| Forward osmosis (FO) is a new membrane separation technology developed in recent years. It has great prospects in the fields of desalination and water treatment etc. with features of low energy consumption and non-pollution. However, the further applications of this technology are limited by the decrease of actual FO water flux due to internal concentration polarization (ICP) ocurres in FO membrane. Therefore, reducing or even eliminating ICP is an approach for the improvement of FO separation efficiency. Thin film composite (TFC) FO membrane is commonly used in FO. ICP is related to the material properties (e.g. hydrophilicity and roughness) and the structural properties (e.g. thickness and porosity) of TFC FO membrane, and the material and the structural properties of TFC FO membrane are influenced by the membrane fabrication conditions.In this paper, Polyamide-Polysulfone (PA-Psf) TFC FO membrane is used as the researching objective. During the studies, the single variable controlled method was conducted and series of PA-Psf TFC FO membranes with different material and structural properties were fabricated by adjusting the membrane fabrication conditions including the polyethylene glycol 400 (PEG-400) concentration in the membrane casting solution, the N-methyl-2-pyrrolidone (NMP) concentration in coagulant bath and the casting thickness of membrane casting solution. The material and structural properties of membranes fabricated were measured and characterized using the equipment such as contact angle goniometer and scanning electron microscopy (SEM), and the separation properties and FO performances of the membranes were evaluated through the ultrafiltration and FO experiments. Through analyzing the obtained experimental data systematically, the formation mechanism of the distinctions in membrane structure and separation properties in FO were investigated, the correlation of membrane fabrication condition, the structural and separation properties of membrane as well as ICP were explored and established. The results of the studies would provide data and theoretical support to the further optimization and application of PA-Psf TFC FO membrane.The results showed that the variation of TFC FO membrane fabrication condition parameters have significant effects on the membrane structure and FO performances. Adding proper amounts of PEG-400 into membrane casting solution could improve the TFC FO membrane structure and properties to some extent. When the PEG-400 concentration was 6 wt.%, the membrane fabricated had a high FO water flux of 47.4 L/m2·h. Adding NMP in coagulant bath resulted in a tendency for finger-like pores to be more and longer in membrane, while pores on the membrane bottom surface tended to shrink. When NMP concentration was 3 wt.%, the membrane fabricated had the best separation performances in FO. The increase of the casting thickness of membrane casting solution lead to a great thickness increase of sponge-like pore layer in membrane, an increase of ICP and a significant drop of membrane water flux in FO. When the casting thickness was 100 μm, the membrane fabricated had the highest water flux and best separation performance in FO. In addition, reducing ICP and improving TFC FO membrane performances in FO could be achived through lessening the thickness of TFC FO membrane support layer, enhancing membrane hydrophilicity, enlarging pore size within membrane, increasing open pore area percentage at membrane bottom surface as well as increasing the portion of finger-like pores in membrane etc.. |
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