IMIS

Iron (Fe) cycling exerts strong control on the mobility and bioavailability of the key nutrient phosphate (PO₄) in soils and sediments. Coprecipitation of PO₄ is known to alter the structure of Fe (oxyhydr)oxides (FeOx), however the environmental fate of PO₄-bearing FeOx is not well-understood. Here, PO₄ -bearing FeOx with 9 mol% coprecipitated PO₄ were prepared byFe(III) hydrolysis and Fe(II) oxidation in the presence of dissolved PO ₄, in addition to pure FeOx synthesized in PO₄-free solutions. The pure and PO₄-bearing FeOx were subsequently exposed to different concentrations of dissolved Fe(II) and sulfide (2 and 10 mmol L⁻¹). Mineral transformations and the fate of PO ₄ were tracked over 7–14 days with wet chemical techniques (including sequential Fe and S extraction) and synchrotron-based Fe K-edge X-ray absorption spectroscopy. Coprecipitation of PO₄ affected the rate and extent of FeOx transformation differently for Fe(II) and sulfide. Poorly-ordered PO₄-bearing FeOx was preserved in the presence of dissolved Fe(II) while pure ferrihydrite was nearly completely transformed into goethite over 7 days. By contrast, coprecipitation of PO₄ rendered FeOx more reactive towards sulfide compared to pure FeOx. Reaction with dissolved sulfide resulted in the formation of non-sulfidized Fe(II) or Fe(II) sulfide under high and low Fe/sulfide ratio, respectively. Under low Fe/sulfide ratio, Fh and PO₄-bearing, poorly-ordered FeOx were nearly completely sulfidized after 14 days. Sulfidation of FeOx led to efficient release of PO₄ into solution, and at low Fe/sulfide ratio more PO₄ was released than expected based on the extent of Fe sulfidation. The results suggest feedback mechanisms of environmental relevance: coprecipitation of strongly-sorbing species such as PO₄ disrupts FeOx structure, which affects FeOx reactivity and the overall nutrient or contaminant retention capacity of soils or sediments differently depending on the ambient redox conditions . Specifically, the switch from reducing to sulfidic conditions may be associated with the disproportionate release of nutrients and contaminants.