Some studies of the diffusion model of phosphate sorption on mica surfaces

Abstract

The work described in this thesis was part of a research effort aimed at understanding the chemistry underlying the fixation of phosphate by soils. The model system chosen was the freshly cleaved 001 face of the clay mineral muscovite. It had been found that the pretreatment of the surface with aluminium or iron (iii) solutions greatly enhanced the uptake of phosphate by the surface. A diffusion model has been proposed to account for this phenomenon. The model proposed that phosphate ions diffused into positively charged spherical colloidal particles of hydrous aluminium oxide attached to the negatively charged surface, the particles having been formed during the pretreatment process. The purpose of the work described in this thesis is to test and study the diffusion model. In order to understand the nature of the colloid-mica bond, attempts were made to move the particles along the surface in an electric field but the experiments failed to give conclusive results. Later experiments with 59Fe treated surfaces suggested that when desorptions were carried out into strong phosphate solutions, the cationic surface phase desorbed by a reversal of charge mechanism. This implied that the colloid-mica bend was of ionic character. An attempt was made to correlate the properties of aluminium-phosphate systems on paper with similar systems on the mica surface. The results of the chromatography experiments were not encouraging, but the electrophoresis experiments led to a tentative correlation. Treatment of colloidal aluminium hydroxide solutions with phosphate before treating the mica with the colloid did not indicate that diffusion was important when the colloid was in the solution phase. The sorption kinetics of phosphate were measured (using 32P tracer) to test the kinetic equations based on the simple diffusion model. The agreement was not sufficient for acceptance of the simple diffusion model without modification. Further experiments also indicated that the diffusion theory required modification. The desorption process was found to be non-reversible. More important was the desorption of the cation from the surface. From 59Fe experiments, it was found that after the initial fast desorption of phosphate, the rates of desorption of cation and anion were similar. Increasing the time of phosphate adsorption caused a lessening in the proportion of phosphate released independently of iron. Then, when desorption was carried out into strong phosphate solution, the entire surface phase desorbed. An experiment with a lower pH desorbing solution of strong phosphate indicated that the effect might be due to reversal of the charge on the particles. A modified diffusion theory was proposed. In this model a fast adsorption of phosphate onto the surface of the hydrous oxide particles was followed by slow diffusion into the particles with replacement of hydroxyl groups. When the solution in contact with the mica was reduced to zero concentration of phosphate, the surface phase desorbed away quickly. The following slow process was ascribed to the loss of particles from the surface caused by turbulence in the solution.