It was assumed that the reaction between phosphate and goethite consisted of a second order forward reaction between phosphate molecules and adsorption sites, opposed by a first order desorption reaction. Because adsorption occurs at a charged surface, it was assumed that the rates were affected by the electrostatic potentials at the surface. A mixed analytic and numeric approach was used in which the rate equation was integrated over small time steps and a computer programme was used to generate the effect of many small steps. Comparison between the model and results for rate of adsorption suggested that the rate‐determining step preceded the electron transfer steps and did not, of itself, involve electron transfer. With this assumption, rate of adsorption could be modelled with the estimation of only one parameter beyond those needed to model the equilibrium values for adsorption. The model gave a good description of the rate of adsorption at three pH values for times up to one hour. It did not describe the slow adsorption over longer periods and this suggested that a different mechanism was involved. Although the main features were reproduced, description of desorption was less satisfactory. This deficiency of the model may occur because it was restricted to the reaction with phosphate and did not model the rate component for the other changes in charge that accompany adsorption.
|Number of pages||10|
|Journal||Journal of Soil Science|
|Publication status||Published - 1 Jan 1981|