The relationship between protons and the aqueous oxidation of pyrite by molecular oxygen

Abstract

The reaction of pyrite (FeS₂) with dissolved oxygen (DO) in acidic media (pH 2.5, 3.0 and 4.0) was studied at 25 °C. In this regard, a series of experimental (potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS) or cyclic voltammetry) and theoretical (quantum calculations) approaches were used. It was found that the proton concentration ([H⁺]) has not a significant influence on the oxidative dissolution of FeS₂. The oxidation current density (j_ox) varies little when the pH increases from 2.5 (j_ox = 1.02 μA cm⁻²) to 4.0 (j_ox = 0.80 μA cm⁻²), the order of reaction with respect to [H⁺] being 0.05 ± 0.10. EIS spectra indicate that the pyrite oxidation with dissolved oxygen is controlled by a surface electron transfer reaction. The results of quantum calculations show that protons spontaneously adsorb to the pyrite surface. The density of states of adsorbed protons is located far from Fermi level (between −0.3 and −0.2 Hartree) indicating that they are not available for subsequent reactions. The results of quantum analysis outline the main reasons why protons do not play a discernible role in the oxidation of pyrite with DO and the formation of acid mine drainage.