Formation of Fe(III)-containing mackinawite from hydroxysulphate green rust by sulphate reducing bacteria

Abstract The interactions between Fe(II–III) hydroxysulphate GR( SO 4 2 - ) and sulphate reducing bacteria (SRB) were studied. The considered SRB, Desulfovibrio desulfuricans subsp. aestuarii ATCC 29578, were added with GR( SO 4 2 - ) to culture media. Different conditions were envisioned, corresponding to various concentrations of bacteria, various sources of sulphate (dissolved SO 4 2 - + GR( SO 4 2 - ) or GR( SO 4 2 - ) alone) and various atmospheres (N2:H2 or N2:CO2:H2). In the first part of the study, CO2 was deliberately omitted so as to avoid the formation of carbonated compounds, and GR( SO 4 2 - ) was the only source of sulphate. Cell concentration increases from ∼4 × 107 to ∼7 × 108 cells/mL in 2 weeks. The evolution with time of the iron compounds, monitored by Raman spectroscopy and X-ray diffraction, showed the progressive formation of a FeS compound, the Fe(III)-containing mackinawite. This result is consistent with the association GR( SO 4 2 - )/SRB/FeS observed in rust layers formed on steel in seawater. In the presence of CO2 and additional dissolved sulphate species, a rapid growth of the bacteria could be observed, leading to the total transformation of GR( SO 4 2 - ) into mackinawite, found in three physico-chemical states (nanocrystalline, crystalline stoichiometric FeS and Fe(III)-containing), and siderite FeCO3.