Testing a mechanistic model. IX. Competition between anions for sorption by soil

A model which describes the reaction of anions and cations with soil when they are added separately was applied to the results of Roy et al. (1986a,b) in which pair‐wise combinations of phosphate, arsenate, and molybdate competed for sorption on three soils. The model postulates that the reaction is between ions in solution and charged surfaces of the soil particles. The surfaces are assumed to be non‐uniform and to differ in their electric potential. Competition comprises both ordinary competition between ions for adsorption sites and the electrostatic interactions which occur because reaction with one anion makes the surface more negative and, therefore, less favourable for reaction with a second anion. The equations of the model were expressed in such a way that the outcome was related to the initial anion concentrations. With appropriate choice of parameters, the model described sorption both when the anions were added singly and when they were added in competing pairs. This shows that the model is comprehensive in that it can be extended to competing pairs of ions. It also shows that simpler descriptions of competition which neglect the electrostatic effects are unlikely to be comprehensive.