To investigate the thermodynamics parameters for adsorption-desorption behavior of metolachlor (nonionic herbicides) and 2,4-D (anionic herbicides) on the selected soils by using a kinetic approach. The objective of this study to investigate the rate constants and other kinetic parameters for the adsorption-desorption behavior and to elucidate the effect of temperature, the experiments were done at 10, 25, 40 ±1 ℃. Data from the experiments revealed that the adsorption-desorption of metolachlor and 2,4-D on the selected soil samples followed the first order rate law, and revealed that adsorption of metolachlor and 2,4-D was spontaneous exothermic, a partly physical and partly chemical in nature to some extent. Values of adsorption rate constant k a were in the range 0.853 -2.263 and 0.765 -2.399 for metolachlor and 2,4-D respectively, desorption rate constant k d were in the range 0.744 -1.303 and 0.705 -1.289 for metolachlor and 2,4-D respectively. By using the Arrhenius equation the activation energy of each adsorption-desorption of both pesticides were calculated. The activation energy E a for adsorption processes followed the range 14.667 -17.361 kJ·mol , and -16.292 to -61.292 J·mol -1 ·K -1 for metolachlor and 2,4-D respectively.

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),

A study was conducted to determine the differences in the adsorption behavior of three pesticides, nonionic atrazine [2-chloro-4-ethyl amino-6-isopropylamino-1,3,5,-triazine], an ionic picloram [4-amino-3,5,6-trichloropicolinic acid] and anilide group propanil [N-3,4-dichlorophenyl) propanaamide] on eight agricultural soil samples. Data from batch equilibrium method revealed that the adsorption of herbicides on the selected soil samples followed the first order rate law. Propanil exhibited the faster rate of accumulation with 24.82% adsorption on the soil solid matrix after 0.5h as compared to that for picloram and atrazine 18.98%, 13.9% respectively. The standard error (S.E) values were from 0.034 to 1.000, 0.007 to 0.983 and from 0.691 to 0.859 for atrazine, picloram and propanil respectively. Linear, Freundlich and Langmuir models were used to describe the adsorption of the three pesticides. Values of distribution coefficient (Kd) indicated moderate to low adsorption for atrazine (mean calculated Kd: 1.1597 mlg-1) while slightly higher for picloram (mean calculated Kd: 1.5779 mlg-1) and highest for propanil adsorption (mean calculated Kd: 12.662 mlg-1) and consequently there is no considerable risk of ground water contamination. Wide variation in adsorption affinities of the soils to the pesticides was observed, Kd values for atrazine varied between 0.951 and 1.53 mlg-1 while for picloram between 0.99 and 2.309 and for propanil between 9.044 and 16.56 mlg-1.