Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite

Details Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite Vibrational spectroscopy of functional group chemistry and arsenate coordination in ettringite

Nov 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3499m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 62, Issue 21-22, pp.3499-3514

Computer Science

12

Scientific paper

The functional group chemistry and coordination of AsO 4 3- -sorption complexes in ettringite [Ca 6 Al 2 (SO 4 ) 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-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.

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