2.4.3. Thermodynamic Study of the Adsorption

Thermodynamic parameters, specifically free energy, enthalpy, and entropy changes of adsorption, were assessed utilizing Vant Hoff's equation expressed as follows [80]:

$$
\ln \text{K}l = -\frac{\Delta \text{G}^{\circ}}{\text{RT}} = \frac{\Delta \text{S}^{\circ}}{\text{R}} - \frac{\Delta \text{H}^{\circ}}{\text{RT}^{\prime}} \tag{7}
$$

where ΔG◦ is free energy of adsorption (J/mol), ΔH◦ is change in enthalpy (J/mol), and ΔS◦ is change in entropy (J/mol/k).

Energy and entropy factors must be considered for every adsorption process in terms of deciding whether the process has taken effect spontaneously. Thermodynamic variable measurements are the exact metrics for the functional operation of the method [81,82]. Consequently, if the adsorption rate temperature progresses, (ΔH◦) > 0, the mechanism is endothermic, or (ΔH◦) < 0, the mechanism is exothermic [83,84].

#### **3. Results and Discussion**

#### *3.1. Characterization of the Activated Carbon*

As displayed in Figure 3, the profile of thermogravimetric analysis (TGA) obtained with argan nutshells clearly shows weight loss occurring as function of temperature increase. This profile is likewise of interest regarding the carbonization temperature range needed for the activated carbon production. In concurrence with the writing [85], the first weight loss of 5.9% is credited to the released of moisture content and volatile matter at a temperature range between 20 ◦C and 100 ◦C. The second decomposition stage of the profile shows a weight loss of 61.9% at a temperature range of 240 ◦C to 370 ◦C and is due to the decomposition of hemicellulose and cellulose. The final stage of the profile exhibited weight loss of 12.6% and is credited to the decomposition of lignin at a temperature above 370 ◦C. Stabilization of the material was seen near 600 ◦C and explains the consideration of this temperature for carbonization.

**Figure 3.** TGA/DSC curve of the argan shells under nitrogen atmosphere.

The FT-IR spectrum of AC, displayed in Figure 4, shows characteristic vibration bands of carbonaceous materials [86]. The figure of spectrum FTIR shows the presence of aromatic amines between 1500 to 1600 cm−1, C-O bonds of Ester between 1210 to 1260 cm−1, the isopropyl group (CH3)2CH- bonds between 990 to 1050 cm<sup>−</sup>1, and C-N bonds of the nitrile derivatives at 834 cm<sup>−</sup>1.

The textural properties of the AC were measured by nitrogen physisorption at 77K. It was evident that AC presented the type II physisorption isotherm (Figure 5) according to IUPAC classification [87], which is characteristic for the microporous materials. The results show that the phosphoric acid obtained the highest specific surface area, highest pore volume, and narrow pore size distribution (Table 3). These properties offer a good potential for the prepared activated carbons to be used as efficient adsorbents.

**Figure 4.** FTIR spectra of activated carbon from argan nutshells.

**Figure 5.** (**a**) Nitrogen adsorption/desorption isotherms; (**b**) pore size distribution with insert in the region of pore diameter between 0 and 100 nm for AC based on argan nutshells.



*3.2. Adsorption of Emergent Contaminants*

3.2.1. Effect of Contact Time and Adsorption Kinetic

We studied the adsorption efficiency of the two emerging contaminants while modifying the contact time 15, 30, 60, 90, 120 and 150 min. Samples for analysis were taken at regular time intervals to determine the percent removal of contaminants. The results obtained are shown in Figure 6.

**Figure 6.** Adsorption of Dic and Caf onto AC based on argan nutshells at different temperatures (Co = 100 mg/L; m = 1 g; T = 30 ◦C; agitation speed = 200 rpm).

Adsorption kinetics of Caf and Dic showed that they were adsorbed rapidly at the investigated conditions, with equilibration already achieved at 90 min of contact for Dic and 60 min for Caf (Figure 6).

The absorbance quantity of Dic and Caf at the equilibrium was 82% and 92%, with experimental uptake capacities of 82.60 mg/g and 93.09 mg/g, respectively.

This information indicates that all adsorption data obtained after these times can be considered as obtained under equilibrium conditions. It is necessary to identify the step that governs the overall removal rate in the above adsorption process. The pseudo-first-order and pseudo-second-order kinetic models were tested to fit the experimental data obtained for Dic and Caf uptake by AC. The kinetic study results are given in Table 4.

**Table 4.** Pseudo-first-order and pseudo-second-order parameters for adsorption of Dic and Caf onto AC based on argan nutshells.


The kinetic data of Dic and Caf adsorption on AC based on argan nutshells was investigated at temperatures of 30 ◦C. The best fitting model was defined by the higher determination coefficient (R2). The pseudo-second-order model was the most suitable for the Dic and Caf adsorption on AC based on argan nutshells data because this model has a R2 value close to 1 compared to pseudo-first-order model. The experimental adsorption capacity for Dic (91.16 mg/g) and for Caf (95.99 mg/g) was also close to the calculated adsorption capacity for Dic (82.60 mg/g) and for Caf (93.09 mg/g) (Figures 7 and 8). This suggests that the adsorption kinetics of emergent contaminants can be well described by the pseudosecond-order kinetic model. This means that the adsorption process is one of chemisorption with various interactions, such as electrostatic attractions, stacking (pi-stacking interactions (attractive, noncovalent interactions between aromatic rings)), hydrogen-bond formation, and Van der Waals forces between the adsorbent and adsorbate [88].

**Figure 7.** Pseudo-first-order kinetic model applied to the adsorption of Caf (**a**) and Dic (**b**) on activated carbon from argan nutshells.

**Figure 8.** Pseudo-second-order kinetic model applied to the adsorption of Caf (**a**) and Dic (**b**) on activated carbon from argan nutshells.
