- Email: [email protected]
Green solvents for the fabrication of polymer inclusion membranes (PIMs)
Journal Pre-proofs Green Solvents for the Fabrication of Polymer Inclusion Membranes (PIMs) Clayton A. Carner, Charles F. Croft, Spas D. Kolev, M. Inês G.S. Almeida PII: DOI: Reference:
S1383-5866(19)34259-5 https://doi.org/10.1016/j.seppur.2019.116486 SEPPUR 116486
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 September 2019 24 December 2019 24 December 2019
Please cite this article as: C.A. Carner, C.F. Croft, S.D. Kolev, M.I.G.S. Almeida, Green Solvents for the Fabrication of Polymer Inclusion Membranes (PIMs), Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116486
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Green Solvents for the Fabrication of Polymer Inclusion Membranes (PIMs) Clayton A. Carner1, Charles F. Croft1, Spas D. Kolev, M. Inês G.S. Almeida * School of Chemistry, The University of Melbourne, Victoria 3010, Australia 1
These authors contributed equally to this work
* Corresponding author. Tel.: +61 3 8344 6813; fax: +61 3 9347 5180 E-mail address: [email protected] (M.I.G.S. Almeida)
ABSTRACT Chemical separation based on polymer inclusion membranes (PIMs) is a “Green Chemistry” alternative to solvent extraction by drastically reducing the use of toxic and volatile solvents which are used predominantly in PIM fabrication. This paper thus assesses the suitability of non-hazardous and renewably sourced “green” solvents (i.e., acetone, ethyl acetate, 2methyltetrahydrofuran, and dihydrolevoglucosenone (CyreneTM)) for the fabrication of PIMs composed of the most commonly used polymers (i.e., poly(vinyl chloride) (PVC), cellulose triacetate and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)) and extractants (i.e., di-(2-ethylhexyl) phosphoric acid and Aliquat 336). It is demonstrated that PVC- and PVDF-HFP-based PIMs fabricated with 2-methyltetrahydrofuran and ethyl acetate, respectively, exhibit extraction performance and stability similar to those of their counterparts fabricated with the conventional solvent tetrahydrofuran, even though their surface morphologies were slightly different. This result indicates that “green” solvents are a viable alternative to conventional “non-green” solvents used in the fabrication of PVC- and PVDFHFP-based PIMs.
Keywords:
Green
solvents;
polymer
inclusion
membranes
(PIMs);
extraction;
dihydrolevoglucosenone (CyreneTM).
1
1. INTRODUCTION The use of solvents is ubiquitous in chemistry as they are vital to many chemistry processes in research and industry [1]. However, it is important to recognise the environmental hazards, toxicity, and non-sustainable sourcing associated with current solvent usage. Although solvent use cannot be entirely eliminated, recent research is being focused on finding “greener” alternatives to current solvents [2, 3]. Solvent extraction is a method commonly used in industry for the separation and recovery of a wide range of chemical species, although it involves copious volumes of solvents that are often toxic, flammable and expensive [4]. Separation based on the use of polymer inclusion membranes (PIMs), however, has proven to be a suitable alternative to conventional solvent extraction [4, 5]. The use of PIMs for the extraction and transport of metal ions and some organic compounds has grown exponentially in recent years mainly due to their high selectivity, durability and reusability [6, 7]. PIMs are composed of a base polymer, with the most common being poly(vinyl chloride) (PVC), cellulose triacetate (CTA) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and an extractant (also known as carrier) with the most common being Aliquat 336 (a mixture of quaternary alkylammonium chlorides) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). In some cases, a plasticizer (e.g., 2-nitrophenyloctylether (NPOE)) or a modifier (e.g., 1-tetradecanol) can also be included in the membrane composition to improve the membrane’s performance. The casting method commonly used for the preparation of PIMs involves the dissolution of all these components into a small volume of an appropriate solvent (i.e., tetrahydrofuran (THF) when PVC or PVDF-HFP is used as a base polymer, or dichloromethane (DCM) when CTA is used). This mixture is then poured into a cast (e.g., a Petri dish or a glass ring placed on a flat glass plate) and the solvent is allowed to evaporate slowly over a suitable period of time (e.g., 24 h). The resultant PIM is considered successful
2
when it is flexible, self-standing and transparent, indicating a good compatibility between the membrane components. Even though the application of PIMs for the extraction and separation of a wide range of analytes, as an alternative to solvent extraction, eliminates the need for the use of large volumes of solvents, small volumes of hazardous and non-sustainable solvents are still required for the PIM fabrication. Hence, it is of great interest to replace these solvents with “greener” and safer alternatives (i.e., renewably sourced, non-hazardous), especially considering the potential of these membranes for industrial applications at a large scale. Following the recommendations reported by Prat et. al. [2] and Pena-Pereira et. al. [3], who have ranked a wide range of solvents in terms of their health, safety and environmental hazards, the
solvents
acetone,
ethyl
acetate,
2-methyltetrahydrofuran
(2-methylTHF)
and
dihydrolevoglucosenone (CyreneTM) were selected as prospective greener alternatives to THF and DCM to dissolve PIM components. Each of these greener alternatives was screened for their suitability in the preparation of PIMs with the most frequently used compositions, i.e., containing PVC, PVDF-HFP or CTA as the base polymer and D2EHPA or Aliquat 336 as the extractant. Those membranes cast successfully were then compared to PIMs prepared using conventional solvents in terms of their ability to extract Zn(II) or SCN-, for those membranes containing D2EHPA or Aliquat 336 as an extractant, respectively. Their stability and surface morphology were also studied.
2. EXPERIMENTAL 2.1. Reagents and solutions PIMs were prepared by weighting given masses of D2EHPA (97%), Aliquat® 336 (88.293.0%), NPOE (>99%), 1-tetradecanol (97%), PVC (high molecular weight), and PVDF-HFP (MW 400,000), all purchased from Sigma Aldrich, as well as CTA, purchased from Acros 3
Organics-Thermo Fisher Scientific. These components were dissolved in THF (without stabiliser, 99.75%, VWR), acetone (99%, VWR), 2-methylTHF (>99%, Sigma Aldrich), ethyl acetate (99%, Chem Supply), or dihydrolevoglucosenone (CyreneTM, >99%, Circa Group). Feed and receiving solutions were prepared using zinc(II) chloride (99.95%, APS Specialty Chemicals), HCl (32% w/w, reagent grade, Thermo Fisher Scientific), potassium thiocyanate (>99.0%, VWR) and 1.0 mol L-1 NaNO3 (99%, Chem Supply), which was also used as conditioning solution for the Aliquat 336-based PIMs. The reagent used to analyse SCN- was composed of iron(III) nitrate nonahydrate (>98%, Sigma Aldrich) and nitric acid (69.5% w/w, Scharlau). For the phase inversion aqueous baths, sodium chloride (99.95%, Fluka Chemicals) and sulfuric acid (98%, RCI Labscan) were used. Deionized water (≥18 M cm, Millipore, Synergy 185) was used for the preparation of all solutions.
2.2. Membrane preparation The casting method was used to prepare PIMs containing PVC or PVDF-HFP as base polymers. This method involves slow solvent evaporation, and thus the following volatile solvents were selected and used: THF, DCM, 2-methylTHF, ethyl acetate, acetone. Using this method, PIMs were prepared by dissolving given masses of the extractant (i.e., D2EHPA or Aliquat 336), the modifier (i.e., 1-tetradecanol, when necessary), and the base polymer (i.e., PVC or PVDF-HFP) in a chosen solvent (2 g per 20 mL) to obtain the following membrane compositions: 40 wt% D2EHPA and 60 wt% base polymer (i.e., PVC [8] or PVDF-HFP); 30 wt% Aliquat 336 and 70 wt% PVDF-HFP [7]; 20 wt% Aliquat 336, 10 wt% 1-tetradecanol and 70 wt% PVC [6]. PIM solutions were then magnetically stirred for 5 h at room temperature. If the mixture had not dissolved, it was heated in a water bath at 50 ⁰C (except for acetone, in 4
which case 40 ⁰C were chosen due to its low boiling point) for at least 12 h with magnetic stirring, and lastly, an additional 10 mL of solvent were added. PVC-based membranes were prepared by pouring the respective PIM solution into a glass ring (internal diameter of 7.6 cm) placed on a smooth glass plate. This was then covered with filter paper and another glass plate, along with an aluminium foil tray to allow slow solvent evaporation. PVDF-HFP-based PIMs were prepared using a casting knife with a gap of 0.5 mm, to spread the PIM solution across a glass plate [7]. PVDF-HFP-based solutions were then covered with an aluminium foil tray to ensure slow solvent evaporation. PIMs were left to cast over 24 h before being removed from the glass plate. CyreneTM was tested to replace DCM in the preparation of CTA-based PIMs. However, because it is not volatile, the phase-inversion method described by O’Bryan et al. [7] was employed for the preparation of these membranes. Following this procedure, CTA-based PIMs were prepared by dissolving given masses of D2EHPA, NPOE (plasticiser) and CTA in CyreneTM (2 g in 30 mL) to obtain a 30 wt% D2EHPA, 10 wt% NPOE, and 60 wt% CTA membrane composition. The PIM solution was then heated for 3-4 days with constant stirring at 50 ⁰C until the components were completely dissolved. This PIM solution was then spread across a glass plate using a casting knife, followed by its submersion into an aqueous bath (i.e., deionized water, 15% w/v NaCl and 0.1 M H2SO4, 20% w/v NaCl and 0.1 M H2SO4, or 0.1 M H2SO4) to release the CyreneTM into the aqueous phase and form the PIM.
2.3. Extraction experiments PIM segments were cut from the centre of the cast membranes (45 mm in diameter) and placed in 100 mL of feed solution: 30.0 mg L-1 Zn(II) at pH 3 (adjusted with HCl solution) for D2EHPA-based PIMs, or 125 mg L-1 SCN- for Aliquat 336-based PIMs. The PIMs fabricated for the extraction of SCN- were conditioned in 1.0 M NaNO3 solution before use by shaking 5
them for 20 h (Ratek Platform Mixer, Thermo Fisher Scientific) at 150 rpm as described by O’Bryan et al. [7] Membrane segments were suspended by customised sample holders and immersed in the respective feed solutions, followed by solution mixing using an orbital shaker (Ratek Platform Mixer, Thermo Fisher Scientific) at 150 rpm. Samples of the feed solution (0.5 mL) were taken at given time intervals and replaced with the same volume of the original feed solution. Extraction experiments were performed in triplicate.
2.4. Stability study The stability of the PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant was assessed to determine if fabricating the PIMs with the respective green solvent had an effect on their reusability. Hence, the membranes were reused three times by performing three cycles of extraction and back-extraction. The extraction procedure was similar to that described in the previous section, except that no samples were collected throughout the 5 h duration of the extraction step. The back-extraction consisted of submerging the PIM loaded with the target species (Zn(II) or SCN- in the case of D2EHPA- or Aliquat 336based PIMs, respectively) for 5 h in 100 mL of receiving solution: 1 mol L-1 HCl to strip Zn(II) and 1 mol L-1 NaNO3 to strip SCN-. Samples were just collected at the end of each backextraction experiment (i.e., at 5 h). The mass of the membranes was measured before the first extraction and after the third back-extraction.
2.5. Analysis of zinc and thiocyanate Zn(II) concentrations in the feed and receiving solutions were determined by atomic absorption spectrometry (AAS, Hitachi Z-2000 240V spectrophotometer with a Hitachi Zn HLA-4s hollow cathode lamp at 213.9 nm). The pH of the feed solutions was measured
6
potentiometrically (using an Ionide IJ44C pH probe connected to a multi-parameter laboratory analyser, Smart Chem-Lab, TPS, Australia) and adjusted to pH 3.0 with either HCl or NaOH. The quantitative analysis of SCN- in the feed and receiving solutions was performed by flow injection analysis, using the same system as described by Cho et al. [6], except that a flow rate of 0.8 mL min-1 was used instead. Calibration was performed daily using standards within the range of 2.50 - 15.0 mg L-1 SCN- prepared in the same matrix as the samples (i.e., deionized water or 1 mol L-1 NaNO3 solution). Samples were diluted with deionised water or 1 mol L-1 NaNO3 solution as needed, and triplicate measurements were taken spectrophotometrically (Pharmacia Novaspec II UV–Vis spectrophotometer, Amersham Pharmacia Biotech, Sweden) at 480 nm.
2.6. Surface morphology The surface morphology of PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant and fabricated with THF was compared to that of those fabricated with ethyl acetate or 2-methylTHF, respectively. Topographic images were taken by atomic force microscopy (AFM) on a Cypher AFM (Asylum research, Oxford instruments). Cantilevers (Tap3000-G Si AFM probes, Budget Sensors) with a resonant frequency of 300 KHz and a constant spring force of 40 N/m were used in the tapping mode in air. The roughness values were calculated using the Igor Pro 6.3.7 software.
7
3. RESULTS AND DISCUSSION 3.1. Green solvents screening Acetone, 2-methylTHF, and ethyl acetate were selected as alternative volatile solvents for the present study based on their sustainability (i.e., produced from renewable sources), nonhazardousness and for being known as greener alternatives to THF or DCM [2, 3]. CyreneTM [9] was also included in this study even though it is not volatile, since it is a bio-based alternative to DCM, and to the best of our knowledge its ability to dissolve the base polymers commonly used in PIMs is unknown. As mentioned above, when preparing PIMs, THF is commonly used to dissolve the base polymers PVC and PVDF-HFP, while DCM is used to dissolve CTA. Hence, the aim of this screening study consisted of finding at least one greener alternative for the dissolution of each base polymer mentioned above. Table 1 displays the suitability of the greener solvents for each PIM containing different base polymers. D2EHPA was used as the extractant in this study because it is one of the most commonly used extractants in PIM applications [4]. The PIM composition of 40 wt% D2EHPA and 60 wt% PVC has been described in previous studies as successful, using THF as the solvent [8], thus the same composition was applied when using PVDF-HFP as the base polymer. However, the same composition could not be applied for the CTA-based PIM cast by using DCM, because the membrane was very rigid. Hence, a small amount of the plasticizer NPOE was added to the membrane composition in order to increase the PIM’s flexibility and resolve this issue. With the aim of keeping the same amount of membrane liquid phase (consisting of the extractant and plasticizer/modifier if used), and base polymer as in the other PIMs, the CTA-based PIM was composed of 30 wt% D2EHPA, 10 wt% NPOE and 60 wt% CTA. These three compositions were thus used as a reference for the assessment of PIMs prepared using the greener alternative solvents.
8
Table 1. Green solvents suitability for the preparation of transparent, homogenous and selfstanding PVDF-HFP-, PVC- or CTA-based PIMs. PIM compositions: 40 wt% D2EHPA and 60 wt% PVDF-HFP; 40 wt% D2EHPA and 60 wt% PVC; and 30 wt% D2EHPA, 10 wt% NPOE and 60 wt% CTA. Green solvent Base
Dissolution conditions
polymer
Homogenous, transparent and self-standing PIM?
Acetone
Ethyl acetate
2-MethylTHF
CyreneTM
PVDF-HFP
Dissolved at RT with 20 mL solvent
No
PVC
ND
NT
CTA
ND
NT
PVDF-HFP
Dissolved at 50 °C with 20 mL solvent Yes
PVC
ND
NT
CTA
ND
NT
PVDF-HFP
ND
NT
PVC
Dissolved at 50 °C with 30 mL solvent Yes
CTA
ND
NT
PVDF-HFP
ND
NT
PVC
Dissolved at 60 °C with 30 mL solvent No
CTA
Dissolved at 60 °C with 30 mL solvent No
ND, not dissolved; RT, room temperature; NT, not tested.
Acetone was unable to dissolve PVC- or CTA-based PIM components even when the temperature and the solvent volume were raised to 40 °C and 30 mL, respectively. Even though it successfully dissolved all components of the PVDF-HFP-based PIM using the initial conditions (i.e., room temperature, 20 mL of solvent), after casting for 24 h the PIMs did not look homogenous (frosted appearance) and were of poor mechanical strength. This was likely due to acetone’s high volatility resulting in its rapid evaporation during the casting process, causing improper setting of the PIM. Regarding ethyl acetate, none of the PIM components were completely solubilised when using the initial conditions mentioned above, although when increasing the temperature to 50 °C all components of the PVDF-HFP-based PIM were 9
successfully dissolved. After casting, this membrane appeared physically similar to its counterpart cast in THF in terms of transparency, flexibility and mechanical resistance (i.e., equally easy to peel off the glass plate without tearing or stretching). 2-MethylTHF could dissolve the PVC-based PIM components with an extra 10 mL of solvent (i.e., 30 mL in total) and heating at 50 °C, and the resulting membranes after casting were similar to those prepared using THF. The remaining PIM components of PVDF-HFP or CTA-based PIMs were not fully solubilised by 2-methylTHF, even when the temperature or the solvent volume were increased. In summary, ethyl acetate and 2-methylTHF were found to be suitable greener alternatives for the preparation of PVDF-HFP-based and PVC-based PIMs, respectively, and were thus chosen and the extraction performance of the corresponding PIMs was further assessed. Testing CyreneTM as a green solvent for the fabrication of PIMs is of great interest because it is not only renewably sourced and not hazardous, but it is also non-toxic. However, CyreneTM is not volatile (boiling point, 200 ⁰C) [10], which prevents the use of the solvent evaporation casting method for the preparation of PIMs (where a volatile solvent is removed by evaporation). Thus, in order to be able to use CyreneTM for PIM casting, the phase inversion method had to be used instead for the removal of the solvent. CyreneTM successfully dissolved all PIM compositions, except for PVDF-HFP, when heated at 60 °C and using a total of 30 mL of solvent (higher temperatures were not attempted to avoid the degradation of the extractant/plasticiser). Nevertheless, after the phase inversion casting in a water bath, the membranes looked opaque, white in colour and not homogenous. It must be noted that, to the best of our knowledge, this is the first time that the use of CyreneTM has been reported for the dissolution of PVC or CTA. Even though the PIMs prepared in CyreneTM were not considered successful, since this was the only screened green solvent able to dissolve the CTA-based PIM components, this solvent was still selected for further studies (Section 3.5) to assess if different
10
aqueous bath conditions (i.e., salinity, pH) would improve the membrane’s physical appearance.
3.2. PIMs extraction performance: Conventional solvents versus greener alternatives In order to assess whether the PIMs cast using the greener solvent alternatives perform in a similar fashion to those cast by conventional solvents, extraction experiments were performed using D2EPHA- and Aliquat 336-based PIMs. The use of D2EHPA for the extraction of Zn(II) using PIMs has been well characterized and optimized [4], thus making this extraction process suitable for the comparison of PIMs cast with the conventional solvent THF and those cast with the suitable greener solvents ethyl acetate and 2-methylTHF. D2EHPA extracts Zn(II) by a cation-exchange process according to Equation (1) [8]:
Zn2+(aq) + 3/2(HD)2 (PIM) ZnD2.HD(PIM) + 2H+(aq)
(1)
where (HD)2 refers to the D2EHPA dimer, and the subscripts (aq) and (PIM) refer to the feed solution and PIM phase, respectively. PVDF-HFP-based and PVC-based PIMs, containing 60 wt% base polymer and 40 wt% D2EHPA, were cast using the conventional solvent THF and their new greener alternative (i.e., ethyl acetate and 2-methylTHF, respectively). These PIMs were then contacted with a solution containing Zn(II) under the same experimental conditions, and the percentage of Zn(II) extracted into each PIM was monitored over time by periodically determining the residual concentration of Zn(II) in the feed solution. The results obtained are presented in Figure 1.
11
Figure 1. Effect of the solvent used to cast D2EHPA-based PIMs containing PVDF-HFP (A) or PVC (B) as the base polymer, on the percentage (%) of Zn(II) extracted relative to its initial amount in the feed solution: ethyl acetate (), 2-methylTHF (), THF (). Membranes’ composition, 40 wt% D2EHPA and 60 wt% PVDF-HFP/PVC; feed solutions, 30 mg L-1 Zn(II) adjusted at pH 3.0. Error bars are ± standard deviation for 1 σn-1 (n =3).
12
It can be observed in Figure 1A that the PVDF-HFP-based PIMs cast in ethyl acetate were found to perform similarly to the PIMs cast in THF in terms of extraction rate and amount extracted (28.11.9 vs 27.90.2 mg Zn(II) g-1, respectively, after 5 h). In regard to the PVCbased membranes (Figure 1B), those cast in 2-methylTHF and THF extracted 17.10.3 and 16.01.6 mg Zn(II) g-1, respectively (after 5 h). Once again, PIMs performed similarly irrespective of the solvent choice. Hence, these results demonstrate that PVDF-HFP- and PVCbased PIMs, containing D2EHPA as the extractant and prepared with their respective greener alternative solvents to THF, showed no statistically significant difference at the 95% confidence level in their Zn(II) extraction performance (Student’s t test: 0.158<2.83 and 1.16<2.40, respectively). For the purpose of proving the appropriateness of these greener solvents for the preparation of PIMs containing other extractants, PIMs containing another commonly used extractant (i.e., Aliquat 336) were also assessed. In this case thiocyanate was used as a model target chemical species, since its extraction and transport using Aliquat 336-based PIMs has been often reported in the literature [6, 7, 11]. Aliquat 336 extracts SCN- by an anion-exchange process according to Equation (2) [12]:
SCN- (aq) + A+NO3- (PIM) A+(SCN-) (PIM) + NO3-(aq)
(2)
where A+NO3- refers to Aliquat 336 after conditioning (where the original chloride ion was replaced with the nitrate ion), and subscripts (aq) and (PIM) refer to the feed solution and the PIM phase, respectively. Aliquat 336-based PIMs were thus cast using ethyl acetate and 2-methylTHF, when PVDFHFP or PVC was applied as the base polymer, respectively, using the same fabrication conditions as described in Table 1. A comparison between the membranes cast using the 13
greener solvents and those prepared with the conventional solvent THF was conducted in terms of extraction of SCN-. The PIM compositions chosen for the corresponding experiments were selected based on previous studies, which reported the following compositions (casting using THF) as optimal for the SCN-extraction: 30 wt% Aliquat 336 and 70 wt% PVDF-HFP [7], and 20 wt% Aliquat 336, 10 wt% 1-tetradecanol and 70 wt% PVC [6]. Figure 2 shows the effect of the casting solvents (THF vs ethyl acetate/2-methylTHF, respectively) on the membranes’ performance in terms of SCN- extraction. These results demonstrate that the extraction ability of PVDF-HFP- and PVC-based PIMs containing Aliquat 336 was also not affected by the use of a different casting solvent. After 5 h, PVDF-HFP-based PIMs cast with ethyl acetate or THF extracted 45.41.1 or 49.03.8 mg SCN- g-1, respectively, and the PVC-based PIMs cast with 2-methylTHF or THF extracted 35.21.1 or 39.64.5 mg SCN- g-1, respectively. According to the Student’s t test, no statistically significant difference was observed at the 95% confidence level (i.e., 3.59<5.94 and 4.34< 6.87, respectively). The conclusion that can be drawn from these experimental results is that neither ethyl acetate nor 2-methylTHF interfere with the PIM formation and its extraction ability and can thus be used as greener alternatives of THF for the preparation of PVDF-HFP and PVC-based PIMs.
14
Figure 2. Effect of the solvent, used to cast Aliquat 336-based PIMs containing PVDF-HFP (A) or PVC (B) as the base polymer, on the percentage of SCN- (%) extracted relative to its initial amount in the feed solution: ethyl acetate (), 2-methylTHF (), or THF (). Membranes’ compositions, 30 wt% Aliquat 336 and 70 wt% PVDF-HFP (A), 20 wt% Aliquat 336, 10 wt% 1-tetradecanol, and 70 wt% PVC (B). Feed solution, 125 mg L-1 SCN-. Error bars are ± standard deviation for 1 σn-1 (n =3).
15
3.3. Stability study It has been demonstrated that PIMs fabricated with a conventional solvent or a greener alternative have similar performance after one single use. However, it is also of interest to know if their performance changes after several uses. Thus, the same PIM compositions studied above and fabricated with THF, ethyl acetate or 2-methylTHF were reused three times by performing three extractions each followed by a back-extraction (i.e., stripping of the target species into a fresh receiving solution). The results obtained for the D2EHPA- and Aliquat 336-based PIMs are presented in Figures 3 and 4, respectively.
Figure 3. Stability of D2EHPA-based PIMs containing PVDF-HFP (A and B) or PVC (C and D) as the base polymer and fabricated with THF (A to D, blue solid bars) or their respective green alternative: ethyl acetate (green bars with diagonal stripes) or 2-methylTHF (green horizontal stripes), respectively. Three consecutive extraction (E1 to E3) and back-extraction (BE1 to BE3) experiments were performed to assess if the PIMs fabricated with the green 16
solvent had similar stability to those fabricated with THF (PIM compositions as in Figure 1). Model target species, Zn(II); feed solution, 30.0 mg L-1 Zn(II) at pH 3 (adjusted with HCl solution); receiving solution, 1 mol L-1 HCl; extraction and back-extraction experiment duration, 5 h. Error bars are ± standard deviation for 1 σn-1 (n =3).
Figure 4. Stability of Aliquat 336-based PIMs containing PVDF-HFP (A and B) or PVC (C and D) as the base polymer and fabricated with THF (A to D, blue solid bars) or their respective green alternative: ethyl acetate (green bars with diagonal stripes) or 2-methylTHF (green horizontal stripes), respectively. Three consecutive extraction (E1 to E3) and back-extraction (BE1 to BE3) experiments were performed to assess if the PIMs fabricated with the green solvent have similar stability to those fabricated with THF (PIM compositions as in Figure 2). Model target species, SCN-; feed solution, 125 mg L-1 SCN-; receiving solution, 1 mol L-1 NaNO3; extraction and back-extraction experiment duration, 5 h. Error bars are ± standard deviation for 1 σn-1 (n =3).
17
Figures 3A to 3C show that the D2EHPA-based PIMs containing PVDF-HFP or PVC as the base polymer, respectively, were very stable over the three consecutive extraction and backextraction cycles regardless of the solvent used to fabricate them. The PVDF-HFP-based membranes cast either with THF or ethyl acetate started to look slightly wrinkled around the edges after the first extraction, although this did not seem to affect their extraction ability. In the case of PVC-based PIMs fabricated with 2-methylTHF or THF, no changes were observed in their physical appearance after the three cycles. Very small mass losses were registered for both PVDF-HFP- or PVC-based PIMs prepared with THF or their respective green alternative (i.e., 4.70.7% vs 4.50.6% for ethyl acetate; 3.40.2% vs 4.00.3% for 2-methylTHF) which should correspond to the loss of D2EHPA. Nevertheless, all PIMs were still able to extract a similar amount of Zn(II) in the second and third extraction step, thus showing that this loss did not affect their stability. It can be observed in Figure 4A that the Aliquat 336-based PIMs containing PVDF-HFP as the base polymer and fabricated with THF or ethyl acetate, both exhibited a slight decrease in their extraction ability as the membranes were reused with the latter this effect being moderately more pronounced. The same was not observed for the PVC-based PIMs (Figures 4C and 4D), which showed to be able to extract similar amounts of SCN- in the first, second and third extraction steps regardless of the solvent used to prepare them. Very small mass losses were again registered with the PVDF-HFP-based PIMs fabricated with THF and ethyl acetate, i.e., 2.30.4% and 1.20.3%, respectively, and the PVC-based PIMs fabricated with THF and 2methylTHF lost 1.40.2% and 2.50.2%, respectively. The physical appearance of the Aliquat 336-based PIMs did not change after the three cycles of extraction and back-extraction (i.e., they were still transparent and flexible, without any deformities). Overall, the stability of the PIMs fabricated with the green solvents was not significantly different from their counterparts prepared with THF. 18
3.4. Surface morphology study Since THF has a lower boiling point than ethyl acetate or 2-methylTHF (i.e., 66 °C vs 77/80 °C, respectively) their evaporation during the casting of the membranes is expected to proceed at different rates and, as a result, could have an effect on the PIM surface morphology. Hence, AFM images of the PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 and fabricated with both conventional and green solvents were taken on both sides of each membrane (side in contact with the glass plate and side in contact with the air) and their respective roughness was calculated. Figure 5 depicts the average roughness of the membranes containing PVDF-HFP and D2EHPA or Aliquat 336 as the base polymer and extractant, respectively, cast with the conventional solvent THF or with its green alternative ethyl acetate.
Figure 5. Average roughness (n=3) of each side of the PVDF-HFP-based PIMs containing D2EHPA or Aliquat 336 as the extractant, fabricated with THF (blue bars) or ethyl acetate (green bars with diagonal stripes).
It can be observed that the average roughness of the PVDF-HFP-based PIMs on the side in contact with the air was generally higher than the roughness of the side in contact with the 19
glass, which means that the PIM surface exposed to the air is rougher due to the evaporation of the solvent. Moreover, the average roughness of PIMs cast with THF were higher than that of PIMs cast with ethyl acetate (for both sides of the membranes). Because ethyl acetate has a higher boiling point than THF it is expected to evaporate at a slower rate during the casting process, thus leading to smoother membrane surfaces. However, this difference in roughness was not significant enough to change the kinetics of extraction as demonstrated in Figures 1A and 2A. The AFM images of the PVC-based PIMs with D2EHPA or Aliquat 336 cast with THF or 2methylTHF revealed very smooth membrane surfaces with no significant differences between the roughness on the air side or the glass side. Table 2 reports on the average roughness values calculated for all PVC-based PIMs.
Table 2. Average roughness of the PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant, fabricated with THF or 2-methylTHF. Membrane liquid phase
Casting solvent
Average roughness (n=5)
D2EHPA (40 wt%)
THF
1.830.87
D2EHPA (40 wt%)
2-methylTHF
5.520.93
Aliquat 336 (20 wt%) and THF 1-tetradecanol (10 wt%)
3.880.93
Aliquat 336 (20 wt%) and 2-methylTHF 1-tetradecanol (10 wt%)
1.560.41
It can be observed that the average roughness of the PIMs containing PVC as the base polymer is about one order of magnitude lower than that of those composed of PVDF-HFP. This could be due to their different properties, with PVDF-HFP being denser and more hydrophobic than 20
PVC (i.e., 1.8 vs 1.4 g cm-3 at 25 °C), thus leading to the formation of membranes with rougher surfaces. Moreover, and even though the average roughness of the PIMs cast with THF differ from those cast with 2-methylTHF, all values were very low (ranging between 1.6 and 5.5 nm), which means that all membranes can be classified as very smooth.
3.5. CTA-based PIMs fabricated with CyreneTM CTA-based membranes are usually dissolved in DCM and cast by solvent evaporation. Since CyreneTM is not volatile, the phase inversion method was implemented instead of the standard evaporation casting method. The initial attempt of casting these membranes using a water bath, resulted in non-homogenous PIMs, thus further optimization of the phase inversion method was performed with the aim of forming successful PIMs by increasing their homogeneity, transparency and mechanical strength. This optimization was carried out by altering the salinity (0% w/v, 15% w/v, or 20% w/v NaCl) and/or the pH (deionized water or 0.1 M H2SO4) of the aqueous bath within which a thin film of the PIM casting solution was immersed. Increasing the salinity (i.e., ionic strength) and/or acidity of the bath was hypothesized to aid the formation of successful membranes by minimizing the leaching out of their acidic extractant D2EHPA by reducing its solubility in the aqueous phase due to the salting out effect. The membranes cast in the aqueous bath composed of 20% w/v NaCl and 0.1 M H2SO4 were found to be more homogenous and self-standing than those cast using other aqueous bath compositions, although they still did not have an appearance similar to a PIM cast using DCM. Despite the lack of complete transparency, these PIMs were able to extract about 10 mg Zn(II) per g of PIM when contacted with a Zn(II) feed solution for 18 h. These results can be viewed as highly promising with respect to the potential of CyreneTM as a green solvent for the preparation of CTA-based PIMs.
21
4. CONCLUSIONS Renewable and non-hazardous solvents, namely acetone, ethyl acetate, 2-methylTHF and CyreneTM, were screened as greener alternatives to THF and DCM for the casting of PIMs. Ethyl acetate and 2-methylTHF were found to be suitable solvents for the preparation of PVDFHFP- and PVC-based PIMs, respectively, containing either D2EHPA or Aliquat 336 as the extractant. Similar extraction performance and stability was observed when comparing PIMs prepared using the conventional solvent THF with their greener alternatives mentioned above. AFM imaging showed differences in surface morphology between PIMs fabricated with green or conventional solvents, however these were not significant enough to affect the kinetics of extraction. These results demonstrate that the fabrication of PIMs containing the base polymers PVDF-HFP or PVC can be performed with bio-based solvents instead of the conventional THF. The water soluble CyreneTM was found for the first time in this study to be capable of dissolving PVC and CTA and because of this solvent’s low volatility the phase inversion technique was used for the casting of the corresponding PIMs. The PVC- and CTA-based PIMs cast with CyreneTM were inhomogeneous and opaque, although since this solvent was the only green alternative able to dissolve the CTA-based PIM components, further optimization to the casting water bath was conducted. Despite the fact that these PIMs did not have the same appearance as their counterparts cast with DCM (by evaporative solvent casting) they could still extract Zn(II), thus demonstrating the potential of CyreneTM as a green solvent for the preparation of CTA-based PIMs.
Conflicts of interest There are no conflicts to declare.
22
Acknowledgements The authors wish to thank Dr Anastasios Polyzos from the School of Chemistry, The University of Melbourne, for valuable suggestions, Circa Group for kindly providing CyreneTM, and Dr Tian Zheng from the Materials Characterisation and Fabrication Platform at The University of Melbourne for her assistance with the AFM imaging.
23
REFERENCES [1] F.G. Calvo-Flores, M.J. Monteagudo-Arrebola, J.A. Dobado, J. Isac-Garcia, Green and biobased solvents, Top. Curr. Chem., 376 (2018) 40. https://doi.org/10.1007/s41061-018-0191-6. [2] D. Prat, J. Hayler, A. Wells, A survey of solvent selection guides, Green Chem., 16 (2014) 4546-4551. [3] F. Pena-Pereira, A. Kloskowskic, J. Namiesnik, Perspectives on the replacement of harmful organic solvents in analytical methodologies: a framework toward the implementation of a generation
of
eco-friendly
alternatives,
Green
Chem.,
17
(2015)
3687-3705.
https://doi.org/10.1039/c5gc00611b. [4] M.I.G.S. Almeida, R.W. Cattrall, S.D. Kolev, Recent trends in extraction and transport of metal ions using polymer inclusion membranes (PIMs), J. Membr. Sci., 415 (2012) 9-23. https://doi.org/10.1016/j.memsci.2012.06.006. [5] S.D. Kolev, M.I.G.S. Almeida, R.W. Cattrall, Polymer inclusion membranes, in: A.K. Pabby, S.S.H. Rizvi, A.M. Sastre (Eds.) Handbook of membrane separations: chemical, pharmaceutical, food and biotechnological applications, CRC Press, Boca Raton, 2015, pp. 721-737. [6] Y. Cho, C.L. Xu, R.W. Cattrall, S.D. Kolev, A polymer inclusion membrane for extracting thiocyanate from weakly alkaline solutions, J. Membr. Sci., 367 (2011) 85-90. https://doi.org/10.1016/j.memsci.2010.10.040. [7] Y. O'Bryan, R.W. Cattrall, Y.B. Truong, I.L. Kyratzis, S.D. Kolev, The use of poly(vinylidenefluoride-co-hexafluoropropylene) for the preparation of polymer inclusion membranes. Application to the extraction of thiocyanate, J. Membr. Sci., 510 (2016) 481-488. https://doi.org/10.1016/j.memsci.2016.03.026. [8] S.D. Kolev, Y. Baba, R.W. Cattrall, T. Tasaki, N. Pereira, J.M. Perera, G.W. Stevens, Solid phase extraction of zinc(II) using a PVC-based polymer inclusion membrane with di(2-
24
ethylhexyl)phosphoric acid (D2EHPA) as the carrier, Talanta, 78 (2009) 795-799. https://doi.org/10.1016/j.talanta.2008.12.047. [9] T. Marino, F. Galiano, A. Molino, A. Figoli, New frontiers in sustainable membrane preparation: Cyrene (TM) as green bioderived solvent, J. Membr. Sci., 580 (2019) 224-234. https://doi.org/10.1016/j.memsci.2019.03.034. [10] J. Sherwood, M. De Bruyn, A. Constantinou, L. Moity, C.R. McElroy, T.J. Farmer, T. Duncan, W. Raverty, A.J. Hunt, J.H. Clark, Dihydrolevoglucosenone (Cyrene) as a bio-based alternative for dipolar aprotic solvents, Chem. Commun., 50 (2014) 9650-9652. https://doi.org/10.1039/c4cc04133j. [11] Y. O'Bryan, Y.B. Truong, R.W. Cattrall, I.L. Kyratzis, S.D. Kolev, A new generation of highly stable and permeable polymer inclusion membranes (PIMs) with their carrier immobilized in a crosslinked semi-interpenetrating polymer network. Application to the transport
of
thiocyanate,
J.
Membr.
Sci.,
529
(2017)
55-62.
https://doi.org/10.1016/j.memsci.2017.01.057. [12] Y. Cho, R.W. Cattrall, S.D. Kolev, A novel polymer inclusion membrane based method for continuous clean-up of thiocyanate from gold mine tailings water, J. Hazard. Mater., 341 (2018) 297-303. https://doi.org/10.1016/j.jhazmat.2017.07.069.
25
Highlights Fabrication of polymer inclusion membranes (PIMs) using green solvents was assessed Tetrahydrofuran (THF) could be replaced by ethyl acetate or 2-methylTHF 2-MethylTHF can be used to fabricate poly(vinyl chloride) (PVC)-based PIMs Ethyl acetate dissolves poly(vinylidene fluoride-co-hexafluoropropylene) PIMs prepared with green solvents exhibited similar extraction performance
26
CRediT author statement Clayton A. Carner: Investigation, Writing - Original Draft Charles F. Croft: Validation, Formal analysis, Investigation, Writing - Review & Editing Spas D. Kolev: Resources, Writing - Review & Editing, Supervision, Funding acquisition M. Inês G.S. Almeida: Conceptualization, Methodology, Validation, Formal analysis, Writing - Review & Editing, Supervision, Project administration.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
27
S1383-5866(19)34259-5 https://doi.org/10.1016/j.seppur.2019.116486 SEPPUR 116486
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 September 2019 24 December 2019 24 December 2019
Please cite this article as: C.A. Carner, C.F. Croft, S.D. Kolev, M.I.G.S. Almeida, Green Solvents for the Fabrication of Polymer Inclusion Membranes (PIMs), Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116486
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Green Solvents for the Fabrication of Polymer Inclusion Membranes (PIMs) Clayton A. Carner1, Charles F. Croft1, Spas D. Kolev, M. Inês G.S. Almeida * School of Chemistry, The University of Melbourne, Victoria 3010, Australia 1
These authors contributed equally to this work
* Corresponding author. Tel.: +61 3 8344 6813; fax: +61 3 9347 5180 E-mail address: [email protected] (M.I.G.S. Almeida)
ABSTRACT Chemical separation based on polymer inclusion membranes (PIMs) is a “Green Chemistry” alternative to solvent extraction by drastically reducing the use of toxic and volatile solvents which are used predominantly in PIM fabrication. This paper thus assesses the suitability of non-hazardous and renewably sourced “green” solvents (i.e., acetone, ethyl acetate, 2methyltetrahydrofuran, and dihydrolevoglucosenone (CyreneTM)) for the fabrication of PIMs composed of the most commonly used polymers (i.e., poly(vinyl chloride) (PVC), cellulose triacetate and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)) and extractants (i.e., di-(2-ethylhexyl) phosphoric acid and Aliquat 336). It is demonstrated that PVC- and PVDF-HFP-based PIMs fabricated with 2-methyltetrahydrofuran and ethyl acetate, respectively, exhibit extraction performance and stability similar to those of their counterparts fabricated with the conventional solvent tetrahydrofuran, even though their surface morphologies were slightly different. This result indicates that “green” solvents are a viable alternative to conventional “non-green” solvents used in the fabrication of PVC- and PVDFHFP-based PIMs.
Keywords:
Green
solvents;
polymer
inclusion
membranes
(PIMs);
extraction;
dihydrolevoglucosenone (CyreneTM).
1
1. INTRODUCTION The use of solvents is ubiquitous in chemistry as they are vital to many chemistry processes in research and industry [1]. However, it is important to recognise the environmental hazards, toxicity, and non-sustainable sourcing associated with current solvent usage. Although solvent use cannot be entirely eliminated, recent research is being focused on finding “greener” alternatives to current solvents [2, 3]. Solvent extraction is a method commonly used in industry for the separation and recovery of a wide range of chemical species, although it involves copious volumes of solvents that are often toxic, flammable and expensive [4]. Separation based on the use of polymer inclusion membranes (PIMs), however, has proven to be a suitable alternative to conventional solvent extraction [4, 5]. The use of PIMs for the extraction and transport of metal ions and some organic compounds has grown exponentially in recent years mainly due to their high selectivity, durability and reusability [6, 7]. PIMs are composed of a base polymer, with the most common being poly(vinyl chloride) (PVC), cellulose triacetate (CTA) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and an extractant (also known as carrier) with the most common being Aliquat 336 (a mixture of quaternary alkylammonium chlorides) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). In some cases, a plasticizer (e.g., 2-nitrophenyloctylether (NPOE)) or a modifier (e.g., 1-tetradecanol) can also be included in the membrane composition to improve the membrane’s performance. The casting method commonly used for the preparation of PIMs involves the dissolution of all these components into a small volume of an appropriate solvent (i.e., tetrahydrofuran (THF) when PVC or PVDF-HFP is used as a base polymer, or dichloromethane (DCM) when CTA is used). This mixture is then poured into a cast (e.g., a Petri dish or a glass ring placed on a flat glass plate) and the solvent is allowed to evaporate slowly over a suitable period of time (e.g., 24 h). The resultant PIM is considered successful
2
when it is flexible, self-standing and transparent, indicating a good compatibility between the membrane components. Even though the application of PIMs for the extraction and separation of a wide range of analytes, as an alternative to solvent extraction, eliminates the need for the use of large volumes of solvents, small volumes of hazardous and non-sustainable solvents are still required for the PIM fabrication. Hence, it is of great interest to replace these solvents with “greener” and safer alternatives (i.e., renewably sourced, non-hazardous), especially considering the potential of these membranes for industrial applications at a large scale. Following the recommendations reported by Prat et. al. [2] and Pena-Pereira et. al. [3], who have ranked a wide range of solvents in terms of their health, safety and environmental hazards, the
solvents
acetone,
ethyl
acetate,
2-methyltetrahydrofuran
(2-methylTHF)
and
dihydrolevoglucosenone (CyreneTM) were selected as prospective greener alternatives to THF and DCM to dissolve PIM components. Each of these greener alternatives was screened for their suitability in the preparation of PIMs with the most frequently used compositions, i.e., containing PVC, PVDF-HFP or CTA as the base polymer and D2EHPA or Aliquat 336 as the extractant. Those membranes cast successfully were then compared to PIMs prepared using conventional solvents in terms of their ability to extract Zn(II) or SCN-, for those membranes containing D2EHPA or Aliquat 336 as an extractant, respectively. Their stability and surface morphology were also studied.
2. EXPERIMENTAL 2.1. Reagents and solutions PIMs were prepared by weighting given masses of D2EHPA (97%), Aliquat® 336 (88.293.0%), NPOE (>99%), 1-tetradecanol (97%), PVC (high molecular weight), and PVDF-HFP (MW 400,000), all purchased from Sigma Aldrich, as well as CTA, purchased from Acros 3
Organics-Thermo Fisher Scientific. These components were dissolved in THF (without stabiliser, 99.75%, VWR), acetone (99%, VWR), 2-methylTHF (>99%, Sigma Aldrich), ethyl acetate (99%, Chem Supply), or dihydrolevoglucosenone (CyreneTM, >99%, Circa Group). Feed and receiving solutions were prepared using zinc(II) chloride (99.95%, APS Specialty Chemicals), HCl (32% w/w, reagent grade, Thermo Fisher Scientific), potassium thiocyanate (>99.0%, VWR) and 1.0 mol L-1 NaNO3 (99%, Chem Supply), which was also used as conditioning solution for the Aliquat 336-based PIMs. The reagent used to analyse SCN- was composed of iron(III) nitrate nonahydrate (>98%, Sigma Aldrich) and nitric acid (69.5% w/w, Scharlau). For the phase inversion aqueous baths, sodium chloride (99.95%, Fluka Chemicals) and sulfuric acid (98%, RCI Labscan) were used. Deionized water (≥18 M cm, Millipore, Synergy 185) was used for the preparation of all solutions.
2.2. Membrane preparation The casting method was used to prepare PIMs containing PVC or PVDF-HFP as base polymers. This method involves slow solvent evaporation, and thus the following volatile solvents were selected and used: THF, DCM, 2-methylTHF, ethyl acetate, acetone. Using this method, PIMs were prepared by dissolving given masses of the extractant (i.e., D2EHPA or Aliquat 336), the modifier (i.e., 1-tetradecanol, when necessary), and the base polymer (i.e., PVC or PVDF-HFP) in a chosen solvent (2 g per 20 mL) to obtain the following membrane compositions: 40 wt% D2EHPA and 60 wt% base polymer (i.e., PVC [8] or PVDF-HFP); 30 wt% Aliquat 336 and 70 wt% PVDF-HFP [7]; 20 wt% Aliquat 336, 10 wt% 1-tetradecanol and 70 wt% PVC [6]. PIM solutions were then magnetically stirred for 5 h at room temperature. If the mixture had not dissolved, it was heated in a water bath at 50 ⁰C (except for acetone, in 4
which case 40 ⁰C were chosen due to its low boiling point) for at least 12 h with magnetic stirring, and lastly, an additional 10 mL of solvent were added. PVC-based membranes were prepared by pouring the respective PIM solution into a glass ring (internal diameter of 7.6 cm) placed on a smooth glass plate. This was then covered with filter paper and another glass plate, along with an aluminium foil tray to allow slow solvent evaporation. PVDF-HFP-based PIMs were prepared using a casting knife with a gap of 0.5 mm, to spread the PIM solution across a glass plate [7]. PVDF-HFP-based solutions were then covered with an aluminium foil tray to ensure slow solvent evaporation. PIMs were left to cast over 24 h before being removed from the glass plate. CyreneTM was tested to replace DCM in the preparation of CTA-based PIMs. However, because it is not volatile, the phase-inversion method described by O’Bryan et al. [7] was employed for the preparation of these membranes. Following this procedure, CTA-based PIMs were prepared by dissolving given masses of D2EHPA, NPOE (plasticiser) and CTA in CyreneTM (2 g in 30 mL) to obtain a 30 wt% D2EHPA, 10 wt% NPOE, and 60 wt% CTA membrane composition. The PIM solution was then heated for 3-4 days with constant stirring at 50 ⁰C until the components were completely dissolved. This PIM solution was then spread across a glass plate using a casting knife, followed by its submersion into an aqueous bath (i.e., deionized water, 15% w/v NaCl and 0.1 M H2SO4, 20% w/v NaCl and 0.1 M H2SO4, or 0.1 M H2SO4) to release the CyreneTM into the aqueous phase and form the PIM.
2.3. Extraction experiments PIM segments were cut from the centre of the cast membranes (45 mm in diameter) and placed in 100 mL of feed solution: 30.0 mg L-1 Zn(II) at pH 3 (adjusted with HCl solution) for D2EHPA-based PIMs, or 125 mg L-1 SCN- for Aliquat 336-based PIMs. The PIMs fabricated for the extraction of SCN- were conditioned in 1.0 M NaNO3 solution before use by shaking 5
them for 20 h (Ratek Platform Mixer, Thermo Fisher Scientific) at 150 rpm as described by O’Bryan et al. [7] Membrane segments were suspended by customised sample holders and immersed in the respective feed solutions, followed by solution mixing using an orbital shaker (Ratek Platform Mixer, Thermo Fisher Scientific) at 150 rpm. Samples of the feed solution (0.5 mL) were taken at given time intervals and replaced with the same volume of the original feed solution. Extraction experiments were performed in triplicate.
2.4. Stability study The stability of the PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant was assessed to determine if fabricating the PIMs with the respective green solvent had an effect on their reusability. Hence, the membranes were reused three times by performing three cycles of extraction and back-extraction. The extraction procedure was similar to that described in the previous section, except that no samples were collected throughout the 5 h duration of the extraction step. The back-extraction consisted of submerging the PIM loaded with the target species (Zn(II) or SCN- in the case of D2EHPA- or Aliquat 336based PIMs, respectively) for 5 h in 100 mL of receiving solution: 1 mol L-1 HCl to strip Zn(II) and 1 mol L-1 NaNO3 to strip SCN-. Samples were just collected at the end of each backextraction experiment (i.e., at 5 h). The mass of the membranes was measured before the first extraction and after the third back-extraction.
2.5. Analysis of zinc and thiocyanate Zn(II) concentrations in the feed and receiving solutions were determined by atomic absorption spectrometry (AAS, Hitachi Z-2000 240V spectrophotometer with a Hitachi Zn HLA-4s hollow cathode lamp at 213.9 nm). The pH of the feed solutions was measured
6
potentiometrically (using an Ionide IJ44C pH probe connected to a multi-parameter laboratory analyser, Smart Chem-Lab, TPS, Australia) and adjusted to pH 3.0 with either HCl or NaOH. The quantitative analysis of SCN- in the feed and receiving solutions was performed by flow injection analysis, using the same system as described by Cho et al. [6], except that a flow rate of 0.8 mL min-1 was used instead. Calibration was performed daily using standards within the range of 2.50 - 15.0 mg L-1 SCN- prepared in the same matrix as the samples (i.e., deionized water or 1 mol L-1 NaNO3 solution). Samples were diluted with deionised water or 1 mol L-1 NaNO3 solution as needed, and triplicate measurements were taken spectrophotometrically (Pharmacia Novaspec II UV–Vis spectrophotometer, Amersham Pharmacia Biotech, Sweden) at 480 nm.
2.6. Surface morphology The surface morphology of PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant and fabricated with THF was compared to that of those fabricated with ethyl acetate or 2-methylTHF, respectively. Topographic images were taken by atomic force microscopy (AFM) on a Cypher AFM (Asylum research, Oxford instruments). Cantilevers (Tap3000-G Si AFM probes, Budget Sensors) with a resonant frequency of 300 KHz and a constant spring force of 40 N/m were used in the tapping mode in air. The roughness values were calculated using the Igor Pro 6.3.7 software.
7
3. RESULTS AND DISCUSSION 3.1. Green solvents screening Acetone, 2-methylTHF, and ethyl acetate were selected as alternative volatile solvents for the present study based on their sustainability (i.e., produced from renewable sources), nonhazardousness and for being known as greener alternatives to THF or DCM [2, 3]. CyreneTM [9] was also included in this study even though it is not volatile, since it is a bio-based alternative to DCM, and to the best of our knowledge its ability to dissolve the base polymers commonly used in PIMs is unknown. As mentioned above, when preparing PIMs, THF is commonly used to dissolve the base polymers PVC and PVDF-HFP, while DCM is used to dissolve CTA. Hence, the aim of this screening study consisted of finding at least one greener alternative for the dissolution of each base polymer mentioned above. Table 1 displays the suitability of the greener solvents for each PIM containing different base polymers. D2EHPA was used as the extractant in this study because it is one of the most commonly used extractants in PIM applications [4]. The PIM composition of 40 wt% D2EHPA and 60 wt% PVC has been described in previous studies as successful, using THF as the solvent [8], thus the same composition was applied when using PVDF-HFP as the base polymer. However, the same composition could not be applied for the CTA-based PIM cast by using DCM, because the membrane was very rigid. Hence, a small amount of the plasticizer NPOE was added to the membrane composition in order to increase the PIM’s flexibility and resolve this issue. With the aim of keeping the same amount of membrane liquid phase (consisting of the extractant and plasticizer/modifier if used), and base polymer as in the other PIMs, the CTA-based PIM was composed of 30 wt% D2EHPA, 10 wt% NPOE and 60 wt% CTA. These three compositions were thus used as a reference for the assessment of PIMs prepared using the greener alternative solvents.
8
Table 1. Green solvents suitability for the preparation of transparent, homogenous and selfstanding PVDF-HFP-, PVC- or CTA-based PIMs. PIM compositions: 40 wt% D2EHPA and 60 wt% PVDF-HFP; 40 wt% D2EHPA and 60 wt% PVC; and 30 wt% D2EHPA, 10 wt% NPOE and 60 wt% CTA. Green solvent Base
Dissolution conditions
polymer
Homogenous, transparent and self-standing PIM?
Acetone
Ethyl acetate
2-MethylTHF
CyreneTM
PVDF-HFP
Dissolved at RT with 20 mL solvent
No
PVC
ND
NT
CTA
ND
NT
PVDF-HFP
Dissolved at 50 °C with 20 mL solvent Yes
PVC
ND
NT
CTA
ND
NT
PVDF-HFP
ND
NT
PVC
Dissolved at 50 °C with 30 mL solvent Yes
CTA
ND
NT
PVDF-HFP
ND
NT
PVC
Dissolved at 60 °C with 30 mL solvent No
CTA
Dissolved at 60 °C with 30 mL solvent No
ND, not dissolved; RT, room temperature; NT, not tested.
Acetone was unable to dissolve PVC- or CTA-based PIM components even when the temperature and the solvent volume were raised to 40 °C and 30 mL, respectively. Even though it successfully dissolved all components of the PVDF-HFP-based PIM using the initial conditions (i.e., room temperature, 20 mL of solvent), after casting for 24 h the PIMs did not look homogenous (frosted appearance) and were of poor mechanical strength. This was likely due to acetone’s high volatility resulting in its rapid evaporation during the casting process, causing improper setting of the PIM. Regarding ethyl acetate, none of the PIM components were completely solubilised when using the initial conditions mentioned above, although when increasing the temperature to 50 °C all components of the PVDF-HFP-based PIM were 9
successfully dissolved. After casting, this membrane appeared physically similar to its counterpart cast in THF in terms of transparency, flexibility and mechanical resistance (i.e., equally easy to peel off the glass plate without tearing or stretching). 2-MethylTHF could dissolve the PVC-based PIM components with an extra 10 mL of solvent (i.e., 30 mL in total) and heating at 50 °C, and the resulting membranes after casting were similar to those prepared using THF. The remaining PIM components of PVDF-HFP or CTA-based PIMs were not fully solubilised by 2-methylTHF, even when the temperature or the solvent volume were increased. In summary, ethyl acetate and 2-methylTHF were found to be suitable greener alternatives for the preparation of PVDF-HFP-based and PVC-based PIMs, respectively, and were thus chosen and the extraction performance of the corresponding PIMs was further assessed. Testing CyreneTM as a green solvent for the fabrication of PIMs is of great interest because it is not only renewably sourced and not hazardous, but it is also non-toxic. However, CyreneTM is not volatile (boiling point, 200 ⁰C) [10], which prevents the use of the solvent evaporation casting method for the preparation of PIMs (where a volatile solvent is removed by evaporation). Thus, in order to be able to use CyreneTM for PIM casting, the phase inversion method had to be used instead for the removal of the solvent. CyreneTM successfully dissolved all PIM compositions, except for PVDF-HFP, when heated at 60 °C and using a total of 30 mL of solvent (higher temperatures were not attempted to avoid the degradation of the extractant/plasticiser). Nevertheless, after the phase inversion casting in a water bath, the membranes looked opaque, white in colour and not homogenous. It must be noted that, to the best of our knowledge, this is the first time that the use of CyreneTM has been reported for the dissolution of PVC or CTA. Even though the PIMs prepared in CyreneTM were not considered successful, since this was the only screened green solvent able to dissolve the CTA-based PIM components, this solvent was still selected for further studies (Section 3.5) to assess if different
10
aqueous bath conditions (i.e., salinity, pH) would improve the membrane’s physical appearance.
3.2. PIMs extraction performance: Conventional solvents versus greener alternatives In order to assess whether the PIMs cast using the greener solvent alternatives perform in a similar fashion to those cast by conventional solvents, extraction experiments were performed using D2EPHA- and Aliquat 336-based PIMs. The use of D2EHPA for the extraction of Zn(II) using PIMs has been well characterized and optimized [4], thus making this extraction process suitable for the comparison of PIMs cast with the conventional solvent THF and those cast with the suitable greener solvents ethyl acetate and 2-methylTHF. D2EHPA extracts Zn(II) by a cation-exchange process according to Equation (1) [8]:
Zn2+(aq) + 3/2(HD)2 (PIM) ZnD2.HD(PIM) + 2H+(aq)
(1)
where (HD)2 refers to the D2EHPA dimer, and the subscripts (aq) and (PIM) refer to the feed solution and PIM phase, respectively. PVDF-HFP-based and PVC-based PIMs, containing 60 wt% base polymer and 40 wt% D2EHPA, were cast using the conventional solvent THF and their new greener alternative (i.e., ethyl acetate and 2-methylTHF, respectively). These PIMs were then contacted with a solution containing Zn(II) under the same experimental conditions, and the percentage of Zn(II) extracted into each PIM was monitored over time by periodically determining the residual concentration of Zn(II) in the feed solution. The results obtained are presented in Figure 1.
11
Figure 1. Effect of the solvent used to cast D2EHPA-based PIMs containing PVDF-HFP (A) or PVC (B) as the base polymer, on the percentage (%) of Zn(II) extracted relative to its initial amount in the feed solution: ethyl acetate (), 2-methylTHF (), THF (). Membranes’ composition, 40 wt% D2EHPA and 60 wt% PVDF-HFP/PVC; feed solutions, 30 mg L-1 Zn(II) adjusted at pH 3.0. Error bars are ± standard deviation for 1 σn-1 (n =3).
12
It can be observed in Figure 1A that the PVDF-HFP-based PIMs cast in ethyl acetate were found to perform similarly to the PIMs cast in THF in terms of extraction rate and amount extracted (28.11.9 vs 27.90.2 mg Zn(II) g-1, respectively, after 5 h). In regard to the PVCbased membranes (Figure 1B), those cast in 2-methylTHF and THF extracted 17.10.3 and 16.01.6 mg Zn(II) g-1, respectively (after 5 h). Once again, PIMs performed similarly irrespective of the solvent choice. Hence, these results demonstrate that PVDF-HFP- and PVCbased PIMs, containing D2EHPA as the extractant and prepared with their respective greener alternative solvents to THF, showed no statistically significant difference at the 95% confidence level in their Zn(II) extraction performance (Student’s t test: 0.158<2.83 and 1.16<2.40, respectively). For the purpose of proving the appropriateness of these greener solvents for the preparation of PIMs containing other extractants, PIMs containing another commonly used extractant (i.e., Aliquat 336) were also assessed. In this case thiocyanate was used as a model target chemical species, since its extraction and transport using Aliquat 336-based PIMs has been often reported in the literature [6, 7, 11]. Aliquat 336 extracts SCN- by an anion-exchange process according to Equation (2) [12]:
SCN- (aq) + A+NO3- (PIM) A+(SCN-) (PIM) + NO3-(aq)
(2)
where A+NO3- refers to Aliquat 336 after conditioning (where the original chloride ion was replaced with the nitrate ion), and subscripts (aq) and (PIM) refer to the feed solution and the PIM phase, respectively. Aliquat 336-based PIMs were thus cast using ethyl acetate and 2-methylTHF, when PVDFHFP or PVC was applied as the base polymer, respectively, using the same fabrication conditions as described in Table 1. A comparison between the membranes cast using the 13
greener solvents and those prepared with the conventional solvent THF was conducted in terms of extraction of SCN-. The PIM compositions chosen for the corresponding experiments were selected based on previous studies, which reported the following compositions (casting using THF) as optimal for the SCN-extraction: 30 wt% Aliquat 336 and 70 wt% PVDF-HFP [7], and 20 wt% Aliquat 336, 10 wt% 1-tetradecanol and 70 wt% PVC [6]. Figure 2 shows the effect of the casting solvents (THF vs ethyl acetate/2-methylTHF, respectively) on the membranes’ performance in terms of SCN- extraction. These results demonstrate that the extraction ability of PVDF-HFP- and PVC-based PIMs containing Aliquat 336 was also not affected by the use of a different casting solvent. After 5 h, PVDF-HFP-based PIMs cast with ethyl acetate or THF extracted 45.41.1 or 49.03.8 mg SCN- g-1, respectively, and the PVC-based PIMs cast with 2-methylTHF or THF extracted 35.21.1 or 39.64.5 mg SCN- g-1, respectively. According to the Student’s t test, no statistically significant difference was observed at the 95% confidence level (i.e., 3.59<5.94 and 4.34< 6.87, respectively). The conclusion that can be drawn from these experimental results is that neither ethyl acetate nor 2-methylTHF interfere with the PIM formation and its extraction ability and can thus be used as greener alternatives of THF for the preparation of PVDF-HFP and PVC-based PIMs.
14
Figure 2. Effect of the solvent, used to cast Aliquat 336-based PIMs containing PVDF-HFP (A) or PVC (B) as the base polymer, on the percentage of SCN- (%) extracted relative to its initial amount in the feed solution: ethyl acetate (), 2-methylTHF (), or THF (). Membranes’ compositions, 30 wt% Aliquat 336 and 70 wt% PVDF-HFP (A), 20 wt% Aliquat 336, 10 wt% 1-tetradecanol, and 70 wt% PVC (B). Feed solution, 125 mg L-1 SCN-. Error bars are ± standard deviation for 1 σn-1 (n =3).
15
3.3. Stability study It has been demonstrated that PIMs fabricated with a conventional solvent or a greener alternative have similar performance after one single use. However, it is also of interest to know if their performance changes after several uses. Thus, the same PIM compositions studied above and fabricated with THF, ethyl acetate or 2-methylTHF were reused three times by performing three extractions each followed by a back-extraction (i.e., stripping of the target species into a fresh receiving solution). The results obtained for the D2EHPA- and Aliquat 336-based PIMs are presented in Figures 3 and 4, respectively.
Figure 3. Stability of D2EHPA-based PIMs containing PVDF-HFP (A and B) or PVC (C and D) as the base polymer and fabricated with THF (A to D, blue solid bars) or their respective green alternative: ethyl acetate (green bars with diagonal stripes) or 2-methylTHF (green horizontal stripes), respectively. Three consecutive extraction (E1 to E3) and back-extraction (BE1 to BE3) experiments were performed to assess if the PIMs fabricated with the green 16
solvent had similar stability to those fabricated with THF (PIM compositions as in Figure 1). Model target species, Zn(II); feed solution, 30.0 mg L-1 Zn(II) at pH 3 (adjusted with HCl solution); receiving solution, 1 mol L-1 HCl; extraction and back-extraction experiment duration, 5 h. Error bars are ± standard deviation for 1 σn-1 (n =3).
Figure 4. Stability of Aliquat 336-based PIMs containing PVDF-HFP (A and B) or PVC (C and D) as the base polymer and fabricated with THF (A to D, blue solid bars) or their respective green alternative: ethyl acetate (green bars with diagonal stripes) or 2-methylTHF (green horizontal stripes), respectively. Three consecutive extraction (E1 to E3) and back-extraction (BE1 to BE3) experiments were performed to assess if the PIMs fabricated with the green solvent have similar stability to those fabricated with THF (PIM compositions as in Figure 2). Model target species, SCN-; feed solution, 125 mg L-1 SCN-; receiving solution, 1 mol L-1 NaNO3; extraction and back-extraction experiment duration, 5 h. Error bars are ± standard deviation for 1 σn-1 (n =3).
17
Figures 3A to 3C show that the D2EHPA-based PIMs containing PVDF-HFP or PVC as the base polymer, respectively, were very stable over the three consecutive extraction and backextraction cycles regardless of the solvent used to fabricate them. The PVDF-HFP-based membranes cast either with THF or ethyl acetate started to look slightly wrinkled around the edges after the first extraction, although this did not seem to affect their extraction ability. In the case of PVC-based PIMs fabricated with 2-methylTHF or THF, no changes were observed in their physical appearance after the three cycles. Very small mass losses were registered for both PVDF-HFP- or PVC-based PIMs prepared with THF or their respective green alternative (i.e., 4.70.7% vs 4.50.6% for ethyl acetate; 3.40.2% vs 4.00.3% for 2-methylTHF) which should correspond to the loss of D2EHPA. Nevertheless, all PIMs were still able to extract a similar amount of Zn(II) in the second and third extraction step, thus showing that this loss did not affect their stability. It can be observed in Figure 4A that the Aliquat 336-based PIMs containing PVDF-HFP as the base polymer and fabricated with THF or ethyl acetate, both exhibited a slight decrease in their extraction ability as the membranes were reused with the latter this effect being moderately more pronounced. The same was not observed for the PVC-based PIMs (Figures 4C and 4D), which showed to be able to extract similar amounts of SCN- in the first, second and third extraction steps regardless of the solvent used to prepare them. Very small mass losses were again registered with the PVDF-HFP-based PIMs fabricated with THF and ethyl acetate, i.e., 2.30.4% and 1.20.3%, respectively, and the PVC-based PIMs fabricated with THF and 2methylTHF lost 1.40.2% and 2.50.2%, respectively. The physical appearance of the Aliquat 336-based PIMs did not change after the three cycles of extraction and back-extraction (i.e., they were still transparent and flexible, without any deformities). Overall, the stability of the PIMs fabricated with the green solvents was not significantly different from their counterparts prepared with THF. 18
3.4. Surface morphology study Since THF has a lower boiling point than ethyl acetate or 2-methylTHF (i.e., 66 °C vs 77/80 °C, respectively) their evaporation during the casting of the membranes is expected to proceed at different rates and, as a result, could have an effect on the PIM surface morphology. Hence, AFM images of the PVDF-HFP- and PVC-based PIMs containing D2EHPA or Aliquat 336 and fabricated with both conventional and green solvents were taken on both sides of each membrane (side in contact with the glass plate and side in contact with the air) and their respective roughness was calculated. Figure 5 depicts the average roughness of the membranes containing PVDF-HFP and D2EHPA or Aliquat 336 as the base polymer and extractant, respectively, cast with the conventional solvent THF or with its green alternative ethyl acetate.
Figure 5. Average roughness (n=3) of each side of the PVDF-HFP-based PIMs containing D2EHPA or Aliquat 336 as the extractant, fabricated with THF (blue bars) or ethyl acetate (green bars with diagonal stripes).
It can be observed that the average roughness of the PVDF-HFP-based PIMs on the side in contact with the air was generally higher than the roughness of the side in contact with the 19
glass, which means that the PIM surface exposed to the air is rougher due to the evaporation of the solvent. Moreover, the average roughness of PIMs cast with THF were higher than that of PIMs cast with ethyl acetate (for both sides of the membranes). Because ethyl acetate has a higher boiling point than THF it is expected to evaporate at a slower rate during the casting process, thus leading to smoother membrane surfaces. However, this difference in roughness was not significant enough to change the kinetics of extraction as demonstrated in Figures 1A and 2A. The AFM images of the PVC-based PIMs with D2EHPA or Aliquat 336 cast with THF or 2methylTHF revealed very smooth membrane surfaces with no significant differences between the roughness on the air side or the glass side. Table 2 reports on the average roughness values calculated for all PVC-based PIMs.
Table 2. Average roughness of the PVC-based PIMs containing D2EHPA or Aliquat 336 as the extractant, fabricated with THF or 2-methylTHF. Membrane liquid phase
Casting solvent
Average roughness (n=5)
D2EHPA (40 wt%)
THF
1.830.87
D2EHPA (40 wt%)
2-methylTHF
5.520.93
Aliquat 336 (20 wt%) and THF 1-tetradecanol (10 wt%)
3.880.93
Aliquat 336 (20 wt%) and 2-methylTHF 1-tetradecanol (10 wt%)
1.560.41
It can be observed that the average roughness of the PIMs containing PVC as the base polymer is about one order of magnitude lower than that of those composed of PVDF-HFP. This could be due to their different properties, with PVDF-HFP being denser and more hydrophobic than 20
PVC (i.e., 1.8 vs 1.4 g cm-3 at 25 °C), thus leading to the formation of membranes with rougher surfaces. Moreover, and even though the average roughness of the PIMs cast with THF differ from those cast with 2-methylTHF, all values were very low (ranging between 1.6 and 5.5 nm), which means that all membranes can be classified as very smooth.
3.5. CTA-based PIMs fabricated with CyreneTM CTA-based membranes are usually dissolved in DCM and cast by solvent evaporation. Since CyreneTM is not volatile, the phase inversion method was implemented instead of the standard evaporation casting method. The initial attempt of casting these membranes using a water bath, resulted in non-homogenous PIMs, thus further optimization of the phase inversion method was performed with the aim of forming successful PIMs by increasing their homogeneity, transparency and mechanical strength. This optimization was carried out by altering the salinity (0% w/v, 15% w/v, or 20% w/v NaCl) and/or the pH (deionized water or 0.1 M H2SO4) of the aqueous bath within which a thin film of the PIM casting solution was immersed. Increasing the salinity (i.e., ionic strength) and/or acidity of the bath was hypothesized to aid the formation of successful membranes by minimizing the leaching out of their acidic extractant D2EHPA by reducing its solubility in the aqueous phase due to the salting out effect. The membranes cast in the aqueous bath composed of 20% w/v NaCl and 0.1 M H2SO4 were found to be more homogenous and self-standing than those cast using other aqueous bath compositions, although they still did not have an appearance similar to a PIM cast using DCM. Despite the lack of complete transparency, these PIMs were able to extract about 10 mg Zn(II) per g of PIM when contacted with a Zn(II) feed solution for 18 h. These results can be viewed as highly promising with respect to the potential of CyreneTM as a green solvent for the preparation of CTA-based PIMs.
21
4. CONCLUSIONS Renewable and non-hazardous solvents, namely acetone, ethyl acetate, 2-methylTHF and CyreneTM, were screened as greener alternatives to THF and DCM for the casting of PIMs. Ethyl acetate and 2-methylTHF were found to be suitable solvents for the preparation of PVDFHFP- and PVC-based PIMs, respectively, containing either D2EHPA or Aliquat 336 as the extractant. Similar extraction performance and stability was observed when comparing PIMs prepared using the conventional solvent THF with their greener alternatives mentioned above. AFM imaging showed differences in surface morphology between PIMs fabricated with green or conventional solvents, however these were not significant enough to affect the kinetics of extraction. These results demonstrate that the fabrication of PIMs containing the base polymers PVDF-HFP or PVC can be performed with bio-based solvents instead of the conventional THF. The water soluble CyreneTM was found for the first time in this study to be capable of dissolving PVC and CTA and because of this solvent’s low volatility the phase inversion technique was used for the casting of the corresponding PIMs. The PVC- and CTA-based PIMs cast with CyreneTM were inhomogeneous and opaque, although since this solvent was the only green alternative able to dissolve the CTA-based PIM components, further optimization to the casting water bath was conducted. Despite the fact that these PIMs did not have the same appearance as their counterparts cast with DCM (by evaporative solvent casting) they could still extract Zn(II), thus demonstrating the potential of CyreneTM as a green solvent for the preparation of CTA-based PIMs.
Conflicts of interest There are no conflicts to declare.
22
Acknowledgements The authors wish to thank Dr Anastasios Polyzos from the School of Chemistry, The University of Melbourne, for valuable suggestions, Circa Group for kindly providing CyreneTM, and Dr Tian Zheng from the Materials Characterisation and Fabrication Platform at The University of Melbourne for her assistance with the AFM imaging.
23
REFERENCES [1] F.G. Calvo-Flores, M.J. Monteagudo-Arrebola, J.A. Dobado, J. Isac-Garcia, Green and biobased solvents, Top. Curr. Chem., 376 (2018) 40. https://doi.org/10.1007/s41061-018-0191-6. [2] D. Prat, J. Hayler, A. Wells, A survey of solvent selection guides, Green Chem., 16 (2014) 4546-4551. [3] F. Pena-Pereira, A. Kloskowskic, J. Namiesnik, Perspectives on the replacement of harmful organic solvents in analytical methodologies: a framework toward the implementation of a generation
of
eco-friendly
alternatives,
Green
Chem.,
17
(2015)
3687-3705.
https://doi.org/10.1039/c5gc00611b. [4] M.I.G.S. Almeida, R.W. Cattrall, S.D. Kolev, Recent trends in extraction and transport of metal ions using polymer inclusion membranes (PIMs), J. Membr. Sci., 415 (2012) 9-23. https://doi.org/10.1016/j.memsci.2012.06.006. [5] S.D. Kolev, M.I.G.S. Almeida, R.W. Cattrall, Polymer inclusion membranes, in: A.K. Pabby, S.S.H. Rizvi, A.M. Sastre (Eds.) Handbook of membrane separations: chemical, pharmaceutical, food and biotechnological applications, CRC Press, Boca Raton, 2015, pp. 721-737. [6] Y. Cho, C.L. Xu, R.W. Cattrall, S.D. Kolev, A polymer inclusion membrane for extracting thiocyanate from weakly alkaline solutions, J. Membr. Sci., 367 (2011) 85-90. https://doi.org/10.1016/j.memsci.2010.10.040. [7] Y. O'Bryan, R.W. Cattrall, Y.B. Truong, I.L. Kyratzis, S.D. Kolev, The use of poly(vinylidenefluoride-co-hexafluoropropylene) for the preparation of polymer inclusion membranes. Application to the extraction of thiocyanate, J. Membr. Sci., 510 (2016) 481-488. https://doi.org/10.1016/j.memsci.2016.03.026. [8] S.D. Kolev, Y. Baba, R.W. Cattrall, T. Tasaki, N. Pereira, J.M. Perera, G.W. Stevens, Solid phase extraction of zinc(II) using a PVC-based polymer inclusion membrane with di(2-
24
ethylhexyl)phosphoric acid (D2EHPA) as the carrier, Talanta, 78 (2009) 795-799. https://doi.org/10.1016/j.talanta.2008.12.047. [9] T. Marino, F. Galiano, A. Molino, A. Figoli, New frontiers in sustainable membrane preparation: Cyrene (TM) as green bioderived solvent, J. Membr. Sci., 580 (2019) 224-234. https://doi.org/10.1016/j.memsci.2019.03.034. [10] J. Sherwood, M. De Bruyn, A. Constantinou, L. Moity, C.R. McElroy, T.J. Farmer, T. Duncan, W. Raverty, A.J. Hunt, J.H. Clark, Dihydrolevoglucosenone (Cyrene) as a bio-based alternative for dipolar aprotic solvents, Chem. Commun., 50 (2014) 9650-9652. https://doi.org/10.1039/c4cc04133j. [11] Y. O'Bryan, Y.B. Truong, R.W. Cattrall, I.L. Kyratzis, S.D. Kolev, A new generation of highly stable and permeable polymer inclusion membranes (PIMs) with their carrier immobilized in a crosslinked semi-interpenetrating polymer network. Application to the transport
of
thiocyanate,
J.
Membr.
Sci.,
529
(2017)
55-62.
https://doi.org/10.1016/j.memsci.2017.01.057. [12] Y. Cho, R.W. Cattrall, S.D. Kolev, A novel polymer inclusion membrane based method for continuous clean-up of thiocyanate from gold mine tailings water, J. Hazard. Mater., 341 (2018) 297-303. https://doi.org/10.1016/j.jhazmat.2017.07.069.
25
Highlights Fabrication of polymer inclusion membranes (PIMs) using green solvents was assessed Tetrahydrofuran (THF) could be replaced by ethyl acetate or 2-methylTHF 2-MethylTHF can be used to fabricate poly(vinyl chloride) (PVC)-based PIMs Ethyl acetate dissolves poly(vinylidene fluoride-co-hexafluoropropylene) PIMs prepared with green solvents exhibited similar extraction performance
26
CRediT author statement Clayton A. Carner: Investigation, Writing - Original Draft Charles F. Croft: Validation, Formal analysis, Investigation, Writing - Review & Editing Spas D. Kolev: Resources, Writing - Review & Editing, Supervision, Funding acquisition M. Inês G.S. Almeida: Conceptualization, Methodology, Validation, Formal analysis, Writing - Review & Editing, Supervision, Project administration.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
27