THEORETICAL STUDIES ON ARSENIC OXIDE AND HYDROXIDE SPECIES IN MINERALS AND IN AQUEOUS SOLUTION

Abstract

In the absence of sulfide, As hydroxide complexes are the major As containing species in hydrothermal solution. As (III) hydroxides are dominant, except under highly oxidizing conditions. In neutral and acid pH at low concentration the neutral monomer, As (OH)3, is the predominant species. Pokrovski et al. (1996) have recently presented Raman spectra for aqueous As-containing solutions which indicate the presence of both monomeric and oligomeric As species. Helz et al. (1995) presented corresponding results for sulfidic As solutions. We have now calculated the structures, stabilities, and vibrational spectra for As (OH)3 and its anions to compare with the results of Pokrovski et al. (1996). We obtain good agreement with their spectral data, giving further support to our earlier interpretation of the As sulfide Raman spectra. We have also refined our methods for describing the structure and spectra of anions in solutions and can now better reproduce changes in spectra at high pH. The concentrated aqueous As solutions studied by Pokrovski et al. (1996) show vibrational spectra similar to those of crystalline and amorphous arsenic oxides which contain oligomeric units. We have calculated the structures, stabilities, and vibrational spectra for a series of As oxyhydroxide oligomers: As2O(OH)4, As2O2(OH)2, As3O3(OH)3, As4O6, and As6O6(OH)3. We find that As3O3(OH)3, As6O6(OH)6, and As4O6 are very stable with respect to As(OH)3. Several features in the Raman spectrum of concentrated aqueous As solutions are assigned to the symmetric breathing modes of the As3O3(OH)3 molecule. Our results indicate that As4O6 unites are hydrolyzed to give As3O3(OH)3 and As(OH)3 units in solution. The IR spectrum of glassy As2O3 is reasonably consistent with the presence of a three-ring species related to As3O3(OH)3, while the NQR spectra may be better interpreted using a six-ring species.