FBE- Çevre Bilimleri Mühendisliği ve Yönetimi Programı- Doktora
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ÖgeDevelopment of high-performance and chlorine-resistant thin-film composite membranes with zwitterionic surfaces for seawater desalination(Institute of Science And Technology, 2019-01-02)Thin film composite (TFC) membranes are usually produced by the interfacial polymerization method. The TFC fabrication method allows one to use different polymers as active and support layers. A TFC membrane with a polyamide active layer provides a good separation barrier for reverse osmosis (RO) and can, therefore, be used for seawater desalination. However, these membranes allow only moderate water fluxes and the polyamide active layer can be damaged when it is exposed to chlorine. This thesis describes the fabrication of novel TFC membranes by using zwitterionic monomers (reactants with both positively and negatively charged functional groups) during the interfacial polymerization process for the production of the membrane active layer. In theory, oppositely charged salts in the feed should be repelled electrically by the fixed ionic groups of a zwitterionic active layer. Furthermore, zwitterionic polymers are highly hydrophilic and thus provide an ideal pathway for water. This thesis aims give a deeper insight into the mentioned effects of zwitterionic groups on membrane performance and desalination technology. It will be argued that the zwitterionic properties have a positive effect on salt rejection, water flux and chlorine resistance. After an introduction into early and recent TFC membrane development efforts in the first chapter of this thesis, the research work is presented in the following two chapters. The second chapter describes the fabrication of the optimal support layer by the phase inversion technique. Commercial RO membranes with high performance for seawater desalination usually have polyamide active layers, which are produced on top of a compaction resistant polysulfone support layer by interfacial polymerization with m-phenylenediamine (MPD) in aqueous and trimesoyl chloride (TMC) in organic phase. In this work, the impact of the support layer pore size on the polyamide active layer polymerization was investigated and the fabrication conditions were optimized. Therefore, six different TFC membrane types with support layers having average pore sizes ranging from 18 nm to 120 nm were fabricated on which the active layers were polymerized. The third chapter of the thesis describes the use of the zwitterionic functional trialkoxysilane monomer (3-sulfopropylbetaine-propyl)-trimethoxysilane (SPPT) as an additional monomer to MPD in the aqueous phase. An interpenetrating polysiloxane-polyamide network is polymerized by using these two monomer for interfacial polymerization with TMC in the organic phase. To fınd the optimal fabrication conditions, membranes were produced at different monomer concentration ratios and compared to control membranes prepared without silane monomers. Several characterization methods were used including Fourier transform infrared (FTIR) spectroscopy, contact angle measurements, streaming zeta potential measurements of the membrane surface and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Cross flow RO tests were performed under seawater desalination conditions (3.2% NaCl feed; 55.2 bar operating pressure) and salt rejection was calculated from permeate conductivity. Chlorination experiments were performed under the same conditions but with addition of 500 ppm active chlorine to the feed. The SEM micrographs in the second chapter show that the polyamide ridge-and-valley structure is more pronounced for active layers of TFC membranes prepared with support layers having larger pores. Cross-flow RO tests reveal that the salt rejection of polyamide TFC membranes systematically increases from 80.5% to 99.0% with decreasing support layer pore size. Convective monomer transport during interfacial polymerization is discussed as a possible reason behind the formation of ear- and ridge-like protuberances, of which the latter can apparently be damaging to the inner active layer. In the third chapter, SEM micrographs of the polysiloxane-polyamide hybrid membranes fabricated with the additional zwitterionic monomer reveal that at high SPPT concentrations the ridge-and-valley structure of the polyamide active layer appears to be filled out with another polymer that has a brittle structure. In the EDS analysis of the scanned areas, the amount of silicon in the modified membranes was measured above the value of the polyamide membrane. The silicon contents of the membranes prepared with higher SPPT/MPD ratios indicate thus a higher incorporation of polysiloxane into the polyamide layer. Comparison of streaming zeta potential measurements display a pH-independent behavior of the zwitterionic membranes due to their strongly acidic and basic functional groups. Membranes produced with a SPPT/MPD ratio of 1 to 10 exhibit an increase in permeate flux from 25 L m-2 h-1 to 33 L m-2 h-1 when compared to the control membrane prepared without SPPT, which is an increase of 31%. Furthermore, salt rejection is not compromised, but slightly increases from 98.8% to 98.9%. On the other hand, the highest resistance to chlorine is observed for membranes produced with a SPPT/MPD ratio of 1 to 1. The proposed electrostatic forces induced by the incorporated zwitterionic groups apparently affect the free volume in the active layer and the repulsion of mobile ions. By using an in-situ polymerization method as in this work, the zwitterionic side groups can be produced not only on the surface but also inside the active layer. It is postulated that the ionic solvation of free water molecules induced by the incorporated zwitterionic groups creates a more hydrophilic pathway for water and a larger free volume in the polyamide network. As a consequence, a higher permeate flux is observed in the modified membranes. Furthermore, the diffusion path of solute ions through the membrane is longer due to electrostatic repulsion between the feed solution and the zwitterionic groups in the active layer. Ion shielding effects that would undermine the electrostatic repulsion are not expected on the permeate side of the active layer, because when solutes diffuse to the permeate side of the membrane, the ionic strength of the permeating solution becomes lower. Even though the free volume of the hybrid polymer is expected to be larger, a high salt rejection can be maintained due to the elongated diffusion path of sodium and chloride ions. Furthermore, an increased rejection of charged chlorine species that are present at high pH, such as hypochlorite (ClO-) and trichloride (Cl3-) is hypothesized. This effect is thought to shield chlorination of the underlying polyamide layer and thus improve the resistance of the membrane material against chlorine. This thesis proposes for the first time the use of trialkoxysilane coupling reagents in interfacial polymerization to produce polyamide-polysiloxane hybrid networks. A review of the literature showed that neither zwitterionic functional trialkoxysilanes, nor other functional trialkoxysilanes had been used as reactants for interfacial polymerization before. Because modification of the TFC membrane fabrication method resulted in the incorporation of zwitterionic polysiloxane polymers inside the polyamide active layer, it is fundamentally different from other methods that involve a coating layer on top of the active layer. An additional coating would result in an additional resistance to membrane flux. In contrast to these methods, the modified membranes produced in this work exhibit a higher flux at certain SPPT/MPD ratios. They also show a significant change in the membrane's active layer morphology, surface properties and chlorine resistance. The lack of a correlation between the streaming zeta potential of the membrane surface and the pH of the streaming solution is, furthermore, a strong indication for the successful integration of the zwitterionic groups into the membrane active layer.