Effect of pH on the Dissolution Rate of Magnetite in Solutions Containing Chelating Agents

Abstract

The dissolution behavior of magnetite powder (as received) and pellet (prepared by sintering) in solutions containing EDTA or citrate was studied at different pHs (0.3-9) and temperatures (30-95°C). The concentration of dissolved Fe increased with time, and is highest at pH 2.4 with EDTA and at pH 3.7 with citrate. A rate equation of dissolution was derived from a consideration that the dissolution reactions consist of the following two coupled processes: (1) the transfer of Fe ions from solid to aqueous phase by reaction with chelating agents and (2) the transfer of lattice O^2- ions by reaction with H⁺ ions to form H₂O. The resulting rate equation in solutions with EDTA (H₄Y) is
d[FeY]/dt=k′k″[H⁺]^8/3α₄([Y]_T-[FeY])/(k′[FeY]+k″[H⁺]^8/3)
where k′ and k″ are the rate constants for the processes (1) and (2), α₄ is the ionization fraction of Y^4-, and [Y]_T is the total concentration of EDTA. The amount of dissolved Fe vs. pH peaks at a specific pH, which can be explained by the rate equation noted above.