TY - JOUR
T1 - Interfacial polymerization using biobased solvents and their application as desalination and organic solvent nanofiltration membranes
AU - Lin, Shiliang
AU - Semião, Andrea J. Correia
AU - Zhang, Yanqiu
AU - Shao, Lu
AU - Lau, Cher Hon
N1 - Funding Information:
We acknowledge financial funding from the Royal Society International Exchange Grant (grant number: IECS\NSFC\201329 ). This work was also supported by the National Natural Science Foundation of China ( 22178076 , 22208072 ).
Funding Information:
Ascribing to lower energy requirements, membrane-based separations are typically considered as environmentally sustainable separation technologies [1,2]. For example, in seawater desalination, a distillation-based process consumes 14.45 to 27.25 kWh/m3 of electricity, compared to consuming 4 to 6 kWh/m3 for reverse osmosis [3]. This is also valid for solvent recovery where distillation-based processes consume 25–32 times more energy for recovering organic solvents from mixtures compared to organic solvent nanofiltration (OSN) with polymer membranes [4,5]. Most polymer membranes deployed in seawater desalination and organic solvent nanofiltration exist as thin-film composites (TFCs). The most common preparation method of such TFCs is to deposit thin polyamide films as selective layers on porous polymer supports to yield polyamide TFCs for desalination and organic solvent nanofiltration [6,7]. In this process, acyl chlorides e.g. trimesoyl chloride (TMC) is dissolved in a water-immiscible organic solvents such as n-hexane [8], toluene [9] and isoparrafins [10] and interact with diamines such as piperazine (PIP) [11], m-phenylenediamine (MPD) [12] dissolved in water. These monomers react with each other at the water-organic solvent interface, forming a polyamide film (Fig. 1). Fully aromatic polyamides such as those containing MPD are preferred for desalination due to higher negative charge [13,14], while semi-aromatic polyamides comprising PIP with lower salt rejection are typically deployed in organic solvent nanofiltration [14,15]. Although the separation process itself is more environmentally sustainable, the process of fabricating polymer membranes is not [16,17]. This is attributed to the use of toxic, hazardous solvents like dimethylformamide (DMF) and n-methyl-2-pyrrolidone (NMP) for fabricating the porous support layer and n-hexane [8], toluene [9] and isoparrafins [10] for the selective layer.The porous support layers for the TFC membranes studied here were fabricated using (Cyrene™) [38] via spray coating. We chose to use PES support layers fabricated from this approach to demonstrate the feasibility of fabricating the entire structure of TFC membranes – selective layer and porous support from more benign, bio-based solvents.Briefly, PES dope solutions were prepared by dissolving 15 wt% PES and 1 wt% PVP i.e., porogen in Cyrene™ at 80 °C. This dope solution was stirred magnetically until complete PES dissolution, forming a viscous and transparent solution. This dope solution was loaded into the solution reservoir of a spray gun. Spraying distance was set to 20 cm above a glass plate placed on the build plate of the 3D printer. The build plate was not heated prior and throughout spray coating. 4 bar of nitrogen was supplied to the spray gun and spray gun movement was controlled by the control circuit and stepper motors. The spray gun moved across the glass plate to ensure full coverage of the printing area. This process was repeated for 6 times at room temperature to produce a PES film with thickness around 200 μm. After the dope solution was coated on top of glass plate, the glass plate was immersed immediately into a coagulation bath, forming a PES support upon non-solvent induced phase separation. The resultant PES supports were stored in pure water at room temperature until further procedures were needed. The pure water permeance of this spray coated PES support was 63.7 L m−2 h−1 bar−1.Briefly, PES membranes fabricated from spray coating were used as porous support layers for all TFCs. A PES membrane was taped to a glass plate, with the top surface facing upwards and placed in an aqueous solution comprising 2 wt% MPD or PIP for 5 min. The amine-loaded PES support was removed from the solution and pressed with a roller to remove excess amine solution, prior to immersion in organic solutions. For n-hexane, the TMC concentration was set at 0.2 wt%, and for CPME and 2-MeTHF the TMC concentrations were 3 wt%. This is due to the PA layer formation ability and crosslinking degree, detailed reasonings of this concentration setting will be explained in result and discussion session below. The resultant TFCs were placed in an oven at 50 °C for 5 min and washed gently with water to remove unreacted and residue amine and TMC.All polyamide-TFC membranes in this study were fabricated by depositing polyamide films onto the surfaces of porous PES supports via interfacial polymerization. Amine concentrations in water were set at 2 wt% for both fully aromatic MPD-TMC polyamide and semi-aromatic PIP-TMC polyamide. Depending on solvent choice, TMC content in n-hexane was fixed at 0.2 wt% and 3 wt% in both CPMD and 2-MeTHF. SEM images of the pristine PES porous support is showed in Fig. S7, and SEM images of all TFCs were shown in Figs. 3 and 4.Surface roughness is one of the key features that can partially determine the final membrane performance [42,44,55,59,64]. Higher surface roughness can provide larger active area [59,64] and bigger pore size [66] for water permeation, but this also means that the membrane is more vulnerable to fouling where permeances will inadvertently be reduced [67]. In this work, we characterised the surface roughness (root mean square, RMS) of both the pristine PES support and all polyamide TFC membranes using an AFM (Fig. 5). The surface roughness of MPD-TMC polyamide film fabricated using n-hexane reached 59.99 ± 12.69 nm. This was identical to similar polyamide films reported elsewhere [15,68]. The surface roughness of MPD-TMC polyamide was slightly reduced to 52.08 ± 10.69 nm when CPME was used as organic phase during interfacial polymerization. This was in line with its corresponding SEM micrograph (Fig. 3b and e), where we did not observe any ridge and valley structures. The surface of MPD-TMC polyamide fabricated with 2-MeTHF as organic solvent was the roughest amongst all membranes studied here, reaching a RMS value of 169 ± 40.78 nm. As shown in Figure 3c and f, this surface roughness was attributed to the high stacks and large nodules of polyamide. Meanwhile, for PIP-TMC polyamides, we observed that the surface roughness of these films increased from 27.24 ± 6.32 nm to 50.77 ± 8.76 nm and 109.7 ± 14.39 nm as we switched the organic phase from n-hexane to CPME and to 2- MeTHF. The surface roughness of our PIP-TMC film derived from n-hexane was comparable to those reported elsewhere [63]. The changes in surface roughnesses of PIP-TMC polyamide films as a function of solvent water miscibility correlated to the morphological changes observed from SEM microscopy (Fig. 4).In this study, we have successfully demonstrated the substitution of a toxic solvent, n-hexane, with more benign solvents such as CPME and 2-MeTHF as the organic phase during interfacial polymerization of polyamides. We showed that these two solvents could be used to fabricate fully and semi-aromatic polyamide and their variants. By using CPME and 2-MeTHF as organic phases, we deposited polyamide selective layers on to the surfaces of porous PES support layers, yielding TFC membranes. Replacing n-hexane with CPME enhanced the fully aromatic MPD-TMC based polyamide TFC with a higher NaCl rejection rate, reaching 97.8% rejection, while the permeance is slightly decreased by 14.2%. Replacing n-hexane with 2-MeTHF enhanced the semi-aromatic PIP-TMC based polyamide TFC with a 3.7-fold higher permeance and 97.1% dye rejection. The increments in solvent permeances did not reduce dye rejection rates during organic solvent nanofiltration. These findings could be beneficial for membrane separations whilst improving the green metrics of membrane fabrication. These promising results could potentially transform the fabrication of TFCs into a more sustainable process.We acknowledge financial funding from the Royal Society International Exchange Grant (grant number: IECS\NSFC\201329). This work was also supported by the National Natural Science Foundation of China (22178076, 22208072).
Publisher Copyright:
© 2023 The Authors
PY - 2024/2
Y1 - 2024/2
N2 - Thin-film composite membranes are widely regarded as more sustainable technologies for desalination and organic solvent nanofiltration. However, the process of fabricating these membranes is not. This is because nhexane, a toxic and hazardous solvent, or other fossil-derived oily solvents are used as the organic phase to fabricate polyamide selective layers of such membranes. Here we replaced fossil-derived solvents with benign, biorenewable solvents that possess better environmental, health and safety metrics – cyclopentyl methyl ether (CPME) and 2-methyltetrahydrofuran (2-MeTHF). A fully aromatic polyamide thin film composite (TFC) membrane fabricated via CPME demonstrated a higher NaCl rejection (97.8%), while the same membrane fabricated using n-hexane only presented 92.4% rejection. Meanwhile a semi-aromatic polyamide TFC membrane fabricated with 2-MeTHF showed an ethanol permeance of 9.87 L m−2 h−1 bar−1 and 97.1% RB rejection, 3.7-fold higher than the TFC fabricated using n-hexane. This demonstrated the feasibility and advantages of replacingtoxic and hazardous solvents that have long been the standard solvents used in membrane fabrication, with benign alternatives. This work could potentially improve the sustainability of membrane fabrication.
AB - Thin-film composite membranes are widely regarded as more sustainable technologies for desalination and organic solvent nanofiltration. However, the process of fabricating these membranes is not. This is because nhexane, a toxic and hazardous solvent, or other fossil-derived oily solvents are used as the organic phase to fabricate polyamide selective layers of such membranes. Here we replaced fossil-derived solvents with benign, biorenewable solvents that possess better environmental, health and safety metrics – cyclopentyl methyl ether (CPME) and 2-methyltetrahydrofuran (2-MeTHF). A fully aromatic polyamide thin film composite (TFC) membrane fabricated via CPME demonstrated a higher NaCl rejection (97.8%), while the same membrane fabricated using n-hexane only presented 92.4% rejection. Meanwhile a semi-aromatic polyamide TFC membrane fabricated with 2-MeTHF showed an ethanol permeance of 9.87 L m−2 h−1 bar−1 and 97.1% RB rejection, 3.7-fold higher than the TFC fabricated using n-hexane. This demonstrated the feasibility and advantages of replacingtoxic and hazardous solvents that have long been the standard solvents used in membrane fabrication, with benign alternatives. This work could potentially improve the sustainability of membrane fabrication.
U2 - 10.1016/j.memsci.2023.122281
DO - 10.1016/j.memsci.2023.122281
M3 - Article
SN - 0376-7388
VL - 692
JO - Journal of Membrane Science
JF - Journal of Membrane Science
M1 - 122281
ER -