Stibiconite (Sb3O6OH), senarmontite (Sb2O3) and valentinite (Sb2O3): Dissolution rates at pH 2–11 and isoelectric points

Batch reactor experiments were carried out in order to derive rate laws for the proton promoted dissolution of the main natural antimony oxide phases, namely stibiconite (idealized composition SbSb2O6OH), senarmontite (cubic Sb2O3) and (metastable) valentinite (orthorhombic Sb2O3) over the range 2 ⩽ pH ⩽ 11, under standard conditions and ionic strength I = 0.01 mol l−1. The rates of antimony release by stibiconite were r = (2.2 ± 0.2) × 10−9 a(H+)0.11±0.01 mol m−2 s−1 for 2.00 ⩽ pH ⩽ 4.74 and r = (4.3 ± 0.2) × 10−10 a(H+)−0.030±0.003 mol m−2 s−1 for 4.74 ⩽ pH ⩽ 10.54. The rates of dissolution of senarmontite were r = (5.3 ± 2.2) × 10−7 a(H+)0.54±0.05 mol m−2 s−1 for 2.00 ⩽ pH ⩽ 6.93 and r = (1.4 ± 0.3) × 10−14 a(H+)−0.53±0.07 mol m−2 s−1 for 6.93 ⩽ pH ⩽ 10.83. The rates of dissolution of valentinite were r = (6.3 ± 0.2) × 10−8 a(H+)0.052±0.003 mol m−2 s−1 for 1.97 ⩽ pH ⩽ 6.85. Above pH = 6.85, valentinite was found to dissolve at a constant rate of r = (2.79 ± 0.05) × 10−8 mol m−2 s−1. Activation energies were determined at selected pH values in the acidic and basic domain, over the temperature range 25–50 °C. The values for stibiconite are −10.6 ± 1.9 kJ mol−1 (pH = 2.00) and 53 ± 14 kJ mol−1 (pH = 8.7). For senarmontite, we found 46.6 ± 4.7 kJ mol−1 (pH = 3.0) and 68.1 ± 6.1 kJ mol−1 (pH = 9.9) and for valentinite 41.9 ± 1.1 kJ mol−1 (pH = 3.0) and 39.0 ± 4.6 kJ mol−1 (pH = 9.9). These activation energies are interpreted in the text. The solubility of stibiconite at 25 °C in the pH domain from 2 to 10 was determined; solubilities decrease from 452.0 μg l−1 (as Sb) at pH = 2.00 to 153.2 μg l−1 at pH = 7.55 and increase again in the basic region, up to 176.6 μg l−1 at pH = 9.92. A graphical synopsis of all the kinetic results, including those of stibnite (Sb2S3) from earlier work, is presented. This allows an easy comparison between the dissolution rates of stibnite and the minerals examined in the present work. The isoelectric point (i.e.p.) of the minerals was determined electrokinetically, as a proxy for the zero point of charge (pHzpc), which was experimentally inaccessible; the measured i.e.p. do not correspond to the dissolution rate minima. I.e.p. of antimony oxides have not yet been documented in the literature. The geochemical implications for the weathering of antimony oxide minerals and stibnite, with particular reference to the mobilization of antimony in the context of an abandoned antimony mine (Goesdorf, Luxembourg), are discussed.