Implications of the iron(II/III)-arsenic ratio on the precipitation of iron-arsenic minerals from pH 2.5 to 10.5

Precipitating arsenic (As) as Fe-As minerals by stepwise neutralization is a common practise to sequester As from acidic industrial effluent (e.g., acidic uranium mill effluents). Despite their importance, the evolution of specific Fe-As minerals during the neutralization process and their stability after formation is poorly understood. This study characterized the Fe-As secondary mineral phases and their ability to sequester As a function of solution Fe redox speciation, Fe/As molar ratio, retention time, and neutralization pH in a series of batch neutralizations of synthetic acidic solution with Ca(OH)2. The presence of Fe(III) in solution facilitates the removal of As and Fe from aqueous solution by precipitating arsenical ferrihydrite, with >99.9% removal of As at pH 4 when Fe/As≥3. The presence of Fe(II) in solution favors the precipitation of ferric arsenate (FA) over arsenical ferrihydrite. During the neutralization of Fe(II)-As(V) solutions under oxic conditions, As removal is limited by the in-situ oxidation of Fe(II) to Fe(III) followed by the predominant precipitation of FA. Neutralization of mixed Fe-As(V) (Fe(II)/Fe(III) = 1) solutions is consistent with a pure Fe(III) system. The mineralogy of the precipitated Fe-As is largely independent of retention time; however, increased retention time can re-dissolve FA formed within 90 min and release As back into solution. Neutralization pH strongly influences the type of Fe-As mineral precipitated. For both Fe(II)-Fe(III) and Fe(III) solutions tested at Fe/As≥3, increased solution pH results in the transformation of FA into arsenical ferrihydrite through incongruent dissolution. The pH required for this transformation is dependent on Fe(II)/Fe(III) and Fe/As molar ratio, and can be as low as 2.5 for Fe(III) solutions with Fe/As = 6. The transformation of FA into arsenical ferrihydrite does not increase the solution As concentration.