Absorption is the process in which a fluid is dissolved by a liquid or a solid (absorbent).
Adsorption is the process in which atoms, ions or molecules from a substance (it could be gas, liquid or dissolved solid) adhere to a surface of the adsorbent.
Desorption is a reversible process of adsorption, only adsorbents like as Activated alumina, Molecular sieve, Porous carbon and some other materials got this property.
Absorption and adsorption are two natural occurring processes that are similar, but are not the same. Here is a basic breakdown of how they are different: absorption occurs when one material' s physical state is absorbed into another material' s physical state, while adsorption occurs when one material physically sticks to another material without changing it' s physical state.
Absorption occurs when a gas turns into a liquid, or a liquid into a solid, etc. This is what separates it from adsorption, the physical state of the molecules have changed. For example if you were to drink a glass of milk, your body would absorb it into your digestive system and eventually into your bloodstream. The earth absorbs the suns rays and has converted its energy into the life sustaining planet we live on today. The roots of plants absorb water when it rains converting into the energy it needs to survive. All of these examples feature one material' s phase being turned into another.
Adsorption occurs when liquid or gas molecules stick to the side of surface, preserving their physical state. This is useful for separating certain molecules from one another. Adsorbents are most commonly found as carbon compounds or oxygen compounds.
Oxygen compound adsorbents are used to make products like silica gel which works to absorb moisture and reduce humidity levels or zeolites which can be tailored to specifically remove certain molecules from the air like carbon dioxide.
Carbon compound adsorbents like activated carbon can be effectively used to treat waste water and gas. Contaminates will get stuck to the pores that are found all over the surface area of activated carbon while the water filters through.
Absorption and adsorption are both sorption processes, they both take in a substance or hold it in place and that is how they are related and why the are so similar, the process, however, is different.