Solubility of hematite revisited: effects of hydration

Measured pH and dissolved ferric iron concentration ([Fe(III)diss]) in contact with well-characterized hematite indicated an equilibrium with hematite immediately after synthesis, but [Fe(III)diss] increased with hydration time to be consistent with the predicted solubility of goethite or hydrous ferric oxide (HFO), hydrated analogues of hematite. X-ray diffraction did not detect structural modification of hematite after 190 days of hydration, but Mössbauer spectroscopy detected hydration that penetrated several crystalline layers. When the hematite suspension was diluted with water, solids were invariably identified as hematite, but [Fe(III)diss] and pH indicated an equilibrium with goethite or HFO. This is the first experimental confirmation that the interfacial hydration of anhydrous hematite results in higher solubility than predicted by bulk thermodynamic properties of hematite. Correspondence of the results with previously published measurements and implications for environmental chemistry of ferric oxides are also discussed.