Elsevier

Journal of Membrane Science

Volume 499, 1 February 2016, Pages 269-281
Journal of Membrane Science

In situ immobilization of silver nanoparticles for improving permeability, antifouling and anti-bacterial properties of ultrafiltration membrane

https://doi.org/10.1016/j.memsci.2015.10.055Get rights and content

Highlights

  • Modification of UF membrane by PDA deposition and in situ reduction of AgNPs.

  • Immobilization of AgNPs on membrane surface as well as top layer cross section.

  • Improved water flux and antifouling property of AgNPs-PDA/PSf membrane.

  • Excellent anti-bacterial and anti-biofouling property of AgNPs-PDA/PSf membrane.

Abstract

In this work, silver nanoparticles (AgNPs) were in situ immobilized on polysulfone (PSf) ultrafiltration membrane via polydopamine (PDA) deposition and in situ reduction of silver ammonia aqueous solution (Ag(NH3)2OH). The adhesive and reductive PDA layer existing on the membrane surface induced the reduction of Ag+ on the site of surface without surface pore blockage, and also favored to the firm attachment and uniform distribution of AgNPs onto the membrane. The diameter and density of the immobilized AgNPs were regulated by changing [Ag(NH3)2]+ ion concentration, which exerted an influence on membrane hydrophilicity and permeation flux. The prepared AgNPs-PDA/PSf membranes were characterized by X-ray photoelectron spectroscopy (XPS), scanning election microscope (SEM), atomic force microscopy (AFM), water contact angle measurement and thermogravimetric analysis (TGA). The results indicated that AgNPs were immobilized on the surface as well as the top layer cross section of the membranes. The increase of Ag+ ion concentration led to an increase in the size and density of AgNPs anchored onto the membrane. Compared with PSf membrane, AgNPs-PDA/PSf (20 mM) membrane showed higher water flux, slower flux decline rate during BSA solution filtration and higher flux recovery ratio after simple water flushing. The AgNPs-PDA/PSf (20 mM) membrane possessed excellent stability with little release of silver during the filtrate operation or static immersion in water for 12 days. Besides, AgNPs-PDA/PSf (20 mM) membrane displayed excellent anti-bacterial and ani-biofouling properties against E. coli and B. subtilis.

Introduction

As an efficient separation technology, ultrafiltration (UF) process has been widely used in chemical, pharmaceutical and food industries [1]. UF membranes with surface pore sizes ranging from 1 to 100 nm is the most important component during the filtration process since it can remove proteins, bacteria and organic particles from a liquid stream. Polysulfone (PSf), polyethersulfone (PES) and polyvinylidene fluoride (PVDF) are commonly used as commercial UF membrane materials owing to their excellent chemical and thermal stability. However, the inherent hydrophobic property of these materials often causes the serious organic fouling and biofouling of the prepared membrane, which are induced by physical or chemical interactions between membrane surface and macromolecules or microorganisms in separation solution [2], [3]. The adhesion of foulant on membrane surface would diminish or block membrane surface pores, leading to a sharp decrease in permeation flux. Besides, the fouling would also deteriorate membrane structure and shorten the membrane lifetime. Therefore, UF membranes with high permeation flux, excellent antifouling and anti-biofouling properties are urgently required for their widespread application.

Several studies were carried out to develop UF membranes with better separation performance [4], [5], [6], [7], [8]. As is well-known, hydrophilic surface modification is an effective method to reduce membrane fouling since the hydrophilic surface could repel the foulant adsorption through repulsive hydration force. Thus, surface modification techniques including surface coating and chemical grafting have been developed to effectively improve membrane hydrophilicity and antifouling property [6], [7], [9], [10].

Compared with other kinds of fouling, biofouling is considered as a bigger obstacle to the broad application of membrane technology. The biofouling is much more difficult to be cleaned due to the self-replicating nature of organisms [11]. In general, the main strategies to control biofouling include anti-adhesion approach for reducing initial adsorption of bacteria, and anti-bacterial approach for suppressing the activity of attached organisms. Silver has a strong antimicrobial potential toward several types of bacteria but low toxicity to human cells [12], [13]. Antimicrobial effect of silver can be increased by manipulating size at nanolevel, and silver nanoparticles (AgNPs) having size in the range of 10-100 nm were reported to have strong bactericidal potential against both Gram-positive and Gram-negative bacteria [14], [15]. Recently, AgNPs are directly incorporated into the casting solution to fabricate the blended membrane with biofouling resistance, which could effectively kill the attached bacteria and prevent their growth [11], [12], [13], [16], [17], [18]. The reported studies have proved the feasibility of AgNPs for providing membrane anti-biofouling property. However, the incorporated AgNPs are mostly embedded in membrane matrix and some of them would be leached out during the filtrate operation owning to their poor compatibility with polymer [12], [13]. The study of Zodrow et al. [13] incorporated AgNPs into the casting solution to fabricate the membranes with microbial control. About 10% Ag loss from the surface was detected after a filtration period, which caused a significant loss of anti-bacterial activity. Besides, it was stated that potential approaches to be explored included encapsulating AgNPs in a polymer and then covalently binding it to membrane polymers either directly or through the use of cross-linkers [13]. In the study of Zhang et al. [18], different amounts of bio-Ag0 were embedded in PES membrane using phase-inversion method to improve the anti-bacterial activity. Surface modification of membranes by using AgNPs deposition was also investigated to solve the biofouling problem of separation membranes [19], [20]. In the study of Ben-Sasson et al. [19], in situ formation of AgNPs was performed by directly reducing AgNO3 on TFC RO composite membrane, and a reduction (up to 17%) in water permeability of modified membrane was observed due to the surface blockage by AgNPs precipitates on the membrane. The study of Park et al. [20] developed sustainable anti-biofouling UF membrane by covalently immobilizing colloidal AgNPs onto PVDF membrane mediated by a thiol-end functional amphiphilic block copolymer linker. The reported procedure was a little complicated including AgNPs synthesis, thiolated PVDF membrane modification and AgNPs assembly. As is well-known, permeation flux is an important property for the application of UF membrane. Thus, it is necessary to develop a facile and versatile strategy for immobilizing AgNPs onto the membrane to achieve high permeation flux as well as excellent antifouling properties after the surface modification.

Recently, dopamine (DOPA) has gained great attention as a promising bio-inspired material for membrane modification since it can spontaneously polymerize under alkaline condition to form a thin polydopamine (PDA) film [21], [22], [23]. Polydopamine is not only a hydrophilic material due to the existence of catechol, quinine and amine groups, but also a bio-inspired polymer having similar capability to mussel’s adhesive foot protein [24]. The entire process of DOPA polymerization is based on the oxidative self-polymerization of dopamine (2-(3,4-dihydroxy-phenyl) ethylamine) onto surfaces with an intermediate step that formed 5,6-dihydroxyindole after oxidation and structural rearrangement [25]. The formed PDA film could firmly adhere to a wide variety of solid materials mainly through covalent bonding and hydrogen bonding [26], thereby usually was used as an efficient and applicable method for surface modification. Moreover, it was reported that dopamine could reduce Ag+ into zero-valent silver (Ag°) by catechol group, resulting in the formation of AgNPs-PDA composite [27], [28], [29]. The study of Sureshkumar et al. [29] investigated the AgNPs immobilization on the surface of PDA pre-modified substrate, such as polyethylene, glass and poly(methyl methacrylate), and the results indicated that the modified substrates exhibited effective anti-bacterial and biocompatible properties.

Several researches have been conducted by using PDA or nanoparticles/PDA film for surface modification of polymeric separation membranes [30], [31], [32], [33], [34]. McCloskey et al. [33] and Cheng et al. [34] successfully utilized PDA to modify PSf and PES UF membranes. The results indicated that PDA could firmly adhere to membrane surface and effectively improve surface hydrophilicity. Besides, since PDA can easily stick to solid materials, it could be served as the “bio-glue” to provide a versatile platform for nanoparticles deposition on membrane surface. Shao et al. [35] designed a facile surface modification via PDA deposition and subsequent hydrolysis of ammonium fluotitanate to obtain TiO2/PDA/PVDF membrane. With the aid of PDA film, TiO2 can be homogeneously distributed and firmly adhered on the membrane surface due to the coordination bond formed between TiO2 and PDA. Although the PDA or nanoparticles/PDA modification could improve membrane antifouling or anti-biofouling properties, a decrease in permeation flux was often observed for the modified membranes, probably due to the blockage of surface pores.

In this work, a facile and effective method for AgNPs immobilization was provided in the field of UF membrane modification, with obtaining stable attachment of AgNPs on membrane surface and preventing the surface pore blockage. With this method, the modified membrane could probably show high permeation flux as well as excellent antifouling and anti-biofouling properties. To achieve this aim, commercial PSf UF membrane was firstly modified by the deposition of PDA layer, and then AgNPs was immobilized on PDA-coated membrane via in situ reduction of silver ammonia aqueous solution (Ag(NH3)2OH), as depicted in Fig. 1. PDA layer is important for the modification of UF membrane as well as the in situ immobilization of AgNPs on porous UF membrane. Firstly, PDA is a hydrophilic material due to the existence of catechol, quinine and amine groups, and thus could offer a certain hydrophilic modification for UF membrane. Secondly, PDA is a bio-inspired polymer having adhesive capability. The [Ag(NH3)2]+ would be preferentially absorbed onto PDA layer due to the metal-binding ability of phenolic hydroxyl groups of PDA [36], and then reduced into AgNPs by catechol groups of PDA, resulting in the formation of AgNPs on the site of surface without surface pore blockage. Moreover, the AgNPs could be firmly immobilized due to the “bio-glue” function of PDA layer. Thus, with the aid of adhesive and reductive PDA layer, AgNPs could be effectively attached and immobilized onto membrane without surface pore blockage, which offers the possibility for improving antifouling property and permeation flux simultaneously. Through regulating the modification parameters, the diameter and density of immobilized AgNPs were also expected to be regulated, which probably exerted an influence on membrane hydrophilicity and permeation flux. The surface chemical composition, surface and cross-section morphology, and hydrophilic property of the modified membranes were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and dynamic water contact angle measurement. UF experiment were conducted to analysis the influence of AgNPs immobilization on the permeation flux and antifouling property of the membranes. AgNPs stability on AgNPs-PDA/PSf membrane was also investigated by detecting the dissolved Ag in the permeate solution or the soak solution using inductively coupled plasma mass spectrometry (ICP-MS). Finally, E. coli and B. subtilis as-representing Gram-negative and Gram-positive bacteria were used to evaluate the anti-bacterial and anti-biofouling properties of the modified membranes.

Section snippets

Materials

Polysulfone (PSf) flat ultrafiltration membrane was purchased from Vontron Technology of China. Tris (hydroxymethyl) aminomethane, 3-hydroxytyramine hydrochloride (98%, dopamine) and polyvinylpyrrolidone (PVP, K30) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Glucose was obtained from Real&lead Chemical Co., Ltd. (Tianjin, China). Silver nitrate, ammonium hydroxide and sodium hydrogen sulfite were purchased from Jiangtian Chemical Technology Co., Ltd. (Tianjin, China).

Chemical characterization of the membranes

The dopamine polymerization mechanism involves the oxidation of catechol in dopamine to quinone by alkaline pH-induced oxidation, and finally resulting in polydopamine [21].

Fig. 2 shows FTIR spectra of PDA powder, PSf membrane and the modified membranes. The characteristic absorption bands corresponding to PDA can be observed at 1610 cm−1 (aromatic rings stretching vibrations and N–H bending vibrations) and 3400 cm−1 (catechol –OH groups and N–H groups) [34]. The characteristic absorption bands

Conclusions

In this work, a facile method was developed for fabricating AgNPs-PDA/PSf membrane to improve surface hydrophilicity, permeation flux, antifouling and anti-biofouling properties of UF membrane. Surface chemical composition analysis and morphological characterization confirmed that AgNPs are immobilized on both membrane surface and top layer cross section. Compared with PSf membrane, the pure water flux of AgNPs-PDA/PSf (20 mM) membrane was increased from 248 to 336 L m−2 h−1. The AgNPs-PDA/PSf

Acknowledgment

This research was supported by National Natural Science Foundation of China (No. 21306130), Science & Technology Pillar Program of Tianjin (No. 10ZCKFSH01700) and Program of Introducing Talents of Discipline to Universities (No. B06006).

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