Environmentally-friendly bioleaching of manganese from pyrolusite: Performance and mechanisms

Currently, bioleaching of manganese (Mn) from Mn-poor ore (e.g. pyrolusite) is facing the challenges of low leaching efficiency and high cost of leachant and process operation. In this study, a new process was developed that uses Microbacterium trichothecenolyticum Y1 (bacteria Y1) and waste molasses to bioleach Mn from pyrolusite or pure MnO₂ (for comparison). Results indicate that the innovative process can be operated at normal temperature/pressure and a weak acidic condition (pH = 3.5) with a much improved Mn bioleaching efficiency (about 98% in 6–8 days) as compared with the previously-reported efficiencies (only 60–80%). The optimal conditions were found as the solid-liquid ratio of 1 kg of mineral powder to 20 L of leaching liquid, pH 3.5, anaerobic, stirring speed of 80 rpm, temperature 45 °C and waster molasses dose of 2.5 g L⁻¹. The associated mechanism was elucidated with the help of different equipment and methods. The leaching process first reduced Mn(IV) in MnO₂ to Mn(II) as MnO in the solid phase and then dissolved Mn(II) into the liquid phase. Bacteria Y1 could produce a large number of tyrosine-like substances in the system, which promoted the decomposition of sugar and Mn(IV) reduction. Weak acid (pH = 3.5) could destroy biofilm and manganese oxides (e.g., MnO) on mineral surfaces to promote the leaching process. Kinetic studies show that the pyrolusite leaching process belongs to mixed control by solid film diffusion control and chemical reaction control. The innovative process may be cost-effective and green, with a great potential for future applications.