Donald Langmuir, John Mahoney
Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 10 4.04 (FeH2AsO4 2+), 109.86 (FeH-AsO4 +), and 10 18.9 (FeAsO4). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log Ksp) are /C0 23.0 ± 0.3 and /C0 25.83 ± 0.07, respectively, at 25 /C176 C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2–8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log K sp of ferric arsenate decreased from /C0 22.80 to /C0 24.67, while that of FO (as Fe(OH) 3) increased from /C0 39.49 to /C0 33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2–3.
@article{4bef6430-b83c-411d-adaa-bdfa8ad9ef4f,
title={Solubility products of amorphous ferric},
author={Donald Langmuir and John Mahoney},
year={2026},
language={en}
}TY - JOUR TI - Solubility products of amorphous ferric AU - Donald Langmuir AU - John Mahoney PY - 2026 LA - en ER -
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