Cation Enhanced Anion Retention at the 001 Face of Muscovite

Abstract

The solid-liquid surface chemistry of the 001 face of a cleaved muscovite crystal was studied with the aim of gaining an understanding of interactions in this system that could also underlie the anion retention phenomena of clay mineral surfaces in soils. Muscovite offered many advantages for the study. The crystals were readily cleaved providing clean smooth surfaces of known structure similar to that of several clay minerals. The effect studied was the enhanced anion retention observed when the surfaces were treated with certain cations. Radiochemical methods were used for the low area systems employed. Most of the work was concerned with ions of Al(III), Fe(III), phosphate, and sulphate as these species are important in soils, and the latter three are available in high specific activity radioisotopes. Techniques developed by previous workers were used in preliminary kinetic experiments but this work was discontinued when non reproduceability became apparent. The uneven adsorption, as shown by autoradiography, indicated contamination of the systems. When stringent precautions were observed, uniform and reproduceable adsorption was obtained. Hydrolysis reactions were shown to be of major importance. A survey of sulphate retention after pretreatments by the cations of a representative selection of metals showed that only multivalent and extensively hydrolysed cations with anion exchanging hydrous oxides and hydroxides caused the effect. The use of a technique developed for studying adsorption by surfaces under controlled conditions and in the absence of possible effects due to the crystal edges, showed that non hydrolysed Al3+ and Fe3+ cations had little effect. It was apparent from studies of anion adsorption, that the Fe(III) species that were effective were non exchangeable. Radioactive Fe(III) species were used to study the cation adsorption which occurred in several forms including exchangeable cations, non exchangeable but acid soluble complexes or aggregates, and non exchangeable and acid insoluble aggregates including colloidal particles and macroscopic particles of hydrous oxide. The formation of hydrous oxide films on glass and mica was also observed. The kinetics of phosphate retention by surfaces treated with FeCl3 solution and with a colloidal suspension of hydrous oxide, were consistent with diffusion into spherical particles. A complete description of the sorption desorption processes was complicated by the only partial reversibility of the reactions with the surface phase. Isotopic dilution techniques were applied to estimate the amounts of cations and anions adsorbed. At the highest phosphate concentrations used, a phosphate to Fe(III) ratio of nearly one was observed. There was sufficient material for the formation of one or two atomic layers, or spaced colloidal particles. For the diffusion interpretation of the kinetic effects, the aggregation of the material into colloidal particles was necessary. Adsorption isotherm for phosphate onto Fe(III) pretreated surfaces indicated different adsorption mechanisms at high and low phosphate concentrations. When the cations and anions were adsorbed from the same solution, precipitation did not appear to be an important effect. In this thesis it was shown that colloidal particles and other hydrolysed species adsorbed by an alumino-silicate surface accounted for the majority of the cation enhanced anion retention properties. These species are almost certainly formed and adsorbed during weathering reactions and so could determine the anion "retaining" and "fixing" properties of soils and clays.