Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe8O8(OH)(6)SO4) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe-II, SO4 and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH similar to 6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO3). Interactions between schwertmannite and Fell at pH similar to 6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (alpha FeOOH). Thermodynamic calculations indicate that this Fe-II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO4-reduction. This lead firstly to the formation of elemental S via H2S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H2S precipitation with Felt. Pyrite (FeS2) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.