Effect of pH on sorption kinetic process of acidic herbicides in a volcanic soil

Kinetic studies of pesticides in soil are of great importance to know the processes and parameters that govern their fate in the environment. The aim of this study was to evaluate the influence of pH on the kinetic sorption process of four acidic herbicides, clopyralid, fluroxypyr, picloram and triclopyr on an Andisol, characterized by its high organic matter content and acidic pH. The pseudo-second-order model appeared to fit the data better than other models (R2>0.999). All herbicides were adsorbed rapidly during the first stage, but their adsorption was affected by pH and their chemical nature. The initial rates of reactions strongly decreased by increasing pH. The initial adsorption rates (h) and the sorption capacity of the herbicides showed the following order fluroxypyr > triclopyr > picloram > clopyralid. At pH 4.0 the maximum amounts of fluroxypyr, triclopyr, picloram, and clopyralid adsorbed were respectively 75.2, 69.7, 40.5 and 11.7 %. The application of Elovich and Weber-Morris models suggests that mass transfer through the boundary layer and to a lesser degree intra-particle diffusion, control sorption kinetics, which appeared to be influenced by pH and chemical nature of the herbicides. The results obtained in this work suggest that soil pH and acidic character of herbicides could strongly affect the bioavailability of these herbicides on soil and their potential leaching.