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

Abstract

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 (<720, <770, <790 cm −1 for HAsO 4 2−, H 2AsO 4 −, and H 3AsO 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.