Citation: Li Wang et al., Water transport in reverse osmosis membranes is governed by pore flow, not a solution-diffusion mechanism. Sci.Adv.9, eadf8488(2023). DOI:10.1126/sciadv.adf8488
For over fifty years, the widely accepted solution-diffusion (SD) model has been used to explain how reverse osmosis (RO) membranes function in desalination. This model assumes that water molecules dissolve and diffuse through the membrane based on a concentration gradient. However, a groundbreaking study led by Professor Menachem Elimelech and his team at Yale, in collaboration with researchers from the University of Wisconsin-Madison and Texas Tech University, has challenged this foundational understanding. Their findings, published in Science Advances, demonstrate that water transport in RO membranes is instead driven by pressure gradients, not by molecular concentration differences.
Challenging the Solution-Diffusion Model
The SD model, established in the 1960s, posited that water molecules dissolve into the polymeric membrane, separate, and diffuse from regions of high concentration to low concentration. A critical assumption of this model is that pressure remains constant across the membrane, an idea that Elimelech and his team found implausible. Their research showed that when water flows through any porous material, pressure must drop across the medium, contradicting the SD model.
Experimental and Computational Findings
To investigate the validity of the SD model, the researchers conducted:
- Nonequilibrium Molecular Dynamics (NEMD) Simulations – These revealed that water molecules travel in clusters through a network of pores in the membrane, contradicting the SD model, which assumes water molecules separate and diffuse individually.
- Permeation Experiments – Using polyamide and cellulose triacetate RO membranes, they tested how water and organic solvents move through the membranes. The results showed that solvent permeance depends on membrane pore size, molecular size, and viscosity—not on solubility, which is a core assumption of the SD model.
These combined approaches demonstrated that water movement is best explained by a pore-flow mechanism, where a pressure gradient inside the membrane drives water transport. The findings indicated that the SD model misrepresents water dynamics at the molecular level and that past attempts to optimize RO membranes based on this model may have been misguided.
Key Implications
More Accurate RO Design: Since RO is the dominant desalination technology, correctly understanding its working mechanism could significantly improve membrane efficiency, reduce energy consumption, and enhance desalination performance.
- New Membrane Development: By shifting focus to pore-flow mechanisms, future RO membranes could be optimized for faster water transport, lower energy costs, and higher salt rejection.
- Better Industrial Applications: The correction of this fundamental misunderstanding can refine the design of membranes not only for desalination but also for applications in wastewater treatment, pharmaceuticals, and energy production.
Conclusion
This research overturns a long-standing theory in membrane science and establishes a new paradigm for understanding water transport in RO membranes. By replacing the SD model with a pressure-driven pore-flow model, it paves the way for more efficient and scientifically grounded desalination technologies, ultimately advancing global water sustainability
