Experimental and theoretical vibrational spectroscopic evaluation of arsenate coordination in aqueous solutions, solids, and at mineral-water interfaces

• Published in: Geochimica et cosmochimica acta 1998-10, Vol.62 (19), p.3285-3300
• Main Authors: Myneni, Satish C.B, Traina, Samuel J, Waychunas, Glenn A, Logan, Terry J
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(98)00222-1

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 2O 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 2O. For all AsO 4 3− complexes, the As-OX symmetric stretching (X = metal, H +, H 2O; ≤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 (