Adsorption of imidazolinone herbicides on smectite-humic acid and smectite-ferrihydrite associations

Adsorption of imazapyr (IMZ), imazethapyr (IMZT), and imazaquin (IMZQ) was studied on two smectite−humic acid and two smectite−ferrihydrite binary systems prepared by treating a Wyoming smectite with a humic acid extracted from soil (4 and 8% w/w of the smectite) and with just-precipitated synthetic ferrihydrite (8 and 16% w/w of the smectite). Adsorption of the three herbicides on the smectite was not measurable at pH >4.5, presumably because of negative charges on the surface of the smectite. Adsorption on the smectite−humic acid systems was also not measurable, presumably because of negative charges on the surface, despite the high affinity of the three herbicides for humic acid, the adsorption order of which was IMZ < IMZT ≪ IMZQ. Adsorption decreased in the order IMZ < IMZT < IMZQ on the smectite−ferrihydrite systems and IMZQ < IMZT < IMZ on ferrihydrite, although here the differences were small. These results show that even though pure smectite cannot adsorb herbicides, it modifies the adsorption capacity of ferrihydrite. The mutual interaction of active phases such as humic acid, ferrihydrite, and smectite alters the characteristics of the resulting surface and hence the adsorption process. Investigations of herbicide adsorption have been seen to produce more reliable results if conducted on polyphasic systems rather than on single soil components.