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

In the absence of sulfide, As hydroxide complexes are the major As con taining species in hydrothermal solution. As(III) hydroxides are domin ant, except under highly oxidizing conditions. In neutral and acid pH at low concentration the neutral monomer, As(OH)(3), is the predominan t species. Pokrovski et al. (1996) have recently presented Raman spect ra 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 calculat ed 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 sup port to OUT earlier interpretation of the As sulfide Raman spectra. We have also refined our methods for describing the structure and spectr a of anions in solutions and can now better reproduce changes in spect ra at high pH. The concentrated aqueous As solutions studied by Pokrov ski et al. (1996) show vibrational spectra similar to those of crystal line and amorphous arsenic oxides which contain oligomeric units. We h ave calculated the structures, stabilities, and vibrational spectra fo r a series of As oxyhydroxide oligomers: As2O(OH)(4), As2O2(OH)(2), As 3O3(OH)(3), As6O6 and AS(6)O(6)(OH)(3). We find that As3O3(OH)(3), AS( 6)O(6)(OH)(6), and As4O6 are very stable with respect to As(OH)(3). Se veral features in the Raman spectrum of concentrated aqueous As soluti ons are assigned to the symmetric breathing modes of the AS(3)O(3)(OH) (4) molecule. Our results indicate that As,Oa 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-ri ng species related to As3O3(OH)(3), while the NQR spectra may be bette r interpreted using a six-ring species. Copyright (C) 1997 Elsevier Sc ience Ltd.