Iron Atom Exchange between Hematite and Aqueous Fe(II)

Abstract

Aqueous Fe­(II) has been shown to exchange with structural Fe­(III) in goethite without any significant phase transformation. It remains unclear, however, whether aqueous Fe­(II) undergoes similar exchange reactions with structural Fe­(III) in hematite, a ubiquitous iron oxide mineral. Here, we use an enriched ⁵⁷Fe tracer to show that aqueous Fe­(II) exchanges with structural Fe­(III) in hematite at room temperature, and that the amount of exchange is influenced by particle size, pH, and Fe­(II) concentration. Reaction of 80 nm-hematite (27 m² g^–1) with aqueous Fe­(II) at pH 7.0 for 30 days results in ∼5% of its structural Fe­(III) atoms exchanging with Fe­(II) in solution, which equates to about one surface iron layer. Smaller, 50 nm-hematite particles (54 m² g^–1) undergo about 25% exchange (∼3× surface iron) with aqueous Fe­(II), demonstrating that structural Fe­(III) in hematite is accessible to the fluid in the presence of Fe­(II). The extent of exchange in hematite increases with pH up to 7.5 and then begins to decrease as the pH progresses to 8.0, likely due to surface site saturation by sorbed Fe­(II). Similarly, when we vary the initial amount of added Fe­(II), we observe decreasing amounts of exchange when aqueous Fe­(II) is increased beyond surface saturation. This work shows that Fe­(II) can catalyze iron atom exchange between bulk hematite and aqueous Fe­(II), despite hematite being the most thermodynamically stable iron oxide