Electrical potential gradient:

Membrane processes use selective membranes for separation of various chemical substances which can be both in suspended or dissolved states and also in gaseous forms. Semipermeable membranes are physical barriers which allow permeation of certain chemical constituents across them and effectively preventing others from a feed mixture. Permeation of chemical substances requires a driving force which can be a pressure gradient, electrical potential gradient or concentration gradient.

Gaseous permeation, reverse osmosis, ultrafiltration and microfiltration processes utilise pressure gradient across the membrane. Dialysis and liquid membrane processes utilise concentration gradient. Electrodialysis process utilises electrical potential gradient as the driving force.

Mechanism of permeation of chemical substances through membranes basically involves solution-diffusion and the permeating species dissolve in the membrane and subsequently diffuse across. Preferential permeation across the membrane arises due to the associative dissolution of the substances within the membrane and difference in their diffusion rates by the membrane matrix. Chemical preference among the membrane and the permeating substance, selective complexation inside the membrane and size compatibility among them might contribute to the separation mechanism.

Osmotic phenomena occur due to the difference in chemical potential between pure solvent and solution and are the flow of solvent from pure solvent to solution through semipermeable membranes. Reverse osmosis includes separation of solvent from solution through application of pressure in excess of the osmotic pressure of the solution.

Dialysis process includes separation of low molecular weight solutes from high molecular weight solutes through using concentration gradient across semipermeable membranes of around pore size and thickness. Electrodialysis procedures include separation of cations and anions using cation and anion selective membranes through application of electrical potential.

Reverse osmosis and electrodialysis processes are majorly used for production of pure water from naturally occurring saline waters and industrial effluents. They are hugely applied for producing drinking water and process water for organization use as well as for effluent treatment. Dialysis is majorly used for artificial purification of blood from patients suffering from kidney failure. A protein metabolic product like urea, uric acid and creatinine, etc. is separated from the blood stream in dialysis process.