Fe3O4 membranes in pervaporative dehydration of ethanol

Fe3O4 membranes in pervaporative dehydration of ethanol

Accepted Manuscript Structure, morphology and separation efficiency of hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol Gabriela Dud...

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Accepted Manuscript Structure, morphology and separation efficiency of hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol Gabriela Dudek, Monika Krasowska, Roman Turczyn, Małgorzata Gnus, Anna Strzelewicz PII: DOI: Reference:

S1383-5866(16)32074-3 http://dx.doi.org/10.1016/j.seppur.2017.03.043 SEPPUR 13629

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

17 October 2016 22 March 2017 22 March 2017

Please cite this article as: G. Dudek, M. Krasowska, R. Turczyn, M. Gnus, A. Strzelewicz, Structure, morphology and separation efficiency of hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.03.043

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Structure, morphology and separation efficiency of hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol

Gabriela Dudek,* Monika Krasowska, Roman Turczyn, Małgorzata Gnus, Anna Strzelewicz Department of Physical Chemistry and Technology of Polymers Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland Phone +48 32 2371427, Fax +48 32 2371509 e-mail: [email protected]

ABSTRACT Pervaporation is a membrane separation technology applied especially for the dehydration of organic solvents. Since its excellent permselectivity toward water, sodium alginate has been identified as the promising membrane material for the pervaporative dehydration of ethanol. Hybrid alginate membranes filled with various amount of magnetite (Fe3O4) and crosslinked using four different agents, i.e. calcium chloride (AlgCa), phosphoric acid (AlgP), glutaraldehyde (AlgGA) and citric acid (AlgC) were prepared and applied in pervaporative dehydration of ethanol. In this paper, the correlation between chemical composition, structure and morphology, and separation properties of membranes is discussed.

KEYWORDS Sodium alginate membranes, pervaporation, ethanol/water separation, ferroferric oxide, fractal analysis

INTRODUCTION Membrane separation technology has attracted great attention in recent years because of the significant role of separation and purification techniques in various branches of science and technology [1-3]. Pervaporation (PV) is a membrane technique for separation of liquid mixture, having certain elements similar to reverse osmosis and gas separation. Pervaporation is advantageous in separation of azeotropes, close-boiling mixtures, thermally sensitive compounds, and removal of species present in low concentrations. Three major fields of applications can be distinguished, namely dehydration of aqueous - organic mixtures, removal of organic compounds from organic - aqueous mixtures and separation of organic - organic mixtures [4-6]. Dehydration of organic solvents is the most important application of pervaporation, since the first commercial application of this method was the dehydration of ethanol using a hybrid system with a distillation column. For the dehydration of organic solvents, hydrophilic membranes are generally used, mainly because of favourable water solubility and diffusivity in hydrophilic matrix, resulting in selective permeation of water. Many researchers focus on developing new polymeric membrane materials. Recently, several reports have been published describing the application of different types of hydrophilic polymers, including poly(vinyl alcohol) (PVA), sodium alginate and chitosan, as membrane materials [7-11]. Sodium alginate (NaAlg) is a natural anionic linear polysaccharide, extracted from seaweed, containing (1-4)-linked β-D-mannuronic acid (M) and α-L-gluronic acid (G) units. This common hydrophilic carbohydrate polymer has been identified as a promising membrane material for pervaporative dehydration. The performance of membranes made from sodium alginate was shown to exceed those of other polysaccharides and even poly(vinyl alcohol). The presence in alginate structure of both carboxylic and hydroxyl groups plays a crucial role in preferential water sorption and diffusion through the membranes. Since alginate has the

excellent permselectivity toward water, it may be a suitable material to offer high separation performance [10-12]. One of the major drawbacks of hydrophilic polymer membranes used in pervaporation dehydration application is the relatively poor membrane stability, especially for the feeding solutions possessing high water contents. In order to increase the membrane stability many efforts are used, including crosslinking the membranes [7-8], utilization of polymer blends [7, 11,13-14], formation of organic – inorganic composites [12,15-19], chemical modification of hydrophilic polymers by grafting [20-22] and formation of a thin and dense separation layer on top of a porous substrate [11,23-24]. Uragami et al. [25] investigated the performance of Alg-DNA blend membranes cross-linked with Ca2+ and Mg2+ ion in the dehydration of an ethanol/water azeotrope. They obtained very high separation factors for H 2O/EtOH selectivity (5500 and 6500) and high permeation rates (0.01 kg·m-2·h-1). Nigiz et al. [26] applied zeolite filled sodium alginate membranes in pervaporative ethanol/water mixtures separation. The addition of zeolite into NaAlg matrix improved the mechanical properties of alginate membranes and increased flux values. Gao et al. [27] prepared the hyaluronic acid/sodium alginate two-active-layer composite membranes for the same dehydration process as Nigiz et al. [26]. It was found that the capping layer with higher hydrophilicity and water retention capacity, and the inner layer with higher permselectivity are able to increase the separation performance of the composite membranes. In our previous research [16-17,28-30] we investigated the performance of chitosan membranes with iron oxide nanoparticles in the process of ethanol dehydration. The results showed that the addition of magnetite particles to chitosan matrix created extra free volumes in polymer, and in consequence, offered space for water molecules to facilitate their permeation through the membranes. In this paper we present the characterization of hybrid alginate membranes filled with various amounts of ferroferric oxides (Fe3O4) cross-linked by

four different agents, i.e. calcium chloride (AlgCa), phosphoric acid (AlgP), glutaraldehyde (AlgGA) and citric acid (AlgC). The structure and morphology of membranes is studied by fractal analysis and the transport and separation properties are evaluated and compared for investigated membranes in the pervaporation process of water/ethanol mixture. We discuss the influence of cross-linking agents, iron oxide powder content and the morphology of investigated membranes on the transport properties of water and ethanol molecules.

EXPERIMENTAL Sodium alginate was obtained from Across. FeCl6 ∙6H2O, sodium acetate, ethylene glycol and 2,2’(ethylenedioxy)bis(ethylamine) (EDBE), that were used in the preparation of iron oxide nanoparticles, were purchased from Sigma-Aldrich. 2.5 wt% glutaraldehyde solution, calcium chloride, phosphoric acid and citric acid, applied as crosslinkers, were purchased from Avantor Performance Materials.

Membrane preparation The scheme of the preparation of sodium alginate membranes is shown in Fig.1. Briefly, 1.5 % sodium alginate solution was prepared by dissolving an appropriate amount of sodium alginate powder in deionised water. This solution was mixed with an appropriate portion of magnetite nanoparticles (0; 5; 10; 15; 20; 25 wt% ). The sodium alginate solution was then cast onto a levelled glass plate and evaporated to dryness at 40oC. Next, after 24 h, the membrane was crosslinked using a suitable crosslinkers, i.e. 2.5 wt% calcium chloride in water, 1.25 wt% glutaraldehyde solution in water, 3.5 vol.% of phosphoric acid or 3.5 wt% citric acid in isopropanol/water mixture. In case of Ca2+ ions (AlgCa), sodium alginate membranes were immersed in calcium chloride solution for 120 min at room temperature. Crosslinked glutaraldehyde membranes (AlgGA) were prepared by 24 h immersing of the dry

membrane in the 50 cm3 of glutaraldehyde solution and subsequent washing with deionised water. In case of crosslinking with phosphoric acid (AlgP), appropriate sodium alginate membranes were immersed in isopropanol–water bath (90/10 vol.%) containing 3.5 vol.% of phosphoric acid for 180 min at room temperature. Crosslinked citric membranes (AlgC) were prepared by immersing of the dry membrane in 50 cm3 3.5 wt% citric acid solution in isopropanol/water mixture (7:3 v/v) for 3 hours, then washing with distilled water until neutral pH. The pristine NaAlg membrane was prepared in the same manner as above except for the addition of iron oxide nanoparticles. The membrane thickness was measured by the micrometre screw gauge and was equal to 28.0 ± 2.0 μm.

Fig. 1

Preparation of iron oxide nanoparticles In a 100ml beaker, a mixture of FeCl6∙6H2O (10mmol), sodium acetate (10mmol), and ethylene glycol (30ml) was stirred vigorously at 50˚C to give a transparent solution. 30mmol of EDBE was added to the resultant solution and the temperature of the reaction mixture was raised to 120˚C. Stirring and heating was continued for additional 2 hours. Then, the mixture was poured into 800ml beaker and ethylene glycol was evaporated until the brown suspension transformed into black colloid. The obtained suspension was cooled to r.t. and finally resulting particles were subjected to magnetic decantation and repeated washing with distilled water, ethanol and acetone. The size distribution of the prepared magnetite nanopowder was measured using a Malvern Instruments Zetasizer Nano ZS90.

Characterization of membrane morphology Membranes morphology was characterized based on the image analysis of the membrane's cross-section. All images were collected using scanning electron microscope Phenom Pro-X. Constructing the rectangles with the smallest area around the particle profile provided the mathematical descriptors of the particle shape. Therefore, if A is the projected area of an object and L is the actual perimeter of the profile and a and b are the lengths of the sides of the minimum area of the embracing rectangle, then the following particles shape descriptors are obtained: elongation factor f1  bulkiness f 

a b

(1)

A ab

surface factor (circularity) f 2 

(2)

L2 4A

(3)

also known as Hausner shape indices [31]. Elongation factor determines the degree of elongation of the examined element in relation to a circle. Circularity for the circle is equal to f1 = 1, for other shapes its value is greater than 1. Irregularity parameter f3 is a coefficient which is both sensitive to profile irregularities and surface elongation. It is defined as follows: f3 

d1 d2

(4)

where d1 and d2 are the diameters of the maximum inscribed and minimum circumscribed circles, respectively. Fractal analysis based on fractal dimension DF and generalized fractal dimension Dq is a useful tool for quantifying the structure and morphology of self-similar objects or structures and is commonly used in many works [32-38]. For the self-similar sets the number of nonempty coverings of covering

in the following way:

scales with the current size

(5) where

is a fractal dimension.Taking the logarithm at the limit

DF  lim  0

ln N   1 ln    

gives (6)

Fractal dimension describes the object giving one number. Various objects can have the same fractal dimension, that is why the generalised fractal dimension is introduced and defined by the formula: N  

1 Dq  q  1 lim  0

ln

P i 1

q

i

(7)

ln 

where N() is the number of covering elements (boxes);  is the size of the covering element (length of the edge); q is a real number (dimension index); and Pi is the probability of finding a point in the ith element. Box Counting Method (BCM) allows calculating both the fractal dimension (DF) and its extension – the generalized fractal dimension (Dq) [34-38]. For q = 0 generalized fractal dimension Dq = D0 = DF is obtained [43]. DF and the Dq were determined with an accuracy of 0.05. Properties of Dq spectra used in this analysis have been described in details in paper [34]. The degree of multifractality

is related to the deviation from a simple

self-similarity and is the difference of the maximum and minimum dimension associated with the least dense and most dense points in the phase space [39] i.e.: (8) Where p is the probability of visiting one region of the square (p≤1/2), and for the remaining region is 1−p. The parameters l1 andl2, where

describe both the stretching and

folding in the phase space and are related to the dissipation. The texture created by the magnetite particles inside a polymer matrix was analysed by means of fractal analysis. The standard statistical analysis was also performed and discussed in section 3.1.

Pervaporation experiments PV experiments were carried out using the apparatus described in previous studies [18] under the following conditions: the effective membrane area was equal to 112·10-4 m2, the reduced pressure on the permeate side was equal to 300 Pa and the permeation temperature was 23◦C. As the feed solution, an aqueous solution of 97 wt% ethanol was used. The permeate was collected in a cold trap cooled with liquid nitrogen. Flux was calculated from the measured weight of liquid collected in the cold traps during a certain time intervals at steady-state conditions. The composition of feed, permeate and retentate was analysed by gas chromatography by means of Perkin Elmer Clarus 500 GC equipped with 30 m elite-WAX ETR column and a flame ionization detector (FID). For each membrane the experiment was repeated three times. The results showed the repeatability of measurements and the errors were of the order of a few percent. The permeation flux of a component i is calculated using the following equation [4-6,40]:

Ji 

mi At

(9)

where mi – weight of a component i in permeate, A – effective membrane area, t – permeation time. Two parameters are used for the description of the separation properties of the membrane, namely separation factor (αAB) and selectivity coefficient (ScAB). Separation factor is calculated by [4-6]:

 AB 

y A / yB x A / xB

where xA, xB – weight fraction of components in the feed [wt%], yA,

(10) yB

– weight fraction of

components in permeate, wt%. Selectivity coefficient is equal to the ratio of permeability of separated components [39-41]:

Sc AB 

PA PB

(11)

In order to compare the separation efficiency of investigated membranes, pervaporation separation index expressed by following equation [40-42] is used:

PSI  J ( AB  1)

(12)

where J – total permeate flux, αAB – separation factor.

RESULTS AND DISCUSSION Characterization of morphology The exemplary SEM images of randomly selected particles of the cross-section of the investigated membranes crosslinked with four different species are presented in Fig. 2. In order to get consistent results, SEM images were taken from at least five different areas representative for the sample.

Fig. 2

The evaluated Hausner shape indices and results of the fractal analysis describing the morphology of hybrid alginate membranes are presented in Table 1 and Table 2, respectively.

TABLE 1

The measured particle size of the magnetite present in the investigated membranes varies from several nanometres to several micrometres. The results show that all evaluated shape descriptors, i.e. elongation factor (f1), bulkiness (f), surface factor (f2) and irregularity parameter (f3) are practically independent on either the type of a crosslinking agent or the

magnetite content and indicate the similar value for all membranes. Thus, elongation factor obtains value 1.04 - 3.57, f equals to 0.38 – 1.00, f2 reaches the values from 1.00 to 3.51 and f3 changes in the range 1.20 - 4.29. The values of evaluated f1, f2, f3 shape coefficients are greater than 1 what indicates that the shape of the magnetite particles is not perfectly spherical, but rather elongated and irregular. This phenomenon could be explained by the independence of ferroferric oxide particles distribution in alginate membranes. It means that the type of investigated crosslinking agent and the amount of iron oxide particles have no effect on the shape nor the size of ferroferric oxide particles loaded in the examined membranes. Independent on the size of the magnetite particles, the shape parameters are similar, what means that the bigger particles aggregate from the nanosized objects and should exhibit self-similarity. The results of fractal analysis for investigated membranes are presented in Table 2. It was found that the hybrid alginate membranes have fractal structure with the value of fractal dimension between 2.65 and 2.76. This fractal structure is stochastic, meaning that it is built out of probabilities and randomness, what is confirmed by the multifractal spectrum Dq (Fig. 2), that shows an asymmetry. The values of D change between 4.52 and 4.94 for different types of Alg membranes. Furthermore, the results of ΔD indicate that the distribution of iron oxide powder in alginate membranes is irregular causing the heterogeneity of the investigated membrane structures. For homogeneous, ideal self-similar structure ΔD equals 0.

TABLE 2

The smallest value of ΔD was obtained for alginate membranes crosslinked by phosphorous acid with 15 wt% or 20 wt% of magnetite powder. These membranes are more homogeneous and self-similar than the other ones. The highest values of ΔD are calculated for

glutaraldehyde crosslinked membranes in which the distribution of iron oxide particles is the most irregular. Fig. 3 shows the spectrum of Dq versus q for membranes crosslinked with different crosslinking agents and containing 20 wt% of magnetite. It was noted that for q> 0 the curves for different membranes overlap while for q< 0 differ from each other. It points the formation of aggregates of magnetite particles in polymer matrix. The possible aggregation of the magnetite particles is also confirmed by the particle size distribution measurements by means of Zetasizer Nano ZS90 (Fig.4). The magnetite powder forms four group of aggregates with the diameter of 40 nm, 250 nm, 450 nm and several m. As it was shown previously, the size of a single particle of magnetite determined by TEM ranges in 10-20 nm [29].

Fig.3

The Dq spectrum also shows that the difference between positive and negative parts of graph is the smallest for membranes crosslinked by phosphorous acid (D = 4.58). On the other hand, the biggest difference is observed for alginate membrane crosslinked with glutaraldehyde (D = 4.89). Due to the fact that the value of ΔD indicates self-similarity, for membranes with a magnetite content of 20 wt% the self-similarity of the membrane morphology decreases in the following order: AlgP>AlgC>AlgCa>AlgGA.

Fig.4

Pervaporation performance of alginate-based membranes The parameters describing transport properties and separation effectiveness of the examined membranes are collected in Table 3.

TABLE 3

The comparison of results evaluated for pristine membranes shows that membranes crosslinked with citric and phosphoric acid are characterized by better selectivity that glutaraldehyde and Ca2+ ions crosslinked ones. Only the PSI index of AlgC is smaller - it is caused by the fact that this parameter strongly depends on the flux, which is low in the case of dense membrane. According to [43] phosphoric and citric acids react in the same way with sodium alginate matrix. In consequence, generated linkage producing both robust and hydrophilic polymer electrolyte complex, which ensures better separation efficiency. In case of sodium alginate membranes crosslinked with Ca2+ ions, the polymer chains form electronegative cavities, capable of holding the cations via ionic interactions, resulting in crosslinking of the chains into a structure resembling an "egg box", contributing to improve the performance of materials [44]. Membranes made of alginate crosslinked with Ca2+ ions or glutaraldehyde also exhibit lower flux and separation effectiveness, compared with the citric or phosphoric acid crosslinked membranes because of the decrease in hydrophilic character of polymer matrix and its low affinity to water, what is especially visible for the AlgGA membrane. After the process of crosslinking, based on the reaction between alginate's hydroxyl and aldehyde groups of GA, formation of an acetal ring and ether linkage is observed. Molecules of GA are miscible in both water and ethanol, indicating good affinity towards both of them. However, created acetal linking has rather better affinity towards alcohol than water [45]. After the addition of iron oxide nanoparticles (magnetite), the values of separation parameters are changed. For the three crosslinkers agents, i.e. calcium chloride, phosphoric acid and citric acid, the increase in selectivity coefficients, separation factor and PSI is observed. The maximum performance is reached between 15 and 20 wt% of magnetite content depending on the type of membranes, i.e. species used for alginate crosslinking. For AlgCa and AlgP, the

highest values of the selectivity coefficient, separation factor and PSI show membranes containing 15 wt% of magnetite, whereas in case of AlgC the maximum of separation effectiveness is reached at 20 wt%. The biggest relative change of aforementioned parameters when compared to the pristine membrane is observed for AlgP (about two times higher). Because of the high flux, the AlgP membrane containing 15 wt% of magnetite shows the highest PSI among all investigated membranes, equal to 33.86 kg·m-2·h-1·μm-1. The another character of changes is observed in case of AlgGA membrane. Only for this membrane, the addition of iron oxide nanoparticles until 20 wt% causes the continuous decrease in separation selectivity, separation factor and PSI. The values of these parameters are recovered for 25 wt% magnetite loaded membrane, reaching similar level as for pristine one. In consequence, only high loaded hybrid AlgGA membranes, even more that 25 wt%, should be considered as potentially useful in pervaporative dehydration of ethanol.

Comparison between alginate membranes separation effectiveness and morphology In membranes with the disordered structure many physical properties become anomalous, e.g. diffusion, and therefore the transport phenomena in such membranes are often poorly described by standard theories. If the disordered system shows self-similarity, the membrane structure can be regarded as a fractal lattice, and the fractal approach may be useful to study its static and dynamic properties [46-47]. The fractal dimension, which is linked with the scaling of mass of membrane and other fractal measures are very suitable for this purpose. Depending on the object, the value of DF may change from 1 to 3 (in 2D: . Dense, non-porous two-dimensional plane is characterized by DF equal to 2. For fractal interfaces DF will be close to 2 for Euclidean-like surface and closer to 3 if the interface is spread in 3D and extended in depth.

The comparison of investigated Alg membranes according to pervaporation separation index (PSI) and the values of ΔD are shown in Figs. 5-7. As it was aforementioned, PSI increases with the magnetite content and reaches its maximum for a given values, different for each type of membrane, namely 15 or 20 wt% of iron oxide. Comparing the trends in changes of PSI and corresponding D values, it can be noticed, that the maximum PSI is always reached for the smallest values of D. The smaller value of this parameter, the distribution of magnetite particles in the polymer matrix is more homogeneous and self-similar. In case of AlgP max. PSI equals to 33.86 kg·m-2·h-1·μm-1 and D is equal to 4.52 (Fig. 5). According to this tendency, AlgGA has the lowest value of PSI, equal to 5.50 kg·m-2·h-1·μm-1. For this membrane, the analysis of structure morphology shows that the evaluated value of ΔD is the highest among all investigated membranes and is equal to 4.89. The similar relation is also observed for the other membranes.

Fig.5 Comparing the max. PSI and corresponding D values for all examined membranes (Fig. 6) one can see, that the dependence between PSI and D is preserved. The AlgP membrane has the lowest D (4.52) and, at the same time, the highest PSI (33.86). On the other hand, the biggest D (4.89) of the AlgGA is accompanied with the smallest PSI (5.50). Moreover, this relation has a linear character (Fig. 7), what suggests that D fractal parameter can be regarded as a measure of separation efficiency of the series of Alg membranes.

Fig.6

Fig.7

Similar trends were observed in the literature. Miyata et al. [48] showed that PMMA-b-PDMS membrane changed its permselectivity with the DF. Annealed membranes with lower DF formed smoother interface with the continuous phase of PDMS, and stayed predominantly ethanol permselective. Recio et al. [49,50] demonstrated that permeability of 6FDA-6FpDA membrane decreased with DF. On the other hand, the ideal selectivity coefficient increased, showing tendency to saturation for the DF value nearly 3. Since the smaller value of D, the distribution of magnetite in polymer matrix is more homogeneous and self-similar and achieved separation efficiency is higher, it seems to be important to pay more attention to prevent the fillers to form aggregates.

CONCLUSIONS In the context of the determination of structure morphology parameters, the shape coefficients i.e. elongation factor (f1), bulkiness (f), surface factor (f2) and irregularity parameter (f3) are evaluated. The results show that the values of these parameters are independent on the kind of investigated membrane. The shape of magnetite particles is not perfectly spherical, but elongated and irregular and they have the similar distribution in polymer matrix forming mainly four aggregates: 40 nm, 250 nm, 450 nm and several m in diameter. The fractal dimension ranging from 2.65 to 2.76 indicates a self-similar structure of Alg membranes. The fractal index ΔD is in a good correlation with the separation efficiency. When this parameter reaches a minimum, the PSI is the highest indicating that such membrane with homogeneous distribution of magnetite in polymer matrix, and self-similarity can be used in pervaporation practise. In accordance with the estimated values of ΔD (4.56 and 4.52 respectively) AlgP membranes containing 10-15 wt% of magnetite are the most homogeneous, self-similar, and show the best ethanol dehydration effectiveness (separation factor of 85.81 and 90.46,

respectively). In conclusion, we have verified that basing on the fractal parameters characterizing self-similarity it is possible to determine the most effective Alg membrane for pervaporative ethanol dehydration process. Furthermore, the degree of multifractality ∆D could be the good parameter for the correlation between membrane's structural properties and anticipated performance.

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FIGURE CAPTIONS FIGURE 1 General scheme of the hybrid AlgCa, AlgP, AlgGA and AlgC membranes preparation FIGURE 2 SEM images of various crosslinked alginate membranes containing 15 wt% magnetite particles: A) AlgCA (magnification 5800x); B) AlgP (magnification 5700x); C) AlgGA (magnification 5700x); D) AlgC (magnification 5700x) FIGURE 3 The Dq versus q spectrum corresponding to various investigated membranes FIGURE 4 The size distribution of iron oxide particles FIGURE 5 The changes in PSI and corresponding ΔD with the magnetite content for AlgP membrane FIGURE 6 The changes in max. PSI and corresponding ΔD for hybrid AlgCa, AlgP, AlgGA and AlgC membranes FIGURE 7 The plot of max PSI versus corresponding ΔD for hybrid AlgCa, AlgP, AlgGA and AlgC membranes

TABLE CAPTIONS TABLE 1 Morphology parameters of crosslinked Alg membranes TABLE 2 Fractal parameters (3D) of magnetite particles in the cross-section of Alg membranes

TABLE 3 Evaluated separation effectiveness of Ca2+ ions (AlgCa), phosphoric acid (AlgP), glutaraldehyde (AlgGA) and citric acid (AlgC) crosslinked alginate membranes: selectivity coefficient (Sc), separation factor (αH2O/EtOH), pervaporative separation index (PSI)

Table 1 Morphology parameters of crosslinked Alg membranes Iron oxide nanoparticles content, wt% Shape Membrane coefficients 5 10 15 20

AlgCa

AlgP

AlgGA

AlgC

f1 f f2 f3 f1 f f2 f3 f1 f f2 f3 f1 f f2 f3

1.20–2.08 0.38-1. 00 1.00-3.51 1.22–2.43 1.04–1.79 0.38-1.00 1.00-3.51 1.20–1.79 1.13–2.80 0.50-1.00 1.00-2.54 1.33–3.37 1.09–1.59 0.38-1.00 1.00-2.54 1.13–1.96

1.08–2.59 0.50-1.00 1.00-2.54 1.26–2.74 1.18–2.00 0.38-1.00 1.00-3.51 1.18–2.18 1.04–3.01 0.50-1.00 1.00-2.54 1.49–3.50 1.29–3.57 0.50-1.00 1.00-2.54 1.35–3.57

1.05–2.53 0.38-1.00 1.00-3.51 1.51–2.57 1.33–2.40 0.38-1.00 1.00-3.51 1.43–2.75 1.06–2.64 0.50-1.00 1.00-2.54 1.33–3.16 1.11–1.65 0.38-1.00 1.00-3.51 1.20–2.32

1.29–2.31 0.38-1.00 1.00-3.51 1.39–4.38 1.11–1.76 0.38-1.00 1.00-3.51 1.43–2.08 1.43–2.41 0.50-1.00 1.00-3.51 2.07–3.31 1.04–2.53 0.38-1.00 1.00-3.51 1.33–3.38

25 1.05–2.17 0.38-1.00 1.00-3.51 1.48–2.26 1.10–1.91 0.38-1.00 1.00-3.51 1.30–2.04 1.08–2.07 0.38-1.00 1.00-3.51 1.18–2.37 1.13–2.83 0.50-1.00 1.00-3.51 1.43–4.29

Table 2 Fractal parameters (3D) of magnetite particles in the cross-section of Alg membranes Membrane AlgCa AlgP AlgGA AlgC

Fractal parameters DF D DF D DF D DF D

Iron oxide nanoparticles content, wt% 5

10

15

20

25

2.69±0.05 4.87±0.05 2.71±0.05 4.68±0.05 2.69±0.05 4.86±0.05 2.70±0.05 4.82±0.05

2.72±0.05 4.78±0.05 2.73±0.05 4.56±0.05 2.73±0.05 4.92±0.05 2.70±0.05 4.77±0.05

2.71±0.05 4.70±0.05 2.74±0.05 4.52±0.05 2.76±0.05 4.94±0.05 2.72±0.05 4.76±0.05

2.65±0.05 4.73±0.05 2.73±0.05 4.58±0.05 2.71±0.05 4.89±0.05 2.72±0.05 4.63±0.05

2.71±0.05 4.81±0.05 2.69±0.05 4.81±0.05 2.69±0.05 4.90±0.05 2.72±0.05 4.70±0.05

Highlights

Hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol.

Fractal analysis for determination the most effective membrane.

Good correlation of multifractality degree with membrane separation efficiency.

Self-similar membrane with homogeneous Fe3O4 distribution useful in PV practise.