Fabrication of novel poly(amide–imide) forward osmosis hollow fiber membranes with a positively charged nanofiltration-like selective layer

Hollow fiber membranes with a positively charged nanofiltration (NF)-like selective layer have been successfully fabricated by utilizing asymmetric microporous hollow fibers made of Torlon® polyamide–imide (PAI) material as the porous substrate followed by polyelectrolyte post-treatment using polyethyleneimine (PEI), which has not been reported previously. The reaction between PAI and PEI has been confirmed by experiments. The membrane structure and surface property are carefully tailored by adjusting polymer dope composition, spinning conditions and the post-treatment parameters. These membranes are, subsequently, characterized by a series of standard protocols in terms of membrane morphology, structure, permeability and salt rejection, and are applied in forward osmosis (FO) processes. The newly developed PAI hollow fiber membranes show a pure water permeability of 2.19–2.25 L/m2 h bar and reasonable NaCl and MgCl2 rejections of 49% and 94% at 1 bar, respectively. It is also found that in FO processes, when using 0.5 M MgCl2 as a draw solution and DI water as the feed in the active layer facing feed water (AL-facing-FW) configuration at 23 °C, the water fluxes of two PAI FO hollow fiber membranes are 8.36 and 9.74 L/m2 h, respectively, and the JS/JV of the two membranes is smaller than 0.4 g/L, which is lower than the data of 0.85 g/L for commercial FO membrane from Hydration Technologies Inc. (HTI). Different from a neutral membrane, the positively charged FO membrane provides double electric repulsions to the salt transfer through the membrane in the AL-facing-FW configuration, leading to a reduction of salt penetration, while in the active layer facing draw solution (AL-facing-DS) configuration, the positive charges facilitate salt transportation. This study provides a simple and effective approach to make hollow fibers suitable for FO applications