chitosan derivatives microparticles for pervaporative dehydration of ethanol

Separation and Purification Technology 234 (2020) 116094

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Robust poly(vinyl alcohol) membranes containing chitosan/chitosan derivatives microparticles for pervaporative dehydration of ethanol

T

Gabriela Dudeka, Roman Turczyna, Krystyna Koniecznyb a b

Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland Faculty of Biology and Environmental Sciences, Cardinal Wyszyński University in Warsaw, Wóycickiego1/3, 01-938 Warszawa, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Poly(vinyl alcohol) membranes Chitosan particles Chitosan derivatives Pervaporation Ethanol/water separation

The series of novel hybrid poly(vinyl alcohol) (PVA) membranes filled with chitosan particles (CS) and its four derivatives: phosphorylated (CS-P), crosslinked with glutaraldehyde (CS-GA), glycidol-modified (CS-G) and sulphated (CS-SO3) chitosan were prepared and tested in the pervaporative separation process of a water/ ethanol mixture. The influence of various types of chitosan particles, as well as their loadings, on transport properties of the membranes was discussed. Consequently, the addition of chitosan particles into the PVA matrix was found to significantly change its hydrophilicity and have a positive impact on the efficiency of ethanol dehydration. The best performance in terms of the highest separation factor (263.3) and PSI (380.3 kg·m−2·h−1) was shown for a PVA membrane filled with the phosphorylated chitosan particles (3 wt% CS-P), which significantly outperformed not only other materials considered in this study, but also other hybrid PVA-based membranes described in the literature.

1. Introduction Pervaporation (PV) is a promising technique employed widely for liquid separation processes, such as the removal of water from organic solvents, the purification of aqueous streams and the separation of organic-organic mixtures. Apart from being environmental friendly, PV offers numerous benefits, including low energy consumption, high selectivity, excellent separation ratio, as well as modular and compact design [1–3]. In a PV process, a separation occurs by the solution-diffusion mechanism and consists of three steps: (a) sorption of a liquid at the upstream side of the membrane surface, (b) diffusion of liquids through the membrane and (c) desorption of the sorbed molecules in a vapour phase at downstream side of the membrane by applying vacuum. Due to their good chemical stability, outstanding physicochemical properties and/or excellent membrane forming ability, in the recent years mainly poly(vinyl alcohol) (PVA) [4–12], chitosan [13–15] and alginate [16–18] have been employed as membrane materials in the dehydration of organic solvents. Indeed, PVA can be indicated as one of the most commonly used polymers in PV process, still attracting more interest for researchers who keep studying PVA extensively as a membrane material. It is a non-toxic, water soluble, bio-compatible and biodegradable synthetic polymer, prepared by a partial or complete hydrolysis of poly(vinyl acetate) [19–21]. The hydroxyl groups present in PVA can form strong hydrogen bonds between intra- and

intermolecular hydroxyl groups, leading to the high affinity of PVA towards water. Therefore, PVA is mainly used as a membrane material in PV for the dehydration of solvents [4–12]. PVA membranes, however, usually exhibit unsatisfactory separation performance due to their instability and swelling phenomenon in the presence of water, as well as their high crystallinity stemming from intermolecular hydrogen bonding. Among various solutions proposed to overcome this problem, controllable incorporation of inorganic nanoparticles into polymer matrix has been considered as one of the most efficient methods to improve membrane’s stability [9–12]. Moreover, the presence of inorganic fillers can effectively adjust the hydrophilic-hydrophobic balance of polymeric membranes, thereby enhancing their permeability and selectivity. Consequently, Wang et al. [4] studied PVA hybrid membranes containing different types of graphitic carbon nitride nanosheets in PV dehydration of ethanol. It was found that for the 90 wt% ethanol/water mixture at 75 °C, with the increase of binding force among polymer/ inorganic interface, the total flux decreased from 4634 to 2328 g·m−2·h−1 and the separation factor increased from 32.4 to 57.9. Also Zhang et al. [5] investigated metal–organic framework/poly (vinyl alcohol) nanohybrid membranes in PV separation of a toluene/nheptane mixture. The obtained results revealed that the separation factor and permeate flux of the optimized modified Cu3(BTC)2/PVA MOF membranes compared with a pristine PVA one, were improved from 8.9 to 17.9 and from 14 to 133 g·m−2·h−1, respectively. Dave et al.

E-mail address: [email protected] (G. Dudek). https://doi.org/10.1016/j.seppur.2019.116094 Received 8 August 2019; Received in revised form 15 September 2019; Accepted 15 September 2019 Available online 16 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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points. For all the membranes, the thickness was equal to 30.0 ± 2.0 μm.

[6] studied a polymeric composite membrane consisting of graphene oxide and PVA, and applied it in the recovery of acetic acid from vinegar wastewater. PV experiment revealed that such a membrane was able to achieve high volumetric flux of water and maximum separation factor of 62.2 at 20% acetic acid concentration. Xia et al. [7] incorporated organosilica into PVA nanohybrid membrane and employed this material for ethanol dehydration (85 vol% ethanol aqueous solution at 40 °C) via PV, achieving the optimal performance of PVA–PTES hybrid membrane with the highest flux of 145 g·m−2·h−1 and the best separation factor of 1026. In our previous work [17], we described a new type of organic matrix/organic filler hybrid membranes based on the alginate matrix and different modified chitosan particles with submicron size. It was found that the presence of chitosan filler greatly enhances membrane performance, in terms of high flux and selectivity, leading to the development of a membrane outperforming numerous conventional materials. Since PVA membranes are known to achieve better performance effectiveness in ethanol dehydration than alginate, and keeping in mind a distinct positive impact of organic filler, in order to further improve the efficiency of PV process we have now prepared a series of novel PVA membranes filled with chitosan (CS) and chitosan derivatives microparticles, i.e. phosphorylated chitosan (CS-P), glycidol-modified chitosan (CS-G), glutaraldehyde crosslinked chitosan (CS-GA) and sulphated chitosan (CS-SO3). In addition, the physico-chemical properties of the resulting membranes were comprehensively studied by FTIR, SEM, contact angle and swelling measurement. We also evaluated and compared the transport and separation properties of investigated membranes in the PV dehydration process of ethanol, as well as discussed the influence of CS particles modification and their content on the transport properties of water and ethanol, as well as on the overall effectiveness of the separation process. Finally, the results obtained in this study were compared with the relevant alginate-based hybrid membranes (Alg_CS) [17] and other hybrid PVA-based membranes described in the literature.

2.3. Preparation of modified chitosan particles Glycidol-modified chitosan (CS-G) particles were prepared based on a Shainoff et al. method described in [22]. In this case, 200 ml of 3 wt% chitosan in 2 vol% acetic acid solution was mixed with 12 ml of glycidol and 100 ml of 1 M NaOH containing sodium borohydride, NaBH4 (2 mg·μL−1) as antioxidant. The mixture was stirred vigorously at 25 °C for 18 h and then washed with distilled water until reaching pH of 7 ± 0.5, finally yielding CS with grafted glycerol moieties (CS-O-CH2CHOH-CH2OH). The resultant CS-G particles were further suspended in 250 ml of deionised water, oxidised with 70 ml of 0.16 M sodium periodate (NaIO4) and kept under slight stirring for 2 h at room temperature [17]. Glutaraldehyde crosslinked chitosan (CS-GA) particles were synthesized using the modified method described by Poon et al. [23]. An aqueous solution of chitosan (3 wt%) was prepared by dissolving 2 g of CS in 100 ml of 2 vol% acetic acid solution. 0.01 M NaOH was added drop-wise to CS solution until the pH reached 5.6. Then, 1.4 ml of 50 wt % glutaraldehyde solution was added to the mixture and vigorously stirred. The slurry was left overnight for the aging process to occur. Next, 2 M NaOH was added drop-wise into the liquid phase for 3 h until a dark brown suspension was produced and the final pH reached 7. The insoluble, crosslinked CS-GA particles were filtered and subsequently washed with several portions of deionised water and cold acetone. Finally, after partial air-drying, the product was crushed and allowed to dry at room temperature in air for further 24 h. Phosphorylated chitosan (CS-P) particles were prepared by a method described by Sakaguchi et al. [24]. 20 g of chitosan, 100 g of urea, and 20 g of 100% orthophosphoric acid were added into 200 ml of dimethylformamide. The mixture was stirred and heated for 1 h at 150 °C. The obtained suspension was cooled to room temperature and centrifuged. The precipitate was than washed thoroughly with deionised water and freeze-dried in a Martin Christ Alpha 2–4 LSC + lyophilisator. Sulphated chitosan (CS-SO3) particles were prepared based on a Gamzazade et al. method [25]. 33 ml of 3 wt% CS solution in 2 vol% acetic acid solution was added to the sulphating complex composed of 4.5 ml of HClSO3 in 30 ml of cooled in advance dimethylformamide (0–4 °C) and vigorously stirred. The sulphation process was held for 60 min at room temperature and was followed by the formation of gel beads of CS-SO3. At the end of the process, the reaction mixture was diluted with deionised water and neutralized by 20 wt% NaOH solution. Finally, CS-SO3 particles were precipitated with ethanol.

2. Experimental 2.1. Materials Poly(vinyl alcohol), PVA (MW = 72 kDa, DH = 97%), chitosan, CS (MW = 600–800 kDa), urea (purity ≥ 98%), glutaric dialdehyde (50 wt % solution in water), sodium hydroxide (purity ≥ 98%), orthophosphoric acid (purity ≥ 100%), dimethylformamide (for analysis), glycidol (purity ≥ 96%), sodium borohydride (purity ≥ 99%), sodium periodate (purity ≥ 99%), and ethanol (96 vol%, pure p.a.) were obtained from Acros Organic. Calcium chloride (purity ≥ 96%), acetic acid (purity ≥ 99%) were purchased from Avantor Performance Materials.

2.4. Membrane characterization 2.2. Membrane preparation Membranes were characterized using scanning electron microscopy (SEM) and FTIR spectroscopy. Contact angles of dry membranes were measured using a MDA1300 Handheld USB Metal Microscope, according to the following procedure: 1 μL droplet of deionised water was used and the contact angle was measured immediately after dropping and after a period of 10 sec. Surface characterization was carried out using Phenom Pro X SEM microscope. FTIR measurements were performed for the membrane films using PerkinElmer Spectrum Two FTIR spectrometer at ambient temperature, in the spectral range of 650–3750 cm−1, and with the spectral resolution of 2 cm−1. The degree of swelling (DS) of the pristine and hybrid PVA membranes with different CS and modified CS particle loading was determined using a sorption test. In this case, membrane samples were immersed in water or ethanol and mass changes of analysed samples were determined during one week using analytical balance. DS was calculated using the following equation:

1.0 wt% PVA solution was prepared by dissolving an appropriate amount of PVA powder in deionised water. This solution was mixed with an appropriate portion of chitosan (CS) or modified CS microparticles, such as phosphorylated chitosan (CS-P), glycidol-modified chitosan (CS-G), glutaraldehyde crosslinked chitosan (CS-GA) and sulphated chitosan (CS-SO3), to obtain the required filler concentration, namely 1; 2; 3; 4 and 5 wt%. The particular solutions were then casted onto the Petri dishes placed on a levelled plate and evaporated to dryness at 60 °C. After 24 h, the membranes were crosslinked using glutaraldehyde by immersing them in the 2.5 wt% glutaraldehyde solution for 15 min at room temperature. The pristine PVA membrane was prepared in the same manner as above, except for the addition of chitosan particles. The membrane thickness was measured using a waterproof precise coating thickness gauge MG-401 ELMETRON, and was estimated as a mean value of at least 10 measurements in different 2

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DS =

Wwet − Wdry Wdry

·100(%)

The solubility coefficient is a measure of the sorption in membrane. It is characterized by splitting of the penetrant between the membrane and the outer phase in equilibrium. This parameter is calculated from following relation:

(1)

where Wwet is the weight of a wet membrane and Wdry is the weight of a dry membrane.

S=

2.5. Pervaporation (PV) experiments

P D

(6)

where PV experiments were carried out using the apparatus described in our previous paper [13] under the same conditions. As the feed, an aqueous solution of 96 vol% ethanol was used. The permeate was collected in a cold trap cooled with liquid nitrogen and the flux was calculated from the weight of condensed permeate during a certain time intervals at steady-state conditions. The composition of all streams, i.e. feed, permeate and retentate, was analysed by gas chromatography using Agilent Technologies 6850 gas chromatograph equipped with an autosampler, Elite-WAX column and a flame ionisation detector (FID). Nitrogen was used as a carrier gas and hydrogen as a fuel gas. To verify if the results are statistically significant, three samples of each type of membrane were fabricated, and for every membrane the PV experiment was repeated three times, giving nine independent measurements for every type of membrane. The permeation flux of component i was calculated using the following equation [26,27]:

mi A·t

Ji =

cm3

S – solubility coefficient ⎡ 3 STP ⎤ ⎣ cm ·cmHg ⎦ D – diffusion coefficient Selectivity coefficient is equal to the ratio of permeation coefficients of separated components [26,27]:

ScAB =

PSI = J (αAB − 1)

3. Results and discussion 3.1. Membrane characterization 3.1.1. FTIR studies FTIR spectra of pristine CS and modified CS particles are shown in Fig. 1. The spectrum of nonmodified CS particles shows characteristic peaks of amide I from acetylated amine groups (C]O stretching) at 1651 cm−1, amide II (NeH bending overlapped by the amide I band) at 1576 cm−1, CH2 wagging coupled with CeOH in plane deformation at 1380 cm−1, ether group (CeOeC stretching) at 1159 cm−1, secondary hydroxyl group (CeOH stretching) and primary hydroxyl and amine groups (CeOH and CeNH2 stretching) at 1065 cm−1 and 1030 cm−1, respectively. The characteristic peaks located at 3275 cm−1 stand for the combined stretching vibration of the hydroxyl OeH, and amine/ amide group NeH. Phosphorylation of CS particles leads to their insolubility in water. The insoluble character of CS-P may be attributed to the existence of

(3)

where xA, xB are the weight fraction of components A and B in the feed [wt%], yA, yB are the weight fractions of components A and B in permeate, wt%. Based on the 1st Fick’s law, the permeation coefficient can be determined according to the formula:

P=

Js ·l Δp

(4)

where P is the permeation coefficient, Barrer =

cm3STP ·cm cm2·s·cmHg

·1010 ,

l is the membrane thickness, cm, Δp is the difference of vapour pressure at both sides of the membrane, cmHg. Js is the diffusive mass flux,

cm3STP cm2·s

The diffusion coefficient is estimated using the method described in [28]. This model assumes that the analysed membranes are not empty before measurement. It also considers the length of the tubing between the permeation cell and the cold traps. According to this method the diffusion coefficient is calculated using the following equation:

D=

−l2 3La

(8)

where J is the total permeate flux, αAB is the separation factor. The results acquired for the materials investigated in this study showed good repeatability and with the SD less than 3%.

where mi is the weight of component i in permeate, A is the effective membrane area, t is the permeation time. Two parameters were used for the description of the separation properties of the membrane, namely separation factor (αAB) and selectivity coefficient (ScAB). Separation factor was calculated using the following equation [26,27]:

αAB

(7)

In order to compare the separation efficiency of investigated membranes, pervaporation separation index expressed by following equation [26,27] was used:

(2)

y /y = A B xA / xB

PA PB

(5)

where

La – the effective total Time Lag calculated as La = La2 − 6.5·La1 La1 – Time Lag for the tubing, s La2 – the asymptotic Time Lag, s l – the thickness of membrane, cm Fig. 1. ATR FTIR spectra of pristine and modified chitosan particles. 3

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inter- or intramolecular salt linkage between CS aminogroups and phosphate groups due to the postulated formation of a poly-ion complex [29]. Introduction of groups such as phosphonic acid or phosphonate onto CS by the reaction of phosphorylating agent with the amino groups is known to increase the chelating properties of CS and is supposed to modify its solubility [30]. In the FTIR spectrum of CS-P, the phosphorylation leads to the appearance of a shoulder at 1374 cm−1 which can be attributed to the P]O asymmetric stretching originated from the phosphates. The peaks found at 1059, 1029 and 972 cm−1 are assigned to the PeOH bond. Protonation of CS amine functionalities is suggested by the presence of two peaks, both attributed to the eNH3+ cation, namely the antisymmetrical deformation at 1650 cm−1 and the symmetric deformation at 1553 cm−1. The initial amide I and II bands are possibly overlapped by these vibrations. The crosslinking of CS membranes with glutaraldehyde leads to the increase in the absorption at 1675 cm−1. As already stated, the mechanism of crosslinking between glutaraldehyde and the free secondary amine on chitosan is proposed to follow a Schiff’s base reaction that results in a C]N imine bond formation. According to Poon et al. [23], there are two possible reactions that may undergo during crosslinking of CS by glutaraldehyde, namely the reaction may occur through (a) an amine-catalyzed aldol addition or (b) become an acetal derivative, where a substitution reaction occurs between the amine on CS. According to Bellamy [31], the C]N peak can show up anywhere between 1620 and 1680 cm−1 depending on the compounds being reacted. Relatively to the intensity of other peaks, the shoulders at 1564 and 1718 cm−1 due to the ethylenic and free-aldehydic bonds can be observed, respectively. The increase in CeH stretching signal at 2938 cm−1 with the decrease in the intensity of peak at 1113 cm−1 because of the presence of aliphatic aminogroups can be also noticed. It indicates that the crosslinking with glutaraldehyde turns the membrane more hydrophobic. In the case of the CS modified with glycidol and oxidised with sodium periodate, the strong, intense band at 1035 cm−1 is observed, which should be associated with the etherification process of vicinal hydroxyl groups as well as hemiacetal and aceal, and the formation of a new CeN bond [22]. Sulphated CS exhibits intense adsorption at 1065, 1152 and 1381 cm−1 that is attributed to the S]O stretching vibration from the sulphate group. Likewise in the phosphorylated chitosan, an additional peak at 1530 cm−1 is attributed to the eNH3+ vibration, arising from the ionic bridges with the SO42– moieties, which probably give also signal at 613 cm−1. The lack of the band corresponding to sulphate ester CeOeS bond deformation, typically observed at around 800 cm−1, is probably caused by its overlapping with other signals. PVA can be crosslinked using aldehyde crosslinking agents such as glutaraldehyde or acetaldehyde. When crosslinking process is conducted in the presence of sulphuric acid, acetic acid or methanol, acetal bridges between the pendant hydroxyl groups of the PVA chains are formed. FTIR spectra of pristine and hybrid PVA membranes filled with CS and modified CS particles are shown in Fig. 2. Pristine PVA membrane exhibits characteristic wide bands referred to the stretching of the hydroxyl OeH groups (3320 cm−1) and attributed mainly to the vibrations of traces of water (1650 cm−1). The stretching vibrations of the eCH2 bonds correspond to the absorption in the 2915 and 2860 cm−1 region, whereas the deformation vibrations of the eCH2 groups appear at 822 cm−1. The most pronounced bands are located in the spectral region of 1000–1450 cm−1. The absorption bands with their maxima at 1035, 1055, 1318 and 1430 cm−1, correspond to the vibrations associated with the CeOH group and CeC skeletal vibrations, respectively [32]. For hybrid PVA membranes, the characteristic peaks of the pristine PVA membranes remain at the same positions but their intensities are different. The spectra of PVA_CS, PVA_CS-GA and PVA_CS-SO3 indicate the increase in the intensity of the band at 3290 cm−1, attributed to hydroxyl group stretching vibrations of PVA with a secondary amide

Fig. 2. ATR FTIR spectra of pristine and hybrid PVA membranes filled with particles of chitosan and its derivatives.

Fig. 3. Average contact angles and degrees of swelling of pristine and hybrid PVA membranes filled with different chitosan particles, n = 10.

group of CS.

3.1.2. Swelling and contact angle measurements The results of swelling and contact angle experiments for pristine and hybrid PVA membranes containing different CS fillers are shown in Fig. 3. The swelling and contact angle measurements give the good estimate of the filler influence on the variation of a bulk and surface hydrophilicity. It is shown that due to the chemical? Nature of a PVA matrix, all investigated membranes exhibit hydrophilic character and the degree of swelling reaches significant values. Correspondingly, the values of contact angle and degree of swelling for pristine PVA membrane are equal to 49.4° and 202%, respectively. The introduction of CS particles into PVA matrix causes a further increase in hydrophilicity due to the increased amount of different hydrophilic groups provided by the filler, mainly eOH and eNH. Moreover, the disturbance in the regularity of the matrix structure results in lowering the crystallinity of PVA and hydrogen bonding network, thus enhancing the availability of the matrix eOH groups. The lowest contact angles are observed for PVA membranes filled with CS-P particles and CS-SO3, which is associated with an increased amount of –OH groups relative to the nonmodified CS particles. Glycidol and glutaraldehyde modified CS particles possess 4

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particles and the cross-sectional views of the hybrid PVA membrane filled with these particles are shown in Fig. 4. CS particles are found to have irregular shapes often in form of flakes, plates, irregular ovals or rectangles, and their sizes are located in the submicron range. Also, CS particles show a bimodal size distribution, i.e. have two populations of particles, namely a population consisting of larger particles with the sizes ca. 1 μm and a fraction of particles with the sizes smaller than 150 nm. It is observed that the smaller particles are settled on the surface of the larger particles forming some kinds of agglomerates. The contribution of the small-size particle population in CS-SO3 is bigger than for other type of modified CS. It is further noticed that the particles of the fillers are evenly distributed throughout the polymer matrix without distinct signs of clustering. Maybe only in case of the PVA_CS-P membrane some evidence of a preferred concentration of the chitosan component on the membrane surface is observed, what was also reported by Koyano et al. [33] for the PVA/chitosan hydrogel materials. It was found that on the air-side surface of such membranes the concentration of chitosan was four times higher than in the bulk of the material. From the cross-sectional view of 3 wt% loaded membranes, the dense structure with rather good compatibility between matrix and filler particles is visible, without pronounced marks of phase separation, defects or holes. 3.2. Pervaporation performance of hybrid alginate membranes The evaluated parameters describing transport properties (flux, permeation, diffusion, solubility and selectivity coefficients) and separation effectiveness (separation factor and pervaporation separation index) of ethanol and water in a PV process through a pristine and all five types of hybrid PVA membranes filled with modified CS particles are presented in Tables S1–S5. As it can be seen, the pristine PVA membrane shows quite good selectivity, especially when compared to the previously investigated pristine alginate membrane [17]. In this case, measured separation efficiency expressed by the separation factor and PSI are equal to 91.4 and 11.8 kg·m−2·h−1, respectively. These values are compatible with the results that have been already presented in literature, e.g. [34]. Generally, hydrophilic membranes have a stronger affinity to water than to ethanol molecules, what stems from the difference between the affinity of the feed components towards the matrix and fillers, the mutual interactions of the components and the way the interactions of one penetrant with membrane moderates the interactions of the others. Generally, PVA is considered to be more hydrophilic than alginate or chitosan, but it should be noted, that any modification, e.g. crosslinking, may vitally change the hydrophilicity of this polymer. A hydrophilic membrane surface is conducive to absorb more water molecules and repel alcohol molecules, which could be beneficial in the PV dehydration process [35]. The swelling experiments confirmed that the bulk hydrophilicity of PVA is higher than in case of Alg, so it is expected that pristine PVA membrane will perform better in water-ethanol separation than Alg one. The formation of hybrid PVA membrane with CS or modified CS particles enhances the separation effectiveness, in terms of PSI, in all investigated cases. The best performance is achieved at ca. 3 wt% of CS particles loading. Further addition of fillers above 5 wt% content leads to the formation of large, free spaces in PVA membranes and enormous increase of flux, what causes the deterioration of membrane separation properties. In this state, hybrid PVA membranes significantly loose their separation ability, since both water and ethanol molecules can easily penetrate throughout the membrane. As it can be seen in Fig. 5 the incorporation of CS filler amends the membrane structure through the interference of the polymer chain packing and induces some fractional free volume, favouring the permeation of the components through the membrane [23,29,30]. This phenomenon is manifested by the increase in the flux. The mere presence of CS particles sharply shifts the flux more than tenfold, which later gradually increases with the increasing content of the filler but it is

Fig. 4. SEM images of chitosan particles used for the preparation of hybrid PVA_CS membranes (magnification 15,000×) and cross-sectional view of PVA_CS membranes with 3 wt% filler content (magnification 5000×).

fewer hydrophilic groups and introduce some hydrophobic e(CH2)ne structural units, so their hydrophilic character is lower compared to CS itself.

3.1.3. Structural properties SEM images of a series of CS, CS-G, CS-GA, CS-P and CS-SO3 5

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or gradually increases for PVA_CS and PVA_CS-G membranes, finally reaching the similar level. Only for PVA_CS-GA the increase in water permeation coefficient is distinctly smaller and slowly decreases for the higher CS-GA particle content. The highest value of the water permeation coefficient reaches 20.11 Barrer for a PVA_CS-P membrane containing 3 wt% of filler. The permeation coefficient of ethanol through hybrid PVA membranes is less affected by the filler content. It keeps almost constant value or slowly increases. Although the changes in the permeation coefficient of ethanol look negligible, if used for the selectivity coefficient calculation (as a ratio between permeation coefficient of both components) they reveal that the best performance is reached for the two PVA membrane type, i.e. these loaded with 3 wt% of CS–SO3 and CS-P. What is more, these results reveal that the membrane systems filled with CS-G and CS-GA particles are rather not promising (see Fig. S1). According to the dual sorption theory, the permeation of any species through the membrane is ruled by two processes, namely diffusion and solubility. By analysing the particular graphs presenting the changes in the diffusion and solubility coefficients (Figs. S2 and S3), one may conclude that in almost all cases the diffusion mechanism is responsible for the permeation process. Moreover, the shape of the curves suggests that the diffusion takes place mainly through the free volume generated near the filler surface and is limited by the interactions between penetrant molecules and the surface of CS particles. Only for the PVA_CSGA membrane, the solubility of the penetrant, especially ethanol, limits the permeation process. In this case, the final value of solubility coefficient of ethanol at 5 wt% exceeds the relevant water solubility coefficient. In contrary, the addition of other type of fillers into PVA matrix causes the decrease in water and ethanol solubility coefficients, which are nearly independent on the amount of filler. The values of water solubility coefficient correspond to the membranes surface hydrophilicity and are well correlated with the estimated contact angle. As a consequence, for more hydrophilic material the water solubility coefficient is bigger. The observed increase in ethanol diffusion coefficient starting from the 3 wt% filler content for several types of chitosan particles is the reason of the drop in the selectivity of the membranes. The practical effectiveness of a component separation could be figured out through the assessment of the separation factor (αH2O/EtOH) and considering the overall efficiency of the pervaporation process through the pervaporative separation index (PSI) which balances both the quality of separation and the speed of this process. The influence of a filler loading on these parameters is presented in the Figs. 7 and 8. As

Fig. 5. The variation of measured total fluxes for PVA membranes with increasing content of modified chitosan.

limited to the 5 wt% by the aforementioned substantial drop in selectivity. The trend of flux changes is very similar for all types of modified CS particles except for the PVA_CS membranes, for which the flux rises slower from 0.13 to 1.11 kg·m−2·h−1, for 5 wt% CS particles content. This behaviour should be related to the fact that the synthesis of modified CS particles introduces some highly hydrophilic groups, e.g. phosphate or sulphate, what turns into increased hydrophilicity of the membrane surface when compared with nonmodified CS. Sunitha et al. [36] reported that the ionic crosslinking present in phosphorylated chitosan membrane increases the flux. Similar conclusion was given by Wang et al. [37], who investigated PVA membranes crosslinked with glutaraldehyde and noticed that the flux increases gradually with the glutaraldehyde content in the membranes due to the changes in membrane structure. Regarding the transport parameters, the addition of CS or modified CS particles enhances membrane hydrophilicity favouring the permeation of water. This behaviour can be clearly seen in Fig. 6presenting the evolution of permeation coefficient of water and ethanol as a function of the filler content. The permeation coefficient of water increases rapidly and then is almost constant (PVA_CS-SO3 and PVA_CS-P)

Fig. 6. The variation of evaluated permeation coefficients of water (filled symbol) and ethanol (blank symbol) for PVA membranes with increasing content of modified chitosan.

Fig. 7. The variation of evaluated separation factors αH2O/EtOH with increasing content of modified chitosan in poly (vinyl alcohol) membranes. 6

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index. 3.3. Comparison of hybrid PVA and Alg membranes contained the same CS fillers The comparison of two series of membranes filled with the same modified CS particles shows the vital role of a polymer matrix (either Alg or PVA) in the design of an efficient membrane system. The first type of membranes has been already described in our previous work [17], and is based on the natural polysaccharide – sodium alginate crosslinked with divalent calcium cations. The second one, presently investigated polymer matrix is based on the synthetic polymer of a great interest in separation technology – poly(vinyl alcohol). Both polymers are hydrophilic because of the presence of eOH groups in their structure. Additionally, alginate possesses anionic carboxylic group in its structure. Since PVA is produced by the hydrolysis of poly (vinyl acetate) (PVAc), it is possible to tune its hydrophilicity by changing the degree of hydrolysis. Low hydrolysed PVA is insoluble in water. By increasing a degree of hydrolysis, more hydroxyl groups becomes exposed what considerably increases hydrophilicity of the material. In contrary, higher degree of hydrolysis reduces the water solubility due to the formation of strong intermolecular hydrogen bonding between polymer chains. Thus, for practical reasons, PVA is always a co-polymer of vinyl acetate and vinyl alcohol units. Also the properties of alginate could vary depending on the source of this natural biopolymer, and the variations include the ratio between β-D-mannuronate and α-L-guluronate units and different sequences or blocks as well molecular mass. The comparison of the fluxes (Fig. 9) shows that in case of PVA membranes they are systematically, barely lower than for the relevant Alg membranes. Only for the pristine membranes this effect is more distinct. As a consequence of the neighbouring group effects, acetate groups adjacent to alcohol groups are more readily hydrolysed and partially hydrolysed PVA exhibits a multi PVAc-co-PVA block-like structure. This kind of structure favours the attractive interactions between eOH groups and gives rise to enhanced polymer arrangement and formation of crystalline domains. Regarding the variety of alginate structure, Alg has only restrained crystallization tendency. Typically, a polymer with high crystallinity restrains the penetration of permeate through more dense, ordered crystallite domains. As mentioned before, membranes filled with CS particles manifest the influence of the matrix crystallinity on the evaluated fluxes to a lower extend than pristine PVA membrane. It is connected with the permeation of water and ethanol molecules that is mostly held through CS particles rather than polymer

Fig. 8. The variation of calculated PSI with increasing content of modified chitosan in poly (vinyl alcohol) membranes.

it was indicated by previous results, the best effectiveness, in terms of αH2O/EtOH and PSI, is always associated with the membranes filled with 3 wt% of CS or modified CS particles. Based on the αH2O/EtOH, however, the membranes could be divided in two groups. The first group, consisting of the PVA_CS-G and PVA_CS-GA membranes, is not effective in the separation of water/ethanol mixture and their αH2O/EtOH (92.2 and 91.8, respectively) is similar to the value of a pristine PVA membrane (91.4). The second group consists of three membranes, namely PVA_CSP, PVA_CS-SO3 and PVA_CS, where PVA loaded with phosphorylated and sulphated chitosan particles are slightly more efficient. The maximum value of αH2O/EtOH, reached for the 3 wt% loaded membranes equals 263.3, 243.3 and 210.3, respectively (see inset in the Fig. 7). Like previously, all membranes can be grouped into two sets based on their PSI. A more effective group consists of PVA_CS-P and PVA_CSSO3 membranes, and the somewhat worse consist of PVA_CS, PVA_CS-G and PVA_CS-GA membranes. All hybrid membranes behave better than pristine PVA in terms of PSI, as it takes into account the flux, which is small in case of a pristine PVA membrane. Likewise, because of small flux PVA_CS membrane is ranked to the second group of membranes with lower PSI (211.4 kg·m−2·h−1). The increase in flux has a positive impact on the PSI, so even the addition of 1 wt% of filler results in an increase of this parameter by about eleven times for the second set of membranes if compared with a pristine PVA membrane. The best result are exhibited by the membranes with the 3 wt% of filler content and PSI equal to 122.1, 136.8 and 211.4 kg·m−2·h−1 for the PVA_CS-GA, PVA_CS-G and PVA_CS membranes, respectively (see inset in the Fig. 8). Further addition of filler has no significant influence on evaluated values of PSI (PVA_CS-G) or entails a gradual drop of this parameter (PVA_CS-GA and PVA_CS), mainly because of the lowering of separation factor. The high value of flux and separation factor of PVA_CS-P and PVA_CS-SO3 membranes is expressed in the highest values of PSI. For these membranes, PSI reaches the values of 380.3 and 327.1 kg·m−2·h−1, respectively, and is ca. 2–3 times bigger than PSI of the less effective membranes. The best results achieved for PVA_CS-P and PVA_CS-SO3 are directly associated with the fact that phosphorylated and sulphated CS particles possess extra hydroxyl groups in the inorganic moieties which strongly enhance the surface and bulk hydrophilic properties of investigated membranes (the lowest contact angles and highest degree of swelling) and preferentially facilitate the diffusion of water with respect to more hydrophobic ethanol molecules. This feature of the aforementioned membranes results in the high values of the evaluated separation factor and pervaporative separation

Fig. 9. The comparison of the highest evaluated fluxes for alginate and poly (vinyl alcohol) membranes filled with the same modified chitosan submicron particles. 7

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Fig. 10. The comparison of the highest evaluated separation factors for alginate and poly (vinyl alcohol) membranes filled with the same modified chitosan submicron particles.

Fig. 11. Comparison of the highest permeation fluxes and the best PSI of investigated membranes with several hybrid PVA membranes used for pervaporative dehydration of ethanol.

matrix. Although exhibiting lower fluxes, PVA membranes are characterized by better separation efficiency. As compared in the Fig. 10, separation factors for all types of PVA membranes are higher than corresponding values found for alginate membranes. The increase in separation factors does not only result from the lowering of fluxes, which are not significant, but the observed extra augmentation in αH2O/ EtOH must be attributed to the properties of matrix and its specific interactions with the particles of organic fillers.

4. Conclusions The new hybrid poly(vinyl alcohol) membranes consisting of PVA matrix and different chitosan particles (CS, CS-P, CS-GA, CS-G, CS-SO3), and crosslinked with glutaraldehyde were prepared for the application in the dehydration of ethanol. The investigated membranes were characterized by several physico-chemical methods and tested in the pervaporative dehydration of 96 vol% ethanol. The swelling and contact angle results clearly indicate a significant increase in both surface and bulk hydrophilicity of hybrid membranes filled with modified chitosan particles. Conducted pervaporative experiments show that the addition of various chitosan particles into PVA matrix has a positive impact on the efficiency of the water/ethanol separation process. The highest fluxes (1.58 and 1.59 kg·m−2·h−1, respectively) are obtained in case of CS-P and CS-G fillers, and are well correlated with the high increase in the hydrophilic properties of membranes containing this two types of chitosan derivatives. Concerning separation effectiveness, the best performance in terms of separation factor and PSI are reached for the membranes containing 3 wt% of fillers. The highest values are noted in case of membranes loaded with CS-P and CS-SO3 particles. The separation factor reaches the values of 263.3 and 243.3 for PVA_CS-P and PVA_CS-SO3 membranes, with 3 wt% of filler, respectively. Based on the calculated PSI, the PVA_CS-P membrane could be pointed as the best one. Evaluated value of PSI at 3 wt% of CS-P filler is equal to 380.3 kg·m−2·h−1 and is the highest among all studied membranes.

3.4. Comparison of PVA hybrid membranes filled with different fillers The performance of PV dehydration process for several alcohol aqueous solutions via PVA-based membranes is summarized in Table 1 and Fig. 11. Taking into account the values of membrane fluxes presented in Table 1, the investigated membranes exhibit medium values of this parameter in the range from 1.01 to 1.50 kg·m−2·h−1. The highest value of flux that equals to 5.00 kg·m−2·h−1 was found for the PVA membrane modified with γ–aminopropyltriethoxysilane [12]. Concerning the separation factor, the best values were obtained in case of PVA membranes filled with organosilica [8] and ZIF-90 [10] (1026 and 1379, respectively). Membranes presented in this work are characterized by the medium values of separation factor (92.1–263.3). Nevertheless, the combination of medium values of estimated fluxes and separation factors leads to the significant values of evaluated PSI, in contrast to the other considered membranes. In this case, PSI takes values in the range of 122.1–380.3 kg·m−2·h−1.

Table 1 Comparison of the separation effectiveness of several PVA-based hybrid membranes applied in the dehydration of alcohol. Filler

T °C

Flux kg·m−2·h−1

Separation factor

PSI kg·m−2·h−1

Ref.

Graphitic carbon nitride Organosilica Fullerenol ZIF-90 Silica γ-aminopropyltriethoxysilane CS CS-P CS-GA CS-G CS-SO3

75 40 25 30 60 50 25 25 25 25 25

2.33 0.14 0.50 0.27 1.33 5.00 1.01 1.45 1.34 1.50 1.35

57.9 1026 70 1379 11 63 210.3 263.3 92.1 92.2 243.3

132.6 143.50 34.5 372.1 13.3 310.0 211.4 380.3 122.1 136.8 327.1

[4] [8] [9] [10] [11] [12] This work This work This work This work This work

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Declaration of Competing Interest

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