# Facile Method by Bentonite Treated with Heat and Acid to Enhance Pesticide Adsorption
## 1 Introduction  
Toxic pesticides have a significant impact on human health as well as aquatic life. Currently, it is a topic of significant concern because of the increasing use of pesticides in agriculture and domestic activities [1; 2; 3; 4; 5; 6; 7]. Therefore, it is necessary to find ways to limit the agrochemical substances discharged into the environment as much as possible, while also acting to eventually eliminate them completely. Adsorption is a simple and convenient method for the removal of compounds compared to other physical, chemical, or biological technologies. It also involves low investment in terms of both initial cost and land required. Various solid substrates offer good adsorptive properties, including zeolites, activated carbon, fly ash, mesoporous silica and clay minerals, which were applied in the removal of organic contaminants from water [2; 8; 9; 10; 11; 12; 13; 14]. Bentonite clay minerals are available not only in Thailand, but their deposits are widespread all over the word. They are an important, low-cost source of montmorillonite and exhibit excellent adsorption capacities as well as high cation exchange capability (CEC) [15; 16]. The major component of bentonite is montmorillonite, a dicotaberal smectite. Montmorillonite is a porous clay mineral consisting of a 2:1-layered structure, with alternating layers of exchangeable cations. The layers consist of octahedral alumina sheetsthat are sandwiched by two tetrahedral silica sheets. Substituting Al\({}^{3+}\) for Si\({}^{4+}\) in the silica tetrahedral sheets and Mg\({}^{2+}\) for Al\({}^{3+}\) in the alumina octahedral sheets produced a net negative charge, which is usually balanced by adsorbed cations. These cations are easily replaced by either organic or inorganic cations, accounting for the unique hydrophilic, tumescent, and adsorption properties of montmorillonite [17; 18]. As in previous reports, the type and concentration of the acid effect on modified clay were used for the removal of organic substance [19; 20; 21; 22]. HCl acid is usually modified for this purpose according to its multifunctional groups such as silanol, so that it can be as efficient adsorbent. Furthermore, mild conditions can stabilise clay structure and the effect of heat used in the modification process was also promoted for the removal of organic compounds [23; 24] Acid and heat modification optimises the physical characteristics of bentonite for the removal of contaminants by causing structural and textural changes within the clay, thus enhancing the adsorption capacity. Acid and thermal treatments are low cost methods that require few chemicals and are simple to apply. Acid-activated bentonite is suitable for the removal or elimination of impurities; it increases both porous surface area and the numbers of silanol groups present in the clay, which are important for the adsorption of organic compounds [25; 26]. Heat treatment significantly alters the surface properties of bentonite; it improves porosity by increasing mesopore volume, increases the number of surface adsorption sites, and increases the number of siloxane groups present while reducing the number of hydroxyl groups present [23; 27; 28; 29; 30]. In addition to these structural changes, both acid and heat treatments alter the textural properties of the clay and improve its dispersibility in water. As mentioned above, the use of heat and acid for treatments can promote pesticide adsorption performance. Thus, we are interested in using these treatments to modify bentonite by combining both treatments rather than solely with heat or acid, and we additionally consider the sequence of each treatment in the modification of bentonite in order to optimise active sites for the adsorption of different types of pesticides such as atrazine (basic), diuron (non-ionic), 2,4-D (acidic) and paraquat (cation) in aqueous media, including the investigation of the simultaneous adsorption of these pesticides to evaluate the potential of these adsorbents for application as multifunctional adsorbents.