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

Abstract

We examined the transformations of Fe and S associated with schwertmannite (Fe 8O 8(OH) 6SO 4) 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, SO 4 and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO 3). Interactions between schwertmannite and Fe II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). 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 SO 4-reduction. This lead firstly to the formation of elemental S via H 2S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H 2S precipitation with Fe II. Pyrite (FeS 2) 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.