An anti-fouling hybrid membrane was prepared by incorporating Ag-SiO2 nanohybrid into a polysulfone (PSf) matrix. The addition of Ag-SiO2 can significantly improve the hydrophilicity, separation property, anti-fouling ability, and especially anti-bacterial activity of hybrid membranes. The optimum performance of the Ag-SiO2/PSf hybrid membrane is achieved when the concentration of Ag-SiO2 is as low as 0.45 wt%. Compared with PSf membrane and SiO2/PSf hybrid membrane, the Ag-SiO2/PSf hybrid membrane displays the best overall properties. The excellent performance of the Ag-SiO2/PSf hybrid membrane can be attributed to the well-tailored structure and unique property of Ag-SiO2 nanohybrid, where nanosized Ag (∼5 nm) can densely and uniformly disperse on the surface of silica spheres. The obtained membrane could be a promising material for water treatment.
Membrane separation technology is increasingly applied in water treatment, thanks to the advantages of high efficiency, easy operation, and low cost. During the lengthy separation process, membrane fouling is an inevitable problem causing deterioration in the membrane performance, such as a reduction of flux rate, an elevated operation maintenance cost, and a decrease in the extent of membrane survival (Drews 2010; Zhang et al. 2012). Membrane fouling is usually defined as the undesirable deposition of retained particles, colloids, macromolecules, salts, etc. at the membrane surface or at the pore wall inside the pores (Rana & Matsuura 2010). Generally the fouling can be classified into three major categories: organic fouling, inorganic fouling and biofouling (Kappachery et al. 2010). Among them, biofouling is regarded as the most complicated one, as a result of the self-replicating nature of biofouling organisms. Even at a low nutrient concentration, a tiny amount of bacteria can grow and multiply rapidly, giving rise to a difficult elimination (Flemming et al. 1997). The main approaches to control biofouling include anti-adhesion and anti-microbial methods (Huang et al. 2012). Developing a hydrophilic membrane surface is a useful anti-adhesion approach to reduce initial adsorption of bacteria. Regarding an anti-microbial approach, some biocides can be incorporated into the membrane and endow it with the ability to effectively kill bacteria attached on the membrane surface and also prevent their growth.
It has been proved that incorporating inorganic nanoparticles into a membrane matrix is a convenient and useful method for the enhancement of anti-fouling properties (Wang et al. 2005; Jeong et al. 2007; Lind et al. 2009; Qiu et al. 2009; Liao et al. 2010; Weng et al. 2011; Celik et al. 2011a, 2011b; Majeed et al. 2012). Some hydrophilic inorganic nanoparticles, such as carbon nanotube, graphene oxide, and zeolite, help the hydrophilicity and, correspondingly, anti-adhesion ability of membranes (Wang et al. 2005; Choi et al. 2006; Jeong et al. 2007; Lee et al. 2008; Lind et al. 2009; Qiu et al. 2009; Liao et al. 2010; Sun et al. 2010; Wu et al. 2010, 2012, 2013a, 2013b; Celik et al. 2011a, 2011b; Majeed et al. 2012); moreover, some specific nanoparticles that exhibit inhibitory and bactericidal effects, directly contribute to the anti-microbial ability of membranes. Especially, silver has attracted much interest due to its high toxicity toward many types of bacteria and low toxicity for humans and animals (Tenover 2006). Various membranes such as polysulfone (PSf), polyethersulfone, polyamide, polyacrylonitrile, chitosan, and cellulose acetate decorated with silver have been fabricated for performance improvement (Chou et al. 2005; Taurozzi et al. 2008; Zodrow et al. 2009; Basri et al. 2010, 2012; Cao et al. 2010; Zhang et al. 2012, 2013; Huang et al. 2012, 2014; Yin et al. 2013; Liu et al. 2013). Furthermore, Mollahosseini et al. (2012) found silver nanoparticle size had a significant effect on the anti-bacteriality of membrane and the hybrid membrane with smaller silver nanoparticles had better properties. Silver nanoparticles of less than 10 nm were more effective for anti-biofouling (Gunawan et al. 2011). However, silver nanoparticles commonly produced by chemical reduction usually encounter a serious problem in that nanoparticles are less stable and tend to aggregate when the average particle size is less than 40 nm. This definitely reduces improvement in membrane performance (Mafune et al. 2000; Jiang et al. 2006).
To overcome the problems mentioned above and thus achieve better membrane performance, Ag-SiO2 nanohybrid was prepared and then used to functionalize polymer membrane. Yu et al. (2013) observed maximal water flux (∼150% higher than that of pure PES membrane) and a good antibacterial effect of Ag-SiO2/polyethersulfone (PES) hybrid membrane prepared at the nanohybrid concentration of 2.68% in the casting solution. Huang et al. (2014) focused on the anti-bacterial and anti-formation properties of Ag-SiO2/PES hybrid membrane, and found the membrane with 2% Ag-SiO2 nanohybrid displayed the best overall performance.
Herein, the microstructure of nanohybrid has a significant effect on the membrane performance; the concentration of nanohybrid is also crucial because a high addition of nanohybrid could easily have some negative effect, such as particle aggregation, membrane structure change, and deterioration of membrane properties. Thus, in this study, the structure of Ag-SiO2 was precisely tailored to take full advantage of this nanohybrid and exploit a systematic research. It should be emphasized that Ag with a particle size of approximately 5 nm can disperse densely and uniformly on the surface of a silica sphere (∼150 nm). The unique structure of Ag-SiO2 nanohybrid can not only effectively reduce the aggregation of nanoscale-sized Ag, but also result in an extremely high hydrophilicity of hybrid particles. PSf is used as the membrane matrix in this study, because it has been widely used as an ultrafiltration membrane material in many industrial fields due to its low cost, superior film-forming ability, good mechanical and anti-compaction properties, and strong chemical and thermal stabilities. Subsequently, low concentrations of Ag-SiO2 nanohybrid (0.15 wt% ∼ 0.75 wt%) were introduced into the PSf matrix to study the improvement of hybrid membrane performance. Transmission electron microscopy (TEM) with energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and UV-vis were employed for the characterization of Ag-SiO2. Then, separation properties including pure water flux and rejection, anti-fouling and antibacterial behaviors were investigated to gain a deep insight into the influence of Ag-SiO2 morphology and loading on the membrane performance. Furthermore, Ag releasing behavior was also tested for the stability of silver in a hybrid membrane.
Commercial PSf, Udel P3500, was purchased from Boji Co., Ltd. Polyvinylpyrrolidone (PVP), tetraethyl orthosilicate (TEOS), aqueous ammonia (25% w/w), and N-methyl-2-pyrrolidone (NMP) were bought from Aladdin Co., Ltd. C2H5OH, NaOH, silver nitrate (AgNO3), and bovine serum albumin (BSA) were purchased from Sinopharm Chemical Reagent Co., Ltd and used as received. Egg albumin with an average molecular weight of 45,000 g/mol was used as a probe molecule for rejection tests and supplied by Sinopharm Chemical Reagent Co., Ltd.
Preparation of inorganic particles
Synthesis of silica particles (SiO2)
To a flask, 50 mL of EtOH, 1 mL of H2O, and 3 mL of aqueous ammonia were added. A volume of 1.5 mL of TEOS was rapidly added to the flask and magnetically stirred (300 rpm) for 3 h at 40 °C. The SiO2 was harvested by centrifugation, washed with copious ethanol, and then dispersed in ethanol at 2–3% silica concentration for subsequent use.
Synthesis of Ag-SiO2 nanohybrid
Ag-SiO2 nanohybrid was synthesized as follows (Deng et al. 2007). A 10 mL quantity of freshly prepared [Ag(NH3)2]+ ion solution was quickly added to 10 g of SiO2/EtOH solution under magnetic stirring at room temperature for 1.5 h. Then, 50 mL of ethanol containing PVP (0.5 mmol/L) was added for stabilization and reduction, and the mixture was stirred at 70 °C for 7 h. The product was collected by centrifugation and finally dried.
The PSf-based hybrid membranes were prepared by incorporating Ag-SiO2 nanohybrid via phase inversion method. The detailed procedure of membrane preparation can be obtained from our recent work (Wu et al. 2012). First, an approximately 15 wt% PSf homogeneous solution with a given amount of Ag-SiO2 nanohybrid (the particle concentrations in the casting solution varied from 0.15 wt% to 0.75 wt%) was prepared. Next, the mixture solution was cast onto a glass plate and immersed in deionized (DI) water at a temperature of 30 °C for at least 24 h. The thickness of the membrane was approximately 100 μm.
Particles were observed by transmission electron microscope (Hitachi H-600). XRD patterns were acquired by a D8 ADVANCE and DAVINCI.DESIGN (Bruker) X'pert diffractometer with Cu Kα radiation. The size distribution of the particles was measured by dynamic light scattering (DLS) using a Zetasizer Nano measurement. UV-visible absorption spectra were recorded using a UV-visible spectrophotometer (Hitachi U-2910).
The surfaces and cross-sections of membrane were observed using a scanning electron microscope (SEM; TESCAN 5136MM) and high-resolution FE-SEM S-4800 equipped with energy-dispersive spectrometer (TEM-QUANTAX 400, Bruker). Cross-sectional membrane samples were obtained by previous freeze fracturing after immersion in liquid nitrogen.
The static contact angle of water on the surface of a polymer membrane was measured by using OCA15 (Dataphysics Co., Germany) for determination of the hydrophilicity. The average value of the contact angle on each polymer membrane was calculated using at least five different locations on each membrane.
Flux and separation experiments
where Cp and Cf are the concentrations of the permeation and feed solutions, respectively. The concentrations of the feed and permeate solutions were determined by an ultraviolet–visible spectrophotometer (Hitachi U-2910) at 205 nm. The results shown were calculated from at least three membranes.
Anti-organic fouling and anti-biofouling test
Measurements for organic fouling resistance of membrane were performed as follows (Wu et al. 2012). First, distilled water passed through the membrane for 60 min at an operation pressure of 0.2 MPa, and the water flux (J1) reached a stable stage. Second, feed solutions (1.0 mg/mL BSA solution, pH was kept at 7.0 with 0.1 M phosphate buffer solution) were filtrated for 60 min at 0.2 MPa, and the flux for protein solution (Jp) was determined. After ultrafiltration of BSA solution, the membranes were simply washed by immersing them into DI water for 15 min under stirring, then the pure water flux of cleaned membranes (J2) was measured again. Several ratios, including flux recovery ratio (FRR), total fouling ratio (Rt), irreversible fouling ratio (Rir), and reversible fouling ratio (Rr), were defined to describe the organic fouling resistance of membranes to analyze the fouling process, and calculated as follows: FRR = J2/J1, Rt = 1−Jp/J1, Rir = (J1−J2)/J1, Rr = (J2−Jp)/J1, Rt = Rir + Rr, respectively.
The anti-biofouling ability of prepared membranes was evaluated by inhibition zone method against Escherichia coli. Before the test, E. coli strain was first cultured in a flask, and then the prepared E. coli solution was pipetted onto a plate and spread throughout the surface. A circular disk of each membrane (d = 19 mm) was placed on the bacterial surface to incubate for 24 h at 37 °C. After that, the inhibition rings that formed around the membranes could serve as an indicator for antibacterial performance, and were visually observed with a digital camera.
Silver releasing test
The releasing behavior of silver nanoparticles from the Ag-SiO2/PSf hybrid membrane was investigated. Hybrid membrane with 0.45 wt% of Ag-SiO2 was chosen for the test. The membrane was cut into 1 cm2 pieces, and the samples were immersed in 10 mL deionized water at room temperature. The water was replaced every 24 h and collected. Finally, all the samples were acidified by 1% HNO3 and analyzed by atomic adsorption spectrometer (Hitachi Z-5000).
RESULTS AND DISCUSSION
Characterization of Ag-SiO2 nanohybrid
Effect of Ag-SiO2 loading on the membrane properties
Characterization of Ag-SiO2/PSf hybrid membrane
For further investigation of the advantages of incorporation of Ag-SiO2 on membrane performance, pristine PSf membrane and SiO2/PSf hybrid membrane were prepared and compared with Ag-SiO2/PSf hybrid membrane in terms of water flux, rejection to egg albumin, contact angle, and anti-fouling ability, respectively. Herein, the contents of inorganic particles in the casting solution are both 0.45 wt%.
Ag-SiO2 nanohybrid was prepared, and then employed to prepare anti-fouling PSf-based hybrid membrane. In the structure of Ag-SiO2, nanosized Ag (∼5 nm) densely and uniformly disperse on the surface of silica spheres, and this fine microstructure contributes to the high hydrophilicity and anti-bacterial ability of nanohybrid. The obtained Ag-SiO2 could significantly improve the hydrophilicity, separation property, organic fouling resistance ability, and especially anti-bacterial activity of Ag-SiO2/PSf hybrid membrane. The results indicate that the hybrid membrane with a low content of Ag-SiO2 (0.45 wt%) displays the best overall properties, which have obvious advantages over PSf membrane and SiO2/PSf hybrid membrane.
The present study was supported by the National Natural Science Foundation of China (No. 11372108), the Natural Science Foundation of Hunan Province, China (No. 14JJ5021), Open Fund Project Innovation Platform of University in Hunan Province, China (No. 20161006), Open Fund Project Innovation Platform of State Key Laboratory of Molecular Engineering of Polymers (Fudan University) (K2017-29) and High-level talents support plan of Xiamen University of Technology (No. YKJ16001R).