SiO2-PNIPAM mixed matrix membrane for saline oil emulsion separation and its cleaning efficiency

Desalination 408 (2017) 1–12

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Preparation of thermoresponsive PVDF/SiO2-PNIPAM mixed matrix membrane for saline oil emulsion separation and its cleaning efficiency H.P. Ngang, A.L. Ahmad, S.C. Low, B.S. Ooi ⁎ School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan 14300, Nibong Tebal, Pulau Pinang, Malaysia

H I G H L I G H T S • • • •

Thermoresponsive SiO2/PNIPAM particles were direct blended into the membrane matrix. Various membranes forming temperature for proper control of cavity size The shrinking-swelling actuation motion will loosen the fouling layer by alternate thermal cleaning method. Improving the cleaning efficiency by controlling the free gaps for particles actuation

a r t i c l e

i n f o

Article history: Received 10 October 2016 Received in revised form 22 December 2016 Accepted 3 January 2017 Available online xxxx Keywords: Mixed matrix membrane Thermoresponsive, poly (Nisopropylacrylamide) Oil emulsion Fouling Actuation force

a b s t r a c t Thermoresponsive polyvinylidene fluoride/silica-poly (N-isopropylacrylamide) (PVDF/SiO2-PNIPAM) mixed matrix membranes were synthesized using diffusion induced phase separation techniques via direct blending method. The effects of SiO2-PNIPAM particle concentrations and surface cavity size on membrane physicochemical properties were investigated through oil emulsion separation using cross-flow ultrafiltration. Compared with pristine PVDF membrane, the PVDF/SiO2-PNIPAM membrane showed higher alternated thermal cleaning ratios (N = 2.04) especially at higher SiO2-PNIPAM particle concentrations. Proper control of cavity size allows better particles actuation and minimized the surface roughness that would induce fouling phenomenon. The flux recovery ratio of 69.91% can be achieved by preparing the membrane using dope solution at 25 °C, coagulation bath temperature at 40 °C at particle dosage of 1.0 wt.% (M25-40/1.0). It was found that the actuation force as a result of temperature swing is responsible for loosening the irreversible fouling layer (gel layer) on the membrane surface. © 2017 Published by Elsevier B.V.

1. Introduction Nowadays, membrane technology has become great significance separation techniques in practical waste water recovery owing to its compact design, lower energy consumption, and higher water quality [1,2]. The most critical limitation of current membranes application is membrane fouling, which generally causes serious flux reduction, reduce the effluent quality, thus reduces the membrane life span [3]. Frequent membrane cleaning or replacement was required to solve membrane fouling problem resulting in higher operating cost [4,5]. Several approaches were done to develop fouling mitigation strategies to enhance the membrane performance. Membrane surface modification by alter their hydrophilicity, surface charge and morphology, are some of the effective ways to mitigate membrane fouling [3,6–8].

⁎ Corresponding author. E-mail addresses: [email protected] (H.P. Ngang), [email protected] (B.S. Ooi).

http://dx.doi.org/10.1016/j.desal.2017.01.005 0011-9164/© 2017 Published by Elsevier B.V.

Polyvinylidene fluoride (PVDF) has become great consideration from researchers throughout the world owing to its superior thermal stability, chemical resistant, and high mechanical strength. It is widely applied in wastewater recovery, production of drinking water, and desalination [9–11]. However, its hydrophobic nature leads to severe fouling problem which become major barrier for membrane application in water recovery process. Recently, modifications of PVDF membranes by blending with inorganic particles become an advance technology for improving their membrane performances. Blended membrane can acquires benefit from both organic and inorganic materials in enhancement of hydrophilicity, permeability as well as selectivity [12]. A number of published studies had investigated the performance of blended PVDF membranes by adding particles, for instance titanium dioxide (TiO2) [13–15], zirconium dioxide (ZrO2) [16], silica (SiO2) [17] and alumina (Al2O3) [18,19]. These studies showed that by blending the hydrophilic inorganic materials into membrane matrix increased the surface hydrophilicity and reduce potential adsorption of foulant. Among all the inorganic materials, silica (SiO2) is widely used owing to its easy

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H.P. Ngang et al. / Desalination 408 (2017) 1–12

operation, good thermal and mechanical stability as well as mild reactivity [20,21]. In a long term operation, once the fouling start to occur, the modification of membrane surface is no more feasible in fouling prevention [8]. In this case, chemical cleaning was commonly applied to effectively remove the deposited irreversible foulant. Nonetheless, if the modified membrane surface could be induced for physical stretches from within, it will help to loosen the irreversible fouling layer, thus enhance the membrane cleaning efficiency without using chemicals. Stimuli-responsive polymers refer to polymer that response to environmental stimuli for instance pH, UV, temperature, magnetic field, and ionic strength have gained much consideration for development of membranes with anti-fouling properties. Poly (N-isopropylacrylamide), PNIPAM is an outstanding thermoresponsive polymer, which shows significant swelling-shrinking in water during the temperature changes around its lower critical solution temperature (LCST). When the temperature is lower than LCST, PNIPAM is completely dissolve in water and swelling owing to the hydrogen bond formed between the amide side chains and water molecules. However, PNIPAM will experience a “coil-to-globule” phase change as a result of disruption of the hydrogen bond and the structured water around the isopropyl groups was release when temperature was heated up above the LCST [22,23]. Various methods to fabricate the temperature-sensitive membranes have been studied for NIPAM grafting onto the pre-prepared membranes, which including high energy radiation [24,25], plasma treatment [22], UV photografting [26] and chemical grafting [27,28], etc. For example, Zhou and the co-researchers [28] had prepared a temperature responsive zirconium oxide (ZrO2) membrane by grafting the temperaturesensitive PNIPAM brushes onto ZrO2 membrane surface via chemical grafting. It was found that the modified membrane was easily clean and showed good anti-fouling properties during the filtration of bovine serum albumin. Modifying the membrane surface by grafting polymerization may not be uniform, which cause the changes in membrane surface pore size and its pore size distribution, thus result in reduced membrane permeability [29]. Preparing thermoresponsive membrane by direct blending method may serve as a facile and practical approach to overcome the above-mentioned shortcoming. Although direct blending method is ease for scaling up production, however, the effectiveness of thermoresponsive particle within PVDF membranes may be impaired due to the limited conformational changes of the PNIPAM within the polymer matrix and the cavity size formed on the membrane surfaces. To our best knowledge, the preparation of membrane via direct blending with controllable cavity sized for thermoresponsive is still lacking. In this study, a relatively simple method was adopted for the preparation of PVDF/SiO2-PNIPAM thermoresponsive mixed matrix membrane, which was synthesized using direct blending method. The effect of particles concentration and surface cavity size on membrane morphology and performance were elucidated. The efficiency of the alternated thermal cleaning of PVDF/SiO2-PNIPAM membranes toward oil fouling was evaluated.

2. Experimental 2.1. Materials Tetraethylorthosilicate (TEOS, 99.999%), N-isopropylacrylamide (NIPAM, ≥99%), 3-methacryloxypropyl trimethoxysilane (MPS, 98%), N,N- methylenebisacrylamide (MBA, 99%), Triton X-100, sea salts were purchased from Sigma Aldrich and used as received. Ethanol, ammonium solution 25%, potassium persulfate (KPS), Dimethylformamide (DMF) were purchased from Merck, Germany. Polyvinylidene fluoride (Solef® PVDF) was purchased from Solvay, France. Crude oil was supplied by Petronas Malaysia. PVDF powder was dried overnight at 70 °C before use to avoid moisture content.

2.2. Preparation of SiO2-MPS particles Stöber method was used to synthesize single-disperse silica particles [30]. The resulting silica particles suspension was directly modified with MPS to introduce C _C onto the SiO2 particles surface. Firstly, 200 mL of ethanol, 22 mL of deionized water and 12 mL of ammonium solution 25% were mixed together using magnetic stirrer at 350 rpm. Next, 12 mL of TEOS was added to the solution mixture at 30 °C. The solution was left to stir for 24 h. Then, 3 mL of MPS in ethanol mixture (1:1) was dropped into the resulting suspension at 0.2 mL/min for 15 min. The reaction was allowed to react for another 24 h. The suspension was then heated up to 80 °C for 2 h to promote covalent bonding. The produced SiO2-MPS were purified by centrifugation and re-suspension in ethanol for 3 times using ultrasonic to wash away the MPS residuals. The SiO2MPS particles were then re-dispersed in deionized water and the SiO2MPS particles suspension concentration was determined using gravimetric method. 2.3. Preparation of thermoresponsive SiO2-PNIPAM particles Thermoresponsive SiO2-PNIPAM particles was synthesized through radical polymerization using SiO2-MPS particles as core material. Briefly, 0.4 g of NIPAM, 0.06 g of MBA and 0.4 wt.% of SiO2-MPS particles suspension was mixed together into total 100 g of water solution. The mixture was sonicated for 15 min to produce homogenous solution. After that, the temperature was elevated to 70 °C for the reaction to be carried out under argon atmosphere. Next, 0.02 g of KPS solution was added into the mixture to initiate the polymerization. The polymerization was continued with vigorous stirring at 70 ± 1 °C for 4 h. The SiO2-PNIPAM solution was left to cool down to 25 °C. The solution was pour into 50 mL centrifuge tube and centrifuged and re-dispersed in deionized water for three cycles to wash away the residual of PNIPAM homopolymer. The SiO2-PNIPAM particles were then re-dispersed in DMF solvent and the particle concentration in the suspension was determined gravimetrically prior to use. 2.4. Membrane preparation The predetermined amount of SiO2-PNIPAM particles suspension was dispersed in DMF solvent. The suspension was subjected to sonicate for 15 min to ensure good particle dispersion. PVDF polymer was added into the SiO2-PNIPAM particles suspension. In this study, the PVDF polymer concentration was fixed at 18 wt.% due to the higher flux recovery ratio as well as lower relative flux reduction, which is having the best antifouling properties among the others (Table S1). The solution was stirred (250 rpm) at 65 ± 2 °C for 4 h to allow fully dissolution of the PVDF powder. The polymer dope was then continued to stir overnight at 40 °C to produce homogenous polymer dope. The solution was pour in a glass bottle and allowed to degas for 10 min. The polymer solution temperature was controlled prior to cast. The polymer dope was cast on the glass plate with membrane support (polyester cloth) using an automatic film applicator (Elcometer 4340, E.U.). The glass plate was immediately dipped into the coagulation bath with predetermined temperature and let the phase inversion occur for 24 h in order for fully leached out of the DMF residual. The detail of membrane synthesis parameter used was summarized in Table 1. The membrane annotation, for example MXX/YY-Z, is referred to membrane casted under different parameters. The M represents membrane, XX refers to the polymer dopes temperature, YY refer to the coagulation bath temperature, whereas Z represents wt.% of SiO2PNIPAM particles that were added into the polymer dopes. 2.5. Preparation of oil emulsion Oil emulsion was prepared freshly prior to the filtration experiment. Briefly, salty solution was prepared by dissolved 32 g of sea salts in 1 L of

H.P. Ngang et al. / Desalination 408 (2017) 1–12 Table 1 Detail of the membrane synthesis parameter used in this study. Membrane

PVDF DMF SiO2-PNIPAM Polymer dope temperature (wt.%) (wt.%) (wt.%) (°C)

Coagulation bath temperature (°C)

M25/25-0 M25/25-0.1 M25/25-0.5 M25/25-1.0 M25/25-2.0 M40/40-1.0 M25/40-1.0

18 18 18 18 18 18 18

25 25 25 25 25 40 40

82 81.9 81.5 81 80 81 81

0 0.1 0.5 1.0 2.0 1.0 1.0

25 25 25 25 25 40 25

distilled water under stirred condition. Subsequently, 50 ppm of oil emulsion was prepared by dispersing 50 mg of crude oil into the salty solution. Surfactant (Triton X-100) was then added into the solutions to stabilize the oil emulsion. The prepared oil emulsion was then dispersed using a blender (Khind BL-1515, power = 300 W) for 2 min at room temperature. Finally, the dispersed solution was sonicated for 5 min to remove any air bubble which might exist during the dispersing process. The oil droplets size distribution was measured using Malvern Zetasizer Nano ZS90 (Malvern Instruments Ltd., U.K.), with refractive index of 1.47 and 1.335 for the oil droplets and dispersant (32 g/L sea salts), respectively. The oil droplet size distribution was found in the range of 38–900 nm with an average particle diameter of 153 ± 4.83 nm (Fig. S1). The oil droplets have larger size compared to the membrane pore size. 2.6. Characterizations 2.6.1. Field emission scanning electron microscope The surface and cross-sectional morphology of the synthesized membranes were scanned by Field Emission Scanning Electron Microscope (FESEM, Quanta Feg 450, FEI, USA) at an accelerating voltage of 5 kV. The membrane was air dried overnight prior to scan. The membranes were dipped into nitrogen liquid and cryogenically cracked to prevent mechanical rupture of the membrane structure. 2.6.2. Pore size distribution The membrane pore size distributions were analysed by Capillary Flow Porometer, Porolux 1000 (Benelux Scientific, Belgium). Perfluoroethers was used as a pore wetting agent. Five pieces of membrane samples with size of 20 mm in diameter were characterized in order to minimize the experimental error. 2.6.3. Fourier transform infrared spectroscopy The functional group on the SiO2-MPS and SiO2-PNIPAM particles were investigated through Fourier transform infrared (FTIR) with attenuated total reflection (ATR) (Thermo Scientific model NICOLET iS10). The FTIR spectra were recorded over the wavenumber range of 4000– 650 cm−1. 2.6.4. Membrane wettability The surface wettability of the membranes was characterized by water contact angle instrument (Rame-Hart Model 300 Advanced Goniometer) based on sessile drop methods. The images were analysed by DROPimage software to obtain the contact angles values. The presented results were average of 10 measurements to minimize the errors. 2.6.5. Membrane surface topography The surface topography and roughness of the membranes were scanned by atomic force microscope, AFM (Park Scientific, Korea, XE100) The membrane area of 20 μm × 20 μm was scanned in dry state by non-contact mode. Similar scan size was used for all samples for comparison purpose.

3

2.6.6. Thermal gravimetric analysis Thermal gravimetric analysis (TGA, Perkin Elmer, USA) was carried out to estimate the polymer loading on the silica surface. All measurements were done on dried powders with nitrogen purging under ambient pressure with 10 °C/min ramp heating from ambient temperature to 800 °C. 2.6.7. Transmission electron microscopy The primary core size of SiO2-MPS and SiO2-PNIPAM particles was observed under Transmission electron microscopy (TEM) (JEOL, JEM20CX) A single droplet of particles dispersion was deposited onto a carbon grid, followed by tungstophosphoric acid hydrate staining, and airdried for 10 min. The immobilized particles onto the carbon grid were then observed under various magnifications. 2.6.8. Dynamic light scattering analysis Thermoresponsive behaviour of SiO2-PNIPAM particles was investigated through dynamic light scattering analysis (Malvern Zetasizer Nano ZS90). The hydrodynamic diameter (Dh) changes for SiO2PNIPAM particles during heating and cooling were measured at various temperature by controlling the chamber temperature from 20 to 50 °C and vice versa. 2.7. Membrane permeation test A cross-flow ultrafiltration unit (Fig. 1) was employed to evaluate the performance of various membranes in the oil emulsion separation. The membrane chamber (Sterlitech, CF042) has an effective geometrical area of 0.0042 m2. The pure water and oil emulsion were re-circulated from feed tank to whole ultrafiltration unit at a constant flow rate of 0.4 L/min by peristaltic pump (Flex-Pro A4 Series). Prior to each filtration process, membranes were subjected to 30 min compression at transmembrane pressure (TMP) of 1.5 ± 0.1 bar. After the membrane compression process, the TMP was lower down to 1.0 ± 0.1 bar and the pure water flux was recorded. The permeation data was collected at a time interval of 1 min and the readings was recorded using electronic balance that connected to computer (Model: Fx-3000i, A&D Company). The filtration of the oil emulsion was performed under the same temperature and TMP conditions for 1 h. One litre of 50 ppm oil emulsion was first prepared and pump through the UF unit. The solution was recirculated through the membrane chamber. TMP was monitored throughout the experiment. After 1 h of oil emulsion filtration, the membranes were rinsed by simple hydraulic flushing by pure water (25 ± 1 °C) for 10 min to remove loosely bound oil droplet. The pure water flux (J2) (L/m2 · h) at 25 ± 1 °C was recorded and the flux recovery ratios (FRR1) was calculated based on Eq. (1) and Eq. (2). The membrane was further cleaned with alternated thermal cleaning process by circulating pure water at 40 °C and 25 °C each for another 10 min to study the thermoresponsive cleaning properties of the PVDF/SiO2-PNIPAM mixed matrix membrane. After cleaning, the pure water flux (J3) (25 ± 1 °C) were measured again and the FRR2 were calculated by using Eq. (3): J¼

V At

ð1Þ

FRR1 ¼

J2  100% J1

ð2Þ

FRR2 ¼

J3  100% J1

ð3Þ

where V (L) represents the volume of water permeates, A (m2) is the area of the membrane, t (h) is time taken for permeate collected, J1 is the initial pure water flux, J2 is the pure water flux after 25 °C washing, and J3 is the pure water flux after 1 cycle of alternated thermal cleaning.

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H.P. Ngang et al. / Desalination 408 (2017) 1–12

Fig. 1. Schematic diagram of cross flow UF unit.

All the presented results are average data obtained from three measurements. The alternated thermal cleaning ratio (N) is defined as followed: N¼

FRR2 FRR1

ð4Þ

The oil concentration in the permeate samples were determined by UV–visible spectrophotometer (UV mini-1240, Shimadzu) at maximum absorbance of wavelength 220 nm. The rejection of the solute, R (%) was calculated based on Eq. (5): R¼

  Cp  100% 1− Cf

The relative flux reduction (RFR) of the membranes was calculated using Eq. (6) to evaluate the membrane's fouling propensity.

RFR ¼ 1−

Jp  100% J w1

ð6Þ

where Jw1 is the initial pure water flux (L/m2 · h), and Jp is the final oil permeate flux ((L/m2.h). 3. Results and discussion

ð5Þ

where cp is the oil content in the permeate (ppm) and cf. is the initial concentration of the oil in the feed (ppm).

3.1. Characterization of the SiO2-PNIPAM particles The particles were characterized for its physicochemical properties such as particle size, thermoresponsiveness and chemical compositions.

Fig. 2. The FTIR spectra of SiO2-MPS and SiO2-PNIPAM particles.

H.P. Ngang et al. / Desalination 408 (2017) 1–12

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trapped inside the particles [33]. A significant weight loss was observed around 350 °C which corresponds to the PNIPAM chain decomposition. The absolute weight loss of SiO2-PNIPAM particles was 46.43% compared with SiO2-MPS particles (only 12.45%). Therefore, 41.12 wt.% of the particles was constituted by the PNIPAM polymer.

Fig. 3. TGA curves of SiO2-MPS and SiO2-PNIPAM particles.

3.1.1. Chemical properties of the SiO2-PNIPAM particles Fig. 2 shows the chemical composition of the SiO2-MPS and SiO2PNIPAM using FTIR spectroscopy. The characteristic peaks at 1060 and 804 cm−1 are associated to S-O symmetric stretching and asymmetrical stretching of SiO2 particles, respectively [31]. The FTIR spectrum of the SiO2-MPS particles showed two significant peaks at 1718 and 1630 cm−1 due to the C _O and C _C groups of 3-methacryloxypropyl trimethoxysilane (MPS) stretching vibrations. After the radical polymerization, two new characteristic peaks appeared at 1644 and 1532 cm−1, which were ascribed to the C _O and N\\H stretching of the secondary amide groups of PNIPAM polymer chains, respectively [32]. Two additional peaks at 2971 and 2933 cm−1 were assigned to the methylene groups. These results confirmed the successful polymerization of PNIPAM onto the SiO2-MPS particles surface.

3.1.2. Degree of PNIPAM crosslinking Thermal gravimetric analysis (TGA) was conducted to evaluate the degree of PNIPAM cross-linked onto the SiO2-MPS particles. Fig. 3 depicts the decay of weight (%) with step increment in heating temperature. As expected, at temperature below 150 °C, a mass loss was observed which is due to the loss of volatiles solvent or moisture

3.1.3. Structure of SiO2-PNIPAM particles Fig. 4 reveals the TEM images for SiO2-MPS and SiO2-PNIPAM particles. They exhibited nearly perfect spherical shape and well defined size. The average particle diameter for the mono-dispersed SiO2-MPS particles was observed to be around 220 nm. A polymer layer appeared around the SiO2-MPS surface after precipitation polymerization of PNIPAM. The diameter of SiO2-PNIPAM particles was about 270 nm and uniformly distributed with cross-linked shell thickness of 25 nm. The SiO2-PNIPAM particles are mono-disperse with single SiO2 core. This observation of PNIPAM layer provides another evidence of the incorporation of PNIPAM onto SiO2-MPS particles.

3.1.4. Thermoresponsive behaviour of SiO2-PNIPAM particles The thermoresponsive behaviour of SiO2-PNIPAM particles were characterized by using Malvern Zetasizer Nano ZS90. The hydrodynamic diameter, Dh changes was measured from temperature 20 °C to 50 °C for heating, and 50 °C to 20 °C for cooling in order to study the thermoresponsive behaviour during heating and cooling cycles. Fig. 5 shows the temperature-induced volume phase transition curves for SiO2-PNIPAM particles. The hydrodynamic diameter (Dh) of the SiO2PNIPAM particles decreased from 528.53 nm to 373.33 nm when the temperature increased from 20 °C to 50 °C. When the temperature is below LCST, PNIPAM is completely solvated in water and swelling owing to the hydrogen bond formed between the amide side chains and water molecules. However, PNIPAM were shrunken and experience a “coil-to-globule” phase change as a result of disruption of the hydrogen bond and the structured water around the isopropyl groups was release when temperature was heated up above the LCST [22,34,35]. While during the cooling process from 50 °C to 20 °C, it was found that the particle was going back to original size with almost negligible hysteresis. This indicated that the temperature sensitivity of the SiO2PNIPAM particles during heating and cooling process was reversible.

Fig. 4. TEM images of (a) SiO2-MPS and (b) SiO2-PNIPAM particles.

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H.P. Ngang et al. / Desalination 408 (2017) 1–12

3.1.5. Distribution of SiO2-PNIPAM particles within PVDF membrane matrix Fig. 6 presents the FESEM images for membranes dosed with different concentration of SiO2-PNIPAM particles. The pristine membrane showed a smoother surface morphology compare to the composite membranes. The SiO2-PNIPAM particles were uniformly dispersed within the membrane matrix with a ring-like gap (cavity) around them because of the shrunken state of the PNIPAM network during membrane drying. The cavity is the space for the thermoresponsive SiO2-PNIPAM particles underwent swelling and shrinking during alternated thermal cleaning process (40 °C/25 °C). The swelling and shrinking process create the actuation force for gel layer loosening. It is important to note that there is no SiO2-PNIPAM particles agglomeration was observed, which means that SiO2-PNIPAM particles were dispersed homogeneously within PVDF membrane matrix. Nonetheless, the surface porosity of the membrane changed significantly with the increased of SiO2-PNIPAM particles concentration. The increased in membrane surface porosity at higher SiO2-PNIPAM particles concentration can be clearly seen from the FESEM images in Fig. 6 (left). FESEM cross-sectional images in Fig. 6 (right) shows that at higher SiO2-PNIPAM particles concentration, the upper membrane sub-layer asymmetric structure changed from finger-like to macrovoid-like structure. The length and diameter of the finger-like structure became larger and irregular in shape, indicating that the nonsolvent influx is higher than the solvent outflux during liquid-liquid demixing at higher SiO2-PNIPAM particles concentration. It was due to the hydrophilic behaviour of SiO2-PNIPAM particles, which enhance the kinetic of water influx during the phase inversion process. 3.1.6. Surface roughness Fig. 7 shows the AFM images of membrane prepared with different concentration of SiO2-PNIPAM particles. The surface roughness values (Rq) are shown in Table 2. The pristine membrane exhibits the Rq value of 63.128 nm, which is lower than the value for particle-dosed membranes. These images indicated that the micro-roughness of the membranes surface increased structurally with the increased of SiO2PNIPAM contents owing to the higher surface porosity as shown in Fig. 6. The surface roughness was governed by the SiO2-PNIPAM particles themselves rather than the polymer nodular structure. Higher roughness will have caused two changes in the composite membrane; which is the increased of effective porosity and a reduce in membrane performance. Increasing membranes roughness could easily trap foulant in the valley, which is an unwanted phenomenon, thus reduce membrane performance and antifouling ability [5,36]. 3.1.7. Membrane pore size distribution Fig. 8 shows membranes pore size distribution for membranes prepared with various SiO2-PNIPAM concentration. The membrane mean pore size was reduced slightly from 39.81 nm (M25/25-0) to 37.46 nm (M25/25-0.1) with the addition of 0.1 wt.% particles. 600

However, the membrane pore sizes start to increase and widely distributed at higher SiO2-PNIPAM concentration, with the mean pore size ranged from 42.60 to 51.39 nm for M25/25-0.5 and M25/25-2.0, respectively as show in Table 2. This phenomenon was due to the fact that the SiO2-PNIPAM particles created more porous structure on the membrane surface and cross sectional area. The phenomenon of pore widening should be avoided in order to prepare membrane with better antifouling properties. 3.1.8. Membrane wettability The membrane contact angles were obtained to evaluate the effect of SiO2-PNIPAM particles toward membrane wettability. Fig. 9 depicts that the addition of 1.0 wt.% of SiO2-PNIPAM particles could reduce the contact angle as compared to pristine membrane. However, further increased of SiO2-PNIPAM particles concentration (2 wt.%) resulted in increasing membrane contact angle. This phenomenon was due to the changes of roughness of membrane ‘microstructure’ as discussed earlier in the AFM images (Fig. 7) [37]. Theoretically, contact angle is mainly affected by membrane surface porosity, roughness and its hydrophilicity. It quantifies the wettability of a solid surface by a liquid via the Young equation which relates the surface tension of a drop of water on a solid is given by the relation γLV cosθ þ γ SL ¼ γ SV

ð7Þ

where γLV, γSL and γSV represent the surface tension of liquid-vapour, solid-liquid, and solid-vapour, respectively. At higher roughness whereby pillar effect is not significant, the increase of roughness could further reduce the contact angle as explained by Wenzel equation as shown in Eq. (8). cosθw ¼ r cosθY

ð8Þ

where θw and θY are the contact angle based on Wenzel's model and Young's relation, respectively. The roughness ratio (r) is defined as the ratio of membrane surface true area to its apparent area. However, in this study, the effect of microstructure roughness could be analysed by using Cassie–Baxter equation that take into consideration the pillar effect, where the surface porosity of the heterogeneous surface was considered. The Cassie–Baxter equation was shown in Eq. (9): n

cosθCB ¼ ∑ f i cosθi;Y i

ð9Þ

where fi is the fraction of the polymer and air gap. As shown in FESEM images in Fig. 6 (left), the surface porosity was increased with the increasing of SiO2-PNIPAM particles concentration. Therefore, the air gap trapped within pores caused the increased of surface contact angle at higher SiO2-PNIPAM particles concentration (2 wt.%) compensating the hydrophilic characteristic due to induced surface roughness. The increasing of contact angle is an unwanted phenomenon as higher surface tension between the water molecules and membrane surface will impair the membrane antifouling properties

Dh (nm)

550

3.2. Permeability and antifouling properties of PVDF/SiO2-PNIPAM mixed matrix membranes for oil emulsion separation

Heating

500

Cooling 450 400 350 20

25

30

35 40 Temperature ( ˚C )

45

Fig. 5. Thermoresponsive behaviour of SiO2-PNIPAM particles.

50

Fig. 10 shows the pure water flux and flux recovery ratio for PVDF/ SiO2-PNIPAM mixed matrix membranes. The pure water flux decreased initially when 0.1 wt.% of SiO2-PNIPAM particles were added into the membrane dopes solution due to the pore blocking by the particles as shown by the reducing pore size in Table 2. However, the pure water flux increased with the further increasing of SiO2-PNIPAM particles concentration which might be due to the two fold reasons namely increasing mean pore size (Table 2) and/or increasing hydrophilicity (Fig. 9). However, in this case, the pore size increment was dominant as compared to membrane hydrophilicity as could be seen in Fig. 9 that highest

H.P. Ngang et al. / Desalination 408 (2017) 1–12

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Fig. 6. FESEM images of membrane surface structure (left) and cross sectional structure (right) for (a) M25/25-0, (b) M25/25-0.1, (c) M25/25-0.5, (d) M25/25-1.0, and (e) M25/25-2.0.

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H.P. Ngang et al. / Desalination 408 (2017) 1–12

Fig. 7. AFM images of PVDF membrane surface for (a) M25/25-0, (b) M25/25-0.1, (c) M25/25-0.5, (d) M25/25-1.0, and (e) M25/25-2.0.

contact angle for M25/25-2.0 which still having high flux. The oil emulsion rejection showed no significant effect for different concentration of SiO2-PNIPAM particles added, which are all above 99% removal efficiency, as shown in Table 2. The membrane relative flux reduction (RFR) at different SiO2PNIPAM particles concentration is shown in Table 2. The membrane fouling tends to increase with the increased of SiO2-PNIPAM particles concentration (rougher surface as showed in Table 3) due to foulant build up within the “valleys” of the rougher surface [5]. As we can see clearly from Fig. 10, the flux recovery ratio after the first stage of pure

water (25 °C) cleaning (FRR1) was reduce from 55.01% (M25/25-0) to 22.8% (M25/25-2.0). This phenomenon can be explained by the changing of surface porosity as discussed earlier. More severe membrane blocking is expected in the membrane with higher surface porosity. Pore blocked foulant is difficult to be clean away by pure water cross flushing. In this case, with the aid of actuation motion from within, it is expected that membrane with the thermoresponsive particles is able to perform the cleaning in a more efficient manner. Table 2 shows the results of alternated thermal cleaning ratio of the PVDF/SiO2-PNIPAM mixed matrix membranes which is the ratio of FRR2/FRR1. PVDF neat

Table 2 Physical properties of PVDF/SiO2-PNIPAM mixed matrix membrane with different SiO2-PNIPAM particles concentration. Membrane

Mean pore size (nm)

Rq (nm)

RFR (%)

M25/25-0 M25/25-0.1 M25/25-0.5 M25/25-1.0 M25/25-2.0

39.81 37.46 42.60 45.99 51.39

63.128 65.261 66.732 70.625 93.372

82.90 89.52 90.57 91.94 93.03

± ± ± ± ±

6.80 7.85 6.25 7.01 9.05

± ± ± ± ±

0.42 0.77 2.43 0.16 1.05

Rejection (%)

Alternate thermal cleaning ratio, N (%)

98.95 99.25 99.20 99.13 99.07

1.17 1.34 1.40 2.04 1.79

± ± ± ± ±

0.54 0.28 0.47 0.52 0.63

H.P. Ngang et al. / Desalination 408 (2017) 1–12

9 80

70

110

M25/25-0 60

M25/25-0.1 M25/25-0.5

FRR (%)

M25/25-2.0

30 20 10

50 70

40 30

50

20

30

10

0 0

0.01

0.02

0.03

0.04 0.05 Pore Size (um)

0.06

0.07

0.08

0.09

10

PWF

60

90

M25/25-1.0 40

FRR2

PWF (L/m2.h)

Frequency

50

FRR1

70

M25/25-0

M25/25-0.1

M25/25-0.5

M25/25-1.0

M25/25-2.0

0

Membrane

Fig. 8. Effect of SiO2-PNIPAM concentration on membrane pore size distribution.

membrane shows a lower alternate thermal cleaning ratio (1.17) as compared to the PVDF/SiO2-PNIPAM mixed matrix membranes with highest ratio (2.04) shown by M25/25-1.0. The higher alternate thermal cleaning ratio is due to the actuation motion of the thermoresponsive SiO2-PNIPAM particles by shrinking and swelling at 40 °C/25 °C, respectively. This mechanical action assisted in foulant detachment from the membrane surface by loosening up the gel layer. Nonetheless, at higher particle concentration, M25/25-2.0 shows a significant decreased in the ratio (1.79). This phenomenon is due to difficulty of releasing the foulant that plug inside the pores via actuation force. 3.3. Free gaps for actuation of particles and its cleaning efficiency It was postulated that the higher free gaps of particles within the cavity could provide enough room for it to response to the alternated temperature changes, which create higher actuation force for foulant removal. However, as discussed earlier, membrane with too big gaps might create unwanted oil droplet blocking phenomenon. Therefore, in this section, three different gaps ratio were carried out to study their antifouling and cleaning efficiency. 3.3.1. Membrane morphology Three types of membranes with different particle to cavity ratio were prepared, namely M25/25-1.0, M40/40-1.0, and M25/40-1.0. Fig. 11 shows that the membrane surface porosity contributed by SiO2PNIPAM particles was able to be controlled by changing different membrane formation temperature. Fig. 12 illustrates the control of cavity and particle size at different formation temperature with the cavity size originated from the initial size of particle occupancy before phase inversion. Compared to M25/25-1.0, M40/40-1.0 showed smaller diameter of the cavity (587.48 nm) as tabulated in Table 3. It is due to the SiO2PNIPAM particles which were shrunken at 40 °C before phase separation gave not much difference in terms of size after it was dried up. As a result, smaller Rp value of M40/40-1.0 (61.31%) as compared to 80

Contact Angle (º)

75 70

Fig. 10. Performance of PVDF/SiO2-PNIPAM membrane with different SiO2-PNIPAM concentration.

M25/25-1.0 (66.64%) was obtained. On the other hand, M25/40-1.0 shows the intermediate Rp value (62.99%) due to the temperature of the polymer dope was controlled to 25 °C to maintain SiO2-PNIPAM particles in swollen state. The polymer dope was cast on glass plate and immediately dipped into the coagulation bath. During phase inversion process, the SiO2-PNIPAM particles start to shrink when contact with the 40 °C coagulation bath, thus resulting in formation of reduced SiO2-PNIPAM particles size on the membrane surface as compared to M25/25-1.0. The mechanism of cavity formation is illustrated in Fig. 12. Fig. 13 shows the pore size distribution of PVDF/SiO2-PNIPAM mixed matrix membranes prepared at different membrane forming temperature. Table 4 shows that the membrane mean pore size was reduced initially from 45.99 nm (M25/25-1.0) to 30.51 nm and 32.50 nm for M40/ 40-1.0 and M25/40-1.0, respectively. This observation shows that the elevated temperature for both dope solution and coagulation bath had significant effect on the cavity size. However, it seems that the coagulation bath temperature had more significant effect in controlling the cavity size. Increasing the coagulation bath temperature decreased the cavity size on the membrane surface. The analysis on surface roughness showed that membrane M25/40-1.0 exhibited the lowest Rq value of 67.708 nm as tabulated in Table 4. It is a desirable property as membrane with lower surface roughness always possesses better antifouling properties as discussed earlier. 3.3.2. Cavity and its role on cleaning efficiency The membrane permeability and antifouling properties of the PVDF/ SiO2-PNIPAM mixed matrix membranes were investigated by filtering oil emulsion under a constant TMP of 1 bar. Fig. 14 shows the pure water fluxes and flux recovery ratio for PVDF/SiO2-PNIPAM mixed matrix membranes at different membrane forming temperature. The pure water flux decreased from 63 L/m2.h to 43.57 L/m2.h for M25/251.0 and M40/40-1.0, respectively. The pure water flux decreased was due to the decreased of membrane mean pore size from 45.99 nm to 30.51 nm, as showed in Table 4. However, the M25/40-1.0 showed better pure water flux (45.29 L/m2.h) compared to M40/40–1.0 in accordance to the slightly bigger mean pore size (32.50 nm). Nonetheless, the oil emulsion removal efficiency was still maintained above 99% for all three prepared membranes at different membrane forming

65 Table 3 Physical properties of PVDF/SiO2-PNIPAM mixed matrix membrane surface.

60 55

Membrane

D1 (nm)

D2 (nm)

Rp (%)

50

M25/25-1.0 M40/40-1.0 M25/40-1.0

240.21 ± 4.18 227.28 ± 5.38 224.56 ± 5.7

720.16 ± 5.44 587.48 ± 12.27 606.79 ± 8.96

66.64 61.31 62.99

M25/25-0

M25/25-0.1

M25/25-0.5

M25/25-1.0

M25/25-2.0

Membrane Fig. 9. Effect of SiO2-PNIPAM particles concentration on membrane contact angle.

D1 = Diameter of SiO2-PNIPAM particles. D2 = Diameter of cavity. Rp = Gaps ratio between SiO2-PNIPAM particles to the cavity ð1− DD12 Þ  100%:

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H.P. Ngang et al. / Desalination 408 (2017) 1–12

Fig. 11. FESEM images of membrane surface structure for (a) M25/25-1.0, (b) M40/40-1.0, and (c) M25/40-1.0.

Fig. 12. Schematic diagram illustrating the cavity size formed at different membrane forming temperature. The dotted line represented the size of cavity.

80 M25/25-1.0 M40/40-1.0 M25/40-1.0

Frequency

60

40

20

0 0

0.02

0.04 Pore Size (um)

0.06

0.08

Fig. 13. Effect of membrane forming temperature on membrane pore size distribution.

temperature. This further proved that membrane forming temperature can effectively control the membrane permeability of PVDF/SiO2PNIPAM mixed matrix membrane. The membrane relative flux reduction (RFR) at different membrane forming temperature is shown in Table 4. No significant change of RFR value for all the membranes indicated that the membrane forming temperature has minimum effect on chemical properties of the surface. This was due to the SiO2-PNIPAM particles lost its hydrophilicity under high salinity oil emulsion separation. The presence of salt ions in oil emulsion could disrupt the structure of water molecules around PNIPAM layer within the membrane matrix (Hofmeister effect) [38,39]. Sodium chloride which is an effective salting-out agent, caused the coated PNIPAM polymer less soluble at high salt concentrations. The water molecules of the PNIPAM hydration layer was released and form a ‘rigid’ structure around the high salts ions [40,41]. Membrane fouling layer by oil emulsion could be classified as free moving layer (loose cake layer) as well as the irreversible stagnant layer (gel layer). The loose fouling layer can be washed away by physical

H.P. Ngang et al. / Desalination 408 (2017) 1–12

11

Table 4 Effect of membrane forming temperature on PVDF/SiO2-PNIPAM membrane's physical properties. Membrane

Mean pore size (nm)

Rq (nm)

RFR (%)

Rejection (%)

Alternate thermal cleaning ratio, N (%)

M25/25–1.0 M40/40–1.0 M25/40–1.0

45.99 ± 7.01 30.51 ± 5.54 32.50 ± 5.84

70.625 80.501 67.708

91.94 ± 0.16 91.64 ± 0.41 91.47 ± 0.17

99.13 ± 0.52 99.29 ± 0.33 99.30 ± 0.40

2.04 1.19 1.31

cleaning using pure water hydraulic cleaning (cross flushing). However, chemical cleaning method or back flushing are needed for cleaning of the gel layer. In this case, an environmental friendly and gentle cleaning procedure to remove the fouled oil layer from the PVDF/SiO2-PNIPAM mixed matrix membrane surface can be realised using the alternated thermal cleaning method. The actuation force of the thermoresponsive particles was due to the hydrophilic/hydrophobic properties of the PNIPAM layer which could be changed by alternating the cleaning temperature. As shown in Fig. 15, the cleaning temperature was initially introduced at 40 °C, the PNIPAM experience a “coil-to-globule” phase change as a result of hydrogen bond disruption and the structured water around the isopropyl groups was release, resulting in shrunken of SiO2-PNIPAM particles. By reducing the temperature to 25 °C which is below LCST temperature, the intermolecular hydrogen bonds between water and polar groups of PNIPAM again solubilize the polymer and become hydrophilic and swollen. This shrinking-swelling motion will loosen the fouling layer by actuation forces thus increase the cleaning efficiency. In this study, the flux recovery ratio (FRR2) was an indicator used to determine the membrane cleaning efficiency after temperature swing. The FRR2 for M25/40-1.0 shows highest recovery (69.55%) compare to M25/25-1.0 and M40/40-1.0, which are 57.38% and 63.93%,

respectively. With respect to this, we would like to highlight the importance of the gaps ratio which acts as a room for the particles to response during temperature swing in order to remove and loosen the foulant. As show in Fig. 11, M25/25-1.0 has the highest gaps among all. Therefore, it possesses highest alternated thermal cleaning ratio (N = 2.04) due to the bigger room for the thermoresponsiveness. On the other hand, the membrane M40/40-1.0 with smallest gap ratio create lower actuation motion (N = 1.19), thus reduce the irreversible foulant removal. Hence, the membrane M25/40-1.0 with intermediate Rp was prepared to further tested for oil emulsion filtration. M25/40-1.0 shows intermediate gaps between SiO2-PNIPAM particles in between M25/25-1.0 and M40/40-1.0 as discussed earlier. The membrane shows highest FRR2 with N = 1.31, which further proved that the gaps room space is important for the SiO2-PNIPAM particles to response with temperature changes. It creates enough actuation force during irreversible foulant removal, at the same time minimize the gap that could be blocked by the oil emulsion, thus increasing the efficiency of the membrane cleaning (Fig. 15). 4. Conclusion A thermoresponsive PVDF/SiO2-PNIPAM mixed matrix membrane was prepared by using direct blending method. Membrane dosed with 1 wt.% SiO2-PNIPAM particles had better thermal cleaning properties due to increased hydrophilicity. Membrane forming at different temperature affected the membrane morphology and performance due to the cavity size as well as porosity. The membrane forming temperatures change the cavity size with different gaps ratio. As a result, the membrane M25/40-1.0 with intermediate gaps ratio shows highest FRR2, which further prove that the gaps room space is important for easy cleaning properties. The actuation motion of the thermoresponsive SiO2-PNIPAM particles is responsible for foulant release from membrane surface. Acknowledgements

Fig. 14. Effect of membrane forming temperature on performance for oil emulsion separation.

The authors are grateful for the funding from Universiti Sains Malaysia (RU Grant) (1001/PJKIMIA/814210), FRGS (203/PJKIMIA/ 6071252) and Membrane Science and Technology Cluster (1001/PSF/ 8610011). In addition, the first author also thanks the Ministry of Higher Education (MyPhD) for financial support.

Fig. 15. Schematic diagram illustrating the thermoresponsive cleaning mechanism of the SiO2-PNIPAM particles on the membrane surface.

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