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

Arsenate (AsO₄^3-) 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 AsO₄^3- coordination, solution pH, type of soluble cations, and H₂O structure at the mineral-fluid interfaces. While the vibrational spectroscopy has been widely used in examining the AsO₄^3- coordination chemistry, insufficient knowledge on the correlation of AsO₄^3- molecular structure and its vibrational spectra impeded the complete spectral interpretation. In this paper, we evaluated the vibrational spectroscopy of AsO₄^3- 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 AsO₄^3- polyhedron is strongly distorted, and its As-O vibrations are affected by protonation and the relative influence on AsO₄^3- structure decreases in the order: H⁺ >> cation ≥ H₂O. For all AsO₄^3- complexes, the As-OX symmetric stretching (X = metal, H⁺, H₂O; ≤820 cm^-1) shifted to lower wavenumbers when compared to that of uncomplexed AsO₄^3-. 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 HAsO₄^2-, H₂AsO₄^-, and H₃AsO₄⁰, 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 AsO₄^3- does not influence the As-OH vibrations significantly, deducing symmetry information from their vibrational spectra is difficult. However, for metal unprotonated-AsO₄^3- 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 AsO₄^3- 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 AsO₄^3- sorption complexes at the solid-water interfaces. The vibrational spectra of other tetrahedral oxoanions are expected to be along similar lines.