Hydration of arsenic oxyacid species

Abstract

The bond distances in hydrated arsenic oxyacid species in aqueous solution have been studied by EXAFS spectroscopy and large angle X-ray scattering, LAXS. These results have been compared to structures in the solid state, as found in an extensive survey of available crystal structures. Protonated oxygen atoms can be distinguished with a longer As–O distance for both arsenic(V) and arsenic(III) species in the crystalline state. However, the average As–O distance for the H_nAsO₄⁽³⁻ⁿ⁾⁻ species (0 ≤ n ≤ 3) remains the same. These average values are slightly shorter, ca. 0.02 Å, than in aqueous solution due to the hydration as determined by EXAFS and LAXS. The K absorption edges for arsenic(V) and arsenic(III) species are separated by 4.0 eV, and the shape of the absorption edges differs as well. Small but significant differences in the absorption edge features are seen between the neutral acids and the charged oxyacid species. The most important arsenic species from an environmental point of view is arsenous acid, As(OH)₃. In addition to arsenous acid, we have used orthotelluric acid, Te(OH)₆, for comparison with arsenous acid and for detailed studies of the hydration of covalently bound hydroxo groups. Arsenous acid cannot be studied with the same accuracy as orthotelluric acid, due to a relatively low solubility of As₂O₃(s) in neutral to acidic aqueous solution. The results from the DDIR studies support the assignment of As(OH)₃ as a weak structure maker analogous to Te(OH)₆, both being neutral weak oxyacids.