Effect of Acidithiobacillus ferrooxidans on pyrolusite bioleaching

Biohydrometallurgy is an increasingly popular ore extraction technology and is especially applicable for low-grade ores. In particular, Acidithiobacillus ferrooxidans (A. ferrooxidans) is by far the most widely used bioleaching microorganism for leaching ores, including for sulfide ores and manganese dioxide ores. At present, many works are focused on the vital facilitating role of A. ferrooxidans in the cycles of sulfur and iron for sulfide ores bioleaching. However, research on the effect of A. ferrooxidans on manganese dioxide ores leaching is limited. The effects of A. ferrooxidans on the electrochemistry behavior of pyrolusite in simulated solutions (9K basic medium, A. ferrooxidans, Fe(Ⅲ), A. ferrooxidans+Fe(Ⅲ)) were investigated using cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. Mott-Schottky curves were utilized to determine the passive film formed on the surface of pyrolusite ore in the presence or absence of bacteria bath solutions. The results show that A. ferrooxidans promotes the redox of MnO₂/Mn²⁺ and triggers the reaction of MnO₂/Mn(OH)₂. A. ferrooxidans accelerates electron exchange between pyrolusite and solution; in the A. ferrooxidans-simulated solution, the charge-transfer reaction resistance of manganese dioxide is 34% lower than that of the control (9K) and 11% lower than that of the Fe(Ⅲ) solution. Germs cause polarization of pyrolusite, leading to an increase in oxidative activity of manganese dioxide. Bacteria facilitate the transformation of MnO₂ to MnO·OH and is beneficial to its diffusion. The indirect action mechanism is adopted to explain the interaction between A. ferrooxidans and pyrolusite. The passive films formed in simulated solutions exhibit p-n-p-n type semiconductor properties at the polarization potential of 0.2 V when pH is 2.0, and the depletion layer of pyrolusite appears between 0.2 and 0.4 V. Introducing A.ferrooxidans to the Fe(Ⅲ)-free solution decreases the donor density and the acceptor density because bacteria contain a variety of groups involved in electron transfer, which accept free electrons or fill holes, prompting the exchange of species between manganese oxide and solution. Admixing A. ferrooxidans to Fe(Ⅲ)-containing solution increases carrier density, reducing the corrosion resistance of membrane. The corrosion rate of pyrolusite increases with the addition of A. ferrooxidans.