An electrochemical study of the oxidative dissolution of iron monosulfide (FeS) in air-equilibrated solutions

Highlights: • The oxidation of FeS was studied using electrochemical techniques. • 1,10-phenanthroline reacts with FeS surface. • 1,10-phenanthroline controls the concentration of surface Fe²⁺. • The rate is controlled by a mix regime (electron transfer and diffusion). - Abstract: The oxidative dissolution of FeS in air-equilibrated solutions at 25 °C and pH between 2.5 and 5 was investigated by electrochemical methods. In order to understand the role played by S-Fe bonds polarized by H⁺ attack in the overall reaction, we added 1,10-phenanthroline (a Fe²⁺ ligand) to the reaction system at initial concentrations from 0 to 1 mmol dm⁻³. It was found that, at all concentrations of 1,10-phenanthroline, the oxidation current density (j_ox) decreases when pH increases from 2.5 to 4. A further increase of pH to 5 causes a slight increase of j_ox. Most probably, the rapid precipitation of Fe(III) oxyhydroxides at pH > 4 induces a local decrease of pH and promotes the oxidative dissolution of FeS. The current density decreases when 1,10-phenanthroline is added in the system, but after a moderate decrease, in the range of 0.1–0.5 mmol dm⁻³ 1,10-phenanthroline, it slightly increases. The oxidation potential of FeS electrode increases when 1,10-phenanthroline is added to the solutions. This behavior can be explained by the mixed potential theory. The largest shift of E_ox is registered at pH 2.5 (303.2 mV) and it decreases when pH increases up to 5 (48.8 mV). The analysis of experimental data highlights the importance of the surface concentration of S-Fe bonds polarized by H⁺ attack ({>Fe²⁺}) in the oxidative dissolution of FeS and confirms that the reaction rate is controlled by a mix regime of electron transfer from surface Fe²⁺ to the cathodic centers and diffusion. We expect that the organic ligands which increase the lability of S-Fe bonds and stabilize the dissolved ferrous iron will control {>Fe²⁺} and implicitly the oxidative dissolution of FeS.