On the adsorption of iron oxide—hydroxide sol on alumina and silica

Abstract

The adsorption of α-FeO (OH) colloidal particles was investigated on coarsely dispersed adsorbents by the radioisotopic tracer method. On the basis of the Langmuir representation, the sol adsorption is usually considered as a pseudo first-order process, and changes in the adsorbed amount with the equilibrium sol concentration can be described by the Langmuir adsorption isotherm. However, the adsorption of the iron oxide-hydroxide sol on aluminum oxide (pH 4.3) was not found to be first order. An expression was derived which made possible the simple treatment of experimental data even in such cases. In the case of adsorption of iron oxide-hydroxide on SiO2 (pH 4.3–5) the amount adsorbed by unit mass of adsorbent (the specific adsorbed amount) changed with the sol/adsorbent ratio at a given equilibrium sol concentration, i.e., the Langmuir adsorption isotherm proved unsuitable for the quantitative description of adsorption.Results suggest that the surface properties of the SiO2 change by changing the sol/adsorbent ratio; the SiO2 becomes positive by adsorbing ferric hydroxo complex species from solution. However, if ferric ions in sufficiently high concentration are added to the ultrafiltrate at pH 3.6 before the adsorption measurement, the SiO2 will acquire a net positive charge; as a consequence, the measured data lie along a straight line by plotting the linearized Langmuir isotherm, i.e., amounts of particles adsorbed at equilibrium will be independent of the sol/adsorbent ratio. The effect of indifferent electrolytes on the iron oxide-hydroxide sol/SiO2 (or silica) system (pH 3.6) is interpreted in terms of interactions between surfaces of like charge. The special effect of the fluoride ion is attributed to secondary processes of a chemical nature and cannot be interpreted simply by the DLVO theory taking into account only interactions between double layers.