Raman spectroscopic study of the hydrogen‐arsenate mineral pharmacolite Ca(AsO3OH)·2H2O—implications for aquifer and sediment remediation

The removal of arsenate anions from aqueous media, sediments and wasted soils is of environmental significance. The reaction of gypsum with the arsenate anion results in pharmacolite mineral formation, together with related minerals. Raman and infrared (IR) spectroscopy have been used to study the mineral pharmacolite Ca(AsO₃OH)· 2H₂O. The mineral is characterised by an intense Raman band at 865 cm^?1 assigned to the ν₁ (AsO₃)^2? symmetric stretching mode. The equivalent IR band is found at 864 cm^?1. The low‐intensity Raman bands in the range from 844 to 886 cm^?1 provide evidence for ν₃ (AsO₃) antisymmetric stretching vibrations. A series of overlapping bands in the 300‐450 cm^?1 region are attributed to ν₂ and ν₄ (AsO₃) bending modes. Prominent Raman bands at around 3187 cm^?1 are assigned to the OH stretching vibrations of hydrogen‐bonded water molecules and the two sharp bands at 3425 and 3526 cm^?1 to the OH stretching vibrations of only weakly hydrogen‐bonded hydroxyls in (AsO₃OH)^2? units. Copyright © 2009 John Wiley & Sons, Ltd.