Double-blade casting technique for optimizing substrate membrane in thin-film composite forward osmosis membrane fabrication

The thin-film composite (TFC) methodology has been explored in the last couple of years to fabricate forward osmosis (FO) membranes. Different tactics have been proposed to mitigate the internal concentration polarization (ICP) phenomenon in the substrate membrane layer. However, such modifications on the substrate membranes would likely alter their top surface morphology and this in turn, would profoundly influence the subsequent process of interfacial polymerization (IP) to form the active layer. In the current work, we presented a facile substrate membrane fabrication strategy - double-blade casting technique, to produce substrate membranes with enhanced structural features to mitigate ICP, yet retaining an ideal top surface for the formation of an intact and highly salt-rejecting active layer. A series of standard protocols have been utilized to characterize the substrate membranes and resultant TFC-FO membranes. Overall the resultant TFC-FO membranes based on the double-blade casted polysulfone substrate membranes showed improvement in water flux, Jv with reduced apparent structural parameter, S values and retained a relatively low reverse salt flux/water flux, Js/Jv ratio. With a 1 M NaCl draw solution and DI water feed, the best TFC-FO membranes achieved a Jv of 31.1 LMH and a Js of 8.5 gMH in the FO orientation, and a Jv of 60.3 LMH and aJs of 17.6 gMH in the PRO orientation. This concept demonstration study may open up many new platforms for flat-sheet substrate membrane fabrication for FO membranes.