Simultaneous Inner- and Outer-Sphere Arsenate Adsorption on Corundum and Hematite

Left: Derived interfacial density profile as a function of height above the Al2O3 (012)-solution interface obtained with in-situ measurements using resonant anomalous X-ray reflectivity.  This plot also shows the arsenic-specific density profile (obtained both with a model-independent and model-dependent approaches), and clearly indicate two distinct As-adsorption species.  Below: Schematic depiction of the adsorption geometry at the Al2O3 (012) surface.  The inner-sphere species is directly bound to the surface as observed previously.  The second outer-sphere (OS) species has no direct bond.  Similar results were also observed on the hematite (012) surface.

Elevated arsenic concentration in groundwater is a major public health concern. Decades of research have led to the conclusion that arsenate adsorbs to iron and aluminum oxide surfaces solely as an inner-sphere species. This implies a robust mechanism for sequestration of arsenic through adsorption to mineral surfaces.

Direct in-situ observations of arsenic adsorption to alumina and hematite surfaces using resonant anomalous X-ray reflectivity have led to new insights into the mechanism of surface binding:

• Arsenate adsorbs simultaneously as an inner-sphere (IS) and outer-sphere (OS) species on both alumina and hematite surfaces. The binding geometry of the IS species is consistent with previous studies, but the OS species was not previously observed.
• The partitioning of arsenate between IS and OS species on each surface was independent of the As-solution concentration (i.e., surface coverage), but was different for the two surfaces.
• Stabilization of the OS species is likely through a combination of electrostatic attraction, hydrogen bonds to the surface oxygens, and through interactions with the interfacial water layer.
• These results raise questions concerning the effectiveness of surface adsorption to metal-oxide surfaces as a mechanism of reducing the bio-availability of arsenate.

Reference

J. G. Catalano, C. Park, P. Fenter, and Z. Zhang, “Simultaneous Inner- and Outer-Sphere Arsenate Adsorption on Corundum and Hematite,” Geochimica et Cosmochimica Acta, 72(8) 1986-2004 (2008).