**Polarization of concentration**

The accumulation of solutes retained on the surface of the membrane leads to the phenomenon of concentration polarization [43], which causes an increase in the concentration of solute at this surface [41].

When the driving force is interrupted and the permeate flow quenched, the concentration polarization is invisible [45].

Assuming that the true concentration is unidirectional along an axis perpendicular to the membrane, the total permeation flux is the sum of the retro-diffusive flux and the convective flux [44].
