A critical review on porous substrates of TFC polyamide membranes: Mechanisms, membrane performances, and future perspectives

Reverse osmosis and nanofiltration membranes commonly adopt a thin film composite (TFC) structure featuring a polyamide rejection layer formed on a porous substrate. Recent studies have shown compelling evidence that the separation performance of TFC membranes is critically influenced by their substrates. Substrates not only affect the formation of polyamide rejection layers but also regulate the transport pathways of water and solutes through the membranes. This review provides a comprehensive summary of the effects of substrate pore structure and chemistry on the water permeance, selectivity, and fouling performance of the resulting TFC membranes. Membrane transport models and formation theories are critically assessed to evaluate the key mechanisms leading to improved membrane performance. In addition, membranes formed on substrates are compared to those formed at free interfaces to provide deeper insights into these mechanisms. The potential use of composite/reactive/gutter layer-coated substrates towards better membrane performance is also highlighted. The comprehensive and mechanistic insights gained in the critical review provide a solid basis to guide the future development and optimization of TFC polyamide membranes.