Sorption and precipitation of iron of Kaolinite. II. Sorption isotherms and their interpretation in terms of Iron(III) ionic equilibria

Abstract

The sorption of iron(III) by kaolinite was studied in different media as a function of iron concentration, pH, and time, with such low concentrations of total iron that precipitation was excluded over most of the pH range examined. The reaction continued at a measurable rate for several weeks but a state of quasi-equilibrium, independent of the medium and of clay : solution ratio, was achieved in about 1 week. The amount of iron sorbed from the most acidic solutions was relatively low but increased markedly with increasing pH. Total iron concentrations measured in equilibrium with the clay were resolved by computer into concentrations of the component ionic species, which were then plotted against the total amount of iron sorbed. Unique relationships were found within specified pH ranges. From pH2O to 2.7, the amount of iron sorbed was correlated with the concentration of the species FeOH2+ and, from about pH 2.7 to 3.6, with the concentration of Fe(OH)2+. Specific sorption of these species by the clay is discussed. At higher pH values and prior to the precipitation of iron, other factors contributed to the sorption of iron. A reaction of monomeric iron species from solution with iron already sorbed by the clay is proposed, leading to the formation of polymers on the clay surface. Under suitable conditions of iron concentration and pH, precipitation of ferric hydroxide, probably a later stage of the polymerization process, occurred on the clay surface, in accordance with a well-defined solubility product relationship.