The synthesis of crystalline scorodite, FeAsO 4 · 2H 2O

Abstract

Crystalline scorodite, FeAsO 4 · 2H 2O, was synthesized by precipitation from 0.3 M Fe(NO 3) 3−25 g/L As 5+ solution pH ∼ 0.7. Temperatures greater than 125 ° C are needed to ensure good scorodite crystallinity, and a temperature of 160°C was used for the current syntheses. Although initial solution pH values in the 0.2 to 1.8 range have little effect on either the structure or composition of the scorodite, As 5+ concentrations greater than 15 g/L are needed to ensure a pure product. About 1%SO 2− 4 could be incorporated in the scorodite structure, and the SO 2- 4 content decreased with increasing solution pH. Upon heating, scorodite decomposes at 100–200°C with the simultaneous release of both of its H 2O molecules to form anhydrous FeAsO 4.