Formation of Layered Fe(II)–Al(III)-Hydroxides during Reaction of Fe(II) with Aluminum Oxide

The reactivity of aqueous Fe­(II) with aluminum oxide in anoxic solutions was investigated with batch kinetic experiments combined with Fe <i>K</i> edge X-ray absorption spectroscopy measurements to characterize Fe­(II) sorption products. Formation of Fe­(II)–Al­(III)-layered double hydroxides with an octahedral sheet structure similar to nikischerite (NaFe^II₆ Al₃(SO₄)₂(OH)₁₈ (H₂O)₁₂) was observed within a few hours during sorption at pH 7.5 and aqueous Fe­(II) concentrations of 1–3 mM. These Fe­(II) phases are composed of brucite-like Fe^II(OH)₂ sheets with partial substitution of Al­(III) for Fe­(II), charge balanced by anions coordinated along the basal planes. Their fast rate of formation suggests that these previously unrecognized Fe­(II) phases, which are structurally and compositionally similar to green rust, may be an important sink of Fe­(II) in suboxic and anoxic geochemical environments, and impact the fate of structurally compatible trace metals, such as Co­(II), Ni­(II), and Zn­(II), as well as redox-reactive species including Cr­(VI) and U­(VI). Further studies are required to assess the thermodynamics, formation kinetics, and stability of these Fe­(II) minerals under field conditions