Adsorption of the herbicides imazapyr, imazethapyr and imazaquin on soils and humic acids

Adsorption of the imidazolinone herbicides imazapyr, imazethapyr and imazaquin have been studied on two humic acids and on ten soils. On humic acids, adsorption was promoted by low pH values and was in the order imazapyr < imazethapyr < imazaquin corresponding to the increasing lipophilicity of the molecules, in agreement with the octanol-water partition coefficient. On soils, the correlations between the Freundlich coefficient K-f of the isothermes and the soil characteristics provided evidence that for each herbicide studied, the soil components mainly involved in the process were organic matter, amorphous iron oxides and iron-organic matter complexes. Adsorption was promoted by low pH values. However, the order of adsorption in soils was imazapyr approximate to imazaquin < imazethapyr though imazaquin exhibited the highest affinity for soil humic acids. An hypothesis for this behaviour could be that the big and rigid quinoline moiety of the imazaquin molecule does not allow it to reach the adsorption sites of natural soils.