Use of Organosmectites to Reduce Leaching Losses of Acidic Herbicides

Pepperman, 1995a; Johnson and Pepperman, 1995b). These control-release formulations also decrease pestiThe modification of smectitic clays with organic cations via cationcides losses by offering protection from other processes exchange reactions produces sorbents with an increased sorption capacity for organic compounds such as acidic herbicides. These organoclays such as volatilization (El-Nahhal et al., 1998; El-Nahhal (OCls) could be used as carriers in controlled release formulations of et al., 1999). Clays have been used as an aid in powdered herbicides to decrease their contamination potential. Various OCls and granular formulations of pesticides for a long time. and two acidic herbicides (bentazone [3-isopropyl-1H-2,1,3-benzothiRecently, there has been an increased interest in the adiazin-4 (3H) one 2,2-dioxide] and dicamba [2-methoxy-3,6-dichlorouse of such natural soil components as possible herbicide benzoic acid]) were selected and herbicide–OCl complexes were precarriers in controlled-release formulations (Margullies pared by either sorbing the herbicides on the OCl from solution or et al., 1993; Margullies et al., 1994; Gerstl et al., 1998; by dry mixing of both components. Those preparations were assayed Gonzalez-Pradas et al., 1999; Cox et al., 2000). A recent as controlled-release formulations under static (water solution) and comprehensive monograph on controlled-release fordynamic (soil column leaching) conditions. Herbicide release in closed mulation for pesticides has been edited by Scher (1999). (static) systems was fast and reached a maximum concentration after 10 to 20 h. The total herbicide released ranged from 20 to 100% of The sorptive capacity of clays for organic compounds the active ingredient initially incorporated in the complex depending can be enhanced by chemical modification, whereby the on the sorption capacity of the OCl for the herbicide and the strength native inorganic-exchangeable cations are replaced with of herbicide–OCl interaction (aging time). Complexes releasing !50% organic cations via ion-exchange reactions (Boyd et al., of the herbicide, corresponding to the most sorptive OCls, may not 1991; Xu et al., 1997). Organoclays have been shown to be appropriated for weed control. Total leaching losses in soil columns be good sorbents for removing polar pesticides from were reduced from 94% for free technical bentazone to 55 to 90% water (Hermosin and Cornejo; 1993; Zhao et al., 1996; for bentazone-OCl complexes, and from 100% for technical dicamba Socias-Viciana et al., 1998; Celis et al., 1999; Aguer et to 50 to 100% for dicamba-OCl complexes. Maximum concentrations al., 2000; Sheng and Boyd, 2000). Brixie and Boyd (1994) in the leaching profiles of the herbicide-OCl complexes were much reported that organoclays have a strong immobilizing efsmaller than for the technical compounds. Bioassays of dicamba-OCl complexes as preemergence herbicide showed the same efficiency as fect on nonionic organic chemicals with low water soluthe technical compound. These results suggest OCls as possible carribility such as benzene derivatives and phenols; other studers in controlled release formulations for very mobile and persistent ies have shown OCls to be good sorbents for diverse acidic herbicides, thereby decreasing their potential for surface and nonpolar organic contaminants (Xu et al., 1997; Nir et ground water contamination. al., 2000). Polar chemicals that are soluble in water and weakly sorbed by soil particles can move rapidly with the infilT increasing use of agrochemicals may result in trating water and hence, are likely to be found in ground serious potential human health and environmental water (Goodrich et al., 1991). Acidic herbicides such as problems, which must be addressed to minimize harmful bentazone and dicamba have these characteristics, and effects. When chemicals such as acidic herbicides are aphave been used as models of very mobile and leachable plied to soil, only a small portion reaches the target site. herbicides (Romero et al., 1995; Ritter et al., 1996). ReMost of the pesticide is subject to processes such as cent studies further suggest the use of OCls to protect sorption, degradation, run off, and leaching. Transport soil and water from acidic herbicides such as bentazone by run off and leaching, can result in contamination of (Carrizosa et al., 2000; Carrizosa et al., 2001), as well surface and ground waters (Kalkhoff et al., 1998; Kolpin as potential carriers in slow-release formulations for et al., 1998). polar (Hermosin et al., 2001) and hydrophobic (El-NahThe use of controlled-release formulations of mobile hal et al., 1998; El-Nahhal et al., 1999) pesticides. In herbicides has been suggested to restrict their moveprevious studies, we have shown OCls to be effective ment in soil, thereby reducing their potential of surface sorbents to remove 2,4-D (Hermosin and Cornejo, and ground water contamination. Several synthetic (cat1992), bentazone (Carrizosa et al., 2000), and dicamba ionic surfactants and organic polymers) and natural (Carrizosa et al., 2001) from water. The sorption capac(plant lignin and starch) materials have been proposed ity of OCls is favored by high layer charge and saturation as supporting agents for pesticides in these formulations with bulky organic cations close to the cation-exchange to reduce movement (Gish et al., 1994; Johnson and capacity (CEC). Hydrophobic interactions with polar contributions are responsible for adsorption of these M.J. Carrizosa, M.C. Hermosin, and J. Cornejo, Instituto de Recursos molecules, which need free polar space between alkylNaturales y Agrobiologia de Sevilla, CSIC, P.O. Box 1052, 41080 ammonium groups in the OCl interlayer (Hermosin and Sevilla, Spain; W.C. Koskinen, USDA-ARS, Soil and Water Management research Unit, 1991 Upper Buford Cir. Rm 439, St. Paul, MN 55108. Received 22 June 2001. *Corresponding author (cornejo@ Abbreviations: CEC, cation-exchange capacity; HPLC, high prefoirnase.csic.es). mace liquid chromatography; OC, organic C; OCl, organoclay; TC, technical compound. Published in Soil Sci. Soc. Am. J. 67:511–517 (2003).