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

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 [Ksp] and the Gibbs free energies of formation [ΔGfo] 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.