4. Membrane transfer and retention

Generally speaking, membrane selectivity can be seen as a more or less pronounced exclusion effect, resulting from :

• size effect (micro and ultrafiltration);

• a charging effect (Donnan effect, for example, in nanofiltration or electrodialysis);

• chemical affinity (reverse osmosis sorption and other processes);

• or dielectric effects (nanofiltration).

The simplest way to represent this effect is to assign it a partition coefficientΦ :