Other
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3285m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 62, Issue 19-20, pp.3285-3300
Other
17
Scientific paper
Arsenate (AsO 4 3- ) is a common species in oxidizing aquatic systems and hydrothermal fluids, and its solubility and partitioning into different mineral phases are determined by the nature of AsO 4 3- coordination, solution pH, type of soluble cations, and H 2 O structure at the mineral-fluid interfaces. While the vibrational spectroscopy has been widely used in examining the AsO 4 3- coordination chemistry, insufficient knowledge on the correlation of AsO 4 3- molecular structure and its vibrational spectra impeded the complete spectral interpretation. In this paper, we evaluated the vibrational spectroscopy of AsO 4 3- in solutions, crystals, and sorbed on mineral surfaces using theoretical (semiempirical, for aqueous species) and experimental studies, with emphasis on the protonation, hydration, and metal complexation influence on the As-O symmetric stretching vibrations. Theoretical predictions are in excellent agreement with the experimental studies and helped in the evaluation of vibrational modes of several arsenate-complexes and in the interpretation of experimental spectra. These vibrational spectroscopic studies (IR, Raman) suggest that the symmetry of AsO 4 3- polyhedron is strongly distorted, and its As-O vibrations are affected by protonation and the relative influence on AsO 4 3- structure decreases in the order: H + >> cation H 2 O. For all AsO 4 3- complexes, the As-OX symmetric stretching (X = metal, H + , H 2 O; 820 cm -1 ) shifted to lower wavenumbers when compared to that of uncomplexed AsO 4 3- . In addition, the As-OH symmetric stretching of protonated arsenates in aqueous solutions shift to higher energies with increasing protonation (<720, <770, <790 cm -1 for HAsO 4 2- , H 2 AsO 4 - , and H 3 AsO 4 0 , respectively). The protonated arsenates in crystalline solids show the same trend with little variation in As-OH symmetric stretching vibrations. Since metal complexation of protonated AsO 4 3- does not influence the As-OH vibrations significantly, deducing symmetry information from their vibrational spectra is difficult. However, for metal unprotonated-AsO 4 3- complexes, the shifts in As-OM (M = metal) vibrations are influenced only by the nature of complexing cation and the type of coordination, and hence the AsO 4 3- coordination environment can be interpreted directly from the splitting of As-O degenerate vibrations and relative shifts in the As-OM modes. This information is critical in evaluating the structure of AsO 4 3- sorption complexes at the solid-water interfaces. The vibrational spectra of other tetrahedral oxoanions are expected to be along similar lines.
Logan Terry J.
Myneni Satish C. B.
Traina Samuel J.
Waychunas Glenn A.
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