A natural application of reverse osmosis
Seabirds use reverse osmosis to desalinate seawater. They possess a membrane in their throats which allows water molecules to get through and stops the salt. This enables them to drink fresh, unsalted water, and spit out the salty waste.
So, how can we replicate this process?
The majority of the commercially manufactured Reverse Osmosis (RO) membranes are usually made from cellulose acetate, polysulfone, and polyamide. The membrane consists of a skin about 0.25 microns and a support layer of about 100 microns. The skin is the active barrier and primarily allows water to pass through.
The amount of dissolved solids in water produced by reverse osmosis is approximately a constant percentage of those in the feed water. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively). A RO system design is based on a certain range of feed water TDS, the percentage of rejection, and percentage of recovery desired. For a given system, the higher the percentage of the recovery or the lower the percentage of rejection, the poorer the quality of product water becomes. The video below shows a Reverse Osmosis Electrolysis Demonstration.
A Scientific Explanation
Reverse osmosis is a complicated process that uses a membrane under pressure to separate relatively pure water (or another solvent) from a less pure solution. When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, water passes through the membrane in the direction of the more concentrated solution as a result of osmotic pressure. If enough counter pressure is applied to the concentrated solution to overcome the osmotic pressure, the flow of water will be reversed.
Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Ions are repelled by dielectric interactions; ions with higher charges are repelled to a greater distance from the membrane surface. The nominal rejection ratio of common ionic salts is 85 - 98%.