Sulfide Slowly Unlocks the Iron Cage That Traps Phosphorus in Lake Sediments

Every summer, in lakes around the world, an invisible chemical switch flips. Deep in the oxygen-starved sediment at the bottom of nutrient-rich waters, phosphorus that managers believed was safely locked away begins to seep back into the water column, feeding algal blooms that choke shorelines, kill fish and drive up the cost of drinking water treatment. For decades, lake restorers have fought this internal phosphorus loading by dosing lakes with iron, betting that the added metal will grab dissolved phosphorus and bury it in a stable mineral form. A new study now shows that this bet can quietly fail, and that the saboteur is a gas with the unmistakable smell of rotten eggs.

Researchers led by Harm van Kuppevelt of the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, working with colleagues at Brandenburg University of Technology, the University of Southern Denmark and Aarhus University, set out to test how long iron-bound phosphorus actually survives in sediment when sulfide is present. Their findings, published in the journal Biogeochemistry, reveal a slow chemical heist in which sulfide strips iron away from phosphorus, converting a durable mineral vault into a leaky one. The work matters because it identifies a measurable early warning signal, porewater sulfide concentration, that could tell lake managers their iron treatment is losing its grip before blooms return.

The mineral at the heart of the story is vivianite, an iron phosphate with the chemical formula Fe(II)3(PO4)2·8H2O. When iron is added to eutrophic lake sediment under anoxic conditions, dissolved phosphorus can precipitate with ferrous iron to form this pale blue-green mineral. Vivianite is prized by restoration ecologists because it is redox-stable: unlike the loosely adsorbed phosphorus that clings to iron oxides and dissolves the moment oxygen disappears, vivianite holds its phosphorus even in oxygen-free sediment. In theory, once phosphorus is locked into vivianite, it should stay buried for years, breaking the feedback loop that keeps eutrophic lakes green long after external nutrient inputs have been reduced.

Theory, however, meets a complication in sulfate-rich waters. Many lakes affected by salinization, seawater intrusion, agricultural runoff or drought-induced water level changes carry elevated sulfate concentrations. In anoxic sediment, microbes respire sulfate instead of oxygen, producing sulfide as a byproduct. Sulfide is a ferocious chemical competitor for iron, binding it into iron sulfide minerals such as amorphous FeS and the far more stable pyrite. If sulfide outcompetes phosphate for the iron in vivianite, the mineral should dissolve, releasing its phosphorus into the porewater and, potentially, back into the overlying lake. Whether and how fast this happens in realistic sediment conditions was, until now, poorly quantified.

To find out, the team built a controlled model system in the laboratory: a 96-day mesocosm experiment using lake sediment that had been amended with iron and phosphorus and deliberately enriched with vivianite during an anoxic pre-incubation period. This ensured that the sediment started with a substantial pool of iron-bound phosphorus in the very mineral form that iron treatments are meant to create. The sediment was then incubated under oxic overlying water at two sulfate levels, one low, below 100 micromoles per liter, and one high, around one millimole per liter, mimicking the range found in freshwater systems under different degrees of sulfate influence.

The experimental design was deliberately multi-pronged, because no single technique can capture the full picture of what happens to iron and phosphorus in sediment. The researchers measured porewater profiles with microsensors and with diffusive gradients in thin films, known as DGT, a technique that samples dissolved solutes at high spatial resolution in the sediment’s microscopic pore spaces. They complemented these measurements with sequential chemical extraction of the solid phase, which separates phosphorus into operationally defined pools of decreasing reactivity, and with scanning electron microscopy coupled to energy dispersive spectroscopy, which reveals the elemental composition of individual mineral grains. X-ray diffraction completed the toolkit by identifying crystalline mineral phases.

The results told a clear and sobering story. Under oxic overlying water, vivianite persisted in the sediment, confirming its reputation as a robust phosphorus sink when sulfide is scarce. But as sulfate-reducing microbes accumulated sulfide in the sediment, the mineral was progressively destabilized. The sulfide drove coupled dissolution-reprecipitation reactions: vivianite dissolved, its ferrous iron was captured by sulfide, and new amorphous iron sulfides formed, some of which matured into pyrite. The phosphorus released in the process did not simply vanish. A portion of it was re-adsorbed onto freshly precipitated iron(III) phases in the oxygenated surface layer, where oxic conditions allowed iron oxides to form and grab dissolved phosphate. Yet this rescue operation was only partial. The net effect in both treatments was a decline in the sediment’s total phosphorus-binding capacity and a measurable loss of total solid-phase phosphorus.

The chemistry behind this transformation is worth appreciating in detail, because it illustrates why sulfidic conditions are so corrosive to iron-based phosphorus retention. Vivianite owes its stability to the strong bonds between ferrous iron and phosphate within its crystal lattice. Sulfide attacks this stability on two fronts. First, dissolved sulfide is a stronger ligand for ferrous iron than phosphate under the relevant conditions, so it thermodynamically favors the formation of iron sulfides. Second, once iron sulfides such as pyrite form, they are kinetically inert, meaning the iron is effectively removed from the phosphorus cycle for good. The dissolution-reprecipitation sequence observed in the mesocosms, in which vivianite-bound iron was converted into amorphous FeS and ultimately pyrite, therefore represents a one-way ratchet: each sulfide molecule that captures an iron atom permanently reduces the sediment’s capacity to hold phosphorus in a redox-stable mineral form.

Importantly, the study also shows that the fate of released phosphorus depends on the redox structure of the sediment. The oxic surface layer acted as a partial safety net, because oxygen diffusing down from the overlying water allowed fresh iron(III) oxides to precipitate and re-bind some of the liberated phosphate. This finding suggests that lakes with a well-oxygenated sediment-water interface may temporarily buffer the phosphorus release triggered by sulfide-driven vivianite dissolution. But the buffer is finite, and the underlying loss of iron-binding capacity continues as long as sulfate reduction proceeds. In lakes that stratify in summer and develop anoxic bottom waters, that safety net disappears precisely when internal loading pressures are highest, raising the risk that sulfide-driven phosphorus release coincides with the season of maximum algal growth.

For lake managers, the practical message is that iron treatments should not be treated as permanent fixes, particularly in systems with rising sulfate loads. The authors highlight that monitoring porewater sulfide concentrations could serve as a practical early warning indicator of declining phosphorus retention capacity. Sulfide is relatively straightforward to measure with microsensors, colorimetric methods or peeper samplers, and rising sulfide levels in sediment porewater would signal that vivianite and other iron-bound phosphorus pools are under chemical attack. Such monitoring could inform decisions about whether repeated iron dosing is needed, whether sulfate inputs from salinization or pollution should be controlled, and whether the expected longevity of a restoration investment needs to be revised downward.

The research also carries a broader environmental warning. Sulfate concentrations in inland waters are increasing worldwide due to seawater intrusion into coastal aquifers, road salt application, mining discharge, acid sulfate soil runoff and reduced dilution during droughts. Each increment of sulfate is a potential increment of sulfide, and each increment of sulfide erodes the iron-phosphorus chemistry on which many restoration strategies depend. The study’s controlled mesocosm results now provide a mechanistic chain of evidence connecting sulfate availability, microbial sulfate reduction, sulfide accumulation, vivianite dissolution, iron sulfide formation and net phosphorus loss from sediment. As climate change and land use intensification push more sulfate into lakes, the fragile mineral vault that keeps phosphorus buried may be opening in far more places than managers currently realize, and the smell of rotten eggs rising from a lake bed may be the first clue.

Subject of Research: The persistence and sulfide-driven destabilization of iron-bound phosphorus, particularly vivianite, in lake sediment under oxic and sulfidic conditions.

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

Article References: van Kuppevelt, H., Hupfer, M., Reitzel, K., Sudo, M. L., & Marzocchi, U. (2026). Investigating the persistence of iron-bound phosphorus in lake sediment under sulfidic conditions. Biogeochemistry, 169(5), Article 55. https://doi.org/10.1007/s10533-026-01375-3

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01375-3

Keywords: vivianite, iron-bound phosphorus, internal phosphorus loading, lake restoration, sulfate reduction, sulfide, phosphorus mobilization, eutrophic lakes, sediment biogeochemistry, pyrite, porewater, mesocosm experiment