Investigating the impact of TFC membrane structure and compaction on performance in hypersaline brine desalination via high-pressure reverse osmosis

Recent works on high-pressure reverse osmosis (HPRO, applied pressure ΔP ≥ 120 bar) seek to understand the impact of compaction on thin-film composite (TFC) polyamide membranes. However, previous studies have primarily focused on commercial membranes to identify key traits for HPRO operation, leaving a gap in guidance on how to fabricate resilient TFC membranes, particularly in terms of the support and polyamide selective layers. In this work, we synthesized four types of TFC membranes with customized support and polyamide structures to determine the optimal composite design for HPRO operation. Our results indicate that a TFC membrane with a dense polyamide layer (featuring low protuberances and a high degree of crosslinking), synthesized atop a sponge-like support layer (17 wt% Polysulfone), exhibits greater resistance to compaction (denoted as TFC-17-L_protub). In SWRO test (ΔP: 55 bar, 35 g/L NaCl feed), the TFC-17-L_protub membrane demonstrated a water permeability of 0.80 Lm⁻²h⁻¹bar⁻¹ with 99.2 % salt rejection. This performance decreased to 0.39 Lm⁻²h⁻¹bar⁻¹ with 98.6 % salt rejection during the desalination of hypersaline brine via HPRO (ΔP: 150 bar, 70 g/L NaCl feed). Post-compaction analysis showed a compression of the support layer's surface, with a 28 % reduction in pore diameter and a 54 % decrease in cross-sectional thickness. In contrast, the polyamide surface morphology and cross-sectional height remained unchanged at approximately 190 nm. This study enhances the understanding of the compaction behavior of TFC membranes under high pressure and explores the potential benefits of incorporating an HPRO stage in the context of SWRO brine management.