Structure, property and performance correlation in RO membranes

In this study, thin and durable thin film composite (TFC) polyamide membranes were prepared on mesh-reinforced polysulfone (PSF) supports for forward osmosis (FO) applications. To understand the influence of mesh-incorporation on the formation of support structures and the final FO performance, mesh-embedded PSF supports were prepared via the phase inversion process from various PSF casting solutions in 1-methyl-2-pyrrolidinone (NMP) solvent (9–18 wt%) by using a polyester (PE) mesh as a mechanical reinforcing material. A crosslinked aromatic polyamide layer was then fabricated on top of each support to form a TFC membrane. For the mesh-embedded PSF supports prepared with relatively low concentration casting solutions (9 and 12 wt%), the PE mesh was oriented towards the bottom of the PSF supports, creating additional macroscopic pores around the mesh lines on the backside surface. However, the PE mesh was moved towards the center of PSF supports when higher viscosity solutions (15 and 18 wt%) were used for the support preparation, and consequently the macroscopic pores near the mesh lines were isolated within the support layers without exposing to the bottom surface. The PE-mesh also adversely affected the formation of a finger-like morphology for higher concentration casting solutions (≥12 wt%), forming a sponge-like morphology in the lower half of the support layers. A desirable support structure with a well-developed finger-like morphology and macroscopic open-pores on the bottom surface was achieved from the mesh-embedded PSF support prepared with 9 wt% casting solution (PSF9). The corresponding TFC membrane (TFC/PSF9) thus exhibited the lowest structural parameter (S=314±29 µm) among the tested samples and presented outstanding FO performance, almost 2.5 times higher water flux (49 LMH) with lower reverse salt flux (7.0 GMH) compared to a commercial CTA membrane in PRO mode evaluation.