Adsorption of herbicides 2,4-D and acetochlor on inorganic–organic bentonites
Introduction
Pesticides are recognized as an important source of potential pollution to soil and water due to their mobility and degradation in soils (Nir et al., 2006, Arias-Estévez et al., 2008). Among these pesticides, 2,4-D (2,4-dichlorophenoxyacetic acid) is an anionic herbicide whereby the water solubility depends on the pH of solution. It is widely used on wheat, maize and rice to control broad-leaf weeds. As most anionic contaminants, 2,4-D is weakly adsorbed by soil particles and poses a threat of surface and ground water contamination. Acetochlor (2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide) is a nonionic herbicide that inhibits the protein synthesis of weeds in cropland. It is widely used in China and many other countries, and considered as an herbicide with a high leaching potential, and easily transported into water bodies (Rebich et al., 2004).
Clay minerals have high specific surface areas associated with their small particle size. After surface modification with inorganic or organic cations, they can be used as adsorbents for many hazardous herbicides to eliminate these pollutants from water (Hermosín and Cornejo, 1992, Pal and Vanjara, 2001, Yaron-Marcovich et al., 2004, Neitsch et al., 2006). The modified clay minerals are also used in formulations of pesticides to control the release of active ingredients and decrease the environmental impact of herbicides (Gerstl et al., 1998, Undabeytia et al., 2000, Fernández Pérez et al., 2000, Lagaly, 2001, Celis et al., 2002, Nir et al., 2006, Flores Céspedes et al., 2007, Li et al., 2008). Recently, inorganic–organic clay minerals, namely clay minerals modified with both inorganic and organic cations, demonstrated higher adsorption of organic pollutants (Bouras et al., 2001, Bouras et al., 2007, Wu et al., 2001, Zhu and Zhu, 2007). The present work is aimed to develop adsorbents having enhanced adsorption of the anionic herbicide 2,4-D and the nonionic herbicide acetochlor. For this purpose, inorganic–organic bentonites were prepared by intercalation of poly(hydroxo aluminum) or poly(hydroxo iron) cations, followed by intercalation of cetyl trimethylammonium (CTMA) ions.
Section snippets
Materials
Technical grade herbicides 2,4-D (98.2% pure) and acetochlor (96.2% pure) (Fig. 1) were purchased from Hangzhou Qingfeng Agrochemicals Co. Ltd., China. The bentonite (denoted Na-bent) consisting mainly of Na+-montmorillonite was purchased from Zhejiang, China. It was produced by ion exchange of calcium-bentonite with Na2CO3 by the manufacturer. The product was washed and dried at 115 °C before use in our experiments. The cation exchange capacity (CEC) of the dried Na-bent was 0.570 mmol g− 1. Cetyl
Characterization of bentonites
The specific surface area of Al-bent and Fe-bent was enhanced in comparison with the original Na-bent (Table 1). The basal spacing was also increased, indicating that the interlayer space of montmorillonite was partly occupied by poly(hydroxo metal) cations. As reported previously (Bottero et al., 1988, Michot and Pinnavaia, 1992), Al13 polycations [Al13O4(OH)24(H2O)12]7+ predominate in the hydrolyzed Al3+ solutions when a [OH−]/[Al3+] molar ratio of 2.0–2.4 was used. These Al13 species may
Conclusions
Pillared bentonites with subsequent adsorption of cetyl trimethylammonium ions showed enhanced 2,4-D adsorption in comparison with organo-bentonites but did not show higher adsorption of acetochlor. Thus, inorganic–organic bentonites are a good choice for removing certain pollutants from water and potential carriers for controlled release.
Acknowledgements
This work was supported by the Natural Science Foundation of Zhejiang Province (Y506288) and the Natural Science Foundation of China (30500387).
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