Concentration polarization

Flux and concentration profiles in the membrane and the surrounding solution. In part a, a driving force (pressure difference, concentration difference ...) is imposed on the system originally in equilibrium: Due to the selectivity of the membrane, the flux of substance 1 in the membrane is higher than its flux in the solution in front of and behind it . A higher flux in the membrane creates the concentration gradient, which is shown in part b. This concentration gradient provides for diffusive mass transport in the membrane opposite to the driving force and on the membrane (in the boundary layer) in the direction of the flux, so that a steady state can be set, in which the flux is the same everywhere . ${\displaystyle J_{1}^{m}}$ ${\displaystyle J_{1}^{s}}$ ${\displaystyle J_{1}^{m}=J_{1}^{s}}$

In filter and membrane technology, the term concentration polarization denotes the formation of a boundary layer on the membrane during the filtration process . This concentration gradient is formed by macromolecules that cannot pass through the membrane. If concentration polarization occurs during a filtration, this becomes noticeable through a reduced transmembrane flow (flux). This effect is reversible - if the membrane is washed, the original flux can almost be restored. If a cross-flow ( tangential flow filtration ) is applied to the membrane , this minimizes the concentration polarization.

Causes and effects

A substance is considered that is in the feed (inlet of a membrane filtration or a membrane module for water treatment) and for which the membrane is impermeable: The flow transports the substance convectively to the membrane and accumulates on it. The concentration of this substance on the membrane would steadily increase with only convective transport. Since the resulting concentration gradient acts as the driving force for a diffusive return transport of the substance away from the membrane, a boundary layer is created on the membrane in which a stationary concentration gradient is established. Concentration polarization leads to a drop in flux and a reduced selectivity of the membrane.

See also

• Fouling (membrane technology)