Valerolactone As Bio-Based Solvent for Nanofiltration Membrane

membranes

Article γ-Valerolactone as Bio-Based Solvent for Nanoﬁltration Membrane Preparation

Muhammad Azam Rasool and Ivo F. J. Vankelecom *

Membrane Technology Group (MTG), Division cMACS, Faculty of Bioscience Engineering, KU Leuven, Celestijnenlaan 200F, P.O. Box 2454, 3001 Leuven, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +32-1632-1594

Abstract: γ-Valerolactone (GVL) was selected as a renewable green solvent to prepare membranes via the process of phase inversion. Water and ethanol were screened as sustainable non-solvents to prepare membranes for nanoﬁltration (NF). Scanning electron microscopy was applied to check

the membrane morphology, while aqueous rose Bengal (RB) and magnesium sulphate (MgSO4) feed solutions were used to screen performance. Cellulose acetate (CA), polyimide (PI), cellulose triacetate (CTA), polyethersulfone (PES) and polysulfone (PSU) membranes were ﬁne-tuned as materials for preparation of NF-membranes, either by selecting a suitable non-solvent for phase inversion or by increasing the polymer concentration in the casting solution. The best membranes were prepared with CTA in GVL using water as non-solvent: with increasing CTA concentration (10 wt% to 17.5 wt%) in the casting solution, permeance decreased from 15.9 to 5.5 L/m2·h·bar while RB rejection remained higher than 94%. The polymer solubilities in GVL were rationalized using Hansen solubility parameters, while membrane performances and morphologies were linked to viscosity measurements and cloudpoint determination of the casting solutions to better understand the kinetic and thermodynamic aspects of the phase inversion process.

Citation: Rasool, M.A.; Vankelecom, Keywords: bio-based solvent; green solvent; polymer solubility; polymeric membranes; nanofiltration I.F.J. γ-Valerolactone as Bio-Based Solvent for Nanofiltration Membrane Preparation. Membranes 2021, 11, 418. https://doi.org/10.3390/ 1. Introduction membranes11060418 Membrane technology offers separations in the chemical industry and water treatment Academic Editor: Enrico Drioli of small molecules (solutes) from solvent or water streams by using membranes and providing an economically viable alternative for separation and purification [1,2]. In Received: 6 May 2021 nanofiltration (NF), the separation process is run under pressure, rejecting molecules with Accepted: 20 May 2021 a molecular weight of 200–1000 Da. Published: 31 May 2021 Several techniques have been applied to prepare polymeric membranes, including temperature and non-solvent induced phase separation (TIPS and NIPS) [3–5]. Among all Publisher’s Note: MDPI stays neutral techniques, NIPS is the most versatile and widely used. In the NIPS process, a film cast with regard to jurisdictional claims in from a polymer solution is immersed in a coagulation bath. Upon immersion, demixing published maps and institutional affil- occurs, resulting in the solidification of the polymer and creation of a porous structure. iations. Controlling the process of demixing in the polymer film allows the desired membrane morphology to be fine-tuned [6–18]. NF membranes are asymmetric and often prepared via NIPS. Asymmetric membranes consist of a thin active separation layer on a much thicker, more open support to provide mechanical strength. NF is used for water softening, micro Copyright: © 2021 by the authors. pollutant removal, dye removal, pretreatment for desalination and heavy metal removal. Licensee MDPI, Basel, Switzerland. As membrane processes are appearing increasingly in industrial applications, it is This article is an open access article becoming more important that membrane preparation itself becomes “green” [19–22]. Mem- distributed under the terms and brane manufacturing currently produces over 50 billion liters of contaminated wastewater conditions of the Creative Commons annually [21]. The European Chemicals Agency (ECHA), classified toxic solvents into a Attribution (CC BY) license (https:// REACH list [2,23]. Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), 1-methyl- creativecommons.org/licenses/by/ 2-pyrrolidone (NMP) and other conventional solvents are usually used in membrane 4.0/).

Membranes 2021, 11, 418. https://doi.org/10.3390/membranes11060418 https://www.mdpi.com/journal/membranes Membranes 2021, 11, x 2 of 19

wastewater annually [21]. The European Chemicals Agency (ECHA), classified toxic sol- Membranes 2021, 11, 418 vents into a REACH list [2,23]. Tetrahydrofuran (THF), N,N-dimethylformamide2 (DMF), of 18 1-methyl-2-pyrrolidone (NMP) and other conventional solvents are usually used in membrane preparation due to the good solubility of common polymers [18,24], but have all been classified as highly concerned solvents by ECHA [2,23]. Industrial use of DMF and preparationTHF is expected due to to the be good banned solubility soon by of the common European polymers Union, [while18,24], NMP but haveis on alla watch been list classiﬁed[25,26]. as highly concerned solvents by ECHA [2,23]. Industrial use of DMF and THF is expectedAccording to be banned to principles soon by the5 and European 7 of the Union,12 principles while NMPof green ison chem a watchistry, listsafer [25 solvents,26]. According to principles 5 and 7 of the 12 principles of green chemistry, safer solvents and auxiliaries and use of renewable feed stock are main aspects of green chemistry and auxiliaries and use of renewable feed stock are main aspects of green chemistry [27,28]. [27,28]. Not much work has been done yet in membrane preparation to implement sus- Not much work has been done yet in membrane preparation to implement sustainable tainable solvents [5,29]. Some alternative solvents have been proposed to substitute DMF solvents [5,29]. Some alternative solvents have been proposed to substitute DMF (by (by dimethyl sulfoxide) and 1,4-dioxane (by acetone) [30]. Green solvents, like me- dimethyl sulfoxide) and 1,4-dioxane (by acetone) [30]. Green solvents, like methyl/ethyl thyl/ethyl lactate [31], ionic liquids [32–34], triethyl phosphate [35] and 𝛾-butyrolactone lactate [31], ionic liquids [32–34], triethyl phosphate [35] and γ-butyrolactone [36] have [36] have been proposed to substitute conventional solvents in phase inversion. been proposed to substitute conventional solvents in phase inversion. 𝛾-Valerolactone (GVL) is a non-toxic solvent with high boiling point (207 °C) [37,38] γ-Valerolactone (GVL) is a non-toxic solvent with high boiling point (207 ◦C) [37,38] and has been applied in chemical processes and as flavor additive in perfumes [37–39]. and has been applied in chemical processes and as ﬂavor additive in perfumes [37–39]. GVL is obtained from acid hydrolysis of cellulose based biomass (wood). It is prepared GVL is obtained from acid hydrolysis of cellulose based biomass (wood). It is prepared from levulinic acid through a catalytic cyclization reaction via dehydration. During this from levulinic acid through a catalytic cyclization reaction via dehydration. During this reaction,reaction, an unstablean unstable intermediate intermediate is formed is formed which which is converted is converted to GVLto GVL on on hydrogenation, hydrogenation, whilewhile levulinic levulinic acid acid is is produced produced from from hydroxymethylfurfural hydroxymethylfurfural throughthrough aa dehydration re- reactionaction [ 40[40].]. A relatedA related lactone-based lactone-based solvent, solvent,γ-butyrolactone, 𝛾-butyrolactone, has already has already been used been for used NF mem- for NF branemembrane preparation preparation via NIPS via and NIPS TIPS and [36 TIPS,41]. [36,41]. In the currentIn the current study, focusstudy, is focus laid onis laid the on applicationthe application of GVL of as GVL potential as potential bio-based, bio-based, sustainable sustainable solvent solvent for the preparationfor the preparation of NF- of membranes,NF-membranes, either by either selecting by selecting a suitable a suitable non-solvent non-solvent for phase for inversion phase inversion or by using or by high using polymerhigh polymer concentrations concentrations in the casting in the solution. casting solution.

2. Materials2. Materials and and Methods Methods 2.1.2.1. Chemicals Chemicals PolysulfonePolysulfone (PSU, (PSU, Udel Udel P-1700 P-1700 LCD) LCD) and and polyimide polyimide (Matrimid) (Matrimid) and and were were provided provided byby Solvay Solvay (Belgium) (Belgium) and and cellulose cellulose triacetate triacetate by by EastmanEastman (Belgium). Cellulose Cellulose acetate, acetate, pol- polyethersulfoneyethersulfone andand GVLGVL werewere purchased purchased from from Sigma-Aldrich Sigma-Aldrich (Belgium). (Belgium). All All polymers polymers ◦ werewere dried dried for for 24 h24 at h 105 at 105C. °C. Molecular Molecular weight weight (MW) (MW) and and the the structure structure of theof the polymers, polymers, feedfeed solutes solutes and and solvent solvent used used in this in this work work are are given given in Table in Table1. 1.

TableTable 1. Molecular 1. Molecular weight weight (MW), (MW), structure structure of polymers, of polymers, feed feed solutes solutes and and solvent solvent used used in this in this work. work.

Polymer/SolventPolymer/Solvent MWMW (kDa) (kDa) Structure Structure

PolyimidePolyimide (PI) (PI) 90–134 90–134 Membranes 2021, 11, x 3 of 19

CelluloseCellulose acetate acetate (CA) (CA) ∼30∼30

Polysulfone (PSU) 21

Cellulose triacetate (CTA) 278–282

Polyethersulfone (PES) ∼35

Magnesium sulfate (MgSO4) 0.12

Rose Bengal (RB) 1.017

𝛾-Valerolactone (GVL) 0.100

MembranesMembranes 2021 2021, ,11 11, ,x x 33 of of 19 19 Membranes 2021, 11, x 3 of 19

Membranes 2021, 11, x 3 of 19

MembranesMembranes 2021 2021, ,11 11, ,x x 33 of of 19 19

CelluloseCellulose acetateacetate (CA)(CA) ∼∼3030 Cellulose acetate (CA) ∼30 MembranesCellulose2021, 11, 418 acetate (CA) ∼30 3 of 18 CelluloseCellulose acetateacetate (CA)(CA) ∼∼3030

Table 1. Cont.

Polymer/Solvent MW (kDa) Structure PolysulfonePolysulfone (PSU)(PSU) 21 21 Polysulfone (PSU) 21 Polysulfone (PSU) 21 PolysulfonePolysulfone (PSU)(PSU) 21 21 Polysulfone (PSU) 21

CelluloseCellulose triacetatetriacetate (CTA)(CTA) 278–282 278–282 Cellulose triacetate (CTA) 278–282 Cellulose triacetate (CTA) 278–282 CelluloseCelluloseCellulose triacetate triacetatetriacetate (CTA) (CTA)(CTA) 278–282 278–282 278–282

Polyethersulfone (PES) ∼∼35 Polyethersulfone (PES) 35 Polyethersulfone (PES) ∼35

PolyethersulfonePolyethersulfone (PES) (PES) ∼35∼35 Polyethersulfone (PES) ∼35 Polyethersulfone (PES) ∼35

MagnesiumMagnesium sulfatesulfate (MgSO(MgSO44)) 0.120.12 MagnesiumMagnesium sulfate sulfate (MgSO (MgSO4)4) 0.120.12 Magnesium sulfate (MgSO4) 0.12 MagnesiumMagnesium sulfatesulfate (MgSO(MgSO44)) 0.120.12

RoseRose BengalBengal (RB)(RB) 1.017 1.017 RoseRose Bengal Bengal (RB) (RB) 1.017 1.017 Rose Bengal (RB) 1.017 RoseRose BengalBengal (RB)(RB) 1.017 1.017

γ-Valerolactone𝛾𝛾-Valerolactone-Valerolactone (GVL) (GVL)(GVL) 0.100 0.100 0.100 𝛾-Valerolactone (GVL) 0.100 𝛾-Valerolactone (GVL) 0.100 𝛾𝛾-Valerolactone-Valerolactone (GVL)(GVL) 0.100 0.100

2.2. Membrane Preparation

All polymers were dissolved in GVL at room temperature and stirred magnetically over 24 h by dissolving 10–20 wt% of the polymer (except for cellulose triacetate (CTA), where 17.5 wt% was found to be the upper concentration limit) in GVL, for details see Table2. A wet casting thickness of 225 µm at a 1.5 m/minute speed on a polyethylene (PE)/polypropylene (PP) non-woven fabric (Novatexx 2413) impregnated with GVL was used to cast the solution. After casting, the ﬁlms were instantaneously immersed in the non-solvent bath. All membranes were kept below 20 ◦C in distilled water until ﬁltration. Membranes 2021, 11, 418 4 of 18

Table 2. Concentration of polymer in the casting solutions, the applied non-solvent and the membrane ID. γ-Valerolactone (GVL) was used as solvent for all membrane preparations.

Membrane ID Polymer Type and Concentration Non-Solvent CA10W 10 wt% CA water CA15W 15 wt% CA water CA20W 20 wt% CA water CA10E 10 wt% CA ethanol CA15E 15 wt% CA ethanol CA20E 20 wt% CA ethanol CTA10W 10 wt% CTA water CTA15W 15 wt% CTA water CTA17.5W 17.5 wt% CTA water CTA10E 10 wt% CTA ethanol CTA15E 15 wt% CTA ethanol CTA17.5E 17.5 wt% CTA ethanol PI10W 10 wt% PI water PI15W 15 wt% PI water PI20W 20 wt% PI water PI10E 10 wt% PI ethanol PI15E 15 wt% PI ethanol PI20E 20 wt% PI ethanol PES10W 10 wt% PES water PES15W 15 wt% PES water PES20W 20 wt% PES water PES10E 10 wt% PES ethanol PES15E 15 wt% PES ethanol PES20E 20 wt% PES ethanol PSU10W 10 wt% PSU water PSU15W 15 wt% PSU water PSU20W 20 wt% PSU water PSU10E 10 wt% PSU ethanol PSU15E 15 wt% PSU ethanol PSU20E 20 wt% PSU ethanol

2.3. Viscosity Measurements The viscosity or rheological measurements for all polymer samples were done on a Anton Paar MCR 501 (Austria) with cone-plate geometries and evaporation blocker, as described in literature [8,10,42].

2.4. Cloudpoint Determination The procedure for cloudpoint determination was adapted from literature, as described elsewhere [8,10,42] using water as non-solvent.

2.5. Filtrations Filtrations were performed at 23 ◦C under pressures from 2 to 16 bar permitting ﬁltrations of 16 membrane coupons simultaneous with a high-throughput ﬁltration set- up [43,44]. Each membrane coupon had an active surface area of 0.000172 m2. Solutions of 35 µM rose Bengal (RB) or 16.7 mM MgSO4 in distilled water (H2O) were taken as feed (Table1). Permeance is measured by the quantity of liquid that passes through the membrane per unit of area, time and pressure. Equation (1) is used to calculate permeance. Retention (rejection) is a dimensionless parameter, expressed in percentage (in % from 0 to 100%) with respect to feed solution. Equation (2) is used to determine retention, in which CF represents initial feed concentration and CP represents permeate concentration. RB-concentrations were measured on a Perkin-Elmer ultraviolet–visible Membranes 2021, 11, 418 5 of 18

(UV/VIS)-spectrophotometer at a wavelength of 548 nm. In the case of MgSO4, a Consort K620 conductometer was used to measure the concentration of permeate and feed.

Vol [L] Permeance L/m2 · h · bar = (1) membrane area [m2] × ∆p[bar] × time(h)

C Retention (R) = 1 − P × 100 [%] (2) CF

2.6. Membrane Morphology For membrane morphology studies, membrane samples were broken in liquid ni- trogen and coated with 2–5 nm gold/palladium as conductive layer using a Jeol-AFC HR-sputter coater. Images were acquired using a JEOL JSM 6010LV scanning electron microscopy (SEM).

2.7. Solubility Parameters The Hansen solubility parameters (HSP) are used to describe the afﬁnity between a polymer and GVL, as discussed earlier [39,42,45,46]. Ra (solubility parameter distance) was calculated using Equation (3), which is a measure for afﬁnity between polymer (1) and solvent (2). q 2 2 2 Ra = 4(δD2 − δD1) + (δP2 − δP1) + (δH2 − δH1) (3)

Values for GVL are taken from literature [39,46–48]. See supporting information, for the details of HSP and RED (Relative energy difference) values (Tables S1–S5). Solubility parameters difference (Ra) of GVL and non-solvent is given in Table S6.

3. Results and Discussion 3.1. Phase Inversion Behavior of Polymer/γ-Valerolactone (GVL) Systems 3.1.1. Introduction To understand the role of polymer concentration in NIPS, both kinetic and thermodynamic aspects are studied in detail to see the effect on final membrane morphology and performance. Kinetics play a role in phase inversion via the diffusion of non-solvent into the polymer solution and of solvent out of the cast polymer film. These diffusion rates obviously depend on the molecular size and the viscosity of the polymer solution. Depending on this solvent/non-solvent exchange rate and the strength of the non-solvent (higher RaNS−P or RaS−NS values) to phase-separate the polymer solution, two different types of demixing processes can be distinguished. In the case of instantaneous demixing, a membrane with a porous skin-layer, often with finger-like or pear-shaped macrovoids over the full cross-section, is generally formed, while denser membranes with a dense skin having sponge-like substructure are formed in delayed demixing [6,49].

3.1.2. S-NS (Solvent and Non-Solvent) and NS-P (Non-Solvent and Polymer) Interaction Distance Parameters All polymers were dissolved in GVL at room temperature with polymer concentrations from 10 wt% to 20 wt%. However, it was impossible to dissolve CTA concentrations higher than 17.5 wt%. Two different non-solvents, i.e., water and ethanol, were used in the coagulation bath to further tune the membranes toward NF-performance. To better understand the role of solvent, non-solvent and polymer in the NIPS process, the interaction distance (Ra) between solvent/non-solvent and between non-solvent/polymer were calculated (Table3) and plotted in Figure1. Membranes 2021, 11, x 6 of 19

All polymers were dissolved in GVL at room temperature with polymer concentrations from 10 wt% to 20 wt%. However, it was impossible to dissolve CTA concentrations higher than 17.5 wt%. Two different non-solvents, i.e., water and ethanol, were used in the coagulation bath to further tune the membranes toward NF-performance. To better understand the role of solvent, non-solvent and polymer in the NIPS process, the interaction distance (Ra) between solvent/non-solvent and between non-solvent/polymer were calculated (Table 3) and plotted in Figure 1.

Table 3. Solubility parameters of the polymers, GVL and non-solvents. Membranes 2021, 11, 418 6 of 18 Ra (𝑵𝑺) −𝑷 Ra (𝑺−𝑷) HSP Values Polymers/Sol- (MPa1/2) (MPa1/2) vent Table𝜹 3.D Solubility parameters𝜹P of the𝜹H polymers, GVL and non-solvents. H2O C2H5OH GVL MPa1/2 MPa1/2 MPa1/2 Ra (NS−P) Ra (S−P) HSP Values CA 18.6 12.7 11.0 30.1 (MPa6.1 1/2) 11.0(MPa 1/2) Polymers/Solvent CTA δ18.4 11.9δ 10.1 δ 32.9 11.1 9.9 D P H H OC H OH GVL PI MPa 20.91/2 11.3MPa 1/2 9.7 MPa1/2 34.7 2 14.2 2 5 13.0 CAPSU 18.619.7 8.3 12.7 8.3 11.035.9 30.113.6 6.1 9.3 11.0 CTAPES 18.4 19.6 10.8 11.9 9.2 10.134.5 32.912.9 11.1 10.5 9.9 PI 20.9 11.3 9.7 34.7 14.2 13.0 PSUGVL 19.715.5 4.7 8.3 6.6 8.3 35.9 13.6 9.3 PESH2O 19.615.5 16.0 10.8 42.3 9.2 34.5 12.9 10.5 GVL 15.5 4.7 6.6 C2H5OH 15.8 8.8 19.4 H2O 15.5 16.0 42.3 C2H5OH 15.8 8.8 19.4

FigureFigure 1. Interaction 1. Interaction distance distance between solvent between and non-solventsolvent and (Ra non-solventS−NS) vs. theinteraction (𝑅𝑎) distance vs. the between interaction non-solvent dis- Ra ) andtance polymer between interaction non-solvent ( NS−P . and polymer interaction (𝑅𝑎 ). Replacing water by ethanol as non-solvent increased the affinity of the polymer for Replacing waterthe non-solvent by ethanol drastically as non-solvent (shift to the increased left in Figure the1). Ethanolaffinity is of thus the a weaker polymer NS and for a the non-solvent drasticallymore delayed (shift demixing to the can left be expected,in Figure which 1). Ethanol is supposed is thus to lead ato weaker a more sponge-likeNS and a more delayed demixingmembrane can structure. be expected, In contrast, which ethanol is supposed clearly has ato stronger lead to affinity a more for sponge- GVL (shift to the bottom in Figure1). This should increase the driving force for NS to enter the like membrane structure.polymer/solvent In contrast, system, ethanol which wouldclearly lead has to a more stronger instantaneous affinity demixing, for GVL inducing (shift to the bottom in Figuremore macrovoids. 1). This should Thesecontradicting increase the impacts driving of thermodynamics force for NS to thus enter render the prediction polymer/solvent system,of expectedwhich membranewould lead structures to more and instantaneous performance very demixing, difficult. inducing more

Membranes 2021, 11, x 7 of 19

Membranes 2021, 11, 418 7 of 18 macrovoids. These contradicting impacts of thermodynamics thus render prediction of expected membrane structures and performance very difficult.

3.2.3.2. PhasePhase DiagramsDiagrams PhasePhase diagramsdiagrams ofof the polymer-GVL-water te ternaryrnary systems systems were were obtained obtained by by the the ti- titrationtration method method via via cloud cloud point point determination, determination, using using GVL GVL as as a asolvent solvent system system and and water wa- teras a as non-solvent a non-solvent until until the visual the visual appearance appearance of turbidity of turbidity in the inpolymer the polymer solution solution (Figure (Figure2) [6,7,50].2)[ 6Cellulose,7,50]. Cellulose acetate (CA)/GVL acetate (CA)/GVL was the most was thestable most system stable among system all among polymers all polymerssince large since amounts large amounts of water of as water non-solvent as non-solvent were required were required to cause to cause turbidity turbidity [51]. [ 51The]. Thestability stability order order of the of thepolymer polymer systems systems toward toward the theaddition addition of non-solvent of non-solvent (water) (water) was was CA CA > CTA > PES > PSU ≥ polyimide (PI) [52–54]. The Ra values of CA, CTA, PES, PSU, > CTA > PES > PSU ≥ polyimide (PI) [52–54]. The 𝑅𝑎S− Pvalues of CA, CTA, PES, PSU, andand PI PI were were above above 9.0. 9.0. Therefore, Therefore, in in theory theory at at least, least, no no solubility solubility was was expected. expected.But But still, still, thesethese polymers polymers were were readily readily soluble soluble in in GVL. GVL.

FigureFigure 2. 2.Phase Phase diagrams diagrams of of the the polymer-GVL-water polymer-GVL-water ternary ternary systems systems obtained obtained by by the the titration titration method method to to determine determine the the cloudcloud point point (right (right ﬁgure figure = = enlarged enlarged zoom). zoom).

3.3.3.3. KineticKinetic Aspects Aspects of of Non-Solvent Non-Solvent Induced Induced Phase Phase Separation Separation (NIPS) (NIPS) Process Process KineticKinetic aspects aspects of of NIPSNIPS cancan partiallypartially bebe understood with rheological (viscosity) meas- mea- surementsurements of of the the casting casting solutions. solutions. The The viscosities viscosities of ofthe the polymer polymer solutions solutions logically logically in- increasedcreased with with increasing increasing polymer polymer concentrations concentrations (Figure (Figure 3).3 Polymer). Polymer chains chains entangle entangle more more in high polymer concentration solutions, and a certain minimal level of entanglement in high polymer concentration solutions, and a certain minimal level of entanglement is is essential to prepare sufficiently strong, defect-free membranes [55–57]. essential to prepare sufficiently strong, defect-free membranes [55–57]. 3.4. Membrane Performance and Morphology 3.4.1. Influence of Cellulose Acetate (CA) The influence of CA concentration was noticeably seen in the permeances of the membranes. A permeance around 1.6 L/m2·h·bar was found for CA10W which logically decreased to 0.70 and 0.30 L/m2·h·bar respectively with RB rejection increasing from 49% to 95% for CA15W and CA20W membranes (Figure4). When water was replaced with ethanol, both permeances and rejections improved. CA15E had a RB- rejection of 96% with a permeance around 2.0 L/m2·h·bar, qualifying very well for NF. CA20E had a lower permeance around 0.35 L/m2·h·bar and a slightly higher rejection of 98%. All CA membranes prepared via NIPS using either water or ethanol as non-solvent had a spongy structure (Figure5).

Membranes 2021, 11, x 8 of 19

100 PES PI PSU CA CTA Membranes 2021, 11, x 10 8 of 19

Membranes 2021, 11, 418 8 of 18 Viscosity(Pa.s)

100 PES PI 0.1 9 12151821 PSU Polymer CA concentration in GVL (wt%) CTA 10 Figure 3. Influence of increasing polymer concentration on rheology of the casting solutions.

3.4. Membrane Performance and Morphology

3.4.1. InfluenceViscosity(Pa.s) of Cellulose Acetate (CA)

The influence1 of CA concentration was noticeably seen in the permeances of the membranes. A permeance around 1.6 L/m2·h·bar was found for CA10W which logically decreased to 0.70 and 0.30 L/m2·h·bar respectively with RB rejection increasing from 49% to 95% for CA15W and CA20W membranes (Figure 4). When water was replaced with ethanol, both permeances and rejections improved. CA15E had a RB- rejection of 96% with a permeance around 2.0 L/m2·h·bar, qualifying very well for NF. CA20E had a lower permeance around0.1 0.35 L/m2·h·bar and a slightly higher rejection of 98%. All CA membranes prepared via 9NIPS using either 12151821 water or ethanol as non-solvent had a spongy structure (Figure 5). Polymer concentration in GVL (wt%)

Figure 3. Inﬂuence of increasing polymer concentration on rheology of the casting solutions. Figure 3. Influence of increasing polymer concentration on rheology of the casting solutions. 6.0 100 3.4. Membrane Performance and Morphology

3.4.1. Influence of Cellulose Acetate (CA) 80 4.5 The influence of CA concentration was noticeably seen in the permeances of the .h.bar)

2 2 membranes. A permeance around 1.6 L/m ·h·bar was found60 for CA10W which logically 2 Ethanol as NS decreased3.0 to 0.70 and 0.30 L/m ·h·bar respectivelyEthanol as with NS RB rejection increasing from 49% to 95% for CA15W and CA20W membranesWater (Fig asure NS 4). When water was replaced with 40

ethanol, both permeances and rejections improved. CA15E hadRB rejection (%) a RB- rejection of 96% with 2 a permeance1.5 around 2.0 L/m ·h·bar, qualifying very well for NF. CA20E had a lower per- Permeance (L/m Permeance meance around 0.35 L/m2·h·bar and a slightly higher rejection20 of 98%. All CA membranes prepared via NIPS using either water or ethanol as non-solvent had a spongy structure

(Figure 5).0.0 0 10 15 20 CA concentration (wt%)

Figure 4. Inﬂuence of cellulose acetate (CA) concentration on membrane performance in terms of rose Bengal (RB) rejection and permeance. Striped lines represent membranes prepared using ethanol as non-solvent6.0 and full lines those with water as non-solvent. 100

80 4.5 .h.bar) 2 60 Ethanol as NS 3.0 Ethanol as NS Water as NS 40 RB rejection (%)

1.5 Permeance (L/m Permeance 20

0.0 0 10 15 20 CA concentration (wt%)

Membranes 2021, 11, x 9 of 19

Membranes 2021, 11, 418 Figure 4. Influence of cellulose acetate (CA) concentration on membrane performance in terms of9 of 18 rose Bengal (RB) rejection and permeance. Striped lines represent membranes prepared using ethanol as non-solvent and full lines those with water as non-solvent.

(a)

CA10W CA15W CA20W

(b)

CA10E CA15E CA20E

FigureFigure 5. 5.Inﬂuence Influence of CA CA concentration concentration on on the the scanning scanning electron electron microscopy microscopy (SEM)-cross-section (SEM)-cross-section morphology morphology of CA mem- of CA membranesbranes cast cast from from using using (a) water (a) water as non-solvent as non-solvent and ( andb) ethanol (b) ethanol as non-solvent. as non-solvent.

3.4.2.3.4.2. InﬂuenceInfluence of Cellulose Triacetate Triacetate (CTA) (CTA) AsAs withwith CA,CA, all CTA-membranes had had sponge-like sponge-like structures. structures. However, However, also also some some macrovoids appeared now when using water as NS. With water as NS, RB rejection Membranes 2021, 11, x macrovoids appeared now when using water as NS. With water as NS, RB rejection10 of 19 slightly slightly increased from 94.4% to 96.8% with increasing polymer concentration but perme- increased from 94.4% to 96.8% with increasing polymer concentration but permeance ance decreased strongly from 15.9 to 5.5 L/m2·h·bar. All membranes thus had very good decreased strongly from 15.9 to 5.5 L/m2·h·bar. All membranes thus had very good permeances with RB rejections over 94%, clearly suitable for NF purposes (see Figure 6). permeances with RB rejections over 94%, clearly suitable for NF purposes (see Figure6). Using ethanol as NS, the spongy morphology of the CTA-membrane did not really change with increasing CTA concentration in the casting solution (see Figure 7). Perme- ance1000 decreased from 158 L/m2·h·bar to 62 L/m2·h·bar on increasing100 CTA concentration, while RB rejection increased slightly but remained very low. These low rejections are in contract with the results of the CA-membranes. 80 100 .h.bar) 2 60

40 Rejection (%) Rejection

1 Permeance(L/m 20 - - - - Ethanol as NS Water as NS 0.1 0 9 121518 CTA concentration (wt%)(%)

Figure 6. Inﬂuence of cellulose triacetate (CTA) concentration on membrane performance in terms Figureof 6. RB Influence rejection of andcellulose permeance. triacetate Striped(CTA) conc linesentration represent on membrane membranes performance prepared in using terms ethanol as of RBnon-solvent rejection and and permeance. full lines Striped those with lines water represent as non-solvent. membranes prepared using ethanol as non- solvent and full lines those with water as non-solvent.

(a)

CTA10W CTA15W CTA17.5W

(b)

CTA10E CTA15E CTA17.5E Figure 7. Influence of CTA concentration on SEM cross-sections of CTA membranes cast from using (a) water as non- solvent and (b) ethanol as non-solvent.

Membranes 2021, 11, x 10 of 19

1000 100

80 100 .h.bar) 2 60

40 Rejection (%) Rejection

1 Permeance(L/m 20 - - - - Ethanol as NS Membranes 2021, 11, 418 Water as NS 10 of 18 0.1 0 9 121518 CTA concentration (wt%)(%) Using ethanol as NS, the spongy morphology of the CTA-membrane did not really change with increasing CTA concentration in the casting solution (see Figure7). Permeance 2 2 Figuredecreased 6. Influence from 158 of cellulose L/m ·h triacetate·bar to 62 (CTA) L/m conc·h·barentration on increasing on membrane CTA performance concentration, in terms while ofRB RB rejection rejection increasedand permeance. slightly Striped but remainedlines represent very membranes low. These prepared low rejections using ethanol are in contractas non- solventwith the and results full lines of thethose CA-membranes. with water as non-solvent.

(a)

CTA10W CTA15W CTA17.5W

(b)

CTA10E CTA15E CTA17.5E

FigureFigure 7. 7. InfluenceInﬂuence of CTACTA concentration concentration on on SEM SEM cross-sections cross-sections of CTAof CTA membranes membranes cast cast from from using using (a) water (a) water as non-solvent as non- solventand (b) and ethanol (b) ethanol as non-solvent. as non-solvent.

3.4.3. Inﬂuence of Polyimide (PI) Using water as NS, PI-membrane morphology remained spongy even with changing polymer concentration. However, the inﬂuence of increasing polymer concentration was seen in membrane performance. As usual, permeances decreased from 76.5 to 10.5 L/m2·h·bar, while RB rejection increased from 16% to 65% but thus remained below the NF-threshold. On increasing the polymer concentration, RB rejections above 97% were realized with permeances around 2.6–1.3 L/m2·h·bar. Although the effect of non-solvent could not be seen in membrane morphology, it is thus very visible in membrane performance (see Figure8). When water was replaced by ethanol as NS, morphology of the PI-membranes did not change either (see Figure9). Membranes 2021, 11, x 11 of 19

3.4.3. Influence of Polyimide (PI) Using water as NS, PI-membrane morphology remained spongy even with changing polymer concentration. However, the influence of increasing polymer concentration was seen in membrane performance. As usual, permeances decreased from 76.5 to 10.5 L/m2·h·bar, while RB rejection increased from 16% to 65% but thus remained below the NF-threshold. On increasing the polymer concentration, RB rejections above 97% were realized with permeances around 2.6–1.3 L/m2·h·bar. Although the effect of non-solvent could not be seen in membrane morphology, it is thus very visible in membrane performance (see Fig- ure 8). When water was replaced by ethanol as NS, morphology of the PI-membranes did Membranes 2021, 11, 418 not change either (see Figure 9). 11 of 18

150 100

- - - - Ethanol as NS 120 80 Water as NS .h.bar) 2 90 60

60 40 RB rejection (%) rejection RB Permeance (L/m 30 20

0 0 10 12 14 16 18 20 PI concentration (wt.%)

Figure 8. Inﬂuence of polyimide (PI) concentration on membrane performance in term of RB rejection Membranes 2021, 11, x and permeance. Striped lines represent membranes prepared using ethanol as non-solvent and12 of full 19 Figure 8. Influence of polyimide (PI) concentration on membrane performance in term of RB rejection andlines permeance. these with waterStriped as non-solvent.lines represent membranes prepared using ethanol as non-solvent and full lines these with water as non-solvent.

(a)

PI10W PI15W PI20W

(b)

PI10E PI15E PI20E

Figure 9. InfluenceInﬂuence of of PI PI concentration on on cross-section cross-section of of the the SEM SEM images images of of PI PI membranes membranes cast cast from from using using ( (aa)) water water as as non-solvent and ( b) ethanol as non-solvent.

3.4.4.3.4.4. Influence Inﬂuence of of Polyethersulfone Polyethersulfone (PES) (PES) WithWith ethanol ethanol as as NS, NS, a asimilar similar change change in inmorphology morphology could could be beobserved. observed. Due Due to the to effectthe effect of NS, of RB NS, rejection RB rejection drastically drastically improved improved above above 95% for 95% PES15E for PES15E and PES20E and PES20E membranes,membranes, with 2.6–1.2 with 2.6–1.2 L/m2·h·bar L/m (Figure2·h·bar (Figure10) permeances. 10) permeances. These membranes These membranes thus qualified thus forqualiﬁed NF. for NF. Using water as NS, a membrane with macrovoids is formed for the lowest PES concentration (Figure 11), but macrovoids totally disappeared and a sponge-like structure was formed on increasing PES concentration in the casting solution. The effect of increasing PES concentration was also seen in a permeance reductions from 151 to 100 L/m2·h·bar. However, RB rejection remained very low.

180 100

150 Water as NS 80

120 .h.bar) 2 60 90

40 60 (%) RB rejection

Permeance (L/m Ethanol as NS 20 30

0 0 10 12 14 16 18 20 PES concentration (wt%)

Membranes 2021, 11, x 12 of 19

(a)

PI10W PI15W PI20W

(b)

PI10E PI15E PI20E Figure 9. Influence of PI concentration on cross-section of the SEM images of PI membranes cast from using (a) water as non-solvent and (b) ethanol as non-solvent.

3.4.4. Influence of Polyethersulfone (PES) With ethanol as NS, a similar change in morphology could be observed. Due to the effect of NS, RB rejection drastically improved above 95% for PES15E and PES20E membranes, with 2.6–1.2 L/m2·h·bar (Figure 10) permeances. These membranes thus qualified for NF. Using water as NS, a membrane with macrovoids is formed for the lowest PES concentration (Figure 11), but macrovoids totally disappeared and a sponge-like structure was formed on increasing PES concentration in the casting solution. The effect of increasing PES concentration was also seen in a permeance reductions from 151 to 100 L/m2·h·bar.

Membranes 2021, 11, 418 However, RB rejection remained very low. 12 of 18

180 100

150 Water as NS 80

120 .h.bar) 2 60 90

40 60 (%) RB rejection

Permeance (L/m Ethanol as NS 20 30

0 0 10 12 14 16 18 20 PES concentration (wt%)

Figure 10. Inﬂuence of polyethersulphone (PES) concentration on membrane performance in term of RB rejection and permeance. Striped lines represent membranes prepared using ethanol as non-solvent and full lines those with water as non-solvent. Membranes 2021, 11, x 13 of 19

Using water as NS, a membrane with macrovoids is formed for the lowest PES concentration (Figure 11), but macrovoids totally disappeared and a sponge-like structure Figurewas formed 10. Influence on increasing of polyethersulphone PES concentration (PES) concen in thetration casting on solution. membrane The performance effect of increasing in term 2 ofPES RB rejection concentration and permeance. was also Striped seen in lines a permeance represent membranes reductions prepared from 151 using to 100 ethanol L/m as· non-h·bar. solventHowever, and full RB lines rejection those remained with water very as non-solvent. low.

(a)

PES10W PES15W PES20W

(b)

PES10E PES15E PES20E

FigureFigure 11. 11. InfluenceInﬂuence of of PES PES concentration concentration on on cross-section cross-section of of the the SEM SEM images images of of PES PES membranes membranes cast cast from from using using (a (a) )water water asas non-solvent non-solvent and and (b (b) )ethanol ethanol as as non-solvent. non-solvent.

3.4.5. Influence of Polysulfone (PSU) When water was replaced by ethanol as NS, very unusual morphologies were found for the lowest concentrations. In particular, the PSU10E membrane looked defective, as also confirmed by its performance (Figure 12). Permeances of the membranes decreased to 1.3 L/m2·h·bar with increasing polymer concentration in the casting solution, while RB- rejection increased to 98.5% (see Figure 12). Using water as NS, a spongy structure was found with a quite obvious denser top layer. PES and PSU membranes are very different with respect to morphology (Figure 13). While the permeance of PSU membranes decreased from 103 to 10.5 L/m2·h·bar with increasing PSU concentration, RB-rejection increased from 5% to 65%, and hence was not high enough for NF.

Membranes 2021, 11, 418 13 of 18

3.4.5. Inﬂuence of Polysulfone (PSU) Membranes 2021, 11, x When water was replaced by ethanol as NS, very unusual morphologies were found 14 of 19 for the lowest concentrations. In particular, the PSU10E membrane looked defective, as also conﬁrmed by its performance (Figure 12). Permeances of the membranes decreased to 1.3 L/m2·h·bar with increasing polymer concentration in the casting solution, while RB-rejection increased to 98.5% (see Figure 12).

150 100

120 80 .h.bar) 2 90 60

60 40 Ethanol as NS RB rejection (%)

Permeance (L/m Permeance Water as NS 30 20

0 0 10 12 14 16 18 20 PSU concentration (wt%)

Figure 12. Inﬂuence of polysulphone (PSU) concentration on membrane performance in term of RB Figurerejection 12. Influence and permeance of polysulphone Striped lines represent(PSU) concentrat membranesion prepared on membrane using ethanol performance as non-solvent in term of RB rejectionand full linesand thosepermeance with water Striped as non-solvent. lines represent membranes prepared using ethanol as non- solvent andUsing full waterlines asthose NS, awith spongy water structure as non-solvent. was found with a quite obvious denser top layer. PES and PSU membranes are very different with respect to morphology (Figure 13). While the permeance of PSU membranes decreased from 103 to 10.5 L/m2·h·bar with increasing PSU concentration, RB-rejection increased from 5% to 65%, and hence was not high enough for NF.

3.5. Overall Comparison The membranes which qualiﬁed for NF (having RB rejection above 90%), were also (a) tested using a MgSO4/H2O feed solution. None of the membranes had MgSO4 rejection above 90%. These membranes are thus clearly suitable for loose NF and a comparison of current membranes with a selection of commercial or membranes from literature is given in Figure 14 (permeances vs. RB-rejection) and Figure 15 (permeances vs. MgSO4 rejections) (for more details, see S.I. Tables S7 and S8).

PSU10W PSU15W PSU20W

(b)

PSU10E PSU15E PSU20E Figure 13. Influence of PSU concentration on cross-section of the SEM images of PSU membranes cast from using (a) water as non-solvent and (b) ethanol as non-solvent.

3.5. Overall Comparison The membranes which qualified for NF (having RB rejection above 90%), were also tested using a MgSO4/H2O feed solution. None of the membranes had MgSO4 rejection above 90%. These membranes are thus clearly suitable for loose NF and a comparison of

Membranes 2021, 11, x 14 of 19

150 100

120 80 .h.bar) 2 90 60

60 40 Ethanol as NS RB rejection (%)

Permeance (L/m Permeance Water as NS 30 20

0 0 10 12 14 16 18 20 PSU concentration (wt%)

Membranes 2021, 11, 418 Figure 12. Influence of polysulphone (PSU) concentration on membrane performance in term14 of of 18 RB rejection and permeance Striped lines represent membranes prepared using ethanol as non- solvent and full lines those with water as non-solvent.

(a)

PSU10W PSU15W PSU20W

(b)

Membranes 2021, 11, x 15 of 19

PSU10E current membranes with a selection PSU15E of commercial or membranes from PSU20E literature is given in Figure 14 (permeances vs. RB-rejection) and Figure 15 (permeances vs. MgSO4 rejec- Figure 13. Influence of PSU concentration on cross-section of the SEM images of PSU membranes cast from using (a) water Figure 13. Inﬂuence of PSU concentrationtions) (for more on cross-section details, see ofS.I. the Tables SEM imagesS7 and ofS8). PSU membranes cast from using (a) water asas non-solvent non-solvent and ( b)) ethanol ethanol as as non-solvent non-solvent..

FigureFigure 14. 14.Rejection-permeance Rejection-permeance plot plot of of nanoﬁltration nanofiltration (NF)-membranes(NF)-membranes from from current current work work and and their their comparison comparison with with a a

selectionselection of commercialof commercial or or literature-reported literature-reported membranes membranes usingusing aa RB/HRB/H2O2O feed feed solution solution..

When aiming for a high-permeance, CTA membranes (CTA10W, CTA 15W and CTA17.5W) are the best option (permeances ranging from 15.9 to 5.5 L/m2·h·bar) with all RB rejections above 90%. When selectivity is more important, PES membranes are pre- ferred with 68% MgSO4 and a permeance of 1.1 L/m2·h·bar. Permeances and MgSO4 rejections are in general low as compared to commercial membranes which typically range from 1.0 to 16.3 L/m2·h·bar and 60.0% to 99.2% rejections [58]. The current membranes had comparable permeance, however, the MgSO4 rejections were lower.

MembranesMembranes 20212021,, 1111,, 418x 1516 ofof 1819

FigureFigure 15.15. Rejection-permeance plot plot for for membra membranesnes (which (which already already qualified qualiﬁed to tobe beloose loose NF NF membranes membranes based based on the on RB- the

RB-data),data), and and their their comparison comparison with with a selection a selection of co ofmmercial commercial and and literature-reported literature-reported membranes membranes using using MgSO MgSO4/H42O/H as2 Ofeed as feedsolution. solution.

WhenCTA10W aiming is the for best a high-permeance,membrane having CTA a permeance membranes around (CTA10W, 15.9 L/m CTA2·h·bar 15W with and a CTA17.5W)94% RB-rejection are the among best option all membranes (permeances prepared ranging from from polymer/GVL 15.9 to 5.5 L/m systems,2·h·bar) while with allPES15E RB rejections is the best above based 90%. on performance When selectivity in terms is of more MgSO important,4 rejection. PES membranes are 2 preferredWhen with sustainability 68% MgSO is4 andconcerned a permeance with me ofmbrane 1.1 L/m preparation,·h·bar. Permeances bio-based and materials MgSO4 rejections(i.e., CA or are CTA) in general with sustainable low as compared solvents to commercial are always membranessuggested over which petroleum-based typically range fromones. 1.0However, to 16.3 L/mthere2 is·h ·currentlybar and 60.0% a tradeoff to 99.2% still between rejections selectivity [58]. The and current sustainability. membranes had comparableTo combine permeance, high rejection however, with thea high MgSO permeance4 rejections seems were lower. challenging for polymer/GVLCTA10W systems is the within best the membrane studied havingparameters a permeance space of polymer around 15.9type, L/m polymer2·h·bar concen- with atration 94% RB-rejection and NS choice. among However, all membranes other parame preparedters like, from e.g., polymer/GVL membrane annealing, systems, whilecoag- PES15Eulation bath is the temperature best based onand performance composition, in co-solvent terms of MgSO addition4 rejection. in the casting solution can still beWhen screened sustainability to furthe isr optimize concerned these with properties. membrane γ preparation,-butyrolactone bio-based (GBL) was materials previ- (i.e.,ously CA used or CTA)in membranes with sustainable preparation, solvents e.g., arepolyvinylidene always suggested flouride over (PVDF) petroleum-based membranes ones.were However,prepared therevia TIPS is currently and polyethereth a tradeoff stillerketone between (PEEK-WC) selectivity membranes and sustainability. via NIPS. However,To combine the use high of GBL rejection was withlimited a high to thes permeancee 2 polymers. seems challengingWhile GVL fornot polymer/GVL only replaced systemsGBL, but within it also provided the studied more parameters opportunitie spaces to of prepare polymer NF type, membranes polymer as concentrationit can be com- andbined NS with choice. common However, membrane other parameterspolymers, like like, CA, e.g., PI, membrane PES, CTA annealing,and PSU, making coagulation it an bathinteresting temperature solvent and for membrane composition, preparation, co-solvent superior addition to in GBL. the casting solution can still be screened to further optimize these properties. γ-butyrolactone (GBL) was previously

used in membranes preparation, e.g., polyvinylidene ﬂouride (PVDF) membranes were prepared via TIPS and polyetheretherketone (PEEK-WC) membranes via NIPS. However, the use of GBL was limited to these 2 polymers. While GVL not only replaced GBL, but it also provided more opportunities to prepare NF membranes as it can be combined with common membrane polymers, like CA, PI, PES, CTA and PSU, making it an interesting solvent for membrane preparation, superior to GBL.

Membranes 2021, 11, 418 16 of 18

4. Conclusions NF membranes based on CA, PI, CTA, PES and PSU have been successfully prepared using GVL as a bio-based green solvent via NIPS. For all polymer types, the NF-criterion with RB-rejections above 90% could be fulfilled by tuning the membrane preparation using water or ethanol as non-solvent. The best membrane, CTA10W, was prepared using water as a non-solvent, from low CTA concentration. It had a permeance of 15.9 L/m2·h·bar and a 94% RB-rejection. Other membranes prepared from PES, PI, PSU and CA had a reasonable permeance with RB-rejection over 90%. Loose NF membranes prepared by using polymer/GVL systems still have potential to be further tuned toward tight NF, but can obviously already serve as a ultrafiltration membrane or as a support layer in thin film composite preparation.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/membranes11060418/s1, Table S1: Hansen solubility parameters and relative energy difference (RED) for CA. Table S2: Hansen solubility parameters and relative energy difference (RED) for PI. Table S3: Hansen solubility parameters and relative energy difference (RED) for PSU. Table S4: Hansen solubility parameters for PES and relative energy difference (RED) for PES. Table S5: Hildebrand/Hansen solubility parameters of GVL and interaction distance for CTA. Table S6: Solubility parameters difference (Ra) of GVL and non-solvent. Table S7: Comparison of the overall membranes performance of current membranes and a selection of lab-made and commercial membranes. Table S8: Comparison of the overall membranes performance of current membranes and a selection of lab-made and commercial membranes using MgSO4 feed solution. References for supporting information are cited in the supplementary materials separately [1–30]. Author Contributions: M.A.R.: Conceptualization, Experimental, Methodology, Investigation, For- mal analysis, Writing—original draft, review, and editing. I.F.J.V.: Resources, Writing—review and editing, Supervision. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Erasmus Mundus Doctorate in Membrane Engineering (EUDIME) program, KU Leuven (C16/17/005) and IWT-STW (Nanomexico AIO/150474/SBO). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: M.A. Rasool acknowledges Aurora Teodorescu (Brasov Romania), currently working at Amawaterways for fruitful discussion, helpful comments and visiting Leuven Belgium during this research work. Conﬂicts of Interest: There are no conﬂict to declare.

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