Extracellular Biosynthesis of High-Purity γ-MnS by the Sulfate-Reducing BacteriumAnaerofilumsp. 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.