Various kinetic expressions were tested with phosphate desorption data from four contrasting soils and found inadequate, but the data supported a model recently proposed by Aharoni and Suzin. This model assumes that chemisorption reactions are kinetically controlled by diffusion and integration of Fick's equation, under appropriate boundary conditions, results in an expression for the time dependence of adsorption or desorption, that can be approximated by a sequence of equations-parabolic, Elovich and exponential-at small, intermediate and large times, respectively. Thus, full or even partial conformation of data to such a sequence would suggest diffusion control of the rate determining step, a suggestion usually relying on a firelationship alone.Since true rate constants could not be derived from the other expressions tested, calculation of physically meaningful activation energies was not possible. An approach to the calculation of activation energies at different stages of the process, based on the model of Aharoni and Suzin. and the difficulties encountered are also discussed.
SUM MARYThe kinetics of phosphate sorption and release from soils are often described by a simplified form of the Elovich equation. However, examination of data obtained with three soils from Greece as well as of published data, based on a modified version of the Elovich equation, showed that the assumptions underlying derivation of the simplified form are not valid in all cases. This modified version is free from any a priori assumptions, describes the kinetics of phosphate reactions with soils in a realistic manner and points out to the existence of more than one stage in the process.
The one‐ and two‐surface Langmuir, the Freundlich, and the Temkin isotherms were fitted to P sorption data for 14 representative alfisols of Greece. Each was found to describe P sorption by these soils with comparable success, with the Freundlich and the two‐surface Langmuir isotherms being slightly superior. The Freundlich equation is characterized by simplicity of form, based on more realistic assumptions, and now capable of rigorous derivation. It can therefore be used in preference to the others, since its parameters, as those of the Langmuir equation, allow comparisons among soils.
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