Forward Solute Transport in Forward Osmosis Using a Freestanding Graphene Oxide Membrane

A graphene oxide membrane (GOM) has the potential to be used in forward osmosis (FO) because it has a high water permeability and low reverse salt flux. To explore suitable applications, we initiated the investigation of the forward solute transport through a freestanding GOM in FO. Both uncharged solutes (PEG 200 and PEG 1000) and charged solutes (NaCl, MgSO4, and MgCl2) were investigated, and the forward solute flux in FO was tested. The Donnan steric pore model (DSPM) was utilized to calculate the forward solute flux of the freestanding GOM in FO when discussing diffusion, convection, and electromigration. Our results showed that the freestanding GOM has a better separation performance for multivalent ions than the monovalent ions in the FO mode. We found an information gap between the calculated and experimental forward solute flux values, especially when charged solutes were used in the feed solution and the electrical double layer (EDL) was thick. We propose that the EDL inside the GOM has a screening effect on the forward ion transport during FO, even in the presence of relatively high water flux. According to our analysis, the forward solute transport for charged solutes is governed by steric exclusion and interfacial Donnan exclusion as well as EDL screening along the nanochannels inside the membrane. Our study provides guidance for the future use of the freestanding GOM during FO for water and wastewater treatment.