Salt and Water Transport in Reverse Osmosis Membranes: Beyond the Solution-Diffusion Model

Area of Science:

• Membrane science
• Physical chemistry
• Chemical engineering

Background:

• The solution-diffusion (SD) model is standard for reverse osmosis (RO) but lacks detail on molecular transport.
• Accurate modeling of water and ion transport is crucial for advancing RO technology.

Purpose of the Study:

• To develop a new ion transport model for RO, termed the solution-friction model.
• To incorporate partitioning and inter-species friction mechanisms into RO transport modeling.
• To challenge the assumptions of the traditional SD model.

Main Methods:

• Developed the solution-friction model using the extended Nernst-Planck equation.
• Incorporated friction terms for ion-membrane, water-membrane, and water-ion interactions.
• Validated the model with experimental salt rejection and permeate flux data.

Main Results:

• The solution-friction model accurately predicts RO performance.
• Salt permeability shows strong dependence on feed concentration and pressure, unlike SD model predictions.
• Cross-membrane transport, not membrane-internal, dominates pressure drop, contradicting SD assumptions.

Conclusions:

• The solution-friction model offers a more mechanistic understanding of RO transport.
• This model highlights the limitations of the SD model in describing complex interactions.
• Findings provide a basis for optimizing RO processes through a deeper mechanistic insight.