Abstract

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 oC. 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 ka were in the range 0.853 - 2.263 and 0.765 - 2.399 for metolachlor and 2,4-D respectively, desorption rate constant kd 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 Ea for adsorption processes followed the range 14.667 - 17.361 kJ•mol-1and 16.053 -31.088 kJ•mol-1 for metolachlor and 2,4-D respectively. Values of equilibrium constant lnKo were in the range 0.036 - 0.608 and 0.018 - 0.869 for metolachlor and 2,4-D respectively. The free energy change Go values were in the range -0.095 to -1.431 kJ•mol-1, and -0.048 to -2.048 kJ•mol-1 for metolachlor and 2,4-D respectively. Values of enthalpy change Ho followed the range -5.954 to -13.353 kJ•mol-1 and -16.998 to -42.127 kJ•mol-1 for metolachlor and 2,4-D respectively. The value of entropy change So followed the range-16.138 to -1.757 J•mol-1•K-1, and -16.292 to -61.292 J•mol-1•K-1 for metolachlor and 2,4-D respectively.

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