The term adsorption refers to the accumulation of a substance at the interface between two phases (liquid–solid interface or gas–solid interface).
The substance that accumulates at the interface is called adsorbate and the solid on which adsorption occurs is adsorbent.
Adsorption can be classified into two types:
chemical sorption and physical sorption.
Chemical adsorption or chemisorption is illustrated by the formation of strong chemical associations between molecules or ions of adsorbate to adsorbent surface, which is generally due to the exchange of electrons [43] and thus chemical sorption generally is irreversible.
Physical adsorption or physisorption is characterised by weak van der Waals intraparticle bonds between adsorbate and adsorbent and thus reversible in most cases