*2.1.5 Adsorption*

Adsorption is well known for the treatment of industrial sewage, so it is characterized by its high purifying capacity [27]. This process is based on the use of a material that will retain pollutants. This material can be applied as a fixed bed or can be used in suspension with the effluent (Fluidized Bed). The adsorption material can be inorganic (rocks, ashes …), or organic (fruit, vegetables, wood, bacteria ….). Moreover, it can be used natural or activated.

The activation of a material can be carried out with a physical process (Pyrolysis, calcination, carbonization), a chemical process (Acids, bases), or the both. Physical activation involves high temperatures (800–1000°C), whereas chemical activation requires low temperatures. The most commonly used chemical agents are potassium hydroxide, sodium bicarbonate, sodium hydroxide, zinc chloride, sulfuric and phosphoric acids. However, phosphoric acid is frequently used comparing to other agents because of its low cost and activation temperature, as well as this acid can be recoverable (< 600°C) [27]. On the other hand, the activation of materials with potassium hydroxide is considered as the most effective [28]. In fact, chemical activation has several advantages, especially the increase in the specific exchange surface and the material porosity [8]. Although the activated carbon is a powerful adsorbent, its use is so limited due to its high cost. For that, this technique will be very attractive and promising if the material will be efficient and inexpensive at the same time. Thus, several attempts have been made to find novel materials such as organic waste, fly ash [29], olive pomace [30], and sawdust [31].

## *2.1.5.1 Adsorption types*

According to the literature, the principle of adsorption is based on the binding forces between ions of adsorbent and adsorbate. Consequently, the involved forces divide adsorption into two types:

