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Ke, Xuebin, Ribbens, Stefan, Fan, Yiqun, Liu, Hongwei, Cool, Pegie, Yang,Dongjang, & Zhu, Huai (2011) Integrating efficient filtration and visible-light photocatalysis by loading Ag-doped zeolite Y particles on aluminiananofiber membrane. Journal of Membrane Science, 375(1-2), pp. 69-74.

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Integrating efficient filtration and visible-light photocatalysis by loading Ag-doped zeolite Y particles on aluminia nanofiber membrane Xuebin Ke a, Stefan Ribbens b, Yiqun Fan c, Hongwei Liu a, Pegie Cool b, Dongjiang Yang a, Huaiyong Zhu a* a Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, QLD 4001,Australia, E-mail: hy.zhu@qut.edu.au b Department of Chemistry, Laboratory of Adsorption and Catalysis, University of Antwerpen, Universiteitsplein 1, B-2160 Wilrijk, Belgium c State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China * Corresponding author. Tel: +61 7 31381581; fax: +61 7 31381804. Email address: hy.zhu@qut.edu.au (H.Y. Zhu).

Abstract

Filtration membrane technology has already been employed to remove various organic

effluents produced from the textile, paper, plastic, leather, food and mineral processing

industries. To improve membrane efficiency and alleviate membrane fouling, an integrated

approach is adopted that combines membrane filtration and photocatalysis technology. In this

study, alumina nanofiber (AF) membranes with pore size of about 10 nm (determined by the

liquid-liquid displacement method) have been synthesized through an in situ hydrothermal

reaction, which permitted a large flux and achieved high selectivity. Silver nanoparticles (Ag

NPs) are subsequently doped on the nanofibers of the membranes. Silver nanoparticles can

strongly absorb visible light due to the surface plasmon resonance (SPR) effect, and thus

induce photocatalytic degradation of organic dyes, including anionic, cationic and neutral

dyes, under visible light irradiation. In this integrated system, the dyes are retained on the

membrane surface, their concentration in the vicinity of the Ag NPs are high and thus can be

efficiently decomposed. Meanwhile, the usual flux deterioration caused by the accumulation

of the filtered dyes in the passage pores can be avoided. For example, when an aqueous

solution containing methylene blue is processed using an integrated membrane, a large flux of

200 L m-2 h-1 and a stable permeating selectivity of 85% were achieved. The combined

photocatalysis and filtration function leads to superior performance of the integrated

membranes, which have a potential to be used for the removal of organic pollutants in

drinking water.

Keywords: alumina nanofiber; membrane; photocatalysis; visible-light; silver nanoparticles

1. Introduction

Poor water quality continues to threaten human health and diminish clean drinking water. At

least 1.2 billion people lack access to safe drinking water, 2.6 billion have little or no

sanitation and millions of people die annually from diseases transmitted through unsafe water

[1]. Because of technology insufficiency, this presents a major challenge to efficient water

purification. For example, treatments with intensive uses of chemicals and energy are

environmentally unsustainable. In comparison, filtration membrane technique offers a

solution for the development of novel separation processes, which has low cost, simple

operation, energy saving and high efficiency [2,3]. However, commercial filtration

membranes are susceptible to be fouled due to the deposition and accumulation of the filtered

contaminants. Particularly, organic compounds are one of the major pollutants during

membrane filtration process for water purification [4,5]. An accumulation of contaminants in

the feed may result in an abnormal permeation or vital damage during the filtration. The

instantaneous permeates may have a very high concentration of contaminants, thus produce

highly dangerous water quality beyond the normal operation. Therefore, it is crucial to search

for new-generation membrane techniques to efficiently overcome existing fouling and achieve

high selectivity without deteriorating permeation flux. This fouling could be partly alleviated

with a combination of membrane filtration and photocatalytic oxidation [6,7]. Indeed, this

combination with photocatalysis allows to permanently remove some contaminants and

maintain the initial performance of the membranes in mild conditions. Most current research

on photocatalytic processes focuses on activation by UV-light irradiation. However, the use of

UV-light has serious drawbacks and may cause potential damage to material, equipment, and

even to operators. Moreover, UV-light accounts for only ~ 4% of the total solar energy

reached the earth [8]. To enhance the efficiency of utilizing sunlight for the photocatalytic

process reactions, we have to find photocatalysts that are able to work under the visible light

(λ > 400 nm).

Silver nanoparticles (Ag NPs) doped on porous materials such as zeolites can be used as

more robust, selective, and stable photocatalysts for UV-irradiation degradation of organics

[9-11]. Small Ag NPs stabilized with the aid of zeolites are active sites of electron-trapping

and play a significant role in the photocatalytic and photochemical reactions [11]. Recently,

the surface plasmon resonance of silver nanoparticles was employed to develop plasmonic

photocatalysts because of their strong absorption of visible light [12-14]. Hence, it is possible

to utilize sunlight for this target, as visible light (wavelength 400-800 nm) constitutes around

43 % of the solar energy received by the earth. In order to activate the reactions under visible

light, deposition of silver nanoparticles will be applied.

Several studies have already indicated that the mesh-like structure formed from threads or

fibers represents the most efficient structure for pressure-driven membrane-filtration

processes [15-20]. Such a structure is able to achieve high selectivity while retaining

extremely high filtration rates. If the photocatalytic property of active nanoparticles is

integrated with filtration property of nanofiber membranes, one can combine the two

functions in one device and such combination may lead to a significant improvement of

filtration property.

In the present study, alumina nanofiber (AF) membranes were loaded with silver-doped

zeolite Y (labeled Ag@Y). Zeolite Y also contributes to concentrate the contaminants due to

its excellent adsorption ability. Whereas the top layer of Ag@Y functioned as visible-light

photocatalysts, the nanofibers served as a separation layer. Such a novel membrane structure

was used for tangential-flow filtration and degradation of dyes under visible light. The

widespread disposal of industrial wastewater containing organic dyes onto soil and water

bodies has led to serious contamination in many countries worldwide. About 15% of the total

global production of dyes is lost during the dyeing process and is released into the

environment as textile effluent [21]. The integrated photocatalytic filtration membrane with a

top layer of Ag@Y exhibited superior performance to that of the membranes with a top layer

of P25 (TiO2) and successfully avoid membrane fouling under visible light irradiation.

2. Experimental

2.1. Preparation of alumina nanofiber filtration membranes

Porous α-alumina supports with a diameter of 30 mm and a thickness of 2 mm were

provided by the Membrane Science and Technology Research Center of Nanjing University

of Technology. The mean pore size of the support is 0.8 μm. Aluminum hydroxide gels that

were precipitated from aluminum salt solution were deposited on the surface of supports, and

then hydrothermally treated for 3 days at 170 oC. Boehmite nanofibers were formed in the

pores of the support by consuming the aluminum hydroxide gels. During the subsequent

calcination at 500 oC, boehmite nanofibers were transferred into alumina nanofibers. The pore

size of alumina nanofiber membranes could easily be tuned to below 10 nm by controlling the

hydrothermal synthesis conditions.

2.2. Preparation of alumina nanofiber membranes loaded with silver-doped zeolite Y

Firstly, zeolite Y powders were immersed into AgNO3 solution (1%, weight ratio) in the

dark and were continuously stirred for 4 hours. The compounds were washed by

demineralized water and then calcined at 450oC. Ag@Y with various concentrations of silver

(weight percent ratio, relative to zeolite Y), denoted as Ag@Y-2, Ag@Y-8, and Ag@Y-32,

was attained, respectively. Secondly, Ag@Y was loaded on the AF membranes via

dip-coating using 0.2 wt% of solution. Subsequently, the membranes were calcined at 450 oC.

Similarly, TiO2 powders (P25) were loaded on the AF membranes by dip-coating using 0.2

wt% of solution.

2.3. Membrane characterization

The pore size distributions of the membranes were determined by the liquid-liquid

displacement method [22,23]. Scanning electron microscope (SEM) for measuring the surface

morphology of membranes was performed on a FEI Quanta200 scanning electron microscope.

The UV-Vis spectroscopy (Cary 5000, Varian Inc., Palo Alto, CA) was used to analyze the

light absorption of powder samples. X-ray photoelectron spectroscopy (XPS) analysis was

performed on a Kratos Analytical Axis Ultra X-ray photoelectron spectrometer.

2.4. Photocatalytic test

Experiments were conducted in a homemade set-up (Scheme 1) of tangential-flow

filtration, in which the permeate could be recycled and the photodegradation be clearly

monitored. The applied cross-flow velocity was 3 ml/min. The operaction conditions were

identical in every experiment in order to make a comparison between different membranes

possible. A 100 W Mercury lamp (B-100 AP High Intensity UV lamp, UVP, UK) was used as

UV light source, which produced mono-dispersed light with a wavelength of 365 nm. A 500

W visible light source was used and a light filter was added to remove the light below 420 nm.

The distance between the lamp and the membrane surface was 10 cm. The initial

concentration of the feeding dye solution was 40 mg/L and the applied solution volume was

60 ml. During the first 40 minutes, the feeding solution was pumped across the membrane in

the dark while a vacuum was applied (0.06 MPa). Every 20 minutes, permeate and retention

were collected and the dye concentrations in them were analyzed using UV-Vis spectroscopy

(Cary 50, Varian Inc., Palo Alto, CA) at a fixed wavelength. After 40 minutes, an

adsorption-desorption equilibrium was established between the dye molecules in the solution

and the surface of the membrane. Then light irradiation was applied for 60 minutes.

The photocatalytic efficiency (ηp) is estimated by calculating the concentration (Ct) of the

total solution relative to initial concentration (C0):

(1)

The permeating selectivity (ηs) is defined as:

(2)

where Cp is concentration of the permeation.

3. Results and Discussions

The mesh-like nanofiber layer assures the high filtration performance of these membranes due

to the prominent structural features [15-20, 24-27]. The permeating flux maintained stable at

200 L m-2 h-1 bar-1 under visible light irradiation (Fig.1) and the permeating selectivity at

about 85% (85% of the dye molecules in the flux were retained by the membrane). A serious

problem of conventional membranes is that the flux passing the membranes decreases

substantially due to the thickening of the filtering cake layer on the membranes. In the present

study, the flux of the AF membranes reduced more than 50% without the irradiation because

of the accumulation of dyes in the passages of the membranes (Curve d in Fig.1). The AF

membranes loaded with P25 catalysts (powder of 30-50 nm particles) can also achieve high

selectivity of about 85% because nanofiber separation layer determines the permeating

selectivity; but the flux decreased dramatically in one hour as P25 catalysts cannot decompose

the dye molecules under visible light irradiation (Curve c in Fig.1). These results demonstrate

that loading the visible light photocatalyst Ag@Y on the filtration membranes with a

mesh-like separation layer of nanofibers can effectively prevent fouling and thus avoid the

frequent cleaning via back-washing and some chemical treatments: the non-photocatalytic

membranes suffer from this which then impedes the application of the membrane separation

technology.

The photocatalytic performance of the AF membranes loaded with photocatalysts was

investigated in detail. Fig.2 shows the photocatalytic efficiency of silver doped AF

membranes under visible light irradiation. After equilibrating for 40 minutes in methylene

blue solution, the retention remained balanced. Under visible light irradiation, the AF

membranes loaded with zeolite Y have no evident photocatalytic activity after the adsorption

equilibrium achieved. Loading Ag@Y with various concentrations did not cause obvious

changes in the filtration performance of the AF membrane. However, they could degrade

about 40% of methylene blue in one hour under visible light. During the photocatalytic

degradation process, the top layer of Ag@Y is in sufficient contact with the cross-flow dye

solution. Therefore, the photocatalytic activity resulted from the loaded Ag@Y catalysts.

The SEM images in Fig. 3a show a top view of the AF membrane and that of loaded

Ag@Y on the membrane. The alumina nanofibers on the membrane have a similar

morphology to that of the alumina nanofibers prepared by a wet-chemistry method [28]. It can

be seen that the loaded Ag@Y particles form a functional layer on the top of AF layer

(Fig.3b). The inserted TEM micrograph indicates that silver in the loaded Ag@Y particles

exists as well-dispersed silver nanoparticles with a uniform size of 4 nm on the surface of

zeolite Y (the insert in Fig. 3b). Fig. 4 displays the pore size distributions of ceramic

membranes. f(r) is the pore radius distribution function, which can quantify the fraction of the

pores in a certain size range. The pore diameter of the porous α-alumina support is up to 800

nm (curve a in radius in Fig. 4), and the synthetic AF membrane mainly has pores about 10

nm (curve b in Fig. 4). With various loadings of Ag@Y or P25 in this research, the pore sizes

were not reduced further.

XPS core-level spectrum of the Ag-3d region was acquired for the Ag@Y sample (Fig.

5). The Ag-3d spectrum, characterized by the spin-orbit splitting (Ag 3d5/2 and Ag 3d3/2

components), has two peaks at 368.2 and 374.1 eV, respectively. It indicates that the silver in

the Ag@Y sample exists in metal state [29]. However, no silver peaks can be identified

through XRD patterns, due to low content (less than 0.03 % in weigh ratio) of the loaded

silver on the support as well as the small particle size.

These silver nanoparticles can absorb visible light and activate chemical reactions

because of their near-field effect and the plasmonic effect [14]. Fig. 6 shows the UV-Vis

spectra of the AF membrane loaded with zeolite Y and Ag@Y. It can be seen that for the AF

membrane no absorption is presented in the visible range and only a shoulder peak exists

between 250 nm and 350 nm. Pure zeolite Y indicates no visible light absorption, either. In

contrast, the AF membrane loaded with Ag@Y exhibits a broad absorption of the visible light

in a range from 350 nm to 600 nm. This absorption is attributed to Ag@Y and responsible for

the observed photocatalytic activity under visible light irradiation (Fig. 2). Given that the

loaded Ag accounted for 2% of zeolite Y, the doped silver NPs exhibit a high visible light

activity.

The well-known commercial TiO2 powder photocatalyst of mixed anatase (75-80%) and

rutile (20-25%) phases, P25, was also loaded, by the same procedure as that of Ag@Y, onto

an AF membrane. The photocatalytic performance of the P25 loaded membrane was tested

and was compared with that of the AF membranes loaded with Ag@Y (Fig. 7). The

photocatalytic activity was investigated under both UV-light (monodispersed light, 365 nm),

and visible light (420-800 nm, polydispersed light with a filtering glass that filtered out the

light of wavelength <420 nm). Under UV irradiation, both membranes loaded with P25 and

Ag@Y exhibited high photocatalytic activity. The superior photocatalytic properties of silver

particles or P25 under UV-light are widely known [30-35]. UV light with a wavelength of 365

nm could excite electrons in the valence band of TiO2 powder photocatalyst to conduction

band and leave electronic holes in the valence band. The photogenerated charges induce the

dye degradation. The silver nanoparticles can also significantly absorb UV irradiation that

causes the inter-band excitation of electrons from 4d to 5sp [14]. The holes left in 4d can

capture electrons from dye molecules adsorbed on the particles and oxidize the dye

molecules.

The visible light irradiation cannot excite the valence band electrons to the conduction

band in the TiO2 photocatalyst so that P25 doped membrane exhibits a negligible activity. In

contrast, Ag@Y loaded membranes exhibited a very high photocatalytic activity under visible

light. It can be attributed to the surface plasmon resonance effect of silver nanoparticles, in

which the electromagnetic field of the incident light couples with the oscillation of the

conduction electrons in the silver particles. This not only significantly enhances the local

electromagnetic fields near the rough surfaces of the silver nanoparticles, but the conduction

electrons (5sp electron) of silver gained energy through the absorption and migrated to higher

energy levels [14]. A small number of the energetic electrons at high level may be captured by

oxygen molecules, while the silver nanoparticles with positive charges (the holes left in the

5sp band) can grab electrons from dye molecules. The dye molecules are then oxidized [14].

The surface plasmon resonance effect can also be employed to conduct photochemical

synthesis under visible light irradiation, not limited to silver nanoparticles [36].

To demonstrate the applicability of the integrated photocatalytic and filtration

membrane for degradation of various synthetic dyes, the membranes were also used to

degrade several kinds of dyes, including anionic, cationic and nonionic dyes. The results are

summarized in Table 1. For these typical dyes, Ag@Y photocatalysts exhibit high

degradation efficiency under visible light irradiation. Particularly for cationic dye, basic blue

3, the degradation is very fast and attains complete conversion in one hour. The corresponding

control experiments without light or without catalysts show no degradation of dyes taking

place. The results of powder photocatalysts under the same reaction conditions are

exemplified as Fig. 8, which exhibits the similar degradation efficiency. In this case, the

absorption intensity at 560 nm demonstrates the concentration of sulfhodamine B. After three

hours’ irradiation, the conversion of dyes attained 75%, similar with that of loaded Ag@Y

photocatalysts on the resulted membranes (Table 1). It indicated that the integrated nanofiber

membranes exhibited high degradation efficiency similar with powder photocatalysts.

Because the integrated photocatalysis and AF membrane allows dyes retained and

concentrated on the surface of AF membrane, then further degraded under visible light

irradiation, the novel material structure and blended process have great potential in the

removal of dye effluents. in combination with the filtration.

4. Conclusions

New alumina nanofiber membranes were easily synthesized via a wet-chemistry method

and visible light photocatalyst of Ag@Y was loaded on the nanofiber membranes. The

combination of the photocatalytic function with high separation property effectively prevents

membrane fouling. The integrated membrane can maintain a large flux of 200 L m-2 h-1 bar-1

and a stable permeating selectivity of 85% during the photocatalytic degradation of dyes.

These membranes avoid the frequent clean via back-washing or chemical treatments, which

are time and energy consuming. More importantly, these new composite membranes possess

high visible-light catalytic activity, which allows full utilization of solar light. This study

presents advances towards active photocatalysis membranes and new applications can be

further explored by various combinations of nanofibers and other functional materials.

Acknowledgements

This work is supported by Australia Research Council (ARC) and X. Ke is indebted to QUT

for a Vice-Chancellor’s Research Fellowship. S. Ribbens thanks the FWO Flanders for

financial support (K.2.177.08.N.01)

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Table 1. Degradation of dyes using Ag@Y doped AF membranes under visible light irradiation

Dyes Dye type Degradation efficiency /%

Irradiation time CAS number

Methylene Blue Anionic 42 1 hour 28983-56-4 Sulfhodamine B Anionic 74 3 hours 3520-42-1 Acid blue 29 Anionic 90 2 hours 5850-35-1 Disperse red 1 [a] Nonionic 82 2 hours 2872-52-8 Basic blue 3[a] Cationic 100 1 hour 33203-82-6

[a] The initial concentration of the used dyes is 20 mg/L.

Figure captions Scheme 1. Experimental setup for photocatalytic degradation of dyes using Ag doped AF membranes. 1. Magnetic stirrer; 2. Feed tank; 3. Pump with flow controller; 4. Light source; 5. Power transformer; 6. Membrane; 7. Membrane photoreactor vessel; 8. Pressure gauge; 9. Vacuum pump; 10. Collector Figure 1. Permeating flux of AF membranes loaded with different photocatalysts for removing methylene blue dye. a) Ag@Y under visible light; b) Ag@Y in the dark; c) P25 under visible light; d) No photocatalyst under visible light.

Figure 2. Photocatalytic degradation of methylene blue dye under visible light irradiation using AF membranes with loadings. a) zeolite Y; b) Ag@Y-32; c) Ag@Y-8; d) Ag@Y-2. Figure 3. SEM images of the AF membrane surface: (a) before loading Ag@Y on the surface and (b) after the Ag@Y loading. The insert in (b) is a TEM micrograph of Ag@Y in which the dark spots are silver NPs.

Figure 4. Pore size distributions of ceramic membranes: (a) porous α-alumina support; (b) AF membrane. With various catalysts loading on the top of AF membranes, the pore size distributions are similar with that in (b). Figure 5. X-ray photoelectronic spectra (XPS) of Ag@Y.

Figure 6. UV-Vis spectra of AF membranes with loads of Ag@Y (a) and zeolite Y (b), and without loading (c); as well as a spectrum of zeolite Y powder (d). Figure 7. Catalytic degradation of methylene blue dyes using Ag@Y doped AF membranes and P25 doped membranes.

Figure 8. Degradation of sulfhodamine B dye using Ag@Y catalysts under visible light irradiation. 60 ml of 40 mg/L sulfhodamine B; 0.1 g Ag/Y catalyst; pH 3.0.

Scheme 1. Experimental setup for photocatalytic degradation of dyes using Ag doped AF membranes. 1. Magnetic stirrer; 2. Feed tank; 3. Pump with flow controller; 4. Light source; 5. Power transformer; 6. Membrane; 7. Membrane photoreactor vessel; 8. Pressure gauge; 9. Vacuum pump; 10. Collector

0 10 20 30 40 50 60

120

160

200

240

Per

mea

te F

lux/

Lm

-2h-1

Bar

-1

Time/min

a

b

c

d

Figure 1. Permeating flux of AF membranes loaded with different photocatalysts for removing methylene blue dye. a) Ag@Y under visible light; b) Ag@Y in the dark; c) P25

under visible light; d) No photocatalyst under visible light.

0 10 20 30 40 50 60

0.6

0.8

1.0

C/C

0

Time/min

a

b

c

d

Figure 2. Photocatalytic degradation of methylene blue dye under visible light irradiation using AF membranes with loadings. a) zeolite Y on AF; b) Ag@Y-32; c) Ag@Y-8; d)

Ag@Y-2.

Figure 3. SEM images of the AF membrane surface: (a) before loading Ag@Y on the surface and (b) after Ag@Y loading. The insert in (b) is a TEM micrograph of Ag@Y in which the

dark spots are silver NPs.

1 10 100 1000

Pore radius / nm

f (r

)a

b

Figure 4. Pore size distributions of ceramic membranes: (a) porous α-alumina support; (b) AF membrane. With various catalysts loading on the top of AF membranes, the pore size distributions are similar with that in (b).

380 375 370 365 360

400

600

800

1000

1200

CP

S

Binding energy/eV

Ag3d5/2

Ag3d3/2

374.1

368.2

Figure 5. X-ray photoelectronic spectra (XPS) of Ag@Y.

200 300 400 500 600 700 8000.0

0.4

0.8

1.2

Ads

orpt

ion

Wavelength/nm

a

bc

d

Figure 6. UV-Vis spectra of AF membranes with loads of Ag@Y (a) and zeolite Y (b), and without loading (c); as well as a spectrum of zeolite Y powder (d).

0

10

20

30

40

Deg

rada

tion

rat

io/%

UV Visible Light

Ag@Y

P25

Figure 7. Catalytic degradation of methylene blue dyes using Ag@Y doped AF membranes and P25 doped membranes.

300 400 500 600 700 8000

1

2

3

4

Abs

orba

nce

Wavelength /nm

0 h1 h2 h3 h

560 nm

Figure 8. Degradation of sulfhodamine B dye using Ag@Y catalysts under visible light

irradiation. 60 ml of 40 mg/L sulfhodamine B; 0.1 g Ag@Y catalyst; pH 3.0.