Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite


Myneni, SCB, SJ Traina, GA Waychunas, and TJ Logan. 1998. “Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite.Geochimica Et Cosmochimica Acta 62: 3499-3514.


The functional group chemistry and coordination of AsO43– -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 =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 =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.