Raman spectroscopic study of arsenic speciation in aqueous solutions up to 275°C

Abstract

Raman spectroscopic measurements were performed on As2O3–H2O solutions (0.02⩽[As]⩽5.2 mol kg-1) at temperatures from 20 to 275°C. At 20°C the spectrum of arsenic solutions of low concentration (0.02–0.5 mol kg-1) is the same over a wide range of pH (0–8). It only exhibits a polarized band at 700 cm-1 with a depolarized shoulder at 655 cm-1. These bands can be attributed to the pyramidal molecule As(OH)30. The spectrum of these solutions does not change significantly with increasing temperature. The only change consists in a small shift of the two bands towards low wavenumbers. In solutions of medium As concentration (1 mol kg-1), a broadening of the 700 cm-1 band towards low wavenumbers is observed. This can be explained by the formation of a non-dehydrated dimeric species which forms via hydrogen bridging bonds. At higher As concentrations (2–5.2 mol kg-1), a new band appears at 525 cm-1, the intensity of which increases with increasing As concentration and temperature. By comparison with the spectrum of fused arsenic oxide, this band can be attributed to As—O—As bonds. This suggests that polymeric species are present in concentrated As solutions. In the most concentrated solutions (4.1 and 5.2 mol kg-1), a new band occurs at 380 cm-1, which can be attributed to the symmetric stretching of the As4O6 tetrahedron. © 1997 John Wiley & Sons, Ltd.