Sustainable Fabrication of Organic Solvent Nanofiltration Membranes

membranes

Review Sustainable Fabrication of Organic Solvent Nanoﬁltration Membranes

Hai Yen Nguyen Thi 1, Bao Tran Duy Nguyen 1 and Jeong F. Kim 1,2,*

1 Department of Energy and Chemical Engineering, Incheon National University, Incheon 22012, Korea; [email protected] (H.Y.N.T.); [email protected] (B.T.D.N.) 2 Innovation Center for Chemical Engineering, Incheon National University, Incheon 22012, Korea * Correspondence: [email protected]

Abstract: Organic solvent nanofiltration (OSN) has been considered as one of the key technologies to improve the sustainability of separation processes. Recently, apart from enhancing the membrane performance, greener fabricate on of OSN membranes has been set as a strategic objective. Consider- able efforts have been made aiming to improve the sustainability in membrane fabrication, such as replacing membrane materials with biodegradable alternatives, substituting toxic solvents with greener solvents, and minimizing waste generation with material recycling. In addition, new promising fabrication and post-modification methods of solvent-stable membranes have been developed exploiting the concept of interpenetrating polymer networks, spray coating, and facile interfacial polymerization. This review compiles the recent progress and advances for sustainable fabrication in the field of polymeric OSN membranes.

Keywords: sustainability; organic solvent nanoﬁltration; green solvents; environmental-friendly polymers; bio-based polymers

1. Introduction With escalating environmental concerns, the concept of sustainability has become more Citation: Nguyen Thi, H.Y.; Nguyen, important. The environmental regulations are getting even more stringent, and industries B.T.D.; Kim, J.F. Sustainable Fabrica- tion of Organic Solvent Nanofiltration are widely implementing membrane technology to improve their process sustainability. Membranes. Membranes 2021, 11, 19. Among different types of membrane technologies, organic solvent nanofiltration (OSN) is a https://dx.doi.org/10.3390/ membrane process with capabilities to discriminate nanometer-sized molecules in organic membranes11010019 solvents [1]. Different from ultrafiltration and reverse osmosis, nanofiltration membranes exhibit pore size distributions of 0.5–2 nm [2]. OSN has been recognized as a sustainable Received: 8 October 2020 separation platform with low energy consumption [3] and, also, as a platform to achieve Accepted: 6 November 2020 process intensification via reaction-separation convergence [4]. Published: 28 December 2020 In order to perform separation in organic media, OSN membranes are required to resist harsh media such as aggressive solvents; simultaneously, OSN membranes must be Publisher’s Note: MDPI stays neu- stable over a wide range of pH, including organic acids and bases [4]. Hence, most research tral with regard to jurisdictional claims efforts have been dedicated on improving the chemical stability of polymeric membranes. in published maps and institutional However, modifications such as crosslinking employ toxic reagents and chemicals that affiliations. generate considerable amount of organic wastes. Ironically, although OSN technology has developed to improve the process sustainability, the fabrication of OSN membrane itself has been far from sustainable [3].

Copyright: © 2020 by the authors. Li- As discussed in the work by Figoli et al. [5], to be “sustainable”, a membrane process censee MDPI, Basel, Switzerland. This must not implicate the use of dangerous chemicals in the membrane production process article is an open access article distributed itself. Moreover, a sustainable development can only be realized when it satisﬁes the current under the terms and conditions of the needs without undermining the ability of next generations to meet their own needs. Ideally, Creative Commons Attribution (CC BY) a process should comply with green principles [6], such as effective waste prevention, better license (https://creativecommons.org/ atom economy, replacement of hazardous chemicals with safer alternatives, and higher licenses/by/4.0/). energy efﬁciency.

Membranes 2021, 11, 19. https://dx.doi.org/10.3390/membranes11010019 https://www.mdpi.com/journal/membranes Membranes 2021, 11, 19 2 of 21

Recently, there has been an encouraging progress with respect to sustainability in the OSN membrane fabrication. There are many techniques to fabricate and to modify membranes, such as phase inversion, interfacial polymerization, sintering, irradiation and track-etching, dip coating, and plasma polymerization [7]. Among the reported methods, the phase inversion technique has been the most employed one due to its versatile feature [8] to control the membrane morphology, pore connectivity, and pore size distribu- tion [9]. There are different types of phase inversion methods, as illustrated in Figure1. The principle behind all the methods is essentially the same. Among these illustrated methods, nonsolvent induced phase separation (NIPS) is the most widely employed one for making commercial membranes due to its versatility, closely followed by the thermally Membranes 2020, 10induced, x phase separation (TIPS) technique. 3 of 23

Figure 1. IllustrationFigure of the phase 1. Illustrationinversion techniqu ofe and the respective phase membrane inversion morphology technique for (A) andNIPS (reprinted respective with permission membrane from) mor- [10], (B) TIPS (reprinted with permission from)phology [11], and for(C) VIPS (A) and NIPS EIPS (reprinted (reprinted with permission with permission from) [12]. from) [10], (B) TIPS (reprinted with permission from) [11], and (C) VIPS and EIPS (reprinted with permission from) [12].

In NIPS, the homogeneous dope solution is first prepared by dissolving a polymer(s) in a suitable solvent(s). Additives could be supplemented to enhance the desired properties of the membrane. The prepared solution is then cast onto a substrate to make a thin film, which is subsequently immersed in a nonsolvent bath (thermodynamically unstable environment—mostly water) for coagulation. The phase separation and solidification Membranes 2020, 10proceeds, x; doi: spontaneously [13]. Notably, the www.mdpi.c NIPSom/journal/m processembranes inevitably generates a significant amount of solvent-contaminated wastewater, which must be treated on-site or off-site before discharging into the environment [13]. In comparison, the TIPS dope solution is prepared at an elevated temperature and the membrane is formed by cooling the cast solution below its solidification point. For vapor/evaporation (VIPS/EIPS), the cast solution comes in contact with a humid environment, where vapor absorbs onto the film with evaporation of the dope solvent in tandem [13]. Depending on the target membrane properties, post-modification or post-treatment steps can be applied using techniques such as crosslinking, coating, and interfacial polymerization. Important components and parameters for each step of the phase inversion method are shown in Figure2. The sustainability of each factor must be considered and improved. Membranes 2020, 10, x 4 of 23

In NIPS, the homogeneous dope solution is first prepared by dissolving a polymer(s) in a suitable solvent(s). Additives could be supplemented to enhance the desired properties of the membrane. The prepared solution is then cast onto a substrate to make a thin film, which is subsequently immersed in a nonsolvent bath (thermodynamically unstable environment—mostly water) for coagulation. The phase separation and solidification proceeds spontaneously [13]. Notably, the NIPS process inevitably generates a significant amount of solvent-contaminated wastewater, which must be treated on-site or off-site before discharging into the environment [13]. In comparison, the TIPS dope solution is prepared at an elevated temperature and the membrane is formed by cooling the cast solution below its solidification point. For vapor/evaporation (VIPS/EIPS), the cast solution comes in contact with a humid environment, where vapor absorbs onto the film with evaporation of the dope solvent in tandem [13]. Depending on the target membrane properties, post-modification or post-treatment steps can be Membranes 2021, 11, 19 applied using techniques such as crosslinking, coating, and interfacial polymerization. Important3 of 21 components and parameters for each step of the phase inversion method are shown in Figure 2. The sustainability of each factor must be considered and improved.

FigureFigure 2. 2. ImportantImportant parameters parameters and comp andonents components involved involvedin each step in of eachthe phase step inversion of the phase inversionmethod. method.

TwoTwo key key componentscomponents in in the the phase phase inversion inversion method method are the arepolymer the polymer and solvent. and Common solvent. Commontypes of polymer types of materials polymer and materials solvents used and to solvents fabricate usedmembranes to fabricate were compiled membranes by Lee et were al. compiled[14], as shown by Lee in Figure et al. 3. [14 Within], as shownthe membra inne Figure technology,3. Within current the OSN membrane membranes technology, reported in currentthe literature OSN membranesare mostly petroleum-derived reported in the polyimid literaturee (PI), are polybenzimidazole mostly petroleum-derived (PBI), and poly(ether poly- imideether (PI), ketone) polybenzimidazole (PEEK) (please refer (PBI),to the list and of poly(etherabbreviations ether after the ketone) Conclusions (PEEK) Section). (please On refer the toother the list hand, of abbreviationscommon solvents after used the to Conclusionsfabricate membranes Section). now On are the mostly other toxic hand, polar common aprotic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methylpyrrolidone solvents used to fabricate membranes now are mostly toxic polar aprotic solvents such (NMP). To maintain the sustainable development of membrane technology, it is imperative to replace asthese dimethylformamide toxic materials with (DMF), environmental-friendly dimethylacetamide alternatives, (DMAc), as the and use of N-methylpyrrolidone these solvents will be (NMP).restricted To maintainafter May the2020 sustainable in Europe [15]. development In fact, this ofhas membrane been an active technology, research topic it is imperativearound the to replaceglobe for these this very toxic reason. materials with environmental-friendly alternatives, as the use of these solventsIn willthe past be restrictedtwo decades, after many May promising 2020 in adva Europencements [15]. have In fact, been this made has by been researchers an active to Membranesresearchrealize 2020 topic,the 10, conceptx around of sustainability the globe for in this membrane very reason. technology. In this review, the reported greener5 of 23 alternatives for membrane polymers and solvents were compiled and compared. Additionally, the recent progress in sustainable membrane fabrication methods were also assessed.

Membranes 2020, 10, x; doi: www.mdpi.com/journal/membranes

FigureFigure 3. Literature 3. Literature survey survey result result of the of (a the) solvents (a) solvents and (b and) polymers (b) polymers employed employed in NIPS in membrane NIPS fabricationmembrane over fabricationthe past five over years the (reprinted past ﬁve with years permission (reprinted from with [14]). permission from [14]).

2. SustainableIn the Membrane past two decades,Materials many promising advancements have been made by researchers to realize the concept of sustainability in membrane technology. In this review, theThere reported has been greener an active alternatives search to for substitute membrane petro-derived polymers and conventional solvents were polymers compiled with andnatural alternativescompared. such Additionally, as bio-based the polymers. recent progress The first ingeneration sustainable of bio-based membrane polymers fabrication employed methods edible cropswere as raw also materials; assessed. however, due to the competition for land used in food production, the focus has gradually shifted to nonedible crops [16]. Some natural products that could be considered as bio- based2. Sustainablepolymers are Membrane collagen, chitosan, Materials and others that could be synthesized by bio-routes [17]. Recently,There Peinemann has been an et activeal. [18] searchreported to an substitute interesting petro-derived work to utilize conventional a sodium alginate polymers (NaAlg) polymerwith naturalto fabricate alternatives composite such membranes. as bio-based This NaAlg polymers. polysaccharide The ﬁrst generation originated offrom bio-based a seaweed cellpolymers wall that contained employed 30% edible to 60% crops alginic as raw acid, materials; which can however, be converted due to to a the water-soluble competition salt. for The modifiedland used polysaccharide in food production, is an unbranched the focus binary has copolymer gradually of shifted1-4-linked to nonedibleβ-D-mannuronic crops acid [16]. (M) andSome α-L-guluronic natural products acid (G). thatThe alginate could be polymer considered has numerous as bio-based advantages, polymers including are collagen, the ability chi- to formtosan, uniform and and others transparent that could films be synthesizedat ambient temperature, by bio-routes their [17 abundance,]. biodegradability, and characteristic liquid-gel behavior in aqueous solutions. The proposed work employed a biodegradable alginate as a membrane polymer, water as the solvent, and calcium chloride as the ionic crosslinking agent. The authors applied the technique called RIPS (new type of phase inversion method) to prepare sodium alginate composite membranes [19], as schematized in Figure 4. The prepared dope solution was cast onto different support materials (polyacrylonitrile (PAN), cellulose, nonwoven polyester, and alumina discs). The cast solution was then left to dry at room temperature instead of immersion in a nonsolvent media. In step (II), the aqueous calcium chloride solution was used as the crosslinking solution, further enhancing the structural stability of the membranes and sustainability of the membrane fabrication. Depending on the desired membrane property, the dried film can be immersed into a nonsolvent bath (step III in Figure 4).

Figure 4. Graphical illustration of the RIPS technique (A) casting a thin film with polymer solution, (B) immersing the cast film in an aqueous calcium chloride bath, (C) immersing the film in a nonsolvent bath, and (D) the final membrane.

Membranes 2020, 10, x; doi: www.mdpi.com/journal/membranes Membranes 2020, 10, x 5 of 23

Figure 3. Literature survey result of the (a) solvents and (b) polymers employed in NIPS membrane fabrication over the past five years (reprinted with permission from [14]). Membranes 2021, 11, 19 4 of 21 2. Sustainable Membrane Materials There has been an active search to substitute petro-derived conventional polymers with natural alternatives such as bio-based polymers. The first generation of bio-based polymers employed edible cropsRecently, as raw materials; Peinemann however, et al.due [ 18to ]the reported competition an for interesting land used in work food toproduction, utilize athe sodium focus alginatehas gradually (NaAlg) shifted polymer to nonedible to fabricate crops [16]. composite Some natural membranes. products that This could NaAlg be considered polysaccharide as bio- originatedbased polymers from are a seaweed collagen, chitosan, cell wall and that others contained that could 30% be tosynthesized 60% alginic by bio-routes acid, which [17]. can be convertedRecently, to a Peinemann water-soluble et al. [18] salt. reported The modified an interesting polysaccharide work to utilize isa sodium an unbranched alginate (NaAlg) binary copolymerpolymer to of fabricate 1-4-linked compositeβ-D-mannuronic membranes. This acid NaAlg (M) and polysaccharideα-L-guluronic originated acid (G). from The a seaweed alginate polymercell wallhas that numerouscontained 30% advantages, to 60% alginic including acid, which the can ability be converted to form to uniform a water-soluble and transparent salt. The filmsmodified at ambient polysaccharide temperature, is an unbranched their abundance, binary copolymer biodegradability, of 1-4-linked and β-D-mannuronic characteristic acid liquid- (M) geland behavior α-L-guluronic in aqueous acid (G). solutions. The alginate polymer has numerous advantages, including the ability to formThe uniform proposed and transparent work employed films at aambient biodegradable temperature, alginate their abundance, as a membrane biodegradability, polymer, waterand characteristic liquid-gel behavior in aqueous solutions. as the solvent, and calcium chloride as the ionic crosslinking agent. The authors applied the The proposed work employed a biodegradable alginate as a membrane polymer, water as the techniquesolvent, and called calcium RIPS chloride (new as type the ionic of phase crosslinking inversion agent. method) The authors to applied prepare the sodium technique alginate called compositeRIPS (new membranestype of phase inversion [19], as method) schematized to prepare in Figuresodium4 alginate. The preparedcomposite membranes dope solution [19], wasas schematized cast onto different in Figure support4. The prepared materials dope (polyacrylonitrile solution was cast onto (PAN), different cellulose, support nonwoven materials polyester,(polyacrylonitrile and alumina (PAN), discs).cellulose, The nonwoven cast solution polyester, was and then alumina left to discs). dry atThe room cast solution temperature was insteadthen left of to immersion dry at room in temperature a nonsolvent instead media. of immersion In step in (II), a nonsolvent the aqueous media. calcium In step chloride(II), the solutionaqueous was calcium used chloride as the crosslinkingsolution was solution,used as the further crosslinking enhancing solution, the further structural enhancing stability the of thestructural membranes stability and of sustainabilitythe membranes ofand the sustainability membrane of fabrication.the membrane Depending fabrication. onDepending the desired on membranethe desired property,membrane theproperty, dried the film dried can film be can immersed be immersed into into a nonsolvent a nonsolvent bathbath (step III III in in FigureFigure4 ).4).

FigureFigure 4.4. Graphical illustration illustration of the of theRIPS RIPS technique technique (A) casting (A) casting a thin film a thin with film polymer with solution, polymer solution,(B) immersing (B) immersing the cast film the in cast an filmaqueous in an calcium aqueous chloride calcium bath, chloride (C) immersing bath, (C the) immersing film in a thenonsolvent film in abath, nonsolvent and (D) the bath, final and membrane. (D) the final membrane.

MembranesThe fabricated2020, 10, x; doi: membranes were surprisingly stable in www.mdpi.c DMF, dimethylom/journal/m sulfoxideembranes (DMSO), and NMP [18]. The membranes also exhibited a competitive and stable OSN performance, as compiled in Table1. In addition, NaAlg membranes showed a good performance for CO2/N2 gas separation, and it can be a very economical polymer compared to commercial membranes like Pebax® and Matrimid® 5218 [16]. Polylactic acid, or polylactide (PLA), is a polyester derived from renewable resources such as starch, roots of tapioca, or sugarcane. In 2010, PLA was the second-most important bioplastic in the world, following the starch-based polymer in terms of consumption vol- ume [20]. Its name of “polylactic acid” is not in-line with IUPAC standard nomenclature, because PLA is not a polyacid (polyelectrolyte) but, rather, a polyester. PLA is biodegradable, with similar characteristics to polypropylene (PP), polyethylene (PE) and polystyrene (PS). Notably, it can be produced from existing manufacturing equipment, which explains why PLA was quickly taken up by the plastic industries. There is a wide range of applications for PLA, such as the production of plastic bottles; ﬁlms; and biodegradable medical devices (screws, pins, rods, and plates, with the expected capability to biodegrade within six to 12 months). Membranes 2021, 11, 19 5 of 21

Table 1. Performance of membranes fabricated with sustainable materials and solvents.

Selective Membrane Solvent for Permeance Highest Rejection No. Support Material Testing Solvent Marker (MW) Ref. Material Fabrication (L/m2·h·bar) (%) 1. NaAlg PAN Water Methanol B12 (1355 g·mol−1) 1.27 ± 0.2 98 ± 2 [18] 2. NaAlg Crosslinked PAN Water DMF B12 (1355 g·mol−1) 0.21 ± 0.1 98 ± 1 [18] 3. NaAlg Cellulose Water Methanol B12 (1355 g·mol−1) 0.38 ± 0.1 95 ± 1 [18] 4. NaAlg Alumina support Water Methanol B12 (1355 g·mol−1) 1.6 ± 0.1 90± 2 [18] 5. NaAlg Alumina support Water DMF B12 (1355 g·mol−1) 0.25 ± 0.02 70 ± 1 [18] 6. NaAlg Alumina support Water DMSO B12 (1355 g·mol−1) 0.15 ± 0.05 76 ± 2 [18] 7. NaAlg Alumina support Water NMP B12 (1355 g·mol−1) 0.11 ± 0.03 80 ± 3 [18] 8. PBI Bamboo ﬁber/PLA DMC, DMAc Water 1068 ± 32 [21] Polypropylene Rose Bengal 9. CTA GVL Water >90 [23] nonwoven (1017 g·mol−1) Polypropylene Rose Bengal 10 CA Methyl lactate Water >90 [23] nonwoven (1017 g·mol−1) Polypropylene Rose Bengal 11. CA GVL Water >90 [23] nonwoven (1017 g·mol−1) Polypropylene Methyl lactate Rose Bengal 12. CA Water 2.4 99.5 [24] nonwoven 2-methyl THF (1017 g·mol−1) 13. PES Polyester nonwoven Cyrene Water 2542.7 [25] 14. PES Polyester nonwoven Cyrene Water 898.4 [25] [10, 15. PVDF PolarClean Water 3000 24] 16. PVDF PolarClean Water Polystyrene 99.99 [26] 17. PVDF-HFP TEP Distilled water NaCl 16.1 99.3 [27] Congo Red 18. Cellulose ((EMIM)(DEP)) Ethanol 19 ± 1 >90 [28] (696 g·mol−1) Congo Red 19. Cellulose ((EMIM)(DEP)) Water 48 ± 3 >99 [28] (696 g·mol−1) Bromothymol Blue 20. Cellulose (EMIM)OAc Ethanol 0.3 94 [29] (624.4 g·mol−1) Bromothymol Blue 21. Cellulose (EMIM)OAc/Acetone Ethanol 8.4 69.8 [29] (624.4 g·mol−1) Membranes 2021, 11, 19 6 of 21

Table 1. Cont.

Selective Membrane Solvent for Permeance Highest Rejection No. Support Material Testing Solvent Marker (MW) Ref. Material Fabrication (L/m2·h·bar) (%) Orange GII 22. TA/Fe3+ PES Water Water 45.6 94.8 [22] (452.4 g·mol−1) Orange GII 23. TA/Fe3+ PES Water Water 34.3 95.5 [22] (452.4 g·mol−1) 24. HPC Water Water 3 ± 0.2 − 38 ± 5 [30] 25. PVDF ATBC Water 538 [11] 26. PVDF DMSO2 Water 1491 [31] 27. PA PE battery separator Hexane, Water Acetone Styrene oligomer (~1000 g·mol−1) ~20 >99 [32] Porous substrate(PE 28. PAES separator, TR-NFM, DMSO DMF Styrene oligomer (1595 g·mol−1) 0.37 ± 0.018 >99 [33] and PET nonwoven) PEGDGE 10% wt PAES membrane with 29. PAES, PES-TA aqueous solutions, Water Methyl violet (407.979 g·mol−1) 15.5 99.8 [6] PES-TA group PEI% wt solution Polystyrene 30. PBI/PDA Polypropylene DMAc, Water DMF 12 ~100 [34] (610 g·mol−1) Polystyrene 31. Bio-phenol coated PI PI membrane Water:EtOH Acetone 1–10 5~100 [35] (390–1550 g·mol−1) Crosslinked PI Sunset Yellow 32. PI DMSO Methanol 11 93 [36] membrane (452 g·mol−1) Membranes 2021, 11, 19 7 of 21

In order to mitigate membrane solid waste discharged to the environment, Szekely et al. [21] introduced another encouraging option to replace petroleum-based nonwoven backing materials (polypropylene or polyethylene terephthalate) commonly employed for flat sheet membranes. The work employed PLA and bamboo fibers to fabricate a porous membrane that can be used as the membrane backing (support) [21]. Bamboo fiber was chosen because of its abundance and low cost for production. The bamboo fiber contains a large amount of hydroxyl and ether groups, while the PLA is rich in hydroxyl groups and ester [21]. Thus, the hydrogen bonding and adhesion between the components can be facilitated. The bamboo fiber was first processed into dry particles in size of 50–150 µm [21], maintaining the original compositions composed of cellulose, hemicellulose, lignin, ash, and other extractives. The mixture of bamboo fiber/PLA was dissolved in dimethyl carbonate (DMC), cast onto a glass substrate using a film applicator, and dried in a vacuum oven for 24 h at 30 ◦C, resulting in a suitable membrane support material similar to other nonwoven backings [21]. For testing the fabricated backing material, a dope solution of PBI in DMAc was prepared to be cast on the bamboo support; then, the filtration performance was tested. Various solvents were used for confirming the chemical stability of the support, such as DMF, NMP, DMSO, DMAc, and acetone, etc., and it was found to be stable in most of them for over six months. [21]. The research result clearly indicated that the bamboo-PLA membrane support could open a new door towards sustainable fabrication with green and biodegradable backing materials [21]. Recently, Lin Fan et al. [22] reported an interesting approach to fabricate thin film composite nanofiltration (TFC-NF) membranes via interfacial polymerization (IP) only in aqueous environments. Tannic acid and iron (III) chloride (FeCl3) dissolved in water acted as two reactive monomers. As a result, a stable coating layer containing metal- polyphenol was formed on a polyethersulfone (PES) porous support via the coordination reaction between tannic acid (TA) and iron ions (Fe3+). This green coating materials for NF membranes can be employed without any toxic reagents nor solvents. They also assessed how the reaction time, concentration of TA, and Fe3+ ions affect the permeation and separation properties of the formed membranes. Additionally, the membrane stability after immersion into different pH media and a NaClO solution was studied. Although the filtration experiment was conducted only for 24 h, the water flux and dye rejection ratio of the membranes remained stable. A conventional IP typically employs an organic layer. In comparison, this work showed that a thin surface layer can be formed at the surface in an aqueous environment. The work showed that TA-Iron(III)/PES composite NF membranes can be developed with green materials and reagents with excellent antioxidant properties after long-term immersion in a NaClO solution.

3. Sustainability for Employed Solvents In pharmaceutical industries where solvents contribute a considerable mass intensity of products (up to 56%), a signiﬁcant amount of research has been carried out to identify green solvents [37]. The undesired and preferred solvents from their perspectives are summarized in Figure5. The key goal is to design and apply a greener synthesis route to avoid environmental impacts. Efforts should be given to move from the left side of the list in Figure5 to the right, and chemical processes will gradually become greener and more sustainable. The solvents in the list were also referred from the work conducted by Alder et al. [38]; they updated and expanded GSK’s solvent sustainability guidelines conducted previously [39]; more solvents were supplemented, and the way in which the health, environment, safety, and waste categories were combined was adjusted. Membranes 2020, 10, x 9 of 23

3. Sustainability for Employed Solvents In pharmaceutical industries where solvents contribute a considerable mass intensity of products (up to 56%), a significant amount of research has been carried out to identify green solvents [37]. The undesired and preferred solvents from their perspectives are summarized in Figure 5. The key goal is to design and apply a greener synthesis route to avoid environmental impacts. Efforts should be given to move from the left side of the list in Figure 5 to the right, and chemical processes will gradually become greener and more sustainable. The solvents in the list were also referred from Membranes 2021, 11, 19 the work conducted by Alder et al. [38]; they updated and expanded GSK’s solvent sustainability8 of 21 guidelines conducted previously [39]; more solvents were supplemented, and the way in which the health, environment, safety, and waste categories were combined was adjusted.

FigureFigure 5. 5.RecommendationsRecommendations for forselecting selecting solvents solvents in industrial in industrial processes processes with withreference reference to GSK’s to solventGSK’s selection solvent selectionguide; each guide; level each lists level undesirable, lists undesirable, usable, and usable, preferred and preferredsolvents, respectively solvents, [38,40].respectively [38,40].

AsAs more more applications applications develop develop using using membrane membrane technology, technology, the environmental the environmental impacts impacts caused bycaused membrane by membrane preparation preparation become mo becomere significant more [15]. significant In membrane [15]. In preparation, membrane solvents preparation, play a crucialsolvents role, play and athe crucial properties role, of and a solvent the properties and its interaction of a solvent with the and polymer its interaction affects the membrane with the morphologypolymer affects and, thethus, membrane performance morphology [41]. Hence, and, identifying thus, performance a green solvent [41]. that Hence, can identifyingdissolve the polymera green solventof interest that is canonly dissolve a prerequisite, the polymer as the ofresulting interest membrane is only a prerequisite,must exhibit asa competitive the result- performanceing membrane to be must adopted exhibit by the a membrane competitive indu performancestry. Furthermore, to be environmental adopted by the regulations membrane now restrictindustry. the Furthermore,use of toxic solvents environmental for membrane regulations production. now Notably, restrict thethe useEuropean of toxic Registration, solvents Evaluation,for membrane Authorization, production. and Notably, Restriction the of European Chemicals Registration, (REACH) has Evaluation, classified DMF, Authorization, DMAc, and NMPand Restrictionas substances of with Chemicals very high (REACH) concerns, has and classified the use of DMF, these solven DMAc,ts andwill be NMP restricted as substances after May 2020with [15]. very Recent high concerns,reports on andgreen the solvent use of alternatives these solvents for membrane will be restricted fabrication after are May summarized 2020 [15 ].in Table 2. Recent reports on green solvent alternatives for membrane fabrication are summarized in Table2. 3.1. Water 3.1.The Water solvents widely employed in the chemical industries are often organic chemicals from petroleumThe solventsproducts widelywith risks employed to health in and the the chemical environment. industries Without are oftena doubt, organic replacing chemicals organic solventsfrom petroleum with water products provides with enormous risks to benefits. health A and research the environment. work by Hanafia Without et al. a[30] doubt, reported replac- the preparationing organic of solvents a porous with membrane water provides from a enormouswater-soluble benefits. biopolymer, A research hydroxypropyl work by Hanafiacellulose (HPC).et al. [ 30HPC] reported was first the dissolved preparation in water of a porousin a concentration membrane of from 20, and a water-soluble a 0.5% wt glutaraldehyde biopolymer, hydroxypropyl cellulose (HPC). HPC was first dissolved in water in a concentration of 20, Membranes 2020, 10, x; doi: www.mdpi.com/journal/membranes and a 0.5% wt glutaraldehyde crosslinker was added in a dope solution after degassing to fix the membrane morphology and to prevent resolubilization. Just before casting the membrane, 1% wt HCl as a catalyst was added to initiate the crosslinking reaction. The phase inversion was induced by elevating the temperature above the lower critical solution temperature (LCST) of the prepared solution. The membrane exhibited porous characteristics when analyzed with SEM. The membrane stability in various organic solvents (DMSO, tetrahydrofuran (THF), methanol, and chloroform) were confirmed showing the effectiveness of chemical crosslinking. This work has opened a new paradigm to fabricate polymer membranes using water as the solvent, following the green chemistry principles [30].

3.2. γ-Valerolactone (GVL) There are some classical solvents that could potentially be considered as green solvents. Mainly, GVL is a solvent with a low melting and high ﬂash point; it has a characteristic herbal odor, and recently, it has been used in ﬂavor, perfume, and food industries [42]. Membranes 2021, 11, 19 9 of 21

GVL exhibits advantages such as a stable liquid form at ambient conditions, very low toxicity (LD50 oral rats = 8800 mg/kg) [43], high miscibility with water, and good stability in neutral media [23,44]. Importantly, GVL can be produced from cellulose-based biomasses [43]. With the objective of exploring bio-based green solvents in membrane preparation, Vankelecom et al. [23] prepared porous membranes via NIPS with common polymers (PI, PES, polysulfone (PSU), cellulose acetate (CA), and cellulose triacetate (CTA)) using GVL and a set of glycerol derivatives [23]. The employed bio-based cosolvents were α,α0- diglycerol, glycerol, monoacetin, glycerol carbonate, and diacetin. The work reported successfully the preparation of microﬁltration (MF) and ultraﬁltration (UF) membranes and, potentially, NF by increasing the dope polymer concentration.

3.3. Methyl Lactate and Ethyl Lactate Figoli et al. (2014) [5] thoroughly reviewed the potential green solvents. Among the candidates, the lactate family is a group of nontoxic and biodegradable solvents with outstanding characteristics [5] that can replace toxic and environmental-unfriendly ones. Lactate esters can be obtained from bio-derived carbohydrate compounds, and the recent development of purification processes lowered the price for lactate products remark- ably [24]. Additionally, it has been employed as an intermediate for polymers and chemicals such as basic dyes and herbicide [24]. Methyl lactate has a high miscibility with water, allowing it to function as a versatile solvent for membrane preparation via NIPS. Vankelecom et al. [24] made use of this solvent’s outstanding green characteristics to fabricate asymmetric cellulose acetate (CA) NF membranes via NIPS. The CA membrane showed a NF performance with 99.5% rejection for rose bengal (1017 MW Da) dye with 2.4-L/(m2·h·bar) permeance in water [24]. Meanwhile, the authors investigated the effects of cosolvent (2-methyl THF) up to 30–50% wt in dope solution, which proportionally made the membrane looser [24]. Ethyl lactate is another important monobasic ester found in small amounts in foods such as chicken, wine, and some fruits [45]. Ethyl lactate can be obtained by the esterification of ethanol and lactic acid [46]. This solvent has attracted considerable attention recently, as it satisfies at least eight of the “twelve principles of green chemistry” [45], with desirable properties of low viscosity, high boiling point, low vapor pressure, and low surface tension [45]. MembranesMembranes 2020 2020, 10, 10, x, x 1414 of of 23 23 Membranes 2020, 10, x 14 of 23 Membranes 2020, 10, x 14 of 23

TableTable 2. 2. Potential Potential green green solvent solvent candidates. candidates. TableTable 2.2. PotentialPotential greengreen solventsolvent candidates.candidates. Membranes 2021, 11, 19 Table 2. Potential green solvent candidates. 10 of 21 SolubleSoluble No.No. SolventSolvent ChemicalChemical Structure Structure Soluble SustainableSustainable Characteristics Characteristics Ref.Ref. No. Solvent Chemical Structure SolublePolymer Sustainable Characteristics Ref. No. Solvent Chemical Structure PolymerPolymerPolymer Sustainable Characteristics Ref. Table 2. PotentialPolymer green solvent candidates. An inorganic, transparent, tasteless, odorless and nearly colorless No. Solvent Chemical Structure SolubleAnAn PolymerAn inorganic,inorganic, inorganic, transparent,transparent, transparent, tasteless,tasteless, tasteless, Sustainable odorlessodorless odorless Characteristics andand and nearlynearly nearly colorlesscolorless colorless Ref. 1.1. WaterWater HPCHPC Anchemicalchemical inorganic, substance substance transparent, with with tasteless,low low risk risk of odorlessof hazard, hazard, and low low nearly cost cost and andcolorless high high [30][30] 1. Water HPC chemical substanceAn inorganic, with low transparent, risk of hazard, tasteless, low odorless cost and and high nearly [30] 1. Water HPC chemical substance with low risk of hazard, low cost and high [30] 1. Water HPC colorless chemicalavailabilityavailability substance with low risk of hazard, [30] availability availabilitylow cost and high availability

γγ-Valerolactone-Valerolactone PI,PI, PES, PES, CA, CA, ColorlessColorless liquid, liquid, bio-based bio-based green green so solvent,lvent, low low toxicity toxicity and and miscible miscible 2.2. γ-Valerolactone PI, PES, CA, Colorless liquid, bio-basedColorless liquid,green bio-basedsolvent, low green toxicity solvent, and low miscible toxicity [23][23] 2. γ-Valerolactone2. γ-Valerolactone (GVL)(GVL) PI,CTA, PES, PI,PSU CA, PES, CA,Colorless CTA, PSU liquid,with bio-based water, stability green sounderlvent, neutral low toxicity media and miscible [[23]23] 2. (GVL)(GVL)(GVL) CTA,CTA,CTA, PSUPSU PSU withwithwithand water,water, water, miscible stabilitystability stability with underunder water, under stabilityneutralneutral neutral undermediamedia media neutral media [23] (GVL) CTA, PSU with water, stability under neutral media

ColorlessColorless and and clear clear substance substance with with a a peculiar peculiar odor, odor, high high boiling boiling Colorless and clearColorless substance and clear with substance a peculiar with odor, a peculiar high odor, boiling high CA,CA, cellulose cellulose Colorlesspointpoint and and slow clearslow volatility substancevolatility rate rate with with with a peculiar biodegradability, biodegradability, odor, high water boilingwater 3.3. MethylMethyl lactate lactate CA, cellulose point and slow volatilityboiling rate point with and slowbiodegradability, volatility rate withwater [23,24][23,24] 3. Methyl3. lactateMethyl lactate CA,derivatives celluloseCA, cellulose derivativespointmiscibility, and slow noncorrosive, volatility rate noncarcinogenic, with biodegradability, and non-ozone- water [23,24][23,24] 3. Methyl lactate derivativesderivativesderivatives miscibility,miscibility,miscibility, noncorrosive,noncorrosive, noncorrosive,biodegradability, nono noncarcinogenic,ncarcinogenic,ncarcinogenic, water miscibility, andand and noncorrosive,non-ozone-non-ozone- non-ozone- [23,24] derivatives miscibility, noncorrosive,noncarcinogenic,depleting noncarcinogenic, andfeatures non-ozone-depleting and non-ozone- features depletingdepletingdepleting featuresfeatures features depleting features

Colorless, sweet smell and clear substance, low toxicity, and Colorless,Colorless,Colorless, sweetsweet sweetColorless, smellsmell smell and sweetand and clearclear clear smell substance,substance, substance, and clearsubstance, lowlow low toxicity,toxicity, toxicity, low toxicity,andand and 4. Ethyl lactate CA agreeableColorless, odor, sweet water smell miscibility, and clear noncorrosive,substance, low noncarcinogenic, toxicity, and [45] 4. Ethyl4.4.4. lactateEthylEthylEthyl lactatelactate lactate CACACA CAagreeableagreeableagreeable odor,odor, odor, waterwater waterand agreeable miscibility,miscibility, miscibility, odor, noncorrosive,noncorrosive, noncorrosive, water miscibility, noncarcinogenic,noncarcinogenic, noncarcinogenic, noncorrosive, [[45][45]45[45]] 4. Ethyl lactate CA agreeable odor, waterandnoncarcinogenic, non-ozone-depleting miscibility, andnoncorrosive, non-ozone-depleting features noncarcinogenic, features [45] andandand non-ozone-depletingnon-ozone-depleting non-ozone-depleting featuresfeatures features and non-ozone-depleting features

Bio-basedBio-based origin, origin, clear clear colorless colorless to to light light yellow yellow liquid, liquid, mild, mild, smoky smoky Bio-basedBio-based origin,origin, clearBio-basedclear colorlesscolorless origin, toto clear lightlight colorless yellowyellow to liquid,liquid, light yellow mild,mild, liquid,smokysmoky Bio-basedketone-likeketone-like origin, odor, odor, clear with with colorless a a compar compar to ativelylightatively yellow high high dynamicliquid, dynamic mild, viscosity viscosity smoky, , 5.5. CyreneCyrene PES,PES, PVDF PVDF ketone-like odor,mild, with smoky a compar ketone-likeatively odor,high withdynamic a comparatively viscosity, [47,48][47,48] 5.5. Cyrene Cyrene PES, PVDF PES, PVDFcomparableketone-like solventodor, with properties a compar toatively NMP without high dynamic nitrogen viscosity or sulfur, [47,48][47,48] 5. Cyrene PES, PVDF comparablecomparablecomparable solventsolvent solventhigh propertiesproperties dynamicproperties viscosity, toto to NMPNMP NMP comparable withoutwithout without nitrogennitrogen nitrogen solvent properties oror or sulfursulfur sulfur [47,48] comparable solvent toproperties NMPheteroatoms withoutheteroatoms to NMP nitrogen without or sulfur nitrogen heteroatoms or sulfur heteroatoms heteroatoms

RhodiasolvRhodiasolv®® Clear,Clear, colorless colorless to to yellow yellow liquid liquid with with slight slight odor, odor, solubility solubility in in 6.6. Rhodiasolv® PI,PI, CA, CA, PVDF PVDF Clear, colorless to yellow liquid with slight odor, solubility in [10,24,26][10,24,26] 6. RhodiasolvPolarclean® PI, CA, PVDF water,Clear, highcolorless solvency to yellow capability, liquid eco-friendly with slight odor, sustainable solubility solvent in [10,24,26] 6. PolarcleanPolarcleanPolarclean PI, CA, PVDF water,water,water, highhigh high solvencysolvency solvency capability,capability, capability, eco-friendlyeco-friendly eco-friendly sustainablesustainable sustainable solventsolvent solvent [10,24,26] Polarclean water, high solvency capability, eco-friendly sustainable solvent

MembranesMembranes 2020 2020, 10, 10, x;, x; doi: doi: www.mdpi.c www.mdpi.com/journal/mom/journal/membranesembranes Membranes 2020, 10, x; doi: www.mdpi.com/journal/membranes Membranes 2020, 10, x; doi: www.mdpi.com/journal/membranes Membranes 2020, 10, x 14 of 23

Table 2. Potential green solvent candidates.

Soluble No. Solvent Chemical Structure Sustainable Characteristics Ref. Polymer

An inorganic, transparent, tasteless, odorless and nearly colorless 1. Water HPC chemical substance with low risk of hazard, low cost and high [30] availability

γ-Valerolactone PI, PES, CA, Colorless liquid, bio-based green solvent, low toxicity and miscible 2. [23] (GVL) CTA, PSU with water, stability under neutral media

Colorless and clear substance with a peculiar odor, high boiling CA, cellulose point and slow volatility rate with biodegradability, water 3. Methyl lactate [23,24] derivatives miscibility, noncorrosive, noncarcinogenic, and non-ozone- depleting features

Colorless, sweet smell and clear substance, low toxicity, and 4. Ethyl lactate CA agreeable odor, water miscibility, noncorrosive, noncarcinogenic, [45] and non-ozone-depleting features Membranes 2021, 11, 19 11 of 21 Bio-based origin, clear colorless to light yellow liquid, mild, smoky ketone-like odor, with a comparatively high dynamic viscosity, 5. Cyrene PES, PVDF [47,48] Table 2. Cont. comparable solvent properties to NMP without nitrogen or sulfur No. Solvent Chemical Structure Soluble Polymerheteroatoms Sustainable Characteristics Ref.

Rhodiasolv® Clear, colorless toClear, yellow colorless liquid to with yellow slight liquid odor, with solubility slight odor, in 6. Rhodiasolv6. ® Polarclean PI, CA, PVDFPI, CA, PVDF solubility in water, high solvency capability, [[10,24,26]10,24,26] Polarclean water, high solvency capability, eco-friendly sustainable solvent MembranesMembranes 2020 2020, 10,, x10 , x eco-friendly sustainable solvent 15 of15 23 of 23

Colorless,Colorless, corrosive corrosive liquid,Colorless, liquid, combustible, combustible, corrosive slowly liquid, slowly combustible, dissolves dissolves in slowly waterin water TriethylTriethyl dissolves in water and sinks in water, no components 7. Triethyl7. phosphate (TEP) CA, PVDF CA, PVDFand sinks in water, no components supposed of persistence, bio- [[27]27] 7. phosphate (TEP) CA, PVDF and sinks in water,supposed no components of persistence, supposed bio-accumulation, of persistence, and bio- toxicity [27] Membranesphosphate 2020, 10 ,(TEP) x; doi: www.mdpi.caccumulation,om/journal/m andembranes toxicity or high persistence in the environment accumulation, and toxicityor or high high persistence persistence in thein the environment environment

PAN,PAN, LowLow melting melting point, point,Low high melting high thermal thermal point, stability, high stability, thermal low low stability,viscosity, viscosity, low low viscosity, low 8. Ionic LiquidsIonicIonic (ILs)Liquids Liquids PAN, Cellulose, PES, CA low chemical reactivity, and negligible vapor pressure [28,29] 8. 8. Cellulose,Cellulose, chemicalchemical reactivity, reactivity, and and neglig negligibleible vapor vapor pressure pressure without without [28,29][28,29] (ILs)(ILs) without ﬂammability Ex) 1-ethyl-3-ethylimidazoliumEx)Ex) 1-ethyl-3- 1-ethyl-3- acetatePES,PES, CA CA flammabilityflammability ethylimidazoliumethylimidazolium acetate acetate

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3.4. Cyrene One of the emerging green solvents for replacing polar aprotic solvents (e.g., NMP, DMAc, and DMF) is the bio-based dihydrolevoglucosenone (Cyrene) [47]. Cyrene currently is a commercially available and nontoxic solvent obtained from renewable waste and a nonfood cellulosic source [25]. According to the work carried out by Sherwood et al. [47], Cyrene can be produced via two simple steps from the biomass, which can cut down on environmental footprints. The computer-aided Kamlet-Abboud-Taft (KAT) solvatochromic parameters indicate that Cyrene is aprotic, with an equivalent π* to those of strongly dipolar aprotic solvents but with a slightly lower β value (hydrogen bond-accepting ability) [47]. Interestingly, the solvent properties of Cyrene are very comparable to NMP without nitrogen or sulfur heteroatoms, which can result in NOx and SOx emissions upon disposal, reducing the environmental impact [47]. The application of Cyrene in membrane preparation was reported by Figoli et al. [48] for preparing PES and PVDF membranes by phase inversion, where the membranes were fabricated via simultaneous VIPS and NIPS methods [48]. In this work, the exposure time to atmospheric relative humidity was changed between 0 and 5 min to gain membranes with different pore sizes and permeability. Pure water permeability tests gave promising data for membranes fabricated via NIPS (∼23,400 L/m2/h/bar) and VIPS-NIPS (~18,700 L/m2/h/bar) [48]. The obtained results indicated the possibility to use Cyrene as a solvent for two of the most used polymers in the membrane industry. Additionally, the experimental results were discussed in connection to the viscosity of the dope solution, membrane morphology, ternary phase diagram, porosity, thickness, and contact angle [48]. Cyrene has also been used by other researchers to prepare PES (with polyvinylpyrrolidone (PVP) used as an additive) flat sheet membranes for water filtration application by NIPS technique [25]. An extensive comparison was made between membranes prepared using Cyrene and NMP. Membranes prepared with bio-based Cyrene were more sustainable, with less polymer losses, tunable pore sizes, and contact angles [25]. The authors claimed that membranes prepared with Cyrene exhibited higher total porosity and larger pore sizes in comparison with those fabricated using NMP [25]. By changing the temperature of the dope solution, which affects the viscosity of the dope solution, the permeability of membranes prepared with Cyrene was easily controlled in the range between NF/RO to MF applications [25]. Further testing of this solvent and other derivatives of Cyrene has been conducted to get more understanding about the replacing capacity of these bio-based solvents for NMP and similar solvents in filtration applications [47].

3.5. Polarclean More recently, another promising eco-friendly alternative to replace polar aprotic solvents (e.g., NMP, DMAc, and DMF), Rhodiasolv® Polarclean (hereafter referred to as Polarclean), was ﬁrst suggested by Lee and Drioli et al. [49] for membrane preparation. The IUPAC name of the solvent is methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate, and it is featured by a high solvent power and low volatility [49]. Polarclean has been employed as a solvent for agrochemicals and as a crystal growth inhibitor in cold solutions. Furthermore, its solubility in water is very high at 490 g/dm−3 at 24 ◦C[49]. Polarclean was ﬁrst applied as an environmentally benign TIPS solvent to prepare PVDF membranes [10]. A comprehensive investigation was conducted to understand the underlying phenomena in the membrane formation kinetics during the TIPS process, and it was shown that the NIPS effect cannot be ignored at the dope-nonsolvent interface upon immersion into the water bath [10]. This phenomenon is now referred to as N-TIPS. Combining NIPS and TIPS (e.g., N-TIPS) is a versatile technique to prepare membranes with narrow pore size distributions. This, however, can lower the membrane performance due to the formation of a dense skin layer by mass exchange at a solvent-nonsolvent interface [26]. Another simple and effective solution was suggested by Jung and et al. [26] by employing a triple spinneret to apply in a transient coating layer to eliminate the NIPS Membranes 2021, 11, 19 13 of 21

effect. The authors investigated the effects of different solvents in forming macro-porous hollow-ﬁber membranes using Polarclean as the dope solvent [26]. The Polarclean solvent was then employed as a NIPS solvent to prepare PI, CA, PES, and PSU membranes by Wang et al. [14]. The work showed that MF, UF, NF, and gas separation membranes can be fabricated, showing the versatility of this environmentally benign Polarclean solvent. In another work carried out by Marino et al. [50], porous PES membranes were successfully fabricated with high pure water permeability. The prepared membranes were fabricated via the combined NIPS and VIPS technique using PolarClean as the solvent. Use of common additives such as PVP and poly(ethylene glycol) (PEG) affected the membrane morphology [50].

3.6. Triethyl-Phosphate (TEP) TEP is a widely used solvent in the industrial scale as an intermediate for insecticide products, as a catalyst for anhydride synthesis, and as an agent for manufacturing rubbers and plastics. TEP appears as a colorless, corrosive liquid with a mildly pleasant odor. Although TEP is considered harmful if swallowed (Regulation No. 1272/2008 by the EU), this substance contains no components associated with bioaccumulation [27]. There are many works that employed TEP for membrane preparation [27,51,52]. It can be applied as both a NIPS and TIPS solvent. Notably, Sufyan Fadhil et al. [27] reported research on fabricating poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) flat sheet membranes using a TEP solvent prepared for direct contact membrane distillation (DCMD) [27]. The authors employed a phase inversion technique with various coagulation bath conditions to optimize the membrane performance for DCMD.

3.7. Ionic Liquid (IL) and Deep Eutectic Solvents (DES) An ionic liquid (IL) is a salt that exists in the state of a liquid, composed of ions and short-lived ion pairs. In the context of green solvents, ILs have two features that can be considered green, depending on their application. First, ILs have negligible vapor pressure (low volatility) without concerns over volatile organic compounds. ILs have been applied in membrane technology rather often recently. Cellulose hollow ﬁbers were fabricated by spinning with a dope solution comprising 1-ethyl-3-methylimidazolium acetate ((EMIM)(Ac)), 1-ethyl-3-ethylimidazolium diethyl phosphate ((EMIM)(DEP)), or 1,3-dimethylimidazolium chloride ((EMIM)(Cl)) [28]. Secondly, ILs allow membrane preparation by a single-step procedure, which means cellulose (nonsoluble in most solvents) can be directly dissolved in the target solvent. A similar work by Sukma et al. [29] reported the preparation of NF cellulose membranes via phase inversion using 1-ethyl- 3-methylimidazolium acetate ((EMIM)OAc) as the solvent and acetone as the volatile cosolvent [29]. Abdellah et al. [51] successfully fabricated a polyester TFC OSN membrane on a cellulose substrate that was prepared using IL as the solvent. In addition, the active layer was composed of quercetin (a polyphenol normally found in blueberries, grapefruit, and black tea) crosslinked with terephthaloyl chloride. In comparison to the currently employed organic solvents, ILs undeniably stand as a potential alternative with excellent thermal stability, low chemical reactivity, and negligible vapor pressure without ﬂammability. However, in spite of such advantages, some concerns remain over the green-ness of the ILs, including unsustainable synthesis protocols [44], high cost, and uncertain biodegradability in many types of ILs [44]. Hence, further research to carefully assess the sustainability of ILs must be performed. On the other hand, deep eutectic solvents (DES) are considered as a new generation of ILs and have emerged as a biodegradable and renewable green solvent. DES are available at a much lower price [53], and they can be produced without generating excessive waste. This solvent group has brought comparable physicochemical properties to those of ILs. In Membranes 2021, 11, 19 14 of 21

comparison to ILs, DES can be easily prepared from the available starting materials with universal dissolution abilities, extensive tenability, and ease of synthesis [54]. Though DES has not been fully employed as a solvent for membrane fabrication yet, some researchers have been uncovering DES as flux boosting and surface cleaning agents for TFC polyamide membranes [54]. A considerable flux increase was obtained after treating polyamide TFC by DES at low temperatures without a noticeable change in the rejection performance. The authors asserted such an improvement is due to the improved wettability on the surface of DES-treated membranes, which enhanced the smoothness of a polyamide TFC membrane in the process of DES treatment [54]. The work reused the DES for activating the surface of polyamide TFC membranes for six consecutive cycles. In another work carried out by Esmaeili [53], DES (containing choline chloride and lactic acid in a molar ratio of 1:9) was used to extract lignin. The DES-lignin 0–1% wt mixture was employed as a hydrophilic adhesion promoter to fabricate antioxidant PES membranes by the phase inversion technique [53]. In this work, NMP was used as the main solvent. The membrane properties were investigated in terms of effect of DES-lignin on the antioxidant activity and membrane performance [53]. Through the obtained results, DES-lignin-modified membranes showed improved hydrophilicity, antioxidant activity, and high pure water permeability, with less negative surface charge in comparison to a pristine PES membrane, while rejection remained almost constant [53]. Although the work used NMP as the dope solvent, other green solvents could be employed in further studies in the future.

3.8. Other Green Solvents (Non-OSN Membranes) Since the TIPS technique proceeds at higher temperatures (better polymer solubility), more possibilities are available to identify green solvents. Notably, one of the recently emerging green solvents is acetyl tributyl citrate (ATBC) without adverse health hazards. ATBC has been reported and widely used as a plasticizer for pharmaceutical coatings and food packaging [11]. ATBC is not miscible with water, and it is considered to be much more environmentally benign in comparison with phthalate-based TIPS solvents [11]. ATBC was employed to fabricate a PVDF membrane with competitive performance. Up to 50% wt of the polymer concentration was used in order to maximize the mechanical strength of the membranes, and the prepared membrane was then stretched to enhance the porosity and permeability [11]. The idea of using universal crystallizable diluent to fabricate a porous membrane by TIPS has been proposed in some reports, and one of the key points herein was seeking for a proper diluent based on the crystallinity and high melting point of polar polymers [31]. Fan et al. [31] employed dimethyl sulfone (DMSO2) as a universal crystallizable TIPS diluent for polar polymer membranes of PVDF, PAN, and CA, because the solvent’s solubility parameter is close to those of the polymers [31]. The tensile strength and elongation of the prepared membranes could be improved if the polymer concentration or cooling rate was increased. Notably, recrystallization and sublimation can be applied to recover and reuse DMSO2. This work can be considered as a green preparation method for microporous polymer membranes via TIPS [31]. Chaouachi et al. [55] reported the use of butyl acetate as a greener solvent to fabricate membranes from recycled low-density polyethylene (LDPE) via the TIPS method [55]. LDPE is a semi-crystalline polymer, soluble only at elevated temperatures in toxic organic solvents such as phthalate-based compounds [55]. The authors made a comparison using xylene as a solvent to assess the difference, and the results indicated that the membrane prepared using butyl acetate showed desirable properties in terms of thickness (~0.363 mm), pore size (~0.14 µm), and contact angle (~120◦). These results were encouraging, because the polymer was valorized from recycled sources [55]. Membranes 2021, 11, 19 15 of 21

Membranes 2020, 10, x 16 of 23 4. Sustainability for Membrane Fabrication Procedure 4.1. Minimizing the Number of Fabrication Steps and Materials 4. Sustainability for Membrane Fabrication Procedure Similar to reverse osmosis technology, the trend in OSN membranes is shifting from 4.1.asymmetric Minimizing the to Number TFC membranes of Fabrication that Steps guarantee and Materials better performance. Recently, a TFC membrane with 10-nm selective layer thickness with ultrafast solvent permeance was reportedSimilar [to56 reverse]. In the osmosis TFC membrane,technology, the the trend selective in OSN layer membranes mostly isgoverns shifting from the asymmetric separation toperformance, TFC membranes and that the guarantee support layer better provides performance. the necessary Recently, mechanicala TFC membrane support with and 10-nm han- selectivedleability layer [2 thickness]. The performance with ultrafast of solvent TFC membranespermeance was can reported be affected [56]. In signiﬁcantly the TFC membrane, by the thesupport selective layer, layer and mostly thus, governs optimization the separation of the performance, support layer and morphology the support islayer also provides important. the necessaryParticularly mechanical for OSN, support the support and handleability layer also must[2]. The be solvent-stable,performance of limitingTFC membranes the number can be of affectedoptions significantly available. by the support layer, and thus, optimization of the support layer morphology is also important. Particularly for OSN, the support layer also must be solvent-stable, limiting the Recently, Lee et al. [32] employed a commercially available PE battery separator as number of options available. a porous support to prepare TFC OSN membranes with impressive performances [32]. Recently, Lee et al. [32] employed a commercially available PE battery separator as a porous A porous PE battery separator is a fascinating support material for OSN applications, support to prepare TFC OSN membranes with impressive performances [32]. A porous PE battery as it exhibits a very high solvent resistance at an ambient temperature with outstanding separator is a fascinating support material for OSN applications, as it exhibits a very high solvent mechanical strengths. resistance at an ambient temperature with outstanding mechanical strengths. TheThe fabrication fabrication procedure procedure is illustrated is illustrated in Figure in Figure 6. It 6can. It be can seen be seenthat, that,in comparison in comparison to the conventionalto the conventional TFC-PI membrane TFC-PI membrane process, the process, TFC-PE themembrane TFC-PE can membrane be prepared can with be prepared just two steps: with just two steps: O plasma for hydrophilization (20 s), followed by interfacial polymerization O2 plasma for hydrophilization2 (20 s), followed by interfacial polymerization (10~15 min). Superior performances(10~15 min). were Superior obtained performances across all ranges were of obtained solvents with across TFC-PE all ranges membranes of solvents compared with to TFC- the conventionalPE membranes TFC-PI compared membranes to [32]. the In conventional comparison, TFC-PIfabrication membranes of the TFC-PI [32 membrane]. In comparison, can take upfabrication to three to four of the days, TFC-PI starting membrane from dissolution can take of the up polymer to three to a to lengthy four days, crosslinking starting reaction from period.dissolution of the polymer to a lengthy crosslinking reaction period.

Figure 6. Comparison of fabrication protocols for (a) the TFC-PI membrane prepared Figure 6. Comparison of fabrication protocols for (a) the TFC-PI membrane prepared on crosslinked polyimideon crosslinked (PI) support polyimide [57,58] and (PI) (b support) proposed [57 TFC-PE,58] and membrane (b) proposed on plasma-treated TFC-PE membrane polyethylene on (PE)plasma-treated support [32] (reprinted polyethylene with permission (PE) support from [ 32[32]).] (reprinted with permission from [32]).

In Inanother another work work by Livingston by Livingston et al. et[33], al. a [ 33robust], a robust TFC OSN TFC membrane OSN membrane was fabricated was fabricated using the spray-coatingusing the spray-coating technique, with technique, excellent stability with excellent in high temperatures stability in highand fast temperatures solvent permeability and fast [33].solvent The details permeability of the proposed [33]. The method details are of schematized the proposed in methodFigure 7. areIt can schematized be seen that in fabrication Figure7. ofIt the can conventional be seen that asymmetric fabrication OSN of themembranes conventional can take asymmetric up to one week OSN and membranes requires 0.11 can kg take of 2 polymerup to one per weekm2 of andmembrane. requires On 0.11 the kgother of polymerhand, the perprop mosedof method membrane. only takes On the half other a day hand, and requiresthe proposed only 0.02 method kg of polymer only takes per m half2 of amembrane. day and requiresThe prepared only membranes 0.02 kg of polymerwere tested per up m to2 15of days membrane. in DMF solution The prepared at different membranes temperatures were [33]. tested up to 15 days in DMF solution at different temperatures [33].

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Membranes 2020, 10, x 17 of 23

FigureFigure 7. Schematic 7. Schematic diagrams diagrams for the convention for the conventionalal fabrication fabrication methods to methods make (a) to commercialized make (a) com- PI organicmercialized solvent nanofiltration PI organic (OSN) solvent membranes, nanoﬁltration (b) poly(ether (OSN) ether membranes, ketone) (PEEK) (b) poly(ether OSN membranes, ether and (cketone)) proposed (PEEK) sequential OSN spraying membranes, and drying and (c fabrication) proposed methods sequential with spraying compact andsteps drying [33] (reprinted fabri- with permissioncation methods from [33]). with compact steps [33] (reprinted with permission from [33]).

ThisThis method method has has several several advantages advantages from from the thegreen green perspective, perspective, such suchas a lower as a lowercarbon carbonfootprint, footprint, lower consumption lower consumption of solvent and of solvent polymers, and and polymers, shorter time and for shorter the coagulation time for the and coagulationwashing steps and [33]. washing steps [33].

4.2. Sustainable Sustainable Post-Modification Post-Modiﬁcation OSNOSN membranes membranes must must show show solvent solvent resistance, resistance, and the and crosslinking the crosslinking technique technique is the most is thewidely most employed widely employedmethod to improve method the to improvepolymer thesolvent polymer stability. solvent Generally, stability. a crosslinking Generally, step a crosslinkingemploys toxic stepdiamines, employs dihalide, toxic diamines,diols, and other dihalide, reactive diols, chemicals and other in organic reactive solvents chemicals at high in organictemperatures solvents [34]. The at high conditions temperatures required [for34 ].crosslinking The conditions are very required harsh and for certainly crosslinking not green. are verySzekely harsh andet al. certainly [34] proposed not green.a unique method to improve the polymer chemical stability without crosslinkingSzekely the et al.backbone. [34] proposed The work a unique employed method the concept to improve of interpenetrating the polymer chemical polymer stabilitynetworks without(IPN) using crosslinking dopamine themonomers backbone. to fabricate The work PBI/polydopamine employed theconcept (PDA) OSN of interpenetrating membranes with polymerunprecedented networks thermal (IPN) and using chemical dopamine stability. monomers Surprisingly, to fabricate the PBI PBI/polydopaminebackbone was not chemically (PDA) OSNcrosslinked, membranes yet exhibited with a unprecedented solvent stability thermalin polar aprotic and chemical solvents (DMF, stability. NMP, Surprisingly, and DMAc), theeven PBIat temperatures backbone wasabove not 100 chemically °C. A close crosslinked,examination revealed yet exhibited that the a solventdopamine stability monomer in polarformed aproticanother solventsintrachain (DMF, network NMP, within and the DMAc), PBI back evenbone, at temperaturesforming a strong above H-bond 100 ◦withC. A the close PBI examinationimidazole groups. revealed Hence, that PBI and the PDA dopamine were physical monomerly crosslinked formed anotherin a form intrachainof semi-IPN. network A density withinfunctional the theory PBI backbone, (DFT) analysis forming revealed a strong that the H-bond H-bond with between the PBI polydopamine imidazole groups. (PDA) and Hence, PBI’s PBIimidazole and PDAgroups were were physically stronger than crosslinked the solv ination aform energy of semi-IPN.by aprotic solvents, A density supporting functional the theoryobserved (DFT) solvent analysis stability. revealed that the H-bond between polydopamine (PDA) and PBI’s imidazoleThe notable groups part were is the stronger sustaina thanbility the achieved solvation for the energy fabrication by aprotic of such solvents, membranes. supporting The key thestep observedto fabricate solvent this membrane, stability. as shown in Figure 8, was the in-situ polymerization of the dopamine withinThe a PBI notable membrane. part is A the dope sustainability solution comprising achieved PBI for and the do fabricationpamine monomers of such membranes.was prepared; then, it was cast and phase-inverted. The dopamine monomer, during this stage, plays a role as an The key step to fabricate this membrane, as shown in Figure8, was the in-situ polymer- additive. The polymerization of the remaining dopamine monomer was carried out in the aqueous ization of the dopamine within a PBI membrane. A dope solution comprising PBI and media of a mixed NaIO4 and Tris buffer solution, yielding a semi-IPN membrane with PDA and PBI. dopamine monomers was prepared; then, it was cast and phase-inverted. The dopamine The stability and performance of the fabricated membranes with different polymerization times were monomer, during this stage, plays a role as an additive. The polymerization of the re- tested at fixed pressures and temperatures. Compared to the pristine PBI membranes, the PDA/PBI- maining dopamine monomer was carried out in the aqueous media of a mixed NaIO IPN membranes showed a robust stability in harsh aprotic solvents with good NF performances. 4 and Tris buffer solution, yielding a semi-IPN membrane with PDA and PBI. The stability and performance of the fabricated membranes with different polymerization times were tested at ﬁxed pressures and temperatures. Compared to the pristine PBI membranes, the PDA/PBI-IPN membranes showed a robust stability in harsh aprotic solvents with good NF performances. Membranes 2021, 11, 19 17 of 21

Membranes 2020, 10, x 18 of 23

Figure 8.Figure Process 8. Process of preparing of preparing a PBI nanofiltration a PBI nanofiltration (NF) membrane (NF) membrane applying applying the interpenetrating the interpenetrating polymerpolymer network network (IPN): ( (IPN):A) casting (A) castingthe membrane the membrane from the from do thepe solution dope solution containing containing PBI and PBI and polydopaminepolydopamine (PDA) and (PDA) (B) and immersing (B) immersing the membrane the membrane in a coagulation in a coagulation bath; bath;membrane membrane was was immersed in (C) NAIO and (D) Tris buffer solution, subsequently. (E,F) illustrate the IPN formed immersed in (C) NAIO4 and (D4 ) Tris buffer solution, subsequently. (E,F) illustrate the IPN formed betweenbetween PBI and PBIPDA and (reprinted PDA (reprinted with permission with permission from [34]). from [34]). In another work of Szekely et al. [35], the performance of the OSN membrane was In anotherenhanced work of by Szekely a coating et withal. [35], bio-phenol the performance materials. of From the OSN the economicalmembrane perspective,was enhanced plant- by a coating withoriginated bio-phenol bio-phenols materials. canFrom be the low-cost economical alternatives. perspective, The plan workt-originated employed bio-phenols tannic acid can be low-cost(originated alternatives. from corkThe oak),work vanillyl employed alcohol tannic (from acid vanilla (originated pods), from eugenol cork (extracted oak), vanillyl from alcohol (fromcloves), vanilla morin pods), (guava eugenol leaves), (extracted and quercetin from cloves), (common morin flavonol (guava amongst leaves), plants).and quercetin First, a (common flavonolPBI membrane amongst was plants). cast from First, a a 19% PBI to membrane 24% wt polymer was cast dope from solution, a 19% to which 24% waswt polymer homoge- dope solution,neously which dissolved, was homogeneously on a polypropylene dissolve nonwovend, on a polypropylene support. The nonwoven pristine membrane support. The was pristine membranethen immersed was then in immersed a solution in containing a solution the containing aforementioned the aforementioned bio-phenols bio-phenols for bio-coating. for bio-coating.This This step step was was followed followed by by immersion immersion into into a NaIOa NaIO4 solution4 solution for for complete complete coating. coating. The The sol- solvent resistancevent resistance of the prepared of the preparedmembranes membranes were investigated were investigated by soaking by them soaking in eight them common in eight organic solventscommon and organicfour green solvents solvents and four[35]. greenThe membrane solvents [35 performance]. The membrane indicated performance that the indi-bio- cated that the bio-phenol coating resulted in a denser membrane with a lower permeance phenol coating resulted in a denser membrane with a lower permeance (22–92%) and higher rejection (22–92%) and higher rejection (12–79%). Importantly, the work also demonstrated that the (12–79%). Importantly, the work also demonstrated that the membrane swelling could be cut down membrane swelling could be cut down by up to 80% after coating. by up to 80% after coating. 5. Conclusions 5. Conclusions Improving the sustainability of membrane fabrication is an important and pressing Improvingissue the in thesustainability field of membrane of membrane technology, fabrication requiring is an more important efforts andand attention.pressing issue Perhaps in the the field of membranemost pressing technology, matter requiring is the fact thatmore the ef useforts of and common attention. polar Perhaps aprotic solvents the most (DMF, pressing NMP, matter is theand fact DMAc) that the will use be of restricted common in polar May 2020.aprotic Although solvents the(DMF, application NMP, and range DMAc) of membrane will be restricted intechnology May 2020. isAlthough very wide, the weapplication mostly focused range of on membrane the sustainability technology of OSN,is very particularly wide, we of polymeric membranes. As compiled in this review, many of the works focused on mostly focused on the sustainability of OSN, particularly of polymeric membranes. As compiled in identifying greener alternatives. However, recently, many greener fabrication protocols this review, many of the works focused on identifying greener alternatives. However, recently, many have also been reported to minimize fabrication mass intensity. greener fabrication protocols have also been reported to minimize fabrication mass intensity.

Membranes 2021, 11, 19 18 of 21

With increasingly strict regulations on environmental protection, research interest should be paid to not only the greener fabrication process of membranes but, also, the whole life cycle of membrane fabrication. It is necessary to carry out comprehensive research by following the “green” concept for the solvent-solute membrane system. Moreover, sustainable fabrication could be more meaningful if experimental and optimal conditions were provided, setting the basis for a sustainable analysis. With updated applications and trends in the fabrication of polymeric membranes, hopefully, encouraging movements will be further carried out in the path of achieving sustainability.

Author Contributions: Conceptualization, H.Y.N.T and J.F.K.; writing—original draft preparation, H.Y.N.T.; writing—review and editing, B.T.D.N. and J.F.K.; and funding acquisition, J.F.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by Incheon National University Research Grant in 2019. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations Abbreviation Full Name (EMIM)(Ac) 1-ethyl-3-methylimidazolium acetate (EMIM)(Cl) 1,3-dimethylimidazolium chloride (EMIM)(DEP) 1-ethyl-3-ethylimidazolium diethyl phosphate (EMIM)OAc 1-ethyl-3-methylimidazolium acetate AGET Activator generated by electron transfer ATBC Acetyl tributyl citrate ATRP Atom transfer radical polymerization CTA Cellulose triacetate DCMD Direct contact membrane distillation DES Deep eutectic solvents DFT Discrete Fourier transform DGE Diglycidyl ether DMC Dimethyl carbonate DMF Dimethylformamide DMSO Dimethyl sulfoxide DMSO2 Dimethyl sulfone EIPS Evaporation induced phase separation EU Europe GVL γ-Valerolactone HPC Hydroxypropyl cellulose IL Ionic liquid IP Interfacial polymerization LCST Lower critical solution temperature LDPE Low-density polyethylene MF Microfiltration MOF Metal-organic frameworks MPD m-phenylenediamine MW Molecular weight NaAlg Sodium alginate NF Nanofiltration NIPS Non-solvent induced phase separation NMP N-methylpyrrolidone N-TIPS NIPS-TIPS OSN Organic Solvent Nanofiltration PA Polyamide PAE Poly(arylene ether sulfone) Membranes 2021, 11, 19 19 of 21

PAES Poly(arylene ether sulfone) PAN Polyacrylonitrile PBI Polybenzimidazole PDA Polydopamine PE Polyethylene PEEK Poly(ether ether ketone) PEG Poly(ethylene glycol) PEI Poly(ethylene imine) PES Polyethersulfone PES-TA Tertiary amine groups PI Polyimide PLA Polylactide PP Polypropylene PS Polystyrene PSU Polysulfone PVDF-HFP Poly(vinylidene fluoride-hexafluoropropylene) PVP Polyvinylpyrrolidone RIPS Reaction induced phase separation SDS Sodium dodecyl sulfate SEM Scanning electron microscopy ST Sodium tartrate TA Tannic acid TEP Triethyl-phosphate TFN Thin-film nanocomposite THF Tetrahydrofuran TIPS Thermally induced phase separation TMC Trimesoyl chloride UF Ultrafiltration VIPS Vapor-induced phase separation

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