In Situ Vivianite Formation in Intertidal Sediments: Ferrihydrite-Adsorbed P Triggers Vivianite Formation

Coastal sediments are a key contributor to oceanic phosphorus (P) removal, impacting P bioavailability and primary productivity. Vivianite, an Fe(II)-phosphate mineral, can be a major P sink in nonsulfidic, reducing coastal sediments. Despite its importance, vivianite formation processes in sediments remain poorly understood. Here, we applied a novel approach to detect and quantify in situ vivianite formation in three intertidal flats. We conducted 7-week long incubations of mesh-bags filled with sediments mixed with (1) ⁵⁷Fe-ferrihydrite, (2) ⁵⁷Fe-ferrihydrite with adsorbed phosphate, and (3) ⁵⁷Fe-ferrihydrite with adsorbed phosphate and some vivianite (natural Fe isotope abundance), which could serve as crystal growth sites. Synthesizing the ferrihydrite from ⁵⁷Fe (96.1%) enabled us to detect transformation products using ⁵⁷Fe-Mössbauer spectroscopy. Vivianite formed only in treatments containing adsorbed phosphate and only at the two sites where vivianite formation was thermodynamically feasible based on porewater chemistry. These results demonstrate vivianite formation within weeks when locally favorable Fe:P ratios exist. Although vivianite comprised a minor fraction of Fe (up to 15%), it represented a significant P pool (up to 72%), emphasizing its role in coastal P burial. Additionally, our results may apply to other environmental systems like limnic sediments.