Type III isotherm. This class of isotherm exhibits a characteristic of weak adsorbate-adsorbent interactions leading to a low capacity of the uptake at low relative pressures. This behavior usually occurs in the adsorption of water on activated carbon because of the non-polar nature of the carbon surface. However, the presence of the oxygenated surface functional groups of activated carbon may act as the primary sites for water to adsorb on, and then this adsorbed molecule further acts as the secondary site for the other water molecules to adsorb on by hydrogen bonding.