3.2.1. Equilibrium Adsorption

The amounts of adsorbed atenolol (qe) against the equilibrium concentration (Ce (mg/L)) at 25 ◦C are presented in Figure 3. The obtained isotherms are of type L and S for the prepared activated carbons and commercial activated carbon, respectively, according to Giles classification and commonly reported for adsorption in the liquid phase [67]. The L-shape isotherm showed a fairly rapid rise in adsorbed quantity as atenolol concentration increases up to saturation which is characterized by a plateau. This indicates a progressive occupancy of the adsorbent surface as a function of concentration up until the entire surface area is coated with a single layer. Such adsorption behavior could be explained by the high affinity of adsorbent– adsorbate at low and moderate concentrations, which then decreases as concentration increases, since vacant adsorption sites decrease as the adsorbent becomes covered.

The S-shape isotherm showed a small sorption at low concentrations of atenolol in the solution and the sorption increased with the solute concentration. This type of isotherm indicates that at low concentrations the surface has a low affinity for the adsorbate, which increases at higher concentrations because of solute–solute attractive forces.

*C* **2022**, *8*, x FOR PEER REVIEW 9 of 16

the adsorbent becomes covered.

the prepared activated carbons and commercial activated carbon, respectively, according to Giles classification and commonly reported for adsorption in the liquid phase [67]. The L-shape isotherm showed a fairly rapid rise in adsorbed quantity as atenolol concentration increases up to saturation which is characterized by a plateau. This indicates a progressive occupancy of the adsorbent surface as a function of concentration up until the entire surface area is coated with a single layer. Such adsorption behavior could be explained by the high affinity of adsorbent–adsorbate at low and moderate concentrations, which then decreases as concentration increases, since vacant adsorption sites decrease as

**Figure 3.** Adsorption isotherms of atenolol on the different activated carbons at 25 °C (symbols: experimental values; continuous lines: fitting to Langmuir, Freundlich and Temkin equations). **Figure 3.** Adsorption isotherms of atenolol on the different activated carbons at 25 ◦C (symbols: experimental values; continuous lines: fitting to Langmuir, Freundlich and Temkin equations).

The S-shape isotherm showed a small sorption at low concentrations of atenolol in the solution and the sorption increased with the solute concentration. This type of isotherm indicates that at low concentrations the surface has a low affinity for the adsorbate, which increases at higher concentrations because of solute–solute attractive forces. Furthermore, it can be observed that the adsorption capacity of the R1-500 and R3- 500 presented the same amount of about 75 mg/g. However, R2-500 displays a higher adsorption capacity, above 110 mg/g and, with a similar adsorption capacity as for the CAC and this result is much higher compared to the literature [46]. This could mean that a high surface area of the adsorbents may not be the only parameter that determines the removal of higher amounts of the pollutant, but also the distribution of porosity could be a deter-Furthermore, it can be observed that the adsorption capacity of the R1-500 and R3-500 presented the same amount of about 75 mg/g. However, R2-500 displays a higher adsorption capacity, above 110 mg/g and, with a similar adsorption capacity as for the CAC and this result is much higher compared to the literature [46]. This could mean that a high surface area of the adsorbents may not be the only parameter that determines the removal of higher amounts of the pollutant, but also the distribution of porosity could be a determining factor in this case [68]. This is confirmed by the porosity data (see Table 5), where the R2-500 presents a combination of both a high volume of micropores and an important mesoporous volume.

mining factor in this case [68]. This is confirmed by the porosity data (see Table 5), where Three isotherms were used to fit the adsorption experimental results including Langmuir (Equation (2)), Freundlich (Equation (3)), and Temkin (Equation (4)) isotherms, taking into account the effects of equilibrium concentration on adsorption capacity. The three model parameters and correlation coefficients (R<sup>2</sup> ) were listed in Table 5.

It can be observed that in general terms, the highest R<sup>2</sup> values were obtained with the Langmuir model for all the tested carbons. The Langmuir adsorption isotherm describes the surface as homogeneous, assuming that there is no lateral interaction between adjacent adsorbed molecules when a single molecule occupies a single surface site. However, considering the shape of the CAC isotherm, this model is not found suitable and could not give the proper information to describe the adsorption process because it does not take into account adsorbate–adsorbate interactions. The maximum monolayer adsorption capacity predicted by the Langmuir model was 98.65, 169.69, and 115.69 mg/g for R1-500, R2-500, and R3-500, respectively.


**Table 5.** Langmuir, Freundlich, and Temkin adsorption parameters obtained from equilibrium isotherms of atenolol for the activated carbons.

The extent of the exponent, n, gives information on the favorability of adsorption. As deduced from the results, the values of 1/n were inferior to one (<1), meaning that the adsorption of atenolol was favorable on all samples. Furthermore, K<sup>F</sup> is a rough index of the adsorption capacity. A high value of K<sup>F</sup> indicates a high adsorption capacity, when the K<sup>F</sup> value increases, the adsorption capacity of the adsorbent increases.
