The Study of Adsorption and Desorption of Picloram on six Agricultural Soils


A study was conducted to determine the differences in the adsorption - desorption behavior of picloram [4-amino-3, 5, 6-trichloropicolinic acid] which is an ionic herbicide on six agricultural soil samples. Employing linearized versions of three kinetic models, namely first-order, Elovich, and power function equations, data from batch equilibrium method revealed that the adsorption - desorption of picloram on the selected soil samples followed the first order rate law. Linear and Freundlich models were used to describe the adsorption of the pesticide. Wide variation in adsorption affinities of the soils to the pesticides was observed, Kd values for picloram varied between 1.406 and 2.151 mlg-1 and KF for picloram between 1.078 and 1.189 mlg-1 for adsorption processes. The value of Kd and KFranged from 0.439 to 0.625 mlg-1, 1.045 to 1.585 mlg-1 respectively for the desorption processes. The desorption processes exhibited with the percent 26.4-94.3% these percentage from adsorbed, as compared to adsorption processes percentage is 16.8-56.9 %.
Considering the experimental ko / ko'= keq, the agreement in the compactions is quite satisfactory. It indicates to a good approximation that adsorption in the systems studied may be viewed as a reaction in which a solute molecule collides with an adsorption site to form the adsorption complex, the desorption constant ko' vary by more than an order of magnitude. The large difference in the equilibrium adsorption arise mainly from the difference in the rate of desorption. Values of Ko by using Bigger equilibrium constant for desorption of picloram on selected soil samples were in the following from 2.673 to 11.395. All desorption isotherms exhibited hysteresis. Higher desorption hystersis (picloram was less readily desorbed),