Adsorption of imidazolinone herbicides on ferrihydrite-humic acid associations

Adsorption of the imidazolinone herbicides imazapyr, imazethapyr and imazaq uin was studied on two binary systems (ferrihydrite-humic acid) prepared by treating ferrihydrite (Fh) immediately after its precipitation with a soil humic acid (HA) at different loadings (4% and X% HA content), and on a bla nk ferrihydrite sample prepared in the same way, but without HA addition. Imidazolinone adsorption on pure Fh and on the 4% Fh-HA decreased with incr easing of the herbicide hydrophobicity (imazaquin < imazethapyr < imazapyr) , whereas on the 8% Fh-HA system the order of adsorption was nearly the sam e, indicating that higher amounts of HA on Fh increase the hydrophobicity o f the surface and thus increases its affinity for the herbicides, specially for imazaquin. Adsorption was also enhanced by low pH values and slight ch anges in pH enhanced the contribution of each component in adsorption. Both the nature of the surface exposed and its ability to interact with her bicides are greatly altered by the mutual interaction of Fh and HA. The res ults show that albeit artificial polyphasic associations are more represent ative of the real situation than single model components and are thus bette r indicators of the behaviour of a given herbicide in the soil.