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

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(AsO4)1 − 0.67x(SO4)x·2H2O 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(AsO4)0.998(SO4)0.01·0.72H2O) 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(AsO4)1 − x(SO4)x(OH)x·(1 − x)H2O, where 0.3 < x < 0.7) at Fe(III)/As(V) molar ratio of 1.67–4.01, 175–225 °C and 4–24 h reaction time. Scorodite was found to be metastable at an Fe(III)/As(V) molar ratio of one, 1 h reaction time and 200 °C. Extension of retention time to 10 h under the same conditions gave predominantly FAsH with minor presence of scorodite, while additional retention time to 24 h resulted in the formation of only FAsH. XRD, Raman and ATR-IR, proved that our FAsH is the same phase with that labeled previously as “Type 1” and the “Phase 4”; similarly our BFAS was identical to the so-called “Type 2” and “Phase 3”. The Fe L-edge XANES analysis showed that scorodite, FAsH and BFAS exhibited an octahedral type of crystal field with a paramagnetic high spin anti-ferromagnetic character as in hematite. Short term leachability tests determined FAsH to be slightly more soluble than scorodite and BFAS after 24 h, while after several TCLP-like contacts there was < 0.1 mg/L arsenic release from all phases. Long term (> 8 months) stability testing of FAsH and BFAS found FAsH to yield somewhat higher arsenic release than BFAS. The latterʹs arsenic release potential was evaluated to be equivalent or slightly better than that of scorodite.