The Adsorption of Phosphate on Mica Surfaces

Abstract

The surface chemistry of the 001 face of cleaved mica sheets was studied with a view to understanding some of the fundamental processes underlying the phenomenon of fixation of phosphate by soils. Radiochemical techniques were developed to make quantitative studies of the adsorption, an important part of these being practical procedures for obtaining sufficient cleanliness and freedeom from airborne contamination. Lack of uniformity of adsorption, as shown by autoradiography, was taken to indicate contamination, and techniques were developed to avid this. Other techniques enabled the continuous monitoring of the sample during adsorption or desorption kinetic experiments. It was shown that adsorption of phosphate on the untreated mica sheets was low, but the adsorption was greatly enhanced if the mica had been treated with aqueous solutions of certain cations such as gallium, aluminium and iron. Form the measurement of the amount of phosphate adsorbed, as a function of the conditions of aluminium treatment, it was concluded that the phosphate could be absorbed by at least three different processes, all of which could be of importance in phosphate fixation by soils. As well as these processes, which occurred on clean, flat, mica surfaces, there were others, involving the edges of mica and sheets, and unknown, but probably organic, films on both mica and air-water surfaces. These could all be of comparable importance in soils. The kinetic measurements of phosphate adsorption and desorption on aluminium-treated mica indicated that many hours were required for attainment of equilibrium, and were quantitatively consistent with the hypothesis that in some cases the adsorption and desorption kinetics were controlled by diffusion of phosphate into particles of some material, possibly a hydrous oxide, adsorbed on the mica. The existence of such particles was supported by the fact that up to one phosphate molecule per two square Angstrom units of mica surface was adsorbed, (and this did not appear to be a value at which the surface was saturated.) Kinetic measurements of 67 Ga sorption processes were consistent with diffusion of gallium through a thin water film, with a diffusion coefficient several orders of magnitude lower than that of single ions in free solution. This may indicate that the gallium was adsorbing as particles, in agreement with the requirements of the phosphate experiments.