VIBRATIONAL SPECTROSCOPY OF FUNCTIONAL GROUP CHEMISTRY AND ARSENATE COORDINATION IN ETTRINGITE - PART 1. EXAFS STUDIES OF THE GEOMETRY OF COPRECIPITATED AND ADSORBED ARSENATE

Abstract

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 =Ca2-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-Ocomplexed 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 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.