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In this study, the glutaraldehyde (GA) crosslinking was adopted in combination of layer-by-layer (LBL) polyelectrolyte deposition to develop a novel hollow fiber NF membrane with low-pressure water softening capability for synthesized hard water containing divalent counterion SO 4 2− . The challenges for the new membrane include (a) the presence of SO 4 2− in the feed water, which tends to deteriorate positively charged NF membrane retention to divalent ions; (b) high salinity feed water up to 10,000 ppm total dissolved salt (TDS); (c) 2 bar operating pressure that requires the membrane to have a good Mg 2+ /Na + selectivity and less mass transfer resistance in the substrate; and (d) better separation performance as compared with the state-of-the-art and commercial NF membranes. Based on the membrane characterization results, the possible structure of the deposited layers and the effect of GA crosslink was proposed. It was found that the crosslinking could tighten the membrane surface pores with increased hydrophilicity while the membrane surface charge was reduced. Therefore, mainly based on the size exclusion, the crosslinked LBL*1.5C membrane showed more than 10 LMH/bar permeability with above 95% rejection to each of 1000 ppm MgCl 2 , MgSO 4 and Na 2 SO 4 solution under 2 bar pressure. It was also proved to be more suitable for the low-pressure water softening application at 2 bar in all tested feed water with the presence of SO 4 2− and TDS up to 10,000 ppm in terms of both permeability and hardness removal efficiency when compared with commercial NF 270 and NF 90 membranes. [ABSTRACT FROM AUTHOR]

The layer-by-layer (LBL) polyelectrolyte deposited membranes have drawn increasing attention in various applications due to the ease of selective layer formation and their stability and versatility. In this study, the LBL deposition was performed at the inner surface of the polyethersulfone (PES) hollow fiber substrate to form composite nanofiltration (NF) membrane. The semi-dynamic deposition procedure was adopted with the aid of syringes. The newly developed inner deposited (id-LBL) membranes were then tested in NF and forward osmosis (FO) applications and the performance were compared with outer surface deposition as well as some literature data. The id-LBL membranes could not only withstand higher operating pressure but also possess superior hardness rejection especially in high concentration mixed salt solutions (more than 95% rejection to Mg 2+ and Ca 2+ in a 5000 ppm total dissolved salt (TDS) mixture under 4.8 bar). As for the FO process, with only two layer deposition, the id-LBL membranes also demonstrated significant performance improvement with increased water flux (up to 70 L/m 2 h using 0.5 M MgCl 2 as draw solution in active layer facing draw solution configuration) and reduced salt leakage (around 0.5 g/m 2 h using 1 M MgCl 2 draw solution in active layer facing feed water configuration). This study suggests that for hollow fiber substrate, the inner surface is more suitable for the formation of the selective layer via LBL deposition than the outer surface. [ABSTRACT FROM AUTHOR]

Abstract: The layer-by-layer (LBL) polyelectrolyte deposition was applied on polyethersulfone (PES) hollow fiber ultrafiltration membrane to prepare nanofiltration (NF) membrane for low pressure water softening application for the first time. The effects of polyelectrolyte type and molecular weight (MW), deposited bilayer number, polyelectrolyte solution pH and supporting electrolyte concentration on the performance of resultant membranes were studied thoroughly. Experiments revealed that the pH of the polyelectrolyte solution and supporting electrolyte concentration presented the most notable effects. For weak poly-cations, Poly(ethylenimine) (PEI) and Poly(allylamine hydrochloride) (PAH), the solution pH affected the membrane performance significantly, and high water permeability and divalent cation rejection were obtained at optimum pH values. Using a low concentration of supporting electrolyte at underneath layers was proved to be an effective way to increase water permeability while maintaining high salt rejection. A salt water flux (SWP) of 12LMH/bar and 94% Mg2+ rejection was obtained by depositing only two bilayers of PSS/PAH polyelectrolytes on the PES substrate, while for the 3000ppm mixed salt solution, the water permeability was 11LMH/bar with Mg2+ and Ca2+ rejection as high as 90%. These promising results demonstrate that LBL technique is potentially applicable for preparing low-pressure water softening hollow fiber membranes. [Copyright &y& Elsevier]