Evaluation of feed concentration effects on salt/ion transport through RO/NF membranes with the Nernst-Planck-Donnan model

Bulk salt concentration within a membrane channel could vary appreciably for different RO/NF membrane processes, and it even changes along a single membrane channel, which may have an important impact on solute transport through the membrane. However, little research has been conducted on the effects of concentration on transport behaviors. The purpose of this study was to demonstrate a valid method from fundamental theories on membrane transport for predicting the effects of feed salt concentration on salt rejection through RO/NF membranes. The salt transport characteristics under different feed salt concentrations were simulated using the Nernst-Planck-Donnan model. This numerical analysis requires the determination of three key parameters relating to membrane properties; namely, effective membrane thickness, effective membrane pore radius, and effective membrane charge density. The first two properties are independent of bulk salt concentration, whereas the third one could be related to bulk salt concentration by means of Freundlich isotherm model. Results of the numerical analysis showed that membrane charge density played an important role in ion/salt transport behavior. The salt retention rate decreased with increasing feed salt concentration, but remained unchanged when the ratio of membrane charge density to feed concentration was kept constant. It is noted that the extended model, obtained by incorporating the relationship between effective membrane charge density and bulk salt concentration into the NernstPlanck-Donnan equation, is useful for describing and predicting membrane salt transport performance through RO/NF membranes.