Accepted Manuscript Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities Ronn Goei, Teik-Thye Lim PII:

S0043-1354(14)00310-8

DOI:

10.1016/j.watres.2014.04.025

Reference:

WR 10627

To appear in:

Water Research

Received Date: 6 September 2013 Revised Date:

6 April 2014

Accepted Date: 13 April 2014

Please cite this article as: Goei, R., Lim, T.-T., Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities, Water Research (2014), doi: 10.1016/j.watres.2014.04.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

ACCEPTED MANUSCRIPT Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities Ronn Goeia,b, Teik-Thye Lima,b,* a

School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore. b Nanyang Environment & Water Research Institute (NEWRI), Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141. Singapore. * Corresponding author: Tel: +65-6790 6933; Fax: +65-6791 0676; E-mail: [email protected].

Abstract

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Ag-decorated TiO2 (Ag-TiO2) photocatalytic membranes have been fabricated by using

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Pluronic P-123 as a pore-forming and structure-directing agent. Six different hierarchical

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architectures were obtained by multilayer coating of different Ag-TiO2 sols. The porous

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structure of the resulting layers could be fine-tuned by altering the amounts of P-123 and

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AgNO3 added during the preparation of TiO2 sols. Physico-chemical and morphological

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properties of different Ag-TiO2 layers were thoroughly investigated. Ag nanoparticles were

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successfully incorporated into the TiO2 matrix. The Ag-TiO2 membranes possessed multi-

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functionality of membrane retention, Ag-enhanced TiO2 photocatalytic activity and anti-

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bacterial action. They were evaluated through experiments using a batch reactor and a

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photocatalytic membrane reactor (PMR). The best performing membrane was able to remove

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up to 1,007 mg m−2 h−1 of Rhodamine B in the PMR. Two phenomena (photocatalytic

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degradation and adsorptive-membrane retention) that were responsible for the RhB removal

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were evaluated. In the batch reactor operated in dark, the membranes were able to remove

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greater than 5-logs of E. coli. The membrane with the highest percentage of Ag incorporated

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was able to remove close to 7-logs of E. coli when operated in the PMR.

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Keywords: Silver nanoparticles; titanium dioxide; anti-bacterial activity; photocatalytic

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membrane reactor; ceramic membrane

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1.

Introduction As a photocatalyst, titanium dioxide (TiO2) possesses many sterling characteristics such

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as excellent chemical and thermal stability, low toxicity, low cost, and high quantum yield

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(Hoffmann et al. 1995, Kwon et al. 2008). Anatase, which is the most photoactive polymorph

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of TiO2, allows the generation of electron-hole pairs under UV irradiation that are able to

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initiate photocatalytic degradation of many noxious environmental pollutants. Furthermore,

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Anatase-TiO2 was also found to possess an effective anti-bacterial property (Carp et al. 2004).

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Immobilizing TiO2 onto a porous support such as membrane would create a synergistic

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coupling between TiO2 photocatalysis and membrane separation processes. The efficacy of

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the supported-TiO2 photocatalysis could be enhanced through the forced permeation of

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pollutants through the TiO2-coated membrane.

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In order to improve the photocatalytic activity of TiO2, incorporation of various metallic

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and non-metallic elements was attempted by previous researchers (Chen and Mao 2007). Ag

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is a favorable metal for incorporation into TiO2 due to its remarkable catalytic, electric, and

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specific optical characteristics (Morones et al. 2005, Roucoux et al. 2002). Also, Ag

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nanoparticle has been reported to exhibit the highest bactericidal activity and

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biocompatibility compared to other bactericidal nanoparticles (Akhavan and Ghaderi 2010,

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Gumy et al. 2006, Li et al. 2008). The synergistic coupling of Ag and TiO2 is attributed to the

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Schottky barrier that is formed due to the difference in their work functions. Ag acts as an

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electron traps that can inhibit the recombination of photo-generated electron and hole pairs

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and thus enhance the overall photocatalytic activity of the composite (Kubacka et al. 2008,

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Seery et al. 2007, Subba et al. 2003, Yu et al. 2005). Furthermore, the incorporation of Ag

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would add an additional functionality of anti-bacterial/anti-biofouling property to the TiO2

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photocatalyst. However, there exists an optimum level of Ag incorporation beyond which

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would result in the photo-hole trapping effect (Yin et al. 2004).

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ACCEPTED MANUSCRIPT Recently, there are several research efforts to incorporate Ag nanoparticles into

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membrane filtration systems. For instance, Krylova et al. (2009) have deposited Ag

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nanoparticles onto different ceramic supports (such as silica, titania, and zirconia) by sol-gel

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method. Liu et al. (2013a) fabricated Ag-loaded nanocomposite nanofiltration (NF) and

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forward osmosis (FO) polymeric membranes via layer-by-layer assembly method. Ma et al.

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(2010) fabricated Ag-TiO2/hydroxyapatite microfiltration (MF) membrane to treat humic acid

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solutions. Liu et al. (2013b) designed a direct observation experiment to study the effect of

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Ag nanoparticles on the bacteria deposition and its associated detachment kinetics from a

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polysulfone membrane. Lei et al. (2011) synthesized Ag/AgCl-coated polyacrylonitrile

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nanofiber membranes that possessed a high photocatalytic activity. Lv and coworkers (Lv et

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al. 2009a, Lv et al. 2009b) have fabricated Ag-decorated porous ceramic composite for the

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inactivation of E. coli. Yin et al. (2013) have attached Ag nanoparticles to the surface of

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polyamide membrane that showed an anti-biofouling property.

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In the present work, we aimed to fabricate different types of Ag-decorated TiO2

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photocatalytic membranes (hereafter referred as Ag-TiO2 membranes) that possess a

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hierarchically porous architecture. Different amounts of Pluronic P-123 triblock copolymer

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were used in the preparation of Ag-TiO2 sols to tailor-made the microstructures of the

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resulting gels. Membrane with a hierarchically porous architecture is preferred because it

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allows maximum flux facilitated by its porosity gradient. In this architecture, the porosity

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gradient was obtained through the coating of multiple photocatalytic layers of different pore

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structures starting from the layer of largest pore size on the support-photocatalyst interface,

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followed by the coating of subsequent layers of decreasing pore sizes. The architecture allows

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TiO2 particles with the most active photocatalytic activity to assemble at the top of the

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membrane. Various characterization techniques were employed to elucidate the physico-

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chemical and morphological characteristics of the resulting membranes. Different

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performance parameters were used to evaluate the photocatalytic activity and anti-bacterial

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action of the membranes. Finally, we aimed to gain an insight into different phenomena that

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govern the performance of the Ag-TiO2 membranes including their various anti-bacterial

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actions. The Ag-TiO2 membranes would also be evaluated for its reusability potential.

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2.

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2.1 Materials and Reagents

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Experimental

The following chemicals were used in the preparation of Ag-TiO2 sol without further

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purification unless it is stated otherwise: titanium tetraisopropoxide (TTIP, Merck), silver

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nitrate ≥98% (AgNO3, Merck), 2-propanol (IPA, Merck), nitric acid 67% (HNO3, Merck),

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and poly(ethylene glycol)−block−poly(propylene glycol)−block−poly(ethylene glycol)

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(Pluronic P-123, Aldrich). Poly(vinyl alcohol) (PVA) 88%−hydrolyzed MW 88,000 g mol−1

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(Acros Organics) was used as pores filler during membrane coating. MilliQ water (18.2 MΩ

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cm at 25°C) was used in the synthesis and other processes. Rhodamine B (RhB, MW =

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479.02 g mol−1, λmax = 553 nm, Merck) was used as the model pollutant for the

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photocatalytic degradation experiment. Membrane retention capability was evaluated using

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polyethylene glycol (PEG, Alfa Aesar) of MW 20,000 g mol−1.

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2.2 Preparation of hierarchically porous Ag-TiO2 membrane

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The macroporous alumina support was fabricated from aluminum oxide powder (α-

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Al2O3, Aldrich) (particle size of 0.05 µm and relative density of 4 g cm−3) according to the

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method used in our previous work (Goei et al. 2013). The diameter and thickness of the

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ceramic alumina support were 23 mm and 2 mm respectively. Ag-TiO2 layers were

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synthesized by the sol-gel method. Firstly, a predetermined amount of P-123 was dissolved in

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IPA to obtain solution A. Solution B was prepared by dissolving AgNO3 in IPA and was

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ACCEPTED MANUSCRIPT stirred continuously at room temperature for 60 min to ensure complete dissolution of AgNO3.

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Solution B was then added drop-wise to the solution A under vigorous stirring. After which,

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HNO3 was added into the mixture. Finally, TTIP and H2O were added drop-wise to the

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mixture for the hydrolysis and condensation reaction under a vigorous stirring. The resulting

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sols were stirred continuously overnight at room temperature to improve its homogeneity.

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The final sols had a molar ratio of IPA:HNO3:TTIP:H2O equals to 60:4:1:4. Variable

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amounts of P-123 and AgNO3 were used in the preparation of different sols to obtain Ag-

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TiO2 sols that contained 0.5, 1.0, 2.0, 5.0, or 10 wt% of P-123 and 0.5, 1.0, 2.0, 3.0 mol% of

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Ag (with respect to TiO2) respectively. The resulted TiO2 layers hereafter are referred as PX-

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AgY where X and Y are the amounts of P-123 and Ag added respectively.

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The hierarchical architecture of Ag-TiO2 membrane was obtained by coating different

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sols on the alumina support. Prior to the coating, the alumina support was coated with 5 wt%

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PVA solution and air dried to prevent possible infiltration of TiO2 sol into the alumina

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support. One side of the alumina support was covered to allow only single-side coating of

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Ag-TiO2 sols. The alumina support was then dipped into the designated Ag-TiO2 sol for 60 s

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using a home-fabricated dip-coater, and withdrawn at a rate of 2 mm s−1. The coated

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membrane was first oven dried (103°C, 15 min), then calcined in the muffle furnace

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(Nabertherm LHT04/16) at 500°C for 1 h (3°C min−1 heating rate). Six different membrane

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specimens of different hierarchical architectures were fabricated and they are summarized in

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Table 1. The first photocatalytic layer was formed through 3 coatings of the similar sol

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resulting into the formation of 3-sub-layers. The intermediate layer which consists of 2-sub-

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layers was formed through 2 coating of another sol. Finally, a thin top-layer was coated once

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with Ag-containing sol. R1, R3, and R5 were decorated with Ag only in the top-most layer

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while R2, R4, and R6 were decorated with Ag in all sub-layers. All the membranes were UV

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irradiated (365 nm, 60 W m−2) for 1 h (after the heat treatment of each coated sub-layers) in

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ACCEPTED MANUSCRIPT order to reduce Ag+ species in the membrane to metallic Ag0. A previously prepared Ag-free

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TiO2 membrane, Plu D, was included in the present study for comparison (Goei et al. 2013).

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In order to characterize the microstructure of the Ag-TiO2, a thick Ag-TiO2 layer was also

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coated onto a Petri dish. The resulting layer was then scrapped off, heat treated, pulverized,

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and stored for its subsequent characterizations.

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2.3 Instruments and Analytical Methods

The mineralogy and crystal structures of the Ag-TiO2 layers were analyzed using an X-

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ray diffractometer (Bruker D8 Advance) with a monochromated high intensity Cu-Kα (λ =

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1.5418 Å) radiation. N2 adsorption/desorption isotherms of the Ag-TiO2 layers were obtained

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using a Quantachrome Autosorb-1 system at liquid N2 temperature of 77 K. The Brunauer-

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Emmett-Teller surface area (SBET) and Barrett-Joyner-Halenda (BJH) pore size distribution

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were derived from the result. X-ray photoelectron spectroscopy (XPS) studies were

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conducted using a Kratos Axis Ultra spectrophotometer with a monochromatic Kα excitation

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source (hv = 1486.71 eV). The binding energies were corrected against an adventitious

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carbon C 1s core level at 284.8 eV.

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Scanning electron microscope (SEM) micrographs and energy-dispersive X-ray (EDX)

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spectra were obtained using a JEOL JSM-7600 FESEM. High resolution transmission

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electron microscope (HRTEM) micrographs were obtained using a JEOL JEM-2010.

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2.4 Experiments with the photocatalytic membrane reactor

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The performance of the Ag-TiO2 membrane was evaluated using a photocatalytic

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membrane reactor (PMR) setup (Fig. 1) which was operated under a dead-end filtration mode

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with constant flux. The membrane cell module (active volume of 15 mL) was fabricated

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using ss316 stainless steel with a Suprasil® quartz top opening that allowed penetration of

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ACCEPTED MANUSCRIPT UV irradiation to the surface of the tightly secured Ag-TiO2 membrane. A high-pressure

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metering pump (5979 Optos Pump 2HM, Eldex Laboratories) was connected on the feed side.

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The permeate flux was monitored using an electronic balance. There were 3 sampling points

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to allow (1) sampling at the inlet of the membrane cell module, (2) sampling at the permeate

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side, and (3) sampling at the cross-flow outlet for the membrane cleaning step during the anti-

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bacterial experiment (Section 2.5.5). The trans-membrane pressure (TMP) was recorded

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using a pressure transducer. Air bubbling was provided at the feed tank to promote mixing

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and to provide oxygen that would assist the photocatalytic reaction. Before commencement

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of each experiment, the PMR setup was rinsed thoroughly with MilliQ water to remove any

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possible impurities.

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The pure water permeability was determined by feeding MilliQ water into the PMR

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setup at various constant fluxes and the corresponding readings of the resulting TMP were

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recorded. PEG solution (20,000 g mol−1, 500 mg L−1) was fed into the PMR setup to

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determine the membrane rejection characteristic. The concentration of the PEG solution was

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determined with a Shimadzu ASI-V TOC analyzer.

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Ag-TiO2 membranes were used to treat 100 mL of 25 µM RhB solution. The PMR setup

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was fed with the RhB solution and was left in the dark for 60 min to achieve RhB adsorption-

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desorption equilibrium. After which, the pump and UV source were turned on to commence

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the experiment. In this experiment, the permeate was re-circulated back to the feed tank.

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Control experiment was performed without UV to quantify the contribution of membrane

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retention to the total RhB removal by the membrane.

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2.5 Anti-bacterial performance of Ag-TiO2 photocatalytic membrane

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2.5.1

Quantification of total-Ag leaching

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ACCEPTED MANUSCRIPT The as-prepared membranes were immersed in MilliQ water (30 mL) to determine the

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total-Ag leaching (i.e. Ag+ ions and Ag nanoparticles) from the membrane. Prior to the total-

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Ag leaching test, all glasswares were soaked in 10% (v/v) HNO3 for 24 h. Triplicate aliquots

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(0.5 mL) were taken at regular time intervals (up to 168 h immersion in MilliQ water) and

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digested in HNO3 (150°C, 1 h) using a Hach DRB 200 digestion unit. Total-Ag leaching from

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the membrane operated in the PMR was also quantified. Each type of Ag-TiO2 membranes

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was secured in the membrane cell module. MilliQ water was fed into the PMR with a

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constant flow rate of 4 mL min−1. Triplicate aliquots (0.5 mL) of the permeate were taken at

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regular time interval (up to 480 min) and digested in HNO3. The total-Ag concentrations in

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the digested aliquots were quantified using an inductively coupled plasma- mass

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spectrophotometer (ICP-MS, Perkin Elmer Elan DRC-e).

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2.5.2

Preparation of microbial strain

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Escherichia coli (E. coli, ATCC® 25922TM), a gram-negative bacteria was selected as the

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model bacteria for the anti-bacterial study. E. coli was inoculated in the tryptic soy broth at

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37°C until their mid-exponential growth phase. E. coli cells were then collected by

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centrifugation followed by multiple washing cycles. After decanting the supernatant, the

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bacteria cells were re-suspended in a 0.9 wt% NaCl solution. The E. coli stock solution of

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approximately 1 × 108 colony-forming units per mL (cfu mL−1) was then prepared for the

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anti-bacterial study.

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2.5.3

Diffusion inhibition zone

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Diffusion inhibition zone of various Ag-TiO2 membranes with different hierarchical

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architectures were analyzed according to the standard Kirby-Bauer approach (Bauer et al.

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1966). The diffusion inhibition zone test provides a fast qualitative indication of the

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ACCEPTED MANUSCRIPT effectiveness of the Ag-TiO2 membrane to inhibit the growth of bacteria cells. An aliquot

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(200 µL, 1 × 108 cfu mL−1) of bacterial inoculums was spread on the triptic soy agar plates.

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Different types of Ag-TiO2 membranes were placed on different agar plates with the

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photocatalytic layer facing down in contact with the agar surface. The diffusion inhibition

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zones were examined after incubation at 37°C for 24 h.

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2.5.4

Anti-bacterial performance in the batch reactor

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Prior to the anti-bacterial experiments, all glasswares were autoclaved (121°C, 15 min)

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and UV-irradiated (254 nm, 15 min) in order to avoid any bacterial contamination. Different

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Ag-TiO2 membranes were immersed in 30 mL of E. coli bacterial suspensions (1 × 108 cfu

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mL−1, pH 7.0 ± 0.1) which was kept in the dark. Control experiments were carried out

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without membrane or using Plu D membrane (no Ag). Aliquots of the E. coli suspensions

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were drawn after 15 min of immersion. Serial dilutions of the aliquots were performed using

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0.9 wt% NaCl solution. 100 µL of the aliquots of appropriate dilutions were streaked on the

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tryptic soy agar plates and incubated at 37°C for 24 h. After which, the colonies were

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enumerated to quantify the amount of active cells. All data were obtained from at least three

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replicates of experimental runs. Each membrane was used for 4 cycles of batch experiments.

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Anti-bacterial performance in the PMR

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Ag-TiO2 photocatalytic membranes were secured on the membrane cell module to

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evaluate its anti-bacterial performance in the PMR. The membrane cell was fed with 120 mL

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of E. coli bacterial suspension (1 × 107 cfu mL−1, pH 7.0 ± 0.1) with a constant flux of 4 mL

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min−1. Aliquots of the permeate were collected at regular time intervals of 5, 15, and 30 min

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respectively. The corresponding flux values were also recorded. The control experiments

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without UV were performed. After 30 min of treatment cycle, the feed was replaced with 120

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ACCEPTED MANUSCRIPT mL of 0.9wt% NaCl solution. The NaCl solution was fed at a constant flux of 8 mL min−1

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while opening the cross-flow valve in order to dislodge any deposited bacterial cells from the

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surface of the membrane. At the end of 15 min cleaning cycle, an aliquot from cross-flow

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outlet was collected. The whole procedure was repeated 4 times in order to discern the

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reusability potential of the membrane. All of the collected aliquots were diluted to the

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appropriate concentration and enumerated on the agar plates. All data were obtained from at

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least three replicates of experimental runs.

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Bacterial repair

In order to investigate the ability of bacteria to undergo repair, once the photocatalytic

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disinfection tests were completed, 5 of the treated bacterial suspension was collected and

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stored in the dark for 48 h. Aliquot of the suspensions were enumerated on the agar plates to

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determine whether the bacteria could undergo repair.

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3.

Results and discussion

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3.1. Physico-chemical characteristics of Ag-TiO2 layers The important physico-chemical characteristics of the as-prepared Ag-TiO2 layers are

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summarized in Table 2. We prepared 30 types Ag-TiO2 layers by varying the amounts of P-

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123 and AgNO3 added into the sols. The difference in the hydrophilicity of the polyethylene

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glycol and polypropylene glycol blocks of P-123 would cause a microphase separation. This

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phenomenon was responsible for formation of the mesoporous micellar structure of the

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resulting Ag-TiO2 layers. The mesoporous structure, which was formed by the attractive

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forces between TiO2 hydrate and P-123, would form during the calcinations step to result in

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the final Ag-TiO2 layers. Besides, EO (ethylene oxide)-type nonionic surfactants including P-

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123 was found to assist the reduction of Ag+ to Ag0 (Andersson et al. 2002).

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ACCEPTED MANUSCRIPT The wide-angle XRD patterns of the representative Ag-TiO2 layers (Fig. S1a) show the

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pulverized Ag-TiO2 with increasing Ag content (from 0.0 up to 3.0 mol% Ag) while the

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amount of P-123 added was kept constant (2.0 wt%). The XRD spectra show five distinctive

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anatase-TiO2 peaks at 25.3°, 37.9°, 48.0°, 54.6°, and 62.8° which correspond to (101), (004),

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(200), (105 and 211), and (204) reflections of crystalline anatase-TiO2 planes, respectively

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(JCPDS 21-1272). No amorphous or other TiO2 polymorphs content can be observed in the

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spectra indicating a successful formation of anatase crystals upon calcinations at 500°C.

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Anatase phase was the most photocatalytically active form among the TiO2 polyforms (Gaya

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and Abdullah 2008). Four distinctive peaks of face-centered cubic metallic Ag at 38.1°, 44.3°,

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64.5°, and 77.4° which correspond to (111), (200), (220), and (311) reflection can be also

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observed in the spectra of Ag-containing sample (JCPDS 04-783). This confirmed the

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successful incorporation of Ag in the TiO2 layers. TiO2 crystallites size are calculated from

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the peak broadening of (101) anatase crystal plane at 25.3° using Scherrer’s equation

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(Scherrer 1918) and are summarized in Table 2a. The sizes of TiO2 crystal decreased as the

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amount of P-123 loading increased. This could be attributed to spatial confinement

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phenomenon of P-123 scaffold (Guldin et al. 2011). The amount of Ag incorporated would

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also affect the resulting TiO2 crystallite size. Ag was found to suppress the growth of the

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anatase crystal due to the inclusion of Ag nanoparticles in the mesoporous TiO2 layers. This

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observation has been reported previously (Ma et al. 2011).

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Further evidence of the ability of P-123 to fine-tune the resulting microstructure of the

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Ag-TiO2 layers can be obtained from the N2 adsorption/desorption isotherms (Fig. S1b-c).

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The result of the porosimetry study is summarized in Table 2b-c. A well developed

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mesoporous material is evidenced from the type-IV adsorption isotherm shown. The sharp

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hysteresis loops indicate that pore sizes in the Ag-TiO2 layers were of narrow distribution.

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We observed that as the amount of P-123 loading increased, the BET surface area and

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ACCEPTED MANUSCRIPT porosity of the resulting Ag-TiO2 layers increased. The increase in the BET surface area and

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the overall porosity of the Ag-TiO2 layers was caused by the pore coalescence and the multi-

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micellar interactions during the calcinations step (Choi et al. 2006a). However, as the amount

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of incorporated Ag increased, the BET surface area and the overall porosity of the Ag-TiO2

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layers decreased. This could be attributed to the inclusion of Ag nanoparticles into the

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mesoporous TiO2 layers. The control of the surface area and the porosity profile of the

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resulting Ag-TiO2 layers is important in order to maximize the density of photocatalytically

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active sites.

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XPS analyses were carried out to understand the chemical states of elements in Ag-TiO2

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layers. Two samples of the pulverized Ag-TiO2 layers (P2.0-Ag0.5 and P2.0-Ag2.0) were

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chosen for the XPS analysis. The high resolution XPS spectra of C 1s, O 1s, Ti 2p, and Ag 3d

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core levels are presented in Fig. 2. There are 2 main peaks observed at O 1s core level,

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namely the dominant peak at 529.6 eV which was the characteristic of metallic oxide and a

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smaller peak at 531.8 eV which could be attributed to the chemisorbed oxygen on the surface

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of Ag-TiO2 layers. The adsorbed oxygen allows the H+ hydroxylation to form –Ti(OH)-O-Ti-

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(OH)- bond which would generate •OH radicals that are beneficial for TiO2 photocatalysis

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(Hoffmann et al. 1995). A separation of 5.7 eV of Ti 2p doublet indicates that Ti exists in the

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Ti4+ oxidation state. Distinct doublet is also observed for Ag 3d core level. A spin-orbit

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splitting energy of 6.0 eV for the 3d doublet of Ag indicates the formation of the metallic Ag0

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(Moulder et al. 1992). No peak associated with other forms of Ag oxide (i.e. AgO or Ag2O) is

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evident indicating the successful photo-reduction of all Ag species to Ag0.

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3.2. Characteristics of Ag-TiO2 membranes

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Fig. 3a-b shows FESEM micrographs of the membrane surface. Generally, relatively

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smooth Ag-TiO2 layers were successfully coated onto the surface of the alumina support. The

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ACCEPTED MANUSCRIPT coated layers showed a minimal presence of cracks and pinholes, which was important to

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ensure uniform permeability and membrane retention performance. The apparent presence of

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well-dispersed Ag nanoparticles (particles that appear brighter in the FESEM micrographs)

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could be observed from the high magnification FESEM micrographs of the membranes. It

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showed a successful dispersion of Ag nanoparticles (~20-30 nm in diameter) onto the TiO2

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matrix. TEM micrograph (Fig. 3c) of the scrapped Ag-TiO2 layer of R2 reveals a dispersion

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of spherical Ag nanoparticles within the cluster of smaller TiO2 nanoparticles. HRTEM

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micrograph of the same membrane (Fig. 3d) confirms the successful formation of the anatase-

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TiO2 crystals and deposition of Ag nanoparticles. Fig. 3e depicts an EDX elemental analysis

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spectrum of the surface of R2. Ag loading was 0.5 mol% at all sub-layers of R2. However,

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the elemental analysis showed a higher Ag concentration of 0.95 mol%. This suggests that

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Ag nanoparticles tended to agglomerate near the surface of the membrane and this is

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favorable for effective anti-bacterial activity of the Ag-TiO2 membranes.

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The hierarchical architectures of the Ag-TiO2 membranes and other important

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characteristics are summarized in Table 1. A few trends could be observed from the

320

characteristics of the membranes of different hierarchical architectures. Firstly, the

321

membranes that incorporate Ag possessed a thinner photocatalytic layer than the membrane

322

without Ag (e.g. 323 nm of R1 vs 598 of Plu D). In the same manner, membranes with Ag

323

incorporated in all of their sub-layers were thinner than the membranes with Ag incorporated

324

only at the top-most layer. This could be attributed to the difference in the viscosity of the

325

sols (i.e.TiO2 sol with higher P-123 and Ag loading tends to be less viscous, thus a thinner

326

layer would be coated).

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Secondly, membranes R2, R4, and R6 showed a higher PEG rejection than their

328

respective R1, R3, and R5 counterparts. The result underlined the influence of the

329

incorporation of Ag on the resulting surface area and overall porosity characteristics of the

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ACCEPTED MANUSCRIPT Ag-TiO2 layers. This would eventually alter the PEG rejection characteristic of the membrane.

331

The 20,000 g mol−1 PEG solution correspond to PEG molecular size of 12 nm (Choi et al.

332

2006b) which is in coherence with the porosity profile obtained from N2 porosimetry analysis.

333

Thirdly, membranes with a more porous photocatalytic layer would exhibit larger pure

334

water permeability. The numerical value of pure water fluxes of different membranes as a

335

function of TMPs (Fig. S2) are listed in Table 1. It can be observed that the pure water

336

permeability flux of the membranes increased linearly with the TMP regardless of the types

337

of their hierarchical architectures. Membranes R1 and R2 showed higher fluxes due to their

338

more porous structure. All the membranes with Ag in all of its sub-layers showed a

339

marginally smaller flux (< 10% difference) when compared with its counterpart which was

340

consistent with the observation from N2 porosimetry analysis. This suggests that by altering

341

the amount of Ag incorporated would only marginally affect the resulting pure water

342

permeability performance. Even the ‘tightest’ membrane (R6) showed a high pure water

343

permeability of 123 L m−2 h−1 Bar−1 which could be attributed to the photo-induced super-

344

hydrophilicity (PSH) of the membrane. The evidence of PSH property of the membrane is

345

shown in the supplementary data (Fig. S3).

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3.3. Photocatalytic degradation of RhB in the PMR reactor

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Fig. 4 shows the specific RhB removal (i.e. the cumulative RhB removal per unit area of

349

membrane over a period of time, mg m−2) of the Ag- TiO2 membrane. The photolysis of RhB

350

at 25 µM under continuous UV irradiation (without the presence of the membrane) was < 5 %.

351

All the membranes with Ag in all of its sub-layers showed a higher specific RhB removal as

352

compared to the membrane with Ag only on its top-most layer. R5 and R6, which contained

353

higher amount of Ag, showed a higher specific RhB removal as compared to their respective

14

ACCEPTED MANUSCRIPT 354

counterparts, R3 and R4. R1 and R2 showed a higher specific RhB removal than the control

355

specimen Plu D. The overall RhB removal could be attributed to two different phenomena: photocatalytic

357

degradation (PC) and adsorptive-membrane retention (Ads-MR). The contribution of each of

358

these phenomena is summarized in Table S1. It can be observed that the RhB removal

359

through the adsorptive-retention by the membrane was lower than the removal through

360

photocatalytic degradation. Membranes R2, R4, and R6 showed a higher removal through

361

adsorptive-retention as compared with its R1, R3, and R5 counterparts respectively.

362

Incorporation of Ag in all sub-layers of R2, R4, and R6 caused RhB to pass through the more

363

tortuous path of the membrane smaller porous architecture. When compared with Plu D, the

364

Ag-TiO2 membrane showed (with the exception of R3) a higher specific RhB removal which

365

was attributed to the Ag-enhanced photocatalytic activity. The incorporation of Ag could

366

enhance the overall photocatalytic activity of the membrane by acting as an electron sink at

367

the Schottky barrier formed at the Ag-TiO2 interface. We found that all the treated solutions

368

showed a final TOC concentration < 1 mg L−1 which was the detection limit of our equipment.

369

This finding indicated > 88% TOC removal in the RhB solution (from the theoretical TOC

370

concentration of 8.4 mg L−1 for 25 µM solution).

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Fouling control is an important characteristic of an efficient membrane process. The Ag-

372

TiO2 membrane exhibited only a small percentage (maximum of 17 %) of flux decline over

373

the duration of 480 min photocatalytic degradation of RhB (Table S1). The higher initial

374

decline of the flux could be attributed to the cumulative adsorptive-retention of RhB

375

molecules onto the membrane. Following which, the steady state flux would be achieved and

376

maintained.

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3.4. Total Ag leaching of Ag-TiO2 membranes

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ACCEPTED MANUSCRIPT The usage of Ag nanoparticles as an anti-bacterial agent might release a significant

380

amount of Ag into the treated water. Significant Ag+ ions release would also have an adverse

381

effect on the aqueous ecosystems and shorten the overall life-span of the Ag-TiO2 membrane.

382

The time-dependent Ag leaching profiles of the Ag-TiO2 membranes are studied (Tables S2).

383

Total Ag releases of R1 and R3 are small (2.0 and 1.5 µg L−1 respectively) which could be

384

attributed to their relatively small Ag content at the top-most layer of the membrane.

385

Generally, the membranes with Ag incorporated in all their sub-layers showed a higher total

386

Ag release. Membrane R4 released more Ag+ than R2 (despite the same amount of Ag

387

loading on their sub-layers) because of its tighter porous structure, thus more Ag

388

nanoparticles were concentrated near the surface of TiO2 clusters. The highest Ag

389

concentration leached out from the Ag-TiO2 membrane (i.e. R6 of 27 µg L−1 after 168 h of

390

immersion) was still 4 times lower than the highest tolerable Ag concentration in drinking

391

water of 100 µg L−1 as prescribed by USEPA regulation and WHO guideline (USEPA 2009,

392

WHO 2011). This showed that the Ag-TiO2 membrane possessed a good chemical stability.

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Total-Ag leaching experiments were also performed in the PMR. Most of the Ag (a

394

maximum of 8 µg L−1 for R6) was leached out from the membrane within the first 120 min of

395

membrane operation (Table S2 and Fig. S4). It was evident that the additional amount of Ag

396

leachate for the last 360 min of operation was marginal (about one-fifth of the amount

397

released in the first 120 min) indicating the chemical stability of the membrane (Fig. S4). The

398

release of Ag+ ions was governed by the diffusion rate of both Ag+ ions and water. At the

399

early stage of the test, the Ag release was mainly contributed from the dissolution of surface

400

Ag nanoparticles into water and the possibility of dislodgment of Ag nanoparticles from the

401

membrane. After which, the rate of release was slower. The slower release of the Ag

402

nanoparticles located inside the TiO2 matrix could be attributed to the slow diffusion rate

403

inside the pores (Ma et al. 2011).

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ACCEPTED MANUSCRIPT 404 405

3.5. Anti-bacterial performance of Ag-TiO2 membranes

406

3.5.1. Diffusion inhibition zone test The results of the diffusion inhibition zone test are presented in Fig. 5. The Ag-free Plu

408

D did not show any apparent inhibition effect against the growth of E. coli. R1 and R3

409

showed small inhibition zones due to their smaller Ag contents. R5 showed an observable

410

inhibition zone (diameter of 27 mm) which highlighted the importance of the amount of Ag

411

incorporated on the anti-bacterial property of the membrane. R2, R4 and R6, which contained

412

Ag in all of their sub-layers, showed the most prominent inhibition effect with diameters of

413

the inhibition zones measured to 29, 33, and 38 mm respectively (Fig. 5). R4 showed a larger

414

inhibition zone as compared to R2 which could be attributed to more Ag+ leaching from the

415

membrane. R6 showed the largest inhibition zone owing to its largest Ag content. It has been

416

reported that the Ag+ possessed more effective anti-bacterial property compared to the Ag

417

nanoparticle (Lv et al. 2009b).

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3.5.2. Performance in batch reactor

Fig. 6 shows log reduction values of E. coli when subjected to the Ag-TiO2 membranes

421

in a batch reactor placed in the dark. The Ag-TiO2 membranes were able to remove minimum

422

5-logs of E.coli from their suspensions within 15 min of contact time. The overall removal of

423

E. coli was caused by the anti-bacterial activity of Ag. This suggests that the membrane

424

exhibited an excellent anti-bacterial activity even when was operated in the dark. R1, R2, R3,

425

R4, R5, and R6 could reduce the E. coli viable cells count up to 5.3, 6.3, 5.2, 6.0, 5.8, and 7.0

426

log scales respectively. R2, R4, and R6 showed a higher E.coli disinfection efficiency due to

427

the incorporation of Ag in all of their sub-layers. This is in a good agreement with the results

428

of the time-dependent Ag leaching study. R1 and R2 showed a slightly higher E. coli removal

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ACCEPTED MANUSCRIPT efficiency than their respective R3 and R4 counterparts. This could be attributed to their more

430

porous structure that allowed more interaction between Ag species and bacteria cells. When

431

treated with Plu D membrane, a 1-log removal of E.coli suggests that anatase-TiO2 possessed,

432

to some extent, an anti-bacterial activity (Kubacka et al. 2009). Repeated usages (up to 4

433

cycles) of the Ag-TiO2 membranes showed a consistent E. coli disinfection efficiency. This

434

indicates a good reusability potential of the membranes.

435

3.5.3. Performance in PMR

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The anti-bacterial performances of the various membranes used in the PMR are

438

illustrated in Figs. 7-9a. Prior to any of the experiments, the PMR was flushed thoroughly

439

with 0.9 wt% NaCl solution. After which, the feed was replaced with E. coli suspension. A

440

control sampling was taken regularly at the sampling point 1 (Fig. 1) to ensure that the

441

concentration of E. coli suspension entering the membrane cell was the same as the

442

concentration at the feed tank. Upon entering the membrane cell, E. coli started to be

443

deposited onto the surface of the membrane. This was evidenced from a declining permeate

444

flux as the experiment progressed (Fig. 7a and b). The fouling was expected as the size of E.

445

coli cell (± 1 µm length) was much larger than the pore size of the Ag-TiO2 membrane

446

(between 4−12 nm depending on the amount of P-123 and Ag loading). The membrane

447

fouling could be caused by the adhesion and accumulation of dead bacteria cells on the

448

membrane surface (Yao et al. 2008). However, as the experiment progressed, the rate of

449

declining flux was progressively reduced. The experiments that were carried out in the

450

presence of UV irradiation exhibited a much smaller flux decline. After 30 min of operation,

451

the feed was replaced by an equi-volume 0.9 wt% NaCl solution and the cross-flow cleaning

452

valve was opened. This was a cross-flow cleaning step that helped to recover the permeate

453

flux by dislodging any deposited dead-bacteria cells. The percentages of flux recovery after

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ACCEPTED MANUSCRIPT each cleaning cycle are shown in Fig. S5. The membranes operated under UV irradiation

455

showed higher flux recoveries. R1 and R2 (possessed more porous structure) exhibited the

456

least flux decline. In general, the membranes with larger porosity exhibited lesser flux decline

457

than their counterparts. From the decreasing rate of flux decline after each operation-cleaning

458

cycle, we could hypothesize that a steady-state sustainable flux would be achieved after a

459

certain number of operation-cleaning cycles.

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The log reduction value of E. coli was evaluated during the PMR operation (aliquots

461

taken from the sampling point 2, Fig. S6) and after each cleaning cycle (aliquots taken from

462

the sampling point 3, Fig. 8). In the presence of UV irradiation (365 nm) alone, only 1.9 ± 0.2

463

log of bacteria cells was killed. Sample aliquots taken from the sampling point 2 during

464

operation should contain a negligible concentration of E. coli (< 0.002% of the initial feed

465

concentration) as most of the bacteria cells (>99.998%) would be retained or inactivated by

466

the membrane. The membranes operating under UV irradiation generally showed a higher E.

467

coli reduction values. Also, the membranes with higher amounts of Ag content showed a

468

higher E. coli reduction values. Sample aliquots (Fig. 8a and b) taken from sampling point 3

469

(after each cleaning cycle) showed a clearer trend. We can easily observe that the membranes

470

operating under UV irradiation showed a higher overall E. coli removal. This could be

471

attributed to three synergistic processes that were taking place, i.e. Ag anti-bacteria activity,

472

Ag-enhanced TiO2 photocatalytic disinfection, and, to the lesser extent, direct UV

473

disinfection. Reduction of viable E. coli counts for experiments that were carried out in the

474

dark could be ascribed to the inherent Ag anti-bacterial activity only.

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To elucidate the contribution of each anti-bacterial action of the Ag-TiO2 membranes,

476

selected results from the PMR experiment (taken at the sampling point 3) are tabulated in Fig.

477

9a. Alumina support disk showed no anti-bacterial activity. Plu D membrane showed some

478

degree of UV-assisted anti-bacterial activity caused by the anatase-TiO2. When operating Plu

19

ACCEPTED MANUSCRIPT D under UV irradiation, a 4-log increase in the overall E. coli removal was achieved. Similar

480

anti-bacterial activity was evidenced from different Ag-TiO2 membranes that were operated

481

under UV irradiation. The membrane with a higher amount of Ag incorporated showed a

482

higher E. coli reduction values (i.e. R2 > R1, R5 >R1, and R6>R5). Therefore, the increase of

483

E. coli removal could be attributed to the Ag anti-bacterial activity and the Ag-enhanced TiO2

484

photocatalytic disinfection.

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The mechanism of anti-bacterial action of the Ag-TiO2 membrane is still only partially

486

understood. The possible anti-bacterial actions of the Ag-TiO2 membranes in PMR are

487

illustrated in Fig. 9b. They are direct UV disinfection (under a powerful UV irradiation),

488

membrane rejection, interaction of bacteria cells with Ag+ ions, direct interaction with Ag

489

nanoparticles, and TiO2 photocatalytic disinfection. The inactivated bacteria could undergo

490

self-repair in the dark. In order to ensure complete inactivation of the bacteria in this study,

491

the ability of bacteria to undergo self-repairs was investigated. It was found that no bacterial

492

growth could be detected in the tested suspension after 48 h dark-incubation period. This

493

suggests that the Ag-TiO2 membranes could cause a permanent damage to the bacterial cells.

495

4.

Conclusion

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We have successfully fabricated Ag-TiO2 photocatalytic membranes using P-123 as the

497

pore forming and structure-directing agent. The different membranes possessed their unique

498

hierarchical architectures that were achieved by multilayer coating of Ag-TiO2 of different

499

porous structures. Fabrication of different hierarchical architectures allowed us to tailor-made

500

the membranes to exhibit optimum operating performance (i.e. high permeability, high

501

reactivity, and good membrane retention). Ag incorporation enhanced the photocatalytic

502

activity of TiO2 by hindering the recombination of photo-generated electrons and holes.

503

Furthermore, the incorporation of Ag imparted anti-bacterial functionality to the membranes.

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ACCEPTED MANUSCRIPT The Ag-TiO2 membrane possessed a multi-functionality of membrane retention, Ag-

505

enhanced TiO2 photocatalytic activity and multiple pathways of Ag anti-bacterial action. We

506

have also demonstrated the reusability potential of the Ag-TiO2 membranes both in the batch

507

and flow-through PMR which would reduce the overall operating cost of the treatment

508

system.

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509 510

Acknowledgement

Financial support of this project was provided by from National Research Foundation

512

(NRF), Singapore, under project EWI RFP 0802-11 which aims to develop novel

513

photocatalyst for water purification and wastewater treatment. R. Goei acknowledges

514

Nanyang Technological University (NTU) for the award of his PhD research scholarship. The

515

authors are grateful to Loo Siew Leng for her assistance in the experiments involving E coli.

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functionalized polypropylene hollow fiber membrane from surface-initiated atom transfer

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radical polymerization. Journal of Membrane Science 319(1-2), 149-157.

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631

Yin, J., Yang, Y., Hu, Z. and Deng, B. (2013) Attachment of silver nanoparticles (AgNPs)

636

onto thin-film composite (TFC) membranes through covalent bonding to reduce

637

membrane biofouling. Journal of Membrane Science 441, 73-82.

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635

638

Yin, S., Hasegawa, H., Maeda, D., Ishitsuka, M. and Sato, T. (2004) Synthesis of visible-

639

light-active nanosize rutile titania photocatalyst by low temperature dissolution-

640

reprecipitation process. Journal of Photochemistry and Photobiology A: Chemistry 163(1-

641

2), 1-8.

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Yu, J., Xiong, J., Cheng, B. and Liu, S. (2005) Fabrication and characterization of Ag-TiO2

643

multiphase nanocomposite thin films with enhanced photocatalytic activity. Applied

644

Catalysis B: Environmental 60(3-4), 211-221.

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ACCEPTED MANUSCRIPT List of Tables and Figures Table 1. Types of Ag-TiO2 photocatalytic membranes fabricated Table 2. Physico-chemical characteristics of synthesized Ag-TiO2 materials: (a) TiO2 crystallite size, (b) BET surface area, and (c) BET porosity Schematic diagram of photocatalytic membrane reactor operating under dead-end

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Fig. 1.

filtration mode. Fig. 2.

High resolution XPS spectra of (a) C 1s, (b) O 1s, (c) Ti 2p and (d) Ag 3d regions of the pulverized Ag-TiO2 layers.

FESEM micrographs of Ag-TiO2 photocatalytic membrane at (a) low and (b) high

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Fig. 3.

magnifications. (c) TEM and (d) HRTEM micrographs of R2 membrane. (The

membrane R2.

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inset depicts the corresponding SAED pattern). (e) EDX of the surface of

Specific RhB removal of Ag-TiO2 photocatalytic membrane (The numerical

Fig. 4

values depict kinetic value based on initial RhB removal rate within 1st hour in mg m−2 h−1). Fig. 5.

Diffusion inhibition zones of Ag-TiO2 photocatalytic membrane against E. coli

Fig. 6.

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after 24 h incubation.

E. coli log reduction value of Ag-TiO2 photocatalytic membrane showing its reusability over 4 cycles of operation in a batch reactor. Error bars represent one standard deviation of at least three replicates of experimental runs. Flux performance of Ag-TiO2 photocatalytic membrane in the dead-end filtration

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Fig. 7.

mode of PMR with inter-cycle cleaning operating (a) with UV and (b) without

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UV. Fig. 8.

E. coli log reduction values of Ag-TiO2 photocatalytic membrane operating in a

photocatalytic membrane reactor: at sampling point (3) after each cross-flow cleaning cycles when operating (a) with and (b) without UV. Error bars represent one standard deviation of at least three replicates of experimental runs.

Fig. 9.

(a) Log reduction values of E. coli at sampling point 3 after cross-flow cleaning cycle for various membranes. Error bars represent one standard deviation of at least three replicates of experimental runs. (b) Possible mechanisms of antibacterial action inside the photocatalytic membrane reactor: 1. Direct UV disinfection (when subjected to a high UV intensity), 2. Membrane rejection, 3.

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ACCEPTED MANUSCRIPT Interaction with Ag+ ions, 4. Interaction with Ag nanoparticles, and 5. Bactericidal

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action of TiO2 (The inset summarizes different actions of bacteria inactivation).

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ACCEPTED MANUSCRIPT Table 1. Types of Ag-TiO2 photocatalytic membranes fabricated Layers coated 1st (bottom)

Types

2nd

TiO2 loading

3rd (top)

3-sub-layers 2-sub-layers

−2

(µg cm )

1-layer

Thickness of

PEG

Ag-TiO2

rejection

layer (nm)

a

(%)

b

Pure water flux (L m−2 h−1 Bar−1)

R1

P10-Ag0.0

P5.0-Ag0.0

P2.0-Ag0.5

58.6 ± 7.1

323 ± 39

R2

P10-Ag0.5

P5.0-Ag0.5

P2.0- Ag0.5

48.3 ± 7.7

260 ± 41

76.4 ± 2.6

146 ± 2

R3

P5.0-Ag0.0

P2.0-Ag0.0

P1.0-Ag0.5

70.3 ± 5.2

341 ± 25

78.8 ± 0.6

142 ± 3

R4

P5.0-Ag0.5

P2.0-Ag0.5

P1.0-Ag0.5

65.8 ± 9.9

298 ± 45

82.5 ± 1.3

129 ± 3

R5

P5.0-Ag0.0

P2.0-Ag0.0

P1.0-Ag1.0

78.5 ± 5.6

355 ± 24

80.1 ± 1.2

134 ± 2

P5.0-Ag1.0

P2.0-Ag1.0

P1.0-Ag1.0

72.9 ± 10.8

325 ± 40

84.8 ± 0.5

123 ± 2

P10-Ag0.0

P5.0-Ag0.0

P2.0-Ag0.0

130.3 ± 10.3

598 ± 38

72.3 ± 1.4

160 ± 1

c

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R6 a

75.2 ± 0.6

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Determined by considering different porosities of each sub-layers and assuming uniform densities of anatase and metallic silver of 3.9 and 10.5 g cm−3 respectively. b Determined using PEG of MW 20,000 g mol−1. c Obtained from our previous work [Goei et al. (2013)] as control.

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159 ± 1

ACCEPTED MANUSCRIPT Table 2. Physico-chemical characteristics of synthesized Ag-TiO2 materials: (a) TiO2 crystallite size, (b) BET surface area, and (c) BET porosity (a) TiO2 Crystallite Size (nm)a

↓ P-123 | Ag →

0.5

1.0

2.0

3.0

14.8

14.6

12.9

10.8

8.9

14.4

13.4

12.8

13.3 7.7 5.5

12.7 6.8 5.3

10.8 6 4.6

5

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0.0

4.5 3.7 (b) BET Surface Area (m2 g−1)

P-123 | Ag

0.0

0.5

1.0

0.0

27.68 ± 0.71

20.15 ± 0.70

0.5

51.29 ± 0.76

1.0

10.5

8.3

9.8 5.2 3.7

7.7 4.7 3.6

3.4

2.6

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(wt%) | (mol%) 0.0 0.5 1.0 2.0 5.0 10

3.0

17.55 ± 0.57

5.83 ± 0.13

1.61 ± 0.01

35.15 ± 1.07

24.07 ± 0.25

13.81 ± 0.08

9.36 ± 0.23

82.19 ± 0.51

46.29 ± 0.57

34.91 ± 0.93

19.97 ± 0.14

10.84 ± 0.09

2.0

127.06 ± 2.95

49.33 ± 0.59

45.64 ± 0.29

36.33 ± 0.40

20.79 ± 0.27

5.0

139.41 ± 1.08

56.08 ± 0.47

53.59 ± 0.61

40.84 ± 0.18

30.51 ± 0.30

10

146.84 ± 1.07

75.52 ± 0.42

68.58 ± 0.52

48.72 ± 0.36

38.32 ± 0.28

2.0

3.0

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2.0

P-123 | Ag

0.0 14.19 ± 0.17

0.5 1.0

29.84 ± 0.19 34.60 ± 0.39

2.0

45.63 ± 0.32

5.0

0.5

1.0

13.22 ± 0.22

11.23 ± 0.13

5.79 ± 0.21

3.97 ± 0.04

28.53 ± 0.23

23.38 ± 0.26

16.35 ± 0.36

12.02 ± 0.14

33.49 ± 0.26

29.86 ± 0.12

18.09 ± 0.17

14.05 ± 0.05

40.52 ± 0.42

37.27 ± 0.43

33.05 ± 0.19

22.82 ± 0.09

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(c) BET Porosity (%)b

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47.98 ± 0.23 46.67 ± 0.33 43.86 ± 0.16 35.18 ± 0.07 32.26 ± 0.06 10 58.20 ± 0.21 51.41 ± 0.42 50.02 ± 0.07 45.70 ± 0.30 21.19 ± 0.10 a Calculated using Scherrer’s equation with error < 1 nm. b Determined assuming uniform densities of anatase and metallic silver of 3.9 and 10.5 g cm−3 respectively.

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Fig. 1. Schematic diagram of photocatalytic membrane reactor operating under dead-end

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filtration mode.

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Fig. 2. High resolution XPS spectra of (a) C 1s, (b) O 1s, (c) Ti 2p and (d) Ag 3d regions of

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the pulverized Ag-TiO2 layers.

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Fig. 3. FESEM micrographs of Ag-TiO2 photocatalytic membrane at (a) low and (b) high magnifications. (c) TEM and (d) HRTEM micrographs of R2 membrane. (The inset depicts the corresponding SAED pattern). (e) EDX of the surface of membrane R2.

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Fig. 4. Specific RhB removal of Ag-TiO2 photocatalytic membrane (The numerical values

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depict kinetic value based on initial RhB removal rate within 1st hour in mg m−2 h−1).

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Fig. 5. Diffusion inhibition zones of Ag-TiO2 photocatalytic membrane against E. coli after

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24 h incubation.

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Fig. 6. E. coli log reduction value of Ag-TiO2 photocatalytic membrane showing its reusability over 4 cycles of operation in a batch reactor. Error bars represent one standard

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deviation of at least three replicates of experimental runs.

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Fig. 7. Flux performance of Ag-TiO2 photocatalytic membrane in the dead-end filtration

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mode of PMR with inter-cycle cleaning operating (a) with UV and (b) without UV.

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Fig. 8. E. coli log reduction values of Ag-TiO2 photocatalytic membrane operating in a photocatalytic membrane reactor: at sampling point (3) after each cross-flow cleaning cycles when operating (a) with and (b) without UV. Error bars represent one standard deviation of

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at least three replicates of experimental runs.

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Fig. 9. (a) Log reduction values of E. coli at sampling point 3 after cross-flow cleaning cycle

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for various membranes. Error bars represent one standard deviation of at least three replicates of experimental runs. (b) Possible mechanisms of anti-bacterial action inside the

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photocatalytic membrane reactor: 1. Direct UV disinfection (when subjected to a high UV intensity), 2. Membrane rejection, 3. Interaction with Ag+ ions, 4. Interaction with Ag nanoparticles, and 5. Bactericidal action of TiO2 (The inset summarizes different actions of bacteria inactivation).

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ACCEPTED MANUSCRIPT Research Highlights • Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture was fabricated. • The effect of P-123 and Ag loading on the resulted pores structure were investigated. • Different membrane architectures were fabricated to optimize the membrane performance.

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• The performance of the membrane was evaluated in the photocatalytic membrane reactor.

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• Ag-TiO2 membrane exhibited an anti-bacterial property with reusability potential.

ACCEPTED MANUSCRIPT Ag-decorated TiO2 photocatalytic membrane with hierarchical architecture: photocatalytic and anti-bacterial activities Ronn Goeia,b, Teik-Thye Lima,b,* a

School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore. b Nanyang Environment & Water Research Institute (NEWRI), Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141. Singapore. * Corresponding author: Tel: +65-6790 6933; Fax: +65-6791 0676; E-mail: [email protected].

Supplementary Data

11

1. XRD and N2 porosimetry result

13 14 15 16

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1 2 3 4 5 6 7 8 9 10

Fig. S1. (a) XRD spectra, (b) N2 adsorption/desorption curves, and (c) pore size distribution of the pulverized Ag-TiO2 layers with different amounts of Ag added.

17

1

ACCEPTED MANUSCRIPT 2. Pure water permeability

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Fig. S2. Pure water permeability of Ag-TiO2 photocatalytic membrane. (The numerical values depict average pure water permeability in L m−2 h−1 Bar−1).

23

3. Photo-induced super-hydrophilicity (PSH)

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25 26 27 28

Fig. S3 Photo-induced super-hydrophilicity of R6-like thin film coated on a borosilicate glass as shown on (a) as coated thin film, (b) upon 1 h of UV irradiation, and after (c) 8 hr, (d) 24 h, and (e) 48 h left in the dark.

29

Ag-TiO2 layers were also coated on a borosilicate glass for the evaluation of their

30

hydrophilicity. The coated layers were identical to those of membrane R6. The glass support 2

ACCEPTED MANUSCRIPT was cleaned thoroughly and dried at the room temperature and then dipped into the

32

designated Ag-TiO2 sol for 60 s using a home-fabricated dip-coater. The coated glass was

33

withdrawn from the sol at a rate of 2 mm s−1. The coated borosilicate glass was then heat-

34

treated by the same method used for the Ag-TiO2 membrane. Contact angle of the TiO2 film

35

coated on glass substrate was measured using video contact angle device (VCA Optima, AST

36

Products). Prior to the contact angle measurement, the Ag-TiO2 coated glass was left in the

37

dark for 24 h to remove any residual photo-induced super-hydrophilicity. The glass slide was

38

then irradiated under UV for 1 h. The contact angle of the TiO2 thin film was recorded at 0, 8,

39

24, and 48 h after the irradiation; between each reading the glass slide was stored in the dark.

40

Droplet volume of 1 µL was dispensed from a 100 µL syringe.

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The coated R6-like thin film shows a contact angle of 51° which subsequently decreased

42

to 8° after 1 h of UV irradiation. This result illustrated the photo-induced super-

43

hydrophilicity property of Ag-TiO2 thin film. When the UV source was switched off, the

44

contact angle increased again to 13°, 17° and 34° after 8, 24 and 48 h respectively.

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Table S1. Performance of Ag-TiO2 membrane in terms of its photocatalytic activity (PC), adsorptive-membrane retention (Ads-MR) and percentage of flux decline Removal (mg m−2) Removal (%) Flux Decline Types (%) PC Ads-MR Total PC Ads-MR

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4. Removal of RhB in PMR

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R1

2048 ± 52

331 ± 26

2379 ± 29

86.1

13.9

13.9

R2

2169 ± 35

351 ± 22

2520 ± 13

86.1

13.9

15.1

R3

1999 ± 56

407 ± 11

2406 ± 46

83.1

16.9

16.4

R4

2048 ± 71

558 ± 23

2606 ± 74

78.6

21.4

16.8

R5

2252 ± 30

486 ± 29

2738 ± 52

82.2

17.8

16.0

R6

2203 ± 52

577 ± 56

2780 ± 29

79.2

20.8

17.1

Plu D

2025 ± 24

329 ± 13

2354 ± 18

86.0

14.0

13.8

3

ACCEPTED MANUSCRIPT 49 50 51 52 Types

5. Total Ag leaching of Ag-TiO2 membrane Table S2. Time-dependent total-Ag leaching of Ag-TiO2 membrane reported in (a) % of initial Ag loading and (b) in µg L−1 (a) Ag loading Cumulative time-dependent total-Ag Cummulative total-Ag leaching (%) in leaching (%) in batch reactor PMR (µg) 0.47 ± 0.03

24 h 3.30 ± 0.02

96 h 6.78 ± 0.09

168 h 12.79 ± 0.23

30 min 2.45 ± 0.02

120 min 2.46 ± 0.02

480 min 2.47 ± 0.03

R2

1.36 ± 0.22

2.27 ± 0.04

5.02 ± 0.08

7.29 ± 0.03

7.56 ± 0.13

13.16 ± 0.22

15.84 ± 0.26

R3

0.76 ± 0.01

2.46 ± 0.03

4.16 ± 0.09

5.74 ± 0.05

4.75 ± 0.08

10.73 ± 0.18

12.95 ± 0.24

R4

1.85 ± 0.28

12.37 ± 0.25

19.52 ± 0.18

26.59 ± 0.21

5.28 ± 0.04

9.21 ± 0.11

11.24 ± 0.15

R5

1.53 ± 0.16

3.64 ± 0.01

15.53 ± 0.20

24.53 ± 0.15

5.86 ± 0.09

9.54 ± 0.12

11.23 ± 0.17

R6

4.09 ± 0.61

6.31 ± 0.02

13.10 ± 0.27

19.82 ± 0.59

5.88 ± 0.08

6.91 ± 0.11

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Cummulative total-Ag leaching (µg L−1)

in PMR

168 h 2.03 ± 0.04

30 min 2.14 ± 0.02

120 min 3.84 ± 0.03

480 min 4.66 ± 0.05

3.30 ± 0.01

3.42 ± 0.06

5.95 ± 0.10

7.16 ± 0.12

1.45 ± 0.01

1.20 ± 0.02

2.71 ± 0.05

3.27 ± 0.06

16.37 ± 0.13

3.25 ± 0.03

5.67 ± 0.07

6.92 ± 0.10

R1

24 h 0.52 ± 0.01

96 h 1.07 ± 0.01

R2

1.03 ± 0.02

2.27 ± 0.04

R3

0.62 ± 0.01

1.05 ± 0.02

R4

7.62 ± 0.16

12.02 ± 0.11

R5

1.86 ± 0.01

R6

8.61 ± 0.02

7.92 ± 0.10

12.52 ± 0.07

2.99 ± 0.04

4.87 ± 0.06

5.73 ± 0.08

17.87 ± 0.37

27.04 ± 0.80

4.00 ± 0.05

8.02 ± 0.10

9.42 ± 0.15

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Cumulative time-dependent total-Ag leaching (µg L−1) in batch reactor

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Fig. S4. Cumulative total-Ag leached from Ag-TiO2 membrane operated in the PMR. 4

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6. Percentage flux recovery after each cleaning cycle.

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58

64 65 66 67

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7. E. coli reduction value of Ag-TiO2 membrane at sampling point 2

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Fig. S5. Percent flux recovery after each cleaning cycle when operation (a) with UV and (b) without UV.

Fig. S6. E. coli log reduction values of Ag-TiO2 photocatalytic membrane operating in a photocatalytic membrane reactor: at sampling point (2) when operating (a) with and (b) without UV. Error bars represent one standard deviation of at least three replicates of experimental runs.

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5