Competitive formation of hydroxycarbonate green rust 1 versus hydroxysulphate green rust 2 in Shewanella putrefaciens cultures

The formation of hydroxysulphate green rust 2, a Fe(II-III) compound commonly found during corrosion processes of iron-based materials in seawater, has not yet been reported in bacterial cultures. Here we used Shewanella putrefaciens, a dissimilatory iron-reducing bacterium to anaerobically catalyze the transformation of a ferric oxyhydroxide, lepidocrocite (gamma-FeOOH), into Fe(H) in the presence of various sulphate concentrations. Biotransformation assails of gamma-FeOOH were performed with formate as the electron donor under a variety of concentrations. The results showed that the competitive formation of hydroxycarbonate green rust 1 (GR1(SO32-)) and hydroxysulphate green rust 2 (GR2(SO42-)) depended upon the relative ratio (R) of bicarbonate and sulphate concentrations. When R greater than or equal to 0.17, GR1(CO42-) only was formed whereas when R < 0.17, a mixture of GR2(SO42-) and GR1(CO32-) was obtained. These results demonstrated that the hydroxysulphate GR2 can originate from the microbial reduction of gamma-FeOOH and confirmed the preference for carbonate oversulphate during green rust precipitation. The solid phases were characterized by X-ray diffraction, transmission Mossbauer spectroscopy and scanning electron microscopy. Diffuse reflectance infrared Fourier transform spectroscopy confirmed the presence of intercalated carbonate and sulphate in green rust's structure. This study sheds light on the influence of dissimilatory iron-reducing bacteria on microbiologically influenced corrosion.