**2. Adsorption phenomenon**

Adsorption is a surface phenomenon with common mechanism for organic and inorganic pollutants removal. When a solution containing absorbable solute comes into contact with a solid with a highly porous surface structure, liquid–solid intermolecular forces of attraction cause some of the solute molecules from the solution to be concentrated or deposited at the solid surface. The solute retained (on the solid surface) in adsorption processes is called ad‐ sorbate, whereas, the solid on which it is retained is called as an adsorbent. This surface ac‐ cumulation of adsorbate on adsorbent is called adsorption. This creation of an adsorbed phase having a composition different from that of the bulk fluid phase forms the basis of separation by adsorption technology.

In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physicsorption (characteristic of weak Van Der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.

As the adsorption progress, an equilibrium of adsorption of the solute between the solution and adsorbent is attained (where the adsorption of solute is from the bulk onto the adsorb‐ ent is minimum). The adsorption amount (qe, mmol g−1) of the molecules at the equilibrium step was determined according to the following equation:

$$\text{Age} = \text{V(Co-Ce)} \,\text{/M} \tag{1}$$

where V is the solution volume (L); M is the mass of monolithic adsorbents (g); and Co and Ce are the initial and equilibrium adsorbate concentrations, respectively.

Other definition of adsorption is a mass transfer process by which a substance is transferred from the liquid phase to the surface of a solid, and becomes bound by physical and/or chemical interactions. Large surface area leads to high adsorption capacity and surface reactivity [10].

#### **2.1. Adsorption isotherms and models**

Efficient techniques for the removal of highly toxic organic compounds from water have drawn significant interest. A number of methods such as coagulation, filtration with coagu‐ lation, precipitation, ozonation, adsorption, ion exchange, reverse osmosis and advanced ox‐ idation processes have been used for the removal of organic pollutants from polluted water and wastewater. These methods have been found to be limited, since they often involve high capital and operational costs. On the other hand ion exchange and reverse osmosis are more attractive processes because the pollutant values can be recovered along with their re‐ moval from the effluents. Reverse osmosis, ion exchange and advanced oxidation processes do not seem to be economically feasible because of their relatively high investment and op‐

Among the possible techniques for water treatments, the adsorption process by solid adsorb‐ ents shows potential as one of the most efficient methods for the treatment and removal of or‐ ganic contaminants in wastewater treatment. Adsorption has advantages over the other methods because of simple design and can involve low investment in term of both initial cost and land required. The adsorption process is widely used for treatment of industrial wastewa‐ ter from organic and inorganic pollutants and meet the great attention from the researchers. In recent years, the search for low-cost adsorbents that have pollutant –binding capacities has in‐ tensified. Materials locally available such as natural materials, agricultural wastes and industri‐ al wastes can be utilized as low-cost adsorbents. Activated carbon produced from these

Adsorption is a surface phenomenon with common mechanism for organic and inorganic pollutants removal. When a solution containing absorbable solute comes into contact with a solid with a highly porous surface structure, liquid–solid intermolecular forces of attraction cause some of the solute molecules from the solution to be concentrated or deposited at the solid surface. The solute retained (on the solid surface) in adsorption processes is called ad‐ sorbate, whereas, the solid on which it is retained is called as an adsorbent. This surface ac‐ cumulation of adsorbate on adsorbent is called adsorption. This creation of an adsorbed phase having a composition different from that of the bulk fluid phase forms the basis of

In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physicsorption (characteristic of weak Van Der Waals forces) or chemisorption (characteristic of covalent

As the adsorption progress, an equilibrium of adsorption of the solute between the solution and adsorbent is attained (where the adsorption of solute is from the bulk onto the adsorb‐

materials can be used as adsorbent for water and wastewater treatment [9].

erational cost.

**2. Adsorption phenomenon**

168 Organic Pollutants - Monitoring, Risk and Treatment

separation by adsorption technology.

bonding). It may also occur due to electrostatic attraction.

An adsorption isotherm is the presentation of the amount of solute adsorbed per unit weight of adsorbent as a function of the equilibrium concentration in the bulk solution at constant temperature. Langmuir and Freundlich adsorption isotherms are commonly used for the description of adsorption data.

The Langmuir equation is expressed as:

$$\text{Ce} \mid qe = \text{1} / b\text{X}m + \text{Ce} / \text{X}m,\tag{2}$$

Where *Ce* is the equilibrium concentration of solute (mmol L−1), *qe* is the amount of solute adsorbed per unit weight of adsorbent (mmol g−1 of clay), *Xm* is the adsorption capacity (mmol g−1), or monolayer capacity, and *b* is a constant (L mmol−1 ).

The Freundlich isotherm describes heterogeneous surface adsorption. The energy distribu‐ tion for adsorptive sites (in Freundlich isotherm) follows an exponential type function which is close to the real situation. The rate of adsorption/desorption varies with the strength of the energy at the adsorptive sites. The Freundlich equation is expressed as:

$$
\log qe = \log k + \text{ 1/ } n \log \text{Ce },\tag{3}
$$

Where *k* (mmol g−1) and 1*/n* are the constant characteristics of the system [11].
