Reductive dissolution of magnetite by solutions containing EDTA and Fe II

Abstract

The dissolution of magnetite particles in solutions containing EDTA and Fe II was studied as a function of the total concentration of EDTA and Fe II; the influence of pH was also studied. The rate shows a Langmuir-type dependence on [FeY 2−] when [EDTA] 0 ⩽ [Fe II] 0. At constant [EDTA] 0, in the range where EDTA is in excess over Fe II, the kinetic order on Fe II is one; Fe II in large excess has no influence on the rate. At constant [Fe II] 0, the rate of dissolution is maximum when the ratio of [EDTA] 0 to [Fe II] 0 is close to 3. These results are interpreted in terms of fast solution and surface complexation processes followed by slow heterogeneous electron transfer from adsorbed FeY 2− to surface >Fe III centers and fast phase transfer of >Fe II. The inhibitory effect of excess EDTA results from competitive adsorption of FeY 2− and EDTA. The rate increases with decreasing pH up to pH 3.1; at this value a maximum is achieved. The pH dependence of rate is the resultant of several factors, the most important being the influence of pH on the adsorption preequilibrium and the need for adsorbed protons adjacent to the reactive site. The stoichiometry of the dissolution reaction is not constant and the ratio of protons consumed to iron released is sensitive to experimental conditions. In the fatest reactions, this ratio is appreciably lower than the limiting value corresponding to the release of unhydrolyzed Fe III species. The implications of this result are discussed.