Pyrite-Arsenopyrite Galvanic Interaction and Electrochemical Reactivity

Comparative voltammetric studies were performed between high purity pyrite mineral (98.86%) and arsenopyrite mineral (content of 85.96% arsenopyrite, FeAsS; 11.84% pyrite, FeS2; 0.98% galena, PbS; 0.06% chalcopyrite, CuFeS2) in order to analyze the galvanic effect on the electrochemical reactivity of two different mineralogical phases when these are associated in the same mineral, using carbon paste electrodes (CPE) in 0.1 M NaNO3 (pH 6.5) as electrolyte. Theoretical studies were performed for a more detailed analysis on the total energy and energy relative of a total optimization of the species involved in the pyrite and arsenopyrite oxidation as pure mineral. The results indicate that the electrochemical reactivity of pyrite in the arsenopyrite mineral was delayed and displaced to more positive potentials with respect to the high purity pyrite mineral electrochemical response, due to a galvanic effect. On the other hand, the voltammetric studies showed the oxidation stages of arsenopyrite mineral, indicating that arsenopyrite was oxidized in a first stage to Fe2+, realgar (As2S2), H3ASO3, and S0. In a second stage, the arsenopyrite and the pyrite were oxidized to FeOOH(s) and S0, followed by oxidation of H3AsO3 to H2AsO4? and S0 to SO4?, besides the scorodite (FeSO42H2O) formation. Meanwhile, at more negative potentials the regeneration of FeAsS and the reduction to elemental arsenic do not occur due to the reduction of arsenate (H2AsO4?) to H3AsO3 and the residual scorodite (FeAsO42H2O) that was not dissolved by chemical reaction. The analysis by scanning electron microscopy and energy dispersive spectrometer to surfaces of the electrodes with the mineral (CPE) modified electrochemically was carried out to support the reaction mechanisms.