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

Arsenate (AsO43-) is a common species in oxidizing aquatic systems and hydr othermal fluids, and its solubility and partitioning into different mineral phases are determined by the nature of AsO43- coordination, solution pH, t ype of soluble cations, and H2O structure at the mineral-fluid interfaces. While the vibrational spectroscopy has been widely used in examining the As O43- coordination chemistry, insufficient knowledge on the correlation of A sO43- molecular structure and its vibrational spectra impeded the complete spectral interpretation. In this paper, we evaluated the vibrational spectr oscopy of AsO43- in solutions, crystals, and sorbed on mineral surfaces usi ng theoretical (semiempirical, for aqueous species) and experimental studie s, with emphasis on the protonation, hydration, and metal complexation infl uence 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 in terpretation of experimental spectra. These vibrational spectroscopic studi es (IR, Raman) suggest that the symmetry of AsO43- polyhedron is strongly d istorted, and its As-O vibrations are affected by protonation and the relat ive influence on AsO43- structure decreases in the order: H+ >> cation grea ter than or equal to H2O. For all AsO43- complexes, the As-OX symmetric str etching (X = metal, H+, H2O; less than or equal to 820 cm(-1)) shifted to l ower wavenumbers when compared to that of uncomplexed AsO43-. 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 HAsO42-, H2AsO4-, and H2AsO40, respectively). The protonated arsena tes in crystalline solids show the same trend with little variation in As-O H symmetric stretching vibrations. Since metal complexation of protonated A sO43- does not influence the As-OH vibrations significantly, deducing symme try information from their vibrational spectra is difficult. However, for m etal unprotonated-AsO43- complexes, the shifts in As-OM (M = metal) vibrati ons are influenced only by the nature of complexing cation and the type of coordination, and hence the AsO43- coordination environment can be interpre ted directly from the splitting of As-O degenerate vibrations and relative shifts in the As-OM modes. This information is critical in evaluating the s tructure of AsO43- sorption complexes at the solid-water interfaces. The vi brational spectra of other tetrahedral oxoanions are expected to be along s imilar lines. Copyright (C) 1998 Elsevier Science Ltd.