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

Abstract

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 AsO 4/(SO 4 + AsO 4) molar ratio in the initial aqueous medium were studied. The rate of precipitation increased with increasing temperature and AsO 4 3− 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 (AsO 4/TO 4) aq ratio (TO 4 ≡ SO 4 + AsO 4) decreased. For (AsO 4/TO 4) aq < 0.2 at 200 °C, arsenic precipitated almost exclusively as natroalunite. For (AsO 4/TO 4) aq between 0.2 and 0.4 at 200 °C, a mixture of natroalunite and mansfieldite was formed. For (AsO 4/TO 4) aq > 0.4, natroalunite was not precipitated, and other arsenate phases such as alarsite, mansfieldite and natropharmacoalumite appeared. AsO 4 substitution in natroalunite in the interval 160–200 °C was practically independent of temperature and reaction time but it increased when increased (AsO 4/TO 4) aq. At 200 °C the substitution type was (AsO 4/TO 4) nat ≅ 0.5(AsO 4/TO 4) aq. The maximum arsenate substitution observed in the natroalunite structure was ~ 15% molar. The introduction 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 Ca 2+/Al 3+/SO 4 2−/AsO 4 3−media.