Enthalpy of Water Adsorption and Surface Enthalpy of Goethite (α-FeOOH) and Hematite (α-Fe2O3)

Goethite, α-FeOOH, and hematite, α-Fe2O3, have high affinity for water, especially when particle size is small. To determine the enthalpy of different types of adsorbed water, we performed water adsorption calorimetry on goethite and hematite with surface areas of 60−270 and 2−150 m2/g, respectively, using a new calorimetric technique combining a microcalorimeter and an automated gas dosing system. Several types of strongly bound water can be distinguished on hematite, depending on heating temperature and surface area. These have enthalpies of adsorption relative to liquid water (ΔHads) equal to −67.1 ± 4.9, −48.6 ± 1.8, and −25.5 ± 4.4 kJ/mol. The last value corresponds to water adsorption on very fine grained hematite and is very close to the water adsorption enthalpy for goethite, ΔHads = −19.4 ± 4.2 kJ/mol. Surface enthalpies for anhydrous surfaces of goethite (0.91 ± 0.09 J/m2) and hematite (1.9 ± 0.3 J/m2) determined experimentally are reported for the first time. The significant difference in surfa...