Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite

The functional group chemistry and coordination of AsO43--sorption complexes in ettringite [Ca6Al2(SO4)(3)(OH)(12). 26H(2)O] 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 =Ca-2-OH exhibit broad overlapping OH bands in the range 3600-3200 cm(-1), prohibiting separation of component vibrational bands. The SO42- 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 AsO43- adsorption, the vibrational spectra of SO42- were least affected, and the OH stretching intensities around 3600 cm(-1) decreased with an increase in AsO43- 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 AsO43-- coprecipitation. The As-O asymmetric stretch of sorbed AsO43- are split and occur as overlapping peaks around 870 cm(-1). The As-O-complexed stretching vibrations are at similar to 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 AsO43- ions are protonated. These spectral features demonstrate that AsO43- 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 AsO43- polyhedra interacts predominantly with =Ca-OH2 and =Ca-2-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. Copyright (C) 1998 Elsevier Science Ltd.