Iron sulfide oxidation in drained coastal lowland soils liberates acidity, Fe and SO4, and leads to the accumulation of secondary Fe(III) minerals, such as schwertmannite (Fe8O8(OH)6SO4). Here we describe new insights on Fe-S biomineralisation and the associated mobility of As following the re-establishment of reducing conditions in reflooded acid sulfate wetlands. Our studies include controlled laboratorybased experiments on model systems as well as field-based observations on acid sulfate wetlands [1–-3].
When subjected to prolonged soil waterlogging as a result of wetland re-establishment, the bacterial reduction of schwertmannite-derived Fe(III) releases abundant Fe2+ (with up to 30 mM in affected groundwater) This process consumes acidity and generates alkalinity thereby driving increases in pH. At near-neutral pH, the presence of mM concentrations of Fe2+ catalyses the very rapid and complete replacement of schwertmannite by goethite [1]. This replacement triggers a shift from a dominance of bacterial Fe(III)- reduction to a dominance of SO4-reduction. This shift can be explained by a partial equilibrium model of the thermodynamic favourability of Fe(III)- versus SO4-reduction. The onset of SO4-reduction leads to the accumulation of iron sulfide minerals, predominantly in the form of 5 – 30 nm nanoparticles of mackinawite (tetragonal FeS).
Schwertmannite in the initially drained acid-sulfate soil has a large capacity to sorb As(V), through exchange with structural SO4. Field-based observations on soil/porewater partitioning and laboratory-based sorption experiments demonstrate that several processes combine to cause significant arsenic desorption and consequent increases in pore-water As concentrations. These desorption processes are driven by increases in the porewater pH, bacterial reduction of As(V) to As(III) and replacement of schwertmannite by goethite via the Fe2+-catalysed pathway.