Nanowire Free-Standing Membrane for Concurrent Filtration and

Downloaded by PENNSYLVANIA STATE UNIV on June 27, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1022.ch014

Chapter 14

TiO2 Nanowire Free-Standing Membrane for Concurrent Filtration and Photocatalytic Oxidation Water Treatment Xiwang Zhang, Alan Jianhong Du, Jiahong Pan, Yinjie Wang and Darren Delai Sun1 School of Civil and Environmental Engineering Nanyang Technological University, Singapore 639798 1 Corresponding Author : [email protected], Tel: +65 6790 6273

A flexible, uniform, and multifunctional TiO2 nanowire membrane was successfully fabricated using a hydrothermal method. XRD results indicate that the TiO2 nanowires forming the membrane consist of anatase TiO2 and Na2(Ti6O13) titanate. These nanowires were 20-100 nm in diameter and had a typical length of several micrometers to tens of micrometers. Under UV irradiation, the membrane showed an excellent performance on concurrent filtration and photocatalytic oxidation degradation of HA in water. A removal rate of 100% and 93.6% for Humic acid and TOC were achieved up to, respectively. The nanowire membrane proves to have remarkable potential for water and wastewater purification, and could be used to eliminate other foulants such as microorganisms and trace organics. It could produce a new flexible dye sensitized solar cell (DSSC) as well.

Introduction The demand for drinking water as a precious resource has increased tremendously in terms of quantity and quality over the past few decades[1]. One of the main strategic solutions recommended for the fulfillment of increasing water demand in quantity and quality is through water reclamation technologies © 2009 American Chemical Society In New Membranes and Advanced Materials for Wastewater Treatment; Mueller, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

219


220 for the conservation and recovery of drinking water. However, the presence of contaminants such as natural organic matters (NOMs), trace organics and microorganisms that accumulate in different sources of raw water creates major problems which make raw water unsuitable for direct human consumption. There is a need for cost effective water treatment prior too water being consumed The alum coagulation/flocculation together with chlorination and ozone oxidation is a conventional technology that has been widely used in water treatment plants for the removal of particles as well as the large molecular weight contaminants found within raw water. Unfortunately, this technology is unable to completely remove the small molecule weight contaminants, thus generating extra volumes of sludge, which require further treatment and safe disposal. The large infrastructure development requires large land space. Furthermore, aluminum presence in the treated water has been suspected to develop the onset of Alzheimer’s diseases[2]. Small molecule weight NOMs such as fulvic acids and humic acids (HA) has been shown to react with the major disinfectants such as chlorine, ozone, chlorine dioxide, chloramines to produce disinfection by-products, such as trihalomethanes (THMs), haloacetic acids (HAAs), bromoform (CHBr3), dibromoacetic acid (DBAA), and 2,4dibromophenol (2,4-DBP) etc, which are carcinogenic compounds[3]. Conventional micro/ultra filtration (MF/UF) membranes are an advanced water treatment processes for producing high quality drinking water with a small footprint size[4]. However, the commercial filtration membranes that available in the industry have high fouling tendencies caused by the deposition and absorption of the contaminants such as NOMs, trace organics and microorganisms, which are the major problems in using the filtration membranes for producing high quality drinking water. There is urgent need in searching for a new generation of membrane which will be able to overcome the existing problems of the conventional filtration membrane fouling and leaking of contaminants within the membrane pore. Nanotechnology has great potential for molecular separation applications by offering more desired structurally control materials for such needs. Titanium dioxide (TiO2) nanosized particle is a popular photocatalyst which attracts much attention from both fundamental research and practical applications for the removal of contaminants from water [5, 6, 7, 8, 9]. It is well known that TiO2 which functions as a photocatalyst would generate electrons and holes when UV light strikes its crystal surface. While the holes are strong oxidants which than can be applied to various environmental problems, such as removal of the contaminants from water through photocatalytic oxidation reactions [10, 11, 12]. Unfortunately, commercially available TiO2 photocatalyst which is in the nano size range, has an inherent and significant drawback [13, 14], that is, the difficulty in separation and recovery of the nanosized particle from water. In order to solve the problem of separation and recovery, it is important to re-design the nanostructured TiO2 photocatalytic material thus not only solving the problem but also further increases the photocatalytic activity of TiO2 In the past 10 years, our research group has put in a large amount of effort to develop a robust, flexible and free-standing multi-functional TiO2

In New Membranes and Advanced Materials for Wastewater Treatment; Mueller, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


221 membrane using nanofabrication technology to overcome the problems that arose from conventional membranes and nanosized TiO2 photocatalyst. The membrane can be formed in four different nanostructes which are nanowire, nanofiber, nanorods and nanotube. It was found that a nanowire membrane is the most inexpensive option out of the four nanostructures. The pioneer work in developing a multi-functional free-standing membrane to overcome the conventional membrane problems was carried out by Anderson and co-workers [15, 16]. They fabricated non-flexible TiO2 photocatalytic ceramic membrane using sintering of TiO2 particles on ϒ-Al2O3 support. Following that many research studies have been taken out to fabricate TiO2 membranes by coating TiO2 films on various supports [17-29]. Unfortunately, photocatalytic efficiency of these coated TiO2 membranes on various supports was lower than that of nanosized TiO2 photocatalysts. Reduction of the filtration production rate, which permeates through such coating or sintering is another drawback. Recently nanotechnology research has shown that one dimensional (1D) nanostructured TiO2, in the form of nanowires, nanofibers, nanorods and nanotubes, can be synthesized by chemical methods[30-36]. Among these chemical methods, hydrothermal method is commonly used due to its uncomplex operation. For example P25 TiO2 powders can be converted into TiO2 nanowires using a stainless steel pressure vessel (autoclave) under controlled conditions such as temperature /pressure with concentrated NaOH solution. Under these controlled conditions, the Ti-O-Ti bonds were broken and then TiO-Na and Ti-OH bonds were formed, which resulted in the formation of TiO2 nanowires [34,37]. Research results also indicate that TiO2 nanowires exhibit superior photocatalytic oxidation efficiency compared with P25 TiO2 due to its larger surface area and presence of quantum size effect (crystal size less than ten nanometer). In the past five years, substantial amount of efforts [38-42] has been devoted to the study of organizing nanowire/fiber/tube into 2D nanomaterials such as carbon nanofilm and strong, transparent, multifunctional carbon nanotube sheet. These 2D nanomaterials brought in new properties such as flexibility, concurrent filtration and multifunctionality while retaining the properties of its 1D nanomaterial. In this section, we introduce a 2D TiO2 nanowire membrane as shown in Figure 1 which is fabricated from TiO2 nanowires suspension synthesized from hydrothermal reaction. The advantages of the TiO2 nanowire membrane are: (1) full surface exposure to UV or solar for self-regeneration through photocatalytic oxidation reaction, which effectively eliminates the membrane fouling problem; (2) concurrent membrane filtration for separation purpose; (3) high surface area, which allows higher adsorption rate of various trace organics and microorganisms for improving water quality; (4) and higher acid/basic and temperature controlled resistance; (5) environmentally friendly and longer membrane life span; (6) flexible property which enables the membrane to be formed into various membrane modules for larger commercial application. We will also investigate a 2D TiO2 nanowire membrane by evaluating its permeability and photocatalytic activity.


222


Figure 1. Images of multifunctional, free-standing, flexible TiO2 nanowire membrane.

Experimental Procedure Synthesis and Characterization of TiO2 nanowire membrane Synthesis: 0.20 g of TiO2 powder (P25, Degussa) was added in with 30mL of 10M NaOH solution in a 45 mL Teflon-lined container in stainless steel vessel and then it was left in 180oC for hydrothermal reaction for 2 days. At the end of the 2 days reaction, white pulp suspension consisting of long nanowires was produced and collected. Following the collected nanowires were washed with distilled water and diluted hydrochloric acid solution (pH 2) for 3 times. 0.1 (wt) % surfactant (F-127) was then added into the washed white pulp nanowire suspension. To make a nanaowire membrane, a suction-filter fitted with a glass filter (Advantec, GC-50, pore size: 0.45 µm) was used to retain the washed white pulp TiO2 nanowire suspension on its surface, to form a TiO2 nanowire membrane. Residual surfactant left in the TiO2 nanowire membrane was washed away with distilled water using the same suction-filter. After the residue was removed, the TiO2 nanowire membrane was left to dry at room temperature. The glass filter was then removed to form a free-standing TiO2 nanowire membrane prior to calcination in a furnace at 700 oC for 2 h with a ramp of 2 oC/min for crystal phase changes. Characterization: The physical properties such as morphologies and the crystal phases of TiO2 nanowire membrane was examined using scanning electron microscopy (SEM), Leica LT7480 and field emission scanning electron microscopy (FESEM, JEOL 6340), powder X-ray diffraction (XRD), Bruker AXS D8 advance and JEOL 2010 transmission electron microscopy (TEM).


223 Permeability of TiO2 nanowire membrane


The permeability of the TiO2 nanowire membrane was evaluated using standard polystyrene microspheres[44, 45], size ranges from 0.05, 0.1, 0.2, 0.5, 1 to 2 μm in diameter obtained from Alfa Aesar. The microspheres was mixed in pure water to obtain 0.0033 (wt) % solution for each Polystyrene microsphere diameter size was filtered in Millipore UF Stirred Cells. TOC analyzer was used to detect the presence of polystyrene microspheres in feed and permeate. The separation factor of the TiO2 nanowire membrane was than determined using the equation.

⎛ C ⎞ Seperation Factor = ⎜⎜1 - permeate ⎟⎟ × 100% Cfeed ⎠ ⎝ Where C

and C feed are the PS solution concentration of permeate collected and the original feed, respectively. permeate

Photocatalytic oxidation and membrane filtration The concurrent photocatalytic oxidation activity and filtration of TiO2 nanowire membrane were investigated by using a dead-end filtration equipment together with UV lamp (11 W Upland 3SC9 Pen-ray lamp, 254nm) and auto data collection system in a concurrent continuous operation mode as shown in Figure 2. The volume of the filter cup was 250 mL. The UV light was above the TiO2 nanowire membrane at a distance of 1cm. HA concentration was measured by determining its absorbance at 436 nm using a UV-visible spectrophotometer. Total organic carbon (TOC) concentration was determined by a Shimadzu TOCVcsh TOC analyzer.

Figure 2. The set-up for concurrent filtration and photocatalytic oxidation. (Reproduced from reference, Zhang et al, Journal of Membrane Science 2008, 313, 44-51)


224


The photocatalytic oxidation activity of the TiO2 nanowire membrane was evaluated in the experiment setup in batch operation mode using humic Acid (HA) obtained from Fluka as a model contaminant. 250 mL HA solution of 15 mg/L was added in to the filtration cup, then the UV lamp was turned on. Samples were collected from the filtration cup using a syringe at intervals of 15 min for analysis. A blank sample using 0.05 g P25 (Degussa) was suspended in water and then deposited on the surface of a glass filter (0.45 µm). Based on our knowledge and experience, photo catalytic oxidation is guided by LangmuirHinshelwood (L-H) kinetics model equation-(1) as shown below. When the chemical concentration Co is a millimolar solution ( Co small) the equation can be simplified to an apparent first-order equation-(2)[46]. A plot of ln Co / Ct versus time will represent a straight line, the slope of which upon linear regression equals the apparent first-order rate constant k app . .

dC kKC = dt 1 + KC

(1)

⎛C ⎞ Ln⎜⎜ o ⎟⎟ = kKt = kapp.t ⎝ Ct ⎠

(2)

r=

Where r is the oxidation rate of the reactant (mg/L.min), C the concentration of the reactant (mg/L), t the illumination time, k the reaction rate constant (mg/L.min), and K is the adsorption coefficient of the reactant (L/min). The concentration of HA solution at 15 mg/L was filtered using the TiO2 nanowire membrane and blank TiO2 sample with/without UV irradiation. Its concentration and TOC in feed and filtrate were measured to calculate their respectively removal rates. The flux of TiO2 nanowire membrane and blank TiO2 sample was chosen at 4, 8, 12 and 16 L/min·m2.

Results and discussion Characterization of the membrane The prepared TiO2 nanowire membrane as shown in Figures 1 and 3 was a 47 mm-diameter membrane. The membrane fabrication had been described in the section of synthesis of TiO2 nanowire membrane. This fabrication method has the following advantages: (1) Homogeneity of the TiO2 nanowire distribution can be formed through process of filtration.


225


(2) The membrane thickness can easily be controlled by adjusting the nanowire suspension concentration and volume of the suspension filtered. (3) The TiO2 nanowire membrane is highly flexible. (4) Calcination above 300 oC ensures the membrane retained its physical shape. Its flexible property enables the membrane to be formed into various membrane modules for larger commercial applications.

(a)

(b)

Figure 3. TiO2 nanowire membrane flexibility. FESEM images of TiO2 nanowire membrane are shown in Figure 4. It can be seen that there is no significant effects on the structure of the TiO2 nanowires after the calcination process from 300 to 700 oC. It also can be seen that TiO2 nanowire membrane was formed by overlapping and interpenetrating of long nanowires with typical lengths in the range of several micrometers to several tens of a micrometer. The “spider web” nonwoven structure can be observed from Figure 4b. TEM image of TiO2 nanowires membrane as shown in Figure 4c revealed that the length of these TiO2 nanowires ranges from 20-100 nm in diameter.

(a)

(b)

(c)

Figure 4. TiO2 nanowire membrane, (a) low magnification FESEM image, (b) high magnification FESEM image, (c) TEM image of the TiO2 nanowire. (Figure 4a, 4b reproduced from reference, Zhang et al, Journal of Membrane Science 2008, 313, 44-51)


226


In order to determine the crystal structure of the nanowire membrane after calcination at different temperatures, the XRD was employed and the results are shown in Figure 5. Clearly, the fabricated nanowires using the method described earlier in section 2.1 are a mixture of anatase TiO2 and titanate. The calcination temperature ranged from 300-500 oC, is able to convert TiO2 and titanate into TiO2-B phase and Na2(Ti12O25), respectively. At the calcinations temperature of 700 oC, the TiO2-B phase was then transformed into anatase and Na2(Ti12O25). When temperature is increased further it will tend to decompose the Na2(Ti6O13) and anatase TiO2. The finding of these XRD results corresponds accordingly with the results reported [35, 42]. As Anatase TiO2 has highest photocatalytic oxidation activity than the other phases of TiO2, therefore TiO2 nanowire membranes were calcinated at 700 oC.

0

(700 C)

0

(500 C)

0

(300 C)

5

15

25

35

45

55

65

75

2 Theta / degree

Figure 5. XRD pattern of the TiO2 nanowire membrane. (Reproduced from reference, Zhang et al, Journal of Membrane Science 2008, 313, 44-51). To indentify the pore size distribution of the TiO2 nanowire membrane, aqueous suspensions of polystyrene microspheres (0.05, 0.1, 0.2, 0.5 and 1μm in diameter, respectively), in the concentration of 0.033(wt)%, had been used. The separation factors of the TiO2 nanowire membrane for these microspheres are shown in Figure 6. These results indicate that the separation factors for 0.2, 0.5, 1 and 2 μm polystyrene microspheres were more than 99%, indicating that these microspheres were unable to pass through the TiO2 nanowire membrane. With the decrease of polystyrene microsphere diameter the separation factor for microspheres decreased as well. The separation factors for 0.1 and 0.05 μm microspheres were 96.3% and 89.5%, respectively, indicating that the TiO2 membrane is capable of achieving partial removal of 0.1 and 0.05 μm microspheres. The pore size of a TiO2 nanowire membrane can be categorized as the diameter of latex microspheres which are 90% retained by the membrane. Therefore, the pore size of the TiO2 nanowire membrane is approximately about 0.05 μm.



227

Figure 6. Separation factors of the TiO2 nanowire membrane for different diameter polystyrene microspheres. Photocatalytic oxidation activity for anti membrane fouling In order to evaluate the multifunctional properties of the TiO2 nanowire membrane, the photocatalytic activity of the TiO2 nanowire membrane was studied in batch operation mode as described in section 2.3. HA under UV irradiation without P25 TiO2 and TiO2 nanowire membrane was also carried out as direct photolysis. P25 TiO2 that was deposited on the glass filter was used as a reference. The removal rate of HA and TOC over the course of the three processes are shown in Figure 7. It can be seen clearly that the TiO2 nanowire membrane shows excellent photocatalytic oxidation activity comparing with P25 TiO2. The photocatalytic oxidation degradation of HA follows the first order kinetics which reveals that the apparent rate constant ( k app . ) for the TiO2 nanowire membrane was 0.022 min-1 (R2 = 0.97), a little lower than that of commercial P25 ( k app . = 0.023 min-1, R2 = 0.98). The TOC removal rate curves as shown in Figure 7 also indicates that both TiO2 nanowire membrane and P25 TiO2 had a similar trend with a close TOC removal efficiency, hence the TOC removal in the solution indicates the mineralization of most HA into carbon dioxide and water. Compared with the photocatalytic oxidation degradation in the presence of either the TiO2 nanowire membrane or P25 TiO2, the degradation of HA by direct photolysis was very slow as the apparent rate constant was only 0.006 min-1 which is one fourth of that of the photocatalytic degradations.


228 100 HA: Photolysis TOC: Photolysis

80

HA: P25


Removal rate (%)

TOC: P25 HA: Nanowire membrane

60

TOC: Nanowire membrane

40

20

0 0

20

40

60

80

Time (t)

Figure 7. HA and TOC removal rates over the courses of photolysis, photocatalytic oxidation of HA via P25 deposited glass filter and TiO2 nanowire membrane. During TiO2 nanowire membrane filtration, HA was deposited on the membrane surface and absorbed by the pore, thus forming a cake layer on the membrane surface and clogging of the membrane pore, which is called membrane fouling. Membrane fouling will result in increase of operation pressure or membrane flux decline. It is the main obstacle in membrane filtration as it causes the reduction in productivity and increase of operation cost. In contrast, the fouling in TiO2 nanowire membrane does not exist. Under UV irradiation, the TiO2 nanowires can be excited to generate high oxidative species, holes (h+) and hydroxyl radicals (OH·). The mechanism of the photocatalytic oxidation degradation reactions is shown as follows: TiO2 + hv (λ

Nanowire Free-Standing Membrane for Concurrent Filtration and

Recommend Documents