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Fabrication of high selectivity blend membranes based on poly vinyl alcohol for crystal violet dye removal
Journal of Environmental Chemical Engineering 8 (2020) 103706
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Fabrication of high selectivity blend membranes based on poly vinyl alcohol for crystal violet dye removal
T
Eman S. Mansora,*, Heba Abdallahb, A.M. Shabana a
Water Pollution Research Department, Environmental Research Division, National Research Centre, Egypt Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Centre, 33 El-Bohouth St. (Former El-Tahrir St.), Dokki, Giza, PO Box 12622., Egypt
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose acetate Polyvinyl alcohol Blend membranes Antifouling Dye removal
Recently membrane technology is considered most effective and promising process for water treatment with high efficiency. So to overcome the brittle problems for cellulose acetate membranes, blend membranes with polyvinyl alcohol were prepared using phase inversion method for crystal violet (CV) dye separation. The characterizations for prepared membranes were performed by scanning electron microscopy (SEM), contact angle, mechanical properties and fouling properties. The prepared membrane from CA/PVA with PVA ratio 6% (M5) has high antifouling properties and good surface wettability. The optimized M5 has molecular weight cut off (800Dalton). The performance of M5 Membrane was carried out using different dye concentrations. Also, long term experiment was performed to prove the efficiency of prepared membrane. CV dye separation increased form 56 % with 0% PVA to reach the maximum 99.9 % with 6% PVA, while the membrane still has better Flux 17 LHM. M5 blend membrane displays good durability property during long term experiments.
1. Introduction Textile industry is one of the greatest industries that produced polluted water due to presence of different kinds of pollution streams resulted from these industries such as textile dyeing and printing [1,2]. The produced wastewater contains huge amount of toxic organic residues. As well as, synthetic dyes that require difficult methods to treat due to the complex aromatic molecular structures that they have, which leading to more difficult to be biodegraded [3,4]. Crystal violet (CV) or methyl violet 10B is a basic dye (Table 1), it is considered carcinogenic, causes severe eye irritation and harm by inhalation. So, it is important to remove these dyes from wastewater [5]. Chemical oxidation, photo-catalysis, precipitation, coagulation and ultrafiltration are most methods were used for dye removal but the treated water still needs post treatment [6–9]. Also adsorption process is considered most commonly approaches in water treatment via the oxidative polymerization owed to simple operation, and wide practicality[10]. Nanofiltration (NF) is a kind of low cost, energy-efficient and environmental friendly membrane technology for dyeing wastewater treatment [9,11]. In addition to membrane bioreactor (MBR) seems to be an alternative process in wastewater treatment for reusing applications [12].
⁎
Most NF membranes operation depends on membrane performance in terms of high water productivity and high rejection percentage, so their limitations are between membrane selectivity and the membrane permeability [11]. Blending method, chemical grafting, and interfacial polymerization are the most common surface modification methods [13]. Blending method technique especially between different polymers is considered an effective method for improving membrane performance. Also coated membranes with different polymers displayed high resistance to protein adhesion like polydopamine or grafting with polyethylene glycol on membrane surface [14]. On the other hand, composite of polymers is another trend for enhancing membrane performance by blending with inorganic fillers, such as SiO2 [15], Al2O3 [16], TiO2 [17], zeolite [18] and metal-organic frameworks, which are considered porous nano-particles and have good compatibility with polymeric chains [17,18]. Polyvinyl alcohol (PVA) is a promising polymer can be used in coating or blending techniques during membranes production. It provides smooth coating film with good hydrophilicity and chemical stability [19]. Also, it has chlorine resistant character and it considered one of polymeric materials that reduce the fouling on the membrane surface [20]. Natural polymers like cellulose acetate (CA) which is a carbohydrate
Corresponding author. E-mail address: [email protected] (E.S. Mansor).
https://doi.org/10.1016/j.jece.2020.103706 Received 1 November 2019; Received in revised form 2 January 2020; Accepted 18 January 2020 Available online 24 February 2020 2213-3437/ © 2020 Published by Elsevier Ltd.
Journal of Environmental Chemical Engineering 8 (2020) 103706
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Table 1 Properties of the studied dye. Characteristics
Textile dye
IUPAC Name Other Name
Tris(4-(dimethylamino)phenyl)methylium chloride Crystal violet or gentian violet or methyl violet 10B or hexamethyl pararosaniline chloride Green to dark-green powder C25H30ClN3 407.99 g·mol−1
Appearance Molecular formula Molecular weight g/mol Molecular structure
Fig. 2. Contact angle values for the prepared membranes.
λ
nanoparticles to enhance the membrane performance [25]. Many studies were carried out to improve PVA properties, one of them was by crosslinking PVA with dialdehyde as in glutaraldehyde [26] formaldehyde [27], and also by crosslinking the polymer using UV-radiation [28]. The aim of this study is usage two blend polymers to enhance the performance of membranes which leading to breakthrough performance for dye removal in terms of permeate quality and flux quantity, also optimization the polymeric matrix. In addition to study physical characteristics of the blend prepared membranes like hydrophilic nature and morphology are elucidated by contact angle measurement. The prepared membranes were characterized using different analysis technique. Different dye concentration on membrane performance is evaluated. The fouling nature of membrane is tested, Long-time performance and durability properties using 20 mg L−1 of crystal violet dye for 72 h by measuring time with dependent water flux. All tests of the membrane are done by dead end nano-filtration process.
590 nm
max
Table 2 Polymer composition for the prepared membranes. Membrane
Polymer composition
Solvent
M1 M2 M3 M4 M5 M6
CA 0 24 24 24 24 24
NMP 0 76 76 76 76 76
PVA 5.0 0.0 0.2 0.4 0.6 0.8
DI H2O 95 0 10 10 10 10
2. Materials and methods 2.1. Materials and chemicals Cellulose acetate with Mw = 100,000 gmol−1 and polyvinyl alcohol with Mw = 90,000 gmol−1 were purchased from Sigma Aldrich. Hydrochloric acid and Sulfuric acid were obtained from Modern Lab chemicals; Ethanol and N- methylpyrirlodine were purchased from sigma Aldrich and Acros Organics (USA) respectively. 1,2,3,4 butane tetra carboxylic acid (BTCA) as cross linker was obtained from MerkGermany (purity > 99 %). Crystal violet dye was purchased from Sigma Aldrich. 2.2. Preparation of PVA membranes Fig. 1. Mechanical properties of the prepared membranes.
5 g PVA was added and dissolved into 95 ml DI water at 90 °C. After PVA was completely dissolved, a dope of concentrated sulfuric acid and butane tetra carboxylic acid were added into the above solution and fully dissolved. After stirring, the polymer solution was poured over a glass plate and subsequently placed in an oven at 50 °C overnight and finally washed with deionized water before further treatment.
polymers composed of repeating units called polysaccharides [21]. According to the polar functional group, backbone structure, molecular weights of these polysaccharides, the physical and chemical properties are determined [22]. CA is considered safe materials, it has good chemical resistance with thermal stability [23]. However, CA membranes can be used for separation applications due to low prices, adequate chlorine resistance and good biocompatibility [24]. Also, the hydrophilic properties of CA membrane surface prevent the blockage of membrane pores. The disadvantage of CA membrane is limited pH using and high temperature can destroy the CA structure. So, the CA membranes can be modified by blending with other polymers or
2.3. Preparation of CA/PVA blend membranes All the blend membranes were prepared by classical phase inversion method and were casted on non –woven polyester as support material. Table 2 shows the composition of CA/PVA casting solutions. For all casting solutions, the CA polymer content to total casting solution was 2
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Scheme 1. The proposed scheme for the prepared CA/PVA membranes.
release of bubbles then was casted. The prepared membranes were kept in deionized water for more than 12 h to remove residual solvent before test.
Table 3 Hydrophilic properties of the prepared membranes. Membrane
Porosity
EWC
Swelling
M1 M2 M3 M4 M5 M6
26 38 21 26 19 30
234 69 42 55 27 64
Swell No swell No swell No swell No swell No swell
2.3.1. Viscosity measurement The viscosity of the studied polymeric solutions was measured using BROOKFIELD AMETEK DV2T viscometer using spindle entry code No. 27. The Rotational Viscometer was measured at room temperature that measures the torque to turn an object in a fluid as a function of that fluid’s viscosity. In addition to measure the shear stress (the force required to move one plane in relation to another) and measure shear rate (the speed of the planes are moving in relation to another)
24 wt., with solvent concentration of 76 wt.. The dope solutions of PVA were 0.2-0.8 wt. were dissolved first in DI water, after complete solubility, drops of sulfuric acid and definite weight of butane tetra carboxylic acid were added to the solution. After that the solvent and CA were added. The polymer matrix was left for 12 h to allow complete Table 4 Viscosity and other properties for the dope polymer solution. Membrane
Viscosity (CP)
Shear Stress (dyn/Cm2)
shear Rate (1/S)
Torque (%)
Speed (rpm)
M1
105 ± 12.0 115 ± 13.5 62310 ± 1.5 62480 ± 3.6 62260 ± 8.0 62900 ± 2.0 64330 ± 3.5 63123 ± 2.0 75910 ± 1.0 75800 ± 1.5 94300 ± 1.5 94100 ± 2.5 94050 ± 4.5 out of range 129200 ± 3 130500 ± 5 139000 ± 6
68 ± 1.4 39 ± 1.0 848 ± 1.5 638 ± 2.5 426 ± 3.0 850 ± 0.8 655 ± 1.5 428 ± 1.0 789 ± 0.4 535 ± 0.8 710 ± 0.5 359 ± 1.0 215 ± 3.0 – 660 ± 1.5 444 ± 2.4 238 ± 3.0
67 ± 0.8 34 ± 0.6 1.4 ± 0.3 1.0 ± 0.9 0.7 ± 1.8 1.36 ± 0.5 1.0 ± 0.6 0.68 ± 0.6 1.0 ± 0.5 0.68 ± 0.5 0.68 ± 0.3 0.34 ± 0.3 0.17 ± 0.4 – 0.5 ± 0.2 0.3 ± 0.3 0.2 ± 0.4
9.5 8.0 99.7 75.0 49.7 99.6 50.0 70.5 93.0 63.0 84.0 43.0 26.0 – 78.0 52.0 28.0
200.0 150.0 4.0 3.0 2.0 4.0 3.0 2.0 3.0 2.0 2.0 1.0 0.5 2.0 1.5 1.0 0.5
M2
M3
M4 M5
M6
3
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contact angle- Data Physics Instrument. The droplets volume was set to 10 μL for all experiments and a photo was taken immediately after the deposition of every drop. The contact angle value was an average of 5 measurements. 2.5. Scanning electron microscope Scanning electron microscope (QUANTA FEG250) was used to determine the morphology of the fabricated membranes. 2.6. Water flux and dye removal Water permeability of membranes was tested by pure water using a dead-end mode setup, with an effective membrane filtration area 12.7 cm2 under pressure of 10 bar for 2 h. After that, water flux was calculated as follows [29]:
Fig. 3. The optimum concentration of CA/PVA s dope polymer.
Flux =
Q AT
(2) 2
Q, A, and_T are quantity of permeate (L), membrane area (m ), and sampling time (h), respectively. Membrane performance was tested using Crystal violet (Merck,Germany) (see Table 1). In order to investigate the effect of dye concentration of feed solution on separation performance, solutions with different concentrations (10, 15, 20 and 25 mg L−1) were prepared and tested. Dye removal efficiency was obtained using Eq. (3):
C Removl (%) = ⎜⎛1 − ⎛ P ⎞ ⎟⎞ 100 ⎝ CF ⎠ ⎠ ⎝ ⎜
⎟
(3) −1
CF and CP are dye concentration (mgL ) in feed and permeate, respectively. UV–vis spectrophotometer (Carry 100) was used to measure the dye concentration.
Fig. 4. FTIR spectrum of neat CA, PVA and CA/PVA blend membrane.
2.6.1. Adsorption of the crystal violet dye at different pH values CA/PVA blend membrane was cut into circle with diameter of 1.7 cm and dipped in 50 mL of CV solution with concentration 20 mg L−1. To investigate the effect of pH values on the adsorption capacity, the pH was set from 3.0 ∼11.0 by changing the solution pH with 0.1 N HCl and 0.1 N NaOH. The CV concentration was measured by taking 3 mL at different time intervals for 90 min (UV–vis spectrophotometer (Carry 100).
2.4. Characterization methods for the prepared membranes 2.4.1. Mechanical testing Stress at break (σ), and elongation at break (ε) were tested on 4 specimens per sample with a length of 100 mm, width of 25 mm, and thickness of about 0.2 mm. The experiment was carried out using an H5KS universal tensile testing machine and the rate was 50 mm min−1. The thickness of the samples was measured by a micrometer with the accuracy of 0.01 mm.
2.6.2. Long-time performance and durability properties The long-term experiment was carried out using M5 for 72 h by continuous filtration test at 10 bar using 20 mg L−1 CV aqueous solution as the feed solution, the properties of the CV dye shown in Table 1, the permeate flux was recorded to determine the decline of flux during process time. Durability property of M5 blend membrane was studied for 3 cycles. The first cycle depends on passing pure water through membrane, then passing the CV dye through membrane and recording the flux. After that, the membrane was washed by mixture of Ethanol and DI-water solution with ratio of 1: 3 times for 30 min. Then the cycles were repeated.
2.4.2. Porosity The membrane porosity was determined by the mass loss of wet membrane after drying. The wet membrane sample weight was measured, then the sample was dried in a dryer up till to the constant weight was determined. Porosity, ε, meaning is the ratio of pore volume to geometrical volume, for the membranes which was calculated using following equation:
ε=
Mw − Md ALP
(1)
where Mw (g) is the wet membrane weight, Md (g) is the dry membrane weight, and A, L, ρ are the wet membrane effective area (cm2), the wet membrane thickness (cm), and the pure water density (gcm−3), respectively. The experiments were repeated for five times to minimize experimental error.
2.7. Studding the fouling properties of the prepared membranes The fouling properties of the prepared membranes were studied using 300 ppm humic acid (HA) as a foulant at 10 bar for 48 h. For more investigation of the antifouling properties, the irreversible fouling ratio (Rirr %) and the reversible fouling ratio (Rr %) have been studied using Eqs. (4–6) [30]. As well the flux recovery ratio (FRR %) were obtained using Eq. (5).
2.4.3. Surface wettability assessment Wettability of the samples was determined by contact angle measurement. The sessile drop method was performed by OCA 15EC 4
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Fig. 5. SEM images for the prepared membranes (C) Cross Section (T) Top surface.
Rr %= (
Rirr
JWC − JP )100 JWI
J − JWC %= ( WI )100 JWI
FRR= (
JWC )100 JWI
determined according to the Stokes-Einstein Eq.(7) [31]: (4)
−3 r= (16.73 (M0.557 W ))10
(7)
(5)
Where r is the Stokes Radius (nm) and Mw is the molecular weight of PEG (Dalton).
(6)
3. Results and discussion
Where JWC is the flux of pure water for cleaning the membrane after filtration of HA and JWI is the flux of pure water for the membrane before HA filtration. JP is the flux of the tested membrane using HA solution and HA concentration was measured by UV absorbance at λ = 254 nm using Agilent Carry 100 UV/Vis spectrophotometer. Rirr (%) and Rr (%) are the irreversible fouling ratio and reversible fouling ratio. The molecular weight cut-off (MWCO) for the optimized membrane was determined by measuring the retention percent of 100 ppm PEG solutions with MWs of 400, 600, 1000, 3000 and 6000 Da. The concentration of PEG was determined using TOC analyzer From the MWCO of the prepared membrane, the pore size can be
3.1. Mechanical properties Stress at maximum load (MPa), and strain at auto break (%) were chosen for membrane mechanical test according to PVA % in the polymeric solution. Fig. 1 indicates that M2 (CA) exhibits low mechanical properties due to its brittleness structure [32]. Blending PVA in the polymeric solution produces membrane that has polymer-plasticizer interactions and leading to decrease of the hardness. Also stress is decreasing with increasing in the strain. This is because blending improves the chains flexibility leading to reduce in membrane stiffness. 5
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Fig. 5. (continued)
contact angle, but with M6 using ratio of 8% reduces the contact angles due to hydrophilicity enhancement and macro voids. It should be noted that M1 (PVA membrane) and M2 (CA membrane) are more hydrophilic in comparison with blend membranes M3-M5, this can be explained by the increase of content of hydroxyl groups with the two neat membranes [35]. Meanwhile the cross-linked phase in blend CA/PVA membrane structure reduces the membrane hydrophilicity as shown in Fig. 2, M5 provides highest contact angle because using the used crosslinker agent BTCA bind and compact the polymer matrix layers together leading to increase in the contact angle [19,36]. While, M6 which has highest ratio of PVA that leads to incomplete crosslinking reaction and the membrane released quickly with the solvent during formation of the membranes and preservation of the membranes. The amount of BTCA as cross linker which was used in this work was constant with all blend membranes and the scheme for the linkage of the two polymeric layers and the cross-linker is shown in scheme 1 .
M3-M6 blend membranes have high stress compared with M1 (PVA) that is because the percentage of CA has an effect on the membrane structure. Furthermore, the strain at break for the blend membranes was increased with increasing the ratio of PVA. This can be explained by the high linkage strength between PVA, CA and BTCA by hydrogen bonds and plasticizer structure of PVA compared to the brittle structure of CA [33,34]. So, the mechanical properties of blend membranes in terms of stress and strain were improved during the testing and it demonstrates that the mechanical properties are governed by the natural polymer.
3.2. Contact angle Fig. 2 indicates that the blend membrane of CA/PVA with PVA ratio 6% (M5) has the highest contact angle (56.5°). Comparatively, the increasing content of PVA leads to raise the 6
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Fig. 5. (continued)
On the other hand, M1 (PVA membrane) has highly water uptake percentage and has swelling properties towards absorption more water molecules inside the membrane. From porosity and water content tests, the addition of PVA to polymeric solution in definite ratio 6% provides blend membranes with best porosity and water content.
3.3. Porosity Table 3. illustrates that M2 (pure CA membrane) has porosity 38 % due to hydrophilicity of CA, while increasing the percentage of PVA to 6% in the polymeric mixture leads to reduce in porosity to 19 % that due to functional groups between CA, PVA and the cross-linker that producing different sites of hydrogen bonding. Increasing PVA to 8% percentage leads to excessive crosslinking or leaching process of uncross linked PVA that can provide voids through membrane structure, accordingly the porosity increased to 30 %. Table 3 indicates that the water uptake of M2 (CA membrane) was 69 %, while increasing PVA percentage from 2 to 8 wt.% reduces the water uptake to be 42, 55, 27, and 64 %, respectively from M3 to M6 depending on blending percentage of PVA. Increasing percentage of PVA to 8% leading to formation of voids and large pores due to leaching process of uncross linked PVA.
3.4. Effect of polymeric solution viscosity The viscosity of the polymeric solution has a correlation with a polymer concentration [37]. The increasing in viscosity is related to increasing in polymer concentration. During experiments, the optimum concentration is determined when the polymer chain entanglement are predictable [38]. The determination of optimum polymer concentration is very important to be as a base of a right selection for polymer concentration in polymeric mixture. 7
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fewer defects [42–44]. The increasing viscosity of the casting solutions for PVA to 8% leads to lost their properties due to the lowest cross-linking and the migration of PVA from the membrane to the coagulation bath, which increases the membrane’s macro void and pore surface once again [42]. 3.5. Chemical structure and functional groups The FTIR spectra of neat PVA, neat CA and blend PVA/CA (M5) were compared for their significant peaks and their interactions shown in Fig. 4. Compared with the neat CA membrane, PVA neat the peak at 3425 cm−1 (−OH resonance vibrations) of M5 reduces obviously. Also the blend membranes displayed absorption bands of both PVA and CA that confirmed the cross linkage of CA layer with PVA layer in the membrane sublayers. For FTIR spectra of pure PVA membrane exhibited the characteristic absorption bands at 3290 cm−1(OH), 2910 cm−1(CH2) and 1240, 1092 cm−1 are related to the asymmetric and symmetric stretching vibration of the CeOCe bond [43]. For CA, the stretching vibration of CeO belongs to carbonyl and carboxyl groups and appears at 1708 cm_1[44]. This FTIR spectrum interpretation confirms the chemical composition of the blend membrane based PVA.
Fig. 6. The effect of PVA Concentration on the flux and rejection for the prepared membranes.
3.6. Scanning electron microscopy (SEM) analysis Fig. 5 illustrates the top surface and cross section of prepared membranes. After the casting the polymeric film on a support. The film immersed in coagulation bath to complete the formation of membrane. The polymers have low miscibility in water accordingly, the exchange occurs between solvent (NMP) and water in different points along the film that leading to form nucleation of the polymer phase and the coagulation occurs to form a membrane [45]. The rate of exchange process effect on the morphology of membranes, since the fast exchange lead to formation of macro-voids in the membrane structure and slow exchange forms a dense structure. Fig. 5, indicates that M1 (pure PVA) has smooth membrane surface with a dense top layer but the formed surface of M2 (CA pure) has pores in the top layer. With increasing PVA concentration within the casting solution, dispersed pores are found on the surface and decrease proportionately in M5. Meanwhile at increasing PVA to 8 wt% in the blend polymer, the membrane exhibits more porous surface as shown in M6. Increasing in PVA concentration can lead to unstable thermodynamic membrane-formation system, which cause ununiform exchange between solvent and water due to flow of un-crosslinked PVA to the coagulation bath making an acceleration in the rate of precipitation [45], Reza et al. has the same observation [46]. The membrane prepared from M1 (pure CA) exhibits a sponge structure meanwhile with increasing the concentration of PVA up to 6% the change in the morphology appears where a finger like cavities and small pores appear in the sub-layer. Increasing PVA content from 0 to 6%, the middle-layer looks as finger like cavities. Compared to the blend membranes M3-M5, higher PVA concentration of 8% (M6) exhibits porous structure due to macrovoids appearance on the membrane skin layer. According to the membrane morphology, the enhancement of the membrane surface selectivity can be performed using blending method between PVA & CA because it can provide best orientation of hydroxyl groups from PVA as a polymer to water during coagulation step [47]. However, hydrophilicity of PVA leads to increase the diffusion between solvent and water during membrane formation process.
Fig. 7. Spectra wave length for the initial dye concentration and after filtration using the prepared membranes.
Table 4 indicates the results of the viscosity of CA/PVA polymeric solution, however increases in PVA concentration leads to increase in viscosity. The trend of the increasing viscosity was clear, that due to the polymer chain entanglement that causes the sudden increasing in the viscosity. The method of determining the critical PVA concentration, was carried out by extrapolating two straight lines that is drawn depending on the slope of the viscosity whereby the viscosity curve becoming more motivated [39,40]. The critical concentration is determined from the crossed point between the two straight lines. In this work, the critical concentration of PVA polymer is 6% as shown in Fig. 3. Chung et al. [41] has reported in their work that using high concentration over optimum concentration of the polymer in polymeric solution, that provides substantial entanglement of chains, leading to dense layer formation with no defects on the membrane surface. Where, they suggested that using polymers concentrations for PES/NMP 35 % led to increase in degree of chain entanglement. In the present work, all polymeric solution compositions produced membranes have a typical asymmetric structure, and the internal structure of membrane whether spongy or finger like structure are related to polymer content and viscosity of the polymeric solution. For high polymeric concentrations the smaller macro voids appeared, so the membrane which has 8% PVA provided high porosity. High viscosity of the polymeric solution leads to delay rate exchange between solvent in the casted polymeric film and water in coagulation bath during membrane formation that provides less porous structure, small pores and
3.7. The effect of PVA Concentration on membrane performance Membrane performance using different PVA Concentration is shown in Fig. 6. The pure CA membrane (M2) has the highest flux 75 LMH and the lowest dye separation. This is owed to the porous top surface that 8
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Fig. 8. Optical images for the prepared membranes after filtration peocess and after washing.
formation a cake on the membrane surface as presented in Fig. 8. This is due to the correlation between dye molecular size and the pores size. Thus, the prepared blend membrane M5 has been chosen for further investigation.
lost his selectivity towards CV dye. Meanwhile, the membrane prepared from pure PVA (M1) exhibits higher separation for CV dye with the lowest flux 5.5 LMH. On the other hand, by increasing PVA concentration from 0 to 6 % the Blend membranes exhibited high selectivity towards CV dye separation that increased form 56 % with 0 PVA to reach the maximum 99.9 % with 6%. This enhancement in rejection percent accompanied with significant decrease in Flux from 75 to 17 LHM and this depicts the change in morphology of the prepared blend membranes as that appeared in SEM images. The M6 (8% PVA) provides higher flux 110 LMH and lower CV dye rejection to be 65 %. That is in accordance with the porosity and permeates water flux (PWF) values of the membrane as shown in Table 3. The changes in the produced membrane performance can be attributed to the changes in the blend morphology as showed in Fig. 7. From the Spectra wave length for the tested dye, the M5 membrane has the highest percent of removal which there is no detection for peak of the λ maximum wave length. Also from the optical images of the prepared membranes after filtration process and after washing, the M5 membrane has the highest cleaning and the lowest agglomeration /
3.8. Adsorption of the crystal violet dye at different pH values It is an important to study the pH of the dye solution to understand the interaction between the membranes and CV, also to obtain more information about the surface nature behavior towards the adsorption process. It affects the structural stability and its color intensity. The adsorption of CV dye using M5 blend membrane over a pH range of 3–11 with 20 mg L−1 dye concentrations is shown in Fig. 9. The percentage adsorption of CV is increased with increase in the pH from 3–11, with the maximum removal at pH 11. The increased adsorption at higher pH is mainly due to enhanced association of the dye cations. It is related to the electrostatic attraction force of the dye compound with M5 surface that is likely to be raised, when the pH increases. The dye CV binds to acid carboxylic groups of membranes by the principle of ion exchange. Because the treatment is carried out in an alkaline medium the carboxyl groups of bound BTCA are found in 9
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Fig. 9. The effect of pH values on the adsorption capacity and filtration process in terms of spectra wave length using M5 membrane.
filtration process and adsorption capacity towards CV dye solution. Also the values for the initial dye absorbance and after filtration process at different pH values indicate the highest performance for the blend membrane M5.
carboxylate form [48]. The minimum removal of CV is found at pH 4 and is probably due to the presence of excess H+ ions competing with the cation groups on the dye for adsorption sites [49]. Moreover, the enhancement of adsorption capacity after incorporation of PVA in the membranes is attributed to the addition of BTCA that provided the hydroxyl-rich chemicals with carboxyl functional groups. Yoon et al. [50] verified that additives of the films with hydroxyl and carboxyl groups were stronger and more flexible than those with only hydroxyl groups. Thus incorporation of PVA with CA in presence of BTCA had important impact on the enhancement of
3.9. The fouling behavior for the prepared membranes To study the cleaning properties, the fouling behavior of the optimized membrane (TFCM7) was studied by filtrating 200 mg L−1 HA solution for 48 h at 10 bar. It can be noticed from Fig. 10 that the CA 10
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Fig. 12. Spectra of different concentration of CV dye before and after the filtration process.
Fig. 10. A): Flux recovery ratio for Pure CA, Pure PVA and the blend membranes. (B):Reversible fouling, irreversible fouling for the prepared membranes.
Fig. 13. Long time performance of M5 blend membrane using 10 mg L−1 CV as the feed solution.
Fig. 11. Effect of different CV dye concentration on M5 blend membrane performance. Fig. 14. Performance of M5 blend membrane interms of flux before washing and after using 20 mg L−1 CVas the feed solution.
neat membrane has the highest Rirr ratio (23 %) and the PVA neat membrane has the lowest Rirr ratio (6.2 %). subsequently by increasing the loading amount of PVA in the polymeric matric, Rirr ratio was enhanced to reach 6.8 % with lowest Rr ratio (11.3 %) for M5 membrane. Also the flux recovery ratio of M5 membrane after rinsing with distilled water is 93 % which implies that HA attached on membrane surface is easily washed away. The results demonstrate that the top layer of the M5 membrane capable of alleviation the deposition of HA on the membrane surface.
3.10. Effect of the dye concentration on membrane performance The dye concentration has a positive effect on membrane behavior so, performance of M5 blend membrane towards different dye concentrations was investigated. Fig. 11, indicates that M5 exhibits rejections of 97.5 %, 98.33 %, 99.2 % and 99.9 % for 10 mg L−1, 15 mg L−1, 20 mg L−1 and 25 mg 11
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be obtained [55]. The mean pore size for the prepared membrane M5 is 0.69 nm. 4. Conclusion CA/PVA blended membranes were successfully prepared using phase inversion process to study the effect of PVA various concentrations on prepared membrane performance. The following conclusions as following from the present work: 1 The mechanical properties in terms of stress and strain were improved after using PVA in membrane preparation and it demonstrates that the mechanical properties are governed by the natural polymer (CA). 2 The prepared membrane from CA/PVA with PVA ratio 6% (M5) has the high antifouling properties and MWCO 800 Da 3 M1 (PVA membrane) and M2 (CA membrane) are more hydrophilic in comparison with blend membranes M3-M5. 4 The porosity decreased with increasing of PVA concentration to 6% in the modified membranes due cross linked functional groups in CA and PVA that providing additional sites for hydrogen bonding. 5 Blend membranes (M3-M5) exhibit lower water content percentage compared with pure CA. 6 The addition of PVA to casting solution in definite ratio 6% provides blend membranes with best porosity and water absorption%. 7 The viscosity curve indicates that the optimum concentration of PVA polymer is 6% 8 With increasing PVA concentration within the casting solution, dispersed pores are found on the membrane surface but decrease proportionately in M5. 9 The prepared M5 blend membrane displays high flux recovery during filtration process, indicating its good durability property.
Fig. 15. Molecular weight cut off for M5 using PEG retention (%).
L−1 aqueous solutions CV dye, along with permeance up to 18.7, 17.9, 17and 16.1 LMH, respectively. It is obvious that membrane has been gotten high rejection for the dye with high concentration that confirmed by the spectra of different concentration of CV dye before and after the filtration process as shown in Fig. 12. At higher concentrations, the molecules were adsorbed easily onto the membrane, which can be observed after the filtration experiments [51]. When the dye concentrations increase, the osmotic pressure increases resulting in the emergence of concentration polarization phenomenon [52]. This phenomenon makes it possible to explain the increase in the retention when the concentration increased. 3.11. Long-time performance and durability properties Fig. 13 indicates long-term experiment to determine the membrane performance for 72 h at 10 bar using 20 mg L−1 CV as feed solution. The flux was constant within the 21 h and then decreased slightly with a small increase in rejection during process time. The recorded flux remained nearly constant with proceeding time (flux > 15 LMH). During operation the initial water permeability reduced due to formation of accumulated dye layer over the membrane surface, then it reached to an equilibrium after certain time of operation [53]. The membrane exhibits approximately fixed flux of 17.5 LMH with rejection of 97.5 %. The results reveal that the M5 blend membrane provides stable performance during long-term experiment. Durability and reusability of M5 blend membrane was studied for 3 cycles and the results were shown in Fig. 14. In the first cycle, the water flux of the membrane was 22 LMH and with CV dye solution reached to 17.4 LMH. Then, after it was rinsed by Et−OH: DI-water solution with 1: 3 times for 30 min, the water flux of the membrane were respectively recovered to 20.9 LMH indicating that the membrane provides a higher flux recovery [54].In the second cycle and third cycle after filtration process and cleaning, the membrane still exhibits good water flux of 20.5 and 2.2 LMH respectively with slightly decrease. In general, the prepared M5 blend membrane displays high flux recovery during filtration process, indicating its good durability property and good reusability for removal of CV dye.
Author contributions section The authorship of this manuscript is Eman S. Mansor1*, Heba Abdallah2, A.M. Shaban1 Eman S. Mansor, Heba Abdallah, A.M. Shaban are responsible of the research idea Eman S. Mansor is responsible for the experimental work Eman S. Mansor and Heba Abdallah contribute in the writing part Eman S. Mansor, Heba Abdallah, A.M. Shaban are responsible of interpretation the analysis A.M. Shaban is the responsible for the final revision. Funding No funding was received for this work. Declaration of Competing Interest No conflict of interest exists. References
3.12. Molecular weight cutoff analysis for M5 membrane
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PEG was selected because of its low interaction with the membrane material to study MWCO for the studied M5 membrane. The molecular weight of PEG corresponding to a retention percent of 90 % on the retention curve is defined as the MWCO of a membrane [30]. As shown in Fig. 15 the retention percent of the optimized NF membrane for PEG has a molecular weight 400,600, 1000, and 3000 is 65 %, 87 %, 96 %, 99 % respectively. Obviously the optimizing condition results in a membrane with a MWCO around 800 Da. As well by substituting the obtained MWCO of M5 to the Stokes-Einstein Eq. (7), the pore size can 12
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Mathew, Porous composite membranes based on cellulose acetate and cellulose nanocrystals via electrospinning and electrospraying, Carbohydr. Polym. 175 (2017) 149–157. [24] C.L. Liu, X. Li, T. Liu, Z. Liu, N. Li, Y. Zhang, X. Feng, Microporous CA/PVDF membranes based on electrospun nanofibers with controlled crosslinking induced by solvent vapor, J. Memb. Sci. 512 (2016) 1–12. [25] N. Li, J. Zheng, P. Hadi, M. Yang, X. Huang, H. Ma, H.W. Walker, B.S. Hsiao, Synthesis and Characterization of a High Flux Nanocellulose–Cellulose Acetate Nanocomposite Membrane, Membranes 9 (2019) 70–85. [26] G. Li, W. Zhang, J. Yang, X. Wang, Time-dependence of pervaporation performance for the separation of ethanol/water mixtures through poly(vinyl alcohol) membrane, J. Colloid Interface Sci. 306 (2007) 337–344. [27] B. Han, J. Li, C. Chen, C. Xu, S.R. 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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Fabrication of high selectivity blend membranes based on poly vinyl alcohol for crystal violet dye removal
T
Eman S. Mansora,*, Heba Abdallahb, A.M. Shabana a
Water Pollution Research Department, Environmental Research Division, National Research Centre, Egypt Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Centre, 33 El-Bohouth St. (Former El-Tahrir St.), Dokki, Giza, PO Box 12622., Egypt
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose acetate Polyvinyl alcohol Blend membranes Antifouling Dye removal
Recently membrane technology is considered most effective and promising process for water treatment with high efficiency. So to overcome the brittle problems for cellulose acetate membranes, blend membranes with polyvinyl alcohol were prepared using phase inversion method for crystal violet (CV) dye separation. The characterizations for prepared membranes were performed by scanning electron microscopy (SEM), contact angle, mechanical properties and fouling properties. The prepared membrane from CA/PVA with PVA ratio 6% (M5) has high antifouling properties and good surface wettability. The optimized M5 has molecular weight cut off (800Dalton). The performance of M5 Membrane was carried out using different dye concentrations. Also, long term experiment was performed to prove the efficiency of prepared membrane. CV dye separation increased form 56 % with 0% PVA to reach the maximum 99.9 % with 6% PVA, while the membrane still has better Flux 17 LHM. M5 blend membrane displays good durability property during long term experiments.
1. Introduction Textile industry is one of the greatest industries that produced polluted water due to presence of different kinds of pollution streams resulted from these industries such as textile dyeing and printing [1,2]. The produced wastewater contains huge amount of toxic organic residues. As well as, synthetic dyes that require difficult methods to treat due to the complex aromatic molecular structures that they have, which leading to more difficult to be biodegraded [3,4]. Crystal violet (CV) or methyl violet 10B is a basic dye (Table 1), it is considered carcinogenic, causes severe eye irritation and harm by inhalation. So, it is important to remove these dyes from wastewater [5]. Chemical oxidation, photo-catalysis, precipitation, coagulation and ultrafiltration are most methods were used for dye removal but the treated water still needs post treatment [6–9]. Also adsorption process is considered most commonly approaches in water treatment via the oxidative polymerization owed to simple operation, and wide practicality[10]. Nanofiltration (NF) is a kind of low cost, energy-efficient and environmental friendly membrane technology for dyeing wastewater treatment [9,11]. In addition to membrane bioreactor (MBR) seems to be an alternative process in wastewater treatment for reusing applications [12].
⁎
Most NF membranes operation depends on membrane performance in terms of high water productivity and high rejection percentage, so their limitations are between membrane selectivity and the membrane permeability [11]. Blending method, chemical grafting, and interfacial polymerization are the most common surface modification methods [13]. Blending method technique especially between different polymers is considered an effective method for improving membrane performance. Also coated membranes with different polymers displayed high resistance to protein adhesion like polydopamine or grafting with polyethylene glycol on membrane surface [14]. On the other hand, composite of polymers is another trend for enhancing membrane performance by blending with inorganic fillers, such as SiO2 [15], Al2O3 [16], TiO2 [17], zeolite [18] and metal-organic frameworks, which are considered porous nano-particles and have good compatibility with polymeric chains [17,18]. Polyvinyl alcohol (PVA) is a promising polymer can be used in coating or blending techniques during membranes production. It provides smooth coating film with good hydrophilicity and chemical stability [19]. Also, it has chlorine resistant character and it considered one of polymeric materials that reduce the fouling on the membrane surface [20]. Natural polymers like cellulose acetate (CA) which is a carbohydrate
Corresponding author. E-mail address: [email protected] (E.S. Mansor).
https://doi.org/10.1016/j.jece.2020.103706 Received 1 November 2019; Received in revised form 2 January 2020; Accepted 18 January 2020 Available online 24 February 2020 2213-3437/ © 2020 Published by Elsevier Ltd.
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Table 1 Properties of the studied dye. Characteristics
Textile dye
IUPAC Name Other Name
Tris(4-(dimethylamino)phenyl)methylium chloride Crystal violet or gentian violet or methyl violet 10B or hexamethyl pararosaniline chloride Green to dark-green powder C25H30ClN3 407.99 g·mol−1
Appearance Molecular formula Molecular weight g/mol Molecular structure
Fig. 2. Contact angle values for the prepared membranes.
λ
nanoparticles to enhance the membrane performance [25]. Many studies were carried out to improve PVA properties, one of them was by crosslinking PVA with dialdehyde as in glutaraldehyde [26] formaldehyde [27], and also by crosslinking the polymer using UV-radiation [28]. The aim of this study is usage two blend polymers to enhance the performance of membranes which leading to breakthrough performance for dye removal in terms of permeate quality and flux quantity, also optimization the polymeric matrix. In addition to study physical characteristics of the blend prepared membranes like hydrophilic nature and morphology are elucidated by contact angle measurement. The prepared membranes were characterized using different analysis technique. Different dye concentration on membrane performance is evaluated. The fouling nature of membrane is tested, Long-time performance and durability properties using 20 mg L−1 of crystal violet dye for 72 h by measuring time with dependent water flux. All tests of the membrane are done by dead end nano-filtration process.
590 nm
max
Table 2 Polymer composition for the prepared membranes. Membrane
Polymer composition
Solvent
M1 M2 M3 M4 M5 M6
CA 0 24 24 24 24 24
NMP 0 76 76 76 76 76
PVA 5.0 0.0 0.2 0.4 0.6 0.8
DI H2O 95 0 10 10 10 10
2. Materials and methods 2.1. Materials and chemicals Cellulose acetate with Mw = 100,000 gmol−1 and polyvinyl alcohol with Mw = 90,000 gmol−1 were purchased from Sigma Aldrich. Hydrochloric acid and Sulfuric acid were obtained from Modern Lab chemicals; Ethanol and N- methylpyrirlodine were purchased from sigma Aldrich and Acros Organics (USA) respectively. 1,2,3,4 butane tetra carboxylic acid (BTCA) as cross linker was obtained from MerkGermany (purity > 99 %). Crystal violet dye was purchased from Sigma Aldrich. 2.2. Preparation of PVA membranes Fig. 1. Mechanical properties of the prepared membranes.
5 g PVA was added and dissolved into 95 ml DI water at 90 °C. After PVA was completely dissolved, a dope of concentrated sulfuric acid and butane tetra carboxylic acid were added into the above solution and fully dissolved. After stirring, the polymer solution was poured over a glass plate and subsequently placed in an oven at 50 °C overnight and finally washed with deionized water before further treatment.
polymers composed of repeating units called polysaccharides [21]. According to the polar functional group, backbone structure, molecular weights of these polysaccharides, the physical and chemical properties are determined [22]. CA is considered safe materials, it has good chemical resistance with thermal stability [23]. However, CA membranes can be used for separation applications due to low prices, adequate chlorine resistance and good biocompatibility [24]. Also, the hydrophilic properties of CA membrane surface prevent the blockage of membrane pores. The disadvantage of CA membrane is limited pH using and high temperature can destroy the CA structure. So, the CA membranes can be modified by blending with other polymers or
2.3. Preparation of CA/PVA blend membranes All the blend membranes were prepared by classical phase inversion method and were casted on non –woven polyester as support material. Table 2 shows the composition of CA/PVA casting solutions. For all casting solutions, the CA polymer content to total casting solution was 2
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E.S. Mansor, et al.
Scheme 1. The proposed scheme for the prepared CA/PVA membranes.
release of bubbles then was casted. The prepared membranes were kept in deionized water for more than 12 h to remove residual solvent before test.
Table 3 Hydrophilic properties of the prepared membranes. Membrane
Porosity
EWC
Swelling
M1 M2 M3 M4 M5 M6
26 38 21 26 19 30
234 69 42 55 27 64
Swell No swell No swell No swell No swell No swell
2.3.1. Viscosity measurement The viscosity of the studied polymeric solutions was measured using BROOKFIELD AMETEK DV2T viscometer using spindle entry code No. 27. The Rotational Viscometer was measured at room temperature that measures the torque to turn an object in a fluid as a function of that fluid’s viscosity. In addition to measure the shear stress (the force required to move one plane in relation to another) and measure shear rate (the speed of the planes are moving in relation to another)
24 wt., with solvent concentration of 76 wt.. The dope solutions of PVA were 0.2-0.8 wt. were dissolved first in DI water, after complete solubility, drops of sulfuric acid and definite weight of butane tetra carboxylic acid were added to the solution. After that the solvent and CA were added. The polymer matrix was left for 12 h to allow complete Table 4 Viscosity and other properties for the dope polymer solution. Membrane
Viscosity (CP)
Shear Stress (dyn/Cm2)
shear Rate (1/S)
Torque (%)
Speed (rpm)
M1
105 ± 12.0 115 ± 13.5 62310 ± 1.5 62480 ± 3.6 62260 ± 8.0 62900 ± 2.0 64330 ± 3.5 63123 ± 2.0 75910 ± 1.0 75800 ± 1.5 94300 ± 1.5 94100 ± 2.5 94050 ± 4.5 out of range 129200 ± 3 130500 ± 5 139000 ± 6
68 ± 1.4 39 ± 1.0 848 ± 1.5 638 ± 2.5 426 ± 3.0 850 ± 0.8 655 ± 1.5 428 ± 1.0 789 ± 0.4 535 ± 0.8 710 ± 0.5 359 ± 1.0 215 ± 3.0 – 660 ± 1.5 444 ± 2.4 238 ± 3.0
67 ± 0.8 34 ± 0.6 1.4 ± 0.3 1.0 ± 0.9 0.7 ± 1.8 1.36 ± 0.5 1.0 ± 0.6 0.68 ± 0.6 1.0 ± 0.5 0.68 ± 0.5 0.68 ± 0.3 0.34 ± 0.3 0.17 ± 0.4 – 0.5 ± 0.2 0.3 ± 0.3 0.2 ± 0.4
9.5 8.0 99.7 75.0 49.7 99.6 50.0 70.5 93.0 63.0 84.0 43.0 26.0 – 78.0 52.0 28.0
200.0 150.0 4.0 3.0 2.0 4.0 3.0 2.0 3.0 2.0 2.0 1.0 0.5 2.0 1.5 1.0 0.5
M2
M3
M4 M5
M6
3
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contact angle- Data Physics Instrument. The droplets volume was set to 10 μL for all experiments and a photo was taken immediately after the deposition of every drop. The contact angle value was an average of 5 measurements. 2.5. Scanning electron microscope Scanning electron microscope (QUANTA FEG250) was used to determine the morphology of the fabricated membranes. 2.6. Water flux and dye removal Water permeability of membranes was tested by pure water using a dead-end mode setup, with an effective membrane filtration area 12.7 cm2 under pressure of 10 bar for 2 h. After that, water flux was calculated as follows [29]:
Fig. 3. The optimum concentration of CA/PVA s dope polymer.
Flux =
Q AT
(2) 2
Q, A, and_T are quantity of permeate (L), membrane area (m ), and sampling time (h), respectively. Membrane performance was tested using Crystal violet (Merck,Germany) (see Table 1). In order to investigate the effect of dye concentration of feed solution on separation performance, solutions with different concentrations (10, 15, 20 and 25 mg L−1) were prepared and tested. Dye removal efficiency was obtained using Eq. (3):
C Removl (%) = ⎜⎛1 − ⎛ P ⎞ ⎟⎞ 100 ⎝ CF ⎠ ⎠ ⎝ ⎜
⎟
(3) −1
CF and CP are dye concentration (mgL ) in feed and permeate, respectively. UV–vis spectrophotometer (Carry 100) was used to measure the dye concentration.
Fig. 4. FTIR spectrum of neat CA, PVA and CA/PVA blend membrane.
2.6.1. Adsorption of the crystal violet dye at different pH values CA/PVA blend membrane was cut into circle with diameter of 1.7 cm and dipped in 50 mL of CV solution with concentration 20 mg L−1. To investigate the effect of pH values on the adsorption capacity, the pH was set from 3.0 ∼11.0 by changing the solution pH with 0.1 N HCl and 0.1 N NaOH. The CV concentration was measured by taking 3 mL at different time intervals for 90 min (UV–vis spectrophotometer (Carry 100).
2.4. Characterization methods for the prepared membranes 2.4.1. Mechanical testing Stress at break (σ), and elongation at break (ε) were tested on 4 specimens per sample with a length of 100 mm, width of 25 mm, and thickness of about 0.2 mm. The experiment was carried out using an H5KS universal tensile testing machine and the rate was 50 mm min−1. The thickness of the samples was measured by a micrometer with the accuracy of 0.01 mm.
2.6.2. Long-time performance and durability properties The long-term experiment was carried out using M5 for 72 h by continuous filtration test at 10 bar using 20 mg L−1 CV aqueous solution as the feed solution, the properties of the CV dye shown in Table 1, the permeate flux was recorded to determine the decline of flux during process time. Durability property of M5 blend membrane was studied for 3 cycles. The first cycle depends on passing pure water through membrane, then passing the CV dye through membrane and recording the flux. After that, the membrane was washed by mixture of Ethanol and DI-water solution with ratio of 1: 3 times for 30 min. Then the cycles were repeated.
2.4.2. Porosity The membrane porosity was determined by the mass loss of wet membrane after drying. The wet membrane sample weight was measured, then the sample was dried in a dryer up till to the constant weight was determined. Porosity, ε, meaning is the ratio of pore volume to geometrical volume, for the membranes which was calculated using following equation:
ε=
Mw − Md ALP
(1)
where Mw (g) is the wet membrane weight, Md (g) is the dry membrane weight, and A, L, ρ are the wet membrane effective area (cm2), the wet membrane thickness (cm), and the pure water density (gcm−3), respectively. The experiments were repeated for five times to minimize experimental error.
2.7. Studding the fouling properties of the prepared membranes The fouling properties of the prepared membranes were studied using 300 ppm humic acid (HA) as a foulant at 10 bar for 48 h. For more investigation of the antifouling properties, the irreversible fouling ratio (Rirr %) and the reversible fouling ratio (Rr %) have been studied using Eqs. (4–6) [30]. As well the flux recovery ratio (FRR %) were obtained using Eq. (5).
2.4.3. Surface wettability assessment Wettability of the samples was determined by contact angle measurement. The sessile drop method was performed by OCA 15EC 4
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Fig. 5. SEM images for the prepared membranes (C) Cross Section (T) Top surface.
Rr %= (
Rirr
JWC − JP )100 JWI
J − JWC %= ( WI )100 JWI
FRR= (
JWC )100 JWI
determined according to the Stokes-Einstein Eq.(7) [31]: (4)
−3 r= (16.73 (M0.557 W ))10
(7)
(5)
Where r is the Stokes Radius (nm) and Mw is the molecular weight of PEG (Dalton).
(6)
3. Results and discussion
Where JWC is the flux of pure water for cleaning the membrane after filtration of HA and JWI is the flux of pure water for the membrane before HA filtration. JP is the flux of the tested membrane using HA solution and HA concentration was measured by UV absorbance at λ = 254 nm using Agilent Carry 100 UV/Vis spectrophotometer. Rirr (%) and Rr (%) are the irreversible fouling ratio and reversible fouling ratio. The molecular weight cut-off (MWCO) for the optimized membrane was determined by measuring the retention percent of 100 ppm PEG solutions with MWs of 400, 600, 1000, 3000 and 6000 Da. The concentration of PEG was determined using TOC analyzer From the MWCO of the prepared membrane, the pore size can be
3.1. Mechanical properties Stress at maximum load (MPa), and strain at auto break (%) were chosen for membrane mechanical test according to PVA % in the polymeric solution. Fig. 1 indicates that M2 (CA) exhibits low mechanical properties due to its brittleness structure [32]. Blending PVA in the polymeric solution produces membrane that has polymer-plasticizer interactions and leading to decrease of the hardness. Also stress is decreasing with increasing in the strain. This is because blending improves the chains flexibility leading to reduce in membrane stiffness. 5
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Fig. 5. (continued)
contact angle, but with M6 using ratio of 8% reduces the contact angles due to hydrophilicity enhancement and macro voids. It should be noted that M1 (PVA membrane) and M2 (CA membrane) are more hydrophilic in comparison with blend membranes M3-M5, this can be explained by the increase of content of hydroxyl groups with the two neat membranes [35]. Meanwhile the cross-linked phase in blend CA/PVA membrane structure reduces the membrane hydrophilicity as shown in Fig. 2, M5 provides highest contact angle because using the used crosslinker agent BTCA bind and compact the polymer matrix layers together leading to increase in the contact angle [19,36]. While, M6 which has highest ratio of PVA that leads to incomplete crosslinking reaction and the membrane released quickly with the solvent during formation of the membranes and preservation of the membranes. The amount of BTCA as cross linker which was used in this work was constant with all blend membranes and the scheme for the linkage of the two polymeric layers and the cross-linker is shown in scheme 1 .
M3-M6 blend membranes have high stress compared with M1 (PVA) that is because the percentage of CA has an effect on the membrane structure. Furthermore, the strain at break for the blend membranes was increased with increasing the ratio of PVA. This can be explained by the high linkage strength between PVA, CA and BTCA by hydrogen bonds and plasticizer structure of PVA compared to the brittle structure of CA [33,34]. So, the mechanical properties of blend membranes in terms of stress and strain were improved during the testing and it demonstrates that the mechanical properties are governed by the natural polymer.
3.2. Contact angle Fig. 2 indicates that the blend membrane of CA/PVA with PVA ratio 6% (M5) has the highest contact angle (56.5°). Comparatively, the increasing content of PVA leads to raise the 6
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Fig. 5. (continued)
On the other hand, M1 (PVA membrane) has highly water uptake percentage and has swelling properties towards absorption more water molecules inside the membrane. From porosity and water content tests, the addition of PVA to polymeric solution in definite ratio 6% provides blend membranes with best porosity and water content.
3.3. Porosity Table 3. illustrates that M2 (pure CA membrane) has porosity 38 % due to hydrophilicity of CA, while increasing the percentage of PVA to 6% in the polymeric mixture leads to reduce in porosity to 19 % that due to functional groups between CA, PVA and the cross-linker that producing different sites of hydrogen bonding. Increasing PVA to 8% percentage leads to excessive crosslinking or leaching process of uncross linked PVA that can provide voids through membrane structure, accordingly the porosity increased to 30 %. Table 3 indicates that the water uptake of M2 (CA membrane) was 69 %, while increasing PVA percentage from 2 to 8 wt.% reduces the water uptake to be 42, 55, 27, and 64 %, respectively from M3 to M6 depending on blending percentage of PVA. Increasing percentage of PVA to 8% leading to formation of voids and large pores due to leaching process of uncross linked PVA.
3.4. Effect of polymeric solution viscosity The viscosity of the polymeric solution has a correlation with a polymer concentration [37]. The increasing in viscosity is related to increasing in polymer concentration. During experiments, the optimum concentration is determined when the polymer chain entanglement are predictable [38]. The determination of optimum polymer concentration is very important to be as a base of a right selection for polymer concentration in polymeric mixture. 7
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fewer defects [42–44]. The increasing viscosity of the casting solutions for PVA to 8% leads to lost their properties due to the lowest cross-linking and the migration of PVA from the membrane to the coagulation bath, which increases the membrane’s macro void and pore surface once again [42]. 3.5. Chemical structure and functional groups The FTIR spectra of neat PVA, neat CA and blend PVA/CA (M5) were compared for their significant peaks and their interactions shown in Fig. 4. Compared with the neat CA membrane, PVA neat the peak at 3425 cm−1 (−OH resonance vibrations) of M5 reduces obviously. Also the blend membranes displayed absorption bands of both PVA and CA that confirmed the cross linkage of CA layer with PVA layer in the membrane sublayers. For FTIR spectra of pure PVA membrane exhibited the characteristic absorption bands at 3290 cm−1(OH), 2910 cm−1(CH2) and 1240, 1092 cm−1 are related to the asymmetric and symmetric stretching vibration of the CeOCe bond [43]. For CA, the stretching vibration of CeO belongs to carbonyl and carboxyl groups and appears at 1708 cm_1[44]. This FTIR spectrum interpretation confirms the chemical composition of the blend membrane based PVA.
Fig. 6. The effect of PVA Concentration on the flux and rejection for the prepared membranes.
3.6. Scanning electron microscopy (SEM) analysis Fig. 5 illustrates the top surface and cross section of prepared membranes. After the casting the polymeric film on a support. The film immersed in coagulation bath to complete the formation of membrane. The polymers have low miscibility in water accordingly, the exchange occurs between solvent (NMP) and water in different points along the film that leading to form nucleation of the polymer phase and the coagulation occurs to form a membrane [45]. The rate of exchange process effect on the morphology of membranes, since the fast exchange lead to formation of macro-voids in the membrane structure and slow exchange forms a dense structure. Fig. 5, indicates that M1 (pure PVA) has smooth membrane surface with a dense top layer but the formed surface of M2 (CA pure) has pores in the top layer. With increasing PVA concentration within the casting solution, dispersed pores are found on the surface and decrease proportionately in M5. Meanwhile at increasing PVA to 8 wt% in the blend polymer, the membrane exhibits more porous surface as shown in M6. Increasing in PVA concentration can lead to unstable thermodynamic membrane-formation system, which cause ununiform exchange between solvent and water due to flow of un-crosslinked PVA to the coagulation bath making an acceleration in the rate of precipitation [45], Reza et al. has the same observation [46]. The membrane prepared from M1 (pure CA) exhibits a sponge structure meanwhile with increasing the concentration of PVA up to 6% the change in the morphology appears where a finger like cavities and small pores appear in the sub-layer. Increasing PVA content from 0 to 6%, the middle-layer looks as finger like cavities. Compared to the blend membranes M3-M5, higher PVA concentration of 8% (M6) exhibits porous structure due to macrovoids appearance on the membrane skin layer. According to the membrane morphology, the enhancement of the membrane surface selectivity can be performed using blending method between PVA & CA because it can provide best orientation of hydroxyl groups from PVA as a polymer to water during coagulation step [47]. However, hydrophilicity of PVA leads to increase the diffusion between solvent and water during membrane formation process.
Fig. 7. Spectra wave length for the initial dye concentration and after filtration using the prepared membranes.
Table 4 indicates the results of the viscosity of CA/PVA polymeric solution, however increases in PVA concentration leads to increase in viscosity. The trend of the increasing viscosity was clear, that due to the polymer chain entanglement that causes the sudden increasing in the viscosity. The method of determining the critical PVA concentration, was carried out by extrapolating two straight lines that is drawn depending on the slope of the viscosity whereby the viscosity curve becoming more motivated [39,40]. The critical concentration is determined from the crossed point between the two straight lines. In this work, the critical concentration of PVA polymer is 6% as shown in Fig. 3. Chung et al. [41] has reported in their work that using high concentration over optimum concentration of the polymer in polymeric solution, that provides substantial entanglement of chains, leading to dense layer formation with no defects on the membrane surface. Where, they suggested that using polymers concentrations for PES/NMP 35 % led to increase in degree of chain entanglement. In the present work, all polymeric solution compositions produced membranes have a typical asymmetric structure, and the internal structure of membrane whether spongy or finger like structure are related to polymer content and viscosity of the polymeric solution. For high polymeric concentrations the smaller macro voids appeared, so the membrane which has 8% PVA provided high porosity. High viscosity of the polymeric solution leads to delay rate exchange between solvent in the casted polymeric film and water in coagulation bath during membrane formation that provides less porous structure, small pores and
3.7. The effect of PVA Concentration on membrane performance Membrane performance using different PVA Concentration is shown in Fig. 6. The pure CA membrane (M2) has the highest flux 75 LMH and the lowest dye separation. This is owed to the porous top surface that 8
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Fig. 8. Optical images for the prepared membranes after filtration peocess and after washing.
formation a cake on the membrane surface as presented in Fig. 8. This is due to the correlation between dye molecular size and the pores size. Thus, the prepared blend membrane M5 has been chosen for further investigation.
lost his selectivity towards CV dye. Meanwhile, the membrane prepared from pure PVA (M1) exhibits higher separation for CV dye with the lowest flux 5.5 LMH. On the other hand, by increasing PVA concentration from 0 to 6 % the Blend membranes exhibited high selectivity towards CV dye separation that increased form 56 % with 0 PVA to reach the maximum 99.9 % with 6%. This enhancement in rejection percent accompanied with significant decrease in Flux from 75 to 17 LHM and this depicts the change in morphology of the prepared blend membranes as that appeared in SEM images. The M6 (8% PVA) provides higher flux 110 LMH and lower CV dye rejection to be 65 %. That is in accordance with the porosity and permeates water flux (PWF) values of the membrane as shown in Table 3. The changes in the produced membrane performance can be attributed to the changes in the blend morphology as showed in Fig. 7. From the Spectra wave length for the tested dye, the M5 membrane has the highest percent of removal which there is no detection for peak of the λ maximum wave length. Also from the optical images of the prepared membranes after filtration process and after washing, the M5 membrane has the highest cleaning and the lowest agglomeration /
3.8. Adsorption of the crystal violet dye at different pH values It is an important to study the pH of the dye solution to understand the interaction between the membranes and CV, also to obtain more information about the surface nature behavior towards the adsorption process. It affects the structural stability and its color intensity. The adsorption of CV dye using M5 blend membrane over a pH range of 3–11 with 20 mg L−1 dye concentrations is shown in Fig. 9. The percentage adsorption of CV is increased with increase in the pH from 3–11, with the maximum removal at pH 11. The increased adsorption at higher pH is mainly due to enhanced association of the dye cations. It is related to the electrostatic attraction force of the dye compound with M5 surface that is likely to be raised, when the pH increases. The dye CV binds to acid carboxylic groups of membranes by the principle of ion exchange. Because the treatment is carried out in an alkaline medium the carboxyl groups of bound BTCA are found in 9
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Fig. 9. The effect of pH values on the adsorption capacity and filtration process in terms of spectra wave length using M5 membrane.
filtration process and adsorption capacity towards CV dye solution. Also the values for the initial dye absorbance and after filtration process at different pH values indicate the highest performance for the blend membrane M5.
carboxylate form [48]. The minimum removal of CV is found at pH 4 and is probably due to the presence of excess H+ ions competing with the cation groups on the dye for adsorption sites [49]. Moreover, the enhancement of adsorption capacity after incorporation of PVA in the membranes is attributed to the addition of BTCA that provided the hydroxyl-rich chemicals with carboxyl functional groups. Yoon et al. [50] verified that additives of the films with hydroxyl and carboxyl groups were stronger and more flexible than those with only hydroxyl groups. Thus incorporation of PVA with CA in presence of BTCA had important impact on the enhancement of
3.9. The fouling behavior for the prepared membranes To study the cleaning properties, the fouling behavior of the optimized membrane (TFCM7) was studied by filtrating 200 mg L−1 HA solution for 48 h at 10 bar. It can be noticed from Fig. 10 that the CA 10
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Fig. 12. Spectra of different concentration of CV dye before and after the filtration process.
Fig. 10. A): Flux recovery ratio for Pure CA, Pure PVA and the blend membranes. (B):Reversible fouling, irreversible fouling for the prepared membranes.
Fig. 13. Long time performance of M5 blend membrane using 10 mg L−1 CV as the feed solution.
Fig. 11. Effect of different CV dye concentration on M5 blend membrane performance. Fig. 14. Performance of M5 blend membrane interms of flux before washing and after using 20 mg L−1 CVas the feed solution.
neat membrane has the highest Rirr ratio (23 %) and the PVA neat membrane has the lowest Rirr ratio (6.2 %). subsequently by increasing the loading amount of PVA in the polymeric matric, Rirr ratio was enhanced to reach 6.8 % with lowest Rr ratio (11.3 %) for M5 membrane. Also the flux recovery ratio of M5 membrane after rinsing with distilled water is 93 % which implies that HA attached on membrane surface is easily washed away. The results demonstrate that the top layer of the M5 membrane capable of alleviation the deposition of HA on the membrane surface.
3.10. Effect of the dye concentration on membrane performance The dye concentration has a positive effect on membrane behavior so, performance of M5 blend membrane towards different dye concentrations was investigated. Fig. 11, indicates that M5 exhibits rejections of 97.5 %, 98.33 %, 99.2 % and 99.9 % for 10 mg L−1, 15 mg L−1, 20 mg L−1 and 25 mg 11
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be obtained [55]. The mean pore size for the prepared membrane M5 is 0.69 nm. 4. Conclusion CA/PVA blended membranes were successfully prepared using phase inversion process to study the effect of PVA various concentrations on prepared membrane performance. The following conclusions as following from the present work: 1 The mechanical properties in terms of stress and strain were improved after using PVA in membrane preparation and it demonstrates that the mechanical properties are governed by the natural polymer (CA). 2 The prepared membrane from CA/PVA with PVA ratio 6% (M5) has the high antifouling properties and MWCO 800 Da 3 M1 (PVA membrane) and M2 (CA membrane) are more hydrophilic in comparison with blend membranes M3-M5. 4 The porosity decreased with increasing of PVA concentration to 6% in the modified membranes due cross linked functional groups in CA and PVA that providing additional sites for hydrogen bonding. 5 Blend membranes (M3-M5) exhibit lower water content percentage compared with pure CA. 6 The addition of PVA to casting solution in definite ratio 6% provides blend membranes with best porosity and water absorption%. 7 The viscosity curve indicates that the optimum concentration of PVA polymer is 6% 8 With increasing PVA concentration within the casting solution, dispersed pores are found on the membrane surface but decrease proportionately in M5. 9 The prepared M5 blend membrane displays high flux recovery during filtration process, indicating its good durability property.
Fig. 15. Molecular weight cut off for M5 using PEG retention (%).
L−1 aqueous solutions CV dye, along with permeance up to 18.7, 17.9, 17and 16.1 LMH, respectively. It is obvious that membrane has been gotten high rejection for the dye with high concentration that confirmed by the spectra of different concentration of CV dye before and after the filtration process as shown in Fig. 12. At higher concentrations, the molecules were adsorbed easily onto the membrane, which can be observed after the filtration experiments [51]. When the dye concentrations increase, the osmotic pressure increases resulting in the emergence of concentration polarization phenomenon [52]. This phenomenon makes it possible to explain the increase in the retention when the concentration increased. 3.11. Long-time performance and durability properties Fig. 13 indicates long-term experiment to determine the membrane performance for 72 h at 10 bar using 20 mg L−1 CV as feed solution. The flux was constant within the 21 h and then decreased slightly with a small increase in rejection during process time. The recorded flux remained nearly constant with proceeding time (flux > 15 LMH). During operation the initial water permeability reduced due to formation of accumulated dye layer over the membrane surface, then it reached to an equilibrium after certain time of operation [53]. The membrane exhibits approximately fixed flux of 17.5 LMH with rejection of 97.5 %. The results reveal that the M5 blend membrane provides stable performance during long-term experiment. Durability and reusability of M5 blend membrane was studied for 3 cycles and the results were shown in Fig. 14. In the first cycle, the water flux of the membrane was 22 LMH and with CV dye solution reached to 17.4 LMH. Then, after it was rinsed by Et−OH: DI-water solution with 1: 3 times for 30 min, the water flux of the membrane were respectively recovered to 20.9 LMH indicating that the membrane provides a higher flux recovery [54].In the second cycle and third cycle after filtration process and cleaning, the membrane still exhibits good water flux of 20.5 and 2.2 LMH respectively with slightly decrease. In general, the prepared M5 blend membrane displays high flux recovery during filtration process, indicating its good durability property and good reusability for removal of CV dye.
Author contributions section The authorship of this manuscript is Eman S. Mansor1*, Heba Abdallah2, A.M. Shaban1 Eman S. Mansor, Heba Abdallah, A.M. Shaban are responsible of the research idea Eman S. Mansor is responsible for the experimental work Eman S. Mansor and Heba Abdallah contribute in the writing part Eman S. Mansor, Heba Abdallah, A.M. Shaban are responsible of interpretation the analysis A.M. Shaban is the responsible for the final revision. Funding No funding was received for this work. Declaration of Competing Interest No conflict of interest exists. References
3.12. Molecular weight cutoff analysis for M5 membrane
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PEG was selected because of its low interaction with the membrane material to study MWCO for the studied M5 membrane. The molecular weight of PEG corresponding to a retention percent of 90 % on the retention curve is defined as the MWCO of a membrane [30]. As shown in Fig. 15 the retention percent of the optimized NF membrane for PEG has a molecular weight 400,600, 1000, and 3000 is 65 %, 87 %, 96 %, 99 % respectively. Obviously the optimizing condition results in a membrane with a MWCO around 800 Da. As well by substituting the obtained MWCO of M5 to the Stokes-Einstein Eq. (7), the pore size can 12
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