Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite

• Published in: Geochimica et cosmochimica acta Vol. 62; no. 21; pp. 3499 - 3514
• Main Authors: Myneni, Satish C.B., Traina, Samuel J., Waychunas, Glenn A., Logan, Terry J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-11-1998
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(98)00221-X

The functional group chemistry and coordination of AsO
4
3−-sorption complexes in ettringite [Ca
6Al
2(SO
4)
3(OH)
12·26H
2O] 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-OH
2, and ≡Ca
2-OH exhibit broad overlapping OH bands in the range 3600–3200 cm
−1, prohibiting separation of component vibrational bands. The SO
4
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 AsO
4
3− adsorption, the vibrational spectra of SO
4
2− were least affected, and the OH stretching intensities around 3600 cm
−1 decreased with an increase in AsO
4
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 AsO
4
3−-coprecipitation. The As-O asymmetric stretch of sorbed AsO
4
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 AsO
4
3− ions are protonated. These spectral features demonstrate that AsO
4
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 AsO
4
3− polyhedra interacts predominantly with ≡Ca-OH
2 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.