The herbicide imazaquin has an acid and a basic ionizable functional group, and its behavior in soil depends on the pH and organic carbon (OC) and oxide contents, mainly in soils with variable charges. The free energy (DG) of 14C-imazaquin molecule sorption was evaluated in surface and subsurface samples of an Anionic "Rhodic" Acrudox (RA), an Anionic "Xanthic" Acrudox (XA), and a Rhodic Kandiudalf (RK), at four different pH levels. The Freundlich equation was fitted to the resulting data to determine the sorption coefficient. Independently of the sampling depth, the stability of the sorbed forms of imazaquin decreased as the pH increased. In other words, imazaquin sorption decreased, i.e., its remaining concentration in the soil solution after the equilibrium increased as the pH increased. For all samples, DG increased abruptly in the pH range that lies between the molecule dissociation constant value (pKa = 3.8) and pKa + 2 (= 5.8). Within this pH range, the percentage of anionic imazaquin forms increased, which entails electrostatic repulsion and water solubility. Among the surface samples, the RK soil showed higher sorption of imazaquin, probably due to its higher OC and clay contents, consequently presenting lower DG values at the different pH levels. However, there was practically no difference among the subsurface samples. DG values emphasized the importance of pesticide pKa, electric potential, and oxide contents in the subsurface soil layer for the understanding of the sorption behavior of ionizable herbicides in highly weathered tropical soils.
pesticide; pH; tropical soil; iron oxide; electric surface potential
