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Abstract: Delamination-free ultrafiltration (UF) dual layer hollow fibers have been successfully fabricated by dry jet-wet spinning technique. Two different polymers of poly(amide-imide) (PAI) and polyethersulfone (PES) were utilized as the selective layer on the outer side and the porous support layer on the inner side, respectively. The dual layer substrates were subsequently modified to develop a nanofiltration (NF)-like skin on the outer layer by polyethyleneimine (PEI) cross-linking followed by multilayer polyelectrolyte depositions. These membranes were characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were applied in FO process. Experiments reveal that the dual-layer NF hollow fiber modified by polyelectrolyte cross-linking and depositions can achieve pure water permeability of 4.1l/m2.h.bar and a high rejection to MgCl2 up to 97%. In FO process, the dual-layer hollow fibers exhibited a water flux of 39 and 21lm−2 h−1 in the orientations of active layer facing draw solution and feed water, respectively, by using 0.5M MgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. It suggests that the combined treatment of polyelectrolyte cross-linking and deposition is an effective way to form a NF-like selective layer for FO dual-layer hollow fiber membranes. [Copyright &y& Elsevier]

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]

Abstract: Forward osmosis (FO) has attracted wide interest for its many potential applications. Recent developments suggest that double-skinned FO membranes may perform superiorly over conventional single-skinned ones. In this design, a dense rejection skin facing the draw solution is responsible for the separation, while a second skin facing the feed solution is designed to prevent foulants entering into the porous support layer sandwiched between the two skins. For the first time, an analytical model is developed for double-skinned FO membranes and is verified experimentally. Model simulations suggest that an optimal double-skinned membrane shall have a feed skin similar to that of a low rejection nanofiltration membrane to minimize the overall hydraulic resistant and to reduce internal concentration polarization. On the other hand, the optimization of the draw skin calls for a compromise between its water permeability and solute rejection. FO water flux is greater for support layers with higher mass transfer coefficients. In addition, it decreases with decreasing draw solution concentration and increasing feed solution concentration. While membrane orientation plays an important role for applications with low feed solution concentrations (with the denser skin preferentially facing the draw solution), this effect becomes negligible at high feed solution concentrations. [Copyright &y& Elsevier]