Iron sulfide mineral found to trap phosphorus in oxygen-poor lake sediments

A study from Concordia University has identified a chemical process that helps keep phosphorus locked in lake sediments even when oxygen levels drop. Published in Scientific Reports, the research shows that mackinawite, an iron sulfide mineral that forms in oxygen-depleted sediment, can bind phosphate under conditions where other phosphorus-trapping minerals become unstable.

Phosphorus is an essential nutrient for aquatic life, but excess amounts from fertilizer runoff, wastewater and shoreline development drive eutrophication and harmful algal blooms. Lake sediments normally store phosphorus using iron-rich minerals that require oxygen to remain stable. When sediments become anoxic, those minerals dissolve and release phosphorus back into the water column, prolonging water-quality problems long after external pollution is reduced.

Lead author Milad Ezzati, a Ph.D. candidate in the Department of Chemistry and Biochemistry, said sediments play a central role in controlling overlying water quality. The team tested whether mackinawite, which is common in organic-rich lake sediments, could continue to trap phosphorus when traditional iron minerals fail. Their experiments confirmed that mackinawite acts as an additional phosphorus sink under anoxic conditions.

Until now, researchers recognized two main mechanisms for permanent phosphorus storage in oxygen-poor sediments: burial of organic matter and formation of the iron-phosphate mineral vivianite. The study adds mackinawite as a third pathway, suggesting phosphorus cycling in lakes is more complex than previously understood.

The researchers also found that natural organic matter competes with phosphate for binding sites on the mackinawite surface, reducing the mineral's retention capacity. This competition means the effectiveness of the new pathway will vary with sediment composition and environmental conditions.

Ezzati said the discovery could improve predictions of how quickly eutrophic lakes recover once phosphorus inputs are cut. Current models assume sediment phosphorus continues cycling back into the water for years; the mackinawite pathway may remove a portion of that phosphorus from the active cycle, potentially accelerating recovery.

The team plans to investigate how different forms of natural organic matter interact with mackinawite and to quantify the importance of this sink relative to other pathways at the whole-lake scale. Ezzati noted that many chemical processes in lake sediments remain poorly understood, and clarifying them will help diagnose current conditions and forecast future ecosystem health.