Arsenic immobilization from aqueous solution by the precipitation of the pseudo-octahedral arsenate-substituted...

A series of the arsenate-substituted natroalunite solid solutions were synthesized by hydrothermal precipitation at 200 °C and pH of 4 and characterized to investigate the AsO₄ substitution for SO₄ in natroalunite as the base of a possible immobilization method for arsenic. The AsO₄ substitution in natroalunite increased when the [AsO₄/(AsO₄ + SO₄)]_aq molar ratios of the initial aqueous solutions increased and the maximum substitution reached ~67% molar for the [AsO₄/(AsO₄ + SO₄)]_aq = 0.26. The XRD analysis confirmed that all hydrothermal synthetical solids for the [AsO₄/(AsO₄ + SO₄)]_aq ≤ 0.26 were characteristic of natroalunite-type phases. The AsO₄ substitution in the natroalunites increased the c lattice parameters, owing to the difference between the SO distance and the AsO distance in the crystal structures. For the [AsO₄/(AsO₄ + SO₄)]_aq = 0.28 at 200 °C and pH of 4, a mixture of natroalunite, amorphous arsenate phase and Na₂SO₄ was formed. The crystals of the arsenate-substituted natroalunites were regular ditrigonal scalenohedron (pseudo-octahedron). The Raman spectra were characterized by two bands centered upon 899-917 cm^-1 and 981-997 cm^-1, which represented the symmetric stretching vibration v₁(AsO₄) and the antisymmetric stretching vibration v₃(AsO₄), respectively. The infrared bands around 868-897 cm^-1 were assigned to the symmetric stretching vibration v₁(AsO₄). The thermal decomposition of the arsenate-substituted natroalunites showed three separated endothermic steps, namely the loss of H₃O⁺, the loss of OH^- and the loss of SO₃ + (As₂O₃ + O₂). The solubility products [K_sp] and the Gibbs free energies of formation [ΔG_f^o] for the arsenate-substituted natroalunites decreased from 10^-81.21 to 10^-109.16 and from -4714.49 kJ/mol to -5352.95 kJ/mol with the increase of the AsO₄ substitution from 0% to 67%, respectively.