Antifouling PVDF membrane prepared by VIPS for microalgae harvesting

Chemical Engineering Science 142 (2016) 97–111

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Antifouling PVDF membrane prepared by VIPS for microalgae harvesting Antoine Venault a,n,1, Melibeth Rose B. Ballad a, Yu-Tzu Huang b, Yi-Hung Liu a, Chi-Han Kao b, Yung Chang a,n,2 a

R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan Research Center for Analysis and Identification and Department of Bioenvironmental Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan b

H I G H L I G H T S







Formation of anti-biofouling PVDF membranes by VIPS. Membranes resist biofouling in static and dynamic conditions. Membranes efficiently resist biofouling by microalgae. Harvesting by membrane filtration competes with centrifugation.

art ic l e i nf o

a b s t r a c t

Article history: Received 13 October 2015 Received in revised form 13 November 2015 Accepted 14 November 2015 Available online 15 December 2015

Provided the use of a water-insoluble modifier, in-situ modification of PVDF membranes is an ideal method for preparing low-biofouling membranes. We report the formation, characterization and lowbiofouling performances of modified PVDF membranes prepared by vapor-induced phase separation for microalgae harvesting. Poly(styrene)-b-poly(ethylene glycol) methacrylate (PS-b-PEGMA) is used as antifouling material. After characterizing the physico-chemical properties of membranes by SEM, AFM, FT-IR, XPS, and tensile tester, their hydrophilicity was assessed. Hydration capability was importantly enhanced with copolymer content. Adsorption of bovine serum albumin (BSA), lysozyme (LY) and fibrinogen (FN) was tested to investigate the resistance of membranes to nano-biofouling. Best results were obtained with membrane prepared from a casting solution containing 4 wt% copolymer (PS-bPEGMA-4). Bacterial attachment tests proved that membranes could also resist micro-biofouling. Flux recovery ratio after filtration of BSA with PS-b-PEGMA-4 membrane was higher than with a commercial hydrophilic PVDF membrane. Applied in microalgae harvesting, it was found that membranes could efficiently resist biofouling by microalgae (FRR¼ 76.9% with PS-b-PEGMA-4), still enabling a rejection ratio over 99.7%. & 2015 Elsevier Ltd. All rights reserved.

Keywords: PVDF membranes PS-b-PEGMA copolymer VIPS process Low-biofouling Microalgae

1. Introduction Five years ago, Rana and Matsuura reviewed the available strategies to modify hydrophobic membranes in order to provide them with antifouling properties [Rana and Matsuura, 2010]. They insisted on the fact that an increase in hydrophilicity of the polymeric material was generally accepted as a prerequisite to achieve biofouling resistance. Therefore, the choice of the surfacen

Corresponding authors. E-mail addresses: [email protected] (A. Venault), [email protected] (Y. Chang). 1 Tel.: þ886 3 265 4113; fax: þ 886 3 265 4199. 2 Tel.: þ886 3 265 4122; fax: þ886 3 265 4199. http://dx.doi.org/10.1016/j.ces.2015.11.041 0009-2509/& 2015 Elsevier Ltd. All rights reserved.

modifier or that of the membrane preparation process should be oriented toward an optimization of the surface and/or bulk hydrophilicity. If we briefly survey recent literature of the scientific community working in this field, three major classes of processes can be distinguished. These processes are (i) surface modification involving low-energy interactions between the membrane and the surface-modifying molecule [Muppalla et al., 2013; Wang et al., 2010] (ii) surface modification by chemical reaction [Yue et al., 2013; Liu et al., 2013] and (iii) matrix (whole membrane material) modification after incorporating a hydrophilic or an amphiphilic copolymer in the solution to be cast, also termed in-situ modification [Higuchi et al., 2002; Li et al., 2006]. The third approach presents a number of important advantages. First, the membrane preparation process is a one-step process.

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Contrary to surface modification processes by coating, grafting onto or grafting from, which all require a modification step of the polymer matrix after membrane formation, the surface-modifying molecule is directly incorporated in the polymer solution when the in-situ modification is at play. Therefore, these in-situ modified membranes are potentially readily scalable. Second, not only the surface is modified, but also the deeper layers of the membrane, which implies that internal fouling is expected to be reduced as well. Nevertheless, the blending method also presents some disadvantages explaining why it is not as popular as other strategies. A first limitation concerns the solubility of the surface-modifying molecule in the polymer solution. If it has limited affinity with the solvent and/or the polymer, no spontaneous mixing can be established. The enthalpic contribution to the free enthalpy of mixing is too high, because of high interaction parameters. Consequently, the difference of free enthalpy of mixing is positive and no spontaneous mixing of the different species (polymer, copolymer, solvent) can be obtained. However, this issue may be addressed by controlling the hydrophobic moieties/hydrophilic moieties ratio in the amphiphilic copolymer. Increasing the number of repeat units of the hydrophobic block or on the contrary, of the hydrophilic block, as it has been done with poly(propylene oxide)-b-poly(sulfobetaine methacrylate) copolymer [Hsiao et al., 2014], should allow not only to mitigate fouling, but also to improve the miscibility of the species. A second disadvantage is the stability of the interactions. Low-energy interactions are established, like in self-assembling process, so that amphiphilic molecules can be partially or totally released either during membrane formation or during membrane filtration. So, if the copolymer is water-soluble, such membranes cannot be used in water treatment for long-term operations. To address this issue, one can incorporate a water-insoluble copolymer that still contains hydrophilic moieties and that is still able to entrap water in significant proportions [Zhao et al., 2008; Ran et al., 2011; Liu et al. 2013]. Last, but not least, control of membrane formation and so, membrane modification, is difficult to achieve. When blending is chosen, membranes are often formed by the wet-immersion process [Sun et al., 2006; Rahimpour et al., 2009; Riyasudheen and Sujith, 2012], in which phase separation is instantaneous for many systems used, owed to fast solvent outflow and nonsolvent inflow [Tsay and McHugh, 1990; Stropnik and Kaiser, 2002]. Regarding the chemical nature of the surface-modifying molecule, constituted of both hydrophobic and hydrophilic moieties, its motion toward the polymer system/air interface is expected, in order to thermodynamically stabilize the membrane. However, membrane formation by wet-immersion is kinetically not favorable to this stabilization, so that surface-modification may not be optimal. This, along with the lack of understanding of the exact role of the additive on membrane formation, often pore-former [Kim and Lee, 1998; Loh et al., 2011; Guillen et al., 2011; Loh and Wang, 2013] but also sometimes pore-suppressor as reminded in a recent review [Guillen et al., 2011], has probably pushed scientists to focus their attention on two-step processes in which membrane formation is perfectly controlled and understood during the first step, before surface-modification. Recently, we proposed to apply the vapor-induced phase separation (VIPS) process to the formation of low-biofouling polyvinylidene fluoride (PDVF) membranes [Venault et al., 2012]. Because of the gas/liquid nature of the interface between the nonsolvent atmosphere and the polymeric system, a supplementary resistance to mass transfers is created, which necessarily slows down mass transfers (diffusive influx of nonsolvent and outflux of solvent). By doing so, thermodynamic stabilization of

the polymeric system (motion of the amphiphilic copolymer) is also improved. It arose in better low-biofouling properties for membranes prepared by VIPS than for those prepared by wetimmersion, since a denser surface coverage was achieved by VIPS. This set of studies was performed with a tri-block poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer. However, this molecule is water-soluble. Therefore, low-biofouling properties were not optimal. Recently, our group also focused on the design and application of di-block copolymers possessing a strong hydrophobic anchoring group and that is water-insoluble [Chiag et al., 2012]. The synthesis of this copolymer, poly(styrene)b-poly(ethylene glycol) methacrylate (PSm-b-PEGMAn), can be controlled in order to regulate the hydrophobic–hydrophilic balance and so, optimize the solubility in matrix polymer/solvent system as well as the final low-biofouling properties. Low-biofouling membranes are of major interests in numerous fields including water and wastewater treatment or food industry, but another potential field of interests could be microalgae harvesting. The interest for microalgae has grown in importance over the past few years owing to their remarkable aptitude to convert carbon dioxide into high value products [Chisti, 2007]. Once cultivated, microalgae should be harvested, which is referred as to the concentration of biomass, and which is often achieved by centrifugation as it is a very fast technique [Barros et al., 2015]. However, traditional centrifugation processes are energydemanding and eventually expensive. Therefore, in an effort to reduce costs without sacrificing performances (in terms of concentration factors), membrane technology has recently attracted considerable interest for microalgae harvesting [Bilad et al., 2014]. Yet, a major question remains unanswered related to the use of membranes for microalgae harvesting, as reminded by Bilad et al. (2014) who insisted on the need for fouling control and the use of antifouling membranes for microalgae harvesting. They reported in their related review that many membranes used were commercial. To the best of our knowledge, not many commercial membranes offer both excellent bulk and antifouling properties. In a very recent work, Hwang et al. applied in-situ modified PVDF membranes for microalgae recovery [Hwang et al., 2015]. They actually utilized novel mixed-matrix membranes containing hydrophilic copolymers such as F127, PVP and PEG400 in an attempt to reduce fouling of membranes by microalgae. Yet, this promising approach could be further improved using hydrophilic water-insoluble copolymer and it is believed that both high microalgae rejection (efficient harvesting) and reduced biofouling could then be reached. Inspired from the above knowledge in membrane formation processes and polymer synthesis, we present in this report our latest low-biofouling membranes formed by VIPS process using PS30-b-PEGMA68 copolymer as a surface-modifying molecule for microalgae harvesting. In this work, we investigate the membrane structure associated to the potential effect of copolymer on membrane formation, the membrane surface chemistry and the anti-biofouling properties. Especially, several proteins and bacteria, considered as major pollutants and biofoulants in waters, will be employed to assess the resistance to biofouling of our membranes. Furthermore, we run cyclic filtration tests, to evaluate the biofouling resistance in dynamic conditions, through the evaluation of flux recovery ratio after protein filtration, and compare performances of our membranes with those of commercial membranes. Finally, we test microalgae harvesting and demonstrate that these microfiltration membranes can offer efficient alternate to existing technologies.

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2. Materials and methods 2.1. Materials Polyvinylidene fluoride polymer (PVDF) (Mw ¼ 150,000 g mol  1) was purchased from Kynars and was washed with methanol and deionised water before use. PSm-b-PEGMAn amphiphilic additive was synthesized in our group, so that hydrophobic segments contain 30 repeat units of polystyrene (PS), and hydrophilic moiety possesses 68 repeat units of poly (ethylene glycol) methacrylate (PEGMA). Final molecular weight of diblock copolymer was 35,656 g mol  1 and its polydispersity index was about 1.28. Nmethylpyrrolidone (NMP), bought from Tedia, was directly used as solvent. Proteins used in this study (fibrinogen, FN, Mw E340,000 g mol  1; lysozyme, LY, Mw E14,300 g mol  1; bovine serum albumin, BSA, MwE66,000 g mol  1) were obtained from Sigma Aldrich. Deionized water was purified with a Millipore water purification system having a minimum resistivity of 18.0 MΩ cm. 2.2. 2 Methods 2.2.1. Preparation of solutions PVDF and PS30-b-PEGMA68 were blended in NMP solvent at 32 °C. The choice of this temperature is based on a previous study which showed that a low dissolution temperature permitted to optimize the mechanical properties of PVDF membranes prepared by VIPS, through the control of gelling process during phase separation. Total weight of solutions was 20 g. Solvent content was kept constant to 77 wt%, PVDF content varied between 19 wt% and 23 wt% while PS30-b-PEGMA68 was in the range 0–4 wt%. Solutions were stirred for at least 72 h, until homogeneous blend was obtained, and then allowed to rest until they stopped bubbling. Formulation of solutions is provided in Table 1. 2.2.2. Preparation of membranes Membranes were prepared by vapor-induced phase separation (VIPS) process (Fig. 1). At first, solutions were cast on a glass plate placed inside a glove box using a metallic casting knife. The initial thickness of solutions was 300 mm. Relative humidity (RH) was set to 7072% and temperature was 3072 °C. These conditions were set 1.5 h before casting the solutions. Exposure time to water vapors was controlled to 20 min, in order to ensure that phase separation over the whole polymeric system's thickness would happen in the humidity chamber and that hydrophobic interactions between PVDF matrix polymer and PS-b-PEGMA copolymer would be optimized. Then, newly formed membranes were immersed in a water bath in order to remove solvent. This step lasted 24 h, during which water was changed once to optimize membrane washing. Finally, drying of matrices at ambient temperature was performed. 2.2.3. Physicochemical characterization of PEGylated PVDF membranes Morphology (surface and cross-section) of PEGylated membranes was first investigated by scanning electron microscope Table 1 Composition of casting solutions, and XPS analysis of the surface chemistry of obtained membranes. Membrane ID

PVDF (wt%)

PS30-b-PEGMA68 (wt%)

NMP (wt%)

Virgin PVDF PS-b-PEGMA-1 PS-b-PEGMA-2 PS-b-PEGMA-3 PS-b-PEGMA-4

23 22 21 20 19

0 1 2 3 4

77 77 77 77 77

99

(SEM). A Hitachi S-3000 instrument was used and the accelerating voltage was set to 7 keV. Membranes were first mounted on sample holders with double-sided adhesive tape and sputtercoated with gold during 150 s, before being placed in the SEM observation chamber. Moreover, atomic force microscopy (AFM) technique was used to examine the surface state of membranes. Images were obtained using a JPK Instruments AG multimode (Germany), provided with a NanoWizard scanner. Images were obtained in air tapping-mode AFM using a commercial Si cantilever (TESP tip) having a  320 kHz resonant frequency. For all experiments, relative humidity was inferior to 40%. Surface chemistry of membranes was investigated by FT-IR and XPS analysis. FT-IR characterization was performed with a Perkin-Elmer Spectrum One spectrophotometer, using Zinc Selenide as an internal reflection element. Final spectra were obtained after averaging a number of 16 scans, captured at a 4 cm  1 resolution. A PHI Quantera SXM/Auger spectrometer with a monochromated Al KR X-ray source (1486.6 eV photons) was used. The analysis was performed using a software supplied by Service Physics, Inc. A hemispherical energy analyzer at pass energies ranging from 50 to 150 eV permitted to measure the energy of emitted electrons. All spectra were obtained at photoelectron take off angles of 45°, with respect to the membrane surface. Moreover, the peak maximum in the C 1s spectrum was set to 284.6 eV, in order to reference the binding energy (BE) scale. In addition, a Shirley background subtraction as well as a series of Gaussian peaks allowed fitting the high-resolution C 1s spectrum. Finally, mechanical properties of PEGylated membranes were determined. Tensile tests were conducted on a DMA 7e instrument (Perkin-Elmer). First, membrane samples (2 cm  0.5 cm) were cut from the as-prepared membrane sheet and their thickness measured at 5 different positions using a thickness tester. They were then mounted between the instrument clamps. The force applied to stress the membrane samples was gradually increased at a rate of 250 mN/min, starting from 60 mN. A Merlins software was used to record and analyze the data, until failure of the membrane samples. The porosity ε (%) of membranes was also evaluated using ethanol (Aldrich, ρEtOH ¼ 0.789 g/mL) as follows. Dry membranes were weighed (WD) and subsequently immersed in ethanol for 24 h. After removing the excess of ethanol from the surface, wet membranes were weighed (WW), and porosity assessed using the formula available in literature [Gu et al., 2006]. The value for the density of PVDF was taken to be ρPVDF ¼1.78 g/mL. That of PS30-b-PEGMA68 was evaluated from the knowledge of the density of styrene and PEGMA, and the number of repeat units forming the copolymer. It was found to be ρPS-bPEGMA ¼1.03 g/mL. As for the average pore diameter, it was evaluated with a capillary flow porometer (CFP-1500-AXEL, PMI) following an experimental protocol earlier described [Kao et al., 2008]. 2.2.4. Assessment of hydrophilic properties of PEGylated PVDF membranes Hydrophilic properties of membranes were assessed by measuring their static water contact angle and their hydration capability. As for water contact angle, an automatic contact angle meter (CA-VP, Kyowa Interface Science Co., Ltd. Japan) was used. A water droplet (4 mL) was dropped onto the surface of the membranes surface using a micropipette and the water contact angle was measured after 5 s. A similar procedure was done at 10 different positions, and the average was taken as the final value for the water contact angle of the sample. As for the hydration capability (mg/cm3), a disk of 1-cm-diameter was cut from the membrane sheet and weighed. Its thickness was also measured before being immersed in DI water for 24 h. After this period, residual water on the surface of the sample was gently wiped out with a soft tissue, and the membrane was then weighed again. The

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PS hydrophobic block PEGMA hydrophilic block

T = 30°C RH = 70% t = 20 min

VIPS chamber

Nonsolvent inflow Solvent outflow

PVDF/PS-b- PEGMA Membrane

PVDF/PS-b -PEGMA Solution Phase separation

Fig. 1. Schematic presentation of the vapor-induced phase separation process applied to the PVDF/PS-b-PEGMA system to prepare PEGylated PVDF membranes.

hydration capability volume, between the was repeated 5 times taken as the final considered.

was defined as the difference, per unit wet weight and the dry one. This operation for each type of membrane, and the average hydration capability of the membrane

2.2.5. Low-biofouling properties of PVDF membranes Assessment of the resistance of membranes to biofouling was done carrying out protein adsorption tests and bacterial attachment tests. For protein adsorption tests, three proteins were considered: bovine serum albumin (BSA), lysozyme (LY) and fibrinogen (FN). The experimental procedures have been detailed elsewhere [Venault et al., 2012]. Briefly for BSA and LY, membrane samples were incubated at about 25 °C in 1 ml of pure ethanol for 30 min, before being immersed 120 min in phosphate-buffered saline (PBS). After removing PBS, membranes were incubated with either BSA or LY solution for 2 h. Protein concentration, before potential adsorption by the membranes, was 1 mg/mL. In order to determine the remaining protein concentration in the incubation solution, the absorbance at 280 nm (for both BSA and LY) was measured (UV–vis spectrophotometer, PowerWave XS, Biotech). Finally, using an appropriate calibration curve, it was possible to deduce the total amount of protein adsorbed onto membranes' surfaces. To ensure reliable data, tests were performed three times. The adsorption of FN was studied using the ELISA test. As no change in the procedure has been performed, compared to our previous published works, one is invited to refer to one of these studies for further details [Chiag et al., 2012; Venault et al., 2012]. Concerning bacterial attachment studies, two species were used: Escherichia coli (EC, gram-negative) as well as E. coli modified with a Green Fluorescent Protein (EC-GFP, gram-negative). The protocol for genetic modification of EC leading to EC-GFP has been reported elsewhere [Hsiao et al., 2014]. Attachment of EC was studied using SEM, while that of EC-GFP was assessed using confocal microscopy. Both bacteria were cultured at 37 °C in a medium containing beef extract (3.0 mg/mL) and peptone (5.0 mg/mL) until the stationary phase was reached, corresponding to a concentration of 108 cells/mL and 107 cells/mL after a 12-h-incubation period for EC and EC-GFP, respectively. Membrane disks (1.3-cmdiameter) were then incubated at 37 °C with 1 mL of bacteria solution for 3 h. Then, membranes were washed 3 times with PBS. Adhesion of EC to surfaces was then immediately observed by SEM (Hitachi S-3000 instrument), after coating membranes with gold for 150 s. As for EC-GFP, membranes were observed by confocal laser scanning microscope (NIKON CLSM A1R instrument) mounted on a resonance scanner with 200  magnification. Images were taken at λex ¼ 488 nm/λem ¼520 nm. 2.2.6. Permeability and resistance to biofouling during filtration In order to evaluate the water permeability of membranes, we used a dead-end cell filtration system connected with a

compression nitrogen cylinder and a reservoir which volume capacity is 5 L (water tank) or 3 L (protein tank). The filtration cell has an inner diameter of 47 mm. All membranes were first immersed in ethanol for 30 min, to allow their swelling. Then, the membrane tested was placed in the filtration cell and an overpressure cycle (1.5 atm) was run for 30 min at ambient temperature (about 25 °C) using DI water. Afterwards, pressure was reduced to 1 atm, and water flux (Jw,0) measured at steady-state, for 1 h, using a Mettler Toledo balance connected to a computer equipped with a Mettler Toledo software, allowing to record online the weight variations of permeate. The water permeability of membranes was then obtained from the water flux. Then, water was replaced by a BSA aqueous solution (1 g/L). Filtration was performed similarly, maintaining the feed pressure to 1 atm. The BSA cycle was run for 1.5 h. Subsequently, membranes were washed with DI water to remove loosely adhering proteins (reversible fouling). Protein solution was replaced by water and water permeation flux (Jw,1) recorded for one hour. Another BSA/water cycle was conducted, leading to Jw,2. Experiments were repeated 3 times for each type of membrane. Notice that a commercial hydrophilic PVDF membrane (pore size: 0.1 μm, Millipore) was also used as a control. The different ratios (flux recovery ratio FRR, reversible flux decline ratio DRr and irreversible flux decline ratio DRir) reported in this study are obtained according to the following equations:
ð1Þ FRR ¼ J w2 =J w0  100% DRr ¼ DRir ¼




 J w2  J BSA2 =J w0  100%

ð2Þ




 J w0  J w2 =J w0  100%

ð3Þ

2.2.7. Microalgae culture Microalgae (Chlorella sp.) was cultured in an artificial wastewater medium according to Feng et al.’s work (2011). The culturing environment was kept at room temperature (25 °C) under a light intensity of 9000–12,000 lx (measured by Lux Meter TM 10000, TOMEI, Tokyo, Japan. The rotational speed in the 1–2 L cultivation Erlenmeyer flask (DRRAN Group, Mainz, Germany) was set to 150 rpm. The initial concentration of inoculation was set at 0.2 optical density, and cultured for around 7 days until the final concentration reached over 1 g dry weight per liter. The measurement of growth and adjustment during cultivation were done according to our previous study [Huang and Su, 2014]. After completion of algal culture, the algae were harvested by filtration. 2.2.8. Microalgae filtration Membranes (diameter: 47 mm) were pre-wetted with ethanol until it becomes translucent and they were placed in dead-end filtration cell. Then, an overpressure water cycle was run at 1 atm, for 30 min. Afterwards, pressure was decreased to 0.5 atm and DI

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Fig. 2. Scanning electron microscopy characterization of virgin and PEGylated PVDF membrane.

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water flux was recorded for 30 min. Then, the DI water feed solution was replaced by a microalgae solution at a concentration of 1.2–1.4 g/L, and flux recorded similarly for 1 h. At the end of the cycle, the membrane was removed from the filtration cell, flushed with DI water and soaked in a DI water bath (500 mL) for 30 min to wash it. A second water-microalgae filtration cycle was run after the washing procedure, according to a similar protocol as that used during the first cycle. Membrane was then washed again, and a final DI water filtration cycle run to evaluate the final flux recovery ratio. In order to evaluate the microalgae retention according to the following equation, the optical density at 440 nm of the permeate was measured using a BioTech instrument:
Ralgae ¼ 1 ODpermeate =ODf eed  100 ð4Þ 2.2.9. Carbon impact The carbon impact was evaluated following the guidance of ISO14040, and the inputs and outputs of each step with four main stages (definition of purpose and scope, inventory analysis, impact assessment, interpretation) were done. The cost of electric power supply, and greenhouse gas emission parameters were according to Taiwan Power Company and Energy Board. SimaPro (SimaPro UK) was applied for standardization and parameter modeling.

3. Results and discussion 3.1. Morphological characterization of PVDF/ PS30-b-PEGMA68 membranes The morphology of membranes was investigated by SEM and AFM and results are provided in Figs. 2 and 3, respectively. All membranes exhibit a nodular structure. Secondly, PS30-bPEGMA68 did not affect much membrane formation in the conditions tested. SEM and AFM images of virgin and PEGylated membranes all unveil nodular structures, with quite similar size of polymer domains (nodule size). Furthermore, there is no change regarding the crystalline polymorph forming these spherulites (Fig. 4a). The β-polymorph dominates, as seen by the presence of the stretching band at 840 cm  1. α-Polymorph, that

would have supported a change of gelling process cannot be detected on the FT-IR spectra of the lower wavenumbers region since no absorption band can be seen at 763 cm  1 [Li et al., 2010]. The type of morphology observed arises mainly from a crystallization-gelling process. As mass transfers are relatively slow in VIPS process, the system remains in the crystallization region of the related quaternary phase diagram long enough to allow the growth of crystalline nuclei, eventually leading to nodules. Therefore, crystallization clearly dominates the formation mechanisms of these membranes. Yet, we have reported earlier that a block copolymer of the same family (but with a different number of hydrophilic and hydrophobic repeat units, different hydrophobic moieties/hydrophilic moieties ratio and different molecular weight) could lead to a change of membrane morphology [Venault et al., 2014a]. However, in that previous work, the total PVDF concentration was kept constant while the copolymer concentration was increased, meaning that the viscosity of the polymeric system increased, when comparing the virgin PVDF solution to the PVDF/PS-b-PEGMA ones. Consequently, viscous forces prevented the growth rate of polymer nodules, finally leading to bicontinuous structures. An essential difference with the present work, besides the characteristic of the copolymer, is the relative polymer/copolymer/solvent composition. Here, the PVDF concentration was decreased while the total dried matter content was kept constant to 23 wt%. As PVDF is replaced by PS30-b-PEGMA68 exhibiting a lower Mw (150,000 g mol  1 vs 35,656 g mol  1), the viscosity of the polymeric system is actually reduced (Appendix A). Therefore, the growth of polymer domains in the PEGylated solution is facilitated compared with that of nodules in the virgin system, eventually explaining the retainment of the nodular structure. Furthermore, the decrease of PVDF concentration and increase of PS30-b-PEGMA68 have consequences on the thermodynamic stability of the polymeric system. Given its amphiphilic nature, PS30b-PEGMA68 enables faster penetration of water within the matrix during phase separation, which in turns leads to a faster membrane formation. Consequently, the growth of polymer nodules is again limited. In summary, the replacement of PVDF by PS30-bPEGMA68 in the casting solution leads to a decreasing of viscosity

PS-b-PEGMA-1

Virgin PVDF

2.261 µ m

1.325 µ m

0 µm

0 µm PS-b -PEGMA-3

2.738 µ m

0 µm

PS-b-PEGMA-4

3.276 µ m

0 µm

Fig. 3. 30 μm  30 μm atomic force microscopy characterization of virgin and PEGylated PVDF membranes.

A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

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PS-b -PEGMA-1

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virgin PVDF

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-1

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-1

Wavenumber (cm )

Wavenumber (cm )

Fig. 4. FT-IR characterization of virgin and PEGylated PVDF membranes. (a) Highlight on the lower wavenumbers region evidencing the nature of the crystalline polymorph forming the membranes; (b) Spectra over the whole wavenumbers range. The oval shapes highlight the major changes on spectra due to the addition of PS-b-PEGMA.

C1s

O1s

[H -C-H, C -O] PS -b-PEGMA-4

60

[O =C,O-C]

[F-C -F]

Carbon

[C-H]

Oxygen

Fluorine

50

Atom content (At%)

PS-b-PEGMA-3

PS -b-PEGMA-2

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Binding energy (eV) Fig. 5. X-ray photoelectron spectroscopy characterization of virgin and PEGylated PVDF membranes (a) Core-level spectra; (b) atom content at the surface of membranes as a function of copolymer concentration in the casting solution.

which facilitates the growth of nodules, while it enhances phase separation kinetics which also hinders crystalline domains growth. These two effects must be taken into account in the analysis of formation mechanisms at play. Their simultaneous analysis permits to reasonably explain why nodules are observed for PEGylated membranes but also why they are not significantly larger than those of virgin membranes.

3.2. Chemical characterization of PVDF/PS30-b-PEGMA68 membranes To support the presence of PS30-b-PEGMA68 onto the surfaces, FT-IR and XPS chemical characterizations were also performed. FT-IR spectra obtained are displayed in Fig. 4. FT-IR analysis of PVDF films is available in literature so that characteristic absorption bands of PVDF will not be reminded here [Boccaccio et al.,

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2002]. Instead, we focus on absorption bands due to the presence of PS30-b-PEGMA68. From the analysis of the spectra, peaks are clearly found for PVDF/PS30-b-PEGMA68 membranes, from a 2 wt% concentration, in the wavenumber range 3500–3750 cm  1, as well as at 1730 cm  1. These signals correspond to the stretching of the –OH group and to that of the carbonyl group held by PS30-bPEGMA68, respectively. Despite a quite low intensity of the peaks, FT-IR tends to confirm the effective modification of membranes by the copolymer. Performing XPS analysis permitted to confirm that PS30-b-PEGMA68 copolymer was present at the surface of membranes. Fig. 5a presents XPS C1s core level spectra while Fig. 5b displays the atom content at the surface of membranes as a function of PS30-b-PEGMA68 content. Characteristics peaks appearing on C1s core level spectra could be assigned. First, one may want to refer to literature to identify peaks on XPS spectra of virgin membranes [Liu et al., 2009]. As for modified membranes, a shoulder can be seen at a binding energy (BE) in the range 284.1– 284.6 eV, assigned to C–H species. Furthermore, two main peaks were observed, as on the spectrum of virgin PVDF membrane. The first one, located at BE of 285.9 eV, corresponds to the contributions of both H–C–H and C–O species while the second large peak, observed at about 290.6 eV was attributed to F–C–F species. Also, Fig. 5b supports results displayed in Fig. 5a. It clearly showed that the oxygen content at the surface of polymer membranes increased with copolymer concentration in the casting solution, since PS30-b-PEGMA68 contains numerous C–O and C¼ O functional groups on its backbone. The regular increase of oxygen content also proves that the in-situ modification process was well controlled. Logically, the fluorine content was found to decrease, while the carbon content was constant. One may have noted the presence of a slight oxygen peak onto the spectrum of PVDF membrane. It arose from the presence of oxygen impurities during the analysis, and obviously not from the presence of oxygen atoms in the membrane structure. The gradual qualitative increase of the intensity of the oxygen peak on XPS spectra, as well as those of hydroxyl and carbonyl groups on FT-IR spectra, reveal a good control of membrane formation process.

to weaker inter-chain interactions during membrane formation. A way to address this issue would be to increase the PVDF concentration in the matrix. It was not done herein, as membranes could still be handled without failure during tests. Furthermore, compressive forces are at play during filtration, rather than tensile forces as in the present test, yet often used to establish the various mechanical properties of polymeric membrane. Literature is not well documented on the mechanical properties of PVDF membranes prepared by phase separation form the vapor phase. However, comparison of our results with a few recent studies such as those of Li et al. (2010) or Peng et al. (2012) unveil that typical tensile stress ranges between 0.5 and 1.2 MPa for membranes prepared from a 20 wt% polymer concentration and exposed 20 min to water vapors. Our results fall in this range. 3.4. Hydration properties of PVDF/PS30-b-PEGMA68 membranes Even though the effect of amphiphilic additive on membrane formation and the surface chemistry of membranes is slight, PS30b-PEGMA68 must have been retained within the porous matrix or at the surface as it is water-insoluble, and stable, as demonstrated for PVDF membranes coated by a similar copolymer and applied in MBR process (Lin et al., 2013). In this respect, hydration properties of PVDF/PS30-b-PEGMA68 membranes were expected to be better than those of virgin membranes. Water-contact angles as well as hydration capability were measured, and related results are presented in Fig. 7. If only a slight decreasing of water contact angle was measured, from 122° to from 118°, hydration capacity was importantly enhanced, up to 500 mg/cm3 for membrane containing 4 wt% PS30-b-PEGMA68. The quite high value of water contact angle, despite the presence of amphiphilic additive, can be explained by the nodular morphology of PVDF/PS30-b-PEGMA68 membranes, as well as by their high porosity. Water contact angle is affected by both the chemical composition and the roughness of a surface. Herein, the physical parameter was not favorable at all,

6x10

3.3. Characterization of the physical properties of PVDF/PS30-bPEGMA68 membranes Tensile stress (MPa)

Essential physical properties including the mean pore diameter, the porosity and the mechanical properties of membranes were evaluated. Membranes with large pores and an average porosity of more than 70% were obtained, suitable for microfiltration (Table 2). Also, the porosity slightly increases with the PS30-bPEGMA68 content, as in the same time, the matrix polymer content (PVDF) decreases. Meanwhile, the average pore size increased, consistently with the decrease of PVDF content. Notice however that the mechanical properties of membranes tended to decrease with copolymer content (Fig. 6). This was due to the difference of Mw between PVDF and PS30-b-PEGMA68. As explained earlier, when switching from virgin PVDF membrane to PEGylated PVDF matrix, the total dry matter content was kept constant to 23 wt% but a lower molecular weight for PS30-b-PEGMA68 was associated

virgin PVDF PS- b-PEGMA-1 PS-b-PEGMA-2 PS-b-PEGMA-3 PS-b-PEGMA-4

5x10

4x10

3x10

2x10

1x10

0 0

50

100

150

200

250

300

Tensile strain (% elongation) Fig. 6. Effect of amphiphilic PS-b-PEGMA copolymer content on mechanical behavior of membranes.

Table 2 Physico-chemical characterization of virgin and PEGylated PVDF membranes. WCA: water contact angle; HC: hydration capability; h: thickness; RMS: roughness deviation coefficient; ε: porosity; Ø: mean pore size (μm); E: Modulus of elasticity; TS: tensile strength at break. Membrane ID

WCA (deg)

HC (mg/cm3)

h (mm)

RMS (nm)

ε (%)

Ø (μm)

E (MPa)

TS (MPa)

Virgin PVDF PS-b-PEGMA-1 PS-b-PEGMA-2 PS-b-PEGMA-3 PS-b-PEGMA-4

122 72 119 72 119 72 118 72 117 72

0 107 4 1627 27 4497 42 5337 20

112 72 105 73 103 71 102 71 105 71

730.3 301.2 619.0 776.1 744.6

72 7 1 71 7 2 75 7 1 77 7 1 79 7 2

0.26 – 0.42 – 0.82

19 13 17 10 8

0.63 0.49 0.42 0.29 0.26

A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

Fig. 7. Effect of amphiphilic PS-b-PEGMA copolymer content on hydration properties of membranes. 140

Relative protein adsorption (%)

FN

BSA

LY

120

100

80

60

40

20

0

vir

gi

n

PV

1

-b

PS

M EG

-P

P -b-

PS

A-

A-

A-

M EG

4

3

2

A-

DF

M EG

P

P

-b

PS

M EG

-b

PS

Fig. 8. Effect of amphiphilic PS-b-PEGMA copolymer content on resistance of membranes to protein adsorption.

as suggested by the values of root mean square (RMS) surface roughness coefficients presented in Table 2, obtained from Fig. 3, and by the high porosity of membranes (Table 2), promoting the entrapment of air. Nevertheless, membranes could readily entrap water after a long contact with aqueous media, as proved by the hydration capacity results measured after a 24-h-immersion in DI water. The swelling behavior associated to large hydration capacity values is due to the establishment of a hydrated layer around the hydrophilic head of the copolymer spread over the surfaces and within the matrix. This property in particular is essential to resist nonspecific biofouling caused by proteins and bacteria. 3.5. Protein adhesion resistance of PVDF/PS30-b-PEGMA68 PVDF membranes Optimal hydration of membranes is a pre-requisite to the formation of anti-adhesive surfaces resisting the adsorption of proteins (Rana and Matsuura, 2010). In this work, in order to evaluate the resistance to protein adsorption, three distinct proteins were used: bovine-serum albumin, lysozyme and fibrinogen. Associated results are displayed in Fig. 8. Firstly, resistance to protein adsorption is improved with copolymer additive content. As PS30b-PEGMA68 allows water binding around hydrophilic moieties and

105

water entrapment within the brushes at the surface of and inside the membrane, hydrophobic–hydrophobic interactions between proteins and PVDF material are hindered. Secondly, a ranking could be established. Resistances to BSA and LY adhesion were the best, while performance concerning FN was not as good with a final adsorption for the supposedly best low-biofouling membrane (PS-b-PEGMA-4) reduced to about 70%. If we consider that protein adsorption arises from a combination of at least four factors that are (i) the topography of the surface, (ii) the hydrophilic/hydrophobic balance of the protein, (iii) its number of hydrophobic moieties per molecule and (iv) its capability to diffuse to the adsorption sites contained at the surface and within the matrix, then the three last parameters have to be investigated in priority in the present case, since similar surface morphologies were obtained (Figs. 1 and 2). Diffusivity of species varies in a complicated fashion with molecular dimensions, but it is accepted that large molecular weight molecules will diffuse slower than smaller ones. FN has the largest molecular weight among all the proteins studied. Therefore, its diffusivity through the pores to the adsorption sites should not be facilitated compared to those of BSA and LY. However, large molecular weight, as it is the case for FN (340,000 g mol  1), implies that more hydrophobic segments/ molecule can potentially interact with the membrane materials which can reasonably explain why the least adsorption reduction was obtained with this protein. It is also worth insisting on the fact that the morphology obtained is not favorable at all for anti-adhesive properties. Smooth surfaces are believed to provide less potential anchorage groups to biofoulants than rougher ones, regardless their chemical nature. Yet, by comparing the present results to data previously reported concerning the preparation of smooth PVDF/PS30-bPEGMA68 membranes using a similar copolymer, one will see that comparable results were obtained herein regarding the adsorption of BSA and LY (Venault et al., 2014b). As copolymer concentration was the same, the improvement observed in the present work arises from the different processes used. Despite the obtaining of rougher surfaces by VIPS, the slowness of mass transfers involved is believed to have promoted the copolymer diffusivity toward the interface during membrane formation. Therefore, the copolymer density onto the surface of membranes surface prepared by VIPS is probably larger than that obtained in the case of membranes prepared by wet-immersion process, and consequently, comparable anti-protein adhesion properties are measured despite the very rough surfaces obtained by VIPS. 3.6. Resistance of PVDF/PS30-b-PEGMA68 membranes to bacterial attachment In water treatment, feed can contain numerous bacteria which can be trapped within the membrane structure by steric hindrance, or interact with the membrane material via hydrophobic interactions between their cell-wall and the polymer membrane. In both case, it leads to the formation of a biofilm, one reason of irreversible fouling. A way to avoid depth filtration mechanism and the entrapment of foulants by molecular sieving is to reduce the pore size of the membranes, for instance by increasing the polymer concentration. But this strategy is not suitable for bacteria as some micro-organisms are deformable and can permeate through smaller pores than their own characteristic dimensions. This is the case for instance of E. coli, a common bacteria used to study the antibacterial properties of materials as it is a gram negative bacterium whose membrane is made of a thin and deformable peptidoglycan layer (Lebleu et al., 2009). Therefore, one has to focus mainly on annihilating the low-energy interactions established between the bacteria and the membrane material. In other words, surface (and bulk) chemistry of PVDF

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Virgin PVDF

PS-b-PEGMA-1

PS-b-PEGMA-2

PS-b-PEGMA-3

PS-b-PEGMA-4

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

10 µm

Fig. 9. Effect of amphiphilic PS-b-PEGMA copolymer content on resistance of membranes to attachment of Escherichia coli.

membranes should be modified. By making use of PS30-bPEGMA68, we showed that a hydrophilic interface could be generated, which was correlated with an important resistance to protein adhesion. It was then believed that the matrices would be able to efficiently resist bacterial attachment. Two species, E. coli (EC) and E. coli with Green Fluorescent Protein (EC-GFP), were tested. EC-GFP is a genetically modified bacteria species, therefore different from EC as its cell-wall has been modified. Fig. 9 presents the SEM images taken right after bacterial attachment using EC, while Fig. 10 displays results related to ECGFP. From SEM images, it is clear that the resistance to E. coli was greatly enhanced using PS30-b-PEGMA68 as a matrix modifier. Three SEM pictures are presented for each membrane, taken at three different locations of the surfaces. Adhesion of bacteria onto virgin PVDF membranes was facilitated by the hydrophobicity of the membrane material and the roughness of the surface. It led to the beginning of biofilm formation. For 1 wt% additive content, EC attachment was still important but biofilm formation was not as severe as for virgin membrane. Important reduction of bacteria adhesion was observed from 3 wt% PS30-b-PEGMA68 content, highlighting the efficiency of membrane modification. Similarly, confocal images revealed that EC-GFP attachment was importantly reduced for membranes containing 3 and 4 wt% copolymer. Additionally, a special arrangement of bacteria into the matrix was observed (as these confocal images arise from depth analysis). Furrows observed could suggest mediation of bacterial attachment by the first colonies entrapped in the membrane occurred or that the copolymer distribution within the membrane material was heterogeneous. The first hypothesis is supported by images of virgin PVDF membrane, as it does not contain any copolymer. The later observation would suggest that a copolymer distribution gradient exists along a same xy plane, but this has never been mentioned, to the best of knowledge. Only a concentration gradient in the z-direction (from permeate to retentate side of the membrane) was previously mentioned (but not clearly evidenced). Another possible explanation would be that bacteria were trapped between clusters of polymeric nodules seen on SEM images (Fig. 2). Further, one should note that there is a close relationship between the hydration capacity of membranes and their ability to

resist bacterial adhesion. Indeed, from 3 wt% PS30-b-PEGMA68 content for which membrane hydration is greatly enhanced, bacteria barely adhered onto their surfaces. Water molecules trapped and bounded within the PEGylated brushed provided an efficient barrier to bacterial adhesion. 3.7. Resistance of PVDF/PS30-b-PEGMA68 membranes to biofouling during BSA filtration After assessing the low-biofouling properties of PVDF membranes in static condition, dead-end filtration tests were carried out, using BSA solution (1 mg mL  1). In these tests, after running an overpressure water cycle, several water-BSA cycles were run, to evaluate the efficiency of the block copolymer to repel proteins, despite the application of a drag force, and thus to optimize flux recovery. Fig. 11 displays the evolution of flux as a function of time, and two representations are provided. Fig. 11a shows the evolution of flux with actual dimensions, while Fig. 11b presents dimensionless flux. As initial water permeability of membranes was different, Fig. 11b enables to readily evaluate the efficiency of block copolymer. In addition Fig. 12 provides essential ratios associated to the antifouling properties of membranes. First, it is seen that membranes exhibit high pure water permeability, in the MF range, clearly in accordance with the porous structure observed in Fig. 2 as well as with the characterization of pore size presented in Table 2. It is also worth noticing the effect of copolymer on initial water permeability: our results indicate that the water flux is increased by a 2.6-fold factor, when comparing virgin PVDF membrane with PS-b-PEGMA-4 membrane. This observation has to be correlated to the important hydrophilic effect of the copolymer (Fig. 7) which promoted water transport, as no obvious structural change was earlier observed. Then, an important decreasing of flux is observed when filtrating BSA solution. After cleaning the membrane and re-testing water permeability, the nonfouling effect of block copolymer is seen in dynamic conditions, as flux recovery for PS-b-PEGMA-4 membrane was measured to be 83% after the first cycle and 98% after the 2nd cycle, while it was only 13% after the 1st cycle and 63% after the 2nd cycle with virgin membrane. The global FRR shown in Fig. 12 was then 81.5% for PS-b-PEGMA-4 membrane while it was only 8.1% for

A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

107

Fig. 10. Effect of amphiphilic PS-b-PEGMA copolymer content on resistance of membranes to attachment of Escherichia coli with GFP.

virgin membrane. Most of biofouling occurring on commercial hydrophilic, virgin PVDF, PS-b-PEGMA-1 and PS-b-PEGMA-2 membranes is irreversible while it is reversible when PS-bPEGMA-3 and PS-b-PEGMA-4 membranes are used (see DRr and DRir). These results and observations support those obtained in static conditions, but are somewhat even more convincing regarding the effect of copolymer on antifouling properties of membranes, as the drag-force involves during filtration facilitates pore blocking of membrane.

3.8. Microalgae filtration using PVDF/PS30-b-PEGMA68 membranes In this test, we were interested in two particular aspects. First, we wanted to evaluate the extent of biofouling by microalgae to assess the nonfouling power of membranes using bioorganisms different from biofoulants often tested such as humic acids (Hwang et al., 2013; Chang et al., 2015) or bacteria (Lebleu et al., 2009; Venault et al., 2014b). Second, we cared for the maximum rejection achievable with such membranes. Indeed, as mentioned

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A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

4

1.4x10

Commercial

Commercial

4

4

Virgin PVDF

1.0x10

Virgin PVDF

PS-b-PEGMA-1

PS-b-PEGMA-1

1.2x10

PS-b-PEGMA-2

4

1.0x10

PS-b-PEGMA-3

3

PS-b-PEGMA-3

8.0x10

Flux(kg/m2.h.atm)

Flux (Kg/m2 h.atm)

PS-b-PEGMA-2 PS-b-PEGMA-4

3

8.0x10

3

6.0x10

PS-b-PEGMA-4

3

6.0x10

3

4.0x10

3

4.0x10

3

2.0x10 3

2.0x10

0.0

0.0 0

30

60

90

120

150

180

0

210

30

60

90

Time (min)

150

180

210

Virgin PVDF

Virgin PVDF

PS-b-PEGMA-1

PS-b-PEGMA-1 PS-b-PEGMA-2

PS-b-PEGMA-2

0.8

Dimensionless flux

PS-b-PEGMA-3

0.8

Commercial

1.0

Commercial

1.0

Dimensionless flux

120

Time(min)

PS-b-PEGMA-4

0.6

0.4

PS-b-PEGMA-3 PS-b-PEGMA-4

0.6

0.4

0.2

0.2 0.0

0.0

0

30

60

90

120

150

180

0

210

30

60

90

Time (min) Fig. 11. Filtration of BSA protein by virgin and PVDF/PS-b-PEGMA membranes: assessment of antifouling properties in filtration operation (a) with dimensions and (b) dimensionless.

DRr

FRR

90

90

80

80

70

70

60

60

50 40

20

20

10

10

0

c

0

gin

vir

P

G PE bS-

4 A-

2 AM P

G PE bS-

DRir

40 30

F

DRr

50

30

D PV

Microalgae filtration

100 DRir

Ratio (%)

Ratio (%)

FRR

l cia er m om

150

180

210

Fig. 13. Filtration of microalgae by virgin and PVDF/PS-b-PEGMA membranes: assessment of antifouling properties in filtration operation (a) with dimensions and (b) dimensionless.

BSA filtration

100

120

Time (min)

M

c

c er m om

ial vir

gin

PV

DF

P

G PE bS-

2 AM P

G PE bS-

4 AM

Fig. 12. Analysis of antifouling properties of membranes after filtration of BSA solution and microalgae solution.

A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

in the introduction of this work, filtration is believed to be a better option than centrifugation, which probably requires a higher energy-demand as shown later, if a high rejection can be reached. The as-prepared membranes have large pores as observed in Figs. 2 and 3, in the lower range of MF, while the size of microalgae ranges between 1 and 10 mm. Therefore, these membranes may permit to achieve a suitable rejection, useful for concentrating microalgae enough and subsequently re-using them in bioenvironmental studies. Filtration results are presented in Fig. 13 and the analysis of flux recovery ratio, reversible flux decline ratio and irreversible flux decline ratio is presented in Fig. 12. Again, we plotted the actual values obtained for flux, but to better assess the nonfouling properties of block copolymer and also better compare membranes, we represented the dimensionless flux over filtration time. Results are very encouraging as far as the flux recovery with modified membranes is concerned. This is pretty clear that PS-bPEGMA importantly mitigates biofouling by microalgae, as it did

with BSA. The maximum flux recovery was obtained for PS-bPEGMA-4 membranes (76.91%), which suggests that surfaces remained pretty clean and that most of biofouling was reversible, unlike that measured for virgin or commercial membrane. Comparison of SEM pictures of virgin PVDF and PS-b-PEGMA-4 membranes before and after filtration (Fig. 14) shows the formation of a cake layer on the surface of virgin PVDF membrane, partially hiding the initial porous structure. On the contrary, there was almost no difference observable regarding the surface of PS-bPEGMA-4 membrane, before and after filtration of microalgae. From there, we could conclude that the first objective was reached. One will have noticed from Fig. 13 that improvement of fouling resistance is gradual in the case of Microalgae filtration, while it was sudden in the case of BSA filtration (Fig. 11). This is ascribed to the different nature of foulants. Microalgae is a microfoulant while BSA, a protein, is a nanofoulant, meaning that different scales are involved in fouling resistance. In other words, as Microalgae are larger, resistance to their attachment is more readily achieved,

Fig. 14. Scanning electron microscopy characterization of virgin and PS-b-PEGMA-4 membranes before and after filtration of microalgae.

100

Rejection ratio (%)

80

60

40

20

0

Co

m

m

er

cia

l Vi

r

n gi

PV

PS

-b

PS

M

M

M

G PE

G PE

G PE

-b

A-

A-

A-

A-

M

G PE

4

3

2

1

DF

PS

-b

109

Feed solution Permeate

-b

PS

Fig. 15. Separation performances of microalgae by commercial and as-prepared VIPS membranes.

110

A. Venault et al. / Chemical Engineering Science 142 (2016) 97–111

Table 3 Evaluation of energy demand and carbon emission for the membrane filtration process used and centrifugation process. Data reported per run. Process

Power consumption (W)

Carbon emis- NOx emission (kg) sion (kg)

SOx (kg)

Membrane filtration Centrifugation

498.4

343.92

12.83

4.54

501.7

346.15

12.91

4.57

while BSA can stick to the membrane at a nanoscale. Therefore, to resist nanofouling, the density of nonfouling moieties should be larger than that good enough to resist microfouling. As for the second objective, Fig. 15 highlights that all membranes tested (commercial hydrophilic PVDF and as-prepared membranes) enabled to reject more than 99.7% of the microalgae. Therefore, membranes chosen are appropriate to efficiently concentrate microalgae, and this technology may offer a suitable alternative to centrifugation. This was eventually supported by calculating the energy demand and the carbon impact of two processes, centrifugation on one hand and filtration on the other hand (Table 3). For centrifugation process, we evaluated the energy demand and carbon impact using a 6000-rpm-rotational speed, a 2-h-process and a room temperature-process, as usually done in our laboratory on a Hitachi type centrifuge (model CF15R). The consumption of total watt per run was 501.7 and 498.4 for centrifugation and filtration, respectively, while emission of carbon dioxide was 344.15 kg and 343.92 kg, respectively. Therefore, membrane harvesting can slightly decrease electric cost, carbon impact and emission of NOx and SOx to 0.6%, compared to harvesting by centrifugation, which is a non-negligible improvement, especially today as global warming is a major environmental concern.

4. Conclusion This work aimed at further investigating the antifouling properties of PVDF/PS-b-PEGMA membranes formed by VIPS process, and their potential as material for microalgae harvesting. After a complete physicochemical characterization of the matrices used, a number of key points were highlighted: 1. Despite a rough nodular structure, membranes prepared from a casting solution containing 4 wt% copolymer (PS-b-PEGMA-4) were extremely efficient to resist nano-biofouling by three different proteins in static conditions. Hence, adsorptions of BSA, lysozyme and fibrinogen were reduced to 7%, 15% and 31% the limitation of virgin membrane; 2. Membranes prevented the adsorption of bacteria (E. coli species), that is, microbiofouling was efficiently mitigated; 3. Biofouling by BSA in dynamic condition was also importantly reduced, as a flux recovery ratio over 90% was measured with PS-b-PEGMA-4, higher than that of the hydrophilic commercial PVDF membrane tested; 4. Membranes efficiently resisted biofouling by microalgae during microalgae harvesting, with a FRR found to be 76.91%, again higher than that obtained with the control membrane; 5. The filtration process with these as-prepared PVDF/PS-bPEGMA membranes used may offer an ideal alternate method to centrifugation, as very high rejection obtained (over 99.7%) leads to very high concentration of microalgae in the retentate. Yet the process used has a lower electric cost and carbon impact than centrifugation, which is of great importance for environmental sound reasons.

Appendix Viscosity measurements are presented in the supporting information section.

Funding This project was funded by the Ministry of Science and Technology, Taiwan (former National Science Council) through the projects NSC103-2221-E-033-074, MOST 104-2221-E-033-066-MY3 and MOST104-2623-E-033-002-ET.

Conflict of interest The authors declare that they have no conflict of interest.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2015.11.041.

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