Spectroscopic evidence for Ni(II) surface speciation at the iron oxyhydroxides-water interface

Understanding in situ metal surface speciation on mineral surfaces is critical to predicting the natural attenuation of metals in the subsurface environment. In this study, we have demonstrated the novel Ni K-edge X-ray absorption spectroscopy (XAS) measurements needed to understand Ni(II) surface speciation in three synthetic iron oxyhydroxides (ferrihydrite, goethite, and hematite). The adsorption of Ni gradually increases with increasing pH from 5 to 8, and the adsorption edge appears at near the point of zero salt effect (PZSE) of the solids. The results of XAS analysis indicate four different Ni innersphere surface species are present. While total Ni surface species in hematite at pH 6.85 surfaces consist of ∼63% face-sharing (interatomic distance of Ni-Fe (R_Ni-Fe) ∼2.9 Å) and ∼37% corner-sharing (R_Ni-Fe ∼4.0 Å) surface species on iron octahedra, a combination of two different edge-sharing (between NiO₆ and FeO₆ octahedra, in chains or in rows) and corner-sharing surface species are observed in goethite and ferrihydrite at pH 5.09-6.89. In ferrihydrite, approximately 70% of surface species are edge-sharing surface species (in chains) (R_Ni-Fe ∼3.0 Å), followed by ∼30% of edge-sharing species (in rows) (R_Ni-Fe ∼3.2 Å) and ∼3-5% of corner-sharing surface species (R_Ni-Fe ∼4.0Å). Goethite contains ∼54% edge-sharing (R_Ni-Fe ∼3.0 Å), ∼26% edge-sharing (R_Ni-Fe ∼3.2 Å), and 20% corner-sharing surface species. These findings indicate that the reactivity and surface speciation of Ni are sensitive to the crystallinity of iron oxyhydroxides. The spectroscopic evidence for multi-Ni surface speciation should be factored into predictions of the transport of Ni in soil-water environments.