Exploration of using thermally responsive polyionic liquid hydrogels as draw agents in forward osmosis

Article abstract of DOI:10.1039/c5ra19018e

Thermally responsive hydrogels based on ionic liquid monomers of tetrabutylphosphonium p-styrenesulfonate (P4444 SS) and tributylhexylphosphonium p-styrenesulfonate (P4446 SS) were prepared by bulk polymerization in the presence of a crosslinker, and explored as draw agents in forward osmosis for the first time. Unlike traditional copolymer hydrogels from N-isopropylacrylamide (NIPAm) and sodium acrylate (SA) or sodium 2-acrylamido-2-methylpropane sulfonate (AMPS), the combination of thermo-sensitivity and the ionic nature in the hydrogel was achieved by subtle structure design of the ionic liquid monomers. These polyionic liquid hydrogels were proved to be able to generate a reasonable water flux from brackish water, and effectively release the desalinated water at temperatures above their Lower Critical Solution Temperature (LCST). In addition, a set of rational criteria for assessing responsive hydrogel draw agents was proposed, for the first time, to improve the hydrogels' comprehensive performance. Our studies revealed that, both intrinsic factors including hydrogel structure, membrane structure and hydrogel particle-particle contact conditions as well as extrinsic parameters such as the hydrogel's area density on the membrane and FO-regeneration cycle duration, would have a profound impact on the desalination efficacy using hydrogels.

Full text of DOI:10.1039/c5ra19018e

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Exploration of using thermally responsive polyionic
liquid hydrogels as draw agents in forward
osmosis†
Cite this: RSC Adv., 2015, 5, 97143
Yufeng Cai,ab Rong Wang,bc William B. Krantz,bd Anthony G. Fanebc
ab
*
and Xiao ‘Matthew’ Hu
Thermally responsive hydrogels based on ionic liquid monomers of tetrabutylphosphonium p-
styrenesulfonate (P4444 SS) and tributylhexylphosphonium p-styrenesulfonate (P4446 SS) were prepared
by bulk polymerization in the presence of a crosslinker, and explored as draw agents in forward osmosis
for the first time. Unlike traditional copolymer hydrogels from N-isopropylacrylamide (NIPAm) and
sodium acrylate (SA) or sodium 2-acrylamido-2-methylpropane sulfonate (AMPS), the combination of
thermo-sensitivity and the ionic nature in the hydrogel was achieved by subtle structure design of the
ionic liquid monomers. These polyionic liquid hydrogels were proved to be able to generate
a
reasonable water flux from brackish water, and effectively release the desalinated water at
temperatures above their Lower Critical Solution Temperature (LCST). In addition, a set of rational criteria
for assessing responsive hydrogel draw agents was proposed, for the first time, to improve the hydrogels'
comprehensive performance. Our studies revealed that, both intrinsic factors including hydrogel
structure, membrane structure and hydrogel particle–particle contact conditions as well as extrinsic
parameters such as the hydrogel's area density on the membrane and FO–regeneration cycle duration,
would have a profound impact on the desalination efficacy using hydrogels.
Received 15th September 2015
Accepted 3rd November 2015
DOI: 10.1039/c5ra19018e
www.rsc.org/advances
regeneration process.5,10,12 Undoubtedly, these would bring in
additional equipment and energy costs for FO desalination.
Introduction
Hydrogels are crosslinked polymers with water entrapped
within the network. Directly immersing hydrogel in brackish
water has been proved to only achieve a low desalination effi-
ciency.14,15 However in FO, a membrane separates hydrogels (the
draw agents) from brackish water so that hydrogels were
swollen by desalinated water due to water chemical potential
gradient across the membrane.16,17 The advantages of hydrogels
as draw agents include no draw agent loss or back diffusion,
and most importantly, hydrogels can easily release desalinated
water by shrinking network under external stimulus like
temperature change, therefore exempting any further tedious
membrane-based water polishing processes. Poly-N-iso-
propylacrylamide (PNIPAm) based hydrogels have been inten-
sively studied as draw agents.18 Neat PNIPAm hydrogel can
swily release entrapped water at temperatures above LCST in
regeneration process due to its thermo-sensitivity. However, its
performance in FO process at temperatures below LCST is quite
poor owing to its non-electrolyte nature. Randomly copoly-
merizing NIPAm with electrolyte, i.e., sodium acrylate, would
annihilate thermo-sensitivity of the copolymer hydrogel19 albeit
the water ux generated in FO process increases.18
Desalination by forward osmosis (FO) has recently attracted
much attention owing to its advantages such as operating at low
hydraulic pressure and low membrane fouling propensity.1,2
In the FO process, water permeates automatically through
a membrane driven by a chemical potential gradient to dilute
the draw agent, and fresh water is then separated from the draw
agent in the regeneration process. Among all the draw agents
developed to date,1,3,4 the majority of them are dissolved5–10 or
dispersed11–13 in water to make an aqueous draw solution.
Therefore, in order to obtain high quality water and minimize
draw agent loss, additional processes including nanoltration
(NF), reverse osmosis (RO) or (membrane) distillation are
indispensable as the nal step regardless of any possible
previous separation by external stimuli in the draw agent
aSchool of Materials Science and Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798. E-mail: asxhu@ntu.edu.sg
bNanyang Environmental & Water Research Institute, 1 Cleantech Loop, Singapore
cSchool of Civil & Environmental Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798
dDepartment of Chemical and Biological Engineering, University of Colorado, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra19018e
Organic and inorganic llers including black carbon parti-
cles,20 graphene21 and Fe3O4 (ref. 22) have been incorporated
into the NIPAm–SA copolymer hydrogels to enhance deswelling
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performance. However, since the NIPAm–SA copolymer hydro-
gel matrix is still not thermally responsive, majority of the water
released was in vapor state due to evaporation under heat, albeit
the heat was smartly indirectly generated from solar energy or
magnetic induction. Recently, electro- and magnetic responsive
hydrogels were also tested as draw agents.23,24 All of the hydro-
gels serving as draw agents face the compromise between
performances in FO process and regeneration process.
However, in almost all the papers published on hydrogel draw
agents, the FO process and regeneration process were studied
separately and never evaluated in a comprehensive FO–regen-
eration cyclic continuous process perspective with few excep-
tions.25,26 For example, FO performance was always evaluated
by water ux generated by hydrogels from completely dry state
that cannot be recovered aer regeneration process. Our group
once came up with PNIPAm-based semi-IPN hydrogel to
increase the water ux in FO process without jeopardizing
thermo-sensitivity, and demonstrated preliminary concept in
assessing hydrogel's FO–regeneration comprehensive perfor-
mance.26 However, the relatively low water ux of ꢀ0.2 LMH still
le room for further improvement. What's more, the absence of
a systematic criterion to judge a hydrogel's comprehensive
performance hinders the progress of exploring more competent
hydrogels as draw agents.
Preparation of ionic liquid monomer
Tetrabutylphosphonium p-styrenesulfonate (P4444 SS) was
synthesized by ion exchange. In a typical synthesis, 20 g of tet-
rabutylphosphonium bromide and 17.7 g of sodium p-styr-
enesulfonate were dissolved in 30 ml of deionized (DI) water
and the homogeneous solution was stirred for 24 hours before
80 ml of dichloromethane was added to extract the ionic liquid.
The dichloromethane phase was washed with 20 ml DI water
for three times. The dichloromethane was evaporated under
high vacuum (<0.1 mbar) at room temperature for 72 hours. The
synthesis of tributyl-hexyl phosphonium p-styrenesulfonate
(P4446 SS) was conducted in a similar method. 20 g of tributyl-
hexyl phosphonium bromide and 16.3 g of sodium p-styr-
enesulfonate were dissolved into 30 ml water. The solution
turned into opaque suspension since the target ionic liquid
P4446 SS is hydrophobic. Aer 24 hours stirring, the ionic liquid
was extracted by 80 ml dichloromethane and washed with 20 ml
DI water for three times. The ionic liquid was puried at high
vacuum for 72 hours. Both P4444 SS and P4446 SS are stored
at ꢁ22 ꢂC to avoid polymerization. The structures and NMR
spectra of these two ionic liquids were demonstrated in Fig. 1.
P4444 SS: 1H NMR, dH (400 MHz, CDCl3): 0.89–0.93 (t, 12H,
CH3), 1.43–1.45 (m, 16H, CH2), 2.23–2.25 (m, 8H, CH2), 5.19–
In this work, we discarded the traditional NIPAm–electrolyte
copolymer hydrogel system and managed to blend hydrophi-
licity and hydrophobicity into sole monomer of a thermally
responsive ionic liquid tetrabutylphosphonium p-styrenesulfo-
nate (P4444 SS). Thermally responsive polyionic liquid hydrogel
can be easily synthesized by polymerizing and crosslinking this
novel ionic liquid, and tuned by copolymerizing with other ionic
liquids. We scrutinized the inuences on the water production
rate from both intrinsic factors including hydrogel structure,
membrane structure as well as hydrogel particle–particle
contact conditions and extrinsic parameters such as hydrogel's
area density on membrane and FO–regeneration cycle duration.
In addition, a systematic judging criterion was proposed for the
rst time to help improve hydrogel draw agents' performances.
Experimental
Materials
N-Isopropylacrylamide (NIPAm, $98%) was purchased from
Wako Pure Chemical Industrials Ltd. (Japan). N,N0-Methyl-
enebis(acrylamide) ($99%), polyvinyl alcohol (PVA, Mw
¼
61 000 g molꢁ1), N,N,N0,N0-tetramethylethylenediamine (TEMED,
99%), benzophenone ($99%) and dichloromethane ($99%)
were purchased from Sigma-Aldrich. Ammonium peroxydisulfate
(98%) and polyethylene glycol dimethacrylate (Mw ¼ 198 g molꢁ1
)
were purchased from Alfa Aesar. Tetrabutylphosphonium
bromide (>99%) and sodium p-styrenesulfonate (>98%) were
purchased from Tokyo Chemical Industry Ltd. Tributyl-hexyl
phosphonium bromide was purchased from Sino Rarechem
Labs. All chemicals were used directly without any purication.
Two kinds of FO membranes were received from Hydration
Technology Innovations (HTI) and synthesized according to
a literature,27 respectively.
Fig. 1 Molecular structures and NMR spectra of (upper) tetrabutyl-
phosphonium p-styrenesulfonate (P4444 SS) and (lower) tributylhexy-
lphosphonium p-styrenesulfonate (P4446 SS).
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5.22 (d, 1H, CH), 5.69–5.73 (d, 1H, CH), 6.63–6.68 (q, 1H, CH), temperature range was from 10 to 75 ꢂC at a heating rate of 3 ꢂC
7.26–7.34 (d, 2H, CH), 7.81–7.83 (d, 2H, CH) ppm. P4446 SS: 1H minꢁ1. Dry nitrogen was used for purging at a ow rate of 50 ml
NMR, dH (400 MHz, CDCl3): 0.83–0.85 (t, 3H, CH3), 0.88–0.90 (t, minꢁ1. The swelling ratio, water ux and deswelling perfor-
9H, CH3), 1.19–1.22 (m, 4H, CH2), 1.39–1.43 (m, 16H, CH2), mance of hydrogel were studied by utilizing gravimetric
2.18–2.21 (m, 8H, CH2), 5.18–5.20 (d, 1H, CH), 5.67–5.71 (d, 1H, method. In FO process, dry hydrogel powders or discs (formed
CH), 6.63–6.68 (q, 1H, CH), 7.26–7.34 (d, 2H, CH), 7.81–7.83 (d, by particles under pressure of 200 MPa) were put onto FO
2H, CH) ppm.
membrane with selective layer facing the hydrogel. The FO
process was conducted at 23 ꢃ 1 ꢂC. The water ux Jv (litre per
square meter per hour, LMH) was determined by the hydrogel
weight increment given time and FO membrane area.
Synthesis of poly ionic liquid hydrogels
The PNIPAm and PNIPAm–PVA semi-IPN hydrogels were
synthesized according to our previous publication.26 For the
synthesis of poly ionic liquid hydrogel poly(tetrabutylphosphonium
p-styrenesulfonate) (PP4444 SS), 44.8 mg (0.226 mmol) of
Jv ¼ DW/(ADtr)
where DW (g) is the weight increase of the hydrogel, A is the
membrane area (m2), Dt (h) is the FO duration and r (g Lꢁ1) is
the water density. 2000 ppm NaCl solution was used feed
solution throughout this work.
crosslinker polyethylene glycol dimethacrylate (Mn ¼ 198 g molꢁ1
)
and 20.6 mg (0.113 mmol) of photo-initiator benzophenone
were dissolved in 5 g (11.3 mmol) P4444 SS. The homogenous
solution was purged with N2 for 15 minutes and the polymeri-
zation was conducted under UV (365 nm, 54 mW cmꢁ2) for 2
hours. The copolymer hydrogel of P4444 SS with 20 mol% of
P4446 SS (PP444-6 SS) were synthesized in a similar method.
The crosslinker and photo-initiator used were still 2 mol%
and 1 mol%, respectively with respect to monomers. The poly-
ionic liquid hydrogel syntheses were shown in Fig. 2. All
the hydrogels were allowed to swell to equilibrium to leach
unreacted molecules and then dried until constant weight
before ball milled into powers ready for use. The dimension
of the dried hydrogel particle is less than 30 mm.
Hydrogel's swelling ratio was dened as SR ¼ mwater/mdry
,
where mwater is the weight of absorbed water within hydrogel
while mdry is weight of dry hydrogel. Hydrogel's deswelling was
conducted by putting 2 g swollen hydrogel (SR ꢀ 3.8 g gꢁ1) on
the surface of 60 ꢂC hotplate and monitoring hydrogel's weight
change with deswelling time. The hydrogel was covered by wiper
(kimwipes) to absorb the released liquid water. The wiper was
weighed to determine the amount of liquid water released and it
was replaced whenever the hydrogel's weight was measured.
Results and discussion
FO performances of the hydrogels
Characterizations
Fig. 3 demonstrated the water ux and swelling ratio proles as
function of FO process duration of each hydrogel in powder
form with cellulose triacetate (CTA) membrane. For each
hydrogel as draw agent, the water ux decays and swelling ratio
ascends as time elapsed in FO process. The PNIPAm hydrogel,
doubtlessly, shows the weakest drawing ability that the initial
water ux generated from dry state was only below 0.8 LMH
The morphology of FO membranes was characterized by FESEM
(JEOL, 6340F). The LCST of each hydrogel was determined by
a differential scanning calorimeter (TA Q10 Modulated DSC,
TA Instruments). The hydrogels were swollen to a swelling ratio
of about 3 g gꢁ1 before the DSC measurement. For each DSC
measurement, approximately 20 mg of swollen hydrogel was
placed into
a hermetically sealed aluminum pan. The
Fig. 2 The synthesis route of polyionic liquid hydrogels. Polyethylene Fig. 3 The water flux and swelling ratio profile of each thermally
glycol dimethacrylate is crosslinker, and benzophenone is the photo- responsive hydrogel from dry state. The hydrogels were in powder
initiator. The polymerization and crosslinking were conducted in bulk form contacting CTA membrane from HTI with an area density of
state without solvent.
0.6 g/4.5 cm2.
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and the plateau water ux aer one hour FO was reduced to a reasonable water ux of ꢀ0.5 LMH even at the swelling ratio of
below 0.1 LMH. The incorporation of linear hydrophilic PVA 4 g gꢁ1. The improved FO performance of polyionic liquid
into PNIPAm network slightly improved the water ux, albeit hydrogels may be accredited to ꢀ20 times higher equilibrium
the swelling ratio of the semi-IPN hydrogel was still only about swelling ratios than those of PNIPAm-based hydrogels, as
one (ꢀ50 wt% water) aer seven hours. This is because any shown in Fig. 4 inlet. Higher equilibrium swelling ratio indi-
hydrophilic modications, e.g., incorporating linear polyvinyl cates more clearance for water chemical potential reduction,
alcohol or polysodium acrylate (PSA) into the PNIPAm network, therefore, imparts hydrogel the ability to generate higher water
cannot overwhelm the dominance of PNIPAm presence in ux at the same swelling ratio. In order to fully exert their
order to preserve thermo-sensitivity and thus the water ux potential capacity in FO process, other aspects inuencing
improvement is limited. In order to further increase thermally water diffusion such as hydrogel particle–particle contact
responsive hydrogels' performance in FO process, a new route conditions and FO membrane have also be scrutinized. PP4444
other than copolymerization or semi-IPN to blending hydro- SS and PP444-6 SS demonstrated improved FO performances
phobicity and hydrophilicity in a hydrogel is needed. Replacing when the hydrogel powders were compressed into disc form
the cationic sodium of electrolyte sodium p-styrenesulfonate and cellulose triacetate membrane from HTI was replaced with
with a more hydrophobic phosphonium cation, the synthesized thin lm composite (TFC) membrane as shown in Fig. S1, ESI.†
ionic liquid P4444 SS has achieved hydrophobicity and hydro- This is because that water diffusion from feed solution to hydrogel
philicity coalition in monomer level, and its thermally respon- has been facilitated by using thinner membrane (Fig. S2, ESI†), and
sive homopolymer hydrogel demonstrated signicantly the compressed interstitial volume between particles in the disc
improved performance in FO process. The initially generated improves the inter-particle contact condition and water diffusion.28
water ux was higher than 2 LMH and aer 7 hours the decayed As shown in Fig. 4, aer optimization in water diffusion facilitation,
plateau ux could be maintained at about 0.5 LMH. Similar with PP4444 SS can generate initial water ux of higher than 4 LMH and
PNIPAm, this thermally responsive polyionic liquid hydrogel maintain it at ꢀ2.5 LMH at swelling ratio of 1 g gꢁ1, while PNIPAm–
with much better FO performance can also be copolymerized PVA in powder form can only generate ꢀ0.1 LMH at the same
with other species to tune the properties. As can be seen swelling ratio with CTA membrane.
in Fig. 3, aer copolymerizing with 20 mol% of hydrophobic
P4446 SS, the swelling ratio as well as water ux of PP444-6 SS
decreased due to declined hydrophilicity.
Regeneration performance of hydrogels
An ideal hydrogel as draw agent in FO not only should generate
a high water ux, but can easily and swily release the absorbed
water to make the desalinated water available. The hydrogel's
swelling ratio reduction at 60 ꢂC and LCSTs were shown in Fig. 5
and 6. The subtle balance between hydrophilicity and hydro-
phobicity imparts thermo-sensitivity to the PP4444 SS with
a LCST of 58.7 ꢂC. Thus, the swelling ratio of PP4444 SS can
A hydrogel's performance in FO process can also be quali-
tatively characterized by plotting water ux generated at various
swelling ratios, as shown in Fig. 4. Obviously, hydrogel with
desired FO performance should generate a very high water ux
from dry state and the water ux declines insignicantly as
swelling ratio increases. The PNIPAm-based hydrogels can only
generate initial water ux of ꢀ1 LMH, which quickly falls below
0.1 LMH when hydrogel is swollen to swelling ratio of only
about 1 g gꢁ1. The PP4444 SS, however, not only generates
higher initial water ux of more than 2 LMH, but maintain
reduce from 3.8 g gꢁ1 to 1.7 g gꢁ1 aer 15 minutes at 60 C. It
ꢂ
is worth noting that the LCST peaks of polyionic ꢂliquid hydrogels
are quite broad,29 deswelling temperature of 60 C falls into the
Fig. 4 The swelling ratio and the corresponding generated water flux
correlation of each thermally responsive hydrogel. Inlet was the Fig. 5 Swelling ratio profiles of each thermally responsive hydrogel in
equilibrium swelling ratio of each hydrogel immersing in DI water.
regeneration process.
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in regeneration process are always contradictory. Increasing
the drawing ability in FO process inevitably jeopardizes the
deswelling properties in regeneration process. As can be seen
in Fig. 3 that the FO performance ascends in the order of PNI-
PAm < PNIPAm–PVA < PP444-6 SS < PP4444 SS, while the
performance in regeneration process descends in the order
of PNIPAm > PNIPAm–PVA > PP444-6 SS > PP4444 SS, as seen
in Fig. 5. This compromise also appeared in all the previous
published papers about hydrogels as draw agents,18,20,22,32 where
the FO and dewatering performances were discussed separately
and never considered together to evaluate the overall perfor-
mance in a comprehensive manner. The question of “which
hydrogel performs better” was never answered in those papers.
Fig. 7 is the schematic of the universal criterion of hydrogel's
comprehensive performance as draw agent in FO desalination.
Since all the thermally responsive hydrogels cannot retain their
completely dry state aer deswelling process, the hydrogel
would start the next FO process with a swelling ratio of SR0
obtained aer the deswelling in regeneration process. All the
water absorbed in the FO process with hydrogel starting from
swollen state (SR0) could be released in regeneration process to
make the FO–regeneration cycle reversible, and the water ux
calculated in this criterion depicts the desalinated water
production rate rather than the water absorbing rate in sole FO
process. For example, aer knowing the tting function of the
correlation between swelling ratio and FO time of each hydrogel
(Fig. S4, ESI†), we can easily calculate the increment of swelling
ratio (DSR) from SR0 in FO process whose duration is TFO. The
duration of regeneration process (Tdes.) should be long enough
to restore SR0. Then the desalinated water production rate can
be calculated given the dry hydrogel weight and total duration
of one FO–regeneration cycle.
Fig. 6 The LCSTs and latent heat during phase change of each
hydrogel.
peak region of PP4444 SS and may not be high enough to guarantee
complete phase transition. Instead of increasing deswelling
temperature to achieve faster hydrogel regeneration, declining
the hydrogels' LCSTs would be a more feasible and attractive
method. Aer copolymerizing with 20 mol% more hydrophobic
P4446 SS, the polyionic liquid hydrogel PP444-6 SS has an
acceptable LCST of ꢀ44 ꢂC and deswelling temperature of 60 ꢂC
locates above the entire phase separation peak. Therefore,
PP444-6 SS has better deswelling performance than PP4444 SS
that it can reach a swelling ratio of 0.85 g gꢁ1 aer 15 minute
deswelling. Actually besides the LCST, the deswelling process of
hydrogels can also be depicted by the phase transition heat.
This energy input is an indication of the extent or severity of the
thermally responsive hydrogels' structural change including
bound water to free water transition and hydrophobic associa-
tion, etc.30 Therefore, hydrogels like PSA and its copolymer with
NIPAm assuming no phase transition would make the majority
of water released at high temperature in vapor form, while
typical thermally responsive PNIPAm and PNIPAm–PVA have
very high transition heat inputs indicating very strong phase
transition at LCST, resulting in a quite high liquid water
recovery (>80%) since the water was majorly squeezed out
(Fig. S3, ESI†). Although the polyionic liquid hydrogels may
have a unique phase transition mechanism,31 the rule that
incorporating more hydrophobic segments into polymer back-
bone reduces LCST also applies. The PP444-6 SS demonstrated
not only reduced LCST but increased transition heat compared
with pure PP4444 SS hydrogel. Therefore, the recovered liquid
water fraction of PP444-6 SS was increased to ꢀ50% albeit still
lower than those of PNIPAm-based hydrogels.
Fig. 7 The schematic illustration of criterion for hydrogel's compre-
hensive performance in reversible FO–regeneration cycles. TFO and
Tdes. are the duration of FO process and regeneration process,
respectively. SR0 is swelling ratio obtainable after regeneration, which
can be found in Fig. 5 for each hydrogel. A is the active membrane area
that is 4.5 cm2 in this work. DSR is the swelling ratio increment during
the FO process and mgel is the weight of dry hydrogel. mwater is the
amount of water absorbed that can also be fully released during
regeneration process in one cycle. Therefore, the apparent water flux
Criterion for assessing hydrogel's comprehensive
performance
The required abilities of hydrogels as draw agents to generate
high water ux in FO process and easily release absorbed water (Japp.) calculated in this criterion is the water production rate.
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Table 1 Influence of hydrogel composition, membrane structure and
hydrogel particle–particle contact conditions on comprehensive
performance as draw agentsa
The inuence of hydrogel's composition, membrane structure
and hydrogel contact condition
We use the established criterion to assess the comprehensive
performance of hydrogels as draw agents for the rest of this
paper. If we x one FO–regeneration cycle within one hour, the
swelling ratio proles of each thermally responsive hydrogel
within one FO–regeneration cycle were demonstrated in Fig. 8
PNIPAm– PP4444 PP4444 PP444-6 PP444-6
PNIPAm PVA
SS
SS*
SS
SS*
TFO (min)
Tdes. (min)
52
8
0.17
) 0.134
51
55
52
53
50
9
5
8
7
10
and the key parameters of the criterion were summarized in SR0 (g gꢁ1
)
0.28
0.161
96.6
0.22
1.70
0.409
245.4
0.55
1.70
0.574
344.4
0.77
0.85
0.459
275.4
0.61
0.85
0.702
421.2
0.94
DSR (g gꢁ1
Table 1. The PNIPAm's inherently poor drawing ability makes
the lowest desalinated water production rate of 0.18 LMH
though the hydrogel's swelling in FO process starts from
a almost dry state (SR0 ¼ 0.17 g gꢁ1). The incorporation of
hydrophilic linear PVA only slightly improves the comprehen-
sive performance with a water production rate of 0.22 LMH.
The PP4444 SS can generate the highest water ux from dry state
as mentioned above, however, its relatively poor deswelling
performance resulted in the highest swelling ratio aer desw-
elling (SR0 ¼ 1.70 g gꢁ1), which in turn paralyzed its FO
performance in cyclic perspective. More balanced FO and
regeneration performances achieved by PP444-6 SS make it the
best draw agent for FO desalination that can achieve a desali-
nated water production rate of 0.61 LMH. In addition, as
mentioned, the water diffusion from brackish water into
hydrogels also hinges on the barriers from membrane structure
and hydrogel particle–particle contact condition. Aer using
a thinner membrane and improving the hydrogel contact
condition by compressing it into disc form, the water produc-
tion rate of PP444-6 SS can be further enhanced to 0.94 LMH.
mwater (mg) 80.4
Japp. (LMH) 0.18
a
The entire one FO–regeneration duration is one hour. Hydrogels
without asteroid mark were using CTA membrane in powder form.
Hydrogels with asteroid mark were using TFC membrane in disc form.
with a smaller area density would increase faster than that of
a hydrogel with a larger one. This has been reported and was
probably due to the inuence of water diffusion path length
through the hydrogel.26,28 On a xed membrane area and within
xed FO time, less amount of hydrogel would lead to larger
swelling ratio increment, while the amount of water is calcu-
lated by the swelling ratio increment multiplying weight of dry
hydrogel. Therefore, there exists a best hydrogel area density
that maximizes the amount of water produced. Again, we tted
the correlation between swelling ratio and FO time of PP444-6
SS (in disc form with TFC membrane) with different area
densities (Fig. S6, ESI†) and xed one FO–regeneration cycle in
one hour to compare the desalinated water production rate.
The swelling ratio proles within one FO–regeneration cycle
was demonstrated in Fig. 9 and the key parameters in the
criterion were summarized in Table 2. It can been observed that
although the area density of 0.09 g/4.5 cm2 can generate the
highest swelling ratio increment, yet the area density of 0.2 g/
4.5 cm2 produces the largest amount of water within one FO–
regeneration cycle and the highest desalinated water
The inuence of hydrogel area density and one FO–
regeneration cycle duration
The hydrogel PP444-6 SS with the best comprehensive perfor-
mance among all the thermally responsive hydrogels was
selected to achieve a higher desalinated water production rate
by tuning the hydrogel area density (weight of hydrogel/area of
membrane). Fig. S5† shows that the swelling ratio of a hydrogel
Fig. 8 Comparison of comprehensive performance of each hydrogel Fig. 9 The influence of hydrogel area density on the comprehensive
in one reversible FO–regeneration cycle within one hour. Area density performance. The hydrogel was PP444-6 SS in disc form contacting
was 0.6 g/4.5 cm2.
TFC membrane.
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Table 2 Influence of hydrogel area density on the water production Table 3 The comprehensive performance of PP444-6 SS with one
rate of PP444-6 SSa FO–regeneration cycle fixed within 30 minutesa
1.0 g/4.5
0.6 g/4.5
cm2
0.2 g/4.5
cm2
0.09 g/4.5
cm2
1.0 g/4.5
cm2
0.6 g/4.5
cm2
0.2 g/4.5
cm2
0.09 g/4.5
cm2
cm2
TFO (min)
Tdes. (min)
56
4
0.85
0.199
199.0
0.44
50
10
0.85
0.702
421.2
0.94
46
14
0.85
2.337
467.4
1.04
42
18
0.85
3.568
321.1
0.71
TFO (min)
Tdes. (min)
27
3
0.85
0.099
99.0
0.44
24
6
0.85
0.380
228.0
1.01
18
12
0.85
1.371
274.2
1.22
16
14
0.85
2.249
202.4
0.90
SR0 (g gꢁ1
)
SR0 (g gꢁ1
)
DSR (g gꢁ1
)
DSR (g gꢁ1
)
mwater (mg)
Japp. (LMH)
mwater (mg)
Japp. (LMH)
a
a
The hydrogel was in disc form contacting TFC membrane.
The hydrogel was in disc form contacting TFC membrane.
production rate of 1.04 LMH. The reduction of area density draw agent back diffusion and no ensuing water polishing
from 0.6 g/4.5 cm2 to 0.2 g/4.5 cm2 not only increased the steps, another merit of thermally responsive hydrogels as draw
desalinated water production rate, but reduced the amount of agents is that theoretically only low grade thermal energy from
required hydrogel that can be a decline in hydrogel synthesis waste heat or solar source is needed in desalination if electricity
cost for possible future scale-up.
consumption for uid motion is neglected. Although this water
Aer the optimization of parameters including hydrogel production rate of 1.22 LMH from polyionic liquid hydrogel is
composition, water diffusion conditions and area density, one 5 times higher than that from PNIPAm-based hydrogel, admit-
FO–regeneration cycle duration is another important parameter tedly, it is still comparatively lower than the water ux generated
for study to aim at the highest possible water production rate. from other draw agent with smaller molecular weights. This
As shown in Fig. 10, we reduced the duration of one FO– is probably originated from severe dilutive concentration
regeneration cycle from one hour to 30 minutes trying to make polarization, and improving the hydrogel–hydrogel and hydro-
more use of the steep section in the swelling ratio–FO time gel–membrane contact conditions may further enhance the
curve to increase water ux. Similarly as seen from the key performance of hydrogels as draw agent. Interestingly, it was
parameters summarized in Table 3, the area density of 0.2 g/ recently proven that shrinking hydrogel's dimension to less
4.5 cm2 still generates the highest water production rate of than 0.3 mm can produce a much higher water ux in FO
1.22 LMH. The distribution of 18 minutes in FO process and process.33 We believe that further optimizing the fabrication
12 minutes in regeneration process in a 30 mintue cycle is also of thermally responsive polyionic liquid hydrogel would
a macroscopic indication of a well balanced performance ach- upgrade its performance, and hopefully make it more compe-
ieved by the PP444-6 SS hydrogel.
tent to other candidates as draw agent.
It is worth noting that the water production rate introduced
in this work is different with the water ux values measured in
FO process from other studies. Besides the advantages of no
Conclusions
The thermally responsive polyionic liquid hydrogels have been
synthesized and for the rst time scrutinized as the draw agents
in forward osmosis desalination. They can absorb water from
feed solution faster than the PNIPAm-based hydrogels at room
temperature, and squeeze absorbed water out at temperatures
higher than the LCST. The thermally responsive ionic liquid can
copolymerize with other species to subtly tune the balance of
performances in FO and regeneration processes. In addition,
other factors that can inuence the FO performances including
the contact conditions between hydrogel particles as well as FO
membrane structure were also improved. Furthermore, we for
the rst time proposed a criterion to assess a hydrogel's
comprehensive performance as draw agent in FO desalination,
which is by comparing the desalinated water production rate in
reversible FO–regeneration cycles. With this criterion, tuning
other extrinsic parameters including hydrogel area density and
one FO–regeneration duration were also found possible to
improve water production rate. Compared with the PNIPAm-
based hydrogels, we have nally increased the water produc-
tion rate by a factor of 5 aer tuning these parameters. We
Fig. 10 The comprehensive performance of PP444-6 when one FO–
regeneration cycle was fixed within 30 minutes. PP444-6 SS was used
in disc form contacting TFC membrane.
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Acknowledgements
The authors would like to thank the Environment and Water
Industry Council (EWI) of Singapore for support via ECMG and
SMTC of the Nanyang Environment and Water Research Insti-
tute (NEWRI) of Nanyang Technological University. Graduate
scholarship awarded by Nanyang Technological University to
the author of CYF is also gratefully acknowledged.
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97150 | RSC Adv., 2015, 5, 97143–97150
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