Journal of Water Process Engineering 55 (2023) 104223
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Journal of Water Process Engineering
journal homepage: www.elsevier.com/locate/jwpe
Occurrence, treatment, and potential recovery of rare earth elements from
wastewater in the context of a circular economy
Delal E. Al Momani a, *, Zainab Al Ansari a, Mariam Ouda b, c, Mohammed Abujayyab b, c,
Mujeeb Kareem b, c, Taofeeqah Agbaje b, d, Banu Sizirici a
a
Department of Civil and Environmental Engineering, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, United Arab Emirates
Department of Chemical Engineering, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, United Arab Emirates
Center of Membranes and Advanced Water Technology, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, United Arab Emirates
d
Center for Catalysis and Separation, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, United Arab Emirates
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rare earth elements
Wastewater
Occurrence
Treatment
Recovery
Rare Earth Elements (REEs) are increasingly consumed and released into the environment, leading to REE
anomalies in aquatic systems. However, REE extraction is associated with detectable radioactive matter and
poses risks to human health, including carcinogenicity, occupational poisoning, and other health effects. REEs
have also been shown to have toxic effects on aquatic organisms. This review discusses the levels and occurrence
of REEs in wastewater treatment plants, state-of-the-art removal and recovery techniques, and the potential for
REE recovery in a circular economy. The study showed that among the light rare earth elements (LREE), Ce had
the highest concentration in wastewater, while Gd had the highest concentration among the heavy rare earth
elements (HREE). Treatment of REE wastewater has successfully reduced its toxicity and enabled the recovery of
valuable components for reuse. In the circular economy, the sustainability of REE mining and application can be
improved by converting REEs from wastewater into industrial resources, but limited laboratory and pilot studies
make the economic viability of REE recovery uncertain. Therefore, more research is needed to understand
wastewater-borne REEs’ fate and behavior, in order to develop sustainable and economically feasible recovery
methods for a circular economy.
1. Introduction
Rare Earth Elements (REEs) are the 15 lanthanides with atomic
numbers from 57 to 71 as well as 27 and 39, as shown in Table A1, which
occur together in nature and possess a similar trivalent oxidation state
due to having the same outer electron configuration of the 5d16s2 orbital
[1–4]. REEs are defined as “rare” because they are not present in
economically available quantities [1]. Some REEs are more abundant
than other non-REEs; for example, cerium (Ce) is the 25th most abun­
dant element on earth, even more than copper, and Lutetium (Lu) is 200
times more abundant than gold [3,5]. However, REEs are not likely to be
concentrated in particular localities, which makes their extraction more
difficult than gold [3,6]. The extraction process is energy-intensive,
expensive, and produces large amounts of waste. Additionally, most of
the world’s REEs are produced in China, which has caused concerns
about supply security and geopolitical risks. The limited availability of
REEs and their extraction difficulties have made them increasingly
expensive and environmentally damaging to extract. Therefore, it is
essential to explore alternative sources of REEs, such as wastewater, to
ensure a sustainable supply.
REE anomalies in aquatic systems have been reported in many areas
around the world [2], and their occurrence has been attributed to both
anthropogenic and industrial activities such as fertilizer and catalyst
production, and mining activities [2,4]. As such, their concentration can
be influenced by these activities. For instance, wastewater from mining
and processing activities can contain high concentrations of REEs, which
can lead to environmental harm. The presence of REEs in aquatic sys­
tems can also serve as a tracer for human activities such as agriculture
and urbanization, providing valuable information on the sources and
impacts of these activities. The fate, occurrence, and removal of REEs
from surface water, groundwater, and drinking water have been
reviewed several times from different perspectives [9–14].
REEs do not usually occur in nature as native elemental metals,
rather, they occur in host minerals like carbonates, oxides, etc. [5].
* Corresponding author.
E-mail address: 100058315@ku.ac.ae (D.E. Al Momani).
https://doi.org/10.1016/j.jwpe.2023.104223
Received 30 May 2023; Received in revised form 1 August 2023; Accepted 26 August 2023
2214-7144/© 2023 Elsevier Ltd. All rights reserved.
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
Apart from the natural sources in mines, REEs are also recovered from
sources such as uranium mining waste, acid mine drainage, electronic
waste etc. [5]. However, one disadvantage of REE extraction from their
ores is the detectable radioactive matter that the rare earth sources
contain which can contaminate the surrounding soil and water sources.
The mining process itself can lead to deforestation, habitat destruction,
and the displacement of local communities [6,7]. The refining process
required to extract the pure REEs from the ore is also energy-intensive
and can result in the production of greenhouse gases and other pollut­
ants. Furthermore, the chemicals used in the refinery process may lead
to respiratory issues, and skin and occupational poisoning of residents
[6,7]. Studies have shown that La, Ce, Praseodymium (Pr) and Nd have
displayed toxic effects on aquatic organisms such as hydra and mussels
[4]. Another effect of REE on marine wildlife is the formation of reactive
oxygen species (ROS), which can impair the organism’s oxidative status
[8]. As such, it is important to find alternative sources of REEs and to
develop more sustainable methods of extraction and refining.
Wastewater treatment plants (WWTP) become a potential source of
REEs. REEs can enter the wastewater stream through industrial efflu­
ents, domestic sources, and stormwater runoff. For example, laundry
detergents, dishwashing detergents, and personal care products like
toothpaste are among the domestic of REEs in wastewater, while in­
dustrial sources include mining and processing activities, as well as
manufacturing processes that use REEs, such as electronics
manufacturing and catalyst production. Various wastewater treatment
technologies can be utilized to remove and recover REEs from waste­
water, resulting in a concentrated stream that can undergo further
processing to create usable products. Among these technologies, one
method is adsorption, where adsorbent materials are employed to cap­
ture REEs from wastewaterThis ability to generate valuable byproducts
enhances the sustainability and cost-effectiveness of wastewater treat­
ment systems.
Among these technologies, one method is adsorption, where adsor­
bent materials are employed to capture REEs from wastewater (Fig. 1).
The mechanism relies on the interaction between the adsorbent material
and the REEs, involving various physical and chemical processes like ion
Fig. 1. General overview of the adsorption mechanism involved in the recovery of REEs from wastewater streams.
2
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
exchange, surface complexation, and precipitation [15]. Activated car­
bon is a common absorbent for REEs recovery from wastewater as it has
numerous pores and functional groups, that can interact with the REEs.
[16]. Through ion exchange, activated carbon can adsorb REEs by
exchanging ions on its surface [17]. In surface complexation, REEs form
complexes with functional groups on their surface leading to their
adsorption [17,18]. Additionally, in precipitation, the REEs react with
the functional groups on the activated carbon’s surface, resulting in the
formation of insoluble REE compounds that can also be adsorbed [18].
Chitosan is another example of an adsorbent material. It is a biopolymer
obtained from chitin, a natural polymer present in crustacean shells. The
surface of chitosan features numerous amino and hydroxyl groups,
enabling interactions with REEs in wastewater using similar physical
and chemical processes mentioned earlier. The mechanism of adsorption
can be influenced by various factors, including the pH of the wastewater,
the concentration of the REEs in the wastewater, and the properties of
the adsorbent material.
The electrochemical process is another method for REEs recovery
from wastewater. it utilizes electrochemical cells or reactors to selec­
tively extract REEs from wastewater, based on their electrochemical
properties (Fig. 2). The mechanism involves the oxidation or reduction
of the REEs to form soluble or insoluble complexes that can be separated
from the wastewater.
In electrochemical recovery, the reactions that occur at the electrode
surface depend on the electrode material and the pH of the wastewater
[19]. The anode reaction involves the oxidation of water to form oxygen
gas, protons (H+), and electrons (e-).
secondary resources, with limited emphasis on their potential recovery
[12],[20]. Furthermore, the few studies that have explored REE recov­
ery have not specifically targeted wastewater as a potential resource
[21–29]. This study presents a unique approach by examining the
feasibility of REE recovery from wastewater by highlighting the state-ofthe-art removal and recovery techniques, with a focus on adsorption and
electrochemical treatment. This approach is particularly significant due
to the increasing demand for REEs in various industrial applications,
coupled with concerns over their limited availability and the environ­
mental impacts associated with traditional mining and extraction
methods. Additionally, the study adopts a circular economy approach by
exploring the potential for REE recovery to contribute to more sustain­
able and environmentally friendly industrial production. Overall, the
study offers a unique and valuable contribution to the field of environ­
mental science and sustainability by providing a comprehensive analysis
of the occurrence, treatment, and potential recovery of REEs from
wastewater.
2. Occurrence of REE in global wastewater treatment plants and
environment and environmental risk
REEs can find their way into wastewater and the environment from
various sources, including natural weathering of rocks and minerals,
agricultural runoff, industrial discharges, and sewage effluent/sludges.
The relative contributions of these sources may vary depending on the
location and local conditions [30–32]. REEs can exist in various chem­
ical forms depending on the pH and other chemical characteristics of the
water. Most commonly, REEs tend to bind to suspended particles and
settle out of the water column. However, some REEs can also remain in
solution and be transported further downstream or into other water
bodies and environments.
2H2 O→O2 + 4H+ + 4e− .
The cathode reaction involves the reduction of ions REE3+ to form
neutral REE atoms.
2REE3+ + 6e− →2REE.
2.1. Sources, fate, and occurrence of REE in wastewater
The overall reaction involves the oxidation of water and the reduc­
tion of REE ions to form oxygen gas, neutral REE atoms, and protons
[19].
The fate of REEs in WWTPs depends on the specific treatment pro­
cesses used. Most treatment plants rely on physical and chemical pro­
cesses, such as sedimentation, coagulation, and filtration, to remove
suspended particles and organic matter. REEs can be adsorbed to these
particles and be removed along with them [30]. However, some may
remain in solution or form complexes with organic matter that can be
more difficult to remove. The fate of each REE in wastewater treatment
processes depends on several factors such as the initial concentration,
pH, oxidation-reduction potential, the presence of other competing ions,
2H2 O + 2REE3+ →O2 + 2REE + 4H+ .
Once the REEs have been captured and recovered, they can be reused
in the manufacturing process, reducing the need for new mining and
extraction. This ability to produce valuable byproducts enhances the
sustainability and cost-effectiveness of WWTP.
Previous research has mainly focused on the removal of REEs from
Fig. 2. Schematic diagram of different electrochemical REEs recovery techniques from wastewater: a) electrodeposition, b) electro-sorption & c) electrodialysis.
3
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
efficiency is low. Er is extensively used in nuclear reactors [37]. The
main source of Er in wastewater is industrial discharges from the elec­
tronics, lighting, and glass manufacturing industries. Er, can be effec­
tively eliminated from wastewater using techniques such as ion
exchange, adsorption, or chemical precipitation [38]. Y is commonly
used in phosphors, ceramics, and superconductors and can be found in
various wastewater streams [36]. It can be easily removed during pri­
mary treatment processes such as sedimentation, coagulation, and
flocculation due to its high adsorption capacity onto the sludge. Finally,
Pm is a radioactive element that is not used commercially and requires
specific handling and disposal procedures, as the resulting sludge is
considered hazardous waste. Due to its high radioactivity and short halflife, there is limited information available on its occurrence and fate in
wastewater treatment processes, and it has not been detected in WWTPs
to date.
The concentrations of REE in wastewater effluent from different
studies are summarized in Table 1, while concentrations in sludge are
summarized in Table 2. Concentrations varying from 1.12 to 439.0 mg/
kg in dry sludge for REE were detected in Xiamen city, China [30]. In
North America, the average per capita load of trace elements and the
amount of trace elements discharged from the Great Lake basin was
reported [39]. HREEs are mainly present in much lower concentrations
compared to LREE. These concentrations vary depending on the specific
sources and characteristics of the wastewater, as well as the sampling
locations and analytical methods employed in the studies. The removal
efficiency of these elements depends on the treatment method used, and
their potential toxicity and radioactivity can pose challenges in the final
disposal of the sludge. Most REEs were found to possibly accumulate in
sludge.
and the type of treatment process used.
Ce is an abundant REE in wastewater, primarily originating from
industrial sources such as metalworking, glass, and automotive in­
dustries where it is commonly used in catalytic converters [32]. It has a
high adsorption capacity onto sludge, resulting in its removal during
primary treatment processes such as sedimentation, coagulation, and
flocculation [33]. However, this can lead to environmental risks due to
its toxicity. During the anaerobic digestion process, Ce can be released
back into the wastewater, emphasizing the need for proper handling and
disposal. La is predominantly present in industrial wastewater, specif­
ically from electronic, metal processing, and manufacturing industries,
due to its primary usage in the production of catalysts and rechargeable
batteries [32]. During the primary treatment processes, La is easily
adsorbed onto the sludge and is partially eliminated during the sec­
ondary treatment processes such as activated sludge and membrane
filtration. Nd in wastewater is primarily sourced from industrial efflu­
ents associated with rare earth processing, electronic manufacturing,
and magnet production. Nd is not removed during primary treatment
processes, but it can possibly be removed in the activated sludge process.
Sm is primarily found in industrial effluents linked to electronics,
magnets, and nuclear applications [34]. It can be eliminated from
wastewater through ion exchange, adsorption, or chemical precipita­
tion, and the resulting sludge can be further processed by incineration or
landfilling. Pr is commonly used in magnets and lighting manufacturing
and is found in wastewater from this industry. Pr tends to be removed
during primary treatment processes, and it can also be removed during
the activated sludge process. Eu can be found in wastewater from in­
dustries such as lighting, electronics, and nuclear industries [34]. It
tends to be removed from wastewater primarily during primary treat­
ment processes because it has a high adsorption capacity onto the
sludge. Proper handling is required for the disposal of the sludge due to
potential concerns with radioactivity. The element Gd is commonly
utilized as a contrast agent in medical imaging and is present in medical
wastewater. Although it can be eliminated during primary treatment
processes, Gd may still exist in the effluent due to its strong bonding with
ligands [35]. Industrial effluents from the lighting, electronics, and glass
manufacturing sectors are the primary source of Tb in wastewater. Dy is
found in wastewater from the magnets manufacturing industry [36]. Dy
tends to be removed during primary treatment processes and can also be
removed during the activated sludge process. Lu is utilized in nuclear
reactors and radiation therapy and fibres optic amplifiers [36]. Its
removal from wastewater can be achieved through ion exchange,
adsorption, or chemical precipitation. However, its elimination
2.2. Occurrence of REE in the environment
Although REEs are naturally present in various geological materials,
including rocks, minerals, and soils, human activities such as mining,
industrial processes, agricultural runoff, and wastewater discharges can
also contribute to their presence in environment.
Different regions have conducted assessments on the presence and
accumulation of REE in the environment. For instance, in 1988, Japan
reported that sludges from chemical industry wastewater treatment
plants and municipal sewage sludges showed varying REE patterns and
were considered contaminated [47]. In China, positive Gd anomalies
were detected in most waters near Pearl River Delta (PRD), indicating
significant human impacts [41]. In Switzerland, sludge samples from 63
Table 1
Minimum and maximum concentrations of light rare earth elements and heavy rare earth elements in different wastewater treatment plant effluents (ng/L).
LREE
[39]
[35]
[40]
[41]
[42]
[43]
[44]
[45]
[32]
[46]
140
141
146
147
153
–
–
17
5.7
1.2
9
15
5.1
5.5
2.4
3040
280
16.3
2.8
100,000
1.6
5300
13
43
3.9
1.6
63
35
10
0.1
3.3
14.3
3.8
7.2
2.8
3530
570
20
3.7
280,000
2
15,000
62
75.3
3.6
0.3
58
5
1.4
0.1
1.5
2.5
1.1
1.2
0.4
270
50
2.5
0.4
–
–
2500
3
6.3
0.4
0.8
41
24
5.8
1.8
6.2
11.3
5.1
4.6
2
1300
220
9.8
1.9
150,000
2
12,000
1
40
6.3
1
4
11.7
4.7
1.2
0.8
0.4
2020
60
2
0.4
45,000
1
3900
85
2.4
0.2
0.3
9.6
5
0.3
2.5
6.8
0.6
0.2
0.1
0.1
79,620
920
0.6
0.2
14,000
0.2
890
56
0.8
0
La
Ref.
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
HREE
139
Ce
Pr
Nd
20.8
1.6
Sm
Eu
4
157
Gd
4.4
286
120
2.3
0.7
86.7
24.4
2.5
1.9
0.6
2380
330
226.5
107.3
49,000
0
5600
25
166
0.4
159
162
166
0.2
57.6
1.3
0.2
0.1
0.4
0.7
0.2
0.1
0.1
0
0
0.5
0.1
–
–
990
2
0.4
0
0.5
42.7
9.2
1.2
0.4
2.4
5.8
1.7
0.9
0.6
140
30
3.5
1.2
39,000
0.3
6000
11
2.6
0.3
0.3
39.5
12
12
0.1
2.3
6.2
1.8
0.7
0.5
80
30
4.8
2.5
22,000
0.3
4200
16
1.5
0.1
Tb
Dy
Er
169
172
0.1
57.1
0.5
2.1
0
0.6
1
0.3
0.1
0.1
280
40
0.9
0.5
2300
–
630
3
0.2
0
0.4
881
4.4
21
0.3
5.2
9.3
2
0.6
0.4
5390
380
8.9
5.3
–
–
4200
24
1.4
0.1
Tm
Yb
175
Lu
0.2
59.2
0.9
4.5
0.1
0.9
1.9
0.4
0.1
0.1
2600
160
1.8
1.1
–
–
670
6
0.3
0
Journal of Water Process Engineering 55 (2023) 104223
D.E. Al Momani et al.
Table 2
Concentration of Rare Earth Elements in wastewater treatment plant sludge (mg/kg).
Light Rare Earth Elements
Min
139
La
–
140
Ce
0.126
141
Pr
0.008
146
Nd
0.030
147
Sm
0.010
153
Eu
0.003
Heavy Rare Earth Elements
157
Gd
0.010
159
Tb
0.004
162
Dy
0.010
166
Er
0.060
169
Tm
0.009
172
Yb
0.040
175
0.007
Ref
[39]
Lu
Max
Min
Max
Max
Min
Max
Min
Average
–
88.9
11.8
28.4
4.8
0.8
38.1
74.2
10.4
38.1
9.33
1.88
255
439
22.2
71.5
118
14.3
112.69
500.74
3.19
13.15
10.33
2.09
3.2
4.74
0.43
1.47
0.28
0.06
13.6
37.9
3.23
13.4
1.86
2.7
4.64
0.62
2.52
0.4
22.3
48.6
5.54
28.6
5.13
1.28
3.7
0.6
2.2
547.0
0.0
132.0
10.6
12.6
6.37
3.17
0.443
2.98
5.47
0.4
2.65
1.56
0.19
1.25
0.34
0.04
0.26
0.13
0.02
0.15
2.47
0.27
1.48
0.88
0.12
0.82
0.54
0.08
0.48
0.32
0.05
0.27
6.4
0.764
3.75
2.18
0.297
1.9
0.0
–
14.6
13.7
8.26
4.09
0.577
3.86
—
——
0.17
0.02
0.13
0.04
0.283
[30]
[33]
WWTPs were assessed for REE [33]. While most WWTPs showed un­
specific inputs with REE patterns similar to the average REE pattern of
soils in Switzerland, a few plants exhibited significant REE variations.
Gadalinium (Gd) which is used as a contrast agent in magnetic reso­
nance imaging, was found to be discharged into surface water at a rate of
90 kg per year from one of the WWTP [33]. The anthropogenic Sm
enters the Rhine River with industrial wastewater north of Worms,
Germany, and can be traced through the Middle and Lower Rhine to the
Netherlands. The Rhine River may carry up to 584 kg of anthropogenic
Sm per year towards the North sea with concentrations close to or well
above levels where ecotoxicological effect [49]. In South Korea, in the
Nakdong River Estuary, the concentrations of REEs were negatively
correlated with salinity, except for sites near WWTPs, where the con­
centration was much higher [50]. Positive La and Sm anomalies were
present in the Han River indicating the impact of a point source. Based
[47]
[48]
on the discharge rate and anthropogenic REE concentrations, the esti­
mated fluxes were on average 952 + 319 kg/yr [44].
While most developed countries have REE contamination awareness,
developing and underdeveloped countries are yet to monitor and assess
their wastewater. Regions such as the Middle East and North Africa have
no studies that measure REE in secondary sources, despite having major
industrial activity. Additionally, the only country in South America that
was studied was Brazil, which indicates a major gap in this region.
Australia and neighboring countries have also not reported the presence
of REEs. Ecotoxicological effects have been documented due to these
sources, hence this shows the significance of international awareness
[49].
Table 3
Toxic effects of some REEs on marine life.
REE
Concentration Studied
Target Organism
Type of
Organism
Toxic Effect
Reference
Gd
50 μg/L
Mytilus galloprovincialis
Increased ROS leading to increased cellular damage.
[8]
Nd, Gd, Yb
EC50 = 1-10 mg/L
Daphnia magna
Increased toxicity measured by decreased mobility
[52]
Nd
2.5-40 μg/L
Mytilus galloprovincialis
100-6400 μg/L
Daphnia magna
Increased ROS leading to cellular damages and loss of
redox balance
Growth inhibition
[53]
Ce, Gd, Lu
Gd
15–120 μg/L
Mytilus galloprovincialis
Marine,
Mussel
Marine,
crustacean
Marine,
Mussel
Marine,
crustacean
Marine,
Mussel
HREEs: Dy, Ho,
Er, Yb, Lu, Ce
La, Y
10−
0.01–100 mg/L
Mytilus galloprovincialis
La
0.1–10 mg/L
Mytilus galloprovincialis
Acute, 48 h 100-50,000
Daphnia carinata
La
Gd
Y, La, Ce, Nd, Sm,
Eu and Gd
Gd
La
Gd
Yt
7
to 10−
5
M
μg/L
Chronic 100 -1000 μg/L
20 μM for P. lividus
500 nM and 5 μM for
H. tuberculata
10− 4 to 10− 7 M
Sphaerechinus granularis
[54]
Induce mussels oxidative stress (ROS) and
neurotoxicity as well as to reduce their metabolic
capacity
Increased developmental defect (DD)
[55]
Marine, Sea
Urchin
Marine,
Mussel
Marine,
Mussel
Marine,
crustacean
Increased developmental defect (DD), La > Y
[57]
Decreased metabolic capacity, increased oxidative
stress and neurotoxicity
Mortality, increased age of maturity, decreased
maturity
[58]
Mediterranean Paracentrotus lividus
Australian Heliocidaris tuberculata
Marine, Sea
Urchin
Negative effect on skeleton development with a similar
morphological response in the two species
[60]
Sphaerechinus granularis
Marine, Sea
Urchin
Marine,
Mussel
Marine
Developmental defects
[61]
Triggers mitochondrial and anti-inflammatory
pathways
Accumulates in the liver and induces antioxidant
enzyme production [60]
[4]
10-1250 μg/L
Dreissena polymorpha
0.015 mM
Goldfish
5
[56]
[59]
[62]
Journal of Water Process Engineering 55 (2023) 104223
D.E. Al Momani et al.
2.3. Environmental risks related to REE in water
the environment as they are produced. Phosphate-based fertilizers are
notable avenues for the entry of REEs into the environment [71], and
wastes from the production of phosphogypsum contain significant
extractable quantities of REEs like Ce, La, and Nd [71], making the
consideration of effective and environmentally sustainable treatment
techniques highly important.
Some of the toxic effects of REEs on marine life reported in the
literature are shown in Table 3. Maximum tolerable limits for REEs in
drinking water were described as lacking from any standards organi­
zation [29]. Notwithstanding, some inferred limits were proposed based
on ecotoxicological calculations [51] and referred to as maximum
permissible concentrations (MPC) in drinking water for some of the
REEs. The MPCs, in units of ng/mL, were 10.1 for La, 22.1 for Ce, 9.1 for
Pr, 1.8 for Nd, 8.2 for Sm, 7.1 for Gd, 9.3 for Dy, and 6.4 for Y. MPCs for
other REEs were not reported. REEs on their own are not understood to
be significantly toxic [29]. In one classification of elements according to
the understanding of their toxicity to living beings and their biological
relevance, REEs were placed under class 4 for those which can be
possibly toxic even at low concentrations [29]. This potential toxicity
can be aggravated if these REEs are released into water from used ma­
terials (e.g., e-wastes) that contain ions like F− , Cl− , HCO−3 , CO2−
3 ,
3−
HPO2−
4 , and PO4 [29]. This highlights the need for effective treatment
methods to remove REE from wastewater.
3.1. Pretreatment technologies for REE wastewaters
Pretreatment technologies are aimed to reduce the toxicity or acidity
of the REE wastewaters by effectively concentrating and removing
components whose presence leads to this toxicity. The relevant treat­
ment techniques for wastewater comprising extractable REE’s typically
entail chemical methods like precipitation and coagulation [9] which
offer good recovery efficiencies but might be limited in environmental
sustainability and techno-economic factors when compared to other
methods as described in Table 4. Electrically assisted methods such as
electrocoagulation and electrodialysis have been helpful techniques in
treating toxic wastewater from which REEs are recovered [72]. Physical
methods such as membrane filtration with an ion-selective nano-filter
have also been applied [73]. One of the factors relevant to the effective
treatment of rare-earth bearing wastewaters was the transmembrane
pressure [74]. Nanofiltration membranes have also been used for the
recovery of highly acidic wastewater from mines [75,76]. Even though
nanofiltration is a pressure-driven physical separation process based on
filtration, the possibility of precipitation on the surface of the membrane
introduces a chemical reaction into its mechanism since such pre­
cipitates are easier to separate and recover for further valorization [76].
Biological methods utilizing micro-organisms such as algae have
been explored for their potential for wastewater pretreatment of toxic
components comprising REE which could be valorized to obtain valu­
able rare earths [64,77]. Microalgae can obtain nutrients from REE
wastewater while maintaining resistance to its toxicity derived from its
acidity and chemical composition [78]. Cells of Saccharomyces cerevisiae,
Kluyveromyces marxianus, and Debaromyces hansenii have also been
evaluated for their effectiveness in absorbing Eu(III) from aqueous sys­
tems in an attempt to develop low-cost bio-absorbents with high metal
uptake capacities [79]. Younger cells were found most effective with this
effectiveness dependent on the fatty acid content and the types of
carboxyl functional groups therein [79]. Algae like Desmodesmus multi­
variabilis and Spirulina have been useful for effecting high-capacity
adsorption of REEs of La and Ce, respectively [12]. Finally, hybrid
methods combining one or two methods or materials have also been
explored. The effectiveness of a high-capacity adsorbent with good
reusability and selectivity was reportedly achieved with a polymer that
incorporated both lanthanum and phosphorus [80].
3. The treatment of REE-bearing wastewaters
The treatment of REE-bearing wastewaters aims to achieve the
reduction of the environmental toxicity of the wastewater before its
release to the environment, isolation or removal of toxic components
from the wastewater, exploitation of its chemical components to obtain
re-usable chemicals, and in rare cases, the utilization of the wastewater
as a valuable by-product in other applications [68].
The intense exploration of REEs activities from the ores has
contributed to the production of toxic wastewater from the mining
process [63]. This toxicity is attributed to its high acidity and the
presence of significant quantities of chemical components used for the
leaching process, notably ammonium and phosphates [64]. Wastewater
from rare earth mining differs from severely toxic radioactive wastes
from the nuclear industry, referred to as trivalent actinides [65,66].
However, these radioactive wastes often come up in the discussions of
treatment aimed at recovering REEs like La(III), Eu(III) [67], and Ce(III)
[66], because of their physicochemical similarities.
In line with the circular economy objectives, industrial wastewater
sludge has been considered as fertilizer material for some soils to utilize
the waste for its nutrients without the need for treatment. However, this
was suspected to contain environmentally toxic contents of REEs and
trace elements [69], further aggravated by the fact that their biological
impacts are not fully understood [70]. Thus, it is indeed clearer that the
consideration of effective treatment of REE wastewater is preferable due
to potential valorization and footprint reduction than utilizing them in
Table 4
Summary of methods applied for REE wastewater pretreatment.
Treatment
class
Description/example
Physical
Membrane filtration e.g. ion-selective nano-filtration;
Adsorption
Chemical
Chemical precipitation and coagulation
Electrically
assisted
Electrocoagulation, and electrodialysis
Biological
Rapid infiltration or the use of algae and micro-algae
including symbiotic systems with bacteria.
Hybrid
Combination of two or more methods or materials e.g.
electrodialysis with membrane distillation or composite of
porous polymers and magnetic adsorbents
Advantages
Could be optimized to control
cost and improve efficiency
Selectivity, stability, and
reusability
Enables high REM recovery;
controllable via pH
adjustment
Optimizable and controllable;
helps prevent fouling with
membranes
High potential for
sustainability; costeffectiveness
Enables superior recovery of
valuable elements reaching
99.99 %
6
Disadvantages
Ref
Limited efficiency
[74]
Costliness of high performing adsorbents
[80]
Significant environmental footprint due to high
chemical amount required; produces large quantity
of sludge requiring costly post-treatment.
[81]
Could require more energy to run
[72].
Slow process, and changes in process conditions
can affect activity of biological agents;
pretreatments required for dried algae
[77,82]
Could be technically challenging to manage
[63,83,84]
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
3.2. Ammonia removal as pollutant from REE wastewater pretreatment
and opportunities for valorization
adsorbent, operating conditions (including temperature and pressure),
pH and ionic strength of the medium, and mass transfer rate efficiency
[95–97].
Regarding adsorption isotherms, both Langmuir and Freundlich
isotherms were found suitable to characterize the adsorption process. It
was found that Langmuir isotherm is suitable to characterize the
monolayer adsorption process of several REEs using a variety of adsor­
bents, such as MWCNT [98], oxidized MWCNT [92], GO [93,94], sul­
fonated GO [93], MWCNT/Iron Oxide Magnetic Composite [98],
chitosan/CNT [99]. Freundlich model could be also used to characterize
the adsorption process occurring on heterogeneous surfaces
[92–94,100].
Recovery of REEs after adsorption is mainly conducted by desorption
which typically involves various separation and extraction techniques
such as acid leaching, solvent extraction, ion exchange, smelting and
bioleaching.
Ammonia is a useful substance for the chemical industry with
emerging applications as fuel for energy generation, export vector for
hydrogen, and as a fuel-cell component [85] potentially useful in
WWTPs and mining plants. Because rare earth mining typically utilizes
ammonium sulfate leaching for extracting REEs from their ores [82], it is
often the case that the resulting REE wastewaters comprise extractable
and utilizable REEs, alongside ammonia, both of which can be recovered
during the treatment of these WWs [77]. It is of interest to ensure its
concentration in effluents from wastewater plants is below 15 mg/l [82]
in line with the “Emission Standard for Pollutants from Rare Earth In­
dustry” GB 26451–2011 [86]. The technical efficiency of the removal of
ammonia from REE wastewaters could be improved by the careful se­
lection of removal reagents. Precipitation of ammonia as magnesium
ammonium phosphate was optimally achieved to 99.4 % recovery when
magnesium and phosphates were each added [87]. The cost-efficiency of
this precipitation process was improved by recycling the reagents
leading to 48.6 % cost savings [87]. The ammonia was recovered in each
cycle until when a low residual concentration of 0.015 g/l was reached
[87], as compared to the concentration of ammonium-rich REE waste­
waters, which could be thousands of times higher at 130 g/l [88].
Microalgae-bacteria symbiosis systems have been also employed for
treating wastewater leading to the obtainment of ammonia [64].
Microalgae usage for REE wastewater pretreatment possesses advan­
tages over the activated sludge method because it can utilize the nitro­
gen in dissolved ammonium and inorganic carbon for the metabolic
needs of the microalgae. However, the slow rate of degradation of the
pollutants as well as a reduction in performance due to pH changes to
less acidic conditions, unpreferable to acidophilic microalgae [88], are
some of the limitations of this process [64]. Over 46.4 kg per day of
ammonia on average was recovered from typical wastewater [77]. This
is an interesting potential because large plants producing ammonia via
the conventional Haber-Bosch process generate 1.25 kg per day under
steady operation [89]. Adjusting pH led to an increase in ammonia
removal rate and made the algae more tolerant to REEs in wastewater
like La, Y, and Sm affecting simultaneous maximum removal rates for
these REEs of 97.9 %, 96.6 %, and 99.1 %, respectively [78]. Under
optimum temperature conditions, utilizing microalgae was reported to
have enabled 9 % higher ammonia removal efficiencies [90]. However,
higher temperatures could inhibit the effective growth of some micro­
algae, as was reported for Chlamydomonas reinhardtii cells at 32 ◦ C [91].
4.1.1. Carbon-based adsorbents
These materials involve carbon nanotubes, carbon dots, graphene
oxide, graphene, and graphite [15]. The use of carbon-based materials in
REEs extraction is mainly controlled by electrostatic forces resulting
from the different functional groups, in which carbon-based materials
are easy to be functionalized and thus have a better adsorption capacity
to REEs [101].
Several studies have described the use of graphene-based adsorbents
to extract REEs [102–104] under different conditions, including pH and
temperature, Table 5. Graphene oxide (GO) was mainly utilized.
Generally, the increase in pH leads to an increase in the adsorption ca­
pacity. The adsorption capacity of Eu was 78 mg/L at pH of 5 [105],
while it was 89.7 mg/L at pH of 7 [102]. The pH affects the adsorption
process in which the surface charge of the used adsorbent depends on
the acidity. This effect is mainly because the surface charge of the
adsorbent should be negative since the adsorption of the REEs occurs by
electrostatic forces. Regarding the impact of temperature, it was stated
that inadequate temperatures might lead to a reduction in the adsorp­
tion efficiency [15].
CNTs have remarkable mechanical and electronic properties, along
with a unique nanostructure [15]. Oxidized MWCNTs were widely used
in REEs recovery due to their cheaper cost and higher efficiency when
compared to oxidized SWCNTs [15]. In some studies, CNTs composites
with other materials were created. For example, a composite of multiwalled carbon nanotubes (MWCNT) and Fe3O4 was created to enhance
the adsorption capacity of REEs. Although this was conducted at a very
low concentration of Eu, it led to 100 % extraction [107].
A variety of solutions are used to recover REE after adsorption. For
example, the desorption of La, Dy, and Dy from oxidized MWCNT was
performed by placing the oxidized MWCNT (after the adsorption
experiment) in a nitric acid solution with a pH of 1.55. When comparing
the initial concentrations of La and Dy, it was found that 71 % of the
initial concentration was recovered [92]. The solution desorption solu­
tion was used to recover Gd from graphene oxide (GO) after the
adsorption process and it provided a desorption process of 85 % [93].
Sodium perchlorate with a concentration of 0.01 M was also used to
recover Eu adsorption on GO/magnetic GO surfaces [94]. In another
study, both hydrochloric acid (0.2 M) and sulfuric acid (0.2 M) were
used separately to desorb Tb from cellulose-based adsorbent. The
desorption percentage using hydrochloric acid was between 63 and 75
%, while it was between 72 and 81 % when using sulfuric acid for
desorption [95].
4. Techniques for recovery of rare earth metals from mine
wastewaters
4.1. Adsorption
Adsorption, although a traditional technique, is a promising process
to extract REEs. It is an effective, affordable, and efficient process based
on attracting the desired element to the active sites of the absorbent. The
adsorbent’s selectivity for the element of interest is crucial, and thus,
several adsorbents have been synthesized and evaluated for their
selectivity or effectiveness to extract the REEs [15].
The adsorption kinetics of REEs were not intensively discussed in the
studies of REEs adsorption. Among the discussed kinetics are the pseudofirst, pseudo-second, Bangham, and Elovich models. For example, the
kinetics of the adsorption process of Tb by cellulose was analyzed using
the experimental data obtained at different temperatures. It was found
that the experimental data at temperatures from 25 to 60 ◦ C fit well with
a pseudo-second-order model based on correlation coefficients obtained
[95]. A similar conclusion was obtained in the case of absorbing Sc by
Posidonia oceanica, a marine alga, where the pseudo-second-order model
was more suitable to fit the experimental data [96]. It was concluded
that the kinetics of each adsorption process depends on the nature of the
4.1.2. Natural adsorbents
Natural adsorbents have attracted researchers due to their advan­
tages, which include being environmentally friendly, cheap, biode­
gradable, and the ability to adsorb REEs. Several natural adsorbents
including cellulose, algae, and chitosan have been used for REEs
extraction [92]. Functionalization was proposed as a potential technique
7
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
Table 5
The use of graphene-based, CNT-based, natural and polymeric-based adsorbents for REEs extraction.
Classification
Adsorbent
REEs
presented in
water
The initial
concentration of
REEs (ug/L)
Adsorbent
concentration
(mg/L)
pH
Temperature,
◦
C
Contact
time (h)
Adsorption capacity
(mg/g)
Reference
Graphenebased
adsorbents
GO
GO
Colloidal GO
Eu
Eu
Nd, La, Y, Gd
100,000
10,000
5000
100
500
1000
5
5.5
6
25
20
25
48
24
0.5
[105]
[94]
[103]
CNT-based
adsorbents
GO
GO
Colloidal GO
CNTs-COOH
MWCNT-Fe3O4
Oxidized MWCNT
Oxidized MWCNT
Ti-MWCNT
Sc
Eu
Eu
Sc
Eu
Ce, Sm
Ce
La, Lu, Tb
300,000
10,000
10,000
300,000
61
10,000
10,000
40,000
5000
1000
1000
5000
600
1000
1000
5000
4
7
7
4
5.5
5
5
5
25
20
20
25
25
30
30
20
4
24
24
4
48
2
2
1
Oxidized MWCNT
Nutshell cellulose
Dy
Eu, Gd, Sm,
La
Tb
Sc
Nd
20,000
200
1200
500
6
4
30
20
2
12
100,000
200,000
250,000
250
1000
2.5
5
5
5.5
25
20
27
2
24
5
78
143
Nd = 189, La = 85.7,
Y = 136, Gd = 226
39.7
89.7
70.2
42.5
100 %
98 %
97 %
La = 5.35, Lu = 3.97
%, Tb = 8.55
98 %
Eu = 5.7, Gd = 5.1,
Sm = 6.1, La = 6
24.7
66.81
38.2
Eu
La
100,000
5000
20,000
1200
2.1
8
25
–
1
3
142
344.8
[113]
[95]
La, Ce, Pr, Nd,
Sm, Eu, Gd,
Tb, Dy, Ho,
Er, Tm, Yb, Lu
50,000
150,000
–
25
6
[96]
Poly(4-vinyl pyridine)Schiff base lanthanide
ion-imprinted polymer
Pr, Nd, Sm,
Eu, Gd
50,000
–
6
25
2
Resorcinolterephthalaldehyde
resin
Polyacrylamide at
perlite
Eu
–
10,000
4
25
24
La = 29, Ce = 32, Pr
= 33, Nd = 35, Sm =
39, Eu = 42, Gd = 51,
Tb = 48, Dy = 51, Ho
= 52, Er = 51, Tm =
50, Yb = 48, Lu = 49
Pr = 125.3, Nd =
126.5, Sm = 127.6,
Eu = 128.2, Gd =
129.1
70
Tb
250,000
10,000
5
25
24
68.3
[116]
Natural
adsorbents
Polymericbased
adsorbents
Carboxylated cellulose
P. oceanica (green algae)
Chlorella vulgaris (green
microalgae)
Chitosan nano particles
Chitosan-Imprinted
Nano Zero-Valent Iron
Nanocomposite
Silica-supported
polydiglycolamide
to increase natural adsorption capacity. For example, the grafting of
carboxylic acid and amino acid on cellulose has resulted in increasing
the adsorption capacity of La to 101 mg/g [117]. Adding a carboxylic
functional group to cellulose has increased the adsorption capacity of Tb
to 24.7 mg/g [111]. The extraction of REEs using algae is mainly due to
the presence of functional groups, including amino, sulfate, and car­
boxylic groups presented in starch, hemicellulose, glycogen, and cellu­
lose contained in the algae [118]. T. Conoides (brown algae) showed a
Tm adsorption capacity of 200.5 mg/g [119]. While chitosan nano­
particles showed 142 mg/g of Eu [113].
Compared to graphene-based adsorbents, natural adsorbents have
shown fewer REEs adsorption capacities. This could be referred to the
high porosity of graphene-based adsorbents and the better interaction
between their surfaces and the desired ions. One exception is with
chitosan-imprinted nano zero-valent Iron nanocomposite, which has
achieved 344.8 mg/g extraction of La [95]. This high adsorption ca­
pacity might conclude the use of nanocomposites could be attractive in
REEs extraction.
[106]
[102]
[104]
[106]
[107]
[100]
[108]
[109]
[98]
[110]
[111]
[112]
[99]
[114]
[115]
adsorption capacity of polymeric microcapsule membrane functional­
ized with an acidic group for Ce, La, Pr, Eu, Sm, Gd, Ho, Y, and Er re­
covery has reached >95 %. [123].
The polymer porosity is an important factor that affects the adsorp­
tion and the kinetics of the material in which a higher porosity results in
better uptake of REEs [124]. Further, small pores lead to an increase in
the surface area of the adsorbent [20]. However, the most important
factor that affects the uptake capacity of REEs is the interaction between
the surface of the adsorbent and the REEs [125]. Examples of the use of
polymers in REEs extraction are shown in Table 5. Ionic imprinted
polymers are also among the promising adsorbents of REEs [126], an
adsorption capacity of 125 mg/g for La was achieved [127]. Although
extracting REEs by adsorption is effective and affordable, it is limited by
the production of wastes (waste adsorbents) and the dependence of
performance on the adsorbent type [9].
4.1.4. Metal-organic frameworks (MOFs)
Metal-organic frameworks (MOFs) are promising porous materials
for REE recovery due to their high surface area, adjustable pore size, and
chemical functionality. MOFs consist of metal ions or clusters linked by
organic ligands to form a porous structure. The unique properties enable
MOFs to selectively capture and separate elements from complex mix­
tures, such as wastewater. MOFs can be tailored for specific applications
[128] and their tunable pore size allows targeted elements to be
captured, selectively while excluding other ions and molecules
[129,130].
The modification of MOFs (such as functionalization, doping, and
4.1.3. Polymeric adsorbents
Polymer-based adsorbents have been widely used in extracting REEs
due to their capabilities of adsorption and instability at high pH values.
Such advantages have been mainly shown by porous polymeric adsor­
bents [120,121]. Cross-linked polymeric adsorbents along with func­
tional groups were proven to be able to bind metal ions, including REEs,
along with providing several advantages, including environmental
compatibility, reuse and regeneration, and ease of operation [122]. The
8
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
post-synthetic modification) can improve their selectivity and efficiency
for REEs recovery. [131,132]. Functionalization enhances MOF’s affin­
ity for REEs by introducing functional groups onto its surface. These
groups, like carboxylic acid, amino, or hydroxyl, can be added through
covalent or non-covalent bonding. Doping involves incorporating
foreign atoms or molecules into the MOF structure to modify its prop­
erties. Dopants may include transitioning metals, lanthanides, or other
elements, and they can be added during or after synthesis. Defect en­
gineering. [133], introduces defects or vacancies into the MOF structure
to enhance its adsorption capacity and selectivity. Hybridization com­
bines MOFs with other materials, like polymers or nanoparticles,
creating hybrid materials with improved properties. These modifica­
tions increase active sites, alter surface chemistry, and optimize pore
size and shape, enhancing selectivity and efficiency in REEs recovery
[134]. Some MOFs such as MIL-101, UiO-66, and ZIF-8, already exhibit
high adsorption capacity and selectivity for REEs such as La3+, Ce3+,
Nd3+, Sm3+, Dy3, and Gd3 [135–138]. These MOFs exhibit diverse
structures and chemical properties, impacting their adsorption perfor­
mance for different REEs. MIL-101’s large pore size and high stability
suit the adsorption of larger REE ions like La and Ce. UiO-66’s narrow
pore size and high hydrophobicity are ideal for adsorbing smaller REE
ions such as Nd and Pr. ZIF-8’s high surface area and thermal stability
make it suitable for various REE ions. The choice of MOF and modifi­
cation method for REE recovery depends on various factors, such as the
type and concentration of REEs in the wastewater, the pH and temper­
ature of the wastewater, and compatibility with downstream processing
methods. Modified MOFs have shown promise in REE removal from
wastewater, although limited quantitative data exists on the recovery
rate of REEs [130,139,140]. In a recent study, nitrogen and sulfur atoms
were introduced into the MOF structure through a solvothermal method
to enhance its selectivity for Gd3+. The modified MOF had a high
adsorption capacity for Gd3+ due to the increased number of active sites
and the strong electrostatic interaction between the Gd3+ ions and the N
and S dopants. A maximum adsorption capacity of 238.10 mg/g, and a
recovery rate of 99.8 % for Gd3+ were reported [141]. In another study,
a modified lanthanum-based MOF with polyethyleneimine (PEI)
demonstrated a high adsorption capacity for Gd3+ (296.7 mg/g at pH
4.0 and 298 K) compared to Fe3O4, and other MOFs, such as MIL-101
and UiO-66. Researchers attributed the high adsorption capacity and
selectivity to the chelation effect of the ligand, which forms coordination
bonds with Gd3 through carboxyl and pyridine groups. The synergistic
effect of the ligand and MOF framework provided numerous adsorption
sites and a large surface area for Gd3+ interaction. The PEI modification
increased the number of adsorption sites for Gd3+ and enhanced the
electrostatic interactions of the MOF [142]. A diimide-based MOF,
called ZJNU-10, was found to have high adsorption capacity and
selectivity for La3+ because of the diimide functional group’s strong
coordination bonds with La3+ ions. ZJNU-10 effectively removed lan­
thanides from real tailing wastewater samples, with a maximum
adsorption capacity of 186.9 mg/g for La3+ [130]. Functionalized SBA15 and MIL-101 (Cr) with aminopropyltriethoxysilane (APTES) and 1,3propane-sultone (PS), respectively were studied for the adsorption of Lu
and Y. Although the recovery rates were not reported, both materials
effectively adsorb the Lu having adsorption capacities of 6.8 and 5.8
mmol/g for SBA-15 and MIL-101 (Cr) respectively. The (APTES func­
tional group on SBA-15 and PS functional group on MIL-101 (Cr)
demonstrated a high affinity for REEs, enabling preferential adsorption.
The functional groups also increased the surface area of the MOFs,
providing more sites for REE adsorption. [143]. Several hybrid processes
incorporating MOFs for REEs recovery were discussed in various studies.
For instance, geopolymer filters covered with MOFs showed high re­
covery rates of 96, 91, 89, 93, 93, 94, 94, 96, 92 % for Ce, Eu, Gd, La, Nd,
Pr, Sm, Tb, and Y respectively, with potential applications in the recy­
cling of electronic waste and other industrial processes [144].
MOF-based membranes were utilized for selective separation and
recovery of REEs from aqueous solutions [145]. These membranes are
prepared by growing MOF crystals on the surface of porous support
[146], demonstrating high selectivity and permeability towards REEs
due to the unique MOF structure. Additionally, MOF-based adsorbents
were combined with carbon nanotubes [147] or other MOFs to form
hybrid adsorbents, exhibiting synergistic effects in adsorption capacity
and selectivity [148]. [148]. These hybrid processes, including solvent
extraction, ion exchange, or precipitation with MOFs, enhance effi­
ciency and selectivity in REEs recovery. They offer advantages like
improved performance, reduced cost, and increased versatility, paving
the way for more efficient and sustainable REEs recovery technologies.
However, the practical application of MOFs for REE recovery still
faces some challenges. One major drawback is their limited stability
under certain conditions, such as high temperatures, acidic or alkaline
environments, or exposure to moisture [18]. This instability can degrade
or collapse the MOF structure, reducing its adsorption capacity and
selectivity for REEs. Additionally, the scalability of MOF synthesis can
hinder large-scale production and commercialization of MOF-based
adsorbents or membranes. The synthesis of MOFs involves complex
and time-consuming procedures, leading to variable yield and purity of
MOF crystals depending on synthesis conditions and precursors. More­
over., the relatively high cost of MOFs compared to materials such as
activated carbon or zeolites, limits their practical applications in REEs
recovery [149]. Lastly, the presence of other metal ions or organic
compounds in the feedstock can compete with REEs for adsorption sites
on the MOF surface, affecting MOFs’ selectivity and overall recovery
efficiency.
Despite these drawbacks, recent advances in MOF synthesis and
engineering have shown great potential for the development of practical
adsorbents for REE recovery.
4.1.5. Covalent-organic frameworks (COFs)
COFs are porous materials made of organic molecules linked by co­
valent bonds to form a crystalline structure. Like MOFs, COFs have high
surface areas and tunable pore size, making them attractive for gas
storage, catalysis, and separation [150]. The most common synthesis
methods for COFs are solvothermal synthesis and microwave-assisted
synthesis [151]. Solvothermal synthesis involves heating a mixture of
organic building blocks and solvents at high temperatures and pressures
to form covalent bonds. Microwave-assisted synthesis employs micro­
wave radiation to promote covalent bond formation between the
building blocks. Other methods like mechanochemistry, electrochemical
synthesis, and liquid-phase synthesis have also been used for COF syn­
thesis, each with its own advantages and limitations. The choice of
synthesis method depends on the specific properties and applications of
the COFs being synthesized.
COFs have shown high adsorption capacity, fast kinetics, and selec­
tivity towards certain metal ions and can be functionalized with specific
ligands to selectively bind to REEs and remove them from the solution
[18]. Functionalized COFs serve as adsorbents to selectively capture the
REEs from aqueous solutions, with subsequent elution using an appro­
priate eluent A nitrogen-rich COFs (NCOFs) demonstrated excellent re­
covery rates of 98.3 % for Y and 96.6 % for Euand remained efficient (>
90 % recovery) even after five cycles of use [152]. The coordination of
the nitrogen atoms in the NCOF contributed to REEs adsorption. The
functionalization of the NCOF with carboxylic acid groups enhanced the
recovery of the REEs by increasing the number of coordination sites
available for the REEs to bind to. Carboxylic acid groups enhanced re­
covery by increasing coordination sites and providing additional
hydrogen bonding sites to facilitate adsorption from water. Modified
sulfonic COFs exhibited high selectivity for the adsorption of La with a
maximum adsorption capacity of 283.1 mg/g and high recovery rates,
ranging from 92 % to 99 %, depending on the specific element. Sulfonic
COFs were found to be easily regenerated and reused for multiple cycles
of adsorption and desorption without significant loss of adsorption ca­
pacity or selectivity [153]. COF with high porosity, large surface area,
and tunable properties preconcentrated REEs with recovery rates
9
Journal of Water Process Engineering 55 (2023) 104223
D.E. Al Momani et al.
ranging from 88.9 % to 104.5 % for the 17 REEs tested including La, Ce,
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, and Tb [154]. The
building block choice also impacted COF performance, with pyridinebased COF having a higher adsorption capacity (14,804 mg/g) for La
ions than triazine-based (135.1 mg/g)[155]. Additionally, honeycombshaped COFs were synthesized in deep eutectic solvents and exhibited
exceptional selectivity with adsorption capacities up to 230 mg/g. The
effects of various factors on the adsorption performance of the COFs,
such as pH, contact time, and initial concentration of REEs were also
investigated. The results suggest that the honeycomb-shaped COFs have
great potential as effective and selective adsorbents for REEs, which are
critical materials for various high-tech applications [156].
Despite the potential of COFs as a material for adsorption and their
success in metal recovery from other sources, there is still limited
research on the use of COFs for the recovery of REEs from wastewater or
secondary resources, and it has yet to be determined whether COF has
the ability to selectively separate REEs [157]. This gap in knowledge
highlights the need for further investigation into the viability of COFs for
REE recovery, particularly given the increasing demand for REEs in
various industries and the need for sustainable methods of their
extraction.
anode is also immersed in the solution. When an electric current is
applied, rare earth metals are reduced and deposited onto the cathode
surface while the anode is oxidized. The deposited metals can then be
scraped off from the cathode surface and further purified. The compo­
sition of the solution, the type of electrode used, and the applied current
density are the main parameters affecting the recovery. Some studies
have reported the feasibility and high recovery rates using electrode­
position from different resources [164]. However, dilute solutions can
result in low metal recovery such as the case in some wastewater streams
[165]. This is attributed to the low concentration of metal ions which
can result in low current efficiency due to competing reactions (such as
water oxidation), and a low mass transfer rate as the diffusion of metal
ions to the electrode surface can be slow leading to a decrease in the rate
of electrodeposition and recovery. This can be improved by preventing
the hydrolysis of REEs and the occurrence of hydrogen evolution re­
actions, which can be achieved by optimizing process parameters, the
use of molten salts, organic solvents, and ionic liquids, and introducing
additives and ligands [23]. Ce and Nd were 92 % and 87 % recovered,
respectively, while the presence of coexisting ions was found to reduce
the recovery efficiency [166]. The effect of various parameters on the
extraction efficiency of neodymium from neodymium magnets using a
salt extraction process was investigated [26]. The efficiency increased
with increasing temperature, time, and salt concentration and reached
up to 98 % under optimized conditions. Meanwhile, the presence of
impurities such as iron, cobalt, and nickel was reported to reduce that
efficiency [26].
4.2. Solvent extraction
Solvent extraction, also known as liquid extraction, implies two
different immiscible liquids, an organic solvent, and an aqueous
mixture, to separate different compounds through the attraction of the
desired component from the aqueous mixture [158]. It is widely applied
due to its effective separation selectivity, extraction ability, and
commercially applicable. However, the loss of ions through phase
disengagement and the finite solubility of the solvents, modifiers, and
extractants, being labor-intensive, time-consuming, and excessive
amounts of solvent required, are among the main limitations of this
process [9]. Solvation extractants, cation exchangers, and anion ex­
changers are the main three classes of extractants utilized for REEs re­
covery [9].
Due to the toxicity of the most used solvents in solvent extraction of
REEs, the use of ionic liquid was proposed as an efficient, clean tech­
nique to extract REEs. It has favorable properties, including low flam­
mability, low vapor pressure, the ability to be used at wide ranges of
temperatures, excellent thermal stability, and environmentally friendly
[159]. Several studies have shown the ability of ionic liquids to act as
effective agents in solvent extraction of REE [160]. A summary of some
solvents incorporated in the use of solvent extraction for REEs extraction
is shown in Table 6.
4.3.2. Electro-sorption
Platforms such as capacitive deionization (CDI) [167], and mem­
brane capacitive deionization (MCDI) are fundamental in electrosorption. As these methods do not include the transfer of electrons be­
tween the electrolyte and the electrode, i.e., a non-Faradic process
where the ion uptake, separation efficiency, and selectivity are limited
[168]. The initial concentration of REE, pH, applied voltage and current
density, electrode material, and the existence of interfering ions and
other organic matters are the main factors affecting the recovery pro­
cess. Sodium diphenylamine sulfonate-modified activated carbon elec­
trode (SDPAS) was used to increase the adsorption capacity for REEs.
The electrode has shown highly effective adsorbent material for La, Nd,
and Ce ions with an adsorption rate of up to 60% [169]. Capacitive
deionization (CDI) with cellulose-derived carbon as electrode materials
showed higher selectivity for Ce with an efficiency of about 76 %
compared to cross-linked cellulose adsorbents [170].
4.3.3. Electrodialysis
Electrodialysis employs two approaches for the separation of ions.
First, electrochemical potential acts as the driving force for the mass
transfer of ions. Second, a membrane allows some ions to pass while
blocking others. Cerium was separated from wastewater by electrodi­
alysis/vacuum membrane distillation hybrid method with a rejection
efficiency by up to 99.9 %. The removal efficiency was improved by
increasing the flow rate, current density, and nitric acid concentration
4.3. Electrochemical recovery of REEs
4.3.1. Electrodeposition
Electrodeposition or electrowinning has been explored for the re­
covery of metals since the 70s [22]. In REEs recovery, the process in­
volves immersing a cathode into a solution containing the metals. An
Table 6
The use of solvent extraction for REEs extraction.
Solvent
REEs
Initial concentration of REEs (ug/
L)
pH
Temperature
(◦ C)
Contact time
(h)
Removal
(%)
Ref
Di-(2,3-dimethylbutyl)-phosphinic acid
Y
Nd
Pr
Gd
Dy
Yb
Ce
960,000
2.65
17
2
[161]
–
1
25
48
98,000
99,482
88
35
50
92
100
4.4
25
0.25
92
[159]
Eu
151,964
4
32
1.5
98.5
[163]
Di-(2-ethylhexyl) phosphoric acid
Tetraoctylammonium oleate (functionalized ionic liquid)
Trioctyl(2-ethoxy-2-oxoethyl)ammonium dihexyl
diglycolamate
10
[162]
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
[83]. Electrodialysis is found to be ideal for recovering low-concentrated
scandium from secondary sources. Li et al. used a system of electro­
chemical membrane process with a titanium electrode coated with Pt/Ir
as an anode and a cathode of stainless steel in a sulfuric acid anolyte/
catholyte solution to recover Sc from wastewater. The results showed
increases in the removal efficiency of Sc3+ of 99 % with increasing the
solution pH and the feed concentration [72]. However, due to the
similar ionic size and chemical behavior, the mechanism of selectivity is
complicated. Electrodialysis was able to recover multiple elements in a
mixture of a wide range of REE from fly ash solution. Neodymium,
dysprosium, and terbium were successfully recovered by 62 %, 74 %,
and 55 %, respectively [27]. Lima and Ottosen also investigated the
recovery of REE from coal fly ashes in different solutions [28]. The
electrodialysis method has proven to be effective for recovering the
desired elements in electrolyte-mixed ions with high selectivity. Ln, Ce,
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Em, Tm, and Yb were all recovered over a
metal electrode in NaClO4 condition [166]. Unlike precipitation, elec­
trodialysis does not need a high solution pH. Also, it eliminates the
production of secondary waste, such as sludge, and the additional cost of
operation. It can operate with diluted systems with lower REE concen­
trations than solvent extraction. It is environmentally friendly and can
be added to the concept of sustainability and circular economy by
converting waste into resources. However, the fouling of the membranes
can reduce the efficiency of the process over time. Furthermore, the high
cost of the membranes used in electrodialysis can make the process
expensive.
Fig. 4. Breakdown of global REE utilization by the ultimate customer in
2021 [177].
elements extracted from wastewater for eventual application in the
production of such magnets, and catalysts, among other important in­
dustrial materials as presented in Fig. 5.
According to U.S. Geological Survey, the prices of REEs exhibit a
wide range of variability [178], with values ranging from <2$ to almost
1800$ per kilogram. Among the REEs used in the production of magnets,
prices have undergone substantial increases in recent years. Conversely,
Ce and La are currently experiencing an oversupply, resulting in a sta­
bilization or decline of their prices. The recycling process recovers only
2 % of the REEs from different secondary resources compared to 90 % of
that for iron and steel [179]. Since the individual prices of REE differ
markedly, the presence of high-value REE heavily influences the prof­
itability considering selected recovery technology. For example, Nd
oxide has a value of $130/kg, while Ce oxide is sold at $1/kg, making
the revenue from selling 1 kg of Nd oxide equivalent to over 100 kg of
Ce. Both compositions of recovered REE from wastewater may vary from
one to another, reflecting the geological background and anthropogenic
activities and affecting its economic feasibility [180]. Wastewater with a
high concentration of REE influences generates more REE and hence
more revenue. However, recovering technology costs can negate the
increased revenue and overall profit.
The recovered REE from wastewater can also be used in developing
wastewater treatment technologies enabling a circular economy appli­
cation. They can be used to enhance the efficiency of various treatment
methods, such as adsorption, coagulation, and membrane filtration.
5. Utilization of REEs from wastewater in the context of a
circular economy
REEs are important components in today’s advanced technologies
such as cellular phones, computer tablets, batteries, magnets, agricul­
ture, military devices, etc. [1,2,4,5,171]. The demand is projected to
reach a market size of 2.5 trillion dollars by 2030 [1]. Recovery of REEs
from wastewater can provide a driving force for a quantum leap from a
linear economy, where materials are used once and then discarded as
waste, which leads to resource depletion, environmental pollution, and
economic inefficiency, to a circular economy approach that seeks to
create closed-loop systems in which resources are kept as long as
possible through continuous reuse and recycling, thereby reducing
waste and conserving resources [172], as shown in Fig. 3. Utilizing
wastewater for REEs recovery helps to create new markets for these
valuable materials, reduces the need for new mining and processing
operations, and creates novel employment opportunities, promoting a
more sustainable and resource-efficient economy [173].
REEs have a wide range of applications across various high-tech in­
dustries, aerospace, medicine, and energy [174–176]. Fig. 4 summarizes
the end-user distribution of REE consumption worldwide according to a
Statista report [177]. According to the report, magnet production was
the largest consumer, accounting for 43.2 % of global REE, while cata­
lyst production was the second. Nd, La, and Ce were the most abundant
Fig. 3. Transition from a linear economy model to recovering REEs from wastewater in a circular economy.
11
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
Fig. 5. Percentage distribution of the most frequently used wastewater borne REEs per end-use applications in 2021 (based on data from [181]).
Recovered REEs can also be used as catalysts in advanced oxidation
processes to degrade organic pollutants [182], just as ferric-based pho­
tocatalysts doped with Nd and Dy have been used for wastewater
treatment [183].
The understanding of the economic viability of rare earth metal re­
covery from wastewater is still limited due to the difficulty in assessing
the economic potential of REE recovery from specific wastewater. This is
due to limited studies and information on economic analysis of any
involved techniques such as capital costs, processing costs, electricity,
utility, labor, and other costs. Another limitation is the absence of
detailed pilot studies on the recovery of REE from wastewater. The
presence of such studies would allow the assessment of process feasi­
bility on a small scale, provide valuable data on performance under realworld conditions, and identify potential issues that could arise upon
scaling up, all of which build confidence in the recovery process.
performance. Coordination absorbents such as COFs and MOFs have
shown high selectivity and capacity for REE recovery. However, their
synthesis and production are still challenging and require further opti­
mization to reduce the cost and improve the scalability for industrial
applications. Organic solvents are commonly employed owing to their
high selectivity, efficiency, and commercial viability. However, the high
cost and the generation of large amounts of organic waste limit their
applicability. Electrodialysis is effective for recovering multiple ele­
ments from a mixture of REEs, more selective than precipitation, and
does not require a high solution pH or produce secondary waste. How­
ever, the fouling and the high cost of the membranes are the main
drawbacks. Future research should focus on developing efficient and
cost-effective recovery methods that can be scaled up for industrial
applications. The use of advanced technologies such as membrane
filtration and the development of hybrid technologies that combine
multiple methods can further improve the efficiency and selectivity of
REE recovery. Moreover, the environmental impact of REE recovery
from wastewater should be carefully evaluated to ensure that the pro­
cess does not cause any adverse effects on the environment. The life
cycle assessment of REE recovery from wastewater can provide valuable
insights into the environmental impact of the process and help identify
areas for improvement.
The circular economy concept can ensure that REEs are converted to
resources for others, minimizing waste and improving sustainability.
REEs recovered from wastewater have potential applications, such as
their incorporation into adsorbents leading to improved adsorption ca­
pacity, but current manufacturing processes limit their scalability. The
economic viability of REE recovery from wastewater is still uncertain
due to limited studies on economic analysis and the absence of pilot
studies. Therefore, more research is needed to better understand the
fate, behavior, and potential impacts of REEs during wastewater treat­
ment processes, as well as to develop sustainable and economically
feasible methods for their recovery and reuse.
6. Conclusions
REEs are critical elements that are widely used in various high-tech
applications, such as electronics, magnets, and batteries. However, the
current production of REEs is highly concentrated in a few countries,
which poses a risk to the global supply chain. REEs presence in WWTPs
highlights the need for effective monitoring and treatment strategies to
minimize their release into the environment. Ce was the most commonly
found LREE, whereas Gd was the most commonly detected HREE.
Improved monitoring of REEs in water bodies is recommended to pre­
vent potential negative impacts they could have, the details of which are
still developing. In addition, the need for the development of regulatory
standards for these REEs in recycled water and drinking water was
highlighted.
Wastewater from various industrial processes, such as mining, met­
allurgy, and electronics, contains significant amounts of REEs. The re­
covery of REEs from wastewater can not only reduce environmental
pollution but also provide a sustainable source of REEs. Pretreatment of
REE wastewater has been effective in reducing its toxicity and recov­
ering valuable components for reuse. Adsorbents have shown great
potential for the recovery of REEs from various sources, including
wastewater. Carbon-based adsorbents, polymeric adsorbents, natural
adsorbents, and coordination absorbents have been studied due to their
high selectivity, low cost, and ease of regeneration. Carbon-based ad­
sorbents have been widely used for REE recovery due to their high
surface area and porosity. However, they suffer from low selectivity and
require pre-treatment to enhance their performance. Polymeric adsor­
bents have shown high selectivity for REEs but suffer from low capacity
and poor stability. Natural adsorbents are renewable and environmen­
tally friendly but require further optimization to enhance their
Funding
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
12
D.E. Al Momani et al.
Journal of Water Process Engineering 55 (2023) 104223
Table A1
Rare earth elements.
1
2
3
4
5
6
7
8
9
REE
Symbol Atomic Number
Cerium
Dysprosium
Erbium
Europium
Gadolinium
Holmium
Promethium
Lanthanum
Lutetium
Ce
Dy
Er
Eu
Gd
Ho
Pm
La
Lu
58
66
68
63
64
67
61
57
71
10
11
12
13
14
15
16
17
REE
Symbol Atomic Number
Neodymium
Praseodymium
Samarium
Terbium
Thulium
Ytterbium
Scandium
Yttrium
Nd
Pr
Sm
Tb
Tm
Yb
Sc
Y
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Data availability
Data will be made available on request.
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