Synthesis of Goethite α‑FeOOH Particles by Air Oxidation of Ferrous Hydroxide Fe(OH)₂ Suspensions: Insight on the Formation Mechanism

Iron oxide and iron oxyhydroxide particles, particularly, the goethite α-FeOOH phase, are environmentally friendly materials and are used in various technological applications as adsorbents, precursors of Fe powders for magnetic recording media, and pigments. In this work, the formation process of α-FeOOH by air oxidation of Fe­(OH)₂ suspensions has been studied. The effects of the air flow rate, as well as of the reactant concentration ratio, <i>R</i> (=[(OH)⁻]/[Fe­(II)]), on the reaction product were analyzed. It has been found that the morphology and the size of the α-FeOOH particles can be modified by means of the air flow rate. Furthermore, by performing a detailed microscopic analysis of the morphology of the initial, intermediate, and final reaction products, we have obtained evidence of epitaxial growth of α-FeOOH on the Fe­(OH)₂ substrate. It is suggested that the similarity between the anion arrangements in both phases facilitates this process. Based on these results, a pathway for the formation of α-FeOOH in highly alkaline medium is proposed in which the size and shape of the initial Fe­(OH)₂ particles plays a significant role in the formation the α-FeOOH particles obtained upon completion of the oxidation process