Iron(III) (oxyhydr)oxides are the major sink for PO(4)^(3-) in freshwater sediments, helping to control the onset of eutrophic conditions when PO(4)^(3-) is in excess; however, this sink is transient as ferrous iron and PO(4)^(3-) are typically released when sediments become anoxic, triggering a self-sustaining reversible cycle as sediments become an internal source of phosphate. Ferrous iron can then react with dissolved sulfides to form iron sulfide minerals such as mackinawite (FeS), whose...