# Facile Method by Bentonite Treated with Heat and Acid to Enhance Pesticide Adsorption
## 3 Results and Discussion
### Adsorption of Pesticides onto Bentonite and Modified Bentonite  
#### 3.2.1 The Effect of pH  
The changes in pH of the solution affect the speciation of adsorbate species, adsorbent surface charge, and degree of pesticide ionisation [41; 42]. Bentonite can adsorb pesticides through two different mechanisms including cation exchange and adsorption [43]. Both mechanisms are pH dependent because most silanol and aluminol groups are protonated on the surface of bentonite under acid conditions. Thus, bentonite surface exists in a typically positive charge at low pH and the charge decreases, eventually becoming negative as pH increases. Figure 6a shows the effect of pH on cationic pesticide (paraquat; PQ\({}^{2+}\)) adsorption by bentonite and modified bentonite. PQ\({}^{2+}\) attaches to the active site through ion exchange, emerging mostly at the interlayer space of bentonite and the modified bentonite samples due to its small size, unlike the ion exchange of protonated atrazine (acidic form) that occurs very well at the face and edge of the adsorbents because of the large size of atrazine. The high adsorption of PQ\({}^{2+}\) appears on bentonite and BC\({}_{500}\) as shown in Figure 6a. The lowest adsorption of PQ\({}^{2+}\) is on BA\({}_{0.5}\)C\({}_{500}\). The high amount of adsorbed PQ\({}^{2+}\) corresponds to the high value of CEC and negative zeta potential (see Tables 1 and 4). All adsorbents display the adsorption capacity of PQ\({}^{2+}\), which increases with an increase in the pH of the solution, as shown in Figure 6a. The adsorption value of PQ\({}^{2+}\) increases drastically at high pH because the surface of the adsorbents becomes more negatively charged, possibly due to the SiOH\({}^{-}\)group on the surface changing to SiO\({}^{-}\) that prefers to adsorb PQ\({}^{2+}\). Figure 6: _Cont._In contrast to the case of 2,4-D adsorption (Figure 6b), 2,4-D has a pKa of 3.55; it can exist in both neutral and anionic form. At a pH of 3-6, an anionic form appears at around 22-99.5% in aqueous solution [44]. 2,4-D adsorption by BA\({}_{0.5}\)C\({}_{500}\) is found to be the highest in the pH range of 2-6, due to the positively charged surface of BA\({}_{0.5}\)C\({}_{500}\) (as in Table 4), preferring to absorb the anion form and including BA\({}_{0.5}\)C\({}_{500}\) with the highest surface area. Thus, BA\({}_{0.5}\)C\({}_{500}\) is an adsorbent suitable for 2,4-D. The lower adsorption capacity of 2,4-D with an increase in pH is probably due to the increased negative charge of the adsorbents. Considering the case of atrazine adsorption by bentonite and modified bentonite shown in Figure 6c, atrazine is possibly in either protonated or neutral form depending on the solution pH. At pH values approaching a pKa of atrazine (pKa = 1.7), both protonated and neutral forms are present in the solution. Protonated atrazine preferentially adsorbs by cation exchange, while un-protonated form tends to adsorb on the active sites. According to the results of the adsorption in the range of pH 4-8, the neutral form of atrazine seems to be the most preferential to BC\({}_{500}\)A\({}_{0.5}\). At pH 2.0, BC\({}_{500}\) exhibits the greatest atrazine adsorption capacity. One explanation is that at pH 2, where atrazine exists in a neutral and protonated form, and the removal of atrazine would be through ion exchange and the adsorption mechanism. BC\({}_{500}\) removes atrazine mainly through ion exchange due to its low surface area. Nonetheless, it has a lower CEC than bentonite, suggesting that the ion exchange process of cationic atrazine probably occurs efficiently at the edge and face of BC\({}_{500}\) instead of at the interlayer spacing (like bentonite), in which a large molecule of  
Figure 6: Adsorption capacity of modified and unmodified bentonite for paraquat (**a**); 2,4-D (**b**); atrazine (**c**); and diuron (**d**) at various pH and at 50 ppm. atrazine is difficult to penetrate, causing very low adsorption for bentonite.