chemical engineering

1. Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Removal of Cs(I) from simulated radioactive wastewater by three forward
osmosis membranes
Xiaojing Liua
, Jinling Wua,⁎
, Jianlong Wanga,b,⁎
a
Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China
b
Beijing Key Laboratory of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, PR China
H I G H L I G H T S
• Cs(I) removal from radioactive waste-
water by three FO membranes was
studied.
• Removal efficiency of Cs(I) by FO was
compared with other membrane pro-
cesses.
• Active layer played an important role
in Cs(I) retention.
• The optimal operational conditions for
Cs(I) retention were examined.
• CTA-ES membrane has potential for Cs
(I) removal from radioactive waste-
water.
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
Keywords:
Forward osmosis
Cs(I) removal
Operational parameters
Radioactive wastewater
A B S T R A C T
The removal of Cs(I) from radioactive wastewater by three forward osmosis (FO) membranes was investigated
using NaCl solution and simulating seawater, as draw solution. The influences of membrane material, membrane
orientation, draw solution concentration and flow velocity on FO performance were examined. Removal effi-
ciency of Cs(I) by FO was compared with other membrane separation processes as well. Active layer played an
important role in Cs(I) retention. CTA membrane achieved high Cs(I) retention of 90.35%–97.15%. TFC
membrane was unable to reject Cs(I) effectively with Cs(I) retention of lower than 48%. Support layer sig-
nificantly affected water flux. ES support layer helped to alleviate concentration polarization and therefore yield
high water flux. Effect of membrane orientation was related to the support layer. AL-FS obtained higher Cs(I)
retention than AL-DS by CTA-NW. AL-FS featured lower Cs(I) retention than AL-DS by CTA-ES and TFC-ES. NaCl
concentration influenced Cs(I) retention by TFC-ES at AL-DS mode. Increasing flow velocity could not enhance
Cs(I) retention at both modes. The optimal operational conditions for Cs(I) retention were achieved by CTA-ES
membrane with NaCl concentration of 1.5 M and flow velocity of 11 cm s−1
at AL-DS. Under the optimal con-
dition, Cs(I) retention reached 96.24%. Therefore, CTA-ES membrane is a potential membrane for Cs(I) removal
from radioactive wastewater by FO process.
1. Introduction
In recent years, a large amount of radioactive wastewater has been
produced and must be purified before discharge. Radionuclides in
radioactive wastewater are cesium, strontium, cobalt, tritium, lantha-
nides, actinides and so on [1]. Among these radionuclides, cesium (Cs)
https://doi.org/10.1016/j.cej.2018.03.046
Received 31 January 2018; Received in revised form 7 March 2018; Accepted 9 March 2018
⁎
Corresponding authors at: Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China (J. Wang).
E-mail addresses: jinlingwu@tsinghua.edu.cn (J. Wu), wangjl@tsinghua.edu.cn (J. Wang).
Chemical Engineering Journal 344 (2018) 353–362
Available online 10 March 2018
1385-8947/ © 2018 Elsevier B.V. All rights reserved.
T

2. is one of most abundant fission products, which is released from nuclear
reactor. Cs is a typical alkaline metal ion and chemically similar to
sodium and potassium, which is more likely to enter in a variety of
environments and easily assimilated by organisms. Once ingested and
accumulated, Cs can be deposited in the soft tissues throughout the
body and induce thyroid cancer. In the isotopes of Cs, 137
Cs is a ha-
zardous isotope to human and ecosystem, because 137
Cs is a beta and
gamma radiation source and its half-life is relatively long about
30 years [2,3]. Compared to other nuclides, Cs is much more difficult to
be removed due to its smaller hydrated radii and the higher diffusion
coefficient [4]. Therefore, selective removal of Cs ions from radioactive
wastewater is a challenging task.
In recent years, a plenty of methods have been used to remove Cs(I)
from aqueous solution such as solvent extraction, ion exchange, ad-
sorption, chemical precipitation evaporative and membrane technology
[5–8]. Solvent extraction, ion exchange and adsorption methods are
based on ionic selectivity and efficiency. Large-scale application of
solvent extraction is restricted due to high costs of chemicals and
equipment. Although ion exchange and adsorption have received lots of
attention for high selectivity, simple operation, high thermal and ra-
diation stabilities, selecting a promising resin or adsorbent with high
capacity and selectivity, high adsorption rate and low cost remains a
challenge [9]. The regeneration process will produce the secondary
pollution and increase overall operating cost. Chemical precipitation
has low selectivity for Cs(I) and can produce a lot of sludge for disposal
[10]. Similarly, evaporative methods also have disadvantages, such as
corrosion, scaling or foaming [10]. Membrane technologies with the
advantages of high Cs(I) retention, low energy consumption, minimized
maintenance and human exposure have received high attention in Cs(I)
removal [5], especially pressure-driven membrane processes, such as
hybrid process of complexation and ultrafiltration (UF) [11], nanofil-
tration (NF) [12], direct contact membrane distillation (DCMD)
[6,13,14], vacuum membrane distillation (VMD) [3] and reverse os-
mosis (RO) [5,15,16]. Although hybrid process of complexation and UF
has the high Cs(I) retention, the complexation may pollute the feed
solution. NF has the low Cs(I) retention and low water flux. The Cs(I)
retentions of DCMD and VMD are nearly 100%, but low water flux
limits its application. RO, with high Cs(I) removal and high water flux,
has been used in real radioactive wastewater treatment.
Forward osmosis (FO), as a new membrane technology, takes ad-
vantage of the osmotic pressure difference as the driving force to
transport the water molecules across a semi-permeable membrane from
feed solution to draw solution, which eliminates the need for an ex-
ternal energy source [17–19]. Compared with RO, FO featured the
equivalent level of nuclide removal. Besides, FO is an energy saving
process with low membrane fouling and absence of high pressure de-
vices, which might be used as an alternative process for Cs(I) removal.
When FO is applied for radioactive wastewater treatment, a large
amount of seawater is available for FO process because most of the
nuclear power plants are located by the seaside. Regeneration of draw
solution could be ignored if the nuclide retention was high enough. The
used draw solution could be discharged back to the sea. In our previous
study, FO has been successfully used in Co(II) removal from aqueous
solution [20]. Therefore, FO in this study was used for Cs(I) separation
from the aqueous solution.
FO performance is dependent of many factors including membrane
properties, draw solution properties, feed solution properties and the
operating parameters [20,21]. Membrane properties referring to
membrane material and membrane orientation play important roles in
FO performance. In FO process, two different kinds of materials: cel-
lulose triacetate (CTA) and polyamide (PA)-based thin film composite
(TFC) have been commercially used, which have different separation
characteristics. In previous studies, Zheng et al. compared the tetra-
cycline retention by CTA and TFC membrane. Retention of CTA mem-
brane was relatively higher than TFC membrane [22]. Jin et al. took
advantage of CTA and TFC membranes to reject the pharmaceuticals.
TFC membranes exhibited superior performance with great retention
(> 94%) [23]. CTA and TFC membranes have also been used to remove
boron in some studies [24–27]. And a double-skinned TFC membrane
displayed superior boron retention (83.9%) [27] higher than CTA
membrane (60%) [26]. In FO process, membrane orientation, including
active layer facing feed solution (AL-FS) and active layer facing draw
solution (AL-DS) modes [28], has an effect on FO performance. The
selection of membrane orientation depends on the feed solution com-
position and concentration [28]. When feed solutions contain higher
fouling/scaling tendencies or higher salinity water (i.e. wastewater
treatment, sulfamethoxazole and carbamazepine [29], haloacetic acids
[30], boron and arsenic [26], seawater desalination, brine concentra-
tion), AL-FS mode was preferable. However, AL-DS mode is more fa-
vorable to treat feed solutions with lower fouling/scaling tendencies or
lower salinity water [28]. Draw solution properties affect the solute
transport and overall process performance. Inorganic (NaCl, KCl,
NH4Cl, CaCl2, NaNO3, KNO3, NH4NO3, Ca(NO3)2 [31], etc.), organic
(switchable polarity solvents [32], phosphazene [33], dimethyl ether
[17], etc.), and high molecular (polyacrylamide [34], polymer hydro-
gels [35], etc.) solutes are chosen as the draw solute for some specific
applications, which display different effects on FO performance because
their different chemistry properties and diffusion rates. Higher draw
solution concentration not only produces higher water flux, but results
in higher reverse solute flux and more severe dilution effect [36]. Not
only draw solution, but feed solution composition and concentration
affects FO performance as well [20]. Temperature and pH of feed so-
lution are important factors affecting pollutant removal by FO
[24,29,37]. High temperature decreases the retention of trace organic
contaminants [29]. However, high pH is helpful to improve retention of
boron acid [24] and sulfamethoxazole [29]. The operating parameters
such as flow velocity influence the external concentration polarization
and therefore FO performance. Increasing flow velocity can reduce the
external concentration polarization and improve water flux [38]. But
flow velocity may also be not significant to affect external concentra-
tion polarization and mass transfer if the flow velocity changed slightly
[24].
Based on the above, the effects of membrane materials, membrane
orientation, draw solution concentration, flow velocity on the FO per-
formance are investigated for this study. Three commercial FO mem-
branes were used in this study. NaCl was employed to be draw solution.
The objective of this study is to investigate the feasibility of FO for Cs(I)
removal and compare the Cs(I) retention by three commercial FO
membranes. Water flux, Cs(I) retention, Cs(I) flux, reverse NaCl flux
were used to evaluate the performance of FO. The operational para-
meters were optimized. And the FO performance for Cs(I) removal was
finally compared to other membrane separation processes.
Fig. 1. The scheme of the experimental set-up.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
354

3. 2. Materials and methods
2.1. FO membrane and characterizations
Three commercial FO membranes (HTI, US) were applied in this
study, including cellulose triacetate with a cast nonwoven support
(CTA-NW), cellulose triacetate with embedded polyester screen support
(CTA-ES) and polyamide (PA)-based thin-film composite with em-
bedded polyester screen support (TFC-ES). These membranes were
asymmetric membranes, which consist of a porous support layer and a
dense active layer.
The original FO membranes were observed with a scanning elec-
tronic microscope (SEM) (FEI QUANTA 200 FEG, USA). The detailed
structure and roughness of the active layer of the membranes were
observed by an atomic force microscope (AFM) (Bruker AXS Dimension,
Germany). Rq, Ra and Rmax were mean-square surface roughness, mean
surface roughness and maximum vertical distance between the highest
and lowest data points respectively, which were used to evaluate the
roughness. A contact angle goniometer (OCA20, Germany) was applied
to measure contact angle by using the sessile drop method. Before the
measurement, vacuum freeze dryer (LGJ-18, China) was applied to dry
membrane samples for 24 h.
2.2. FO operation
The scheme of experimental set-up is shown in Fig. 1. A membrane
cell had two symmetric channels of 2 cm in height. The effective
membrane area was 40.5 cm2
. For the experiments, feed solution and
draw solution circulated counter currently by using two gear pumps
(BT600-2J, China). Feed solution rested on a digital balance (ML6001,
Table 1
Surface characteristics of CTA-NW, CTA-ES and TFC-ES membranes.
Membrane CTA-NW CTA-ES TFC-ES
Membrane layer Active Support Active Support Active Support
Hydrophobicity Contact angle (°) 79.1 110.8 65.6 72.9 46.8 57.5
Roughness Rq 3.1 5.2 44.2
Ra 2.4 4.1 36.1
Rmax 27.0 41.8 301.1
Fig. 2. Surfaces and cross-sections of the three membranes by SEM: (a) active layer of CTA-NW, (b) cross-section of CTA-NW, (c) support layer of CTA-NW, (d) active layer of CTA-ES, (e)
cross-section of CTA-ES, (f) support layer of CTA-ES, (g) active layer of TFC-ES, (h) cross-section of TFC-ES and (i) support layer of TFC-ES.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
355

4. Mettler Toledo, China) and a data logging system recorded the changes
of weight in order to determine the water flux. Each FO experiment was
conducted for 3 h at 25 ± 2 °C.
Various draw solution concentrations and flow velocities were ap-
plied for FO runs, which were investigated at AL-FS and AL-DS modes
for the three membranes. Draw solution is NaCl solution and its initial
concentration varied from 0.5 to 2 M. During the experiments, the draw
solution was continuously diluted. A concentrated NaCl solution (5 M)
was intermittently added to maintain NaCl concentration in draw so-
lution a constant. The flow velocities varied from 2 to 11 cm s−1
on both
sides.
Fig. 3. Active layers of the three membranes: (a) CTA-NW, (b) CTA-ES and (c) TFC-ES by AFM.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
356

5. 2.3. Determination of water flux, Cs(I) retention and flux, reverse NaCl flux
Four indicators, including water flux, Cs(I) retention, Cs(I) flux and
reverse NaCl flux, were used to assess FO performance.
The water flux is calculated as:
=
J
m
ρ A t
Δ
· ·Δ
w
m (1)
where Jw is the water flux, Δm is the measured weight interval for
permeated water, ρ is the density of water. Am is the effective area of
membrane, Δt is the measured time interval.
The Cs(I) retention can be defined as:
= −
R
C V
V C
(%) 1
·
·
f,DS f,DS
p i,FS (2)
where R is the Cs(I) retention, Ci,FS, Cf,DS are the initial and final con-
centration of Cs(I) in the feed solution and draw solution respectively,
VP, Vf,DS are permeated water volume and final draw solution volume
respectively.
The Cs(I) flux is calculated as:
=
J
C V
A t
·
·Δ
Cs(I)
f,DS f,DS
m (3)
where JCs(I) is the Cs(I) flux.
The reverse NaCl flux is calculated by the flowing equation:
=
−
J
C V C V
A t
· ·
·Δ
NaCl
fNa,FS f,FS iNa,FS i,FS
m (4)
where JNaCl is the reverse NaCl flux, CiNa,FS is the initial NaCl con-
centration in feed solution, Vi,FS is the initial feed solution volume,
CfNa,FS is the final NaCl concentration in feed solution, Vf,FS is the final
feed solution volume.
2.4. Chemical reagents and analytical methods
Non-radioactive isotope of CsCl and NaCl were purchased from
Sinopharm (Beijing, China). Feed solution was CsCl solution containing
0.15 mmol L−1
(20 mg L−1
) Cs(I). All chemicals were of analytical re-
agent grade in this experiment.
Cs(I) and Na(I) concentration were detected by atomic absorption
spectrophotometry (HITACHI ZA3000, Japan). Three parallel samples
were used to ensure the reliability of the experiment.
3. Results and discussion
3.1. Membrane characterization
In order to study the influence of membrane on Cs(I) flux, char-
acteristics of the three membranes were analyzed in terms of contact
angle (Table 1) and surface morphology. Membrane surface was ob-
served by SEM (Fig. 2) and AFM (Fig. 3). From Fig. 2, support layers of
all the three membranes were coarse and complex. CTA-ES and TFC-ES
membranes had embedded screen support layer (ES) (Fig. 2f, i). CTA-
NW membrane had a different support layer from the other two
membranes, which was nonwoven (NW) with lots of small openings
inside (Fig. 2c). Compared to the support layers, the active layers were
relatively smooth and dense for all the three membranes. CTA active
layer was more smooth, more hydrophobic and less negatively-charged
than TFC layer (Table 1).
(a)
(b)
CTA-NW CTA-ES TFC-ES
0
10
20
30
40
50
Water
flux
(L
m
-2
h
-1
)
Membrane materials
0.5 M NaCl
1.0 M NaCl
1.5 M NaCl
2.0 M NaCl
AL-FS
CTA-NW CTA-ES TFC-ES
0
10
20
30
40
50
AL-DS
Water
flux
(L
m
-2
h
-1
)
Membrane materials
0.5 M NaCl
1.0 M NaCl
1.5 M NaCl
2.0 M NaCl
Fig. 4. Effect of NaCl concentration on water fluxes at AL-FS (a) and AL-DS modes (b) by
three membranes with feed solution of 0.15 mmol L−1
Cs(I) and flowrate of 11 cm s−1
.
(a)
(b)
0.5 1.0 1.5 2.0
0
5
90
95
100
AL-FS
NaCl concentration (M)
Cs(I)
retention
(%)
CTA-NW
CTA-ES
TFC-ES
0.5 1.0 1.5 2.0
0
25
90
95
100
AL-DS
NaCl concentration (M)
Cs(I)
retention
(%)
CTA-NW
CTA-ES
TFC-ES
Fig. 5. Effect of NaCl concentration on Cs(I) retentions by three membranes at AL-FS (a)
and AL-DS modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and flow velocity of
11 cm s−1
.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
357

6. 3.2. Effect of NaCl concentration
3.2.1. Water flux
Water fluxes of the three membranes with NaCl concentration at
both modes were shown in Fig. 4.
At AL-FS mode, water fluxes of CTA-NW, CTA-ES, TFC-ES were
5.4–9.8, 9.7–19.3 and 7.7–22.0 L m−2
h−1
(Fig. 4a), respectively. With
NaCl concentration increased, water fluxes of the three membranes
increased. With various NaCl concentrations, CTA-ES and TFC-ES fea-
tured higher water fluxes than CTA-NW. Among the three membranes,
the non-woven support layer of CTA-NW was more complex and thicker
than the support layer of the CTA-ES and TFC-ES (Fig. 2b, c), which
retarded the NaCl diffusion and resulted in the severe internal con-
centration polarization in support layer. Therefore, the effective trans-
membrane osmotic pressure of CTA-NW membrane was lowest among
the three membranes. Moreover, the non-woven support layer of CTA-
NW membrane was more hydrophobic and thicker than the other
membranes (Table 1), which led to higher resistance for water diffusion
through the membrane. Therefore, the CTA-NW membrane featured the
lowest water flux.
At AL-DS mode, water fluxes of CTA-NW, CTA-ES, TFC-ES were
10.4–21.9, 17.0–38.0, 15.9–29.8 L m−2
h−1
(Fig. 4b). CTA-NW fea-
tured the lowest water flux. CTA-ES obtained the highest. It was related
to the support layer complex. The complex nonwoven support layer
aggravated the internal concentration polarization degree and therefore
reduced water flux.
AL-DS achieved higher water flux than AL-FS, indicating that the
dilutive external concentration polarization at AL-DS was less severe
than the dilutive internal concentration polarization at AL-FS.
Regardless of membrane materials and orientation, water flux
increased with increasing NaCl concentration. However, water flux
could not proportionally increase with increasing NaCl concentration,
because increasing water flux aggravated internal concentration po-
larization in support layer, which limited increase of water flux. Some
other researchers have also found that the water flux had a negative
effect on the draw solution performance at high concentration when FO
was applied for desalination [38].
3.2.2. Cs(I) retention
The effect of NaCl concentration on Cs(I) retentions by the three
membranes at both modes were shown in Fig. 5.
At AL-FS mode, Cs(I) retention by CTA-NW, CTA-ES, TFC-ES
membranes were 93.63–97.15%, 93.52–95.56%, 0–1.57% (Fig. 5a),
respectively. It is noteworthy that CTA active layer rejected more Cs(I)
than membrane with TFC active layer. Cs(I) retention by TFC-ES
membrane was almost zero, which indicated that TFC-ES was unable to
reject Cs(I). Since Cs(I) retention was mainly the result of size exclusion,
the nominal pore size of the TFC-ES membrane could be larger than
those of CTA membranes and Cs(I) ion size. The Cs(I) retention of CTA-
NW membrane was higher than that of CTA-ES membrane. The stronger
hydrophobicity of CTA-NW membrane (Table 1) weakened the pene-
tration of water and the adsorption of Cs(I) onto the membrane.
Therefore, the CTA-NW membrane featured the highest Cs(I) retention.
At AL-DS mode, Cs(I) retentions by CTA-NW, CTA-ES, TFC-ES
membranes were 94.93–95.77%, 95.42–96.24%, 26.31–48.00%
(Fig. 5b), respectively. Likewise, Cs(I) retention by TFC-ES membrane
was far lower than CTA-ES and CTA-NW. By TFC-ES, Cs(I) retention
increased with increasing NaCl concentration. However, by CTA-NW
and CTA-ES, Cs(I) retention remained unchanged with increasing NaCl
(a)
(b)
0.5 1.0 1.5 2.0
0.0
0.1
0.2
2.0
3.0
4.0
5.0
AL-FS
NaCl concentration (M)
Cs(I)
flux
(mmol
m
-2
h
-1
)
CTA-NW
CTA-ES
TFC-ES
0.5 1.0 1.5 2.0
0.0
0.1
0.2
1.0
2.0
3.0
4.0
5.0
AL-DS
NaCl concentration (M)
Cs(I)
flux
(mmol
m
-2
h
-1
)
CTA-NW
CTA-ES
TFC-ES
Fig. 6. Effect of NaCl concentration on Cs(I) fluxes by three membranes at AL-FS (a) and
AL-DS modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and flow velocity of
11 cm s−1
.
(a)
(b)
CTA-NW CTA-ES TFC-ES
0
2
4
6
8
10
12
Reverse
NaCl
flux
(g
m
-2
h
-1
)
Membrane materials
0.5 M NaCl
1.0 M NaCl
1.5 M NaCl
2.0 M NaCl
AL-FS
CTA-NW CTA-ES TFC-ES
0
2
4
6
8
10
12
AL-DS
Reverse
NaCl
flux
(g
m
-2
h
-1
)
Membrane materials
0.5 M NaCl
1.0 M NaCl
1.5 M NaCl
2.0 M NaCl
Fig. 7. Effect of NaCl concentration on reverse NaCl fluxes of three membranes at AL-FS
(a) and AL-DS modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and flow velocity of
11 cm s−1
.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
358

7. concentration. Cs(I) retention of CTA-ES was higher than CTA-NW
membrane. Since nonwoven support layer hindered the Cs(I) diffusion
at AL-DS mode, which increased the trans-membrane Cs(I) concentra-
tion and therefore reduced Cs(I) retention. AL-FS achieved higher Cs(I)
retention than AL-DS by CTA-NW membrane, which may be attributed
to severe concentrative internal concentration polarization at AL-DS
mode because of complex support layer. However, higher Cs(I) reten-
tions by CTA-ES and TFC-ES membrane were achieved at AL-DS, which
was attributed to the enhanced electrostatic repulsion, because support
layer was more negative than active layer [22].
3.2.3. Cs(I) flux
The effect of NaCl concentration on Cs(I) fluxes by three membranes
at both modes were shown in Fig. 6.
At AL-FS mode, Cs(I) flux by CTA-NW, CTA-ES, TFC-ES membranes
were 0.03–0.05, 0.09–0.12, 3.01–3.24 mmol m−2
h−1
(Fig. 6a), re-
spectively. Cs(I) flux of TFC-ES was extremely high, which agreed with
lowest Cs(I) retention of TFC-ES membrane. Cs(I) fluxes by three
membranes with NaCl concentration were almost unchanged.
At AL-DS mode, Cs(I) flux by CTA-NW, CTA-ES, TFC-ES membranes
were 0.06–0.15, 0.11–0.17, 1.71–2.24 mmol m−2
h−1
, respectively. Cs
(I) fluxes of three membranes slightly increased with increasing NaCl
concentration, because high NaCl concentration enhanced the osmotic
pressure difference on both sides of the membrane and resulted in the
increase of water flux. The increasing water flux aggravated the internal
concentration polarization and improved the effect of convection and
Cs(I) diffusion.
In general, CTA membranes had much lower Cs(I) fluxes than TFC
membrane. AL-DS obtained higher Cs(I) fluxes than AL-FS by CTA-ES
and CTA-NW. In contrary, lower Cs(I) flux by TFC-ES membrane could
be achieved at AL-DS mode.
3.2.4. Reverse NaCl flux
Fig. 7 shows reverse NaCl fluxes by three membranes with various
NaCl concentrations on at both modes. At AL-FS mode, the reverse NaCl
flux of CTA-NW, CTA-ES and TFC-ES were 0.67–0.91, 2.03–4.22,
1.21–3.03 g m−2
h−1
(Fig. 7a), respectively. The corresponding reverse
NaCl fluxes were 1.23–2.09, 3.28–9.78, 2.16–5.56 g m−2
h−1
(Fig. 7b)
at AL-DS, respectively. At both modes, CTA-ES obtained the highest
reverse NaCl flux, however CTA-NW the lowest. The reverse NaCl fluxes
of the membranes with ES support layer were higher than those with
the NW support layer. At AL-FS, severe internal concentration polar-
ization occurred, which caused the trans-membrane NaCl concentration
reduced, especially in CTA-NW membrane. Therefore, the reverse NaCl
flux of CTA-NW membrane was lowest among three membranes. In
addition, NaCl easily passed through the CTA-ES membrane with the
simple ES support layer structure resulted in a high reverse NaCl flux.
Reverse NaCl flux increased with increasing NaCl concentration for
three membranes at both modes, which was attributed to that the in-
creased NaCl concentration improved the trans-membrane pressure
difference of NaCl and finally increased the reverse NaCl flux.
3.3. Effect of flow velocity
3.3.1. Water flux
Fig. 8 presents the effects of flow velocity on water fluxes by three
membranes at both modes. At AL-FS mode, the water flux by CTA-NW,
CTA-ES, TFC-ES were 5.93–8.35, 10.78–15.33, 6.97–10.75 L m−2
h−1
(Fig. 8a), respectively. The corresponding water flux at AL-DS mode
were 9.71–11.32, 15.02–21.67, 9.93–21.05 L m−2
h−1
(Fig. 8b), re-
spectively. CTA-ES membrane obtained highest water flux and CTA-NW
(a)
(b)
CTA-NW CTA-ES TFC-ES
0
5
10
15
20
25
30
Water
flux
(L
m
-2
h
-1
)
Membrane materials
2 cm s-1
flow velocity
5 cm s-1
flow velocity
8 cm s-1
flow velocity
11cm s-1
flow velocity
AL-FS
CTA-NW CTA-ES TFC-ES
0
5
10
15
20
25
30
AL-DS
Water
flux
(L
m
-2
h
-1
)
Membrane materials
2 cm s-1
flow velocity
5 cm s-1
flow velocity
8 cm s-1
flow velocity
11cm s-1
flow velocity
Fig. 8. Effect of flow velocity on water fluxes by three membranes at AL-FS (a) and AL-DS
modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and NaCl concentration of 1.0 M.
(a)
(b)
2 5 8 11
0
20
40
90
95
100
AL-FS
Flow velocity (cm s-1
)
Cs(I)
retention
(%)
CTA-NW
CTA-ES
TFC-ES
2 5 8 11
0
20
40
90
95
100
AL-DS
Flow velocity (cm s-1
)
Cs(I)
retention
(%)
CTA-NW
CTA-ES
TFC-ES
Fig. 9. Effect of flow velocity on Cs(I) retentions by three membranes at AL-FS (a) and AL-
DS modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and NaCl concentration of 1.0 M.
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
359

8. membrane had lowest water flux. The water flux increased with in-
creasing flow velocity at both modes, because high flow velocity alle-
viated external concentration polarization on both sides and kept the
feed solution and draw solution remain effective osmotic pressure.
3.3.2. Cs(I) retention
The effect of flow velocity on Cs(I) retentions by three membranes
at both modes were shown in Fig. 9.
At AL-FS mode, Cs(I) retentions by CTA-NW, CTA-ES, TFC-ES
membranes were 91.31–92.98%, 94.02–94.59% and 0 (Fig. 9a), re-
spectively. The corresponding Cs(I) retentions were 90.35–94.13%,
93.67–93.97% and 28.12–33.32% (Fig. 9b), respectively at AL-DS. At
both modes, Cs(I) retention of CTA-ES were highest. Cs(I) retention of
TFC-ES were lowest. Cs(I) retentions by three membranes increased
insignificantly with the increasing flow velocity. Although high flow
velocity alleviated external concentration polarization on both sides
(Fig. 8), Cs(I) retention was uninfluenced (Fig. 9).
3.3.3. Cs(I) flux
The effect of flow velocity on Cs(I) fluxes by three membranes at
both modes were presented in Fig. 10.
At AL-FS mode, the average Cs(I) fluxes by CTA-NW, CTA-ES, TFC-
ES membranes were 0.08, 0.11 and 2.11 mmol m−2
h−1
(Fig. 10a),
respectively. At AL-DS mode, the average Cs(I) fluxes by CTA-NW, CTA-
ES, TFC-ES membranes were 0.13, 0.16 and 1.53 mmol m−2
h−1
(Fig. 10b), respectively. At both modes, the Cs(I) fluxes by TFC-ES
membrane were highest and CTA-ES membrane featured the lowest Cs
(I) flux. The Cs(I) flux of TFC-ES at AL-DS mode increased significantly
with the increasing flow velocity, which agreed with the change of
water flux (Fig. 8b). Therefore, Cs(I) transferred through TFC-ES
membrane by convection at AL-DS mode. Cs(I) fluxes of CTA mem-
branes including CTA-ES and CTA-NW increased insignificantly with
increasing flow velocity.
3.3.4. Reverse NaCl flux
Reverse NaCl fluxes with the various flow velocities by three
membranes at both modes were shown in Fig. 11. At AL-FS mode, re-
verse NaCl fluxes of CTA-NW, CTA-ES and TFC-ES membranes were
0.68–0.86, 2.21–3.15 and 0.88–1.57 g m−2
h−1
, (Fig. 11a) respectively.
The corresponding reverse NaCl flux were 1.11–1.50, 3.64–5.61,
1.50–3.64 g m−2
h−1
(Fig. 11b), respectively.
CTA-ES membrane featured the highest reverse NaCl flux and CTA-
NW membrane featured the lowest reverse NaCl flux. At AL-FS, when
the flow velocity increased from 2 to 8 cm s−1
, reverse NaCl flux in-
creased from 2.21 to 3.15 g m−2
h−1
. However, the reverse NaCl flux
(a)
(b)
2 5 8 11
0.0
0.1
0.2
1.0
2.0
3.0
4.0
5.0
AL-FS
Flow velocity (cm s-1
)
Cs(I)
flux
(mmol
m
-2
h
-1
)
CTA-NW
CTA-ES
TFC-ES
2 5 8 11
0.0
0.1
0.2
1.0
2.0
3.0
4.0
5.0
AL-DS
Flow velocity (cm s-1
)
Cs(I)
flux
(mmol
m
-2
h
-1
)
CTA-NW
CTA-ES
TFC-ES
Fig. 10. Effect of flow velocity on Cs(I) fluxes by three membranes at AL-FS (a) and AL-DS
modes (b) with feed solution of 0.15 mmol L−1
Cs(I) and NaCl concentration of 1.0 M.
(a)
(b)
CTA-NW CTA-ES TFC-ES
0
2
4
6
8
10
Reverse
NaCl
flux
(g
m
-2
h
-1
)
Membrane materials
2 cm s-1
flow velocity
5 cm s-1
flow velocity
8 cm s-1
flow velocity
11cm s-1
flow velocity
AL-FS
CTA-NW CTA-ES TFC-ES
0
2
4
6
8
10
AL-DS
Reverse
NaCl
flux
(g
m
-2
h
-1
)
Membrane materials
2 cm s-1
flow velocity
5 cm s-1
flow velocity
8 cm s-1
flow velocity
11cm s-1
flow velocity
Fig. 11. Effect of flow velocity on reverse NaCl fluxes by three membranes at AL-FS (a)
and AL-DS modes (b) with the feed solution of 0.15 mmol L−1
Cs(I) and the NaCl con-
centration of 1.0 M.
Table 2
Cs(I) removal by FO and other membrane technologies.
Technology Cs(I) removal (%) Water flux (L m−2
h−1
) Reference
Complexation + UF ∼100 42 [11]
NF 75.50 35–40 [12]
RO 98 – [5]
RO 99.74 – [15]
RO 94.63 93.13 [16]
DCMD ∼100 15 [6]
DCMD ∼100 19.62 [13]
DCMD ∼100 0.17 (kg m−1
h−1
) [14]
VMD 99.76–99.83 6.14 [3]
FO 96.24 38.03 This study
X. Liu et al. Chemical Engineering Journal 344 (2018) 353–362
360

9. maintained unchanged when the flow velocity was higher than
8 cm s−1
. This phenomenon indicated that flow velocity slightly alle-
viated the external concentration polarization when the flow velocity
was lower than 8 cm s−1
.
At AL-DS mode, the reverse NaCl flux increased from 3.64 to
5.61 g m−2
h−1
with the increasing flow velocity, which implied that
increasing flow velocity was able to alleviate the dilutive external
concentration polarization and increase the trans-membrane Na(I)
concentration resulting in the high reverse NaCl flux.
3.4. Comparison of FO with the other membrane technologies for Cs(I)
separation
Table 2 shows performance of FO process and other membrane
technologies for for Cs(I) separation. From Table 2, DCMD achieved the
highest Cs(I) removal of nearly 100% [6,13,14]. The Cs(I) retention by
FO reached 97.15% in this study, which exceeded the Cs(I) removal
with by RO (94.63%) [16] and NF (75.50%) [12]. As to the water flux,
RO achieved the highest flux of 93.13 L m−2
h−1
[16]. The complexa-
tion + UF [11] and NF [12] featured the water fluxes of 42 and
35–40 L m−2
h−1
, respectively. Water flux in this study was
38.03 L m−2
h−1
by FO, which was lower than the water flux of com-
plexation + UF and NF, but higher than the water flux of MD
(6.14–19.62 L m−2
h−1
) [3,6,13,14]. The electrical energy of RO pro-
cess for seawater desalination is about 3.02 kWh/m3
[39]. However,
the FO desalination process required an electrical power of only less
than 0.25 kWh/m3
[39]. Compared to RO, FO achieved higher Cs(I)
retention with lower energy consumption and relatively lower mem-
brane fouling. The shortage of FO is the lower water flux.
4. Conclusions
The CTA membranes achieved high Cs(I) retention of
90.35%–97.15%. The TFC membrane was unable to reject Cs(I) effec-
tively and Cs(I) retention was lower than 48.00%. The NW support
layer hindered NaCl confusion, which resulted in severe concentration
polarization and low water flux. Support layer of ES helped to alleviate
concentration polarization. Effect of membrane orientation was related
to the support layer materials. The Cs(I) retention by CTA-NW mem-
brane at AL-FS mode was higher than AL-DS mode. The Cs(I) retention
by CTA-ES and TFC-ES membrane at AL-FS mode was lower than AL-DS
mode. Water flux and reverse NaCl flux at AL-DS mode was higher than
those at AL-FS mode regardless of membrane. Cs(I) retention and flux
by TFC-ES membrane were significantly affected by increasing NaCl
concentration at AL-DS mode. At AL-FS mode, Cs(I) retention and flux
were almost uninfluenced by increasing NaCl concentration. The water
flux and reverse NaCl flux increased with the increasing NaCl con-
centration at both modes. Cs(I) retentions by three membranes were
insignificantly affected by the increase of flow velocity. The increasing
flow velocity increased the water flux and the reverse NaCl flux. The
optimal operational conditions for Cs(I) retention were achieved by
CTA-ES membrane with NaCl concentration of 1.5 M and the flow ve-
locity of 11 cm s−1
at AL-FS mode. Under the optimal condition, Cs(I)
retention reached 96.24%. Water flux was 33.34 L m−2
h−1
.
Acknowledgements
The research was supported by the National Natural Science
Foundation of China (51578307), the National Key Research and
Development Program (2016YFC1402507), the National S&T Major
Project (2013ZX06002001) and the Program for Changjiang Scholars
and Innovative Research Team in University (IRT-13026).
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