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 AsO substitution for SO in natroalunite as the base of a possible immobilization method for arsenic. The AsO substitution in natroalunite increased when the [AsO/(AsO + SO)] molar ratios of the initial aqueous solutions increased and the maximum substitution reached ~67% molar for the [AsO/(AsO + SO)] = 0.26. The XRD analysis confirmed that all hydrothermal synthetical solids for the [AsO/(AsO + SO)] ≤ 0.26 were characteristic of natroalunite-type phases. The AsO 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 [AsO/(AsO + SO)] = 0.28 at 200 °C and pH of 4, a mixture of natroalunite, amorphous arsenate phase and NaSO 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 and 981-997 cm, which represented the symmetric stretching vibration v(AsO) and the antisymmetric stretching vibration v(AsO), respectively. The infrared bands around 868-897 cm were assigned to the symmetric stretching vibration v(AsO). The thermal decomposition of the arsenate-substituted natroalunites showed three separated endothermic steps, namely the loss of HO, the loss of OH and the loss of SO + (AsO + O). The solubility products [K] and the Gibbs free energies of formation [ΔG] for the arsenate-substituted natroalunites decreased from 10 to 10 and from -4714.49 kJ/mol to -5352.95 kJ/mol with the increase of the AsO substitution from 0% to 67%, respectively.