New insights into the interaction between surface-charged membranes and positively-charged draw solutes in the forward osmosis process

Abstract

In this study, a flat-sheet polyamide thin film composite (TFC) membrane with different surface charges has been designed as a forward osmosis (FO) membrane while a positively-charged organic chitooligosaccharide (COS) applied as a draw solution (DS). The flat-sheet TFC FO membrane consists of three layers (from top to bottom): the polyamide selective layer, electrospun cellulose triacetate (CTA) nanofibers and mechanically robust polyacrylonitrile (PAN) nanofibers coated with dopamine (DPA)/polyethyleneimine (PEI). The DPA/PEI modification on the PAN nanofibers has proven to be efficient and resulted in a more positively-charged surface with improved hydrophilicity. Meanwhile, the polyamide layer formed by the interfacial polymerization (IP) method was negatively charged on top of the FO membrane. The interaction between surface-charged membranes and positively-charged draw solutes in the FO process has been investigated comprehensively. The result showed that hydrophilicity and electrostatic repulsion could alleviate the fouling generated by organic DS, while hydrophobicity and electrostatic attraction could magnify the fouling. Finally, the positively-charged DS was recycled by the positively-charged PEI NF membranes with a reasonably high special water flux and salt rejections. Therefore, this study shows that the combination of the surface-charged membranes and positively-charged COS DS has the desirable water flux, little reverse salt flux (RSF) and fouling effects. In conclusion, the developed membrane is providing a new way to design practical FO processes.