The need for improved understanding of the phase relationship in the system Fe–AsO4–SO4 involving ferric arsenate, scorodite, and ferrihydrite motivated a series of synthesis experiments at 40 °C and 70 °C, and pH 0.5 to 1.5, from solutions with variable compositions. The precipitates were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM with selected-area electron diffraction, SAED), and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques. Amorphous ferric arsenate is the initial metastable phase forming from solutions with As/S molar values of as low as 0.1, as a precursor to scorodite. The onset of scorodite formation is exponentially delayed with increased pH and at lower temperatures. High-resolution TEM images reveal that amorphous ferric arsenate occurs as randomly oriented stubby nanoparticles showing no changes in size and crystallinity with increased synthesis. Simultaneous fitting of the As Kedge and Fe K-edge EXAFS spectra of a series of ferric arsenate precipitates resulted in local structural parameters that are similar to those reported earlier, supporting a local structure made of short polymers containing chains of corner-linked octahedra instead of the framework model or a scorodite-like local structure. The EXAFS modeling results can also be conceptualized as short chains of alternating octahedra and tetrahedra without the Fe–Fe linkages of the original chain model, suggesting that there is no unique EXAFS solution. Experimental results indicate that As can be fixed as scorodite through precipitation under ambient conditions from arsenic-rich effluents in the pH range 0.5–4.5. Improved understanding of the kinetics of phase transformations and their dependence on pH and temperature would facilitate the development of new stabilization technologies involving scorodite or ferrihydrite (or both) by improved controls on the behavior of arsenic in mine wastes. Stabilization of As through controlled precipitation of ferrihydrite during incongruent dissolution of scorodite at pH greater than about 2, which would maximize the uptake of As and optimize the kinetics of FeO6 polymerization without the presence of ferric arsenate, would be the most desirable outcome.

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