A study on sorption and desorption kinetics of potassium in some selected surface soils of West Bengal

Five surface soils of different agro-climatic zones of West Bengal with varying potassium supplying power and different clay mineralogical make-up were characterized in terms of relevant physico-chemical properties and various forms of soil K. The K sorption and desorption kinetics in these soils were examined by using the batch technique. The K sorption and desorption kinetic data in the present soils at two temperatures, were fitted to several kinetic models. The K adsorption and desorption rate coefficient (k_a and k_d, respectively) were computed from the respective first-order kinetic equations. The k_a values were the highest and the lowest for the Kalyani and the Abas soils, respectively. The k_d values, on the other hand, were the highest and the lowest for the Abas and the Polba soils, respectively. These k_a and k_d values were well related with the nature and the content of the clay minerals of the given soils. The activation energy for K desorption (E_d) was higher than the corresponding activation energy for K adsorption (E_a) in case of the Kalyani and the Karbala soils, and the reverse was true for the Abas, Polba and Digha soils. The standard free energy change of K adsorption at mean temperature (ΔG⁰_mean), standard enthalpy change of K adsorption at mean temperature (ΔH⁰_mean), standard entropy change of K adsorption at mean temperature (ΔS⁰_mean) were determined from the first-order sorption-desorption kinetic data and were correlated with the relevant soil properties. A comparison among different kinetic equations with the aid of the standard error of estimate (S.E.) and the coefficient of determination (R²) values indicated that the parabolic diffusion and the power-function equations described the adsorption and desorption kinetics of K of the soils equally well as did the first-order kinetic equations. The constants of different kinetic equations were computed and interrelated. The Elovich and the zero-order kinetic equations were less successful in the present study.