The Hydrothermal Formation of Hydronium Alunite from Aqueous Al₂(SO₄)₃ Solution and the Characterization of the Solid

Raman spectra have been measured for aqueous Al₂(SO₄)₃ solution from room temperature up to hydrothermal conditions at steam saturation. An aluminium(III) sulfato complex, in which the sulfate acts possibly as a bidentate or tridentate bridging ligand, is formed in solution, in addition to the monodentate aluminium(III) sulfato complex. At lower temperatures the monodentate aluminium(III) sulfato complex is the only ion pair to be identified. Under hydrothermal conditions it was possible to observe the first hydrolysis step of the aluminium(III) aqua-ion by measuring the relative intensity of bands due to SO₄²⁻ and HSO₄⁻ (W. W. Rudolph, Z. Phys. Chem. 194 (1996) 73) according to the coupled equilibrium reaction [Al(OH₂)₆]³⁺ + SO₄²⁻ ⇔ [Al(OH₂)₅OH]²⁺ + HSO₄⁻.

A white precipitate in equilibrium with the solution at 184 °C could be characterized as (H₃O)Al₃(SO₄)₂(OH)₆ (hydronium alunite) by X-ray diffraction, Raman and infrared spectroscopy. SEM pictures were taken to characterize the morphology of the hydronium alunite. Additionally, hydrothermal synthesis experiments have been carried out to study the formation and composition of hydronium alunite. The pure potassium end member, alunite sensu stricto has also been prepared and characterized for comparison with the hydronium alunite.