Coprecipitation of Iron and Aluminum during Titration of Mixed Al3+, Fe3+, and Fe2+ Solutions

Abstract

Ahstract--Potentiometric titration analysis was used to examine the hydrolysis behavior of Fe 2§ Fe 3+, and AP § in pure solution and in mixture, in order to evaluate the potential for coprecipitation and mixed solid-phase formation. Mixtures of Fe 3+ and AP § did not interact during neutralization; base consumed in their respective buffer regions was equivalent to the total metal added. Fe2+-At 3§ solutions, however, showed excess base consumption in the A13§ buffer region, indicating hydrolysis of Fe 2§ at lower than normal pH. Ferric/ferrous iron analyses of systems at the A1 endpoint (pH 5.5) showed amounts of oxidized Fe equivalent to the excess base consumption (-10% of total Fe), with substantial amounts of Fe z+ sorbed to or occluded within A1 polymers present. Increased electrolyte levels or the presence of SO42- inhibited oxidation and sorption of Fe z§ on A1 surfaces, suggesting that Fe hydrolysis and oxidation was catalyzed at the surfaces. Increasing ml 3+ :Fe a§ ratios in the titrated solutions also increased the amount of Fe z§ coprecipitation, supporting a surface-mediated reaction mechanism. Ferrous iron oxidation was sensitive to O2 levels, which also affected the amount ofcoprecipitation. These findings suggest that surface-facilitated oxidation of Fe 2§ may be important in the formation of mixed Fe-AI mineral phases in dilute soil solutions.