3.2.2. Adsorption Isotherm

The adsorption isotherm study was done to describe the interactions between Dic and Caf on AC prepared from argan nutshells. It is important for the interpretation of the surface properties, the adsorption capacities of AC, and to complete the adsorption isotherm study that the equilibrium data were fitted to the Langmuir model and the Freundlich model [89,90]. The Langmuir and Freundlich parameters of Dic and Caf adsorption on AC were calculated using Equations (S5) and (S6) in the Supplementary information. The isotherm parameters are listed in Table 5, Based on the comparison of the correlation coefficient (R2) values of Dic and Caf adsorbed on AC (Figures 9 and 10).

**Table 5.** Parameters of Langmuir and Freundlich models of Dic and Caf onto AC based on argan shells.


**Figure 9.** Langmuir isotherm of Dic (**a**) and Caf (**b**) on AC based on argan nutshells.

**Figure 10.** Freundlich isotherm of Dic (**a**) and Caf (**b**) on AC based on argan nutshells.

Figure 9 describes the linear equations 1/Qe versus 1/Ce of Dic and Caf on AC based on argan nutshells. The Qm and K*l* values are presented in Table 5, which show contaminants adsorption on the heat-resistant activated Langmuir angle and the calculated values of the parameters.

Based on Table 5, the correlation coefficient of the linear regression equation (R2) of the Langmuir isotherm adsorption model is reasonable for the adsorption of Dic and Caf by activated carbon based on argan nutshells with values of 0.996, and 0.990, respectively. The maximum adsorption capacities (Qm) of Dic and Caf by activated carbon based on Argan nutshells calculated from the Langmuir model are 126.16 mg/g, and 210.65 mg/g, respectively.

When the experimental equilibrium data are appropriately described by the Langmuir model, it is essential to calculate the separation factor [91]. It was originally proposed that the essential characteristics of the Langmuir isotherm model could be indicated in terms of a dimensionless constant separation factor or equilibrium parameter R*l*, which is defined as follows:

$$\text{Rl} = \frac{1}{1 + (\text{K}l \ast \text{Co})} \tag{8}$$

where R*l* is a constant separation factor (dimensionless) of a solid–liquid adsorption system, K*l* is the Langmuir equilibrium constant, and Co is the initial concentration.

The results show that R*l* values for Dic were 0.169 and for Caf 0.266. All of the values between zero and one indicate the suitability of the Langmuir isotherm model for the description of the adsorption process of Dic and Caf.

Figure 10 describes the linear equation log (Qe) versus log (Ce), thereby determining the constants K*f* and n, as shown in Table 5.

Table 5 shows the adsorption process of Dic and Caf on activated carbon based on argan nutshells, according to the Freundlich isotherm model with values of 0.85 and 0.97, respectively. These indicate that the Freundlich isotherm model is not suitable for describing the contaminants adsorption process by the adsorbents.

The results show that n values for Dic were 1.501 and for Caf were 1.076. They were both superior to one, indicating that the adsorption isotherms are poorly modelled by the Freundlich equation.

Furthermore, the Langmuir isotherm model has a higher regression coefficient R2 than the Freundlich model (Table 5), indicating the Langmuir model provides a better description of AC (based on argan nutshells) adsorption process in Dic and Caf. Therefore, these results suggest monolayer adsorption of AC on the surface of the adsorbent.

Table 1 shows a comparison of absorbance capacity of Dic and Caf on various adsorbents reported in the literature, since the absorbance capacity of contaminants adsorbed varies as a function of different parameters (Initial concentration, contact time, etc.). Nevertheless, AC from argan nutshells presented high capacities for Dic and Caf, comparable or even higher than the ones obtained with other activated carbons derived from agricultural waste (Table 1).

To understand the mechanisms associated with the adsorption of Dic and Caf by the AC from argan nutshells, it is important to evaluate a potential practical application of adsorbents related to the removal of this type of contaminant.

The results presented in Table 6 also highlight that the surface area is not always the important feature in the removal of these adsorbate molecules.


**Table 6.** Adsorption capacities and surface area of different contaminants using AC based on argan fruits shells compared to the literature data.

As mentioned before, textural properties were not the main factors in the adsorption of Dic and Caf since the AC obtained from argan nutshells presented a higher surface area and pore volume did not perform better regarding adsorption capacity of Dic and Caf. The large micropores developed on AC from argan nutshells do not provide an optimum size for adsorbates adsorption, which can explain the minor impact of surface area (Table 6). In fact, the role of the microporous network in the interaction with pharmaceutical molecules was previously demonstrated: If the critical dimension of the adsorbate molecule is close to the width of the micropores there will be an enhanced interaction and packing of the molecules [93].

#### 3.2.3. Effect of Temperature and Thermodynamic Study

The effect of temperature on the adsorption phenomenon was studied by varying this parameter from 10 ◦C to 30 ◦C using a thermostat bath to maintain the temperature at the desired value. The tests were carried out by stirring 1 g of activated carbon based on argan shells with 100 mg of each contaminant (diclofenac and caffeine) in 1 L of the solution.

Initially, thermodynamic parameters such as Gibbs free energy (ΔG◦), enthalpy (ΔH◦), and entropy (ΔS◦) for diclofenac and caffeine adsorptions were determined by the slope and intercept in Ln(K) versus 1/T plot (Figure 11) that allowed for calculating the values of ΔH◦ and ΔS◦ in both matrices. The results are shown in Table 7.

**Figure 11.** Plot of ln(Kc) versus temperature (1/T) for thermodynamic parameter calculation for the adsorption of Dic (**a**) and Caf (**b**) on AC based on argan nutshells.


**Table 7.** Thermodynamic parameters relating to the adsorption of contaminants (Dic and Caf) on activated carbon based on argan nutshells.

The negative values of the three parameters ΔH◦, ΔG◦, and ΔS◦ of diclofenac indicate that the reaction is spontaneous and exothermic and that the order of distribution of the contaminant molecules on the adsorbent is large compared to that in solution. Furthermore, an examination of the standard enthalpy values of the adsorption (<40 kJ/mol) shows that it is physisorption. In the case of diclofenac, the negative ΔS◦ value shows that adsorption occurs with increasing order at the solid–solution interface. The negative values of ΔG◦ increase with temperature and indicate an increase in disorder during adsorption, and the randomness increases at the solid–solution interface during this binding process. This can be explained by the redistribution of energy between the absorbent and the absorbate.

The positive value of ΔH◦ confirms the endothermic nature of the adsorption process of caffeine (values lower than 40 Kj/mol). Therefore, the adsorption regarding the matrices occurs by physisorption. Indeed, ΔS◦ presented positive values, which agrees with a dissociative mechanism. Moreover, the positive value of ΔS◦ shows the increased randomness at the solid–solution interface during the adsorption. It might display an increment of the degrees of freedom for the caffeine molecules in the solution. Additionally, Table 7 shows more negative ΔG◦ values as the temperature increased; these indicate that the adsorption process is spontaneous, and spontaneity increases with an increase in temperature.

## *3.3. Statistical Analysis*

Results are reported as the means of four replicates. Data obtained were subjected to one-way analysis of variance (ANOVA) for assessing the significance of quantitative changes in the variables as a result of biochar treatments. The statistical analysis was done by the Statistical Package for Social Science (SPSS 23.0).

According to the statistical analysis (Table 8), the effect of the dose shows that there is a significant difference (*p* > 0.05) between the means of the adsorption capacities of caffeine and diclofenac by the activated carbons from argan nutshells (AC). On the other hand, the statistical analysis of the effect of the initial concentration shows that the test is significant at the 5% level. Furthermore, the statistical analysis of the effect of contact time shows that the test is highly significative at the 1% value; there is a significant difference between the mean adsorption capacities of caffeine and diclofenac by the activated carbons from argan nutshells (AC). Moreover, the statistical analysis of the effect of temperature shows that the highly significant test at the 5% threshold shows a significant difference between the means of adsorption capacities of caffeine and diclofenac by the activated carbons from argan nutshells (AC).

**Table 8.** Analysis of variance (F-test) of the effects on the adsorption of caffeine and diclofenac by the activated carbons from argan nutshells (AC).


Values are averages ± standard deviation of triplicate analysis. Data obtained were subjected to one-way analysis of variance (ANOVA). NS: Non-significant (*p* > 0.05). S: Significant (*p* < 0.05). AC: Activated carbons from argan nutshells.
