Effects of hyper-enriched reactive Fe on sulfidisation in a tidally inundated acid sulfate soil wetland

Solid phase Fe and S fractions were examined in an acid sulfate soil (ASS) wetland undergoing remediation via tidal inundation. Considerable diagenetic enrichment of reactive Fe(III) oxides (HCl- and dithionite-extractable) occurred near the soil surface (0?C0.05 m depth), where extremely large concentrations up to 3534 ??mol/g accounted for ~90% of the total Fe pool. This major source of reactive Fe exerts a substantial influence on S cycling and the formation, speciation and transformation of reduced inorganic S (RIS) in tidally inundated ASS. Under these geochemical conditions, acid volatile sulfide (AVS; up to 57 ??mol/g) and elemental sulfur (S⁰; up to 41 ??mol/g) were the dominant fractions of RIS in near surface soils. AVS?CS to pyrite?CS ratios exceeded 2.9 near the surface, indicating that abundant reactive Fe favoured the accumulation of AVS minerals and S⁰ over pyrite. This is supported by the significant correlation of poorly crystalline Fe with AVS?CS and S⁰?CS contents (r = 0.83 and r = 0.85, respectively, P < 0.01). XANES spectroscopy provided direct evidence for the presence of a greigite-like phase in AVS?CS measured by chemical extraction. While the abundant reactive Fe may limit the transformation of AVS minerals and S⁰ to pyrite during early diagenesis (~5 years), continued sulfidisation over longer time scales is likely to eventually lead to enhanced sequestration of S within pyrite (with a predicted 8% pyrite by mass). These findings provide an important understanding of sulfidisation processes occurring in reactive Fe-enriched, tidally inundated ASS landscapes.