Hydrothermal reaction chemistry and characterization of ferric arsenate phases precipitated from Fe2(SO4)3–As2O5–H2SO4 solutions

Abstract The hydrothermal reaction chemistry and characterization of high temperature (150–225 °C) ferric arsenate phases produced from sulfate media was studied. In particular, the effect of Fe (III)/As (V) molar ratio, temperature, and time on the phases formed was examined. Three major arsenate-bearing phases were produced in our studies: (a) sulfate-containing scorodite (Fe(AsO 4 ) 1 − 0.67 x (SO 4 ) x ·2H 2 O where x  ≤ 0.20) at an Fe(III)/As(V) molar ratio of 0.7–1.87, 150–175 °C and 2–24 h reaction time; (b) ferric arsenate sub-hydrate (FAsH; Fe(AsO 4 ) 0.998 (SO 4 ) 0.01 ·0.72H 2 O) at Fe(III)/As(V) molar ratio of 0.69–0.93, 200–225 °C and 10–24 h reaction time; (c) basic ferric arsenate sulfate (BFAS; Fe(AsO 4 ) 1 −  x (SO 4 ) x (OH) x ·(1 −  x )H 2 O, where 0.3  x  8 months) stability testing of FAsH and BFAS found FAsH to yield somewhat higher arsenic release than BFAS. The latter's arsenic release potential was evaluated to be equivalent or slightly better than that of scorodite.