Improving fouling resistance of thin-film composite polyamide reverse osmosis membrane by coating natural hydrophilic polymer sericin
Introduction
During the past two decades, reverse osmosis (RO) process, which uses polymeric semi-permeable membranes to achieve molecular separation, has become the main technology for the desalination of saltwater, the production of drinking and ultra-pure water and the reclamation of wastewater [1], [2]. The dominated reverse osmosis membrane in current use is thin-film composite (TFC) aromatic polyamide membrane, which is usually fabricated by forming a dense aromatic polyamide barrier layer on a porous polysulphone support through interfacial polarization of an aromatic polyamine with one or more aromatic polyacyl halides [3]. Despite its advantages such as high water permeability and ion rejection, resistance to pressure compaction, wide operation temperature and pH ranges, and high stability to biological attack, the TFC polyamide RO membrane faces a major challenge of fouling, which is mainly due to the buildup of the material being rejected and manifests itself as a decline in flux with operation time [4], [5].
It is known from literatures that foulants can adsorb to the membrane surface through hydrophobic interaction, hydrogen bonding, van der Waals attraction, Lewis acid–base interaction and electrostatic interaction [6], [7], and the surface characteristics of the TFC polyamide reverse osmosis membrane such as hydrophilicity, roughness and charge are known to be strongly related to fouling [8], [9]. Membranes with a smooth hydrophilic surface of similar charge to the foulant seem to possess good antifouling property [10], [11], [12], [13]. Therefore, many research efforts have been devoted to modify the surface characteristics of the TFC polyamide RO membrane through coating hydrophilic materials so as to improve the antifouling property.
For example, neutral polymer polyvinyl alcohol (PVA) was coated on the surface of TFC polyamide reverse osmosis membrane by Kim et al. [14]. It was found that the modified membrane had a smooth surface with decreased surface negative charge and exhibited improved fouling resistance in treating dyeing wastewater. The coating of synthetic polyether–polyamide block copolymer on the surface of TFC polyamide reverse osmosis membrane was conducted by Louie et al. [15]. It was reported that the protective coating layer resulted in a smooth, neutral and hydrophilic surface with improved fouling resistance, but a declined flux. In consideration of the fact that most of the thin-film composite polyamide membrane surfaces are negatively charged, polyethyleneimine (PEI) was coated on the surface of TFC polyamide reverse osmosis membrane by electrostatic self deposition [16]. The surface charge was effectively reversed from negative to positive after modification and the modified membrane exhibited good fouling resistance to cationic foulants as the result of the electrostatic repulsion. In our previous studies [17], [18], hydrophilic thermo-responsive copolymers poly(N-isopropylacrylamide-co-acrylic acid) and poly (N-isopropylacrylamide-co-acrylamide) were used to modify commercial TFC polyamide RO membrane through surface coating technique. It was found that the modified membrane had a more hydrophilic surface of thermo-responsive property and showed not only improved antifouling property but also enhanced physical cleaning efficiency. More recently, the deposition of polyelectrolytes on the surface of TFC polyamide reverse osmosis membrane was performed by Ishigami et al through layer-by-layer assembly [19]. The membrane surface became more hydrophilic and smoother after modification and showed reduced fouling when filtrated with bovine serum albumin (BSA) aqueous solution.
In summary, the surface characteristics of the commercial TFC polyamide reverse osmosis membrane could be effectively modified for improved fouling resistance through coating a thin surface layer of hydrophilic material. However, the coating layer tended to offer additional resistance to water permeation and/or to decrease the membrane surface negative charge, and thereby decreasing water permeability and/or salt rejection. Therefore, research interest still remains in the modification of commercial thin-film composite polyamide reverse osmosis membranes for improved antifouling and separation properties through coating a surface layer of functional material.
Accordingly, in this study, the natural hydrophilic polymer sericin, a water-soluble globular protein having polar side groups of hydroxyl, carboxyl and amino groups [20], was used to modify the commercial thin-film composite polyamide reveres osmosis membrane for improved antifouling property through surface coating followed by cross-linking with glutaraldehyde (GA). Membrane surface chemical structure, morphological structure, hydrophilicity and charge were characterized to analyze the changes that resulted from the deposition of sericin layer. The permeation properties of the unmodified and sericin-coated TFC polyamide reverse osmosis membranes were evaluated through cross-flow permeation tests by investigating the pure water permeability and salt permeability coefficient. Additionally, cross-flow fouling experiments with bovine serum albumin (BSA) aqueous solution were also carried out to investigate the antifouling properties of the membranes in terms of the time-dependant flux, cumulative volume of the permeate produced and hydraulic resistance to water permeation.
Section snippets
Materials
A commercial flat-sheet thin-film composite polyamide reverse osmosis membrane was obtained from the Development Center of Water Treatment Technology (Hangzhou, China). It was manufactured through interfacial polymerization of m-pheylenediamine with trimesoyl chloride on a reinforced polysulfone porous substrate. The membrane had a cross-linked polyamide barrier layer, as shown schematically in Fig. 1.
Natural polymer sericin (average Mw = 10,000 g/mol, chemical structure schematically shown in
Membrane surface properties
In this study, surface modification of TFC polyamide reverse osmosis membrane was carried out through dip-coating sericin aqueous solution followed by cross-linking with glutaraldehyde (GA). The natural polymer sericin adsorbed to the membrane surface mainly by hydrogen bonding and was insolublized through in situ cross-linking with GA. The coating of sericin layer on the membrane surface was confirmed by ATR-FTIR analysis. As shown in Fig. 3, the deposition of sericin layer on the surface of
Conclusions
In this work, modification of the commercial TFC polyamide RO membrane for improved fouling resistance was carried out with the natural polymer sericin through surface coating technique. The following conclusions can be drawn from the experimental results.
- (1)
The application of sericin surface coating layer on the commercial TFC polyamide reverse osmosis membrane resulted in improved surface hydrophlicity, smoothed surface morphology and enhanced negative charge, which were desirable for the
Acknowledgments
The authors gratefully acknowledge the financial supports of the National Nature Science Foundation of China (NNSFC) (Grant No. 21276242), the National High-tech R&D Program of China (863 Program) (Grant No. 2012AA03A601), 521 Personnel Training Plan of Zhejiang Sci-Tech University and Zhejiang Provincial Key Innovation Team (No. 2010R50038).
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