Chemical Engineering Journal 236 (2014) 314–322

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Chemical Engineering Journal

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Fabrication of nanostructured TiO2 hollow fiber photocatalyticmembrane and application for wastewater treatment

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.09.059

⇑ Corresponding author at: Department of Chemical Engineering, Faculty ofEngineering, Monash University, Clayton, Vic 3800, Australia. Tel.: +61 3 9905 1867.

E-mail address: Xiwang.Zhang@monash.edu (X. Zhang).

Xiwang Zhang a,b,⇑, David K. Wang a, Diego Ruben Schmeda Lopez a, João C. Diniz da Costa a

a The University of Queensland, FIMLab – Films and Inorganic Membrane Laboratory, School of Chemical Engineering, Brisbane, Qld 4072, Australiab Department of Chemical Engineering, Faculty of Engineering, Monash University, Clayton, Vic 3800, Australia

h i g h l i g h t s

� A facile spinning–sintering method is developed for the TiO2 hollow fiber membrane.� Concurrent separation and photocatalytic oxidation are achieved by the membrane.� Calcination temperature is the key factor of the membrane properties.� The membrane fouling is alleviated by the photocatalytic oxidation.

a r t i c l e i n f o

Article history:Received 16 February 2013Received in revised form 11 September 2013Accepted 14 September 2013Available online 25 September 2013

Keywords:TiO2

Hollow fiberPhotocatalytic membraneWastewater treatment

a b s t r a c t

Nanostructured TiO2 hollow fiber photocatalytic membranes were fabricated in this study via a facilespinning–sintering method. The morphology, crystal phase, porosity and mechanical strength of themembranes were characterized by SEM, XRD, N2 sorption and bending test, respectively. The dimensionsof the sintered hollow fibers are approximately 1.0–1.2 mm in the outer diameter and 150 lm in thick-ness. Finger-like macrovoids and sponge-like mesopores form inside the membranes whilst the TiO2

layer is denser near the outer wall of the membranes. The calcination temperature has a significantimpact on the properties of the membranes. With increasing the calcination temperature, the pore sizeand photocatalytic activity decrease while the mechanical robustness increases. The photocatalytic activ-ity, permeability and separation efficiency of the membranes were evaluated using both Acid Orange 7(AO7) and raw sewage as pollutants. The membrane calcined at 900 �C has a good balance betweenmechanical properties and photocatalytic activity. A high organic removal rate of 90.2% is achieved bythe filtration and photodegradation functionalities of the membrane. The water flux of 12.2 L m�2 h�1

for the membranes exposed UV irradiation is 2.2 times higher than that in the absence of it, which indi-cates lower levels of membrane fouling. The results provide vital insights into the development of pho-tocatalytic membranes and their applications in water treatment.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Membrane technology has attracted a significant research inter-est in the last few decades. Compared to conventional separationtechnologies, it offers superior separation efficiency, smaller foot-print and easier maintenance [1]. Among various membranes, inor-ganic membranes have received considerable attention due totheir excellent thermal, chemical and mechanical stability, andreusability over conventional polymeric membranes [2]. Further-more, apart from the separation properties, inorganic membranescould also offer other attractive functions, such as catalysis,

adsorption and oxidation, if the right choice of inorganic functionalmaterials are used as membrane materials [3].

Titanium dioxide (TiO2) is widely investigated for its chemicaland thermal stability, and excellent photocatalytic activity. UnderUV irradiation, electrons are promoted into the conduction bandof TiO2 which leaves holes in the valence band. Subsequently, holesand electrons diffuse to the surface of TiO2 particle reacting withthe hydroxyl groups and oxygens to generate hydroxyl radicalsand superoxides, which can be utilized to destroy organics, bacte-ria and viruses [4]. As such, a large number of studies have alreadydemonstrated that the photocatalytic properties of TiO2 nanoparti-cles dispersed in a slurry system can efficiently degrade organicpollutants and disinfect water [5,6]. However, a separation stepis still required for the slurry system to reclaim the TiO2 photocat-alysts from the treated water, which increases its operationalcost [7]. This problem could be resolved by employing TiO2

X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322 315

photocatalytic membranes, which simplifies the overall photocat-alytic system design and process [8].

Since the pioneering work on the fabrication of TiO2 photocata-lytic membranes by Dionysius and Quan’s groups via sol–gel meth-od [9–11], a number of photocatalytic membranes have beenreported. For instance, a free-standing TiO2 nanotube flat mem-brane fabricated via anodization of Ti film was reported by Albuet al. [12]. The dye pollutants were effectively rejected and de-graded by the TiO2 nanotube membrane. Recently, we have devel-oped free-standing TiO2 nanowire flat membranes via a facilefiltration method which show excellent performance on photocat-alytic degradation of organic pollutants and disinfection [8]. In an-other work, we synthesized a TiO2 nanotube membrane usingliquid phase deposition via an environmentally friendly method,in which TiO2 nanotubes were grafted into the channels of aAl2O3 microfitration (MF) [8]. Ma and Quan modified the sol–geldip-coating method to enhance the performance of TiO2 mem-brane via doping with Ag and Si and HAP coupling [13–15]. Fur-thermore, a visible light responsive TiO2 membrane wasfabricated via co-doping C, N and Ce through a weak alkalinesol–gel process by Cao and co-workers [16]. Most recently, Liudeposited Ag nanoparticles on TiO2 nanofiber and subsequentlyformed a Ag/TiO2 flat membrane on glass fiber substrate for disin-fection under solar light irradiation [17].

To date, most of the reported photocatalytic membranes are flatsheet membranes. The fabrication of TiO2 hollow fiber membranestill remains a challenge, especially a nanostructured free-standinghollow fiber membrane. Regarding the geometry of the membrane,hollow fibers have the highest surface area per volume ratio, whichmakes them very appealing because of the possibility of obtainingsmall membrane modules with large surface areas and thus achiev-ing small foot print [18–20]. In this study, for the first time, a TiO2

nanostructured hollow fiber membrane was fabricated via a facilemethod. First of all, the effect of the key parameter, calcination tem-perature, on the performance of the membranes was systemicallyinvestigated. Then the photocatalytic activity, permeability and sep-aration efficiency of the membrane were evaluated using Acid Or-ange 7 (AO7) and sewage as pollutants. This study provides newinsights into the development of TiO2 multifunctional membranesand their potential in water purification applications.

2. Experimental sections

2.1. Chemicals and materials

Unless otherwise specified, all chemicals and reagents used inthis study were of ACS grade, and were used without further puri-fication. P25 TiO2 nanoparticles were provided by Evonik Indus-tries AG, Germany. P25 TiO2 is a mixture of anatase and rutileTiO2 (70%:30%), and its BET surface area and average particle sizeof P25 is of 50 m2 g�1 and 20–30 nm, respectively. More detailsabout P25 TiO2 were reported in previous study [21].

2.2. Fabrication of hollow fiber membrane

The TiO2 hollow fiber membranes were fabricated via a spin-ning-sintering technique. In a typical experiment, P25 TiO2 nano-particles were added to a mixture of polyether imide (PEI) andsolvent (1-methyl-2-pyrrolidinone (NMP)) in the ratio of18:25:75 (w/w) to create a spinning dope. PEI was selected asthe binder to produce dope mixtures, particularly because PEIhas been proved to deliver superior ceramic hollow fibers [19].The mixture was constantly stirred for a period of 24 h to ensurea uniform mixture and then degassed by vacuum. The spinningdope mixture was then extruded using a tube-in-orifice spinneret

(OD = 2.5 mm and ID = 0.8 mm) to form a thin TiO2/PEI hollow fi-ber. Phase inversion was induced from the inner side of the hollowfiber by using DI water. The phase inversion was completed byleaving the hollow fiber immersed in DI water for 24 h. The pres-sure in the spinning dope and air gap between the spinneret andthe coagulation bath used for the spinning process was of 4 barsand 50 mm, respectively. DI water was used as the bore fluid atthe rate of 40 ml/min. Then the TiO2/PEI hollow fiber was calcinedat a temperature in the range of 700 to1400 �C for 8 h with a heat-ing/cooling rate of 5 �C min�1. PEI was removed due to its pyrolysisduring calcination and consequently the pure TiO2 hollow fibermembranes were obtained.

2.3. Membrane characterizations

The morphological structure of the prepared TiO2 hollow fibermembranes were examined using a field-emission scanning elec-tron microscope (FESEM, JEOL 6610). The crystal structure andphase composition were analyzed by a powder X-ray diffractionsystem (XRD, Bruker AXS D8 advance, Cu Ka radiation) after themembranes were grinded into powders. The Brunner–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) poresize were measured on a Micromeritics TriStar 3000 (N2). The pyr-olyzation of PEI during calcination was monitored by a thermalgravimetric analyzer (TGA, Shimadu, TGA-50). Computed tomogra-phy (CT) scans of the membranes were carried out on a 3D X-raymicroscopy (VersaXRM-500, Xradia, USA) with a resolution of1.57 lm/pixel and a field of view of 1.5 mm � 1.5 mm � 1.5 mm.The percentage of macroporosity of the CT cross-sectional imagewas calculated by using a Jasc Paint Shop Pro 8 software to deter-mine the percentage of the dark pixels against the total pixels in afixed representative area. A three point bending test was per-formed on an Instron 5543 universal testing machine to measurethe mechanical strength of the TiO2 membranes. A strain rate of1 mm min�1 was applied for the testing. The maximal bendingstress was calculated using the following expression for simpleshapes, in this case adapted for a tube:

r ¼ 8FLD

pðD4 � d4Þð1Þ

where r is the bending stress (MPa), F is the load applied (N), L isthe span (mm), D is the outer diameter (mm) and d is the innerdiameter (mm) of the hollow fiber.

2.4. Membrane evaluations

The photocatalytic activity and permeability of the TiO2 hollowfiber membrane were evaluated using a custom made photocatal-ysis setup as shown in Fig. 1. Four UV-A lamps (SYLVANIA BlackliteF8 W/BL350, emit at 330–370 nm) were used as the UV lightsource and the power of each lamp was 8 W. Acid Orange 7(AO7) was used as a model pollutant due to its excellent stabilityunder UV irradiation [22]. A glass bottle of 25 ml was used as thephotocatalytic reactor vessel which was surrounded by the fourUV lamps in concentric fashion. To minimize the heating effect ofthe UV lamps, the reactor was placed in the center of the set-upmeasured at 15 cm away from the UV lamps. As such, it was esti-mated that about 5% of the UV light emitted from the four lampsreached the reactor. The calcined membranes of 5 cm long (mem-brane surface area: 1.9–2.1 cm2) were submerged in 20 ml of AO7at a concentration of 20 mg L�1 contained in the reactor. Darkadsorption of 30 min was conducted prior to switching on theUV lamps. During the photocatalytic degradation, the temperatureof AO7 solution gradually increased from room temperature(20 �C) to about 45 �C. To avoid the evaporation of solution, the

Fig. 1. Schematics of photocatalytic reaction setup (a) and membrane filtration setup (b).

316 X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322

photocatalytic reactor was covered by a glass plate. In addition,previous study showed that solution temperature does not haveapparent impact on photocatalytic degradation of organic pollu-tants in the range from 20 to 80 �C [23]. Hence, it is reasonableto neglect the impact of solution temperature increase in thisstudy. The UV–vis spectrum of the samples was recorded from220 nm to 620 nm by a UV–vis spectrophotometer (Evolution220, Thermo Scientific). The concentration of AO7 was determinedby measuring its absorbance at a fixed wavelength (485 nm)according to the established calibration curve. The photocatalyticactivity of the membrane can be reflected by the percent removalof AO7 which is calculated based on the below equation.

RAO7ð%Þ ¼C0 � Ct

C0� 100 ð2Þ

RAO7 is the percent removal of AO7; C0 is the AO7 concentrationin the solution before photocatalytic reaction; Ct is the AO7 con-centration at reaction time t.

The permeability of the membrane was first evaluated by mea-suring the membrane flux of DI water in a dead end filtration mode[24]. As shown in Fig. 1, one end of the membrane is sealed withsilicone gel and the other end is connected to a permeate collectorwhich is connected to a vacuum pump. A vacuum of 0.9 bar is ap-plied across the membrane as the driving force. The membrane fluxis calculated based on the volume change of the feed solution asdescripted in the following equation:

J ¼ V0 � Vt

A� tð3Þ

where J is the membrane flux, V0 is the volume of feed solution be-fore filtration (at time zero) and Vt is the volume of the feed solutionat time t. A is the membrane surface area. In this study, the t is 1 h.

The anti-fouling property of the membrane was evaluated usingsewage as feed solution. The sewage was collected from the St. Lu-cia Campus of The University Queensland, which containedapproximately 600 mg L�1 of total suspended solid (TSS) and500 mg L�1 of total organic carbon (TOC). More details about thewater quality of the sewage are referred to literature [25]. The sew-age was first degraded by the membrane for 30 min under UV irra-diation. Then, the UV lamps were switched off and started thefiltration of the treated sewage once its temperature returned backto room temperature. A control experiment without UV irradiationwas also conducted. After 1 h of filtration, UV–vis spectrum of thepermeate was recorded. The percent removal of organic pollutantsin the sewage by the membranes is calculated by:

RS ð%Þ ¼ 1� Ap420 þ Ap254

Ar420 þ Ar254

� �� 100 ð4Þ

where RS is the percent removal of organic pollutants in the sewage,Ap420 and Ap254 are the absorbance of the permeate at 420 and

254 nm, respectively. Ar420 and Ar254 are the absorbance of theraw sewage at the same respective wavelengths.

3. Results and discussion

3.1. Membrane characterization

Fig. 2 shows representative SEM images of the as-prepared TiO2

hollow fiber membranes before calcination. The membrane is of2.0 ± 0.1 mm in external diameter and 400 ± 100 lm in thickness.It can be seen that the morphology of the uncalcined membraneis similar to that of the conventional polymeric membranes as re-ported elsewhere [26], as both were prepared via phase-inversionmethod. Two distinct domains of structure are observed. Isolatedround microvoids measuring 100–500 nm and hole-like macrov-oids are seen near the outer surface of the membrane while asym-metric finger-like macrovoids spanning across over half of themembrane thickness originate from the inner wall. The diametersof these macrovoids are measured to be about 10–50 lm. Themicrovoid and macrovoid formation usually occur during mem-brane fabrication via phase inversion method which is governedby the thermodynamic aspects of chemical potential gradient,and a combination of local surface instability, material and stressimbalance [27]. These voids can serve as water channels duringmembrane filtration, which could reduce the membrane resis-tance. It has been observed in several studies [28,29] that the addi-tion of nanoparticles in spinning dope changes the rate of phaseseparation during phase inversion process, consequently changesthe morphology of the membrane. Nanoparticles can play variousroles as thermodynamic enhancer or rheological deterrent whichis depended on their dosage. The thermodynamic enhancementis usually dominated at low dose of nanoparticles, which reducesthe compatibility of polymer with solvent as well as increasesthe penetration velocity of water, hence resulting in a porousstructure. In contrast, rheological hindrance becomes dominantonce the dosage of nanoparticles is higher than a threshold, anda less porous structure is formed. The threshold is normally be-tween 3% and 10%. In this study, the dosage of P25 TiO2 nanopar-ticles is higher than this threshold. Hence, a rheologicalhindrance effect is expected to reduce the porosity of the preparedTiO2/PEI membranes, which could lead to an improved mechanicalstrength of the membrane. The high magnification images (Fig. 2band c) clear show that the TiO2 nanoparticles were uniformly dis-persed, which is the result of control homogenous mixing of theTiO2 and PEI dope mixture prior to spinning the hollow fibers.The selection of PEI as the binder for this process was based onthe high affinity between the binder and the ceramic serving as agood dispersant for the TiO2 nanoparticles [30].

The uncalcined TiO2 membrane was analyzed by TGA and DTGto monitor the weight change of the membrane composite with

Fig. 2. SEM images of uncalcined TiO2 hollow fiber membrane at (a) low magnification of cross section, (b) high magnification of cross section, (c) low magnification of outersurface and (d) high magnification of outer surface.

X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322 317

increasing temperature. As shown in Fig. 3, the weight of the TiO2

membrane decreases slightly with increasing temperature until400 �C. This is attributed to the loss of water inside the porousmembrane. The weight loss accelerates once the temperaturepasses 400 �C, indicating the onset of the pyrolysis of PEI leadingto a dramatic weight loss at 520 �C. With further increase of thetemperature, the rate of weight loss decreases slightly which is evi-denced by the small shoulder in the DTG curve. A maximum rate ofweight loss is observed at 543 �C which accounts for about 40% ofthe total mass. After 568 �C, no weight change is found between568 and 1000 �C, which indicates that the pyrolysis of PEI hadreached completion.

Since calcination temperature has a big impact on the crystalphase of TiO2 which is closely related to its photocatalytic activity,the TiO2 membranes calcined at different temperatures were char-acterized by XRD. As shown in Fig. 4, for the TiO2 membrane cal-cined at 700 �C, the XRD pattern shows that the peaks assignedto rutile phase become sharper and more intense at the expenseof the anatase phase compared to that of the original P25 nanopar-ticles. By calculating the ratio of the diffraction peak intensities of

Fig. 3. TGA and DTG of TiO2 hollow fiber membrane.

Fig. 4. XRD patterns of TiO2 hollow fiber membranes calcined at differentemperature.

t

the anatase and rutile phases, the weight fraction of the rutilephase can be determined via Eq. (5):

RS ¼1

1þ 0:8 IaIr

ð5Þ

where x is the weight fraction of the rutile phase, Ia is the diffractionpeak intensity of the anatase (101) plane, Ir is the diffraction peakintensity of the rutile (110) plane [31]. The results show that theweight fraction of the rutile phase increases to 70.4% from an initial

Fig. 5. Maximal bending press of TiO2 hollow fiber membranes calcined at differenttemperature.

318 X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322

fraction of 18.2% (P25) after calcination at 700 �C. It is observed thatincreasing the calcination temperature to 800 �C results in the ana-tase phase almost disappearing.Some unidentified diffraction peakswere observed when the calcination temperature reached 900 oC.Although some studies [32,33] showed that phase transition fromanatase to rutile of TiO2 is often inhibited by other impurities, theresult in this work indicates that PEI binder does not have a signif-icant impact on this transition.

The mechanical strength of the TiO2 membranes was evaluatedvia measuring their maximal bending stress (MBS). As shown inFig. 5, the calcination temperature has a significant impact onthe mechanical strength. The membrane calcined at 700 �C ismechanically weak and its MBS was measured approximately2.5 MPa which is similar to MBS of the 800 �C membrane. The rea-son may be that these membranes are still composed of individualTiO2 nanoparticles after calcination. When the calcination temper-ature reached 900 �C, the MBS of the membrane doubled, whichindicates that the TiO2 nanoparticles started to sinter together.After the membrane was calcined at the maximum temperature

Fig. 6. Computed tomography of TiO2 hollow fiber membrane c

of 1400 �C, its mechanical strength significantly increased as indi-cated by a 10-fold increase in its MBS.

To better understand the evolution of the mechanical strengthand porosity of the TiO2 membrane with calcination temperature,computed tomography scan of the membranes was carried out.As displayed in Fig. 6, the size of the macrovoids gradually reducedwith increasing calcination temperature. The percentage of themacroporosity of the membrane decreased from 69.5% at 700 �Cto 40.5% at 1400 �C, which indicates that the membrane densityis proportional to its calcination temperature. Previous study [34]reported that the mechanical strength of porous materials is clo-sely related to their macroporosity via an exponential relationship.This is indeed also observed in this study whereby the measuredMBS values scale exponentially with decreasing porosity and thusas a function of the calcination temperature.

To further investigate the impact of calcination temperature, themorphology of the TiO2 membranes were characterized by SEM.Fig. 7(a–f) shows SEM images of the membrane calcined at 900 �C.The outer diameter is measured at 1.2 ± 0.2 mm and the thicknessat 180 ± 20 lm, which is about one-third in the overall size reduc-tion. This is mainly attributed to (i) the pyrolysis of the PEI binderbetween 500 and 600 �C, followed by (ii) sintering and coalescingof titania particles at high temperatures. The calcined TiO2 mem-branes forms mesoporous structures due to the interspaces betweenthe precursor P25 nanoparticles. As shown in Fig. 7(b), TiO2 nano-particles are sintered together to some extent, which confers im-proved mechanical properties. The porosity of TiO2 membranedecreased along in the lateral direction as shown in Fig. 7(c). Theouter layer of the membrane (Fig. 7(e)) became denser comparedto the inner layer (Fig. 7(d)), which is closely associated with theasymmetric structure formed during phase inversion process. Afterthe full removal of the PEI binders, the structure still remainedasymmetric. TiO2 nanoparticles are observed in Fig. 7(f) to form adense homogenous surface layer, which in principle is attractive asa rejection layer. Further, the microvoids initially found on the outerwall of the uncalcined membrane have disappeared after calcina-

alcined at (a) 700 �C, (b) 800 �C, (c) 900 �C and (d) 1400 �C.

Fig. 7. SEM images of TiO2 hollow fiber membrane calcined at 900 �C at (a) low magnification of cross section, (b) high magnification of the internal structure, (c) highmagnification of cross section, (d) low magnification of the internal structure, (e) low magnification and (f) high magnification of the outer surface.

X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322 319

tion, which indicates that these nanoparticles underwent somerestructuring during the calcination process.

By increasing the calcination temperature further, the morphol-ogy of the TiO2 membrane has another phase transformation asdisplayed by the SEM images in Fig. 8(a–f) for the TiO2 membranecalcined at 1400 �C. In comparison, the outer diameter is furtherreduced to 1.0 ± 0.2 mm along with the thickness obtained at140 ± 20 lm. The finger-like macrovoids are still present, however,their diameters reduce to 10–20 lm. The larger hole-like macrov-oids disappeared or rearranged into some small macrovoids in theorder of 1–10 lm. High magnification image shows that the TiO2

nanoparticles were completely sintered together. The images ofthe surface of the TiO2 membrane outer layer as shown inFig. 8(e and f) indicate that after calcination at 1400 �C, the mem-branes became dense and no pore is visible which is in good corre-lation with the N2 sorption results.

The BET surface area and pore size of the TiO2 membranes cal-cined at different temperatures were also analyzed. The results areshown in Table 1. Compared to P25 nanoparticle powder, the sur-face area of the TiO2 membrane is lower, which decreases signifi-cantly with increasing calcination temperature. Since P25 TiO2

nanoparticles are essentially nonporous, the measured pore sizesof the TiO2 membrane are mainly attributed to the interparticlepores. In addition, a higher calcination temperature results in com-paratively smaller pore sizes due to TiO2 particles coalescing to-

gether. As a consequence, the membrane calcined at 1400 �Cbecame almost dense.

3.2. Membrane testing

The photocatalytic activity of the TiO2 membranes was evalu-ated by photocatalytic degradation of AO7 as a model pollutant.Fig. 9 presents the comparison of AO7 degradation after 1 h of reac-tion time by photolysis and the TiO2 hollow fiber membranes pre-pared at various calcination temperatures. A negligible degradationof AO7 (1.2%) by photolysis is observed in the absence of TiO2

membrane, which indicates that AO7 is UV-stable and is not de-graded by UV irradiation alone due to its strong UV resistance[35]. The AO7 degradation significantly increased to 66.7% in thepresence of TiO2 membrane calcined at 700 �C. By raising the cal-cination temperature to 800, 900 and 1400 �C, the AO7 percent re-moval decreased to 41.5%, 25.1% and 2.5%, respectively. Inprinciple, the degradation rate of AO7 is mainly dominated bythe surface area and crystal phase of the TiO2 membrane. Themembrane external surface area is the active UV light contact areafor the photodegradation of organic pollutants from wastewater.Since the dimension of the hollow fiber membranes is controlledby the same extruding orifice during fabrication, the external sur-face areas of the hollow fiber exposed to UV light are similar, withsmall variations from 1.9 to 2.1 cm2 as the sintering temperature

Fig. 8. SEM images of TiO2 hollow fiber membrane calcined at 1400 �C at (a) low magnification of cross section, (b) high magnification of the internal structure, (c) highmagnification of cross section, (d) low magnification of the internal structure, (e) low magnification of the outer layer and (f) high magnification of the outer layer.

Table 1BET surface area and BJH pore size of TiO2 membranes calcined at differenttemperature.

Calcination temperature(�C)

BET surface area(m2 g�1)

BJH average pore size(nm)

Uncalcined P25a 50 _700 15.3 7.5900 5.0 5.61400 0.2 –

a Based on the results reported in our previous article [21].

320 X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322

increases from 700 to 1400 �C. Hence, the significant decrease inthe degradation rates cannot simply be attributed to theminor changes in their overall surface area as a result of increasingcalcination temperature. Therefore, the AO7 degradation in thisstudy is strongly dependent on the crystal phase of the TiO2 mem-branes, particularly the anatase phase which was more prevalentat 700 �C, and almost disappeared for samples calcined at 900 �C,whilst becoming brookite at 1400 �C. These results are consistentwith the previous studies [36] which demonstrated that the photo-catalytic activity of the three TiO2 crystal phases decreases in theorder of anatase > rutile > brokite. Therefore, there is clearly atrade-off between the photocatalytic activity of the membraneand its mechanical strength by increasing the sintering tempera-

ture which reduces the photocatalytic activity. However, the phasetransition of the TiO2 membrane from anatase to rutile and brook-ite could be restrained if the calcination is carried out under aninert atmosphere or the TiO2 is doped with iron or silica as com-posites [37,38]. This may be one of the solutions to improve theTiO2 membranes having both high photocatalytic activity andmechanical strength.

In order to verify the performance of the TiO2 hollow fibers forreal world applications, these membranes were also exposed toresidential raw sewage. The membranes were initially exposed toDI water to measure their water permeability. The TiO2 mem-branes calcined at 700 and 800 �C were mechanically weak andthe water permeation test failed as the hollow fibers could notwithstand the transmembrane pressure and fractured. For thestronger membrane calcined at 900 �C, the membrane flux washigh at 87.7 L m�2 h�1. No water permeation was observed forthe membrane calcined at 1400 �C, thus confirming its dense struc-ture as per results listed in Table 1. Hence, only the membrane cal-cined at 900 �C was further evaluated for its separation efficiencyusing residential raw sewage. Fig. 10 shows the UV–vis spectraof the raw sewage and the collected permeates in the absenceand presence of UV irradiation. The percent removal of organicpollutants in the absence of UV irradiation by membrane filtrationalone was 84.5%. The sewage consisted of large amount of

Fig. 9. Percent removal of AO7 by TiO2 hollow fiber membranes calcined at 700,800, 900, 1400 �C and photolysis in the absent of the membrane.

Fig. 10. Absorption spectra of the raw sewage and the permeates after filtration bythe TiO2 membrane (calcined at 900 �C) in the absence and presence of UVirradiation.

X. Zhang et al. / Chemical Engineering Journal 236 (2014) 314–322 321

dissolved or suspended pollutants, which results in a severe mem-brane fouling during the filtration of sewage. The membrane fluxwas only 5.6 L m�2 h�1, a 93.6% reduction compared to that of DIwater. This indicates that the pollutants in the raw sewage rapidlyform a cake layer on the membrane surface and thus blocking themembrane pores significantly. This phenomenon was also ob-served during polymeric membrane filtration of sewage as re-ported elsewhere [39].

In waste water filtration, organic pollutants are the major fou-lants since they are easily adsorbed on the membrane surface oron the wall of the membrane pores. Nevertheless, in the presenceof UV irradiation, the flux of the TiO2 membrane is higher than thatin the absence of UV irradiation, being of 12.2 L m�2 h�1. This valueis relatively large if compared against zeolite [40,41] or silica mem-branes [42] for water processing. The higher membrane flux in thepresence of UV irradiation means lesser membrane fouling. Since itis well known that the reactive hydroxyl radicals, being the stron-gest oxidants in the aqueous solution, can be generated by TiO2 viaUV irradiation [22], the organics on the TiO2 membrane surfacecould be destroyed by these radicals. Our previous study [8,24]showed that the cake layer formation and pore blocking are allevi-ated in the presence of UV irradiation, which also leads to a reduc-tion in membrane fouling, the so-called anti-fouling. Moreover, thepercent removal of the organics reached 90.2% in the presence ofUV irradiation, indicating a better quality of the permeate, whichis also contributed to the photocatalytic oxidation of organic pollu-tants by the TiO2 membrane.

4. Conclusions

A facile method for spinning TiO2 and PEI mixture followed bycalcination was developed to fabricate nanostructured TiO2 hollowfiber membranes with photocatalytic properties for waste watertreatment. SEM and computed tomography showed that a hierar-chical porous structure was formed across the membrane fromthe inner wall to the outer wall. The calcination temperature hada significant impact on the properties of the TiO2 membranes,including the morphology, crystal phase, photocatalytic activity,mechanical strength and permeability. The membrane calcined at900 �C delivered the best combined performance of catalytic activ-ity and mechanical properties. The membrane flux and organicpercent removal in sewage treatment by the membrane increasedto 12.2 L m�2 h�1 and 90.2% in the presence of UV irradiation,respectively, which was attributed to the photocatalytic degrada-tion of the organics on the membrane surface.

Acknowledgments

The authors acknowledge funding support by Australia Re-search Council to X. Zhang (DP110103533) and J.C. Diniz da Costa(DP110101185), and The University of Queensland (ECR605203,NSRSF605709). The authors also acknowledge the facilities, andthe scientific and technical assistance, of (i) the Australian Micros-copy & Microanalysis Research Facility at the Centre for Micros-copy and Microanalysis and (ii) to Dr. T.D. Nguyen and Prof. A.Nguyen at the X-ray CT scanner Facility at the School of ChemicalEngineering, The University of Queensland. X. Zhang speciallythanks for the fellowships provided by Australia Research Counciland Monash University.

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