In order to improve the permeate flux of photocatalytic membranes, we present an approach for coupling TiO2 with ceramic hollow fiber membranes. The ceramic hollow fiber membranes with high permeate flux were fabricated by a controlled wet-spinning process using polyethersulfone (PESf) and ceramic powder as precursors and 1-methyl-2-pyrrolidinone as solvent, and the subsequent TiO2 coating was performed by a dip-coating process using tetra-n-butyl titanate as precursor. It has been found that the PESf/ceramic powder ratio could influence the structure of the membranes. Here the as-prepared TiO2 hollow fiber membranes had a pure water flux of 4,450 L/(m2·h). The performance of the TiO2 hollow fiber membrane was evaluated using humic acid (HA) as a test substance. The results demonstrated that this membrane exhibited a higher permeate flux under UV irradiation than in the dark and the HA removal efficiency was enhanced. The approach described here provides an operable route to the development of high-permeable photocatalytic membranes for water treatment.
Fresh water resources available on earth account for only about 0.26% of the total water resources. Unfortunately, part of the surface water has been polluted by the drainage of industrial wastewater, the emissions of an agricultural nonpoint source, or the discharge of domestic sewage effluent (Sujaul et al. 2013; Miao et al. 2015; Schreiber et al. 2015). Recently, TiO2 photocatalysis has attracted great interest because it is environment-friendly and provides a promising pathway for potential applications in breaking down many kinds of organic pollutants, such as persistent toxic substances, dyes, pesticides, and herbicides under UV light irradiation. (Hoffmann et al. 1995; Chen & Mao 2007; Yu et al. 2009a, b; Chong et al. 2010). However, there are issues with TiO2 photocatalysis involving the recycling and reuse of the TiO2; these are inherent drawbacks that restrict practical application of this technology (Yu et al. 2001; Zhang et al. 2006).
Membrane separation achieves the separation, purification, and concentration of different components by using membrane selectivity; this has the advantages of high efficiency, energy savings, and convenient operation (Molinari et al. 2002; Sholl & Johnson 2006). However, the traditional membrane has the drawback of rapid membrane fouling. Considering the advantages of combining TiO2 photocatalysis and membrane separation, a TiO2-based photocatalytic membrane has been proposed as a possible solution that would provide the dual functions of membrane separation and photocatalytic pollutant degradation concurrently (Choi et al. 2006; Zhang et al. 2006; Albu et al. 2007). In particular, the photocatalysis function could greatly reduce membrane fouling. (Ma et al. 2009; Zhang et al. 2014)
Until now, most of the reported TiO2-based photocatalytic membranes have been limited to single-channel tube membranes or flat sheet membranes (Wang et al. 2008; Ma et al. 2010), whose permeate flux is relatively low. Hollow fiber membranes with high specific surface area have shown more promise in increasing the permeate flux of membranes (Kingsbury & Li 2009; Yu et al. 2009a, b; Leo et al. 2011; Haworth et al. 2012; Wu et al. 2013; Fan et al. 2015). For instance, Razmjou et al. (2012) reported that the pure water flux of a TiO2 nanoparticle-modified polyethersulfone ultrafiltration hollow fiber membrane was about 60 L/(m2·h). Zhang et al. (2014) reported that a nanostructured TiO2 hollow fiber photocatalytic membrane exhibited the membrane flux of 87.7 L/(m2·h). Especially, a porous yttria-stabilized zirconia hollow fiber membrane, although without the photocatalytic property, has been reported to show a extremely high pure water permeate flux of 10.89 m3/(m2·h·bar) at a transmembrane pressure difference of 1.5 bar (Zhang et al. 2015). Motivated by this finding above, we expect that the permeate flux of TiO2 hollow fiber membranes could still be enhanced by optimizing the hollow fiber formation process. Therefore, in the present work, the TiO2 hollow fiber membrane was prepared by dry-jet wet-spinning and dip-coating, and the photocatalytic activity of this membrane was evaluated using humic acid (HA) as a test substance.
MATERIALS AND METHODS
Chemicals and materials
Ceramic powder and polyethersulfone (PESf) were provided by Shanghai Kaidu Industrial Development Co., Ltd. 1-Methyl-2-pyrrolidinone (NMP), tetra-n-butyl titanate ([CH3(CH2)3O]4Ti), ethanol (C2H5OH), and diethanolamine (NH(C2H4OH)2) were obtained from Tianjin Fuyu Fine Chemical Co., Ltd. HA was provided by MP Biomedicals, Inc. (Eschwege, Germany).
Preparation of ceramic hollow fiber membranes
The TiO2 hollow fiber membranes were obtained by coating TiO2 on the ceramic hollow fiber membranes using a dip-coating process. A TiO2 colloidal solution was prepared by adding 17 mL of tetra-n-butyl titanate and 4.5 mL of diethanolamine to 67 mL of absolute ethyl alcohol, and continuously stirring for 2 h. Subsequently, a mixture of 10 mL anhydrous ethanol and 1 mL pure water was added, and continuously stirred for 1 h (Yu et al. 2001). A dip coater (SYDC-100, Shanghai SAN-YAN Technology Co., Ltd, China) was used for coating TiO2 on the ceramic hollow fiber membranes. The dipping time was set to 15 min and the lifting speed was 1 mm/s. In this study, the impregnation of ceramic hollow fiber membranes with TiO2 solution was repeated 5, 7 or 9 times. The final products were calcined at 500 °C for 2 h with a heating/cooling rate of 2 °C/min.
where and are the permeate and the feed concentrations of HA, respectively.
RESULTS AND DISCUSSION
Although the membrane shows attractive prospects for water treatment, the fouling often restricts its practical application. The degree of membrane fouling could be investigated by monitoring the permeate flux variation. As shown in Figure 12, the fouling of the TiO2 hollow fiber membrane could be demonstrated by the changes in permeate flux of HA without and with UV irradiation. In the case of without UV irradiation, the permeate flux of HA declined from 290 initially to 85 L/(m2·h) after 30 min, suggesting the cake layer of HA formed due to surface adsorption by filtration alone. However, the permeate flux reached 250 L/(m2·h) with UV irradiation, indicating the coating of TiO2 was beneficial to resist fouling, due to its distinguished photocatalytic ability to mineralize HA, ensuring the attractive performance of the TiO2 hollow fiber membrane.
A TiO2 hollow fiber membrane was fabricated by a controlled wet-spinning process, followed by a subsequent dip-coating process. It is shown that the optimized fabrication process is very effective in controlling membrane performance. In this study, using the TiO2 hollow fiber membrane, the pure water flux reached 4,450 L/(m2·h). The permeate flux was significantly enhanced in the presence of UV irradiation, and increasing the UV light intensity resulted in increased pure water flux. Meanwhile, the TiO2 hollow fiber membrane was applied for treating an aqueous HA solution. Compared with the 31.4% rejection of HA by filtration alone, 83.8% of HA removal efficiency was achieved. The HA removal was improved under UV irradiation, likely due to enhanced performance of filtration along with photocatalytic degradation of HA simultaneously. In addition, the use of TiO2 hollow fiber membranes provided a better reduction of membrane fouling in the presence of UV light irradiation. Considering the high permeate flux and enhanced performance, the TiO2 hollow fiber membranes may find various applications in water treatment.
This work was supported by the National Natural Science Foundation of China (51478075) and the Natural Science Foundation of Liaoning Province of China (2014020149).