The Formation and Transformation of Iron Monosulfides Under Varying Biogeochemical Conditions

Abstract:

Iron monosulfides (FeS) are a precursor to pyrite formation, potential acid sulfate soil materials, and trace metal scavengers. While insoluble, black-colored FeS concentrations form and remain stable under anaerobic conditions, they disappear upon oxidation and consequentially lower soil pH. Although it is understood that the formation of FeS requires the presence of S, Fe, organic C, sulfate reducing bacteria (SRB), and strongly reducing soil conditions, the exact rate of FeS formation and transformation still remains unclear. Our objective was to determine the conditions necessary for the formation of FeS in soils and how the soil chemistry could be impacted by drought-like conditions and resultant oxidation of FeS. To do so, we constructed a 120-day wet-incubation experiment which consisted of 80 mesocosms containing soils with four ratios of Fe to S (Fe>S, Fe=S, Fe<S, control), across five treatment types (C, SRB, C and SRB, autoclaved soil plus C and SRB, only nanopure water). The percentage of FeS formed in the soil morphology and detection of H₂S was assessed once weekly, while reducing soil conditions were assessed at 2, 4, and 6 weeks via IRIS films. After the wet incubation period was complete, soils were re-aerated for two weeks to assess the percent of FeS change in the soil morphology and the potential acidity produced. Preliminary findings show that FeS concentrations form in as short as 2-weeks and the maximum % FeS in the soil morphology is as high as 90% by week 5. There is a direct relationship between FeS formation and H₂S production, particularly in soils with Fe>S and Fe=S, and soil treatments containing C. Results from this study will improve the understanding of FeS formation and transformation under dynamic redox conditions. These results are valuable to soil and wetland managers for the improvement of wetland identification and function, and to inform the management of potential acid sulfate soil materials under wet-dry cycles.