62
To impart anti-
biofouling properties to the adsorbents, post-modication reac-
tions are performed to introduce antibacterial compounds (e.g.,
neomycin) onto the adsorbents to attenuate long-term perfor-
mance degradation due to marine biofouling. Also, the coated
compounds should be tightly crosslinked onto the adsorbents so
that they are not easily worn away aer multiple cycles of
adsorption–desorption.
58
Besides being water-stable, an ideal MOF for uranium
extraction from seawater should possess high selectivity to uranyl
ions (especially against vanadium, the strongest competing
ion).
55,64
Molecular imprinting technology has been proposed to
improve the selectivity and binding affinity of the adsorption sites
to uranyl ion. For instance, a uranyl-imprinted template could be
inserted into the carboxyl and hydroxyl functional groups of
a UiO-66 derived framework to guide the evolution of the most
suitable nanocage structure for uranium capture (Fig. 3A and B).
57
The template is subsequently removed by treatment with HNO
3
to
form the coordinating space for uranyl ion adsorption (evidenced
by an increase in pore size from 14 Å to 15.9 Å aer elution of the
template). Because of the tted coordination structure for uranyl
ion capture, the resultant MOF could achieve a uranium
adsorption capacity of 7.35 mg g
−1
in natural seawater,
Table 1 (Contd. )
Type Material
U extraction efficiency/
adsorption capacity Feed solution (U conc.) Ref., year
POP POP-oNH
2
-AO 4.36 mg g
−1
aer 56 d Natural seawater 75, 2018
POP-AOF >98% aer 5 d Natural seawater (3.3 ppb) 76, 2021
AO-PIM-1 >95% aer 6 h Natural seawater (3.3 ppb) 77, 2016
PAN-functionalized PAF
(PPN-6-PAO)
4.81 mg g
−1
aer 42 d U-spiked seawater (80 ppb) 78, 2016
Molecular imprinted PAF
(MIPAF-11c)
35.44 mg g
−1
aer 1 h Simulated seawater (7.1
ppm)
79, 2018
PAF-1-NH(CH
2
)
2
AO 36.5 mg g
−1
aer 7 d U-spiked seawater (8 ppm) 80, 2019
PAF-170-AO 8.92 mg g
−1
aer 60 d Natural seawater (3.3 ppb) 81, 2020
MISS-PAF-1 5.79 mg g
−1
aer 56 d Natural seawater (4.4 ppb) 82, 2019
Biological entities Super uranyl-binding
protein
60–90% aer 0.5 h Synthetic seawater (3.2
ppb)
83, 2014
UUS-1 9.46 mg g
−1
aer 2 d Natural seawater (3.1 ppb) 84, 2019
Natural crab carapace 1.38 mg g
−1
aer 36 d Natural seawater 85, 2022
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representing an 18-fold higher selectivity against vanadium (0.4
mg g
−1
).
57
Besides restricting the entry of competing ions,
another study surmised that the coordination interaction in the
template reinforced the binding affinity with uranyl ions.
56
However, the size of the prefabricated uranyl complex may not
always be tting for the cross-linking sites in the material, in
which case the immobilization could lead to changes in the
coordination structure of the uranyl-coordinated complex.
57
Nevertheless, the intrinsic microporous nature of MOFs
imposes some restrictions on the diffusion of guests, resulting
Fig. 2 (A) Uranyl ion capture by KMS-1 through selective intercalation. Reproduced from ref. 36 with permission from American Chemical
Society, copyright 2012. (B) Demonstration of the fabrication of PAO fibers with an interconnected open-pore architecture with its adsorption
performance in natural seawater outlined in (C) reproduced from ref. 45 with permission from the Royal Society of Chemistry, copyright 2020.
(D) Schematic illustration of the formation of a PAO fiber with a 3D hierarchical structure. The fibers can be visualized by transmission electron
microscopy (TEM) (E) and scanning electron microscopy (SEM). (F) Reproduced from ref. 47 with permission from the Royal Society of Chemistry,
copyright 2019. (G) Schematic illustration of the synthesis of cyclized PAN amidoxime fibers. Reproduced from ref. 49 with permission from
Elsevier, copyright 2022.
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in low utilization of adsorption sites in the host framework. To
overcome this limitation, some researchers have proposed to
integrate macropores into porous solids to design exquisite
architectures that promote mass transfer and accessibility of
the surface areas and shortened diffusion paths that lead to full
exploitation of adsorption sites.
90
Mollick et al. recently
extended this concept to uranium extraction from seawater by
anchoring coordinative functional moieties onto ionic micro-
porous MOFs.
65
The MOF adsorbent was synthesized by rst
functionalizing UiO-66-NH
2
through imidazole 2-carbox-
yldehyde, and thereaer ZIF-90 was grown over UiO-66-NH
2
to
form the hybrid adsorbent (Fig. 3C). It was postulated that the
large ionic macropores speeded up the ion diffusion process,
while the surface functional groups selectively trap uranyl ions
by establishing a favourable six-membered ring. Here, the
strong affinity towards uranyl ions is provided by various
functional groups lining the pore wall, eventually allowing the
Fig. 3 (A) Illustration of a uranyl-imprinted MOF (MUU
im
). The removal of the imprinted uranyl via elution results in MUU
re
, which could be used
to adsorb uranyl ions to form MUU
ad
. The corresponding SEM images and uranium concentration in the material are shown in (B). Reproduced
from ref. 57 with permission from Wiley-VCH, copyright 2022. (C) Illustration of the multi-step synthesis of an ionic macroporous MOF and (D)
the adsorption performance in natural seawater. Reproduced from ref. 65 with permission from the Royal Society of Chemistry, copyright 2022.
(E) Graphical view of COF-TpDb-AO (blue: N, gray: C, red: O, and H is omitted) with (F) a schematic illustration of chelating groups in COFs with
uniform pore morphology that permit unrestricted access of ions to the chelating sites. Reproduced from ref. 66 with permission from Wiley-
VCH, copyright 2018.
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MOF to achieve fast uranium adsorption kinetics in natural
seawater (6 mg g
−1
within 2 d, see Table 1 and Fig. 3D).
As compared to MOFs, COFs also possess robust structures
linked by covalent bonds, rendering them favourable platforms
to accommodate chelating functionalities into ordered periodic
arrays for uranium capture. Here, the high tunability of COFs
arises from the broad chemical and topological varieties in
a modular structure.
70,91
An early study by Sun et al. studied the
possibility of preparing an amidoxime-functionalized COF
(COF-TpDb-AO, Fig. 3E) by condensing 2,5-diaminobenzonitrile
(Db) with triformylphloroglucinol (Tp) prior to hydroxylamine
treatment in methanol at 70 °C for 4 h.
66
Possessing a BET
surface area of 826 m
2
g
−1
, the COF displayed a notable
adsorption capacity of 127 mg g
−1
aer 90 min in uranium-
spiked seawater. The remarkably fast kinetics is postulated to
arise from the dened ordered channels that enabled the high
exposure and utilization of adsorption sites (i.e., the chelating
groups on the open 1D channels and their proximity in the
adjacent layers in the 2D extended polygons; see Fig. 3F). Since
then, more studies have explored methods to synthesize COFs
with higher adsorption capacity, higher selectivity, and excel-
lent recyclability for uranium extraction in natural seawater
(Table 1).
67,68
An emerging method involves the synthesis of COFs with
photocatalytic extraction properties to tap into sunlight as an
energy source.
70–74
COFs are ideal candidates for photocatalytic
applications due to the availability of a vast array of linkers that
could function as light absorption components.
73
For instance,
the photoelectric effects of COFs could produce electropositive
holes that are electrostatically attracted to [UO
2
(CO
3
)
3
]
4−
in
seawater. On exposure to light, the adsorbed U(VI) undergoes
photocatalytic reduction to form insoluble U(IV) species, which
facilitates the regeneration of binding sites for further adsorp-
tion of soluble U(VI) ions.
70–72,92
Some COF photocatalysts could
also generate reactive oxygen species (e.g.,cO
2
,cOH, and
1
O
2
)
that destroy marine bacteria, with recent studies demonstrating
a33–42% enhancement in extraction capacity (Table 1).
70–72
It is
worth noting that photocatalytic uranium extraction is at its
infancy stage because of three reasons: (i) most experiments
were conducted using simulated light, which is not represen-
tative of the performance in natural sunlight that exhibits
longer wavelength and lower intensity, (ii) the underlying
mechanisms of uranium reduction are unclear in terms of the
reduction route and intermediate uranium species, and (iii)
competitive adsorption in seawater makes it difficult to
enhance the adsorption capacity. Readers are referred to ref.
72
and
93
for detailed discussions on this subject matter.
The third class of nanostructured materials discussed in this
review is porous organic polymers (POPs), highly cross-linked
amorphous polymers possessing intrinsic porosities.
94,95
As
compared to MOFs, POPs are non-crystalline in nature and their
non-uniform pores are also not well dened. POP adsorbents
for uranium extraction are typically synthesized via the affixa-
tion of amidoxime ligands onto polymers with intrinsic
microporosity (PIMs) or porous aromatic frameworks (PAFs)
(see Table 1). In layman terms, PIMs are POPs that possess
micropores (width <2 nm) whereas PAFs are composed of
aromatic linkers.
96
The customary strategy to elevate the
adsorption capacity of POPs is to enhance the affinity between
amidoxime ligands and uranyl ions (for example, by intro-
ducing auxiliary amino substituents to create uranyl-specic
nano-traps).
75,76
The rationale for using PIMs lies in their high
permanent porosity that allows the fast diffusion of uranyl ions
to the adsorption sites (e.g., amidoxime PIM-1 with a pore size of
2.5 nm possesses a BET surface area and total pore volume of
531 m
2
g
−1
and 0.28 cm
3
g
−1
, respectively).
77
On the other hand, PAFs are porous solids characterized by
rigid aromatic open-frameworks that are linked by covalent
bonds between light elements (e.g., C, N, and O).
97
A diverse
range of PAFs can be synthesized via various organic coupling
reactions and building monomers to form designable skeletons
that encapsulate amidoxime-functionalized ligands.
79,80
Coupled with their high chemical stability and compatibility
with commercial polymers, PAFs have been widely studied as
adsorptive materials for uranium extraction (see Table 1). Yue
et al. rst demonstrated the facile synthesis of an amidoxime-
functionalized PAF (PPN-6-PAO) for uranium extraction from
seawater by graing acrylonitrile monomers onto a PAF skel-
eton through an ATRP reaction.
78
However, the adsorption
capacity of PPN-6-PAO was evaluated in simulated seawater
(80 ppb U; see Table 1). Since then, a few studies have unveiled
the possibility of designing PAFs with higher adsorption
capacity in natural seawater (Table 1). The ordinary approach is
to create PAF skeletons with a higher density of adsorption
ligands, but the challenge lies in the ability to enhance the
accessibility of the adsorption sites to uranyl ions. Thus, many
studies have proposed the incorporation of rigid fragments into
the PAF skeleton to furnish an open architecture that promotes
more contact between the adsorption sites and uranyl ions.
81,82
Similar to MOFs, molecular imprinting technology has shown
success in the synthesis of highly selective PAFs for uranium
extraction, with recent studies reporting high UO
22+
/V
3+
selec-
tivity ratios of 113–171 in seawater.
79,82
2.1.4 Genetically engineered proteins and biological enti-
ties. The last class of adsorptive materials elaborated in this
section is proteins and biological entities. Computational
studies together with advances in biochemical techniques have
opened the doors to the engineering of natural biological enti-
ties for uranium extraction.
98
A notable study by Zhou et al.
successfully developed a computational screening program to
identify proteins with uranyl-binding sites in the Protein Data
Bank.
83
This research work led to the discovery of a uranyl-
selective binding motif and the subsequent development of
a super uranyl-binding protein via genetic modication of
Methanobacterium thermoautotrophicum. The engineered
protein displayed high affinity to uranyl ions with >10 000-fold
higher selectivity against other metal ions such as VO
22+
,Na
+
,
and Mg
2+
, achieving up to 90% uranium recovery in seawater
(Table 1). Some bacterial strains and cells (e.g.,aBacillus vele-
zensis strain termed UUS-1) have also been explored for
uranium extraction because of their intrinsic anti-microbial
activity on top of their selectivity to uranyl ions.
84
The former
trait is important for application in seawater due to the
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pervasive problem of marine biofouling (to be discussed in the
subsequent sections).
2.2 Technologies for uranium extraction
In this sub-section, uranium extraction from seawater and
SWRO brines is discussed with respect to the adsorption
mechanisms and associated technologies that enable them to
be scaled-up for practical usage. The section starts with a revisit
of the most established technology to date –conventional
adsorption –to provide readers with a background on the
earliest technology explored for uranium extraction. Next, we
discuss the emergence of newer technologies that have evolved
in the past 10 years to overcome the limitations of conventional
adsorption. Specically, adsorptive membranes and electro-
chemical approaches will be presented together with a critical
assessment of their merits and limitations to provide readers
with a glimpse into where their potential lies. The challenges
and opportunities that are common to both uranium and
lithium extraction will be discussed in Section 4.
2.2.1 Conventional adsorption. Many types of composite
adsorbents have been developed for uranium recovery thus far,
including but not limited to hydrogels, bers, sponges, and
microspheres. As shown in Table 2, the extraction performance
of the composite adsorbents can be evaluated in static or
dynamic mode. The former involves the immersion of the
adsorbent in a xed volume of solution, whereas the latter
exposes the adsorbent in a owing stream with enhanced mass
transfer (Fig. 4A). Typically, composite adsorbents that display
high uranium adsorption capacities and fast kinetics possess
a large specic surface area and excellent hydrophilicity that
serve to enhance the mass transfer of uranyl ions to the
adsorption sites.
99–101
Top-down approaches are regularly used
to fabricate the adsorbents by introducing suitable functional
groups (e.g., acrylonitrile) onto substrate materials via cross-
linking reactions prior to amidoximation.
102
Commercial
substrate materials are oentimes utilized due to their low price
and decent mechanical strength.
103,104
Recently, Wang et al.
outlined the feasibility of graing amino groups onto bamboo
strips to create nano-pockets for uranyl ion capture.
105
The
composite adsorbent displayed an adsorption capacity of 1.09
mg g
−1
and a U/V selectivity of 1.8 in seawater (Table 2). The
latter is postulated to arise from the pertinent diameter of the
nano-pocket (12.07 Å) that enabled the selective capture of
uranyl ions (diameter: 10.37 Å).
Most of the composite adsorbents explored thus far
comprise porous macrostructures in which internal adsorption
sites are not easily accessible. As a result, the concentration of
free uranyl ions around the adsorption sites is drastically
reduced, thereby limiting the uranium adsorption capacity. The
low ion accessibility also results in a long duration of the
adsorption process, requiring a few weeks or even a few months
to reach saturation in seawater. Because of this reason, hydrogel
adsorbents are favoured due to their 3D network composed of
large amounts of water (Fig. 4B–D). Specically, the hydrophilic
and loose structures of hydrogels aid in the facile migration of
uranyl ions into their inner structures, thereby promoting
maximal utilization of adsorption sites.
109,110
To further speed
up the adsorption process, some researchers have designed
MOF/COF-based hydrogels with well-dened porous channels
that serve to expose more adsorption sites and expedite the
mass transfer of uranyl ions.
112,113
From a practical perspective, besides possessing high
adsorption capacities and fast kinetics, a good adsorbent
should allow fast desorption of adsorbed uranium to regenerate
the adsorption sites.
114
Acids are typically used for elution and
an ideal adsorbent should display regeneration efficiencies
>90% within the rst few cycles. A late study by Tang et al. has
also outlined the possibility of fabricating thermo-responsive
hydrogels that permit the desorption of uranyl ions at
elevated temperatures without the use of harsh chemicals.
111
In
addition, for application in seawater in which the marine
bacteria concentration is high (e.g.,10
8
cfu mL
−1
), the ideal
adsorbent should be hydrophilic and charged-balanced to
inhibit the adhesion of marine foulants that could block the
adsorption sites.
100,101
On top of that, many studies have
unveiled the possibility of introducing anti-biofouling proper-
ties to the adsorbents by crosslinking anti-bacterial compounds
onto the surface of the latter (Fig. 4D).
58,124
There is also
increasing interest in imparting photocatalytic properties to the
adsorbents by generating reactive oxygen species that could
destroy marine bacteria (with up to a 93.4% inhibition rate re-
ported in seawater).
107,108
Some studies have hitherto estimated
that the cost to extract 1 kg of uranium from seawater using the
best adsorbents lies around 150–300 US$.
102,114
To further
reduce the extraction cost, not only should the fabrication cost
be kept low, but the adsorbent needs to be mechanically robust
to guarantee a long service life (reusable for >30 cycles).
106,126
2.2.2 Membrane scaffold-based adsorption. Adsorptive
membranes have gained increasing attention for uranium
extraction from seawater because of their ability to amalgamate
the merits of adsorbent particles (high selectivity for uranyl
ions) and membranes (a scaffold to encapsulate the adsor-
bents). In the fabrication process, the adsorbents (e.g., ami-
doxime particles) are incorporated into the membrane matrix
that is composed of polymeric materials (refer to Table 3). In
this sub-section, we briey elaborate on three overarching types
of membrane designs for uranium extraction.
2.2.2.1 Composite ber-based membranes. The customary
types of membrane scaffold that have been employed for
uranium extraction are composite microbers and nanobers
because of their high processability and scalability. The rst
method to fabricate composite bers is electrospinning, which
uses a high voltage to draw out nanobers over a set distance.
155
The random assembly of ultrane bers on a large-scale mat
produces a nanobrous membrane in which the pores are the
inter-ber spaces at the micro- to nano-metre scale (Fig. 5A). Xie
et al. rst demonstrated a two-nozzle electrospinning technique
to fabricate hybrid nanobrous membranes, comprising PAO
and polyvinylidene uoride (PVDF) bers for uranium extrac-
tion.
127
The authors reasoned that the addition of PVDF dis-
rupted the electrostatic attraction between PAO bers (due to
the strong polarity of the former) and hence created a more
uniformly dispersed nanober network (up to a 31% increase in
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porosity).
127
However, electrospun membranes could only ach-
ieve an adsorption capacity of 1.6 mg g
−1
in uranium-spiked
seawater (Table 3).
In the past ve years, continued research efforts on the
optimization of fabrication conditions have developed electro-
spun nanobrous membranes with better mechanical strength
and adsorption capacity for seawater application (Table 3).
128,156
Recently, Liu et al. unveiled a multi-scale computational
modelling methodology to guide the design of high-
performance polymeric adsorbents for uranium extraction.
130
PAN polymeric chains were lengthened with block copolymers
in a droplet-ow microuidic platform prior to electrospinning.
The electrospun bers (diameter: 800 nm) were then subjected
to hydroxylamine treatment to obtain PAO membranes. By
bridging the gap between the specic metal–ligand interaction
at the molecular level and the spatial conformational properties
of PAO chains at the mesoscopic level, this work outlined the
importance of modelling to guide the design of high-
performance electrospun membranes for seawater uranium
extraction (adsorption capacity: 11.4 mg g
−1
; Table 3).
130
The second method to produce composite bers is blow-
spinning (also known as air-jet spinning), which uses a high-
Table 2 Summary of recent studies on the fabrication of composite adsorbents for uranium recovery from seawater and SWRO brine. The
adsorption performances are reported in terms of the adsorption capacity or (%) extraction after a time period
Substrate
Adsorptive
material/ligand(s)
U adsorption
capacity/performance Feed solution (U conc.) Testing mode Ref., year
Aerogel PAO and chitosan 9.29 mg g
−1
aer 30 d Natural seawater (3.3
ppb)
Dynamic 106, 2020
Co single atom loaded PAO 9.7 mg g
−1
aer 49 d Natural seawater Static 107, 2022
UiO-66-NH
2
and black
phosphorus quantum dots
6.77 mg g
−1
aer 42 d Natural seawater Dynamic 108, 2021
Hydrogel PAO 6.42 mg g
−1
aer 7 d Natural seawater Static 109, 2021
PAO and polyvinyl alcohol
(PVA)
3.12 mg g
−1
aer 21 d Natural seawater (3.3
ppb)
Dynamic 110, 2023
UO
22+
imprinted polymeric
network
59.69 mgg
−1
aer 3 d Natural seawater (3.3
ppb)
Dynamic 111, 2021
MOF-based polyelectrolytes 6.99 mg g
−1
aer 35 d Natural seawater (4.21
ppb)
Dynamic 112, 2020
Thiazole-linked COF 4.15 mg g
−1
aer 10 d Natural seawater Dynamic 113, 2021
Polymeric peptide 7.12 mg g
−1
aer 21 d Natural seawater (3.3
ppb)
Dynamic 114, 2021
Chitosan with glyphosine 9.18 mg g
−1
aer 14 d Natural seawater Static 115, 2023
Zwitterion functionalized
graphene oxide
6.21 aer 30 d Natural seawater Static 116, 2023
Super uranyl-binding
protein
9.2 mgg
−1
aer 1 h Seawater (3.2 ppb) Static 117, 2017
DNA-based 6.06 mg g
−1
aer 6 d Natural seawater (3.35
ppb)
Dynamic 25, 2020
Bacterial debris in PVA 1.18 mg g
−1
aer 28 h Natural seawater (3.8
ppb)
Static 118, 2022
Polymer gel or lms Amidoximated ethylene
acrylic acid
5.3 mgg
−1
aer 2 d Simulated seawater (3
ppb)
Static 119, 2022
PAO 1.39 mg g
−1
aer 42 d SWRO brine (4.8 ppb) Dynamic 120, 2021
Melamine sponge or
foam
PIDO/alginate 5.84 mg g
−1
aer 56 d Natural seawater Dynamic 121, 2019
UiO-66-NH
2
and
polydopamine (PDA)
4.17 mg g
−1
aer 28 d Natural seawater Dynamic 122, 2021
Unfolded proteins 541 mg g
−1
aer 3 d U-spiked seawater (8
ppm)
Dynamic 123, 2022
Silver ion doped ZIF-8 12.24 mgg
−1
aer 12 h Natural seawater (3.69
ppb)
Static 124, 2020
Bamboo materials Amidoxime strips 0.97 mg g
−1
aer 30 d Natural seawater Static 104, 2021
Amidoxime and amino
groups
1.09 mg g
−1
aer 30 d Natural seawater Static 105, 2022
Amidoxime 6.37 mg g
−1
aer 30 d Simulated seawater Static 101, 2021
Others Polyethylenimine and
guanidyl (hemp bers)
>85% aer 1 d Simulated seawater
(3.05 ppb)
Static 103, 2020
Amidoxime (microspheres) 7 mg g
−1
aer 1 d Simulated seawater Dynamic 100, 2021
PDA-Fe
3
O
4
(nanoparticles) 99.8% aer 7 d Natural seawater (2.8
ppb)
Static 99, 2017
UUS-1 (nanoparticles) 7.03 mg g
−1
aer 36 h Natural seawater Static 125, 2023
Amidoxime (nanotubes) 9.01 mg g
−1
aer 30 d Natural seawater Static 102, 2019
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speed pressurized airow to stretch the polymer solution along
the airow direction to produce ultrane bers (Fig. 5B–D). As
compared to electrospinning, blow-spinning does not require
a high voltage source and the collector need not be electrically
conductive. The thickness of the collected nanobrous mat can
be tens of centimetres, which is difficult to achieve via electro-
spinning since the morphology and thickness of the electro-
spun mat are contingent on the stability of the high voltage
electrostatic eld, which inevitably changes with the deposition
of nanobers. For blow-spinning, the amidoximation step
occurs in the dope solution prior to spinning (Fig. 5B). To
fabricate a 3D nanobrous mat with high porosity and large
specic surface area, the ber diameter and packing density of
blow-spun nanobrous membranes are typically optimized by
working parameters during the spinning process (i.e., injection
rate, air pressure, and working distance).
137
One of the earliest studies by Wang et al. demonstrated the
potential of a multi-nozzle blow-spinning platform to fabricate
nanobrous membranes on a large scale (Fig. 5C).
133
The
membranes not only showed decent adsorption capacity (8.7
mg g
−1
) in natural seawater (Table 3), but also possessed
sufficient mechanical strength for at least 8 cycles of adsorp-
tion–desorption with elution efficiencies of ∼99% (see Fig. 5D).
In the past ve years, some studies have proposed the inclusion
of additives to the dope solution (e.g., montmorillonite,
135
black
phosphorus powder,
134
and PEG
137
) that seek to fabricate
Fig. 4 (A) A schematic illustration of the synthesis of amidoximated cellulose microspheres and dynamic testing of its adsorption performance in
afiltration column. Reproduced from ref. 100 with permission from Elsevier, copyright 2021. (B) Cross-sectional image of a pristine wet hydrogel
in laser scanning confocal microscope characterization. Reproduced from ref. 109 with permission from Wiley-VCH, copyright 2021. (C) The
uranium adsorption performance of a bifunctional polymeric peptide hydrogel in natural seawater and (D) a schematic illustration of its uranyl-
binding and anti-fouling mechanisms. Reproduced from ref. 114 with permission from Springer Nature, copyright 2021.
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nanobrous membranes with enhanced hydrophilicity and
weaker surface negative charge or to impart photocatalytic and
photoelectric effects to the membrane scaffold. Empirical
evidence suggests that these traits could enhance the uranium
adsorption capacity and selectivity of the latter, as well as
reduce its biofouling propensity in seawater.
136
Table 3 Recent advances in adsorptive membranes for uranium extraction from seawater
Membrane type Adsorbent material
Parent membrane
material
U adsorption
capacity/performance
Feed solution
(U conc.)
Testing
condition Ref., year
Electrospun
nanobers
PAO PAN and PVDF 1.6 mg g
−1
aer 1 d U-spiked seawater
(330 ppb)
Static 127, 2015
PAO PAN 6.65 mg g
−1
aer 1 d U-spiked seawater
(330 ppb)
Static 128, 2022
PAO PAN and silver
nanoparticles
11.89 mg g
−1
aer 1 d U-spiked seawater
(350 ppb)
Static 129, 2022
PAO PAN and block
copolymer
11.4 mg g
−1
aer 28 d Natural seawater
(3.3 ppb)
Dynamic 130, 2022
Bayberry tannin Gelatin and PVA 1.4 mgg
−1
aer 1 d Simulated seawater
(3 ppb)
Static 131, 2019
Poly(cyclic imide
dioxime)
PVDF 11.39 mg g
−1
aer 87 d Natural seawater
(3.1 ppb)
Static 132, 2022
Blow-spun
nanobers
Polyimide-dioxime PAN 8.7 mg g
−1
aer 56 d Natural seawater
(3.3 ppb)
Dynamic 133, 2018
PAO and black
phosphorus
PAN 11.76 mg g
−1
aer 56 d Natural seawater Static 134, 2020
PAO PAO/PAN and
montmorillonite
9.59 mg g
−1
aer 56 d Natural seawater Dynamic 135, 2019
PAO PAN and ZIF-8 11.17 mg g
−1
aer 25 d Natural seawater Dynamic 136, 2021
PAO PAN 10.31 mg g
−1
aer 35 d Natural seawater Dynamic 137, 2020
PAO and chitosan PAN 4.91 mg g
−1
aer 10 d Natural seawater
(3.6 ppb)
Dynamic 138, 2021
Protein bers Super uranyl-
binding protein
Spidroin 12.33 mg g
−1
aer 3.5 d Natural seawater
(3.9 ppb)
Dynamic 139, 2019
Dual super uranyl-
binding protein
Spidroin 17.45 mg g
−1
aer 3 d Natural seawater Dynamic 140, 2020
Hydrogel
membrane
PAO Supramolecular
network with zinc
chloride
9.23 mg g
−1
aer 28 d Natural seawater
(3.1 ppb)
Dynamic 141, 2020
PAO Polyacrylamide 4.87 mg g
−1
aer 28 d Natural seawater
(3.3 ppb)
Dynamic 142, 2019
PAO PAN and
glutaraldehyde
8.56 mg g
−1
aer 56 d Natural seawater Static 143, 2021
PAO Graphene oxide
hybrid sheets
13.63 mg g
−1
aer 32 d Natural seawater Dynamic 144, 2022
Flat-sheet
membrane
Polyphenol Polyamide 27.8 mgg
−1
aer
ltering 10 L seawater
Natural seawater
(3.29 ppb)
Dynamic 145, 2019
Polyphenol and
polyethyleneimine
PVDF 6.86 mg g
−1
aer 20 d Natural seawater Dynamic 146, 2023
Poly(amidoxime-
ethyleneimine)
Polyamide 0.015 mg g
−1
aer 2 d Natural seawater
(4.58 ppb)
Static 147, 2022
PAO Bacterial cellulose 6.73 mg g
−1
aer 28 d Natural seawater
(3.3 ppb)
Dynamic 148, 2022
PAO Cellulose 1.22 mg g
−1
aer
ltering 10 L seawater
Simulated seawater
(330 ppb)
Dynamic 149, 2020
Lysozyme and
phytic acid
Polypropylene 1.15 mg g
−1
aer 15 d Natural seawater
(3.3 ppb)
Dynamic 150, 2023
Amidoxime and
PIMs
Microporous
polymers
9.03 mg g
−1
aer 28 d Natural seawater
(3.32 ppb)
Dynamic 151, 2022
PAO and chitosan PAN 60 mgg
−1
Simulated seawater
(5.28 ppb)
Dynamic 152, 2022
PAO Sulfonated cellulose
with polyamide
epichlorohydrin
8.78 mg g
−1
aer 25 d Natural seawater
(3.3 ppb)
Dynamic 153, 2021
PAO PAO and
glutaraldehyde
9.35 mg g
−1
aer 35 d Natural seawater Dynamic 154, 2020
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Unlike electrospinning and blow-spinning techniques dis-
cussed thus far, biomimetic spinning is the sole method
utilized to fabricate spidroin-based protein bers. The rationale
for using articial silk bers lies in their higher mechanical
strength as compared to polymeric bers. Yuan et al. rst
explored the possibility of fabricating a chimeric spidroin-based
nanober by fusing a super uranyl-binding protein (SUP) with
spidroin prior to biomimetic spinning (Fig. 5E–G).
139
Aer 3.5
days of equilibration, the protein ber demonstrated a break-
through adsorption capacity of 12.33 mg g
−1
in natural seawater
(Table 3 and Fig. 5H). One year later, they unveiled the possi-
bility of replacing the spidroin in the chimeric protein to form
Fig. 5 (A) SEM image of an electrospun PAN membrane prior to amidoximation. Reproduced from ref. 130 with permission from Springer
Nature, copyright 2022. (B) A schematic illustration outlining the pre-amidoximation of PAN prior to blow-spinning. (C) The elution performance
using two eluents is shown as a function of time. (D) Reproduced from ref. 133 with permission from Wiley-VCH, copyright 2018. (E) The protein
domain composition of a fusion protein. (F) A schematic illustration of the biomimetic spinning process with an SEM image of the resultant fiber in
a dry state. (G) SEM images of the fiber morphology after uranium extraction. The corresponding energy-dispersive X-ray spectroscopy (EDS)
images are shown. (H) The adsorption performance of the biomimetic-spun fibers in natural seawater. Reproduced from ref. 139 with permission
from Wiley-VCH, copyright 2019.
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dual-SUP via genetic fusion.
140
The high adsorption capacity
(17.45 mg g
−1
in seawater) of the protein ber was attributed to
the abundant coordination sites available for uranyl ion chela-
tion, whereas the superior tensile strength (56 MPa) and elas-
ticity (4-fold length extension prior to break) were postulated to
arise from the uniform distribution of tension throughout the
ber network.
2.2.2.2 Hydrogel-based membranes. Hydrogel-based
membranes are hydrated selective barriers constructed by
crosslinked polymer chains capable of retaining large amounts
of water by swelling. When compared against composite bers,
the major advantage of hydrogel-based membranes is their well-
dened 3D semi-interpenetrating network that effectively
disperses amidoxime particles and hence maximizes the
Fig. 6 (A) A schematic illustration of the fabrication of a hydrogel membrane and the mechanism for uranium adsorption. Reproduced from ref.
142 with permission from Wiley-VCH, copyright 2019. (B) SEM and EDS mapping of a metal-phenolic network (MPN) membrane after uranium
extraction, and the (C) extraction performance after filtering 10 L of natural seawater. (D) A material cost analysis of an MPN membrane system in
three real-world applications. Reproduced from ref. 145 with permission from the Royal Society of Chemistry, copyright 2019. (E) Schematic
depiction of a hierarchical porous membrane (right: specific binding sites are provided by amidoxime functionalities), with representative (F)
cross-sectional SEM and (G) digital images (before and after uranium extraction). (H) Uranium adsorption kinetics of the hierarchical porous
membrane and two solution-cast control membranes (feed: 32 ppm U-spiked solution). Reproduced from ref. 151 with permission from Springer
Nature, copyright 2022.
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exposure of adsorption sites. Hydrogel membranes are typically
fabricated via reaction moulding or polymerization, in which
a variety of cross-linkers could be used to embed amidoxime
functionalities (e.g., bi-/multi-valent cations and uorescent
polymers; see Table 3 and Fig. 6A).
141–143,157
For example, Yan
et al. demonstrated that up to 96 wt% of PAOs could be incor-
porated into an ion-crosslinked supramolecular Zn
2+
-PAO
hydrogel membrane. Aer four weeks of immersion in
seawater, the membrane displayed a uranium adsorption
capacity of 9.23 mg g
−1
.
141
The adsorption of uranyl ions was
manifested by a drastic decrease in membrane pore diameter
(from 5–10 mmto50–100 nm) in surface SEM images.
142
2.2.2.3 Self-supporting membranes. Self-supporting
membranes, in this context, refer to continuous membranes
that are made up of adsorbents or chelating groups incorpo-
rated in a support matrix. In this section, we focus mainly on
at-sheet microporous membranes that have demonstrated
potential for uranium extraction (Table 3). A common approach
to fabricate self-supporting membranes is to introduce
chelating groups onto the surface of commercial membranes
via crosslinking reactions.
145–150
Luo et al. demonstrated the
feasibility of a glutaraldehyde-based covalent cross-linking
strategy to introduce polyphenol functional groups capable of
forming metal-phenolic networks with uranyl ions in seawater
(Fig. 6B).
145
In marine eld-testing at the Yellow Sea, a three-
layer membrane could extract 84% of uranium with a high
ux of 190 L m
−2
h
−1
under atmospheric pressure (Table 3).
Moreover, the membrane displayed high uranium selectivity
over competing ions and stable performances throughout 10
cycles of adsorption–desorption (Fig. 6C). The former is
surmised to arise from the high valency of the uranyl ions that
interacted with the electron pairs of the hydroxyl groups of
polyphenols. More importantly, the uranium-saturated
membranes could be easily regenerated by treatment with
0.1 M HNO
3
via the protonation of polyphenol moieties and
disruption of the uranium–phenolic complex. Preliminary
techno-economic analysis revealed the uranium extraction cost
to be 275 US$ per kg if the membrane is integrated with desa-
lination systems (Fig. 6D).
The second prevalent method to fabricate at-sheet
membranes for uranium extraction is solution-casting, in
which a polymeric solution undergoes phase inversion and
solidication to form a porous membrane.
152,158
Yang et al.
recently demonstrated that a porous membrane comprising
amidoxime-PIM-1 particles could achieve a uranium extraction
capacity of 6.63 mg g
−1
aer one week of dynamic testing in
seawater.
151
They attributed the high adsorption capacity and
fast kinetics to the hierarchically branched structure of the
membrane that promoted mass transfer of uranyl ions (Fig. 6E
and F). Consequently, adsorption sites throughout the
membrane were fully exploited for uranium capture (Fig. 6G
and H). In general, membranes with a long service life need to
be durable to withstand shear forces for long periods of time
under dynamic conditions. For this reason, some studies have
advocated the bottom-up fabrication of membranes with 3D
polymeric networks that serve to evenly distribute tensile forces
throughout the network. Empirical evidence suggests that
a tensile strength of ∼17 MPa could endow at-sheet
membranes with stable performances for up to 10 cycles of
adsorption–desorption.
153,154
2.2.3 Electrochemical methods. In parallel to the develop-
ment of adsorption technologies based on surface-based phys-
icochemical nature, there is increasing interest in electrically
driven adsorption processes because of their ability to achieve
faster kinetics than the former (Table 4). The key highlight of
electrochemical approaches is the presence of an electric eld
that could accelerate the diffusion of uranyl ions (Fig. 7A),
making it possible to overcome the adsorption–desorption
equilibrium encountered in physicochemical adsorption that is
controlled by thermodynamics. Thus far, capacitive electrode
processes based on electrosorption in electrical double layers
(EDLs) have shown promise for uranium extraction from
seawater. In the charging step of a capacitive electrode process,
an electrical potential is applied across a pair of electrodes, and
the potential difference creates an electric eld that propels
charged species towards oppositely charged electrodes where
they are immobilized (Fig. 7B and C).
159
Upon reaching their
nite capacitance, the discharging step occurs in which the
polarity of the electrodes is reversed to discharge ions. A sepa-
rate recovery solution is inserted between the electrodes to
collect the released metal ions, thereby generating a metal-
enriched stream. Capacitive electrode processes such as
capacitive deionization (CDI) utilize porous carbon materials to
achieve considerable adsorption capacity in which most of the
electrosorption occurs within the micropores of the electrode.
160
However, pristine porous carbon materials do not demonstrate
Table 4 Recent advances in electrochemical methods for uranium extraction from seawater
Type Adsorptive groups on electrode U adsorption capacity/performance Feed solution (U conc.) Ref., year
Electro-sorption Boron phosphide nanosheets 60.8 mg g
−1
aer 8 h Natural seawater (3.3 ppb) 161, 2020
Functional bovine serum albumin 6.6 mg g
−1
aer 18 d Simulated seawater (3.3 ppb) 162, 2023
Graphene-based PAFs 16 mg g
−1
aer 56 d Natural seawater (3.3 ppb) 163, 2021
Polyphenylacetylene (PPA)-incorporated PAF 16.5 mg g
−1
aer 90 d Natural seawater (3.3 ppb) 164, 2020
Electro-deposition Amidoxime 1.62 mg using 4 L seawater Natural seawater (3 ppb) 165, 2017
3D functionalized reduced GO foam 99% extraction aer 14 h U-spiked seawater (3 ppm) 166, 2021
Chitosan 43 mg g
−1
aer 1 d U-spiked seawater (5 ppm) 167, 2018
Electro-catalytic Amidoxime-functionalized Fe-N
x
-C-R 1.2 mg g
−1
aer 1 d Natural seawater (3.5 ppb) 168, 2021
Amidoxime-functionalized In-N
x
-C-R 12.7 mg g
−1
aer 2 d Natural seawater 169, 2022
S-terminated MoS
2
nanosheets 29.5 mgaer 0.5 h U-spiked seawater (330 ppb) 170, 2022
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selectivity to uranyl ions. To impart the required selectivity,
surface functionalization of the electrode is performed in which
adsorptive materials with affinity to uranyl ions are loaded onto
carbon felt substrates.
161–163
In the past ve years, there has been a new thrust to generate
electrodes with fast uptake kinetics and high selectivity to
uranyl ions. For instance, Wang et al. designed a graphene-
based PAF adsorbent by inserting phenyl-based nanopillars
into the interspacing between graphene sheets prior to deco-
ration with a uranyl-specic bis-salicylaldoxime functionality
(Fig. 7D).
163
The adsorbents were then loaded onto carbon felt
substrates via dip coating and air-drying at 70 °C. At a negative
potential of −1.3 V, the system could achieve an adsorption
capacity of 16 mg g
−1
in natural seawater and a U/V selectivity
exceeding 60 in U-spiked seawater (see Table 4 and Fig. 7E). The
high selectivity was considered to arise from the tailor-made
interspacing of the adsorbents (13 Å) that were ne-tuned to
the dimensions of UO
22+
ions (6.04–6.84 Å). This facilitated the
fast unipolar ionic transport of UO
22+
ions to the adsorption
sites while other competing ions such as vanadyl were excluded.
Nevertheless, enrichment of uranyl ions in the micrometer-
sized adsorbent particles is generally difficult due to the
minuscule EDL thickness of the external electric eld (<1 nm).
To overcome this limitation, a recent study has demonstrated
the possibility of integrating polyphenylacetylene (PPA)
conductive chains into the pores of PAFs to provide an
expanded electric eld that permits higher utilization of
adsorption sites.
164
Following electrosorption, the adsorbed uranyl ions could
further undergo electrodeposition to form solid uranium
compounds. Aer the deposited product reaches a specied
thickness, they can be scrapped offthe electrode.
167
In 2017, Liu
Fig. 7 (A) A schematic depiction of the five steps of uranium extraction using the half-wave rectified alternating current electrochemical method.
The ions in seawater (step I) start to migrate according to the external electric field and bind onto the electrode surface (step II). In step III, the
adsorbed uranyl ions are reduced to UO
2
. Upon removal of the bias, non-adsorbed ions are dispersed back into seawater (step IV). Continued
application resulted in the growth of UO
2
particles (step V). Reproduced from ref. 165 with permission from Springer Nature, copyright 2017. (B)
SEM images of a pristine bovine serum albumin (BSA)-coated electrode and (C) after uranium extraction to show the adsorbed uranium.
Reproduced from ref. 162 with permission from Elsevier, copyright 2023. (D) A schematic illustration of a unipolar 2D ion channel with enhanced
uranyl ion conductivity. (E) Adsorption performance of MIGPAF-13 in U-spiked seawater (8 ppm U). Reproduced from ref. 163 with permission
from American Chemical Society, copyright 2021.
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et al. exemplied the possibility of amassing uranium oxide
(UO
2
) particles from seawater using a half-wave rectied alter-
nating current electrochemical method.
165
Surface functionali-
zation of the pristine carbon electrode was performed by
loading amidoxime groups onto the latter. As shown in Fig. 7A,
upon the application of a negative bias, the electric eld guided
the migration of uranyl ions onto amidoxime functionalities on
the carbon electrode. Continued application of the bias entailed
electrodeposition, in which uranyl ions bound by amidoxime
groups are reduced and deposited on the electrode surface as
UO
2
. To prevent water splitting and adsorption of undesired
species, the bias was removed to re-disperse the competing ions
back into the feed solution while UO
2
stayed bound to the
amidoxime groups. As the process continues, the adsorbed UO
2
species slowly grows to form UO
2
particles. In 4 L of natural
seawater feed, the system could achieve a three-fold higher
recovery of uranium (1.62 mg) than physicochemical adsorption.
As shown in Table 4, the third type of electrochemical method
entails the use of electrocatalysts to accelerate the electrode-
position of charge-neutral solid uranium products. For
instance, Wang et al. employed a reduced graphene oxide foam
electrode functionalized with sulfur and nitrogen to generate H
2
and OH
−
via water splitting.
166
The OH
−
anions generated were
surmised to polymerize with UO
22+
cations to form charge-
neutral uranium oxide hydroxide (UO
2
(OH)
2
) on the electrode
surface. Other studies have also outlined the feasibility of
loading amidoxime-functionalized electrocatalysts onto carbon
felt electrodes to induce electrodeposition of uranium
products.
168,169
3. Recent advances in lithium
extraction from seawater and SWRO
brines
3.1 Materials and technologies for lithium extraction
In this section, we discuss the progress made in lithium
extraction by examining the principal materials and accompa-
nying technologies. Unlike uranium extraction where all of the
technologies are based on the overarching process of adsorp-
tion, lithium extraction technologies are more diversied in
that adsorption is not the sole process governing lithium
recovery. We categorized the recovery methods into ve over-
arching types: conventional adsorption, membrane scaffold-
based adsorption, liquid–liquid extraction, electrochemical
intercalation, and electrodialysis. To facilitate a clear under-
standing for readers, we will discuss the progress made in
lithium extraction by examining the materials and accompa-
nying technologies concurrently. In particular, we will touch on
the various methods to incorporate materials into technology
platforms, together with a critical evaluation of their merits and
limitations. We rst begin the discussion with conventional
adsorption since it is the earliest studied technology in the past
40 years. Next, we touch on the remaining four technologies by
introducing their inherent advantages that could overcome the
limitations of conventional adsorption. The merits and current
challenges of these newer technologies are also evaluated, and
future opportunities that are similar to uranium extraction are
furnished in Section 4.
3.1.1 Conventional adsorption. Currently, the most widely
adopted method for lithium extraction from aqueous sources is
adsorption because of its ease of operation and cost effective-
ness.
171,172
In this sub-section, we briey elaborate on the
promising materials for lithium adsorption, namely, metal-
based adsorbents and crown ethers because of their moderate
Li
+
/Na
+
and Li
+
/Mg
2+
selectivities for application in seawater.
Metal-based adsorbents can be broadly categorized into
aluminium (Al)-based, manganese (Mn)-based, and titanium
(Ti)-based (Fig. 8A). At the current stage, Al-based adsorbents
exhibit the highest potential for industrial applications due to
their high technological maturity but the selective uptake
performance of Li
+
needs further improvement.
173
On the other
hand, Mn-based adsorbents show excellent Li
+
adsorption
performances, but do not possess the necessary stability and
robustness for long-term application due to the gradual disso-
lution of Mn from their structure. The third type, Ti-based
adsorbents, is the newest entry and is found to not exhibit the
limitations of Al- or Mn-based adsorbents. Ti-based adsorbents
are currently studied for lithium extraction from salt-lake
brines, but their applicability for seawater remains to be
validated.
174,175
In the past 40 years, the prevalent material that has been
investigated for lithium extraction is inorganic hydrogen
manganese dioxide (HMO) lithium ion-sieves because of their
high stability, facile synthesis, and low cost.
177
HMO is an ion-
sieve oxide that is derived from the corresponding lithium
manganese oxide (LMO) precursor in which Li
+
is eluted from
the latter's crystal site by treatment with hydrochloric acid
(HCl).
178
The cavity formed aer elution is the adsorption site
that accommodates Li
+
. The high selectivity of HMO to Li
+
is
attributed to the small pore windows of MnO
2
that only allow
Li
+
ions to penetrate its spinel structure. Since the principal
mechanism governing lithium adsorption involves the
exchange of Li
+
in seawater and H
+
ions in HMO, metal cations
with larger ionic diameters or that have a higher free energy of
hydration than Li
+
cannot enter the adsorption sites due to
steric hindrance effects.
179
Upon saturation of adsorption sites,
the adsorbents will be subjected to desorption to elute Li
+
,
thereby obtaining a Li-enriched solution that is adequate for
post-processing steps to obtain solid Li
2
CO
3
.
The conventional method for lithium extraction involves the
use of particulate lithium-ion sieves (LISs) in packed column
systems. However, this technology suffers from severe pressure
drops incurred as well as the loss of adsorbents during opera-
tion. In addition, the powdery nature of lithium ion-sieves
makes them difficult to handle, less recyclable, and suscep-
tible to performance degradation in acid desorption. Thus,
some researchers have proposed the binding of HMO powders
with crosslinking compounds to form small capsules, granules,
or tubular structures for higher processability.
180
For example,
Hong et al. demonstrated the feasibility of a two-step process to
fabricate highly porous HMO-Al
2
O
3
composites in which LMO
powder and alumina gel were rst mixed in a 1 : 4 mass ratio
and subsequently expelled from a screw extruder (diameter: 0.5
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mm).
181
The extruded mixture was then cut into tiny cylinders (2–
3 mm) prior to calcination at 500 °C for 4 h to form milli-tubular
structures. The HMO-Al
2
O
3
composite exhibited a Li
+
uptake of
6.5 mg g
−1
in real seawater feed with adsorption isotherms
revealing the heterogeneous nature of lithium adsorption.
181
In another study, Meng et al. demonstrated a facile method
of synthesizing porous hydrogels by embedding l-MnO
2
nano-
particles (∼25 nm diameter) into an interpenetrating network
comprising polypyrrole (PPy) and polyvinyl alcohol (PVA).
176
Here, it is hypothesized that the l-MnO
2
particles were
uniformly crosslinked with polymer chains by coordination,
chelation, and electrostatic interactions (Fig. 8B). This work
narrowed down the optimal loading amount of l-MnO
2
to be
25 wt%, which was determined by the trade-offbetween the
adsorption capacity and mechanical strength of the hydrogel
(i.e., a low loading rate would result in an inferior adsorption
capacity, whereas a high loading rate could compromise its
mechanical integrity). In adsorption tests using seawater, the l-
MnO
2
interpenetrating hydrogel possessed a 3-fold higher
adsorption capacity (20.6 mg g
−1
) than the l-MnO
2
-incorpo-
rated PVA gel (control), revealing that the superb performance
was likely contributed by the higher exposure of Li
+
adsorption
sites in the porous framework (Fig. 8C–E).
176
Besides lithium ion-sieves, crown ethers are the second type
of material that has shown potential for lithium adsorption
from seawater. As a family of macrocyclic ligands characterized
by their [–CH
2
–CH
2
–O–] structural unit, crown ethers possess
exible structures in which their cavity sizes and metal ion
affinities can be ne-tuned to impart selectivity towards Li
+
. For
example, the chelating ability of crown ethers can be tuned by
altering the size of the crown ring and side chain groups,
whereas the ability to form complexes with metal ions through
Fig. 8 (A) A schematic outline of the structural changes upon pre-lithiation of anatase TiO
2
and subsequent ion exchange for lithium extraction.
Reproduced from ref. 175 with permission from Elsevier, copyright 2023. (B) A schematic illustration of the fabrication of a l-MnO
2
inter-
penetrating hydrogel via crosslinking and polymerization. Corresponding SEM (C) and TEM (D) images outline its porous structure and the
presence of l-MnO
2
. (E) The adsorption performance of the hydrogel in terms of its selectivity to metal ions in seawater (inset depicts its flexibility
after 90 days of immersion in LiCl solution). Reproduced from ref. 176 with permission from Elsevier, copyright 2022.
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ion–dipole interactions is attributed to the negatively polarized
oxygen atoms at the pore entrance and macrocyclic structure.
190
Typically, the binding mechanism follows the size-t rule. For
instance, benzo-15-crown-5 (BC15) and benzo-18-crown-6
(BC18) could form complexes with Li
+
due to the size compat-
ibility between their macrocyclic rings (1.7–2.2 Å and 2.6–3.2 Å,
respectively) and the ionic diameter of Li
+
(1.6 Å). In chro-
matographic column tests using real seawater (∼0.178 ppm Li),
the maximum adsorption capacities of BC15 and BC18
composite adsorbents were 6.5 mg g
−1
and 12 mg g
−1
, respec-
tively.
191
Besides the size compatibility factor, ion dehydration
and intrapore diffusion also determine the crown ether's
selectivity towards Li
+
.
190
3.1.2 Membrane-scaffold based adsorption. Membrane-
based processes have been one of the most widely studied
technologies to recover lithium from aqueous sources due to
their better energetics and lower footprint when compared
against chemical exchange or stripping methods.
32,192
Here, the
membrane either acts as (i) a high-precision sieve that allows
the selective permeation of Li
+
over other ions (resulting in a Li
+
-
enriched permeate solution, Fig. 9A) or (ii) a membrane
adsorber capable of sequestering Li
+
ions (Fig. 9B). The rst
approach of selective permeation is contingent on the single-
ion species selectivity of the membrane and is used in extrac-
tion from salt-lake brines or geothermal waters in which the
initial concentration of lithium is hundreds of ppm.
11
Nano-
ltration (NF) membranes are typically used to enrich the
concentration of lithium (∼500–7000 ppm) prior to the precip-
itation of solid lithium compounds. The desired properties for
NF membranes are high water permeability and high Li
+
/Mg
2+
selectivity to ensure that the process can be operated at low
applied pressures (<8 bar). Readers are referred to the literature
for detailed discussions on the desired traits of such NF
membranes.
193–196
On the other hand, the selective adsorption
approach is more applicable in the extraction of lithium from
low concentration sources such as seawater or SWRO brines
(∼0.1–1 ppm).
197
Fig. 9 (A) Schematic illustration of a Li
+
selective membrane in a two-pass nanofiltration (NF) process. Reproduced from ref. 194 with permission
from Wiley-VCH, copyright 2021. (B) Schematic illustration of a Li
+
adsorptive cellulose membrane with (C) SEM and (D) TEM images outlining its
porous structure and distribution of adsorbent materials, respectively. (E) The adsorption performance of the adsorptive membrane is presented
in terms of the metal ion extraction efficiency and distribution coefficient (K
d
). Reproduced from ref. 178 with permission from American
Chemical Society, copyright 2020. Schematic illustrations of solution-casting (F) and layer-by-layer deposition. (G) Reproduced from ref. 198
with permission from the Royal Society of Chemistry, copyright 2022.
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In the past 10 years, there have been many research efforts
that have attempted to develop adsorptive membranes for
lithium extraction from seawater and SWRO brines.
184,199
During the fabrication process, the adsorbent particles (e.g.,
LISs) are incorporated into the membrane matrix (Fig. 9C–E).
Flat-sheet membranes are typically fabricated via solution
casting in which the dope solution consisting of a polymer and
solvent mixture undergoes phase inversion to form a solidied
membrane (Fig. 9F). A study has also outlined the possibility of
fabricating polyelectrolyte multi-layered membranes by incor-
porating lithium-chelating agents into the polymeric blend
prior to layer-by-layer deposition (Fig. 9G).
200
Recently, Tang
et al. demonstrated that an HMO-incorporated cellulose lm
could extract 99.4% of Li
+
in seawater whereas the extraction
efficiency of competing ions (K
+
,Ca
2+
, and Sr
2+
) was less than
4% (see Table 5 and Fig. 9E).
178
The high selectivity was postu-
lated to arise from the larger free energy of hydration of
competing ions which hindered their access to the adsorption
sites in the membrane. The authors highlighted that the reus-
ability of the membrane (up to 8 cycles) was contingent on the
robustness of the membrane scaffold (tensile strength of ∼15
MPa).
178
One year later, this was corroborated by Qiu et al., who
highlighted that the mechanical strength of the cellulose
acetate scaffold was crucial to prevent the dissolution of HMO
adsorbents and, hence, to endow the membrane with long-term
stability.
183
The Li
4
Mn
5
O
12
-doped cellulose acetate membrane
possessed a high adsorption capacity of 23.26 mg g
−1
and was
able to selectively recover >98% of Li
+
from seawater (see
Table 5).
Besides at-sheet membranes, electrospun nanobrous
membranes have also been widely studied for lithium
extraction. The benet of electrospinning lies in its ability to
fabricate a 3D nanobrous mat with uniform pore size distri-
bution, controllable pore size, and high interconnectivity of
pores on a large-scale.
201
In fact, empirical evidence suggests
that electrospun membranes are more ideal for lithium
extraction as compared to at-sheet membranes because of the
higher specic area and porosity available to expose impreg-
nated adsorbents to the feed solution.
202,203
To maximize
accessibility of the adsorption sites, uniform dispersion of
adsorbents in the bers is required. Typically, nanobers with
small diameters are desired because of their greater specic
surface area (i.e., the pores formed between the bers will result
in more macropores). However, nanobers that are too thin
might not have sufficient mechanical strength, resulting in
defect formation on the nanobrous mat.
178
A recent study has
narrowed down that a nanober diameter of ∼90 nm is optimal
considering the trade-offbetween specic surface area and
mechanical strength.
204
Thus far, electrospun adsorptive
membranes have shown promise to recover lithium from
seawater and SWRO brines.
179,205
For instance, Park et al.
showed that PAN electrospun membranes (loaded with 60 wt%
HMO particles) could concentrate Li
+
in SWRO brine by 486
times whereas the enrichment factor of competing ions
was ∼7.
205
To impart long-term mechanical stability to electrospun
membranes, the nanobers are sometimes subjected to an
additional cross-linking step to stabilize their structure and
minimize the leaching of adsorbents. For example, Limjuco
et al. proposed an aerosol cross-linking step to stabilize elec-
trospun bers consisting of crown ether diol adsorbents
(50 wt%) embedded in a PVA matrix.
206
The cross-linker (4 vol%
Table 5 Recent advances in composite adsorbents, adsorptive membranes, and extractants for lithium extraction
Type Material
Li extraction
performance Feed solution (Li conc.) Testing condition Ref., year
Composite
adsorbents
HMO-Al
2
O
3
composite 6.5 mg g
−1
aer 3 d Li-spiked seawater
(30 ppm)
Static 181, 2018
l-MnO
2
hydrogels 94% extraction aer 2 d Seawater (0.17 ppm) Static 176, 2022
HMO lm 98% extraction aer
12 h
Synthetic seawater
(0.17 ppm)
Static 182, 2023
Adsorptive
membranes (loaded
with HMO)
Cellulose 1 mg g
−1
aer 2 d Natural seawater
(0.21 ppm)
Static 178, 2020
Cellulose acetate 98.8% extraction aer
1d
Synthetic seawater
(0.17 ppm)
Static 183, 2021
Graphene oxide and
amino-b-cyclodextrin
7-Fold enrichment aer
100 cycles
Seawater (0.17 ppm) Static 184, 2021
Polyvinylpyrrolidone
(PVP) electrospun bers
18.8 mg g
−1
aer 1 d Natural seawater
(0.17 ppm)
Static 179, 2020
PSf electrospun bers 10.62 mg g
−1
aer 3 d SWRO brine (3.3 ppm) Dynamic 185, 2016
PAN electrospun bers 8.45 mg g
−1
aer 2 h Li-spiked seawater
(2.95 ppm)
Dynamic 186, 2017
Liquid–liquid
extraction
Calix[4]arene
derivatives
100% extraction aer
4 s of residence time
Natural seawater
(0.182 ppm)
N/A 187, 2019
b-diketone derivatives 55% extraction
aer 30 min
SWRO brine N/A 188, 2016
Organophosphates and
b-diketones
100% extraction
aer 10 min
SWRO brine (2 ppm) N/A 189, 2023
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glutaraldehyde) was sprayed onto electrospun nanobers at
ambient temperature for 5 h in which 0.3 vol% HCl was used to
catalyse the acetalization reaction (Fig. 10A). Using seawater
(7 ppm Li
+
) as the feed solution, the adsorptive membrane
achieved a 520-fold higher enrichment of Li
+
with respect to
other competing ions in seawater.
206
The membrane also
showed ∼100% extraction of adsorbed Li
+
aer ve desorption
runs. However, whilst static adsorption tests demonstrate the
promise of adsorptive membranes in lithium extraction, their
potential for continuous lithium extraction remains to be vali-
dated. In the past six years, some studies have studied the
potential of electrospun adsorptive membranes in continuous
ltration tests in which the convective mass transfer of Li
+
to the
adsorbent surface is enhanced.
200,203
Park et al. demonstrated the use of polysulfone (PSf)-based
electrospun nanobers incorporated with lithium ion-sieves
(Li
0.67
H
0.96
Mn
1.58
O
4
) to recover lithium in a microltration
process (Fig. 10B and C and Table 5).
185
In the ltration tests,
the membrane showed a high water permeability of 18 341 L
m
−2
h
−1
bar
−1
(under 1 bar operating pressure) and maintained
a dynamic Li
+
adsorption capacity of 10.62 mg g
−1
. A complete
cycle of adsorption and desorption took about 11 h. Upon
reaching equilibrium, 100 mL of 0.5 M of HCl was used to
desorb Li
+
ions, and it was shown that the concentration of Li
+
Fig. 10 (A) Schematic illustration of the post-treatment of an electrospun membrane via aerosol cross-linking. Reproduced from ref. 206 with
permission from American Chemical Society, copyright 2017. (B) Surface characterization of a membrane on a field emission-electron probe
micro-analyser (FE-EPMA) with EDS mapping. The adsorbent particles in the membrane are visualized in the high-resolution TEM image. (C)
Reproduced from ref. 185 with permission from Elsevier, copyright 2016. (D) The metal ion concentrations in the feed and stripping solutions in
a cyclic adsorption process using seawater as the initial feed. Reproduced from ref. 186 with permission from Elsevier, copyright 2017.
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in SWRO brine could be enriched from 3.32 ppm to 109 ppm
aer ve cycles.
185
In another study, Chung et al. utilized PAN-
based nanobrous membranes incorporated with LISs to
recover lithium from seawater under a low transmembrane
pressure of ∼0.03 bar (Table 5).
186
To maximize the dynamic Li
+
adsorption capacity, they highlighted that the seawater owrate
must be optimized to ensure that the residence time is kept at
$0.12 min. Aer 10 cycles of adsorption–desorption, the Li
+
concentration was enriched 13-fold from ∼30 ppm to 396 ppm,
representing a 155–1552 times higher enrichment than other
competing ions in seawater (see Fig. 10D).
186
3.1.3 Liquid–liquid extraction. Liquid–liquid extraction
involves the selective removal of compounds or metal
complexes from a mixture using a solvent on the condition that
they are immiscible. As compared to the adsorption-based
approaches discussed thus far, lithium complexes can be
extracted from seawater if the former is more soluble in the
solvent than in the latter. Besides dispersing extractants in the
solvent (i.e., the stripping phase), the former could also be
incorporated into polymer inclusion membranes that are
sandwiched between seawater and the stripping phase.
207
Thus
far, calix[4]arene and b-diketone extractants have shown
promise to extract lithium from seawater or SWRO brine due to
their selectivity towards Li
+
over competing ions.
188,208
For
example, Kurniawan et al. demonstrated the potential of a tri-
propyl-monoacetic acid derivative of calix[4]arene to extract
Li
+
in a microuidic reactor.
187
At a concentration of 20 mM in
chloroform, the extractants could extract 100% of Li
+
ions from
seawater within 4 s of residence time. In stark contrast, only
2.6% of Na
+
ions were extracted despite their ∼63 000-fold
higher concentration (11 385 ppm) in seawater. Aer saturation,
the extractants could be easily regenerated by treatment with
16 mM HCl in water as the stripping agent.
To further expand the materials available for extraction,
there is a new thrust to utilize strapped calix[4]pyrrole as
a contact ion-pair receptor to extract LiCl, LiBr, and Li
2
SO
4
because of the abundance of Cl
−
,Br
−
, and SO
42−
ions in
seawater (Fig. 11A–C). As an extension of calix[4]pyrroles,
strapped calix[4]pyrroles comprise a macrocycle core bridged by
one or more tethers and have received increasing interest for
lithium extraction from seawater due to their enhanced affini-
ties and greater selectivities toward ion-pairs such as LiCl.
209,210
For example, phenanthroline-strapped calix[4]pyrroles could
extract LiCl preferentially (100% selectivity) over KCl and NaCl
from an aqueous phase to a chloroform phase.
210
Another study
has outlined the possibility of processing calix[4]pyrroles into
porous organogels that are capable of capturing LiCl amongst
other salts (KCl, NaCl, CaCl
2
, and MgCl
2
).
211
In terms of recy-
clability potential, 96% of the captured LiCl could be released in
methanol aer 20 h of immersion. Nevertheless, this work
outlined the promise of organogel extractants in organic
solvents which avoided the hydration energy penalty in aqueous
solutions. For application in seawater, future studies would
need to study the possibility of extracting LiCl from aqueous
solutions.
3.1.4 Electrochemical methods. One intrinsic disadvantage
of the adsorption and extraction methods discussed thus far is
their chemically intensive nature (e.g., acid-based desorption is
required to elute the adsorbed lithium ions). For this reason,
electrochemical methods have been studied as an alternative
because of their lower chemical usage. Also, the presence of an
applied voltage (as a driving force) could shorten the time
required to capture the same amount of Li as compared to
passive adsorption processes.
214
3.1.4.1 Electrochemical intercalation. In this paper, we focus
on lithium extraction methods using faradaic electrodes. In
layman terms, faradaic processes involve transfer of electronic
charge across the EDL at the interface between the electrode
and liquid electrolyte. Here, ion intercalation (or insertion) is
the core mechanism governing selective Li
+
capture.
33
There are
currently two prevalent types of cell architectures for lithium
extraction. The rst type is a ow-by cell, in which two parallel
at plate electrodes are vertically submerged into the feed
solution and affixed with a power supply. However, Li
+
adsorption is diffusion-limited due to the existence of the thick
boundary layer (∼100 mm) together with meagre accessibility of
adsorption sites within the electrodes. The second type is a ow-
through cell which typically displays higher ion adsorption
efficiency because of the convective mass transport of water
through the pores of the electrodes.
215
In this section, we review two mainstream types of electrodes
for Li extraction from seawater. Lithium iron phosphate
(LiFePO
4
, Fig. 11D), a polyanionic material, is a frequent elec-
trode material utilized to recover Li
+
because of its low cost,
good structural and cycling stability, and suitable working
potentials within stable windows at all pHs (Fig. 11E).
216
As
shown in Fig. 11D, the LiO
6
and FeO
6
sheets are arranged in
parallel and connected together with PO
4
. Also, the 1D olivine
structure of FePO
4
presents a low Li
+
migration barrier and
thermodynamic preference for Li
+
over Na
+
(the strongest
competing ion in seawater).
217
The rst step of Li
+
electro-
chemical intercalation is dehydration and charge transfer at the
EDL (Fig. 11F). For instance, the selective intercalation of Li
+
over Mg
2+
and Ca
2+
into the FePO
4
host lattice is achieved
because of the signicantly lower hydration enthalpy of the
former (e.g.,DH
hyd
=−519 kJ mol
−1
for Li
+
vs. −1921 kJ mol
−1
for Mg
2+
).
213
In the second step, the ions need to migrate in the
crystal lattice to be stored in the interstitial sites. Here, due to
the stronger bonding and smaller migration barrier (E
m
)ofLi
+
,
it is favourably intercalated into the FePO
4
lattice over
competing ions in seawater (e.g.,E
m
=170 meV for Li
+
vs. 290
meV for Na
+
vs. 572 meV for Ca
2+
).
213
Both steps are manifes-
tations of the thermodynamic and kinetic factors regulating the
selective intercalation of Li
+
over competing ions.
However, an intrinsic drawback of LiFePO
4
is its low Li
+
diffusivity and poor electrical conductivity due to its 1D channel
structure.
229,230
To overcome this limitation, surface coating and
structural modication of the electrodes have been
proposed.
212,231
Kim et al. demonstrated that a surface coating of
5 wt% polydopamine on carbon-overlayed LiFePO
4
electrodes
could ameliorate its electrical conductivity and wettability
(Fig. 12A).
212
Because of the enhanced wetting, the ion concen-
tration and mobility at the electrode surface were enhanced,
providing Li
+
with more energy to surmount the energy barrier
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for insertion into FePO
4
. In an electrochemical cell using
seawater feed, the concentration of Li
+
could be enriched 4330
times more than that of Na
+
(Table 6).
212
Post-extraction char-
acterization of the electrodes revealed that the insertion effi-
ciency (that is, Li
+
occupancy per available site in the lattice
host) of the electrode was ∼99.3%.
212
In essence, the high
enrichment factor of Li/Na is attributed to the fact that almost
all available adsorption sites in the electrodes were occupied by
Li
+
ions (Fig. 12B).
LiFePO
4
electrodes are also susceptible to the co-
intercalation of competing ions (e.g.,Na
+
and Mg
2+
), and thus
researchers have explored methods to enhance their selectivity
towards Li
+
.
233
Recently, Liu et al. unveiled that a TiO
2
-coated
LiFePO
4
electrode integrated with a pulsed electrochemical
method could extract lithium from seawater with ultrahigh Li/
Na selectivity for at least 10 cycles.
218
Despite the 60 000-fold
lower concentration of Li
+
(0.18 ppm) as compared to Na
+
(10
800 ppm), the extraction ratio of Li/Na was approximately 1 : 1,
corresponding to a Li/Na molar selectivity of 18 000 in natural
seawater collected from Half Moon Bay (USA).
218
The authors
highlighted two critical factors for achieving such a superb
performance. First, to enlarge the interface contact area
between the working electrode and electrolyte (seawater),
a hydrophilic TiO
2
layer was coated onto the electrodes via
atomic layer deposition. It was highlighted that the coating
possessed similar Li
+
diffusivity as FePO
4
and a thickness of
3 nm to minimize the activation barrier for Li
+
diffusion.
Second, a pulsed electrochemical method was adopted to allow
the electrode to rest periodically, resulting in the redistribution
of Li
+
and Na
+
in the electrode. The more uniform Li/Na content
is poised to improve electrode homogeneity and reduce the
overpotential to propel the intercalation of Li
+
into the crystal
structure of FePO
4
, thereby increasing its selectivity and struc-
tural stability.
218
However, this work employed a three-electrode
Fig. 11 (A) Chemical structure of a hemispherand-strapped calix[4]pyrrole (receptor 1) capable of selectively extracting Li
+
in (B). (C) The
extracted lithium complex [1 LiCl$H
2
O$MeOH] could be visualized in the single crystal X-ray diffraction structure (front view). Reproduced from
ref. 209 with permission from American Chemical Society, copyright 2016. (D) Schematic illustration of the selective intercalation of Li
+
into the
structure of FePO
4
. (E) The electrochemical stability window of water at different pHs and operating windows of some electrode materials for Li-
ion batteries. Reproduced from ref. 212 with permission from American Chemical Society, copyright 2015. (F) A schematic illustration outlining
the two main steps during electrochemical intercalation. Reproduced from ref. 213 with permission from the National Academy of Sciences,
copyright 2022.
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cell that is not representative of practical working conditions
and the low Li
+
concentration in seawater would result in
a change in Li insertion potential towards the hydrogen evolu-
tion potential (according to the Nernst equation). Future studies
would need to address these fundamental issues before the
proof-of-concept in this work can be realized on an industrial
scale.
The second type of electrode material is LiMn
2
O
4
, which
exhibits a few advantages as compared to LiFePO
4
, such as
a faster Li
+
transport rate and less discernible Na
+
or Mg
2+
co-
intercalation.
219
To achieve higher lithium extraction rates,
smaller LiMn
2
O
4
particles are preferred to maximize the rate of
Li
+
supply to the electrode. For lithium extraction from
seawater, empirical evidence suggests that LiMn
2
O
4
particles
with a diameter of ∼300 nm are optimal to maximize the
interface area density of the electrode and electrolyte surface
(Fig. 12C).
220,232
Nevertheless, lithium extraction is still chal-
lenging due to kinetic limitations imposed by the low Li
+
concentration in seawater, resulting in low current, high resis-
tance, and an overpotential. To circumvent this limitation, Yu
et al. demonstrated seawater lithium extraction via auidic
electrochemical method employing a MnO
2
-coated working
electrode (Fig. 12D).
220
First, a constant current of 0.5 mA was
applied between a carbon felt cathode and MnO
2
working
electrode to induce the selective intercalation of Li
+
into the
crystal lattice of MnO
2
(cell 1). Upon reaching a cut-offvoltage of
1.0 V, the seawater stream was turned offand a constant current
was applied between the lithiated MnO
2
electrode and anode to
release captured Li
+
into the anolyte. Coupled with O
2
reduction
at the anode, the LiOH product was formed and collected in cell
2. Overall, this ow architecture ensures continuous Li
+
replenishment on the electrodes to maintain Li
+
concentrations
at pseudo-constant levels adequate for extraction.
234
The elec-
trochemical cell demonstrated fast kinetics with preliminary
calculations revealing an energy consumption of 7.2 W h to
recover 1 g of Li
+
(see Table 6).
220
Due to the presence of potentially corrosive ions in seawater
(e.g.,Cl
−
), the ideal electrode should not only possess high Li
+
adsorption capacity and selectivity, but also high electro-
chemical stability for a long service life.
235
However, LiMn
2
O
4
electrodes intrinsically suffer from performance degradation in
long-term tests due to the co-elution of manganese with Li
+
during acid treatment (e.g., 29% adsorption capacity loss aer
200 cycles of adsorption–desorption).
233,236
Hence, some recent
studies have proposed the coating of electrodes with a protec-
tive layer to prevent manganese dissolution (on the condition
that the coating does not compromise Li
+
mobility).
222,237
For
instance, Fang et al. demonstrated the coating of a LiMn
2
O
4
electrode with PPy via a two-step process involving in situ
polymerization and high temperature annealing.
222
Using
simulated seawater as the electrolyte, the PPy-coated electrode
exhibited ∼26% higher discharge capacity (117 mA h g
−1
) than
the virgin LiMn
2
O
4
electrodes at the initial stage, with no
noticeable performance degradation aer 50 cycles of adsorp-
tion–desorption (Table 6).
3.1.4.2 Electrodialysis. Electrodialysis is an electrochemical
separation technology capable of recovering valuable metal ions
from aqueous sources such as seawater and brackish
water.
238,239
In an electrodialysis cell, multiple pairs of anionic
exchange membranes and cationic exchange membranes are
placed alternately between one pair of cathode and anode,
thereby forming multiple concentrating and desalting cham-
bers. Under the impetus of an electric eld, cations and anions
in the feed water are conveyed through oppositely charged ion-
exchange membranes (IEMs) but rejected by the same charged
IEMs, resulting in accumulation and depletion of ions in the
concentrated and dilute chambers, respectively. In this sub-
section, we introduce three types of electrodialysis systems for
lithium extraction.
Selective electrodialysis. To capture lithium ions in
seawater, standard IEMs are replaced with monovalent-selective
IEMs because the latter are selective to monovalent ions over
divalent ions.
240
This selectivity stems from two mechanisms: (i)
charge rejection, in which a thin oppositely charged layer is
employed to impede the permeation of divalent ions and (ii)
a cross-linked layer that inhibits the transport of divalent ions
due to the larger hydrated size of the latter.
241
For Li
+
extraction
from SWRO brine, a study has narrowed down that an applied
voltage of 7.0 V was optimal to maximize extraction efficiency.
242
Typically, the mass transfer of Li
+
through the CEM (from the
dilute to the concentrate chamber) will increase with applied
voltage. However, at excessively high applied voltage, undesir-
able leakage of Ca
2+
and Mg
2+
through CEMs might occur,
resulting in a lower separation efficiency between Li
+
and
divalent ions. To enable a sharper separation between Li
+
and
Mg
2+
, the initial volume ratio of the concentrating and desalting
chambers has been narrowed down to ∼0.6–1.0 for selective
electrodialysis.
242,243
Nevertheless, we would like to point out
that other monovalent cations (K
+
and Na
+
) will compete more
rmly with divalent cations (Ca
2+
and Mg
2+
) to migrate through
the monovalent selective CEMs.
244
The high concentrations of
competing ions would result in excessive energy consumption
and low lithium extraction efficiency. Hence, CEMs with higher
selectivities are needed to enable a sharper separation between
Li
+
and other monovalent ions.
Electrodialysis with superconductor membranes. The
second type of electrodialysis conguration involves supercon-
ductor membranes (also known as solid-state electrolytes, refer
to Fig. 13A–C and Table 6). First reported by Hoshino in 2015,
the superconductor membrane can be thought of as a molec-
ular sieve that only permeates Li
+
in seawater while blocking the
transport of other cations (e.g.,Na
+
and Mg
2+
).
245
To ensure
a low potential gap between the anode and cathode, the
superconductor membrane typically possesses a lithium ionic
conductivity >10
−4
Scm
−1
.
245,246
A typical electrochemical cell
consists of two inert electrodes, a liquid organic electrolyte,
a superconductor membrane and seawater (i.e., the supercon-
ductor membrane is placed between the anode and cathode
chambers, see Fig. 13D). Under the action of an electric eld, Li
+
in seawater will selectively permeate through the supercon-
ductor membrane and transfer to the organic electrolyte, while
other cations are blocked by the membrane and remain on the
anode side. Since the Li
+
migration speed is controlled by the
applied voltage, the current density can be increased to speed
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up lithium extraction.
247
However, undesired side reactions
could occur at high current density, resulting in low extraction
rates. Yang et al. outlined that a current density of 240 mAcm
−2
was optimal to maximize the lithium extraction rate from
seawater (Table 6).
223
In short, depending on the feed condi-
tions, membrane used and system design, a comprehensive
optimization in terms of the current density and operating
parameters is needed to achieve the maximum lithium extrac-
tion rate.
Recently, Li et al. utilized an LLTO superconductor
membrane to enrich the lithium concentration in seawater
from 0.21 ppm to 9013 ppm aer ve cycles of continuous
concentration lasting ∼100 h (see Table 6).
224
The 43 000-fold
increase in the lithium concentration enabled the direct
precipitation of battery-grade Li
3
PO
4
in powder form (>99%
purity). Such a superb performance hinges on the exceptionally
high Li/Mg and Li/Na selectivities of the membrane (>45 000
and 16 000 at the rst stage, respectively, Fig. 13E). Theoreti-
cally, the size of pore windows of the membrane should be
between the size of Li
+
and competing ions (e.g.,Mg
2+
and Na
+
),
such that they readily permeate Li
+
while blocking the passage
of the latter.
224,247
However, for some types of membrane
materials such as LLTO, the size of the pore window in the
framework can be slightly smaller than the hydrated Li
+
ion
size. LLTO possesses a perovskite crystal structure in which the
percolation of Li
+
occurs in the vacancies of the LLTO frame-
work (Fig. 13C). Although the size of Li
+
(1.18 Å) is larger than
that of the square windows in the framework (1.07 Å), it was
surmised that the thermal vibrations of the TiO
6
octahedron
resulted in a slight enlargement of the latter to the extent that it
permitted the selective passage of Li
+
over other ions.
224
Also, the membrane should be thin to enable ultrafast
transport of Li
+
but mechanically robust to sustain stable
performances. It was highlighted that a 0.065 mm thick LLTO
Fig. 12 (A) Schematic illustration of the synthesis of polydopamine (pD)-coated FePO
4
(pD-c-FP) from LiFePO
4
(LFP) and carbon-coated FePO
4
(c-FP). (B) Molar ratios of the pD-c-FP electrode and the control electrode (c-FP) after Li extraction. Reproduced from ref. 212 with permission
from American Chemical Society, copyright 2015. (C) SEM image showing ball milled LiMn
2
O
4
powders. Reproduced from ref. 232 with
permission from American Chemical Society, copyright 2020. (D) A schematic setup of a fluidic electrochemical extraction system for lithium
extraction from seawater. The system comprises an O
2
evolution cathode, a MnO
2
working electrode, and an O
2
reduction anode. Reproduced
from ref. 220 with permission from American Chemical Society, copyright 2020.
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membrane possessed a tensile stress and strain of 110 MPa and
0.066%, respectively, which was sufficient to sustain the stable
extraction of lithium for more than 2000 h. Nevertheless, the
energy consumption to extract 1 kg of lithium was 76.3 kW h
due to the high voltage requirement (3.25 V).
224
Another recent
study has underlined that the durability of the membrane was
contingent on its chemical stability against seawater and the
non-aqueous electrolyte.
247
Holistically, these approaches of
utilizing superconductor membranes might not be competitive
from an industrial perspective due to the energy required to
move large volumes of seawater in order to obtain reasonable
amounts of lithium. The high cost of superconductor
membranes and their susceptibility to cracking are additional
issues that may restrict the realization of this technology on an
industrial scale.
225
On the other hand, Zhao et al. demonstrated the potential of
an electrochemical cell employing a 500 mm thick LLAZO
superconductor membrane for lithium extraction from seawater
in the South China Sea (see Table 6).
226
The membrane was
synthesized via spark plasma sintering and possessed an ionic
conductivity of 1.5 ×10
−4
Scm
−1
at 308 K. At a current density of
0.76 mA cm
−2
, the driving voltage of the electrochemical cell was
4.88 V, demonstrating a lithium extraction efficiency of 1980 mg
m
−2
h
−1
. However, the membrane was susceptible to biofouling
aer immersion in seawater for 12 h and hence periodic in situ
cleaning of the superconductor membrane was performed using
a mixture of Cl
2
and O
2
collected atthe anode (each cycle lasted 5
min). The regain of cell efficiency aer cleaning was manifested
by a decrease in working voltage (0.23 V) at a constant current
density of 0.2 mA cm
−2
. Holistically, we would like to point out
the existence of safety issues for this electrodialysis congura-
tion. In particular, some types of liquid organic electrolytes are
ammable (e.g.,LiClO
4
) and could decompose at high temper-
ature or working potential.
249,250
In attempts to overcome the aforementioned issues imposed
by the organic electrolyte, some researchers have proposed the
use of all-solid electrodes in the eld of lithium battery
design.
251,252
Nevertheless, one major challenge for lithium
extraction is minimizing the large interfacial resistance
between the lithium metal electrode and superconductor
membrane (this is contingent on a good physical contact
between them, Fig. 13F). Recently, Zhang et al. proposed the use
of a lithiated Li
x
TiO
2
lm in a solid-state electrochemical device
to recover lithium from seawater.
227
The TiO
2
layer was rst
coated onto a LLAZO superconductor membrane prior to the
annealing of thin lithium foil (0.2 mm) at 423 K for 3 h
(Fig. 13G). During the charging process, Li
+
selectively perme-
ated through the LLAZO membrane and deposited on the
lithium electrode (Fig. 13H). In electrochemical tests using
natural seawater, the device achieved a lithium extraction effi-
ciency of 1100 mg m
−2
h
−1
and exhibited cycling stability for 80
cycles (see Table 6).
227
Sandwiched liquid-membrane electrodialysis. As an alter-
native to solid membranes, electrodialysis using liquid
membranes has also been explored for lithium extraction from
seawater. The basis of this technology is to amalgamate liquid-
membrane extraction and electrodialysis in which the
composite membrane consists of a Li
+
-impregnated liquid
carrier.
253,254
For example, PP13-TFSI (C
11
H
20
F
6
N
2
O
4
S
2
) is known
to be an effective organic liquid because of its high lithium
conductivity induced by the TFSI functional group.
228
The ideal
ionic liquid should facilitate the selective permeation of Li
+
over
competing ions in seawater (from the anode to the cathode
side), resulting in Li
+
enrichment in the latter. At an applied
voltage of 2.0 V, an electrodialysis cell employing PP13-TFSI
ionic liquid membranes could extract 22.2% of Li
+
in seawater
(Table 6).
228
However, a direct consequence of this design is the
reduction in the Li
+
concentration in the recovery stream,
Table 6 Recent advances in electrochemical methods for lithium extraction from seawater
Type Material Li extraction performance Feed solution (Li conc.) Ref., year
Electrochemical
intercalation
Polydopamine-coated
LiFePO
4
4330-Fold Li to Na
enrichment
Natural seawater (0.183 ppm) 212, 2015
TiO
2
-coated LiFePO
4
18 000-Fold Li to Na
enrichment
Natural seawater (0.18 ppm) 218, 2020
Li
x
Mn
2
O
4
37 mg g
−1
d
−1
Synthetic seawater (1 ppm) 219, 2022
LiMn
2
O
4
20.6 mg g
−1
aer 0.8 h Synthetic seawater (7 ppm) 220, 2020
LiMn
2
O
4
1800-Fold Li enrichment SWRO brine (0.242 ppm) 221, 2020
Polypyrrole-coated
LiMn
2
O
4
37.1 mg g
−1
aer 1.5 h Synthetic seawater (170 ppm) 222, 2022
Electrodialysis
(superconductor
membranes)
Li
1+x
Al
y
Ge
2−y
(PO
4
)
3
(LAGP) 570 mg m
−2
h
−1
Seawater 223, 2018
Li
0.33
La
0.56
TiO
3
(LLTO) 43 000-Fold enrichment
aer 5 cycles
Natural seawater (0.21 ppm) 224, 2021
Li
1.3
Al
0.3
Ti
1.7
(PO
4
)
3
(LATP)
composite
3-Fold enrichment aer 5 h Seawater (0.198 ppm) 225, 2023
Li
6.75
La
3
Al
0.25
Zr
2
O
12
(LLAZO)
1980 mg m
−2
h
−1
Natural seawater (0.2 ppm) 226, 2020
Li
x
TiO
2
lm on LLAZO 1100 mg m
−2
h
−1
Natural seawater (0.2 ppm) 227, 2021
Electrodialysis (liquid-
membrane)
PP13-TFSI ionic liquid 22.2% extraction aer 2 h Natural seawater (0.17 ppm) 228, 2013
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resulting in a continuously decreasing extraction efficiency with
time (due to the reduction in driving force). Solvent leakage and
membrane swelling also compromise the long-term stability of
the membrane. Lastly, the highly toxic nature of ionic liquids is
another limitation hindering the deployment of this technology
for practical operations.
4. Challenges and future perspectives
In this section, we seek to paint a more complete picture for
readers of the current challenges and future opportunities for
uranium and lithium extraction from seawater. A summary of
the different technologies, current developmental stage, and
proposed opportunities for future directions of investigation is
provided in Fig. 14. To date, conventional adsorption
demonstrates the highest technology readiness level (TRL)
given its ease of operation and low cost (Fig. 14). In particular,
conventional adsorption using amidoxime bers and LISs was
engineered and successfully operated in marine environments
for uranium and lithium extraction, respectively.
42,44,171
However, conventional adsorption using these old-time mate-
rials is still plagued by the Achilles heel of competitive
adsorption and marine biofouling that renders it difficult to
achieve breakthrough performance. For this reason, we reckon
that novel adsorptive materials design will continue to make
headway in advancing uranium and lithium extraction tech-
nologies. Besides achieving higher adsorption capacities, care-
ful attention should be paid to their regeneration performances
and long-term stability.
Fig. 13 (A) Cross-sectional SEM image of a lithium superconductor membrane. (B) The structure of a Li
1+x
Al
x
Ge
2−x
(PO
4
)
3
superconductor
membrane. Reproduced from ref. 248 with permission from American Chemical Society, copyright 2022. (C) Crystal structure of LLTO in ball and
stick mode (left) with an illustration of the percolation of Li
+
in the lattice (right). (D) Schematic diagram of a three compartment cell that is used to
enrich lithium from seawater. (E) The plot of the passing amount with the stage number that quantifies the progressive permeation of various ions
through the membrane (inset shows the passing amount in a lower range). Reproduced from ref. 224 with permission from the Royal Society of
Chemistry, copyright 2021. (F) Cross-sectional SEM image outlining the poor interfacial contact at the garnet solid-state electrolyte and Li metal
interface (inset shows a digital photo of melted Li metal on the garnet surface). (G) Cross-sectional SEM image showing good interfacial contact
in the presence of a thin TiO
2
layer with a corresponding illustration of the electrochemical device shown in (H). Reproduced from ref. 227 with
permission from Elsevier, copyright 2021.
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In parallel with materials development, signicant progress
has been made in the deployment of adsorptive materials into
technology platforms. Besides conventional adsorption,
another technology that is common to both uranium and
lithium extraction is adsorptive membranes that has gained
increasing attention over the past 10 years. Membrane tech-
nology is widely favoured because of its high manufacturability,
low energy consumption and footprint, and most importantly,
its high scalability using roll-to-roll assembly that is well
established in the membrane industry.
255
Electrochemical
approaches are another class of technology that has evolved in
the past 10 years to overcome the challenge of sluggish kinetics
for uranium and lithium extraction from seawater.
33,165
Here, it
is worth noting that there are fundamental differences in terms
of their mechanisms for uranium and lithium extraction.
Electrochemical approaches for uranium extraction include
electrosorption and electrodeposition that are based on the
overarching process of adsorption (Fig. 14). Hence, improve-
ment in the extraction efficiency ultimately boils down to the
evolution of materials with higher adsorption capacity. On the
other hand, electrochemical lithium extraction methods are
based on intercalation or electrodialysis processes that are not
related to adsorption. We reckon that advances in electrode
materials and membranes will play a critical role in advancing
performance standards achievable by electrochemical lithium
extraction technologies (Fig. 14). Overall, there remain many
Fig. 14 Assessment of the current challenges and future opportunities of (A) uranium and (B) lithium extraction from seawater and SWRO brines.
The different technologies are assessed via their technology readiness level (TRL) in the form of a spider diagram. The strengths and weaknesses
of the technologies are also indicated.
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research opportunities to expand this emerging and exciting
eld of uranium and lithium extraction on both the material
and technology development levels (this will be discussed in
Sections 4.1 and 4.2).
4.1 Next-generation adsorbent materials and engineering
designs
Amid the promising materials and technologies available, the
ultralow concentration remains the Achilles heel of uranium
and lithium extraction from seawater and SWRO brine. Moving
forward, we map out several short- and long-term goals in three
different domains to help realize the vision of capitalizing
offshore uranium and lithium resources for securing a sustain-
able energy future (Fig. 15). First, materials research and engi-
neering will continue to hold the key to unlocking this
potential. To this end, some of the best uranium and lithium
extractions from seawater are still using old-time amidoxime-
based and LIS-based adsorbents, respectively. Notwith-
standing that these adsorbents are still the current state-of-the-
art, from a long-term perspective, there is a need to expand our
current capability and put in place other novel chemistries and
materials that could possibly improve extraction capacity as well
as kinetics. The use of computational simulation and machine
learning tools could potentially help accelerate this process. In
this data-driven age where published literature, online data-
bases and repositories are readily accessible, there is no
shortage of data availability and knowledge for materials design
and synthesis. The great challenge is how to strategically capi-
talize on this wealth of resources to carry out rational design of
critical adsorbents that could be a game-changer.
Recently, the use of density functional theory (DFT) compu-
tation has gained signicant traction as a high-throughput tool
to study materials properties such as atomic and electronic
structures as well as stability and chemical reactivity from rst
principles.
256,257
This is joined by the growing interest in machine
learning (ML), which –when combined with DFT computation –
serves as a powerful predictive framework for materials
discovery. A properly trained and optimized ML model by
exploiting sizable DFT data from either empirical results or
existing literature and databases can enable greater computa-
tional accuracy at a shorter lead time.
258
To date, such frame-
works have already been used to carry out successful predictions
for a plethora of materials including crystalline solids, 2D
materials and alloys.
259–263
We reckon that embracing such a data-
driven approach is the way forward to rationally design next-
generation adsorbent materials for ultralow concentration
uranium and lithium extraction from seawater.
For now, focusing efforts on biosorption and designing
hierarchical structures for greater ion diffusivity and accessi-
bility would likely see more immediate and tangible results.
Biosorption is a physicochemical process that removes or binds
desired substances in liquid streams by using bio-derived
materials. Compared to chemisorption, biosorption has the
advantages of high and selective uptakes and anti-microbial
properties, apart from being greener and more sustainable,
which make it compelling for sustainable lithium and uranium
extraction from seawater.
264
As discussed in Section 2.1.4, the
use of uranyl-binding proteins and bacterial strains has already
shown promise for uranium extraction from seawater, owing to
their unique selectivity towards uranium.
83,84
For lithium
extraction, Tsuruta screened 63 species of microorganisms and
found that both Arthrobacter nicotianae IAM12342 and Brevi-
bacterium helovolum IAM1637 could accumulate 0.87 and 0.68
mg g
−1
of Li from a 0.5 ppm LiCl solution, respectively.
265
Sel-
vamani et al. also constructed a recombinant Escherichia coli
with surface-displayed a lithium-binding peptide, which
demonstrated 0.32 mg g
−1
of Li accumulated from a 0.7 ppm
LiCl solution.
266
Molecular modelling revealed that oxygen
atoms on the carbonyl groups of proline and glycine in the
Fig. 15 Roadmap of uranium and lithium extraction from seawater, showing different short- and long-term strategies in three domains to secure
a sustainable energy future.
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peptide were likely responsible for lithium binding. Seemingly,
all the evidence of bacteria showing Li accumulation from
solutions with low Li concentration –similar to that of SWRO
brine –suggests the promise of biosorption for seawater
extraction. And, considering that biosorption is at present not
as widely explored as chemisorption, the potential is there to
create a short-term impact in this direction.
As for chemisorption, gearing towards novel porous mate-
rials and nanobrous scaffolds to construct hierarchical struc-
tures for increasing contact surface areas and ion accessibility
will help elevate performances of current chemistry and
adsorbents used. To date, porous nanomaterials such as MOFs,
COFs and POPs have already been employed as porous plat-
forms to host amidoxime functional groups for uranium
extraction (see Section 2.1), while electrospun nanobrous
scaffolds have provided the high specic surface area and
porosity necessary for greater contact and exposure of feed
solutions to the LIS-based adsorbents for lithium extraction (see
Section 3.1.2). There is still room for improvement by amal-
gamating these two strategies to realize hierarchically struc-
tured designs with two or more levels of pore systems. By this,
we mean leveraging the large macropores arising from spaces
between intertwined nanobers to induce better ow distribu-
tion of seawater for increasing mass transport to the macro-
pores –or even smaller micropores –of the host nanomaterials.
The outcome should see greater ion accessibility to active sites
for better adsorption performances. Improving mass transport
and ion accessibility should also at the same time allow for
more effective elution, leading to better recyclability and
applicability of the adsorbents.
4.2 SWRO brine: a more practical solution to seawater
extraction
To address the challenge of ultralow concentration extraction
beyond advancing adsorption chemistry and engineering,
a more straightforward solution is to simply increase the
concentration of uranium and lithium. In this regard, SWRO
desalination brine is technically a better resource than seawater
itself. This proposition is attractive from three perspectives.
First, SWRO brine could provide between 5.5–8.3 ppb and 0.28–
0.43 ppm of uranium and lithium, respectively, on the account
that most single-to two-stage SWRO plants these days have the
capacity to deliver an overall water recovery rate between 40 and
60%.
267
This indicates that the uranium and lithium concen-
trations in SWRO brine can potentially reach up to 2.5 times
that of seawater without the need for additional energy input
and/or capital investment. Second, in the wake of rapid climate
change and urbanization, water scarcity is likely to exacer-
bate,
268,269
which means that the demand for desalination will
increase in the coming decades. Today, desalination supplies
drinking water to around 300 million people worldwide and
produces around 142 million m
3
per day of SWRO brine as a by-
product in the process.
270–272
This amount of SWRO brine is
projected to increase with increasing demand for desalination,
placing the global desalination industry in a good position to
provide the required SWRO brine to support uranium and
lithium extraction at scale. Third, in a typical desalination
process, pre-treatment processes are always installed upstream
to prep the seawater for the SWRO process. Generally, two types
of pre-treatment, conventional and membrane processes, are
used with the aim of reducing microorganisms including
bacteria and microalgae, colloidal contaminants, total organic
carbon (TOC) and silt density index (SDI) of raw seawater to
satisfy the downstream SWRO feed water requirement and
reduce fouling propensity.
273
In particular, the SDI of the
seawater, which measures the fouling capacity in the RO
system, will drop to a typical value of <3 aer pre-treatment,
274
suggesting that SWRO brine may be a relatively cleaner resource
than raw seawater for uranium and lithium extraction.
Notwithstanding the merits of using SWRO brine as opposed
to seawater, there are two critical challenges that need to be
overcome in order to enable the use of SWRO brine for extrac-
tion. First, despite being cleaner, the pre-treated seawater will
subsequently be reduced into SWRO brine, which means
residual foulants that remain aer pre-treatment will eventually
be concentrated. It is therefore important to systematically
study the impact of biofouling caused by these concentrated
foulants during extraction so as to understand the true potential
for biofouling mitigation using SWRO brine. Second, like
seawater, SWRO brine is a complex matrix. Whilst the concen-
tration of uranium and lithium will be higher in SWRO brine as
compared to seawater, the concentration of other ions will
likewise be greater in the former as water is recovered in an
SWRO process. This means that there could be more severe
competitive adsorption by other cations, which will undermine
the capture efficiency of the extraction process. Hence, from
a long-term outlook, antifouling properties and higher selec-
tivity towards uranyl and lithium cations must be imbued into
the design of next-generation adsorbent materials to incentivize
the switch from seawater to SWRO brine. Strategically speaking,
substituting SWRO brine for seawater will not only help the
desalination sector tackle the recurring issue of brine
management, but also create a more dynamic ecosystem to
recover uranium and lithium sustainably, which will be dis-
cussed in the next section.
4.3 Economic feasibility: the most important technology
driver
From a technological standpoint, extracting uranium and
lithium from seawater is technically possible. However, the
most important driver still lies in its economic feasibility. To
this end, the cost of seawater uranium extraction is estimated to
be as low as 220 and 280 US$ per kg,
275
which is even then much
higher than the spot price of ∼120 US$ per kg in May 2023.
276
The cost of electricity for extracting lithium from seawater using
the electrochemical method is also recently estimated to be 5
US$ per kg, while that from other types of brine is found to be
between 2 and 7 US$ per kg of LiCO
3
.
218,224,277
Incorporating
other expenses that include materials and operational costs
could easily push the cost beyond the recent commodity price of
between 40 and 45 US$ per kg of LiCO
3
.
278
It should be noted
that commodity pricing, which is closely tied to product purity
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and market uctuation, is an indispensable part of the equa-
tion. Benchmarking of commodity pricing must be built into
detailed economic analysis to provide a more comprehensive
evaluation of the protability and economics of seawater
extraction.
The current consensus is that seawater extraction is unlikely
to be economically attractive due to the ultralow concentration
of uranium and lithium, lack of mature technologies at a high
technological readiness level (TRL), and uncertainty in extrac-
tion efficiency as well as product purity and uniformity.
279,280
Switching to SWRO brine may produce unexpected results, but
this proposition remains elusive at present given the scarce
literature that supports this claim. Herein, we recommend
staying forward-looking to wait for future opportunities that
could reduce the barrier for SWRO brine extraction. Notably,
alongside the future expansion of the desalination sector, the
volume of brine produced per day is expected to scale accord-
ingly (see Section 4.2). This implies that the quantity of uranium
and lithium extracted could increase to a substantial amount to
benet from the signicant economies of scale that could
render SWRO brine extraction protable.
281
Furthermore, with desalination becoming increasingly
widespread, SWRO brine disposal will eventually be a serious
environmental issue that could drive the cost of desalination up
as the variable cost for brine management and disposal
increases.
271,272
To address this challenge, there are a multitude
of valuable and high concentration elements that could be
recovered from SWRO brine. And, among these elements,
bromine, chlorine, magnesium and sodium are of particular
interest as their extraction from SWRO brine has already been
demonstrated to be economically feasible through a combina-
tion of extraction techniques.
282
Co-extracting these protable
elements could potentially reconcile the cost-prohibitive
economics of uranium and lithium extraction.
Hence, as we move forward in these directions, there is
a need to holistically look into techno-economic analysis and
life-cycle assessment of uranium and lithium extraction that
put the value chain of desalination in mind. Prospectively, cost
calculations must be integrated with ongoing advances in
SWRO desalination such as energy-efficient membranes, low-
pressure nanoltration seawater pre-treatment, and zero
liquid discharge using novel hybrid processes.
270
We reckon
that only by synergizing with the considerable space for future
cost improvements of SWRO desalination could we nd
a potential window to create a win–win situation for both the
desalination and energy industries, leading to a long-term
sustainable uranium and lithium seawater extraction.
5. Conclusive remarks
Securing long-term supplies of uranium and lithium is crucial
for clean energy production as well as energy storage and
renewable energy applications. In this paper, we assert that
uranium and lithium extraction from seawater is a viable
solution to minimize resource shortages in the future.
Furthermore, extracting uranium and lithium from seawater
would appreciably reduce reliance on land-based mining that
causes environmental degradation. This paper presents recent
advances in materials and technologies for uranium and
lithium extraction from seawater and SWRO brine, focusing on
their adsorptive capabilities at ultralow concentrations with
a high background of competing ions. The platforms that are
used to architect the adsorptive materials are evaluated in terms
of their extraction performance, scalability, and durability for
long-term application. In particular, surface-centric strategies
to impart fouling resistance to the adsorptive materials,
membranes, and electrode materials are highlighted for miti-
gating performance degradation due to fouling and corrosion
effects.
This paper furnishes useful guidelines to expedite the
development of materials and technologies for achieving
a more economical and sustainable extraction of uranium and
lithium. Moving forward, we need to discriminate between
materials and technologies that are unique for application in
SWRO brine. This will allow us to better tap into the value of the
latter as a resource to achieve sustainability at the water-energy
nexus. As extraction technologies become more effective in the
coming decades, it is also important to keep in mind the
dynamic nature of metal market prices due to changing market
conditions. Hence, techno-economic analysis and life-cycle
assessments will be essential to evaluate the costs and bene-
ts of extracting these metal resources from the oceans. This
review is anticipated to spur innovative ideas in the interdisci-
plinary research domains of materials science, membrane
engineering, and separation and recovery technologies to meet
the growing demands of water, uranium, and lithium for the
benet of all mankind.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by the National Research Founda-
tion, Singapore and implemented by the Public Utilities Board
(PUB), Singapore's National Water Agency, under its Competitive
Funding for Water Research Funding Initiative (grant award
CWR-2101-0010). We would also like to thank the National
Research Foundation, Singapore, and PUB, under its RIE2025
Urban Solutions and Sustainability (USS) (Water) Centre of
Excellence (CoE) Programme which provides funding to the
Nanyang Environment & Water Research Institute (NEWRI) of
the Nanyang Technological University, Singapore (NTU).
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