Antimonate and arsenate speciation on reactive soil minerals studied by differential pair distribution function analysis

- Ferrihydrite (Fh) and δ-MnO2 adsorb more Sb(V) than As(V), whereas montmorillonite adsorbs Sb(V) and As(V) similarly.
- Differential PDF (d-PDF) analysis shows As(V) adsorbs only in double corner-sharing (2C) geometries to Fh and δ-MnO2.
- Sb(V) binds in single edge-sharing (1E) and 2C geometries to Fh and δ-MnO2 but only in 2C geometries to montmorillonite.
- Atomic pairs indexed with the d-PDF at R > 6 Å show that sorption of Sb(V) by Fh induces less disorder to the mineral than sorption of As(V) by Fh.
- Differences in Sb(V) and As(V) interactions with mineral surfaces are linked to their distinct coordination environments.

The pentavalent oxyanions antimonate (Sb(V)) and arsenate (As(V)) are toxic metalloids in environmental systems. In this study, we compare their sorption and surface speciation on 2-line ferrihydrite (Fh), δ-MnO2 and Na-montmorillonite (Mont) at pH 5.5. The Sb(V) and As(V) sorption affinity and the highest measured surface excess (mmol:g) show that mineral reactivity towards Sb(V) or As(V) sorption increases in the order of Mont < δ-MnO2 < Fh. In addition, Sb(V) showed greater uptake relative to As(V) by both Fh and δ-MnO2. Using differential pair distribution function (d-PDF) analysis of high-energy X-ray scattering data, we found that the larger SbO6 octahedron (RSb-O = 1.95 Å) can bind in both single edge-(1E) and double corner-sharing (2C) geometries to Fh and δ-MnO2, whereas the smaller AsO4 tetrahedron (RAs-O = 1.69 Å) binds only in the 2C geometry. Thus, a greater number of surface sites support Sb(V) adsorption relative to As(V) adsorption on Fh and δ-MnO2. Conversely, Mont supports only 2C adsorption geometries, which is consistent with the similar uptake behaviors of Sb(V) and As(V) on this mineral. Finally, we indexed atomic pairs from adsorbed Sb(V) and As(V) at R > 6 Å in the d-PDFs. The analysis of the intermediate-ranged surface structure of our samples substantiated our interpretations of oxyanion adsorption geometries and provided information regarding changes in the underlying mineral surface upon ion adsorption. Our results show that the differences between Sb(V) and As(V) interactions with reactive soil minerals, including their sorption geometries, are linked to their distinct first-shell coordination environments.