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Effect of Peg on Polysulfone Support and Reverse Osmosis Membrane Property

A study on the thin film composite on polysulfone support membranes prepared by different casting solution was conducted. The following conclusions can be drawn from this work, the membrane flux increased with increasing PEG concentration and the pore size and the porosity on the membrane surface became bigger by the increase PEG concentration; the permeability of thin film composite membrane was effected by the membrane continuity and the amount of polyamide formed within PSf membrane pores.


Introduction
Water resources become to a crucial problem directly related to the sustainable development of the society and economy after human civilization entering a new century [1]. Recently, reverse osmosis (RO) has become a promising water technology, utilizing the selective permeability of semi permeable membrane to eliminate impurity and the supply the clean water [2][3].
The typical RO TFC membranes consist of two parts [4][5]: (1) support substrate(50 m), the polysulfone (PSF) ultrafiltration membrane is the most common use for supporting, (2) thin barrier layer (200 nm), the desalination layer is generated during the interfaci--al polymerization (IP) interaction formed on the substrate. Researchers focus on the properties of thin barrier layer and neglect the effect of PSF substrate on the water permeability and salt rejection of RO membrane. Until recently, the importance of supporting membrane received more attention [6][7][8]. Ghosh et al reported that the pore structure and chemistry have a significant influence on interfacial polymerization reaction [7]. However, the manually preparation of PSF supporting membrane could no guarantee the identical casting speed and residence time nascent membrane both in air and coagulation bath [9][10].
In the paper, the preparation of PSF supporting membrane is precisely controlled by a semi-automatic film casting machine, the effect of concentration of PEG-600 on the structure and chemistry property of both supporting membrane and ultra-thin barrier layer additive was investigated, the optimal preparation parameters was confirmed which could provide valuable data and theory foundation on processes optimization of RO membrane.

Polysulfone (PSf) Membrane Formation
The PSf was measured and separated into 2 pieces and then dried at 125 °C for 3 hours to get rid of the moisture in the PSf particle. A piece of PSf was added into an airtight bottle in which a specified amount of DMF was in a stirring state at 25 °C. The solution was heated to 80 °C after PSf completely dissolved and the rest of the PSf was added and agitated for several hours. PEG was finally added into the above solution when the effects of the PEG additive were studied. The specific composition of casting solutions was provided in Table 1. The casting solution was kept still overnight for degassing. The membrane was formed by spreading the solution over the non-woven support fabric using semi-automatic film casting machine with a gap of 120 μm and then, the non-woven support was immediately immersed into a DI water bath. The PSf membrane was washed thoroughly and stored in the DI water.

TFC membrane formation
The PSf membrane was immersed into an aqueous solution contained MPD and other additives for 15 s. The excess aqueous solution was then eliminated from the skin surface via an air knife. After then, the membrane was immersed into the TMC-Isopar G solution for 15 s to initiate the polymerization reaction. The resulting TFC membranes were subsequently heat cured for 6 min at 90 °C. The TFC membranes were finally washed thoroughly to remove any residual solution from the surface by DI water.

Characterization of polysulfone and polyamide membrane
Separation performance of PSf membrane and TFC membrane was evaluated using a cross-flow filtration system with an effective membrane area of 70 cm2 at 25 °C. For the PSf membrane, the pure water flux was performed under the pressures of 0 and 0.3 MPa, respectively. For the TFC membrane, the flux and the selectivity were characterized by a 2000 ppm NaCl feed solution under 1.55 MPa. Salt rejections were determined from the conductivity of the feed and permeate water. The surface and cross-sectional morphology of membranes is visualized by scanning electron microscopy, SEM (Nova Nano SEM 430, FEI Company, Hitachi, Japan). The chemical structure of PA-TFC membranes was characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR).

Effect of PSf Concentration on PSf Membrane
The water flux changes of PSf membranes prepared with different PSf concentration are presented in Figure 2. It indicated that the flux of the PSf membrane decreased significantly with the increasing PSf concentration under both 0 MPa and 0.3 Mpa, the PSf-1 membrane was the most permeable membrane with the flux of 960 LMH (0 Mpa) and 2400 LMH (0.3 Mpa), whereas the PSf-4 membrane has the lowest flux values of 415 LMH (0 Mpa) and 1048 LMH (0.3 Mpa), respectively. As shown in SEM micrographs, a typical asymmetric membrane structure was observed in all the cross-sectional images, the top-layer of the membrane was dense and the sub-layer where macrovoids existed was sparse. Furthermore, the membrane structure became denser and the quantity of the macrovoids was suppressed with the increasing PSf concentration, the membrane changed to sponge-like structure with few macrovoids when PSf concentration reached 18%.The changes of the membrane structure and permeability could be attributed to the transport rates, the inflow rate of non-solvent (water) and the outflow rate of the solvent (DMF).  The flux changes of the PSf membrane after adding PEG-600 were presented in Figure 3. From the plot we could clearly observed that the membrane flux increased with increasing the concentration of PEG. The structure of membrane still showed a dense top-layer and sparse sub-layer feature. Moreover, the pore size and the porosity on the membrane surface became bigger by the increase PEG concentration. The structure changes of the membranes could also be considered as the result of the

Effect of PSf Membrane on TFC Membrane
The surface morphology and chemical composition of the TFC membrane on the PSf-1 support membrane are analyzed by SEM and ATR-FTIR as shown in Figure 3.(a) and (b). We could observe a nodular membrane surface under the experimental condition which is different from the classic ridge-and-valley surface features in the literatures. In order to further characterize TFC thin-film layer, ATR-FTIR spectroscopy was used to identify the main functional groups, indicated that a successful polymerization process had occurred. It clearly appeared that the PSf membrane has significant effect on the TFC membrane performance. According to Figure 6.(c), the TFC-5 membrane possessed the highest water flux 10.7 LMH with 94.8% salt rejection, TFC-1 membrane had the highest salt rejection 99.1% and the flux was just little lower than that of TFC-5. For the PSf membrane prepared with PEG, the aqueous solution diffuse into the surface pores more easily because of the lager pore size and hydrophilic. However, when the polymerization reaction began, MPD diffused slowly out of the pores due to favorable interactions with polar functional groups of PEG. As a result, amount of polyamide formed deeper within PSf membrane pores and no continuous and complete polyamide formed on the surface especially when the PEG was added to 15%. This factor increased the overall path length for water transport and caused the less salt rejection and water flux.

Conclusion
The concentration of PEG additive lead to a structural change from dense top-layer and sparse sub-layer feature to a symmetric, sponge type with no macro-void structure, which could be considered as the result of the different diffusion rates. The polyamide formed deeper within PSf membrane pores result in the increased the overall path length for water transport and the less salt rejection and water flux.

Acknowledgments
This work was financially supported by National Natural Science Foundation of China (Grant No. 51804240), Key Industrial Chain Project of Shaanxi Province (Grant No. 2017ZDCXL-GY-07-03).