Binding and release of phosphate accompanying iron mineral transformations in aquatic sediments

This dissertation investigates the mechanisms and kinetics governing phosphate mobility in the iron-sulfide-phosphate (Fe-S(-II)-PO₄) ternary system, with implications for eutrophication management through iron-rich water treatment residuals (Fe-WTR). As phosphorus fertilizer demands continue to rise alongside escalating phosphate rock prices, developing sustainable phosphorus management approaches has become essential for balancing agricultural productivity with environmental protection. The first investigation characterized reduction kinetics of iron-rich water treatment residuals (Fe-WTR) from 27 Dutch drinking water treatment plants, revealing significant variability related to structural properties and production conditions. High pH and dissolved phosphorus during treatment favored formation of Fe precipitates with low structural order and high reactivity. Ferrihydrite-specific X-ray diffraction parameters and OH/Fe ratios emerged as potential indicators for predicting Fe-WTR reactivity, enabling tailored selection for environmental applications. Subsequently, phosphate release mechanisms during iron (oxyhydr)oxide (FeOx) sulfidation were examined using lepidocrocite (γ-FeOOH) as a model compound. Experiments in flow-through reactors revealed phosphate release proceeded faster than FeOx consumption, with the discrepancy more pronounced at higher pH. A counterintuitive finding emerged: slower sulfide oxidation rates enhanced phosphate mobilization through ligand-exchange mechanisms by maintaining higher dissolved sulfide concentrations, suggesting important implications for modeling benthic phosphate fluxes. The third study demonstrated that iron sulfide (FeS) can effectively supply iron for vivianite (Fe₃(PO₄)₂·8H₂O) formation over days at pH 6-8 when dissolved sulfide is removed. The transformation proceeds via FeS dissolution followed by vivianite precipitation, with FeS dissolution as the rate-limiting step. Competing FeS aging, more pronounced at higher pH but inhibited by phosphate adsorption, significantly influences vivianite formation rates and extent, indicating FeS can contribute to phosphate immobilization when sulfide concentrations decrease. The final investigation examined direct vivianite formation during FeOx sulfidation. Thermodynamic calculations confirmed vivianite formation feasibility at typical sediment porewater conditions, but experimental results revealed key kinetic constraints: rapid FeS precipitation limiting dissolved Fe(II) availability; slow FeOx-sulfide reactions at low sulfide concentrations; insufficient solid-bound Fe(II) release rates in flow-through conditions; and high supersaturation requirements for efficient vivianite nucleation. Nevertheless, the study demonstrated that vivianite precipitation can occur when excess reactive FeOx is present, suggesting a previously unconsidered pathway for phosphate sequestration in FeOx-rich sediments experiencing transient sulfide production. Collectively, these findings advance understanding of the complex dynamics within the Fe-S(-II)-PO₄ system, revealing how the interplay between thermodynamic feasibility and kinetic constraints governs phosphate mobility and immobilization in aquatic environments. The elucidated mechanisms provide a scientific foundation for optimizing Fe-WTR applications in lake restoration projects and developing sustainable phosphorus management strategies that align with circular economy principles.