Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite

The functional group chemistry and coordination of AsO₄^3-sorption complexes in ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O] were evaluated as a function of sorption type (adsorption, coprecipitation) and pH using Raman and Fourier Transform infrared (FTIR) spectroscopies. The reactive functional groups of ettringite, ≡Al-OH, ≡Ca-OH₂, and ≡Ca₂-OH exhibit broad overlapping OH bands in the range 3600-3200 cm^-1, prohibiting separation of component vibrational bands. The SO₄^2- polyhedra of the channels are present in three crystallographically different sites and exhibit weakly split S-O asymmetric stretch at 1136 cm^-1 (with several components) and symmetric stretch at 1016, 1008, and 989 cm^-1. During AsO₄^3- adsorption, the vibrational spectra of SO₄^2- were least affected, and the OH stretching intensities around 3600 cm^-1 decreased with an increase in AsO₄^3- sorption. In contrast, the S-O symmetric stretch at 1016 and 1008 cm^-1 were almost completely removed, and the OH vibrations were relatively unaffected during AsO₄^3-coprecipitation. The As-O asymmetric stretch of sorbed AsO₄^3- are split and occur as overlapping peaks around 870 cm^-1. The As-O(complexed) stretching vibrations are at ~800 cm^-1. The low pH samples (pH = 10.3-11.0) exhibit distinct As-OH stretching vibrations at 748 cm^-1, indicating that some of the sorbed AsO₄^3- ions are protonated. These spectral features demonstrate that AsO₄^3- directly interacts with ettringite surface sites during adsorption and substitute inside the channels during coprecipitation (preferentially for two of the three sites). The energy position of the As-O symmetric stretch vibrations suggest that the AsO₄^3- polyhedra interacts predominantly with ≡Ca-OH₂ and ≡Ca₂-OH sites rather than with ≡Al-OH sites. Sorption of more than one type of As species was evident in low pH (<11.0) samples.