Here's a Wikipedia page explaining 'adsorption': http://en.wikipedia.org/wiki/Adsorption While 'absorb' is a normal English word, only scientists would use 'adsorb'. If you tried to use it in an everyday context, people would think that you were making a mistake and getting the word 'absorb' wrong.

The distinction is technical : when a material is absorbed, it is distributed through the physical structure of the agent that absorbs it. The word is also used to refer to knowledge being absorbed by someone. When a material is adsorbed, it usually bonds or binds to the surface of the adsorbing agent. This word is only used in a technical context.