**1.1 Overview of adsorption process**

Adsorption is a surface phenomenon in which a solution containing the adsorbate gets adsorbed on the surface of an adsorbent. Adsorption phenomenon can be of two types; one is physiosorption, in which the adsorbate binds to adsorbent due to van der Waals forces, and other is chemisorption, which occurs due to chemical reactions between adsorbate and adsorbent. Physiosorption is reversible, weak and is usually endothermic, while chemisorption is irreversible, selective and exothermic [26–28].

## **1.2 Adsorption isotherm and models**

Adsorption isotherms are representations that estimate the amount the solute that is adsorbed on the surface of the adsorbent per unit weight as a function of equilibrium concentration at a constant temperature. The most commonly used are Langmuir and Freundlich isotherms that describe the adsorption process [29]. Some other models are also used such as Redlich and Peterson [30], Radke and Prausnitz [31], Sips [32], Toth [33] and Koble and Corrigan [34].
