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PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property
Journal Pre-proofs Novel (4,4-diaminodiphenyl sulfone coupling modified PES/PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property Fariba oulad, Sirus Zinadini, Ali Akbar Zinatizadeh, Ali Ashraf Derakhshan PII: DOI: Reference:
S1383-5866(19)33462-8 https://doi.org/10.1016/j.seppur.2019.116292 SEPPUR 116292
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
4 August 2019 12 October 2019 4 November 2019
Please cite this article as: F. oulad, S. Zinadini, A. Akbar Zinatizadeh, A. Ashraf Derakhshan, Novel (4,4diaminodiphenyl sulfone coupling modified PES/PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property, Separation and Purification Technology (2019), doi: https://doi.org/ 10.1016/j.seppur.2019.116292
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Novel (4,4-diaminodiphenyl sulfone coupling modified PES/PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property Fariba oulad1, Sirus Zinadini1,*, Ali Akbar Zinatizadeh1, Ali Ashraf Derakhshan1 1- Environmental Research Center, Department of Applied Chemistry, Razi University, Kermanshah, Iran.
* Corresponding author: Tel: (+98) 8334274554, Fax: (+98) 8334274554, Email: [email protected]
1
Abstract In this work, firstly coupling modified PES (CPES) was synthesized by efficient coupling methodology using 4,4-diaminodiphenyl sulfone as precursor starting and the membranes were produced at five various compositions via the classical phase inversion method. The results of membrane performance shown that incorporation of CPES can be greatly enhanced permeation flux of pure water, so that the CPES membrane exhibited the pure water flux (PWF) of 134.05 kg /m2.h, that was much higher than PWF of original PES membrane (17.4 kg /m2.h). FTIR spectroscopy, SEM analyses, AFM and WCA measurement method were utilized to evaluate membranes performance and morphology. The contact angle results confirmed that the surface hydrophilicity improved in the attendance of CPES for all of modified membranes compared to bare PES membrane. From the results of this study, the coupling treatment as a cost-effective, simple and feasible procedure has a great potential for production of different kinds of membranes with exceptional permeation flux, special fouling resistance capability and superior dye rejection ability.
Keywords Coupling method, PES membrane, nanofiltration, antifouling, hydrophilicity, Direct Red 16.
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Nomenclature A
Membrane effective area (m2)
AFM
Atomic force microscopy
CPES
Coupling modified polyethersulfone
Cf
Feed concentration
Cp
Permeate concentration
DR16
Direct red 16
DMAc
Dimethylacetamide
FRR
Flux recovery ratio
FTIR
Fourier transform infrared spectroscopy
Jp
Flux of foulant (kg/m2.h)
Jw,1
Initial pure water flux (kg/m2.h)
Jw,2
Secondary pure water flux (kg/m2h)
L
Membrane thickness
M
Permeate quantity (kg)
M1
Membrane with 25 wt.% CPES
M2
Membrane with 50 wt.% CPES
M3
Membrane with 75 wt.% CPES
M4
Membrane with 100 wt.% CPES
Mw
The weights of the wet membrane
Md
The weights of the dry membrane
NF
Nanofiltration
PES
Polyethersulfone
PVP
Polyvinyl pyrrolidone
PWF
Pure water flux (kg/m2h)
Q
The permeate water volume per unit time (m3/s)
R
Rejection percentage
Rir
Irreversible fouling ratio
rm
Membrane mean pore radius
Rr
Reversible fouling ratio
Sa
mean roughness (nm) 3
SEM
Scanning electron microscopy
Sq
root mean square of the Z data (nm)
Sz
mean difference between highest peaks and lowest valleys (nm)
WCA
Water contact angle (0)
Ɛ
Overall porosity
η
Water viscosity
ρ
The pure water density (g/cm3)
ΔP Δt
operation pressure (MPa) permeation time (h)
1. Introduction Inadequate access to energy, natural resources and water shortage are a critical problems faced recently by humanity [1-3]. Membrane process as an effective strategy has a terrific potential for water treatment. In the coming decade the application of membrane separation for preparation of fresh water is attracted remarkable interests [4-6]. The membrane- based processes have a great benefit such as, low operation temperature, energysaving, eco-friendly, easy scale-up, economical and high efficiency [7-9]. However, membrane processes have a major drawback such as, fouling phenomenon, that can be influenced the water permeation flux and the membrane lifetime and as well as, degenerated the performance of membrane [10-14]. In order to overcome the drawbacks of the membrane- based processes, different procedures were applied including self-assembly [15-16], chemical treatment [17-18], coating [19-21], grafting [22-24], plasma treatment [25-27] and blending with amphiphilic materials [28-31]. Self-assembly is well-known as a prevalent method for the formation of many complex systems and structures of super-molecular [38]. Benganiet et al. [34] were used the self-assembly of zwitterionic copolymer as selective layers for fabricating polymeric membranes. The modified thin film composite membranes exhibited size-based selectivity, high permeability and fantastic antifouling properties during filtration of oil emulsions and protein solutions. These membranes 4
demonstrate terrific potential for wastewater treatment, pharmaceuticals purification and biomolecule separations, due to presentation of sharp selectivity, high flux and special antifouling performance of these membranes. In the coating procedure, the hydrophilic functional groups as the thin selective layer are physically settled on the surface of membrane [12]. New thermo-responsive composite membranes via dip coating of poly (2-ethyl-2-oxazoline) were reported by changmai et al. [36]. Efficacy of temperature, concentration of polymer and dip coating time on the performance of prepared membranes were estimated. The results indicated that, temperature had a considerable efficacy on the filtration rate. Enhancement of temperature become leading to remarkable increment of the flux values. Similar results were observed for varying dip coating time and polymer concentration that, with increase these parameters permeability of membrane diminished. In order to appear distinguished membranes properties via the grafting of monomers and preserving the surface of membrane from different agents, the approach of graft polymerization was suggested [12]. Surface initiated atom transfer radical polymerization (SI-ATRP) procedure were used by Lee et al. [35] for grafting poly (sulfobetaine methacrylate) (PSBMA) brushes on the internal and external surface of poly (vinylidene fluoride) (PVDF) ultrafiltration membranes. The results revealed that for attainment of antifouling membrane, a thick brush layer of zwitterionic polymer onto the interior pore of membrane is necessary. Nevertheless, the small pore size of PVDF UF membrane incur restriction to form thick brush layer inside the pore surfaces. They offered self-assembly of zwitterionic copolymers as a promising procedure, that do not have above mentioned problems and may be created significantly variability in the fouling resistant of UF membranes. To improve membrane properties, blending method as an adaptable and constructive procedure has been developed widely [12]. Behboudi and co-workers [32] prepared high performance polyvinyl chloride / poly carbonate (PVC/ PC) ultrafiltration nanocomposite membrane via nonsolvent induced phase separation (NIPS) method. The results indicated that when the PC content in the UF PVC membranes enhanced up to 50% primarily the distribution of pore size of membranes have shifted onto smaller pores and afterwards, have shifted back onto larger pores. The PVC/PC blend membranes in comparison with pristine membranes exhibited the improvement of BSA rejection, antifouling performance and water permeation flux, owing to the existence of 5
PC in the membrane. A novel thin-film nanocomposite (TFN) membranes was successfully prepared by Dong et al. [33] through in situ incorporation of zeolite nanoparticle for interfacial polymerization of polyamide layer. It was shown that, the new thin-film nanocomposite membranes contained a similar surface hydrophilicity and higher surface roughness in comparison of conventional thin-film nanocomposite membranes and control membrane. The addition of hydrous aluminum oxide (HAO) as an inorganic nanofiller on performance of flat sheet polysulfone (PSf) ultrafiltration membrane was evaluated by Gohari et al. [37]. HAO- modified membrane indicated the high permeate flux of 1194 L/m2 h bar (151 L/m2 h bar bare- PSf), high oil rejection and fouling resistance against oil layer formation on the top surface of membrane. Exhibition of the flux recovery ratio of 67% by PSf/HAO-2 (HAO: PSf = 2) membrane confirmed awesome antifouling properties via restraint of oil layer adsorption or deposition. The HAOmodified membrane exhibited acceptable oil rejection and high permeability while, complete oil rejection (100%) indicated by optimum PSf/HAO-2. Diazonium-induced grafting as a useful and popular procedure have been developed recently, for modification of various types of carbons, polymers, metal oxides, electrodes and silicon. Main purpose of these modification was appeared properties such as stability of interface between substrate and the external environment, wettability and adhesive properties. Use of diazonium chemistry as a convenient way has been used widely to reach above-mentioned purposes [39-43]. Despite many benefit of diazonium-induced grafting, but unfortunately has been seldom applied to membrane treatment, so that to our knowledge only two research has presented surface modification of membrane using this procedure [44,45]. The purpose of present work was to display the capability of diazonium-induced grafting in surface modification of polyethersulfone (PES) nanofiltration membrane. For access of this target, at first step, PES was modified with 4,4-diaminodiphenyl sulfone via coupling method and afterwards, CPES/PES membranes were prepared via the phase inversion method. The water permeability, hydrophilicity, morphology and antifouling resistance of CPES/PES membranes were characterized in detail. SEM, AFM, FTIR, overall porosity and WCA measurements was utilized to assess nanofiltration performance of modified membranes. The rejection of Direct Red was measured to estimate the performance of CPES/PES composite nanofiltration membranes.
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Fouling- resistance properties of fabricated membranes were examined using powdered milk solution.
2. Experimental 2.1. Materials Polyethersulfone (PES Ultrason E6020P) was supplied from BASF company, Germany. N, Ndimethylacetamide (DMAc) as solvent and Polyvinylpyrrolidone (PVP) as pore former from Merck were applied in casting solution. 4, 4- diaminodiphenyl sulfone was selected as precursor starting for diazonium-induced grafting. Sodium nitrite 97% and hydrochloric acid 37% were used as received. Direct Red 16 was received from Alvan Sabet Co., Iran and utilized without any further purification. All chemicals reagents applied in this research were of reagent grade. Deionized water was utilized in this work.
2.2. Synthesize of the coupling modified Polyethersulfone (CPES) 10 mmol 4, 4- diaminodiphenyl sulfone was dissolved to 40 mL deionized water by means of addition of 100 mL HCl 0.5M. Simultaneous 10 mmol Sodium nitrite in the 15 mL deionized water Was dissolved. In order to generate aryl diazonium salts, after reach to 5 ºC temperature, the solution of sodium nitrite was added gradually at the solution of 4, 4- diaminodiphenyl sulfone. The above-mentioned solution stirred up at 800 rpm and 6 g PES was added slowly and the resulted solution have to keep at 5 ºC for 240 min. After completion of reaction the resulted CPES was washed several times by deionized water and ethanol and ultimately dried at 70 ºC. Schematically surface modification of PES by means of diazonium coupling procedure exhibited in Figure 1.
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Figure 1. Digital photograph of bare PES (a) and coupling modified PES (b).
2.3. Preparation of blended CPES/PES membranes The asymmetric blended CPES/PES membranes were prepared via the classical phase inversion induced by immersion precipitation technique, that shown schematically in Figure 2. The formulations of the fabricated membranes are explained in detail in Table1. Exact value of polymers and PVP were dissolved in DMAC under stirring to make homogenous solution. Then the resulted dop solutions were carried out on a flat glass plate with a gap of 150µm by means of home-made casting knife and immediately, soaked into deionized water as a coagulation bath at a room temperature. In order to confidence of the complete phase inversion, the prepared membranes were dipped in fresh deionized water for at least 24 h. Lastly, the membrane are drying between filter papers at a room temperature for 24 h.
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Figure 2. Schematically preparation of CPES/PES blended membrane through diazonium coupling approach. Table 1. Composition of prepared membranes. Membrane type Unfilled PES(M0) M1 M2 M3 M4
PES(wt.%) 20 15 10 5 -
CPES(wt.%) 5 10 15 20
PVP(wt.%) 1 1 1 1 1
DMAC(wt.%) 79 79 79 79 79
2.4. Membrane characterization Fourier transform infrared spectroscopy (FTIR, Bruker spectrometer TENSOR27) was applied to investigate the functional groups on PES. To investigate structure of mixed matrix membranes, scanning electron microscopy (Philips SEM, XL30) was used. Measurement of surface roughness 9
and specifications of surface topography of membrane were performed by atomic force microscopy image (AFM, Nanosurf Mobile S scanning probe-optical microscope, Switzerland). Measurement of water drop contact angle for assessment of membrane hydrophilicity was performed by goniometer (G10, KRUSS, Germany). In order to enhance precision of experiments accomplishment of each measurement were at least 6 times.
2.5. Overall porosity The overall porosity (Ɛ) was definition the ratio of pore volume to geometrical volume, that was procurable by gravimetric method, that was obtained by: Ɛ=
(mw ―
md) ρ
(1)
A×L
Where mw and md are the weights of the wet membrane and dry membrane respectively. A (m2) is the effective area of the membrane, L (m) is the thickness of membrane samples and ρ (g/cm3) is the pure water density [8, 30, 46].
2.6. Pore size mean pore radius (rm) was definite by Guerout–Elford–Ferry equation: rm
(2.9 1.75 ) 8 l Q A P
(3.8)
(2)
where η is the viscosity of water (8.9×10-4 Pa.s), Q is the permeate water volume per unit time (m3/s), and ΔP is the operational pressure (0.3 MPa).
2.7. Nanofiltration experiments The permeation experiments of Unfilled and embedded membranes in terms of water permeability, powdered milk fouling tests and dye rejection were estimated via batch type, dead end stirred cell nanofiltration apparatus (effective area of 12.56 cm2, volume capacity of 125mL). The system feed side was pressurized by means of nitrogen gas. All membrane samples were first pretreated at a high pressure drop of 4 bar with deionized water for about 30 min. Afterwards, the pressure was diminished to the trans membrane pressure of 3 bar. The water permeability flux Jw (kg /m2.h) of Unfilled and embedded membranes is calculated from:
10
jw =
M A ⋅ Δt
(3)
Where M(kg), A (m2) and Δt(h) are permeate quantity, the effective membrane area and sampling time interval respectively. In the next step, in order to rejection measurement of the nanofiltration membrane, DR16 dye were applied. The dye rejection test was determined using a 50 mg/L feed solution. The rejection can be calculated by: Cp
(4)
R(%)=(1- Cf )×100
Where cp and cf are particular component concentration of permeate and feed solution concentration respectively [47,48]. Fouling resistance ability of prepared membranes were characterized using powdered milk solution with high proteins concentration (8000 mg/L), that cause promptly crucial fouling. After powdered milk filtration, protein-fouled membrane was rinsed with deionized water for 15 min to remove reversible fouling and procure the water flux recovery ratio. Consequently, the test involves 3 step: measuring PWF for 30 min Jw,1 (kg /m2.h), measurement of powdered milk permeability for 120 min calculated Jp (kg /m2.h) and as well as renewed measuring of PWF for another 30 min Jw,2 (kg /m2.h) at 3 bar operational pressure. The flux recovery ratio was defined as follows: FR R (% ) (
J w ,2 J w ,1
(5)
) 100
Generally, high flux recovery value is the exhibition of special fouling-resistance property and high cleaning efficiency of nanofiltration membranes [49-51]. As well as, reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated to estimate the fouling phenomenon in details using the following expressions: [52] J w2 J p Rr J w1
(6)
100 11
(7)
J J R ir w 1 w 2 100 Jw 1
3. Results and discussion 3.1. Characterization of coupling modified PES The FTIR spectra of the coupling modified PES and pure PES exhibits in Figure 3. The little alterations in the characteristic peaks of PES were emerged that these alterations can be attributed to coupling treatment of PES. Two peaks around 3067 and 3441 cm-1 are related to symmetric and asymmetric stretching vibration peaks of –NH2 bonds respectively. The appeared peak at 1075 cm1
is characteristic of the C-N stretching peak of the CPES groups. The existence of the
aforementioned peaks confirms successful synthesis of coupling modified PES using coupling procedure. The FTIR spectra depicted, that the embedding coupling modified PES can be improved hydrophilicity.
Figure 3. The FTIR spectra of pure PES and coupling modified PES.
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3.2. Hydrophilicity and permeability measurement It is well-known that the fouling resistance property and permeability of membranes have a close relationship with surface polarity [53]. Therefore, the membrane surface hydrophilicity in term of water contact angle was measured to evaluate the surface wettability of CPES/PES blend membranes. The results of water contact angle measurement of CPES/PES blend membranes presented in Figure 4. As observed clearly, the bare PES membrane possess the highest water contact angle of 75.57 . It can be related to hydrophobic nature of bare PES membrane. The water contact angle reduced gradually with the increasing of the weight ratios of coupling modified PES from 0% to 100 % and the lowest water contact angle of 66.11 was measured for CPES membrane (M4, 100%). In fact, the higher weight percentage of coupling modified PES (M4 membrane) causes change the nature of bare PES membrane from hydrophobic to hydrophilic. The reduction of water contact angle may be attributed to increment of hydrophilicity that resulted from existence of countless number of hydrophilic functional groups (NH2, SO2, N=N) on the coupling modified PES surface. Dynamic wettability of the CPES/PES blend membranes could be estimated in term of permeated pure water flux. The pressure-assisted procedure and the pre-wetted procedure are the prevalent approaches, that used to measurement of pure water permeation flux [54,55]. In this current research, we applied pressure-assisted procedure to measurement of permeated pure water flux of CPES/PES blend membranes using a dead-end permeation cell and the results are depicted in Figure 5. The results of hydrophilicity are consistent with the pure water flux values for all of membranes, that previously had been observed by other researchers [12,30]. The PWF of the membranes enhanced with increment of coupling modified PES contents. So that, the CPES membrane (M4) with the highest hydrophilicity (the lowest water contact angle, 66.11 ) indicated the highest pure water flux (134.05 kg /m2.h). In addition to, the minimum PWF (17.4 kg /m2.h) was measured for bare PES membrane that can be justified by poor hydrophilicity nature of PES. From the affecting factors of pure water flux of membrane can be called membrane hydrophilicity, viscosity of dope solution, porosity and pore size. The overall porosity and mean pore radius exhibited in Figure 6 and 7 respectively. As can be seen patently, these parameters increased gradually with enhancement of coupling modified PES contents from 0 to 100 %. The results revealed the positive impact of coupling modified PES in CPES/PES blend membrane for reaching of higher porosity and pore size [56]. Increment of the 13
weight ratio of coupling modified PES in casting solution lead to enhancement of hydrophilicity, due to the intensify of functional groups induced by coupling treatment procedure. On the other hand, increasing of coupling modified PES contents incur also reduction of viscosity of casting solutions. so that, the M4 casting solution (casting solution involving 100 % CPES) had the lowest viscosity in compared to other casting solutions and M0 casting solution (pure casting solution) possessed the highest viscosity. The increase of hydrophilicity and the decrease of viscosity induced by coupling treatment lead to accelerates the rate exchange of solvent and non-solvent throughout the phase-inversion process. The rapid interchange of solvent and non-solvent during phase-inversion incur the larger pore radius formation consequently, the higher overall porosity in the surface of membrane and ultimately, improvement of the water permeability.
Figure 3. The results of water contact angle measurement of the prepared membrane.
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160 134.05
120 100 80 57.81 60
45.4
38.4
40 17.4 20 0 Unfilled-PES
M1
M2
M3
M4
Figure 5. The results of permeation flux of pure water of prepared membranes at 3 bar.
100 90 80
83.6
85.8
M1
M2
89.3
89.8
M3
M4
68.2
70 Porosity (%)
Pure water flux (kg/m2.h)
140
60 50 40 30 20 10 0 Unfilled-PES
15
Figure 6. The overall porosity of CPES/PES blend membranes. 16
14.3
14
Mean pore radius (nm)
12
9.4 10
8.3 7.6
8 6
5.1
4 2 0 Unfilled-PES
M1
M2
M3
M4
Figure 7. Mean pore radius of CPES/PES blend membranes.
3.3. Morphology analysis The cross-sectional morphology of CPES/PES blended membranes with various CPES contents are shown in SEM images (Figure 8). As observed clearly, all of the prepared membranes possess the similar hierarchical characteristic of asymmetric structure with dense top-layer on a finger-like porous sub-layer. The finger-like pores of membranes at presence of CPES become wider and actually, with CPES addition the micro-voids lengths were elongated. The membranes porosity and mean pore radius values (pore structure) were increased with enhancement of CPES content (Figure 6 and Figure 7 supporting information). The alteration of membranes pore structure could be justified by hydrophilic nature of CPES that, elevated the rate of mass transfer between the solvent and non-solvent in the coagulation bath during phase separation process. Finally, the larger pore channels were formed. It is also worth noting that, some lateral pore structures are emerged particularly in 75 and 100 wt.% CPES that,
16
incur permeability improvement. The similar lateral pores were obtained in fabrication of graphene oxide embedded PES membranes by Zinadini et al. [30].
Figure 8. The cross-section SEM micrographs of blended membranes.
3.4. Dye removal assessment 17
The retention of DR16 with concentration of 50 mg/L was investigated in a dead end cell, to investigate nanofiltration performance of the CPES/PES blend membranes, that a schematic representation of module is demonstrated in Figure 9. The results of dye rejection after 30 min filtration at trans-membrane pressure of 3 bar are depicts in Figure 10. As can be seen, all of the CPES/PES blend membranes be able to remove dye from water with high efficiency, that this capability is exhibition of typical property of nanofiltration membranes. Nevertheless, the CPES/PES blended membranes indicated higher rejection capability compared to bare PES membrane. These results can be inspired by increment of Donnan exclusion effect between the surface of membrane and some groups of the dye, due to attendance of same groups in the structure of both membrane and solute [13]. On the other hand, enhancement of the electrostatic effects resulted from presence of hydrophilic groups in the coupling modified membranes is possible, that assist with Donnan exclusion effect in removing DR16 molecules [52]. Actually, incorporation of coupling modified PES can be greatly enhanced rejection capability of CPES/PES blended membranes. All of the modified blended membranes removed color more than 90 %. Among of four CPES/PES composite membranes, M3 (membrane with 75 % CPES) demonstrated the highest dye rejection of 99 %. In contrast, the dye rejection of M4 (membrane with 100 % CPES) was slightly lower than M3, owing to the permeation flux of M4 was much larger than M3. In fact, with the enhancement of CPES content, from 75% to 100%, the mean pore radius was increased from 9.4 to 14.3 nm, that ultimately incur the reduction of dye rejection and increment of pure water flux of M4. These results revealed that the performance of CPES/PES modified membranes in the current study had a special effect in textile effluent treatment and can be industrialized in feature.
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Figure 9. Schematic illustration of DR16 rejection of prepared membranes in the dead-end filtration module.
19
120
Dye rejection (%)
100
88
95.6
94.2
99.2
96.4
80
60
40
20
0 Unfilled-PES
M1
M2
M3
M4
Figure 10. The results of dye rejection performance of CPES/PES modified membranes.
3.5. Fouling resistance capability Fouling resistance capability of nanofiltration membranes is a key factor for practical application. Electrostatic interactions, van der waals attractions, hydrogen bonding and hydrophobic interactions can be incurring adsorption of foulants on the surface of membrane and as a result creation of fouling phenomenon. So, an efficacious procedure to declin fouling of membrane is to elevate repulsive interactions and to make least adsorptive interactions between membrane surface and foulants. On the other hand, enhancement of membrane hydrophilicity possesses a positive impact on the amelioration of the fouling resistance property [8, 57]. To prove this effect, the antifouling evaluation of CPES/PES modified membranes was conducted in dead-end permeation cell (Figure 11). The flux recovery ratio (FRR) of the bare and blended membranes is illustrated in Figure 12. Assessment of membrane fouling by powdered milk solution with concentration of 8000 mg/L for the bare PES membrane resulted in substantial diminution of the permeability (67.82 % of FRR). Actually, the main reason of permeability reduction of bare PES membrane is the low membrane hydrophilicity, that cause deposition of foulants on the membrane surface. As observed clearly, all of the CPES/PES blended membranes exhibited the FRR value more than 90 %, that this result was consistent with superior fouling resistance capability of these membranes.
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These achievements suggested, that embedding coupling modified PES in all of weight ratios (25, 50, 75 and 100 %) incur enhancement of filtration performance (antifouling properties) of the CPES/PES blended membranes especially, M3 and M4. As can be seen, the observed results of FRR was accordant with trend of membrane hydrophilicity (Figure 4 and Figure 5 supporting information). In fact, the enhancement of the FRR values was inspired by the improvement of membrane surface hydrophilicity, that caused mitigation of the hydrophobic interactions between foulants and membranes. Addition of the coupling modified PES and its attendance on the blended membranes may be regarded as a reason for augmentation of hydrophilicity, that leads to higher FRR values and as a result higher antifouling properties. The hydrophilic CPES/PES composite membranes indicated the amazing fouling resistance capability against powdered milk solution, that may be attributed to stronger interaction of these membranes with molecules of water than protein molecules. Nevertheless, it is worth nothing, which these hydrophilic CPES/PES composite membranes contained hydrogen bonding sites, that be able to adsorb water molecules and desorb foulants easily. In order to, further realization of antifouling behaviour of CPES/PES composite membranes, fouling resistance parameters such as reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated and indicated in Figure 13. Addition of coupling modified PES in all of weight percentage (25, 50, 75 and 100 %) cause increment of reversible resistances and reduction of irreversible resistances of CPES embedded membranes. The irreversible fouling ratio (Rir) of the PES membranes was remarkably decreased from 32.18 % for the bare PES membrane to 1.18 % for 75 wt. % CPES embedded membrane (M3). The results revealed that all of CPES/PES blend membranes possessed a lower Rir and higher (Rr, FRR) in comparison of bare PES membrane after cleaning process (washing with deionized water), that these results proved exceptional antifouling capability of the whole CPES/PES blend membranes. This behaviour can be attributed to higher hydrophilicity and hydratability of these membranes active layer, that this feature induced by hydrophilic functional groups (NH2, SO2, N=N) on the CPES surface via coupling treatment. These results recommended that coupling treatment is a feasible procedure for production of membranes with special fouling resistance capability and exceptional permeability flux of pure water.
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In order to assessment of membranes surface morphology, the AFM image analysis was conducted and indicated in Figure 14. The surface roughness parameters of CPES/PES embedded membranes are listed with details in Table 2. As can be observed clearly, the roughness parameters of modified membranes were reduced gradually with increment of CPES content. The results revealed that the surface roughness altered to be smoot by blending CPES. In fact, the existence of CPES on the membrane surface incur significant enhancement of membranes surface hydrophilicity, diminution of interaction between membrane surface and foulants and as well as, tangible subtraction of roughness parameters of modified membranes.
Figure 11. Schematic representation of protein rejection of CPES/PES embedded membranes.
22
120 98.83 100
91.8
92.56
M1
M2
97
FRR ratio (%)
80 67.82 60
40
20
0 Unfilled-PES
M3
M4
Figure 12. The water flux recovery ratio of CPES/PES blended membranes after protein filtration. 100
Fouling Resistance ratio (%)
90
Irreversible Fouling 7.42
3.021
Reversible Fouling 7.42 1.18
80 70 60 32.18 50
89.14
40
73.4
73.39
76
M1
M2
M3
30 20 10
16.7
0 Unfilled-PES
M4
Figure 13. Fouling resistance ratio of CPES/PES blended membranes.
23
(a)
(b)
(c)
(d)
(e)
24
Figure 14. AFM images of the bare(a), M1(b), M2(c), M3(d), M4(e) membranes.
Table 2. The results of membranes Surface roughness parameters. Membrane
Sa(nm)
Sq(nm)
Sz(nm)
bare
21.251
28.671
245.15
M1
9.42
12.146
119.33
M2
10.772
13.538
118.64
M3
8.7459
11.197
94.289
M4
8.4018
9.9359
51.132
Conclusion The CPES/PES blended membranes were prepared via phase inversion technique, using 4,4diaminodiphenyl sulfone as precursor starting for coupling treatment of PES. The influence of CPES in different weight percentages on the structures, morphology and performance of CPES/PES blended membranes was investigated and afterwards, compared with bare PES membrane. The results revealed that the pure water permeation flux of CPES/PES blended membranes was enhanced significantly with increment of CPES contents that can be attributed to high hydrophilicity induced by CPES. This trend incurs enhancement of water molecules diffusion through the membranes and consequently, causes increment of permeated pure water flux. The assessment of fouling behavior with utilization of powdered milk solution indicated that the CPES/PES blended membranes in all of the weight percentage possessed superior fouling resistance properties in comparison of bare PES membrane. Also, the prepared membranes exhibited terrific dye rejection capability of DR16. The coupling treatment as a promising approach had an excellent potential for preparation of high performance blended membranes.
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Highlights 1-Coupling modified PES (CPES) was synthesized using aryldiazonium methodology. 2-CPES mixed matrix membrane exhibited special permeability of 134.05 kg /m2.h. 3- Antifouling properties of CPES/PES membranes was improved (FRR more than 91%). 4- CPES/PES membranes are shown high performance dye rejection (more than 94%). 3-This procedure is convenient for modification of various membranes.
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Graphical abstract
34
Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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S1383-5866(19)33462-8 https://doi.org/10.1016/j.seppur.2019.116292 SEPPUR 116292
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
4 August 2019 12 October 2019 4 November 2019
Please cite this article as: F. oulad, S. Zinadini, A. Akbar Zinatizadeh, A. Ashraf Derakhshan, Novel (4,4diaminodiphenyl sulfone coupling modified PES/PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property, Separation and Purification Technology (2019), doi: https://doi.org/ 10.1016/j.seppur.2019.116292
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© 2019 Published by Elsevier B.V.
Novel (4,4-diaminodiphenyl sulfone coupling modified PES/PES) mixed matrix nanofiltration membranes with high permeability and anti-fouling property Fariba oulad1, Sirus Zinadini1,*, Ali Akbar Zinatizadeh1, Ali Ashraf Derakhshan1 1- Environmental Research Center, Department of Applied Chemistry, Razi University, Kermanshah, Iran.
* Corresponding author: Tel: (+98) 8334274554, Fax: (+98) 8334274554, Email: [email protected]
1
Abstract In this work, firstly coupling modified PES (CPES) was synthesized by efficient coupling methodology using 4,4-diaminodiphenyl sulfone as precursor starting and the membranes were produced at five various compositions via the classical phase inversion method. The results of membrane performance shown that incorporation of CPES can be greatly enhanced permeation flux of pure water, so that the CPES membrane exhibited the pure water flux (PWF) of 134.05 kg /m2.h, that was much higher than PWF of original PES membrane (17.4 kg /m2.h). FTIR spectroscopy, SEM analyses, AFM and WCA measurement method were utilized to evaluate membranes performance and morphology. The contact angle results confirmed that the surface hydrophilicity improved in the attendance of CPES for all of modified membranes compared to bare PES membrane. From the results of this study, the coupling treatment as a cost-effective, simple and feasible procedure has a great potential for production of different kinds of membranes with exceptional permeation flux, special fouling resistance capability and superior dye rejection ability.
Keywords Coupling method, PES membrane, nanofiltration, antifouling, hydrophilicity, Direct Red 16.
2
Nomenclature A
Membrane effective area (m2)
AFM
Atomic force microscopy
CPES
Coupling modified polyethersulfone
Cf
Feed concentration
Cp
Permeate concentration
DR16
Direct red 16
DMAc
Dimethylacetamide
FRR
Flux recovery ratio
FTIR
Fourier transform infrared spectroscopy
Jp
Flux of foulant (kg/m2.h)
Jw,1
Initial pure water flux (kg/m2.h)
Jw,2
Secondary pure water flux (kg/m2h)
L
Membrane thickness
M
Permeate quantity (kg)
M1
Membrane with 25 wt.% CPES
M2
Membrane with 50 wt.% CPES
M3
Membrane with 75 wt.% CPES
M4
Membrane with 100 wt.% CPES
Mw
The weights of the wet membrane
Md
The weights of the dry membrane
NF
Nanofiltration
PES
Polyethersulfone
PVP
Polyvinyl pyrrolidone
PWF
Pure water flux (kg/m2h)
Q
The permeate water volume per unit time (m3/s)
R
Rejection percentage
Rir
Irreversible fouling ratio
rm
Membrane mean pore radius
Rr
Reversible fouling ratio
Sa
mean roughness (nm) 3
SEM
Scanning electron microscopy
Sq
root mean square of the Z data (nm)
Sz
mean difference between highest peaks and lowest valleys (nm)
WCA
Water contact angle (0)
Ɛ
Overall porosity
η
Water viscosity
ρ
The pure water density (g/cm3)
ΔP Δt
operation pressure (MPa) permeation time (h)
1. Introduction Inadequate access to energy, natural resources and water shortage are a critical problems faced recently by humanity [1-3]. Membrane process as an effective strategy has a terrific potential for water treatment. In the coming decade the application of membrane separation for preparation of fresh water is attracted remarkable interests [4-6]. The membrane- based processes have a great benefit such as, low operation temperature, energysaving, eco-friendly, easy scale-up, economical and high efficiency [7-9]. However, membrane processes have a major drawback such as, fouling phenomenon, that can be influenced the water permeation flux and the membrane lifetime and as well as, degenerated the performance of membrane [10-14]. In order to overcome the drawbacks of the membrane- based processes, different procedures were applied including self-assembly [15-16], chemical treatment [17-18], coating [19-21], grafting [22-24], plasma treatment [25-27] and blending with amphiphilic materials [28-31]. Self-assembly is well-known as a prevalent method for the formation of many complex systems and structures of super-molecular [38]. Benganiet et al. [34] were used the self-assembly of zwitterionic copolymer as selective layers for fabricating polymeric membranes. The modified thin film composite membranes exhibited size-based selectivity, high permeability and fantastic antifouling properties during filtration of oil emulsions and protein solutions. These membranes 4
demonstrate terrific potential for wastewater treatment, pharmaceuticals purification and biomolecule separations, due to presentation of sharp selectivity, high flux and special antifouling performance of these membranes. In the coating procedure, the hydrophilic functional groups as the thin selective layer are physically settled on the surface of membrane [12]. New thermo-responsive composite membranes via dip coating of poly (2-ethyl-2-oxazoline) were reported by changmai et al. [36]. Efficacy of temperature, concentration of polymer and dip coating time on the performance of prepared membranes were estimated. The results indicated that, temperature had a considerable efficacy on the filtration rate. Enhancement of temperature become leading to remarkable increment of the flux values. Similar results were observed for varying dip coating time and polymer concentration that, with increase these parameters permeability of membrane diminished. In order to appear distinguished membranes properties via the grafting of monomers and preserving the surface of membrane from different agents, the approach of graft polymerization was suggested [12]. Surface initiated atom transfer radical polymerization (SI-ATRP) procedure were used by Lee et al. [35] for grafting poly (sulfobetaine methacrylate) (PSBMA) brushes on the internal and external surface of poly (vinylidene fluoride) (PVDF) ultrafiltration membranes. The results revealed that for attainment of antifouling membrane, a thick brush layer of zwitterionic polymer onto the interior pore of membrane is necessary. Nevertheless, the small pore size of PVDF UF membrane incur restriction to form thick brush layer inside the pore surfaces. They offered self-assembly of zwitterionic copolymers as a promising procedure, that do not have above mentioned problems and may be created significantly variability in the fouling resistant of UF membranes. To improve membrane properties, blending method as an adaptable and constructive procedure has been developed widely [12]. Behboudi and co-workers [32] prepared high performance polyvinyl chloride / poly carbonate (PVC/ PC) ultrafiltration nanocomposite membrane via nonsolvent induced phase separation (NIPS) method. The results indicated that when the PC content in the UF PVC membranes enhanced up to 50% primarily the distribution of pore size of membranes have shifted onto smaller pores and afterwards, have shifted back onto larger pores. The PVC/PC blend membranes in comparison with pristine membranes exhibited the improvement of BSA rejection, antifouling performance and water permeation flux, owing to the existence of 5
PC in the membrane. A novel thin-film nanocomposite (TFN) membranes was successfully prepared by Dong et al. [33] through in situ incorporation of zeolite nanoparticle for interfacial polymerization of polyamide layer. It was shown that, the new thin-film nanocomposite membranes contained a similar surface hydrophilicity and higher surface roughness in comparison of conventional thin-film nanocomposite membranes and control membrane. The addition of hydrous aluminum oxide (HAO) as an inorganic nanofiller on performance of flat sheet polysulfone (PSf) ultrafiltration membrane was evaluated by Gohari et al. [37]. HAO- modified membrane indicated the high permeate flux of 1194 L/m2 h bar (151 L/m2 h bar bare- PSf), high oil rejection and fouling resistance against oil layer formation on the top surface of membrane. Exhibition of the flux recovery ratio of 67% by PSf/HAO-2 (HAO: PSf = 2) membrane confirmed awesome antifouling properties via restraint of oil layer adsorption or deposition. The HAOmodified membrane exhibited acceptable oil rejection and high permeability while, complete oil rejection (100%) indicated by optimum PSf/HAO-2. Diazonium-induced grafting as a useful and popular procedure have been developed recently, for modification of various types of carbons, polymers, metal oxides, electrodes and silicon. Main purpose of these modification was appeared properties such as stability of interface between substrate and the external environment, wettability and adhesive properties. Use of diazonium chemistry as a convenient way has been used widely to reach above-mentioned purposes [39-43]. Despite many benefit of diazonium-induced grafting, but unfortunately has been seldom applied to membrane treatment, so that to our knowledge only two research has presented surface modification of membrane using this procedure [44,45]. The purpose of present work was to display the capability of diazonium-induced grafting in surface modification of polyethersulfone (PES) nanofiltration membrane. For access of this target, at first step, PES was modified with 4,4-diaminodiphenyl sulfone via coupling method and afterwards, CPES/PES membranes were prepared via the phase inversion method. The water permeability, hydrophilicity, morphology and antifouling resistance of CPES/PES membranes were characterized in detail. SEM, AFM, FTIR, overall porosity and WCA measurements was utilized to assess nanofiltration performance of modified membranes. The rejection of Direct Red was measured to estimate the performance of CPES/PES composite nanofiltration membranes.
6
Fouling- resistance properties of fabricated membranes were examined using powdered milk solution.
2. Experimental 2.1. Materials Polyethersulfone (PES Ultrason E6020P) was supplied from BASF company, Germany. N, Ndimethylacetamide (DMAc) as solvent and Polyvinylpyrrolidone (PVP) as pore former from Merck were applied in casting solution. 4, 4- diaminodiphenyl sulfone was selected as precursor starting for diazonium-induced grafting. Sodium nitrite 97% and hydrochloric acid 37% were used as received. Direct Red 16 was received from Alvan Sabet Co., Iran and utilized without any further purification. All chemicals reagents applied in this research were of reagent grade. Deionized water was utilized in this work.
2.2. Synthesize of the coupling modified Polyethersulfone (CPES) 10 mmol 4, 4- diaminodiphenyl sulfone was dissolved to 40 mL deionized water by means of addition of 100 mL HCl 0.5M. Simultaneous 10 mmol Sodium nitrite in the 15 mL deionized water Was dissolved. In order to generate aryl diazonium salts, after reach to 5 ºC temperature, the solution of sodium nitrite was added gradually at the solution of 4, 4- diaminodiphenyl sulfone. The above-mentioned solution stirred up at 800 rpm and 6 g PES was added slowly and the resulted solution have to keep at 5 ºC for 240 min. After completion of reaction the resulted CPES was washed several times by deionized water and ethanol and ultimately dried at 70 ºC. Schematically surface modification of PES by means of diazonium coupling procedure exhibited in Figure 1.
7
Figure 1. Digital photograph of bare PES (a) and coupling modified PES (b).
2.3. Preparation of blended CPES/PES membranes The asymmetric blended CPES/PES membranes were prepared via the classical phase inversion induced by immersion precipitation technique, that shown schematically in Figure 2. The formulations of the fabricated membranes are explained in detail in Table1. Exact value of polymers and PVP were dissolved in DMAC under stirring to make homogenous solution. Then the resulted dop solutions were carried out on a flat glass plate with a gap of 150µm by means of home-made casting knife and immediately, soaked into deionized water as a coagulation bath at a room temperature. In order to confidence of the complete phase inversion, the prepared membranes were dipped in fresh deionized water for at least 24 h. Lastly, the membrane are drying between filter papers at a room temperature for 24 h.
8
Figure 2. Schematically preparation of CPES/PES blended membrane through diazonium coupling approach. Table 1. Composition of prepared membranes. Membrane type Unfilled PES(M0) M1 M2 M3 M4
PES(wt.%) 20 15 10 5 -
CPES(wt.%) 5 10 15 20
PVP(wt.%) 1 1 1 1 1
DMAC(wt.%) 79 79 79 79 79
2.4. Membrane characterization Fourier transform infrared spectroscopy (FTIR, Bruker spectrometer TENSOR27) was applied to investigate the functional groups on PES. To investigate structure of mixed matrix membranes, scanning electron microscopy (Philips SEM, XL30) was used. Measurement of surface roughness 9
and specifications of surface topography of membrane were performed by atomic force microscopy image (AFM, Nanosurf Mobile S scanning probe-optical microscope, Switzerland). Measurement of water drop contact angle for assessment of membrane hydrophilicity was performed by goniometer (G10, KRUSS, Germany). In order to enhance precision of experiments accomplishment of each measurement were at least 6 times.
2.5. Overall porosity The overall porosity (Ɛ) was definition the ratio of pore volume to geometrical volume, that was procurable by gravimetric method, that was obtained by: Ɛ=
(mw ―
md) ρ
(1)
A×L
Where mw and md are the weights of the wet membrane and dry membrane respectively. A (m2) is the effective area of the membrane, L (m) is the thickness of membrane samples and ρ (g/cm3) is the pure water density [8, 30, 46].
2.6. Pore size mean pore radius (rm) was definite by Guerout–Elford–Ferry equation: rm
(2.9 1.75 ) 8 l Q A P
(3.8)
(2)
where η is the viscosity of water (8.9×10-4 Pa.s), Q is the permeate water volume per unit time (m3/s), and ΔP is the operational pressure (0.3 MPa).
2.7. Nanofiltration experiments The permeation experiments of Unfilled and embedded membranes in terms of water permeability, powdered milk fouling tests and dye rejection were estimated via batch type, dead end stirred cell nanofiltration apparatus (effective area of 12.56 cm2, volume capacity of 125mL). The system feed side was pressurized by means of nitrogen gas. All membrane samples were first pretreated at a high pressure drop of 4 bar with deionized water for about 30 min. Afterwards, the pressure was diminished to the trans membrane pressure of 3 bar. The water permeability flux Jw (kg /m2.h) of Unfilled and embedded membranes is calculated from:
10
jw =
M A ⋅ Δt
(3)
Where M(kg), A (m2) and Δt(h) are permeate quantity, the effective membrane area and sampling time interval respectively. In the next step, in order to rejection measurement of the nanofiltration membrane, DR16 dye were applied. The dye rejection test was determined using a 50 mg/L feed solution. The rejection can be calculated by: Cp
(4)
R(%)=(1- Cf )×100
Where cp and cf are particular component concentration of permeate and feed solution concentration respectively [47,48]. Fouling resistance ability of prepared membranes were characterized using powdered milk solution with high proteins concentration (8000 mg/L), that cause promptly crucial fouling. After powdered milk filtration, protein-fouled membrane was rinsed with deionized water for 15 min to remove reversible fouling and procure the water flux recovery ratio. Consequently, the test involves 3 step: measuring PWF for 30 min Jw,1 (kg /m2.h), measurement of powdered milk permeability for 120 min calculated Jp (kg /m2.h) and as well as renewed measuring of PWF for another 30 min Jw,2 (kg /m2.h) at 3 bar operational pressure. The flux recovery ratio was defined as follows: FR R (% ) (
J w ,2 J w ,1
(5)
) 100
Generally, high flux recovery value is the exhibition of special fouling-resistance property and high cleaning efficiency of nanofiltration membranes [49-51]. As well as, reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated to estimate the fouling phenomenon in details using the following expressions: [52] J w2 J p Rr J w1
(6)
100 11
(7)
J J R ir w 1 w 2 100 Jw 1
3. Results and discussion 3.1. Characterization of coupling modified PES The FTIR spectra of the coupling modified PES and pure PES exhibits in Figure 3. The little alterations in the characteristic peaks of PES were emerged that these alterations can be attributed to coupling treatment of PES. Two peaks around 3067 and 3441 cm-1 are related to symmetric and asymmetric stretching vibration peaks of –NH2 bonds respectively. The appeared peak at 1075 cm1
is characteristic of the C-N stretching peak of the CPES groups. The existence of the
aforementioned peaks confirms successful synthesis of coupling modified PES using coupling procedure. The FTIR spectra depicted, that the embedding coupling modified PES can be improved hydrophilicity.
Figure 3. The FTIR spectra of pure PES and coupling modified PES.
12
3.2. Hydrophilicity and permeability measurement It is well-known that the fouling resistance property and permeability of membranes have a close relationship with surface polarity [53]. Therefore, the membrane surface hydrophilicity in term of water contact angle was measured to evaluate the surface wettability of CPES/PES blend membranes. The results of water contact angle measurement of CPES/PES blend membranes presented in Figure 4. As observed clearly, the bare PES membrane possess the highest water contact angle of 75.57 . It can be related to hydrophobic nature of bare PES membrane. The water contact angle reduced gradually with the increasing of the weight ratios of coupling modified PES from 0% to 100 % and the lowest water contact angle of 66.11 was measured for CPES membrane (M4, 100%). In fact, the higher weight percentage of coupling modified PES (M4 membrane) causes change the nature of bare PES membrane from hydrophobic to hydrophilic. The reduction of water contact angle may be attributed to increment of hydrophilicity that resulted from existence of countless number of hydrophilic functional groups (NH2, SO2, N=N) on the coupling modified PES surface. Dynamic wettability of the CPES/PES blend membranes could be estimated in term of permeated pure water flux. The pressure-assisted procedure and the pre-wetted procedure are the prevalent approaches, that used to measurement of pure water permeation flux [54,55]. In this current research, we applied pressure-assisted procedure to measurement of permeated pure water flux of CPES/PES blend membranes using a dead-end permeation cell and the results are depicted in Figure 5. The results of hydrophilicity are consistent with the pure water flux values for all of membranes, that previously had been observed by other researchers [12,30]. The PWF of the membranes enhanced with increment of coupling modified PES contents. So that, the CPES membrane (M4) with the highest hydrophilicity (the lowest water contact angle, 66.11 ) indicated the highest pure water flux (134.05 kg /m2.h). In addition to, the minimum PWF (17.4 kg /m2.h) was measured for bare PES membrane that can be justified by poor hydrophilicity nature of PES. From the affecting factors of pure water flux of membrane can be called membrane hydrophilicity, viscosity of dope solution, porosity and pore size. The overall porosity and mean pore radius exhibited in Figure 6 and 7 respectively. As can be seen patently, these parameters increased gradually with enhancement of coupling modified PES contents from 0 to 100 %. The results revealed the positive impact of coupling modified PES in CPES/PES blend membrane for reaching of higher porosity and pore size [56]. Increment of the 13
weight ratio of coupling modified PES in casting solution lead to enhancement of hydrophilicity, due to the intensify of functional groups induced by coupling treatment procedure. On the other hand, increasing of coupling modified PES contents incur also reduction of viscosity of casting solutions. so that, the M4 casting solution (casting solution involving 100 % CPES) had the lowest viscosity in compared to other casting solutions and M0 casting solution (pure casting solution) possessed the highest viscosity. The increase of hydrophilicity and the decrease of viscosity induced by coupling treatment lead to accelerates the rate exchange of solvent and non-solvent throughout the phase-inversion process. The rapid interchange of solvent and non-solvent during phase-inversion incur the larger pore radius formation consequently, the higher overall porosity in the surface of membrane and ultimately, improvement of the water permeability.
Figure 3. The results of water contact angle measurement of the prepared membrane.
14
160 134.05
120 100 80 57.81 60
45.4
38.4
40 17.4 20 0 Unfilled-PES
M1
M2
M3
M4
Figure 5. The results of permeation flux of pure water of prepared membranes at 3 bar.
100 90 80
83.6
85.8
M1
M2
89.3
89.8
M3
M4
68.2
70 Porosity (%)
Pure water flux (kg/m2.h)
140
60 50 40 30 20 10 0 Unfilled-PES
15
Figure 6. The overall porosity of CPES/PES blend membranes. 16
14.3
14
Mean pore radius (nm)
12
9.4 10
8.3 7.6
8 6
5.1
4 2 0 Unfilled-PES
M1
M2
M3
M4
Figure 7. Mean pore radius of CPES/PES blend membranes.
3.3. Morphology analysis The cross-sectional morphology of CPES/PES blended membranes with various CPES contents are shown in SEM images (Figure 8). As observed clearly, all of the prepared membranes possess the similar hierarchical characteristic of asymmetric structure with dense top-layer on a finger-like porous sub-layer. The finger-like pores of membranes at presence of CPES become wider and actually, with CPES addition the micro-voids lengths were elongated. The membranes porosity and mean pore radius values (pore structure) were increased with enhancement of CPES content (Figure 6 and Figure 7 supporting information). The alteration of membranes pore structure could be justified by hydrophilic nature of CPES that, elevated the rate of mass transfer between the solvent and non-solvent in the coagulation bath during phase separation process. Finally, the larger pore channels were formed. It is also worth noting that, some lateral pore structures are emerged particularly in 75 and 100 wt.% CPES that,
16
incur permeability improvement. The similar lateral pores were obtained in fabrication of graphene oxide embedded PES membranes by Zinadini et al. [30].
Figure 8. The cross-section SEM micrographs of blended membranes.
3.4. Dye removal assessment 17
The retention of DR16 with concentration of 50 mg/L was investigated in a dead end cell, to investigate nanofiltration performance of the CPES/PES blend membranes, that a schematic representation of module is demonstrated in Figure 9. The results of dye rejection after 30 min filtration at trans-membrane pressure of 3 bar are depicts in Figure 10. As can be seen, all of the CPES/PES blend membranes be able to remove dye from water with high efficiency, that this capability is exhibition of typical property of nanofiltration membranes. Nevertheless, the CPES/PES blended membranes indicated higher rejection capability compared to bare PES membrane. These results can be inspired by increment of Donnan exclusion effect between the surface of membrane and some groups of the dye, due to attendance of same groups in the structure of both membrane and solute [13]. On the other hand, enhancement of the electrostatic effects resulted from presence of hydrophilic groups in the coupling modified membranes is possible, that assist with Donnan exclusion effect in removing DR16 molecules [52]. Actually, incorporation of coupling modified PES can be greatly enhanced rejection capability of CPES/PES blended membranes. All of the modified blended membranes removed color more than 90 %. Among of four CPES/PES composite membranes, M3 (membrane with 75 % CPES) demonstrated the highest dye rejection of 99 %. In contrast, the dye rejection of M4 (membrane with 100 % CPES) was slightly lower than M3, owing to the permeation flux of M4 was much larger than M3. In fact, with the enhancement of CPES content, from 75% to 100%, the mean pore radius was increased from 9.4 to 14.3 nm, that ultimately incur the reduction of dye rejection and increment of pure water flux of M4. These results revealed that the performance of CPES/PES modified membranes in the current study had a special effect in textile effluent treatment and can be industrialized in feature.
18
Figure 9. Schematic illustration of DR16 rejection of prepared membranes in the dead-end filtration module.
19
120
Dye rejection (%)
100
88
95.6
94.2
99.2
96.4
80
60
40
20
0 Unfilled-PES
M1
M2
M3
M4
Figure 10. The results of dye rejection performance of CPES/PES modified membranes.
3.5. Fouling resistance capability Fouling resistance capability of nanofiltration membranes is a key factor for practical application. Electrostatic interactions, van der waals attractions, hydrogen bonding and hydrophobic interactions can be incurring adsorption of foulants on the surface of membrane and as a result creation of fouling phenomenon. So, an efficacious procedure to declin fouling of membrane is to elevate repulsive interactions and to make least adsorptive interactions between membrane surface and foulants. On the other hand, enhancement of membrane hydrophilicity possesses a positive impact on the amelioration of the fouling resistance property [8, 57]. To prove this effect, the antifouling evaluation of CPES/PES modified membranes was conducted in dead-end permeation cell (Figure 11). The flux recovery ratio (FRR) of the bare and blended membranes is illustrated in Figure 12. Assessment of membrane fouling by powdered milk solution with concentration of 8000 mg/L for the bare PES membrane resulted in substantial diminution of the permeability (67.82 % of FRR). Actually, the main reason of permeability reduction of bare PES membrane is the low membrane hydrophilicity, that cause deposition of foulants on the membrane surface. As observed clearly, all of the CPES/PES blended membranes exhibited the FRR value more than 90 %, that this result was consistent with superior fouling resistance capability of these membranes.
20
These achievements suggested, that embedding coupling modified PES in all of weight ratios (25, 50, 75 and 100 %) incur enhancement of filtration performance (antifouling properties) of the CPES/PES blended membranes especially, M3 and M4. As can be seen, the observed results of FRR was accordant with trend of membrane hydrophilicity (Figure 4 and Figure 5 supporting information). In fact, the enhancement of the FRR values was inspired by the improvement of membrane surface hydrophilicity, that caused mitigation of the hydrophobic interactions between foulants and membranes. Addition of the coupling modified PES and its attendance on the blended membranes may be regarded as a reason for augmentation of hydrophilicity, that leads to higher FRR values and as a result higher antifouling properties. The hydrophilic CPES/PES composite membranes indicated the amazing fouling resistance capability against powdered milk solution, that may be attributed to stronger interaction of these membranes with molecules of water than protein molecules. Nevertheless, it is worth nothing, which these hydrophilic CPES/PES composite membranes contained hydrogen bonding sites, that be able to adsorb water molecules and desorb foulants easily. In order to, further realization of antifouling behaviour of CPES/PES composite membranes, fouling resistance parameters such as reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated and indicated in Figure 13. Addition of coupling modified PES in all of weight percentage (25, 50, 75 and 100 %) cause increment of reversible resistances and reduction of irreversible resistances of CPES embedded membranes. The irreversible fouling ratio (Rir) of the PES membranes was remarkably decreased from 32.18 % for the bare PES membrane to 1.18 % for 75 wt. % CPES embedded membrane (M3). The results revealed that all of CPES/PES blend membranes possessed a lower Rir and higher (Rr, FRR) in comparison of bare PES membrane after cleaning process (washing with deionized water), that these results proved exceptional antifouling capability of the whole CPES/PES blend membranes. This behaviour can be attributed to higher hydrophilicity and hydratability of these membranes active layer, that this feature induced by hydrophilic functional groups (NH2, SO2, N=N) on the CPES surface via coupling treatment. These results recommended that coupling treatment is a feasible procedure for production of membranes with special fouling resistance capability and exceptional permeability flux of pure water.
21
In order to assessment of membranes surface morphology, the AFM image analysis was conducted and indicated in Figure 14. The surface roughness parameters of CPES/PES embedded membranes are listed with details in Table 2. As can be observed clearly, the roughness parameters of modified membranes were reduced gradually with increment of CPES content. The results revealed that the surface roughness altered to be smoot by blending CPES. In fact, the existence of CPES on the membrane surface incur significant enhancement of membranes surface hydrophilicity, diminution of interaction between membrane surface and foulants and as well as, tangible subtraction of roughness parameters of modified membranes.
Figure 11. Schematic representation of protein rejection of CPES/PES embedded membranes.
22
120 98.83 100
91.8
92.56
M1
M2
97
FRR ratio (%)
80 67.82 60
40
20
0 Unfilled-PES
M3
M4
Figure 12. The water flux recovery ratio of CPES/PES blended membranes after protein filtration. 100
Fouling Resistance ratio (%)
90
Irreversible Fouling 7.42
3.021
Reversible Fouling 7.42 1.18
80 70 60 32.18 50
89.14
40
73.4
73.39
76
M1
M2
M3
30 20 10
16.7
0 Unfilled-PES
M4
Figure 13. Fouling resistance ratio of CPES/PES blended membranes.
23
(a)
(b)
(c)
(d)
(e)
24
Figure 14. AFM images of the bare(a), M1(b), M2(c), M3(d), M4(e) membranes.
Table 2. The results of membranes Surface roughness parameters. Membrane
Sa(nm)
Sq(nm)
Sz(nm)
bare
21.251
28.671
245.15
M1
9.42
12.146
119.33
M2
10.772
13.538
118.64
M3
8.7459
11.197
94.289
M4
8.4018
9.9359
51.132
Conclusion The CPES/PES blended membranes were prepared via phase inversion technique, using 4,4diaminodiphenyl sulfone as precursor starting for coupling treatment of PES. The influence of CPES in different weight percentages on the structures, morphology and performance of CPES/PES blended membranes was investigated and afterwards, compared with bare PES membrane. The results revealed that the pure water permeation flux of CPES/PES blended membranes was enhanced significantly with increment of CPES contents that can be attributed to high hydrophilicity induced by CPES. This trend incurs enhancement of water molecules diffusion through the membranes and consequently, causes increment of permeated pure water flux. The assessment of fouling behavior with utilization of powdered milk solution indicated that the CPES/PES blended membranes in all of the weight percentage possessed superior fouling resistance properties in comparison of bare PES membrane. Also, the prepared membranes exhibited terrific dye rejection capability of DR16. The coupling treatment as a promising approach had an excellent potential for preparation of high performance blended membranes.
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Highlights 1-Coupling modified PES (CPES) was synthesized using aryldiazonium methodology. 2-CPES mixed matrix membrane exhibited special permeability of 134.05 kg /m2.h. 3- Antifouling properties of CPES/PES membranes was improved (FRR more than 91%). 4- CPES/PES membranes are shown high performance dye rejection (more than 94%). 3-This procedure is convenient for modification of various membranes.
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Graphical abstract
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Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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