TiO2 Nanofibrous Membrane for Water

Chapter 14

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Hierarchical Hybrid K-OMS-2/TiO2 Nanofibrous Membrane for Water Treatment Tong Zhang and Darren Delai Sun* School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798 *E-mail: [email protected]. Tel: +65 6790 6273

This chapter presents a novel free-standing nanofibrous microfiltration membrane made by hierarchical hybrid K-OMS-2/TiO2 nanowires for the removal of organic pollutants in water. The K-OMS-2/TiO2 nanowire was around 10 µm in length with the high density secondary TiO2 hair-like structures (20-40 nm) anchored on surfaces of primary underlying K-OMS-2 nanowires. At 30 L/m2·h (LMH) membrane flux, the color and total organic carbon (TOC) removal rates were 90 % and 50%, respectively. Clearly, the organics (AO 7) will be filtered directly by membrane pores with external pressure, and the smaller ones would be adsorbed onto the nanowires via an adsorption process that is greatly enhanced by the hydrophobic nature of the supporting K-OMS-2 nanowires, as well as large surface area of the hierarchical nanowires. The organic pollutants on the photocatalytic membrane were simultaneously degraded by the PCO process under a UV light, alleviating membrane pore blocking by small organic molecules, thus maintaining a constant permeate flux.

Introduction Membrane technologies have been widely applied in water treatment, dairy, food, pharmaceutical, bioengineering, chemical, nuclear-energy and electronic industries due to their excellent performance on the removal of pollutants, relative low energy cost, well arranged process conductions, and no addition of chemicals. Membranes for water purification can offer high quality clean water with small © 2013 American Chemical Society In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


footprint and high flexibility for scaling up. Most membranes used in industrially are polymer based membrane (1–5). However, the membrane fouling caused by deposition of organics and inorganics on membrane surfaces is the major obstacle. In addition, these organic membranes are typically unsuitable for very high-temperature applications (2, 4). Thus, there is an urgent need in searching for a multifunctional membrane which is able to purify water and concurrently remove fouling from the membrane surfaces. Nanofabrication technology has great potential for molecular separation applications by offering more structurally controlled materials for such needs. For the past 15 years, our research group has put in a large amount of effort to develop a robust, flexible and free-standing multifunctional TiO2 membrane using nanofabrication technology to overcome the fouling problem that arose from conventional polymeric membranes (6, 7). The pioneer work in developing a multifunctional free-standing, flexible membrane to overcome the polymeric membrane problem was carried out by Zhang and co-workers (6, 7). This two-dimensional (2D) nanostructured TiO2, in the form of nanowires membrane was synthesized by hydrothermal method (7). The 2D nanomaterial brought in new properties such as flexibility and concurrent filtration while retaining the properties of its 1D nanomaterial. Inorganic nanofibrous/nanowires membranes have received increasing attention in the fields of catalysis, adsorption, sensors and filtration (8–10). Owing to their great mechanical resistance and reusability, inorganic membranes with different properties and functions have been successfully fabricated for water purification (11–13). As the main function of a membrane is to discriminate species, it comes into contact within one phase and transports them to the other. Selectivity and permeability becomes the most important properties of a membrane. Selectivity is expressed as a separation factor, which is governed by the surface properties of a membrane, while permeability describes the rate at which species are transported across a membrane (14). The two properties are determined by the structural and morphological properties of the membrane, such as the pore size, pore structure, wettability and porosity. Thus, applications of inorganic nanofibrous membranes in separation technology will depend upon the ability to prepare membranes with desired pore size and pore structure. However, controlling of pore size and pore structure suffers from complicated technical requirements as well as high operational costs and thus severely restricting the applications of membranes. To address this problem, other techniques can be considered in the filtration process to maintain high performences of the membrane, such as adsorption and photocatatic oxidation. Membranes with higher organic adsorption capacity would enhance rejection of organic molecules; while the introduction of photocatalysts as membrane components would endow a membrane with self-cleaning property. It is desirable to synthesize hybrid nanofibrous membranes with efficient adsorption and photocatalytic activity, which can further enhance the selectivity and permeability of the membranes. In this chapter, we introduce a 2D hierarchical hybrid K-OMS-2/TiO2 nanowires membrane. The advantages of the 2D hierarchical hybrid K-OMS-2/TiO2 nanowires membrane are: (1) full surface exposure to UV or solar for self-regeneration through TiO2 photocatalytic oxidation (PCO) reaction, which 268 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

effectively eliminates the membrane fouling problem; (2) concurrent membrane filtration and PCO oxidation; (3) high surface area and hybrid structure, which allows higher adsorption rate and better PCO oxidation of various trace organics for improving water quality; (4) higher acid/basic and temperature controlled resistance; (5) flexible property which enables the membrane to be formed into various membrane modules for larger commercial application. A free-standing and flexible manganese oxide based nanofibrous TiO2 hair-like membrane will be investigated by evaluating its permeability and photocatalytic activity.


Materials and Methods In order to obtain the basic supporting material of the membrane, K-OMS-2 nanowires were synthesized via a hydrothermal method (15). In a typical procedure, 12 mmol of K2SO4, 12 mmol of K2S2O8, and 8 mmol of MnSO4·H2O were dissolved in 70 ml of deionized water. The solution was then transferred to a 125 ml Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated in an oven at 250 °C for 4 days. The resulting black precipitate was suspended in 1000 ml deionized water, and stirred vigorously for 12 h. After thorough washing with deionized water to remove remaining ions present in the product, the sample was dried at 105 °C for 24 h. To synthesize the K-OMS-2/TiO2 heterojunctions, 50 mg of the synthesized K-OMS-2 nanowires, 400 mg titanous sulfate, and 0.4 ml concentrated H2SO4 (98%) were dissolved in 70 ml deionized water. After homogenization by an ultrasonic homogenizer for 10 mins, the solution was transferred to the 125 ml Teflon-lined autoclave again and heated at 105 °C for 20 h. Then, the product was washed several times before being separated using a centrifuge, and the resulting product was dried in vacuum for 48 h. To synthesize the K-OMS-2/TiO2 nanofibrous membrane, suspension of the synthesized hierarchical K-OMS-2/TiO2 nanowires was vigorously stirred for 10 min. Then, the suspension was filtered on a vacuum-filtration setup with a glass membrane (ADVANTEC, GC-50, 0.45 µm), and the hierarchical nanowires would form a compact cake layer on the membrane. The glass membrane was then placed in an oven for drying at 105 °C. After removing the glass membrane, a freestanding membrane was formed. The membrane was further pressurized under 5 bar at 120 °C on a customized hot press for 2 mins. Finally, the membrane was calcinated at 550 °C for 1 h. K-OMS-2 nanowire membrane was also fabricated via a similar process for comparison.

Results and Discussion Since organic pollutants have higher affinity towards the hydrophobic K-OMS-2 nanowires, this material can be used as a good adsorbent for the removal of organic pollutants (16). Furthermore, the nanowires can also act as a template, and other nanostructures (e.g. TiO2) can be deposited onto the surface to form hierarchical heterojunctions. FESEM and TEM images of K-OMS-2 nanowires are shown in Figure 1a and 1b respectively, indicating that the K-OMS-2 269 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


nanowires have diameters of about 40-100 nm and are over several hundreds of microns long. Figure 1c and 1d shows that high density secondary TiO2 nanostructures were uniformly anchored on surfaces of the primary underlying K-OMS-2 nanowires, making a hybrid inorganic heterojunctions. It can be seen from Figure 1d that the TiO2 nanostructures are of lengths ranging from 20-40 nm with much smaller diameter as compared to the K-OMS-2 nanowire. The BET surface area of the synthesized K-OMS-2 nanowires and K-OMS-2/TiO2 heterojunctions are recorded at 25.08 m2/g and 111.58 m2/g, respectively. It is noteworthy that the loading of TiO2 nanostuctures on the K-OMS-2 nanowires has greatly increased the specific area of the material. This phenomenon can be attributed to the presence of K-OMS-2 scaffold, which acts as a dispersing template to downsize the TiO2 nanostructures during the synthesis process (17).

Figure 1. (a) FESEM image of K-OMS-2 nanowires. (b) TEM image of K-OMS-2 nanowire. (c)FESEM image of K-OMS-2/TiO2 heterojunctions. (d)TEM image of K-OMS-2/TiO2 heterojunctions. 270 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. (a)XRD patterns of the synthesized K-OMS-2 nanowires and K-OMS-2/TiO2 heterojunctions. (b) EDX spectrum of the synthesized K-OMS-2/TiO2 heterojunctions. (Reproduced from reference (17). Copyright 2012 Royal Society of Chemistry). XRD analysis displays the crystal phase of the synthesized materials. All diffraction peaks of the upper curve in Figure 2a can be perfectly indexed to the K-OMS-2 crystalline phase (JCPDS 44-1386). The XRD pattern of synthesized K-OMS-2/TiO2 nanostructures shows that TiO2 structures are present in the synthesized K-OMS-2/TiO2 heterojunction, indicating the successful coating of TiO2 nanostructure. The diffraction peaks at 27°, 36° and 55° indicate that the 271 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


rutile phase of TiO2 (JCPDS 21-1276) is present in the synthesized material, and the diffraction peaks at 25° and 48° evidenced the co-existence of anatase TiO2 (JCPDS 21-1272). Moreover, from EDX result in Figure 2b, we can confirm that the external surface of the synthesized K-OMS-2/TiO2 heterojunction contains K, Mn, Ti, and O.

Figure 3. (a) Digital photo of the K-OMS-2 membrane. (b) Digital photo of the K-OMS-2/TiO2 membrane. (c) Top view FESEM image of the K-OMS-2 membrane. (d) Top view FESEM image of the K-OMS-2/TiO2 membrane. (Figure 4c is reproduced from reference (18); Figure 4d are reproduced from reference (17). Copyright 2012 Royal Society of Chemistry). The digital photos of the synthesized free-standing membranes are shown in Figure 3a and 3b. The thickness of the membrane can be adjusted by adjusting the dosage of the materials, and the diameter of the membrane can be controlled by our filtration apparatus. In this work, the thickness and diameter of the membrane are about 300 μm and 40 mm, respectively. Figure 3c and 3d shows the top view FESEM images of K-OMS-2 membrane and K-OMS-2/TiO2 membrane, respectively. Both of them have relatively flat topology with no observed cracks or pinholes. From Figure 3d, it can be found that the open porous network was formed by overlapping and interweaving of the ultra long hierarchical K-OMS-2/TiO2 heterojunctions, which enhance the permeability of the membrane in a filtration 272 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


process. Furthermore, the pore size of the synthesized K-OMS-2/TiO2 membrane is around 0.1 μm, which classify the membrane under the microfiltration category. The synthesized K-OMS-2/TiO2 membrane was investigated by XPS to identify the surface elemental compositions and binding energies, and the result indicates the sample contains K, Mn, O and Ti. The high-resolution XPS spectrum of Mn 2p taken on the surface of K-OMS-2/TiO2 heterojunctions is shown in Figure 4a and Gaussian curve fitting of Mn 2p3/2 was performed to describe the oxidation states of Mn more specifically. The peaks are centred at 641.7 eV and 643.0 eV, indicating the presence of Mn3+ and Mn4+ respectively (19). It is noteworthy that the mixed-valences of Mn is important in electron transfer and lead to conductivity of resultant material, and thus enhance the photocatalytic activity of the synthesized material (20). Figure 4b shows that the peaks for Ti 2p3/2 and Ti 2p1/2 are centered at 458.8 eV and 464.4 eV, representing the presence of a Ti4+ oxidation state in the sample (21), which further confirmed the presence of TiO2 on the surface of K-OMS-2 nanowires. Importantly, the presence of TiO2 can endow the synthesized membrane with self-cleaning property under UV light or solar light irradiation.

Figure 4. High-resolution XPS spectra of Mn 2p (a) and Ti 2p region (b) taken on the K-OMS-2/TiO2 membrane. To investigate the performance of the synthesized membrane, the adsorption capacity of the synthesized K-OMS-2/TiO2 heterojunctions were firstly evaluated using acid orange 7 (AO 7) as a pollutant. The K-OMS-2 nanowire and TiO2 nanostructure were also used for comparison. As shown in Figure 5, the synthesized K-OMS-2/TiO2 exhibited much higher adsorption capacity than TiO2 which was prepared using an identical procedure as that of K-OMS-2/TiO2. The BET surface area of the synthesized K-OMS-2, TiO2 and K-OMS-2/TiO2 are 25.08 m2/g, 72.74 m2/g and 111.58 m2/g respectively. The increased BET surface area of K-OMS-2/TiO2 may partly explain the enhanced adsorption capacity. Figure 5 also shows that K-OMS-2 nanowires possess excellent adsorption capacity for the removal of AO 7, which can be attributed to the hydrophobic nature of the nanowires. Previous studies indicated that the surrounding environment of the constructed TiO2 nanostructures can be affected by the hydrophobic supporting materials (22). Thus, the hydrophobic nature of K-OMS-2 nanowires may be another important factor for the enhanced adsorption capacity. 273 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 5. Adsorption of AO 7 onto the prepared K-OMS-2, TiO2 and K-OMS-2/TiO2 at pH 5. (Reproduced from reference (17). Copyright 2012 Royal Society of Chemistry).

The performance of the synthesized K-OMS-2/TiO2 nanofibrous membrane was evaluated using a dead-end filtration equipment (Figure 6). When a flux of 30 L/m2·h was applied in the system, only 8.7 % of AO 7 was removed using the synthesized nanofibrous membrane alone, which can be attributed to the adsorption equilibrium of the membrane was gradually reached during the filtration process. When UV irradiation was concurrently applied to the membrane, an AO 7 removal rate of 96.3 % was achieved, owing to combined adsorption and PCO effects. However, only 54.1 % of total organic carbon (TOC) was removed in this process due to the incompletely photocatalytic mineralization of the AO 7 molecules. In addition, flux is another factor which can affect the removal rate of AO 7 and TOC. According to our study, with the increase of flux, the removal rate of AO 7 and TOC decreased, and 60 L/m2·h was the critical point in establishing an optimal balance between photocatalytic degradation of AO 7 and flux in our experimental conditions. 274 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 6. The setup of concurrent filtration, adsorption and photocatalytic oxidation. (Reproduced from reference (17). Copyright 2012 Royal Society of Chemistry).

Conclusion In this study, we synthesized a novel free-standing nanofibrous microfiltration membrane via a vacuum-filtration process. Importantly, the synthesized membrane can reject organic pollutants via a combination effect of interception and adsorption. A concurrent filtration, adsorption and photocatalytic oxidation experiment showed that the nanofibrous membrane possessed excellent performance in the removal of organic pollutants. The synthesized membrane may have potential in extensive applications of membrane filtration and water purification technologies.

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TiO2 Nanofibrous Membrane for Water

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