## 3 Results and Discussion
### Effect of Experimental Parameters
#### 3.1.4 Effect of the Initial pH of Methylparaben Solution  
The pH of the adsorbate solution is a parameter that has a very important influence on the adsorption process by determining the ionization state of the functional groups and, consequently, the charge of both the adsorbate and the adsorbent. The effect of the initial pH of the methylparaben solution on its adsorption on activated carbon and activated olive stones is shown in Figure 1d. It can be observed that the adsorption capacity of methylparaben on both adsorbents decreases with the increase in pH, with this decrease being more pronounced at basic pH values, especially in the case of activated carbon.  
Methylparaben has a pKa value of 8.17 [7], which means that at values below this pH, it has no charge, and at higher values, it has a negative charge as a consequence of the ionization of the phenolic hydroxyl group.  
The commercial activated carbon used has a zero charge point of 4.7 [56], which means that it will have a positive charge at pH values below 4.7 and a negative charge at higher pH values. Activated olive stones have a zero charge point of 3.5 [28], being positively charged at pH values below 3.5 and negatively charged at pH values above 3.5. With regard to the functional groups present on the surface of these adsorbents, the presence of carboxyl, phenol, anhydride, lactose, quinone and ether groups has been described in both activated carbon [57; 58] and activated olive stones [28; 59].  
At pH 3, as methylparaben is in a neutral state and the two adsorbents are positively charged, some electrostatic interactions may occur between the positive zones of the adsorbents and the oxygens of the hydroxyl and ester groups of methylparaben (oxygens with a certain density of negative charge due to their inductive-I effect). This results in  
Figure 1: Effect of experimental parameters on the MPB adsorption onto AC and AOS (**a**): initial methylparaben concentration; (**b**) adsorbent dose; (**c**) stirring speed; (**d**) initial pH of methylparaben solution).  
adsorption capacity values of 89.66 mg/g for activated carbon and 18.26 mg/g for activated olive stones.  
At pH 7, methylparaben is in a neutral state and the two adsorbents are negatively charged, with the possibility of establishing hydrogen bridge interactions between the hydrogen of the hydroxyl group of the methylparaben and the negatively charged groups of the adsorbents, but with no possibility of the electrostatic interactions described in the previous paragraph. All this causes the adsorption capacity to decrease to values of 77.18 mg/g for AC (13.92% decrease) and 15.95 mg/g for AOS (12.65% decrease).  
Finally, at pH 11, both methylparaben and the adsorbents are negatively charged, so there will be a significant electrostatic repulsion between the adsorbate and adsorbents, resulting in a significant decrease in the adsorption capacity of methylparaben, which decreases to values of 39.32 mg/g in AC (40.05% decrease in the value at pH 7 and 56.15% decrease in the value at pH 3) and of 10.03 mg/g in AOS (37.12% decrease in the value at pH 7 and 45.07% decrease in the value at pH 3).