# Facile Method by Bentonite Treated with Heat and Acid to Enhance Pesticide Adsorption
## 4 Conclusions  
Bentonite was modified by either acid, heat, or both for the removal of different types of pesticides from an aqueous solution. It was found that BC\({}_{500}\)A\({}_{0.5}\) is the best and specific to atrazine adsorption with a very large amount compared to other adsorbents as a result of its small particle size with high edges and faces containing a high number of active sites (Al Lewis-acid sites, Si-OH, and Si-OH\({}_{2}\)\({}^{+}\) groups). It is remarkable that BC\({}_{500}\)A\({}_{0.5}\) is highly specific in the adsorption of atrazine mainly due to participation and strong interaction between a lone pair electron on the nitrogen of the pesticide to Al Lewis-acid site of the adsorbent and additional moderate interaction is hydrogen bond formation of amine groups to Si-OH and Si-OH\({}_{2}\)\({}^{+}\) groups. BC\({}_{500}\) shows higher diuron adsorption than the others adsorbent due to its high hydrophobicity in siloxanes; consequently, the interaction of BC\({}_{500}\) and diuron maybe mainly hydrophobic. Additionally, BA\({}_{0.5}\)C\({}_{500}\) is suitable for 2,4-D. The 2,4-D-adsorption may be induced by the positive zeta potential \begin{table}
\begin{tabular}{c c c c c} \hline
**Pesticide** & **Absorbents** & **q\({}_{m}\)** & **pH** & **Reference** \\ \hline Atrazine & Zeolite-X & q\({}_{m}\) = 11.86 mg g\({}^{-1}\) & Not adjust & [2] \\  & Biochars (SBB) & q\({}_{m}\) = 3.05 mg g\({}^{-1}\) & & [46] \\  & carbon nanotubes & q\({}_{e}\) = 17.35 mg g\({}^{-1}\) & 6 & [9] \\  & (MWCNTs-O) & q\({}_{m}\) = 11.76 mg g\({}^{-1}\) & 4.5 & [49] \\  & & & & \\  & Combination with Heat 500 \({}^{\circ}\)C and activated & q\({}_{m}\)= 15.87 mg g\({}^{-1}\) & 6.5 & This work \\  & 0.5 HCl on Bentonite (BC\({}_{500}\)A\({}_{0.5}\)) & q\({}_{m}\) = 0.97 mmol g\({}^{-1}\) & & [50] \\ \hline Diuron & Activated carbon & q\({}_{m}\) = 0.97 mmol g\({}^{-1}\) & & [45] \\  & CTA-TixHy-montm & q\({}_{e,\text{exp}}\) = 3.08 mg g\({}^{-1}\) & & [45] \\  & Heat treatment (500 \({}^{\circ}\)C) of bentonite (BC\({}_{500}\)) & q\({}_{m}\) = 1.92 mg g\({}^{-1}\) & 6.5 & This work \\ \hline
2,4-D & Graphene (FGN) & q\({}_{e}\) = 19.95 mg g\({}^{-1}\) & & [51] \\  & Combination with Heat 500 \({}^{\circ}\)C and activated & q\({}_{m}\) = 0.39 & 6.5 & This work \\  & 0.5 HCl on Bentonite (BA\({}_{0.5}\)C\({}_{500}\)) & q\({}_{e,\text{exp}}\) = 6.25 mg g\({}^{-1}\) & & [52] \\ \hline Paraquat & Treated diatomaceous earth & q\({}_{m}\) = 17.54 mg g\({}^{-1}\) & 6.5 & This work \\  & Bentonite & q\({}_{m}\) = 94.34 mg g\({}^{-1}\) & 6.5 & This work \\ \hline \end{tabular}
\end{table} 
Table 13: Comparison of q\({}_{m}\) values from the Langmuir isotherm model of adsorbents and modified adsorbents for the removal of pesticides. compensation indicating an interaction of anionic 2,4-D and the positively charged surface of the adsorbent. Finally, it seems apparent that high paraquat adsorption capacity was found for all adsorbents. However, bentonite is the most favourite for paraquat as a result of the main removal of paraquat through ion exchange which is easily occurring due to a small size of paraquat and bentonite presents a high value of CEC, whereas BA\({}_{0.5}\)C\({}_{500}\) showed the lowest capacity because of BA\({}_{0.5}\)C\({}_{500}\) with low CEC and swelling. BC\({}_{500}\)A\({}_{0.5}\) was recently developed by combining steps of bentonite for simultaneous pesticide removal from an aqueous solution, because it could remove a higher total amount of pesticides than the other adsorbents, indicating that it could be used as a good multifunctional adsorbent. The adsorption of all pesticides fits well with the Langmuir adsorption model compared to the Freundlich model, and the adsorption kinetics follow a pseudo-second order (chemisorption).