Arsenate substitution in natroalunite: A potential medium for arsenic immobilization. Part 1: Synthesis and compositions

Arsenate-for-sulfate substitution in natroalunite has been investigated as the basis of a potential method for arsenic immobilization. The effects of temperature, reaction time and AsO4/(SO4 + AsO4) molar ratio in the initial aqueous medium were studied. The rate of precipitation increased with increasing temperature and AsO43− concentration, and it was practically complete in < 30 min at > 180 °C. Natroalunite formation was faster and preferential than mansfieldite precipitation; the latter decreased when the temperature increased and the (AsO4/TO4)aq ratio (TO4 ≡ SO4 + AsO4) decreased. For (AsO4/TO4)aq < 0.2 at 200 °C, arsenic precipitated almost exclusively as natroalunite. For (AsO4/TO4)aq between 0.2 and 0.4 at 200 °C, a mixture of natroalunite and mansfieldite was formed. For (AsO4/TO4)aq > 0.4, natroalunite was not precipitated, and other arsenate phases such as alarsite, mansfieldite and natropharmacoalumite appeared. ubstitution in natroalunite in the interval 160–200 °C was practically independent of temperature and reaction time but it increased when increased (AsO4/TO4)aq. At 200 °C the substitution type was (AsO4/TO4)nat ≅ 0.5(AsO4/TO4)aq. The maximum arsenate substitution observed in the natroalunite structure was ~ 15% molar. troduction of gypsum in the initial medium ends with the precipitation of anhydrite and natroalunite with a Ca-for-Na substitution about 4–6% molar and a similar degree of arsenate-for-sulfate substitution. In absence of Na+, Ca/As-rich alunite-type phases such as schlossmacherite or arsenocrandallite were not precipitated at reasonable rates in Ca2+/Al3+/SO42−/AsO43−media.