## 3 Results and Discussion
### Resticide Adsorption Studies  
#### 3.3.1 Kinetic Adsorption Studies  
The adsorption kinetics of the pesticides onto the synthesized activated carbons are depicted in Figure 3a-d.  
Slow adsorption rates were observed for the three pesticides, achieving equilibrium times ranging from 24 to 144 h. As can be seen in Figure 3, in the first stage, rapid adsorption occurred because adsorption sites on the surface are easily accessed and occupied by the micropollutant; in the next stage, the diffusional effect comes into play (conditioned by the steric hindrance caused by the adsorbate molecules), due to the relatively high molecular size of the pesticides used compared to the medium available pore size of the activated carbons, requiring in this stage more time to diffuse until reaching equilibrium. According to this, longer equilibrium times were found when the molecular volume of the pesticides  
Figure 3: Adsorption kinetics of the three pesticides onto the (**a**) AC-ZnCl\({}_{2}\), (**b**) AC-FeCl\({}_{3}\), (**c**) AC-Fe(NO\({}_{3}\))\({}_{3}\), and (**d**) AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) activated carbons.  
used was larger, following the order ACT (266 A\({}^{3}\)) \(<\) IMD (271 A\({}^{3}\)) \(<\) THM (303 A\({}^{3}\)). For this reason, the slowest equilibrium time (144 h) was obtained for the larger pesticide, THM.  
In the same way, very high adsorption removal values were obtained at equilibrium time. The best results were achieved with the AC-ZnCl\({}_{2}\) sample. Generally, it was observed that better adsorption capacities were obtained with those activated carbons that showed a higher specific surface area (Table 2); therefore, it is clear that the textural properties of the materials played a key role in their adsorption performance. An exception was observed for THM removal using the AC-Fe(NO\({}_{3}\))\({}_{3}\) and AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) samples, where a simultaneous contribution of adsorption and reaction phenomena was observed (Figure 3c,d). In both cases, a first stage related to the adsorption process and a second stage where the adsorption process occurred simultaneously with a possible degradation-mediated by-reaction were observed; this phenomenon may be due to the high iron content found in these carbonaceous materials.  
Pseudo-first-order and pseudo-second-order kinetic models were applied to assess the experimental kinetic data. The pseudo-first-order (PPO) kinetic model, also known as the Lagergren equation, assumes that physisorption limits the adsorption rate on the adsorbent particles and can be expressed by the following equation:  
\[\ln(q_{e}-q)=\ln q_{e}-k_{1}\cdot t \tag{2}\]  
where \(k_{1}\) (h\({}^{-1}\)) is the first-order reaction rate equilibrium constant, and \(q\) (mg/g) and \(q_{e}\) (mg/g) denote the adsorption capacity at any time and at the equilibrium time, respectively.  
On the other hand, the pseudo-second-order (PSO) kinetic model assumes that chemisorption is the rate-limiting step of the adsorption process, and it is expressed as follows:  
\[\frac{t}{q}=\frac{1}{k_{2}\cdot q_{e}^{2}}+\frac{t}{q_{e}} \tag{3}\]  
where \(k_{2}\) (g/mg\(\cdot\)h) is the rate constant of the pseudo-second-order kinetic model.  
As shown in Table 4, considering the experimental and estimated adsorption capacities and R\({}^{2}\) values, both the pseudo-first-order and pseudo-second-order kinetic models well described the adsorption of pesticides onto the synthesized activated carbons. This suggests that physical and chemical interactions simultaneously contribute to and control the adsorption of the pesticides on the surfaces of the activated carbons employed. Generally, physical adsorption, or physisorption, involves van der Waals forces, while chemical adsorption, or chemisorption, occurs when electrons are shared or transferred through covalent bonds [39].