The solution-diffusion model for water transport in reverse osmosis: What went wrong?

Reverse osmosis (RO) stands as the state-of-the-art desalination technology, owing to its high energy efficiency and low cost. Further improvements in the performance of this technology require a fundamental understanding of transport mechanisms in RO membranes. For decades, the solution-diffusion model has been the prevalent approach to describe water transport mechanism in RO membranes. In this model, water first partitions into the membrane and then diffuses down a concentration gradient of water within the RO membrane. However, recent experimental and theoretical findings pose serious challenges to the fundamental assumptions of this theory. In this perspective, we discuss seven critical flaws in the solution-diffusion model and explain why the model fails to describe water transport in RO membranes. Instead, through our careful analyses, we determine that the pore-flow model, in which a pressure gradient drives water flow through membrane pores or interconnected free volumes, represents the true mechanism for water transport. We conclude by highlighting the need for research efforts to better understand the frictional interactions between water, salt, and membrane and their impact on water transport. Overall, a fundamental understanding of the water transport mechanism will inform the development of next-generation RO desalination membranes.