THE PYRITIZATION OF BASIC FERRIC SULFATES IN ACID SULFATE SOILS: A MICROBIOLOGICAL INTERPRETATION

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Abstract
The aim of this investigation was to suggest a mechanism whereby the basic ferric sulfates, which occur in acid sulfate soils as a result of the microbial oxidation of pyrite in the original sedimentary parent material, can be microbiologically transformed back to pyrite when the soils are flooded. Three basic ferric sulfates were tested and it was found that in the presence of lactate and Desulfovibrio desulfuricans, 10 g of each mineral were reduced within 12 wk to mackinawite (FeS). Additional incubation, to a total of 33 wk, resulted in no further sulfidation. However, in the presence of elemental sulfur, the sulfidation process resumed and mackinawite was soon transformed into greigite (Fe3S4) and then, following an aging process of increased temperature and pressure, pyrite (FeS2) was produced. Under simulated flooding conditions of sea water and decomposing seaweed, the sulfates were converted to a black X-ray amorphous sulfide which is known to change to mackinawite and/or greigite. The ability of H2S-oxidizing bacteria to form elemental sulfur in sedimentary pyrite-forming systems is reviewed and interpreted as a factor in producing the necessary sulfur. Basic aluminum sulfates were stable to microbial reduction. The X-ray pattern for ammoniojarosite was refined.