Surface modification of ultrafiltration membranes by grafting glycine-functionalized PVA based on polydopamine coatings
Introduction
Due to severe water shortages, membranes have been used increasingly in water purification and wastewater treatment. However, membrane fouling is the primary issue that impedes their application. Surface hydrophilic modification is a popular method used to improve the antifouling performance of membranes [1], [2], [3]. Many physical and chemical modification methods can be used to alter the characteristics of membrane surfaces and to improve the performances of these surfaces [4]. These methods include blending with a surface-modification additive, coating or adsorption, a chemical reaction with a contacting solution, plasma or corona treatment, and surface grafting after high-energy irradiation [5].
During a study inspired by biological adhesion, dopamine helped reduce oil/water emulsion-induced fouling because it produces a firm adhesive coating on solid surfaces [6], [7], [8]. In 2007, Lee et al. reported that in an aerobic aqueous solution, dopamine could form very thin surface-adherent polydopamine (PD) films on a solid surface through self-polymerization [9]; consequently, a PD film formed through a self-polymerization of dopamine under aerobic conditions [10]. Recently, a new method has been developed to prevent membrane fouling, gaining significant attention during studies of water purification by membranes [11], [12]. However, some researchers reported that PD coatings were not as effective as they expected [13], [14].
PD coatings on membrane surfaces containing reactive groups can provide a versatile platform for secondary reactions and can immobilize functional molecules with specific functional groups, such as NH2 or SH [15]. Consequently, some researchers attempted to graft hydrophilic material onto the membrane surface to improve its hydrophilicity and to increase its antifouling abilities. Jiang et al. prepared a hydrophilic membrane by immersing polyethylene (PE) porous membranes into aqueous dopamine solutions before heparin was covalently immobilized onto the reactive PD layer [16]. The results indicated that these modifications significantly suppressed the adhesion of platelet, enhancing the anticoagulation activity of the membranes. Zhu et al. immobilized bovine serum albumin (BSA) onto porous polyethylene (PE) membranes while using strongly attached PD as a spacer [17]. Their results revealed that the hydrophilicity of PE membrane improved significantly after coating with the PD and binding with the BSA. In addition, amine-terminated polyethylene glycol (mPEG-NH2), which is strongly hydrophilic, was used to improve the membrane surface functionalization after applying the PD coating. Bryan et al. coated the surfaces of reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF) membranes with PD before grafting them with poly(ethylene glycol) (PEG) [7], [18]. The membranes modified with PD exhibited a systematic reduction in protein adhesion [19], and the PD-g-PEG-modified membranes were slightly more resistant toward fouling when filtering oily water [20].
PVA is widely to modify the surfaces of membranes due to its high hydrophilicity, excellent chemical properties, thermal stability and low cost [21], [22], [23]. This material is produced through the polymerization of vinyl acetate and the hydrolysis of polyvinyl acetate, which replaces the ester groups with hydroxyl groups via ester exchange. Commercial PVA can be classified based on the extent of hydrolysis or hydrolysis degree as fully hydrolyzed (97.5–99.5%) and partially hydrolyzed (87–89%) materials. The hydrolysis degree of PVA significantly affects its water solubility. Partially hydrolyzed PVA is more hydrophobic than the fully hydrolyzed material because it still contains ester groups. The partially hydrolyzed PVA dissolves readily in water at ambient temperatures, while fully hydrolyzed PVA is soluble only at high temperatures. Due to its good solubility in water, the partially hydrolyzed PVA was suitable for coating membrane surfaces during hydrophilic modifications [22], [24], [25], [26].
Because mPEG-NH2 can be immobilized on the PD layer on membrane surface through nucleophilic addition [7], [9], we proposed that glycine-functionalized PVA could be grafted to a membrane surface coated with PD, thus improving the hydrophilicity of the membrane surface. The glycine-functionalized PVA could be prepared through an esterification reaction between PVA and amino acid while using an inorganic acid as a catalyst [27], as shown in Fig. 1.
In this paper, glycine-functionalized PVA, which was synthesized through an esterification between PVA and amino acids, was used to modify the surfaces of PES ultrafiltration membranes. Polyethersulfone (PES), which has good chemical resistance, thermal stability, mechanical and film-forming properties, is broadly manufactured for industrial applications. However, the intrinsic poor hydrophilic property limited the performance and application of PES membranes. When PES UF membranes were used in water treatment, unavoidable water flux decrease and fouling occurred in filtration. The glycine-functionalized PVA was grafted on the membrane surface by the covalent bonds between NH2 group and the coated PD layer. The membrane surface was characterized using Fourier transform infrared (FT-IR) spectrometry and surface morphology analysis to confirm that the PVA layer was present. The membrane modified with mPEG-NH2 was compared to glycine-functionalized PVA in this investigation. An oil/water emulsion was separated to test the antifouling ability of the PD-g-PVA-modified membrane.
Section snippets
Materials
Glycine was used as the amino acid during this study. The glycine, dimethyl sulfoxide (DMSO), PVA (1788, molecular weight 77,000, hydrolysis degree 88%), Trizma hydrochloride (i.e., tris buffer), alcohol and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. The dopamine hydrochloride and mPEG-NH2 (CH3O(CH2CH2O)nCH2CH2NH2, MW = 10,000 Da, CAS: 80506-64-5) were purchased from Aladdin (Shanghai, China). The acetone and isopropanol (IPA) were obtained from Lingfeng Chemical Co.,
FTIR analysis of PVA and glycine-functionalized PVA
The FTIR spectra of the net PVA and glycine-functionalized PVA are shown in Fig. 2. The peaks corresponding to the typical OH and CH (2910–2942 cm−1) bands were obvious in the spectra of the pure PVA [26]. The adsorption peaks at approximately 1708 cm−1 and 1631 cm−1 are more obvious in the glycine-functionalized PVA than the net PVA, which can be attributed to the formation of the COC and CO groups after the reaction between the hydroxyl group of the PVA and glycine [21]. The strength of the peak
Conclusions
The characteristics and performance of PES UF membranes modified with PD-g-PVA were investigated in this paper. The presence of glycine-functionalized PVA grafts on the PD-modified membrane was confirmed using FT-IR and pure water flux measurements. During the AFM analysis, the roughness of the PD-g-PVA-modified membrane surface decreased obviously after it was modified. The proposed structure of glycine-functionalized PVA layer immobilized on the membrane surface is horizontal. A significant
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant No. 51478099, 51108266) and the Fundamental research Funds for the Central Universities ((Grant No. 2232015A3-04).
References (42)
- et al.
Development of antifouling reverse osmosis membranes for water treatment: A review
Water Res.
(2012) - et al.
Novel nanocomposite Kevlar fabric membranes: Fabrication characterization, and performance in oil/water separation
Appl. Surf. Sci.
(2014) - et al.
Application and modification of poly(vinylidene fluoride) (PVDF) membranes - A review
J. Membrane. Sci.
(2014) - et al.
Exploring the synergetic effects of graphene oxide (GO) and polyvinylpyrrodione (PVP) on poly(vinylylidenefluoride) (PVDF) ultrafiltration membrane performance
Appl. Surf. Sci.
(2014) - et al.
Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation
J. Membr. Sci.
(2013) - et al.
Influence of polydopamine deposition conditions on pure water flux and foulant adhesion resistance of reverse osmosis, ultrafiltration, and microfiltration membranes
Polymer
(2010) - et al.
A facile strategy to enhance PVDF ultrafiltration membrane performance via self-polymerized polydopamine followed by hydrolysis of ammonium fluotitanate
J. Membr. Sci.
(2014) - et al.
Short-term adhesion and long-term biofouling testing of polydopamine and poly(ethylene glycol) surface modifications of membranes and feed spacers for biofouling control
Water Res.
(2012) - et al.
Impact of feed spacer and membrane modification by hydrophilic, bactericidal and biocidal coating on biofouling control
Desalination
(2012) - et al.
Surface modification of PE porous membranes based on the strong adhesion of polydopamine and covalent immobilization of heparin
J. Membrane. Sci.
(2010)