Effect of solid-solution ratio on anion adsorption on hydrous metal oxides

Abstract

Goethite (I?-FeOOH) is one of the most common forms of iron oxide in the natural environment and is a strong adsorbent for many anions of environmental interest. These adsorption reactions control the mobility of the anions in the environment. The reactions are usually modeled by assuming that the anion forms a surface complex with the metal ion in the solid. However, the results of many studies on anion adsorption on goethite have shown that the reaction is more complex than a simple surface complex formation, and may involve precipitation as well as surface complexation. One of the results that is inconsistent with surface complex formation is the effect of solids concentration in which the adsorption isotherm are much higher at low solids concentration than at high. In a surface complexation model, the solid-solution ratio should have no effect on the adsorption isotherm. In contrast, in actual experiments, the adsorption isotherm is much greater at low solids concentration than at high. In this study the effect of the solids concentration (solid-solution ratio) is investigated. The results are consistent with a two phase reaction: a very rapid reaction at low surface coverage which is not influence by the solids concentration, followed by a much slower reaction which is influenced by solids concentrations. The first reaction is probably true surface complexation and achieves about 70% of the calculated maximum surface coverage. The second reaction is probably surface precipitation.