Simultaneous Cr(VI) reduction and methylene blue removal by Bacillus sp. JH2-2 isolated from mining site soil

A Bacillus sp. strain (JH2-2), isolated from soil at an abandoned mine site, reduced Cr(VI) to Cr(III) under aerobic conditions. At pH 7, the strain reduced essentially all of the Cr(VI) in M9 minimal medium to Cr(III) at initial concentrations ≤50 mg Cr(VI) L⁻¹ within 100 h. The X-ray diffraction pattern of the Cr(III) precipitate matched chromium (III) hydrogen phosphate (CrH₂P₃O₁₀·2H₂O). The JH2-2 strain showed high tolerance to other heavy metal(loid)s, with minimal inhibitory concentrations in liquid medium of (mg L⁻¹): As (500), Cd (100), Cu (350), Ni (300), Zn (200), and Pb (1800). JH2-2 also promoted decolorization of methylene blue dye. Decolorization was faster in the presence of 10 mg Cr(VI) L⁻¹ than in the absence of Cr(VI). A lag in decolorization at 30 and 50 mg Cr(VI) L⁻¹ is likely due to initial toxicity and inhibition of bacterial growth. The chemistry of MB is complicated by its reduction to colorless leucomethylene blue, which can reoxidize to MB. However, aeration of the solution did not restore measurable MB, supporting removal of the dye via biosorption. Results indicate the bioremediation potential of Bacillus sp. JH2-2 for simultaneous Cr(VI) reduction and methylene blue removal from contaminated water.