Nanostructured Ceramic Photocatalytic Membrane

0 downloads 0 Views 7MB Size Report
Dec 9, 2017 - reservoir under continuous stirring at a constant pressure of 1.5 bar ..... reduced graphene oxide/TiO2 for photocatalytic ultrafiltration water ...
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



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/).