Study of Aqueous Al_2(SO_4)_3 Solution under Hydrothermal Conditions: Sulfate Ion Pairing, Hydrolysis, and Formation of Hydronium Alunite

摘要

Raman spectra have been measured for aqueous Al_2(SO_4)_3 solutions from 25 to hydrothermal conditions at 184 deg C under steam saturation. The Raman spectrum at 184deg C contained four polarized bands in the S-O stretching wavenumber range, which suggest that a new sulfato complex,where sulfate acts as a bridging ligand (possibly bidentate or tridentate), is formed in solution, in addition to a 1:1 aluminium (III) sulfato complex, where sulfate is monodentate, which is the only ion pair identified at room temperature. Under hydrothermal conditions, it was possible to observe the hydrolysis of aluminum(III) aqua ion by measuring the relative intensity of bands due to SO_4~(2-) and HSO_4~-, according to the coupled equilibrium reaction [Al(OH_2)_6]~(3+)+ SO_4~(2-) <=>[Al(OH_2)_5OH]~(2+) + HSO_4~-. The precipitate in equilibrium with the solution at 184 deg C could the characterized as a hydronium alunite, (H_3O)Al_3(SO_4)_2(OH)_6, by chemical analsis, X-ray diffraction, and Raman and infrared spectroscopy.