J Polym Eng 2015; x(x): xxx–xxx

*Corresponding author: Woei Jye Lau, Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia, e-mail: lwoeijye@utm.myMei Jiun Lee: Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia; and Faculty of Engineering and Science, Setapak Campus, Universiti Tunku Abdul Rahman, Jalan Genting Klang, 53300 Setapak, Kuala Lumpur, MalaysiaChi Siang Ong, Be Cheer Ng and Ahmad Fauzi Ismail: Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, MalaysiaSoon Onn Lai: Faculty of Engineering and Science, Setapak Campus, Universiti Tunku Abdul Rahman, Jalan Genting Klang, 53300 Setapak, Kuala Lumpur, Malaysia

Q1:Might it be better to sup-ply the entire address in English for consistency?

Mei Jiun Lee, Chi Siang Ong, Woei Jye Lau*, Be Cheer Ng, Ahmad Fauzi Ismail and Soon Onn Lai

Degradation of PVDF-based composite membrane and its impacts on membrane intrinsic and separation properties

DOI 10.1515/polyeng-2015-0064Received February 21, 2015; accepted June 28, 2015

Abstract: In this work, an attempt was made to evalu-ate the effects of ultraviolet (UV) irradiation period on the intrinsic and separation properties of composite membrane composed of organic polyvinylidene fluoride and inorganic titanium dioxide (TiO2) nanoparticles by exposing the membrane to UV-A light for up to 250 h. The changes on membrane structural morphologies and chemical characteristics upon UV exposure were stud-ied by field- emission scanning electron microscope and Fourier transform infrared, respectively. It was observed that some cracks and fractures were formed on the mem-brane outer surface when it was exposed to 120-h UV light. Further increase in UV irradiation time to 250  h had caused membrane structure to collapse, turning it into powder form. Filtration experiments showed that the permeate flux of irradiated membrane was significantly increased from 10.89 L/m2 h to 21.84 L/m2 h ( > 100% flux increment) while oil rejection decreased with increasing UV exposure time from 0  h to 120 h. Furthermore, the mechanical strength and thermal stability of irradiated membrane were also reported to decrease with increas-ing UV exposure time, suggesting the negative impacts of UV light on the membrane overall stability. This research

is of particular importance to evaluate the suitability and sustainability of polymeric membrane, which is widely considered as the host for photocatalyts and used for wastewater treatment process under UV irradiation.

Keywords: chemical composition; degradation; PVDF; spectroscopy; UV exposure.

1 IntroductionContaminated wastewaters with aromatic and aliphatic compounds produced from oil and gas industries have a severe and widespread impact on the world energy and economy [1]. There is an urgent need to find an effective way to reduce the amount of contaminant releases to our natural water body systems. Although oily wastewater has been treated by several conventional techniques including centrifugation [2, 3], electrocoagulation [4], membrane fil-tration [5–7] and flotation process [8, 9], these techniques are associated with high operating cost and lack of capa-bility in removing oil molecules below 10 μm in size.

Advanced oxidation process (AOP) has been con-sidered as a promising technique due to its advantages of degrading recalcitrant, toxic and non-biodegradable compounds. Of the AOP methods available, submerged membrane photocatalytic reactor (sMPR) has drawn immense attention owing to its capability in degrading organic pollutants and producing treated solution simul-taneously [10–19]. Furthermore, sMPR offers great advan-tages such as low operating cost, ease of maintenance and high efficiency in removing organic substances [10]. In general, there are two distinctive configurations for sMPR, namely (i) reactor with catalyst suspended in the feed solu-tion and (ii) reactor with catalyst immobilized in/on the membrane [11]. The latter option is more favorable owing to its simplicity in reactor design, cost-effectiveness and efficient catalyst recovery. However, one inherent problem for this configuration is the possible deterioration of the polymeric membrane material when membrane is directly

2      M.J. Lee et al.: Degradation of PVDF-based composite membrane

exposed to ultraviolet (UV) light during treatment process. The immobilized photocatalysts might absorb UV light energy, causing membrane aging and further altering its surface morphology and separation performance.

In general, the initiation of the hydrocarbon com-pound (RH) photodegradation takes place following the generation of free radicals upon UV illumination (Eq. 1). Those hydrocarbon radicals (R·) generated would then react with oxygen to form peroxy radicals (ROO·) (Eq. 2), which subtract hydrogen molecules from polymer chain (Eq. 3), resulting in hydroperoxides (ROOH) and other hydrocarbon radicals. The decomposition of hydroper-oxides can then lead to backbone cleavage of O-O bonds followed by β-scission of the polymer chain. The radical propagations can be terminated when two hydrocarbon radicals combine (Eqs. 4–5), which results in signifi-cant surface morphology changes and formation of low-chained molecular compounds (ROOR or RR) [20, 21].

Initiation: R H H R→ ⋅+ ⋅ (1)

2Propagation: R O RO O⋅+ → ⋅ (2)

ROO RH ROOH R⋅+ → + ⋅ (3)

Termination: ROO R ROOR⋅+ ⋅→ (4)

R R RR⋅+ ⋅→ (5)

It has been known that the UV energy is the main factor for most of the damages on membrane surface, as the high radiation energy tends to induce chain cleavage in the mol-ecules, leading to not only structural deterioration but also chemical changes in polymeric membranes [22–24]. For instance, Rupiasih et al. [23] found that some cracks and fractures appeared on the surface of the membrane made of polysulfone (PSf) when the membrane was exposed to UVC light (with intensity of 0.28 W/m2) for only 45–60 min. The surface damages are elucidated to the dissociation of bonds, i.e. C-S and C-O groups of polymeric materials upon UV exposure. The changes in membrane surface properties have also been found to significantly increase pure water flux of membrane from 900 L/m2 h reported in the control membrane to  > 3000 L/m2 h in the membrane irradiated with 60-min UV light. Similar morphological changes were also observed by Kushwaha et al. [24] on poly(2,2′-ethylene-5,5′-bibenzimidazole) (PBIE) membrane surface after UV irradiation. It was further reported that UV irradiation has strong impact on membrane surface chemistry, owing to the significant bond dissociation occurring for both car-bonyl and hydroxyl groups that corresponded to the PBIE polymer. These findings were further supported by Jyothi et al. [25] where the surface of PSf/titanium dioxide (TiO2) mixed matrix membrane suffered from random cracks after

only 40 min of UV exposure. Although most of the studies show that there are significant changes on the membrane surface after UV irradiation, very limited researches have been concerned with the membrane separation performance with respect to the membrane microstructure, morphologi-cal changes and chemical composition under prolonged UV exposure. It is of importance to provide new insight into how the changes in polymeric membrane intrinsic characteristics upon long-term UV exposure would affect the performance stability of membrane in photocatalytic membrane process. Different types of polymers have been previously used to fab-ricate photocatalytic membranes, but the sustainability of using polymer-based membrane process under UV irradia-tion has rarely been reported [23, 24].

In this study, an attempt is made to evaluate the intrin-sic properties and performance stability of the composite membrane composed of organic polyvinylidene fluoride (PVDF) and inorganic TiO2 photocatalyts by exposing the membrane to UV light for up to 250 h. This type of com-posite membrane has been previously evaluated for the treatment process of oily wastewater, but the entire exper-imental period (under UV irradiation) was not more than 10  h [18]. In terms of membrane surface properties and separation performances, no significant changes were experienced for relatively short period of UV exposure. The aim of this study is to provide valuable information on how the long-term effect of UV irradiation on polymer membrane would affect membrane structural morpholo-gies and further its separation performance. The research is of particular importance to evaluate the suitability and sustainability of the polymeric membrane that is widely considered as the host for photocatalyts and used for wastewater treatment process under UV irradiation.

2 Materials and methods

2.1 Materials

PVDF (Kynar® 760) pellets purchased from Arkema Inc., Philadelphia, USA, were used as the main mem-brane forming material. N,N-dimethylacetamide (DMAc) (Merck,  > 99%) was used as solvent to dissolve polymer without further purification. Polyvinylpyrrolidone (PVP) (molecular weight: 40,000 g/mol) (molecular structure shown in Figure 1) purchased from Sigma Aldrich and titanium dioxide (TiO2) (Degussa P25, a mixture of 75% anatase and 25% rutile with BET surface area of 50 m2/g, average particle size ~21  nm and energy band gap of 3.18 eV) from Evonik were used as the photocatalyst.

Q2:Please supply the manu-facturer’s name, city, state and country for all reagents, devices, and software mentioned throughout

M.J. Lee et al.: Degradation of PVDF-based composite membrane      3

Two 36-W UV-A lamps (Model: PL-L 36W/10/4P, Philips) were located at the top and bottom of the chamber. PVDF-TiO2 hollow fiber membranes were placed in the middle of the chamber with distance of around 4  cm to the top and bottom UV lamps. Total light intensity of approxi-mately 170 mW/cm2 (from two UV lamps) was recorded at the middle of the chamber by a UVX radiometer (Model: UVP Inc., Upland, CA, USA) with a UV-A sensor (Model: UVX-36, UVP Inc., Upland, CA, USA). The membrane expo-sure period intervals for UV irradiation were set at 0, 40, 60, 120, 170, 200, 225 and 250 h. For each interval, several fibers were taken and used for further analysis.

2.4 Ultrafiltration experiment

The separation performance was assessed by placing two U-shaped hollow fiber membrane modules at the bottom of a submerged tank containing approximately 14 l of oily wastewater. The synthetic oily wastewater was prepared by mixing 1000  ppm crude oil obtained from Terengganu Crude Oil Terminal (Location: RE110, Malaysia) and sodium dodecylbenzenesulfonate (SDS) at the ratio of 9:1 under an agitation speed of 50  Hz using a high-speed blender (Model: BL 310AW, Khind) for approximately 2  min at room temperature. To mini-mize the fouling effect, a constant air flow rate of 5 l/min generated by an air compressor (Model: 2HP single cylin-der 24 l tank, Orimas) was used to generate air bubbles within the submerged tank through air diffuser installed underneath the membrane modules. Water permeate was produced using a peristaltic pump (Model: 77200-60, Masterflex l/S, Cole Parmer) by creating a vacuum condition on the permeate side. Both the vacuum pres-sure and pump flow rate were kept constant at -15 inHg and 15 ml/min, respectively, throughout the experimen-tal period. A 10-ml sample was taken from the permeate for sample analysis, and the remaining permeate was recycled back to the tank. Three measurements were made for each sample, and then the average value was reported. The membrane water flux (J) was determined according to Eq. (6).

QJAt

=

(6)

where J is the water flux (L/m2 h), Q is the quantity of the permeate (L), A is the effective membrane area (m2), and t is the time (h) to obtain the quantity of Q. The membrane oil rejection was then calculated using the following equation.

2.2 Preparation of membrane and membrane module

Eighteen weight percent of PVDF was added into pre-weighed DMAc solvent after being dried for 24  h in an oven at 50°C. The solution was then mechanically stirred at 600 rpm until all the polymeric pellets were completely dissolved. It was followed by the addition of 2 wt% of TiO2 nanoparticles and 5 wt% of PVP into the mixture. The dope solution was then ultrasonicated to remove any air bubbles trapped in the solution prior to the spinning process.

PVDF-TiO2 hollow fiber membranes were fabricated using dry-jet wet spinning method as described else-where [16]. The as-spun hollow fibers were immersed into water bath for 2  days to remove residual solvent. Prior to air drying, the fibers were post-treated using 10 wt% glycerol aqueous solution for 1 day to minimize fiber shrinkage and pore collapse. Lastly, the hollow fibers were dried at room temperature for 3 days before module fabrication.

A bundle of 60 hollow fibers with approximate length of 28 cm was then potted into each PVC tube using epoxy resin (E-30CL Locite® Corporation, USA). The membrane module was then left for hardening at room temperature before its protruding parts were cut and fixed into a PVC adaptor to complete the module preparation.

2.3 Membrane UV exposure chamber

Figure 1 illustrates a membrane UV exposure chamber made of aluminum with size of 33 cm (W) × 53 cm (L) × 10 cm (H).

Figure 1: Schematic diagram of the UV exposure chamber: (A) top UV lamp, (B) bottom UV lamp, (C) aluminum foil tray and (D) PVDF-TiO2 hollow fibers.

4      M.J. Lee et al.: Degradation of PVDF-based composite membrane

P

F

1- 100C

RC

= ×

(7)

where R is the oil rejection (%), Cp and CF are the concen-tration of oil in the permeate (ppm) and the feed (ppm), respectively. The oil concentration in the permeate and feed was determined using a UV-vis spectrophotometer (Model: DR5000, Hach) measured at a wavelength of 278 nm at which the maximum absorption occurs.

2.5 Membrane characterization

The membrane surface and cross-sections of the samples were examined using field-emission scanning electron microscope (FESEM) (Model: SU8020, Hitachi). The dry membrane samples were immersed in liquid nitrogen and fractured, followed by sputter-coating with platinum using a sputtering device. An energy dispersive X-ray (EDX) spec-troscope using an acceleration voltage of 10 kV and mag-nification of 60,000 ×  was used for elemental analysis in order to confirm the polymer degradation on membrane surface. Fourier transform infrared (FTIR) spectra of the membrane samples were performed using attenuated total reflectance Fourier transform infrared spectroscope (ATR-FTIR) (Model: Nicolet 5700, Thermo Electron Scientific

Instruments Corporation). These spectra were recorded at resolution between 650 and 4000 cm-1. Thermal gravimetric analysis (TGA) of the membrane samples was recorded by thermogravimetry analyzer (Model: TGA/SDTA85, Mettler Toledo). The membrane sample was cut into small pieces, weighed and placed into a pre-weighed aluminum cru-cible. The samples were heated from 30°C to 800°C at a heating rate of 10 °C/min, under nitrogen atmosphere, with a nitrogen flow rate of 20 ml/min. Fiber tensile test was performed at room temperature on a tensile tester (Model: LRX 2.5KN, LLYOD). The gauge length of membrane sample was fixed at 50 mm, and the gauge running speed was set at 10 mm/min. The mechanical properties of the membrane were then evaluated with respect to tensile strength and elongation-at-break using NEXTGEN software.

3 Results and discussion

3.1 Effect of UV irradiation on membrane intrinsic properties

Figure 2 shows the FESEM images of the outer surface of the hollow fiber membranes after exposing to UV light for different periods. The results revealed that cracks and

A B

C D

Figure 2: FESEM surface images of PVDF-TiO2 membranes after exposure to UV of (A) 0 h, (B) 40 h, (C) 60 h and (D) 120 h.

M.J. Lee et al.: Degradation of PVDF-based composite membrane      5

fractures on the irradiated membrane surface became more obvious with increasing UV exposure time. Selective layer (dense structure) of the membrane was significantly altered after it was exposed to UV light of 120 h. In this study, membrane UV exposure experiments were con-ducted for a maximum period of 250 h, but the characteri-zation on the membrane properties was unable to perform for the membrane exposing to UV light of  > 120 h. This is because the cylindrical-shaped hollow fiber membrane was turned to powder form at high UV exposure time

( > 120 h). The propagation of surface cracks at the early stage of UV exposure (40 h) can be ascribed to the absorp-tion of UV energy on membrane surface which generates a large amount of free radicals that causes membrane surface starts to deform. It is also possible that the ther-mal-induced expansion and contraction between surface and interior of the irradiated membrane under continu-ous UV irradiation are the factors causing the formation of cracks and fractures as discussed in a previous work [26]. Figure 3 further compares the changes in membrane

A

B

C

Figure 3: FESEM images (left – 0 h and right – 120 h) of cross-sectional morphologies of PVDF-TiO2 membranes with different magnifica-tions, (A) 100 × , (B) 600 ×  and (C) 5000 × .

6      M.J. Lee et al.: Degradation of PVDF-based composite membrane

cross-sectional morphology at different UV irradiation period. Clearly, the wall thickness of the membrane was significantly reduced from 153 μm in the control mem-brane to  < 120 μm in the membrane exposed to 120-h UV light. The images also revealed that UV light only affects the morphology of the membrane outer surface, as there is barely change in the membrane structure near its lumen. The reduced membrane wall thickness might have influ-ence not only on the membrane surface chemical proper-ties but also on its mechanical stability.

As shown in Table 1, the decreasing contents of C and F element with increasing UV irradiation have provided additional evidence to support the physical changes in membrane outer surface as presented in Figure 2. Lower C and F contents are likely due to the degradation of the PVDF polymer (-(C2H2F2)n-) under UV irradiation.

3.2 Effect of UV irradiation on membrane chemical composition

To further understand the effect of UV exposure on PVDF-TiO2 composite membranes, FTIR analysis was performed and the results are shown in Figure 4. It can be seen that

Figure 4: FTIR spectra of PVDF-TiO2 composite membrane at differ-ent UV exposure time.

Table 1: EDX results of PVDF-TiO2 membrane at different UV expo-sure time.

Elements (wt%)  0 h  40 h  60 h  120 h   > 120 h

F   49.92  47.76  39.88  37.28  n/aC   38.29  35.12  28.56  26.64  n/aO   6.88  9.02  16.63  18.21  n/aTi   4.92  8.11  14.93  17.86  n/a

there are some chemical changes in the irradiated mem-brane for 120 h compared with that of without UV irradia-tion. The peaks at 1043.4 cm-1 and 1109.7 cm-1 are assigned to stretching vibration of C-N group owing to the pres-ence of polyvinylpyrrolidone (PVP) (-(C6H9NO)n-), which was used as a pore former to enhance flux performance in composite membrane [27, 28]. The dissociation of C-N group is due to the absorption of UV energy on membrane outer surface that causes the degradation of the organic compound (PVP and/or PVDF) into inorganic products. Furthermore, the peak at 996 cm-1 is assigned to stretching vibration of C-C group in the PVDF-TiO2 membrane [29]. The scission of this C-C group is caused by the photodeg-radation of PVDF polymer upon UV illumination.

Nevertheless, there are no significant chemical changes between α- and β-phase of PVDF main polymer with increasing UV exposure time. Those peaks at 763 cm-1, 876  cm-1 and 1180  cm-1 are assigned to α-phase of PVDF, while those in the range of 1400–1200  cm-1 and several peaks at 1072 cm-1 and 840 cm-1 are attributed to β-phase of PVDF polymer [30]. Our findings were consistent with the report of Gu et al. [31] in which PVDF membranes blended with and without poly(ethyl acrylate) (PMMA-co-PEA) copolymer were prepared and subjected to UV exposure. The chemical changes in the membranes were mainly attributed to the degradation of the co-polymer rather than that of the PVDF main polymer. Although the mechanisms involved have not been clearly established, it is believed that the absorption of UV energy does, to some extent, alter the surface chemistry of the polymeric membrane, damag-ing their repeating structural units. It must be also pointed out that the IR beam penetration depth during FTIR analy-sis might exceed the top surface of altered PVDF-based membrane, covering internal parts beneath.

3.3 Effect of UV irradiation on membrane flux and oil rejection

In this section, the filtration performances of both pris-tine and irradiated PVDF-TiO2 composite membranes were evaluated using synthetic oily wastewater contain-ing 1000  ppm of oily wastewater. The membranes were tested under submerged condition using self-customized submerged reactor, as described in our previous work [18]. Table 2 shows the membrane permeate flux as a function of UV exposure time. It was observed that the permeate flux of irradiated membranes was significantly increased from 10.89 L/m2 h to 21.84 L/m2 h with increas-ing UV exposure time from 0 h to 120 h, recording  > 100% flux increment. The increased water flux of membrane

M.J. Lee et al.: Degradation of PVDF-based composite membrane      7

UV irradiation time. With increasing UV exposure time, the PVDF-TiO2 membrane demonstrated a reduction in the tensile strength and elongation-at-break. The deterioration in membrane mechanical properties can be well-correlated to the formation of cracks on the membrane surface as well as reduced wall thickness, as evidenced in Figures 2 and 3, respectively. Although the mechanisms involved have not been clearly established, it is believed that the surface defects and reduction in membrane wall thickness would act as weak points of the entire membrane integrity and further results in high tendency of membrane structure col-lapse [33]. Cui et al. [26] also found that increasing UV expo-sure time for a bio-renewable polymer from 250 h to 2000 h could lead to deleterious effect on polymer mechanical strength following the formation of surface cracks on the studied polymer during photodegradation. Apart from the weakening mechanical properties, the membrane thermal property with respect to decomposition temperature (Td) was reported to decrease from 406°C to 390°C after only 120-h UV irradiation. The lower thermal stability of irra-diated membrane can be ascribed to the change in struc-tural morphologies of PVDF-TiO2 composite membrane as a result of free radical-induced degradation. Similar pattern was observed by Kushwaha et al. [24] in which a significant decrease of Td was experienced for poly(2,2′-ethylene-5,5′-bibenzimidazole) (PBIE) membrane irradiated for  > 250-h UV light. They attributed the deterioration in thermal sta-bility of irradiated membrane to the change in molecular structure and molecular weight of PBIE as oxidized by strong radicals produced from UV irradiation.

4 ConclusionThe properties and performance stability of PVDF-TiO2 composite membrane irradiated for 250  h have been studied. The formation of cracks and fractures was observed on the outer surface layer of membrane when it was exposed to 120-h UV light. Further increase in UV irradiation time to 250 h had caused membrane structure

can be related to the formation of cracks and/or decreased membrane wall thickness as evidenced in Figures 2 and 3. As a consequence, it reduces water transport resistance and increases water permeability. Similar findings were also reported by You and Wu [32] in which they found that higher permeate flux was obtained for the membrane irradiated with longer period of UV light. However, they attributed the increased water flux to the enhanced hydro-philicity and antifouling properties of the UV-irradiated membrane, instead of polymer deterioration. According to them, upon UV irradiation, the larger organic matters deposited on membrane surface are degraded into smaller organic matters by the formation of OH· radicals, causing them to be easily removed and thereby improving the anti-fouling properties and increasing water flux.

With respect to membrane separation efficiency, the oil rejection increased from 85% to 98% when UV expo-sure time was increased from 0  h to 60  h but gradually decreased to 92% when UV exposure time was further increased to 120 h. It can be explained by the effect of oil droplet size on the membrane surface. As reported in our previous studies [33, 34], the oil droplet size dis-tribution of the synthesized solution was in the range of 0.4–2.6 μm, with a mean particle diameter of 1.08 μm (±0.16). This allowed some smaller oil particles to perme-ate through the membrane surface at higher UV exposure time ( > 60 h), owing to the increases of surface defects (as shown in Figure 2) that appeared on the irradiated mem-brane outer layer, thus resulting in the deterioration of oil rejection but with higher water flux.

3.4 Effect of UV irradiation on membrane properties with respect to mechanical strength and thermal stability

Table 3 presents the mechanical and thermal properties of the studied PVDF-TiO2 membrane exposed to different

Table 2: Effect of UV irradiation on permeate flux and oil rejec-tion as a function of UV exposure time (operating conditions: temperature = 25°C, oil concentration = 1000 ppm, air bubble flow rate = 5 l/min and vacuum pressure = -15 inHg).

UV exposure time (h)  Permeate flux (L/m2 h)  Oil rejection (%)

0   10.89  8540   11.54  9760   16.03  98120   21.84  92 > 120a   n/a  n/a

aMembrane module was not prepared, as hollow fiber membrane turned into powder after  > 120-h UV irradiation.

Table 3: Effect of UV irradiation on mechanical strength and thermal stability of PVDF-TiO2 membrane as a function of UV expo-sure time.

UV exposure time (h)

  Tensile strength (MPa)

  Elongation-at-break (%)

  Td (°C)

0   1.93  326%  40640   1.90  160%  42160   1.82  128%  406120   1.73  120%  390

Q3:Reference “You et al. [32]” has been changed to “You and Wu [32]” to match with the reference list. Please check and confirm

Q4:Please check the edit made to the sentence “According to them, … increasing water flux.” retained its intended meaning

8      M.J. Lee et al.: Degradation of PVDF-based composite membrane

to collapse, turning it into powder form. The degradation of membrane was further supported by the reduced mem-brane wall thickness and lower C and F contents at high UV exposure time. Furthermore, the surface chemistry of the composite membranes was altered by the dissociation of C-N and C-C bonds under UV irradiation. Those changes in membrane physical appearance and chemical com-position had resulted in deleterious effect to both mem-brane mechanical strength and thermal stability. With respect to the membrane performance, the permeate flux of irradiated membrane was significantly increased from 10.89 L/m2 h to 21.84 L/m2 h, while oil rejection decreased with increasing UV exposure time. Overall, the findings of this work are of particular importance to the research-ers who have considered the polymeric-based membranes as the host for photocatalyts and used the membranes for photocatalytic process under UV irradiation.

5 Future worksAlthough PVDF is generally rated as good UV-resistant polymer, the results from our current work have shown that the properties of membranes made of this polymer were severely altered under prolonged UV exposure time. There-fore, in order to improve the sustainability and practicability of the photocatalytic membrane in the wastewater treatment process, other types of polymers that exhibit much better UV resistance should be explored. As reported in a technical report published by Cole Parmer [34], polyetherimide and polyimide polymer, which have been used as the construc-tion materials for the advanced instruments manufacturing in the outer space, could be the most potential polymeric materials for photocatalytic membrane making.

Acknowledgments: The authors gratefully acknowledge the financial support by the LIMPID FEP-7 Collabora-tive European Project Nanocomposite Materials for Pho-tocatalytic Degradation of Pollutants (Project number: NMP3-SL-2012-310177).

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Q5:Please sup-ply missing elements in references and confirm if details provided are correct, such as year of publication and author names

Q6:Please sup-ply publisher location for refs. [11, 12, 33]

Q7:As per journal style et al. not allowed. Please confirm the list of author names in ref. [27]