Investigating the persistence of iron-bound phosphorus in lake sediment under sulfidic conditions

Abstract Iron (Fe) additions are widely used to suppress internal phosphorus (P) loading in eutrophic lakes by promoting the formation of vivianite (Fe(II) 3 (PO 4 ) 2 ·8H 2 O), a redox-stable Fe–P mineral that can sequester P in anoxic sediments over the long term. However, vivianite is unstable under sulfidic conditions, but the remobilization potential of P is unclear. In a 96-day mesocosm experiment, we examined how sulfate availability affects Fe-bound P persistence in a controlled model system of Fe-amended lake sediment. Sediment amended with Fe-P and enriched with vivianite during an anoxic pre-incubation was subsequently incubated under oxic overlying water at either low (<100 µmol L -1 ) or high (~1 mmol L -1 ) sulfate concentrations. Porewater profiles measured with microsensors and diffusive gradients in thin films (DGT), combined with solid-phase analyses by sequential extraction, scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD), revealed that vivianite persisted in sediment under oxic water conditions but was progressively destabilized as sulfide accumulated through microbial sulfate reduction. Sulfide drove coupled dissolution–reprecipitation reactions that transformed vivianite-bound Fe into amorphous Fe sulfides and pyrite. As a result, total sediment P-binding capacity declined. Although a portion of released P was re-adsorbed onto freshly precipitated Fe(III) phases in the oxic surface layer, this only partially compensated for the net loss of P-binding capacity, and total solid-phase P declined in both treatments. These findings highlight that P retention is linked to sulfide availability, and that monitoring porewater sulfide concentrations may serve as a practical early warning indicator of declining P retention capacity.