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

The interactions between Fe(II-III) hydroxysulphate GR(SO42-) and sulphate reducing bacteria (SRB) were studied. The considered SRB. Desulfovibrio desulfuricans subsp. aestuarii ATCC 29578, were added with GR(SO42-)to culture media. Different conditions were envisioned, corresponding to various concentrations of bacteria, various sources of sulphate (dissolved SO42- + GR(SO42-) or GR(SO42-) alone) and various atmospheres (N-2:H-2 or N-2:CO2:H-2). In the first part of the study, CO2 was deliberately omitted so as to avoid the formation of carbonated compounds, and GR(SO42-) was the only source of sulphate. Cell concentration increases from similar to 4 x 10(7) to similar to 7 x 10(8) 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(SO42-)/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(SO42-) into mackinawite, found in three physico-chemical states (nanocrystalline, crystalline stoichiometric FeS and Fe(III)-containing), and siderite FeCO3. (C) 2009 Elsevier Ltd. All rights reserved.