Solution-Diffusion model for water transport in reverse osmosis- What went wrong?

The attached study critically examines the Solution-Diffusion (SD) model which has been traditionally used to explain water transport in reverse osmosis (RO) membranes. The authors argue that the SD model, which assumes water diffuses across the membrane driven by a concentration gradient, does not accurately represent the actual mechanisms at play in RO membranes. They advocate for the pore-flow model, which posits that water transport is driven by a pressure gradient across the membrane’s pores.

The SD model has been the prevalent framework for describing water movement in RO processes, involving water partitioning into the membrane and diffusing down its concentration gradient. However, recent experimental and theoretical findings have exposed significant flaws in this model:

1. **RO Membranes are Porous:** Contrary to the SD model’s assumption of nonporous membranes, advanced techniques like positron annihilation lifetime spectroscopy (PALS) and wide-angle X-ray scattering (WAXS) have shown that RO membranes have a network of sub-nanometer pores, making them porous.

2. **Clustered Water Molecules:** Instead of dispersed individual water molecules as assumed in the SD model, water tends to move in clusters through membrane pores, as demonstrated by molecular simulations.

3. **No Concentration Gradient:** The assumption of a concentration gradient driving water diffusion is questioned. Experimental and simulation data indicate a lack of such a gradient within the membrane.

4. **Misuse of Diffusion Concept:** The application of the diffusion concept in the SD model does not align with observed transport behaviors, which are more consistent with viscous flow.

5. **Independent Treatment of Water and Salt:** The SD model treats the transport of water and salt as independent processes, ignoring their interdependence and the frictional interactions between them.

The authors conclude that the pore-flow model, supported by the solution-friction model, more accurately describes the dynamics of water transport in RO membranes. This model acknowledges the role of pressure-driven flow through membrane pores and considers the interactions between water, salt, and the membrane material. By correcting these fundamental misunderstandings, the study calls for a shift in research focus towards developing RO membranes based on accurate transport mechanisms, which could enhance the efficiency and effectiveness of desalination technology.

Reference: Hanqing Fan, Mohammad Heiranian, Menachem Elimelech, The solution-diffusion model for water transport in reverse osmosis: What went wrong?, Desalination, Volume 580, 2024, 117575, ISSN 0011-9164,

Published on 25th March 2024