Our understanding of reverse osmosis has just been proven wrong

Reverse osmosis is the key process to making clean drinking water acceptable to all. A new study reveals that our understanding of the process is wrong.
Tejasri Gururaj
Representational image of seawater.
Representational image of seawater.


Access to clean drinking water depends on the process of desalinating seawater. This is done via the process of reverse osmosis.

Now a new study demonstrates that the diffusion mechanism, which is the underlying principle of reverse osmosis, is wrong. Researchers from Yale University reveal that it is pore flow that governs reverse osmosis instead.

The study published in Science Advances used solvent permeation experiments and non-equilibrium molecular dynamics simulations to prove their claim. 

What is reverse osmosis?

Reverse osmosis is a process for water purification by removing impurities, minerals, and other contaminants through a semi-permeable membrane. The process involves applying pressure to force water molecules through a membrane that allows only pure water molecules to pass through while unwanted dissolved and suspended particles are removed.

The first observation of reverse osmosis came as early as 1748 by French physicist Jean-Antoine Nollet, but it wasn't used to produce fresh water until the 1950s. Additionally, the process is also used for wastewater treatment and the production of ultraclean water for electronics manufacturing and pharmaceuticals.

The explanation of reverse osmosis has been attributed to solution diffusion. According to this theory, reverse osmosis separates impurities and contaminants from pure water through solution and diffusion driven by concentration gradients and membrane pore characteristics.

Reverse osmosis is driven by a pressure gradient

The solution-diffusion theory has been accepted for more than 50 years since it was proposed in 1967. Now, Prof. Menachem Elimelech and colleagues suggest that this theory is wrong. The computer simulations revealed that a pressure gradient was driving the water transport within the membranes, not a water concentration gradient.

They also noticed that the water molecules traveled in clusters through a network of pores in the membrane. The team also noticed that the size of the membrane pores, the size of the water molecules, and the viscosity of the water all affected how water permeated the membrane.

To confirm their simulation results, the team further conducted permeation experiments using water, organic solvents, and a reverse osmosis membrane. The results of the experiment were in agreement with the simulations, hence confirming their hypothesis. They concluded that solvent and water transport is driven by a pressure gradient, which they demonstrated using the solution-friction model. 

The results of this study offer new insight into a process that seemed to have been well-established nearly 50 years ago. The results can be used to improve and fine-tune the process of reverse osmosis and take the world one step closer to making clean drinking water accessible to everyone.

Study abstract:

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

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