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Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase
Journal Pre-proof Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase Yusak Hartanto, Maxime Corvilain, Hanne Mariën, Julie Janssen, Ivo F.J. Vankelecom PII:
S0376-7388(19)33503-3
DOI:
https://doi.org/10.1016/j.memsci.2020.117869
Reference:
MEMSCI 117869
To appear in:
Journal of Membrane Science
Received Date: 15 November 2019 Revised Date:
8 January 2020
Accepted Date: 19 January 2020
Please cite this article as: Y. Hartanto, M. Corvilain, H. Mariën, J. Janssen, I.F.J. Vankelecom, Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117869. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement Yusak Hartanto: Conceptualization, Methodology, Validation, Formal analysis, Writing – Original Draft, Writing – Review & Editing, Visualization Maxime Corvilain: Conceptualization, Methodology, Formal analysis, Writing – Original Draft, Visualization Hanne Mariën: Conceptualization, Methodology Julie Janssen: Investigation, Visualization Ivo F.J. Vankelecom: Resources, Supervision, Writing – Review & Editing
Graphical Abstract
Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase
Yusak Hartantoa, Maxime Corvilaina, Hanne Mariëna, Julie Janssena, and Ivo F.J. Vankelecoma*
a
cMACS - Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy for
Sustainable Solutions, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Corresponding author: [email protected]
1
Abstract Ionic liquids (ILs) were used as the organic reagent phase in interfacial polymerization (IFP) to prepare thin film composite (TFC) membranes with a crosslinked polyimide (PI) as support for use in forward osmosis (FO). In a very straightforward and environmentally benign method, phase inversion, PI cross-linking, and amine monomer impregnation steps were all performed simultaneously. To prepare suitable supports, PI concentration and solvent/co-solvent (NMP/THF) ratios in the casting solution were optimized. The crosslinking of the support allows contact with a broad range of organic reagent phases during IFP. The effect of triethylamine and sodium dodecyl sulphate as additives in the coagulation bath on the performance of the final membranes was studied. The best hexanebased TFC-FO membrane was obtained on supports prepared from 14 wt.% PI solutions containing a 3:1 ratio of NMP:THF and in the presence of 0.1 wt. % SDS. The IL, 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) was selected as IL organic phase to replace hexane. The resultant TFC membrane had a performance similar to that of a TFC synthesized using hexane as organic phase but did not require any additives. To reduce IL costs, a mixture of ([C4mim][Tf2N]/hexyl acetate was applied in which the optimal monomer concentrations were 0.5 wt.% MPD and 0.3 w/v.% TMC. This TFC membrane outperformed all other membranes in this work with a selectivity (Js/Jw) of 0.25 g/L and a normalized water flux (Jw/∆ ) of 0.41 LMH/bar for a 0.5 M NaCl draw solution in FO mode. Keywords: interfacial polymerization; polyimide support; ionic liquids; forward osmosis; thin-film composite
2
1. Introduction Forward osmosis (FO) is an attractive membrane separation technology which relies on the osmotic pressure difference as separation driving force. Compared to pressure driven membrane process, there are several potential advantages for FO, such as low energy cost [1], reduced fouling propensity [2], and higher product yield [3]. Thin film composite (TFC) membranes are current state-of-the-art FO membranes. These membranes consist of a thin selective polyamide (PA) layer deposited on top of a porous support via interfacial polymerization (IFP) [4–6]. Current IFP makes use of organic solvents, such as hexane, heptane or isopar, which have serious environmental impacts and are dangerous to human health [7,8]. Furthermore, evaporation is responsible for hazards as well as for a significant loss of these solvents during membrane manufacturing. Ionic liquids (ILs) have emerged as green solvents with a low vapor pressure. They have been investigated in many research fields, such as polymer synthesis [9], catalysis [10], and batteries [11]. ILs have also been applied as casting solvents for phase inversion membranes [12], as additives in IFP [13], as FO draw agents [14] and to prepare gas separation membranes [15]. Water-immiscible ILs, like 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]), were recently introduced as organic phase to replace neurotoxic hexane in IFP of PA-based TFC-membranes [16]. The required m-phenylenediamine (MPD) concentration during membrane preparation was 20 times lower than in conventional IFP-membrane preparation. Furthermore, typical IFP additives, such as sodium dodecylsulphate (SDS) or triethylamine (TEA), were not needed to achieve comparable FO membrane performance. In contrast to conventional organic phases, the IL phase was less hazardous, easily recycled and potentially applied via spraying. This all drastically reduces the solvent intensity of the IL-based IFP process. Moreover, fouling
3
propensity of the membranes was drastically reduced and flux/selectivity performance was improved [17]. In this study, IL-assisted IFP was employed to prepare FO membranes. Polyimide (PI) was chosen as support material due to its excellent chemical, thermal, and mechanical stability. To prepare suitable supports for FO, an optimization of the phase inversion process was first performed. Furthermore, a simplified (SIM) method simultaneously combining phase inversion, PI crosslinking and m-phenylenediamine impregnation was employed to prepare the TFC FO membrane on a crosslinked PI support to create a fast and easy preparation method [18]. In addition, the crosslinking of the support guarantees excellent membrane stability in a wide range of solvents, including ionic liquids or IL/solvent mixtures, during IFP. Thus, leaving more freedom to optimize the IFP in the new reagent system. In addition, it would potentially allow for the application of very powerful solvent activation [19,20]. The hexane phase was substituted by [C4mim][Tf2N] as well as its mixture with a cheaper and environmentally friendly, solvent, i.e. hexyl acetate, to further improve the FO performance of the membrane and to reduce the cost of the IL-based TFC membrane preparation.
2. Experimental 2.1.
Materials PI (Matrimid 9725 US) was purchased from Huntsman. Hexanediamine (HDA, 99.5%,
Acros), m-phenylenediamine (MPD, 99+%, Acros), triethylamine (TEA, 99+%, Sigma– Aldrich), sodium dodecyl sulphate (SDS, 99%, Acros), and 1,3,5-benzenetricarbonyl chloride (TMC, 98%, Acros) were used for TFC-membrane synthesis. N-Methylpyrrolidone (NMP, ≥99%, Honeywel Riedel-de-Haën), tetrahydrofuran (THF, 99.9+%, Sigma–Aldrich), nhexane (99+%, Chem-Lab), anisole (99%, Sigma Aldrich), methyl-tert-butylether, (98%, Sigma Aldrich), hexyl acetate (99%, Acros), butyl butyrate (98%, Acros), nonyl aldehyde
4
(95%, Acros), 3-phenylpropionaldehyde (95%, Acros), 3-octanone (99%, Sigma-Aldrich), 2nonanone (99%, Sigma-Aldrich), and 2-decanone (98%, Acros) were used as received. 1butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide
(C4MIM
Tf2N),
(99+%,
Iolitec) was dried for 16 h at 80 °C under vacuum before use. Sodium chloride (99.8%, VWR) was used to prepare feed and draw solutions in RO and FO evaluation. 2.2.
Synthesis of crosslinked PI supports PI was dried overnight in an oven at 100°C. Polymer casting solutions were prepared
by stirring PI (10-15 wt%) in NMP or PI (14 wt.%) in different NMP/THF ratios (3:1, 4:1, 5:1 and 9:1). The solutions were then left unstirred overnight for degassing. The support was prepared using the phase inversion method [21]. The PI-solution was cast on a glass plate with a casting knife having a height of 200 µm using an automatic casting device (Braive Instruments, Belgium). Before the film was immersed in a coagulation bath, an evaporation time of 30 s was applied. The temperature and humidity conditions were 20 oC and 30%, respectively. The coagulation bath consisted of MPD (0.1–2.0 wt%), and HDA (0.5 wt%) in Milli-Q water (1) to perform phase inversion, (2) to impregnate the support with MPD and (3) to cross-link it with HDA simultaneously, as described elsewhere [22]. Additives, such as base TEA (2.0 wt%) and surfactant SDS (0.1 wt%), were also introduced in the coagulation bath when relevant. The resultant support was taken out of the coagulation bath after 5 min to perform the IFP. 2.3.
Preparation of PA-based TFC-membranes The impregnated and cross-linked support was put on a metal plate and the excess
amine solution was removed by with rubber wiper. Subsequently, a solution of TMC (0.10.5% w/v) in hexane, C4MIM Tf2N or a 50/50 mixture of C4MIM Tf2N/hexylacetate was poured on top of the support. After 1 min reaction, the excess organic solution was discharged by rinsing the TFC membrane with hexane for the hexane-based TMC solution or
5
with acetone for the case of the IL-based TMC solutions. Afterwards, the TFC membrane was dried for 1 min in air at room temperature before storing it in deionized (DI) water. 2.4.
Membrane characterization The morphology of the PA top-layer and the pore structure of the support were
characterized via scanning electron microscopy (SEM, JEOL JSM-6010LV). The samples were coated with a conductive gold/palladium layer (JEOL JFC-1300 auto fine coater) before imaging. Transmission electron microscopy (TEM) was used to study the PA selective layer in more detail. The membrane samples were embedded in an araldite resin (Polyscience) and subsequently cut into very thin slices (±70 nm, Reichert Ultracut E microtome). The measurements were performed with a JEOL ARM-200F at 80 kV. An average thickness of the PA top layer was calculated by measuring at least 30 equidistant spots along the cross section of PA layer using a public domain image-processing program (ImageJ 1.46), developed by National Institute of Mental Health, Bethesda, Maryland, USA. Atomic force miscroscopy (AFM) was employed to determine the roughness of the top layer of the TFC membranes. The measurements were performed with an Agilent 5500 AFM with NCSHR probes of NanoAndMore GmbH in tapping mode. The cantilever consists of Si and has a spring constant of 40-50 Nm-1 and a nominal tip apex radius of less than 5 nm. The samples were measured over an area of 25 mm2 and analyzed with the WSxM software [23]. The reported root mean square (RMS) roughness was average of from three different locations. 2.5.
RO and pure water permeances (PWP) In order to determine the intrinsic properties of the PA layer, such as salt rejection
(R, %), water (A, L m-2 h-1 bar-1(LMH/bar)), and solute permeance (B, LMH), the prepared TFC membranes were tested in a RO experiment with a feed solution of 2 g/l NaCl at 14 bar. The RO experiments were carried out in a dead-end, high-throughput filtration equipment to allow for a statistical analysis of the data obtained [24]. Eight membranes can be tested
6
simultaneously in this particular set-up with an effective membrane surface area of 1.77 cm2 each. The filtration cell was stirred to minimize concentration polarization. Permeates were collected after a period of time (∆t). R, A, and B were calculated using Eq. (2) – (4), respectively.
% = 1−
=
=
(1)
× 100
(2) ∆ ∆ −∆
1−
∆ −∆
=
∆
1−
(3)
Cf is the solute concentration in the feed, Cp is the solute concentration in the permeate, V is the volume of permeate (L), Am is the effective membrane area (m2), ∆t is the permeation time (h), ∆P is the applied pressure, and ∆π is the osmotic pressure difference across the membrane. The values of retention and permeance are averages of two duplicate tests of three different samples, randomly cut from the same membrane sheet. Pure water permeances (L/ (m² h bar)) were measured in a dead-end cell at a pressure of 1 bar, with an effective membrane surface area of 1.77 cm². 2.6.
FO The TFC membranes were placed in a custom-made, high-throughput, lab-scale
cross-flow FO cell that allows the simultaneous testing of 4 membranes with different feed and draw solutions. The cross-flow cell consisted of 4 rectangular channels with a 55 mm length, a 15 mm width, and a 2 mm depth, each containing a membrane with an active surface of 8.25 cm2. A 0.5 M NaCl solution was used as draw solution and DI water was used as feed solution, which created a theoretical bulk osmotic pressure difference of 22 bar. Membranes were tested in both FO (active layer facing feed solution) and PRO (active layer
7
facing draw solution) configuration under a co-current flow of 250 mL/min with mesh spacers. The Reynolds number was in the range of 300 – 400, reflecting laminar flow. ECP moduli were calculated according to McCutcheon et al. [25] and these were around 0.9 – 1, indicating the effective mixing and good hydrodynamic condition of the set-up. The absolute volumetric change ∆V (L) of the feed with time was recorded. The experiment was carried out for 5 h and the average water flux Jw (LMH) was estimated by: =
∆
(5) ∆
The concentration of the feed solution was measured at different times and the reverse salt flux Js (g m-2 h-1) was calculated by:
=
− ∆
(6)
where C0 and V0 are the initial salt concentration and volume of feed, respectively; Ct and Vt are the salt concentration and the volume of feed at time t, respectively.
3. Results and discussion First of all, an appropriate support layer had to be prepared, often which the actual IFP from IL-phases could be studied. 3.1.
Effect of polymer concentration in the casting solution The concentration of the polymer casting solution largely defines the rate of the de-
mixing during the phase-inversion and the final porosity of the support [26,27]; a high polymer concentration impedes the diffusion of solvent and non-solvent, delaying the demixing and resulting in a denser zone at the top part of the support [28]. To realize adequate PA layer formation, an optimal PI concentration was sought in the 10-15 wt.% range. In this range, the support kept a finger-like pore morphology and no differences were seen in the typical ridge-and-valley structures of the final TFC-membrane surfaces, as shown in Fig.1.
8
However, the PA layer thickness was lower for an 11 wt% polymer concentration (48.9 ± 30.9 nm) than for 14 wt% (145.5 ± 62.0 nm), in agreement with previous findings that a lower surface porosity of the support led to larger pores, thus producing a rougher and thicker PA layer [29].
Fig. 1 (a) SEM images of cross-sections of TFC-membranes, (b) SEM images of PA surfaces, (c) TEM images of cross-sections of TFC-membranes, prepared from 11 wt% (top row) and 14 wt% (bottom row) PI. Arrows indicate the PA-layers.
Both in FO and PRO mode, (Fig. 2) the water flux and reverse salt flux decreased and levelled off at the higher PI concentration in the casting solution, consistent with results from PWP and RO experiments (Fig. S1). PWP tends to decrease as the polymer casting solution concentration increases from 1324 LMH/bar to 131 LMH/bar, while the salt rejection showed the opposite trend. The water fluxes in PRO mode were consistently higher (11.2 – 13.7 LMH) compared to the FO mode (8.4 – 12.5 LMH) due to the ICP phenomenon, while the reverse salt flux was slightly higher in PRO mode (3.9 gMH) than in FO mode (2.3 gMH) at higher polymer casting solution concentration [30]. Further analysis of the RO and FO performance (Table S1) clearly showed that the lower polymer casting concentration resulted in a significantly lower membrane selectivity compared to the membranes prepared from higher polymer casting concentrations, as indicated by their B/A and Js/Jw values. 9
Fig. 2 The effect of PI concentration in the casting solution on water fluxes and reverse salt fluxes of FO TFC-membranes evaluated in (a) FO and (b) PRO mode. Test conditions: DI water as feed and 0.5 M NaCl as draw solution.
3.2.
Effect of THF as co-solvent in the polymer casting solution Since a casting solution concentration of 14 wt.% showed the best water flux and
reverse salt flux, this concentration was selected to study the effect of co-solvent addition. The SEM and TEM images of the cross-sections and top-surfaces of the TFC-membranes with high (9:1) and low (3:1) NMP/THF ratios are shown in Fig. 3. Both porous supports displayed a finger-like morphology with a thickness of 100 ± 5.0 µm. As anticipated, the support prepared by adding more THF to the casting solution had a denser upper layer than the one prepared with less THF. This is due to the evaporation of THF which causes a localized increase of polymer concentration in the upper layer of the cast polymer film during phase inversion [31]. While both TFC-membrane surfaces exhibited a typical ridge-andvalley structure for the PA-layer, a thicker layer was formed on top of the support prepared at the lower NMP/THF ratio: 65.9 ± 41.8 nm and 145.5 ± 62.0 nm, respectively. Besides the thicker PA layer, the membrane prepared with the higher THF content in the casting solution also had a visually rougher morphology, as confirmed by AFM images of the TFC membranes (Fig. S2). The root mean square roughness (RMSR) of the membrane support was lower for the higher NMP:THF ratio (RMSR of 34.97 ± 1.12 nm for 9:1 ratio vs. 58.03 ± 6.73 nm for 3:1 ratio). The thicker and rougher PA morphology can be explained by the
10
decrease of the surface pore diameter as a result of the localized increase of polymer concentration during membrane preparation. This consequently results in more intense eruptions of the diamine solution during IFP, which in turns yields thicker and rougher PA layers [29].
Fig. 3 SEM images of (a) cross-sections, (b) polyamide surfaces and (c) TEM images of crosssections of TFC-membranes prepared with different NMP:THF ratios of 9:1 (top row) and 3:1 (bottom row). Arrows indicate the PA layer.
In RO, (Fig. 4) the water permeance decreased from 0.42 LMH/bar to 0.23 LMH/bar as the NMP/THF ratio increased, while the salt rejection displayed the opposite trend, rising from 61.5% to 92.0%. This was due to the formation of a denser upper layer of the support and a thicker PA layer which acted as additional membrane resistance for the support prepared with the higher NMP/THF ratio. In FO, water fluxes for both TFC membranes were lower than in PRO mode due to the ICP. The water fluxes for FO mode for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 9.56 LMH and 9.64 LMH, respectively. On the other hand, the water fluxes for PRO mode for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 12.46 LMH and 12.02, respectively. The reverse salt flux of the TFC membrane prepared from a support with the higher NMP/THF 11
ratio was higher (5.21 gMH) than from the support prepared with a lower ratio of NMP/THF (2.37 gMH) in FO mode, which was in agreement to the RO experiment. However, there was no significant difference of reverse salt fluxes for both membranes in PRO mode. The reverse salt fluxes for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 3.75 gMH and 4.10 gMH, respectively. Further analysis of RO and FO performance parameters presented in Table S2 shows that increasing the amount of THF improves the TFC membrane selectivity, as confirmed by the values of the B/A ratio for RO, and specific solute selectivity (Js/Jw) and normalized water flux (Jw/∆π) for FO.
Fig. 4 (a) RO characterization (test conditions: 2 g/L NaCl and ∆P = 14 bar), and FO characterization (test conditions: DI water as feed and 0.5 M NaCl as draw solution): (b) FO mode and (c) PRO mode, of TFC membranes based on supports with different NMP/THF ratio.
3.3.
Effect of additives in the coagulation bath IFP-additives can be introduced in the non-solvent bath to further improve the
performance of the final membranes. Sodium dodecyl sulphate (SDS) can act as a surfactant 12
during IFP. It contributes to lowering the surface tension between the aqueous and organic phases, and increases the wettability of the support, which promotes the transport of the amine monomer. Triethylamine (TEA) can improve the IFP reaction by acting as a proton acceptor and as a catalyst. No significant difference can be observed between the pristine TFC and the one with SDS addition as shown in Fig. 5. However, a more nodular PA layer was present when SDS and TEA were added simultaneously to the coagulation bath. This nodular morphology results from the alteration of the interfacial reaction zone and the formation of a less defective PA layer [32].
Fig. 5 Surface morphology of final TFC membranes without additives, with 0.1 wt% SDS, and with 0.1wt% SDS + 2.0 wt% TEA.
The performance of TFC membranes evaluated in RO and FO is shown in Fig. 6. In RO, adding SDS to the coagulation bath resulted in a slight increase in water permeance from 0.23 LMH/bar to 0.27 LMH/bar with a slight increase of salt rejection from 92.0 % to 96.2 %. The addition of SDS might cause the PA to be formed at the water-hexane interface, but also in the organic layer, which could lead to PA having more free volume in comparison to the pristine membrane [33]. Simultaneous addition of SDS and TEA to the coagulation bath resulted in a further increase in water permeance to 0.39 LMH/bar followed with a salt rejection decrease to 93.4%. TEA acts as an acylation catalyst by capturing proton formed as a side product of polycondensation between MPD and TMC. This will promote IFP, resulting in less defective PA layer. When TEA is combined with SDS addition, the PA layer will have less defect while maintaining certain free volume, resulting in a synergistic enhancement for both membrane permeance and salt rejection. The salt selectivity trend observed in RO,
13
however, was not observed in either FO or PRO mode. This could be due to the influence of applied pressure in RO which compressed the PA layer resulting in higher salt rejections. In the absence of an applied hydraulic pressure in FO, the PA layer would then be more permeable to salt. The FO performance presented in Table S3 further confirmed that the membrane prepared with SDS addition has the best selectivity and normalized water flux.
Fig. 6 (a) RO evaluation (2 g/l NaCl, ∆P of 14 bar) and FO evaluation: (b) FO mode and (c) PRO mode (DI water as feed, 0.5 M NaCl as draw solution) of TFC-membranes without additives, with 0.1 wt% SDS, and with 0.1wt% SDS + 2.0 wt% TEA in the coagulation bath.
3.4.
Ionic liquid as organic phase for IFP Hexane and C4mimTf2N have very different physical properties, drastically altering the
conditions in the reaction zone (Table S4). As a result, different monomer concentrations are required for the formation of an optimal PA layer. The optimum monomer concentration for RO and SRNF purposes, based on a previous study, was selected as starting point (i.e. 0.1 wt% MPD in water and 0.5 w/v% TMC in IL compared to 2 wt% MPD in water and 0.1
14
w/v%TMC in hexane) [16]. The lower MPD concentration in the IL phase was the result of an enhanced transport of the MPD monomer across the water/IL interface to the reaction zone due to the amphiphilic character of the IL.
Fig. 7 FO performance of TFC-membranes: (a) FO Mode and (b) PRO Mode, with hexane or [C4mim][Tf2N] as organic phase (0.5 M NaCl as draw solution, DI water as feed).
In FO mode, (Fig. 7) both TFC membranes had similar water fluxes (8.85 LMH for hexane-based membrane and 8.88 LMH for IL-based membrane) but the membrane which was interfacially polymerized using IL as reagent phase had a slightly higher reverse flux (2.89 gMH) than the hexane-based membrane (2.34 gMH). In PRO mode, on the other hand, the hexane-based membrane had a higher water and reverse salt flux. The water fluxes of hexane-based membrane and IL-based membrane were 12.0 LMH and 10.3 LMH, respectively while the reverse salt fluxes of hexane-based membrane and IL-based membrane were 4.1 gMH and 3.4 gMH, respectively. 3.5.
IL/solvent mixtures as reagent phase for IFP To further improve the performance of this IL-based membrane and to further lower
preparation cost, a mixture of IL with an organic solvent was used. The organic solvent should be miscible with the IL while still immiscible with water, should contain no reactive functional groups, and be less harmful than hexane. Table 1 shows the properties of different solvents employed in bulk IFP tests to investigate the formation of a stable PA film. The
15
addition of an apolar organic solvent to the IL would increase the interfacial tension and would decrease the viscosity of the organic phase. 0.5 wt% MPD concentration in water and 0.4 w/v% TMC concentration in IL/organic solvent were chosen as starting point. The mixture with hexyl acetate resulted in a stable PA film and a 50/50 IL/hexylacetate ratio was chosen for TFC-membrane preparation, since this solvent also scores well with respect to cost, as well as to safety, environmental and safety scores. Table 1 Selection of organic solvent for mixing with IL Solubility
Solubility
Safety
Health
Env.
Cost
Formation
in water
in IL
score*
score*
Score*
(€/l)**
PA film
IL
-
-
-
-
-
-
-
Anisole
-
+
4
1
5
52
-
Methyl-tert-butylether
-
+
8
3
5
124
-
Hexyl acetate
-
+
3
1
5
83
Stable
Butyl butyrate
-
+
3
1
5
51
Unstable
Nonyl aldehyde
-
-
1
2
5
280
-
3-Phenylpropionaldehyde
-
+
1
2
3
661
-
3-Octanone
-
+
3
1
5
41
-
2-Nonanone
-
+
1
2
5
187
Stable
2-Decanone
-
+
1
1
7
-
Unstable
Hexane
-
-
8
7
7
-
-
Solvents*
* The hazard rating based on CHEM21 solvent guide calculation. Color legends: green = recommended, yellow = problematic and red = hazardous [34]. **Costs are obviously approximation, based on purchase of small lab-scale quantities.
Next, a range of MPD and TMC monomer concentrations was evaluated and the surface morphologies of resulting PA layers are shown in Fig. 8. The membranes showed a typical ridge-and-valley morphology at low and moderate MPD concentrations combined with various TMC concentrations, but not using high MPD concentrations. Membranes D, E, and F combined a slightly higher retention (~ 94.5%) with a significantly higher permeance (~0.40 LMH/bar) in RO (Fig. 9) than the hexane-based membrane (0.23 LMH/bar for water permeance and 92.0% for salt rejection), thus confirming the great potential of this mixed IL/organic solvent reaction system. The MPD:TMC ratio in the reaction zone determines the
16
degree of cross-linking of the PA layer. Membranes A, B, and C were prepared with a 0.1 wt% MPD concentration (low MPD:TMC ratio), which resulted in high retentions (94.3 % 96.7%) and low permeances (0.13 LMH/bar – 0.25 LMH/bar), while membranes G, H, and I, synthesized with 1 wt. % MPD (high MPD:TMC ratio), had low retentions (31.4 % - 73.6 %) and high permeances (0.42 LMH/bar – 0.63 LMH/bar). The MPD concentration thus played the most important role in determining the RO performance.
Fig. 8 SEM images of TFC membrane surfaces prepared from mixtures of IL/hexyl acetate with different MPD and TMC concentrations.
The best membranes (D, E, and F) from the in RO evaluation were further tested in FO. Membrane D displayed a higher water flux (9.1 LMH) while maintaining similar reverse salt flux (2.30 gMH) as the reverse salt flux for hexane-based membrane (2.34 gMH). Membranes E and F, prepared using a higher TMC concentration, showed lower water fluxes than membrane D with slightly higher reverse salt flux for membrane F. The water fluxes of membrane E and membrane F were 7.90 LMH and 7.53 LMH, respectively while the reverse salt fluxes of membrane E and membrane F were 2.31 gMH and 2.72 gMH, respectively. In
17
PRO mode (Fig. 9c), membrane D had, by far, the best performance in terms of water flux (17.0 LMH) and reverse salt flux (3.57 gMH), while membranes E and F had lower fluxes and higher reverse salt fluxes. The water fluxes of membrane E and membrane F were 12.6 LMH and 16.2 LMH, respectively while the reverse salt fluxes of membrane E and membrane F were 5.70 gMH and 7.40 gMH, respectively. Recalculating these results in Table 2, it is clearly shown that the efficiency in RO (B/A ratio) was best for membranes synthesized with the IL as organic phase, while the specific solute flux selectivity (Js/Jw) of membrane D was similar to the hexane-based TFC and lower than the IL-based TFC. Furthermore, membrane D exhibited the highest normalized water flux (0.41 LMH/bar) and was therefore the best FO membrane in this study.
Fig. 9 (a) RO performance (2 g/l NaCl, ∆P of 14 bar) and FO performance (DI water as feed, 0.5 M NaCl as draw solution) of TFC-membranes in: (b) FO mode and (c) PRO mode, using hexane and
18
50/50 mixture of IL/hexylacetate as the organic phases with different TMC and MPD concentrations. Membrane codes refer to the annotations in Fig. 8. Table 2 B/A ratio, specific solute flux selectivity (Js/Jw), and normalized water flux (Jw/∆π) (FO mode, 0.5 M NaCl, DI water as feed) of TFC membrane prepared using different organic phase during interfacial polymerization.
Hexane
B/A (bar) 1.10
Js/Jw (g/L) 0.24
Jw/∆π (LMH/bar) 0.38
IL only
0.42
0.30
0.39
D
0.72
0.25
0.41
E
0.59
0.29
0.36
F
0.74
0.36
0.34
Organic Phase
IL + hexyl acetate
3.6.
Comparison of membrane performance with literature The newly developed TFC membranes are compared to recently synthesized TFC FO
membranes from literature, as shown in Table 3. Although TFC FO membranes with polyimide support in this work had lower water flux than commercial TFC FO membranes from HTI, the solute specific selectivity of the former membranes is lower than the later ones. By examining the solute specific selectivity (Js/Jw), the TFC FO membrane prepared from an IL/hexyl acetate mixture led to the best performance (third entry). Table 3 Comparison of the performance of TFC FO membranes from this work with recent literature data Membrane
Orientation
Jw (LMH)
Js (gMH)
Js/Jw (g/L)
FO
8.9
2.3
0.26
PRO
12.0
4.1
0.34
FO
8.9
2.9
0.33
PRO
10.3
3.4
0.33
FO
9.1
2.3
0.25
PRO
17.0
3.6
0.21
FO
13.9
26.8
1.93
Ref
TFC – XLPI (Hexane)
TFC – XLPI (IL) This work
TFC – XLPI (D) TFC – HTI
19
PRO
21.3
15.0
0.70
FO
11.8
6.9
0.58
PRO
14.6
10.2
0.69
FO
11.0
11.7
1.06
PRO
23.0
29.3
1.27
FO
9.7
5.1
0.53
PRO
13.0
7.5
0.58
FO
39.0
9.8
0.25
PRO
46.0
13.0
0.28
TFC – CTA
[35]
TFC – PES/sPEEK
[31]
TFC – PAN
[36]
TFC – DPE
[37]
* DI water as feed solution and 0.5 M NaCl solution as draw solution were used as evaluation condition.
4. Conclusions A cross-linked PI support allowed to prepare excellent osmotic TFC membranes using ILs and IL/solvent combinations as organic reagent phase. The polymer concentration and addition of a volatile co-solvent to the PI casting solution during the phase inversion process, was screened, resulting in optimal 14 wt.% PI casting solution concentration and an NMP/THF ratio of 3:1. The addition of SDS during IFP enhanced the water flux of the membrane but the simultaneous addition of SDS and TEA had an adverse impact on the reverse salt flux of the membrane. The IL ([C4mim][Tf2N]) was selected as organic phase during the IFP process to render membrane preparation less hazardous and potentially improve the membrane performance. This led to osmotic TFC membranes with similar performances to membranes conventionally prepared from hexane as organic media. The membrane preparation costs could be further reduced by applying a 50/50 mixture with C4mimTf2N/hexyl acetate. Moreover, this TFC membrane even outperformed all other membranes. It is thus clear that combinations of ILs with solvents as mixed reagent phase for IFP have great potential to prepare high-performance membranes for RO as well as for FO and PRO
20
Acknowledgements Maxime Corvilain recognizes KU Leuven for a doctoral scholarship. We are also grateful for the financial funding from the OT (11/061) and C16/17/005 from KU Leuven and the I.A.P.-P.A.I. grant (IAP 7/05 FS2) from the Belgian Federal Government. Thanks to An Vandoren and Prof. Johan Billen from the Biology department of KU Leuven for the help with the TEM sample preparation, and to A.Volodin from the Laboratory of Solid-State Physics and Magnetism of KU Leuven for the AFM measurements.
References [1]
Y. Cai, X.M. Hu, A critical review on draw solutes development for forward osmosis, Desalination. (2016). doi:10.1016/j.desal.2016.03.021.
[2]
S. Lee, C. Boo, M. Elimelech, S. Hong, Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO), J. Memb. Sci. (2010). doi:10.1016/j.memsci.2010.08.036.
[3]
W. Xu, Q. Chen, Q. Ge, Recent advances in forward osmosis (FO) membrane: Chemical modifications on membranes for FO processes, Desalination. (2017). doi:10.1016/j.desal.2017.06.007.
[4]
M. Corvilain, C. Klaysom, A. Szymczyk, I.F.J. Vankelecom, Formation mechanism of sPEEK hydrophilized PES supports for forward osmosis, Desalination. (2017). doi:10.1016/j.desal.2017.05.037.
[5]
C. Klaysom, T.Y. Cath, T. Depuydt, I.F.J. Vankelecom, Forward and pressure retarded osmosis: Potential solutions for global challenges in energy and water supply, Chem. Soc. Rev. (2013). doi:10.1039/c3cs60051c.
[6]
C. Klaysom, S. Hermans, A. Gahlaut, S. Van Craenenbroeck, I.F.J. Vankelecom, Polyamide/Polyacrylonitrile (PA/PAN) thin film composite osmosis membranes: Film
21
optimization, characterization and performance evaluation, J. Memb. Sci. (2013). doi:10.1016/j.memsci.2013.05.037. [7]
I. Soroko, Y. Bhole, A.G. Livingston, Environmentally friendly route for the preparation of solvent resistant polyimide nanofiltration membranes, Green Chem. (2011). doi:10.1039/c0gc00155d.
[8]
J. Da Silva Burgal, L. Peeva, A. Livingston, Towards improved membrane production: Using low-toxicity solvents for the preparation of PEEK nanofiltration membranes, Green Chem. (2016). doi:10.1039/c5gc02546j.
[9]
A. Eftekhari, T. Saito, Synthesis and properties of polymerized ionic liquids, Eur. Polym. J. (2017). doi:10.1016/j.eurpolymj.2017.03.033.
[10] J. Hulsbosch, D.E. De Vos, K. Binnemans, R. Ameloot, Biobased Ionic Liquids: Solvents for a Green Processing Industry?, ACS Sustain. Chem. Eng. (2016). doi:10.1021/acssuschemeng.6b00553. [11] Y. Li, J. Sniekers, J. Malaquias, X. Li, S. Schaltin, L. Stappers, K. Binnemans, J. Fransaer, I.F.J. Vankelecom, A non-aqueous all-copper redox flow battery with highly soluble active species, Electrochim. Acta. (2017). doi:10.1016/j.electacta.2017.03.039. [12] D.Y. Xing, N. Peng, T.S. Chung, Formation of cellulose acetate membranes via phase inversion using ionic liquid, [BMIM]SCN, As the solvent, Ind. Eng. Chem. Res. (2010). doi:10.1021/ie1007085. [13] L. Yung, H. Ma, X. Wang, K. Yoon, R. Wang, B.S. Hsiao, B. Chu, Fabrication of thinfilm nanofibrous composite membranes by interfacial polymerization using ionic liquids as additives, J. Memb. Sci. (2010). doi:10.1016/j.memsci.2010.08.033. [14] Y. Cai, W. Shen, J. Wei, T.H. Chong, R. Wang, W.B. Krantz, A.G. Fane, X. Hu, Energy-efficient desalination by forward osmosis using responsive ionic liquid draw 22
solutes, Environ. Sci. Water Res. Technol. (2015). doi:10.1039/c4ew00073k. [15] D. Nikolaeva, I. Azcune, M. Tanczyk, K. Warmuzinski, M. Jaschik, M. Sandru, P.I. Dahl, A. Genua, S. Loïs, E. Sheridan, A. Fuoco, I.F.J. Vankelecom, The performance of affordable and stable cellulose-based poly-ionic membranes in CO2/N2 and CO2/CH4 gas separation, J. Memb. Sci. (2018). doi:10.1016/j.memsci.2018.07.057. [16] H. Mariën, L. Bellings, S. Hermans, I.F.J. Vankelecom, Sustainable Process for the Preparation of High-Performance Thin-Film Composite Membranes using Ionic Liquids as the Reaction Medium, ChemSusChem. (2016). doi:10.1002/cssc.201600123. [17] H. Mariën, I.F.J. Vankelecom, Optimization of the ionic liquid-based interfacial polymerization system for the preparation of high-performance, low-fouling RO membranes, J. Memb. Sci. (2018). doi:10.1016/j.memsci.2018.03.071. [18] S. Hermans, E. Dom, H. Mariën, G. Koeckelberghs, I.F.J. Vankelecom, Efficient synthesis of interfacially polymerized membranes for solvent resistant nanofiltration, J. Memb. Sci. (2015). doi:10.1016/j.memsci.2014.11.046. [19] M.F. Jimenez Solomon, Y. Bhole, A.G. Livingston, High flux membranes for organic solvent nanofiltration (OSN)-Interfacial polymerization with solvent activation, J. Memb. Sci. (2012). doi:10.1016/j.memsci.2012.08.030. [20] H. Mariën, I.F.J. Vankelecom, Transformation of cross-linked polyimide UF membranes into highly permeable SRNF membranes via solvent annealing, J. Memb. Sci. (2017). doi:10.1016/j.memsci.2017.06.080. [21] A.K. Hołda, I.F.J. Vankelecom, Understanding and guiding the phase inversion process for synthesis of solvent resistant nanofiltration membranes, J. Appl. Polym. Sci. (2015). doi:10.1002/app.42130.
23
[22] S. Hermans, H. Mariën, E. Dom, R. Bernstein, I.F.J. Vankelecom, Simplified synthesis route for interfacially polymerized polyamide membranes, J. Memb. Sci. (2014). doi:10.1016/j.memsci.2013.10.005. [23] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: A software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. (2007). doi:10.1063/1.2432410. [24] P. Vandezande, L.E.M. Gevers, J.S. Paul, I.F.J. Vankelecom, P.A. Jacobs, High throughput screening for rapid development of membranes and membrane processes, J. Memb. Sci. (2005). doi:10.1016/j.memsci.2004.11.002. [25] J.R. McCutcheon, M. Elimelech, Influence of concentrative and dilutive internal concentration polarization on flux behavior in forward osmosis, J. Memb. Sci. (2006). doi:10.1016/j.memsci.2006.07.049. [26] P. Vandezande, X. Li, L.E.M. Gevers, I.F.J. Vankelecom, High throughput study of phase inversion parameters for polyimide-based SRNF membranes, J. Memb. Sci. (2009). doi:10.1016/j.memsci.2008.12.068. [27] P. Vandezande, L.E.M. Gevers, I.F.J. Vankelecom, Solvent resistant nanofiltration: Separating on a molecular level, Chem. Soc. Rev. (2008). doi:10.1039/b610848m. [28] A. Tiraferri, N.Y. Yip, W.A. Phillip, J.D. Schiffman, M. Elimelech, Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure, J. Memb. Sci. (2011). doi:10.1016/j.memsci.2010.11.014. [29] A.K. Ghosh, E.M.V. Hoek, Impacts of support membrane structure and chemistry on polyamide-polysulfone interfacial composite membranes, J. Memb. Sci. (2009). doi:10.1016/j.memsci.2009.03.024. [30] S. Zhao, L. Zou, D. Mulcahy, Effects of membrane orientation on process performance 24
in forward osmosis applications, J. Memb. Sci. (2011). doi:10.1016/j.memsci.2011.08.020. [31] R. Thür, M. Corvilain, C. Klaysom, Y. Hartanto, I.F.J. Vankelecom, Tuning the selectivity of thin film composite forward osmosis membranes: Effect of co-solvent and different interfacial polymerization synthesis routes, Sep. Purif. Technol. 227 (2019). doi:10.1016/j.seppur.2019.06.009. [32] A.K. Ghosh, B.H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, J. Memb. Sci. (2008). doi:10.1016/j.memsci.2007.11.038. [33] A. Rahimpour, S.S. Madaeni, Y. Mansourpanah, The effect of anionic, non-ionic and cationic surfactants on morphology and performance of polyethersulfone ultrafiltration membranes for milk concentration, J. Memb. Sci. (2007). doi:10.1016/j.memsci.2007.03.029. [34] D. Prat, A. Wells, J. Hayler, H. Sneddon, C.R. McElroy, S. Abou-Shehada, P.J. Dunn, CHEM21 selection guide of classical- and less classical-solvents, Green Chem. (2015). doi:10.1039/c5gc01008j. [35] Q.Y. Wu, X.Y. Xing, Y. Yu, L. Gu, Z.K. Xu, Novel thin film composite membranes supported by cellulose triacetate porous substrates for high-performance forward osmosis, Polymer (Guildf). (2018). doi:10.1016/j.polymer.2018.08.017. [36] H.E. Kwon, S.J. Kwon, S.J. Park, M.G. Shin, S.H. Park, M.S. Park, H. Park, J.H. Lee, High performance polyacrylonitrile-supported forward osmosis membranes prepared via aromatic solvent-based interfacial polymerization, Sep. Purif. Technol. (2019). doi:10.1016/j.seppur.2018.11.053. [37] S.J. Kwon, S.H. Park, M.G. Shin, M.S. Park, K. Park, S. Hong, H. Park, Y.I. Park, J.H.
25
Lee, Fabrication of high performance and durable forward osmosis membranes using mussel-inspired polydopamine-modified polyethylene supports, J. Memb. Sci. (2019). doi:10.1016/j.memsci.2019.04.074.
26
Supporting Information
Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase
Yusak Hartantoa, Maxime Corvilaina, Hanne Mariëna, Julie Janssena, and Ivo F.J. Vankelecoma*
a
cMACS - Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy for
Sustainable Solutions, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Corresponding author: [email protected]
27
Fig. S1 Effect of PI casting solution concentration on (a) pure water permeability (PWP) and (b) RO performance (2 g/L NaCl, ∆P = 14 bar)
28
Fig. S2 AFM images of the top surfaces of TFC membranes prepared using 14 wt% PI with (a) 9:1 and (b) 3:1 NMP/THF ratio.
29
Table S1 B/A ratio, specific solute flux (Js/Jw), and the normalized water flux (Jw/∆π) of TFC membranes based on supports with different polymer concentration (FO mode, 0.5 M NaCl, DI water as feed). PI concentration (wt%) 10
B/A (bar) 3.04
Js/Jw (g/L) 0.87
Jw/∆π (LMH/bar) 0.56
11
3.59
0.48
0.57
12
0.90
0.34
0.54
13
0.50
0.28
0.45
14
0.88
0.25
0.43
15
0.44
0.27
0.38
Table S2 B/A ratio, specific solute flux (Js/Jw) and normalized water flux (Jw/∆π) of TFC membranes based on supports prepared from a 9:1 and 3:1 NMP:THF ratio (FO mode, 0.5 M NaCl, DI water as feed). NMP:THF ratio 9:1
B/A (bar) 0.68
Js/Jw (g/L) 0.54
Jw/∆π (LMH/bar) 0.43
3:1
0.88
0.25
0.43
Table S3 B/A ratio, specific solute flux (Js/Jw), and the normalized water flux (Jw/∆π) of TFC membranes without additives, with 0.1 wt% SDS, and with 0.1 wt% SDS + 2.0 wt% TEA in the coagulation bath (FO mode, 0.5 M NaCl, DI water as feed).
Pristine
B/A (bar) 1.10
Js/Jw (g/L) 0.24
Jw/∆π (LMH/bar) 0.38
SDS
0.50
0.25
0.43
SDS + TEA
0.89
0.61
0.43
Additives
Table S4 Properties of hexane and C4mimTf2N at 20 °C. Physical Properties Viscosity (mPa s)
Hexane 0.31
[C4mim][Tf2N] 63.05
Density (g/cm3)
0.66
1.44
MPD solubility (wt%)
0.07 – 0.1
>1
Interfacial tension with water (mN/m)
50.80
13.69
30
Highlights •
An IL was applied as organic reagent phase in IFP for FO membranes.
•
A simplified method was employed to prepare a TFC FO membrane.
•
No additives were needed when IL was used as organic phase in IFP.
•
IL-based TFC membranes had comparable performance to conventional TFC membrane.
•
An IL/hexyl acetate as organic phase resulted in a superior TFC FO membrane.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0376-7388(19)33503-3
DOI:
https://doi.org/10.1016/j.memsci.2020.117869
Reference:
MEMSCI 117869
To appear in:
Journal of Membrane Science
Received Date: 15 November 2019 Revised Date:
8 January 2020
Accepted Date: 19 January 2020
Please cite this article as: Y. Hartanto, M. Corvilain, H. Mariën, J. Janssen, I.F.J. Vankelecom, Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117869. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement Yusak Hartanto: Conceptualization, Methodology, Validation, Formal analysis, Writing – Original Draft, Writing – Review & Editing, Visualization Maxime Corvilain: Conceptualization, Methodology, Formal analysis, Writing – Original Draft, Visualization Hanne Mariën: Conceptualization, Methodology Julie Janssen: Investigation, Visualization Ivo F.J. Vankelecom: Resources, Supervision, Writing – Review & Editing
Graphical Abstract
Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase
Yusak Hartantoa, Maxime Corvilaina, Hanne Mariëna, Julie Janssena, and Ivo F.J. Vankelecoma*
a
cMACS - Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy for
Sustainable Solutions, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Corresponding author: [email protected]
1
Abstract Ionic liquids (ILs) were used as the organic reagent phase in interfacial polymerization (IFP) to prepare thin film composite (TFC) membranes with a crosslinked polyimide (PI) as support for use in forward osmosis (FO). In a very straightforward and environmentally benign method, phase inversion, PI cross-linking, and amine monomer impregnation steps were all performed simultaneously. To prepare suitable supports, PI concentration and solvent/co-solvent (NMP/THF) ratios in the casting solution were optimized. The crosslinking of the support allows contact with a broad range of organic reagent phases during IFP. The effect of triethylamine and sodium dodecyl sulphate as additives in the coagulation bath on the performance of the final membranes was studied. The best hexanebased TFC-FO membrane was obtained on supports prepared from 14 wt.% PI solutions containing a 3:1 ratio of NMP:THF and in the presence of 0.1 wt. % SDS. The IL, 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) was selected as IL organic phase to replace hexane. The resultant TFC membrane had a performance similar to that of a TFC synthesized using hexane as organic phase but did not require any additives. To reduce IL costs, a mixture of ([C4mim][Tf2N]/hexyl acetate was applied in which the optimal monomer concentrations were 0.5 wt.% MPD and 0.3 w/v.% TMC. This TFC membrane outperformed all other membranes in this work with a selectivity (Js/Jw) of 0.25 g/L and a normalized water flux (Jw/∆ ) of 0.41 LMH/bar for a 0.5 M NaCl draw solution in FO mode. Keywords: interfacial polymerization; polyimide support; ionic liquids; forward osmosis; thin-film composite
2
1. Introduction Forward osmosis (FO) is an attractive membrane separation technology which relies on the osmotic pressure difference as separation driving force. Compared to pressure driven membrane process, there are several potential advantages for FO, such as low energy cost [1], reduced fouling propensity [2], and higher product yield [3]. Thin film composite (TFC) membranes are current state-of-the-art FO membranes. These membranes consist of a thin selective polyamide (PA) layer deposited on top of a porous support via interfacial polymerization (IFP) [4–6]. Current IFP makes use of organic solvents, such as hexane, heptane or isopar, which have serious environmental impacts and are dangerous to human health [7,8]. Furthermore, evaporation is responsible for hazards as well as for a significant loss of these solvents during membrane manufacturing. Ionic liquids (ILs) have emerged as green solvents with a low vapor pressure. They have been investigated in many research fields, such as polymer synthesis [9], catalysis [10], and batteries [11]. ILs have also been applied as casting solvents for phase inversion membranes [12], as additives in IFP [13], as FO draw agents [14] and to prepare gas separation membranes [15]. Water-immiscible ILs, like 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]), were recently introduced as organic phase to replace neurotoxic hexane in IFP of PA-based TFC-membranes [16]. The required m-phenylenediamine (MPD) concentration during membrane preparation was 20 times lower than in conventional IFP-membrane preparation. Furthermore, typical IFP additives, such as sodium dodecylsulphate (SDS) or triethylamine (TEA), were not needed to achieve comparable FO membrane performance. In contrast to conventional organic phases, the IL phase was less hazardous, easily recycled and potentially applied via spraying. This all drastically reduces the solvent intensity of the IL-based IFP process. Moreover, fouling
3
propensity of the membranes was drastically reduced and flux/selectivity performance was improved [17]. In this study, IL-assisted IFP was employed to prepare FO membranes. Polyimide (PI) was chosen as support material due to its excellent chemical, thermal, and mechanical stability. To prepare suitable supports for FO, an optimization of the phase inversion process was first performed. Furthermore, a simplified (SIM) method simultaneously combining phase inversion, PI crosslinking and m-phenylenediamine impregnation was employed to prepare the TFC FO membrane on a crosslinked PI support to create a fast and easy preparation method [18]. In addition, the crosslinking of the support guarantees excellent membrane stability in a wide range of solvents, including ionic liquids or IL/solvent mixtures, during IFP. Thus, leaving more freedom to optimize the IFP in the new reagent system. In addition, it would potentially allow for the application of very powerful solvent activation [19,20]. The hexane phase was substituted by [C4mim][Tf2N] as well as its mixture with a cheaper and environmentally friendly, solvent, i.e. hexyl acetate, to further improve the FO performance of the membrane and to reduce the cost of the IL-based TFC membrane preparation.
2. Experimental 2.1.
Materials PI (Matrimid 9725 US) was purchased from Huntsman. Hexanediamine (HDA, 99.5%,
Acros), m-phenylenediamine (MPD, 99+%, Acros), triethylamine (TEA, 99+%, Sigma– Aldrich), sodium dodecyl sulphate (SDS, 99%, Acros), and 1,3,5-benzenetricarbonyl chloride (TMC, 98%, Acros) were used for TFC-membrane synthesis. N-Methylpyrrolidone (NMP, ≥99%, Honeywel Riedel-de-Haën), tetrahydrofuran (THF, 99.9+%, Sigma–Aldrich), nhexane (99+%, Chem-Lab), anisole (99%, Sigma Aldrich), methyl-tert-butylether, (98%, Sigma Aldrich), hexyl acetate (99%, Acros), butyl butyrate (98%, Acros), nonyl aldehyde
4
(95%, Acros), 3-phenylpropionaldehyde (95%, Acros), 3-octanone (99%, Sigma-Aldrich), 2nonanone (99%, Sigma-Aldrich), and 2-decanone (98%, Acros) were used as received. 1butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide
(C4MIM
Tf2N),
(99+%,
Iolitec) was dried for 16 h at 80 °C under vacuum before use. Sodium chloride (99.8%, VWR) was used to prepare feed and draw solutions in RO and FO evaluation. 2.2.
Synthesis of crosslinked PI supports PI was dried overnight in an oven at 100°C. Polymer casting solutions were prepared
by stirring PI (10-15 wt%) in NMP or PI (14 wt.%) in different NMP/THF ratios (3:1, 4:1, 5:1 and 9:1). The solutions were then left unstirred overnight for degassing. The support was prepared using the phase inversion method [21]. The PI-solution was cast on a glass plate with a casting knife having a height of 200 µm using an automatic casting device (Braive Instruments, Belgium). Before the film was immersed in a coagulation bath, an evaporation time of 30 s was applied. The temperature and humidity conditions were 20 oC and 30%, respectively. The coagulation bath consisted of MPD (0.1–2.0 wt%), and HDA (0.5 wt%) in Milli-Q water (1) to perform phase inversion, (2) to impregnate the support with MPD and (3) to cross-link it with HDA simultaneously, as described elsewhere [22]. Additives, such as base TEA (2.0 wt%) and surfactant SDS (0.1 wt%), were also introduced in the coagulation bath when relevant. The resultant support was taken out of the coagulation bath after 5 min to perform the IFP. 2.3.
Preparation of PA-based TFC-membranes The impregnated and cross-linked support was put on a metal plate and the excess
amine solution was removed by with rubber wiper. Subsequently, a solution of TMC (0.10.5% w/v) in hexane, C4MIM Tf2N or a 50/50 mixture of C4MIM Tf2N/hexylacetate was poured on top of the support. After 1 min reaction, the excess organic solution was discharged by rinsing the TFC membrane with hexane for the hexane-based TMC solution or
5
with acetone for the case of the IL-based TMC solutions. Afterwards, the TFC membrane was dried for 1 min in air at room temperature before storing it in deionized (DI) water. 2.4.
Membrane characterization The morphology of the PA top-layer and the pore structure of the support were
characterized via scanning electron microscopy (SEM, JEOL JSM-6010LV). The samples were coated with a conductive gold/palladium layer (JEOL JFC-1300 auto fine coater) before imaging. Transmission electron microscopy (TEM) was used to study the PA selective layer in more detail. The membrane samples were embedded in an araldite resin (Polyscience) and subsequently cut into very thin slices (±70 nm, Reichert Ultracut E microtome). The measurements were performed with a JEOL ARM-200F at 80 kV. An average thickness of the PA top layer was calculated by measuring at least 30 equidistant spots along the cross section of PA layer using a public domain image-processing program (ImageJ 1.46), developed by National Institute of Mental Health, Bethesda, Maryland, USA. Atomic force miscroscopy (AFM) was employed to determine the roughness of the top layer of the TFC membranes. The measurements were performed with an Agilent 5500 AFM with NCSHR probes of NanoAndMore GmbH in tapping mode. The cantilever consists of Si and has a spring constant of 40-50 Nm-1 and a nominal tip apex radius of less than 5 nm. The samples were measured over an area of 25 mm2 and analyzed with the WSxM software [23]. The reported root mean square (RMS) roughness was average of from three different locations. 2.5.
RO and pure water permeances (PWP) In order to determine the intrinsic properties of the PA layer, such as salt rejection
(R, %), water (A, L m-2 h-1 bar-1(LMH/bar)), and solute permeance (B, LMH), the prepared TFC membranes were tested in a RO experiment with a feed solution of 2 g/l NaCl at 14 bar. The RO experiments were carried out in a dead-end, high-throughput filtration equipment to allow for a statistical analysis of the data obtained [24]. Eight membranes can be tested
6
simultaneously in this particular set-up with an effective membrane surface area of 1.77 cm2 each. The filtration cell was stirred to minimize concentration polarization. Permeates were collected after a period of time (∆t). R, A, and B were calculated using Eq. (2) – (4), respectively.
% = 1−
=
=
(1)
× 100
(2) ∆ ∆ −∆
1−
∆ −∆
=
∆
1−
(3)
Cf is the solute concentration in the feed, Cp is the solute concentration in the permeate, V is the volume of permeate (L), Am is the effective membrane area (m2), ∆t is the permeation time (h), ∆P is the applied pressure, and ∆π is the osmotic pressure difference across the membrane. The values of retention and permeance are averages of two duplicate tests of three different samples, randomly cut from the same membrane sheet. Pure water permeances (L/ (m² h bar)) were measured in a dead-end cell at a pressure of 1 bar, with an effective membrane surface area of 1.77 cm². 2.6.
FO The TFC membranes were placed in a custom-made, high-throughput, lab-scale
cross-flow FO cell that allows the simultaneous testing of 4 membranes with different feed and draw solutions. The cross-flow cell consisted of 4 rectangular channels with a 55 mm length, a 15 mm width, and a 2 mm depth, each containing a membrane with an active surface of 8.25 cm2. A 0.5 M NaCl solution was used as draw solution and DI water was used as feed solution, which created a theoretical bulk osmotic pressure difference of 22 bar. Membranes were tested in both FO (active layer facing feed solution) and PRO (active layer
7
facing draw solution) configuration under a co-current flow of 250 mL/min with mesh spacers. The Reynolds number was in the range of 300 – 400, reflecting laminar flow. ECP moduli were calculated according to McCutcheon et al. [25] and these were around 0.9 – 1, indicating the effective mixing and good hydrodynamic condition of the set-up. The absolute volumetric change ∆V (L) of the feed with time was recorded. The experiment was carried out for 5 h and the average water flux Jw (LMH) was estimated by: =
∆
(5) ∆
The concentration of the feed solution was measured at different times and the reverse salt flux Js (g m-2 h-1) was calculated by:
=
− ∆
(6)
where C0 and V0 are the initial salt concentration and volume of feed, respectively; Ct and Vt are the salt concentration and the volume of feed at time t, respectively.
3. Results and discussion First of all, an appropriate support layer had to be prepared, often which the actual IFP from IL-phases could be studied. 3.1.
Effect of polymer concentration in the casting solution The concentration of the polymer casting solution largely defines the rate of the de-
mixing during the phase-inversion and the final porosity of the support [26,27]; a high polymer concentration impedes the diffusion of solvent and non-solvent, delaying the demixing and resulting in a denser zone at the top part of the support [28]. To realize adequate PA layer formation, an optimal PI concentration was sought in the 10-15 wt.% range. In this range, the support kept a finger-like pore morphology and no differences were seen in the typical ridge-and-valley structures of the final TFC-membrane surfaces, as shown in Fig.1.
8
However, the PA layer thickness was lower for an 11 wt% polymer concentration (48.9 ± 30.9 nm) than for 14 wt% (145.5 ± 62.0 nm), in agreement with previous findings that a lower surface porosity of the support led to larger pores, thus producing a rougher and thicker PA layer [29].
Fig. 1 (a) SEM images of cross-sections of TFC-membranes, (b) SEM images of PA surfaces, (c) TEM images of cross-sections of TFC-membranes, prepared from 11 wt% (top row) and 14 wt% (bottom row) PI. Arrows indicate the PA-layers.
Both in FO and PRO mode, (Fig. 2) the water flux and reverse salt flux decreased and levelled off at the higher PI concentration in the casting solution, consistent with results from PWP and RO experiments (Fig. S1). PWP tends to decrease as the polymer casting solution concentration increases from 1324 LMH/bar to 131 LMH/bar, while the salt rejection showed the opposite trend. The water fluxes in PRO mode were consistently higher (11.2 – 13.7 LMH) compared to the FO mode (8.4 – 12.5 LMH) due to the ICP phenomenon, while the reverse salt flux was slightly higher in PRO mode (3.9 gMH) than in FO mode (2.3 gMH) at higher polymer casting solution concentration [30]. Further analysis of the RO and FO performance (Table S1) clearly showed that the lower polymer casting concentration resulted in a significantly lower membrane selectivity compared to the membranes prepared from higher polymer casting concentrations, as indicated by their B/A and Js/Jw values. 9
Fig. 2 The effect of PI concentration in the casting solution on water fluxes and reverse salt fluxes of FO TFC-membranes evaluated in (a) FO and (b) PRO mode. Test conditions: DI water as feed and 0.5 M NaCl as draw solution.
3.2.
Effect of THF as co-solvent in the polymer casting solution Since a casting solution concentration of 14 wt.% showed the best water flux and
reverse salt flux, this concentration was selected to study the effect of co-solvent addition. The SEM and TEM images of the cross-sections and top-surfaces of the TFC-membranes with high (9:1) and low (3:1) NMP/THF ratios are shown in Fig. 3. Both porous supports displayed a finger-like morphology with a thickness of 100 ± 5.0 µm. As anticipated, the support prepared by adding more THF to the casting solution had a denser upper layer than the one prepared with less THF. This is due to the evaporation of THF which causes a localized increase of polymer concentration in the upper layer of the cast polymer film during phase inversion [31]. While both TFC-membrane surfaces exhibited a typical ridge-andvalley structure for the PA-layer, a thicker layer was formed on top of the support prepared at the lower NMP/THF ratio: 65.9 ± 41.8 nm and 145.5 ± 62.0 nm, respectively. Besides the thicker PA layer, the membrane prepared with the higher THF content in the casting solution also had a visually rougher morphology, as confirmed by AFM images of the TFC membranes (Fig. S2). The root mean square roughness (RMSR) of the membrane support was lower for the higher NMP:THF ratio (RMSR of 34.97 ± 1.12 nm for 9:1 ratio vs. 58.03 ± 6.73 nm for 3:1 ratio). The thicker and rougher PA morphology can be explained by the
10
decrease of the surface pore diameter as a result of the localized increase of polymer concentration during membrane preparation. This consequently results in more intense eruptions of the diamine solution during IFP, which in turns yields thicker and rougher PA layers [29].
Fig. 3 SEM images of (a) cross-sections, (b) polyamide surfaces and (c) TEM images of crosssections of TFC-membranes prepared with different NMP:THF ratios of 9:1 (top row) and 3:1 (bottom row). Arrows indicate the PA layer.
In RO, (Fig. 4) the water permeance decreased from 0.42 LMH/bar to 0.23 LMH/bar as the NMP/THF ratio increased, while the salt rejection displayed the opposite trend, rising from 61.5% to 92.0%. This was due to the formation of a denser upper layer of the support and a thicker PA layer which acted as additional membrane resistance for the support prepared with the higher NMP/THF ratio. In FO, water fluxes for both TFC membranes were lower than in PRO mode due to the ICP. The water fluxes for FO mode for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 9.56 LMH and 9.64 LMH, respectively. On the other hand, the water fluxes for PRO mode for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 12.46 LMH and 12.02, respectively. The reverse salt flux of the TFC membrane prepared from a support with the higher NMP/THF 11
ratio was higher (5.21 gMH) than from the support prepared with a lower ratio of NMP/THF (2.37 gMH) in FO mode, which was in agreement to the RO experiment. However, there was no significant difference of reverse salt fluxes for both membranes in PRO mode. The reverse salt fluxes for the supports prepared with the NMP/THF ratio of 9:1 and 3:1 were 3.75 gMH and 4.10 gMH, respectively. Further analysis of RO and FO performance parameters presented in Table S2 shows that increasing the amount of THF improves the TFC membrane selectivity, as confirmed by the values of the B/A ratio for RO, and specific solute selectivity (Js/Jw) and normalized water flux (Jw/∆π) for FO.
Fig. 4 (a) RO characterization (test conditions: 2 g/L NaCl and ∆P = 14 bar), and FO characterization (test conditions: DI water as feed and 0.5 M NaCl as draw solution): (b) FO mode and (c) PRO mode, of TFC membranes based on supports with different NMP/THF ratio.
3.3.
Effect of additives in the coagulation bath IFP-additives can be introduced in the non-solvent bath to further improve the
performance of the final membranes. Sodium dodecyl sulphate (SDS) can act as a surfactant 12
during IFP. It contributes to lowering the surface tension between the aqueous and organic phases, and increases the wettability of the support, which promotes the transport of the amine monomer. Triethylamine (TEA) can improve the IFP reaction by acting as a proton acceptor and as a catalyst. No significant difference can be observed between the pristine TFC and the one with SDS addition as shown in Fig. 5. However, a more nodular PA layer was present when SDS and TEA were added simultaneously to the coagulation bath. This nodular morphology results from the alteration of the interfacial reaction zone and the formation of a less defective PA layer [32].
Fig. 5 Surface morphology of final TFC membranes without additives, with 0.1 wt% SDS, and with 0.1wt% SDS + 2.0 wt% TEA.
The performance of TFC membranes evaluated in RO and FO is shown in Fig. 6. In RO, adding SDS to the coagulation bath resulted in a slight increase in water permeance from 0.23 LMH/bar to 0.27 LMH/bar with a slight increase of salt rejection from 92.0 % to 96.2 %. The addition of SDS might cause the PA to be formed at the water-hexane interface, but also in the organic layer, which could lead to PA having more free volume in comparison to the pristine membrane [33]. Simultaneous addition of SDS and TEA to the coagulation bath resulted in a further increase in water permeance to 0.39 LMH/bar followed with a salt rejection decrease to 93.4%. TEA acts as an acylation catalyst by capturing proton formed as a side product of polycondensation between MPD and TMC. This will promote IFP, resulting in less defective PA layer. When TEA is combined with SDS addition, the PA layer will have less defect while maintaining certain free volume, resulting in a synergistic enhancement for both membrane permeance and salt rejection. The salt selectivity trend observed in RO,
13
however, was not observed in either FO or PRO mode. This could be due to the influence of applied pressure in RO which compressed the PA layer resulting in higher salt rejections. In the absence of an applied hydraulic pressure in FO, the PA layer would then be more permeable to salt. The FO performance presented in Table S3 further confirmed that the membrane prepared with SDS addition has the best selectivity and normalized water flux.
Fig. 6 (a) RO evaluation (2 g/l NaCl, ∆P of 14 bar) and FO evaluation: (b) FO mode and (c) PRO mode (DI water as feed, 0.5 M NaCl as draw solution) of TFC-membranes without additives, with 0.1 wt% SDS, and with 0.1wt% SDS + 2.0 wt% TEA in the coagulation bath.
3.4.
Ionic liquid as organic phase for IFP Hexane and C4mimTf2N have very different physical properties, drastically altering the
conditions in the reaction zone (Table S4). As a result, different monomer concentrations are required for the formation of an optimal PA layer. The optimum monomer concentration for RO and SRNF purposes, based on a previous study, was selected as starting point (i.e. 0.1 wt% MPD in water and 0.5 w/v% TMC in IL compared to 2 wt% MPD in water and 0.1
14
w/v%TMC in hexane) [16]. The lower MPD concentration in the IL phase was the result of an enhanced transport of the MPD monomer across the water/IL interface to the reaction zone due to the amphiphilic character of the IL.
Fig. 7 FO performance of TFC-membranes: (a) FO Mode and (b) PRO Mode, with hexane or [C4mim][Tf2N] as organic phase (0.5 M NaCl as draw solution, DI water as feed).
In FO mode, (Fig. 7) both TFC membranes had similar water fluxes (8.85 LMH for hexane-based membrane and 8.88 LMH for IL-based membrane) but the membrane which was interfacially polymerized using IL as reagent phase had a slightly higher reverse flux (2.89 gMH) than the hexane-based membrane (2.34 gMH). In PRO mode, on the other hand, the hexane-based membrane had a higher water and reverse salt flux. The water fluxes of hexane-based membrane and IL-based membrane were 12.0 LMH and 10.3 LMH, respectively while the reverse salt fluxes of hexane-based membrane and IL-based membrane were 4.1 gMH and 3.4 gMH, respectively. 3.5.
IL/solvent mixtures as reagent phase for IFP To further improve the performance of this IL-based membrane and to further lower
preparation cost, a mixture of IL with an organic solvent was used. The organic solvent should be miscible with the IL while still immiscible with water, should contain no reactive functional groups, and be less harmful than hexane. Table 1 shows the properties of different solvents employed in bulk IFP tests to investigate the formation of a stable PA film. The
15
addition of an apolar organic solvent to the IL would increase the interfacial tension and would decrease the viscosity of the organic phase. 0.5 wt% MPD concentration in water and 0.4 w/v% TMC concentration in IL/organic solvent were chosen as starting point. The mixture with hexyl acetate resulted in a stable PA film and a 50/50 IL/hexylacetate ratio was chosen for TFC-membrane preparation, since this solvent also scores well with respect to cost, as well as to safety, environmental and safety scores. Table 1 Selection of organic solvent for mixing with IL Solubility
Solubility
Safety
Health
Env.
Cost
Formation
in water
in IL
score*
score*
Score*
(€/l)**
PA film
IL
-
-
-
-
-
-
-
Anisole
-
+
4
1
5
52
-
Methyl-tert-butylether
-
+
8
3
5
124
-
Hexyl acetate
-
+
3
1
5
83
Stable
Butyl butyrate
-
+
3
1
5
51
Unstable
Nonyl aldehyde
-
-
1
2
5
280
-
3-Phenylpropionaldehyde
-
+
1
2
3
661
-
3-Octanone
-
+
3
1
5
41
-
2-Nonanone
-
+
1
2
5
187
Stable
2-Decanone
-
+
1
1
7
-
Unstable
Hexane
-
-
8
7
7
-
-
Solvents*
* The hazard rating based on CHEM21 solvent guide calculation. Color legends: green = recommended, yellow = problematic and red = hazardous [34]. **Costs are obviously approximation, based on purchase of small lab-scale quantities.
Next, a range of MPD and TMC monomer concentrations was evaluated and the surface morphologies of resulting PA layers are shown in Fig. 8. The membranes showed a typical ridge-and-valley morphology at low and moderate MPD concentrations combined with various TMC concentrations, but not using high MPD concentrations. Membranes D, E, and F combined a slightly higher retention (~ 94.5%) with a significantly higher permeance (~0.40 LMH/bar) in RO (Fig. 9) than the hexane-based membrane (0.23 LMH/bar for water permeance and 92.0% for salt rejection), thus confirming the great potential of this mixed IL/organic solvent reaction system. The MPD:TMC ratio in the reaction zone determines the
16
degree of cross-linking of the PA layer. Membranes A, B, and C were prepared with a 0.1 wt% MPD concentration (low MPD:TMC ratio), which resulted in high retentions (94.3 % 96.7%) and low permeances (0.13 LMH/bar – 0.25 LMH/bar), while membranes G, H, and I, synthesized with 1 wt. % MPD (high MPD:TMC ratio), had low retentions (31.4 % - 73.6 %) and high permeances (0.42 LMH/bar – 0.63 LMH/bar). The MPD concentration thus played the most important role in determining the RO performance.
Fig. 8 SEM images of TFC membrane surfaces prepared from mixtures of IL/hexyl acetate with different MPD and TMC concentrations.
The best membranes (D, E, and F) from the in RO evaluation were further tested in FO. Membrane D displayed a higher water flux (9.1 LMH) while maintaining similar reverse salt flux (2.30 gMH) as the reverse salt flux for hexane-based membrane (2.34 gMH). Membranes E and F, prepared using a higher TMC concentration, showed lower water fluxes than membrane D with slightly higher reverse salt flux for membrane F. The water fluxes of membrane E and membrane F were 7.90 LMH and 7.53 LMH, respectively while the reverse salt fluxes of membrane E and membrane F were 2.31 gMH and 2.72 gMH, respectively. In
17
PRO mode (Fig. 9c), membrane D had, by far, the best performance in terms of water flux (17.0 LMH) and reverse salt flux (3.57 gMH), while membranes E and F had lower fluxes and higher reverse salt fluxes. The water fluxes of membrane E and membrane F were 12.6 LMH and 16.2 LMH, respectively while the reverse salt fluxes of membrane E and membrane F were 5.70 gMH and 7.40 gMH, respectively. Recalculating these results in Table 2, it is clearly shown that the efficiency in RO (B/A ratio) was best for membranes synthesized with the IL as organic phase, while the specific solute flux selectivity (Js/Jw) of membrane D was similar to the hexane-based TFC and lower than the IL-based TFC. Furthermore, membrane D exhibited the highest normalized water flux (0.41 LMH/bar) and was therefore the best FO membrane in this study.
Fig. 9 (a) RO performance (2 g/l NaCl, ∆P of 14 bar) and FO performance (DI water as feed, 0.5 M NaCl as draw solution) of TFC-membranes in: (b) FO mode and (c) PRO mode, using hexane and
18
50/50 mixture of IL/hexylacetate as the organic phases with different TMC and MPD concentrations. Membrane codes refer to the annotations in Fig. 8. Table 2 B/A ratio, specific solute flux selectivity (Js/Jw), and normalized water flux (Jw/∆π) (FO mode, 0.5 M NaCl, DI water as feed) of TFC membrane prepared using different organic phase during interfacial polymerization.
Hexane
B/A (bar) 1.10
Js/Jw (g/L) 0.24
Jw/∆π (LMH/bar) 0.38
IL only
0.42
0.30
0.39
D
0.72
0.25
0.41
E
0.59
0.29
0.36
F
0.74
0.36
0.34
Organic Phase
IL + hexyl acetate
3.6.
Comparison of membrane performance with literature The newly developed TFC membranes are compared to recently synthesized TFC FO
membranes from literature, as shown in Table 3. Although TFC FO membranes with polyimide support in this work had lower water flux than commercial TFC FO membranes from HTI, the solute specific selectivity of the former membranes is lower than the later ones. By examining the solute specific selectivity (Js/Jw), the TFC FO membrane prepared from an IL/hexyl acetate mixture led to the best performance (third entry). Table 3 Comparison of the performance of TFC FO membranes from this work with recent literature data Membrane
Orientation
Jw (LMH)
Js (gMH)
Js/Jw (g/L)
FO
8.9
2.3
0.26
PRO
12.0
4.1
0.34
FO
8.9
2.9
0.33
PRO
10.3
3.4
0.33
FO
9.1
2.3
0.25
PRO
17.0
3.6
0.21
FO
13.9
26.8
1.93
Ref
TFC – XLPI (Hexane)
TFC – XLPI (IL) This work
TFC – XLPI (D) TFC – HTI
19
PRO
21.3
15.0
0.70
FO
11.8
6.9
0.58
PRO
14.6
10.2
0.69
FO
11.0
11.7
1.06
PRO
23.0
29.3
1.27
FO
9.7
5.1
0.53
PRO
13.0
7.5
0.58
FO
39.0
9.8
0.25
PRO
46.0
13.0
0.28
TFC – CTA
[35]
TFC – PES/sPEEK
[31]
TFC – PAN
[36]
TFC – DPE
[37]
* DI water as feed solution and 0.5 M NaCl solution as draw solution were used as evaluation condition.
4. Conclusions A cross-linked PI support allowed to prepare excellent osmotic TFC membranes using ILs and IL/solvent combinations as organic reagent phase. The polymer concentration and addition of a volatile co-solvent to the PI casting solution during the phase inversion process, was screened, resulting in optimal 14 wt.% PI casting solution concentration and an NMP/THF ratio of 3:1. The addition of SDS during IFP enhanced the water flux of the membrane but the simultaneous addition of SDS and TEA had an adverse impact on the reverse salt flux of the membrane. The IL ([C4mim][Tf2N]) was selected as organic phase during the IFP process to render membrane preparation less hazardous and potentially improve the membrane performance. This led to osmotic TFC membranes with similar performances to membranes conventionally prepared from hexane as organic media. The membrane preparation costs could be further reduced by applying a 50/50 mixture with C4mimTf2N/hexyl acetate. Moreover, this TFC membrane even outperformed all other membranes. It is thus clear that combinations of ILs with solvents as mixed reagent phase for IFP have great potential to prepare high-performance membranes for RO as well as for FO and PRO
20
Acknowledgements Maxime Corvilain recognizes KU Leuven for a doctoral scholarship. We are also grateful for the financial funding from the OT (11/061) and C16/17/005 from KU Leuven and the I.A.P.-P.A.I. grant (IAP 7/05 FS2) from the Belgian Federal Government. Thanks to An Vandoren and Prof. Johan Billen from the Biology department of KU Leuven for the help with the TEM sample preparation, and to A.Volodin from the Laboratory of Solid-State Physics and Magnetism of KU Leuven for the AFM measurements.
References [1]
Y. Cai, X.M. Hu, A critical review on draw solutes development for forward osmosis, Desalination. (2016). doi:10.1016/j.desal.2016.03.021.
[2]
S. Lee, C. Boo, M. Elimelech, S. Hong, Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO), J. Memb. Sci. (2010). doi:10.1016/j.memsci.2010.08.036.
[3]
W. Xu, Q. Chen, Q. Ge, Recent advances in forward osmosis (FO) membrane: Chemical modifications on membranes for FO processes, Desalination. (2017). doi:10.1016/j.desal.2017.06.007.
[4]
M. Corvilain, C. Klaysom, A. Szymczyk, I.F.J. Vankelecom, Formation mechanism of sPEEK hydrophilized PES supports for forward osmosis, Desalination. (2017). doi:10.1016/j.desal.2017.05.037.
[5]
C. Klaysom, T.Y. Cath, T. Depuydt, I.F.J. Vankelecom, Forward and pressure retarded osmosis: Potential solutions for global challenges in energy and water supply, Chem. Soc. Rev. (2013). doi:10.1039/c3cs60051c.
[6]
C. Klaysom, S. Hermans, A. Gahlaut, S. Van Craenenbroeck, I.F.J. Vankelecom, Polyamide/Polyacrylonitrile (PA/PAN) thin film composite osmosis membranes: Film
21
optimization, characterization and performance evaluation, J. Memb. Sci. (2013). doi:10.1016/j.memsci.2013.05.037. [7]
I. Soroko, Y. Bhole, A.G. Livingston, Environmentally friendly route for the preparation of solvent resistant polyimide nanofiltration membranes, Green Chem. (2011). doi:10.1039/c0gc00155d.
[8]
J. Da Silva Burgal, L. Peeva, A. Livingston, Towards improved membrane production: Using low-toxicity solvents for the preparation of PEEK nanofiltration membranes, Green Chem. (2016). doi:10.1039/c5gc02546j.
[9]
A. Eftekhari, T. Saito, Synthesis and properties of polymerized ionic liquids, Eur. Polym. J. (2017). doi:10.1016/j.eurpolymj.2017.03.033.
[10] J. Hulsbosch, D.E. De Vos, K. Binnemans, R. Ameloot, Biobased Ionic Liquids: Solvents for a Green Processing Industry?, ACS Sustain. Chem. Eng. (2016). doi:10.1021/acssuschemeng.6b00553. [11] Y. Li, J. Sniekers, J. Malaquias, X. Li, S. Schaltin, L. Stappers, K. Binnemans, J. Fransaer, I.F.J. Vankelecom, A non-aqueous all-copper redox flow battery with highly soluble active species, Electrochim. Acta. (2017). doi:10.1016/j.electacta.2017.03.039. [12] D.Y. Xing, N. Peng, T.S. Chung, Formation of cellulose acetate membranes via phase inversion using ionic liquid, [BMIM]SCN, As the solvent, Ind. Eng. Chem. Res. (2010). doi:10.1021/ie1007085. [13] L. Yung, H. Ma, X. Wang, K. Yoon, R. Wang, B.S. Hsiao, B. Chu, Fabrication of thinfilm nanofibrous composite membranes by interfacial polymerization using ionic liquids as additives, J. Memb. Sci. (2010). doi:10.1016/j.memsci.2010.08.033. [14] Y. Cai, W. Shen, J. Wei, T.H. Chong, R. Wang, W.B. Krantz, A.G. Fane, X. Hu, Energy-efficient desalination by forward osmosis using responsive ionic liquid draw 22
solutes, Environ. Sci. Water Res. Technol. (2015). doi:10.1039/c4ew00073k. [15] D. Nikolaeva, I. Azcune, M. Tanczyk, K. Warmuzinski, M. Jaschik, M. Sandru, P.I. Dahl, A. Genua, S. Loïs, E. Sheridan, A. Fuoco, I.F.J. Vankelecom, The performance of affordable and stable cellulose-based poly-ionic membranes in CO2/N2 and CO2/CH4 gas separation, J. Memb. Sci. (2018). doi:10.1016/j.memsci.2018.07.057. [16] H. Mariën, L. Bellings, S. Hermans, I.F.J. Vankelecom, Sustainable Process for the Preparation of High-Performance Thin-Film Composite Membranes using Ionic Liquids as the Reaction Medium, ChemSusChem. (2016). doi:10.1002/cssc.201600123. [17] H. Mariën, I.F.J. Vankelecom, Optimization of the ionic liquid-based interfacial polymerization system for the preparation of high-performance, low-fouling RO membranes, J. Memb. Sci. (2018). doi:10.1016/j.memsci.2018.03.071. [18] S. Hermans, E. Dom, H. Mariën, G. Koeckelberghs, I.F.J. Vankelecom, Efficient synthesis of interfacially polymerized membranes for solvent resistant nanofiltration, J. Memb. Sci. (2015). doi:10.1016/j.memsci.2014.11.046. [19] M.F. Jimenez Solomon, Y. Bhole, A.G. Livingston, High flux membranes for organic solvent nanofiltration (OSN)-Interfacial polymerization with solvent activation, J. Memb. Sci. (2012). doi:10.1016/j.memsci.2012.08.030. [20] H. Mariën, I.F.J. Vankelecom, Transformation of cross-linked polyimide UF membranes into highly permeable SRNF membranes via solvent annealing, J. Memb. Sci. (2017). doi:10.1016/j.memsci.2017.06.080. [21] A.K. Hołda, I.F.J. Vankelecom, Understanding and guiding the phase inversion process for synthesis of solvent resistant nanofiltration membranes, J. Appl. Polym. Sci. (2015). doi:10.1002/app.42130.
23
[22] S. Hermans, H. Mariën, E. Dom, R. Bernstein, I.F.J. Vankelecom, Simplified synthesis route for interfacially polymerized polyamide membranes, J. Memb. Sci. (2014). doi:10.1016/j.memsci.2013.10.005. [23] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: A software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. (2007). doi:10.1063/1.2432410. [24] P. Vandezande, L.E.M. Gevers, J.S. Paul, I.F.J. Vankelecom, P.A. Jacobs, High throughput screening for rapid development of membranes and membrane processes, J. Memb. Sci. (2005). doi:10.1016/j.memsci.2004.11.002. [25] J.R. McCutcheon, M. Elimelech, Influence of concentrative and dilutive internal concentration polarization on flux behavior in forward osmosis, J. Memb. Sci. (2006). doi:10.1016/j.memsci.2006.07.049. [26] P. Vandezande, X. Li, L.E.M. Gevers, I.F.J. Vankelecom, High throughput study of phase inversion parameters for polyimide-based SRNF membranes, J. Memb. Sci. (2009). doi:10.1016/j.memsci.2008.12.068. [27] P. Vandezande, L.E.M. Gevers, I.F.J. Vankelecom, Solvent resistant nanofiltration: Separating on a molecular level, Chem. Soc. Rev. (2008). doi:10.1039/b610848m. [28] A. Tiraferri, N.Y. Yip, W.A. Phillip, J.D. Schiffman, M. Elimelech, Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure, J. Memb. Sci. (2011). doi:10.1016/j.memsci.2010.11.014. [29] A.K. Ghosh, E.M.V. Hoek, Impacts of support membrane structure and chemistry on polyamide-polysulfone interfacial composite membranes, J. Memb. Sci. (2009). doi:10.1016/j.memsci.2009.03.024. [30] S. Zhao, L. Zou, D. Mulcahy, Effects of membrane orientation on process performance 24
in forward osmosis applications, J. Memb. Sci. (2011). doi:10.1016/j.memsci.2011.08.020. [31] R. Thür, M. Corvilain, C. Klaysom, Y. Hartanto, I.F.J. Vankelecom, Tuning the selectivity of thin film composite forward osmosis membranes: Effect of co-solvent and different interfacial polymerization synthesis routes, Sep. Purif. Technol. 227 (2019). doi:10.1016/j.seppur.2019.06.009. [32] A.K. Ghosh, B.H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, J. Memb. Sci. (2008). doi:10.1016/j.memsci.2007.11.038. [33] A. Rahimpour, S.S. Madaeni, Y. Mansourpanah, The effect of anionic, non-ionic and cationic surfactants on morphology and performance of polyethersulfone ultrafiltration membranes for milk concentration, J. Memb. Sci. (2007). doi:10.1016/j.memsci.2007.03.029. [34] D. Prat, A. Wells, J. Hayler, H. Sneddon, C.R. McElroy, S. Abou-Shehada, P.J. Dunn, CHEM21 selection guide of classical- and less classical-solvents, Green Chem. (2015). doi:10.1039/c5gc01008j. [35] Q.Y. Wu, X.Y. Xing, Y. Yu, L. Gu, Z.K. Xu, Novel thin film composite membranes supported by cellulose triacetate porous substrates for high-performance forward osmosis, Polymer (Guildf). (2018). doi:10.1016/j.polymer.2018.08.017. [36] H.E. Kwon, S.J. Kwon, S.J. Park, M.G. Shin, S.H. Park, M.S. Park, H. Park, J.H. Lee, High performance polyacrylonitrile-supported forward osmosis membranes prepared via aromatic solvent-based interfacial polymerization, Sep. Purif. Technol. (2019). doi:10.1016/j.seppur.2018.11.053. [37] S.J. Kwon, S.H. Park, M.G. Shin, M.S. Park, K. Park, S. Hong, H. Park, Y.I. Park, J.H.
25
Lee, Fabrication of high performance and durable forward osmosis membranes using mussel-inspired polydopamine-modified polyethylene supports, J. Memb. Sci. (2019). doi:10.1016/j.memsci.2019.04.074.
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Supporting Information
Interfacial polymerization of thin-film composite forward osmosis membranes using ionic liquids as organic reagent phase
Yusak Hartantoa, Maxime Corvilaina, Hanne Mariëna, Julie Janssena, and Ivo F.J. Vankelecoma*
a
cMACS - Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy for
Sustainable Solutions, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Corresponding author: [email protected]
27
Fig. S1 Effect of PI casting solution concentration on (a) pure water permeability (PWP) and (b) RO performance (2 g/L NaCl, ∆P = 14 bar)
28
Fig. S2 AFM images of the top surfaces of TFC membranes prepared using 14 wt% PI with (a) 9:1 and (b) 3:1 NMP/THF ratio.
29
Table S1 B/A ratio, specific solute flux (Js/Jw), and the normalized water flux (Jw/∆π) of TFC membranes based on supports with different polymer concentration (FO mode, 0.5 M NaCl, DI water as feed). PI concentration (wt%) 10
B/A (bar) 3.04
Js/Jw (g/L) 0.87
Jw/∆π (LMH/bar) 0.56
11
3.59
0.48
0.57
12
0.90
0.34
0.54
13
0.50
0.28
0.45
14
0.88
0.25
0.43
15
0.44
0.27
0.38
Table S2 B/A ratio, specific solute flux (Js/Jw) and normalized water flux (Jw/∆π) of TFC membranes based on supports prepared from a 9:1 and 3:1 NMP:THF ratio (FO mode, 0.5 M NaCl, DI water as feed). NMP:THF ratio 9:1
B/A (bar) 0.68
Js/Jw (g/L) 0.54
Jw/∆π (LMH/bar) 0.43
3:1
0.88
0.25
0.43
Table S3 B/A ratio, specific solute flux (Js/Jw), and the normalized water flux (Jw/∆π) of TFC membranes without additives, with 0.1 wt% SDS, and with 0.1 wt% SDS + 2.0 wt% TEA in the coagulation bath (FO mode, 0.5 M NaCl, DI water as feed).
Pristine
B/A (bar) 1.10
Js/Jw (g/L) 0.24
Jw/∆π (LMH/bar) 0.38
SDS
0.50
0.25
0.43
SDS + TEA
0.89
0.61
0.43
Additives
Table S4 Properties of hexane and C4mimTf2N at 20 °C. Physical Properties Viscosity (mPa s)
Hexane 0.31
[C4mim][Tf2N] 63.05
Density (g/cm3)
0.66
1.44
MPD solubility (wt%)
0.07 – 0.1
>1
Interfacial tension with water (mN/m)
50.80
13.69
30
Highlights •
An IL was applied as organic reagent phase in IFP for FO membranes.
•
A simplified method was employed to prepare a TFC FO membrane.
•
No additives were needed when IL was used as organic phase in IFP.
•
IL-based TFC membranes had comparable performance to conventional TFC membrane.
•
An IL/hexyl acetate as organic phase resulted in a superior TFC FO membrane.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: