Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/16/1997
Publication Date: N/A
Citation: Interpretive Summary: Contamination of surface and ground water by soil-applied pesticides occurs through runoff events and leaching processes, respectively. Both processes may carry soil particles whose sorption capacity make the particles a carrier of organic contaminants from point of application. Soil clay fraction as a whole (mineral and organic components) has been shown to be responsible for the sorption of many soil applied pesticides. This study determined the influence of different components of the clay fraction of five low organic matter (OM) content soils on the sorption of the new insecticide imidacloprid. Sorption of imidacloprid by mixtures of different model soil colloidal components was also determined. Sorption of imida- cloprid on the soil clay fractions and model sorbents show the importance of OM in sorption of this insecticide; sorption on soil clays decreases after OM removal and increases in model sorbents with the presence of OM. Sorption data also indicate that certain types of clay minerals are also active in sorption of imidacloprid by soil colloids, although mineral surfaces may be obscured by its association with OM. Using this data we have determined the mechanism of imidacloprid sorption to soil and users of the chemical can use this information to reduce the risk of potential for contamination of surface and ground water. For instance, by using low rates of application in soils with typical organic matter and clay contents, imidacloprid would have a low potential to leach through soil.
Technical Abstract: Soil clay (mineral and organic components) has been shown to be responsible for sorption of many pesticides. This study assessed the influence of the different components of the clay fraction of five soils and model sorbents (montmorillonite, iron oxide and humic acid) and their binary and ternary associations on sorption of the insecticide imidacloprid (1-[(6-chloro-3- pyridinyl)-methyl]-N-nitro-2-imidazolidinimine). Sorption was very high on a clay fraction containing a high content of montmorillonite with low laminar charge and relatively high organic carbon content. When the organic matter of the soil clays was reduced by H2O2, sorption decreased, although Koc increased. The increase in Koc was greater on clays whose surface areas substantially increased after H2O2 treatment. This indicates a contribution of the surfaces of mineral components of the soil clay in sorption of imidacloprid. Imidacloprid sorbed on both montmorillonite and humic acid, whereas sorption was very low on iron oxide. Sorption distribution coefficient Kd on montmorillonite increased from 15 to 180 when pH was decreased from 3.5 to 1.0, suggesting protonation of the -NH group of the imidazol ring. Sorption on binary and ternary associations of these model sorbents indicated that organic matter is the most important single component affecting sorption of imidacloprid, although there is some contribution of the mineral surfaces.