Atrazine and picloram adsorption in organic horizon forest samples under laboratory conditions

Adsorption of two herbicides, atrazine and picloram, displaying different sorption characteristics, were evaluated for O (organic) horizon samples collected from SMZs (streamside management zones) in Piedmont (Ultisol) of Georgia, USA. Samples were randomly collected from within 5 SMZs selected for a study of surface flow in field trials. The five SMZs represented five different slope classes, 2, 5, 10, 15 and 20%. Results indicate that 0 horizons have the potential for sorbing atrazine from surface water moving through forested SMZs. Atrazine adsorption was nearly linear over a 24-hour period. Equilibrium adsorption, determined through 24-hour laboratory tests, resulted in a Freundlich coefficient of 67.5 for atrazine. For picloram, negative adsorption was observed in laboratory experiments. This seemed to be due to interference with ELISA analyses; however, this was not confirmed. The adsorption coefficient (Kd) obtained for atrazine in 0 horizons was greater than it would have been expected for mineral soil (from 1 to 4). Picloram was not sorbed in 0 horizons at any significant degree. Although there is a significant potential for the direct adsorption of soluble forms of herbicides in SMZs, the actual value of this adsorption for protecting water is likely to be limited even for relatively strongly sorbed chemicals, such as atrazine, due to relatively slow uptake kinetics. Este trabalho avaliou a adsorção de dois herbicidas - atrazina e picloram - com diferentes características de sorção, em amostras florestais de horizonte orgânico (HO) coletadas em matas ciliares de manejo (MCM) localizadas em uma região de Piedmond (Ultisol), no estado da Geórgia, EUA. As amostras foram coletadas aleatoriamente dentro de cinco áreas de mata ciliar distintas, em classes de declividade de 2, 5, 10, 15 e 20%, as quais foram selecionadas para desenvolver experimentos de campo sobre o transporte de herbicidas por escoamento superficial. Os resultados indicam que o HO apresenta potencial para adsorver atrazina na solução do escoamento superficial ao longo da mata ciliar. Em um período de 24 horas, o processo de adsorção de atrazina apresentou tendência linear. O equilíbrio de adsorção, determinado em testes em laboratório com duração de 24 horas, pode ser expresso pela curva de Freundlich com coeficiente f de 67,5 para a atrazina. O comportamento do picloram nos experimentos em laboratório foi diferente, apresentando tendência de adsorção negativa. Apesar de ainda não ser