Extracellular biosynthesis of high-purity γ-MnS by the sulfate-reducing bacterium anaerofilum sp. through selective precipitation of a Mn²⁺-eriochrome black t chelate complex

Biosynthesis of MnS has received increasing attention due to its potential applications in a p-type semiconductor and optoelectronic devices. However, the concentration of PO₄³⁻ and OH⁻ in media has to be seriously controlled with bioreduction of SO₄²⁻ into S²⁻ being performed by resting cells in pure solutions of MnSO₄ because both PO₄³⁻ and OH⁻ adversely affect the biosynthesis process, resulting in a very low yield of MnS. In this work, selective precipitation is first applied for biosynthesis of γ-MnS based on the steric-hindrance effect of a metal complex of Mn²⁺ and eriochrome black T (EBT) at high concentrations of PO₄³⁻ and OH⁻. The results show that Mn²⁺-EBT was capable of eliminating the reaction between PO₄³⁻/OH⁻ and Mn²⁺, leading to selective precipitation between biogenic S²⁻ and Mn²⁺ for biosynthesis of γ-MnS. The dominant form of prepared γ-MnS was a tetrakaidecahedron prismoid-like crystal with an upper width of ca. 1.0 μm, a bottom width of ca. 4.0 μm and a height of ca. 4.0 μm, having an evident absorption peak at 320 nm, a coercivity of 37.404 Oe and a retentivity of 2.695 × 10⁻³ emu·g⁻¹. The γ-MnS exhibited optical and magnetic activity, suggesting a wide application potential.