Solubility of nanocrystalline scorodite and amorphous ferric arsenate: Implications for stabilization of arsenic in mine wastes

Solubility experiments were performed on nanocrystalline scorodite and amorphous ferric arsenate. Nanocrystalline scorodite occurs as stubby prismatic crystals measuring about 50 nm and having a specific surface area of 39.88 ± 0.07 m2/g whereas ferric arsenate is amorphous and occurs as aggregated clusters measuring about 50–100 nm with a specific surface area of 17.95 ± 0.19 m2/g. Similar to its crystalline counterpart, nanocrystalline scorodite has a solubility of about 0.25 mg/L at around pH 3–4 but has increased solubilities at low and high pH (i.e. <2 and >6). Nanocrystalline scorodite dissolves incongruently at about pH > 2.5 whereas ferric arsenate dissolution is incongruent at all the pH ranges tested (pH 2–5). It appears that the solubility of scorodite is not influenced by particle size. The dissolution rate of nanocrystalline scorodite is 2.64 × 10−10 mol m−2 s−1 at pH 1 and 3.25 × 10−11 mol m−2 s−1 at pH 2. These rates are 3–4 orders of magnitude slower than the oxidative dissolution of pyrite and 5 orders of magnitude slower than that of arsenopyrite. Ferric arsenate dissolution rates range from 6.14 × 10−9 mol m−2 s−1 at pH 2 to 1.66 × 10−9 mol m−2 s−1 at pH 5. Among the common As minerals, scorodite has the lowest solubility and dissolution rate. Whereas ferric arsenate is not a suitable compound for As control in mine effluents, nanocrystalline scorodite that can be easily precipitated at ambient pressure and temperature conditions would be satisfactory in meeting the regulatory guidelines at pH 3–4.