Design Strategies for Forward Osmosis Membrane Substrates with Low Structural Parameters—A Review

Abstract

This article reviews the many innovative strategies that have been developed to specifically design the support layers of forward osmosis (FO) membranes. Forward osmosis (FO) is one of the most viable separation technologies to treat hypersaline wastewater, but its successful deployment requires the development of new membrane materials beyond existing desalination membranes. Specifically, designing the FO membrane support layers requires new engineering techniques to minimize the internal concentration polarization (ICP) effects encountered in cases of FO. In this paper, we have reviewed several such techniques developed by different research groups and summarized the membrane transport properties corresponding to each approach. An important transport parameter that helps to compare the various approaches is the so-called structural parameter (-value); a low -value typically corresponds to low ICP. Strategies such as electrospinning, solvent casting, and hollow fiber spinning, have been developed by prior researchers—all of them aimed at lowering this -value. We also reviewed the quantitative methods described in the literature, to evaluate the separation properties of FO membranes. Lastly, we have highlighted some key research gaps, and provided suggestions for potential strategies that researchers could adopt to enable easy comparison of FO membranes.

In‐situ generated ZnO nanoparticles for enhanced performance of thin film nanocomposite membrane in forward osmosis process

“…According to Blanco's theory 31 and Ren's research, 32 polymer‐lean phase growth and coagulation have been boosted by decelerating phase inversion, eventuated in the sizeable finger‐like pores. It is accepted that the ICP phenomenon can be reduced in more hydrophilic and porous support layers 33 . The cross‐sectional SEM images of modified support layers evidently reveal that the PSf 1%‐in‐situ has wider pores than the others, resulting in enhanced water transport and ultimately facilitating the water flux passing through the membrane.…”

“…It is accepted that the ICP phenomenon can be reduced in more hydrophilic and porous support layers. 33 The cross-sectional SEM images of modified support layers evidently reveal that the PSf 1%-in-situ has wider pores than the others, resulting in enhanced water transport and ultimately facilitating the water flux passing through the membrane.…”

Beyond Symmetry: Exploring Asymmetric Electrospun Nanofiber Membranes for Liquid Separation

“…The porous support layer, despite being bypassed by the selective layer, impedes the contact of the latter with the perturbed draw solution, amplifying this concentration imbalance. [ 87 ]…”

“…[83] The topography and transport resistance of the substrate substantially influences mass permeation and diffusion actions within the selective layer. [84] This resistance can be significantly reduced on account of the specific characteristics of the electrospun nanofibrous support layer, which exhibits low tortuosity and an open interconnected pore structure. Thus, the electrospinning technology can effectively facilitate practical nanofibrous design and reduce mass transfer resistance in the process mentioned.…”

“…The porous support layer, despite being bypassed by the selective layer, impedes the contact of the latter with the perturbed draw solution, amplifying this concentration imbalance. [87] The structural parameter S of the FO membrane defines the average length of water molecules transported across the membrane, which can intuitively measure the severity of the ICP phenomenon in the FO membrane. [88] The S parameter is proportional to the membrane bulk tortuosity 𝜏 (positively), thickness L (positively), and bulk porosity 𝜖 (negatively) as in Equation 2).…”