Concentration polarization phenomena and dialysis process:

Concentration polarization is a phenomenon occurring within reverse osmosis process because of the build up of a layer of highly concentrated solution near the membrane surface as compared to the bulk feed solution. This occurs since water permeation by the membrane leaves the more concentrated solution layer near the membrane surface that must diffuse back into the bulk liquid layer. Concentration polarization raises the osmotic pressure of the salt solution near the membrane that causes a reduction within water flux across the membrane and an increase in the solute diffusion by the membrane.

The significant parameters characterizing dialysis procedures are the diffusive solute flux (J_D) across the membrane, that is directly associated to concentration difference across the membrane (?C) , the solute diffusion coefficient in the membrane phase (D_m) the partition coefficient for the solute along with respect to the membrane and the external solution (K) and inversely proportional to the membrane thickness (?x). The term (KD_m /?x) is collectively known as membrane diffusive permeability (P_m). The higher the value of P_m , the faster will be its rate of permeation through the membrane. Because P_m is inversely related the membrane thickness, the thickness of the membrane requires to be less to achieve faster rate of diffusion. For this purpose, dialysis membranes are made as thin as possible consistent along with the requirement of mechanical strength.