Arsenate substitution in natroalunite: A potential medium for arsenic immobilization. Part 2: Cell parameters and stability tests

Abstract

The effect of AsO 4 substitution in natroalunite ((Na,H 3O)Al 3(SO 4) 2-x(AsO 4) x(OH) 6 − x , x ≤ 0.3) has been investigated by Rietveld refinement of the XRD patterns. Cell refinements confirmed a structural type R-3 m Z = 3, characteristic of alunite-type phases. AsO 4-for-SO 4 substitution slightly increased the c unit-cell parameter, in consistency with the cell expansion due to the differences between As-O1 and S-O1 distances in the crystal structure. The a cell parameter was practically independent of the AsO 4 substitution. The arsenic leaching and stability of arsenical natroalunite compared with other related phases (mansfieldite, alarsite, natropharmacoalumite and amorphous Al-arsenate) was studied in the pH interval 1 to 12. The dissolution of arsenical natroalunite was congruent at pH < 3 and pH 12 and incongruent at pH between 4 and 11, showing the maximum stability at pH 5 to 8, in which the As solubilization (24 h) were 0.01–0.05 mg/L. These values can be considered suitable for the inertization of wastes containing large SO 4 2−/AsO 4 3− ratios. In short term leaching tests (24 h), the mixture of arsenical natroalunite and mansfieldite showed a similar behavior to arsenical natroalunite. However, precipitates containing alarsite or amorphous Al-arsenate showed As solubilization two or three orders of magnitude greater. Natropharmacoalumite was very stable for 72 h of leaching in pH range 1 to 12. In acid medium, this phase exchanged H 3O + for Na +. The stability and acid-exchange properties of natropharmacoalumite could make it potentially attractive for arsenic disposal, especially when it can exist in contact with acidic soils or wastes.