# Facile Method by Bentonite Treated with Heat and Acid to Enhance Pesticide Adsorption
## 3 Results and Discussion
### Adsorption Isotherms  
Pesticide adsorption data were evaluated with Langmuir and Freundlich isotherm models. The Langmuir isotherm was used to determine the monolayer and homogeneous adsorption on a surface [33]. The Freundlich isotherm is an empirical equation that assumes the adsorption surface is heterogeneous during the course of the adsorption process or multilayer sorption [46]. Linear forms of the Langmuir and Freundlich adsorption isotherms shown in Equations (2) and (3), respectively, were applied to quantitatively evaluate sorption performance:  
\[\frac{\mathrm{C_{e}}}{\mathrm{q_{e}}}=\frac{\mathrm{C_{e}}}{\mathrm{q_{m}}}+ \frac{1}{\mathrm{q_{m}K_{L}}} \tag{2}\]  
where \(\mathrm{C_{e}}\) is the equilibrium concentration of the pesticide in a bulk solution (mg L\({}^{-1}\)), \(\mathrm{q_{e}}\) is the amount of pesticide adsorbed per unit mass of adsorbent at equilibrium (mg g\({}^{-1}\)), \(\mathrm{q_{m}}\) is the maximum adsorption capacity (mg g\({}^{-1}\)), and \(\mathrm{K_{L}}\) is the Langmuir constant (L mg\({}^{-1}\)):  
\[\mathrm{logq_{e}}=\mathrm{logK_{F}}+\frac{1}{\mathrm{n}}\mathrm{logC_{e}} \tag{3}\]  
where \(\mathrm{K_{F}}\) (units: (mg g\({}^{-1}\))(L mg\({}^{-1}\))\({}^{1/\mathrm{n}}\)) is the Freundlich constant, a characteristic of the system. The constant is indicative of the relative adsorption capacity of adsorption, while the term 1/n represents adsorption intensity. The detailed parameters of these different isotherm equations are also listed in Tables 5-8. All correlation coefficients (R\({}^{2}\)) exceed 0.9, suggesting that all models fit the experimental results well. The atrazine adsorption at pH 6 fits the Langmuir model better than the Freundlich model and the calculated maximum monolayer (\(\mathrm{q_{m}}\)) of adsorbed atrazine on BC\({}_{500}\)A\({}_{0.5}\) is higher than the other adsorbents. Moreover, neutral diuron adsorption at pH 6 shows the best fit to the Langmuir model compared to the Freundlich isotherm model for BC\({}_{500}\), BA\({}_{0.5}\)C\({}_{500}\), and BC\({}_{500}\)A\({}_{0.5}\). Moreover, diuron at pH 6 was preferentially adsorbed on bentonite and the modified bentonite surfaces via monolayer adsorption. In the case of bentonite and BA\({}_{0.5}\) for diuron adsorption at pH 6, it is fit both for the Langmuir and the Freundlich isotherm models. In the case of anion and cation, pesticides would be described with 2,4-D and paraquat pesticide, respectively. The data for 2,4-D adsorption at pH 6 the adsorption is only anion 2,4-D. The R\({}^{2}\) value confirms the adsorption equilibrium data fitted well with the Langmuir model. The \(\mathrm{q_{m}}\) of 2,4-D adsorbed on BA\({}_{0.5}\)C\({}_{500}\) is higher than that on the other adsorbents in this experiment. Moreover, the adsorption isotherm for the removal of paraquat from aqueous solution, the R\({}^{2}\) values from the Langmuir isotherm of all adsorbents are close to 1, indicating that the surface of each adsorbent is unique for paraquat adsorption in monolayer adsorption. We selected the adsorbent that is capable of adsorbing the maximum amount of each pesticide to study the adsorption isotherms. The adsorption isotherms of bentonite and the selected modified bentonite at 303, 313, and 323 K are shown in the supplementary information in Figure S3 and Tables 9-12. The results indicate that atrazine, diuron and 2,4-D adsorptions increase with higher temperatures, being the cause of increased pesticide diffusion rates onto the active sites of the adsorbents, but with little significant effect for paraquat adsorption. The maximum adsorption monolayer capacity (q\({}_{\text{m}}\)) and the Langmuir constant (K\({}_{\text{L}}\)), calculated from the slope and intercept of the linear plot, are shown in Tables 9-12.