Template-etched sodium alginate hydrogel as the sublayer to improve the FO performance with double barriers for high metal ion rejection.

• Novel forward osmosis membrane with porous hydrogel sublayer. • Formation of double barriers for high rejection of heavy metal ions. • Sublayer promoting the formation of denser and smoother FO membrane active layer. • Enhanced forward osmosis and antifouling performance. In the current research, preparation of a forward osmosis membrane that can effectively reject heavy metals in wastewater treatment would prove to be highly beneficial. Here, an innovative forward osmosis (FO) membrane with a "sandwich" structure (formed via the template method) was achieved for the first time through the fabrication of a porous hydrogel sublayer. A porous hydrogel matrix with excellent water retention capacity was formed through the crosslinking reaction of sodium alginate and calcium chloride in the presence of zinc oxide (ZnO) nanoparticles, followed by etching to remove the ZnO nanoparticles in the sublayer to produce the porous structure. The water flux of the modified membrane was quintuple compared with the that of the original membrane, with no reduction on the salt rejection performance. The surface polyamide layer and sublayer acted as double barriers and achieved a rejection rate of over 98% for heavy metal ions (Cd2+, Cu2+, Pb2+, Ni2+), when the glucose solution was used as the draw solution. The inclusion of the porous sublayer can provide deeper insights into the fabrication of high-performance FO membranes, and expand their applications. [ABSTRACT FROM AUTHOR]