Separation and Purification Technology 142 (2015) 137–148

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Separation and Purification Technology

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Effect of process parameters on fouling and stability of MF/UF TiO2

membranes in a photocatalytic membrane reactor

http://dx.doi.org/10.1016/j.seppur.2014.12.0471383-5866/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +48 91 449 47 30.E-mail address: sylwia.mozia@zut.edu.pl (S. Mozia).

Sylwia Mozia a,⇑, Kacper Szymanski a, Beata Michalkiewicz a, Beata Tryba a, Masahiro Toyoda b,Antoni W. Morawski a

a West Pomeranian University of Technology, Szczecin, Institute of Chemical and Environment Engineering, ul. Pułaskiego 10, 70-322 Szczecin, Polandb Oita University, Department of Applied Chemistry, Dannoharu 700, Oita 870-1192, Japan

a r t i c l e i n f o

Article history:Received 20 October 2014Received in revised form 19 December 2014Accepted 20 December 2014Available online 13 January 2015

Keywords:Photocatalytic membrane reactorCeramic membraneUltrafiltrationMicrofiltrationFoulingStability

a b s t r a c t

The investigations on the influence of TiO2 photocatalyst (Aeroxide� TiO2 P25) loading, feed cross-flowvelocity (vF) and transmembrane pressure (TMP) on the fouling and stability of ceramic membranes ina photocatalytic membrane reactor are presented. Two ultrafiltration membranes with molecular weightcut-off of 5 kDa (Filtanium 5) and 100 kDa (Filtanium 100), and one microfiltration membrane (Filtanium0.2) with maximum pore size of 0.2 lm were used. Regardless of the applied vF (3–6 m/s), TMP (1–3 bar)and TiO2 P25 loading (0.5–1.5 g/dm3) no permeate flux decline was observed when the UF membraneswere used. On the opposite, an increase of the flux for 5–10% compared to pure water flux due to abrasionof the membranes separation layer by TiO2 P25 particles was found. In case of MF membrane a significantinfluence of vF and TMP on permeate flux was observed. Application of vF = 3 m/s led to a significantmembrane fouling while at vF = 6 m/s the permeate flux exceeded pure water flux in the whole rangeof TMP. The fine UF Filtanium 5 membrane lost its separation properties due to abrasion by TiO2 P25 par-ticles, whereas the performance of the ordinary UF Filtanium 100 membrane did not change during 100 hof operation in the PMR. Additionally, the influence of a commercial TiO2 ST01 (Ishihara Sangyo, Japan)and laboratory made A700 (anatase) and A800 (rutile) on the permeate flux through Filtanium 100 mem-brane were assessed. The difference between the lowest and the highest values of the permeate fluxesmeasured for various TiO2 materials did not exceed 12%.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Every year huge amounts of wastewater containing toxicorganic pollutants are produced worldwide. Conventional waterand wastewater treatment technologies are usually ineffective inremoval of these chemicals and the pollutants enter the environ-ment thus contaminating water sources. Therefore, numerousattempts have been undertaken to develop new methods of waterand wastewater treatment. A very promising solution could beapplication of photocatalytic membrane reactors (PMRs). PMRsare hybrid systems coupling photocatalysis and a membrane pro-cess in one unit. Photocatalysis allows the organic pollutants tobe decomposed and mineralized to H2O, CO2 and mineral salts. Amembrane enables separation of the photocatalyst from the reac-tion medium and its further reuse. Additionally, the membranecould serve as a barrier for the molecules present in the solution,

both initial compounds and products or by-products formed dur-ing the decomposition [1].

Two main types of PMRs can be differentiated: (i) with a photo-catalyst in a suspension and (ii) with a photocatalyst immobilizedon the membrane surface or within its structure [1]. Most of thePMRs described in literature are slurry reactors equipped withpolymer membranes [2–4]. However, polymer membranes havesome serious drawbacks when their application in PMRs is consid-ered. The most important are low resistance to UV light and theaction of hydroxyl radicals. The limited stability of commercialpolymeric membranes under UV irradiation was confirmed byMolinari et al. [5], whereas Chin et al. [6] reported the membranesdamage during their exposure to 200 mmol/dm3 H2O2 solution. Toincrease the lifetime of the polymeric membranes in the hybridphotocatalysis-membrane systems it is necessary to place themseparately from a photoreactor. Ceramic membranes exhibit prop-erties that make them more appropriate for the application in thePMRs, namely high chemical, pH, temperature and pressure resis-tance [7]. Ceramic membranes can be made of various materials,including TiO2, Al2O3 or ZrO2. Particularly, the pure titania

138 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

membranes are widely used in microfiltration (MF) and ultrafiltra-tion (UF) systems because they exhibit good fouling resistance dueto highly hydrophilic properties of their surface [8]. Despitenumerous advantages of ceramic membranes, the literaturereports on their application in PMRs are very limited [7,9–12].Moreover, most of papers refer to a preparation of the membraneswith immobilized TiO2 layer and evaluation of their photocatalyticefficiency in the hybrid systems [9,10]. PMRs equipped with suchmembranes were applied for decomposition of model dyes includ-ing Methyl Orange [9], Acid Orange 7 [10,12], Acid Red 4 [11] andRhodamine B [13]. The literature data on slurry PMRs equippedwith ceramic membranes is even much more scanty. A pilot scalePMR with ceramic MF membranes and TiO2 in suspension wasapplied for treatment of Colorado River water [14]. The authorsdescribed mainly the efficiency of the removal of pharmaceuticalsand endocrine disrupting compounds; however, did not presentany details on the permeate flux variations during the experiment.Shi et al. [15] presented results of the treatment of waste seawaterfrom shrimp farms in a photocatalytic membrane reactor equippedwith a-Al2O3 membrane with pore size of 0.05 lm. Except fromTiO2 photocatalysis they examined combined systems, i.e. photo-catalysis enhanced with H2O2 and photocatalysis in the presenceof Fenton reagent. The authors reported that the permeate fluxunder 0.5 bar was 432 dm3/m2 h. Another researchers [16] studiedfouling and regeneration of two Al2O3 MF membranes with thepores diameter of 0.2 and 1.0 lm for the recovery of fine TiO2 par-ticles from acid wastes. It must be stressed, however, that the sys-tem was not a PMR but just an ordinary MF configuration. Theauthors observed that the permeate flux increased with increasingfeed cross flow velocities (vF) in the range of 1–3.5 m/s. However,at higher velocities it slightly decreased. That was explained bythe removal of larger particles by scouring action of the cross-flowstream as the vF was increased. As a result, smaller particles weredeposited on a membrane which led to the formation of a moredense filtration cake with a higher resistance. Application of back-flush was found to be an efficient method of the TiO2 cake removalonly in case of the 1 lm pore-size membrane [16].

The present state of the art in the area of PMRs with polymericmembranes shows that application of TiO2 in a slurry contributesto permeate flux decline due to deposition of TiO2 on a membranesurface [1]. The severity of membrane fouling caused by TiO2

depends on process parameters such as photocatalyst loading,transmembrane pressure and feed cross flow velocity [17]. Gener-ally, the permeate flux improvement with increasing cross-flowvelocity was observed [17,18]. Moreover, it was found that whenthe TiO2 concentration was increased from 0.001 to 3.0 g/dm3

the flux decline was proportional to TiO2 loading [17–19].The reports concerning membrane fouling caused by TiO2 parti-

cles in PMRs utilizing ceramic membranes are hardly to be found.Moreover, there are no papers describing the stability of ceramicmembranes in PMRs. There is also lack of the papers presentingthorough investigations on the influence of process parameterssuch as TiO2 amount, transmembrane pressure or feed cross-flowvelocity on the ceramic membranes performance in PMRs.

The present work describes the studies on the influence of pro-cess parameters on the performance of TiO2 micro- and ultrafiltra-tion membranes during filtration of TiO2 suspension underconditions prevailing in a slurry photocatalytic membrane reactor.Moreover, the effect of the photocatalyst type on the permeate fluxbehavior was evaluated. Additionally, the membranes stability interms of separation properties was assessed during long termoperation in the PMR. The ceramic membranes made of titaniumdioxide were chosen due to the fact that they were made of a mate-rial (TiO2) exhibiting similar physico-chemical properties as thephotocatalysts used in the experiments. It should be noted, how-ever, that the membranes were not the photocatalytic membranes.

The membrane module was separated from the photoreactor andtherefore the only role of a membrane in the system was separa-tion of photocatalyst particles.

2. Experimental

2.1. Materials

Three single-channel tubular ceramic membranes with brandname of ‘‘Filtanium’’ (TAMI Industries, France) were applied in theexperiments. Two of them were UF membranes with the molecularweight cut-off, MWCO (according to manufacturer) of 5 kDa (Filta-nium 5) and 100 kDa (Filtanium 100), and the third one was an MFmembrane with the maximum pore size of 0.2 lm (Filtanium 0.2).The membranes were made of titanium dioxide. The membraneslength was 25 cm and the external/internal diameters amountedto 10 mm/6 mm. The effective membrane area was 0.0047 m2. Amembrane was placed in a stainless steel housing having lengthof 250 mm and diameter of 15 mm (TAMI Industries, France).Before the experiments the brand new membranes were cleanedchemically with application of NaOH and H3PO4 solutions accord-ing to the procedure recommended by the manufacturer.

During the first part of the investigations TiO2 Aeroxide� P25(Evonik, Germany) was used as a photocatalyst. The TiO2 loadingamounted to 0.5, 1.0 or 1.5 g/dm3. In the second part of theresearch three additional photocatalysts were applied: a commer-cially available ST-01 anatase-phase TiO2 (Ishihara Sangyo, Japan)and the laboratory-made A700 (anatase) and A800 (rutile) sam-ples. The A700 and A800 photocatalysts were prepared by calcina-tion of the industrial grade TiO2, obtained by the sulfate technologyat the Chemical Factory ‘‘Police’’ S.A. (Poland). The calcination tem-perature amounted to 700 and 800 �C, respectively. A detailedcharacteristics of the photocatalysts can be found in our previousworks [20,21].

Polyethylene glycols (PEG) with an average molecular weight(MW) of 4000 and 6000 g/mol (Sigma–Aldrich) and a series of dex-trans with an average MW of 5000, 70,000, 110,000 g/mol (PolfaKutno) and 2,000,000 g/mol (Sigma–Aldrich) were used to deter-mine the separation properties of the membranes. The concentra-tion of the compounds in the feed was set at 1 g/dm3.

All solutions applied in the experiments were prepared usingpure water (0.066 lS/cm, Elix 3, Millipore).

2.2. Methods

Experimental setup of the photocatalytic membrane reactor ispresented in Fig. 1. The installation was equipped with two flowthrough photocatalytic reactors (V = 0.84 dm3 each) containingUV-C lamps (Philips TUV 16 W, kmax = 254 nm; UV light intensity:1.54 W/m2). An air-operated double diaphragm pump SANDPIPER�

S05 (WARREN RUPP, INC., Mansfield, USA.) was applied to deliverfeed to the membrane module. At the beginning of each experi-ment the feed solution (7 dm3) was introduced to the feed tank.After that the UV lamps were switched on and the pump wasstarted. During the process the solution in the feed tank was con-tinuously mixed by a motorized mechanical stirrer. In the experi-ments in which the Filtanium 5 and Filtanium 100 membraneswere applied the transmembrane pressure (TMP) was changedfrom 1 to 3 bar. In case of the Filtanium 0.2 the TMP was in therange of 0.5–3 bar. The feed cross-flow velocity (vF) was set at 3,4.5 or 6 m/s. The change of vF was achieved by changing the pumpdelivery rate for a defined transmembrane pressure. The tempera-ture of the feed was maintained at 20 ± 1 �C. The membrane wasworking in the ‘‘inside-out’’ mode, i.e. the feed flowed in the lumenside of the membrane and the permeate was collected from the

Fig. 1. Schematic diagram of the laboratory-scale photocatalytic membrane reactor used in the experiments (P1, P2, P3 – manometers, R1, R2 – rotameters, FT – feed tank,S – stirrer, P – pump, UV1, UV2 – photoreactors with UV-C lamps, MM – membrane module, H – heater, C – cooler).

S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148 139

shell side of the module. The permeate flux was estimated by mea-suring the volume of permeate which passed through the mem-brane during a period of time. Both the retentate and permeatewere recycled to the feed tank in order to maintain constant con-centration of the photocatalyst. The experiments were repeatedat least twice in order to confirm the reproducibility of the results.

In case of the MF membrane a backflush procedure using per-meate as a flushing medium was applied. The backflush pressurewas 4 bar. The backflush frequency was 30:1, which means thatthe backpulse duration was 1 s and the backpulse intervalamounted to 30 s.

The morphology of the surface of the brand new membranesand the TiO2 filtration cake was analyzed using the laser scanningmicroscopy (LSM) and atomic force microscopy (AFM) techniques.The LSM measurements were performed with application ofVK9700 microscope (Keyence, Japan). The LSM membrane rough-ness (LSM-Ra) was calculated using the VK Analyzer software.The AFM analysis was performed using NanoScope V Multimode8 scanning probe microscope (Bruker Corp.). The measurementswere carried out in the ScanAsyst mode using the silicon nitrideScanAsyst – Air probe. The AFM membrane roughness (AFM-Ra)was calculated with application of the NanoScope Analysissoftware.

The morphology of the membranes and the thickness of theTiO2 filtration cake was examined using Hitachi SU8020 Ultra-HighResolution Field Emission Scanning Electron Microscope (UHR FE-SEM) equipped with Energy Dispersive Spectroscopy system (EDSNSS 312, Thermo Scientific).

The FTIR/DRS spectra were recorded using Jasco FTIR 4200 spec-trometer (Japan) equipped with a diffuse reflectance accessory(Harrick, USA).

The XRD patterns were recorded using X’Pert PRO diffractome-ter with Cu Ka radiation (k = 1.54056 Å). TiO2 anatase over rutileratio was calculated from [19]:

Anatase contentA ¼ 1=ð1þ 1:26ðIR=IAÞÞ ð1Þ

where IA and IR are the diffraction intensities of the (101) anataseand (110) rutile crystalline phases at 2h = 25.3� and 27.4�,respectively.

The average anatase crystallite diameter D (nm) was calculatedusing Scherrer’s equation [19]:

D ¼ Kk=b cos h ð2Þ

where K = 0.9 is a shape factor for spherical particles, k is the wave-length of the incident radiation (k = 1.54056 Å), h is half of the dif-fraction angle (rad) and b is the line width at half-maximum height.

The Brunauer–Emmett–Teller (BET) surface area of the powderswas determined on the basis of nitrogen adsorption–desorptionmeasurements at 77 K conducted in Quadrasorb SI (Quantachrome,USA) apparatus. The samples were degassed at 80 �C prior to nitro-gen adsorption measurement. The BET surface area was deter-mined by multipoint BET method using the adsorption data.

The particle size distribution of the photocatalysts was analyzedby LA-950 Laser Diffraction Particle Size Distribution Analyzer(Horiba, Japan). During the measurement the samples were circu-lated without any sonication in order not to destroy aggregatesformed in water suspensions of the photocatalysts.

The concentrations of polyethylene glycol and dextrans in feedand permeate were determined using high performance liquidchromatograph (HPLC) LaChrom Elite (Hitachi, Japan) equippedwith refractive index (RI) detector L-2490 and the PolySep-GFC-P4000 column. The mobile phase was ultrapure water. The rejec-tion coefficient was calculated on a basis of the equation:

Rð%Þ ¼ ð1� Cpermeate=CfeedÞ � 100% ð3Þ

where Cpermeate and Cfeed represent the concentrations of dextran orPEG in permeate and feed, respectively. The rejection of the modelmolecules was determined in the absence of TiO2 in the feed.

3. Results and discussion

3.1. Characterization of TiO2 photocatalysts

Table 1 presents selected physico-chemical parameters of thephotocatalysts applied in the experiments: the commercially avail-able TiO2 Aeroxide� P25 and ST-01 as well as laboratory madeA700 and A800. A detailed description of the A700 and A800 phot-ocatalysts can be found in our previous works [20,21].

In Fig. 2 the particle size distribution (PSD) of the photocatalystsis shown. During PSD determination the sonication of the sampleswas not applied in order to avoid destruction of the aggregateswhich are formed in water. The P25 and ST01 photocatalysts

Table 1Selected physico-chemical parameters of the photocatalysts.

Photocatalyst Specific surfacearea SBET (m2/g)

Anatase overrutile ratio (A:R)

Crystallite size ofanatase (nm)

A700 30 94:6 32A800 6 2:98 51P25 51 80:20 22ST-01 252 100:0 8

Fig. 2. Particle size distribution of various photocatalysts: P25, ST01, A700 andA800.

140 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

exhibited the most uniform particle size distribution among all ofthe TiO2 materials used, with the mean particle size of 4.2 and6.6 lm, respectively. The A700 contained a portion of particleswith the diameters being lower than those in case of P25 andST01, however, in the PSD graph a second peak corresponding tomuch larger particles can also be observed. The mean particle sizescalculated for both peaks amounted to 3.4 lm and 33 lm, respec-tively. Similarly, a bimodal particle size distribution was found incase of A800. However, the diameters of the particles measuredfor this material were much larger than those determined in caseof other samples. The mean particle size calculated for the firstand the second peak amounted to 4.9 lm and 95.5 lm,respectively.

The analysis of the FTIR-DRS spectra (Fig. 3) revealed significantdifferences in the intensity of the bands corresponding to –OHvibrations (a broad band between 3000 and 3700 cm�1). The inten-sity of these bands decreased in the following order:

Fig. 3. FT-IR/DRS spectra of various photocatalysts.

ST01 > P25 > A700 > A800. Higher intensity of the bands can beattributed to a higher hydrophilicity of a material. According to thisassumption, the ST01 was the most hydrophilic whereas A800exhibited the lowest hydrophilicity from all of the materials used.

3.2. Characterization of brand new membranes

Fig. 4 presents SEM cross sections of the membranes applied inthe study. All the membranes were made of TiO2. The Filtanium 5consisted of 4 layers: a porous support with a thickness of ca.2 mm, two intermediate layers: the first one built of larger TiO2

particles (thickness of ca. 40 lm), and the second one consistingof smaller particles (thickness of ca. 15 lm), and the top layer com-posed of fine particles (thickness of ca. 1.5 lm). The Filtanium 100membrane was also composed of 4 layers, with approximate thick-ness of 2.2 mm, 50 lm, 10 lm and 1.5 lm, respectively. The TiO2

particles forming the skin layer were, however, much larger in caseof Filtanium 100 (Fig. 4d) compared to Filtanium 5 (Fig. 4b). TheFiltanium 0.2 was built of 3 layers: the porous support exhibitedthickness of ca. 2.3 mm, the intermediate layer was ca. 30 lm thickand the separation layer thickness was ca. 10 lm.

The brand new membranes were also characterized by the sur-face roughness (LSM-Ra) determined on a basis of LSM measure-ments. The highest LSM-Ra value was observed in case of the MFmembrane (0.165 lm, S.D. = 0.016 lm). That was associated withthe large diameter of the pores on the membrane surface. TheLSM roughness of Filtanium 5 membrane amounted to 0.082 lm(S.D. = 0.022 lm), whereas the LSM-Ra calculated for Filtanium100 was 0.051 lm (S.D. = 0.008 lm).

Figs. 5–7 present changes of permeate flux during filtration ofpure water through the membranes. In case of Filtanium 5 mem-brane the experiment was carried out for 16 h, divided for 2 days(Fig. 5). The flux measured during the first day was lower than thatobserved during the last 8 h of UF when it stabilized at a level of142 dm3/m2 h. In case of Filtanium 100 membrane (Fig. 6) theultrafiltration of pure water was carried out for 25 h. A continuousdecrease of the permeate flux was observed in the initial 12 h ofthe experiment. During this time the flux was lowered for ca.50%, i.e. from 1000 dm3/m2 h to ca. 500 dm3/m2 h. The permeateflux got stabilized at a level of 494 dm3/m2 h.

The longest time necessary for the permeate flux stabilizationwas observed in case of Filtanium 0.2 membrane (Fig. 7). MF ofpure water through this membrane was conducted for 34 h. Atthe beginning of the process the permeate flux rapidly decreasedfrom ca. 6600 dm3/m2 h to 4250 dm3/m2 h just within 1 h. The sta-ble permeate flux was found to be 2196 dm3/m2 h.

A significant decrease of PWF through a ceramic membraneduring conditioning stage was also reported by Chevereau et al.[22]. The authors observed that the flux was decreasing for about500 h until it reached a stable value. They concluded that the sur-face hydration of TiO2 is a very slow process, during which the for-mation of hydroxyl groups and adsorption of molecular water onthe membrane occurs. Mendret et al. [10] also found that the purewater flux through a brand new ceramic membrane was decreas-ing in the initial step of filtration. However, it got stabilized muchfaster than in the previously described example, i.e. after 4 h of theprocess. The authors reported also that when they used the samemembrane, but firstly immersed in water for 30 days, the permeateflux during filtration of pure water was stabilized just at the begin-ning of the process. They concluded that the membrane that waskept dry gave higher permeate flux than the membrane immersedfor a long time in pure water because in the former case the feedsolution must flow for a defined time to assure the wetting ofmembrane pores. The authors also stressed that this phenomenonis not often described and explained in literature [10].

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 4. SEM cross sections of (a and b) Filtanium 5, (c and d) Filtanium 100 and (e and f) Filtanium 0.2 membranes (1 – support, 2, 3 – intermediate layers, 4 – top layer).

Fig. 5. Changes of permeate flux during UF of pure water through Filtanium 5membrane at TMP of 3 bar. Insert: dependence of stabilized pure water flux (PWF)on TMP.

Fig. 6. Changes of permeate flux during UF of pure water through Filtanium 100membrane at TMP of 3 bar. Insert: dependence of stabilized pure water flux (PWF)on TMP.

S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148 141

Based on the analysis of the results shown in Figs. 6 and 7another explanation for the permeate flux decline can also befound. During filtration the unbound TiO2 particles present in thesurface layer of the membranes were being moved under theaction of the pressure and the flowing feed. That led to a highermembrane rigidity and decreased porosity. As a result the perme-ate flux decline was observed. During the night the membrane was

kept wet but was no longer under the action of the pressure. There-fore, at the beginning of the second day of water filtration the per-meate flux was a bit higher than at the end of the first day of theexperiment, what is especially visible in case of Filtanium 100and Filtanium 0.2 (Figs. 6 and 7). Such an increase of the flux mightbe explained by the membrane decompression.

A linear dependence between PWF and transmembrane pres-sure was observed for all the examined membranes (Figs. 5–7).

Fig. 7. Changes of permeate flux during UF of pure water through Filtanium 0.2membrane at TMP of 3 bar. Insert: dependence of stabilized pure water flux (PWF)on TMP.

142 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

The permeance in case of Filtanium 5 amounted to 47 dm3/m2 hbar, in case of Filtanium 100 it reached 165 dm3/m2 h bar and forthe Filtanium 0.2 membrane it amounted to 732 dm3/m2 h. Thestable PWF values were used as a reference in further discussion.

The separation properties of the membranes were determinedusing PEG and dextrans solutions. The rejection of PEG with anaverage molecular weight (MW) of 4000 and 6000 g/mol by the Fil-tanium 5 membrane reached 97% and 99%, respectively. The rejec-tion coefficient (R%) of dextran having MW of 5000 g/mol waslower and amounted to 83%. In case of Filtanium 100 membranethe efficiency of separation of dextrans having 70,000 g/mol and110,000 g/mol was equal to 95% and 99%, respectively. The separa-tion properties of the MF Filtanium 0.2 membrane were evaluatedwith application of dextran with MW of 2,000,000 g/mol. The R%value in this case was equal to 17%.

The rejection coefficients determined for the brand new mem-branes served as a point of reference during assessment of thedegree of the membranes damage by the abrasive action of TiO2

particles.

Fig. 8. SEM microphotographs of cross-sections of the Filtanium 5 and Filtanium 100 me1.5 g TiO2P25/dm3).

3.3. Influence of TiO2 loading on permeate flux through UF membranesin the PMR

In order to evaluate the influence of TiO2 loading on the perme-ate flux through Filtanium 5 and Filtanium 100 membranes a set ofexperiments in which photocatalyst concentration amounted to0.5, 1.0 or 1.5 g/dm3 was performed. The experiments were con-ducted at TMP of 3 bar and the feed cross-flow velocity of 3 m/s.During the process the UV lamps were switched on. No significantdifferences between the permeate fluxes measured under theapplied conditions were observed. The flux in case of the Filtanium5 membrane was about 150–155 dm3/m2 h, whereas in case of Fil-tanium 100 membrane it was in the range of 530–550 dm3/m2 h,regardless of TiO2 doses. It was also noticed that the permeatefluxes during the PMR operation were higher than those measuredfor pure water (Figs. 5 and 6).

The literature data on PMRs utilizing polymeric membranesshow that when the photocatalyst concentration increases, thepermeate flux decrease takes place as a result of a cake layerbuild-up on the membrane surface [17–19]. The results obtainedin the present work are in disagreement with these data, i.e. nopermeate flux decline through the UF membranes due to the pres-ence of TiO2 was observed. This might lead to a conclusion that theconditions applied in the discussed experiments prevented fromTiO2 cake deposition on the membrane surface even at higher pho-tocatalyst loadings. To verify this supposition the samples of mem-branes after PMR operation at TMP of 3 bar and vF of 3 m/s wereanalyzed using SEM. The TiO2 P25 concentration was 1.5 g/dm3.The SEM microphotographs of the membranes cross sections areshown in Fig. 8. It can be observed that in case of both membranesthe surface was covered with a filtration cake layer which meansthat the supposition was wrong. The TiO2 cake had a similar thick-ness in case of both UF membranes, i.e. 37 lm (S.D. = 3 lm) for Fil-tanium 5 and 45 lm (S.D. = 3 lm) for Filtanium 100.

Thus, other explanation for the obtained results should befound. Taking into account that the experiments were realizedunder the conditions prevailing in PMRs, i.e. in the presence of

mbranes after experiments in the PMR (process parameters: vF = 3 m/s, TMP = 3 bar,

S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148 143

UV irradiation, the superhydrophilicity of TiO2 was taken into con-sideration. During UV irradiation TiO2 particles gain superhydro-philic properties [23] and this phenomenon can last even a fewhours after the end of the exposition to UV light [24]. Since inthe experiments performed in this study the TiO2 suspension wascontinuously circulated between the photoreactors and the mem-brane module, the photocatalyst was able to maintain its superhy-drophilic properties. The positive influence of the superhydrophiliceffect on the permeate flux was previously reported by otherresearchers [25,26]. Ma et al. during their investigations [25] foundthat Si-doped TiO2/Al2O3 composite UF membrane after irradiationwith UV light exhibited higher pure water permeability than themembrane kept in the dark. Moustakas et al. [26] suggested thatsuperhydrophilic properties of TiO2 could restrict from depositionof the photocatalyst particles on a membrane surface. Instead, athin layer of superhydrophilic TiO2 particles could be formed ona membrane thus increasing its hydrophilicity which further leadsto the improvement of the permeability.

In order to verify the hypothesis that the increase of the perme-ate flux observed during PMR operation was associated with thesuperhydrophilic effect of TiO2, the additional experiments with-out the presence of UV irradiation were carried out. The obtained

Fig. 9. LSM images of the surface of the Filtanium 5 (a) and Filtanium 100 (b) membranparameters: vF = 3 m/s, TMP = 3 bar, 1.5 g TiO2P25/dm3).

data revealed that the permeate fluxes measured in the presenceand in the absence of UV light were very similar. Taking this intoaccount it can be concluded that the superhydrophilic effect ofTiO2 was not the reason for the observed permeate flux increase.

Nonetheless, considering the results obtained during filtrationof the photocatalyst, it can be stated that the TiO2 cake layer shouldexhibit a porous structure. Otherwise, the resistance of the filtra-tion cake might be so high that the flux decline instead of improve-ment could be observed. Indeed, the SEM analysis confirmed theporous structure of the cake (Fig. 8). The separation layer of bothFiltanium 5 and Filtanium 100 membranes was built of much finerparticles (Fig. 4b and d) than the TiO2 cake (Fig. 8). The PSD analy-sis (Fig. 2) revealed that the diameter of TiO2 aggregates was in therange of 0.7–17.4 lm with the mean size of 4.2 lm. The resistanceof the layer comprising large TiO2 aggregates and exhibiting lowthickness (45 lm) was significantly lower than the membraneresistance and therefore the presence of the deposit did not causethe permeate flux decrease during the filtration.

On a basis of LSM analysis (Fig. 9) it was found that the rough-ness of the TiO2-covered membranes was higher than that of theunused ones. However, no significant differences between theroughness of the filtration cake calculated for both UF membranes

es before (left image) and after (right image) the experiments in the PMR (process

144 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

was observed. The LSM-Ra in case of Filtanium 5 amounted to0.220 lm (S.D. = 0.012 lm) and in case of Filtanium 100 it was0.252 lm (S.D. = 0.021 lm).

To sum up, the obtained results revealed that despite the pres-ence of the TiO2 filtration cake on the surface of Filtanium 5 andFiltanium 100 membranes no permeate flux decrease took place.Instead, an increase of the flux compared to PWF was found. How-ever, the increase cannot be explained in terms of the superhydro-philic effect of TiO2. In view of the above, the most probableexplanation for the permeate flux improvement is the abrasion ofa membrane skin layer by the photocatalyst particles. As a resultsome new pores were formed or some existing pores wereenlarged. A more detailed discussion on the abrasion of the mem-branes by the photocatalyst particles will be presented later.

3.4. Influence of feed cross-flow velocity on permeate flux in the PMR

During the investigations on the influence of feed cross flowvelocity on the permeate flux through the UF and MF membranesthe vF of 3, 4.5 or 6 m/s was applied. Since no effect of TiO2 loadingon permeate flux was observed in case of both UF membranes (Sec-tion 3.3), the concentration of 1.5 g/dm3 was applied in the dis-cussed experiments. The UV lamps remained switched on duringthe whole process in order to assure the conditions prevailing inPMRs. For both Filtanium 5 and Filtanium 100 membranes therewas no significant effect of the velocity on the permeate flux. Themean permeate flux in the experiments with Filtanium 5 mem-brane was about 150 dm3/m2 h. For Filtanium 100 membrane thepermeate flux ranged from 545 to 570 dm3/m2 h and was insignif-icantly higher at vF = 6 m/s. It was also observed that under all theconditions applied in the study the permeate fluxes were higherthan PWF values, similarly as in the experiments described in theprevious section.

Different results were obtained in case of the MF Filtanium 0.2membrane (Fig. 10). Since that membrane had much larger poresthan the UF ones, what created a danger of pore blockage byTiO2 particles, the additional experiments with application of back-flushing (BF) were realized except from the standard filtration. Thebackflushing frequency was set at 30:1, which means that after30 s of filtration a 1 s backflush with permeate was applied. Ascan be seen in Fig. 10, in case of the Filtanium 0.2 the feed cross-flow velocity had a significant influence on the permeate flux. Ingeneral, the flux was decreasing with a decrease of vF. Moreover,at cross flow velocities of 4.5 and 6 m/s the permeate flux washigher than PWF, whereas at 3 m/s the flux decreased significantlybelow the value measured for pure water. It is well noticeable thatthe increase of the flux in case of the higher vF values was muchmore significant than the increase observed for the UF membranes.At the feed cross flow velocity of 6 m/s the permeate flux was

Fig. 10. Influence of the feed cross-flow velocity on the permeate flux throughFiltanium 0.2 membrane (process parameters: TMP = 3 bar, 1.5 g TiO2P25/dm3,backflushing (BF) frequency – 30:1).

higher than the PWF for ca. 1650 dm3/m2 h, whereas at 4.5 m/s –for ca. 510 dm3/m2 h (Fig. 10). In case of the lowest vF a decreaseof the flux for ca. 770 dm3/m2 h below the PWF value wasobserved. The significant increase of the permeate fluxes at vF of4.5. and 6 m/s compared to PWF can be attributed to the abrasionof the surface of Filtanium 0.2 membrane by the photocatalyst par-ticles, similarly as it was found in case of the UF membranes. Ascan be seen in Fig. 4f the top layer of Filtanium 0.2 was built of par-ticles with a non-uniform distribution. It can be assumed that dur-ing filtration the particles having finer sizes underwent erosionwhat led to the opening of new pores or enlargement of the exist-ing ones. As a result the permeate flux increase was found. Theabrasion cannot be also excluded in case of vF = 3 m/s. However,at this cross flow velocity a significant fouling of the membraneby the photocatalyst particles occurred and therefore the fluxincrease caused by the abrasive action of TiO2 particles cannot beseen. Moreover, taking into consideration that the increase of theflux was more significant at vF = 6 m/s compared to that at 4.5 m/s it can also be concluded that at the latter cross flow velocitythe membrane fouling took place as well. However, the severityof this phenomenon in that case was not as high as at vF = 3 m/ssince the flux remained higher than PWF.

The application of the backflush was found to be ineffective inthe discussed experiments. Although the BF frequency was high,the permeate flux was similar to the flux measured during theexperiments without membrane backflushing. No significant influ-ence of BF on permeate flux restoration during MF of processingacid wastewater containing fine TiO2 particles through a 0.2 lmpore-size a-Al2O3 membrane was also found by Zhao et al. [16].On the opposite, the authors observed a high efficiency of backflu-shing in case of a membrane with nominal pore size of 1 lm. Thedifferences in the results were attributed to the lower flux declineand higher resistances of the membrane and fouling layer in case ofthe membrane with 0.2 lm pores compared to that with 1 lmpores.

The data presented in Fig. 10 show that selection of a properfeed cross flow velocity is a more efficient way of improvementof permeate flux than the application of the membrane backflush.

Figs. 11 and 12 present SEM and LSM images of the cross sectionand the surface of the Filtanium 0.2 membrane after the experi-ments realized at vF = 3 m/s in the PMR. Similarly as in case of bothUF membranes a formation of a TiO2 cake was also observed for theMF membrane (Fig. 11a). However, the cake thickness was signifi-cantly lower (14 lm, S.D. = 1 lm) in case of MF compared to UFmembranes. An explanation of these results needs further investi-gations. Nonetheless, based on the present data it might be con-cluded that in case of the Filtanium 0.2 membrane thephotocatalyst particles were not only deposited on the membranesurface but also entered its pores what affected the filtration cakethickness. Such a conclusion can be drawn on a basis of the SEMcross section of the membrane taken at higher magnification(Fig. 11b). In this microphotograph the presence of small TiO2 par-ticles, probably P25, between the larger TiO2 particles forming themembrane structure can be seen. However, it must be remem-bered that due to the fact that both materials, i.e. the membraneand the photocatalyst were made of TiO2 the unequivocal differen-tiation of the particles origin is difficult.

The surface roughness (LSM-Ra) of the TiO2 cake formed on themembrane surface was similar to that measured for both UF mem-branes and amounted to 0.233 lm (S.D. = 0.010 lm). The LSMimages presented in Figs. 9 and 12 show the samples after theexperiments realized at the same process parameters, i.e. atvF = 3 m/s, TMP = 3 bar and with application of 1.5 g TiO2/dm3. Tak-ing this into consideration, it might be concluded that the rough-ness of the surface of TiO2 cake formed under the same processconditions is independent of the membrane type.

(a)

(b)

Fig. 11. SEM micrographs of the cross-section of the Filtanium 0.2 membrane afterthe experiments in the PMR (process parameters: vF = 3 m/s, TMP = 3 bar, 1.5 gTiO2P25/dm3).

Fig. 13. Influence of transmembrane pressure on permeate flux during experimentsrealized with Filtanium 5 and Filtanium 100 membranes (process parameters:vF = 3 m/s; 1.5 g TiO2P25/dm3).

S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148 145

3.5. Influence of transmembrane pressure on permeate flux in the PMR

Fig. 13 presents the influence of transmembrane pressure onthe permeate flux measured for both UF membranes during PMRoperation. For comparison purpose the PWF values are also shown.As was already discussed in the previous sections, in case of the UF

Fig. 12. LSM images of the surface of the Filtanium 0.2 membrane before (left image)TMP = 3 bar, 1.5 g TiO2P25/dm3.

membranes no significant influence of the feed cross flow velocityon the permeate flux was observed, therefore, in Fig. 13 the datacollected at vF of 3 m/s are only presented.

The increase of TMP during the experiments realized in the PMRled to a linear increase of the permeate flux for both UF mem-branes, despite application of the lowest cross flow velocity fromall the examined values. However, the slope of the observed rela-tionship was different from the one representing the correlationdetermined for pure water. The fluxes measured during the exper-iments with TiO2 were slightly higher (ca. 5–10%) than PWF for allthe examined pressures. The observed increase of the flux can beexplained by the abrasion of the skin layer of the membranes asmentioned earlier.

The influence of the transmembrane pressure on the permeateflux during experiments realized with application of MF Filtanium0.2 membrane is shown in Fig. 14. In case of this membrane the vF

value had a significant influence on the permeate flux, thereforethe results obtained at various cross flow velocities are shown. Atthe lowest TMP applied (i.e. 0.5 and 1 bar) the permeate fluxesmeasured during PMR operation were higher than PWF. However,at the lowest vF (i.e. 3 m/s) an increase of the pressure up to 2 barresulted in a decline of the flux compared to pure water flux. More-over, in case of that cross flow velocity the fluxes measured at TMP

and after (right image) experiments in the PMR. Process parameters: vF = 3 m/s,

Fig. 14. Influence of transmembrane pressure on permeate flux during experimentsrealized with Filtanium 0.2 membrane (process parameters: vF = 3–6 m/s; 1.5 gTiO2P25/dm3).

146 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

of 2 and 3 bar were almost the same. On the opposite, a linearincrease of the permeate flux with increasing TMP was found whenthe vF of 6 m/s was applied. In case of the medium cross flow veloc-ity used (i.e. 4.5 m/s) the permeate flux increased almost linearlyup to TMP of 2 bar. However, further rise of the pressure contrib-uted only insignificantly to the flux improvement. The observedresults can be explained in terms of the critical flux phenomenon.The critical flux, in brief, corresponds to the permeate flux belowwhich fouling is not promptly observed [27]. The data presentedin Fig. 14 show that only in case of the highest cross flow velocitythe critical flux was not reached in the investigated system.

3.6. Stability of ceramic membranes during PMR operation

In general, ceramic membranes are known to exhibit very goodresistance to the harmful action of oxidative species or very low/very high solution pH. However, it must be considered that the fil-tration of a photocatalyst suspension at high feed cross-flow veloc-ities, usually applied in ceramic membrane systems, creates a riskof destruction of the membrane surface due to abrasion. This couldresult in a loss of separation properties of the membrane thus con-tributing to shortening of its lifetime.

The stability of the UF membranes used in the experiments wasdetermined on a basis of changes of their separation properties interms of dextran and polyethylene glycol (PEG) rejection after 50,70 and 100 h of PMR operation. The results are summarized inFig. 15.

In case of Filtanium 5 the rejection of both dextran (5000 g/mol)and PEG (6000 g/mol) was continuously decreasing in time. Theefficiency of dextran separation decreased from 83% at the begin-ning of the process to 58% after 100 h of experiments realized in

Fig. 15. Changes of separation properties of Filtanium 5 and Filtanium 100membranes during their operation in PMR.

the presence of TiO2. What is interesting, after the same time ofoperation the Filtanium 5 membrane completely lost the separa-tion properties towards PEG. It is remarkable that in case of the Fil-tanium 100 membrane much higher stability towards TiO2 actionwas observed. In fact, no decrease of the efficiency of dextran(70,000 g/mol) rejection in time was found.

The Filtanium 5 membrane is defined by the manufacturer as a‘‘fine-UF’’ membrane, in contrast to the Filtanium 100 which is anordinary UF membrane. The Filtanium 5 exhibited more dense skinlayer built of smaller particles than the Filtanium 100 (Fig. 4) inorder to assure lower MWCO value. The results shown in Fig. 15indicate that the resistance of such a separation layer to thedestruction by abrasive TiO2 is very low.

In order to investigate whether the structure of the surface ofthe Filtanium 5 membrane changed after its application in thePMR the AFM images of the as-received and the destroyed sampleswere compared (Fig. 16). Before the measurement the sample ofthe membrane taken from the PMR was cleaned gently with asponge in order to remove TiO2 particles. However, despite thisoperation some remains of the filtration cake were still presenton the membrane surface as can be observed in Fig. 16b. TheAFM images were taken at the scanned area of 5 lm � 5 lm and1 lm � 1 lm, respectively. It can be observed that the topographyof the membrane surface changed significantly after its operationin the PMR. Namely, the surface became much more rough. Theaverage roughness of the brand new membrane amounted to3.5 nm (S.D. = 0.5 nm) and 2.0 nm (S.D. = 0.2 nm) for the scannedareas of 5 lm � 5 lm (AFM-Ra5) and 1 lm � 1 lm (AFM-Ra1),respectively. In case of the used membrane the AFM-Ra5 amountedto 14.9 nm (S.D. = 1.4 nm) whereas the AFM-Ra1 was equal to10.7 nm (S.D. = 1.2 nm). It can be seen that the calculated rough-ness values were dependent on the scanned area. An increase ofthe membrane surface roughness with increasing scanned areawas already discussed by Boussu et al. [28] and Johnson et al.[29]. The authors [28] stressed that it is crucial that the same scansize range is used when comparing the surface roughness for dif-ferent samples.

The observed increase in the Filtanium 5 roughness after theexperiment in the PMR compared to the brand new membrane(Fig. 16) might lead to a conclusion that the gentle skin layer pres-ent on this membrane surface (Fig. 4) was scraped by the TiO2 P25particles during the filtration process. As a result, the membranelost its separation properties (Fig. 15).

The loss of separation properties due to the gradual abrasion ofthe membrane skin layer can explain the increase of the permeateflux through Filtanium 5 observed during the experiments dis-cussed earlier. However, in case of the other UF membrane, Filtani-um 100, although the permeate flux was also higher than PWF theseparation properties did not deteriorate. Therefore, it might besupposed that only some finer particles building the membraneskin layer were eroded. That led to opening of some new poreswith diameters so small that they contributed to the value of thepermeate flux but did not change the separation characteristicsof the membrane.

In order to characterize the separation properties of the MF Fil-tanium 0.2 membrane the model solution of 2,000,000 g/mol dex-tran was used. As expected, the retention of the model compoundby the brand new membrane was very low and amounted to 17%only. However, the efficiency of dextran separation increased intime of the process and after 50 h of operation amounted to 34%.The results lead to a conclusion that during the process the depo-sition of TiO2 particles in the membrane pores occurred, whichconfirms the supposition stated earlier on a basis of Fig. 11b.

Taking into consideration the obtained results it was concludedthat the most promising membrane for the application in the PMRis Filtanium 100. The usage of the ultrafine ceramic membrane is

Fig. 16. AFM images of the brand new Filtanium 5 membrane (a and c) and the membrane after the experiments in PMR (b and d). Process parameters: vF = 3 m/s,TMP = 3 bar, 1.5 g TiO2 P25/dm3.

S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148 147

not recommended due to its low resistance to the destruction byharsh factors such as abrasive TiO2 particles. The MF Filtanium0.2 membrane could be possibly used on condition that the opti-mum cross flow velocity and TMP are selected; however, it mustbe taken into account that TiO2 particles deposition in the mem-brane pores might occur and this would affect its separationcharacteristics.

Fig. 17. Influence of the type of a photocatalyst on permeate flux through Filtanium100 membrane (process parameters: TMP = 1–3 bar, vF = 3 m/s, 1.5 g TiO2/dm3).

3.7. Influence of the type of photocatalyst on the permeate flux in thePMR

In order to determine the influence of the photocatalyst type onthe performance of ceramic membranes in the PMR a set of exper-iments with application of P25, ST01, A700 and A800 titaniumdioxide was realized. The Filtanium 100 membrane was selectedfor these investigations due to its high resistance to fouling andabrasion by P25 TiO2 particles. Based on the results described inthe previous sections the vF of 3 m/s and TiO2 concentration of1.5 g/dm3 were applied. The obtained results are summarized inFig. 17. It can be seen that despite various properties of the TiO2

materials (Section 3.1) the photocatalyst type had a negligibleinfluence on the permeate flux. The percentage difference betweenthe lowest and the highest flux at various TMP values was similarand amounted to 10–12% for 1–3 bar, respectively. An explanationfor the obtained results can be the high cross flow velocity appliedin the experiments. As was observed in case of TiO2 P25 photocat-alyst (Fig. 8) the filtration cake formed under such conditions was

thin (45 lm) and very porous, therefore its contribution to theoverall filtration resistance was insignificant. It is also worth notingthat regardless of the TiO2 type used all the measured permeatefluxes were above the PWF.

4. Conclusions

The obtained results revealed that the performance of ceramicUF membranes in a photocatalytic membrane reactor differed sig-nificantly from that observed in case of a ceramic MF membrane.

148 S. Mozia et al. / Separation and Purification Technology 142 (2015) 137–148

During the experiments realized with application of UF mem-branes no influence of feed cross flow velocity and TiO2 P25 photo-catalyst concentration on the permeate flux was found in a range ofinvestigations. In case of Filtanium 0.2 MF membrane a deteriora-tion of permeate flux with decreasing feed cross-flow velocity wasobserved. At cross flow velocity of 3 m/s the flux decreased signif-icantly below the value measured for pure water. That was attrib-uted to the membrane pore blockage by photocatalyst particles. Itwas concluded that selection of a proper feed cross flow velocitywas a more efficient way of improvement of permeate flux throughthe MF membrane than the application of the membranebackflush.

An increase of permeate flux during PMR operation comparedto pure water flux (PWF) was found in case of both UF membranes,regardless of process conditions. When the MF membrane wasapplied such an increase was observed at feed cross flow velocitiesvF P 4.5 m/s. It was concluded that the most probable explanationfor the permeate flux improvement was the abrasion of a mem-brane skin layer by the photocatalyst particles. Nonetheless, theabrasion affected the separation characteristics only in case ofthe fine UF Filtanium 5 membrane. That membrane lost completelyits separation properties towards PEG after 100 h of operation. Nosignificant changes of dextran rejection by Filtanium 100 wasobserved after 100 h of experiment. In case of the MF membranean increase of dextran rejection in time was found which wasattributed to the deposition of TiO2 particles in the membranepores.

The influence of the photocatalyst type on the permeate fluxthrough Filtanium 100 membrane was found to be insignificant.The results can be explained in terms of the high cross flow veloc-ity applied in the experiments. Under such conditions the filtrationcake formed on the membrane was thin and porous thus its contri-bution to the overall filtration resistance was negligible.

A particular attention should be paid to the selection of mem-branes for PMRs not only with reference to their resistance to dam-age by oxidative species or UV radiation but also in terms of theresistance to abrasion by photocatalyst particles. It should be takeninto consideration that ceramic membranes can also be damagedby abrasive action of photocatalyst particles.

Acknowledgements

This research was supported by The National Science Center(Poland) under Project No. 2011/03/B/ST5/01053.

The authors would like to thank Dr. Katarzyna Wilpiszewskafrom Polymer Institute, WPUT, Szczecin for her assistance in LMSanalyses.

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