Evaluating phosphate recovery as vivianite from lake sediments

Vivianite (Fe₃(PO₄)₂·8H₂O) plays a significant role in phosphorus sequestration within anaerobic lake sediments and has recently gained interest as a recoverable phosphate resource from wastewater due to its paramagnetic properties. This study examines vivianite formation and detectability in the context of dredging-based remediation of the eutrophic Lake Ormstrup (Denmark), focusing on whether iron(II) dosing to dredged lake sediment can enhance vivianite production for magnetic recovery. Vivianite was detected only when iron and phosphate were added simultaneously, indicating that phosphate availability, not iron supply, is the key constraint in dredged sediments. Furthermore, low native vivianite concentrations prevented reliable detection using Mössbauer spectroscopy. To improve vivianite quantification, alternative Mössbauer fitting approaches and bipyridine extractions were compared. However, iron-spiked samples showed no measurable increase in Fe(II)-P relative to controls, highlighting persistent analytical challenges. These findings show limited potential for vivianite-based recovery from dredged sediment in Lake Ormstrup. Nevertheless, the results also provide new insights into carbonates and organics likely outcompeting phosphate for iron binding, outline methodological improvements needed to detect vivianite in low-concentration environments, and highlight differences in Fe/P/S ratios in lake sediments compared to those in digested sludge.