membranes

Article Impacts of on the Stability and Separation Performance of Polymeric PVDF-Based Membranes at Mild and High Concentrations: An Experimental Study

Kayode H. Lasisi 1,2 , Weihao Yao 1,2, Temitope F. Ajibade 1,2, Huali Tian 1,2, Fang Fang 1 and Kaisong Zhang 1,* 1 Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China; [email protected] (K.H.L.); [email protected] (W.Y.); [email protected] (T.F.A.); [email protected] (H.T.); ff[email protected] (F.F.) 2 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-592-6190782

 Received: 23 October 2020; Accepted: 24 November 2020; Published: 27 November 2020 

Abstract: This study investigated the effects of an aqueous acidic solution at typical concentrations on polymeric polyvinylidene fluoride (PVDF)-based membranes. Flat-sheet PVDF-based membranes were completely embedded in sulfuric acid at varying concentrations. The effect of the acid concentration after a prolonged exposure time on the chemical, mechanical and physical properties of the membrane were checked via FE-SEM, EDX (Energy-Dispersive Spectrometer), FTIR, XRD, tensile strength, zeta potential, contact angle, porosity, pure water flux measurement and visual observation. The result reveals prompt initiation of reaction between the PVDF membrane and sulfuric acid, even at a mild concentration. As the exposure time extends with increasing concentration, the change in chemical and mechanical properties become more pronounced, especially in the morphology, although this was not really noticeable in either the crystalline phase or the functional group analyses. The ultimate mechanical strength decreased from 46.18 0.65 to 32.39 0.22 MPa, ± ± while the hydrophilicity was enhanced due to enlargement of the pores. The flux at the highest concentration and exposure period increased by 2.3 times that of the neat membrane, while the BSA (Bovine Serum Albumin) rejection dropped by 55%. Similar to in an alkaline environment, the stability and performance of the PVDF-based membrane analyzed in this study manifested general deterioration.

Keywords: polyvinylidene fluoride; embedded membranes; sulfuric acid; stability; deterioration

1. Introduction For several decades, membrane technology has proven efficient in the process of water treatment and separation techniques via the effective use of polymeric membrane materials [1–7]. These polymeric membranes have been reckoned to be cheaper and more efficient than other membranes because of their flexibility in spinning them into either spiral-wound or hollow-fiber modules. They have also displayed improved filtration performance and efficiency [8–11]. Most of the widely used commercial polymeric membranes materials for fabrication are polyimide (PI), polysulfone (PS), cellulose acetate (CA), polyamide (PA), polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF) and polyethylene (PE). Out of these aforementioned materials, PVDF has held and still holds the greatest prospect in this class of especially for both microfiltration (MF) and ultrafiltration (UF) applications owing to its numerous advantages [12–14]. As a semi-crystalline having

Membranes 2020, 10, 375; doi:10.3390/membranes10120375 www.mdpi.com/journal/membranes Membranes 2020, 10, 375 2 of 22 both a crystalline and amorphous phase, it exhibits high thermal stability, good mechanical properties, simple production flexibility and excellent chemical and oxidation resistances against strong acids, bases and oxidants [15–21]. In addition, selecting solvents for PVDF is effortless when compared with PTFE and PP, thus making it the preferred option in membrane applications [14,22]. One major setback of PVDF membranes, despite all their exciting characteristics, is their high hydrophobicity which stands as a great compromise between membrane selectivity and water flux, thus increasing fouling occurrences [10,23,24]. Several studies have been carried out to improve on this limitation via several methods such as surface modification/physical adsorption [25–29], hydrophilic polymer blending [30–32], plasma or chemical modification [33–35], graft polymerization [23,35,36], inorganic particle blending [37–39] and others. PVDF, as a versatile polymeric membrane, has been used both at laboratory and industrial scales under various conditions. Some of these applications include their use in mild or harsh environments via their exposure to chemicals which are harmful to the membrane materials and can alter their structure. This, after a prolonged period of operation, often leads to membrane degradation and a reduction in membrane stability [40,41]. Generally, more attention has been given to the overall system performance of PVDF membranes rather than checking the membranes’ material stability after long use in a harsh environment. Some studies have revealed the degradability and the performance level of these materials when pre-treated with organic or inorganic acids or bases during casting solution preparation or the coagulation process [42,43], while others have investigated the effect of these chemical solutions on the material properties and performance when post-treated or etched in either an acidic or alkaline environment under mild or severe conditions [44–46]. Ross et al. [47] examined the alkaline degradation of PVDF membranes by X-ray photoelectron spectroscopy, secondary mass spectrometry and Raman spectroscopy in order to determine the composition of the modified layer of the membrane using an alkali. The study revealed the formation of a surface layer consisting of bounded C=C bonds with the inclusion of oxygen functionalities in the treatment. Both the formation of C=C bonds and the presence of oxygen validated the defluorination and oxygenation which occurred during alkaline treatment. Puspitasari et al. [48] investigated the effect of (NaClO) solutions on PVDF membranes in both the short- and long-term: there were chemical changes in the functional groups of the membrane, as revealed via Fourier transform infrared (FTIR) analysis, which thus significantly improved the membrane surface’s hydrophilicity. Abdullah and Bérubé [10] assessed the effect of extended NaClO exposure on PVDF-based micro-/ultra-filtration membranes at different concentrations for varying periods of time; they discovered that the period of the exposure had massive impact on the evolutions in the physical and chemical properties of the membrane than NaClO concentration. Hashim et al. [49] used and investigated the use of a sodium hydroxide (NaOH) solution and hydrofluoric (HF) acid as a post-treatment method for removing impurities from PVDF hollow-fiber membranes after formation. Experimental results showed that HF acid treatment was more effective at removing the impurities from the PVDF membrane compared with treatment with the NaOH solution. The mechanical properties of the hollow-fiber membrane treated with HF acid was found to be almost unaffected, while the one treated with the NaOH solution exhibited poor mechanical properties. In addition, Rabuni et al. [24] declared that exposure of a PVDF membrane to an alkaline solution caused some modification in the membrane’s texture, thus leading to structural degradation. This resulted from the chain splitting of the PVDF macromolecular skeleton via the dehydrofluorination reaction. They added that frequent chemical cleaning has an adverse effect on the membrane’s integrity and service life. Wang et al. [50] used analysis of variance to investigate the behavioral change of polyvinylidene fluoride (PVDF) membranes when cleaned with NaClO within a stipulated soaking period. The surface free energy and fouling propensity were also assessed. The analysis of variance showed that varying the NaClO soaking time had a significant effect on the membranes’ hydrophilicity and permeability. They further concluded that there was a reduction in the mechanical strength of the membrane as a result of Membranes 2020, 10, 375 3 of 22

NaClO chemical cleaning, which had a potentially harmful impact on the membrane’s stability, thus supporting the assertion of Rabuni et al. [24]. Despite the fact that some past studies have explored the effect of chemical exposure on the properties of PVDF-based membranes, most of the reports focused on the degradation mechanism in line with membrane stability and performance in an alkaline environment. Very limited studies have considered the effect in the acidic region. As summarized in the previous paragraph describing some of the previous work carried out, to the best of our knowledge, there has not been a detailed investigation on the impact of both mild and severe aqueous acidic concentrated solutions (either organic or inorganic acid) on the physicochemical and mechanical properties of flat-sheet polymeric PVDF-based membranes, subjected to an extended exposure period. However, it is worth mentioning that some earlier works were performed under extreme conditions in which high alkaline concentrations were used, either with or without a catalyst, for a longer exposure time. A systematic investigation of actual industry-related applications is still lacking, particularly in acidic conditions. In this study, we carefully investigate the effects of a sulfuric acid solution, both at mild and harsh concentrations, on PVDF membranes’ stability and performance at different exposure times. The terms mild and harsh are used based on the studies of Tanninen et al. [51] and Platt et al. [52]. This was further achieved via a morphology study, X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR) analysis, porosity, tortuosity, contact angle (hydrophilicity), zeta potential (surface charge), mechanical strength test, permeation flux and rejection studies.

2. Experimental Conditions

2.1. Materials A flat-sheet PVDF membrane with a molecular weight cut-off (MWCO) of 670,000–700,000 Da was kindly supplied by a renowned flat-sheet membrane manufacturer in Fujian Province, China. The preparation method for the PVDF membrane was not disclosed, as the supplier regarded this as proprietary information. The chemicals, including sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%) and sodium hydroxide (NaOH, 100%), were all analytical grade, purchased from Beijing Chemical Co. (Beijing, China) and used as received for both the experiments and characterization. Bovine serum albumin (BSA) was obtained from Sigma-Aldrich (Selangor, Malaysia). All the washing and solution preparation was done using Millipore deionized (DI) water of 18 MΩ cm resistivity at 25 ◦C.

2.2. Membrane Preparation and Embedding The process of embedding membranes in sulfuric acid solution is shown in Scheme1. In general, upon reception of the PVDF membrane in a rolled form, the membrane was initially gently spread out and thoroughly washed to remove any form of solvent impurities and preservatives. Thereafter, it was stored overnight in deionized water before the commencement of the experiment. The membrane was sliced into smaller sizes in order to allow for effective contact with the sulfuric acid solution when embedded. A sulfuric acid aqueous solution was prepared at five different concentrations (1.0, 2.0, 4.0, 8.0 and 15 mol/L) and allowed to cool down to room temperature before the embedding the sliced membranes. The periods of exposure were 7, 28, 56, 84 and 112 days. After the completion of each exposure period, the membranes were carefully rinsed and immersed in deionized water for 1 h in order to allay the reaction and also eliminate residual chemicals remaining on the membrane’s surface before checking the permeation flux and other characteristics. A pristine membrane was also assessed and characterized. For easy evaluation and explanation, the membranes are denoted as PVDF-neat and PVDF-x, where x represents 1, 2, 4, 8 and 15 for various concentrations of the acid solution at 1.0, 2.0, 4.0, 8.0 and 15 mol/L, respectively. Membranes 2020, 10, 375 4 of 22 Membranes 2020, 10, x FOR PEER REVIEW 4 of 22

SchemeScheme 1. 1.Embedding Embedding process process of of the theas-received as-received polyvinylidenepolyvinylidene fluoride fluoride (PVDF (PVDF)) membrane membrane in inan an aqueousaqueous sulfuric sulfuric acid acid solution. solution.

2.3.2.3. Membrane Membrane Characterization Characterization

2.3.1.2.3.1. Morphology Morphology TheThe surface surface morphologies morphologies of of the the neatneat andand embeddedembedded membranes membranes were were determined determined by by a field a field emissionemission scanning scanning electron electron microscope microscope (FE-SEM)(Hitachi(FE-SEM)(Hitachi S4800, S4800, Tokyo, Tokyo, Japan) Japan) with with an an accelerating accelerating voltagevoltage of of 5.0 5.0 kV kV at at various various magnifications. magnifications. Before the images images were were captured, captured, the the samples samples were were positionedpositioned on ona metal a metal holder holder and and sputtered sputtered with with a thin a thin layer layer of gold of gold under under a vacuum a vacuum to reduce to reduce sample chargingsample undercharging the under electron the beam.electron The beam. obtained The obtained FE-SEM FE images-SEM images were then were analyzed then analyzed using using ImageJ softwareImageJ (freely software available, (freely open-source available, open software-source with software a Java-based with a public Java-based domain public image domain processing image and analysisprocessing program) and attached analysis toprogram) the FE-SEM attached machine. to the A FEwell-defined-SEM machine. stepwise A well procedure-defined of stepwise acquiring poreprocedure size data of using acquiring ImageJ pore is size provided data using in other ImageJ studies is provided [53,54 ].in Inother brief, stu thedies FE-SEM [53,54]. In images brief, the were firstFE adjusted-SEM images and convertedwere first adjusted to 8-bit binaryand converted form by to altering 8-bit binary their form brightness by altering and contrasttheir brightness for better imageand resolution,contrast for and better to achieveimage resolution, feasible image and to analysis. achieve Thefeasible binary image image analysis. was thenThe analyzedbinary image using thewas ‘analyze’ then analyzed tool built using into the the ‘analyze’ software. tool built Furthermore, into the software. the relative Furthermore, elemental the compositions relative elemental of the compositions of the membrane surface were also obtained by mapping with an energy-dispersive membrane surface were also obtained by mapping with an energy-dispersive spectrometer (EDX) spectrometer (EDX) attached to the FE-SEM machine. attached to the FE-SEM machine. 2.3.2. X-ray Diffractogram Analysis 2.3.2. X-ray Diffractogram Analysis The crystalline phases and structural changes in both the neat and embedded membranes were The crystalline phases and structural changes in both the neat and embedded membranes were obtained using an XRD-X’Pert PRO (PANalytical, The Netherlands). The membranes were gently obtained using an XRD-X’Pert PRO (PANalytical, The Netherlands). The membranes were gently placed on a thick plexiglass sample holder which was loaded on the diffractometer. The analysis was placed on a thick plexiglass sample holder which was loaded on the diffractometer. The analysis done using 퐶푢퐾∝ radiation (λ = 1.5418 Å ), with the generator working at 40 kV and 40 mA. The λ wasscanning done using was performedCuK∝ radiation within (the= range1.5418 of 5 Å), < 2θ with < 90 the with generator a scanning working step of 0.02 at 40°. kV and 40 mA. The scanning was performed within the range of 5 < 2θ < 90 with a scanning step of 0.02◦. 2.3.3. Fourier Transform Infrared Spectroscopy Analysis 2.3.3. Fourier Transform Infrared Spectroscopy Analysis The Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700 FTIR, Thermo Fisher Scientific Inc.,The Waltham, Fourier transformMA, USA) infraredwas used spectroscopy to qualitatively (FTIR) investigate (Nicolet the 6700 functional FTIR, Thermogroups on Fisher the neat Scientific and Inc., Waltham, MA, USA) was used to qualitatively investigate the functional groups on the neat and

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1 embedded membrane surfaces with a resolution of 4 cm− and 32 scans per spectrum within a wave 1 number range of 400–4000 cm− . Prior to analysis, all samples were dried at room temperature for 24 h. The spectra were collected and processed using OMNIC software with an attenuated reflectance unit.

2.3.4. Thickness, Porosity and Tortuosity The thickness of the membranes was measured using a digital micrometer (293–252, Mitutoyo, Kanagawa, Japan, with a precision of 0.01 mm). Ten various points were randomly selected on each ± membrane for measurement, and their average values and standard deviation were reported. The porosity of both the neat and embedded membranes ε (%) was determined using the gravimetric method of measurement, which is defined as the ratio of the volume of pores to the total volume of the membrane. The wet membranes to be tested were sliced into small 3 by 3 cm pieces and weighed on a digital weighing balance (Mettler Toledo ME204E, Columbus, OH, USA) with 0.001 ± readability [28]. Before measurement, the superficial water on the membrane surfaces was gently mopped off. These membranes were kept in an oven for 24 h at 60 ◦C before measuring the dry weight. The porosity was estimated by Equation (1).

(WW WD)/ρW ( ) = − ε % W W W 100 (1) W− D + D × ρW ρP where WW is the weight of the wet membrane (g), WD is the weight of the dry membrane (g), ρW the 3 3 pure water density (0.998 g/cm ) and ρP is the polymer density (1.780 g/cm ). Membrane tortuosity τ is the intrinsic ability of a membrane to withstand harsh conditions. It has an inverse relationship with porosity and it is dimensionless [55]. It was calculated by Equation (2).

(2 ε)2 τ = − (2) ε

2.3.5. Static Contact Angle Measurement The surface hydrophilicity of both the neat and embedded membranes was measured using a contact angle analyzer (DSA100, KRUSS, Hamburg, Germany). Dry membranes were sliced into 1.5 by 1.5 cm pieces and each was taped onto 250 150 10 mm flexible plexiglass. Through capillary × × action, 3 µL of deionized water was dropped from a microsyringe with a stainless steel needle onto the membrane’s surface at room temperature. The contact angle was determined as the angle between the surface and a tangent to the curved surface of the drop. Five measurements at different points on each membrane surface were averaged to obtain an accurate contact angle.

2.3.6. Zeta Potential The surface charges of both the neat and embedded membranes were tested three times (to get average values) using a streaming potential analyzer (SurPASS 3, Anton Paar, North Ryde, Australia) with a 0.010 mol/L KCl electrolyte solution. Before the zeta potential was determined, each membrane was sliced into two rectangular samples using the clamping cell with an adjustable gap of 10 20 mm. × The pH value of 7.0 0.5 was used as a flowing liquid to adjust the 0.05 mol/L HCl and NaOH solutions ± at a room temperature of 25 1 C; a gap height of about 100 µm was maintained. ± ◦ 2.3.7. Mechanical Strength The ultimate tensile strength and elongation of both the neat and embedded membranes were tested using a Shimadzu universal tensile testing machine at a room temperature of 25 1 C with a ± ◦ loading velocity of 50 mm/min and gauge length of 100 mm. Before measurement, the membranes were sliced into 2 by 10 cm pieces. Three measurements were obtained for each membrane and averaged to Membranes 2020, 10, x FOR PEER REVIEW 6 of 24

208 2.3.7. Mechanical Strength 209 The ultimate tensile strength and elongation of both the neat and embedded membranes were 210 tested using Shimadzu universal tensile testing machine at room temperature of 25 ± 1˚C with a 211 loading velocity of 50 mm/min and guage length of 100 mm. Before measurement, membranes were Membranes 2020, 10, 375 6 of 22 212 sliced into sizes of 2 by 10 cm. Three measurements were obtained for each membrane and 213 averaged to obtain an accurate measurement. The deviation between the membrane values and 214 theirobtain average an accurate is less measurement.than 10%. The deviation between the membrane values and their average was less than 10%. 215 2.3.8. Physical Observation and Examination 2.3.8. Physical Observation and Examination 216 The embedded membranes were checked and examined thoroughly after the completion of 217 each immersionThe embedded time membranesfor any sign wereof discoloration, checked and blistering, examined surface thoroughly attack after and the cracking. completion of each immersion time for any sign of discoloration, blistering, surface attack and cracking. 218 2.4. Permeation and Rejection Studies 2.4. Permeation and Rejection Studies 219 The membrane permeation performance was measured using a self-designed laboratory scale 220 dead-endThe membranefiltration cell permeation (Model 8010, performance Millipore was Corp, measured USA) usingwith effective a self-designed membrane laboratory area of scale 12.56 in a 221 cmdead-end2. Figure filtration 1 illustrates cell (Modelthe schematic 8010, Millipore process of Corp, the filtration Burlington, system. MA, USA) All filtration with an eexperimentsffective membrane were 2 222 conductedarea of 12.56 under cm nitrogen. Figure 1 gas illustrates atmosphere the schematic and a stirrin processg rate of of the 300 filtration rpm to system. prevent All deposition filtration 223 contaminantexperiments at were ambient conducted temperature under a nitrogenof 25 1 gas ⁰C. atmosphere Before measurement and a stirring was rate taken, of 300 the rpm membranes to prevent deposition of contaminants at an ambient temperature of 25 1 C. Before each measurement was 224 were first pre-compacted using deionized water at transmembrane± ◦ pressure (TMP) of 0.15 MPa for taken, the membranes were first pre-compacted using deionized water at a transmembrane pressure 225 30 minutes to minimize compaction effects and acquire a stable flux. Thereafter, the test was carried (TMP) of 0.15 MPa for 30 min to minimize compaction effects and acquire a stable flux. Thereafter, 226 out at TMP of 0.10 MPa and the pure water flux was measured for 10 minutes to achieve a constant the test was carried out at TMP of 0.10 MPa and the pure water flux was measured for 10 min to 227 value using a self-compiling computer program at real time. The permeation flux (J) in L/m2h was achieve a constant value using a self-compiling computer program in real time. The permeation flux (J) 228 calculated by Eq. (3). in L/m2h was calculated by Equation (3). V 229 J = (3) (3) A∆t 230 wherewhereV is V the is the volume volume of permeated of permeated water water (L), (L),A is A the is the effective effective membrane membrane area area (m 2(m) and2) and∆t is∆t theis 231 thepermeation permeation time time (h). (h). 232

1-Compressed Nitrogen gas, 2-Control valve, 3-Pressure guage, 4-Vent valve, 5-Feed container, 6-Amicon filtration cell unit, 7-Membrane sheet, 8-Rotameter, 9-Automated stirrer, 10-Permeate hose, 11-Permeate collector, 12-Electronic digital weighing balance, 13- Self compiling computer program. 233

234 Figure 1. SchematicFigure process 1. Schematic of the processdead-end of the filtration dead-end system filtration setup. system setup. 235 236 TheThe membrane membrane’s separation separation performance performance was was evaluated evaluated by by feeding feeding BSA a BSA solution solution of of 0.3 0.3g/L g/L into into 237 thethe filtration filtration cell cell after testingtesting thethe purepure water water flux flux of of the the membrane. membrane. The The concentration concentration of bothof both the the feed 238 feedand and permeate permeate were were determined determined using using a UV-Vis a UV spectrophotometer-Vis spectrophotometer (Shimadzu (Shimadzu UV-2450, UV- Tokyo,2450, Japan) Japan) 239 atat a awavelength wavelength of of 280 280 nm. nm. Two Two samples samples were were tested tested for for each each membrane membrane and and the the average average value value was was 240 recorded.recorded. The The rejection rejection capability capability (R) (R )of of the the BSA BSA was was calculated calculated by by E Equationq. (4). (4). ! Cp R(%) = 1 100 (4) − C f ×

where Cp and C f are the concentrations of both the permeate and the feed solutions, respectively. Membranes 2020, 10, 375 7 of 22

3. Results and Discussion

3.1. Membrane Morphology and Elemental Composition To understand the influence of sulfuric acid on the membranes’ structure at various concentrations after the completion of each immersion day, the membranes’ top-section FE-SEM images were carefully examined at 20,000 magnification. The FE-SEM image of the neat membrane (stored in deionized × water) was also captured at the completion of each exposure day to check if there were any significant changes in the membrane’s structure. Uniformly distributed pores, spread across the surface of the membrane, were more pronounced on exposure Day 7, 24 and 56, as shown in Figure2 for A. For the embedded membranes, comparing the exposure time at varying acid concentrations in B–F, larger pores were observed on the membranes exposed to higher concentrations of acid than at lower concentrations. This largely influenced the porosity and permeated flux of the membranes, thus decreasing the hydrophobicity of the PVDF membranes. In addition, most of the acidified membranes at different concentrations and exposure times displayed several embossments (B–F), which were more severe on the surface of the PVDF-8 and PVDF-15 membranes. It is hypothesized that these pronounced embossments resulted from the chemical reaction occurring on the membranes’ surface caused by the acid concentration solution with a low pH. On the other hand, it should be noted that FE-SEM cross-section images of both the neat and embedded membranes are absent in this study because the nonwoven support of the PVDF membranes was extremely difficult to break, even in liquid nitrogen. Attempts made to view the few fractured layers under the SEM machine proved abortive. In addition, the pore size measurement was conducted using the known scale of the FE-SEM image in ImageJ. The average mean pore size of the PVDF-neat membranes was approximately 0.50 to 1.0 µm, while those of the embedded membranes ranged from approximately 1.25 to 2.0 µm. The elemental composition of representative membrane surfaces was further investigated by EDX analysis, as shown in Figure3. It can be observed that compared with the neat membrane, a few metal contaminants such as Ti, Rb, Al, Ca and Si, which were not discovered in the neat membrane, were present in the embedded membranes at different concentrations. These were more evident in the PVDF-2 and PVDF-4 membranes on exposure Days 56 and 84, respectively, but these contaminants tended to diminish the longer the membranes stayed in the acid at higher concentrations. This observation could result from the prolonged exposure of the membrane to acid, which could lead to chemical reactions that present elemental impurities due to deterioration. Furthermore, these chemical reactions at different concentrations resulted in a loss of hydrogen and fluorine, which results in conjugation along the chains [24]. The main element found both in the neat and embedded membranes based on weight and atomic percentage composition is fluorine, followed by carbon, with a significant quantity of oxygen. This result is, however, expected, as the chemical composition of PVDF contains mostly carbon and fluorine [56,57]. Membranes 2020, 10, 375 8 of 22

Membranes 2020, 10, x FOR PEER REVIEW 8 of 22

A

B

C

Figure 2. Cont.

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Membranes 2020, 10, x FOR PEER REVIEW 9 of 22

D

E

F

Figure 2. SEM images of the top surface of PVDF-Neat (A), PVDF-1 (B), PVDF-2 (C), PVDF-4 (D), FigurePVDF 2.-8SEM (E) and images PVDF of-15 the (F) topmembranes surface ofon PVDF-Neat(a) Day 7, (b)( DAay), PVDF-128, (c) Day (B ),56 PVDF-2, (d) Day (84C), and PVDF-4 (e) Day (D ), PVDF-8112. (E) and PVDF-15 (F) membranes on (a) Day 7, (b) Day 28, (c) Day 56, (d) Day 84 and (e) Day 112.

Membranes 2020, 10, 375 10 of 22 Membranes 2020, 10, x FOR PEER REVIEW 10 of 22

Figure 3. ElementalElemental composition composition analysis analysis of ofrepresentative representative membrane membrane samples samples:: (a) PVDF (a) PVDF-neat,-neat, (b) (PVDFb) PVDF-1-1 on D onay Day7, (c) 7, PVDF (c) PVDF-2-2 on Day on 28 Day, (d) 28,PVDF (d)-4 PVDF-4 on Day on56, Day(e) PVDF 56, (e-8) on PVDF-8 Day 84 on and Day (f) 84PVDF and- (15f) PVDF-15on Day 112 on. Day 112.

3.2. XRD Analysis To determinedetermine thethe alterationalteration in thethe crystallinecrystalline phase of the membranesmembranes embedded in didifferentfferent acid concentrations,concentrations, XRD analysis was carried out.out. It has been established that PVDF polymerspolymers have fivefive various crystalline forms known as the α,, β, γ, δ and ε phases [[5566,,5588] but,but, according to previous findings,findings, theythey areare usuallyusually availableavailable as as either either the theα αor orβ βphases phases and and can can periodically periodically be be present present in in the theγ phaseγ phase [59 [59–61–6].1 PVDF]. PVDF can can also also be be systematically systematically arranged arranged in in di ffdifferenterent crystal crystal structures, structures depending, depending on theon the processing processing of theof the material material [62 [62–64–64].] The. The peaks peaks at at di diffractioffractionn angle angle 2 2θθ is is 18.01 18.01°,◦, 20.0120.01°◦ and 26.5826.58°◦,, representing the didiffractionsffractions inin planesplanes (020),(020), (110)(110) andand (021)(021) respectively.respectively. X-rayX-ray didiffractionffraction spectraspectra collected for for the the embedded embedded membranes membranes at di atff erent different exposure exposure periods period of 28,s 56of and 28, 84 56 days and are 84 provided days are inprovided Figure in S1, Fig whileure S1 Figure, while4 showsFigure the4 shows peaks the for peak exposures for exposure Day 7 at Day di ff7 erentat different concentrations concentrations have approximatelyhave approximately the same the di sameffraction diffraction angles 2 anglesθ of 17.79, 2θ of 20.76, 17.79, 22.79, 20.76, and 22.79, 26.01 forand the 26.01 neat for membrane, the neat whichmembrane, correspond which to correspond the crystal to planes the crystal of (100), planes (020), of(110) (100), and (020), (021), (110)respectively, and (021) representing, respectively, the characteristicrepresenting theα phase characteristic of PVDF. α The phase peak of whichPVDF. appearedThe peak atwhich 2θ = appeared20.76, originating at 2θ = 20.76, from originating the crystal planefrom the (110), crystal reveals plane the (110), existence reveals of a theβ phase. existence Inaddition, of a β phase. a close In addition, observation a close of the observation characteristic of the 2θ peakscharacteristic for PVDF-1, 2θ peaks PVDF-2, for PV PVDFDF-1, 4, PVDF PVDF-8-2, PVDF and PVDF-15 4, PVDF- are8 and almost PVDF uniform-15 are almost with that uniform of the with neat membrane.that of the neat Similarly, membrane. the peak Similarly, for PVDF-neat the peak at for di PVDFfferent-neat concentrations at different on concentrations exposure Day on 112 exposure (shown inDay Figure 112 (4shown) has di inff ractionFigure angles4) has diffraction 2θ of 17.72, angles 20.84, 22.792θ of and17.72, 25.99. 20.84, These 22.79 values and 25.99. reveal Th someese values slight direvealfferences some when slight compared difference withs when the di comparedffraction angles with the 2θ obtaineddiffraction for angles exposure 2θ obtained Day 7. Although for exposure other membranesDay 7. Although at diff othererent membranes acid concentrations at different exhibit acid similar concentrations peaks to exhibit the neat similar membrane, peaks some to the slight neat stretchingmembrane was, some noticed, slight thus stretching indicating was thenoticed, effect th ofus the indicating extreme conditions. the effect of Summarily, the extreme there conditions. were no seriousSummarily, changes there detected were no in serious the crystal changes structure detected of the in PVDF the crystal membranes, structure even of the at highPVDF concentrations membranes, even at high concentrations of the sulfuric acid solution after a long period of exposure. Furthermore, all the spectra obtained for both the neat and acidified membranes were almost similar. This outcome

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Membranes 2020, 10, x FOR PEER REVIEW 11 of 22 of the sulfuric acid solution after a long period of exposure. Furthermore, all the spectra obtained for bothshows the that neat XRD and analysis acidified may membranes be less effective werealmost at identifying similar. changes This outcome made in shows the crystalline that XRD structure analysis mayby sulfuric be less acid effective attack. at A identifying similar observation changes made was mad in thee by crystalline [9] in the structureNaOH treatment by sulfuric of hollow acid attack.-fiber APVDF similar membranes. observation was made by [9] in the NaOH treatment of hollow-fiber PVDF membranes.

FigureFigure 4.4. XRD crystalline crystalline phase phase an analysisalysis of of neat neat and and embedded embedded membranes membranes at atexposure exposure period periodss of ( ofa) (7a )days 7 days (representing (representing the the minimum minimum exposure exposure period) period) and and (b (b) )112 112 days days (representing the maximummaximum exposureexposure period).period).

3.3. FTIR Spectroscopy 3.3. FTIR Spectroscopy To analyze the chemical change in the functional group of the membranes, and to further validate To analyze the chemical change in the functional group of the membranes, and to further the crystalline phase properties of the membrane surface layers obtained via XRD, attenuated total validate the crystalline phase properties of the membrane surface layers obtained via XRD, reflection Fourier transform infrared spectroscopy (ATR-FTIR) was applied. In addition, the effect attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was applied. In of acid attack on the PVDF membranes at different concentrations, in terms of structural changes in addition, the effect of acid attack on the PVDF membranes at different concentrations, in terms of the chemical groups, was analyzed. The FTIR spectra acquired for the PVDF-neat and embedded structural changes in the chemical groups, was analyzed. The FTIR spectra acquired for the PVDF- PVDFneat and membranes embedded at PVDF various membranes concentrations at vari (Figureous concentrations5a,b) generally (Figure show 5a,b some) generally of the common show some IR absorption bands at approximately 762, 858, 970, 1070 and 1149 cm 1, which is identical to the α of the common IR absorption bands at approximately 762, 858, 970,− 1070 and 1149 cm−1, which is 1 phasesidentical of to the the PVDF α phases crystal, of whilethe PVDF two othercrystal, minor while absorption two other bandsminor noticed absorption at 840 bands and 1404noticed cm −at 840are envisaged as representing the β-phase. This is in line with the observations of some previously studied and 1404 cm−1 are envisaged as representing the β-phase. This is in line with the observations of some 1 workspreviously [50,60 studied,65]. In works addition, [50,60,65] the various. In addition, absorption the bandsvarious observed absorption between bands 1450 observed and1950 between cm− can likely be attributed to the dehydrofluorination occurring on the degraded membrane’s surface, 1450 and1950 cm−1 can likely be attributed to the dehydrofluorination occurring on the degraded causingmembrane the’s formation surface, causing of more the carbon–carbon formation of double more carbon bonds– (–CFcarbon= CH–), double while bond thes (– absorptionCF = CH–),bands while between 2850 and 2950 cm 1 are attributed to stretched C–H bonds [9,57]. The degradation of PVDF the absorption bands between− 2850 and 2950 cm−1 are attributed to stretched C–H bonds [9,57]. The membranesdegradation resultingof PVDF membranes in dehydrofluorination resulting in dehydrofluorination generally displayed agenerally broad peak displayed in the samea broad region. peak Conjugatedin the same region. double Conjugated bonds are also double formed, bonds which are also can formed aid the, attachmentwhich can aid of somethe attachment specific reactants, of some thusspecific introducing reactants, a functionalthus introducing group toa thefunctional membrane group [49 ].to Therethe membrane are some other[49]. There noticeable are some absorption other 1 bandsnoticeable between absorption 2950 and bands 3600 between cm− in 2950 Figure and5 a,b,3600 with cm−1 twoin Figure particular 5a,b, pairswith two of bands particular at 2968 pairs and of 1 1 3428bands cm at− 2968, and and 2971 3428 and cm 3431−1, cmand− 2971for Daysand 3431 7 and cm 112,−1 for respectively, Days 7 and representing 112, respectively, the hydroxyl representing group. Thesethe hydroxyl peaks are group. associated These with peaks the action are associated of oxidation with or pre-treatment the action of products oxidation such or as pre sulfuric-treatment acid, nitricproducts acid, such carboxylic as sulfuric acids acid, and alcoholsnitric acid, [57 ,carboxylic65–67]. The acids intensity and alcohols of the hydroxyl [57,65–67 peaks]. The was intensity observed of tothe increase hydroxyl as peaks the concentration was observed of to the increase acid present as the concentration in the embedded of the membranes acid present increased, in the emb onedded both exposuremembranes Days increase 7 andd 112,, on although both exposure these changes Days 7 wereand 112 not, visiblyalthough noticeable. these changes This further were not proves visibly the inertnessnoticeable. and This good further chemical proves stability the inertness of PVDF and membranes. good chemical Similar stability spectra of PVDF were membranes. acquired for Similar PVDF spectra were acquired for PVDF membranes embedded in various acid concentrations on exposure Days 28, 56 and 84, as shown in Figure S2.

Membranes 2020, 10, 375 12 of 22 membranes embedded in various acid concentrations on exposure Days 28, 56 and 84, as shown in

FigureMembranes S2. 2020, 10, x FOR PEER REVIEW 12 of 22

Figure 5. 5.FTIR FTIR spectra spectra of neat of neat and embedded and embedded membranes membranes at exposure at exposure periods of period(a) 7 dayss of (representing (a) 7 days the(representing minimum the exposure minimum period) exposure and (b period)) 112 days and (representing (b) 112 days the(representing maximum the exposure maximu period).m exposure period). 3.4. Membrane Thickness and Porosity 3.4. MembraneThe thickness Thickness of a membrane and Porosity gives insight into the flux and strength it may likely exhibit. Table1 presentsThe thethickness thickness of a with membrane the respective gives insight standard into deviations the flux and of thestrength neat and it may embedded likely exhibit. membranes. Table It varies from 192.4 3.95 to 196.4 7.07 µm. Before the membranes were immersed in different acid 1 presents the thickness± with the± respective standard deviations of the neat and embedded concentrations, the thickness was measured to be 196.4 7.07 µm at room temperature. This thickness membranes. It varies from 192.4 ± 3.95 to 196.4 ± 7.07 µm.± Before the membranes were immersed in wasdifferent slightly acid higher concentrations, than those the of thickness the embedded was measured membranes, to be as 196.4 they ± followed 7.07 µm at the room decreasing temperature. order ofThis PVDF-neat thickness > wasPVDF-2 slightly> PVDF-1 higher than> PVDF-4 those of> thePVDF-8 embedded> PVDF-15. membran Ines addition,, as they a followed sharp drop the wasdecreasing noticed order in the of thicknessPVDF-neat of > membrane PVDF-2 > PVDF-1PVDF-1 > immediately PVDF-4 > PVDF after-8 immersion > PVDF-15. in In 1 moladdition,/L acid a concentrationsharp drop was but noticed was later in justifiedthe thickness by the of corresponding membrane PVDF thickness-1 immediately reduction after in other immersion membranes, in 1 withmol/L the acid exception concentration of PVDF-2. but Thiswas implieslater justif thatie thed by reduction the corresponding in the membranes’ thickness thickness reduction with in respect other tomembranes different concentrations, with the exception will increase of PVDF the- permeate2. This implies flux, thus that leading the reduction to the cost in of the higher membrane heat losss’ andthickness mechanical with respect properties to different [20]. The concentration porosity ofs the will membranes increase the was permeate further flux checked, thus usingleading the to drythe andcost wetof higher method. heat Table loss and1 shows mechanical that the properties porosity of[20] the. The embedded porosity membranes of the membranes increased was with further an increasechecked inusing acid the concentration dry and wet at method. the completion Table 1 ofshows each immersionthat the porosity period. of Athe noticeable embedded increment membranes was observedincreased inwith the porosityan increase of the in neatacid membranes concentration after at Daythe completion 7 of the exposure of each period, immersion which period. indicates A anoticeable sudden enlargement increment was of observed the membrane’s in the porosity pores, causedof the neat by acidmembranes attack on after the D membranes’ay 7 of the exposure surface. However,period, which some indicate percentages a sudden variations enlargement were noticed of the in themembrane porosity’s particularly pores, caused for by PVDF-1 acid attack and PVDF-2 on the onmembrane exposures’ Daysurface. 28; this However is hypothesized, some percentage to have resulted variations from were a minute noticed distortion in the porosity in the membrane’sparticularly structurefor PVDF caused-1 and byPVDF the sudden-2 on exposure acid attack Day on 28; an this inhomogeneous is hypothesized membrane to have spot. resulted Moreover, from slighta minute and gradualdistortion increments in the membrane were seen’sin structure the membrane caused porosity by the on sudden other acidexposure attack days on asan the inhomogeneous concentration ofmembrane acid increased, spot. Moreover, but these were slight not and as pronouncedgradual increment as on exposures were seen Day in 7. the The membrane overall porosity porosity of theon neat and embedded membranes ranged from 58.6 0.42 to 73.2 0.51%. This outcome further explains other exposure days as the concentration of acid± increased, but± these were not as pronounced as on theexposure SEM images Day 7. inThe Figure overall2, which porosity reveal of the that neat the porosityand embedded changed membranes with increasing ranged acid from concentration 58.6 ± 0.42 into most73.2 ± cases, 0.51%. as mostThis outcome of the pore further sizes wereexplains seen the to beSEM visibly images enlarged in Figure under 2, thewhich electron reveal machine. that the porosity changed with increasing acid concentration in most cases, as most of the pore sizes were seen to be visibly enlarged under the electron machine.

Membranes 2020, 10, 375 13 of 22

Table 1. Membrane porosity and thickness.

µ Membrane Porosity (%) Thickness ( m) ID 7 Days 28 Days 56 Days 84 Days 112 Days Before After PVDF-neat 58.6 0.42 58.6 0.42 58.6 0.42 58.6 0.42 58.6 0.42 196.4 7.07 196.4 7.07 ± ± ± ± ± ± ± PVDF-1 65.3 0.47 66.3 0.46 62.3 0.40 65.3 0.47 61.4 0.46 196.4 7.07 195.2 4.85 ± ± ± ± ± ± ± PVDF-2 66.5 0.56 63.2 0.28 65.3 0.27 66.6 0.24 65.9 0.25 196.4 7.07 195.7 5.19 ± ± ± ± ± ± ± PVDF-4 67.0 0.13 64.7 0.16 65.7 0.52 66.6 0.29 68.5 0.43 196.4 7.07 193.4 2.55 ± ± ± ± ± ± ± PVDF-8 69.1 0.20 68.9 0.10 69.3 0.18 68.4 0.19 71.0 0.16 196.4 7.07 192.9 3.42 ± ± ± ± ± ± ± PVDF-15 68.8 0.30 69.7 0.28 68.7 0.42 71.8 0.33 73.2 0.51 196.4 7.07 192.4 3.95 ± ± ± ± ± ± ±

3.5. Membranes’ Mechanical Strength and Tortuosity The lifespan of a membrane depends greatly on its mechanical strength. It has been reported that the properties of the polymer in use, in terms of membrane structure, morphology and degree of crystallinity, are imperative when assessing the mechanical properties of a membrane [24,68]. For an occasional assessment of the strength of a membrane, tensile strength and ultimate elongation are nonaggressive tests which can be adopted. In this work, these two parameters were measured to determine the effects of sulfuric acid on the membranes, at the lowest and highest concentrations and their respective exposure periods, to check the mechanical stability of the PVDF membranes. Those of the neat membranes were also checked for comparison and the results are shown in Figure6a,b. The ultimate strength of the PVDF-neat membranes decreased from 46.18 0.65 to 42.10 0.41 and ± ± from 32.39 0.22 MPa for PVDF-1 and PVDF-15, respectively, as they were in contact with the acid ± solution, and these reductions occurred gradually as the exposure days were extended. Similarly, the ultimate elongation, which represents the pertinacity of the membrane material, decreased in a stepwise manner from 24.61 0.05% in the neat membrane to 21.78 0.27% and 18.77 0.38% for both ± ± ± PVDF-1 and PVDF-15 at the highest concentration and exposure period. As there was a decrease in the pertinacity of the membrane, this implies that the sulfuric acid caused some potential change in the membrane’s mechanical properties, especially at a high concentration, which could eventually lead to the mechanical instability of the membranes. Furthermore, the result obtained for porosity validated this outcome, as the mechanical properties of a membrane material do not depend only on the polymeric properties but also on the pore topology [68], although, these changes were not really noticeable in the XRD and FTIR analyses. The tortuosity of the membranes was mathematically derived through the membranes’ porosity. It is an intrinsic property which reveals the degree of the membranes’ ruggedness or twistedness when subjected to harsh conditions [55]. Figure6c depicts the ruggedness of both the neat and embedded membranes. The results obtained have an inverse relationship with the membrane porosity. As expected, the tortuosity of the embedded membranes decreased gradually, both with increasing acid concentration and exposure period. In addition, the effects of both sulfuric acid concentration and exposure period were mild on the embedded membranes, as there were inconsiderable differences from the value obtained at the most extreme acidic conditions (i.e., PVDF-15) on Day 112 and the PVDF-neat membrane’s value. The overall tortuosity of the embedded membranes at all levels of concentration ranged from 2.20 to 2.96. Furthermore, the PVDF-8 and PVDF-15 membranes exhibited the least tendency to withstand extreme conditions, which affirms the weakening action taking place in the fiber of the membrane, caused by the acid reaction at extended concentrations and a longer exposure time. Membranes 2020, 10, 375 14 of 22 Membranes 2020, 10, x FOR PEER REVIEW 14 of 22

50 26 Day 7 Day 7 Day 28 Day 28 45 Day 56 24 Day 56 Day 84 Day 84 Day 112 40 Day 112 22

35 20

30

18

Ultimate Strength (MPa) Strength Ultimate Ultimate elongation (%) elongation Ultimate 25

16 20 PVDF-Neat PVDF-1 PVDF-15 PVDF-Neat PVDF-1 PVDF-15 Membranes Membranes (a) (b) 4.5 Day 7 4.0 Day 28 Day 56 3.5 Day 84 Day 112 3.0

2.5

2.0

Tortuosity 1.5

1.0

0.5

0.0 PVDF-Neat PVDF-1 PVDF-2 PVDF-4 PVDF-8 PVDF-15 Membranes (c)

Figure 6. MechaniMechanicalcal properties of representative membrane samples:samples: ( (aa)) u ultimateltimate strength of the membranes;membranes; (b) ultimateultimate elongation ofof thethe membranes;membranes; ((cc)) tortuositytortuosity ofof thethe membranes.membranes.

3.6. Membrane Hydrophilicity To check check alterations alterations in the the surfacesurface properties and also improveimprove the antifouling property of membranes, the wetting ability of the the membranes’ membranes’ surfacesurface is is a a major major determinant. determinant. This is often characterized through the contact angle measurement to check for either the hydrophobicity or hydrophilicity of the membrane [[20]20].. The contact angles of the neat and embeddedembedded membranes are presented in Figure7 7.. TheThe contactcontact angleangle ofof thethe neatneat PVDFPVDF membranemembrane waswas measuredmeasured alongsidealongside allall thethe embedded membranes membranes on on each each day day of of testing. testing. As As seen seen from from the the result, result, the the PVDF PVDF-neat-neat membrane membrane on onall allthe the days days of ofimmersion immersion in indeionized deionized water water presented presented the the highest highest contact contact angle angle values values,, ranging from 78.1 to 81.0◦° with a minor difference difference of 2.9◦°. This This outcome outcome shows shows the the hydrophobic hydrophobic nature of the PVDF membrane. The higher the contact angle of a membrane, the higher the hydrophobicity of its surface; the PVDF membrane, having a contact angle of 80 5 , is categorized as hydrophobic [50]. surface; the PVDF membrane, having a contact angle of 80 ± 5°◦, is categorized as hydrophobic [50]. However, a critical look at the result shows that sulfuric acid exposure has a great impact on the surface properties of the membrane. The PVDF PVDF-1-1 membrane has lower contact angle values than the neat membranes for allall the days of immersion, thus exhibiting high hydrophilicity. Further exposure of the membranes to higher concentrations of 2 and 4 molmol/L/L (PVDF-2(PVDF-2 and PVDF-4)PVDF-4) decreased the contact angle values from 69.9 to 56.0◦° and 66.0 to 49.7 °,◦, respectively respectively,, for all imm immersionersion and exposure periods. This thus further enhance enhancedd the hydrophilic nature of the PVDF membrane. In addition, with an extended exposure to sulfuric acid at concentrationsconcentrations of 8 and 15 mol/L, mol/L, the contact angle followed the previous pattern by showing further reductions (thus increasing hydrophilicity) but with few differences. In general, the contact angle values of the PVDF membrane for all the acid concentrations

Membranes 2020, 10, 375 15 of 22

theMembranes previous 2020, pattern10, x FOR byPEER showing REVIEW further reductions (thus increasing hydrophilicity) but with15 of few 22 differences. In general, the contact angle values of the PVDF membrane for all the acid concentrations considered, with the inclusion of the neat PVDF,PVDF, decreaseddecreased systematicallysystematically with some insignificantinsignificant exceptionsexceptions for PVDF-2PVDF-2 and PVDF-4PVDF-4 on exposure DaysDays 56 and 84, where the contact angle values were relatively close. The The overall overall observed observed pattern pattern is is some somewhatwhat expected expected,, as as the the SEM SEM imagery imagery revealed revealed a asimilar similar pattern pattern (Figure (Figure 2)2 )by by showing showing that that the the pore pore s sizesizes of thethe membranemembrane becamebecame largerlarger inin moremore concentrated acidic solutions,solutions, thus making thethe membranes’membranes’ surfacesurface moremore hydrophilic.hydrophilic.

Day 7 80 Day 28 Day 56

) Day 84

 (

70 Day 112

e

l

g

n

a

t

c 60

a

t

n

o C 50

40 PVDF-Neat PVDF-1 PVDF-2 PVDF-4 PVDF-8 PVDF-15

Membranes Figure 7. ContactContact angles of neat and embedded membranes at different different acid concentrations.concentrations. 3.7. Membrane Surface Charge 3.7. Membrane Surface Charge The charge present on the surface of a membrane is a major determinant in evaluating the The charge present on the surface of a membrane is a major determinant in evaluating the antifouling performance of that membrane [69]. Zeta potential is widely used to provide vital antifouling performance of that membrane [69]. Zeta potential is widely used to provide vital information on the surface charge properties of a membrane. In this study, the zeta potential of the neat information on the surface charge properties of a membrane. In this study, the zeta potential of the and embedded membranes was determined via the streaming potential method. The result presented neat and embedded membranes was determined via the streaming potential method. The result in Figure8 shows that all the membranes demonstrated negatively charged surfaces, with the values presented in Figure 8 shows that all the membranes demonstrated negatively charged surfaces, with ranging from 15.17 to 36.19 mV. It was observed that the zeta potential of membranes calculated at the values ranging− from− −15.17 to −36.19 mV. It was observed that the zeta potential of membranes low concentrations of acid were negative and became less negative as their concentration increased. calculated at low concentrations of acid were negative and became less negative as their This reduction in zeta potential with an increase in concentration could be explained by the protonation concentration increased. This reduction in zeta potential with an increase in concentration could be effect caused by the sulfuric acid attack on the membranes’ surface during the period of immersion. explained by the protonation effect caused by the sulfuric acid attack on the membranes’ surface At higher concentrations (i.e., in a solution with a low pH value), there is the possibility of more during the period of immersion. At higher concentrations (i.e., in a solution with a low pH value), contact ion pairs, which will not seriously contribute to the membranes’ surface charge, but when the there is the possibility of more contact ion pairs, which will not seriously contribute to the concentration decreases, there is an increase in the absorption of cations; thus, a less negative surface membranes’ surface charge, but when the concentration decreases, there is an increase in the charge is realized. Furthermore, a less negative zeta potential is observed for neutral surfaces tending absorption of cations; thus, a less negative surface charge is realized. Furthermore, a less negative to attract anions. The slight variation observed in the zeta potential of the neat PVDF membrane is zeta potential is observed for neutral surfaces tending to attract anions. The slight variation observed speculated to be caused by some impurities that developed over time on the surface of the membrane in the zeta potential of the neat PVDF membrane is speculated to be caused by some impurities that stored in deionized water. The dependency of the antifouling properties of the membranes on both developed over time on the surface of the membrane stored in deionized water. The dependency of the charge on the membrane surface and that on the foulant under selected operational conditions the antifouling properties of the membranes on both the charge on the membrane surface and that has been established by [70]. In addition, the electrostatic interaction existing between the of the on the foulant under selected operational conditions has been established by [70] . In addition, the foulant in the feed solution and the charge on the membranes’ surface is an important factor, because electrostatic interaction existing between the ions of the foulant in the feed solution and the charge when the zeta potential of a membrane is more negative, the tendency for fouling will reduce on the on the membranes’ surface is an important factor, because when the zeta potential of a membrane is surface of that membrane; this is as a result of the electrostatic repulsion between the foulant in the more negative, the tendency for fouling will reduce on the surface of that membrane; this is as a result feed solution and the membrane’s surface [45]. of the electrostatic repulsion between the foulant in the feed solution and the membrane’s surface [45].

Membranes 2020, 10, 375 16 of 22 Membranes 2020, 10, x FOR PEER REVIEW 16 of 22

0 Day 7

-5 Day 28 Day 56 -10 Day 84 Day 112 -15

-20

-25

Zeta Potential (mV) Potential Zeta -30

-35

-40 PVDF-Neat PVDF-1 PVDF-2 PVDF-4 PVDF-8 PVDF-15 Membranes in various acid concentration (mol/L)

FigureFigure 8. SurfaceSurface zeta zeta potential potential at at different different time timess of immersion of membranes embedded in in varying sulfuricsulfuric acid acid concentration concentrations.s. 3.8. Physical/Visual Examination 3.8. Physical/Visual Examination The embedded membranes were inspected physically to observe if there were any changes in The embedded membranes were inspected physically to observe if there were any changes in color. Two concentration boundaries (PVDF-1 and PVDF-15) of the said membranes were selected to color. Two concentration boundaries (PVDF-1 and PVDF-15) of the said membranes were selected to represent the short- and long-term exposure conditions. Figure9 depicts the visual appearance of the represent the short- and long-term exposure conditions. Figure 9 depicts the visual appearance of the PVDF-neat, PVDF-1 and PVDF-15 membranes. No distinct change in color was observed between the PVDF-neat, PVDF-1 and PVDF-15 membranes. No distinct change in color was observed between neat membrane and the embedded ones, except that some grouped minor blistering and cracking was the neat membrane and the embedded ones, except that some grouped minor blistering and cracking noticed on the surface of some of the embedded membranes at the end of exposure Day 7. On the was noticed on the surface of some of the embedded membranes at the end of exposure Day 7. On contrary, there was a slight partial discoloration of the PVDF-15 membrane at the end of exposure the contrary, there was a slight partial discoloration of the PVDF-15 membrane at the end of exposure Day 112, while some more pronounced blisters than those on exposure Day 7 were observed. Prior to Day 112, while some more pronounced blisters than those on exposure Day 7 were observed. Prior immersion, the neat membrane was white in appearance but later changed slightly to light yellow to immersion, the neat membrane was white in appearance but later changed slightly to light yellow after immersion (Figure9f). It is noteworthy that despite the slight changes that occurred particularly after immersion (Figure 9f). It is noteworthy that despite the slight changes that occurred particularly on the surface of the membranes, they did not affect their chemical properties, as no serious stretches on the surface of the membranes, they did not affect their chemical properties, as no serious stretches were observed in their peaks in Figures4 and5. Furthermore, the temperature of the treating or were observed in their peaks in Figures 4 and 5. Furthermore, the temperature of the treating or immersing solution can influence the color change of membranes [40,56]. In this case, the temperature immersing solution can influence the color change of membranes [40,56]. In this case, the temperature of the aqueous acid solution was set at 25 2 C, which partly contributed to the indistinct changes of the aqueous acid solution was set at 25 ± 2 °◦C, which partly contributed to the indistinct changes that occurred. that occurred.

Membranes 2020, 10, 375 17 of 22 Membranes 2020, 10, x FOR PEER REVIEW 17 of 22

FigureFigure 9. 9.Physical Physical inspection inspection of of the the PVDF membranes at at the the end end of of exposure exposure Day Day 7 for7 for (a) ( a neat) neat membranes,membranes, ( b(b)) PVDF-1,PVDF-1, ( (cc)) and and PVDF PVDF-15,-15, (d) (andd) and the end the of end Day of 112 Day for 112neat formembranes neat membranes (e) PVDF- (e) PVDF-11 and ( andf) PVDF (f)- PVDF-15.15. Note: GB, Note: SC and GB, DC SC represent and DC grouped represent blisters, grouped slight blisters,cracks and slight discoloration cracks and, discoloration,respectively. respectively.

3.9.3.9. Membrane Membrane Water Water Permeability Permeability and and BSABSA RejectionRejection It isIt is generally generally noted noted that that the the pure pure waterwater fluxflux of a membrane is is greatly greatly influenced influenced by by its its surface surface hydrophilicity,hydrophilicity, pore pore size size and and porosity porosity [[70,7170,71].]. From Table Table 1 andand Figure 7, showing the the porosity porosity and and contactcontact angle angle results, results respectively,, respectively, it it is is seen seen thatthat the hydrophilicity and and the the porosity porosity of of the the embedded embedded membranesmembranes gradually gradually increased increased with with increasedincreased acidacid concentrationsconcentrations but but with with a afew few exceptions. exceptions. In Inthe the samesame manner, manner, the the permeation permeation results results in in FigureFigure 1010 show a similar similar trend trend,, as as most most of of the the membranes membranes on on exposure Days 7 and 28 displayed high fluxes, with an exceptional increase in membranes on exposure Days 7 and 28 displayed high fluxes, with an exceptional increase in membranes on exposure exposure Day 28. This outcome indicates that the PVDF membranes’ properties were altered Day 28. This outcome indicates that the PVDF membranes’ properties were altered significantly, significantly, even with exposure to low acidic concentrations. The fluxes of the membranes exposed even with exposure to low acidic concentrations. The fluxes of the membranes exposed to 1 mol/L to 1 mol/L almost doubled after exposure Days 7 and 28, with a percentage increase of more than almost doubled after exposure Days 7 and 28, with a percentage increase of more than 90%, while 90%, while these values almost tripled at 8 and 15 mol/L concentrations. This further increase could thesebe explained values almost by the tripled protonation at 8 and effect 15 mol caused/L concentrations. by the sulfuric Thisacid furtherattack on increase the membrane could bes’ explainedsurface by(which the protonation was earlier e ff explainedect caused in by Section the sulfuric 3.7), resulting acid attack in intense on the degradation membranes’ of surface the membrane (which wass’ earlierstructure explained via pore in Section enlargement. 3.7), resulting Meanwhile, in intense an abrupt degradation decrease of was the membranes’ seen in the fluxes structure of all via the pore enlargement.membranes Meanwhile, at all levels an of abrupt acid concentration decrease was on seen exposure in the fluxes Day of 56. all This the membranesunusual reduction at all levels is ofhypothesized acid concentration to be oncaused exposure by the Day contamination 56. This unusual of Al reductionand Si resulting is hypothesized from the chemical to be caused reaction by the. contaminationHowever, ther ofe Al was and a further Si resulting increase from in the the chemical water flux reaction. for all acid However, concentration there was levels a further as the increase days in theof exposure water flux extended. for all acid This concentration trend is akin to levels the FE as- theSEM days images of exposure captured extended.in Figure 2 This, which trend validated is akin to thethe FE-SEM anomaly images in the captured results of inthe Figure embedded2, which membrane validated flux, the as anomaly there was in an the enlargement results of the of the embedded pores membranein the membranes flux, as there on exposure was an enlargementDays 7 and 28 of, which the pores slightly in the shrank membranes on exposure on exposure Day 56 and Days enlarged 7 and 28, whichagain slightly on exposure shrank D onay exposures 84 and Day 112. 56The and neat enlarged membrane again on on all exposure exposure Days days 84 (although and 112. Theit was neat membraneimmersed on in allwater) exposure showed days a flux (although with an itaverage was immersed value of 545.5 in water) Lm−2h showed−1. Generally, a flux increase with ans in average the acidic or alkaline2 concentration1 and immersion time largely contribute to large flux increments, value of 545.5 Lm− h− . Generally, increases in the acidic or alkaline concentration and immersion time making the highest flux to be 1243.4 Lm−2h−1. In addition, the decrease in the contact 2angle1 values at largely contribute to large flux increments, making the highest flux to be 1243.4 Lm− h− . In addition,

Membranes 2020, 10, x FOR PEER REVIEW 18 of 22

higher concentrations of sulfuric acid, as seen in Figure 7, is a validation of the tuning of the PVDF membrane’s structure to become more hydrophilic. The BSA rejection ratios of the PVDF-neat, PVDF-1 and PVDF-15 membranes are presented in Table 2. These membranes are selected as representatives of the neutral, minimum and maximum values for the three identified membranes. The neat membrane (which is the same for all the exposure periods) gives the best rejection performance of 93.2%. Meanwhile, the rejection performance of the membranes decreased after their exposure to 1 mol/L concentration of the acid during the period of exposure. In addition, a further increase in the concentration of the acid up to 15 mol/L resulted in a Membranes 2020, 10, 375 18 of 22 more drastic reduction in the separation efficiency, with only the exception of membranes on exposure Day 56, which had better rejection performances for both the 1 and 15 mol/L concentration thethan decrease exposure in D theays contact 84 and angle112. The values reduction at higher in the concentrations rejection performance of sulfuric is acid, envisaged as seen to in be Figure caused7, isby a the validation alteration of thein the tuning membrane of the’s PVDF surface membrane’s morphology structure and properties to become due more to deterioration. hydrophilic.

1500 Day 7

Day 28 )

1 Day 56

- 1250 h

. Day 84 2 - Day 112

1000

m

.

L

(

x u

l 750

f

r

e t

a 500

w

e

r u

P 250

0 PVDF-Neat PVDF-1 PVDF-2 PVDF-4 PVDF-8 PVDF-15 Membranes

FigureFigure 10.10. Permeate fluxflux of neat and embedded membranes in varying sulfuric acid concentrationsconcentrations atat didifferentfferent immersionimmersion periods.periods.

The BSA rejectionTable ratios 2. Rejection of the PVDF-neat, study of neat PVDF-1 and embedded and PVDF-15 PVDF membranes membranes. are presented in Table2. These membranes are selected as representatives of the neutral, minimum and maximum BSA Rejection (%) values for theMembrane three identified ID membranes. The neat membrane (which is the same for all the exposure periods) gives the best rejection7 Days performance 28 Days of 93.2%. 56 Meanwhile, Days 84 the Days rejection 112 performance Days of the membranesPVDF decreased-Neat after 93.2 their ± 0.78 exposure 93.2 to ± 10.78 mol /L93.2 concentration ± 0.78 93.2 of ± the0.78 acid 93.2 during ± 0.78 the period of exposure. InPVDF addition,-1 a further70.7 ± 1.63 increase 65.95 in ± the 3.04 concentration 76.8 ± 1.98 of55.7 the ± acid 1.34 up 49.9 to 15 ± mol1.77/ L resulted in a more drasticPVDF reduction-15 57.0 in the ± 1.60 separation 55.9 ± e 1.35fficiency, 56.9 with ± 1.84 only 45.8 the ± exception 1.27 31.4 of ± membranes 2.55 on exposure Day 56, which had better rejection performances for both the 1 and 15 mol/L concentration than4. Conclusions exposure Days 84 and 112. The reduction in the rejection performance is envisaged to be caused by theThe alteration effect of in sulfuric the membrane’s acid on the surface chemical, morphology mechanical and and properties physical due properties to deterioration. of a typical flat- sheet PVDF membrane, immersed at different concentrations and exposure times, was assessed using Table 2. Rejection study of neat and embedded PVDF membranes. various analytical techniques. The outcome of the study reveals that the flat-sheet PVDF membranes were immediately attacked by the sulfuric acid aqueousBSA Rejection solution (%) , even at the lowest concentration. Membrane ID Further gradual deterioration7 Days in the 28 properties Days of 56 theDays membran 84 Dayse were observed 112 Days at higher concentrationPVDF-Neats and exposure 93.2 time0.78s. The 93.2 morphology0.78 93.2 and0.78 pure water 93.2 flux0.78 measurement 93.2 0.78 s revealed ± ± ± ± ± PVDF-1 70.7 1.63 65.95 3.04 76.8 1.98 55.7 1.34 49.9 1.77 these gradual changes, which ±occur on the surface± of the± membrane. However± , the ±changes in their PVDF-15 57.0 1.60 55.9 1.35 56.9 1.84 45.8 1.27 31.4 2.55 crystalline phase and the functional± group ±from the XRD± and the FTIR± analyses, respectively± , were not significantly noticed, the decrease in the mechanical strength further attest to the fact that the 4. Conclusions

The effect of sulfuric acid on the chemical, mechanical and physical properties of a typical flat-sheet PVDF membrane, immersed at different concentrations and exposure times, was assessed using various analytical techniques. The outcome of the study reveals that the flat-sheet PVDF membranes were immediately attacked by the sulfuric acid aqueous solution, even at the lowest concentration. Further gradual deterioration in the properties of the membrane were observed at higher concentrations and exposure times. The morphology and pure water flux measurements revealed these gradual changes, which occur on the surface of the membrane. However, the changes in their crystalline phase and the Membranes 2020, 10, 375 19 of 22 functional group from the XRD and the FTIR analyses, respectively, were not significantly noticed, the decrease in the mechanical strength further attest to the fact that the membranes’ soundness was compromised. The zeta potential result shows the predominant charge on the immersed membranes, caused by the sulfuric acid attack, at both at low and high concentrations of acid, to be negative. In addition, the porosity, contact angle and visual examination results further affirm the impact of the acid on the PVDF membrane. The increase in the pure water flux and the corresponding decrease in the BSA rejection study show the inherent tendencies of deterioration which can occur on flat-sheet PVDF membranes which are subjected to use in an acidic environment.

Supplementary Materials: The following are available online at http://www.mdpi.com/2077-0375/10/12/375/s1, Figure S1: X-ray diffraction spectra for neat and embedded membranes on exposure Days 28, 56 and 84. Figure S2: FTIR spectra of neat and embedded membranes on exposure Days 28, 56 and 84. Table S1: Diffraction angles 2θ of the characteristic peaks. Author Contributions: Conceptualization, K.H.L. and K.Z.; funding acquisition, K.Z.; investigation, K.H.L.; methodology, K.H.L., W.Y. and F.F.; project administration, F.F. and K.Z.; resources, H.T. and F.F.; supervision, K.Z.; validation, T.F.A. and H.T.; writing—original draft, K.H.L.; writing—review and editing, W.Y., T.F.A. and H.T. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by grants from the Bureau of International Cooperation (132C35KYSB20160018), the Chinese Academy of Sciences (CAS) and the Chinese Academy of Sciences—The World Academy of Sciences (CAS-TWAS) president’s fellowship program for developing countries. Acknowledgments: The authors appreciate Qin Liu for her valuable comments. Conflicts of Interest: The authors declare no conflict of interest.

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