applied sciences Article
Nanostructured Ceramic Photocatalytic Membrane Modified with a Polymer Template for Textile Wastewater Treatment Rizwan Ahmad 1,† , Jin Kyu Kim 2,† , Jong Hak Kim 2, * 1 2
* †
ID
and Jeonghwan Kim 1, *
Department of Environmental Engineering, Inha University, Inharo-100, Namgu, Incheon 402751, Korea;
[email protected] Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea;
[email protected] Correspondence:
[email protected] (J.H.K.);
[email protected] (J.K.); Tel: +82-32-860-7502 (J.K.) These authors contributed equally to this work.
Received: 5 November 2017; Accepted: 5 December 2017; Published: 9 December 2017
Abstract: Photocatalytic ceramic membranes have attracted considerable attention for industrial wastewater treatment. However, morphological control of the membrane surface to improve its photocatalytic reactivity for the degradation of organic pollutants remains a challenge. Herein, we report a new nanostructured TiO2 /Al2 O3 composite ceramic membrane prepared from a poly(oxyethylene methacrylate) (POEM) template through a sol–gel method and its photocatalytic performance in the treatment of a model dye compound. The POEM polymeric template allowed the homogeneous distribution of catalytic sites, i.e., the TiO2 layer, on the Al2 O3 membrane surface, resulting in improved organic dye degradation along with effective fouling mitigation. The immobilization of a TiO2 layer on the Al2 O3 membrane support also significantly enhanced the membrane adsorption capacity toward dye organic compounds. An organic removal efficiency of over 96% was achieved with the TiO2 /Al2 O3 composite membrane under Ultraviolet (UV) irradiation. In addition, the self-cleaning efficiency of the TiO2 /Al2 O3 composite membrane was remarkably improved by the degradation of organic foulants on the membrane under UV illumination. Keywords: photocatalytic membrane; antifouling; polymer template; TiO2 ; adsorption
1. Introduction The demand for advanced water treatment technologies is increasing for the treatment of high-strength wastewater, such as industrial wastewater including complex water pollutants. Membrane technology is a promising way to enhance wastewater treatment efficiency as it can produce great effluent (permeate) quality with a smaller footprint than that afforded by conventional wastewater treatment processes [1–3]. However, the polymeric membranes widely adopted in wastewater treatment applications display relatively low thermal and chemical resistance. There has been an upsurge of interest in ceramic membranes for the treatment of high-strength wastewater, for which polymeric membranes are not suitable [4,5]. Ceramic membranes consisting of metal oxides usually exhibit high membrane porosity, membrane permeability, and narrow distribution of the membrane pore size. These unique properties allow for superior separation characteristics and antifouling behavior than those of polymeric membranes. Photocatalytic membrane reactors (PMRs) combine photocatalytic reactors with membrane filtration units [6,7]. As the photocatalytic membrane is combined with oxidation promoters, such as UV irradiation, the membrane plays an important role as a support for the photocatalytic materials and as a selective barrier to degrade organic contaminants. The TiO2 materials are often considered
Appl. Sci. 2017, 7, 1284; doi:10.3390/app7121284
www.mdpi.com/journal/applsci
Appl. Sci. 2017, 7, 1284
2 of 12
as coating materials on ceramic support for the photocatalytic membrane because of their high photocatalytic activity and thermal/chemical stability at low environmental toxicity [8–10]. The TiO2 coating on the ceramic support has been reported to provide high water permeability with relatively low fouling rate to polymeric membrane [11,12]. The photo-degradation activity of nanostructured ceramic membranes functionalized in situ have been found to be very effective for wastewater treatment [9]. Thus, the photocatalytic layer on the membrane is expected to reduce the extent of membrane fouling while producing an excellent permeate quality due to the selective photocatalytic degradation of organic compounds on the catalytic membrane surface [13]. In addition, the intrinsic properties of the ceramic membrane against certain chemical reagents allow for the photocatalytic membrane to withstand the concomitant attack of the strong oxidants produced during photocatalytic processes, e.g., hydroxyl radicals [14]. The hierarchically porous nanostructure of the catalyst film on the ceramic membrane plays a critical role in determining the photocatalytic membrane performance. The morphology of the photocatalyst film on the membrane support has been found to be an important factor determining the membrane surface reactivity [15]. The introduction of surface-directing agents on the ceramic support has allowed the control of its porous structure [6,8,15,16]. For example, the porous structure of a photocatalyst film was enhanced using a Pluronic block copolymer for high organic dye removal efficiency [16]. Choi et al. [17] found that the mesoporous and narrow pore size distribution of photocatalyst films improve the wastewater treatment efficiency significantly. A grafted copolymer-based composite membrane exhibited excellent properties in terms of organic-compound degradation efficiency [15]. Homopolymers can also be used with great potential to improve the photocatalytic membrane reactivity. The photocatalytic activity of TiO2 composite membranes has been significantly enhanced by introducing hydrophobic homopolymers consisting of polyvinyl chloride (PVC) as a polymer template in the membrane support [6]. Nevertheless, developing photocatalytic membranes with uniform distribution of catalytic materials on ceramic support with fine-tuned nanostructure is still major challenge. Herein, we report the development of a nanostructured TiO2 /Al2 O3 composite ceramic membrane using a poly(oxyethylene methacrylate) (POEM) template through a sol–gel method to improve the photocatalytic membrane reactivity for textile wastewater treatment application. The prepared membrane was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The adsorption behavior and antifouling performance were also systematically evaluated by filtering a model dye solution. 2. Materials and Methods 2.1. Materials Poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol) methyl ether methacrylate, number average molecular weight (Mn)= 500 g/mol), poly(vinyl pyrrolidone) (PVP, average molecular weight = 40,000 g/mol), titanium(IV) isopropoxide (TTIP, 97%), and a hydrogen chloride solution (37 wt %) were purchased from Sigma-Aldrich, Youngin, Korea. Absolute ethanol and tetrahydrofuran (THF) were bought from J. T. Baker. The alumina support (α-Al2 O3 , thickness = 2 mm, diameter = 30 mm, pore size = 100 nm) was purchased from Nano Pore Materials Co., Ltd., Seoul, Korea. Milli-Q deionized water (resistivity: 18.2 MΩ and pH: 6.8) from ultrapure water production system (STS-8l, Human Science, Seoul, Korea) was used in experimental works performed in this study. 2.2. Membrane Fabrication To prevent the penetration of the polymer/TiO2 precursor mixture solution into the membrane pores, a PVP ethanol solution (10 wt %) was spin-coated on the bare Al2 O3 membrane at 500 rpm for 20 s and subsequently dried at 50 ◦ C in an oven for the complete removal of ethanol. To prepare the TiO2 precursor solution, HCl was slowly added to TTIP dropwise under vigorous stirring. Deionized
Appl. Sci. 2017, 7, 1284
3 of 12
for 20 s and subsequently dried at 50 °C in an oven for the complete removal of ethanol. To prepare Appl. 7, 1284 3 of 12 the Sci. TiO2017, 2 precursor solution, HCl was slowly added to TTIP dropwise under vigorous stirring.
Deionized water was added to the solution and stirred for at least 30 min. POEM (0.03 g) was dissolved in 1.5 mL of THF (2 wt. %) and mixed with 0.2 mL of the as‐prepared TiO2 precursor water was added to the solution and stirred for at least 30 min. POEM (0.03 g) was dissolved in solution over 3 h for the sol–gel process. The mixed polymer/TiO2 precursor mixture solution was 1.5 mL of THF (2 wt %) and mixed with 0.2 mL of the as-prepared TiO2 precursor solution over 3 h spin‐coated on the PVP‐coated Al2O3 membrane, followed by calcination at 450 °C for 30 min. After for the sol–gel process. The mixed polymer/TiO2 precursor mixture solution was spin-coated on calcination, the organic materials such as POEM, PVP, and the solvents were removed and a the PVP-coated Al2 O3 membrane, followed by calcination at 450 ◦ C for 30 min. After calcination, crystalline TiO2 film was obtained on the Al2O3 membrane. the organic materials such as POEM, PVP, and the solvents were removed and a crystalline TiO2 film was obtained on the Al2 O3 membrane. 2.3. Experimental Set‐Up 2.3. Experimental Set-Up The laboratory‐scale, dead‐end PMR is shown in Figure 1. The experimental set‐up consisted of a membrane module integrated with a UV lamp, feed reservoir, and data acquisition system. A The laboratory-scale, dead-end PMR is shown in Figure 1. The experimental set-up consisted of a Congo red module dye solution (100 mg La−1UV ) was used to simulate membrane integrated with lamp, feed reservoir,the dye and datawastewater. acquisition The feed solution system. A Congo −1. The test solution was placed in a 5 L feed was prepared by diluting a stock solution of 500 mg L − 1 red dye solution (100 mg·L ) was used to simulate the dye wastewater. The feed solution was reservoir by under continuous constant of solution 1.5 bar applied by a 1 . The test prepared diluting a stock stirring solutionat ofa 500 mg·L−pressure was placed in pressurized a 5 L feed oxygen tank to push the feed solution toward the membrane module equipped with a disk‐type reservoir under continuous stirring at a constant pressure of 1.5 bar applied by a pressurized oxygen ceramic membrane (effective of 4.5 cm2). module The aim of oxygen gas awas to enhance the tank to push the feed solutionsurface towardarea the membrane equipped with disk-type ceramic photocatalytic activity as it area can of act as cm an and reduce the the photocatalytic probability of 2 ).electron membrane (effective surface 4.5 The aimacceptor of oxygen gasthus wascan to enhance electron hole recombination. A UV‐C lamp (λ max = 254 nm, Philips TUV 4W SLM, Seoul, Korea) was activity as it can act as an electron acceptor and thus can reduce the probability of electron hole installed inside Athe membrane to irradiate UV light on membrane surface directly. The recombination. UV-C lamp (λmodule max = 254 nm, Philips TUV 4W SLM, Seoul, Korea) was installed thickness of water channel between UV lamp and membrane surface was approximately 2 cm. The inside the membrane module to irradiate UV light on membrane surface directly. The thickness of permeate water produced by the membrane in a permeate and water channel between UV lamp and membranemodule surfacewas was collected approximately 2 cm. Thevessel permeate weighed on an electronic balance (Ohaus Corporation, Pine Brook, NJ, USA). The data acquisition water produced by the membrane module was collected in a permeate vessel and weighed on an system was used to monitor the permeate weight on the electronic balance with the filtration time. electronic balance (Ohaus Corporation, Pine Brook, NJ, USA). The data acquisition system was used −2 h−1) was calculated by the permeate volume (L) divided by the The permeate flux (LMH) (L mon to monitor the permeate weight the electronic balance with the filtration time. The permeate flux 2 effective membrane surface area (m − 2 − 1 (LMH) (L m h ) was calculated by) and filtration time (h). The concentration of Congo red in the the permeate volume (L) divided by the effective membrane membrane permeate was analyzed by measuring its absorbance (λ ) at 496 nm using a UV‐Visible surface area (m2 ) and filtration time (h). The concentration of Congomaxred in the membrane permeate spectrophotometer (SCINCO, S‐3100, Seoul, Korea). Pure deionized water from an ultrapure water was analyzed by measuring its absorbance (λmax ) at 496 nm using a UV-Visible spectrophotometer production system (STS‐8l, Human Seoul, Korea) used in all the production experimental work (SCINCO, S-3100, Seoul, Korea). PureScience, deionized water fromwas an ultrapure water system performed in this study. (STS-8l, Human Science, Seoul, Korea) was used in all the experimental work performed in this study.
Figure 1. Experimental setup of the photocatalytic membrane reactor (PMR) with direct ultraviolet Figure 1. Experimental setup of the photocatalytic membrane reactor (PMR) with direct ultraviolet (UV) irradiation on membrane surface. (UV) irradiation on membrane surface.
Appl. Sci. 2017, 7, 1284
4 of 12
2.4. Photocatalytic Performance of the Membranes The adsorption capacity of the bare Al2 O3 and TiO2 /Al2 O3 composite membranes was examined by filtering a feed solution under dark conditions (i.e., without UV illumination). To evaluate the antifouling properties and photocatalytic activity of the membranes, filtrations were performed under UV illumination in dead-end filtration mode. All experimental works were carried out at least three times to confirm result reproducibility. The membranes were cleaned chemically after each experiment by placing them in a 1 mole/L NaOH solution (98% Beads, SAMCHUN Pure Chemicals Co., Ltd. Seoul, Korea) for 5 min and then in deionized water for 12 h. In order to know effect of temperature on membrane permeability, the permeability measured experimentally was compared with the one calculated by Darcy’s equation below at elevated temperature in permeate line under UV radiation at the end of filtration. ∆P J= (1) µRm where J represents membrane permeate flux (L m−2 h−1 ), ∆P is the trans-membrane pressure (bar), Rm is the membrane resistance (m−1 ), and µ represents viscosity (Pa·s) of pure water at specific temperature. 2.5. Analytical Methods for Membrane Characterization The surface and cross-section morphologies of the TiO2 film on the Al2 O3 membrane and the bare Al2 O3 membrane were investigated by field emission SEM (FE-SEM, SUPRA 55VP, Carl Zeiss, Germany/FE-SEM, JSM-7001F, JEOL Ltd., Tokyo, Japan). The crystalline phase of the composite membranes was characterized by XRD (generator: 40 kV, 40 mA, D8 ADVANCE with DAVINCI, BRUKER, Bremen, Germany, wavelength (λ): Cu kα1 (Å)-1.5418 Å, two theta range: 10–80◦ ). The morphology and crystallinity of the TiO2 structure detached from the fluorine-doped tin oxide (FTO) glass by scratching the slide glass were analyzed by high resolution-TEM (HR-TEM, JEM-3010, JEOL, Tokyo, Japan). The 3D surface morphology of the TiO2 film on the Al2 O3 membrane was characterized by AFM (XE-Bio, Park Systems, Suwon, Korea). 3. Results and Discussion 3.1. Membrane Morphology The nanostructure morphology of a membrane has a great effect on its photocatalytic performance for wastewater treatment. Polymer templates are considered effective structure-directing agents for the control of the nanostructure morphology. They not only effectively induce the organized structure of a membrane, but also have no negative impact on the membrane performance due to its complete degradation upon calcination. Since the hydrophilic TiO2 precursor TTIP has high affinity toward hydrophilic materials, it interacts strongly with the hydrophilic POEM chains. A POEM/TTIP hybrid was thus successfully formed during the sol–gel process. Since the low-viscosity POEM/TTIP solution can easily penetrate the macroporous Al2 O3 membrane, its presence will influence the nanostructure morphology and, subsequently, the membrane performance. Our previous study showed that polymeric template/TTIP solution could be penetrated into the pore structure of macroporous Al2 O3 support and it caused non-uniform photocatalytic film on the ceramic support [15]. With the help of a PVP coating on the Al2 O3 membrane [6,15], the penetration of the POEM/TTIP solution deep inside the membrane was prevented and a TiO2 thin film was thereby successfully obtained. During calcination at 450 ◦ C, TTIP was transformed into crystalline TiO2 , while the organic POEM, PVP, and solvents were all completely decomposed. The synthesized TiO2 film on the Al2 O3 membrane was found to be highly crystalline and well organized [15,18]. The surface of the TiO2 structure on the FTO glass and Al2 O3 membrane was characterized by SEM analysis (Figure 2). A uniform TiO2 film prepared with the aid of POEM was successfully synthesized on a flat FTO glass substrate. Owing to the nonporous substrate and strong interactions
Appl. Sci. 2017, 7, 1284 Appl. Sci. 2017, 7, 1284
5 of 12 5 of 12
between TTIP and POEM, the TiO2 film exhibits a well‐organized morphology, as shown in Figure between TTIP and POEM, the TiO2 film exhibits a well-organized morphology, as shown in Figure 2a. 2a. The bare membrane with a pore size of approximately 100 nm is clearly observed in Figure 2b. The bare2Omembrane with a pore size of approximately 100 nm is clearly observed in Figure 2b. Large Al 3 crystals are also observed in the magnified surface image in Figure 2c. The morphology Large Al2 O3 crystals are also observed in the magnified surface image in Figure 2c. The morphology of the POEM‐directed TiO 2 structure on the Al 2O3 membrane in Figure 2d is similar to that of the of the POEM-directed TiO structure on the Al O membrane in Figure is2 similar to that offrom the bare 2 2 3 bare membrane. This is attributable to the formation of a very thin 2d TiO film resulting the membrane. This is attributable to the formation of a very thin TiO film resulting from the low-viscosity 2 low‐viscosity POEM/TTIP solution. In the enlarged image shown in Figure 2e, the Al2O3 crystals POEM/TTIP solution. with In theuniform enlargedand image shown in Figure the Al2 O3 crystals appear appear well covered well‐connected TiO22e, nanoparticles consisting of well the covered with uniform and well-connected TiO nanoparticles consisting of the POEM/TTIP hybrid. 2 POEM/TTIP hybrid.
Figure 2. Surface scanning electron microscopy (SEM) image of a (a) Poly(oxyethylene methacrylate) Figure 2. Surface scanning electron microscopy (SEM) image of a (a) Poly(oxyethylene methacrylate) (POEM)‐directed TiO2 film on FTO glass; (b,c) the bare Al2O3 membrane; and (d,e) the (POEM)-directed TiO2 film on FTO glass; (b,c) the bare Al2 O3 membrane; and (d,e) the POEM-directed POEM‐directed TiO2 film on the Al2O3 membrane. TiO2 film on the Al2 O3 membrane.
The distribution of TiO2 on Al2O3 membrane was investigated with EDS analysis (Figure 3). The distribution of TiO2 on Al2 O3 membrane was investigated with EDS analysis (Figure 3). The Ti elemental mapping image of TiO 2 structure prepared by the POEM template revealed that The Ti elemental mapping image of TiO structure prepared by2O the POEM template revealed that 2 TiO2 was homogeneously distributed on the entire surface of Al 3 membrane. The aggregation of TiO2 was homogeneously distributed onprevented the entirefor surface of Alphotocatalysis The aggregation 2 O3 membrane. photocatalysts on membrane should be effective and high permeate of photocatalysts on membrane should be prevented for effective photocatalysis and high permeate flux. It is widely known that the uniform distribution of photocatalyst without aggregation flux. It is widely known that the uniform distribution of photocatalyst without aggregation increases increases the surface area, thereby leading to the large number of active sites for photocatalysis. In the surface area, thereby leading to the large number of active sites for photocatalysis. In addition, addition, it is favorable for high permeate flux. Thus, the uniform distribution of TiO 2 on membrane it is favorable for high permeate flux. Thus, the uniform distribution of TiO on membrane without 2 without severe aggregation might contribute to the enhanced performance of photocatalytic severe aggregation might contribute to the enhanced performance of photocatalytic membrane. membrane. The cross‐sectional SEM images in Figure 4a,b show the macroporous structure of the The cross-sectional SEMFigure images4c,d in Figure 4a,bthat show macroporous the bare Al bare Al2O3 membrane. reveals the the cross‐section of structure the TiO2of ‐deposited Al22O O33 membrane. Figure 4c,d reveals that the cross-section of the TiO -deposited Al O membrane is not 2 2 3 membrane is not significantly different from that of the bare membrane as a result of the deposition significantly different from that of the bare membrane as a result of the deposition of a very thin TiO2 of a very thin TiO2 film, as seen in the magnified image in Figure 4d. film, as seen in the magnified image in Figure 4d.
Appl. Sci. 2017, 7, 1284 Appl. Sci. 2017, 7, 1284 Appl. Sci. 2017, 7, 1284
6 of 12 6 of 12 6 of 12
Figure 3. (a) Surface SEM image image and and (b) (b) the the corresponding corresponding EDS EDS elemental elemental (Ti) (Ti) mapping mapping image image of of Figure 3.3. (a) Surface (a) SurfaceSEM SEM image and (b) the corresponding EDS elemental (Ti) mapping image of Figure POEM‐directed TiO 2 films on Al 2 3 membrane. POEM-directed TiO films on Al O membrane. 3 POEM‐directed TiO22 films on Al22O3 membrane.
Figure 4. 4. Cross Cross sectional sectional SEM SEM images images of of the the (a,b) (a,b) bare bare Al Al2O 2O membrane and and (c,d) (c,d) POEM‐directed POEM‐directed Figure 3 3membrane Figure 4. Cross sectional SEM images of the (a,b) bare Al2 O3 membrane and (c,d) POEM-directed TiO2 2 film on the Al 2 O 3 membrane. TiO TiO2 film on the Al2O3 membrane. film on the Al2 O3 membrane.
TEM and and HR‐TEM HR‐TEM analyses analyses were were performed performed to to characterize characterize the the crystalline crystalline properties properties and and TEM TEM and HR-TEM analyses were performed to characterize the crystalline properties and morphology of the TiO 2 film. The TEM image in Figure 5a confirmed that the POEM‐templated morphology of the TiO2 film. The TEM image in Figure 5a confirmed that the POEM‐templated morphology ofis the TiO2 film. The TEMand image in Figure 5a confirmed that theThe POEM-templated TiO2 2 structure structure is composed of uniform uniform and well‐interconnected nanoparticles. The HR‐TEM and and TiO composed of well‐interconnected nanoparticles. HR‐TEM TiO structure is composed of uniform and well-interconnected nanoparticles. The HR-TEM and selected area electron diffraction (SAED) pattern in Figure 5b reveal that the TiO 2 structure is highly 2 selected area electron diffraction (SAED) pattern in Figure 5b reveal that the TiO2 structure is highly selected area electron diffraction (SAED) pattern in Figure 5b reveal that the TiO2 structure is highly crystalline with a d‐spacing value of approximately 0.35 nm, consistent with the (101) plane of the crystalline with a d‐spacing value of approximately 0.35 nm, consistent with the (101) plane of the crystalline with a d-spacing valuethese of approximately 0.35that nm,the consistent with the TiO (101) plane of the anatase TiO TiO phase. In general, general, these results indicate indicate that the POEM‐assisted POEM‐assisted TiO2 2 structure structure is anatase 2 2 phase. In results is anatase TiO phase. In general, these results indicate that the POEM-assisted TiO structure is highly highly crystalline and well organized. 2 2 highly crystalline and well organized. crystalline and well organized.
Appl. Sci. 2017, 7, 1284 Appl. Sci. 2017, 7, 1284 Appl. Sci. 2017, 7, 1284
7 of 12 7 of 12 7 of 12
Figure 5. (a) Transmission electron microscopy (TEM) image and (b) high resolution transmission Figure 5. 5. (a)Transmission Transmission electron electron microscopy microscopy (TEM) (TEM) image image and and (b) (b) high high resolution resolution transmission transmission Figure electron (a) microscopy (HR‐TEM) image (inset: selected area electron diffraction (SAED) pattern) of electron microscopy microscopy (HR-TEM) electron diffraction (SAED) pattern) of the electron (HR‐TEM) image image (inset: (inset: selected selected area area electron diffraction (SAED) pattern) of the TiO2 film templated by POEM. TiO2 film templated by POEM. the TiO 2 film templated by POEM.
For detailed characterization of the crystalline properties, XRD analysis was conducted for the For detailed characterization of the crystalline properties, XRD analysis was conducted for the For detailed TiO characterization of the crystalline XRD was conducted the POEM‐directed 2 film deposited on the Al2O3properties, membrane, as analysis shown in Figure 6. The for sharp POEM‐directed TiO 2 film deposited on the Al 2O 3 membrane, as shown in Figure 6. The sharp POEM-directed TiO film deposited on the Al O membrane, as shown in Figure 6. The sharp 2 2 32O3 phases. The highly crystalline Al2O3 patterns diffraction peaks are attributed to the TiO 2 and Al diffraction peaks are attributed to the TiO 2 and Al O33 phases. The highly crystalline Al O33 The patterns diffraction are6 attributed the TiO2of and Al22O phases. The highly crystalline Al22O patterns marked in peaks Figure match the toα‐phase alumina (Al 2O3) (JCPDS Card No. 10‐0173). TiO2 marked in Figure 6 match the α‐phase of alumina (Al 2O 3) (JCPDS Card No. 10‐0173). The TiO 2 marked in Figure 6 match the α-phase of alumina (Al O ) (JCPDS Card No. 10-0173). The TiO peaks 2 3 2 TiO2 peaks at 2θ = 25.5, 37.7, and 61.2° correspond to the (101), (004), and (115) planes of anatase ◦ peaks at 2θ = 25.5, 37.7, and 61.2° correspond to the (101), (004), and (115) planes of anatase TiO 2 at 2θ = 25.5, 37.7 No. and 61.2 correspond to the (101), (004), (115) known planes ofto anatase TiO2 (JCPDS Card (JCPDS Card 21‐1272). The anatase phase is and widely be more effective for (JCPDS Card No. 21‐1272). The anatase phase is widely known to be more effective for No. 21-1272). The anatasethan phase widely known to be more effective for photocatalytic reactions photocatalytic reactions the isrutile and brookite phases [9,19]. From the XRD results, it was photocatalytic reactions than the rutile and brookite phases [9,19]. From the XRD results, it was than the rutile and brookite phases [9,19]. From the XRD results, it was concluded that the organic concluded that the organic species were completely removed and that a highly crystalline anatase concluded that the organic species were completely removed and that a highly crystalline anatase species were completely removed and that a highly crystalline anatase TiO2 structure was obtained TiO2 structure was obtained after calcination at 450 °C. TiO 2 structure was obtained after calcination at 450 °C. after calcination at 450 ◦ C.
Figure 6. X-ray diffraction (XRD) pattern of the POEM-directed TiO2 2film on the Al2 2O membrane. Figure 6. X‐ray diffraction (XRD) pattern of the POEM‐directed TiO film on the Al O33 membrane. Figure 6. X‐ray diffraction (XRD) pattern of the POEM‐directed TiO2 film on the Al2O3 membrane.
The three-dimensional morphology and structure of the POEM-directed TiO2 2film on the Al2 O The three‐dimensional morphology and structure of the POEM‐directed TiO film on the Al 2O 33 The three‐dimensional morphology and structure of the POEM‐directed TiO 2 film on the Al2O3 membrane membrane and and the the bare bare Al Al2 2O O33 membrane membrane were were investigated investigated by by AFM AFM analysis. analysis. The The topographical topographical membrane and the bare Al2Oroughness 3 membrane were investigated by AFM analysis. The topographical images root-mean-square (Rrms of) the were obtained, as shown Figure in 7. images and and root‐mean‐square roughness (R) rms of membranes the membranes were obtained, as inshown images and root‐mean‐square roughness (R rms) of the membranes were obtained, as shown in The Rrms value for bare Al2 O3 membrane was 24.83 nm, resulting from its macroporous morphology. Figure 7. The R rmsthe value for the bare Al 2O3 membrane was 24.83 nm, resulting from its macroporous Figure 7. The R rms value for the bare Al2O3 membrane was 24.83 nm, resulting from its macroporous morphology. With the introduction of a TiO 2 film templated by POEM, the value for the TiO2/Al2O3 morphology. With the introduction of a TiO2 film templated by POEM, the value for the TiO2/Al2O3
Appl. Sci. 2017, 7, 1284
8 of 12
Appl. Sci. 2017, 7, 1284
8 of 12
With the introduction of a TiO2 film templated by POEM, the value for the TiO2 /Al2 O3 membrane was reduced to 5.56 nm. The decreased roughness is thus the result of theis formation a uniform membrane was reduced to 5.56 surface nm. The decreased surface roughness thus the ofresult of the TiO Al2 O3 membrane. formation of a uniform TiO 2 structure on the macroporous Al 2O3 membrane. 2 structure on the macroporous
(a)
(b)
Figure 7. Atomic force microscopy (AFM) images of the (a) bare Al2 O3 membrane and Figure 7. Atomic methacrylate) force microscopy (AFM) images the 2O3 membrane and (b) (b) Poly(oxyethylene (POEM)-directed TiO2 of film on(a) the bare Al2 O3Almembrane. Poly(oxyethylene methacrylate) (POEM)‐directed TiO2 film on the Al2O3 membrane.
3.2. Pure Water Permeability and Antifouling Properties 3.2. Pure Water Permeability and Antifouling Properties Table 1 shows the permeability of the bare Al2 O3 and TiO2 /Al2 O3 composite membranes 3 and TiO2/Al2O3 composite membranes for DI for DITable 1 shows the permeability of the bare Al water after 2.5 h of filtration. A significant2Odecline in the membrane permeability was water after 2.5 h of filtration. A significant decline in the membrane permeability was observed observed after depositing a TiO2 layer on the Al2 O3 membrane support. In the absence of UV − 2 − 1 − 1 after depositing a TiO 2 layer on the Al2O3 membrane support. In the absence of UV illumination, the illumination, the permeability of the Al2 O3 membrane was 65.2 L m h bar , which was reduced 1 bar −1Al the 2O3 membrane was 65.2 L m−2 h−1 bar−1, which was reduced to 9.9 L m−2 h−1 topermeability 9.9 L m−2 h−of for the POEM-directed TiO2 /Al2 O3 composite membrane. Such a reduction of −1 for the POEM‐directed TiO2/Al2O3 composite membrane. Such a reduction of the membrane barmembrane the permeability is attributed to the increased membrane resistance against the water flow permeability attributed to the increased the flow upon upon significantis reduction of the membrane poremembrane size [20,21].resistance However,against exposure to water UV illumination significant reduction of the membrane pore size [20,21]. However, exposure to UV illumination enhanced the membrane permeability of both membranes. The permeability of the bare Al2 O3 enhanced and the membrane of both membranes. The permeability of the 2O3 membrane TiO2 /Al2 O3permeability composite membrane under UV illumination increased to bare 71.5 Al and membrane and TiO 2O3 composite membrane under UV illumination increased to 71.5 and 14.2 14.2 L m−2 h−1 bar−12/Al , respectively. Such a membrane permeability enhancement can be resulted −2 h−1 bar−1, respectively. Such a membrane permeability enhancement can be resulted from L m from improvement of the membrane wettability or higher water temperature induced by the UV improvement of The the TiO membrane wettability or higher water temperature induced by the UV irradiation [15,22]. 2 coating layer can improve surface hydrophilicity under UV radiation irradiation [15,22]. The TiO 2 coating layer can improve surface hydrophilicity under UV radiation due to the production of the hydroxyl groups on membrane [23]. In addition, temperature increase due to the production of the hydroxyl groups on membrane [23]. In addition, temperature increase can reduce water viscosity, thereby increasing membrane permeability [22]. To distinguish this effect, can reduce water viscosity, thereby increasing was membrane permeability [22]. To one distinguish this membrane permeability measured experimentally compared with the calculated by Darcy’s effect, membrane permeability measured experimentally was compared with the calculated one by equation (Equation (1)) at elevated temperature under UV irradiation at the end of filtration. For the Darcy’s equation (Equation (1)) at elevated temperature under UV irradiation at the end of filtration. bare membrane under UV irradiation, there is no difference in permeability between measured and For the bare under there is no that difference in permeability between 2 h−irradiation, 1 bar−1 ). This calculated valuemembrane (71.4 vs. 71.5 L m−UV indicates temperature played a dominant −2 h−1 bar−1). This indicates that temperature played measured and calculated value (71.4 vs. 71.5 L m role in determining membrane permeability. For the TiO2 /Al2 O3 composite membrane, membrane a dominant role in determining membrane permeability. For the TiO /Althe 2O3 composite membrane, permeability measured was 14.2 L m−2 h−1 bar−1 , but the one calculated2at temperature increased −2 −1 −1 membrane permeability measured was 14.2 L m h bar , but the one calculated the − 2 − 1 − 1 by UV irradiation was 10.5 L m h bar . Considering membrane permeability measured at with −2 h−1 bar−1. Considering membrane temperature increased by UV irradiation was 10.5 L m − 2 − 1 − 1 the composite membrane (9.9 L m h bar ), contribution of temperature increase to membrane −1), contribution of permeability measured the composite membrane (9.9 L m−2 h−1 bar permeability was relatively with low compared to the bare membrane. Measurements of water temperature temperature increase to membrane permeability was relatively low compared to Increased the bare ◦ ◦ in the permeation line revealed a 5 ± 1 C increase with bare Al2 O3 membrane (about 31 C). membrane. Measurements of water temperature in the permeation line revealed a 5 ± 1 °C increase water temperatures reduce the viscosity, thereby increasing the membrane permeability [22]. For the with bare Al2O3 membrane (about 31 °C). Increased water temperatures reduce the viscosity, TiO 2 /Al2 O3 composite membrane, the water temperature increase was relatively small as about thereby the membrane permeability [22]. explanation For the TiOis2/Al 2O3 composite membrane, the ◦ ◦ C). A possible 3 C underincreasing UV illumination (about 29 that the TiO2 film on the Al2 O3 water temperature increase was relatively small as about 3 °C under UV illumination (about 29 °C). membrane can support absorbing the UV light and reduce heat dissipation [15,22]. A possible explanation is that the TiO2 film on the Al2O3 membrane can support absorbing the UV light and reduce heat dissipation [15,22]. Table 1. Comparison of pure water permeability for bare Al2O3 membrane and Poly(oxyethylene methacrylate) (POEM)‐directed TiO2/Al2O3 composite membrane during filtration (with and without ultraviolet (UV) illumination).
Appl. Sci. 2017, 7, 1284
9 of 12
Table 1. Comparison of pure water permeability for bare Al2 O3 membrane and Poly(oxyethylene methacrylate) (POEM)-directed TiO2 /Al2 O3 composite membrane during filtration (with and without ultraviolet (UV) illumination).
Appl. Sci. 2017, 7, 1284
9 of 12
−1 bar−1 ) Permeability (L m−2 h−2 −1 −1
2
Without UV Without UV With UV With UV
Permeability (L m h bar ) Bare Al22O TiO Composite Membrane 2 /Al 2 O23O Bare Al O33 Membrane Membrane TiO 2/Al 3 Composite Membrane 65.2 9.9 9.9 65.2 71.5 14.2 71.5 14.2
Figure 8 compares the effect of the permeate flux on the membrane fouling for both the bare Figure 8 compares the effect of the permeate flux on the membrane fouling for both the bare Al2 O3 Al2O3 and TiO2/Al2O3 composite membranes in the absence and presence of UV illumination after and TiO2 /Al2 O3 composite membranes in the absence and presence of UV illumination after filtration filtration for 1 day. In the absence of UV illumination, the extent of the flux decline was lower for for 1 day. In the absence of UV illumination, the extent of the flux decline was lower for the TiO2 /Al2 O3 the TiO2/Al2O3 composite membrane than for the bare Al2O3 membrane. The flux decline was ~89.6% composite membrane than for the bare Al2 O3 membrane. The flux decline was ~89.6% lower on lower on the TiO2/Al2O3 composite membrane than on the bare Al2O3 membrane (1.25 vs. 0.13 LMH the−1TiO2 /Al2 O3 composite membrane than on the bare Al2 O3 membrane (1.25 vs. 0.13 LMH h−1 ). h ). The higher fouling rate in the bare Al2O3 membrane results from the higher probability of pore The higher fouling rate in the bare Al2 O3 membrane results from the higher probability of pore fouling fouling in the larger pores of the bare membrane than in the smaller pores of the TiO2/Al2O3 in the larger pores of the bare membrane than in the smaller pores of the TiO2 /Al2 O3 composite composite membrane [23]. The previous results also confirmed that the membrane permeability membrane [23]. The previous results also confirmed that the membrane permeability decreased decreased significantly after immobilizing a TiO2 layer on the Al2O3 membrane support, as shown significantly after immobilizing a TiO2 layer on the Al2 O3 membrane support, as shown in Table 1. in Table 1. The benefits of applying the POEM template become obvious upon exposing the The benefits of applying the POEM template become obvious upon exposing the membranes to UV membranes to UV illumination. In fact, no flux decline was observed for the POEM‐directed illumination. In fact, no flux decline was observed for the POEM-directed TiO2 /Al2 O3 composite TiO2/Al2O3 composite membrane under UV illumination. In contrast, severe membrane fouling was membrane under UV illumination. In contrast, severe membrane fouling was observed in the bare observed in the bare Al2O3 membrane under UV illumination, with a ~51.2% decay in the permeate Al2 O3 membrane under UV illumination, with a ~51.2% decay in the permeate flux relative to that in flux relative to that in the absence of UV illumination (2.57 vs. 1.25 LMH h−1). The higher fouling − 1 the absence of UV illumination (2.57 vs. 1.25 LMH h ). The higher fouling rate on the bare Al2 O3 rate on the bare Al2O3 membrane under UV illumination is attributed to the formation of a cake membrane under UV illumination is attributed to the formation of a cake layer on the membrane layer on the membrane surface [15]. Although photolysis phenomena are likely involved during surface [15]. Although photolysis phenomena are likely involved during organic degradation on organic degradation on the bare Al2O3 membrane [24,25], photolysis itself may not be sufficient to the bare Al2 O3 membrane [24,25], photolysis itself may not be sufficient to remove the cake layer on remove the cake layer on the membrane. the membrane.
Figure 2 O23O3membrane Figure 8. 8. Comparison Comparison of of the the membrane membrane fouling fouling rate rate for for the the bare bare AlAl membrane and and Poly(oxyethylene methacrylate) (POEM)-directed TiO2 /Al2 O3 composite membrane during filtration Poly(oxyethylene methacrylate) (POEM)‐directed TiO2/Al2O3 composite membrane during filtration in the presence (With UV) and absence (Without UV) of ultraviolet (UV) illumination (filtration in the presence (With UV) and absence (Without UV) of ultraviolet (UV) illumination (filtration time: time: 24 h). 24 h).
3.3. Organic Dye Removal Efficiency Figure 9 shows the dye adsorption curves for both the bare Al2O3 membrane and TiO2/Al2O3 composite membrane. In the absence of UV illumination, the TiO2/Al2O3 composite membrane shows a higher adsorption capacity than the bare Al2O3 membrane, which was almost saturated after 9 h of filtration. The TiO2/Al2O3 composite membrane presents a higher dye organic removal
Appl. Sci. 2017, 7, 1284
10 of 12
3.3. Organic Dye Removal Efficiency Figure 9 shows the dye adsorption curves for both the bare Al2 O3 membrane and TiO2 /Al2 O3 composite membrane. In the absence of UV illumination, the TiO2 /Al2 O3 composite membrane shows a higher adsorption capacity than the bare Al2 O3 membrane, which was almost saturated after 9 h of filtration. The TiO2 /Al2 O3 composite membrane presents a higher dye organic removal10 of 12 efficiency Appl. Sci. 2017, 7, 1284 (~20%) than the bare Al2 O3 membrane after the same filtration time. The higher adsorption capacity of the membrane TiO2/Al2O3 composite membrane arises mainly from complementary of theadsorption TiO2 /Al2capacity O3 composite arises mainly from complementary cavities and rebinding cavities and rebinding sites on the membrane assisted by the POEM template [26]. The advantages sites on the membrane assisted by the POEM template [26]. The advantages of the presence of such a of the presence of such a well‐organized TiO2 film were further observed upon carrying out the well-organized TiO2 film were further observed upon carrying out the membrane filtration under UV membrane filtration under UV illumination (Figure 9b). About 96% of dye removal efficiency was illumination (Figure 9b). About 96% of dye removal efficiency was observed with the TiO2 /Al2 O3 observed with the TiO2/Al2O3 composite membrane after 24 h of filtration. The catalytic active sites composite membrane after 24 h of filtration. The catalytic active sites offered by the POEM template offered by the POEM template favor the membrane photocatalytic activity. In contrast, only ~40% of favorthe organic dye was removed by the bare Al the membrane photocatalytic activity. In contrast, only ~40% of the organic dye was removed by 2O3 membrane under UV illumination [15]. However, the bare Al2 O3 membrane under UV illumination [15]. However, ~89% of organic removal efficiency ~89% of organic removal efficiency was achieved after 6.5 h membrane filtration, probably due to was achieved 6.5 h membrane filtration, probably due[25]. to photolysis occurring photolysis after phenomena occurring on the membrane surface Moreover, phenomena the formed cake layer on may act as a secondary membrane, thus improving the organic dye removal by membrane filtration the membrane surface [25]. Moreover, the formed cake layer may act as a secondary membrane, thus [15]. improving the organic dye removal by membrane filtration [15].
Figure 9. Organic dyedye removal efficiency of bare AlAl andPOEM‐directed TiO POEM-directed TiO Figure 9. Organic removal efficiency of bare 2O33 membrane membrane and 2/Al22/Al O3 2 O3 2O composite membrane in the (a) absence and (b) presence of UV illumination. composite membrane in the (a) absence and (b) presence of UV illumination.
4. Conclusions 4. Conclusions A nanostructured photocatalytic TiO2/Al2O3 composite membrane was successfully fabricated
A nanostructured photocatalytic TiO2 /Al2 O3 composite membrane was successfully fabricated using a POEM polymer template through a sol–gel process. The hydrophilic POEM chains interact usingstrongly with the TiO a POEM polymer template through a sol–gel process. The hydrophilic POEM chains interact 2 precursor (TTIP) to form uniform POEM/TTIP hybrids. Upon calcination at strongly the TiO (TTIP) to form uniform POEM/TTIP hybrids. Upon POEM calcination 2 precursor into 450 with °C, TTIP is transformed highly crystalline anatase TiO2 while the organic is at 450 ◦ completely C, TTIP is transformed into highly crystalline anatase TiO while the organic POEM is completely decomposed. The well‐organized structure and 2morphology of the TiO2 film were decomposed. The well-organized structure and morphology of the TiO2 film were confirmed by confirmed by HR‐TEM, SEM, XRD, and AFM analyses. A homogenous distribution of the catalytic sites on the membrane surface was achieved, exhibiting a higher adsorption capacity toward Congo HR-TEM, SEM, XRD, and AFM analyses. A homogenous distribution of the catalytic sites on the red (a surface model organic dye) than that of the bare Al 2O3 membrane. Under UV illumination, membrane was achieved, exhibiting a higher adsorption capacity toward Congo red (athe model TiO 2/Al2O3 composite membrane showed 96% organic removal efficiency along with effective organic dye) than that of the bare Al2 O3 membrane. Under UV illumination, the TiO2 /Al2 O3 composite fouling mitigation. membrane showed 96% organic removal efficiency along with effective fouling mitigation. Acknowledgments: This work was supported by an Inha University research grant.
Acknowledgments: This work was supported by an Inha University research grant. Author Contributions: Jeonghwan Kim and Jong Hak Kim conceived and designed the experiments; Rizwan
Author Contributions: Jeonghwan Kim and Jong Hak Kim conceived and designed the experiments; Ahmad and and Jin Jin Kyu Kim performed the the experiments; Rizwan Ahmad, Jin Kyu Jong Hak Rizwan Ahmad Kyu Kim performed experiments; Rizwan Ahmad, Jin Kim, Kyu Kim, Jong Kim Hak and Kim and Jeonghwan Kim analyzed the data; Jeonghwan Kim, Jong Hak Kim, Rizwan Ahmad and Jin Kyu Kim wrote Jeonghwan Kim analyzed the data; Jeonghwan Kim, Jong Hak Kim, Rizwan Ahmad and Jin Kyu Kim wrote the paper. the paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
References
Appl. Sci. 2017, 7, 1284
11 of 12
References 1. 2.
3.
4.
5. 6. 7.
8.
9.
10. 11. 12.
13.
14.
15.
16.
17.
18.
Liu, G.; Han, K.; Ye, H.; Zhu, C.; Gao, Y.; Liu, Y.; Zhou, Y. Graphene oxide/triethanolamine modified titanate nanowires as photocatalytic membrane for water treatment. Chem. Eng. J. 2017, 320, 74–80. [CrossRef] Zhao, H.; Li, H.; Yu, H.; Chang, H.; Quan, X.; Chen, S. CNTs–TiO2 /Al2 O3 composite membrane with a photocatalytic function: Fabrication and energetic performance in water treatment. Sep. Purif. Technol. 2013, 116, 360–365. [CrossRef] Aslam, M.; McCarty, P.L.; Shin, C.; Bae, J.; Kim, J. Low energy single-staged anaerobic fluidized bed ceramic membrane bioreactor (AFCMR) for wastewater treatment. Bioresour. Technol. 2017, 240, 33–41. [CrossRef] [PubMed] Ko, C.-C.; Chen, C.-H.; Chen, Y.-R.; Wu, Y.-H.; Lu, S.-C.; Hu, F.-C.; Li, C.-L.; Tung, K.-L. Increasing the performance of vacuum membrane distillation using micro-structured hydrophobic aluminum hollow fiber membranes. Appl. Sci. 2017, 7, 357. [CrossRef] Hofs, B.; Ogier, J.; Vries, D.; Beerendonk, E.F.; Cornelissen, E.R. Comparison of ceramic and polymeric membrane permeability and fouling using surface water. Sep. Purif. Technol. 2011, 79, 365–374. [CrossRef] Ahmad, R.; Kim, J.K.; Kim, J.H.; Kim, J. Effect of polymer template on structure and membrane fouling of TiO2 /Al2 O3 composite membranes for wastewater treatment. J. Ind. Eng. Chem. 2017, 57, 55–63. [CrossRef] Ma, N.; Zhang, Y.; Quan, X.; Fan, X.; Zhao, H. Performing a microfiltration integrated with photocatalysis using an Ag-TiO2 /HAP/Al2 O3 composite membrane for water treatment: Evaluating effectiveness for humic acid removal and anti-fouling properties. Water Res. 2010, 44, 6104–6114. [CrossRef] [PubMed] Goei, R.; Lim, T.-T. Asymmetric TiO2 hybrid photocatalytic ceramic membrane with porosity gradient: Effect of structure directing agent on the resulting membranes architecture and performances. Ceram. Int. 2014, 40, 6747–6757. [CrossRef] Ahmad, R.; Ahmad, Z.; Khan, A.U.; Mastoi, N.R.; Aslam, M.; Kim, J. Photocatalytic systems as an advanced environmental remediation: Recent developments, limitations and new avenues for applications. J. Environ. Chem. Eng. 2016, 4, 4143–4164. [CrossRef] Kwon, S.; Fan, M.; Cooper, A.T.; Yang, H. Photocatalytic applications of micro- and nano-TiO2 in environmental engineering. Crit. Rev. Env. Sci. Technol. 2008, 38, 197–226. [CrossRef] Gao, Y.; Hu, M.; Mi, B. Membrane surface modification with TiO2 –graphene oxide for enhanced photocatalytic performance. J. Membr. Sci. 2014, 455, 349–356. [CrossRef] Romanos, G.E.; Athanasekou, C.P.; Katsaros, F.K.; Kanellopoulos, N.K.; Dionysiou, D.D.; Likodimos, V.; Falaras, P. Double-side active TiO2 -modified nanofiltration membranes in continuous flow photocatalytic reactors for effective water purification. J. Hazard. Mater. 2012, 211–212, 304–316. [CrossRef] [PubMed] Athanasekou, C.P.; Romanos, G.E.; Katsaros, F.K.; Kordatos, K.; Likodimos, V.; Falaras, P. Very efficient composite titania membranes in hybrid ultrafiltration/photocatalysis water treatment processes. J. Membr. Sci. 2012, 392, 192–203. [CrossRef] Athanasekou, C.P.; Morales-Torres, S.; Likodimos, V.; Romanos, G.E.; Pastrana-Martinez, L.M.; Falaras, P.; Dionysiou, D.D.; Faria, J.L.; Figueiredo, J.L.; Silva, A.M.T. Prototype composite membranes of partially reduced graphene oxide/TiO2 for photocatalytic ultrafiltration water treatment under visible light. Appl. Catal. B Environ. 2014, 158, 361–372. [CrossRef] Ahmad, R.; Kim, J.K.; Kim, J.H.; Kim, J. Well-organized, mesoporous nanocrystalline TiO2 on alumina membranes with hierarchical architecture: Antifouling and photocatalytic activities. Catal. Today 2017, 282, 2–12. [CrossRef] Goei, R.; Dong, Z.; Lim, T.-T. High-permeability pluronic-based TiO2 hybrid photocatalytic membrane with hierarchical porosity: Fabrication, characterizations and performances. Chem. Eng. J. 2013, 228, 1030–1039. [CrossRef] Choi, H.; Stathatos, E.; Dionysiou, D.D. Sol–gel preparation of mesoporous photocatalytic TiO2 films and TiO2 /Al2 O3 composite membranes for environmental applications. Appl. Catal. B Environ. 2006, 63, 60–67. [CrossRef] Zheng, M.-P.; Gu, M.-Y.; Jin, Y.-P.; Wang, H.-H.; Zu, P.-F.; Tao, P.; He, J.-B. Effects of PVP on structure of TiO2 prepared by the sol-gel process. Mater. Sci. Eng. B 2001, 87, 197–201. [CrossRef]
Appl. Sci. 2017, 7, 1284
19.
20. 21. 22. 23.
24. 25. 26.
12 of 12
Li, G.; Richter, C.P.; Milot, R.L.; Cai, L.; Schmuttenmaer, C.A.; Crabtree, R.H.; Brudvig, G.W.; Batista, V.S. Synergistic effect between anatase and rutile TiO2 nanoparticles in dye-sensitized solar cells. Dalton Trans. 2009, 45, 10078–10085. [CrossRef] [PubMed] Alem, A.; Sarpoolaky, H.; Keshmiri, M. Titania ultrafiltration membrane: Preparation, characterization and photocatalytic activity. J. Eur. Ceram. Soc. 2009, 29, 629–635. [CrossRef] Choi, H.; Stathatos, E.; Dionysiou, D.D. Photocatalytic TiO2 films and membranes for the development of efficient wastewater treatment and reuse systems. Desalination 2007, 202, 199–206. [CrossRef] Guo, B.; Pasco, E.V.; Xagoraraki, I.; Tarabara, V.V. Virus removal and inactivation in a hybrid microfiltration–UV process with a photocatalytic membrane. Sep. Purif. Technol. 2015, 149, 245–254. [CrossRef] Mendret, J.; Hatat-Fraile, M.; Rivallin, M.; Brosillon, S. Hydrophilic composite membranes for simultaneous separation and photocatalytic degradation of organic pollutants. Sep. Purif. Technol. 2013, 111, 9–19. [CrossRef] Ahmad, R.; Kim, J.K.; Kim, J.H.; Kim, J. In-situ TiO2 formation and performance on ceramic membranes in photocatalytic membrane reactor. Membr. J. 2017, 27, 328–335. [CrossRef] Kamel, D.; Sihem, A.; Halima, C.; Tahar, S. Decolourization process of an azoïque dye (congo red) by photochemical methods in homogeneous medium. Desalination 2009, 247, 412–422. [CrossRef] Zhao, K.; Feng, L.; Lin, H.; Fu, Y.; Lin, B.; Cui, W.; Li, S.; Wei, J. Adsorption and photocatalytic degradation of methyl orange imprinted composite membranes using TiO2 /calcium alginate hydrogel as matrix. Catal. Today 2014, 236, 127–134. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
