# Facile Method by Bentonite Treated with Heat and Acid to Enhance Pesticide Adsorption
## 3 Results and Discussion
### Kinetic Experiments  
Kinetics studies on the adsorption of pesticide onto bentonite and modified bentonite were performed. The results are shown in Figure 9. Measurements were made using 20 mg adsorbent at 30 \({}^{\circ}\)C, with atrazine concentrations of 50 mg L\({}^{-1}\). All adsorbents exhibited a high affinity for atrazine during the first 50 min, and equilibrium was reached within 200 min.  
Pseudo-first and pseudo-second-order models were employed to correlate the kinetics data. A pseudo-first-order kinetics adsorption model was suggested for the sorption of solid/liquid systems and is expressed in an integrated and linear form in Equation (6) [47]:  
\[\log(q_{e}-q_{t})=\log q_{e}-\frac{k_{1}}{2.303}t \tag{6}\]  
where \(k_{1}\) is the adsorption rate constant (min\({}^{-1}\)), \(q_{e}\) and \(q_{t}\) are the atrazine adsorption loadings (mg g\({}^{-1}\)) at equilibrium and at time t (min), respectively. If the pseudo-first-order kinetics is applicable, a plot of log (\(q_{e}-q_{t}\)) versus t should provide a straight line, from which \(k_{1}\) and the predicted value for \(q_{e}\) can be determined from the slope and intercept of the plot.  
The pseudo-second-order kinetics model is expressed by Equation (7) [48]:  
\[\frac{t}{q_{t}}=\frac{1}{q_{e}}t+\frac{1}{k_{2}q_{e}^{2}} \tag{7}\]  
where \(k_{2}\) (g mg\({}^{-1}\) min\({}^{-1}\)) is the pseudo-second-order adsorption rate constant. A plot of t/q\({}_{t}\) against (Figure 9) yields a straight line, from which the rate constant \(k_{2}\) and \(q_{e}\) can be calculated. The pseudo-second-order model is based on the assumption that the rate-limiting step may be chemisorption, which involves valence forces by electron sharing or electron exchange between the adsorbent and the adsorbate. Tables S6-S9 in  
Figure 9: Pseudo-second order plots of atrazine (**a**); diuron (**b**); 2,4-D (**c**); and paraquat (**d**) adsorption onto bentonite and modified bentonite.  
the supplementary information shows maximum adsorption capacities of q\({}_{e}\), k\({}_{1}\), k\({}_{2}\) and the correlation coefficient R\({}^{2}\), calculated from the pseudo-first and pseudo-second-order models. The maximum adsorption capacity, q\({}_{e}\), calculated from the pseudo-second-order equation, is in accordance with the experimentally determined values. The R\({}^{2}\) values for the pseudo-second-order are greater than those in the pseudo-first-order model. These results suggest that adsorption follows the pseudo-second-order model. The indication is that the adsorption of pesticides occurs by a chemisorption mechanism between pesticide and adsorbent by innersphere complexes.  
The Langmuir maximum adsorption capacity for the pesticides obtained from this work was compared with other reports from the literature. Table 13 confirms that the efficiency of bentonite and modified bentonite is similar to that of the other adsorbents, but the adsorbent of BC\({}_{500}\)A\({}_{0.5}\) is the best for atrazine removal due to the smallest particle size and the highly active region of Al Lewis-acid site which is available for easily bound atrazine via a Lewis acid-base complex. Therefore, the fixation of atrazine in BC\({}_{500}\)A\({}_{0.5}\) is more promoted than zeoliteX, biochars (SBB) and iron nanoparticles (INPs). While ion exchange is a primary process in removing cationic pesticide (paraquat), bentonite is therefore more efficient than diatomaceous earth. For diuron adsorption, BC\({}_{500}\) is considered a preferred diuron for hydrophobic interaction that BC\({}_{500}\) provides high hydrophobic space. Consequently, it shows a similar result to using activated carbon.