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

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 SO₄^2- 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 O₂ levels, 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.