Photocatalytic Membranes for Efficient Water

Article pubs.acs.org/IECR

Hybrid Ultrafiltration/Photocatalytic Membranes for Efficient Water Treatment G. E. Romanos,* C. P. Athanasekou, V. Likodimos, P. Aloupogiannis, and P. Falaras Division of Physical Chemistry, Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, NCSR Demokritos, 153 10 Agia Paraskevi Attikis, Athens, Greece ABSTRACT: The development of hybrid materials exhibiting the simultaneous action of photocatalysis and membrane filtration can lead to improved water treatment processes. Photocatalysis has the potential to solve problems related to the fouling of membranes, the generation of toxic condensates, and the existence of very small and harmful organic pollutants in the permeate effluent. On the other hand membranes, especially the ceramic ones, are appropriate supports for the deposition of thin photocatalytic layers due to their high affinity with the photocatalyst (e.g., TiO2) and the possibility to further stabilize and activate the deposit with calcination. In addition, membranes exhibit two surfaces that come into contact with the polluted water and can be exploited for the photocatalyst deposition. Thus, with appropriate design of the membrane module it is possible to illuminate both membrane surfaces and develop very efficient photocatalytic ultrafiltration processes. Such processes must involve “double sided active photocatalytic membranes”, where the pollutant undergoes two sequential photodegradation steps, the first in contact with the feed surface and the second in contact with the permeate surface of the membrane. Moreover the asymmetric pore structure of ceramic membranes assures proper mixing of the fluid and better contact with the porous photocatalytic layers. In this work double side active photocatalytic ultrafiltration (UF) membranes were developed by means of different chemical vapor deposition (CVD) techniques. Their performance in the elimination of methyl orange from water was elucidated by means of a prototype photocatalytic membrane reactor under continuous flow, applying UV irradiation on both membrane surfaces. Important aspects of membrane technology such as the evolution of water permeability and the energy consumption were compared with the standard and highly efficient nanofiltration (NF) process and the results indicated the beneficial effects of the hybrid UF/photocatalytic process. TiO2 nanofibers through glass filters followed by hot pressing13 or liquid phase pressurization,14 the hydrothermal growth of free-standing TiO2 nanowire membranes,15 the anodization of titanium films sputtered on to stainless steel substrates,16 the embedment of TiO2 nanoparticles into the matrix of polymeric membranes, either electrospun fibers,1718 or flat membranes prepared by recasting,19 the development of TiO2 layers with rapid atmospheric plasma spray coating,20 and the fabrication of free-standing and flow through TiO 2 nanotube membranes.21−25 In most of the above studies the photocatalytic activity of the membranes was evaluated in batch experiments and methyl orange (MO)10,12,19,21 or methylene blue (MB)10,13,17,18,25 were applied as the model pollutants. Pharmaceuticals,15 humic acid,14,23 and other dyes such as reactive black 520 (RB5) and RGB24 have also been tested. Even in the few cases where a hybrid photocatalytic/membrane filtration process was applied,13,20,25 this was done in the presence of fouling prone molecules such as humic acid and cyclodextrines with the purpose to exemplify the antifouling characteristics of the produced membranes and not to highlight

1. INTRODUCTION The first attempts to combine photocatalysis with membrane technology were limited to the application of membranes as filters for achieving the separation and reuse of the photocatalyst nanoparticles.1−6 Moreover, the use of a suspended photocatalyst for the elimination of pollutants from the condensed retentate effluent of nanofiltration (NF) processes7 or from the effluents of membrane bioreactors (MBR)8,9 has often been reported. Both approaches faced the usual problems of membrane fouling, photocatalyst deactivation, and nanoparticles agglomeration. Therefore, with the purpose to improve and further simplify these hybrid photocatalysis/ filtration water treatment processes, research efforts have started to develop photocatalytic membranes as standalone elements exhibiting both photocatalytic and separation efficiency. In this way complex cleaning procedures that cancel the continuous operation of a membrane filtration process, such as forward and reverse flushing, backwashing, air scouring, and back permeation can be avoided. Moreover, the photocatalyst is fully stabilized on the substrate surface or incorporated in the substrate matrix and consequently does not agglomerate. What is more important, there is no need for any post- or pretreatment stage, a feature that constitutes the photocatalytic membrane reactor approach as a very attractive process for up scaled industrial water treatment applications. Several methods have been recently developed and optimized for the manufacturing of titania based photocatalytic membranes. Among them are dip coating,1011 or spin coating12 of porous supports using TiO2 precursor sols, the filtration of © 2013 American Chemical Society

Special Issue: Recent Advances in Nanotechnology-based Water Purification Methods Received: Revised: Accepted: Published: 13938

December March 11, March 12, March 12,

22, 2012 2013 2013 2013

dx.doi.org/10.1021/ie303475b | Ind. Eng. Chem. Res. 2013, 52, 13938−13947

Industrial & Engineering Chemistry Research

Article

Figure 1. CVD reactor including the differential pressure transducer for online monitoring the differential pressure across the membrane.

Table 1. Membranes Tested and the Fluid Flow Conditions in the Photocatalytic Membrane Reactor material code

material description

total feed flux (mL/(min cm2))

permeate fluxa (mL/(min cm2))

feed pressure (bar)

Mem1 Mem1m Mem2 Mem2m

γ-alumina UF with 5 nm pore size Mem1 modified with nanoparticles deposition γ-alumina UF with 10 nm pore size Mem2 modified with physisorption and surface reaction

0.0536 0.0536 0.107 0.0536

0.015 0.0096 0.0393 0.0429

14.8 14.2 1.6 2.8

a

The permeate flux values represent the average of the values obtained during the experiment.

ends 3 cm each side) exhibited an asymmetric pore structure, consisting of two intermediate α-alumina layers and a rough macroporous α-alumina support and carried a γ-alumina UF layer on their external surface. When developing photocatalytic membranes, the selection of a ceramic based support brings many advantages compared to a polymer based one, such as the affinity with the photocatalyst and the sustainability at the elevated temperatures required for the CVD deposition and possibly for the transformation of amorphous TiO2 precursor to the photocatalytically active phase (e.g., anatase). The CVD reactor applied for the development of the photocatalytic UF membranes is illustrated in Figure 1. Each membrane was accommodated in the CVD reactor cell which was maintained at the desired temperature inside a tubular furnace. The external surface of the membrane was 28 cm2, and the internal was 20 cm2. A nitrogen (N2) gas stream of 200 mL/min, after being enriched with vapor or droplets of titanium tetraisopropoxide (TTIP) inside a glass bubbler at room temperature, was further introduced in the reactor on either side of the membrane depending on the position of the three-way valve V1 (Figure 1). In this manner it was possible to deposit the photocatalyst on both membrane surfaces (e.g., external and internal) sequentially, without the need to dismantle the membrane from the CVD reactor. Titania (TiO2) deposition was performed following two approaches, and the produced photocatalytic membranes are abbreviated as Mem1m and Mem2m, respectively (Table 1). In the first approach, by keeping the reactor temperature at 450 °C, we facilitated the pyrolitic decomposition of TTIP and the formation of TiO2 nanoparticles in the gas phase. The grown nanoparticles could be further transferred and deposited on the membrane surface under thermophoretic or diffusion driving forces, depending on the temperature profile of the reactor e.g. the difference in temperature between the gas phase sweeping the reactor and the membrane surface. The second approach involved physisorption of the TTIP vapor on the surface and pores of the membrane at room temperature followed by decomposition and surface polymerization reactions in oxygen (O2) atmosphere at temperatures up to 350 °C. This procedure was applied sequentially for several times in order to develop multiple TiO2 layers. The first and second procedures were involved to modify γ-alumina

the potentiality of such an hybrid process for water treatment applications that could successfully replace nanofiltration. Currently, there is a shortage in investigations related to the intensification of the hybrid photocatalysis/filtration process whereas the photocatalyst deposition is usually limited to one of the two surfaces of disk shape supports which are not appropriate for up-scaled applications. In this context, we have directed our research effort to the development of photocatalytic membrane manufacturing methods that could be easily transferred to a higher scale without further implications to the environment, e.g. the generation of large amount of concentrated polluted sols for disposal. To this end, chemical vapor deposition (CVD) techniques were developed which provide the capability to deposit layers of photocatalytic nanoparticles on both surfaces of tubular UF membranes, either directly from the gas phase or through adsorption and surface reactions. These techniques were further optimized to allow for monitoring the evolution of the photocatalyst deposition, thus providing the possibility to control the porosity of both the photocatalytic layer and porous support. More specific the membrane characteristics were optimized by efficiently controlling the permeability of the carrier gas (N2) that transferred the alkoxide vapor into the reactor. Gas diffusion into nanopores is analogous to the cubic power of the pore dimension (Knudsen regime)26 and consequently the permeability reduction due to deposition can be directly related to the pore size reduction. The evaluation of important parameters of the photocatalytic and ultrafiltration processes such as pollutant adsorption, retention and photodegradation, water recovery, permeability evolution, and energy consumption was for the first time implemented in a continuous flow process operating in the tangential membrane mode (crossflow), using a prototype photocatalytic purification device that permitted the illumination of each membrane surface either simultaneously or separately. In this way, it was possible to discriminate between the retention, adsorption, and photocatalytic efficiency of the composite titania/UF membranes.

2. EXPERIMENTAL SECTION 2.1. Membrane Development. Tubular, ceramic UF membranes, under the brand name of Inopor, were purchased from Fraunhofer Institute for Ceramics and Systems (IKTS). The membranes, (length 15 cm, OD 1 cm, ID 0.7 cm, glazed 13939



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Figure 2. (a) Flow path of the fluid inside the reactor. (b) Detailed cross-sectional scheme of the reactor cell showing its components. (c) External view of the reactor. (d) Internal view. (e) Illumination system with the reactor in the middle.

membranes with pore size of 5 (Mem1) and 10 nm (Mem2), respectively. 2.2. Membrane Characterization. The SEM analysis was performed with a Jeol JSM 7401F Field Emission Scanning Electron Microscope equipped with Gentle Beam mode and the new r-filter was employed to characterize the surface morphology of the developed membranes. Gentle Beam technology can reduce charging and improve resolution, signal-to-noise, and beam brightness, especially at low beam voltages (down to 100 V). In order to verify the deposition of TiO2 nanoparticles on the membranes’ surface and investigate their structural characteristics, micro-Raman measurements were performed on a RenishawinVia Reflex microscope with an Ar+ ion laser (λ = 514.5 nm) as an excitation source. 2.3. Photocatalytic Performance Evaluation: Batch Experiments. Batch photocatalytic experiments were applied in order to evaluate the methyl orange degradation efficiency of the membrane that was developed with the pyrolitic CVD approach (Mem1m). The obtained results were further compared with those reported in the recent literature. The experiments were conducted by immersing the TiO2 membrane into a borosilicate glass reactor containing 25 mL of 20 μM methyl orange solution in water. The irradiation during the

batch photocatalytic experiments was achieved with four UV lamps (9 W UV lamps (Phillips-UVA (PUVA) PL-S/PL-L), which were placed at a distance of 5 cm around the glass reactor. The lamps emitted near-UV radiation with a peak at 365 nm. In such a configuration it was not possible to irradiate both membrane surfaces due to the high thickness of the alumina support (2 mm). Thus, the irradiation intensity as measured solely on the external membrane surface was 2.1 mW/cm2. To further demonstrate the higher photocatalytic efficiency that can be achieved by having the photocatalyst deposited on both membrane surfaces, a borosilicate glass tube, of similar to the tubular membrane dimensions, was CVD treated for the same period and under the same pyrolitic CVD conditions applied for developing the TiO2 membrane. In this way, due to the transparency of the glass substrate and of the externally deposited TiO2 layer, it was possible to achieve the irradiation of both photocatalytic surfaces. 2.4. Photocalatylic Ultrafiltration Experiments. A prototype photocatalytic membrane reactor was designed and constructed for the performance evaluation of the developed membranes in a photocatalytic ultrafiltration setup. The reactor is already patented as a “photocatalytic purification device”27 (Figure 2). It provides us with the capability to investigate important characteristics which are related not only to the 13940



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membrane technology (permeability, fouling, water recovery, pollutant retention, energy for pumping) but also to the photocatalytic technology (photocatalyst deactivation/regeneration, degradation efficiency, sorption capacity, energy consumption by the irradiation sources) and gives the possibility to evaluate the performance of the hybrid titania/ UF membranes. Experiments can be performed under continuous flow conditions, in the flow-through or tangentialflow membrane mode, with UV illumination applied either concurrently on both membrane surfaces or separately on each of them. Four (4) near-UV radiation sources (9 W, 315−380 nm) with a peak at 365 nm were applied for irradiating the external surface of the membrane with an intensity of 2.1 mW/ cm2. The inner membrane surface was irradiated by an array of 10 miniature light emitting diodes (LEDs) emitting near-UV radiation (360−420 nm) with a peak at 383−392 nm and intensity of 0.5 mW/cm2. In Figure 2a, and b, we provide a detailed schedule of the fluid flow path in the tangential mode that was applied in this work for evaluating both the hybrid titania/UF photocatalytic membranes and the corresponding unmodified ones. It can be seen that the polluted water stream is fed in the reactor and after sweeping the external surface of the tubular membrane, exits from the retentate port of the reactor cell (retentate), and is directed toward a back pressure regulator device (BPR). The BPR maintains a constant pressure in the annular space of the membrane while allowing a fraction of the fluid stream to exit from the retentate side of the reactor. Under the action of this constant pressure, another fraction of the water stream is forced to flow through the pore structure of the membrane toward its opposite side (bore). The permeating water slips down the internal surface of the membrane and exits through the permeate port of the reactor cell. Collection of both permeate and retentate effluents was followed by quantitative analysis with UV at 466 nm in order to define the concentration of methyl-orange pollutant (C). The initial concentration (C0) of methyl orange pollutant in the feed was 20 μM. In the more detailed schedule of Figure 4b, it can be seen that the reactor cell accommodates also a second Plexiglas tube between the external one and the membrane. This tube constricts the fluid volume coming in contact with the external membrane surface leading to lower ratio of fluid/ photocatalyst amount. Moreover, this extra tube generates an additional flow channel where stabilized photocatalysts can be accommodated and act as photocatalytic pretreatment elements just before the contact of the fluid with the external membrane surface. The experimental conditions applied in this work are summarized in Table 1. The photocatalytic efficiency of the UF membranes (modified or not) in the degradation of methyl orange under continuous flow conditions is presented and comparison is made in terms of important characteristics of membrane technology such as water permeability and recovery. It must be noted that for a continuous flow process (as the one presented in this work) the photocatalytic performance is expressed over the volume of treated fluid and not over the irradiation time as is usual in batch processes.

Figure 3. Evolution of N2 permeability during the pyrolitic CVD, nanoparticle deposition (blue), and physisorption and surface reactions (magenta). The vertical line denotes the point at which the deposition is directed from the external to the internal membrane surface (switching of the position of the three-way valve V1).

means of TTIP physisorption and surface reactions (Mem2m). The vertical line in Figure 3 denotes the point where the threeway valve (V1) changes position to direct the TTIP enriched N2 stream from the external to the internal membrane surface. At the same time, the three way valves V2 and V3, which are connected with the measurement ports of the differential pressure transducer, change position to make possible the monitoring of permeability evolution during the CVD treatment of the internal membrane’s surface. The mass (mg) of deposited TiO2, depicted in the horizontal axis (Figure 3), is calculated from the duration of the CVD treatment, the N2 flow rate, and the vapor pressure of TTIP at room temperature, with the hypothesis that all the consumed TTIP vapor is transferred on the membrane surface either in the form or TiO2 nanoparticles or in the form of layers consisting of TiO2 nanocrystallites. Up to the vertical line (Figure 3), it corresponds to the net amount of TiO2 deposited on the external surface (28 cm2) and from the vertical line forward, to the net amount of TiO2 deposited on the internal surface (20 cm2) of the membrane. It can be seen (Figure 3) that the deposition of TiO2 nanoparticles on the external surface of the membrane causes the reduction of N2 permeability down to the 46% of the initial value. The Knudsen diffusion mechanism holds for pore sizes of 2−10 nm and for the pressure and temperature conditions involved in the CVD process. In this respect the permeability factor is analogous to the cubic power of the pore dimension,26 e.g. Pe (cm2/s) ∝ d3, where d is the pore diameter. Thus, we can conclude that the pore size of the membrane developed with the first approach (pyrolitic at 450 °C-Mem1m) was reduced from 5 down to 3.9 nm. With similar calculations, we concluded that the second approach (physisorption/surface reaction at 350 °C-Mem2m) had led to a pore size reduction from 10 down to 5.1 nm. Thus, it can be stated that the elucidated pore size for the membrane developed with the pyrolitic approach (3.9 nm) represents the interparticle space of the formed TiO2 layer. The average size of the deposited nanoparticles (15 nm) as defined by SEM analysis (see Figure 4) is high enough for their introduction into the pores (5 nm) of the beneath located γalumina layer. On the other hand in the case of the physisorption/surface reaction approach, TiO2 layers of stacked nanocrystallites are developed sequentially on the pore surface of the 10 nm γ-alumina pores provoking a size reduction down to 5.1 nm.

3. RESULTS AND DISCUSSION 3.1. Membrane Morphology. In Figure 3 we present the evolution of N2 permeability during the deposition of TiO2 nanoparticles on the external and internal surface of the tubular (5 nm) membrane sequentially (Mem1m), as well as the N2 permeability evolution for the membrane (10 nm) modified by 13941



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Figure 4. (a−d) SEM analysis of the very small (diffusion) TiO2 nanoparticles deposited on the external surface of membrane Mem1m. These pictures correspond to the morphologies obtained from different locations of the membrane surface with a distance of 2 cm in between them. (e) SEM analysis of TiO2 nanoparticles deposited under positive thermophoretic driving force on the external surface of a γ-alumina membrane. (f) EDS analysis of the surface of membrane Mem1m.

deposited TiO2 nanoparticles, the SEM micrograph (Figure 4e) presents a photocatalytic membrane which was also developed with the pyrolitic CVD method. In this case, however the temperature of the gas phase was 4 °C higher than this of the supporting surface and consequently larger nanoparticles of different morphology are also deposited. Figure 4f presents the EDS analysis obtained in different locations of the membrane surface, where the domination of the Ti signal over the Al one confirms the extended deposition of the photocatalyst over the pristine membrane. Figure 5 shows the micro-Raman spectra obtained on the external surface of the photocatalytic membranes Mem1m and Mem2m at a region corresponding to the center of the CVD reactor. The Raman active modes of the anatase TiO2 phase, which is the most photocatalytically active phase of titania were clearly identified for both membranes. Specifically, the anatase

At this point we should emphasize the potentiality of the pyrolitic CVD method to effectively control the size and dispersion of the deposited TiO2 nanoparticles. At first, appropriate CVD conditions have been set in order to keep the temperature on the membrane surface 3−4 °C higher than this of the gas phase sweeping the reactor. Under these conditions, only the smallest among the TiO2 nanoparticles which are formed in the gas phase after pyrolitic TTIP decomposition could overcome the counter acting thermophoretic force and deposit on the membrane surface (diffusion nanoparticles). Indeed, as it can be seen in the SEM analysis of the external surface of the membrane (Figure 4a−d), the average TiO2 nanoparticle size was of the order of 15 nm. Moreover the deposition was uniform along the length of the membrane tube (Figure 4a−d). To further evidence the effect of temperature difference on the morphology and size of the 13942



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particles with size of 15−16 nm for Mem1m, while smaller TiO2 nanoparticles of 7−8 nm can be inferred for the titania layers on the Mem2m surface, in good agreement with the N2 permeability and SEM analysis. 3.2. Photocatalytic Efficiency of the Membranes: Batch Experiment. The photocatalytic efficiency of the membrane developed with the pyrolitic CVD approach (Mem1m) is presented in Figure 6a in comparison to the efficiency of the glass tube with deposited TiO2 layers. Included also is the γ-alumina membrane which did not exhibit any photocatalytic action. What is important to note is that the glass tube showed higher methyl orange degradation efficiency compared to the photocatalytic membrane (Mem1m) due to the UV accessibility on both its TiO2 deposited sides. Furthermore, the histograms of Figure 6b, pertain to the methyl orange photodegradation efficiency of the new photocatalytic UF membrane (and glass tube in comparison with this of other photocatalytic membranes developed via sol− gel dip coating10 and spin coating12 techniques and via the approach of recasting a polymer that incoporates TiO2 nanoparticles (Nafion/TiO2).19 The efficiency of self-standing TiO2 nanotube membranes is also included.21 As it can be seen, the developed in this work CVD membrane (Mem1m) is more efficient compared to the self-standing TiO2 nanotube and sol− gel spin coating membranes and slightly less efficient compared to the Nafion/TiO2 and sol−gel dip coating membranes. It is a fact that the photocatalytic efficiency of a deposited material depends on a variety of factors such as the method of its deposition, the porosity, the characteristics of the testing solution (pH, temperature, pollutant concentration), the chemical affinity of the pollutant with the material, and, most important, the irradiation intensity achieved on the photocatalytic surface and the amount of the photocatalyst per volume of the pollutant solution. In this context it is very difficult to rank the efficiency of several materials especially when dealing with different pollutants. For this reason we

Figure 5. Micro-Raman spectra acquired on the external surfaces of Mem1m and Mem2m at 514.5 nm. Letters (A) depict the anatase TiO2 Raman modes.

Raman bands were observed at 144 (Eg), 197 (Eg), 396 (B1g), 516 (superposition of A1g and B1g), and 638 (Eg) cm−1 on the Mem1m surface.28 However, appreciable shift and broadening were observed for the Mem2m membrane, most prominent for the intense, low frequency Eg mode. That mode shifted from 144 cm−1 for Mem1m to 148 cm−1 for Mem2m, while its fullwidth at half-maximum (fwhm) increased from 10 to 18 cm−1, respectively. This variation is characteristic for the presence of size effects arising from the optical phonon confinement in TiO2 nanomaterials that result in both broadening and blue shift of the anatase Raman modes.28 Using the calculated position and widths of the low frequency anatase Eg mode and the corresponding values observed for Mem1m and Mem2m membranes,29 we estimate the formation of anatase nano-

Figure 6. (a) Methyl orange degradation efficiency of the photocatalytic membrane Mem1m and of the glass tube with TiO2 on both surfaces prepared via similar to Mem1m CVD conditions. Included is the control experiment with the γ-alumina membrane. (b) Comparison of the methyl orange (MO) photocatalytic degradation efficiency of membrane Mem1m and the glass tube with that reported in the recent literature for TiO2 membranes developed with different techniques. 13943



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Figure 7. (a) Comparison between the efficiency of the hybrid photocatalytic membranes and that of the pristine UF membrane in the removal of methyl orange from the permeate effluent. (b) Condensation of the retentate effluent due to size exclusion of methyl orange. (c) Irradiation on both membrane surfaces causing elimination of the retentate condensation. In all figures C0 is the initial concentration of methyl orange in the feed stream, and C is the concentration in the permeate or retentate effluent. The abscissa corresponds to the volume of water (mL) collected from the permeate or retentate side of the membrane.

surface and inside the pores of microchannelled Al2O3−ZrO2 supports.10 However, we should note that the amount of the photocatalyst deposited with the CVD method is fully controllable and depends on the CVD treatment period. Therefore, a larger amounts of the photocatalyst can be easily deposited if required in order to increase the photocatalytic efficiency. 3.3. Photocatalytic Ultrafiltration Performance of the Membranes. The graphs included in Figure 7a and b demonstrate the efficiency of the membrane developed with the TiO2 nanoparticle deposition approach (at 450 °C; Mem1m) in comparison with that of the pristine γ-alumina (5 nm) membrane (Mem1). Results were obtained both in the absence of irradiation and by irradiating solely the internal surface of the tubular membranes. The vertical axis represents the concentration of methyl orange in the permeate and retentate effluents (C) normalized over its initial concentration in the feed stream (C0), whereas the horizontal axis represents the volume (mL) of the methyl orange solution which was collected from the retentate or permeate effluent of the membrane module. It can be seen that, in the absence of irradiation, the developed membrane (Mem1m) is much more

selected methyl orange as the model pollutant for our photocatalytic tests. Methyl orange is among the most commonly used probe molecules for photocatalytic testing. On the other hand it was very difficult to find the exact irradiation intensity applied in the studies reported in the recent literature. In most of the cases the only available information concerned the specifications of the UV sources and their distance from the reactor. However the irradiation intensity applied in our experiments, 2.1 mW/cm2, is moderate to low. For this reason, we can state that compared to other materials our membranes have been tested under the less favorable irradiation conditions. In this regard, the main reason for the lower efficiency of our membranes as compared with the sol− gel dip coating derived and the Nafion/TiO2 membranes was the lower amount of deposited photocatalyst. As showed in Figure 3, the pyrolitic CVD derived membrane (Mem1m) carried 34 mg of deposited photocatalyst on its external surface of 28 cm2 (e.g., 1.2 mg/cm2) and another 20 mg of deposited photocatalyst on its internal surface of 20 cm2 (e.g., 1 mg/cm2) when the more efficient Nafion/TiO2 membrane was the one with a 15% w/w TiO2 content19 and the sol−gel dip coating derived membranes accommodated TiO2 fibers on the external 13944



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Figure 8. (a) Comparison between the efficiency of the photocatalytic membrane developed with the physisorption/surface reaction method and the pristine UF membrane in the removal of methyl orange from the permeate effluent. (b) Dilution of the retentate effluent due to the higher amount and higher surface area of TiO2. (c) Comparison between the efficiency of the membrane developed with the TiO2 nanoparticles deposition and the membrane developed with the physisorption/surface reaction method.

Concerning the efficiency in the removal of methyl orange from the permeate effluent (82%), this was not as high as in the case of the membrane (Mem1m) developed with the TiO2 nanoparticle deposition approach (55%). The reason behind this is the higher flux of water through the membrane pores (see Table 1). In this respect the contact time of the water falling film (Figure 2a) with the photocatalytic layer deposited on the internal side of the tubular membrane was considerably reduced. However, when dealing with photocatalytic membrane filtration and not with simple membrane filtration, the total process efficiency has to be taken into account. Applying a mass balance calculation between the feed, retentate, and permeate streams of the reactor, it was possible to calculate the absolute methyl orange amount which was eliminated with each of the two photocatalytic membranes. The respective plots are presented in Figure 8c. It is evident that for a total feed volume of 400 mL, the Mem2m membrane removed an amount of methyl orange four times higher than compared to membrane (Mem1m). When comparing between the efficiencies of the two membranes presented in Figures 7a and 8a, the above result seems abnormal. The main reason is that due to the geometrical characteristics of the reactor (see Figure 2a) the ratio fluid volume/photocatalyst mass on the external surface of the membrane is much higher than the respective one on the internal surface. Membrane Mem2m, due to its higher permeability, treated about a 4.5 times higher volume (see also Table 1) of water with its internal surface relative to membrane Mem1m, and for this reason, it exhibited higher methyl orange removal efficiency. 3.4. Process Efficiency. Permeability evolution and energy consumption compared to a standard NF process (1 nm, pore size) are presented in Figure 9. It can be seen that the hybrid

efficient in the retention of the pollutant due to its smaller pore size (3.1 vs 5 nm). It reduces the concentration of methyl orange in the permeate effluent down to the 90% of the feed concentration and causes a slight condensation of the retentate effluent. Upon irradiation on the internal surface, the concentration of methyl orange in the permeate drops below the 40% of the concentration in the feed, while during the photocatalytic test and more specifically, after collecting a volume of 50 mL from the permeate, there is a small decrease of the efficiency that can be attributed to the consumption of the adsorption centers on the TiO2 that was deposited on the external surface of the membrane. In Figure 7c, it can be seen that upon irradiation on both sides the condensation of the retentate effluent is almost eliminated, an issue which is of high importance for membrane technology especially when dealing with highly toxic compounds. Moreover the adsorption phenomena occurring on the external membrane surface cease to have an effect on the concentration of methyl orange in the permeate effluent. This is due to the direct decomposition of methyl orange in the presence of irradiation. Figure 8a and b presents the corresponding results for the membrane developed with the physisorption/surface reaction approach (at 350 °C; Mem2m). The most interesting characteristic of this membrane is that upon irradiation on both of its sides it provokes dilution rather than condensation of the retentate effluent and this is something caused by its higher permeability compared to Mem1m. In addition, the physisorption/surface reactions approach resulted to codeposition inside the pore structure, thus generating a membrane with a much higher photocatalytic surface. 13945



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ing. Moreover by appropriate design of the membrane module, it is possible to irradiate both membrane surfaces. In such a process pollutant condensation in the retentate is avoided and, simultaneously, cleaner effluents are obtained from the permeate side. Thus, photocatalytic ultrafiltration processes involving double side active TiO2 membranes may exhibit similar performance to this of the highly efficient standard nanofiltration, operating however at lower energy consumption without the generation toxic byproducts.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +302106503644. Fax: +302106511766. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported the European Commission (Clean Water-Grant Agreement no. 227017). Clean Water is a Collaborative Project cofunded by the Research DG of the European Commission within the joint RTD activities of the Environment and NMP Thematic Priorities.

Figure 9. (a) Energy consumption, water recovery, and pollutant rejection efficiency of the developed photocatalytic UF membranes in comparison to a standard NF process. (b) Permeability evolution of the photocatalytic membrane in the dark and under UV irradiation.

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REFERENCES

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photocatalytic membranes are much more efficient in the elimination of methyl orange and perform with lower energy consumption and higher water recovery. Energy consumption is presented in units of kilowatt hours per 100 m3. For the calculation of the energy loss due to the light sources, a total power of 36 W was applied. A very important feature is the permeability increase during the photocatalytic experiments. This proves the antifouling properties of the photocatalytic membranes and reveals phenomena of photoinduced hydrophilicity.30

4. CONCLUSIONS In this work, significant advantages of the photocatalytic ultrafiltration technology involving CVD derived TiO 2 membranes have been evidenced. Upon irradiation, organic pollutants adsorption on the feed surface is reduced and the photocatalytic membranes can perform for large periods without any degradation of their flux capacity. Another important advantage arises from the control of the retentate effluent condensation. The standard nanofiltration and ultrafiltration processes usually conclude to waste streams which are highly concentrated in toxic pollutants and need further physicochemical treatments to make possible their disposal in the environment. The photocatalytic degradation efficiency limits the concentration of the organic compounds in the retentate effluent to the same or below the level of the concentration in the feed stream. In this way it is possible to retrofit the retentate back to the feed and proceed with a continuous flow filtration process that ends up to the complete elimination of the pollutant. An important issue that should be emphasized is that of the exploitation of both membrane surfaces for the photocatalytic degradation of the pollutant. It has been shown that with the application of CVD, the deposition of very efficient anatase phase TiO2 layers was attainable in a way that can be easily transferred to up-scaled photocatalytic membrane manufactur13946



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(13) Liu, L.; Liu, Z.; Bai, H.; Sun, D. D. Concurrent filtration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane. Water Res. 2012, 46, 1101. (14) Bai, H.; Liu, Z.; Sun, D. D. A hierarchically structured and multifunctional membrane for water treatment. Appl. Catal.-B Environ. 2012, 111−112, 571. (15) Hu, A.; Zhang, X.; Oakes, K. D.; Peng, P.; Zhou, Y. N.; Servos, M. R. Hydrothermal growth of free standing TiO2 nanowire membranes for photocatalytic degradation of pharmaceuticals. J. Hazard. Mater. 2011, 189, 278. (16) Chaudhary, A.; Choudhury, T. H.; Naik, S.; Raghavan, S. Adherent nanoporous anatase TiO2 membranes on stainless steel substrates. J. Am. Ceram. Soc. 2012, 95, 64. (17) Lombardi, M.; Palmero, P.; Sangermano, M.; Varesano, A. Electrospun polyamide-6 membranes containing titanium dioxide as photocatalyst. Polym. Int. 2011, 60, 234. (18) Pant, H. R.; Pant, B.; Pokharel, P.; Kim, H. J.; Tijing, L. D.; Park, C. H.; Lee, D. S.; Kim, H. Y.; Kim, C. S. Photocatalytic TiO2-RGO/ nylon-6 spider-wave-like nano-nets via electrospinning and hydrothermal treatment. J. Membr. Sci. 2013, 429, 225. (19) Ding, X.; Zhou, S.; Jiang, L.; Yang, H. Preparation, photocatalytic activity and mechanism of nano-Titania/Nafion hybrid membrane. J. Sol-Gel Sci. Tech. 2011, 58, 345. (20) Lin, Y.-F.; Tung, K.-L.; Tzeng, Y.-S.; Chen, J.-H.; Chang, K.-S. Rapid atmospheric plasma spray coating preparation and photocatalytic activity of macroporous titania nanocrystalline membranes. J. Membr. Sci. 2012, 389, 83. (21) Fang, D.; Luo, Z.; Huang, K.; Lagoudas, D. C. Effect of heat treatment on morphology, crystalline structure and photocatalysis properties of TiO2 nanotubes on Ti substrate and freestanding membrane. Appl. Surf. Sci. 2011, 257, 6451. (22) Choi, W.-Y.; Park, N.-S.; Wiesner, M. R.; Kim, J.-O. Preparation and characterization of fouling-resistant photocatalytic metal membrane embedded with self-organized TiO2 nanotube. Water Sci. Technol. 2010, 62, 963. (23) Choi, W.-Y.; Chung, J.; Cho, C.-H.; Kim, J.-O. Fabrication and photocatalytic activity of a novel nanostructured TiO2 metal membrane. Desalination 2011, 279, 359. (24) Zhu, L.-W.; Zhou, L.-K.; Li, H.-X.; Wang, H.-F.; Lang, J.-P. Onepot growth of free-standing CNTs/TiO2 nanofiber membrane for enhanced photocatalysis. Mater. Lett. 2013, 95, 13. (25) Free-standing open-ended TiO2 nanotube membranes and their promising through-hole applications. Liao, J.; Lin, S.; Pan, N.; Li, D.; Li, S.; Li, J. Chem. Eng. J. 2012, 211−212, 343. (26) Pilatos, G.; Vermisoglou, E. C.; Romanos, G. E.; Karanikolos, G. N.; Boukos, N.; Likodimos, V.; Kanellopoulos, N. K. Closer Look Inside Nanotubes: Pore Structure Evaluation of Anodized Alumina Templated Carbon Nanotube Membranes Through Adsorption and Permeability Studies. Adv. Funct. Mater. 2010, 20, 2500. (27) Falaras, P.; Romanos, G.; Aloupogiannis, P. Photocatalytic purification device, patent application EP 2409954 A1; 2010. (28) Kelly, S.; Pollak, F. H.; Tomkiewicz, M. Raman spectroscopy as a morphological probe for TiO2 aerogels. J. Phys. Chem. B 1997, 101, 2730. (29) Balaji, S.; Djaoued, Y.; Robichaud, J. Phonon confinement studies in nanocrystalline anatase-TiO2 thin films by micro Raman spectroscopy. J. Raman Spectrosc. 2006, 37, 1416. (30) Kontos, A. I.; Kontos, A. G.; Tsoukleris, D. S.; Vlachos, G. D.; Falras, P. Superhydrophilicity and photocatalytic property of nanocrystalline Titania sol-gel films. Thin Solid Films. 2007, 515, 7370.

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Photocatalytic Membranes for Efficient Water

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