Significant roles of substrate properties in forward osmosis membrane performance: A review

Abstract

Forward osmosis (FO) has gained considerable interest as an attractive membrane-based desalination technology due to its reversible fouling and potentially low costs. Thin-film composite (TFC) membranes, composed of a microporous substrate and a selective polyamide layer, have been widely used for FO. Compared with the substrate in pressure-driven TFC membranes (e.g., reverse osmosis and nanofiltration membranes), the substrate in TFC FO membranes has a more significant impact on membrane performance due to its extra effect on the membrane structural parameter that determines internal concentration polarization (ICP). Many researchers have not differentiated the substrate surface properties from the substrate bulk properties, resulting in misleading conclusions. This review clarifies the individual roles of the substrate surface and the substrate bulk in TFC FO membrane performance. We analyze how the substrate surface properties, such as the surface hydrophilicity, porosity, pore size, pore size distribution and pore number density, impact the PA layer formation during IP, thereby influencing the PA layer permeability/selectivity and thus FO performance. We also evaluate how the substrate bulk properties, such as the bulk porosity, tortuosity and pore structure, affect the membrane structural parameter, thereby influencing ICP and thus FO membrane performance. A desirable substrate surface should have reasonably high hydrophilicity (e.g., water contact angles of 40°–60°), high porosity, small pore size, narrow pore size distribution and high pore number density. A desirable substrate bulk should have high porosity, low tortuosity and interconnected pore structures, thereby minimizing the membrane structural parameter and thus ICP. This review highlights the significant roles of the substrate surface and bulk properties in FO membrane performance. It provides significant insights into developing all kinds of high-performance TFC membranes (e.g., FO, reverse osmosis and nanofiltration) by tailoring the substrate properties.