Influence of pH on the surface properties of magnetite and hematite: A comprehensive spectroscopic and potentiometric study

Iron oxides, due to their abundance and surface reactivity, are essential materials in environmental and industrial applications, where their behaviour in aqueous systems governs processes such as catalysis, remediation, and mineral beneficiation. The influence of pH on the surface properties and reactivity of magnetite and hematite was investigated by using X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT), and potentiometric titrations. Under acidic conditions, the positively charged surface enhances water adsorption and promotes the formation of surface hydroxyl groups, while also stabilizing carbonate species derived from dissolved CO 2 . In contrast, basic conditions result in reduced water adsorption and the absence of CO 2 uptake due to electrostatic repulsion at the negatively charge interface. XPS analysis identifies three distinct oxygen species on the iron oxide surfaces: lattice oxygen, surface hydroxyl groups, and adsorbed water and/or carbonate groups. DRIFT spectra corroborate these findings, showing increased water adsorption and hydroxyl formation at low pH. Moreover, exposure of magnetite to strongly acidic conditions (pH 0) lead to preferential Fe 2+ dissolution and oxidation, resulting in its transformation into vacancy-rich maghemite (γ-Fe 2 O 3 ). Potentiometric titrations confirm a 2-pK model for both oxides, yielding pKa values of 5.7 and 8.7 for hematite and 5.8 and 8.2 for magnetite. Differences in water adsorption behaviour between the two oxides are attributed to their structural characteristics and specific surface areas. These insights are highly relevant for enhancing catalytic performance, improving iron ore beneficiation, and optimizing industrial processes involving iron oxides.