Coprecipitation of Iron and Aluminum During Titration of Mixed AP+, Fe3+, and Fe2+Solutions

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Abstract
Potentiometric titration analysis was used to examine the hydrolysis behavior of Fe2+Fe3+, and Al3+in pure solution and in mixture, in order to evaluate the potential for coprecipitation and mixed solid-phase formation. Mixtures of Fe3+and Al3+did not interact during neutralization; base consumed in their respective buffer regions was equivalent to the total metal added. Fe2+-Al3+solutions, however, showed excess base consumption in the Al3+buffer region, indicating hydrolysis of Fe2+at lower than normal pH. Ferric/ferrous iron analyses of systems at the Al endpoint (pH 5.5) showed amounts of oxidized Fe equivalent to the excess base consumption (∼10% of total Fe), with substantial amounts of Fe2+sorbed to or occluded within Al polymers present. Increased electrolyte levels or the presence of SO42-inhibited oxidation and sorption of Fe2+on Al surfaces, suggesting that Fe hydrolysis and oxidation was catalyzed at the surfaces. Increasing Al3+: Fe2+ratios in the titrated solutions also increased the amount of Fe2+coprecipitation, supporting a surface-mediated reaction mechanism. Ferrous iron oxidation was sensitive to O2levels, which also affected the amount of coprecipitation. These findings suggest that surface-facilitated oxidation of Fe2+may be important in the formation of mixed Fe-Al mineral phases in dilute soil solutions.