Release of Arsenic from Arsenopyrite Dissolution

The kinetics of arsenopyrite oxidative dissolution was studied by means of long-term, flow-through dissolution experiments, assessing the effects of different environmental factors, such as pH (1-13), dissolved oxygen (0.2-8.7 mg L-1), sulphate and iron content (up to 0.01M), and temperature (25-70 C). The objective is to quantify the As release from arsenopyrite dissolution to evaluate the extent of As mobility and arsenic pollution where this mechanism dominates the arsenic release.

Along the pH range studied, precipitation of iron oxy-hydroxide an arsenic-oxygen phases on arsenopyrite surface generated a coating that delimited arsenic release into solution: at pH 1-4 arsenic was released stoichiometrically at the same rate than Fe, whereas sulphur release was lower. When pH increased (5-6) release of arsenic and iron was lower than sulfur release. The deficit of aqueous iron could be attributed to partial iron-oxy(hydroxides) precipitation at this pH range that, in turn, could adsorb some As, yielding an aqueous As deficit. At pH 7-11 precipitation of iron-oxy(hydroxides) and As-O phases occurred all over the arsenopyrite surface and totally controlled the As release through this massive coating, which in turn, could adsorb some As. Nonetheless, as pH increased to 12, sorption capacity of the iron-oxy(hydroxides) decreased as well as arsenopyrite dissolution might increase with pH. At pH > 12 the arsenic release was lower than sulfur release (S/As > 5) and arsenic diffused through a distinct coating formed.

Hence, As release at acidic pH is explained by taking into account dissolution of arsenopyrite, whereas at basic pH depends on dissolution of arsenopyrite and oxygen diffusion through different Fe-oxy(hydroxide) and As-O coatings. The shrinking core model (Wen, 1968) is used to propose an arsenopyrite dissolution rate law that is useful to predict the As release into solution.

Wen C.Y. (1968) Noncatalytic solid-fluid reaction models. Ind. Eng. Chem. 60: 34-54.