Significance of Co-ion Partitioning in Salt Transport through Polyamide Reverse Osmosis Membranes

Area of Science:

• Membrane science and technology
• Water treatment technologies
• Physical chemistry of interfaces

Background:

• Salt permeability of polyamide reverse osmosis (RO) membranes increases with feed salt concentration.
• This increase is often attributed to salt partitioning, but experimental data shows partitioning decreases with concentration, creating a contradiction.

Purpose of the Study:

• To investigate the dependence of total ion and co-ion partitioning coefficients on salt concentration and solution pH in polyamide RO membranes.
• To clarify the relationship between ion partitioning behavior and salt transport through RO membranes.

Main Methods:

• Measured salt partitioning using quartz crystal microbalance (QCM).
• Calculated co-ion partitioning using modified Donnan theory.
• Applied the good co-ion exclusion (GCE) model to predict salt permeability.

Main Results:

• Co-ion partitioning increased fourfold, while total ion partitioning decreased by 60% with increasing NaCl concentration (100-800 mM).
• The dependence of partitioning on concentration was more pronounced at higher pH.
• The GCE model, using co-ion partitioning, accurately predicted experimental salt permeabilities.

Conclusions:

• Co-ion partitioning, not total salt partitioning, is the dominant factor governing salt transport in polyamide RO membranes.
• Understanding co-ion partitioning is crucial for predicting and optimizing RO membrane performance.