## 3 Results and Discussion
### Immersion Enthalpies of Activated Carbons as a Function of Phenol Concentration  
Figure 9 shows the change in adsorption enthalpy of activated carbons as a function of the phenol adsorbed on the samples G, GN, and GNA. The immersion enthalpy determined from the calorimetric results is the energy transferred in the form of heat by water adsorption, displacement of this, and, finally, the adsorption of phenol [4; 23; 24]. In other words, the heat determined involves the interactions of water and phenol, phenol and the activated carbon surface, and water and the surface of the activated carbon. When the process of immersion in water is performed, the heat transferred by the interactions between molecules of water and water molecules and the surface of the activated carbon can be determined. When the enthalpy of immersion in a solution is determined, it is possible to evaluate the solute-solvent and solute-surface interactions. These processes are involved during the adsorption. By subtracting the enthalpy of immersion in water from the enthalpy of immersion in solution, the value obtained is called the enthalpy of adsorption. This is the result presented in Figure 8.  
Figure 9 shows that for sample G, the values of enthalpy indicate that the adsorption is an exothermic process. When the phenol is less than 1 mmol\(\cdot\)L\({}^{-1}\), as the phenol concentration increases, the enthalpy increases; this behaviour could be attributed to the formation of the monolayer. For concentrations of phenol between 2 and 8 mmol\(\cdot\)L\({}^{-1}\), a plateau presents, which could indicate that the heat produced by the adsorption is distributed between the molecules of phenol and water in the  
Figure 8: Comparison between the adsorption in the calorimeter until equilibrium was reached and the maximum capacity of adsorption Q\({}_{0}\) from the Langmuir model. interface to search for a stable arrangement. For concentrations of phenol adsorbed above 8 mmol-L\({}^{-1}\), there is an increase in the amount adsorbed. This could be associated with the formation of other layers of phenol on the surface. It should be noted that this behaviour shows that the interaction of water with the surface of the activated carbon is low, and therefore the heat produced corresponds to the interactions of phenol and its surface. For the sample oxidised with nitric acid (sample GN), it can be observed that for quantities of phenol adsorbed below 1 mmol-L\({}^{-1}\), the values of enthalpy obtained are positive. This indicates that the presence of acid groups on the activated carbon surface favours the interaction of water with this by the formation of hydrogen bonds. Therefore, the process of the adsorption of phenol requires energy to remove the water molecules of the surface. For quantities of phenol adsorbed above 2 mmol-L\({}^{-1}\), the behaviour is similar to that which occurs with the sample G. In terms of the sample GNA, the behaviour is different, since the enthalpy values are positive in the entire range of adsorption. This could be an indication that the interactions of the water and the surface are stronger, and therefore more energy will be required to desorb water molecules and adsorb the phenol molecules. Taking into account that the activated carbons are heterogeneous surfaces, the immersion enthalpy in water and benzene were compared in order to determine the degree of hydrophobicity of the activated carbons surface. Table 5 presents the results obtained for the immersion enthalpies for samples in water and benzene, and the calculation of the hydrophobic factor.