A study of the oxidation of ferrous hydroxide in slightly basic solution to produce gamma-FeOOH

During the reaction of Fe(II) with oxygen, three regions of nearly constant pH are observed, each separated by two regions where there is an abrupt pH change. Data for the oxidation of a suspension with [Fe(II)]/[OH-] = 7/12 and 0.22 M Fe(II) indicate that the oxidation to Fe(III) and the associated hydrolysis to produce a proton are rapid compared to the reaction that consumes the proton generated during the first step. It is suggested that the oxidation occurs in the aqueous solution, and that the slow step is due to the proton reacting with a solid Fe(II) species to produce soluble Fe(II). Fe3O4 is formed at, or near, the first step in the oxidation-pa curve, and then redissolves as oxidation continues. The amount of Fe(II) and Fe(III) in the solid and solution have been determined at several stages of the oxidation, and these are consistent with the amount of oxygen consumed during the reaction.