Concentration polarization

Concentration polarization is a term used in the scientific fields of electrochemistry and membrane science.

[1] Here polarization is understood as the shift of the electrochemical potential difference across the cell from its equilibrium value.

[2][3] the changes in concentration (emergence of concentration gradients in the solution adjacent to the electrode surface) is the difference in the rate of electrochemical reaction at the electrode and the rate of ion migration in the solution from/to the surface.

[vague] In membrane science and technology, concentration polarization refers to the emergence of concentration gradients at a membrane/solution interface resulted from selective transfer of some species through the membrane under the effect of transmembrane driving forces.

Thus, concentration polarization phenomenon is inherent to all types of membrane separation processes.

In the case of dialysis and electrodialysis, the concentrations of selectively transported dissolved species are reduced at the upstream membrane surface compared to the bulk solution.

1a shows the concentration profile near and within a membrane when an external driving force is just applied to an initially equilibrium system.

becomes much lower than the bulk concentration, the resistance of the depleted solution becomes quite elevated.

[5] Concentration polarization strongly affects the performance of the separation process.

Lower rate of separation under the same external driving force means increased power consumption.

Moreover, concentration polarization leads to: Thus, the selectivity of separation and the membrane lifetime are deteriorated.

Generally, to reduce the concentration polarization, increased flow rates of the solutions between the membranes as well as spacers promoting turbulence are applied [5, 6].

This technique results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer, which is defined as the region in the vicinity of an electrode or a membrane where the concentrations are different from their value in the bulk solution.

Electroconvection is defined [8] as current-induced volume transport when an electric field is imposed through the charged solution.

[13] Fruitful ideas are transferred from microfluidics: novel conceptions of electro-membrane devices for water desalination in overlimiting current range have been proposed.

Fig. 1. Fluxes and concentration profiles in a membrane and the surrounding solutions. In Fig. a , a driving force is applied to a system initially at equilibrium: the flux of a selectively permeating species in the membrane, , is higher than its flux in solution, . Higher flux in the membrane causes decreasing concentration at the upstream membrane/solution interface, and increasing concentration at the downstream interface ( b ). Concentration gradients gives rise to diffusion transport, which increases the total flux in solution and decreases the flux in the membrane. In steady state, .