Energetics of bulk and nano-akaganeite, β-FeOOH: Enthalpy of formation, surface enthalpy, and enthalpy of water adsorption

Akaganeite, β-FeOOH, is a commonly occurring ferric mineral in the environment and is a sorbent, ion exchanger, and catalyst. It is often fine-grained (nanophase) and frequently contains excess water. Its enthalpy of formation was studied by solution calorimetry in aqueous HCl. The enthalpy of water adsorption was studied by a new calorimetric technique combining a Calvet microcalorimeter and an automated gas dosing system, used for surface adsorption measurements. Akaganeite samples with surface areas of 30-280 m ²/g were used. Sample characterization was performed by X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer-Emmett-Teller method, scanning electron microscopy, and transmission electron microscopy. Surface enthalpy and enthalpy of water adsorption are reported for the first time. By adsorbing water, akaganeite decreases its effective surface enthalpy from 0.44 J/m ² to 0.34 J/m ². The enthalpy of formation of akaganeite can vary by 10-12 kJ/mol as a function of the surface area. The standard enthalpy of formation of akaganeite with zero surface area was refined and is -554.7 ± 1.9 kJ/mol. Thus, the standard enthalpy of formation and surface enthalpy of akaganeite are between those of goethite and lepidocrocite. The more metastable the polymorph, the lower its surface energy.